**Part 6**

**Geoscience, Mineralogy** 

768 Scanning Electron Microscopy

Yu, L.H.; Fu, X.H. (2006). The Influences of Annealed Temperatures on The Properties and

Zannetti, R.; Celotti, G.; Fichera A.; & Francesconi, R. (1969). The Structural Effects of

Zheng, J.P.; Kwok, H.S. (1993). Preparation of indium tin oxide films at room temperature

ISBN 1882-0778

2006), pp. 16-19. ISBN 1005-1198

pp.137-142. ISBN 0003-3146

pp.99-104. ISBN 0040-6090

Annealing, *Applied Physics Letter*, Vol. 73, No.11, (September 1998). pp.1514-1517.

Microstructure of Ba0.5Sr0.5TiO3 Thin Film, *Advanced Ceramics*, Vol. 27, No. 3, (March

Annealing Time and Temperature on the Paracrystal-Crystal Transition in Isotactic Polypropylene, Die Makromolekulare Chemie, Vol. 128, No.1, (October 1969).

by pulsed laser deposition, *Thin Solid films*, Vol. 232, No. 1, (September 1993).

**38** 

*USA* 

**Microstructural and Mineralogical** 

+ SiO2 (soluble clay silica) → calcium-silicate-hydrate (2)

2- + 26H2O → Ca6[Al(OH)6]2.(SO4)3.26H2O (4)

+Al2O3 (soluble clay alumina) → calcium-aluminate-hydrate (3)

**Calcium-Based Stabilizers** 

Pranshoo Solanki1 and Musharraf Zaman2

*1Illinois State University 2University of Oklahoma* 

(1)

**Characterization of Clay Stabilized Using** 

The properties of clays can be significantly improved by treating with calcium-based stabilizers (or additives) such as hydrated lime (or lime), Portland cement, cement kiln dust (CKD), and class C fly ash (CFA). In the presence of water, the calcium ions released from these stabilizers reduce the thickness of double diffused layer through cation-exchange and flocculation-agglomeration reactions. This is primarily responsible for improvement in workability through reduction of adsorbed water and decrease in plasticity index. In longterm, pozzolanic reactions occur between the calcium ions of the stabilizer and the silica and alumina of the clay minerals resulting in the formation of cementitious products such as calcium-silicate-hydrates (C-S-H), calcium-aluminate-hydrates (C-A-H), and calcium-

The hyphens indicate that the composition is indefinite. The specific composition is defined by pH level, solubility of silica and alumina, clay mineralogy, and curing conditions among other reasons. C-S-H, formed by the hydration of C3S, is also commonly known as tobermorite gel. The tobermorite gel is poorly crystalline, with only a few broad, weak bands in its X-ray diffraction pattern (Mohamed, 2002). C-A-H is formed as platelets with hexagonal symmetry. The morphology of both C-A-H and C-A-S-H resembles that of tobermorite gel. In the presence of calcium sulfate, an additional product of hydration, known as ettringite (Ca6[Al(OH)6]2.(SO4)3.26H2O) is formed (Eq. 4). Ettringite mineral

The formation of aforementioned cementitious products in the soil-stabilizer matrix are responsible for increase in the internal friction and shear strength of the stabilized soil.

aluminum-silicate-hydrates (C-A-S-H). The reaction may be written as:

Ca(OH)2 (ionization of lime) → Ca2+ + 2(OH)-

consists of a prismatic crystal and a hexagonal cross section.

6Ca2+ + 2Al(OH)4- +4OH- + 3SO4

**1. Introduction** 

Ca2+ + OH-

Ca2+ + OH-

## **Microstructural and Mineralogical Characterization of Clay Stabilized Using Calcium-Based Stabilizers**

Pranshoo Solanki1 and Musharraf Zaman2 *1Illinois State University 2University of Oklahoma USA* 

### **1. Introduction**

The properties of clays can be significantly improved by treating with calcium-based stabilizers (or additives) such as hydrated lime (or lime), Portland cement, cement kiln dust (CKD), and class C fly ash (CFA). In the presence of water, the calcium ions released from these stabilizers reduce the thickness of double diffused layer through cation-exchange and flocculation-agglomeration reactions. This is primarily responsible for improvement in workability through reduction of adsorbed water and decrease in plasticity index. In longterm, pozzolanic reactions occur between the calcium ions of the stabilizer and the silica and alumina of the clay minerals resulting in the formation of cementitious products such as calcium-silicate-hydrates (C-S-H), calcium-aluminate-hydrates (C-A-H), and calciumaluminum-silicate-hydrates (C-A-S-H). The reaction may be written as:

$$\text{Ca(OH)}\_{2} \text{ (ionization of lime)} \rightarrow \text{Ca}^{2+} + 2 \text{(OH)}\text{-}\tag{1}$$

$$\text{Ca}^{2+} + \text{OH}^{\cdot} + \text{SiO}\_{2} \text{ (soluble clay silica)} \rightarrow \text{calcium-silicate-hydrate} \tag{2}$$

$$\text{Ca}^{2+} + \text{OH} \cdot + \text{Al}\_2\text{O}\_3 \text{ (solubble clay alumina)} \rightarrow \text{calcium-aluminate-hydrate} \tag{3}$$

The hyphens indicate that the composition is indefinite. The specific composition is defined by pH level, solubility of silica and alumina, clay mineralogy, and curing conditions among other reasons. C-S-H, formed by the hydration of C3S, is also commonly known as tobermorite gel. The tobermorite gel is poorly crystalline, with only a few broad, weak bands in its X-ray diffraction pattern (Mohamed, 2002). C-A-H is formed as platelets with hexagonal symmetry. The morphology of both C-A-H and C-A-S-H resembles that of tobermorite gel. In the presence of calcium sulfate, an additional product of hydration, known as ettringite (Ca6[Al(OH)6]2.(SO4)3.26H2O) is formed (Eq. 4). Ettringite mineral consists of a prismatic crystal and a hexagonal cross section.

$$\text{\#Ca^{2+}} + 2\text{Al(OH)} \cdot \text{\textdegree +4OH} \cdot \text{\textdegree +3SO} \cdot \text{\textdegree +26H} \cdot \text{\textdegree \to Ca} \cdot \text{[Al(OH)} \cdot \text{\textdegree (SO4)} \cdot \text{\textdegree 26H} \cdot \text{\textdegree O} \tag{4}$$

The formation of aforementioned cementitious products in the soil-stabilizer matrix are responsible for increase in the internal friction and shear strength of the stabilized soil.

Microstructural and Mineralogical Characterization

stabilized soils.

of Clay Stabilized Using Calcium-Based Stabilizers 773

and ground granulated blast furnace slag (GGBFS). The highest swell potential of untreated soil was explained by the presence of the highest percentage of sodium smectite clay mineral along with palygorskite and illite in soil. The fabric of the untreated soil was found composed of dense clay matrices with no appearance of aggregations and increasing amount of pore spaces. However, stabilization resulted in the formation of aggregations and few connectors. It was found that higher amount of sodium ions and lower amount of calcium ions promotes swelling and vice versa. Further, the XRD results showed a general reduction in all the clay minerals' peak intensities particularly in the case of CBPD treated samples. This study addressed most of the properties that will be evaluated in the present study. However, it was carried out on predominantly silty soil stabilized with nontraditional additives. It is also important to note that the mineralogical and textural characteristics of fat clay used in the present study are different than silty soil. Also, nontraditional additives are not commonly available and are expensive. This makes it necessary

Stutzman (2004) studied the bulk and surface phase composition of hydraulic cement using SEM in conjunction with XRD. Direct imaging of hydraulic cements obtained through SEM yielded complete picture of both bulk and surface phase compositions. Mass percent and volume percent were calculated by analyzing resulting composite image from SEM. A good agreement was found between mass percentages obtained by SEM imaging and percentages based upon quantitative XRD. The finer grained phases (e.g., gypsum, tricalcium aluminate, and ferrite) showed much higher surface areas per unit mass than the coarser-grained phases such as alite and belite. But no attempt was made to compare the microstructures of

In another laboratory study, Kolias et al. (2005) investigated the effectiveness of using high calcium fly ash and cement in stabilizing lean and fat clays. Strength tests in compression, indirect tension, and flexure modes were conducted on stabilized samples. Additionally, thermogravimetric-single differential thermal analysis (TG-SDTA) and XRD tests were conducted on selected samples to study the hydraulic compounds. The study showed the potential benefit of stabilizing clays with high calcium fly ash. However, it was found that the effectiveness of stabilization is dependent on the type of soil, the amount of stabilizer and the curing time. The study of formation of hydraulic products showed that a significant amount of tobermorite gel is formed due to stabilization leading to a denser and more stable structure of the samples. The mechanical properties such as strength and modulus of elasticity showed considerable enhancement due to stabilization. This study, however, did

Horpibulsuk et al. (2010) analyzed the strength development in cement-stabilized silty clay based on microstructural considerations. A qualitative and quantitative study was conducted on the microstructure using a SEM, mercury intrusion pore size distribution measurements, and thermal gravity analysis. A total of three zones of improvement namely, active, inert, and deterioration zones, were observed. The active zone was found to be most effective for stabilization where the cementitious products increased with cement content and filled the pore space. In the inert zone, both pore size distribution and cementitious products change insignificantly with increasing cement. In deterioration zone, the water is not adequate for hydration because of the excess of cement input. It was found that in short stabilization period, the volume of large pores (> 0.1 µm) increases because of input of

to investigate locally available additives such as lime, fly ash, and CKD.

not examine the microstructure of lime- and CKD-stabilized soils.

However, the efficacy of stabilization depends on the soil mineralogy, type and amount of stabilizer, and curing conditions (e.g., time, temperature, moisture). Although several researchers studied the improvement in engineering properties of stabilized soil at macro level (e.g, shear strength, unconfined compressive strength, swell behavior), very few studies discussed the changes in soil-stabilizer matrix at micro level. Consequently, the primary objective of this study is to examine the changes in microstructure and mineralogy of soil due to stabilization with calcium-based stabilizer. Additionally, at macro level the modulus of elasticity values are evaluated for both raw and stabilized specimens and correlated with microstructural and mineralogical characteristics.

## **2. Literature review**

Several investigations were carried out to study the changes in the microstructural and mineralogical characteristics of stabilized soils. Rajasekaran et al. (1995) studied the influence of sodium hydroxide on the fabric of lime treated marine clays using scanning electron microscopy (SEM) technique. It was found that lime stabilization is very effective for marine clays. Adding sodium hydroxide additive resulted in better formation of pozzolanic compounds. Formation of cementitious compounds such as C-A-H and C-S-H due to the soillime reactions were observed in all lime treated soils which were further confirmed by using X-Ray Diffraction (XRD) technique. The study conducted by using SEM indicated that there is an overall improvement in the structure of the soil system resulting in a porous system and aggregate formation. However, this study was limited to only lime-stabilization.

In another study, Lav and Lav (2000) investigated the effects of cement- and limestabilization on class F fly ash in terms of change in chemical composition, crystalline structures, and hydration products. The unconfined compressive strength (UCS) of samples was also evaluated over time to observe the effect of stabilization. Cement- and limestabilized fly ash produced similar hydration products. None of these produced was recognizable by XRD except weak calcium hydroxide (CH) and calcium carbonate due to carbonation. The improvement in microstructure was found initiating from fly ash particles serving as nucleation centers for hydration or pozzolanic reaction products. The increasing strength gain was attributed to growth of hydrates in the voids between the particles. However, no attempt was made to study the influence of aforementioned additives on soil.

Ghosh and Subbarao (2001) studied the physicochemical and microstructural developments of fly ash-lime- and fly ash-lime-gypsum-stabilized materials. Different analytical techniques namely, XRD, differential thermal analysis, SEM, and Energy Dispersive Spectroscopy (EDS) were used for studying the microstructure. The SEM micrographs revealed evidence of the development of a compact matrix after three months of curing time and a densified compact network of pozzolanic reaction products of fly ash-lime-gypsum with the increase in the curing period to ten months. The XRD analysis results indicated appearance of new peaks of low intensity in the modified fly ash specimens. Some of these peaks were not recognized as part of any new crystalline phases. Similar to previous study, this study was limited to fly ash-lime mixtures and no attempt was made to study the influence of these additives on soil.

In a laboratory study, Al-Rawas (2002) investigated the microfabric and mineralogical aspects of the expansive soil using cement by-pass dust (CBPD), copper slag, slag-cement,

However, the efficacy of stabilization depends on the soil mineralogy, type and amount of stabilizer, and curing conditions (e.g., time, temperature, moisture). Although several researchers studied the improvement in engineering properties of stabilized soil at macro level (e.g, shear strength, unconfined compressive strength, swell behavior), very few studies discussed the changes in soil-stabilizer matrix at micro level. Consequently, the primary objective of this study is to examine the changes in microstructure and mineralogy of soil due to stabilization with calcium-based stabilizer. Additionally, at macro level the modulus of elasticity values are evaluated for both raw and stabilized specimens and

Several investigations were carried out to study the changes in the microstructural and mineralogical characteristics of stabilized soils. Rajasekaran et al. (1995) studied the influence of sodium hydroxide on the fabric of lime treated marine clays using scanning electron microscopy (SEM) technique. It was found that lime stabilization is very effective for marine clays. Adding sodium hydroxide additive resulted in better formation of pozzolanic compounds. Formation of cementitious compounds such as C-A-H and C-S-H due to the soillime reactions were observed in all lime treated soils which were further confirmed by using X-Ray Diffraction (XRD) technique. The study conducted by using SEM indicated that there is an overall improvement in the structure of the soil system resulting in a porous system and

In another study, Lav and Lav (2000) investigated the effects of cement- and limestabilization on class F fly ash in terms of change in chemical composition, crystalline structures, and hydration products. The unconfined compressive strength (UCS) of samples was also evaluated over time to observe the effect of stabilization. Cement- and limestabilized fly ash produced similar hydration products. None of these produced was recognizable by XRD except weak calcium hydroxide (CH) and calcium carbonate due to carbonation. The improvement in microstructure was found initiating from fly ash particles serving as nucleation centers for hydration or pozzolanic reaction products. The increasing strength gain was attributed to growth of hydrates in the voids between the particles. However, no attempt was made to study the influence of aforementioned additives on soil. Ghosh and Subbarao (2001) studied the physicochemical and microstructural developments of fly ash-lime- and fly ash-lime-gypsum-stabilized materials. Different analytical techniques namely, XRD, differential thermal analysis, SEM, and Energy Dispersive Spectroscopy (EDS) were used for studying the microstructure. The SEM micrographs revealed evidence of the development of a compact matrix after three months of curing time and a densified compact network of pozzolanic reaction products of fly ash-lime-gypsum with the increase in the curing period to ten months. The XRD analysis results indicated appearance of new peaks of low intensity in the modified fly ash specimens. Some of these peaks were not recognized as part of any new crystalline phases. Similar to previous study, this study was limited to fly ash-lime mixtures and no attempt was made to study the

In a laboratory study, Al-Rawas (2002) investigated the microfabric and mineralogical aspects of the expansive soil using cement by-pass dust (CBPD), copper slag, slag-cement,

aggregate formation. However, this study was limited to only lime-stabilization.

correlated with microstructural and mineralogical characteristics.

**2. Literature review** 

influence of these additives on soil.

and ground granulated blast furnace slag (GGBFS). The highest swell potential of untreated soil was explained by the presence of the highest percentage of sodium smectite clay mineral along with palygorskite and illite in soil. The fabric of the untreated soil was found composed of dense clay matrices with no appearance of aggregations and increasing amount of pore spaces. However, stabilization resulted in the formation of aggregations and few connectors. It was found that higher amount of sodium ions and lower amount of calcium ions promotes swelling and vice versa. Further, the XRD results showed a general reduction in all the clay minerals' peak intensities particularly in the case of CBPD treated samples. This study addressed most of the properties that will be evaluated in the present study. However, it was carried out on predominantly silty soil stabilized with nontraditional additives. It is also important to note that the mineralogical and textural characteristics of fat clay used in the present study are different than silty soil. Also, nontraditional additives are not commonly available and are expensive. This makes it necessary to investigate locally available additives such as lime, fly ash, and CKD.

Stutzman (2004) studied the bulk and surface phase composition of hydraulic cement using SEM in conjunction with XRD. Direct imaging of hydraulic cements obtained through SEM yielded complete picture of both bulk and surface phase compositions. Mass percent and volume percent were calculated by analyzing resulting composite image from SEM. A good agreement was found between mass percentages obtained by SEM imaging and percentages based upon quantitative XRD. The finer grained phases (e.g., gypsum, tricalcium aluminate, and ferrite) showed much higher surface areas per unit mass than the coarser-grained phases such as alite and belite. But no attempt was made to compare the microstructures of stabilized soils.

In another laboratory study, Kolias et al. (2005) investigated the effectiveness of using high calcium fly ash and cement in stabilizing lean and fat clays. Strength tests in compression, indirect tension, and flexure modes were conducted on stabilized samples. Additionally, thermogravimetric-single differential thermal analysis (TG-SDTA) and XRD tests were conducted on selected samples to study the hydraulic compounds. The study showed the potential benefit of stabilizing clays with high calcium fly ash. However, it was found that the effectiveness of stabilization is dependent on the type of soil, the amount of stabilizer and the curing time. The study of formation of hydraulic products showed that a significant amount of tobermorite gel is formed due to stabilization leading to a denser and more stable structure of the samples. The mechanical properties such as strength and modulus of elasticity showed considerable enhancement due to stabilization. This study, however, did not examine the microstructure of lime- and CKD-stabilized soils.

Horpibulsuk et al. (2010) analyzed the strength development in cement-stabilized silty clay based on microstructural considerations. A qualitative and quantitative study was conducted on the microstructure using a SEM, mercury intrusion pore size distribution measurements, and thermal gravity analysis. A total of three zones of improvement namely, active, inert, and deterioration zones, were observed. The active zone was found to be most effective for stabilization where the cementitious products increased with cement content and filled the pore space. In the inert zone, both pore size distribution and cementitious products change insignificantly with increasing cement. In deterioration zone, the water is not adequate for hydration because of the excess of cement input. It was found that in short stabilization period, the volume of large pores (> 0.1 µm) increases because of input of

Microstructural and Mineralogical Characterization

reaction taking place during the specimen preparation for XRD.

thus, producing increase in strength and modulus values.

of Clay Stabilized Using Calcium-Based Stabilizers 775

production of Portland cement. The physical and chemical properties of the stabilizing agents are presented in Table 1. The X-Ray Fluorescence (XRF) analysis was conducted using a Panalytical 2403 spectrometer on specimens obtained by using fused bead preparation method. The fused bead preparation technique consists of dissolving the specimen in a solvent called a flux at high temperature (>1000°C) in a platinum crucible and to cast it in a casting-dish. It is evident from Table 1 that the calcium oxide content in hydrated lime is 68.6%. This can be explained using the stoichiometry of the chemical

Ca(OH)2 → CaO + H2O (5)

 (74)=40x1+16x2+1x2 (56)=40x1+16x1 (18)=1x2+16x1 Using above chemical equation, it can be shown that 95.9% of Ca(OH)2 (reactant) will produce approximately 72% of CaO (product). Further, the free lime content (i.e., any lime not bound up in glassy phase compounds such as tricalcium silicate and tricalcium aluminate) was determined in accordance with ASTM C 114 (Alternate Test Method B, ammonium acetate titration). Although CFA is having a very low lime content (0.2%), specimens stabilized with CFA showed enhancement in strength and modulus values as will be discussed later in this chapter. It is speculated that during the reaction process some lime is liberated from the bound state which takes part in the cementitious reactions and

Chemical compound/Property Percentage by weight, (%)

ether method (Cerato and Lutenegger 2001); UCS: Unconfined compressive strength;

Table 1. Chemical and physical properties of soil and additives

\*Ca(OH)2 decomposes at 512oC; \*\*Before ignition

Silica (SiO2)a 0.6 37.7 14.1 63.4 Alumina (Al2O3)a 0.4 17.3 3.1 21.5 Ferric oxide (Fe2O3)a 0.3 5.8 1.4 9.1 Calcium oxide (CaO)a 68.6 24.4 47 0.1 Calcium hydroxide (Ca(OH)2)a 95.9\*\* … … … Magnesium oxide (MgO)a 0.7 5.1 1.7 1.2 Sulfur trioxide (SO3)a 0.1 1.2 4.4 0 Alkali content (Na2O + K2O)a 0.1 2.2 1.7 3.0 Loss on ignitionb 31.8\* 1.2 27 … Free limeb 46.1 0.2 6.7 … Percentage passing No. 325c 98.4 85.8 94.2 87.2 pH (pure material)d 12.58 11.83 12.55 4.17 Specific surface area (m2/gm)e 17.0 6.0 12.0 118.5 28-day UCS (kPa) … 708 17 207 aX-ray Fluorescence analysis; bASTM C 114; cASTM C 430; dASTM D 6276; eEthylene glycol monoethyl

Lime CFA CKD Soil

coarser particles while the volume of small pores (< 0.1 µm) decreases because of the solidification of the hydrated cement. With time, the large pores are filled with the cementitious products; thus, the small pore volume increases, and the total pore volume decreases resulting in the development of strength with time. This study, however, was limited to only cement and no attempt was made to compare the microstructure of soil stabilized with other additives.

In a recent study, Chaunsali and Peethamparan (2011) characterized a nontraditional binding material containing cement kiln dust (CKD) and ground GGBFS. The CKD used in this study contained low free lime and high sulfate and alkali content, and proved effective in accelerating the hydration of GGBFS. The strength rate development was found to be dependent on the curing conditions but eventually all the samples achieved similar compressive strengths independent of the curing conditions. The microstructural and mineralogical examinations showed that the strength development was mainly due to the formation of C-S-H. Additionally, aluminum and magnesium incorporated C-S-H phases were also identified in CKD-GGBFS blends. The formation of ettringite appeared as a contributing factor in the development of early age strength in CKD-GGBFS binder.

It is clear from the aforementioned literature review that there is a lack of detailed comparative studies on microstructure and mineralogy of expansive soils stabilized with various stabilizers. Therefore, there was a need to undertake a detailed investigation to fully characterize the microstructure and mineralogy of soil stabilized with locally available additives.

## **3. Materials and sources**

In the present study, one fat clay and three cementitious additives are used. This section describes the fundamental properties including grain size distribution, index properties and chemical compositions of the soils and additives.

#### **3.1 Native soils**

The soil used in this study is Carnasaw series soil. According to the Unified Soil Classification System (USCS), Carnasaw series soil is classified as fat clay (CH) with an average liquid limit of approximately 58% and a plasticity index (PI) of 29 in accordance with ASTM D 4318 test method. The gradation tests revealed percent passing No. 200 sieve (< 0.075 mm) and clay content (< 0.002 mm) as 94% and 48%, respectively. A summary of the physical and chemical properties of the selected soil is presented in Table 1.

#### **3.2 Cementitious additives**

As noted earlier, three different cementitious additives, namely, hydrated lime, CFA, and CKD were used. Hydrated lime was supplied by the Texas Lime Company, Cleburne, Texas. It is a dry powder manufactured by treating quicklime (calcium oxide) with sufficient water to satisfy its chemical affinity with water, thereby converting the oxides to hydroxides. CFA from Lafarge North America (Tulsa, Oklahoma) was brought in wellsealed plastic buckets. It was produced in a coal-fired electric utility plant, American Electric Power (AEP), located in Muskogee, Oklahoma. CKD used was provided by Lafarge North America located in Tulsa, Oklahoma. It is an industrial waste collected during the

coarser particles while the volume of small pores (< 0.1 µm) decreases because of the solidification of the hydrated cement. With time, the large pores are filled with the cementitious products; thus, the small pore volume increases, and the total pore volume decreases resulting in the development of strength with time. This study, however, was limited to only cement and no attempt was made to compare the microstructure of soil

In a recent study, Chaunsali and Peethamparan (2011) characterized a nontraditional binding material containing cement kiln dust (CKD) and ground GGBFS. The CKD used in this study contained low free lime and high sulfate and alkali content, and proved effective in accelerating the hydration of GGBFS. The strength rate development was found to be dependent on the curing conditions but eventually all the samples achieved similar compressive strengths independent of the curing conditions. The microstructural and mineralogical examinations showed that the strength development was mainly due to the formation of C-S-H. Additionally, aluminum and magnesium incorporated C-S-H phases were also identified in CKD-GGBFS blends. The formation of ettringite appeared as a

contributing factor in the development of early age strength in CKD-GGBFS binder.

microstructure and mineralogy of soil stabilized with locally available additives.

the physical and chemical properties of the selected soil is presented in Table 1.

It is clear from the aforementioned literature review that there is a lack of detailed comparative studies on microstructure and mineralogy of expansive soils stabilized with various stabilizers. Therefore, there was a need to undertake a detailed investigation to fully characterize the

In the present study, one fat clay and three cementitious additives are used. This section describes the fundamental properties including grain size distribution, index properties and

The soil used in this study is Carnasaw series soil. According to the Unified Soil Classification System (USCS), Carnasaw series soil is classified as fat clay (CH) with an average liquid limit of approximately 58% and a plasticity index (PI) of 29 in accordance with ASTM D 4318 test method. The gradation tests revealed percent passing No. 200 sieve (< 0.075 mm) and clay content (< 0.002 mm) as 94% and 48%, respectively. A summary of

As noted earlier, three different cementitious additives, namely, hydrated lime, CFA, and CKD were used. Hydrated lime was supplied by the Texas Lime Company, Cleburne, Texas. It is a dry powder manufactured by treating quicklime (calcium oxide) with sufficient water to satisfy its chemical affinity with water, thereby converting the oxides to hydroxides. CFA from Lafarge North America (Tulsa, Oklahoma) was brought in wellsealed plastic buckets. It was produced in a coal-fired electric utility plant, American Electric Power (AEP), located in Muskogee, Oklahoma. CKD used was provided by Lafarge North America located in Tulsa, Oklahoma. It is an industrial waste collected during the

stabilized with other additives.

**3. Materials and sources** 

**3.2 Cementitious additives** 

**3.1 Native soils** 

chemical compositions of the soils and additives.

production of Portland cement. The physical and chemical properties of the stabilizing agents are presented in Table 1. The X-Ray Fluorescence (XRF) analysis was conducted using a Panalytical 2403 spectrometer on specimens obtained by using fused bead preparation method. The fused bead preparation technique consists of dissolving the specimen in a solvent called a flux at high temperature (>1000°C) in a platinum crucible and to cast it in a casting-dish. It is evident from Table 1 that the calcium oxide content in hydrated lime is 68.6%. This can be explained using the stoichiometry of the chemical reaction taking place during the specimen preparation for XRD.


Using above chemical equation, it can be shown that 95.9% of Ca(OH)2 (reactant) will produce approximately 72% of CaO (product). Further, the free lime content (i.e., any lime not bound up in glassy phase compounds such as tricalcium silicate and tricalcium aluminate) was determined in accordance with ASTM C 114 (Alternate Test Method B, ammonium acetate titration). Although CFA is having a very low lime content (0.2%), specimens stabilized with CFA showed enhancement in strength and modulus values as will be discussed later in this chapter. It is speculated that during the reaction process some lime is liberated from the bound state which takes part in the cementitious reactions and thus, producing increase in strength and modulus values.


aX-ray Fluorescence analysis; bASTM C 114; cASTM C 430; dASTM D 6276; eEthylene glycol monoethyl ether method (Cerato and Lutenegger 2001); UCS: Unconfined compressive strength; \*Ca(OH)2 decomposes at 512oC; \*\*Before ignition

Table 1. Chemical and physical properties of soil and additives

Microstructural and Mineralogical Characterization

found in the soil used in this study.

**4.1.4 Specific surface area** 

**4.2 Additive properties 4.2.1 Free-lime content** 

**4.2.2 Specific surface area** 

**4.2.3 Loss on ignition** 

of Clay Stabilized Using Calcium-Based Stabilizers 777

geotechnical structures built on sulfate rich soils stabilized with calcium-based additive (Hunter, 1988; Mitchell and Dermatas, 1990; Petry and Little, 1992; Rajendran and Lytton, 1997; Rollings et al., 1999; Puppala et al., 2004). According to current understanding, "low to moderate" and high sulfate soils are those with sulfate less than 2,000 ppm and more than 2,000 ppm, respectively (Kota et al., 1996; Mitchell and Dermatas, 1990; Puppala et al., 2002; Rao and Shivananda, 2005). In this study, soluble sulfate content in the soil was measured using the Oklahoma Department of Transportation procedure for determining soluble sulfate content: OHD L-49 (ODOT, 2006). No detectable sulfate content (< 200 ppm) was

Surface phenomena have an important influence on the behavior of fine-grained soils; they affect many physical and chemical properties (Cerato and Lutenegger, 2002). The specific surface area (SSA), refers to the area per unit mass of soil, may be a dominant factor in controlling the fundamental behavior of many fine-grained soils (Gomez, 2009). The mineralogy of fine-grained soils is the dominant factor in determining the effect of SSA. For this study, only total SSA measurement was conducted using the polar liquid Ethylene Glycol Monoethyl Ether (EGME) method (Cerato and Lutenegger, 2002) and results are

In calcium-based stabilizers (e.g., Portland cement, CFA, CKD) most of the lime (CaO) is bound up in compounds such as tricalcium silicate and tricalcium aluminate. The unreacted lime that is not combined in any of these compounds is called free-lime, which is expected to play a major role in stabilization (Collins and Emery, 1983; Misra, 1998; Zaman et al., 1998; Ferguson and Levorson, 1999; Miller and Azad, 2000; Miller and Zaman, 2000; Sezer et al., 2006; Khoury and Zaman, 2007; Peethamparan and Olek, 2008). Free-lime content was determined by conducting titration in accordance with ASTM C 114 alternative test method B and results are presented in Table 1. It is clear that lime is having the highest free-lime

The specific surface area (SSA) of additives, as measured by using the ethylene glycol monoethyl ether (EGME) method (Cerato and Lutenegger, 2002), were 17.0, 6.0, and 12.0 m2/gm, respectively, for lime, CFA and CKD. It can be seen that lime and CFA had the highest and the lowest SSA values, respectively. A higher SSA indicates more reactivity of

A higher loss on ignition (LOI) value indicates high carbonates for CFA/CKD and high hydroxides for lime. Some researchers reported that high LOI indicates low free-lime content for CKDs, making CKDs less reactive, and therefore lower improvements (Bhatty et

presented in Table 1. The Carnasaw series soil showed a SSA value of 118.5 m2/gm.

content of 46.7% followed by 6.7% for CKD and 0.2% for CFA.

additive (Nalbantoglu and Tuncer, 2001; Sreekrishnavilasam et al., 2007).

## **4. Factors affecting cementitious stabilization**

The effectiveness of cementitious stabilization depends on properties of both soil and additive (AFJMAN, 1994, Al-Rawas et al., 2002, Parsons et al., 2004, Evangelos, 2006). A description of the pertinent factors intrinsic to the soils and additives which influence the efficiency of cementitious stabilization is presented herein.

## **4.1 Soil properties**

## **4.1.1 Gradation and plasticity index**

Several researchers (e.g., Diamond and Kinter, 1964; Haston and Wohlgemuth, 1985; Prusinski and Bhattacharja, 1999; Little, 2000; Qubain et al., 2000; Kim and Siddiki, 2004; Mallela et al., 2004; Puppala et al., 2006; Consoli et al., 2009) recommended use of lime with fine-grained soils. However, CFA (see e.g., McManis and Arman, 1989; Chang, 1995; Misra, 1998; Zia and Fox, 2000; Puppala et al., 2003; Bin-Shafique et al., 2004; Phanikumar and Sharma, 2004; Nalbantoglu, 2004; Camargo et al., 2009; Li et al., 2009) and CKD (e.g., McCoy and Kriner, 1971; Baghdadi and Rahman, 1990; Zaman et al., 1992; Sayah, 1993; Miller and Azad, 2000; Miller and Zaman, 2000; Parsons and Kneebone, 2004; Sreekrishnavilasam et al., 2007; Peethamparan et al., 2008; Gomez, 2009) is used successfully with both fine- and coarse-grained soils. Lower effectiveness of lime with coarse-grained soil can be attributed to scarcity of pozzolana (silicious and aluminacious material) in coarse-grained soils which is required for pozzolanic (or cementitious) reactions. Little (2000) and Mallela et al. (2004) recommend a soil with a minimum clay content (< 0.002 mm) of 10% and a plasticity index of 10 for lime-stabilization. In this study, Carnasaw series soil fulfils this requirement with a clay content of 48%. Also, mineralogical analyses conducted using XRF revealed that the soil used in this study is having high (85%) amount of pozzolana, as presented in Table 1.

### **4.1.2 Cation exchange capacity**

Cation Exchange Capacity (CEC) is the quantity of exchangeable cations required to balance the charge deficiency on the surface of the clay particles (Mitchell, 1993). During ionexchange reaction of soil with cementitious additive, cation of soil (e.g., Na+, K+) is replaced by cation of additive (Ca2+) and the thickness of double diffused layer is reduced. Hence, the replacement of cations results in an increase in workability and strength of soil-additive mixture. The rate of exchange depends on clay type, solution concentrations and temperature (Gomez, 2009). In soil stabilization studies, CEC values have been used to a limited extent to explain the effectiveness of soil stabilization (Nalbantoglu and Tuncer, 2001; Al-Rawas et al., 2002; Nalbantoglu, 2004; Gomez, 2009).

In this study, CEC was measured by sodium acetate method in accordance with the EPA 9081 test method (Chapman, 1965). As evident from Table 1, Carnasaw soil showed CEC value of 5.2 meq/100 gm.

#### **4.1.3 Sulfate content**

Primary "sulfate-induced heaving" problems arise when natural sulfate rich soils are stabilized with calcium-based additives (Puppala et al., 2004), also known as "sulfate attack." This heave is known to severely affect the performance of pavements, and other geotechnical structures built on sulfate rich soils stabilized with calcium-based additive (Hunter, 1988; Mitchell and Dermatas, 1990; Petry and Little, 1992; Rajendran and Lytton, 1997; Rollings et al., 1999; Puppala et al., 2004). According to current understanding, "low to moderate" and high sulfate soils are those with sulfate less than 2,000 ppm and more than 2,000 ppm, respectively (Kota et al., 1996; Mitchell and Dermatas, 1990; Puppala et al., 2002; Rao and Shivananda, 2005). In this study, soluble sulfate content in the soil was measured using the Oklahoma Department of Transportation procedure for determining soluble sulfate content: OHD L-49 (ODOT, 2006). No detectable sulfate content (< 200 ppm) was found in the soil used in this study.

## **4.1.4 Specific surface area**

776 Scanning Electron Microscopy

The effectiveness of cementitious stabilization depends on properties of both soil and additive (AFJMAN, 1994, Al-Rawas et al., 2002, Parsons et al., 2004, Evangelos, 2006). A description of the pertinent factors intrinsic to the soils and additives which influence the

Several researchers (e.g., Diamond and Kinter, 1964; Haston and Wohlgemuth, 1985; Prusinski and Bhattacharja, 1999; Little, 2000; Qubain et al., 2000; Kim and Siddiki, 2004; Mallela et al., 2004; Puppala et al., 2006; Consoli et al., 2009) recommended use of lime with fine-grained soils. However, CFA (see e.g., McManis and Arman, 1989; Chang, 1995; Misra, 1998; Zia and Fox, 2000; Puppala et al., 2003; Bin-Shafique et al., 2004; Phanikumar and Sharma, 2004; Nalbantoglu, 2004; Camargo et al., 2009; Li et al., 2009) and CKD (e.g., McCoy and Kriner, 1971; Baghdadi and Rahman, 1990; Zaman et al., 1992; Sayah, 1993; Miller and Azad, 2000; Miller and Zaman, 2000; Parsons and Kneebone, 2004; Sreekrishnavilasam et al., 2007; Peethamparan et al., 2008; Gomez, 2009) is used successfully with both fine- and coarse-grained soils. Lower effectiveness of lime with coarse-grained soil can be attributed to scarcity of pozzolana (silicious and aluminacious material) in coarse-grained soils which is required for pozzolanic (or cementitious) reactions. Little (2000) and Mallela et al. (2004) recommend a soil with a minimum clay content (< 0.002 mm) of 10% and a plasticity index of 10 for lime-stabilization. In this study, Carnasaw series soil fulfils this requirement with a clay content of 48%. Also, mineralogical analyses conducted using XRF revealed that the soil used in this study is having high (85%) amount of pozzolana, as presented in Table 1.

Cation Exchange Capacity (CEC) is the quantity of exchangeable cations required to balance the charge deficiency on the surface of the clay particles (Mitchell, 1993). During ionexchange reaction of soil with cementitious additive, cation of soil (e.g., Na+, K+) is replaced by cation of additive (Ca2+) and the thickness of double diffused layer is reduced. Hence, the replacement of cations results in an increase in workability and strength of soil-additive mixture. The rate of exchange depends on clay type, solution concentrations and temperature (Gomez, 2009). In soil stabilization studies, CEC values have been used to a limited extent to explain the effectiveness of soil stabilization (Nalbantoglu and Tuncer,

In this study, CEC was measured by sodium acetate method in accordance with the EPA 9081 test method (Chapman, 1965). As evident from Table 1, Carnasaw soil showed CEC

Primary "sulfate-induced heaving" problems arise when natural sulfate rich soils are stabilized with calcium-based additives (Puppala et al., 2004), also known as "sulfate attack." This heave is known to severely affect the performance of pavements, and other

**4. Factors affecting cementitious stabilization** 

efficiency of cementitious stabilization is presented herein.

**4.1 Soil properties** 

**4.1.1 Gradation and plasticity index** 

**4.1.2 Cation exchange capacity** 

value of 5.2 meq/100 gm.

**4.1.3 Sulfate content** 

2001; Al-Rawas et al., 2002; Nalbantoglu, 2004; Gomez, 2009).

Surface phenomena have an important influence on the behavior of fine-grained soils; they affect many physical and chemical properties (Cerato and Lutenegger, 2002). The specific surface area (SSA), refers to the area per unit mass of soil, may be a dominant factor in controlling the fundamental behavior of many fine-grained soils (Gomez, 2009). The mineralogy of fine-grained soils is the dominant factor in determining the effect of SSA. For this study, only total SSA measurement was conducted using the polar liquid Ethylene Glycol Monoethyl Ether (EGME) method (Cerato and Lutenegger, 2002) and results are presented in Table 1. The Carnasaw series soil showed a SSA value of 118.5 m2/gm.

## **4.2 Additive properties**

#### **4.2.1 Free-lime content**

In calcium-based stabilizers (e.g., Portland cement, CFA, CKD) most of the lime (CaO) is bound up in compounds such as tricalcium silicate and tricalcium aluminate. The unreacted lime that is not combined in any of these compounds is called free-lime, which is expected to play a major role in stabilization (Collins and Emery, 1983; Misra, 1998; Zaman et al., 1998; Ferguson and Levorson, 1999; Miller and Azad, 2000; Miller and Zaman, 2000; Sezer et al., 2006; Khoury and Zaman, 2007; Peethamparan and Olek, 2008). Free-lime content was determined by conducting titration in accordance with ASTM C 114 alternative test method B and results are presented in Table 1. It is clear that lime is having the highest free-lime content of 46.7% followed by 6.7% for CKD and 0.2% for CFA.

#### **4.2.2 Specific surface area**

The specific surface area (SSA) of additives, as measured by using the ethylene glycol monoethyl ether (EGME) method (Cerato and Lutenegger, 2002), were 17.0, 6.0, and 12.0 m2/gm, respectively, for lime, CFA and CKD. It can be seen that lime and CFA had the highest and the lowest SSA values, respectively. A higher SSA indicates more reactivity of additive (Nalbantoglu and Tuncer, 2001; Sreekrishnavilasam et al., 2007).

#### **4.2.3 Loss on ignition**

A higher loss on ignition (LOI) value indicates high carbonates for CFA/CKD and high hydroxides for lime. Some researchers reported that high LOI indicates low free-lime content for CKDs, making CKDs less reactive, and therefore lower improvements (Bhatty et

Microstructural and Mineralogical Characterization

of Clay Stabilized Using Calcium-Based Stabilizers 779

the pH values of soil-additive mixtures were determined to investigate whether pH would

The pH results of raw soil, raw additive and soil-additive mixtures are presented in Table 2 and are used as the primary guide for determining the amount of additive required to stabilize each soil. It is clear that Carnasaw soil is acidic with a pH value approximately 4.17. Also, it was found that raw lime, CFA and CKD had a pH value of 12.58, 11.83 and 12.55, respectively. The pH values of raw CFA and CKD are consistent with the results reported by other researchers (e.g., Miller and Azad, 2000; Sear, 2001; Parsons et al., 2004; Peethamparan and Olek, 2008; Gomez, 2009). The pH trend of raw additives is similar to the

For all the soil-additive mixtures, pH values increase with the increase in the percentage of additive and show an asymptotic behavior after a certain percentage. In the current study, an increase of less than 1% in pH with respect to raw soil is assumed as starting point of the asymptotic behavior. As evident from Table 2, pH values started showing an asymptotic behavior with 5% lime. Additionally, soil never attained an asymptotic behavior with CFA and CKD contents up to 17.5%. This can be attributed to the acidic behavior of Carnasaw soil which requires higher amounts of moderately basic CFA and CKD for neutralization. Based on the aforementioned observations, it was decided to select 9% of lime and 15% of

In this study, a total of 16 specimens were prepared for evaluating modulus of elasticity. The procedure consists of adding a specific amount of additive to the raw soil desired. The amount of additive (9% for lime and 15% for CFA and CKD) was added based on the dry weight of the soil. The additive and soil were mixed manually for uniformity. After the blending process, a desired amount of water was added based on the optimum moisture content (OMC). The mixture was then compacted in a mold having a diameter of 101.6 mm and a height of 203.2 mm to reach a dry density of between 95%-100% of the maximum dry unit weight (MUW). After compaction, specimens were cured at a temperature of 23.0 ± 1.7oC and a relative humidity of approximately 96% for 28 days. A total of four replicates were prepared for each additive type and tested for modulus of elasticity in accordance with

As noted earlier, modulus of elasticity test was conducted in accordance with the ASTM D 1633 test method. Specimens were loaded in a MTS frame at a constant strain rate of 0.63% (of sample height) per minute, which is equivalent to 1.27 mm (0.05 in.) per minute for the specimen configuration used here. Deformation values were recorded during the test using LVDTs fixed to opposite sides of and equidistant from piston rod with a maximum stroke length of ±12.7 mm (±0.5 in). The load values were obtained from a load cell having a capacity of 22.7 kN (5,000 lb). Each specimen was subjected to two unloading-reloading cycles. Straight lines were drawn through the first two unloading-reloading curves (secant

reflect the effectiveness of soil stabilization with lime, CFA or CKD.

trend of available free-lime content in additive, as shown in Table 1.

CFA and CKD for laboratory performance evaluation.

**5. Experimental methodology** 

**5.1 Specimen preparation** 

ASTM D 1633 test method.

**5.2 Modulus of elasticity** 

al., 1996; Miller and Azad, 2000). In the laboratory, LOI was evaluated by igniting additive inside a muffle furnace at a temperature of 950oC (1742oF) in accordance with ASTM C 114 test method for hydraulic cements. As evident from results presented in Table 1, lime and CFA produced highest and lowest LOI values of 31.8% and 1.2%, respectively. On the other hand, approximately 27% of CKD is lost on ignition.

#### **4.2.4 Percent passing No. 325 sieve**

Several researchers noticed increased reactivity of additive with increase in amount of additive passing No. 325 (45 µm) sieve (NCHRP, 1976; Bhatty et al., 1996; Zaman et al., 1998; Zheng and Qin, 2003; Khoury, 2005). The percentage of passing No. 325 sieve for lime, CFA and CKD determined in accordance with ASTM C 430 test method are 98.4, 85.8 and 94.2, respectively. It is clear that lime is finest among all the additives used in this study.

### **4.2.5 pH and pH response**

The elevated pH level of soil-lime mixture is important because it provides an adequate alkaline environment for ion-exchange reactions (Little, 2000). In the laboratory, pH is determined using the method recommended by ASTM D 6276 for lime-stabilization, which involves mixing the solids with de-ionized (DI) water, periodically shaking samples, and then testing with a pH meter after 1 h. The procedure specifies that enough lime must be added to a soil-water system to maintain a pH of 12.4 after 1 h. This ensures that adequate lime is provided to sustain the saturation during the 1-h period (Prusinski and Bhattacharja, 1999).


aIncrease in pH w.r.t. pH value of raw soil; Bold values represent minimum additive content providing asymptotic behavior (< 1% increase)

Table 2. Variation of pH values with soil and additive type

Several researchers (e.g., Haston and Wohlgemuth, 1985; Prusinski and Bhattacharja, 1999; IRC, 2000; Little, 2000; Qubain et al., 2000; Mallela et al., 2004; Puppala et al., 2006; Consoli et al., 2009) used pH values on soil-lime mixture as an indicator of reactivity of lime. However, only limited studies (see e.g., Miller and Azad, 2000; Parsons et al., 2004; Peethamparan and Olek, 2008; Gomez, 2009) evaluated pH response of soil-CFA or soil-CKD mixtures. Hence,

al., 1996; Miller and Azad, 2000). In the laboratory, LOI was evaluated by igniting additive inside a muffle furnace at a temperature of 950oC (1742oF) in accordance with ASTM C 114 test method for hydraulic cements. As evident from results presented in Table 1, lime and CFA produced highest and lowest LOI values of 31.8% and 1.2%, respectively. On the other

Several researchers noticed increased reactivity of additive with increase in amount of additive passing No. 325 (45 µm) sieve (NCHRP, 1976; Bhatty et al., 1996; Zaman et al., 1998; Zheng and Qin, 2003; Khoury, 2005). The percentage of passing No. 325 sieve for lime, CFA and CKD determined in accordance with ASTM C 430 test method are 98.4, 85.8 and 94.2,

The elevated pH level of soil-lime mixture is important because it provides an adequate alkaline environment for ion-exchange reactions (Little, 2000). In the laboratory, pH is determined using the method recommended by ASTM D 6276 for lime-stabilization, which involves mixing the solids with de-ionized (DI) water, periodically shaking samples, and then testing with a pH meter after 1 h. The procedure specifies that enough lime must be added to a soil-water system to maintain a pH of 12.4 after 1 h. This ensures that adequate lime is provided to sustain the saturation during the 1-h period (Prusinski and Bhattacharja,

> pH value

0 4.17 --- 0 4.17 --- 0 4.17 --- 1 9.22 121.1 2.5 5.19 24.5 2.5 7.05 69.1 3 12.23 193.3 5 5.93 42.2 5 8.8 111.0 5 **12.54 200.7** 7.5 6.55 57.1 7.5 10.11 142.4 6 12.55 201.0 10 7.79 86.8 10 10.88 160.9 7 12.55 201.0 12.5 8.32 99.5 12.5 11.28 170.5 9 12.57 201.4 15 8.86 112.5 15 11.62 178.7 100 12.58 201.7 17.5 9.47 127.1 17.5 11.98 187.3 100 4.17 --- 100 4.17 -- aIncrease in pH w.r.t. pH value of raw soil; Bold values represent minimum additive content providing

Several researchers (e.g., Haston and Wohlgemuth, 1985; Prusinski and Bhattacharja, 1999; IRC, 2000; Little, 2000; Qubain et al., 2000; Mallela et al., 2004; Puppala et al., 2006; Consoli et al., 2009) used pH values on soil-lime mixture as an indicator of reactivity of lime. However, only limited studies (see e.g., Miller and Azad, 2000; Parsons et al., 2004; Peethamparan and Olek, 2008; Gomez, 2009) evaluated pH response of soil-CFA or soil-CKD mixtures. Hence,

CFA Additive

% Increasea Content (%)

CKD

% Increasea

pH value

respectively. It is clear that lime is finest among all the additives used in this study.

hand, approximately 27% of CKD is lost on ignition.

Lime Additive

Table 2. Variation of pH values with soil and additive type

% Increasea Content (%)

**4.2.4 Percent passing No. 325 sieve** 

**4.2.5 pH and pH response** 

pH value

asymptotic behavior (< 1% increase)

1999).

Additive Content (%)

the pH values of soil-additive mixtures were determined to investigate whether pH would reflect the effectiveness of soil stabilization with lime, CFA or CKD.

The pH results of raw soil, raw additive and soil-additive mixtures are presented in Table 2 and are used as the primary guide for determining the amount of additive required to stabilize each soil. It is clear that Carnasaw soil is acidic with a pH value approximately 4.17. Also, it was found that raw lime, CFA and CKD had a pH value of 12.58, 11.83 and 12.55, respectively. The pH values of raw CFA and CKD are consistent with the results reported by other researchers (e.g., Miller and Azad, 2000; Sear, 2001; Parsons et al., 2004; Peethamparan and Olek, 2008; Gomez, 2009). The pH trend of raw additives is similar to the trend of available free-lime content in additive, as shown in Table 1.

For all the soil-additive mixtures, pH values increase with the increase in the percentage of additive and show an asymptotic behavior after a certain percentage. In the current study, an increase of less than 1% in pH with respect to raw soil is assumed as starting point of the asymptotic behavior. As evident from Table 2, pH values started showing an asymptotic behavior with 5% lime. Additionally, soil never attained an asymptotic behavior with CFA and CKD contents up to 17.5%. This can be attributed to the acidic behavior of Carnasaw soil which requires higher amounts of moderately basic CFA and CKD for neutralization. Based on the aforementioned observations, it was decided to select 9% of lime and 15% of CFA and CKD for laboratory performance evaluation.

## **5. Experimental methodology**

### **5.1 Specimen preparation**

In this study, a total of 16 specimens were prepared for evaluating modulus of elasticity. The procedure consists of adding a specific amount of additive to the raw soil desired. The amount of additive (9% for lime and 15% for CFA and CKD) was added based on the dry weight of the soil. The additive and soil were mixed manually for uniformity. After the blending process, a desired amount of water was added based on the optimum moisture content (OMC). The mixture was then compacted in a mold having a diameter of 101.6 mm and a height of 203.2 mm to reach a dry density of between 95%-100% of the maximum dry unit weight (MUW). After compaction, specimens were cured at a temperature of 23.0 ± 1.7oC and a relative humidity of approximately 96% for 28 days. A total of four replicates were prepared for each additive type and tested for modulus of elasticity in accordance with ASTM D 1633 test method.

## **5.2 Modulus of elasticity**

As noted earlier, modulus of elasticity test was conducted in accordance with the ASTM D 1633 test method. Specimens were loaded in a MTS frame at a constant strain rate of 0.63% (of sample height) per minute, which is equivalent to 1.27 mm (0.05 in.) per minute for the specimen configuration used here. Deformation values were recorded during the test using LVDTs fixed to opposite sides of and equidistant from piston rod with a maximum stroke length of ±12.7 mm (±0.5 in). The load values were obtained from a load cell having a capacity of 22.7 kN (5,000 lb). Each specimen was subjected to two unloading-reloading cycles. Straight lines were drawn through the first two unloading-reloading curves (secant

Microstructural and Mineralogical Characterization

69

0

50

100

Modulus of Elasticity (MPa)

improvement.

of Clay Stabilized Using Calcium-Based Stabilizers 781

a specimen holder. The holder containing specimen was oven dried for approximately 15 minutes prior to testing. This holder was then mounted on a Rigaku D/Max X-ray diffractometer for analysis. This diffractometer is equipped with bragg-brentano parafocusing geometry, a diffracted beam monochromator, and a conventional copper target X-ray tube set to 40 kV and 30 mA. The measurements were performed from 4o to 70o (2θ range), with 0.05o step size and 5 seconds count (dwell time) at each step. Data obtained by the diffractometer were analyzed with Jade 3.1, an X-ray powder diffraction analytical software, developed by Materials Data, Inc. (Jade, 1999). Generated diffractograms (using the peaks versus 2θ and d-spacing) were used to determine the presence of minerals. Figure

1 (b) shows a photographic view of Rigaku D/Max X-ray diffractometer.

**6. Presentation and discussion of modulus of elasticity test results** 

137

Fig. 2. Variation of modulus of elasticity values with additive type

134

Raw Soil 9% Lime 15% CFA 15% CKD

Additive type

Attempts were made to observe the effect of additive properties, namely, free-lime content, alkali content, loss on ignition, specific surface area (SSA), pH, and passing No. 325 sieve, on the modulus of elasticity. The effect of these additive properties on normalized modulus of elasticity (modulus of elasticity value/percent additive) is depicted in Figure 3. Here, it is clear that the normalized modulus of elasticity value increases with the free-lime content. The Carnasaw soil specimens exhibited an increase of approximately 13 to 15 as the free-lime content increased from 6.7% (CKD) to 46.1% (lime). A decrease in normalized modulus of elasticity values with alkali content can be observed; however, increase in normalized modulus of elasticity values with loss on ignition was observed. This trend is contrary to the behavior reported by other researchers for different type of CKDs (e.g., Bhatty et al., 1996; Miller and Azad, 2000; Peethamparan and Olek, 2008). For example, Bhatty et al. (1996) reported that CKDs

202

The variation of modulus of elasticity values with the additive content is shown in Figure 2. It is clear from Figure 2 that the modulus of elasticity of stabilized specimens is influenced by the type of additive. For example, an increase of approximately 99%, 94% and 193% in modulus of elasticity values was observed for 9% lime-, 15% CFA- and 15% CKD-stabilized specimens, respectively. Overall, 15% CKD-stabilized specimens showed the highest

modulus) and the average slope of these lines is the modulus of elasticity of the stabilized clay specimen.

#### **5.3 Mineralogical studies**

To facilitate the macro-behavior comparison and explanation, the mineralogical study techniques namely SEM and EDS were employed to qualitatively identify the microstructural developments in the matrix of the stabilized soil specimens. The SEM technique was employed using a JEOL JSM 880 microscope to qualitatively identify the microstructural developments in the matrix of the stabilized soil specimens. After the modulus of elasticity test, specimens were broken and mix was air-dried for approximately two days. Three representative tiny pieces were mounted on stubs (1 cm, i.e., 0.4 in. wide discs having a pin-mount on the base of the disc). The samples were not electrically conductive; therefore, they were initially coated by Iridium to maintain conductivity. The quality of images was not satisfactory, so it was decided to use gold-palladium alloy for the process instead of Iridium coating. Hence, pieces were coated with a thin layer (≈ 5 nm) of an alloy of goldpalladium by sputter coating technique to provide surface conductivity. A JEOL JSM 880 scanning electron microscope operating at 15 kV was used to visually observe the coated specimens. The JEOL JSM 880 was fitted with an energy-dispersive X-ray spectrometer (EDS). The EDS was used to analyze chemical compositions of the specimen. In this technique, electrons are bombarded in the area of desired elemental composition; the elements present will emit characteristic X-rays, which are then recorded on a detector. The micrographs were taken using EDS2000 software. It must be noted that SEM study allows only a tiny area of raw and stabilized specimen to be examined (unlike engineering laboratory specimens). However, it is believed to be representative of the reaction process of stabilized specimens. Figure 1 (a) shows a photographic view of JEOL JSM 880 setup along with Hummer VI triode sputter coater.

Fig. 1. (a) JEOL JSM 880 setup for SEM, Hummer VI triode sputter coater with sample (corner picture), and (b) Rigaku D/Max X-ray diffractometer

The XRD tests were performed on raw soil and stabilized specimens. Two-day air dried mix was pulverized with a mortar and pestle, sieved through a U.S. standard No. 325 sieve (45 μm). Then, the powder finer than 45 μm was collected, mixed with methanol, and placed on

modulus) and the average slope of these lines is the modulus of elasticity of the stabilized

To facilitate the macro-behavior comparison and explanation, the mineralogical study techniques namely SEM and EDS were employed to qualitatively identify the microstructural developments in the matrix of the stabilized soil specimens. The SEM technique was employed using a JEOL JSM 880 microscope to qualitatively identify the microstructural developments in the matrix of the stabilized soil specimens. After the modulus of elasticity test, specimens were broken and mix was air-dried for approximately two days. Three representative tiny pieces were mounted on stubs (1 cm, i.e., 0.4 in. wide discs having a pin-mount on the base of the disc). The samples were not electrically conductive; therefore, they were initially coated by Iridium to maintain conductivity. The quality of images was not satisfactory, so it was decided to use gold-palladium alloy for the process instead of Iridium coating. Hence, pieces were coated with a thin layer (≈ 5 nm) of an alloy of goldpalladium by sputter coating technique to provide surface conductivity. A JEOL JSM 880 scanning electron microscope operating at 15 kV was used to visually observe the coated specimens. The JEOL JSM 880 was fitted with an energy-dispersive X-ray spectrometer (EDS). The EDS was used to analyze chemical compositions of the specimen. In this technique, electrons are bombarded in the area of desired elemental composition; the elements present will emit characteristic X-rays, which are then recorded on a detector. The micrographs were taken using EDS2000 software. It must be noted that SEM study allows only a tiny area of raw and stabilized specimen to be examined (unlike engineering laboratory specimens). However, it is believed to be representative of the reaction process of stabilized specimens. Figure 1 (a) shows a photographic view of JEOL JSM 880 setup along

clay specimen.

**5.3 Mineralogical studies** 

with Hummer VI triode sputter coater.

(a) (b)

(corner picture), and (b) Rigaku D/Max X-ray diffractometer

Fig. 1. (a) JEOL JSM 880 setup for SEM, Hummer VI triode sputter coater with sample

The XRD tests were performed on raw soil and stabilized specimens. Two-day air dried mix was pulverized with a mortar and pestle, sieved through a U.S. standard No. 325 sieve (45 μm). Then, the powder finer than 45 μm was collected, mixed with methanol, and placed on a specimen holder. The holder containing specimen was oven dried for approximately 15 minutes prior to testing. This holder was then mounted on a Rigaku D/Max X-ray diffractometer for analysis. This diffractometer is equipped with bragg-brentano parafocusing geometry, a diffracted beam monochromator, and a conventional copper target X-ray tube set to 40 kV and 30 mA. The measurements were performed from 4o to 70o (2θ range), with 0.05o step size and 5 seconds count (dwell time) at each step. Data obtained by the diffractometer were analyzed with Jade 3.1, an X-ray powder diffraction analytical software, developed by Materials Data, Inc. (Jade, 1999). Generated diffractograms (using the peaks versus 2θ and d-spacing) were used to determine the presence of minerals. Figure 1 (b) shows a photographic view of Rigaku D/Max X-ray diffractometer.

## **6. Presentation and discussion of modulus of elasticity test results**

The variation of modulus of elasticity values with the additive content is shown in Figure 2. It is clear from Figure 2 that the modulus of elasticity of stabilized specimens is influenced by the type of additive. For example, an increase of approximately 99%, 94% and 193% in modulus of elasticity values was observed for 9% lime-, 15% CFA- and 15% CKD-stabilized specimens, respectively. Overall, 15% CKD-stabilized specimens showed the highest improvement.

Fig. 2. Variation of modulus of elasticity values with additive type

Attempts were made to observe the effect of additive properties, namely, free-lime content, alkali content, loss on ignition, specific surface area (SSA), pH, and passing No. 325 sieve, on the modulus of elasticity. The effect of these additive properties on normalized modulus of elasticity (modulus of elasticity value/percent additive) is depicted in Figure 3. Here, it is clear that the normalized modulus of elasticity value increases with the free-lime content. The Carnasaw soil specimens exhibited an increase of approximately 13 to 15 as the free-lime content increased from 6.7% (CKD) to 46.1% (lime). A decrease in normalized modulus of elasticity values with alkali content can be observed; however, increase in normalized modulus of elasticity values with loss on ignition was observed. This trend is contrary to the behavior reported by other researchers for different type of CKDs (e.g., Bhatty et al., 1996; Miller and Azad, 2000; Peethamparan and Olek, 2008). For example, Bhatty et al. (1996) reported that CKDs

Microstructural and Mineralogical Characterization

of CKD indicated presence of Ca, Si, Mg, S, and K minerals.

**7.1 Raw soil and additives powder** 

conductive.

of Clay Stabilized Using Calcium-Based Stabilizers 783

Figure 4 (a) shows the SEM micrographs of raw Carnasaw soil sample at high magnification (10,000 times). It is clear that the raw soil has a discontinuous structure, where the voids are more visible because of the absence of hydration products. The EDS results showed majority of silicon (Si) and aluminium (Al) minerals and trace amounts of potassium (K), iron (Fe) and sodium (Na) minerals in the raw soil. The raw additives used in this study were also studied using the SEM/EDS methods. Figures 4 (b), (c) and (d) show SEM/EDS of raw lime, CFA and CKD powder, respectively. As evident from Figure 4 (b), raw lime is an amorphous powder consisting mainly of calcium (Ca) compounds. This is in agreement with the XRF results reported in Table 1. On the other hand, CFA and CKD are more complex compounds. The EDS results indicated presence of Ca, Al, Si, Fe, sulphur (S), phosphorous (P), titanium (Ti), and magnesium (Mg) minerals in CFA. Whereas EDS results

The SEM micrographs of raw CFA showed that CFA is composed of different size spherical particles (or cenosphere); however, CKD micrographs showed particles with poorly defined shapes. The gold (Au) and palladium (Pd) peaks that appeared in all EDS spectra is due to the gold-palladium sputter coating used on SEM samples for making them electrically

(a)

(b)

containing less than 6% alkalis and low LOI values are reactive and produces higher strength. This difference in behavior could be attributed to other factors such as free-lime content that might have influenced in enhancing the effectiveness of the additives. Although CFA had higher alkali content and lower LOI than lime, it also had lower freelime content (0.2% for CFA versus 46.1% for lime).

Further, it is clear from Figure 3 that percent passing No. 325 sieve influences the Mr values. An increase in percent passing No. 325 sieve from 85.8% (CFA) to 98.4% (lime) increased the normalized modulus of elasticity values from 9 to 15 for P-soil. This can be attributed to increase in fine contents in the soil and thus increased surface area for pozzolanic reactivity. Normalized modulus of elasticity values versus SSA of additive are shown in Figure 3. It is clear that normalized modulus of elasticity increases with increase in SSA. The fact that the additive particles have a larger surface to interact with the soil can explain this behavior. Larger SSA values imply more available surface for soil-additive interaction resulting in more cementitious products and thus higher gain in modulus values. The pH value of additive also plays an important role in enhancing the modulus values, as evident from Figure 3. An increase in normalized modulus values with pH can be observed from Figure 3. Lime-stabilized specimens having highest pH value of 12.58 produced the highest modulus value followed by CKD- (pH = 12.55) and CFA- (pH = 11.83) stabilized specimens. As discussed earlier, high pH value causes silica from the clay minerals to dissolve and, in combination with Ca2+ form calcium silicate and calcium aluminate hydrate (Eades, 1962; Diamond and Kinter, 1964).

Fig. 3. Variation of normalized modulus of elasticity values with different properties of additives

#### **7. Microstructure and mineralogical characteristics**

As noted earlier, mineralogical studies namely SEM and EDS were conducted on all the raw soils, raw additives powder, raw additives paste and 28-day cured stabilized Carnasaw soil specimens to study the influence of stabilization on microstructure and mineralogical characteristics.

## **7.1 Raw soil and additives powder**

782 Scanning Electron Microscopy

containing less than 6% alkalis and low LOI values are reactive and produces higher strength. This difference in behavior could be attributed to other factors such as free-lime content that might have influenced in enhancing the effectiveness of the additives. Although CFA had higher alkali content and lower LOI than lime, it also had lower free-

Further, it is clear from Figure 3 that percent passing No. 325 sieve influences the Mr values. An increase in percent passing No. 325 sieve from 85.8% (CFA) to 98.4% (lime) increased the normalized modulus of elasticity values from 9 to 15 for P-soil. This can be attributed to increase in fine contents in the soil and thus increased surface area for pozzolanic reactivity. Normalized modulus of elasticity values versus SSA of additive are shown in Figure 3. It is clear that normalized modulus of elasticity increases with increase in SSA. The fact that the additive particles have a larger surface to interact with the soil can explain this behavior. Larger SSA values imply more available surface for soil-additive interaction resulting in more cementitious products and thus higher gain in modulus values. The pH value of additive also plays an important role in enhancing the modulus values, as evident from Figure 3. An increase in normalized modulus values with pH can be observed from Figure 3. Lime-stabilized specimens having highest pH value of 12.58 produced the highest modulus value followed by CKD- (pH = 12.55) and CFA- (pH = 11.83) stabilized specimens. As discussed earlier, high pH value causes silica from the clay minerals to dissolve and, in combination with Ca2+ form calcium silicate and calcium aluminate hydrate (Eades, 1962;

0 10 20 30 40 50 60 70 80 90 100

Passing No. 325 Sieve

Free Lime Loss on Ignition

Alkali Content Specific Surface Area

pH

Property of Additive

Fig. 3. Variation of normalized modulus of elasticity values with different properties of

As noted earlier, mineralogical studies namely SEM and EDS were conducted on all the raw soils, raw additives powder, raw additives paste and 28-day cured stabilized Carnasaw soil specimens to study the influence of stabilization on microstructure and mineralogical

**7. Microstructure and mineralogical characteristics** 

lime content (0.2% for CFA versus 46.1% for lime).

Diamond and Kinter, 1964).

Normalized Modulus of Elasticity

(Value / % Additive)

additives

characteristics.

Figure 4 (a) shows the SEM micrographs of raw Carnasaw soil sample at high magnification (10,000 times). It is clear that the raw soil has a discontinuous structure, where the voids are more visible because of the absence of hydration products. The EDS results showed majority of silicon (Si) and aluminium (Al) minerals and trace amounts of potassium (K), iron (Fe) and sodium (Na) minerals in the raw soil. The raw additives used in this study were also studied using the SEM/EDS methods. Figures 4 (b), (c) and (d) show SEM/EDS of raw lime, CFA and CKD powder, respectively. As evident from Figure 4 (b), raw lime is an amorphous powder consisting mainly of calcium (Ca) compounds. This is in agreement with the XRF results reported in Table 1. On the other hand, CFA and CKD are more complex compounds. The EDS results indicated presence of Ca, Al, Si, Fe, sulphur (S), phosphorous (P), titanium (Ti), and magnesium (Mg) minerals in CFA. Whereas EDS results of CKD indicated presence of Ca, Si, Mg, S, and K minerals.

The SEM micrographs of raw CFA showed that CFA is composed of different size spherical particles (or cenosphere); however, CKD micrographs showed particles with poorly defined shapes. The gold (Au) and palladium (Pd) peaks that appeared in all EDS spectra is due to the gold-palladium sputter coating used on SEM samples for making them electrically conductive.

Microstructural and Mineralogical Characterization

of Clay Stabilized Using Calcium-Based Stabilizers 785

(a)

(b)

**3**

1

(c)

Fig. 5. SEM/EDS of (a) Lime, (b) CFA, and (c) CKD Pastes

**2**

Fig. 4. SEM/EDS of (a) Raw Soil (b) Lime, (c) CFA, and (d) CKD Powder

#### **7.2 Raw additives paste**

The representative SEM micrographs of raw lime, CFA and CKD paste samples that had been subjected to 28 day curing and air dried for approximately two days are presented in Figures 5 (a) through (c). Figures 4 (b) and 5 (a) show very similar microstructure and EDS results, as expected in lime due to its negligible self-cementing properties. At a higher magnification (x 30,000 times), a flower-like structure of calcium hydroxide crystals is evident. Figure 5 (b) shows the SEM micrographs of raw CFA paste at a magnification level of 5,000. Overall, the microstructure of CFA paste had a relatively finer matrix with cenospheres covered with cementitious products. Based on EDS at different locations, the microstructure consisted of C-A-S-H like crystals with variable amounts of Al, Si, and Mg incorporated phases and traces of Fe and Ti as impurities. The Ca/Si ratio was observed to be approximately 2. The microstructure of CKD paste (Figure 5 (c)) is clearly denser and compact as compared to the microstructure of raw CKD powder (Figure 4 (d)). At a higher magnification (x 20,000 times), C-S-H gel is evident. The microstructure in combination with EDS spectrum gave an indication of the formation of C-S-H phases with Ca/Si ratio of less than 1. Please note that the EDS spectrum presented in this study were collected from a fractured surface of specimen and not from the polished smooth surface of specimen. Hence, the heights of the peaks were used as a qualitative measure rather than a quantitative measure of different crystal phases.

(c)

(d)

The representative SEM micrographs of raw lime, CFA and CKD paste samples that had been subjected to 28 day curing and air dried for approximately two days are presented in Figures 5 (a) through (c). Figures 4 (b) and 5 (a) show very similar microstructure and EDS results, as expected in lime due to its negligible self-cementing properties. At a higher magnification (x 30,000 times), a flower-like structure of calcium hydroxide crystals is evident. Figure 5 (b) shows the SEM micrographs of raw CFA paste at a magnification level of 5,000. Overall, the microstructure of CFA paste had a relatively finer matrix with cenospheres covered with cementitious products. Based on EDS at different locations, the microstructure consisted of C-A-S-H like crystals with variable amounts of Al, Si, and Mg incorporated phases and traces of Fe and Ti as impurities. The Ca/Si ratio was observed to be approximately 2. The microstructure of CKD paste (Figure 5 (c)) is clearly denser and compact as compared to the microstructure of raw CKD powder (Figure 4 (d)). At a higher magnification (x 20,000 times), C-S-H gel is evident. The microstructure in combination with EDS spectrum gave an indication of the formation of C-S-H phases with Ca/Si ratio of less than 1. Please note that the EDS spectrum presented in this study were collected from a fractured surface of specimen and not from the polished smooth surface of specimen. Hence, the heights of the peaks were used as a qualitative measure rather than a

Fig. 4. SEM/EDS of (a) Raw Soil (b) Lime, (c) CFA, and (d) CKD Powder

**7.2 Raw additives paste** 

quantitative measure of different crystal phases.

Fig. 5. SEM/EDS of (a) Lime, (b) CFA, and (c) CKD Pastes

Microstructural and Mineralogical Characterization

of Clay Stabilized Using Calcium-Based Stabilizers 787

(a)

**1**

(b)

(c) Fig. 6. SEM/EDS of 28-Day Cured 9% Lime-Stabilized Caranasaw Soil Specimens (a) C-S-H,

(b) C-A-S-H, and (c) CH

**2 3**

#### **7.3 Carnasaw soil with 9% lime**

To study the microstructure of 9% lime-stabilized Carnasaw soil specimens, 28-day modulus of elasticity tested specimens were examined using SEM micrographs. Figure 6 (a) shows the microstructure at a magnification level of 10,000, which when compared with the raw soil micrograph of Figure 4 (a) shows marked change in morphology. From Figure 6 (a), it is clear that the raw soil structure has transformed from a particle based form to a more integrated composition due to cementitious reactions. At a higher magnification (x 25,000 times), the cementing phases could clearly be seen. Further, EDS pattern was used as a basis to monitor the changes occurring in the chemical composition at selected locations within the Carnasaw soil after stabilization with 9% lime. As evident from Figure 6 (a), analysis on the cementing phases showed presence of Ca and Si with high Ca/Si ratio (>7), which is an indication of the presence of C-S-H (xCaO.ySiO2.zH2O).The other two peaks not marked in Figure 6 (a) belong to Au-Pd coating The cementing phases, due to gradual crystallization of the new secondary minerals, caused an increase in the modulus of elasticity of the stabilized soil, as discussed in Section 6. Similar observations were reported by other researchers (see e.g., Locat et al., 1996; Ghosh and Subbarao, 2001; Nalbantoglu, 2006; Kavak and Akyarh, 2007). Figure 6 (b) shows micrograph of same specimen with EDS spectra collected at three different locations. Three locations from which the EDS was secured are marked as 1, 2 and 3 in the micrographs. All three EDS spectra indicated presence of C-A-S-H with different Ca/Si ratios. For example, the Ca/Si ratio is higher at location 1 (0.8) as compared to location 2 (0.3). This gives an indication that C-A-S-H is at different levels of development at different locations, as expected.

Figure 6 (c) shows EDS spectra at a magnification level of 5,000 taken from a different location. A flower-like structure of calcium hydroxide crystals is evident, which indicates presence of un-reacted hydrated lime in the stabilized specimen. This is in agreement with the micrograph presented in Figure 5 (a) for the lime paste.

## **7.4 Carnasaw soil with 15% CFA**

The SEM micrographs of Carnasaw soil stabilized with 15% CFA are presented in Figures 7 (a) through (d). Figure 7 (a) reveals the formation of cementing products, with lamellar form, adjacent to the fly ash particles. The EDS analysis showed presence of Ca and Si indicating presence of C-S-H, the main cementing product responsible for strength gain (Choquette et al., 1987; Lav and Lav, 2000). The Ca/Si ratio of C-S-H phases identified in the CFA-stabilized soil was qualitatively determined to be approximately 3. Also, two additional peaks of Au and Pd appeared because specimens were sputter coated with alloy of gold-palladium. In viewing these samples, one would notice that the spherical particles of fly ash are joined strongly to the clay particles in its surrounding (Chang, 1995). It was also apparent that the fly ash particles served as nucleation sites for the growth of the hydration products (or coatings), as shown in Figure 7 (b). Formation of ettringite, Ca6[Al(OH)6]2.(SO4)3.26H2O, was also observed in the form of heaps of rod-like crystals (Figure 7 c). This observation was further confirmed by conducting EDS analysis, which suggested presence of Ca, Al and S with traces of Si and Ti as impurities. No areas were found showing normal ettringite spectra without traces of Si and Ti. Similar structure, as shown in Figure 2.25 (c), was reported as ettringite by other researchers (e.g., Mitchell and Dermatas, 1992; Intharasombat, 2003).

To study the microstructure of 9% lime-stabilized Carnasaw soil specimens, 28-day modulus of elasticity tested specimens were examined using SEM micrographs. Figure 6 (a) shows the microstructure at a magnification level of 10,000, which when compared with the raw soil micrograph of Figure 4 (a) shows marked change in morphology. From Figure 6 (a), it is clear that the raw soil structure has transformed from a particle based form to a more integrated composition due to cementitious reactions. At a higher magnification (x 25,000 times), the cementing phases could clearly be seen. Further, EDS pattern was used as a basis to monitor the changes occurring in the chemical composition at selected locations within the Carnasaw soil after stabilization with 9% lime. As evident from Figure 6 (a), analysis on the cementing phases showed presence of Ca and Si with high Ca/Si ratio (>7), which is an indication of the presence of C-S-H (xCaO.ySiO2.zH2O).The other two peaks not marked in Figure 6 (a) belong to Au-Pd coating The cementing phases, due to gradual crystallization of the new secondary minerals, caused an increase in the modulus of elasticity of the stabilized soil, as discussed in Section 6. Similar observations were reported by other researchers (see e.g., Locat et al., 1996; Ghosh and Subbarao, 2001; Nalbantoglu, 2006; Kavak and Akyarh, 2007). Figure 6 (b) shows micrograph of same specimen with EDS spectra collected at three different locations. Three locations from which the EDS was secured are marked as 1, 2 and 3 in the micrographs. All three EDS spectra indicated presence of C-A-S-H with different Ca/Si ratios. For example, the Ca/Si ratio is higher at location 1 (0.8) as compared to location 2 (0.3). This gives an indication that C-A-S-H is at different levels of development at

Figure 6 (c) shows EDS spectra at a magnification level of 5,000 taken from a different location. A flower-like structure of calcium hydroxide crystals is evident, which indicates presence of un-reacted hydrated lime in the stabilized specimen. This is in agreement with

The SEM micrographs of Carnasaw soil stabilized with 15% CFA are presented in Figures 7 (a) through (d). Figure 7 (a) reveals the formation of cementing products, with lamellar form, adjacent to the fly ash particles. The EDS analysis showed presence of Ca and Si indicating presence of C-S-H, the main cementing product responsible for strength gain (Choquette et al., 1987; Lav and Lav, 2000). The Ca/Si ratio of C-S-H phases identified in the CFA-stabilized soil was qualitatively determined to be approximately 3. Also, two additional peaks of Au and Pd appeared because specimens were sputter coated with alloy of gold-palladium. In viewing these samples, one would notice that the spherical particles of fly ash are joined strongly to the clay particles in its surrounding (Chang, 1995). It was also apparent that the fly ash particles served as nucleation sites for the growth of the hydration products (or coatings), as shown in Figure 7 (b). Formation of ettringite, Ca6[Al(OH)6]2.(SO4)3.26H2O, was also observed in the form of heaps of rod-like crystals (Figure 7 c). This observation was further confirmed by conducting EDS analysis, which suggested presence of Ca, Al and S with traces of Si and Ti as impurities. No areas were found showing normal ettringite spectra without traces of Si and Ti. Similar structure, as shown in Figure 2.25 (c), was reported as ettringite by other researchers (e.g., Mitchell and

**7.3 Carnasaw soil with 9% lime** 

different locations, as expected.

**7.4 Carnasaw soil with 15% CFA** 

Dermatas, 1992; Intharasombat, 2003).

the micrograph presented in Figure 5 (a) for the lime paste.

Fig. 6. SEM/EDS of 28-Day Cured 9% Lime-Stabilized Caranasaw Soil Specimens (a) C-S-H, (b) C-A-S-H, and (c) CH

Microstructural and Mineralogical Characterization

**3**

(d) reaction shell of CFA particle

**8.5 Carnasaw soil with 15% CKD** 

(Figure 8 (c)).

products.

**1** 

**2**

of Clay Stabilized Using Calcium-Based Stabilizers 789

**1**

(d) Fig. 7. (Cont'd). SEM/EDS of 28-Day Cured 15% CFA-Stabilized Caranasaw Soil Specimens

**<sup>2</sup> <sup>3</sup>**

The SEM micrographs, as illustrated in Figures 8 (a) through (e) show significant changes in the microstructure of raw soil when mixed with CKD and cured for 28 days. It could be observed that flat clay structure surface observed in Figure 4 (a) is covered with cementitious reaction products, as shown in Figure 8 (a). Figure 8 (a) shows the C-A-S-H (xCaO.yAl2O3.zSiO2.wH2O) phase development which contains distinct peaks of Ca, Si and Al elements based on the EDS analysis, consistent with observation reported by Chaunsali and Peethamparan (2010). The Ca/Si ratio of approximately 3 is also evident from Figure 8 (a). The SEM micrograph at a different location revealed presence of C-S-H phase with Ca/Si ratio less than 1 (Figure 8 (b)). Additionally, C-S-H phases with very high Ca/Si ratio (>10) are also evident from the SEM micrograph and EDS spectra taken at different locations

Figure 8 (d) shows micrographs of rose-shaped and web-shaped hydration coatings and bonds developed in 15% CKD-stabilized Carnasaw soil. Another prominent feature of the microstructure of 15% CKD-stabilized soil was the presence of needle-shaped ettringite crystals (Figure 8 (e)). The presence of ettringite crystals in CKD-stabilized soil is consistent with the observations reported by Peethamparan et al. (2008), Moon et al. (2009), and Chaunsali and Peethamparan (2011). Hence, improved modulus of elasticity exhibited by CKD-stabilized soil specimens after curing could be attributed to aforementioned reaction

Further, the SEM micrographs revealed that most of the fly ash particles were covered with a reaction shell as seen in Figure 7 (d). The approximate chemical composition of the outer shell was determined at location 1 and 3 by the EDS analysis and a typical composition is presented in pattern marked as point and 1 and 3. The composition of the shell was slightly different from that of the un-reacted inner fly ash surface which is shown in spectrum 2. The higher Ca peak in 1 and 3 compared to spectrum 2 suggests the initiation of reaction products (e.g., C-A-S-H) formation on the surface of fly ash particle. It should be noted that the exact quantitative composition cannot be obtained using the EDS analysis of the stabilized specimens.

Fig. 7. SEM/EDS of 28-Day Cured 15% CFA-Stabilized Caranasaw Soil Specimens (a) C-S-H, (b) hydration coatings, and (c) ettringite crystal

Further, the SEM micrographs revealed that most of the fly ash particles were covered with a reaction shell as seen in Figure 7 (d). The approximate chemical composition of the outer shell was determined at location 1 and 3 by the EDS analysis and a typical composition is presented in pattern marked as point and 1 and 3. The composition of the shell was slightly different from that of the un-reacted inner fly ash surface which is shown in spectrum 2. The higher Ca peak in 1 and 3 compared to spectrum 2 suggests the initiation of reaction products (e.g., C-A-S-H) formation on the surface of fly ash particle. It should be noted that the exact quantitative composition cannot be obtained using the EDS analysis of the

(a)

(b)

(c)

(b) hydration coatings, and (c) ettringite crystal

Fig. 7. SEM/EDS of 28-Day Cured 15% CFA-Stabilized Caranasaw Soil Specimens (a) C-S-H,

stabilized specimens.

Fig. 7. (Cont'd). SEM/EDS of 28-Day Cured 15% CFA-Stabilized Caranasaw Soil Specimens (d) reaction shell of CFA particle

## **8.5 Carnasaw soil with 15% CKD**

The SEM micrographs, as illustrated in Figures 8 (a) through (e) show significant changes in the microstructure of raw soil when mixed with CKD and cured for 28 days. It could be observed that flat clay structure surface observed in Figure 4 (a) is covered with cementitious reaction products, as shown in Figure 8 (a). Figure 8 (a) shows the C-A-S-H (xCaO.yAl2O3.zSiO2.wH2O) phase development which contains distinct peaks of Ca, Si and Al elements based on the EDS analysis, consistent with observation reported by Chaunsali and Peethamparan (2010). The Ca/Si ratio of approximately 3 is also evident from Figure 8 (a). The SEM micrograph at a different location revealed presence of C-S-H phase with Ca/Si ratio less than 1 (Figure 8 (b)). Additionally, C-S-H phases with very high Ca/Si ratio (>10) are also evident from the SEM micrograph and EDS spectra taken at different locations (Figure 8 (c)).

Figure 8 (d) shows micrographs of rose-shaped and web-shaped hydration coatings and bonds developed in 15% CKD-stabilized Carnasaw soil. Another prominent feature of the microstructure of 15% CKD-stabilized soil was the presence of needle-shaped ettringite crystals (Figure 8 (e)). The presence of ettringite crystals in CKD-stabilized soil is consistent with the observations reported by Peethamparan et al. (2008), Moon et al. (2009), and Chaunsali and Peethamparan (2011). Hence, improved modulus of elasticity exhibited by CKD-stabilized soil specimens after curing could be attributed to aforementioned reaction products.

Microstructural and Mineralogical Characterization

(d) hydration coatings, and (e) ettringite crystals

were reported by Chaunsali and Peethamparan (2010).

**9. XRD results** 

of Clay Stabilized Using Calcium-Based Stabilizers 791

(d)

(e)

Fig. 8. (Cont'd). SEM/EDS of 28-Day Cured 15% CKD-Stabilized Caranasaw Soil Specimens

The XRD patterns of the raw soil and additives powder is presented in Figure 9. The raw soil showed presence of clay minerals namely, illite, (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]) and kaolinite (Al2Si2O5(OH)4) which are expected in a fat clay with a PI value of 29. The presence of elements (Si, Al, Na, Fe, K) in aforementioned minerals of raw soil is also in agreement with elements observed from, EDS spectrum (Figure 4 (a)). The XRD pattern of raw lime powder indicates only presence of calcite (CaCO3) and calcium hydroxide (Ca(OH)2). The diffractogram of CFA powder revealed presence of three minerals namely, quartz (SiO2), mullite (Al6Si2O13), and merwinite (Ca3Mg(SiO4)2). This is in agreement with XRF results presented in Table 1. Similar minerals were reported by other researchers for class C fly ash (e.g., McCarthy, 2000; Chaunsali and Peethamparan, 2010). The CKD diffractogram showed presence of calcite, quick lime (CaO), quartz, and anhydrite (CaSO4). Similar observations

The XRD patterns of raw and stabilized specimens are presented in one figure for comparison purpose (Figure 10). In general, there was a reduction in the peak intensity of most of the stabilized specimens, as can be seen by the reduction in peak heights of some of the peaks, particularly for the samples stabilized with 9% lime. This could be attributed to cementitious reactions between additive and clay minerals resulting in the reduction of the clay mineral intensities. Similar behavior was reported by Al-Rawas (2002). None of the peaks of the raw soil disappeared due to stabilization. It was noted that with the addition of 15% CFA, the peaks shifted away to the right from their original positions while

Fig. 8. SEM/EDS of 28-Day Cured 15% CKD-Stabilized Caranasaw Soil Specimens (a) C-A-S-H, (b) C-S-H, and (c) C-S-H

(a)

(b)

**x 25,000** 

 (c) Fig. 8. SEM/EDS of 28-Day Cured 15% CKD-Stabilized Caranasaw Soil Specimens (a) C-A-

S-H, (b) C-S-H, and (c) C-S-H

Fig. 8. (Cont'd). SEM/EDS of 28-Day Cured 15% CKD-Stabilized Caranasaw Soil Specimens (d) hydration coatings, and (e) ettringite crystals

## **9. XRD results**

The XRD patterns of the raw soil and additives powder is presented in Figure 9. The raw soil showed presence of clay minerals namely, illite, (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]) and kaolinite (Al2Si2O5(OH)4) which are expected in a fat clay with a PI value of 29. The presence of elements (Si, Al, Na, Fe, K) in aforementioned minerals of raw soil is also in agreement with elements observed from, EDS spectrum (Figure 4 (a)). The XRD pattern of raw lime powder indicates only presence of calcite (CaCO3) and calcium hydroxide (Ca(OH)2). The diffractogram of CFA powder revealed presence of three minerals namely, quartz (SiO2), mullite (Al6Si2O13), and merwinite (Ca3Mg(SiO4)2). This is in agreement with XRF results presented in Table 1. Similar minerals were reported by other researchers for class C fly ash (e.g., McCarthy, 2000; Chaunsali and Peethamparan, 2010). The CKD diffractogram showed presence of calcite, quick lime (CaO), quartz, and anhydrite (CaSO4). Similar observations were reported by Chaunsali and Peethamparan (2010).

The XRD patterns of raw and stabilized specimens are presented in one figure for comparison purpose (Figure 10). In general, there was a reduction in the peak intensity of most of the stabilized specimens, as can be seen by the reduction in peak heights of some of the peaks, particularly for the samples stabilized with 9% lime. This could be attributed to cementitious reactions between additive and clay minerals resulting in the reduction of the clay mineral intensities. Similar behavior was reported by Al-Rawas (2002). None of the peaks of the raw soil disappeared due to stabilization. It was noted that with the addition of 15% CFA, the peaks shifted away to the right from their original positions while

Microstructural and Mineralogical Characterization

sieve, specific surface area, and pH.

**10. Conclusions** 

surface.

**11. Acknowledgment** 

**12. References** 

of Clay Stabilized Using Calcium-Based Stabilizers 793

1. The results from pH tests showed that 5% lime provide an asymptotic behavior (less than 1% increase in pH w.r.t raw soil pH) in Carnasaw soil-lime mixtures. No such asymptotic behavior was observed for Carnasaw soil stabilized with CFA and CKD. 2. All three additives improved the modulus of elasticity values of Carnasaw soil (fat clay) specimens; however, degree of improvement varied with the type of additive. 3. The normalized modulus of elasticity values is better correlated with additive properties – free-lime content, alkali content, loss on ignition, percent passing No. 325

4. In general, microscopic analyzes confirm that the addition of lime or CFA or CKD to soil induces beneficial reactions and significant improvements in stiffness. Also, it could be concluded that the formation of reaction products such as C-S-H, C-A-S-H and

5. The lime-stabilized specimens indicated presence of both C-S-H and C-A-S-H phases in addition to unreacted calcium hydroxide. Additionally, C-A-S-H at different levels of

6. The SEM and EDS results showed presence of C-S-H, C-A-S-H and ettringite crystals in both CFA-and CKD-stabilized specimens. The fly ash particles served as nucleation sites for the growth of the hydration products with reaction products initiating from the

7. The Ca/Si ratio in C-S-H and C-A-S-H phases was found vary between 0.3 – 12 at different locations of the stabilized specimen. The presence of higher Ca/Si ratio phases in the microstructure of the stabilized specimen could be due to possible secondary

8. The XRD results showed a general reduction in all clay minerals' peak intensities particularly in the case of lime-stabilized samples. However, none of the peaks of the

The authors are thankful to the Oklahoma Department of Transportation (ODOT) and Oklahoma Transportation Center (OTC) for providing funds for this project. Technical assistance from Dr. Preston Larson (University of Oklahoma) and Tim Rawlsky (Lafarge

Air Force Manual (AFJMAN) (1994). "Soil Stabilization for Pavements," *Technical Manual No. 5-822-14*, Departments of the Army and Air Force, Washington, DC. Al-Rawas, A. A. (2002). Microfabric and mineralogical studies on the stabilization of an

Al-Rawas, A. A., Taha, R., Nelson, J. D., Al-Shab, T. B., and Al-Siyabi, H. (2002). "A

expansive soil using cement by-pass dust and some types of slags. *Canadian* 

Comparative Evaluation of Various Additives Used in the Stabilization of Expansive Soils," *ASTM Geotechnical Testing Journal*, Vol. 25, No. 2, pp. 199 – 209.

ettringite contributed to strength development of stabilized soil.

cementitious reactions between soil and Ca2+ ion of additive.

*Geotechnical Journal*, Vol. 39, Issue 5, pp. 1150-1167

development at different locations was identified.

raw soil disappeared due to stabilization.

North America) is gratefully acknowledged.

Based on the study presented, in this chapter the following conclusions can be derived:

maintaining the same patterns. This could be due to instrument distortion during the XRD scanning process or conditions of the sample. The XRD pattern of CKD-stabilized specimens revealed additional peaks of calcite formed due to cementitious reactions.

Fig. 9. XRD pattern for raw soil, lime, CFA and CKD powders [CC-calcite (CaCO3), Aanhydrite (CaSO4), QL-quicklime (CaO), Q-quartz (SiO2), Mu-mullite (Al6Si2O13), Mwmerwinite (Ca3Mg(SiO4)2), CH-calcium hydroxide (Ca(OH)2), I-illite, K-kaolinite)

Fig. 10. XRD pattern for 9% lime-, 15% CFA- and 15% CKD-stabilized specimens [CC-calcite (CaCO3), Q-quartz (SiO2), I-illite, K-kaolinite)

## **10. Conclusions**

792 Scanning Electron Microscopy

maintaining the same patterns. This could be due to instrument distortion during the XRD scanning process or conditions of the sample. The XRD pattern of CKD-stabilized specimens

**Q CC**

**Mu Mu**

**CC CC CH CH CH**

**CC**

**CC**

4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 Diffraction Angle 2θ (degrees)

4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 Diffraction Angle 2θ (degrees)

Fig. 10. XRD pattern for 9% lime-, 15% CFA- and 15% CKD-stabilized specimens [CC-calcite

**<sup>I</sup> <sup>K</sup> <sup>Q</sup> <sup>Q</sup> <sup>Q</sup> <sup>Q</sup>**

**Q**

(CaCO3), Q-quartz (SiO2), I-illite, K-kaolinite)

**CC**

Fig. 9. XRD pattern for raw soil, lime, CFA and CKD powders [CC-calcite (CaCO3), Aanhydrite (CaSO4), QL-quicklime (CaO), Q-quartz (SiO2), Mu-mullite (Al6Si2O13), Mwmerwinite (Ca3Mg(SiO4)2), CH-calcium hydroxide (Ca(OH)2), I-illite, K-kaolinite)

**CC**

**Q Q Q**

Raw Soil

Raw Soil

Soil with 9% lime

Soil with 15% CFA

Soil with 15% CKD

Raw Lime

Raw CFA

Raw CKD

revealed additional peaks of calcite formed due to cementitious reactions.

**CC**

**CC A QL**

**Q**

**CC CH**

**<sup>Q</sup> Mw**

**<sup>I</sup> <sup>K</sup> <sup>Q</sup>**

**CH**

Based on the study presented, in this chapter the following conclusions can be derived:


## **11. Acknowledgment**

The authors are thankful to the Oklahoma Department of Transportation (ODOT) and Oklahoma Transportation Center (OTC) for providing funds for this project. Technical assistance from Dr. Preston Larson (University of Oklahoma) and Tim Rawlsky (Lafarge North America) is gratefully acknowledged.

## **12. References**


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**39** 

*1,3Sweden 2Denmark* 

**The Use of ESEM in Geobiology** 

*3Stockholm University, Department of Geological Sciences,* 

*1Swedish Museum of Natural History, Department of Palaeozoology,* 

Geobiology is an interdisciplinary field of research that explores the interaction between the biosphere and the geosphere and/or the atmosphere. It involves researchers from numerous fields such as paleontology, microbiology, mineralogy, geochemistry, biochemistry, sedimentology and genetics. Geobiological research cover a wide range of areas like, for example, the origin and evolution of life, environmental microbiology, microbe-mineral interactions, molecular ecology and detection of biomarkers. It is responsible for at least two major subdisciplines: geomicrobiology (the study of microbe-mineral interactions) and astrobiology (a discipline focused on the conditions for life in the universe, including the

A major part of most geobiological research is focused on interactions between microorganisms and minerals or other substrates. This includes everything from microbes in soil and sediments, via fossilized microorganisms in rock and minerals, to extremophiles at hydrothermal vents. There are numerous methods, protocols and instruments that are used for this kind of research but one of the most basic methods and commonly used is the Scanning Electron Microscope (SEM). SEM is easy operated and can give high resolved images down to micro meter size, which is a requirement when analysing microorganisms. Coupled with, for example, an energy dispersive X-ray spectroscopy (EDS or EDX) detector it becomes a valuable tool for elemental analysis which is a critical part of geobiological research. Microbe-mineral interactions most commonly result in micro-sized biomineralizations or amorphous precipitates that may contain important information about the metabolism and life-cycles of the microbes, redox chemistry in the microbial habitat, and

Geobiological samples usually involve living species of microorganisms or fragile materials like fossilized microorganisms, organic matter or hydrated minerals like clays that collapse in conventional SEM. Environmental Scanning Electron Microscopy (ESEM) is a modification of conventional SEM originally developed for the study of biological samples but has also become more frequently used within geobiological related research. With its gaseous environment in the specimen chamber as well as other technical applications ESEM makes it possible to study wet and uncoated specimens in their natural state. This is a

**1. Introduction**

search for life on other planets).

paleoenvironmental conditions.

substantial advantage opposed to conventional SEM.

Magnus Ivarsson1,2 and Sara Holmström2

*2Nordic Center for Earth Evolution (NordCEE),* 


## **The Use of ESEM in Geobiology**

## Magnus Ivarsson1,2 and Sara Holmström2

*1Swedish Museum of Natural History, Department of Palaeozoology, 2Nordic Center for Earth Evolution (NordCEE), 3Stockholm University, Department of Geological Sciences, 1,3Sweden 2Denmark* 

## **1. Introduction**

798 Scanning Electron Microscopy

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Geobiology is an interdisciplinary field of research that explores the interaction between the biosphere and the geosphere and/or the atmosphere. It involves researchers from numerous fields such as paleontology, microbiology, mineralogy, geochemistry, biochemistry, sedimentology and genetics. Geobiological research cover a wide range of areas like, for example, the origin and evolution of life, environmental microbiology, microbe-mineral interactions, molecular ecology and detection of biomarkers. It is responsible for at least two major subdisciplines: geomicrobiology (the study of microbe-mineral interactions) and astrobiology (a discipline focused on the conditions for life in the universe, including the search for life on other planets).

A major part of most geobiological research is focused on interactions between microorganisms and minerals or other substrates. This includes everything from microbes in soil and sediments, via fossilized microorganisms in rock and minerals, to extremophiles at hydrothermal vents. There are numerous methods, protocols and instruments that are used for this kind of research but one of the most basic methods and commonly used is the Scanning Electron Microscope (SEM). SEM is easy operated and can give high resolved images down to micro meter size, which is a requirement when analysing microorganisms. Coupled with, for example, an energy dispersive X-ray spectroscopy (EDS or EDX) detector it becomes a valuable tool for elemental analysis which is a critical part of geobiological research. Microbe-mineral interactions most commonly result in micro-sized biomineralizations or amorphous precipitates that may contain important information about the metabolism and life-cycles of the microbes, redox chemistry in the microbial habitat, and paleoenvironmental conditions.

Geobiological samples usually involve living species of microorganisms or fragile materials like fossilized microorganisms, organic matter or hydrated minerals like clays that collapse in conventional SEM. Environmental Scanning Electron Microscopy (ESEM) is a modification of conventional SEM originally developed for the study of biological samples but has also become more frequently used within geobiological related research. With its gaseous environment in the specimen chamber as well as other technical applications ESEM makes it possible to study wet and uncoated specimens in their natural state. This is a substantial advantage opposed to conventional SEM.

The Use of ESEM in Geobiology 801

elements in the ocean and the lithosphere, metabolism of hydrocarbons and transformation of organic carbon, fractionation of stable isotopes etc (e.g. Lindsay and Brasier, 2002; Tice and Lowe, 2004; Lowe and Tice, 2007; Furnes et al., 2008). But it is not only biological activity that influences geological processes. It works both ways and geological processes control and influence the microbial ecology as well, something that is explicit in, for example, extreme environments (Huber et al., 2007). The discovery of the subsurface biosphere has deepen the knowledge of life´s distribution, adaptability and variety (Pedersen, 1993). Earths surface is no longer the limit for habitability. The subsurface is just as colonized and may contain as much as one third of the Earths biomass (Gold, 1992; Staudigel et al., 2004). Life in extreme environments and in the subsurface further show that Earth itself may not be the limit for life but that it may extend beyond. Astrobiology has shown that the conditions for life exist on other planets as well, and that life could have originated or been transported there, and

Experimental approaches to examine specimens in chambers filled with water or atmospheric gas with conventional and scanning transmission types of electron microscopes were reported of as early as the 1940s (Ardenne and Beischer, 1940; Abrams and McBain, 1944; Swift and Brown, 1970; Parsons et al., 1974). Such experiments used different kinds of "environmental cells" where gas was introduced temporarily during the examination, however, neither of these experiments succeeded in creating a stable environmental cell for routine analysis. In 1970 the first images of wet specimens in an SEM were published by Lane (1970), who injected a jet of water vapor over the point of observation at the specimen surface. The gas diffused in the chamber without any damage to the instrument. The need for differentially pumped chambers to allow for the transfer of the electron beam from the high vacuums in the gun area to the high pressures in the specimen chamber forced developments during the 1970s. Shah and Beckett (1977) reported of differentially pumped chambers to maintain botanical specimens conductive for signal detection, and Robinson (1974) reported of improvements by combining a backscattered electron detector with differential vacuum pumping and introduction of water vapour around 600 Pa at the freezing point of temperature. In 1978, Gerasimos Danilatos, a Greek-Australian physicist started to work with Robinson at the University of New South Wales in Sydney, Australia, and designed the original ESEM that were operable at room temperature and high pressures up to 7000 Pa (Danilatos and Robinson, 1979). During the 1980s and early 1990s Danilatos developed and optimized the design of the ESEM (Danilatos, 1981, 1985, 1988, 1990a, b; Danilatos and Postle, 1983). He reported the construction of an ESEM capable of working at any pressure from vacuum up to one atmosphere, optimization of the use of differential pumping systems combined with electron backscatter detectors, the idea of the environmental gas itself as detection medium, and the invention of the gaseous detection

The functioning of an ESEM is in many ways identical to a conventional SEM and it is assumed that the reader is familiar with the operation of a SEM. Basically, an ESEM is a

possibly be able to sustain (Farmer and Des Marais, 1999).

**3.1 History** 

device (GDD).

**3.2 Description of ESEM** 

**3. Environmental Electron Scanning Microscope (ESEM)** 

The main difference between ESEM and SEM is that the first has a specimen chamber where the specimen can be imaged while gas is present. The specimen chamber is designed to maintain water in its liquid phase and for that a minimum water vapour pressure of 609 Pa (6.09 mbar or 4.579 Torr) is required at Oº C. This creates new possibilities: A) Hydrated specimens can be examined in contrast to SEM where specimens are desiccated by the vacuum. Thus biological specimens can be maintained fresh and live. B) Specimens do not require preparation techniques used in SEM such as the deposition of a thin gold or carbon coating. Such techniques sometimes require vacuum and can disturb the samples. Biological samples also need to be dehydrated before coating which is a time consuming process. The gas in ESEM is electrically conductive due to the ionization, which prevents that negative charge accumulates and this is the reason why specimens do not need to be coated prior to examination. Thus, with ESEM specimens can be examined faster and more easily, without complex and time consuming preparation methods and without modifications or in worst case damage to the sample surface by preparation work and exposure to vacuum. The aim with this paper is to give a brief background to geobiology and ESEM, and to show the advantages of ESEM over conventional SEM in the study of geobiology.

#### **2. Geobiology: The link between geology and biology**

Geobiology as an independent discipline is relatively new and has attracted a lot of attention during the last decade with the result of an increased number of active researchers, foundation of international scientific journals as well as centers and institutions worldwide devoted to geobiological research. However, the link between life and geological processes can be traced back as far as the foundational text in modern geology by James Hutton (1788). He documented quite ordinary observations that anyone could have done about erosion of land into the oceans by rivers and the presence of fossilized shells in sedimentary rocks in the mountains of Scotland. What Hutton managed to do was to put these observations in a context where he questioned the surface of Earth as a sustainable habitat for life. In the early part of the 20th century Vernadsky (1926) further explored the connection between life and geological processes, and Baas Becking (1934) coined the term geobiology and outlined many geobiological processes much as we see them today.

The last decades have involved a growing awareness of the close connection between the physical world and life sciences. Earth as a system is complex and not as black and white as previously thought. The traditional way when studying the Earth system in dividing it into separated disciplines like geology, biology, chemistry or physics are not always the most practical approach. An interdisciplinary perspective and awareness when looking at the Earth system is almost a requirement to understand it and move forward in Earth sciences. Geobiology is a result of interdisciplinary thinking within geology and biology and their subdisciplines, and geobiology as a science has shown that there is no distinct boundary between the both. They are tightly connected and interact with each other on many levels, both at the present but also throughout Earth´s history (Knoll, 2003). The evolution of life has been intimately connected with the mineral evolution (Hazen et al., 2008), the rise of continents (Rosing et al., 2006), emergence of the aerobic biosphere (Melezhik et al., 2005), formation of fossil fuel and ore formations (Southam & Saunders, 2005). Ever since their emergence on Earth microbes have played an important role as geological agents involved in mineral growth and dissolution, rock and mineral weathering and alteration, mobilization of metals, cycling of

The main difference between ESEM and SEM is that the first has a specimen chamber where the specimen can be imaged while gas is present. The specimen chamber is designed to maintain water in its liquid phase and for that a minimum water vapour pressure of 609 Pa (6.09 mbar or 4.579 Torr) is required at Oº C. This creates new possibilities: A) Hydrated specimens can be examined in contrast to SEM where specimens are desiccated by the vacuum. Thus biological specimens can be maintained fresh and live. B) Specimens do not require preparation techniques used in SEM such as the deposition of a thin gold or carbon coating. Such techniques sometimes require vacuum and can disturb the samples. Biological samples also need to be dehydrated before coating which is a time consuming process. The gas in ESEM is electrically conductive due to the ionization, which prevents that negative charge accumulates and this is the reason why specimens do not need to be coated prior to examination. Thus, with ESEM specimens can be examined faster and more easily, without complex and time consuming preparation methods and without modifications or in worst case damage to the sample surface by preparation work and exposure to vacuum. The aim with this paper is to give a brief background to geobiology and ESEM, and to show the

Geobiology as an independent discipline is relatively new and has attracted a lot of attention during the last decade with the result of an increased number of active researchers, foundation of international scientific journals as well as centers and institutions worldwide devoted to geobiological research. However, the link between life and geological processes can be traced back as far as the foundational text in modern geology by James Hutton (1788). He documented quite ordinary observations that anyone could have done about erosion of land into the oceans by rivers and the presence of fossilized shells in sedimentary rocks in the mountains of Scotland. What Hutton managed to do was to put these observations in a context where he questioned the surface of Earth as a sustainable habitat for life. In the early part of the 20th century Vernadsky (1926) further explored the connection between life and geological processes, and Baas Becking (1934) coined the term geobiology and outlined

The last decades have involved a growing awareness of the close connection between the physical world and life sciences. Earth as a system is complex and not as black and white as previously thought. The traditional way when studying the Earth system in dividing it into separated disciplines like geology, biology, chemistry or physics are not always the most practical approach. An interdisciplinary perspective and awareness when looking at the Earth system is almost a requirement to understand it and move forward in Earth sciences. Geobiology is a result of interdisciplinary thinking within geology and biology and their subdisciplines, and geobiology as a science has shown that there is no distinct boundary between the both. They are tightly connected and interact with each other on many levels, both at the present but also throughout Earth´s history (Knoll, 2003). The evolution of life has been intimately connected with the mineral evolution (Hazen et al., 2008), the rise of continents (Rosing et al., 2006), emergence of the aerobic biosphere (Melezhik et al., 2005), formation of fossil fuel and ore formations (Southam & Saunders, 2005). Ever since their emergence on Earth microbes have played an important role as geological agents involved in mineral growth and dissolution, rock and mineral weathering and alteration, mobilization of metals, cycling of

advantages of ESEM over conventional SEM in the study of geobiology.

**2. Geobiology: The link between geology and biology** 

many geobiological processes much as we see them today.

elements in the ocean and the lithosphere, metabolism of hydrocarbons and transformation of organic carbon, fractionation of stable isotopes etc (e.g. Lindsay and Brasier, 2002; Tice and Lowe, 2004; Lowe and Tice, 2007; Furnes et al., 2008). But it is not only biological activity that influences geological processes. It works both ways and geological processes control and influence the microbial ecology as well, something that is explicit in, for example, extreme environments (Huber et al., 2007). The discovery of the subsurface biosphere has deepen the knowledge of life´s distribution, adaptability and variety (Pedersen, 1993). Earths surface is no longer the limit for habitability. The subsurface is just as colonized and may contain as much as one third of the Earths biomass (Gold, 1992; Staudigel et al., 2004). Life in extreme environments and in the subsurface further show that Earth itself may not be the limit for life but that it may extend beyond. Astrobiology has shown that the conditions for life exist on other planets as well, and that life could have originated or been transported there, and possibly be able to sustain (Farmer and Des Marais, 1999).

## **3. Environmental Electron Scanning Microscope (ESEM)**

#### **3.1 History**

Experimental approaches to examine specimens in chambers filled with water or atmospheric gas with conventional and scanning transmission types of electron microscopes were reported of as early as the 1940s (Ardenne and Beischer, 1940; Abrams and McBain, 1944; Swift and Brown, 1970; Parsons et al., 1974). Such experiments used different kinds of "environmental cells" where gas was introduced temporarily during the examination, however, neither of these experiments succeeded in creating a stable environmental cell for routine analysis. In 1970 the first images of wet specimens in an SEM were published by Lane (1970), who injected a jet of water vapor over the point of observation at the specimen surface. The gas diffused in the chamber without any damage to the instrument. The need for differentially pumped chambers to allow for the transfer of the electron beam from the high vacuums in the gun area to the high pressures in the specimen chamber forced developments during the 1970s. Shah and Beckett (1977) reported of differentially pumped chambers to maintain botanical specimens conductive for signal detection, and Robinson (1974) reported of improvements by combining a backscattered electron detector with differential vacuum pumping and introduction of water vapour around 600 Pa at the freezing point of temperature. In 1978, Gerasimos Danilatos, a Greek-Australian physicist started to work with Robinson at the University of New South Wales in Sydney, Australia, and designed the original ESEM that were operable at room temperature and high pressures up to 7000 Pa (Danilatos and Robinson, 1979). During the 1980s and early 1990s Danilatos developed and optimized the design of the ESEM (Danilatos, 1981, 1985, 1988, 1990a, b; Danilatos and Postle, 1983). He reported the construction of an ESEM capable of working at any pressure from vacuum up to one atmosphere, optimization of the use of differential pumping systems combined with electron backscatter detectors, the idea of the environmental gas itself as detection medium, and the invention of the gaseous detection device (GDD).

#### **3.2 Description of ESEM**

The functioning of an ESEM is in many ways identical to a conventional SEM and it is assumed that the reader is familiar with the operation of a SEM. Basically, an ESEM is a

The Use of ESEM in Geobiology 803

The basic principle of an ESEM is, thus, to have a high pressure and gaseous specimen chamber, and a high vacuum electron optics area separated from each other but still connected to allow for the transfer of the electron beam. The two regions are separated by at least two small pressure limiting apertures, one aperture that separates the high vacuum region of the electron gun and an intermediate cavity. The second aperture separates the intermediate cavity and the high pressure specimen cavity. Gas leaking from the specimen chamber through the aperture to the intermediate cavity is instantly removed by a pump system. This is called differential pumping. Gas that escapes further into the high vacuum area of the electron optics is similarly removed by a pump to maintain required vacuum. Additional pumping

An electron beam generated in the vacuum of the upper column will on its way through the intermediate cavity and in the specimen chamber come in contact with an increasing amount of gas molecules. This will result in a gradual loss of electrons due to electron scattering by the gas molecules and eventually total loss of the beam. However, the electrons are scattered over a broad skirt-like area around the focused spot and since the skirt width is orders of magnitude greater than the spot width, the skirt only contribute background noise and the amount of electrons in the original focused spot is enough for imaging of the specimen. The remaining electron beam is, however, only a fraction of what it was in the upper column and merely enough for imaging. The particular conditions of pressure, distance and beam voltage is crucial for signal detection and the operation of an ESEM is centered on refining the instrument for optimum performance and achieving precision for the instrument to operate close to its physical limit (Danilatos, 2009). By doing this it is possible to use an ESEM in much the same way as a SEM. Secondary and backscattered electrons, X-rays and cathodoluminescence is generated as in a SEM and can be detected with slight modifications to the detectors. The main difference regarding detectors is that the conventional secondary electron detector of SEM, the Everhart-Thornley detector, can not be used in the presence of gas and thus the gaseous detection device (GDD) has been developed. The principle of the GDD is that the environmental gas itself is used for beam transmission and as a detector of the electrons (Danilatos, 1997, 1990a), compared to the Everhart-Thornley detector where light guide the transmitted electrons. In a GDD the signals emanating from the beam specimen-interaction interact with the surrounding gas in the form of gaseous ionization and excitation. The ionized gas is then collected by electrodes

In conventional SEM negative charge is accumulated as the electron beam impinge on the surface of the specimen. This tends to deflect the electron beam from the scanned point with the result of charging artefacts on the image which greatly disturbs the imaging and analyses. This is normally eliminated in conventional SEM by coating the specimen prior to examination by a thin layer of usually gold or carbon. The gas in ESEM is electrically conductive due to the ionization, which prevents that negative charge accumulates and this

is the reason why specimens do not need to be coated prior to examination.

stages may be added to achieve an even higher vacuum in the electron optics area.

**3.2.2 Differential pumping** 

**3.2.3 Electron beam transfer** 

and the signal is amplified for its purpose.

**3.2.4 Specimen charging** 

SEM that can operate at the low pressure of a usual SEM through to, at least, the pressure required to observe liquid distilled water. An ESEM, just like a SEM, employs a scanned electron beam and electromagnetic lenses to focus and direct the beam on the specimen surface. A very small focused electron spot is scanned in a raster form over a small specimen area and the beam electrons interact with the specimen surface layer and produce various signals. These signals are collected with appropriate detectors and can be monitored in the form of images, graphs, digital recordings etc. Beyond these common principles, the ESEM deviates substantially from a SEM in several respects briefly outlined below.

Fig. 1. Schematic illustration of an ESEM lens. Cross section showing the high vacuum region and low pressure region as well as the direction of the gas flow. From the XL30 Options Manual, FEI and Philips (1998).

#### **3.2.1 Separated regions**

The ESEM must have, just like a conventional SEM, a high vacuum region (usually with pressure less than 10-2 Pa) for the generation and focusing of the electron beam. In a SEM the high vacuum region and the region of the specimen is the same but in an ESEM these two regions must be separated (Fig. 1). The specimen chamber in an ESEM is designed to maintain water in its liquid phase and for that a minimum water vapour pressure of 609 Pa (6.09 mbar or 4.579 Torr) is required at Oº C.

SEM that can operate at the low pressure of a usual SEM through to, at least, the pressure required to observe liquid distilled water. An ESEM, just like a SEM, employs a scanned electron beam and electromagnetic lenses to focus and direct the beam on the specimen surface. A very small focused electron spot is scanned in a raster form over a small specimen area and the beam electrons interact with the specimen surface layer and produce various signals. These signals are collected with appropriate detectors and can be monitored in the form of images, graphs, digital recordings etc. Beyond these common principles, the ESEM

deviates substantially from a SEM in several respects briefly outlined below.

Fig. 1. Schematic illustration of an ESEM lens. Cross section showing the high vacuum region and low pressure region as well as the direction of the gas flow. From the XL30

The ESEM must have, just like a conventional SEM, a high vacuum region (usually with pressure less than 10-2 Pa) for the generation and focusing of the electron beam. In a SEM the high vacuum region and the region of the specimen is the same but in an ESEM these two regions must be separated (Fig. 1). The specimen chamber in an ESEM is designed to maintain water in its liquid phase and for that a minimum water vapour pressure of 609 Pa

Options Manual, FEI and Philips (1998).

(6.09 mbar or 4.579 Torr) is required at Oº C.

**3.2.1 Separated regions** 

#### **3.2.2 Differential pumping**

The basic principle of an ESEM is, thus, to have a high pressure and gaseous specimen chamber, and a high vacuum electron optics area separated from each other but still connected to allow for the transfer of the electron beam. The two regions are separated by at least two small pressure limiting apertures, one aperture that separates the high vacuum region of the electron gun and an intermediate cavity. The second aperture separates the intermediate cavity and the high pressure specimen cavity. Gas leaking from the specimen chamber through the aperture to the intermediate cavity is instantly removed by a pump system. This is called differential pumping. Gas that escapes further into the high vacuum area of the electron optics is similarly removed by a pump to maintain required vacuum. Additional pumping stages may be added to achieve an even higher vacuum in the electron optics area.

#### **3.2.3 Electron beam transfer**

An electron beam generated in the vacuum of the upper column will on its way through the intermediate cavity and in the specimen chamber come in contact with an increasing amount of gas molecules. This will result in a gradual loss of electrons due to electron scattering by the gas molecules and eventually total loss of the beam. However, the electrons are scattered over a broad skirt-like area around the focused spot and since the skirt width is orders of magnitude greater than the spot width, the skirt only contribute background noise and the amount of electrons in the original focused spot is enough for imaging of the specimen. The remaining electron beam is, however, only a fraction of what it was in the upper column and merely enough for imaging. The particular conditions of pressure, distance and beam voltage is crucial for signal detection and the operation of an ESEM is centered on refining the instrument for optimum performance and achieving precision for the instrument to operate close to its physical limit (Danilatos, 2009). By doing this it is possible to use an ESEM in much the same way as a SEM. Secondary and backscattered electrons, X-rays and cathodoluminescence is generated as in a SEM and can be detected with slight modifications to the detectors. The main difference regarding detectors is that the conventional secondary electron detector of SEM, the Everhart-Thornley detector, can not be used in the presence of gas and thus the gaseous detection device (GDD) has been developed. The principle of the GDD is that the environmental gas itself is used for beam transmission and as a detector of the electrons (Danilatos, 1997, 1990a), compared to the Everhart-Thornley detector where light guide the transmitted electrons. In a GDD the signals emanating from the beam specimen-interaction interact with the surrounding gas in the form of gaseous ionization and excitation. The ionized gas is then collected by electrodes and the signal is amplified for its purpose.

#### **3.2.4 Specimen charging**

In conventional SEM negative charge is accumulated as the electron beam impinge on the surface of the specimen. This tends to deflect the electron beam from the scanned point with the result of charging artefacts on the image which greatly disturbs the imaging and analyses. This is normally eliminated in conventional SEM by coating the specimen prior to examination by a thin layer of usually gold or carbon. The gas in ESEM is electrically conductive due to the ionization, which prevents that negative charge accumulates and this is the reason why specimens do not need to be coated prior to examination.

The Use of ESEM in Geobiology 805

As mentioned in sections 1 and 2 geobiological research is performed at the intersection where geology and biology meet. One could ask why the use of ESEM in geobiology differs from conventional geological or biological use of ESEM, and of course, they sometimes overlap. Microorganisms are studied with ESEM within microbiology (e.g. Bergmans et al., 2005; Ahmad, 2010), and minerals, sediments and substrates is studied with ESEM within traditional geology (e.g. Donald, 2003; Reed, 2005; Huiming et al., 2011). However, the main difference is that within geobiology the interaction of these two fields is examined. Geobiology represent the point at which life starts to interact with the physical world and the outcome of this is usually very fragile such as living, encrusted or fossilised microorganisms, amorphous, hydrated minerals or substrates like clays, and combinations of these like biomineralisations or mineral trapping biofilms and EPS (extra cellular polymeric substances). ESEM has been used in various geobiological studies (e.g. Little et al., 1991; Douglas and Douglas, 2000; Nealson et al., 2002; Hallberg and Ferris, 2004; Waters, 2008), and in the following sections we will try to illustrate some of its applications. In example I live microorganisms (bacteria and fungus) from soils collected at various locations in Sweden will be studied. In example II drilled rock samples from the oceanic seafloor in the Pacific Ocean will be used to illustrate the exploration of the deep subseafloor biosphere and how the interaction between microorganisms and mineral substrates can be studied, and also how sensitive hydrated minerals can be if they are not treated in a proper

In this study an XL30 environmental scanning electron microscope with a field emission gun (XL30 ESEM-FEG) was used. The ESEM was equipped with an Oxford x-act energy dispersive spectrometer (EDS), backscatter electron detector (BSE) and a secondary electron detector (SE). The acceleration voltage was 20, 15 or 10 kV depending on the nature of the sample and the instrument was calibrated with a cobolt and a carbon standard. Peak and

The high vacuum mode was in some tests used as an equivalent to the conditions of a

Microorganisms used in geobiological related research is usually collected from natural environments, isolated and grown in laboratory for further studies or experiments. This is a time consuming and expensive procedure but beyond the scope of this paper and, thus, will

First of all, we have used a plant-growth-promoting rhizobacteria. These bacteria have been isolated from soil samples where they existed in symbiosis with fungi and then grown on an agar plate (Fig. 2). With the aim of ESEM we are able to view the bacteria at micrometer scale and study their morphology. The bacteria are coccoidal and their main diameter range between ~1 to 3 µm. It is also possible to observe such feature as mitosis (cell division)

element analyses were made using INCA Suite 4.11 software.

**4. The use of ESEM in geobiology** 

way.

**4.1 Instrument** 

conventional SEM.

(Fig. 2C).

**4.2 Example I: Live microorganisms** 

not be described in further detail.

## **3.2.5 Disadvantages**

Even though ESEM involve several substantial advantages over conventional SEM there are a few disadvantages. Some of these disadvantages can be limited by instrument design and the disadvantages differ between various instrument manufacturers. The main disadvantage is the distance in the specimen chamber over which the electron beam remains usable in the gaseous environment. The useful distance is a function of accelerating voltage, beam current, nature and pressure of the gas, and of the aperture diameter used. The distance varies from ~10mm to less than 1mm depending on the gas pressure. Another result of the limitation of useful specimen distance is the limitation of magnification. At very high pressure the distance becomes so small that the field of view is limited by the aperture width. The vacuum in a conventional SEM result in a superior magnification range compared to an ESEM.

Fig. 2. ESEM images of plant-growth-promoting rhizobacteria. A) Image showing an overview of the bacteria grown on an agar plate. B) A close up showing the coccoidal morphology of the bacteria. C) Image showing mitosis.

The presence of gas may also generate various disturbances in certain applications, like, for instance, the resolution of the image. This issue can, however, be limited by altering chamber pressure and accelerating voltage. It is needed for each instrument to find the most useable combination and correlation between the parameters.

Fig. 3. ESEM images of an ectomychorrizal fungus. Note the frequent branching and anstomoses between branches. In B EPS has been precipitated in the fungal mycelium.

Even though ESEM involve several substantial advantages over conventional SEM there are a few disadvantages. Some of these disadvantages can be limited by instrument design and the disadvantages differ between various instrument manufacturers. The main disadvantage is the distance in the specimen chamber over which the electron beam remains usable in the gaseous environment. The useful distance is a function of accelerating voltage, beam current, nature and pressure of the gas, and of the aperture diameter used. The distance varies from ~10mm to less than 1mm depending on the gas pressure. Another result of the limitation of useful specimen distance is the limitation of magnification. At very high pressure the distance becomes so small that the field of view is limited by the aperture width. The vacuum in a

conventional SEM result in a superior magnification range compared to an ESEM.

B

Fig. 2. ESEM images of plant-growth-promoting rhizobacteria. A) Image showing an overview of the bacteria grown on an agar plate. B) A close up showing the coccoidal

Fig. 3. ESEM images of an ectomychorrizal fungus. Note the frequent branching and anstomoses between branches. In B EPS has been precipitated in the fungal mycelium.

EPS

C

The presence of gas may also generate various disturbances in certain applications, like, for instance, the resolution of the image. This issue can, however, be limited by altering chamber pressure and accelerating voltage. It is needed for each instrument to find the most

morphology of the bacteria. C) Image showing mitosis.

A B

Anastomosis

useable combination and correlation between the parameters.

**3.2.5 Disadvantages** 

A

## **4. The use of ESEM in geobiology**

As mentioned in sections 1 and 2 geobiological research is performed at the intersection where geology and biology meet. One could ask why the use of ESEM in geobiology differs from conventional geological or biological use of ESEM, and of course, they sometimes overlap. Microorganisms are studied with ESEM within microbiology (e.g. Bergmans et al., 2005; Ahmad, 2010), and minerals, sediments and substrates is studied with ESEM within traditional geology (e.g. Donald, 2003; Reed, 2005; Huiming et al., 2011). However, the main difference is that within geobiology the interaction of these two fields is examined. Geobiology represent the point at which life starts to interact with the physical world and the outcome of this is usually very fragile such as living, encrusted or fossilised microorganisms, amorphous, hydrated minerals or substrates like clays, and combinations of these like biomineralisations or mineral trapping biofilms and EPS (extra cellular polymeric substances). ESEM has been used in various geobiological studies (e.g. Little et al., 1991; Douglas and Douglas, 2000; Nealson et al., 2002; Hallberg and Ferris, 2004; Waters, 2008), and in the following sections we will try to illustrate some of its applications. In example I live microorganisms (bacteria and fungus) from soils collected at various locations in Sweden will be studied. In example II drilled rock samples from the oceanic seafloor in the Pacific Ocean will be used to illustrate the exploration of the deep subseafloor biosphere and how the interaction between microorganisms and mineral substrates can be studied, and also how sensitive hydrated minerals can be if they are not treated in a proper way.

### **4.1 Instrument**

In this study an XL30 environmental scanning electron microscope with a field emission gun (XL30 ESEM-FEG) was used. The ESEM was equipped with an Oxford x-act energy dispersive spectrometer (EDS), backscatter electron detector (BSE) and a secondary electron detector (SE). The acceleration voltage was 20, 15 or 10 kV depending on the nature of the sample and the instrument was calibrated with a cobolt and a carbon standard. Peak and element analyses were made using INCA Suite 4.11 software.

The high vacuum mode was in some tests used as an equivalent to the conditions of a conventional SEM.

#### **4.2 Example I: Live microorganisms**

Microorganisms used in geobiological related research is usually collected from natural environments, isolated and grown in laboratory for further studies or experiments. This is a time consuming and expensive procedure but beyond the scope of this paper and, thus, will not be described in further detail.

First of all, we have used a plant-growth-promoting rhizobacteria. These bacteria have been isolated from soil samples where they existed in symbiosis with fungi and then grown on an agar plate (Fig. 2). With the aim of ESEM we are able to view the bacteria at micrometer scale and study their morphology. The bacteria are coccoidal and their main diameter range between ~1 to 3 µm. It is also possible to observe such feature as mitosis (cell division) (Fig. 2C).

The Use of ESEM in Geobiology 807

chamber. Figures 4C-E illustrate how the quality of the images change with varied pressure. Wet mode resulted in poor contrast, high vac resulted in charging artifacts, but the best image quality was achieved with low vacuum at 2mbar. At that pressure and in that mode a high quality image could be produced as well as an element spectrum that is of great advantage. We tried to document the microorganisms in an ordinary SEM as an illustrative example but failed to produce images due to charging artefacts. However, conventional SEM is not a realistic option to study these samples. The microorganisms would collapse and to prevent that a dehydration process and coating would be required, which is time consuming and

Element analysis with EDS is also possible to perform in ESEM mode on the live microorganisms and associated biomineralizations without gold or carbon coating (Table 1). In live material and in biomineralizations analyses of the carbon content is sometimes of

A B C

O 21.71 57.87 46.63

P 2.79 20.38 22.08

K 8.96 6.12 3.36

Ca 4.45 0.99 Fe 4.87 26.94 Total 100.00 100.00 100.00

*Granulatus*, C) biomineralization in the hyphal network of *Suillus Granulatus*. It is difficult to identify the mineral but it appears to be a Fe and P-rich oxide. Normally a mineral phase

would destroy the samples and a lot of information that they contain.

highest priority and would be impossible to do with carbon coating.

C 24.72

A B C

Na 14.18 1.81 Mg 1.95

S 1.65 1.64

Cl 25.98 0.91

Table 1. EDS data in wt %. A) A plant-growth-promoting rhizobacteria, B) *Suillus* 

with such high Fe content needs to be coated before analysed by EDS.

**4.2.1 EDS analysis** 

Fig. 4. ESEM images of *Suillus Granulatus* grown in a liquid media. A) Close up of a hypha. B) An overview image showing the mycelium. C-E) Images showing the same hyphae at various conditions: C) low vacuum (2mbar), D) 1mbar, and E) wet mode.

C D E

Secondly, an ectomychorrizal fungus grown on an agar plate has been used (Fig. 3). This fungal mycelium show characteristic fungal morphologies as frequent branching hyphae and anastomosis between branches (Fig. 3A). The diameter of the hyphae varies between a few micro meter to ~10 µm. It is also possible to see production of EPS on the fungal mycelium (Fig. 3B).

Thirdly, a fungus, *Suillus Granulatus*, grown in liquid media has been used. It is a fungus with traditional fungal morphology (Fig. 4). It is characterized by long, curvi-linear hyphae, 3-10 µm in diameter and several hundred µm in length, creating a complex mycelium. Biomineralizations are frequently occurring in this mycelium and ESEM mode makes it possible to analyse them with EDS without coating, which may disturb the analysis (Fig C in Table 1).

Figures 4C-D show one of the issues that may occur with ESEM. The presence of gas in the specimen chamber results in a blurry and unfocused image at high magnifications. To achieve the best quality of the images we experimented by altering the pressure in the specimen

A

C D E

B

Fig. 4. ESEM images of *Suillus Granulatus* grown in a liquid media. A) Close up of a hypha. B) An overview image showing the mycelium. C-E) Images showing the same hyphae at

Secondly, an ectomychorrizal fungus grown on an agar plate has been used (Fig. 3). This fungal mycelium show characteristic fungal morphologies as frequent branching hyphae and anastomosis between branches (Fig. 3A). The diameter of the hyphae varies between a few micro meter to ~10 µm. It is also possible to see production of EPS on the fungal

Thirdly, a fungus, *Suillus Granulatus*, grown in liquid media has been used. It is a fungus with traditional fungal morphology (Fig. 4). It is characterized by long, curvi-linear hyphae, 3-10 µm in diameter and several hundred µm in length, creating a complex mycelium. Biomineralizations are frequently occurring in this mycelium and ESEM mode makes it possible to analyse them with EDS without coating, which may disturb the analysis (Fig C in

Figures 4C-D show one of the issues that may occur with ESEM. The presence of gas in the specimen chamber results in a blurry and unfocused image at high magnifications. To achieve the best quality of the images we experimented by altering the pressure in the specimen

various conditions: C) low vacuum (2mbar), D) 1mbar, and E) wet mode.

mycelium (Fig. 3B).

Table 1).

chamber. Figures 4C-E illustrate how the quality of the images change with varied pressure. Wet mode resulted in poor contrast, high vac resulted in charging artifacts, but the best image quality was achieved with low vacuum at 2mbar. At that pressure and in that mode a high quality image could be produced as well as an element spectrum that is of great advantage. We tried to document the microorganisms in an ordinary SEM as an illustrative example but failed to produce images due to charging artefacts. However, conventional SEM is not a realistic option to study these samples. The microorganisms would collapse and to prevent that a dehydration process and coating would be required, which is time consuming and would destroy the samples and a lot of information that they contain.

#### **4.2.1 EDS analysis**

Element analysis with EDS is also possible to perform in ESEM mode on the live microorganisms and associated biomineralizations without gold or carbon coating (Table 1). In live material and in biomineralizations analyses of the carbon content is sometimes of highest priority and would be impossible to do with carbon coating.


Table 1. EDS data in wt %. A) A plant-growth-promoting rhizobacteria, B) *Suillus Granulatus*, C) biomineralization in the hyphal network of *Suillus Granulatus*. It is difficult to identify the mineral but it appears to be a Fe and P-rich oxide. Normally a mineral phase with such high Fe content needs to be coated before analysed by EDS.

The Use of ESEM in Geobiology 809

Fig. 6. Optical micrographs. A) Showing a vesicle in basalt from ODP Leg 197 with calcite crystals and montmorillonite grown on the vein walls and an assemblage of fossilized microorganisms. B) Close up of the network of fossilized microorganisms seen in A. C) Image that shows how the fossilized filaments have penetrated into the mineral substrate.

Due to the nature of the samples with partly filled veins or vesicles, and the fragile nature of the microfossils the samples are not prepared as thin sections but the veins and the vesicles are sawed to small cubes (~1x1 cm in diameter) from the original drill cores. Attempts were made to expose as much as possible of the content of the vesicles but differences in height are difficult to avoid and the samples are far from being as horizontal as the surface of thin sections. Coating these samples is also not an option due to their nature as sawed cubes and

The zeolites and the clays are both hydrated minerals and contain crystalline H2O. Analysing them in SEM would result in desiccation and eventually crystal collapse.

due to the fragile nature of the fossilized microorganisms.

**4.3.1 Preparation** 

## **4.3 Example II: Hydrated minerals and fossilized microorganisms**

Samples of subseafloor basalts from the Emperor Seamounts in the Pacific Ocean, drilled and collected during Ocean Drilling Program (ODP) Leg 197, are used to present the advantages of ESEM in the study of hydrated minerals, fossilized microorganisms and interactions between the both. For a detailed description of the sampling sites, mineralogy and biogenicity of the microfossils see Ivarsson et al. (2008a, b) and Ivarsson and Holm (2008). Briefly, the samples consist of veins and vesicles in basalts. These veins and vesicles are partly filled by hydrothermally formed secondary mineralisations of calcite, zeolites and clays (Fig. 5). The vein walls are usually coated with a montmorillonite phase ((Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·*n*H2O) and successively with carbonates (CaCO3) or zeolites species such as phillipsite (K2(Ca0.5, Na)4(Al6Si10O32)·12H2O), chabazite (Ca2(Al4Si8O24)·12H2O), and tetranatrolite (Na16(Al16Si24O80)·16H2O). Zeolites and calcite seldom occur in the same void probably due to local differences in the composition of the hydrothermal fluids but they do exist in the same system with interconnected veins and vesicles. In addition, several of these veins and vesicles contain complex networks of fossilized filamentous microorganisms (Figure 6A-C). These microfossils are composed of a similar montmorillonite phase as is found on the vein walls.

Fig. 5. Optical micrograph. A vein in basalt from the Emperor Seamounts in the Pacific Ocean, drilled and collected during Ocean Drilling Program (ODP) Leg 197. The vein contain secondary mineralisations of calcite, zeolites and montmorillonite (clay), and fossilized microorganisms on the vein walls.

Samples of subseafloor basalts from the Emperor Seamounts in the Pacific Ocean, drilled and collected during Ocean Drilling Program (ODP) Leg 197, are used to present the advantages of ESEM in the study of hydrated minerals, fossilized microorganisms and interactions between the both. For a detailed description of the sampling sites, mineralogy and biogenicity of the microfossils see Ivarsson et al. (2008a, b) and Ivarsson and Holm (2008). Briefly, the samples consist of veins and vesicles in basalts. These veins and vesicles are partly filled by hydrothermally formed secondary mineralisations of calcite, zeolites and clays (Fig. 5). The vein walls are usually coated with a montmorillonite phase ((Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·*n*H2O) and successively with carbonates (CaCO3) or zeolites species such as phillipsite (K2(Ca0.5, Na)4(Al6Si10O32)·12H2O), chabazite (Ca2(Al4Si8O24)·12H2O), and tetranatrolite (Na16(Al16Si24O80)·16H2O). Zeolites and calcite seldom occur in the same void probably due to local differences in the composition of the hydrothermal fluids but they do exist in the same system with interconnected veins and vesicles. In addition, several of these veins and vesicles contain complex networks of fossilized filamentous microorganisms (Figure 6A-C). These microfossils are composed of a

Fig. 5. Optical micrograph. A vein in basalt from the Emperor Seamounts in the Pacific Ocean, drilled and collected during Ocean Drilling Program (ODP) Leg 197. The vein contain secondary mineralisations of calcite, zeolites and montmorillonite (clay), and

fossilized microorganisms on the vein walls.

**4.3 Example II: Hydrated minerals and fossilized microorganisms** 

similar montmorillonite phase as is found on the vein walls.

Fig. 6. Optical micrographs. A) Showing a vesicle in basalt from ODP Leg 197 with calcite crystals and montmorillonite grown on the vein walls and an assemblage of fossilized microorganisms. B) Close up of the network of fossilized microorganisms seen in A. C) Image that shows how the fossilized filaments have penetrated into the mineral substrate.

### **4.3.1 Preparation**

Due to the nature of the samples with partly filled veins or vesicles, and the fragile nature of the microfossils the samples are not prepared as thin sections but the veins and the vesicles are sawed to small cubes (~1x1 cm in diameter) from the original drill cores. Attempts were made to expose as much as possible of the content of the vesicles but differences in height are difficult to avoid and the samples are far from being as horizontal as the surface of thin sections. Coating these samples is also not an option due to their nature as sawed cubes and due to the fragile nature of the fossilized microorganisms.

The zeolites and the clays are both hydrated minerals and contain crystalline H2O. Analysing them in SEM would result in desiccation and eventually crystal collapse.

The Use of ESEM in Geobiology 811

system – while the hydrothermal system was active and the volcanism still was active. This is a strong argument for interpreting the microfossils as syngenetic with the rock, the secondary mineralizations and the hydrothermal activity, and not being a modern

Fig. 8. ESEM images of fossilized microorganisms. A) ESEM image showing the well crystalline clay phase the microfossils consist of. B) ESEM image after the microfossil have been subject to high vacuum. Note how the clay phase has been desiccated and appear

A B

A B C D

 A B C D Mg 7.96 3.65 11.40 Na 3.61 0.84 Al 2.99 12.27 8.43 Si 18.95 9.65 30.13 22.62 K 1.15 0.44 Ca 1.07 4.37 1.12 Fe 32.36 46.26 9.48 O 40.73 36.37 48.47 45.67 Total 100.00 100.00 100.00 100.00 Table 2. EDS data in wt%. A) Close up of a fossilized microfossil showing the structure of the clay phase. B) The clay phase of the vein walls. C) Zeolite. D) Cross section of a fossilized microorganism that dissolved the zeolite producing a tunnel structure while it was alive.

contaminant.

collapsed.

However, by studying the minerals with a gaseous atmosphere in ESEM they maintain their mineral structure and collapse is avoided. This is a substantial advantage over conventional SEM. Another advantage is that the samples do not need to be coated, which in this case is impossible due to the sensitivity of the samples.

## **4.3.2 Fossilized microorganisms**

The ESEM analysis of the fossilized microorganisms gives us a much more detailed description of the morphology, occurrence, preservation and composition of these structures than optical microscopy do (Fig 7). The filaments are up to several hundred µm in length and 5- 20 µm in width depending on where in the network they occur, smaller diameter closer to the attachment in the minerals and wider diameters further from the attachment. They branch frequently and in some cases anastomoses between branches occur. They consist of an inner part and an outer part. Both the inner and the outer part are usually 5 to 10 µm in width depending on the total diameter of the filament. They mainly consist of a clay phase that compositionally corresponds to montmorillonite. It is possible to see that the filamentous networks are attached directly onto the vein walls but also penetrating calcite or zeolite crystals. The morphology and occurrence of these fossilized microorganisms resolved by ESEM correspond to fungal morphology rather than filamentous prokaryotes, thus, with the aim of ESEM it is possible to characterize the microfossils and determine what type of microorganisms they once were.

Fig. 7. ESEM images. Images showing how the fossilized microorganisms occur in the veins.

### **4.3.3 Hydrated minerals**

The minerals in the investigated veins and vesicles consist of clays and zeolites, two groups of minerals that contain crystalline H2O in their crystal structure. The ESEM images show how the montmorillonite looks on a micro meter scale and it allow relative good EDS analyses despite the differences in focal depth (Fig. 8, Table 2). Montmorillonite form in aqueous environments thus the fact that the microfossils consist of montmorillonite indicate that the microorganisms lived while the vesicles were circulated by hydrothermal fluids. This makes it possible to constrain a time window when the microorganisms existed in the

However, by studying the minerals with a gaseous atmosphere in ESEM they maintain their mineral structure and collapse is avoided. This is a substantial advantage over conventional SEM. Another advantage is that the samples do not need to be coated, which in this case is

The ESEM analysis of the fossilized microorganisms gives us a much more detailed description of the morphology, occurrence, preservation and composition of these structures than optical microscopy do (Fig 7). The filaments are up to several hundred µm in length and 5- 20 µm in width depending on where in the network they occur, smaller diameter closer to the attachment in the minerals and wider diameters further from the attachment. They branch frequently and in some cases anastomoses between branches occur. They consist of an inner part and an outer part. Both the inner and the outer part are usually 5 to 10 µm in width depending on the total diameter of the filament. They mainly consist of a clay phase that compositionally corresponds to montmorillonite. It is possible to see that the filamentous networks are attached directly onto the vein walls but also penetrating calcite or zeolite crystals. The morphology and occurrence of these fossilized microorganisms resolved by ESEM correspond to fungal morphology rather than filamentous prokaryotes, thus, with the aim of ESEM it is possible to characterize the microfossils and determine what

Fig. 7. ESEM images. Images showing how the fossilized microorganisms occur in the veins.

The minerals in the investigated veins and vesicles consist of clays and zeolites, two groups of minerals that contain crystalline H2O in their crystal structure. The ESEM images show how the montmorillonite looks on a micro meter scale and it allow relative good EDS analyses despite the differences in focal depth (Fig. 8, Table 2). Montmorillonite form in aqueous environments thus the fact that the microfossils consist of montmorillonite indicate that the microorganisms lived while the vesicles were circulated by hydrothermal fluids. This makes it possible to constrain a time window when the microorganisms existed in the

impossible due to the sensitivity of the samples.

**4.3.2 Fossilized microorganisms** 

type of microorganisms they once were.

A B

**4.3.3 Hydrated minerals** 

system – while the hydrothermal system was active and the volcanism still was active. This is a strong argument for interpreting the microfossils as syngenetic with the rock, the secondary mineralizations and the hydrothermal activity, and not being a modern contaminant.

Fig. 8. ESEM images of fossilized microorganisms. A) ESEM image showing the well crystalline clay phase the microfossils consist of. B) ESEM image after the microfossil have been subject to high vacuum. Note how the clay phase has been desiccated and appear collapsed.


Table 2. EDS data in wt%. A) Close up of a fossilized microfossil showing the structure of the clay phase. B) The clay phase of the vein walls. C) Zeolite. D) Cross section of a fossilized microorganism that dissolved the zeolite producing a tunnel structure while it was alive.

The Use of ESEM in Geobiology 813

Fig. 9. ESEM images. Images showing the interaction between the mineral substrate and the fossilized microorganisms. In B and C it is possible to see that the microorganisms produced

C

The ambition with this chapter was to show the advantage of ESEM in geobiological related research over conventional SEM. We have choosen samples that represent that specific intersection in nature where biology meets geology. We have seen how live microorganisms collected and isolated from soil samples can be viewed and analysed in ESEM. We have seen how the deep subseafloor biosphere can be explored by the study of fossilized microorganisms in rock samples. We have further seen how microorganisms interact with their close environment by, for instance, dissolving minerals to scavenge elements for their metabolism, but we have also seen how microorganisms form minerals by precipitation. All this have been made possible by the use of ESEM. The samples used in this study are extremely sensitive and fragile and procedures for ordinary SEM analysis such as desiccation and coating would have substantially damaged the samples. We have in this chapter used modern or quite young samples (even though the samples from the Emperor

the tunnels in the mineral by dissolution. In B dissolved patches are viewable where

fossilized microorganisms have been attached but later removed.

A B

**5. Concluding remarks** 

Tests were done in high vacuum to analyse these mineral phases but failed due to charging artifacts. However, ESEM mode made it possible to produce high resolved images and EDS analyses. Figure 8 illustrate how a fossilized filament consisting of montmorillonite looks like in ESEM and how a fossilized filament look like after being subject to high vacuum mode which is equivalent to SEM mode.

#### **4.3.4 Microbe-mineral interactions**

In optical microscopy it was possible to observe that the microorganisms had penetrated the zeolite and to some extent the calcite crystals during their existence with the result of long micro sized tunnel structures. Microbially produced cavities or tunnel-like structures in minerals are either produced by mechanical force or by chemical dissolution (McLoughlin et al., 2010). It is not possible to determine in optical microscopy how the tunnel-structures in our samples have been produced, however, in ESEM images it is possible to see that the minerals are clearly dissolved at the margins of these structures (Fig. 9). It is also possible to view dissolved patches on the mineral surface where the microfossils have been attached but for some reason been removed (Fig. 9B). Several types of microorganisms are known to produce long, curvi-linear structures in minerals due to directed dissolution of the minerals by the production of acids and siderophores. The cause of mineral dissolution could be just to expand the microbial habitat but there could also be other causes like searching for elements or compounds within the minerals that the microorganisms could use for their metabolism. In subseafloor environments iron and manganese are elements with redox potential that microorganisms commonly use (Edwards et al., 2005). Iron oxidising autotrophic bacteria as well as manganese oxidising heterotrophic bacteria are common at Seamounts in the Pacific Ocean i.e. the Loihi Seamount which is the active seamount of the Emperor-Hawaiian chain today (Emerson and Moyer, 2002; Templeton et al., 2005). The production of tunnel structures in volcanic glass from subseafloor pillow lavas have also been interpreted to be caused by the oxidation of iron since volcanic glass contain high amounts of reduced iron easy to be oxidised by microbes. However, zeolites do not contain iron or any other element that could be used by microorganisms. Thus the question remains why the microorganisms once dissolved the zeolites to such an extent. One explanation could be that they actually did obtain elements or compounds that they could use in their metabolism by boring through the minerals. Zeolites are well known for their capacity to adsorb various elements and compounds like metals, hydrocarbons and molecular hydrogen within their crystal framework of molecular-sized channels (Sheta et al., 2003; Langmi et al., 2003). Zeolites are frequently used in industrial processes as ion exchangers, catalysts and molecular sieves, and it is most likely that zeolites in subseafloor settings adsorb compounds like Fe, CH4 or H2 from hydrothermal fluids which microorganisms can scavenge when dissolving the minerals. The question is whether zeolites can adsorb enough compounds and elements with redox potential to make microbial mining worth the effort or not.

In conclusion, it was possible by the aim of ESEM to characterize the fossilized microorganisms, perform element analyses of the hydrated minerals and study the interaction between the microorganisms and the mineral phases which gives us information about the living conditions of the microorganisms and perhaps even their metabolism, which is interesting in a geobiological context.

Tests were done in high vacuum to analyse these mineral phases but failed due to charging artifacts. However, ESEM mode made it possible to produce high resolved images and EDS analyses. Figure 8 illustrate how a fossilized filament consisting of montmorillonite looks like in ESEM and how a fossilized filament look like after being subject to high vacuum

In optical microscopy it was possible to observe that the microorganisms had penetrated the zeolite and to some extent the calcite crystals during their existence with the result of long micro sized tunnel structures. Microbially produced cavities or tunnel-like structures in minerals are either produced by mechanical force or by chemical dissolution (McLoughlin et al., 2010). It is not possible to determine in optical microscopy how the tunnel-structures in our samples have been produced, however, in ESEM images it is possible to see that the minerals are clearly dissolved at the margins of these structures (Fig. 9). It is also possible to view dissolved patches on the mineral surface where the microfossils have been attached but for some reason been removed (Fig. 9B). Several types of microorganisms are known to produce long, curvi-linear structures in minerals due to directed dissolution of the minerals by the production of acids and siderophores. The cause of mineral dissolution could be just to expand the microbial habitat but there could also be other causes like searching for elements or compounds within the minerals that the microorganisms could use for their metabolism. In subseafloor environments iron and manganese are elements with redox potential that microorganisms commonly use (Edwards et al., 2005). Iron oxidising autotrophic bacteria as well as manganese oxidising heterotrophic bacteria are common at Seamounts in the Pacific Ocean i.e. the Loihi Seamount which is the active seamount of the Emperor-Hawaiian chain today (Emerson and Moyer, 2002; Templeton et al., 2005). The production of tunnel structures in volcanic glass from subseafloor pillow lavas have also been interpreted to be caused by the oxidation of iron since volcanic glass contain high amounts of reduced iron easy to be oxidised by microbes. However, zeolites do not contain iron or any other element that could be used by microorganisms. Thus the question remains why the microorganisms once dissolved the zeolites to such an extent. One explanation could be that they actually did obtain elements or compounds that they could use in their metabolism by boring through the minerals. Zeolites are well known for their capacity to adsorb various elements and compounds like metals, hydrocarbons and molecular hydrogen within their crystal framework of molecular-sized channels (Sheta et al., 2003; Langmi et al., 2003). Zeolites are frequently used in industrial processes as ion exchangers, catalysts and molecular sieves, and it is most likely that zeolites in subseafloor settings adsorb compounds like Fe, CH4 or H2 from hydrothermal fluids which microorganisms can scavenge when dissolving the minerals. The question is whether zeolites can adsorb enough compounds and elements with redox potential to make microbial mining worth the effort or

In conclusion, it was possible by the aim of ESEM to characterize the fossilized microorganisms, perform element analyses of the hydrated minerals and study the interaction between the microorganisms and the mineral phases which gives us information about the living conditions of the microorganisms and perhaps even their metabolism,

mode which is equivalent to SEM mode.

**4.3.4 Microbe-mineral interactions** 

not.

which is interesting in a geobiological context.

Fig. 9. ESEM images. Images showing the interaction between the mineral substrate and the fossilized microorganisms. In B and C it is possible to see that the microorganisms produced the tunnels in the mineral by dissolution. In B dissolved patches are viewable where fossilized microorganisms have been attached but later removed.

## **5. Concluding remarks**

The ambition with this chapter was to show the advantage of ESEM in geobiological related research over conventional SEM. We have choosen samples that represent that specific intersection in nature where biology meets geology. We have seen how live microorganisms collected and isolated from soil samples can be viewed and analysed in ESEM. We have seen how the deep subseafloor biosphere can be explored by the study of fossilized microorganisms in rock samples. We have further seen how microorganisms interact with their close environment by, for instance, dissolving minerals to scavenge elements for their metabolism, but we have also seen how microorganisms form minerals by precipitation. All this have been made possible by the use of ESEM. The samples used in this study are extremely sensitive and fragile and procedures for ordinary SEM analysis such as desiccation and coating would have substantially damaged the samples. We have in this chapter used modern or quite young samples (even though the samples from the Emperor

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### **6. Acknowledgment**

We would like to thank Marianne Ahlbom at the Department of Geological Sciences, Stockholm University, for assistance, technical guidance and discussions during the ESEM sessions.

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**40** 

*Norway* 

**How Log Interpreter Uses** 

**SEM Data for Clay Volume Calculation** 

Guggenheim and Martin (1995), define the clay as a naturally occurring aluminum silicate composed dominantly of fine-grained minerals. Several other definitions and classifications based on the grain size, pore size, sedimentation, lattice, and other properties can be found in the literature. The attention here is, however, not related to these definitions instead it is about "how to calculate reservoir clay volume using Scanning Electron Microscope (SEM)

The rock that predominantly contains clay minerals is by definition called Shale. Shales are mostly fine-grained clastic sedimentary rocks composed of clays and fragments of other minerals i.e., carbonates and siliciclastics. Most of the clay minerals contain variable amounts of water trapped in the mineral structure (Wiki definition). Clay volume is one of the key parameters used to correct porosity and water saturation for the effects of clay

A problematic aspect of evaluating hydrocarbon bearing reservoirs is to accurately model the effects of clay and shale components on reservoir properties. Clay minerals and other sedimentary materials as detrital shale components in the form of shale lamina, structural clasts and dispersed shale matrix can be deposited in the sedimentary rocks. Clay minerals are also commonly present as diagenetic clays, including pore-filling clays, pore-lining clays

Accurate quantification of the clay content, distribution and clay type in the reservoir rock using core and wireline log data is rather complicated by a number of factors including: rock heterogeneity, mud filtrate invasion and fluid contamination, alteration of clay microstructure and wettability by mud invasion. On the other hand, a number of geological

Traditionally, a log interpreter i.e., petrophysicist, uses wireline log data including clay indicator logs of gamma rays and spectral gamma rays in combination with other porosity and resistivity logs to estimate reservoir clay volume. Integrated analyses of these logs together with geological information are generally used for clay volume calculation. Clay volume in homogeneous reservoir rocks can be estimated by conventional methods however, a mixture of clay minerals, quartz, and calcite notably complicates the interpretation of wireline log data. Rock heterogeneity influences the resistivity logs and

and petrophysical methods have been developed to run the clay volume calculation.

bound water in petrophysical evaluation (Crain, 2000).

and pore-bridging clays (Wilson and Pittman 1977).

**1. Introduction** 

data".

Mohammadhossein Mohammadlou and Mai Britt Mørk *Norwegian University of Science and Technology, NTNU, Trondheim,* 


## **How Log Interpreter Uses SEM Data for Clay Volume Calculation**

Mohammadhossein Mohammadlou and Mai Britt Mørk *Norwegian University of Science and Technology, NTNU, Trondheim, Norway* 

## **1. Introduction**

818 Scanning Electron Microscopy

Waters, M.S.; Sturm, C.A.; El-Naggar, M.Y.; Luttge, A.; Udwadia, F.E.; Cvitkovitch, D.G.;

(December 1998), FEI Company and Philips.

Goodman, S.D. & Nealson, K.H. (2008) In search for the microbe/mineral interface: quantitative analysis og bacteria on metal surfaces using vertical scanning interferometry. *Geobiology*, Vol. 6, No. 3 (June 2008), pp. 254-262, ISSN 1472-4677. XL30 Options Manual (1998). *XL30 ESEM Series, Scanning Electron Microscope*, No 2.6,

> Guggenheim and Martin (1995), define the clay as a naturally occurring aluminum silicate composed dominantly of fine-grained minerals. Several other definitions and classifications based on the grain size, pore size, sedimentation, lattice, and other properties can be found in the literature. The attention here is, however, not related to these definitions instead it is about "how to calculate reservoir clay volume using Scanning Electron Microscope (SEM) data".

> The rock that predominantly contains clay minerals is by definition called Shale. Shales are mostly fine-grained clastic sedimentary rocks composed of clays and fragments of other minerals i.e., carbonates and siliciclastics. Most of the clay minerals contain variable amounts of water trapped in the mineral structure (Wiki definition). Clay volume is one of the key parameters used to correct porosity and water saturation for the effects of clay bound water in petrophysical evaluation (Crain, 2000).

> A problematic aspect of evaluating hydrocarbon bearing reservoirs is to accurately model the effects of clay and shale components on reservoir properties. Clay minerals and other sedimentary materials as detrital shale components in the form of shale lamina, structural clasts and dispersed shale matrix can be deposited in the sedimentary rocks. Clay minerals are also commonly present as diagenetic clays, including pore-filling clays, pore-lining clays and pore-bridging clays (Wilson and Pittman 1977).

> Accurate quantification of the clay content, distribution and clay type in the reservoir rock using core and wireline log data is rather complicated by a number of factors including: rock heterogeneity, mud filtrate invasion and fluid contamination, alteration of clay microstructure and wettability by mud invasion. On the other hand, a number of geological and petrophysical methods have been developed to run the clay volume calculation.

> Traditionally, a log interpreter i.e., petrophysicist, uses wireline log data including clay indicator logs of gamma rays and spectral gamma rays in combination with other porosity and resistivity logs to estimate reservoir clay volume. Integrated analyses of these logs together with geological information are generally used for clay volume calculation. Clay volume in homogeneous reservoir rocks can be estimated by conventional methods however, a mixture of clay minerals, quartz, and calcite notably complicates the interpretation of wireline log data. Rock heterogeneity influences the resistivity logs and

How Log Interpreter Uses SEM Data for Clay Volume Calculation 821

echo train and one from the partially polarized echo train, are merged together to build a T2 distribution from 0.3 ms to over 3000 ms for estimating the total porosity. In other words, the sum of the volume fractions occupied by water, oil, and gas will be the total NMR porosity. Traditional NMR tools miss the very fast T2 decay times; however, in modern tools

the T2 spectra include all types and sizes of pores and cracks (Coates et al., 1999).

Fig. 1. Well log data, reservoir zonation and lithology.

thereby water saturation as well. The focus here, however, is to discuss the use of SEM data in combination with wireline logs to calculate reservoir clay volume.

## **2. Conventional shale volume calculation**

Generally, porosity logging tools (neutron, density and sonic) display higher porosity measurements in clay-rich rocks, whereas much of the porosity is neither effective for hydrocarbon accumulation nor for hydrocarbons transmission through the rock. The shale porosity has to be removed from the total porosity to obtain effective reservoir porosity. The clay correction is therefore an essential step to carry out before estimating reservoir porosity and saturation. Figure 1 displays the gamma-ray, spectral gamma-ray, and calculated clay volume from these logs for a reservoir in the offshore Norwegian Barents Sea. Presence of siliciclastic fines and diagenetic minerals (e.g. dolomite) within carbonate breccias has resulted in a heterogeneous carbonate reservoir in the prospect area. Interactive Petrophysics (IP) software was used for wireline log interpretation and petrophysical analysis of the well.

Carbonate rocks, depending on the diagenesis and sedimentary environment, are generally uranium-rich compared to siliciclastic rocks (Luczaj and Goldstein, 2000). A large amount of uranium may precipitate during and after carbonate rock deposition. The process of uranium precipitation is more abundant in the dolomitic sediments. The easy solution to distinguish the uranium-rich from non-uranium rocks is to compare the two most well known logs of natural gamma ray spectrometry (NGS) against natural gamma ray log (GR). Natural gamma-ray (NGS) spectrometry allows estimation of the elemental concentrations of potassium (K), uranium (U), and thorium (Th), which can be used to interpret sediment composition, clay volume, and diagenesis.

Comparison of these logs in Figure 1 shows uranium-rich carbonate rocks in zones 2 and 3 compared to the more silica-rich intervals of zone 5. The higher uranium content originates from uranium-rich organic matter in the reservoir rock. To calculate the shale volume of the reservoir, the spectral gamma-ray log is used. It is also used to verify the calculated shale volume from the gamma-ray log. In Figure 2, track numbers 4, 5, and 6 show the results of the clay volumes for all reservoir sections in this well. The results are shown for potassium, thorium and corrected gamma-ray logs respectively. The shale and clean (sand) lines are drawn on the potassium, thorium, and corrected gamma-ray logs to estimate the bounding values for zero and 100% shale volume. The bounding values to the corrected gamma-ray response are assumed to be 25 API for zero shale volume and 85 API for 100% shale volume. (Crain Petrophysical Handbook). According to the results in figure 2, discrepancy of the results is considerable. The higher uncertainty in clay volume calculation impacts more effectively the reservoir porosity and saturation estimation afterward.

## **3. NMR log application for shale volume calculation**

The NMR log T2 distribution and its characteristics are dependent on the fluid content and pore properties of the formation, and independent of mineral composition. It is by default used for pore size distribution, capillary bound water and free fluid estimation. Basically, modern NMR tools determine the fluid content in the pore space near the wellbore in the order of few inches into the formation. They measure the total rock porosity regardless of the pore fluid type (Dunn et al., 1998). The two T2 distributions, one from the fully polarized

thereby water saturation as well. The focus here, however, is to discuss the use of SEM data

Generally, porosity logging tools (neutron, density and sonic) display higher porosity measurements in clay-rich rocks, whereas much of the porosity is neither effective for hydrocarbon accumulation nor for hydrocarbons transmission through the rock. The shale porosity has to be removed from the total porosity to obtain effective reservoir porosity. The clay correction is therefore an essential step to carry out before estimating reservoir porosity and saturation. Figure 1 displays the gamma-ray, spectral gamma-ray, and calculated clay volume from these logs for a reservoir in the offshore Norwegian Barents Sea. Presence of siliciclastic fines and diagenetic minerals (e.g. dolomite) within carbonate breccias has resulted in a heterogeneous carbonate reservoir in the prospect area. Interactive Petrophysics (IP) software was used for wireline log interpretation and petrophysical

Carbonate rocks, depending on the diagenesis and sedimentary environment, are generally uranium-rich compared to siliciclastic rocks (Luczaj and Goldstein, 2000). A large amount of uranium may precipitate during and after carbonate rock deposition. The process of uranium precipitation is more abundant in the dolomitic sediments. The easy solution to distinguish the uranium-rich from non-uranium rocks is to compare the two most well known logs of natural gamma ray spectrometry (NGS) against natural gamma ray log (GR). Natural gamma-ray (NGS) spectrometry allows estimation of the elemental concentrations of potassium (K), uranium (U), and thorium (Th), which can be used to interpret sediment

Comparison of these logs in Figure 1 shows uranium-rich carbonate rocks in zones 2 and 3 compared to the more silica-rich intervals of zone 5. The higher uranium content originates from uranium-rich organic matter in the reservoir rock. To calculate the shale volume of the reservoir, the spectral gamma-ray log is used. It is also used to verify the calculated shale volume from the gamma-ray log. In Figure 2, track numbers 4, 5, and 6 show the results of the clay volumes for all reservoir sections in this well. The results are shown for potassium, thorium and corrected gamma-ray logs respectively. The shale and clean (sand) lines are drawn on the potassium, thorium, and corrected gamma-ray logs to estimate the bounding values for zero and 100% shale volume. The bounding values to the corrected gamma-ray response are assumed to be 25 API for zero shale volume and 85 API for 100% shale volume. (Crain Petrophysical Handbook). According to the results in figure 2, discrepancy of the results is considerable. The higher uncertainty in clay volume calculation impacts more

The NMR log T2 distribution and its characteristics are dependent on the fluid content and pore properties of the formation, and independent of mineral composition. It is by default used for pore size distribution, capillary bound water and free fluid estimation. Basically, modern NMR tools determine the fluid content in the pore space near the wellbore in the order of few inches into the formation. They measure the total rock porosity regardless of the pore fluid type (Dunn et al., 1998). The two T2 distributions, one from the fully polarized

effectively the reservoir porosity and saturation estimation afterward.

**3. NMR log application for shale volume calculation** 

in combination with wireline logs to calculate reservoir clay volume.

**2. Conventional shale volume calculation** 

composition, clay volume, and diagenesis.

analysis of the well.

echo train and one from the partially polarized echo train, are merged together to build a T2 distribution from 0.3 ms to over 3000 ms for estimating the total porosity. In other words, the sum of the volume fractions occupied by water, oil, and gas will be the total NMR porosity. Traditional NMR tools miss the very fast T2 decay times; however, in modern tools the T2 spectra include all types and sizes of pores and cracks (Coates et al., 1999).

Fig. 1. Well log data, reservoir zonation and lithology.

How Log Interpreter Uses SEM Data for Clay Volume Calculation 823

determination of clay-bound water volume is possible.The clay-bound water T2 cut-off and clay type/shale volume estimation are important parameters in reservoir characterization. The shale volume estimation is more complicated when the rock is composed of a mixture of clay minerals and tiny fragments of other minerals. Greater amounts of fragments of nonclay minerals increase the uncertainty of obtaining an accurate clay bound T2 cut-off value for clay-bound water (Chitale et al., 1999). Although estimation of the T2 cut-off has been discussed by many authors, there is still uncertainty in the optimum choice for the value. With reference to the work of Matteson et al. (2000), the standard relaxation time of 3.3 ms is

The calculated shale volumes from gamma-ray, spectral gamma-ray (thorium and potassium only), and NMR logs (Figure 7, track 4,5,6 and 7)) show that the calculated volumes are not identical from these methods. Depending upon the chosen gamma-ray and spectral gamma-ray values for the clean and shaly rock samples, the results for the shale volume can differ significantly with different methods. The results from the spectral gamma-ray log in tracks 4 and 5 are in good agreement within the whole reservoir section except in zone 5. The uranium-corrected gamma-ray log, however, shows shale-free reservoir except in zone 5 and parts of zones 2 and 4. To estimate shale volume from the NMR log, representative clayey intervals (100% clay volume) and the amount of clay-bound water in those intervals estimated from the NMR log are classified as references for particular clay types in the well. The amount of clay-bound water estimated from the NMR log response at any other point in the well is compared to that in the selected reference clay in order to estimate the shale volume at that point. There may be errors in this estimate if the selected reference clay was inappropriate. The estimated shale volume from the NMR log is significantly less than that from the gamma-ray log indicators (track 6, Figure 2), presum-

ably due to the errors in conversion of clay-bound water to the shale volume.

radioactive minerals.

A clear response to the presence of clay in the T2 distribution is seen right at the top of Figure 2 in zone 1, the top seal of marine claystones. However, assuming that the top shale seal is totally composed of clay minerals, the NMR estimated shale volume is also not accurate within this interval. Selection of too low a T2 cut-off for clay-bound water or too rapid a decay of the clay-bound water could explain the errors. In addition to the clays, viscose hydrocarbons (tar) relax at much shorter relaxation times than the clay-bound water T2 cut-offs. The NMR log interpretation can be more complicated where both clay and tarmats are present in the reservoir rock. Retrieved fluid samples from the wireline formation tester and recovered hydrocarbons from core material obtained from the studied well have not shown any tar in this reservoir. Overall, zones 4 and 5 of the reservoir show uncorrelated results compared to the rest of the well. Assuming a formation rich in potassium minerals coupled with high gamma-ray response, the rock type could be either shale, a mixture of shale and other potassium-bearing rocks, or other rock types rich in

To identify the clay minerals and potassium sources of zones 1 and 5, the thorium– potassium reference cross-plot of Schlumberger (1985) and corresponding cross-plot of the well data are shown in Figure 3. According to these plots, potassium sources in zone 5 are glauconitic or feldspathic sandstone together with illite and mica, whereas the clay minerals of zone 1 are mixed-layer clay and/or illite. The result gives valuable information about the source of the clays but it could not provide a volumetric response for the shale volume of

used, which is appropriate where the clay type is not kaolinite.

Fig. 2. Shale volume calculation from different methods: track-4 shale volume from potassium log; track-5 shale volume from thorium log; track-6 shale volume from uraniumcorrected gamma-ray; track-7 shale volume from NMR log.

The NMR T2 distribution displays a distinct peak at small relaxation times, proving the existence of clay minerals in the logged intervals. To categorize the contribution to porosity from clay, a T2 cut-off value for clay-bound water is applied to the T2 distribution over the reservoir section. Morriss et al. (1997) proposed a T2 clay-bound water cut-off of 3 ms by studying 45 siliciclastic reservoirs. In a supplementary study by Prammer et al. (1996) different T2 cut-off values were found for particular clay types at shorter decay times. Knowing the clay type and implementing an appropriate T2 cut-off value, more accurate

Fig. 2. Shale volume calculation from different methods: track-4 shale volume from

corrected gamma-ray; track-7 shale volume from NMR log.

potassium log; track-5 shale volume from thorium log; track-6 shale volume from uranium-

The NMR T2 distribution displays a distinct peak at small relaxation times, proving the existence of clay minerals in the logged intervals. To categorize the contribution to porosity from clay, a T2 cut-off value for clay-bound water is applied to the T2 distribution over the reservoir section. Morriss et al. (1997) proposed a T2 clay-bound water cut-off of 3 ms by studying 45 siliciclastic reservoirs. In a supplementary study by Prammer et al. (1996) different T2 cut-off values were found for particular clay types at shorter decay times. Knowing the clay type and implementing an appropriate T2 cut-off value, more accurate determination of clay-bound water volume is possible.The clay-bound water T2 cut-off and clay type/shale volume estimation are important parameters in reservoir characterization. The shale volume estimation is more complicated when the rock is composed of a mixture of clay minerals and tiny fragments of other minerals. Greater amounts of fragments of nonclay minerals increase the uncertainty of obtaining an accurate clay bound T2 cut-off value for clay-bound water (Chitale et al., 1999). Although estimation of the T2 cut-off has been discussed by many authors, there is still uncertainty in the optimum choice for the value. With reference to the work of Matteson et al. (2000), the standard relaxation time of 3.3 ms is used, which is appropriate where the clay type is not kaolinite.

The calculated shale volumes from gamma-ray, spectral gamma-ray (thorium and potassium only), and NMR logs (Figure 7, track 4,5,6 and 7)) show that the calculated volumes are not identical from these methods. Depending upon the chosen gamma-ray and spectral gamma-ray values for the clean and shaly rock samples, the results for the shale volume can differ significantly with different methods. The results from the spectral gamma-ray log in tracks 4 and 5 are in good agreement within the whole reservoir section except in zone 5. The uranium-corrected gamma-ray log, however, shows shale-free reservoir except in zone 5 and parts of zones 2 and 4. To estimate shale volume from the NMR log, representative clayey intervals (100% clay volume) and the amount of clay-bound water in those intervals estimated from the NMR log are classified as references for particular clay types in the well. The amount of clay-bound water estimated from the NMR log response at any other point in the well is compared to that in the selected reference clay in order to estimate the shale volume at that point. There may be errors in this estimate if the selected reference clay was inappropriate. The estimated shale volume from the NMR log is significantly less than that from the gamma-ray log indicators (track 6, Figure 2), presumably due to the errors in conversion of clay-bound water to the shale volume.

A clear response to the presence of clay in the T2 distribution is seen right at the top of Figure 2 in zone 1, the top seal of marine claystones. However, assuming that the top shale seal is totally composed of clay minerals, the NMR estimated shale volume is also not accurate within this interval. Selection of too low a T2 cut-off for clay-bound water or too rapid a decay of the clay-bound water could explain the errors. In addition to the clays, viscose hydrocarbons (tar) relax at much shorter relaxation times than the clay-bound water T2 cut-offs. The NMR log interpretation can be more complicated where both clay and tarmats are present in the reservoir rock. Retrieved fluid samples from the wireline formation tester and recovered hydrocarbons from core material obtained from the studied well have not shown any tar in this reservoir. Overall, zones 4 and 5 of the reservoir show uncorrelated results compared to the rest of the well. Assuming a formation rich in potassium minerals coupled with high gamma-ray response, the rock type could be either shale, a mixture of shale and other potassium-bearing rocks, or other rock types rich in radioactive minerals.

To identify the clay minerals and potassium sources of zones 1 and 5, the thorium– potassium reference cross-plot of Schlumberger (1985) and corresponding cross-plot of the well data are shown in Figure 3. According to these plots, potassium sources in zone 5 are glauconitic or feldspathic sandstone together with illite and mica, whereas the clay minerals of zone 1 are mixed-layer clay and/or illite. The result gives valuable information about the source of the clays but it could not provide a volumetric response for the shale volume of

How Log Interpreter Uses SEM Data for Clay Volume Calculation 825

of calculated shale volume in zone 5 (Figure 7), most of the samples are chosen from this zone. Backscattered electron imaging of polished thin sections is used to map mineral distributions in a grey scale with intensity related to the average atomic number of the minerals (Reed, 2005). Mineral identification was supplemented by energy dispersive spectroscopy analysis, and X-ray mapping of element distributions was done to further identify and quantify the mineral distributions. A backscattered electron image of a sample of sandy carbonate rock in zone 5 is shown in Figure 4a. To estimate the proportion of one particular mineral, the image is processed and one component, e.g., potassium feldspar, is characterized (Figure 4b). The energy dispersive spectrum of potassium feldspars (black in Figure 4b) is shown in Figure 4c. The proportion of elements (K, Si, Al, and O) in this figure verify that the selected grains are potassium feldspar. Energy dispersive spectroscopy is used to recognize every selected mineral in a thin section sample by its elemental

composition, but X-ray mapping of the section provides a map of each element.

Fig. 4. Details of scanning electron microscope imaging of one sample from zone 5. (a) Backscattered electron image. (b) Processed image for the characterization of potassium feldspar, showing up as black. (c) Energy dispersive spectrum of a selected clay mineral to

identify it.

the rock. Further calibration of the logs and petrographical analysis of the rock samples are necessary to quantify the reservoir shale volume.

Fig. 3. (a) Schlumberger reference crossplot of thorium/potassium; and (b) corresponding crossplots for zones 1 and 5 to identify the clay types.

## **4. Scanning electron microscope (SEM) analyses**

To identify the rock mineralogy and quantify the mineral volume fractions, SEM analysis was carried out on the selected reservoir samples. Because of inconsistencies in the amount

the rock. Further calibration of the logs and petrographical analysis of the rock samples are

Fig. 3. (a) Schlumberger reference crossplot of thorium/potassium; and (b) corresponding

To identify the rock mineralogy and quantify the mineral volume fractions, SEM analysis was carried out on the selected reservoir samples. Because of inconsistencies in the amount

crossplots for zones 1 and 5 to identify the clay types.

**4. Scanning electron microscope (SEM) analyses** 

necessary to quantify the reservoir shale volume.

of calculated shale volume in zone 5 (Figure 7), most of the samples are chosen from this zone. Backscattered electron imaging of polished thin sections is used to map mineral distributions in a grey scale with intensity related to the average atomic number of the minerals (Reed, 2005). Mineral identification was supplemented by energy dispersive spectroscopy analysis, and X-ray mapping of element distributions was done to further identify and quantify the mineral distributions. A backscattered electron image of a sample of sandy carbonate rock in zone 5 is shown in Figure 4a. To estimate the proportion of one particular mineral, the image is processed and one component, e.g., potassium feldspar, is characterized (Figure 4b). The energy dispersive spectrum of potassium feldspars (black in Figure 4b) is shown in Figure 4c. The proportion of elements (K, Si, Al, and O) in this figure verify that the selected grains are potassium feldspar. Energy dispersive spectroscopy is used to recognize every selected mineral in a thin section sample by its elemental composition, but X-ray mapping of the section provides a map of each element.

Fig. 4. Details of scanning electron microscope imaging of one sample from zone 5. (a) Backscattered electron image. (b) Processed image for the characterization of potassium feldspar, showing up as black. (c) Energy dispersive spectrum of a selected clay mineral to identify it.

How Log Interpreter Uses SEM Data for Clay Volume Calculation 827

the total gamma-ray response based on the three radioactive components of the spectral gamma-ray signal, however potassium-rich shaly sand could be inaccurately interpreted as shale by this equation. Therefore, the contribution of potassium feldspar has to be removed from the spectral gamma-ray log to estimate an accurate shale volume in the selected samples. Using Ellis and Singer's (2008) approximation and estimated potassium values from the SEM samples, the corrected gamma-ray log values were calibrated for selected samples and subsequent shale volumes. The new potassium-corrected log was then used with the thorium log to estimate a revised shale volume in zone 5. Figure 7 shows the calculated shale volumes from all the above-mentioned methods. The calculated clay volumes using potassium, thorium, and uranium-corrected gamma-ray (CGR) and the integration of SEM, CGR and NMR logs are shown in Figure 7 from tracks 4 to 7, respectively. In track 7, the final corrected shale volume has been plotted for the whole

Fig. 6. Backscattered electron image of the sample section in zone 5 showing illite, mica, and

interval.

feldspar.

The X-ray elemental mapping of the selected thin section in Figure 4 also confirms the abundance of potassium feldspar in this section, but not clays. Figure 5 shows the elemental map of Na, O, K, C, S, Si, Mg, Ca, Ti, Al, and Fe for the selected sample in Figure 4. The volume fraction of potassium feldspar from this analysis is calculated as 16–23 %. The map of feldspar content in other thin sections in zone 5 shows that the potassium volume increases toward the bottom of the formation. A lesser amount of illite/mica is estimated in comparison with the estimated volume from the conventional method. The clay types and volume estimations from SEM analysis are also consistent with the Schlumberger crossplot. The results show that the clay minerals of the sample from zone 5 are illite and mica with a volume fraction of 15–21 % (Figure 6).

Fig. 5. The backscattered electron image of the thin sample section in Figure 4 with details of elemental mapping for Na, O, K, C, S, Si, Mg, Ca, Ti, Al, and Fe.

A few thin sections cannot be representative of the whole reservoir section because they have been selected from a heterogeneous interval. However, SEM analysis together with the NMR and spectral gamma-ray logs facilitates shale volume calculation. SEM data provide shale volume and clay types at the selected depths which can be correlated with the spectral gamma-ray log to extract the shale and potassium feldspar volume at those depths. Comparison of these results with the NMR-estimated shale volumes shows that the NMR log underestimates the shale volume. In contrast, the spectral gamma-ray log overestimates the shale volume.

The potassium readings on the spectral gamma-ray log in zone 5 and part of zone 2 come from a mixture of clay minerals and potassium feldspar. As discussed by Ellis and Singer (2008) with reference to the gamma-ray response in shaly rocks, the correlation between clay minerals and thorium is largest, because of the potassium association with other components of the shale, such as feldspars. They also established an empirical equation for

The X-ray elemental mapping of the selected thin section in Figure 4 also confirms the abundance of potassium feldspar in this section, but not clays. Figure 5 shows the elemental map of Na, O, K, C, S, Si, Mg, Ca, Ti, Al, and Fe for the selected sample in Figure 4. The volume fraction of potassium feldspar from this analysis is calculated as 16–23 %. The map of feldspar content in other thin sections in zone 5 shows that the potassium volume increases toward the bottom of the formation. A lesser amount of illite/mica is estimated in comparison with the estimated volume from the conventional method. The clay types and volume estimations from SEM analysis are also consistent with the Schlumberger crossplot. The results show that the clay minerals of the sample from zone 5 are illite and mica with a

Fig. 5. The backscattered electron image of the thin sample section in Figure 4 with details of

A few thin sections cannot be representative of the whole reservoir section because they have been selected from a heterogeneous interval. However, SEM analysis together with the NMR and spectral gamma-ray logs facilitates shale volume calculation. SEM data provide shale volume and clay types at the selected depths which can be correlated with the spectral gamma-ray log to extract the shale and potassium feldspar volume at those depths. Comparison of these results with the NMR-estimated shale volumes shows that the NMR log underestimates the shale volume. In contrast, the spectral gamma-ray log overestimates

The potassium readings on the spectral gamma-ray log in zone 5 and part of zone 2 come from a mixture of clay minerals and potassium feldspar. As discussed by Ellis and Singer (2008) with reference to the gamma-ray response in shaly rocks, the correlation between clay minerals and thorium is largest, because of the potassium association with other components of the shale, such as feldspars. They also established an empirical equation for

elemental mapping for Na, O, K, C, S, Si, Mg, Ca, Ti, Al, and Fe.

volume fraction of 15–21 % (Figure 6).

the shale volume.

the total gamma-ray response based on the three radioactive components of the spectral gamma-ray signal, however potassium-rich shaly sand could be inaccurately interpreted as shale by this equation. Therefore, the contribution of potassium feldspar has to be removed from the spectral gamma-ray log to estimate an accurate shale volume in the selected samples. Using Ellis and Singer's (2008) approximation and estimated potassium values from the SEM samples, the corrected gamma-ray log values were calibrated for selected samples and subsequent shale volumes. The new potassium-corrected log was then used with the thorium log to estimate a revised shale volume in zone 5. Figure 7 shows the calculated shale volumes from all the above-mentioned methods. The calculated clay volumes using potassium, thorium, and uranium-corrected gamma-ray (CGR) and the integration of SEM, CGR and NMR logs are shown in Figure 7 from tracks 4 to 7, respectively. In track 7, the final corrected shale volume has been plotted for the whole interval.

Fig. 6. Backscattered electron image of the sample section in zone 5 showing illite, mica, and feldspar.

How Log Interpreter Uses SEM Data for Clay Volume Calculation 829

The spectral gamma-ray log works on the same principle, except that the gamma rays are assigned to three different energy bins, showing the concentrations of K, U, and Th in the formation. Generally, carbonate rocks show higher uranium measurements on the spectral gamma-ray log because of the presence of organic matter. Uranium is, therefore, removed prior to the use in clay volume calculation.The sources of potassium also have to be distinguished because potassium is present in shaly and non-shaly rocks. In this study, the NMR log is used as an easy tool to check the reliability of the shale volume calculation from gamma-ray and spectral gamma-ray logs. Inconsistency between the shale volumes estimated from these methods is significant in the lowermost reservoir section. To solve the problem, SEM analysis was done to identify the mineralogy and mineral volume fractions. Backscattered electron images and X-ray mapping of selected samples show a noticeable contribution of potassium feldspar in zone 5. The potassium feldspar content of the rock influences both the gamma-ray and spectral gamma-ray logs and thus the shale volume calculation. The spectral gamma-ray log of potassium was therefore corrected on the basis of SEM information. Subsequently, the SEM information was used as a reference point for the calibration of the spectral gamma-ray log to estimate the shale volume. The uniformity of the spectral gamma-ray logs within zone 5 used for calibration over the entire zone 5. The NMR log was also used to verify the calculation to some extent. The volume of potassium feldspar was then removed from the total potassium reading in the spectral gamma-ray log to obtain the actual shale volume of the rock. The results show a considerable reduction in the estimated shale volume of the reservoir rock. Overestimation of shale volume has a direct impact on the reservoir evaluation by causing underestimation of the effective

Authors thank Norwegian University of Science and Technology and Statoil for support and

access to petrophysical data, and Stephen Lippard for his advice on the manuscript.

porosity and, consequently, hydrocarbon volume.

CBW Clay-bound water (decimal fraction)

Depth Depth from rotary table (m) Dt Sonic transit time (μs/ft-1)

HCGR Compensated gamma-ray (API)

HFK Spectral gamma-ray log potassium (%) HTHO Spectral gamma-ray log thorium (ppm)

HURA Spectral gamma-ray log uranium (ppm) NMR\_SHALE Shale volume from NMR log (decimal fraction)

NPHIC Neutron log (corrected) (decimal fraction)

HCGR\_SHALE Shale volume from HCGR log (decimal fraction)

HTHO\_SHALE Shale volume from thorium log (decimal fraction)

Name Description (unit) BS Bit size (inch)

HCAL Caliper log (inch)

nmrT2cutoff T2 cut-off (ms)

**6. Acknowledgements** 

**7. Nomenclature** 


Fig. 7. Shale volume calculation by using HCGR, thorium-potassium ratio, thorium, and integration of SEM, thorium, and potassium.

## **5. Summary**

Although the gamma-ray log has traditionally been used for the analysis of shaly formations, the shale volume estimation from this measurement is, to a greater or lesser extent, inaccurate. The GR log responds to the natural-gamma radiation from the formation. The spectral gamma-ray log works on the same principle, except that the gamma rays are assigned to three different energy bins, showing the concentrations of K, U, and Th in the formation. Generally, carbonate rocks show higher uranium measurements on the spectral gamma-ray log because of the presence of organic matter. Uranium is, therefore, removed prior to the use in clay volume calculation.The sources of potassium also have to be distinguished because potassium is present in shaly and non-shaly rocks. In this study, the NMR log is used as an easy tool to check the reliability of the shale volume calculation from gamma-ray and spectral gamma-ray logs. Inconsistency between the shale volumes estimated from these methods is significant in the lowermost reservoir section. To solve the problem, SEM analysis was done to identify the mineralogy and mineral volume fractions. Backscattered electron images and X-ray mapping of selected samples show a noticeable contribution of potassium feldspar in zone 5. The potassium feldspar content of the rock influences both the gamma-ray and spectral gamma-ray logs and thus the shale volume calculation. The spectral gamma-ray log of potassium was therefore corrected on the basis of SEM information. Subsequently, the SEM information was used as a reference point for the calibration of the spectral gamma-ray log to estimate the shale volume. The uniformity of the spectral gamma-ray logs within zone 5 used for calibration over the entire zone 5. The NMR log was also used to verify the calculation to some extent. The volume of potassium feldspar was then removed from the total potassium reading in the spectral gamma-ray log to obtain the actual shale volume of the rock. The results show a considerable reduction in the estimated shale volume of the reservoir rock. Overestimation of shale volume has a direct impact on the reservoir evaluation by causing underestimation of the effective porosity and, consequently, hydrocarbon volume.

## **6. Acknowledgements**

Authors thank Norwegian University of Science and Technology and Statoil for support and access to petrophysical data, and Stephen Lippard for his advice on the manuscript.

## **7. Nomenclature**

828 Scanning Electron Microscopy

Fig. 7. Shale volume calculation by using HCGR, thorium-potassium ratio, thorium, and

Although the gamma-ray log has traditionally been used for the analysis of shaly formations, the shale volume estimation from this measurement is, to a greater or lesser extent, inaccurate. The GR log responds to the natural-gamma radiation from the formation.

integration of SEM, thorium, and potassium.

**5. Summary** 



### **8. References**


Chitale, D.V. Day, P. I. and Coates, G. R. [1999] Petrophysical implications of laboratory

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Darling, T. [2005] Well logging and formation evaluation. Gulf professional publishing,

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Ellis, D, V. and Singer, J. M. [2008] Well logging for earth scientists. Springer, 2nd edition

Guggenheim, S. and Martin RT [1995] Definition of clay minerals. Joint report of the AIPEA nomenclature and CMS nomenclature committees. Clays Clay Miner 43:255-256. Luczaj, J. A. and Goldstein R. H. [2000] Diagenesis of the lower Permian Krider Member,

Matteson, A. Tomanic, J.P. Herron, M.M. Allen, D.F. and Kenyon, W.E. [2000] NMR relaxation of clay/brine mixtures. SPE Reservoir Eval. & Eng. 3 (5), 408-413. Mohammadlou, M., Mork M. B., Langeland, H., [2010] Quantification of shale volume from borehole logs calibrated by SEM analysis: a case study. First break, 28, 21-29. Reed, S. J. B. [2005] Electron microscope analysis and scanning electron microscopy in

Ruppel, S. C. [1992] Styles of deposition and diagenesis in Leonardian carbonate reservoirs in West Texas. Annual exhibition and technical conference, SPE 24691. Schlumberger [1985] Log interpretation charts. Schlumberger, New York, USA, 207 pp. Schlumberger [1989] Schlumberger log principles and applications. Schlumberger, Wireline

Straley, C. et al. [1994] Core Analysis by Low Field NMR. SCA-9404 presented at the 1994

Prammer, M.G. et al. [1996] Measurements of clay-bound water and total porosity by

Tiab, D. and Donaldson E. C. [2004] Petrophysics, theory and practice of measuring

reservoir rock and fluid transport properties. Gulf professional publishing, 2nd

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PHIE Effective porosity (decimal fraction) PHIT Total porosity (decimal fraction) RHOC Density log (corrected) (g cm-3) Vanhydrite Anhydrite volume (decimal fraction) VDol Dolomite volume (decimal fraction) VLime Limestone volume (decimal fraction) VSand Sandstone volume (decimal fraction) VWL Wey clay volume (decimal fraction)

Conference and Exhibition, Houston, Texas.

Oxford OX2 8DP, UK, 302 pp.

& Testing, Texas, USA, 230 pp.

Netherlands, 698 pp.

773.

**8. References** 

## *Edited by Viacheslav Kazmiruk*

Today, an individual would be hard-pressed to find any science field that does not employ methods and instruments based on the use of fine focused electron and ion beams. Well instrumented and supplemented with advanced methods and techniques, SEMs provide possibilities not only of surface imaging but quantitative measurement of object topologies, local electrophysical characteristics of semiconductor structures and performing elemental analysis. Moreover, a fine focused e-beam is widely used for the creation of micro and nanostructures. The book's approach covers both theoretical and practical issues related to scanning electron microscopy. The book has 41 chapters, divided into six sections: Instrumentation, Methodology, Biology, Medicine, Material Science, Nanostructured Materials for Electronic Industry, Thin Films, Membranes, Ceramic, Geoscience, and Mineralogy. Each chapter, written by different authors, is a complete work which presupposes that readers have some background knowledge on the subject.

Photo by Rost-9D / iStock

Scanning Electron Microscopy

Scanning Electron

Microscopy

*Edited by Viacheslav Kazmiruk*