**1. Introduction**

Characterization studies are essential to gain practical knowledge about materials. They can also be used to correlate structure with properties. There is a host of characterization techniques used to identify materials. They include scanning electron microscopy, transmission electron microscopy, X-ray diffraction, TEM, XRD, X-ray fluorescence, NMR (nuclear magnetic resonance) and synchrotron techniques. However, no single technique can give a full analysis of the material being characterized. In practical research work, conclusions have to be drawn by using 2 or 3 characterization techniques. This chapter examines the use of various characterization techniques for researching geopolymers.

Aluminosilicate binder gel has an amorphous structure. Some years back, a study of amorphous structures was not possible since characterization techniques had not advanced to that extent. But, now it is possible to study amorphous structures, and hence characterization of geopolymers, which can occur in the amorphous state, is possible. Most of the literature in geopolymers concentrates on cements and substitutions/additions to cement. An additional advantage is the high compressive strength of geopolymers. Many geopolymers are manufactured using binders, and most of these binders exist in an amorphous state. These binders are

obtained by the reaction between an alkali source and a solid aluminosilicate powder. The aluminosilicate can be one among metakaolin, fly ash and/or blast-furnace slag. These geopolymers are increasingly being used as construction materials to replace Portland cement [1–3]. Geopolymers based on natural zeolite have also been studied and found to have good adhesion to concrete. These geopolymers have been characterized using XRD and SEM [4]. Fly ash has also been activated by alkali to form a geopolymer, but there are concerns regarding mechanical properties of this material [5]. Geopolymers based on phosphoric acid and illito-kaolinitic clay have been synthesized with reasonably good compression strength [6]. Hence, among all the geopolymers studied, kaolinite-based geopolymers seem to hold promise for the future, and this geopolymer has been used mainly for description of characterization.

Characterization methods have been classified into (a) microscopy (b) X-raybased tomography and fluorescence (c) and other modern methods of characterization which include imaging, nuclear magnetic resonance, FTIR spectra and synchrotron. Each division of characterization has been described briefly before taking up case studies. Selective characterization techniques have been described with metakaolin as an example. Other geopolymers have also been characterized as and when required. During the description of selective characterization techniques, the use of characterization in metakaolin has been dealt with first, and then some more examples of characterization of other geopolymers have been studied as and when required.

#### **2. Microscopy**

Microscopy is commonly used in research and lab studies to throw light upon the detailed features of a material. Optical micrographs are generally used to study the metallurgical microstructures. But, in the case of geopolymers, we are concerned with the pore formation, distribution and other more intricate features. So either scanning electron microscope (SEM) or transmission electron microscope (TEM) is used in recent research. **Figure 1** summarizes the types of rays that are produced when an electron hits a sample target.

Electron beams strike the surface of the sample in all cases of electron microscopy. It would be worthwhile to have a short discussion of what happens when the electron beam strikes the surface. The **Figure 1** shown above gives a gist of the type of rays emitted after striking a sample. The backscattered electrons are used in SEM backscattered image. Unscattered electrons are used in TEM. Auger electrons are used in Auger spectroscopy, and emitted X-rays are used in EDS (electron dispersive spectra) and EPMA (electron probe microanalysis).

#### **2.1 SEM imaging**

Scanning electron microscopy has been widely used to study fractured surfaces. The images give an idea of whether the fracture is ductile or brittle. Sometimes, it is possible to have a mixed mode failure too (**Figure 2**).

#### **2.2 TEM imaging**

**Figure 3** shown above gives in brief the differences between SEM and TEM. Here, the specimens have to be prepared to thin slices of less than 100 nm thickness. Again, similar to SEM, specimen preparation is of utmost importance. Vacuum has to be maintained in the TEM, and any flaw in the maintenance of

**25**

**Figure 2.**

*Working principle of SEM—source [7].*

**Figure 1.**

*Summary of Some Selected Characterization Methods of Geopolymers*

vacuum will reflect in the performance of the TEM. Usually, dislocation density and

The electron gun is a source of electrons. The electrons are focused with the help of condenser lens and objective lens. There is a chamber to hold the workpiece. Care should be taken to prepare the workpiece very carefully and specialists are required for SEM sample preparation. The chamber consists of a backscatter detector and a

second phase precipitation can be clearly seen in TEM images.

secondary detector. So the SEM can be operated under two modes.

*DOI: http://dx.doi.org/10.5772/intechopen.82208*

*Different types of reflected and transmitted rays.*

*Summary of Some Selected Characterization Methods of Geopolymers DOI: http://dx.doi.org/10.5772/intechopen.82208*

#### **Figure 1.** *Different types of reflected and transmitted rays.*

*Geopolymers and Other Geosynthetics*

characterization.

when required.

**2. Microscopy**

**2.1 SEM imaging**

**2.2 TEM imaging**

when an electron hits a sample target.

sive spectra) and EPMA (electron probe microanalysis).

possible to have a mixed mode failure too (**Figure 2**).

obtained by the reaction between an alkali source and a solid aluminosilicate powder. The aluminosilicate can be one among metakaolin, fly ash and/or blast-furnace slag. These geopolymers are increasingly being used as construction materials to replace Portland cement [1–3]. Geopolymers based on natural zeolite have also been studied and found to have good adhesion to concrete. These geopolymers have been characterized using XRD and SEM [4]. Fly ash has also been activated by alkali to form a geopolymer, but there are concerns regarding mechanical properties of this material [5]. Geopolymers based on phosphoric acid and illito-kaolinitic clay have been synthesized with reasonably good compression strength [6]. Hence, among all the geopolymers studied, kaolinite-based geopolymers seem to hold promise for the future, and this geopolymer has been used mainly for description of

Characterization methods have been classified into (a) microscopy (b) X-raybased tomography and fluorescence (c) and other modern methods of characterization which include imaging, nuclear magnetic resonance, FTIR spectra and synchrotron. Each division of characterization has been described briefly before taking up case studies. Selective characterization techniques have been described with metakaolin as an example. Other geopolymers have also been characterized as and when required. During the description of selective characterization techniques, the use of characterization in metakaolin has been dealt with first, and then some more examples of characterization of other geopolymers have been studied as and

Microscopy is commonly used in research and lab studies to throw light upon the detailed features of a material. Optical micrographs are generally used to study the metallurgical microstructures. But, in the case of geopolymers, we are concerned with the pore formation, distribution and other more intricate features. So either scanning electron microscope (SEM) or transmission electron microscope (TEM) is used in recent research. **Figure 1** summarizes the types of rays that are produced

Electron beams strike the surface of the sample in all cases of electron microscopy. It would be worthwhile to have a short discussion of what happens when the electron beam strikes the surface. The **Figure 1** shown above gives a gist of the type of rays emitted after striking a sample. The backscattered electrons are used in SEM backscattered image. Unscattered electrons are used in TEM. Auger electrons are used in Auger spectroscopy, and emitted X-rays are used in EDS (electron disper-

Scanning electron microscopy has been widely used to study fractured surfaces. The images give an idea of whether the fracture is ductile or brittle. Sometimes, it is

**Figure 3** shown above gives in brief the differences between SEM and TEM. Here, the specimens have to be prepared to thin slices of less than 100 nm thickness. Again, similar to SEM, specimen preparation is of utmost importance. Vacuum has to be maintained in the TEM, and any flaw in the maintenance of

**24**

#### **Figure 2.** *Working principle of SEM—source [7].*

vacuum will reflect in the performance of the TEM. Usually, dislocation density and second phase precipitation can be clearly seen in TEM images.

The electron gun is a source of electrons. The electrons are focused with the help of condenser lens and objective lens. There is a chamber to hold the workpiece. Care should be taken to prepare the workpiece very carefully and specialists are required for SEM sample preparation. The chamber consists of a backscatter detector and a secondary detector. So the SEM can be operated under two modes.

#### **Figure 3.** *Difference between light microscope, SEM and TEM.*

**Figure 4** depicts uniform distribution of metakaolin and possibly a ductile fracture. The SEM image above shows a wide variety of shapes of fly ash, tending towards a spherical shape. There seems to be agglomeration and there also seem to be some cracks. Cracks often lead to brittleness. This SEM micrograph of a fly ash geopolymer seems to indicate mixed mode of failure (**Figures 5** and **6**).

Again, this SEM shows an even wider distribution of particles and a very large particle size distribution too. It appears, from the cracks seen and the fragments in the SEM that in this case, there has been a brittle fracture. This possibility is supported by the fact that most concrete fails in a brittle fashion. Addition of geopolymer/substitution of various geopolymeric elements like fly ash could change the morphology and influence fracture to some extent.

Research in these areas is still in the nascent stage. However, it is worth mentioning here that considering the danger that Mother Earth is facing under the deluge of huge amounts of metallic and ceramic waste, it would be a very worthwhile task to look for alternatives to concrete or make some substitutions to concrete to make it more environmentally friendly. It is here that geopolymeric materials could help (**Figure 7**).

**27**

**Figure 7.**

**Figure 5.**

**Figure 6.**

*SEM micrograph of fly ash geopolymer [10].*

*Summary of Some Selected Characterization Methods of Geopolymers*

*SEM image of geopolymer fly ash after heating at 820 Celsius—courtesy of Temujin et al. [9].*

Here, since the geopolymer is clay-based, the SEM shows a ductile type of fracture. Curing may be necessary to improve strength and bonding. Usage without curing may lead to lower tensile strength. Microcrack formations are seen in the SEM, but these are too small to be of any importance or create any immediate

*SEM micrograph clay-based geopolymer brick cured at 85 Celsius for 24 hours [11].*

*DOI: http://dx.doi.org/10.5772/intechopen.82208*

**Figure 4.** *SEM image of metakaolin; source—Wiki Image [8].*

*Summary of Some Selected Characterization Methods of Geopolymers DOI: http://dx.doi.org/10.5772/intechopen.82208*

#### **Figure 5.**

*Geopolymers and Other Geosynthetics*

**Figure 4** depicts uniform distribution of metakaolin and possibly a ductile fracture. The SEM image above shows a wide variety of shapes of fly ash, tending towards a spherical shape. There seems to be agglomeration and there also seem to be some cracks. Cracks often lead to brittleness. This SEM micrograph of a fly ash

Again, this SEM shows an even wider distribution of particles and a very large particle size distribution too. It appears, from the cracks seen and the fragments in the SEM that in this case, there has been a brittle fracture. This possibility is supported by the fact that most concrete fails in a brittle fashion. Addition of geopolymer/substitution of various geopolymeric elements like fly ash could change the

Research in these areas is still in the nascent stage. However, it is worth mentioning here that considering the danger that Mother Earth is facing under the deluge of huge amounts of metallic and ceramic waste, it would be a very worthwhile task to look for alternatives to concrete or make some substitutions to concrete to make it more environmentally friendly. It is here that geopolymeric materials could help

geopolymer seems to indicate mixed mode of failure (**Figures 5** and **6**).

morphology and influence fracture to some extent.

*Difference between light microscope, SEM and TEM.*

**26**

**Figure 4.**

*SEM image of metakaolin; source—Wiki Image [8].*

(**Figure 7**).

**Figure 3.**

*SEM image of geopolymer fly ash after heating at 820 Celsius—courtesy of Temujin et al. [9].*

#### **Figure 6.** *SEM micrograph of fly ash geopolymer [10].*

**Figure 7.** *SEM micrograph clay-based geopolymer brick cured at 85 Celsius for 24 hours [11].*

Here, since the geopolymer is clay-based, the SEM shows a ductile type of fracture. Curing may be necessary to improve strength and bonding. Usage without curing may lead to lower tensile strength. Microcrack formations are seen in the SEM, but these are too small to be of any importance or create any immediate

danger of failing to the geopolymer. Another study of ground-granulated blast-furnace slag (GGBS)-amended fly ash was conducted by Sharma AK et al. This can be used as soil conservative. The SEM shown below shows a fairly ductile fracture. The interface condition is good and porosity is not seen. This indicates that there is good contact between soil particles and cementitious mix. C-S-H and calcium oxide formations have also been confirmed in this study. SEM evidence has shown that fly ash mixed with GGBS has the potential to improve the properties of expansive soil with a minimum requirement of chemical additives such as lime (**Figure 8**).

In **Figure 9**, clustering can be seen. The bright-field transmission electron micrograph of a slag-based geopolymer is shown in **Figure 9**. The figure shows the clustering of slag. This may have a deleterious effect on properties.

In **Figure 10(a)** Medri et al. [8] tested two metakaolins manufactured industrially by the company Imerys with two different kiln technologies. One called M1000 is calcined in a rotary kiln and characterized by rounded massive aggregates of lamellar particles. The second, called M1200S, calcined in a flash kiln, is made up of fine lamellar particles with lower agglomeration. **Figure 10(b)** reports that the geopolymer structure is characterized by gel (amorphous) phase, and also some crystalline phases are present as in most geopolymers.

TEM image shows agglomeration of slag, which can be seen with SEM also, but SEM can be used only for surface studies, whereas TEM can be used to find details of subsurface.

**Figure 8.** *C-S-H bonds and aggregation [10].*

**29**

**Figure 11.**

**Figure 10.**

*based geopolymer [13].*

*Summary of Some Selected Characterization Methods of Geopolymers*

**3. X-ray tomography and fluorescence microscopy**

characterized by X-ray tomography [14].

*histogram depicting volume of pixels of the volume of interest.*

Many different cross-sectional views are created suing X-rays. These are then assembled to create a 3D image of the object. The size of the pixels which are created in this manner so created are in micrometers and hence the word, microtomography. It should be noted that the 3D model is a virtual model and is not in real time. These pixel sizes have also resulted in the terms high-resolution X-ray tomography, micro-computed tomography (micro-CT or μCT) and similar terms. In today's

*(a) SEM images of natural and synthetic metakaolins (Cui et al.) [12] and (b) pore structure in coal ash-*

**Figure 11** illustrates the features of a slag-based geopolymer using the X-ray microtomograph. Furthermore, histogram, **Figure 11**, also depicts particle size distribution. In general, fly ash-based polymers have been studied by Das et al. and

The X-ray tomography shows the distribution of phases. Slag particles can be clearly seen as a white product. There also seem to be some cracks, which, if allowed to propagate, could lead to premature failure of the component in use (**Figure 12**). According to The Royal Chemical Society, X-Ray Fluorescence is an imaging technique where a beam of X-rays is directed at the specimen-Rays are emitted

*X-ray microtomography scan of a sodium silicate-activated binder (80% slag/20% metakaolin, activat) (b) a* 

terminology, tomography automatically implies computer tomography.

*DOI: http://dx.doi.org/10.5772/intechopen.82208*

**Figure 9.** *Bright-field image of slag-based geopolymer.*

**Figure 10.**

*Geopolymers and Other Geosynthetics*

danger of failing to the geopolymer. Another study of ground-granulated blast-furnace slag (GGBS)-amended fly ash was conducted by Sharma AK et al. This can be used as soil conservative. The SEM shown below shows a fairly ductile fracture. The interface condition is good and porosity is not seen. This indicates that there is good contact between soil particles and cementitious mix. C-S-H and calcium oxide formations have also been confirmed in this study. SEM evidence has shown that fly ash mixed with GGBS has the potential to improve the properties of expansive soil with a minimum requirement of chemical additives such as lime (**Figure 8**). In **Figure 9**, clustering can be seen. The bright-field transmission electron micrograph of a slag-based geopolymer is shown in **Figure 9**. The figure shows the

In **Figure 10(a)** Medri et al. [8] tested two metakaolins manufactured industrially by the company Imerys with two different kiln technologies. One called M1000 is calcined in a rotary kiln and characterized by rounded massive aggregates of lamellar particles. The second, called M1200S, calcined in a flash kiln, is made up of fine lamellar particles with lower agglomeration. **Figure 10(b)** reports that the geopolymer structure is characterized by gel (amorphous) phase, and also some

TEM image shows agglomeration of slag, which can be seen with SEM also, but SEM can be used only for surface studies, whereas TEM can be used to find details

clustering of slag. This may have a deleterious effect on properties.

crystalline phases are present as in most geopolymers.

of subsurface.

**28**

**Figure 9.**

**Figure 8.**

*C-S-H bonds and aggregation [10].*

*Bright-field image of slag-based geopolymer.*

*(a) SEM images of natural and synthetic metakaolins (Cui et al.) [12] and (b) pore structure in coal ashbased geopolymer [13].*
