Alkaline Chemistry in Polymers and Composites

#### **Chapter 3**

## An Overview of Alkali Treatments of Hemp Fibres and Their Effects on the Performance of Polymer Matrix Composites

*Tom Sunny and Kim L. Pickering*

#### **Abstract**

The alkali treatment is aimed to modify the surface chemistry of natural plant fibres effectively through several factors. This treatment has been carried out at ambient and high temperature. Natural plant fibres treated with alkali have been seen to have benefits such as improved separation of fibres from fibre bundles, improved removal of unwanted surface constituents, increased tensile strength and stiffness, better thermal stability, and enhanced interfacial adhesions compared to other standard treatments. Hemp fibres are an attractive reinforcement for natural plant fibres as they are environmentally friendly compared to other natural plant fibres and exhibit good mechanical properties. This chapter mainly provides an overview of alkali treatments on hemp fibres.

**Keywords:** alkali treatment, hemp fibres, polymer matrix composites

#### **1. Introduction**

Projections of continuing demand for materials across the world is driving the development of more sustainable materials. In addition, the low energy consumption requirements and recyclability found within the spectrum of natural fibre composites have led to increased interest in improving these sustainable materials. Although the use of natural fibre composite materials has been documented in early civilisations, growing environmental concerns coupled with technological advancements have encouraged the expansion of their use in recent times. However, there are still significant issues, including their limited mechanical performance, that limit the ability to compete for future use.

A major area of recent technological development has been that of natural plant fibre composites (NPFCs). The main constituents of NPFCs include plant fibres as the reinforcement and often polymer-based matrix. Natural plant fibres (NPFs) are broadly classified as non-wood fibres and wood fibres, of which non-wood fibres, such as flax, hemp, jute, kenaf and harakeke (Phormium-tenax), are stronger. Among natural plant fibres, hemp fibres are an attractive alternative reinforcement to synthetic fibres due to their favourable mechanical properties as well as availability. **Table 1** represents the specific properties comparison of hemp and glass fibres. As can be seen, hemp fibres have a higher specific Young's modulus. Most of the


#### **Table 1.**

*Specific properties comparison of hemp and glass fibres [1, 2].*

interior parts in automobiles are mainly designed for low density and high stiffness [1], and hemp fibres are well-suited for this application [3]. Additionally, compared to other natural fibres, hemp fibres are more valuable for the bio-based economy due to environmental benefits such as being grown without pesticides and a high yield of technical fibres.

Plant fibres are lignocellulosic, and the presence of numerous hydroxyl groups make them hydrophilic in nature [4]. Polymeric matrices, although generally hydrophobic, are preferred for NPFCs due to their low density and ability to process at low temperatures [5]. There are usually limited interactions between the hydrophilic natural fibres and hydrophobic polymer matrices, which commonly leads to their poor mechanical performance [4]. Additionally, a weak fibre-matrix interface increases the moisture uptake of these composites, which affects their long-term performance [6].

Physical treatment methods such as corona, plasma and heat treatment often require highly sophisticated equipment. Therefore, chemical treatments are commonly seen in the literature. The most popular treatment is alkali. The treatment is aimed to modify surface chemistry effectively through several factors. Natural plant fibres treated with alkali have been seen to have benefits such as improved separation of fibres from fibre bundles, improved removal of unwanted surface constituents, increased tensile strength and stiffness, better thermal stability, and enhanced interfacial adhesions compared to other standard treatments.

#### **2. Fibre and matrix selections for composites**

#### **2.1 Fibre Selection**

It is very important to incorporate high strength reinforcing fibres in order to manufacture high-performance plant fibre composites. The mechanical properties of plant fibres depend upon many factors other than botanical type. These include chemical composition and structure, harvesting time, extraction method, treatment and storage conditions. Among the different types of plant fibres, bast fibres have the highest specific moduli and tensile strengths, which is considered to be mainly due to their higher cellulose content and their cellulose microfibrils aligned more in fibre direction [2]. **Table 2** shows the mechanical properties of some NPFs.

Hemp is one of the most utilised bast fibres. It exhibits high tensile strength ranging between 550 and 1110 MPa, specific Young's moduli ranging between 39 and 47 GPa/gcm−3 [2], and also environmentally friendly since it can be grown without pesticides and herbicides [7]. Hemp is being considered as a suitable NPF reinforcement for use in the present research because of its local availability and good mechanical properties.


*An Overview of Alkali Treatments of Hemp Fibres and Their Effects on the Performance… DOI: http://dx.doi.org/10.5772/intechopen.100321*

**Table 2.**

*Mechanical properties of some of the NPFs [2, 5].*

#### **2.2 Industrial hemp fibre**

Industrial hemp is the term utilised for hemp grown for industrial use, selected such that it naturally attains a tetrahydrocannabinol (psychoactive chemical) content below 0.6%. It is a fast growing annual plant, which has a height of up to 5 m (1.2-5 m) and stem diameter between 4 and 20mm [5]. It has separate male plants and female plants. Male plants are taller, more slender and with a small number of leaves surrounding the flowers. Female plants are characterised as shorter, stockier and have more leaves meeting at each inflorescence.

A dried hemp stalk is shown in **Figure 1**. Dried hemp stalk. Each dried stalk consists of a hollow core (called 'hurd') which contributes 65 to 70% of the total weight. The bast fibre, of composite interest, is located between the hurd and epidermis, which contributes 25 to 30% of the total dry weight of a stalk [6]. Apart from the general classification of plant fibres, the bast fibres are of two types: primary and secondary bast fibres. Primary bast fibres are larger, stronger and contain more cellulose. The bast fibres are bonded together as fibre bundles. These can be separated into single fibres through alkali treatments. The average hemp fibre length and the average fibre width are 25 mm (5 to 55 mm) and 25 μm (10 to 51) μm, respectively [5]. Humans have used hemp for food, textiles, paper, fabric and fuel oil for thousands of years. Industrial hemp fibre applications include a wide range of composites for automotive, insulation materials and construction.

#### **2.3 Matrix selection**

The matrix is important in a NPFC, as it holds the plant fibres together within the composite. It can protect the fibres from adverse environments (e.g. water,

**Figure 1.** *Dried hemp stalk.*


#### **Table 3.**

*Properties of common thermoplastic polymers used in NPFCs [12, 13].*

chemicals and impact properties) and transfers the applied load to the fibres. NPFCs include either a thermoset or thermoplastic polymer matrices [8].

Thermosets cannot be melted once cured, while tthermoplastics can be repeatedly melted by the application of heat and solidify on cooling. This repeatability is one of the main advantages of thermoplastics, as they can be recycled without much affecting their physical properties. Some thermosets used as matrices include unsaturated polyester, epoxy and vinyl ester. Commonly used thermoplastics include polypropylene (PP), polyethylene (PE) and polystyrene (PS). The selection of matrices in NPFCs are normally limited to those that can be processed at less than 200°C, although it is possible to use a maximum of 240°C for a short duration [2, 9].

Thermoplastic matrices offer several advantages compared with thermosetting matrices. These include recyclability, easier control in processing, high impact resistance, low cost, greater resistance to moisture and some industrial solvents and flexibility in design (molecules in a linear chain can slide over each other) compared to thermoset matrices (cross-linked) [10, 11]. The properties of some of the common thermoplastics used are listed in **Table 3**.

Polypropylene (PP) and polyethylene (PE) are the most widely used thermoplastic matrix in NPFCs, particularly for non-structural applications, because of its low density, low water absorption, excellent processability, good mechanical and electrical properties, good biological and chemical resistance, and good impact resistance and dimensional stability [2, 11–16].

#### **3. Interfacial bonding between the fibre and the matrix**

The strength of the interface has a significant influence on composite properties, which depends on the mechanism and amount of interaction. The mechanisms of interfacial bonding can be mechanical interlocking (rough fibre surface), chemical bonding (presence of chemical functional groups) and inter-diffusion bonding (interaction between atoms and molecules). There are possibilities of multiple bonding mechanisms occurring at an interface at the same time [2]. The interface strength also depends on the density of bonds. As already discussed, for NPFCs, there is usually limited interfacial bonding at the interface due to polar fibres and non-polar polymer matrices. This, in turn, affects the stress-transfer efficiency of NPFCs from the matrix to the fibre, thereby limiting the mechanical properties.

Most literature on interfacial bonding of NPFCs focuses on chemical treatments and coupling agents [4, 5, 17–22]. The main objective in conducting these treatments is to improve wettability and potential for chemical bonding of the fibre surface with the matrix, thereby providing interfacial strength (effective stress

*An Overview of Alkali Treatments of Hemp Fibres and Their Effects on the Performance… DOI: http://dx.doi.org/10.5772/intechopen.100321*

transfer across the interface) [23]. Wettability of the fibre by the matrix is most essential for the matrix-fibre adhesion, which can be assessed from the surface energy of the fibre and the matrix. The surface energy of the reinforcements should be greater than that of the matrix for the occurrence of fibre-matrix adhesion [24]. However, NPFs, due to their hydrophilicity, absorb atmospheric moisture when exposed to the ambient environment. This lowers their surface free energy, which may even result in their surfaces possessing lower surface energy than that of matrices. Coupling agents (also known as compatibilisers) act as a bridge between the fibre and the matrix and bond them together. Many studies have been carried out to achieve improved interfacial strength by different treatment methods on NPFCs.

#### **3.1 Chemical methods**

Chemical treatments involve reactions between fibres and reagents, including alkali, acetyl, silane, benzyl, acryl, stearic acid, maleic anhydride, permanganate, peroxide, isocyanate, titanate, and zirconate [2, 4, 5, 25, 26]. The most popular treatments are alkali, acetyl and silane [2]. The majority of these treatments are aimed to modify surface chemistry. However, alkali treatment, which has been found to be the best method [27], is effective through a number of factors; NPFs treated with alkali have been seen to have benefits such as improved separation of fibres from fibre bundles, improved removal of unwanted surface constituents, increased tensile strength and stiffness, better thermal stability as well as improved interfacial adhesions compared to other common treatments [2, 16–18, 28–30]. Here, we will be discussing the most used chemical treatment, which is the alkali treatment.

#### *3.1.1 Alkali treatment*

Among different chemical treatments, the alkali treatment with sodium hydroxide (NaOH) is one of the most widely used treatments. This treatment removes hemicellulose, lignin, pectin, wax and fat from the NPFs. The removal of hemicellulose, lignin, pectin (cementing materials) from the NPFs results in fibre separation and enhances exposure of hydroxyl groups on the fibre surfaces, thereby improving interfacial bonding and fibre roughness and increasing thermal stability [2, 18, 19]. Modest treatments have been seen to bring about increased cellulose crystallinity which is considered to be because of the removal of the abovementioned amorphous materials, whereas harsher treatments have been shown to convert crystalline cellulose to amorphous cellulose and possibly result in chain scission **Figure 2** [31].

The chemical reaction reported by some researchers, which occurs between fibre cell wall and NaOH (sodium hydroxide), are represented in **Figure 3**. The hydroxyl (OH) groups in the fibre break down and react with water molecules (H-OH). The water molecules are thus driven out. The remaining reactive groups in the fibre (i.e., Fibre cell-O) may form Fibre-cell-O<sup>−</sup> Na+ groups between the cellulose molecular chains, which could significantly improve tensile properties of the fibres. However, alkali treatment is commonly carried out to remove the cementing materials.

Different researchers have carried out these alkali treatments in different ways, including at ambient temperature (AT) and high temperature (HT). AT treatments have many advantages, such as simplicity, low cost and can be easily carried out in large volumes, compared to HT treatment which requires fully controlled methods. Oushabi et al. investigated the effect of alkali treatment on date palm fibres with various concentrations of NaOH (0 wt %, 2 wt %, 5 wt %, 10 wt %) at 25 °C for one hour and found an increase in tensile strength of date palm fibres compared to raw

**Figure 2.**

*Change in crystalline cellulose structure before (left side) and after treatment (right side).*

#### **Figure 3.**

*Chemical reaction between fibre cell and NaOH [4].*

fibres [32]. Mishra et al. reported that alkali treatment at 30°C for one hour with 5 wt% NaOH concentration resulted in better strength for sisal/glass fibre polyester hybrid composites compared to 10 wt% NaOH [33]. Mohanty et al. carried out alkali treatment for sisal fibres at 30°C with 5 wt% NaOH for one hour and reported a slight improvement in mechanical properties of sisal/polypropylene composites [34]. **Table 4** lists some of the recent works on AT and HT alkali treatment of hemp fibres. As it can be seen, for different high temperature treatments significant improvement in average tensile strength was reported for hemp fibres treated with 5 wt% NaOH and 2 wt% Na2SO3 (sodium sulphate) at 120°C with a holding time of 60 minutes compared to 10 wt% NaOH and untreated fibres [18].

Among the two different alkali treatments (high temperature and ambient temperature) for hemp fibres, it has been reported that most of the high temperature treatments maintained or increased the fibre tensile strength (or reduction reported for tensile strength was lower) compared to that of untreated fibre, whereas most reported ambient temperature treatments reduced the fibre tensile strength [40]. The removal of weak components has explained the increase in tensile strength of the high temperature treated fibre, and thus, the remaining material is stronger. Furthermore, the removal of weak components from the fibre cell walls could be leading to close packing and alignment of cellulose chains. The close compaction could have enhanced the adhesion between cellulose microfibrils, thereby providing better fibre tensile strength towards the loading direction compared to untreated fibres.

The ambient temperature treatments reported removed some of the weak components, resulting in a significant reduction in fibre strength, suggesting that cellulose degradation had occurred during these treatments. Literature has reported that chemical reagents first react with the chain ends at the surface of the crystallites, as they cannot diffuse into the crystalline region, thus limiting crystalline damage to open some of the hydrogen-bonded cellulose chains. The chemical reagent then diffuses into the crystalline region, reacting with the cellulose and generating more amorphous cellulose (cellulose degradation) [41, 42].

*An Overview of Alkali Treatments of Hemp Fibres and Their Effects on the Performance… DOI: http://dx.doi.org/10.5772/intechopen.100321*


#### **Table 4.**

*Some of the recent works on alkali treatment of hemp fibres or composites produced [4, 17–19, 29, 35, 36].*

#### **4. Conclusions**

While considering preliminary treatments for industrial hemp fibres, high temperature alkali treatments seem best to produce strong and stiff fibres because low temperature treatments are most likely to bring about degradation of the crystalline cellulose chains in the microfibrils or bonding between cellulose microfibrils before sufficient removal of weak components from the fibres.

#### **Declarations**

This research received no specific grants from any agency in the public, commercial, or not-for-profit sectors. However, the authors would like to thank the University of Waikato's composite research group for their support. The authors declare that there is no conflict of interest. Ethical approval was not required for this study.

*Alkaline Chemistry and Applications*

#### **Author details**

Tom Sunny\* and Kim L. Pickering School of Engineering, University of Waikato, New Zealand

\*Address all correspondence to: tomsunny54@gmail.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*An Overview of Alkali Treatments of Hemp Fibres and Their Effects on the Performance… DOI: http://dx.doi.org/10.5772/intechopen.100321*

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*An Overview of Alkali Treatments of Hemp Fibres and Their Effects on the Performance… DOI: http://dx.doi.org/10.5772/intechopen.100321*

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#### **Chapter 4**

## Structure and Properties of Alkaline Cement and Concrete and Choice of Factors That Affect Service Life

*Oles Lastivka*

### **Abstract**

The chapter covers the results of influence of changes in proportions between Portland cement and slag content, along with different quantities of alkali components in the cement systems on their heat of hydration and the effect of this during their hardening on character of internal stresses development and crack opening in concrete. The correlation between heat of hydration of the cements during hardening, strength of the concrete and stress development, and the dependence of this relationship on technical and technological factors were received. A new structurebased model for the durability (cracking) assessment of the alkaline cement concrete with heat of hydration and deformation properties during hardening as the input was received.

**Keywords:** alkaline cement, granulated blast-furnace slag, sodium carbonate, sodium metasilicate pentahydrate, concrete, chemical shrinkage, internal stress, modeling deformation

#### **1. Introduction**

One of the key objectives of contemporary material science and engineering is to develop new types of effective cement and concrete, which ensure a synthesis of long-lasting artificial conglomerates with high physical, mechanical characteristics and performance. It is well known that concrete is a proven and reliable building material, which is used all over the world in many types of buildings. Special concretes are used for constructing contemporary high-rise buildings and their structural elements, which are expected to have high performance in the service environment, particularly in urban conditions. However, rather high cement contents (400–500 kg/m<sup>3</sup> ) and low water-cement ratios in such concretes can result in crack formations due to thermal stresses and shrinkage deformations, thereby lowering the durability of these structures.

It is known that thermal stresses in concrete depend on the exothermic nature of cement hydration and temperature gradient between the core and surface of structural elements [1]. The temperature gradients result in deformations, which, due to space limitations, can lead to compressive stresses in one part of the structure and

stretching in another part. If these stresses exceed the limit of stretching strength, then cracks will appear and expand over the structure's surface. The possibility of forecasting and controlling heat evolution in concrete during hardening allows avoiding the formation of temperature cracks [2]. This explains the necessity of an enhanced study of the concrete's hardening processes at the early stages, depending on a set of recipe and technological factors.

One of the methods of avoiding the thermal crack formation in concrete structures is the use of cements with active mineral additives, such as fly ash and silica fume. These materials result in an increase in strength due to pozzolanic reaction, reduction in heat evolution during cement hydration and improvement in the concrete durability [3–5]. As before-mentioned, Portland cement remains the main cementitious material in concrete for creating such high-strength concretes, which, however, has certain disadvantages in its production, including high energy consumption and adverse environmental effects. In this respect, the development of technologies and research projects aimed at shortening greenhouse gas emissions and reducing energy consumption becomes essential [6].

From this point of view, alkaline cements developed by the scientific school of Prof. Glukhovsky in Ukraine are one of the most advanced materials. These are represented by five types in accordance with the national standard of Ukraine. The important characteristic of such cements is the possibility of using up to 90% of industrial wastes as the raw material while ensuring not only the strength and durability that are normally obtained with traditional Portland cements but also that is commonly associated with high-strength cements. Such cement systems allow a reduction in energy consumption during cement production, decrease the pollution of the environment and protect natural resources.

The prospects of using alkaline slag cement as one of the types of alkali cement in concretes have been confirmed by more than 50 years of research experience in this area [7–9]. Its use has been found to produce concretes with low heat evolution, high early strength, better dimensional stability and long-term durability. However, the alkaline slag Portland cement has not been widely investigated as part of the system of cements described earlier.

In order to ensure the long-term durability of concretes based on alkali slag cement, an investigation was carried in which the development of its early structure formation was studied in terms of heat of hydration, deformation and crack formation. The mix compositions were decided based on information available in the existing literature on its technical and technological properties, with the slag content varying from 50 to 100% of the total binder content and a variable content of alkali component in the cement.

#### **2. Materials and methods**

#### **2.1 Mix compositions**

**Table 1** reports the composition of cement mixes investigated in this chapter. The alkaline slag Portland cement (ASPC) used in this research was manufactured with the ground granulated blast-furnace slag (GGBS) content varying from 50 to 88% and the remainder consisting of Portland cement (CEM I). Another set of experiments was carried out with ground Portland cement clinker used to replace CEM I at a GGBS content of 50%. A third set of mixes was termed alkaline slag cement (ASC), which consisted of 100% GGBS content and alkaline activator materials (see the section on materials for details of the alkaline materials used), in accordance with reference [10]. CEM II/A-S 42.5 (PC) was used as a control

*Structure and Properties of Alkaline Cement and Concrete and Choice of Factors… DOI: http://dx.doi.org/10.5772/intechopen.101288*


*\*References for cementitious materials are based on the following notations: first the cementing materials, then the silicate content, followed by the carbonate content. PC stands for the CEM II/A-S control. \*\*The content of chemicals taken over 100% of the composition of cement.*

#### **Table 1**

*Experimental variables and mix compositions.*

composition. For all 11 mixes, the water-to-cement ratio was 0.5 and the sand-tocement ratio was 3.

Details of the full set compositions of concrete mixes investigated are given in **Table 2**. The nine compositions of concrete mixes were as follows: cement – 400 kg/m<sup>3</sup> , sand – 680 kg/m<sup>3</sup> , aggregate fraction 5 to 10 mm – 360 kg/m<sup>3</sup> and fraction 10 to 20 mm – 790 kg/m<sup>3</sup> . Changing composition of cement in concrete mixes was an experimental variable factor.

#### **2.2 Materials**

In both ASPC and ASC, granulated blast-furnace slag was used (basicity module Мb = 1.1, 95% of glass phase). CEM I and Portland cement clinker were used as the


**Table 2.** *Compositions of concrete mixes.*


**Table 3.**

*The chemical composition of cement components.*

components of ASPC. The control mix consisted of 100% CEM II/A-S 42.5. The chemical composition of cement components is reported in **Table 3**.

The alkaline components used in ASPCs were sodium carbonate (Na2CO3) and sodium metasilicate pentahydrate (Na2SiO3\*5Н2O). The ASPC was produced as an "all in one" product (dry mix of all components), along with sodium lignosulfonate (LST) admixture to ensure setting time and strength. To assist inter-grinding of slag and clinker, etylhidrosyloksan polymer was used, which prevented absorption of moisture by cement and maintained the properties of cement.

The grinding of ASC was done to obtain a fineness characterized by a specific surface area of 470 m<sup>2</sup> /kg. For Portland cement clinker, the fineness, characterized by the specific surface of 430 m<sup>2</sup> /kg, was ensured with the use of the grinder and the air-jet sieve shown in **Figure 1**.

#### **3. Test methods**

#### **3.1 X-ray diffraction (XRD) analysis**

Paste specimens weighing about 5 g were made from 5φ 5 cm test cylinders, and then, they were wrapped in polythene sheets at 7 and 28 days of curing. The curing temperature was 20 ( 2)°C. After curing, the specimens were put into bottles filled with pure isopropanol in order to stop their hydration. Then, the specimens were filtered from the isopropanol and dried in the desiccators in a vacuum. Part of the dried samples was ground in an agate mortar. Particles passing a 63-μm sieve were used for X-ray diffraction. The X-ray diffraction analysis was

**Figure 1.** *Laboratory grinder and air-jet sieve for collecting ground PC clinker.*

conducted using an X-ray diffractometer (Bruker D2 Phaser Benchtop) with Cu Ka1 radiation and a 2Ɵ scanning range of 7–60°. The XRD scans were performed at a 0.05° interval per second.

#### **3.2 Calorimetry**

The chemical reactions involved in cement hydration are globally exothermal, and the associated heat output can be obtained either numerically or experimentally. Numerical prediction of the heat of hydration of cement requires a knowledge of its chemical composition, which in turn can be used for calculating the percentages of the clinker components C3S, C2S, C4AF and C3A according to Bogue's formula or similar [11, 12]. The heat generation potential of each of these components has been thoroughly studied in the past, and so currently several well-known models exist [12, 13]. However, for composite cements, numerical predictions of any of these models often fail due to the complex chemical interactions that can occur [14]. Therefore, heat of hydration of alkaline cements is determined using experimental tests based on the isothermal calorimeter method. In this test, an eight-channel isothermal calorimeter TAM Air was used, and tests were carried out at a temperature of 20 ( 2)°C and using pastes made of 0.4 water-cementitious material ratio. The heat of hydration of cement was determined during 7 days of hardening.

#### **3.3 Chemical shrinkage**

The method for studying chemical shrinkage of cement pastes is normally done according to ASTM C1608 [15]. It consists of a flask that contains the paste, on top of which a capillary is connected and filled with water (**Figure 2**). The water level is monitored using a webcam connected to a laptop. A few drops of oil with a red colorant are added to the water in the capillary to avoid evaporation of the water. The colored oil drops are also used as tracers in the image analysis of the pictures of the capillary taken with the webcam. The flasks are immersed in a thermostatic water bath and maintained at 20°C. Chemical shrinkage of cement is investigated within 28 days of curing.

#### **3.4 Compressive strength**

The test was carried out according to BS EN 196-1:2005. Compressive strength of mortars specimens was determined by three prismatic specimens 40 40 mm in

#### **Figure 2.**

*Setup developed for the measure of chemical shrinkage (left) and the device used to measure the change in volume by means of dilatometry (right).*

cross section and 160 mm in length after hardening at temperature 20 ( 2)°C and humidity 95 ( 5)% at the age of 2, 7 and 28 days for each mix. Steam room with temperature and humidity controlled is used for curing the various specimens of mortars. The mortar specimens were cured immediately after casting, demolded the next day and cured at room temperature until compressive strength test. The compressive strength result represents the average of three tests with an error deviation of less than 7%.

The compressive strength of the concrete specimens was determined by crushing three cubes of 100 mm size after hardening at temperature 20 ( 2)°C and humidity 95 ( 5)% at the age of 7, 14 and 28 days for each mix. The test was carried out according to BS EN 12390–3:2009. Constant rate of loading of specimens was within the range 0.4 MPa/s (N/mm<sup>2</sup> s).

#### **3.5 Drying shrinkage**

The drying shrinkage tests were carried out following the procedures in ISO 1920-8-2009 [16]. Gauge studs of stainless steel were placed at the end surfaces of the specimens, with each partially embedded the sample for 15 mm and the line joining them coinciding with the main axis of the sample. Shrinkage deformations were evaluated using concrete specimens with size 75 75 280 mm, which were demolded 24 h after casting and moist cured at temperature 20 ( 2)°C and humidity 95 ( 5)% until the age of 2 days. After the moist curing period, the specimens were stored at a temperature of approximately 20 ( 2)°C and relative humidity of 60 ( 5)% until the completion of the test. The first measurement of drying shrinkage of concrete specimens was done at the age of 48 ( 0.25) h.

#### **3.6 Creep deformation**

The creep tests were carried out following the procedures in ISO 1920-9-2009 [17]. Creep deformations were evaluated using concrete specimens of cylinders with size 100 100 400 mm, which were demolded 24 h after casting, and moist cured at temperature 20 ( 2)°C and humidity 95 ( 5)% until the age of 7 days. After the moist curing period, the specimens were stored at a temperature of approximately 20 ( 2)°C and relative humidity of 60 ( 5)% until the completion of the test. The first measurement of creep deformation of concrete occurred at the age of 14 days.

#### **4. Description of the main results**

#### **4.1 Cement characterization**

The investigations with the use of CEM I were carried out in order to evaluate the possibility of substituting clinker additive for CEM I in the composition of alkaline cements. The system "slag – CEM I - alkaline component" was considered, which included 50–100% of slag during the type change and the loss of alkaline component and CEM I (**Table 4**). It has been demonstrated that the substitution of Portland cement clinker for CEM I in the composition of alkali cement does not deteriorate the key properties of the binders. Depending on the slag and CEM I contents when using sodium carbonate within the limits of 2.5–5%, the cement is characterized by the beginning of setting at ≥55 minutes, and the strength of 9.2–19.7 MPa after 2 days and 40–46 MPa after 28 days. When using the sodium metasilicate, the beginning of setting is extended, and the cement strength is


*Structure and Properties of Alkaline Cement and Concrete and Choice of Factors… DOI: http://dx.doi.org/10.5772/intechopen.101288*

*\*References for cementitious materials are based on the following notations: first the cementing materials, then the silicate content, followed by the carbonate content. PC stands for the CEM II/A-S control.*

#### **Table 4.**

*Setting time and strength of alkaline cement and CEM II/A-S.*

increased at an early age within the whole range of the slag component content. At the same time, the use of the combination of sodium metasilicate pentahydrate with sodium carbonate ash as alkali components allows to extend the beginning of setting and to increase the cement strength.

It is possible to assert that with the increase of the slag component content, the alkaline cement strength decreases at the early hardening stages as compared with the PC. However, after 7 days of hardening, the strength indices are raised to the level of PC.

So, the compositions of alkaline cement with the range of slag content (50–100%) and CEM I (12–50%) are obtained, which comply with the requirements [10] in accordance with the investigated properties and are related to class 42.5 cements.

#### **4.2 Heat of hydration for cement**

The thermodynamic calculations [18] show that the basicity of hydration products and the hydration heat depend on the output Portland cement phases. The decrease of the number of Са2+ ions and the ratios of СаО/SiO2 and СаО/Al2O3 in the calcium silicate and aluminate groups reduce the importance of hydration heat effects. These thermodynamic provisions are confirmed by the experimental investigations of alkaline cements (**Figures 3** and **4**). For example, the high basicity of PC facilitates the formation of Са(ОН)2, ettringite and highly basic calcium silicate hydrates at the initial hardening stages (**Figure 3**). Their formation is accompanied by significant heat effects - the value of hydration heat reaches 400 J/g (**Figure 4**). At the same time, the lowering of the dispersed phase basicity in alkaline cements at the expense of СаО facilitates the increase of forming the low basic silicate hydrates in the composition of hydration products. The heat of their formation is lower as

#### **Figure 3.**

*XRD patterns of alkaline cement and CEM II/A-S after 7-day hydration. Notations: PC-CEM II/A-S control, S—Slag, P—Portland cement, Si—Silicate, C—Carbonate.*

#### **Figure 4.**

*Influence of alkaline cement and CEM II/A-S on the hydration rate (a) and released heat of hydration (b).*

compared with the heat of highly basic silicate hydrates, which makes up 200– 350 J/g. In this respect, the hydration heat of alkaline cements is lower as compared with the hydration heat of PC.

The intensity and completeness of heat of hydration during hydration of binders reduce progressively with the decrease of СаО content at the expense of Portland cement (**Figure 4**). The value of the first exo-effect varies from 1.4 mW/g for PC to 0.6 mW/g for ASC. The duration of induction period increases from 10 h for PC to 60 h for ASC.

The substitution of sodium carbonate with sodium metasilicate and the mixture of soda with sodium metasilicate was investigated in order to evaluate the impact of alkaline component nature on the value of specific heat of hydration. It has been demonstrated that alkaline cement (sodium carbonate in the amount of 5.5% has been added to its composition) is characterized with the lowest specific heat of hydration indices. The specific heat of hydration is 210 J/g after 7 days, while for another two compositions, the specific heat of hydrations is 230–240 J/g.

The continuous cooling transformation investigations conducted helped specify that irrespective of slag content, nature and quantity of alkaline component, the alkaline cement is characterized with low specific heat of hydration indices, while high strength indices are achieved both at the early hardening stages and in the standard age of 28 days.

*Structure and Properties of Alkaline Cement and Concrete and Choice of Factors… DOI: http://dx.doi.org/10.5772/intechopen.101288*

#### **4.3 Chemical shrinkage of cement pastes**

The volumetric changes in the cement stone gel, which depend on the system mineralogical composition, cement grinding fineness, conditions and time of hardening, are the key reasons for chemical shrinkage of the binding systems. The shrinkage value at the micro-level depends, first of all, on the ratio of crystalline and gel phases in hydration products of the binders on the density of these compounds. The results of chemical shrinkage are represented in **Figure 5**.

It is shown that the chemical shrinkage of the PC pastes is higher as compared with the alkaline cements. For example, PC composition pastes are characterized by shrinkage within the limits of 0.82 mL/g after 28 days of hardening. When using the alkaline cement SSi0C5.5 composition, the shrinkage deformations decrease down to 0.41 mL/g. The deformations decrease with the increase of slag content and the relevant increase of alkaline composition in the cement. For example, the shrinkage deformations are equal to 0.65 mL/g in the system, with 50% slag and 2.5% sodium carbonate content. The shrinkage deformations decrease down to 0.40–0.43 mL/g with the increase of slag content up to 88–100% and the relevant increase of alkaline component in the cement. At the same time, the substitution of sodium carbonate with sodium metasilicate in the cement composition reduces shrinkage deformations of the binders down to 0.38 mL/g.

Therefore, the use of alkaline cement systems facilitates the decrease of chemical shrinkage indices by 15–65% as compared with the PC-based system.

#### **4.4 Concrete strength**

The heavy concrete composition (see Section 2), which includes the binder in an amount of 400 kg/m<sup>3</sup> , was used to determine the effect of cement composition on the compressive strength of concrete.

The investigation results (**Table 5**) show that with the increase of slag content in the cement, and the concrete strength is reduced by 5–15% in the early stage. However, after 14 days of hardening, the concrete strength, based on alkaline cement within the whole range of the slag component content (50–100%), reaches or becomes equal to the strength of concrete with PC. After 28 days of hardening the concrete strength, based on alkaline cement, equals 48.8–51.4 MPa.

It is worth noting that in accordance with the results obtained, it is possible to identify decreasing early strength of concretes and reduction of specific heat of hydration indices with the increase of slag component in cement composition. Therefore, the increase of slag component is accompanied by improvement of one

**Figure 5.** *Chemical shrinkage of alkaline cement and CEM II/A-S.*



index (reduction of specific heat of hydration) and deterioration of the other (decrease of the early strength). That is why it is necessary to search the optimal cement type for concrete in structures, by considering the quantitative changes in combination of properties. This can be done by considering an index—Coefficient of constructive heat (Cch) [19]:

$$\mathbf{Cch} = \mathbf{R}\_{7(28)} / \mathbf{Q}\_7,\tag{1}$$

where R7(28)—compressive strength at 7 and 28 days; Q7—heat of hydration at 7 days.

Actually, the Cch attests the efficiency of using the binder: The more the strength and the less the heat of hydration, the more are the Cch values and the more the effectiveness of cement use in concrete.

The Cch comparison for the concretes based on alkaline cements after 28 days of hardening (**Figure 6**) testifies to the effectiveness of slag content in the cement within the range of 50–100%, with which the factor gets the values within the range of 1.4–1.8, while for the PC the factor is 1.2.

#### **4.5 Modeling of internal stress in concrete**

The thermal stress condition of the concrete was investigated by means of forecasting method using ELCUT software solution [20]. A cast *in situ* wall 3 m wide, 50 m long and 7 m high, concreted at an ambient temperature of 20°C, was used as a block sample for the simulation. The thermal and physical indices of concrete (heat capacity, heat conductivity, heat transfer factor) are accepted in accordance with references and regulatory data [21]. The following tolerances are approved for minimizing the crack formation: The allowable temperature difference between the structure's core and side surface (horizontal gradient) is 18°С; the one between the structure's core and upper/lower surfaces (vertical gradients) is 16°С.

The following relationship was used for modeling the heat evolution of concrete:

$$\mathbf{Q}(\mathbf{t}) = \mathbf{Q} \ast \left(\mathbf{1} - e^{-kQ} \ast e^{-nQ}.\right. \tag{2}$$

*Structure and Properties of Alkaline Cement and Concrete and Choice of Factors… DOI: http://dx.doi.org/10.5772/intechopen.101288*

**Figure 6.**

*Dependence Cch from cement composition.*

**Figure 7.** *Temperature distribution of modeled concrete block based on PC (a) and SSi7C0 (b).*

where Q—integral heat evolution of concrete (kJ/m<sup>3</sup> ); τ—time (h); kQ і nQ dimensionless coefficients determined method Monte Carlo. Eq. (2) was used for modeling of temperature and the resulting stresses in the concrete block.

PC and SSi7C0 compositions (**Table 4**) of the concrete are selected for simulating temperature distributions along the wall. The simulation results are shown in **Figures 7** and **8**.

The results show that when using PC, the concrete with the maximum heat evolution, the maximum structure's core temperature is 50°С. At the same time, the core temperature of the structure from SSi7C0 composition of the concrete is 30°С. The temperature gradient between the core and vertical surface varies from 25°С for the concretes with maximum heat evolution to 10°С for the concrete with minimum heat evolution (SSi7C0 composition).

The stress envelopes, which occur under the relevant temperatures, were built on the basis of the calculated temperature fields. The following concrete characteristics were taken into account for building the envelopes: Young's modulus E = 30 GPa; Poisson's ratio = 0.3; shear modulus G = 12 GPa.

In accordance with the stress envelopes (**Figure 8**), the maximum stretching stresses occur at the upper and side block planes; at the same time, the block center

**Figure 8.** *Internal stresses of modeled concrete blocks based on cement PC (a) and SSi7C0 (b).*

is compressed. The concrete (SSi7C0 composition) with the minimum heat evolution allows to reduce twice both stretching and compressing stresses in the concrete block as compared with the similar stresses for concrete with the maximum heat evolution (PC composition). Therefore, the maximum stretching stresses are approximately 4 MPa for SSi7C0 composition of the concrete; at the same time, they are equal to 8.5 MPa for PC composition of the concrete.

#### **4.6 Drying shrinkage of the concrete**

The drying shrinkage deformation is the most widespread type of shrinkage, which occurs in the material that is already hardened, and which can cause cracking in concrete, for example, along the prestressed reinforcement, or in the products with a large open surface, and relevantly can deteriorate the quality of structures and their durability. The shrinkage appearance is stipulated, first of all, by water removal from the cement gel, which is not bound by molecular forces with a hard phase [22]. The investigation results of drying shrinkage, as well as mass losses of concrete samples, are shown on the basis of investigated cements in order to determine the concrete deformation condition (**Figure 9**).

It has been identified that the concrete samples, based on the PC and alkaline cements, are characterized by almost the same indices of shrinkage deformation and mass loss after 28 days of hardening. For example, when using SPSi3C0 composition of the concrete, the shrinkage makes up 0.064 mm/m after 28 days of hardening. The shrinkage deformations increase up to 0.072 mm/m with the increase of slag content up to 69% in the cement. However, the shrinkage decreases down to 0.061 mm/m when using the cement with maximum slag content. As a comparison, the shrinkage deformations of concrete, based on the PC, make up 0.065 mm/m.

#### **4.7 Creep deformation of the concrete**

The creep mechanism of concrete is rather difficult. It is not fully known until now. The most probable creep mechanism can be explained by water removal from С-S-H gel and cracking due to application of loads. Because the cement component of concrete makes a significant impact on the creep, the impact of the composition of alkaline cement on the formation of concrete creep deformation is specified. The results are shown in **Figure 10**.

*Structure and Properties of Alkaline Cement and Concrete and Choice of Factors… DOI: http://dx.doi.org/10.5772/intechopen.101288*

**Figure 9.** *(a) Drying shrinkage and (b) loss of mass of concrete.*

**Figure 10.** *Creep deformation of concrete.*

It has been demonstrated that after 12 days of loading, the creep deformation indices of the concrete samples, based on the PC and alkaline cement, are almost the same for the concrete of common strength classes. For example, when using PC composition of the concrete, the creep deformation value is 0.046 mm/m. When using alkaline cement compositions of concrete, the creep deformation values are within the range of 0.041–0.06 mm/m.

#### **4.8 Modeling of width of crack opening in concrete**

A combined effect of such factors as thermal stresses and concrete shrinkage is considered during the crack width investigation in the concrete blocks. The inner stresses (4–9 MPa) and shrinkage deformations (0.061–0.072 mm/m) of the concrete are accepted, taking into account the aforementioned results.

The values obtained (**Figure 11**) testify that the lowest crack width index on the block's surface is typical for proposed SSi7C0 composition of the concrete, with the lowest heat evolution and shrinkage; such a concrete is capable of having a temporary crack opening at the level of 0.03–0.09 mm under the different reinforcement ratios. At the same time, PC composition of the concrete is capable of crack opening at the level of 0.13–0.24 mm under the same conditions depending on the reinforcement rate.

Therefore, the results of concretes obtained, based on the alkaline cement, which has the variable ratio of slag components and the alkali component nature, allow forecasting of their durability. Further, the prospects of widespread use are related to solving ecological problems and reducing energy consumption during production.

**Figure 11.** *Simulated crack opening in concrete blocks.*

#### **5. Conclusions**

On the basis of the investigation carried out and results discussed in this chapter on the use of both alkaline slag Portland cement and alkaline slag cement, and both paste and concrete samples investigated, the following conclusions have been made:


Therefore, the obtained investigation results of concretes, based on the alkaline cement, which has the variable ratio of components and the alkaline component nature, allow forecasting of their higher durability, compared with concrete based on CEM II/A-S, according to the heat of evolution and internal stress of concrete at early stages of hardening and their influence on the formation of cracks.

*Structure and Properties of Alkaline Cement and Concrete and Choice of Factors… DOI: http://dx.doi.org/10.5772/intechopen.101288*

### **Author details**

Oles Lastivka

Scientific Research Institute for Binders and Materials, Kyiv National University of Construction Architecture, Kyiv, Ukraine

\*Address all correspondence to: oles.lastivka@gmail.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Section 4
