*3.1.3 Role of initial alkalinity (Na2O/Al2O3 ratio)*

The alkalis in the alkali-activated aluminosilicate cements participate in all stages of structure formation. In particular, rate and degree of activation of the aluminosilicate component is determined by contents of alkalis [3, 31, 36]. A quantity of alkalis is to be chosen in view of formation of a required phase composition which determines the required properties. For example, the anhydrous aluminosilicates appear in small amounts in the systems with high content of alkali (Na2O/ Al2O3 = 1.0) at 600°C, whereas in the low-alkali compositions (Na2O/Al2O3 = 0.5) the crystallization begins only at 1000°C (**Table 4**) [36, 37].

### *3.1.4 Role of a cation type and direction of phase transformations*

**Figures 9** and **10** demonstrate the XRD scans and SEM images of the microstructure of a cleaved fragment of the artificial stone (K2O/R2O = 0:0.3, (Na2O+K2O)/Al2O3 = 1 and SiO2/Al2O3 = 5) after curing at 80°С. At the ratio SiO2/ Al2O3 = 5, a phase composition of the artificial stone is characterized by the zeolitelike hydration products of the heulandite and phillipsite types (**Figure 9**), and the


#### **Table 4.**

*Phase composition of the fly ash 1-based cements vs. quantity of Na2O.*

#### **Figure 9.**

*The XRD scans of the metakaolin-based cements after drying at 80°С (SiO2/Al2O3 = 5 and K2O/R2O, respectively: a – 0; b – 0.15; c – 0.3). Q – Quartz; N – Natrolite; A – Zeolite-Na-A; P*<sup>0</sup> *– Phillipsite-Na-K; H – Heulandite-Na; H*<sup>0</sup> *– Heulandite-K.*

*Genesis of Structure and Properties of the Zeolite-Like Cement Matrices of the System… DOI: http://dx.doi.org/10.5772/intechopen.97520*

#### **Figure 10.**

*The SEM images of the microstructure of a cleaved fragment of the metakaolin-based cements after drying at 80°С (SiO2/Al2O3 = 5 and K2O/R2O, respectively: a – 0; b – 0.15; c – 0.3)(2500).*

microstructure of the cement matrix displays a large number of sub-crystal phases (**Figure 10**).

The phase composition of the alkali-activated aluminosilicate cements (at K2O/ R2O = 0) is characterized by the zeolite-like hydration products of the following type: natrolite, zeolite-Na-A and heulandite-Na. When potassium ions are replaced by the sodium ions, other zeolite-like hydration products of the potassium heulandite and sodium-potassium phillipsite types appear as well. The presence of the natrolite phase is not observed, the crystallinity of the hydration products increases.

The cements made using sodium compounds (sodium-based cements) are the most widely used cements among the possible alkali-activated aluminosilicate cements due to the lowest price of the alkaline component. The potassium compounds show in some cases better results than the sodium compounds. In particular, potassium aluminosilicates are more thermally resistant compared to the sodium ones: for example, a temperature of thermal destruction (fusion) of orthoclase (K2OAl2O36SiO2) is 1170°C (against 1118°C of albite (Na2OAl2O36SiO2)). A wellknown fact is that the binary systems like the sodium/potassium system are significantly less thermally resistant compared to pure systems: eutectic solutions are formed in such systems and these mixed systems fuse at the significantly lower temperatures [37]. The same refers to other binary systems (Na/Ca or K/Ca) and a ternary system (Na/K/Ca). It is important in view of wide application of the Cacontaining compounds as cement modifiers: this technology helps to solve a lot of problems associated with ordinary cements [20] but, being applied for making heat-resistant materials, the increased content of Ca may cause low temperature of sintering and high thermal shrinkage. It is to be noted that this rule is only in power at limited contents of CaO (approx. below 10%), because at the higher calcium contents the system begins revealing other behaviour and, in fact, is not itself the alkali-activated aluminosilicate cement.


#### **Table 5.**

*Phase composition of the fly ash 1-based cements after high temperature curing vs. cation type.*

Formation of the phases in case of the fly ash-based cement, depending upon a type of the cation and curing temperature is shown in **Table 5**: the first phases which appear during heating (at approx. 600–900°C) are low-silica feldspathoids (nepheline in case of the sodium-based cements and leucite in case of the potassium-based ones), whereas high-silica feldspars appear at the higher temperatures (900–1200°C). Thus, the earlier collected data on alkaline activation of various clays [4, 28] showed that a phase composition of the synthesized aluminosilicates of alkali metals could also be regulated by changing a temperature of high-temperature curing. Nepheline is a main reaction product of the system composed of a clay activated by sodium compound after curing at 750–900°C, and albite – after curing at the higher temperatures (900–1000°C). When clays are activated by the potassium compounds, leucite is the main reaction product after high temperature curing [28].

Sodium and calcium compounds result in the synthesis of plagioclases (binary sodium-calcium feldspars such as labradorite). Such results were obtained in the potassium (KOH)- and calcium (OPC clinker)-modified cements [20, 21, 27, 29, 30], in which leucite (a potassium feldspathoid) and labradorite (a sodium-calcium feldspar) are the main phases after high- temperature curing (**Figure 11**). This composition, N-K-C-A-S-H, has an amorphous structure after curing at high temperature (800°C). With temperature increase up to 900°C, according to DTA examination (**Figure 12**), this amorphous phase begins to crystallize. After curing at 1000°C, according to XRD examination, leucite and labradorite occur.

After 1200°C, redistribution of shares of the above listed reaction products begins and, in accordance with their thermodynamic stability at this temperature, a quantity of labradorite increases, whereas that of leucite decreases.

Concluding the above, the formation of a microstructure of the alkali-activated aluminosilicate cements may be expressed by the following scheme:

#### **3.2 Properties of the alkali-activated aluminosilicate cement-based materials**

Formation of properties of the materials made using the zeolite-like cement matrices depends upon a cement composition and curing parameters applied to these materials.

The most important characteristics of these materials are mechanical strength (usually evaluated by a residual strength after high-temperature exposure) and stability of volume (evaluated by a thermal shrinkage). Other characteristics that are also important in some cases are resistance against crack formation, thermal resistance and a temperature of deformations under load.

*Genesis of Structure and Properties of the Zeolite-Like Cement Matrices of the System… DOI: http://dx.doi.org/10.5772/intechopen.97520*

#### **Figure 11.**

*The XRD scans of the metakaolin-based cement with 10% OPC clinker by mass after curing at 800°C (1), 1000°C (2) and 1200°C (3). (Chamotte (Ch) as a filler)).*

**Figure 12.** *The TGA, DTG and TG of the hydrated metakaolin-based cements. (Chamotte as a filler): 1 – Without OPC clinker; 2 – With 10% OPC clinker by mass.*

Compressive strength and thermal shrinkage change with the temperature increase. When thermally stable zeolite-like phases are the main reaction products of the cement matrices, compressive strength and thermal shrinkage change slowly within a temperature range of 200–800°C. Above 800°C, a liquid phase starts to appear. As a result, the strength increases and the shrinkage also increases too (**Figure 13**). Once the thermal shrinkage reaches the values of 1–2%, the materials lose their functionality because of crack formation, instability of volume, etc., despite still high values of compressive strength. A temperature at which a limit value of thermal shrinkage is reached is a maximum use temperature for a material. The use temperature depends chiefly upon alkalinity and a cation type of the alkali compound. The best heat resistant alkali-activated aluminosilicate cement-based materials may withstand temperatures up to 1000°C).

Heat-resistant materials require a stability of volume changes within a wide temperature range. The shrinkage/expansion processes during exposure to temperatures can be regulated by a choice of an optimal cement composition and appropriate heat-resistant fillers, for example, chamotte [48]. The study of interrelation between a cement composition, curing parameters, phase composition of the reaction products and properties of the cement-based materials was carried out on the compositions: cement: chamotte = 1:1 by mass (**Figure 14**). Also, an interrelation between a relative intensity of the crystalline phase formation and the above properties was studied. In this experiment, a maximum height of the peak in the XRD patterns (fixed for the analcime main peak at 0.343 nm in the XRD pattern of the autoclaved fly ash 1 – based cement with SiO2/Al2O3 = 4) was taken as 100% (**Figure 15**).

The study of interrelation between cement composition, curing conditions, phase composition of the microstructure and properties of the artificial stone was carried out on the compositions (cement:chamotte = 1:1). It was also found that the 1:1 ratio for the composition "fly ash-based cement – chamotte" was optimal for meeting the requirements with regard to workability retention of the mix and

#### **Figure 13.**

*Compressive strength (a) and thermal shrinkage (b) of the alkali-activated aluminosilicate cement-based materials vs. curing temperature.*

*Genesis of Structure and Properties of the Zeolite-Like Cement Matrices of the System… DOI: http://dx.doi.org/10.5772/intechopen.97520*

#### **Figure 14.**

*Compressive strength (a), residual strength (b), and shrinkage (c) after high-temperature curing at 800°C (c) molar ratio Al2O3/SiO2 = 2, 4, 6, 8 of the cements and mode of pre-curing. (A – Autoclave curing at 174°C, S – Steam curing at 80°C, none – Without pre-curing).*

compressive strength of the materials both at normal and high temperatures. The higher contents of chamotte the worser are these properties, however, in this case high contents of chamotte help to control shrinkage at high temperatures.

A material made using the alkali-activated aluminosilicate cements with crystalline phases is less inclined to sharp changes in plastic deformations. A crystallization in a ceramic matrix of the compound of the R2O�Аl2O3 � (2÷6)SiO2 composition, which is analogous to natural nepheline, caliophilite, albite, and orthoclase, is sufficient to produce the materials with properties of ceramics.

Facing materials can be produced using mixed alkali-alkaline earth-activated aluminosilicates of the mNa2O�(1-m) CaO�Al2O3�nSiO2 composition. A conclusion was made that with the increase in the content of the phase of the albite composition (m ! 1) in the reaction products the materials with properties similar to those of facade tiles (water absorption below 10%), and with the increase of the phase of the anorthite composition (m ! 0) – with the properties similar to those of wall ceramics can be produced.

High intensity of crystal formation (higher than 10%) in the compositions after autoclave pre-curing side by side with insufficient degree of crystallization

#### **Figure 15.**

*Relationship between the relative intensity of crystallization and properties of the cement –chamotte compositions: Density, compressive strength after normal temperature curing, residual strength and thermal shrinkage after curing at 800°C.*

results in sharp deterioration of the performance properties. In case of low degree of crystallization the structure is characteristic of a weak crystalline framework.

*Genesis of Structure and Properties of the Zeolite-Like Cement Matrices of the System… DOI: http://dx.doi.org/10.5772/intechopen.97520*


**Table 6.**

*Optimal mixes of the heat resistant composite alkali-activated aluminosilicate cement-based materials.*

Taking into consideration the higher reaction ability of metakaolin compared to the fly ash at initial stages of interaction, two systems of heat resistant composite materials were proposed (**Table 6**). These optimized compositions have the initial setting time at 80°C – up to 4 hours, compressive strength – up to 89 MPa, residual strength after high-temperature curing – up to 245% and thermal shrinkage – up to 4.2%. The developed compositions were used for making heat-resistant composite materials for high use temperatures without pre-treatment.
