**3.1 The structure formation processes in the cement of the R2O-Al2O3-SiO2-H2O system**

#### *3.1.1 Influence of curing conditions*

Ten cement compositions were chosen for this study (**Table 2**). Each cement composition was cured using one out of five curing regimes mentioned above. The XRD and DTA results allowed identifying the fields of crystallization of the aluminosilicate-based hydration products (**Figure 1**).

Thus, the phase compositions of the reaction products of the metakaolin- and fly ash-based cements are similar. The phase composition of the reaction products of the autoclaved fly ash-based cements is represented mainly by analcime, zeolite-Р and zeolite-R. These conclusions coincide well the data obtained on the model systems and reported in [5, 6] and those obtained with the metakaolin-based cements [12].

A synthesis of hydroxysodalite which is formed due to high contents of Nacation in the steam cured fly ash-based cement compositions with SiO2/Al2O3 = 4 and those after drying correlates well with the data obtained on similar systems with high alkali concentrations and after curing in normal conditions [5, 6, 35–37]. When using the low-alkali cement compositions with SiO2/Al2O3 = 6–8


#### **Table 2.**

*Ratio of the components in a cement matrix.*

*Type of hydration products vs. aluminosilicate component of the cements.*

*The 27Al MAS-NMR spectra of (a) typical metakaolin, (b) metakaolin-based Na-geopolymer from metakaolin with Si/Al ratio of 2.15, (c) typical fly ash, and (d) Na-geopolymer from NaOH activation of fly ash (20 hours, 85°C) [38].*

simultaneously with a low-temperature curing such as steam curing or drying, the accepted exposition time is not enough for the zeolite crystallization.

A speed of the mineral (in our case, zeolite) formation is usually measured by a height of the XRD peaks. These peaks in case of the fly ash-based cements are much more higher compared to those in case of the metakaolin-based cements.

This also follows from the Si MAS-NMR and the Al MAS-NMR spectra (**Figures 2** and **3**) [38–46].

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

**Figure 3.**

*The 29Si MAS-NMR spectra of (a) typical metakaolin, (b) metakaolin-based Na-geopolymer with Si/Al ratio of 1.65, (c) typical fly ash, and (d) fly ash-based Na-geopolymer (20 hours, 85°C) [39].*

The greater intensively of the peaks 93 and 98 ppm in the fly ash-based cement compared to the signal at 84.6 and 89 ppm in the metakaolin-based cement is an indication that the aluminosilicate gel that is formed (N-A-S-H) in the fly-ash based cement has a higher silica content.

After drying of both metakaolin- and fly ash-based cements, a minor phase was fixed at first as a "phase Z" [18], but further studies on carbonation finally showed that these peaks could be identified as a trona mineral (Na2CO3NaHCO32H2O). After steam curing, another minor phase with main peaks (0.276, 0.237 nm) was identified [18]. These peaks can be an evidence of formation of another sodium carbonate, Na2CO3H2O (due to carbonation process). These two phases may sometimes co-exist, however, their formation takes place under a different mechanism: the higher W/S ratios, the higher amounts of the trona are formed. A synthesis of the sodium carbonate hydrate takes place only at low W/S ratios and high alkalinity under hydrothermal conditions (steam curing, curing in an air tight mould).

In order to study a phase composition of dehydration products the cement specimens (**Table 2**) were, after pre-curing, subjected to high temperature curing (800°C). On the contrary to the phase composition after hydration, which is dependent, above all, on a mode of pre-curing, the phase composition after dehydration depends only on the cement composition and does not depend on the mode of pre-curing. So, in the cements with SiO2/Al2O3 = 4 a phase composition of the reaction product is represented by nepheline (**Figure 4**). The degree of crystallization does not depend on the mode of pre-curing, and depends on a type of the aluminosilicate component: a maximal degree of crystallization was observed in case of the metakaolin compared to the fly ashes. This can be attributed to the fact that the metakaolin is the most chemically pure component. The impurities from the fly ashes resulted in the reduced contents of the anhydrous aluminosilicates and in the formation of additional amount of hematite.

#### **Figure 4.**

*The XRD scans of the metakaolin-based cements with SiO2/Al2O3 = 4 after high temperature curing at 800°C. Aluminosilicate component: Metakaolin (1–3), fly ash 1 (4–6), fly ash 2 (7–9). Pre-curing mode: Steam curing (1, 4, 7), drying (2, 5, 8), without pre-curing (3, 6, 9) N – Nepheline, Ab – Albite, Kr – Cristobalite, F – Hematite, FA – Residue of unreacted fly ash.*

## *3.1.2 The role of type of the aluminosilicate component and SiO2/Al2O3 ratio*

**Figures 5** and **6** demonstrate XRD scans and SEM images of the microstructure of a cleaved fragment of the artificial stone depending on the ratio SiO2/Al2O3 = 2:7 in the original composition of the zeolite-like cement matrices after drying at 80°C (under condition that K2O/R2O = 0.15 and (Na2O+K2O)/Al2O3 = 1).

A phase composition of the hydration products at low of SiO2/Al2O3 ratios (2:3) after drying at 80°С is represented by zeolite-Na-A (d/n = 0.699; 0.365; 0.336; 0.293 nm), natrolite (d/n = 0.287; 0.243; 0.138 nm), ussingite (d/n = 0.492; 0.347; 0.295 nm). Amorphous phases of the metakaolin-based cement and particles of the non-reacted metakaolin can be clearly seen in the SEM images (**Figure 6**).

The alkali-activated aluminosilicate cements with the SiO2/Al2O3 ratios =4:5 have the zeolite-like hydration products of the following type: zeolite-Na-A (d/ n = 0.699; 0.365; 0.336; 0.293 nm), heulandite-Na (d/n = 0.509; 0.392; 0.296 nm), heulandite-K (d/n = 0.342; 0.281; 0.273 nm), and phillipsite-Na-K (d/n = 0.498; 0.408; 0.269 nm).

As it follows from **Figures 7** and **8**, with the temperature increase the degree of crystallinity increases as well.

The 3D-polymeric crystalline or semi-crystalline aluminosilicates of alkali metals are main hydration products of the cements under study at low temperatures (usually below 200°C). Correspondingly, they refer to "zeolites" or "zeolite precursors". Thermal stability of these phases depends on their structure: some zeolite structures are known to be resistant to heating, and some of them decompose in the process of heating. Among various zeolites able to crystallize in the cement matrix,

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

#### **Figure 5.**

*The XRD scans of the metakaolin-based cements after drying at 80°С (K2O/R2O = 0.15 and SiO2/Al2O3, respectively: a – 2; b – 3; c – 4; d – 5; e – 6; f – 7. Q – Quartz; N – Natrolite; a – Zeolite-Na-a; U – Ussingite; P*<sup>0</sup> *– Phillipsite-Na-K; H – Heulandite-Na; H*<sup>0</sup> *– Heulandite-K.*

#### **Figure 6.**

*The SEM images of the metakaolin-based cements after drying at 80°С (K2O/R2O = 0.15 and SiO2/Al2O3, respectively: a – 2; b – 3; c – 4; d* � *5; e – 6; f – 7 (*�*2500).*

only certain zeolites remain their structure until 700–800°C. Thermally stable structures are: sodalite network (hydroxysodalite), analcime, chabazite structures (zeolite-R, herchelite), faujasite family (zeolites-Na-X and Na-Y), mordenite [5–7].

**Figure 7.**

*The XRD scans of the metakaolin-based cement (K2O/R2O = 0.15 and SiO2/Al2O3 = 5) after drying at temperatures, °С: a – 20; b – 40; c – 60; d – 80. Legend: Q – Quartz; A – Zeolite Na-A; P*<sup>0</sup> *– Phillipsite-Na-K; H – Heulandite-Na; H*<sup>0</sup> *– Heulandite-K.*

**Figure 8.**

*The SEM images of the microstructure of the metakaolin-based cement (K2O/R2O = 0.15 and SiO2/Al2O3 = 5) after drying at temperatures, °С: a – 20; b – 40; c – 60; d – 80 (*�*2500).*

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

Once these phases (or N-A-S-H gels of similar structure) are synthesized in the cement matrix, they slightly improve their crystallinity during heating until 200–400°C, and then keep their structure up to approx. 800°C and after this re-crystallize with the formation of structurally similar nepheline or albite. Other zeolitic structures, for example, zeolites-Na-A or P, decompose at temperatures within the range of 120–300°C, making impossible to use them in the materials that are resistant at high temperatures.

In studying the influence of the cement composition on the phase composition after heating above the temperature of dehydration, a conclusion was drawn on a correlation between the SiO2/Al2O3 molar ratio in the initial cement compositions and their dehydration products (**Table 3**). With increase of the ratio from 2 to 8, the reaction products transform in the following direction: nepheline (Na2OAl2O32SiO2) – albite (Na2OAl2O36SiO2) – α-cristobalite (SiO2) (**Figure 4**). This tendency in general coincides well with the data obtained on the model systems of the alkali-activated aluminosilicate cements [18, 47]. So, a feldspathoid nepheline (SiO2/Al2O3 = 2 in the aluminosilicate framework) appears in a phase composition of the cements with SiO2/Al2O3 = 2–4, irrespectively of a type of the aluminosilicate component used. A maximum XRD- measured quantity of nepheline was found in case when a SiO2/Al2O3 ratio was 2. The increase of the ratio up to 6–8 resulted in the additional formation of α-cristobalite (d = 0.411; 0.252; 0.206; 0.163 nm). The higher contents of amorphous silica, the higher speed of α-cristobalite formation.

In the fly ash-based cements with the SiO2/Al2O3 ratio = 6, the reaction products are represented by high-silica albite and some amounts of nepheline (**Table 3**). Differences in a phase composition of the metakaolin-based and fly-ash-based cements can be attributed to high contents of main oxides in a non-active form such as quartz and mullite which reduce a real stoichiometric ratio in these two cements.

As mentioned above, a type of the aluminosilicate component used (metakaolin or fly ash) does not affect the differences: once heated to high temperatures (600–1200° C), a microstructure of the cements under study tends to transform into that with the phases resistant to high temperatures. In contrast to the OPC-based cements, no any instable phase was identified. However, metakaolin, a purer aluminosilicate component, gives the higher purity and higher content of the formed high-temperature phases formed. In case of the fly ash the lower quantities of the zeolite-like products, however, with a more variable composition due to small quantities of such oxides as Fe2O3, MgO, CaO, TiO2 etc. contained in the fly ash are formed.


#### **Table 3.**

*The SiO2/Al2O3 molar ratio of the cements under study after high temperature curing (1200°C).*
