Thermal Evolution of Geopolymer in the Process of High-Temperature Treatment

More and more attention had been given to geopolymers (GPs) over the last decades because of an increasing urgency to search for high-performance and/or environment-friendly alternatives to traditional Portland cement. In addition, geopolymer technology could also provide an innovative approach to prepare advanced ceramic products by overcoming problems faced in the conventional preparation technology. With only the need to go through appropriate thermal treatment proce-dure, geopolymers could be directly in situ transformed into advanced ceramics such as leucite or pollucite with adjustable microstructures, mechanical properties, coefficient of thermal expansion, and melting points. In the process of high-temperature treatment, multiple parameters, such as the composition of geopolymer, treatment temperature, thermal insulation, etc., would affect the phase composition and microstructure of the resulting products. In the present chapter, two kinds of mixed-alkali metal ion-activated geopolymer systems, Cs (1- x ) Li x GP (where x = 0, 0.1, 0.2, and 0.3) and Cs (1- x ) Na x GP (where x = 0, 0.1, 0.2, 0.3, and 0.4), respectively, were designed and prepared. Phase composition, microstructure evolution, and thermal expansion behaviors of the ceramics derived from the geopolymers were characterized and the effects of ion substitution on the thermal evolution of geopolymer were evaluated.


Introduction
More and more attention had been given to geopolymers over the last decades because of an increasing urgency to search for high-performance and/or environment-friendly alternatives to traditional Portland cement [1][2][3][4][5][6]. They were typically synthesized at ambient or elevated temperature by alkali-activated process between alkali-activated solutions and aluminosilicates. And the source of aluminosilicates was very extensive and inexpensive, such as metakaolin, fly ash, blast furnace slag, etc. [7][8][9][10][11]. Most notably, there was almost no emission of greenhouse gases throughout the preparation process, which made geopolymer technology more competitive in terms of environmental and economic advantages.
From their composition, geopolymers were a class of amorphous materials consisting of cross-linked [AlO 4 ] − and [SiO 4 ] tetrahedra units, and the negative charge aluminum in fourfold coordination was balanced by alkali metal powders were sintered without the occurrence of any reaction between the samples and the crucible.
The phase composition of samples was analyzed using X-ray diffraction (XRD, 40 KV/100 mA, D/max-γB CuKa, Rigaku Corp., Japan) method to obtain the X-ray diffraction spectra at 2θ = 10°-90° with a scanning speed of 4°/min. Slow step-scans with a step width of 0.02 and a step time of 3 s were carried out to determine the shifts of X-ray spectrum.
The microstructure of geopolymers before and after heat treatment was investigated by a scanning electron microscope (SEM, 30 KV, Helios Nanolab600i, FEI Co., USA). Energy dispersive spectrometer (EDS) was also adopted to study the elemental arrangement and phase distribution. Transmission electron microscope (TEM, 300 KV, Talos, FEI Company) was also employed to analyze its microstructure. For ceramics derived from geopolymer, TEM samples were ion-milled at low temperature, using a Fischione ion mill (Model 2; Export, PA).

Results and discussions
3.1 Thermal analysis Figure 1 displays the results of thermogravimetry (TG) analysis as to Cs (1-x) Li x GP and Cs (1-x) Na x GP systems, respectively. Both systems showed similar TG curves. There was significant mass loss as the temperature increased until 800°C. The escape of free water, including water adsorbed on the surface and pore solution, accounted for the mass loss in the interval below 300°C. The mass loss between 300 and 800°C could be mainly attributed to the condensation and polymerization of hydroxyl connected with Si/Al atom, which resulted in the formation of oxo-bridge in the framework of geopolymer [13,36]. As the temperature rose further, the TG curves remained essentially unchanged, suggesting almost no mass loss in this stage. The total mass loss increased regularly with the increase of Li + /Na + content. Take Cs (1-x) Na x GPs for example, Na + had a smaller ionic radius than that of Cs + , which means Na + ion had a higher hydration energy than that of Cs + ion [37]. Therefore, the amount of hydration water associated with Na + was much more than that with Cs + ion and, so, it was easy to see why the mass losses increased with increase in Na + ion substitution amount in the Cs (1-x) Na x GP system. Figure 2 displays thermal shrinkage curves of these two geopolymer systems. The process of thermal shrinkage could be divided into four stages [14,38,39]: (I) Structural resilience (RT ~100°C): due to only free water lost in this interval, the dimensions of the tested samples could be maintained substantially, and the corresponding densification degree of the tested samples did not change significantly synchronously; (II) Dehydration (100-300°C): the causes of the shrinkage and deformation in this interval could be attributed to the capillary contraction induced by the escape of water from micro-and nano-pore solutions; (III) Dehydroxylation (300-800°C): the condensation and polymerization between T-OH groups (T = Al/Si) caused the water escape and shrinkage in the interval, with more weight damage and shrinkage at this stage than those in stage (II); (IV) Viscous sintering (above 800°C): the shrinkage in this stage was caused by the generation of molten amorphous glass phase, and the presence of molten amorphous glass phase would facilitate sintering and densification, which also means the maximum shrinkage behavior would occur in this stage [13,19,40,41]. When the treatment temperature climbed to 1200°C, almost no shrinkage was observed, suggesting that the shrinkage process was fully completed.
However, after Cs + ion was substituted, significant differences are observed in Stage IV. For Cs (1-x) Li x GP samples, they all showed two sintering steps in Stage IV. Taking Cs 0.9 Li 0.1 GP for example, as depicted in Figure 3, there were two sintering steps: Region i: 850-1050°C and Region ii: 1050-1300°C. On raising Li + ion content from 0 to 30 mol%, the onset temperature of two steps decreased gradually, from 850 to 730°C for Region i and from 1050 to 970°C for Region ii, respectively. In general, the melting points of the MAlSi 2 O 6 decline with decreasing ionic radius of M + ion (M = Li, Na, K, Cs). Meanwhile, onset sintering temperatures for NaGP, KGP, and CsGP were 650°C [36], 850°C [14], and 1200°C [13], respectively, indicating that geopolymer containing M+ ion (MGP) also showed a similar trend with MAlSi 2 O 6 . Therefore, it was reasonable to deduce that LiGP would exhibit the lowest onset sintering temperature, resulting in the lower temperature in Region i of Stage IV. Region ii might be due to melting of Li-based aluminosilicates. It should be pointed out that after doping with Li, all the samples show lower overall thermal shrinkage than pure CsGP, implying that the presence of Li facilitated sintering of Cs (1-x) Li x GPs to dense microstructure. The same trend could also be observed in the Cs (1-x) Na x GP system. Figure 4 provides XRD patterns of Cs (1-x) Li x GPs and corresponding ceramic products derived from it. For Cs (1-x) Li x GPs, all the samples show a typical amorphous character with a broad amorphous hump from 20° to 30° 2θ, and almost no changes in phase composition had been observed based the XRD results of Cs (1-x) Li x GPs with different Li + content. However, a substantial amount of pollucite was observed after  pure CsGP was treated at 1300°C for 2 h. In addition, a small amount of spodumene was also observed after Cs + ion was partially substituted by Li + ion, and the content of spodumene was proportional to the Li + ion substitution in Cs (1-x) Li x GPs system. The results of quantitative analysis indicated that the content of spodumene was 8.8 and 14.2 wt.% corresponding to x = 0.2 and 0.3 in Cs (1-x) Li x GPs system, respectively. However, the spodumene content was too small to calculate using XRD data when Li + ion content was 10 mol%.

Phase composition
Unlike Cs (1-x) Li x GPs (x > 0), the phase composition of Cs (1-x) Na x GPs (x > 0) was not completely amorphous after the introduction of Na + ion, and the crystallinity of Cs (1-x) Na x GPs gradually increased as more Cs + ion was substituted. The XRD results indicated the presence of pollucite in the unheated samples partially substituted by Na + ion (Figure 5a), which suggested that the presence of Na + ion would have contributed to the crystallization of pollucite. As for the crystallization of pollucite, a possible reason could be attributed to the formation of a zeolite containing sodium, aluminum, and silicon, such as analcime (NaAlSi 2 O 6 ·H 2 O). The crystallization temperature of zeolite was very low, about 300°C, and the crystallization temperature will become lower (~120°C) for the samples with low Si/Al  ratio under the hydrothermal condition [42,43]. The geopolymer was nanoporous together with numerous capillaries, filled with pore solution. This structure was like a hydrothermal reaction environment. So, it was possible to produce some fine zeolite nucleus if the reaction time was long enough, although the reaction temperature was low. Analcime and pollucite have similar crystal structure and both were tetragonal and belong to I41/acd (space group). Meanwhile, lattice constant of analcime was a~13.727 and c~13.686, which was close to that of pollucite, a~13.677 and c~13.691 Å. Therefore, it was highly possible that the zeolite nucleus could have served as the nucleation site for pollucite, which contributed to pollucite formation. So, crystalline pollucite could be present in Cs (1-x) Na x GPs when x > 0, which was not the same as the phenomenon of Cs (1-x) Li x GPs system.
For the resulting ceramics derived from Cs (1-x) Na x GPs (x > 0), only crystalline phases of pollucite were observed in the corresponding XRD pattern, which were different with the Cs (1-x) Li x GPs system. So, the difference between Cs (1-x) Li x GPs and Cs (1-x) Na x GPs also indicated that Na + may have only existed in the form of amorphous glass phase.
Based on the result of slow step-scans of ceramics derived from Cs (1-x) Li x GPs, it was possible to find that with increase in Li + ion substitution, almost no deviation occurred for the characteristic peaks (4 0 0) and (3 2 1) corresponding to pollucite (Figure 6), which proved that the lattice parameters of pollucite did not change as more and more Li + ions were introduced [33,44]. This implied that Li + ions did not occupy Cs crystallographic sites of the pollucite framework and phase separation occurred during heating. Diphasic compositions of product ceramics derived from Cs (1-x) Li x GPs also conform to the aforementioned two-step sintering behavior observed in thermal shrinkage results.
In contrast, for the heated samples derived from Cs (1-x) Na x GPs, the peaks (4 0 0) corresponding to pollucite shifted to the high-angle region with increase in sodium substitution (Figure 7), which suggested that the lattice parameters of pollucite decreased with increases in sodium content [33,44]. The decline of pollucite's lattice parameters proved that Na + partially occupied/substituted the crystallographic sites of Cs + in the pollucite crystal structure during high-temperature processing. The difference between Cs (1-x) Na x GP and Cs (1-x) Li x GP systems could be attributed to the difference between the size of the ions and the form of the substituted ions. Compared with Li + ion, the size of Na + ion was closer to that of Cs + ion. In the Cs (1-x) Li x system, Li + ion was only in the form of spodumene in the heated samples. The different forms of Na + and Li + ions also suggested that the Na + ion had a higher degree of freedom  than Li + ion in the mass transfer process. So, it was much easier to occupy/substitute the crystallographic sites of Cs + ion in the pollucite crystal structure with Na + ion. Figure 8 displays the microstructure of as prepared Cs (1-x) Li x GPs. The particles are too small to be observed clearly for pure CsGP. For other Cs (1-x) Li x GPs (x ≥ 0.1), the average particle sizes (APSs) are close to ~120, ~200, and ~250 nm corresponding to x = 0.1, 0.2, and 0.3, respectively, which could also be attributed to the difference in hydration energy between Li + ion and Cs + ions. By comparison, the hydration energy of Li + was higher than that of Cs + , which means that there were more [AlO 4 ] and [SiO 4 ] associated with Li + ion than that around Cs + ion in the framework of the geopolymer. Therefore, higher Li + ion contents in the geopolymer always resulted in particles with larger APSs [37]. The same variation trend was also observed in Cs (1-x) Na x GPs system, as shown in Figure 9.  As for Cs (1-x) Li x GPs system, as shown in Figure 10, these precipitates coarsened substantially and all the geopolymers developed a smooth, glassy texture after heating to 1300°C. The coarsening was consistent with the considerable shrinkage observed over the sintering temperature range [45]. Closed pore formations were also observed coincident with significant coarsening and surface area reduction. Pollucite crystals could not be directly observed on any of the fracture surfaces of pure CsGP ceramics despite their noticeable presence in the XRD. As for other Cs (1-x) Li x GPs (x ≥ 0.1), the corresponding ceramic product derived from it contain large numbers of spherical particles surrounded by a glassy matrix, with the size of coarsening increasing with Li + ion content. The mean diameters of spherical particles were close to ~100 (x = 0.1), ~250 (x = 0.2), and ~400 nm (x = 0.3), respectively. The back-scattered electron (BSE) image suggested the presence of diphasic compositions in corresponding ceramic products derived from Cs (1-x) Li x GPs (x ≥ 0.1) (Figure 10e). EDS spectra showed that cesium content at point A (Figure 10f) was much higher than that at point B (Figure 10g). Because EDS was a semi-quantitative analytical measure and Li + ion could not be detected by it, so, the results of EDS suggested that spherical particle (point A) and glassy matrix (point B) corresponded to pollucite and spodumene, respectively.

Microstructure evolution
From the fracture morphology after hydrofluoric acid (HF) corrosion, the existence of pollucite grains could be clearly observed in ceramic derived from pure GsGP (Figure 11a). In contrast, a polydisperse distribution of pollucite crystals and pores left after dissolution of molten glass phase during etching after the introduction of Cs + ion, and the coarsening trend of pollucite grains were also obvious (Figure 11).   Figure 12 shows TEM analysis of the ceramic derived from Cs 0.7 Li 0.3 GP. The grain boundaries could be observed clearly, and the selected area electron diffraction (SAED) patterns of area A indicate the existence of spodumene crystallite in the resulting products, which may have arisen from the recrystallization of molten spodumene during cooling. Meanwhile, SAED patterns of area B also proved the corresponding grain should be pollucite, and the spodumene phase, with no fixed shape, mainly distributed among pollucite grains or in the grain junction area.
The microstructure morphologies of ceramic products derived from Cs (1-x) Na x GPs are given in Figure 13. By comparison, the particles coarsened significantly after   being treated at 1300°C than that of corresponding unheated Cs (1-x) Na x GP samples. It could be observed that the surface of pollucite grains was covered by smooth and glassy texture. The presence of amorphous glass phase would be conducive to the densification of resulting ceramic products [45].
After etching the samples in 4 wt% HF acid for 20 s, pollucite crystals could be clearly observed in ceramics derived from heated Cs (1-x) Na x GPs when x ≤ 0.3, as shown in Figure 14. It could be clearly observed that the particle size gradually  decreased with increasing Na + ion substitution. Compared with ceramics derived from pure CsGP, many pores could be observed, which should be attributed to the dissolution of the amorphous glass phase in the process of static etching. Due to the content of amorphous glass phase and the Na + ion substitution showed a proportional relationship. So, the residual amorphous glass phase will be more in the sample contained a higher Na + ion substitution. Take Cs 0.6 Na 0.4 GPs for example, the vast majority of pollucite grains could not be observed clearly on the corroded surface of corresponding ceramic products due to the higher content of amorphous glass phase.   Figure 15 shows the TEM analysis of the ceramic evolved from treated Cs 0.6 Na 0.4 GP. Similar to Cs (1-x) Li x GPs system, the presence of crystal grains could be clearly observed in this diphase composition system. The SAED patterns of area A (Figure 15b) suggested the crystal grain to be pollucite. The results of the map-scan suggested that Cs + ions were distributed only in the interior of crystal grain, which is consistent with the TEM results, as shown in Figure 16. However, Na + showed a completely different distribution. Map-scan results revealed that although the vast majority of Na + distributed among pollucite grains, there was still a small portion present in the interior of pollucite grains, which further demonstrated that Na + ions partially occupied Cs crystallographic sites of the pollucite framework during high-temperature processing. Combining the SAED patterns of area B (Figure 15c) and the result of the map-scan indicated again that the Na + ion mainly existed in the form of amorphous glass phase and distributed among the pollucite grains.
In heated Cs (1-x) Li x GPs system, the particle size of pollucite coarsened with increases in Li substitution. However, Cs (1-x) Na x GPs system showed a distinctly different variation tendency, in which the particle size of pollucite grains gradually decreased with increasing Na + ion substitution. The phenomenon of grain refinement in heated Cs (1-x) Na x GPs system may have a direct relationship with the presence of zeolite nucleus in corresponding unheated samples. The zeolite nucleus formed in the unheated Cs (1-x) Na x GP samples could serve as the nucleation site of pollucite and accelerate the crystallization process of pollucite. As already mentioned, the number of zeolite nucleus was proportional to Na + ion introduction. Therefore, the growth of pollucite grains was more dispersed as the nucleation sites increased.

Sintering mechanism
A possible sintering mechanism of Cs (1-x) Li x GP system can be suggested in light of the theory of reactive liquid-phase sintering [46]. The composition of Cs (1-x) Li x GP can be expressed as (1-x)Cs 2 O·xLi 2 O·Al 2 O 3 ·4SiO 2 ·wH 2 O. In fact, both Li + and Cs + ions were present as silicates, that is, x 1 Li 2 O·y 1 SiO 2 and x 2 Cs 2 O·y 2 SiO 2 (the complexity of composition and structure of silicate indicated that x 1 , y 1 , x 2 , and y 2 did not have a certain value). Therefore, it was reasonable to employ Li 2 O and Cs 2 O to express the form of Li + and Cs + ions in this system.
It well known that alkaline metal oxides often participated in the sintering process as a sintering aid in many low-temperature sintering systems [46,47]. In addition, as a typical network-modifying agent, the presence of Li 2 O or Li + ion could promote the densification process of sintering [48][49][50][51]. Similarly, the reaction between the alkaline metal oxide dopant and the matrix phase was studied and results showed that the mechanism was exactly the same in many cases. Hence, as for the Cs (1-x) Li x GP system, Li + was an intermediate cation with a stronger electric field intensity in the glass-ceramic system than that of Cs + ion [52]. Thus, with increases in temperature of high-temperature processing, the reaction between Li 2 O, Al 2 O 3, and SiO 2 first occurred at a lower temperature, as in Eq. (1).
The presence of liquid spodumene (LiAlSi 2 O 6 ) and the simultaneous reaction between Li 2 O and the matrix phase decreased the viscosity of the geopolymer system, which doubtlessly accelerated the kinetics of mass-transport processes during the low-temperature sintering. The changes in the sintering processes could reflect from the variation tendency in thermal shrinkage curves with different lithium contents (Figure 3). As the processing temperature rose further, the reaction between Cs 2 O, Al 2 O 3, and SiO 2 would occur at a higher temperature compared with the onset temperature of spodumene, as in Eq.   The presence of liquid spodumene increased the rates of mass transfer and diffusion more easily, conducive to nucleation and crystallization of pollucite (CsAlSi 2 O 6 ). This was consistent with the decrease in crystallization temperature and the grain coarsening behavior observed above with increasing lithium substitution. Meanwhile, the degree of densification was greatly improved as well. Therefore, the presence of Li + ion contributed to the crystallization and sintering densification via aiding the liquid-phase processes.
The sintering process of the Cs (1-x) Na x GP system was similar to that of the Cs (1-x) Li x GP system. The results above have already proved that Cs (1-x) Na x GP ceramics only contained pollucite and amorphous glass phase. Therefore, the main difference in the sintering process between Cs (1-x) Li x GP and Cs (1-x) Na x GP systems lied in the low-temperature area. The reaction between Na 2 O, Al 2 O 3, and SiO 2 in the Cs (1-x) Na x GP system would occur first at a lower temperature, as in Eq. (3). Na 2 O (s) + SiO 2 (s) + Al 2 O 3 (s) → amorphous glass (l) ( In addition to this lower temperature, the sintering process of the two systems also experienced almost the same process at higher temperature intervals.

Thermal expansion behavior
The results of the dilatometric measurement for Cs (1-x) Li x GP and Cs (1-x) Na x GP ceramics with correction are shown in Figure 17. The thermal expansion of ceramics derived from Cs (1-x) Li x GP and Cs (1-x) Na x GP systems showed an increasing trend with increase in temperature, which could be attributed to the larger atomic spacing at elevated temperatures.
The average thermal expansion coefficient (CTE) was defined as the average value of the relative length change in the temperature range of T 1 < T < T 2 , as described by the following Eq. (4): where L 0 , L 1 , and L 2 are the lengths of the specimen at temperatures of T 0 (30°C), T 1 , and T 2 , respectively. From Figure 14, both curves have two very different slopes before and after ~150°C, indicating that there was a phase transition for pollucite from tetragonal to cubic [24,25]. Thus, the average CTE of Cs (1-x) Li x GPs was computed based on data in the interval from 150 to 900°C. DOI: http://dx.doi.org /10.5772/intechopen.81610 As for Cs (1-x) Li x GP ceramics, as depicted in Figure 18, it could be observed that the average CTE decreased from 4.80 × 10 −6 K −1 (x = 0) to 3.61 × 10 −6 K −1 (x = 0.3). The results obtained from our previous study on the K (1-x) Cs x GP system proved that increase in Cs + ion introduction would decrease the average CTE of the resulting leucite ceramic. The reason could be attributed to the substitution between Cs + and K + in the leucite framework [33]. However, as for Cs (1-x) Li x GP ceramics, the XRD results indicated almost no changes in lattice constants of pollucite, which suggested that the integrity of pollucite crystal cell was unchanged after partial substitution by Li + ion. XRD quantitative analysis suggested that the content of spodumene in resulting ceramic products was proportional to the amount of Li + ion substitution. By comparison, spodumene had a lower CTE (1.0 × 10 −6 K −1 ) than that of pollucite. Therefore, the presence of spodumene could significantly reduce CTE of the whole system, and the higher content of spodumene meant lower CTE. Another reason could be attributed to the role of molten spodumene, possibly as a buffer phase exists between pollucite crystals. Hence, with the increase in Li substitution or spodumene content, the average CTE of Cs (1-x) Li x GP ceramics decreased evidently.
However, the calculated results showed that the average CTE of Cs (1-x) Na x GP ceramic products showed a completely different variation pattern compared with Cs (1-x) Li x GP ceramic products, increasing from 4.80 × 10 −6 K −1 (x = 0) to 7.26 × 10 −6 K −1 (x = 0.4) with the rise of sodium substitution.
For K (1-x) Cs x GPs system involved in our previous studies, the average CTE of the leucite ceramic decreased with increase in the amount of Cs + ion introduced. The reason could be ascribed to Cs + occupying/substituting K + crystallographic sites in the leucite framework, which led to the increase in content of stabilized cubic leucite in resulting ceramic products [33]. However, there was no evidence to suggest any substitution at the lattice level between Li + and Cs + in the Cs (1-x) Li x GP system. Besides, as a diphase system, pollucite and spodumene were present as independent components in the resulting products [38]. Therefore, one could conclude that the decline of average CTE was mainly due to the fact that CTE of spodumene was far lower than that of pollucite.
As for Cs (1-x) Na x GP ceramics, XRD results suggested that the substitution between Na + and Cs + ions caused the decline of lattice constant of pollucite. According to the Ikuo et al. reports, the increase in Na + in the crystal lattice could decrease the thermal expansion of the pollucite, and with the increase in Na + ion substitution, the average CTE of ceramics derived from heated Cs (1-x) Na x GPs decreased [24][25][26]. However, compared with Cs (1-x) Li x GPs system, the CTE of Cs (1-x) Na x GP ceramics showed an opposite trend with increasing Na + ion substitution. The average CTE of Cs (1-x) Na x GP ceramics continued to rise with the increase in content of Na + ion introduction. It is well known that amorphous glass phase containing Na + ion always had a higher CTE (9~10 × 10 −6 K −1 ) than that of pollucite (~4.80 × 10 −6 K −1 ) [53][54][55][56]. By contrast, the effect on CTE caused by the presence of the amorphous glass phase was much stronger than the impact caused by the substitution of Na + for Cs + in lattice level. So, the presence of amorphous glass phase could account for the rise of average CTE. The content of amorphous glass phase was proportional to the amount of Na + ion substitution. Therefore, the average CTE of ceramics derived from Cs (1-x) Na x GPs increased almost linearly with the increasing sodium content.

Conclusions
In this chapter, the effects of ion substitution on the thermal evolution of cesium-based geopolymers on heating were studied. The following conclusions could be obtained based on the current results.
i. The introduction of Li + or Na + could favor the formation of pollucite in Cs (1-x) M x GPs (x ≥ 0.1), respectively. A possible reason could be attributed to the formation of zeolite nucleus, which could serve as the nucleation sites for pollucite. This also provided a novel method for preparing pollucite at low temperature, even at room temperature.
ii. Cs (1-x) M x GPs with different content of ionic replacement showed a similar thermal evolution process. The introduction of Li + or Na + changed the sintering mechanism through decreased onset temperature for viscous sintering stage. With the introduction of more Li + or Na + , the amorphous glass phase in resulting products would be conducive to its densification process. The introduction of Li + ion led to particle coarsening in part because of the rapid formation of a liquid phase on heating and perhaps because the hydration energy of Li + is greater than that of Cs + . However, the particle size of pollucite grain gradually decreased with the increase in Na + contents in Cs (1-x) Na x GP ceramics, which had a close relationship with the role of Na + ion in the nucleation process of pollucite.
iii. Compared with traditional way, geopolymer technology could be an excellent alternative to fabricate ceramics or ceramic matrix composites in an in-situ convert way. In the Cs (1-x) Li x GP ceramics, the introduction of Li + results in the formation of a multiphase system, including pollucite, spodumene, and amorphous glass phase, and Li + only existed inside the spodumene grains. In contrast, two forms of Na + ions were present in the ceramic products derived from Cs (1-x) Na x GPs. Therefore, a small portion was present in the pollucite grains due to Na + occupying/substituting Cs + crystallographic sites of the pollucite framework, while the rest were mostly present in the amorphous glass phase and distributed among the pollucite grains.
iv. The average CTE of ceramics derived from Cs (1-x) Li x GPs and Cs (1-x) Na x GPs showed a completely different variation pattern. The reason could be attributed to the difference between two kinds of substituted ion species, which resulted in different phase composition in the corresponding resulting products.
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