**1. 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 [1–6]. 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. [7–11]. 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 [AlO4] <sup>−</sup> and [SiO4] tetrahedra units, and the negative charge aluminum in fourfold coordination was balanced by alkali metal

or alkaline earth metal cation, such as Na<sup>+</sup> , K<sup>+</sup> , Mg2+, and Ca2+ [12–15]. As for geopolymers, alkali metal hydroxides were involved in many processes during the alkali-activated reaction or geopolymerization reaction, including accelerating the dissolution of aluminosilicates, stabilizing the solution species and colloids, reducing the electrostatic repulsion between the anions, and promoting gel formation and rearrangement [16–18]. Therefore, as for alkali metal hydroxide, the OH<sup>−</sup> group in it mainly accounted for creating the reactive precursors, while the alkali cations would play an important role in catalyzing gel formation, acting as a structure-directing agent. Since the alkali metal was one of the most easily alterable constituents in the framework of geopolymer, it was very valuable to investigate the effects of alkali cation species on the evolution of microstructure and properties of the geopolymer system [19]. Fernández-Jiménez et al. proved that Na<sup>+</sup> was much more conducive to the coagulation and precipitation process than K<sup>+</sup> , and the resulting geopolymer gels would consist of a much wider diversity of Q<sup>n</sup> species in the case of Na<sup>+</sup> [20–22]. Duxson et al. suggested that larger cations were more inclined to bind by the gel in mixed-alkali-activated systems that contained Na<sup>+</sup> and K<sup>+</sup> [23]. Ikuo et al. reported that the thermal expansion coefficient (CTE) of the cubic cesium leucite-based compounds decreased with much smaller ions occupying/substituting the crystallographic sites of Cs<sup>+</sup> , such as Na<sup>+</sup> or Li<sup>+</sup> substitutional ions [24–26].

Pollucite (CsAlSi2O6) exhibits a unique thermal expansion curve that has two stages: the average CTE is ~12.5 × 10<sup>−</sup><sup>6</sup> K<sup>−</sup><sup>1</sup> from 298 to 473 K and ~2.2 × 10<sup>−</sup><sup>6</sup> K<sup>−</sup><sup>1</sup> from 473 to 1473 K [24, 27]. In recent years, many reports have given evidence that the thermal expansion of pollucite decreased with ionic substitution at the Cs+ sites by smaller alkali metal ions, such as Na<sup>+</sup> and Li+ [24–33]. But, the mechanisms about how thermal expansion properties of pollucite are affected have not yet been studied clearly. In general, there were three main methods to prepare pollucite, including the solid-state reaction method, sol-gel method, and ion exchange method from leucite [34]. However, solid-state reaction always required high sintering temperatures and easily suffered from problems of cesium volatilization, agglomeration, furnace contamination, etc. Ion exchange and sol-gel techniques were also uncompetitive due to their high cost and severe preparation conditions. Moreover, it was almost impossible to get fully densified pollucite ceramic products because the volatilization of cesium was inevitable in the high-temperature environment during its preparation process. Therefore, only limited researches have been made on pollucite glass ceramic until now. In contrast, geopolymer technology could be an excellent alternative to prepare pollucite ceramic products through in situ conversion due to its ability to from crystalline ceramic phase after proper thermal treatment [35].

Therefore, as a part of our continuing research, a series of ion-substituted cesium-based geopolymer samples, 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), were prepared. Phase composition, microstructure evolution, and thermal expansion behavior of ceramics derived from Cs(1-*x*)Li*x*GP and Cs(1-*x*)Na*x*GP were characterized, with the aim of investigating the effect of ion substitution on the thermal evolution of the geopolymer.

#### **2. Experiments and characterization**

Simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) (Netzsch STA 409, Germany) were carried out under Ar gas flow (20 mL/min) with temperature up to 1200°C at a heating rate of 5°C/min in alumina crucibles. Sample

**101**

**Figure 1.**

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

temperature, using a Fischione ion mill (Model 2; Export, PA).

The total mass loss increased regularly with the increase of Li+

the amount of hydration water associated with Na+

ion substitution amount in the Cs(1-*x*)Na*x*GP system.

*Thermal gravimetric curves of Cs(1-x)LixGPs (a) and Cs(1-x)NaxGPs (b).*

ion had a higher hydration energy than that of Cs+

powders were sintered without the occurrence of any reaction between the samples

The phase composition of samples was analyzed using X-ray diffraction (XRD, 40 KV/100 mA, D/max-*γ*B CuKa, Rigaku Corp., Japan) method to obtain the X-ray diffraction spectra at 2*θ* = 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

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

**Figure 1** displays the results of thermogravimetry (TG) analysis as to Cs(1-*x*)Li*x*GP

ion and, so, it was easy to see why the mass losses increased with increase in Na+

**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

/Na+

was much more than that with

had a smaller ionic radius than that of Cs+

content. Take

ion [37]. Therefore,

, which

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.

*DOI: http://dx.doi.org/10.5772/intechopen.81610*

and the crucible.

shifts of X-ray spectrum.

**3. Results and discussions**

Cs(1-*x*)Na*x*GPs for example, Na+

means Na+

Cs+

**3.1 Thermal analysis**

*Thermal Evolution of Geopolymer in the Process of High-Temperature Treatment DOI: http://dx.doi.org/10.5772/intechopen.81610*

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-*γ*B CuKa, Rigaku Corp., Japan) method to obtain the X-ray diffraction spectra at 2*θ* = 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).

### **3. Results and discussions**

#### **3.1 Thermal analysis**

*Geopolymers and Other Geosynthetics*

that Na<sup>+</sup>

than K<sup>+</sup>

Na<sup>+</sup>

sity of Q<sup>n</sup>

that contained Na<sup>+</sup>

thermal treatment [35].

geopolymer.

or Li<sup>+</sup>

or alkaline earth metal cation, such as Na<sup>+</sup>

species in the case of Na<sup>+</sup>

and K<sup>+</sup>

substitutional ions [24–26].

sites by smaller alkali metal ions, such as Na<sup>+</sup>

**2. Experiments and characterization**

stages: the average CTE is ~12.5 × 10<sup>−</sup><sup>6</sup>

, K<sup>+</sup>

was much more conducive to the coagulation and precipitation process

, and the resulting geopolymer gels would consist of a much wider diver-

cations were more inclined to bind by the gel in mixed-alkali-activated systems

coefficient (CTE) of the cubic cesium leucite-based compounds decreased with much smaller ions occupying/substituting the crystallographic sites of Cs<sup>+</sup>

Pollucite (CsAlSi2O6) exhibits a unique thermal expansion curve that has two

from 473 to 1473 K [24, 27]. In recent years, many reports have given evidence that the thermal expansion of pollucite decreased with ionic substitution at the Cs+

and Li+

K<sup>−</sup><sup>1</sup>

about how thermal expansion properties of pollucite are affected have not yet been studied clearly. In general, there were three main methods to prepare pollucite, including the solid-state reaction method, sol-gel method, and ion exchange method from leucite [34]. However, solid-state reaction always required high sintering temperatures and easily suffered from problems of cesium volatilization, agglomeration, furnace contamination, etc. Ion exchange and sol-gel techniques were also uncompetitive due to their high cost and severe preparation conditions. Moreover, it was almost impossible to get fully densified pollucite ceramic products because the volatilization of cesium was inevitable in the high-temperature environment during its preparation process. Therefore, only limited researches have been made on pollucite glass ceramic until now. In contrast, geopolymer technology could be an excellent alternative to prepare pollucite ceramic products through in situ conversion due to its ability to from crystalline ceramic phase after proper

Therefore, as a part of our continuing research, a series of ion-substituted cesium-based geopolymer samples, 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), were prepared. Phase composition, microstructure evolution, and thermal expansion behavior of ceramics derived from Cs(1-*x*)Li*x*GP and Cs(1-*x*)Na*x*GP were characterized, with the aim of investigating the effect of ion substitution on the thermal evolution of the

Simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) (Netzsch STA 409, Germany) were carried out under Ar gas flow (20 mL/min) with temperature up to 1200°C at a heating rate of 5°C/min in alumina crucibles. Sample

geopolymers, alkali metal hydroxides were involved in many processes during the alkali-activated reaction or geopolymerization reaction, including accelerating the dissolution of aluminosilicates, stabilizing the solution species and colloids, reducing the electrostatic repulsion between the anions, and promoting gel formation and rearrangement [16–18]. Therefore, as for alkali metal hydroxide, the OH<sup>−</sup> group in it mainly accounted for creating the reactive precursors, while the alkali cations would play an important role in catalyzing gel formation, acting as a structure-directing agent. Since the alkali metal was one of the most easily alterable constituents in the framework of geopolymer, it was very valuable to investigate the effects of alkali cation species on the evolution of microstructure and properties of the geopolymer system [19]. Fernández-Jiménez et al. proved

, Mg2+, and Ca2+ [12–15]. As for

[20–22]. Duxson et al. suggested that larger

from 298 to 473 K and ~2.2 × 10<sup>−</sup><sup>6</sup>

[24–33]. But, the mechanisms

, such as

K<sup>−</sup><sup>1</sup>

[23]. Ikuo et al. reported that the thermal expansion

**100**

**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

**Figure 1.** *Thermal gravimetric curves of Cs(1-x)LixGPs (a) and Cs(1-x)NaxGPs (b).*

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 Cs0.9Li0.1GP 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 MAlSi2O6 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 MAlSi2O6. 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.

#### **3.2 Phase composition**

**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° 2*θ*, 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

**103**

Li+

**Figure 4.**

**Figure 3.**

was also observed after Cs+

ion content was 10 mol%.

substituted by Na<sup>+</sup>

of spodumene was proportional to the Li+

*Partial thermal shrinkage curves of Cs0.9Li0.1GP.*

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

pure CsGP was treated at 1300°C for 2 h. In addition, a small amount of spodumene

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

Unlike Cs(1-*x*)Li*x*GPs (*x* > 0), the phase composition of Cs(1-*x*)Na*x*GPs (*x* > 0)

XRD results indicated the presence of pollucite in the unheated samples partially

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 (NaAlSi2O6·H2O). 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

ion (**Figure 5a**), which suggested that the presence of Na+

was not completely amorphous after the introduction of Na<sup>+</sup>

linity of Cs(1-*x*)Na*x*GPs gradually increased as more Cs+

*XRD patterns of Cs(1-x)LixGPs (a) and ceramics derived from it (b).*

ion was partially substituted by Li+

ion, and the content

ion, and the crystal-

ion

ion was substituted. The

ion substitution in Cs(1-*x*)Li*x*GPs system.

*DOI: http://dx.doi.org/10.5772/intechopen.81610*

**Figure 2.** *Thermal shrinkage curves of Cs(1-x)LixGPs (a) and Cs(1-x)NaxGPs (b).*

*Thermal Evolution of Geopolymer in the Process of High-Temperature Treatment DOI: http://dx.doi.org/10.5772/intechopen.81610*

**Figure 3.** *Partial thermal shrinkage curves of Cs0.9Li0.1GP.*

*Geopolymers and Other Geosynthetics*

shrinkage process was fully completed.

However, after Cs+

**3.2 Phase composition**

different Li+

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

in Stage IV. For Cs(1-*x*)Li*x*GP samples, they all showed two sintering steps in Stage IV. Taking Cs0.9Li0.1GP for example, as depicted in **Figure 3**, there were two sintering

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 MAlSi2O6 decline with decreasing ionic radius of M+

(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 MAlSi2O6. 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° 2*θ*, and almost no changes in phase composition had been observed based the XRD results of Cs(1-*x*)Li*x*GPs with

content. However, a substantial amount of pollucite was observed after

steps: Region i: 850–1050°C and Region ii: 1050–1300°C. On raising Li+

(*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

ion was substituted, significant differences are observed

ion content

ion

**102**

**Figure 2.**

*Thermal shrinkage curves of Cs(1-x)LixGPs (a) and Cs(1-x)NaxGPs (b).*

**Figure 4.** *XRD patterns of Cs(1-x)LixGPs (a) and ceramics derived from it (b).*

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%.

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<sup>+</sup> 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<sup>+</sup> 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 (NaAlSi2O6·H2O). 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<sup>+</sup> 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<sup>+</sup> 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<sup>+</sup> 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

**105**

than Li+

**Figure 7.**

**Figure 6.**

the crystallographic sites of Cs+

**3.3 Microstructure evolution**

hydration energy of Li+

difference in hydration energy between Li<sup>+</sup>

more [AlO4] and [SiO4] associated with Li<sup>+</sup>

framework of the geopolymer. Therefore, higher Li+

*Slow step-scan XRD patterns of ceramics derived from Cs(1-x)LixGPs.*

observed in Cs(1-*x*)Na*x*GPs system, as shown in **Figure 9**.

*(a) The (4 0 0) peak in a 2θ of 25–27 (b) Lattice parameters of pollucite.*

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

ion in the mass transfer process. So, it was much easier to occupy/substitute

**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

always resulted in particles with larger APSs [37]. The same variation trend was also

was higher than that of Cs<sup>+</sup>

ion and Cs+

ion than that around Cs<sup>+</sup>

ion in the pollucite crystal structure with Na+

ion.

ions. By comparison, the

ion contents in the geopolymer

ion in the

, which means that there were

*DOI: http://dx.doi.org/10.5772/intechopen.81610*

**Figure 5.** *XRD patterns of Cs(1-x)NaxGPs (a) and ceramics derived from it (b).*

*Thermal Evolution of Geopolymer in the Process of High-Temperature Treatment DOI: http://dx.doi.org/10.5772/intechopen.81610*

**Figure 6.** *Slow step-scan XRD patterns of ceramics derived from Cs(1-x)LixGPs.*

**Figure 7.** *(a) The (4 0 0) peak in a 2θ of 25–27 (b) Lattice parameters of pollucite.*

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.

#### **3.3 Microstructure evolution**

**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<sup>+</sup> ion and Cs+ ions. By comparison, the hydration energy of Li+ was higher than that of Cs<sup>+</sup> , which means that there were more [AlO4] and [SiO4] associated with Li<sup>+</sup> ion than that around Cs<sup>+</sup> 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**.

*Geopolymers and Other Geosynthetics*

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

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

Based on the result of slow step-scans of ceramics derived from Cs(1-*x*)Li*x*GPs, it

ions were introduced [33, 44]. This implied that Li+

partially occupied/substituted the crystallographic sites

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

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

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

 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

ion was only in the form of spodumene in the heated samples. The different forms

ion was closer to that of Cs+

ions also suggested that the Na+

*XRD patterns of Cs(1-x)NaxGPs (a) and ceramics derived from it (b).*

may have only existed in the form of

ion substitution, almost no deviation

ion. In the Cs(1-*x*)Li*x* system,

ion had a higher degree of freedom

ions did

not the same as the phenomenon of Cs(1-*x*)Li*x*GPs system.

and Cs(1-*x*)Na*x*GPs also indicated that Na<sup>+</sup>

was possible to find that with increase in Li<sup>+</sup>

observed in thermal shrinkage results.

amorphous glass phase.

as more and more Li<sup>+</sup>

parameters proved that Na+

and Li+

ion, the size of Na+

of Cs+

with Li+

of Na+

Li+

**104**

**Figure 5.**

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<sup>+</sup> 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<sup>+</sup> 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.

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<sup>+</sup> ion, and the coarsening trend of pollucite grains were also obvious (**Figure 11**).

**Figure 8.**

*Microstructure of Cs(1-x)LixGPs, (a) x = 0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3 (observed from fracture surface).*

**107**

**Figure 10.**

**Figure 9.**

*fracture surface).*

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

**Figure 12** shows TEM analysis of the ceramic derived from Cs0.7Li0.3GP. 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.

*Microstructure of ceramics derived from Cs(1-x)LixGPs, (a) x = 0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3, (e) BSE* 

*image of (d), (f) EDS spectrum of point A, (g) EDS spectrum of point B.*

*Microstructure of Cs(1-x)NaxGPs, (a) x = 0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3, (e) x = 0.4 (observed from* 

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

*DOI: http://dx.doi.org/10.5772/intechopen.81610*

*Thermal Evolution of Geopolymer in the Process of High-Temperature Treatment DOI: http://dx.doi.org/10.5772/intechopen.81610*

#### **Figure 9.**

*Geopolymers and Other Geosynthetics*

of coarsening increasing with Li<sup>+</sup>

the introduction of Cs<sup>+</sup>

obvious (**Figure 11**).

As for Cs(1-*x*)Li*x*GPs system, as shown in **Figure 10**, these precipitates coarsened

ion content. The mean diameters of spherical

ion, and the coarsening trend of pollucite grains were also

ion could not be detected

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

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

by it, so, the results of EDS suggested that spherical particle (point A) and glassy

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

*Microstructure of Cs(1-x)LixGPs, (a) x = 0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3 (observed from fracture* 

matrix (point B) corresponded to pollucite and spodumene, respectively.

EDS was a semi-quantitative analytical measure and Li<sup>+</sup>

**106**

**Figure 8.**

*surface).*

*Microstructure of Cs(1-x)NaxGPs, (a) x = 0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3, (e) x = 0.4 (observed from fracture surface).*

#### **Figure 10.**

*Microstructure of ceramics derived from Cs(1-x)LixGPs, (a) x = 0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3, (e) BSE image of (d), (f) EDS spectrum of point A, (g) EDS spectrum of point B.*

**Figure 12** shows TEM analysis of the ceramic derived from Cs0.7Li0.3GP. 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

#### **Figure 11.**

*Microstructure of ceramics derived from Cs(1-x)LixGPs, (a) x = 0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3 etched in 4 wt% HF at room temperature for 20 s.*

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

#### **Figure 12.**

*TEM images of ceramics derived from Cs(1-x)LixGPs, (a) x = 0.3, (b) and (c): SAED patterns of point A and B, respectively.*

**109**

**Figure 14.**

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

ion substitution. Compared with ceramics derived

ion substitution. Take Cs0.6Na0.4GPs for

ion substitution showed a

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

*Microstructure of ceramics derived from Cs(1-x)NaxGPs, (a) x = 0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3, (e)* 

proportional relationship. So, the residual amorphous glass phase will be more

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

*Microstructure of ceramics derived from Cs(1-x)NaxGPs, (a) x = 0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3, (e)* 

*DOI: http://dx.doi.org/10.5772/intechopen.81610*

decreased with increasing Na<sup>+</sup>

**Figure 13.**

*x = 0.4.*

amorphous glass phase.

in the sample contained a higher Na<sup>+</sup>

*x = 0.4 etched in 4 wt% HF at room temperature for 20 s.*

to the content of amorphous glass phase and the Na<sup>+</sup>

*Thermal Evolution of Geopolymer in the Process of High-Temperature Treatment DOI: http://dx.doi.org/10.5772/intechopen.81610*

#### **Figure 13.**

*Geopolymers and Other Geosynthetics*

**108**

**Figure 12.**

**Figure 11.**

*B, respectively.*

*TEM images of ceramics derived from Cs(1-x)LixGPs, (a) x = 0.3, (b) and (c): SAED patterns of point A and* 

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

*Microstructure of ceramics derived from Cs(1-x)LixGPs, (a) x = 0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3 etched in* 

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

densification of resulting ceramic products [45].

*4 wt% HF at room temperature for 20 s.*

*Microstructure of ceramics derived from Cs(1-x)NaxGPs, (a) x = 0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3, (e) x = 0.4.*

decreased with increasing Na<sup>+</sup> 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<sup>+</sup> ion substitution showed a proportional relationship. So, the residual amorphous glass phase will be more in the sample contained a higher Na<sup>+</sup> ion substitution. Take Cs0.6Na0.4GPs 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 14.**

*Microstructure of ceramics derived from Cs(1-x)NaxGPs, (a) x = 0, (b) x = 0.1, (c) x = 0.2, (d) x = 0.3, (e) x = 0.4 etched in 4 wt% HF at room temperature for 20 s.*

**Figure 15** shows the TEM analysis of the ceramic evolved from treated Cs0.6Na0.4GP. 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<sup>+</sup> 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<sup>+</sup> distributed among pollucite grains, there was still a small portion present in the interior of pollucite grains, which further demonstrated that Na<sup>+</sup> 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<sup>+</sup> 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.
