**3.4 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

#### **Figure 15.**

*TEM images of ceramics derived from Cs(1-x)NaxGPs, (a) x = 0.4, (b) and (c): SAED patterns of point 1 and 2, respectively.*

**111**

Li<sup>+</sup>

**Figure 16.**

Eq. (1).

and Cs<sup>+</sup>

and Cs2O to express the form of Li<sup>+</sup>

cases. Hence, as for the Cs(1-*x*)Li*x*GP system, Li<sup>+</sup>

*HAADF-STEM and STEM-EDX images illustrating the element distribution.*

temperature of spodumene, as in Eq. (2).

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

Cs(1-*x*)Li*x*GP can be expressed as (1-*x*)Cs2O·*x*Li2O·Al2O3·4SiO2·*w*H2O. In fact, both

(the complexity of composition and structure of silicate indicated that *x1*, *y1*, *x2*, and *y2* did not have a certain value). Therefore, it was reasonable to employ Li2O

and Cs<sup>+</sup>

a stronger electric field intensity in the glass-ceramic system than that of Cs<sup>+</sup>

[52]. Thus, with increases in temperature of high-temperature processing, the reaction between Li2O, Al2O3, and SiO2 first occurred at a lower temperature, as in

Li2O(s) + SiO2(s) + Al2O3(s) → LiAlSi2O6(*l*) (1)

The presence of liquid spodumene (LiAlSi2O6) and the simultaneous reaction between Li2O 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 Cs2O, Al2O3, and SiO2 would occur at a higher temperature compared with the onset

Cs2O(s) + SiO2(s) + Al2O3(s) → CsAlSi2O6(*s*) (2)

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 Li2O or Li<sup>+</sup> ion could promote the densification process of sintering [48–51]. 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

ions were present as silicates, that is, *x1*Li2O·*y1*SiO2 and *x2*Cs2O·*y2*SiO2

ions in this system.

was an intermediate cation with

ion

*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 16.** *HAADF-STEM and STEM-EDX images illustrating the element distribution.*

Cs(1-*x*)Li*x*GP can be expressed as (1-*x*)Cs2O·*x*Li2O·Al2O3·4SiO2·*w*H2O. In fact, both Li<sup>+</sup> and Cs<sup>+</sup> ions were present as silicates, that is, *x1*Li2O·*y1*SiO2 and *x2*Cs2O·*y2*SiO2 (the complexity of composition and structure of silicate indicated that *x1*, *y1*, *x2*, and *y2* did not have a certain value). Therefore, it was reasonable to employ Li2O and Cs2O to express the form of Li<sup>+</sup> and Cs<sup>+</sup> 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 Li2O or Li<sup>+</sup> ion could promote the densification process of sintering [48–51]. 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<sup>+</sup> was an intermediate cation with a stronger electric field intensity in the glass-ceramic system than that of Cs<sup>+</sup> ion [52]. Thus, with increases in temperature of high-temperature processing, the reaction between Li2O, Al2O3, and SiO2 first occurred at a lower temperature, as in Eq. (1).

$$\text{Li}\_2\text{O}(\text{s}) + \text{SiO}\_2(\text{s}) + \text{Al}\_2\text{O}\_3(\text{s}) \rightarrow \text{LiAlSi}\_2\text{O}\_6(l) \tag{1}$$

The presence of liquid spodumene (LiAlSi2O6) and the simultaneous reaction between Li2O 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 Cs2O, Al2O3, and SiO2 would occur at a higher temperature compared with the onset temperature of spodumene, as in Eq. (2).

$$\text{Cs}\_2\text{O}(\text{s}) + \text{SiO}\_2(\text{s}) + \text{Al}\_2\text{O}\_3(\text{s}) \rightarrow \text{CsAlSi}\_2\text{O}\_6(\text{s})\tag{2}$$

*Geopolymers and Other Geosynthetics*

decreased with increasing Na+

**3.4 Sintering mechanism**

of zeolite nucleus was proportional to Na+

suggested that Cs<sup>+</sup>

majority of Na<sup>+</sup>

**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

completely different distribution. Map-scan results revealed that although the vast

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**)

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

pollucite grains was more dispersed as the nucleation sites increased.

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

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

*TEM images of ceramics derived from Cs(1-x)NaxGPs, (a) x = 0.4, (b) and (c): SAED patterns of point 1 and* 

is consistent with the TEM results, as shown in **Figure 16**. However, Na+

and the result of the map-scan indicated again that the Na<sup>+</sup>

ions were distributed only in the interior of crystal grain, which

ion substitution. The phenomenon of grain refinement

ion introduction. Therefore, the growth of

distributed among pollucite grains, there was still a small portion

showed a

ion mainly existed in the

**110**

**Figure 15.**

*2, respectively.*

The presence of liquid spodumene increased the rates of mass transfer and diffusion more easily, conducive to nucleation and crystallization of pollucite (CsAlSi2O6). 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<sup>+</sup> 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 Na2O, Al2O3, and SiO2 in the Cs(1-*x*)Na*x*GP system would occur first at a lower temperature, as in Eq. (3).

$$\text{Na}\_2\text{O(s)} + \text{SiO}\_2\text{(s)} + \text{Al}\_2\text{O}\_3\text{(s)} \rightarrow \text{ amorphous glass (l)}\tag{3}$$

In addition to this lower temperature, the sintering process of the two systems also experienced almost the same process at higher temperature intervals.

#### **3.5 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 T1 < T < T2, as described by the following Eq. (4):

$$
\Delta \mathbf{a\_{a}} = (L\_{2} - L\_{1}) / L\_{0} \text{ (\$T\_{2}\$ - \$T\_{1}\$) = \$\Delta L / L\_{0} \text{ } \Delta T \tag{4}
$$

**113**

**Figure 18.**

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

As for Cs(1-*x*)Li*x*GP ceramics, as depicted in **Figure 18**, it could be observed that

The results obtained from our previous study on the K(1-*x*)Cs*x*GP system proved that

 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

leucite ceramic. The reason could be attributed to the substitution between Cs+

spodumene in resulting ceramic products was proportional to the amount of Li+ ion substitution. By comparison, spodumene had a lower CTE (1.0 × 10<sup>−</sup><sup>6</sup>

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

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

For K(1-*x*)Cs*x*GPs system involved in our previous studies, the average CTE of the

occupying/substituting K+

and Cs+

ions caused the decline of lattice constant of pollucite.

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

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

As for Cs(1-*x*)Na*x*GP ceramics, XRD results suggested that the substitution

decrease the thermal expansion of the pollucite, and with the increase in Na+

substitution, the average CTE of ceramics derived from heated Cs(1-*x*)Na*x*GPs decreased [24–26]. 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+

*Thermal expansion properties of Cs(1-x)LixGP ceramics (a) and Cs(1-x)NaxGP ceramics (b).*

(*x* = 0.4) with the rise of sodium substitution.

with Cs(1-*x*)Li*x*GP ceramic products, increasing from 4.80 × 10<sup>−</sup><sup>6</sup>

leucite ceramic decreased with increase in the amount of Cs+

any substitution at the lattice level between Li<sup>+</sup>

According to the Ikuo et al. reports, the increase in Na+

K<sup>−</sup><sup>1</sup>

ion introduction would decrease the average CTE of the resulting

ion. XRD quantitative analysis suggested that the content of

(*x* = 0) to 3.61 × 10<sup>−</sup><sup>6</sup>

K<sup>−</sup><sup>1</sup>

(*x* = 0.3).

 K<sup>−</sup><sup>1</sup> )

K<sup>−</sup><sup>1</sup>

(*x* = 0) to

ion introduced. The

crystallographic sites in

in the Cs(1-*x*)Li*x*GP system.

in the crystal lattice could

ion

ion

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

the average CTE decreased from 4.80 × 10<sup>−</sup><sup>6</sup>

increase in Cs<sup>+</sup>

substitution by Li<sup>+</sup>

decreased evidently.

K<sup>−</sup><sup>1</sup>

reason could be ascribed to Cs+

lower than that of pollucite.

and Cs+

7.26 × 10<sup>−</sup><sup>6</sup>

between Na+

and K<sup>+</sup>

where *L*0, *L*1, and *L*2 are the lengths of the specimen at temperatures of *T*<sup>0</sup> (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.

**Figure 17.** *Linear coefficient of thermal expansion of Cs(1-x)LixGP (a) and Cs(1-x)NaxGP.*

#### *Thermal Evolution of Geopolymer in the Process of High-Temperature Treatment 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<sup>−</sup><sup>6</sup> K<sup>−</sup><sup>1</sup> (*x* = 0) to 3.61 × 10<sup>−</sup><sup>6</sup> K<sup>−</sup><sup>1</sup> (*x* = 0.3). The results obtained from our previous study on the K(1-*x*)Cs*x*GP system proved that increase in Cs<sup>+</sup> ion introduction would decrease the average CTE of the resulting leucite ceramic. The reason could be attributed to the substitution between Cs+ and K<sup>+</sup> 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<sup>+</sup> 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<sup>−</sup><sup>6</sup> K<sup>−</sup><sup>1</sup> ) 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<sup>−</sup><sup>6</sup> K<sup>−</sup><sup>1</sup> (*x* = 0) to 7.26 × 10<sup>−</sup><sup>6</sup> K<sup>−</sup><sup>1</sup> (*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 [24–26]. 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

*Geopolymers and Other Geosynthetics*

Therefore, the presence of Li<sup>+</sup>

**3.5 Thermal expansion behavior**

described by the following Eq. (4):

at elevated temperatures.

densification via aiding the liquid-phase processes.

The presence of liquid spodumene increased the rates of mass transfer and diffusion more easily, conducive to nucleation and crystallization of pollucite (CsAlSi2O6). 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.

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 Na2O, Al2O3, and SiO2 in the Cs(1-*x*)Na*x*GP system would occur first at a lower temperature, as in Eq. (3).

Na2O(s) + SiO2(s) + Al2O3(s) → amorphous glass (*l*) (3)

In addition to this lower temperature, the sintering process of the two systems

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

The average thermal expansion coefficient (CTE) was defined as the average value of the relative length change in the temperature range of T1 < T < T2, as

where *L*0, *L*1, and *L*2 are the lengths of the specimen at temperatures of *T*<sup>0</sup> (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.

*α*a = (*L*2−*L*1)/*L*0 (*T*2−*T*1) = ∆*L*/*L*<sup>0</sup> ∆*T* (4)

also experienced almost the same process at higher temperature intervals.

ion contributed to the crystallization and sintering

**112**

**Figure 17.**

*Linear coefficient of thermal expansion of Cs(1-x)LixGP (a) and Cs(1-x)NaxGP.*

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<sup>−</sup><sup>6</sup> K<sup>−</sup><sup>1</sup> ) than that of pollucite (~4.80 × 10<sup>−</sup><sup>6</sup> K<sup>−</sup><sup>1</sup> ) [53–56]. 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.
