## 3 Results
### ESEM/EDS Studies
#### 3.3.3 Microscopic Analysis of Calcined Alkali-Brick
Upon calcination at 1150 \({}^{\circ}\)C for 24 h, the ESEM micro-graph of heated alkali-brick, represented in Figure 5 shows severe changes in the morph  
Figure 4: ESEM micrograph of alkali-activated brick aggregates after calcination at 1000 \({}^{\circ}\)C for 1 day (**A**); ESEM/EDS mapping image reconstituted from the spatial distributions of Al, Si, Na, and K (**B**); Spatial distribution of (extra)-framework elements (**C**). As can be seen in this figure, aluminosilicate minerals generated through the combination of illicit and zeolitic particles were heterogeneously dispersed in a glass matrix; both the glass regions and ceramic regions could be distinguished on the material surfaces. Next, we considered two zones (cycles 1 and 2, shown in Figure 5), representative of glassy and ceramic states, which were targeted arbitrarily to evaluate their elemental composition. From the ESM/EDS data, we then attempted to establish a corresponding formula by taking silicon as the reference element. Thus, our ESEM/EDS analysis of cycle-1 region led to an elemental composition close to that of silica (SiO\({}_{2}\)): \(\langle S\rangle_{1.000}\)O\({}_{2.089}\)A\({}_{0.039}\)Na\({}_{0.036}\)K\({}_{0.009}\)Mg\({}_{0.007}\). Indeed, this chemical formula corresponds to a silica glass containing low levels of both alumina and Na-K-Mg oxides: SiO\({}_{2}\)0.020 Al\({}_{2}\)O\({}_{3}\)0.018 Na\({}_{2}\)O0.004K\({}_{2}\)O0.007 MgO. As for the cycle-2 region containing small white inclusions, its ESEM/EDS analysis revealed an elemental composition: \(\langle Si\rangle_{1.000}\)O\({}_{3.382}\)Al\({}_{0.830}\)Na\({}_{0.161}\)K\({}_{0.058}\)Mg\({}_{0.027}\). This chemical formula corresponds to a glass material with the following stoichiometry: SiO\({}_{2}\)0.415 Al\({}_{2}\)O\({}_{3}\)0.081 Na\({}_{2}\)O0.029K\({}_{2}\)O0.027 MgO. By considering our XRD findings (that showed the existence of both analemic-leucite solid solutions ((Na\({}^{+}\),K\({}^{+}\))\({}_{2}\)Al\({}_{2}\)Si\({}_{4}\)O\({}_{12}\)) and multilite (3 Al\({}_{2}\)O\({}_{2}\)SiO\({}_{2}\)) in cured brick), one may propose the following stoichiometric expression: 0.356 SiO\({}_{2}\)0.110(Na\({}^{+}\),K\({}^{+}\))\({}_{2}\)Al\({}_{2}\)Si\({}_{4}\)O\({}_{12}\)0.102(3 Al\({}_{2}\)O\({}_{3}\)2 SiO\({}_{2}\)) 0.027 MgO. It is worth noting that, in the latter formula, one could not exclude the partial incorporation of MgO inside the multilite structure, as suggested previously [78]. From results reported by different authors [79; 80] and those obtained here by combining XRD, TGA/DSC, and ESEM/EDS, it was possible to propose a mechanism for the thermal evolution of brick zeolites (Na-A and Na-P) at temperatures up to 1150 \({}^{\circ}\)C. Note that the contribution of illite to the thermal transformations of alkali-brick was not taken into consideration in the following chemical schemes. At rising temperatures (up to 1000 \({}^{\circ}\)C), the decomposition of brick zeolites should take place globally as follows (the chemical formula of zeolite LTA was simplified as 12x[Mg\({}_{8}\)Al\({}_{8}\)Si\({}_{8}\)O\({}_{32}\)]):  
\[3/2[\text{M}_{6}\text{Al}_{6}\text{Si}_{10}\text{O}_{32}]_{\text{Na-P}}\to 3 \text{MAlSi}_{2}\text{O}_{6}+3\text{ ``Al}_{2}\text{O}_{3}\text{ 2 SiO}_{2}\text{''}+3\text{SiO}_{2}\text{(amor.)}+3\text{M}_{2}\text{O} \tag{4}\]  
\[3/2[\text{M}_{8}\text{Al}_{8}\text{Si}_{8}\text{O}_{32}]_{\text{LTA}}\to 4 \text{MAlSi}_{2}\text{O}_{6}+2\text{''Al}_{2}\text{O}_{3}\text{ -}\text{SiO}_{2}\text{''}+2\text{Al}_{2}\text{O}_{3}+4\text{M}_{2}\text{O} \tag{5}\]  
Figure 5: ESEM micrograph of alkali-activated brick aggregates after calcination at 1150 \({}^{\circ}\)C for 1 day. (Red circles representative of glassy/ceramic states and green numbers are targeted zones for ESEM/EDS analysis). \[\text{Al}_{2}\text{O}_{3}+\text{SiO}_{2}\rightarrow\text{SiAl}_{2}\text{O}_{5( \text{spinel})} \tag{6}\]  
where M represents an extra-framework mono-valent cation like Na\({}^{+}\) or K\({}^{+}\), or adsorbed cations like Cs\({}^{+}\) and Rb\({}^{+}\).