## 3 Results
### ESEM/EDS Studies
#### 3.3.3 Microscopic Analysis of Calcined Alkali-Brick  
Upon calcination of alkali-activated brick at 400 \({}^{\circ}\)C and 600 \({}^{\circ}\)C for 24 h, ESEM analysis revealed that cubic and spherical micro-particles of zeolites (Na-A and Na-P) retained the same sizes and shapes as those typically observed in starting alkali-brick (as shown in Figure 3A,B). In addition, quantitative ESEM/EDS analysis indicated no stoichiometric changes in silicon, aluminum, or oxygen, which represent the major structural elements of zeolites. Nevertheless, obtained ESEM/EDS results did not necessarily prove that chemical transformations of zeolitic aluminosilicate frameworks did not occur during the thermal process. Indeed, according to some authors [77], at moderate temperatures, cubic and spherical (zeolites) micro-crystals had to be considered as \"micro-reactors\" in which chemical/structural transformations, nucleation, and crystal growths remained mostly localized inside each reactive crystal (as already observed for cubic micro-crystals of zeolite NH\({}_{4}\)A [77]). Therefore, not surprisingly, one would not expect to observe any stoichiometric changes in such 'closed' zeolitic systems, at least at temperatures of 400 \({}^{\circ}\)C and 600 \({}^{\circ}\)C. However, it is worth noting that additional percentages of potassium were measured as extra-framework cations on zeolitic particles after curing them at 600 \({}^{\circ}\)C, i.e., 0.19-1.24 at.% (instead of 0.09-0.38 at.% in unheated alkali-brick). K increases in zeolitic extra-frameworks seemed to coincide with the thermal phenomena observed in the temperature interval of 400-700 \({}^{\circ}\)C based on TGA and DSC; these thermal events were attributed to the dehydroxylation process of illite, as described in Figure 2.  
Upon calcination at 800 \({}^{\circ}\)C for 24 h, ESEM images of heated alkali-brick displayed: (i) some 'apparent' preservations of the morphology/size/shape of zeolitic micro-crystals at this temperature stage; and (ii) in contrast, melted and degraded zones in which illite predominated (Figure S2). In the brick calcined at 800 \({}^{\circ}\)C, K percentages on zeolitic specimens were found to increase significantly when compared to those on alkali-brick cured at 600 \({}^{\circ}\)C, i.e., 1.10-2.69 at.% (instead of 0.19-1.24 at.% at 600 \({}^{\circ}\)C). This observation suggested that a possible K transfer from a liquid (glass) phase generated at this temperature toward zeolitic specimens ought to occur in the cured material. This hypothesis seemed to agree well with our TGA and DSC analyses, which revealed thermal events corresponding to illite dehydroxylation, followed by the gradual transformation of dehydroxylated illite into aluminosilicate glass (containing potassium ions) at 800 \({}^{\circ}\)C. Such thermal decomposition of illite enabled K transfer into zeolites. To further support this mechanistic scheme, the spatial ESEM/EDS distribution of potassium on the brick surface was compared with those of major elements constituting the zeolite framework (i.e., Si, Al, O) and that of extra-framework (Na). The results clearly indicated a 'net' positive correlation between Al, Si, Na, and K (Figure S2C), showing the predominance of potassium in the brick zeolite. Meanwhile, there was a still negative correlation between Al, Si, Na, and K in the alkali-brick samples cured at 600 \({}^{\circ}\)C for the following reasons: (i) an absence or too weak degradation of illite; and (ii) a too low (24 h) reaction time (Figure 3C). Upon calcination at 1000 \({}^{\circ}\)C for 24 h, ESEM analysis revealed some glassy aspects of cured alkali-brick (Figure 4A). By examining spatial ESEM/EDS element distributions in detail, one may note stronger K enrichments in aluminosilicate-rich regions (see Figure 4C) than those measured on zeolitic particles of alkali-brick heated at 800 \({}^{\circ}\)C (see Figure S2C).