## 3 Results
### ESEM/EDS Studies
#### 3.3.2 Microscopic Analysis of Alkali-Activated Brick
With this in mind, a mass loss of 0.6 w% (as observed by TGA) within the temperature range 150-400 \({}^{\circ}\)C resulted mainly from the removal or desorption of crystalline H\({}_{2}\)O trapped in brick-zeolite frameworks; one could therefore roughly evaluate the number of water molecules in the predominating zeolitic structure of LTA zeolite. We found (H\({}_{2}\)O)\({}_{\rm 18-21}\); this value differed somewhat from that proposed by [PERSON] and [PERSON] [60] for LTA, i.e., (Na)\({}_{\rm 95}\)(Al)\({}_{\rm 96}\)(Si)\({}_{\rm 96}\)(O)\({}_{\rm 384}\)(H\({}_{2}\)O)\({}_{\rm 39}\), but was nevertheless close enough to that deduced from elemental ESM/EDS analysis of cubic particles (see the above formula), i.e., 390.9 (\(\pm\)20.5) \(-\)384 = 6.6-27.1, considering only superior limits. On the other hand, the spatial K distribution image represented in Figure 3C indicates relatively high levels of potassium in certain regions of the targeted surfaces. The localization of K-rich zones suggested the presence of illite in the brick material, as already confirmed by XRD (Figure 1). To support this notion, we decided to target different potassium-rich zones in several brick grains and analyze them by employing ESEM/EDS microscopy. The amounts of major elements within alkali-brick (Si, Al, O, Na, and K) were determined for every pixels, constituting artificially targeted surfaces with the aim of reconstituting spatial element distributions, as illustrated in Figure 3B,C. Owing to such virtual reconstitutions, element-distribution images (Figure 3C) allowed us to better distinguish Si/Al/O/Na-rich zones and Al/Si/O/K-rich zones (Figure 3B) and to evidence the (co)existence of zeolites and illite in alkali brick. The general formula of illite minerals is given by [75]:  
\[\rm(K)_{\rm 1.0-1.5}Al_{4}[Si_{\rm 6.5-7.0}Al_{\rm 1.0-1.5}O_{\rm 20}](OH)_{4} \tag{1}\]  
In their 2:1 layer structure, the presence of negative charges, which are created by a part of illite-Al in tetrahedral sites, allows the occupancy to occur of monovalent cations (mainly K\({}^{+}\)) in interlayer sites. With the aim of appraising the chemical composition of brick illite, ESEM/EDS micro-analyses were carried out on several alkali-brick grains. Byanalyzing about 15 K-rich aggregates which were selected randomly on various brick zones, the atomic percentages of Si, Al, O, K, Na, and Fe were determined and averaged. Elemental analysis led to the following chemical expression:  
\[\text{(K)}_{3.29\pm 0.36}\ \text{(Na)}_{1.09\pm 0.32}\ \text{(Fe)}_{1.45\pm 0.22}\ \text{Al}_{13.83\pm 1.36}\ \text{Si}_{16.73\pm 0.79}\ \text{O}_{61.10\pm 0.15} \tag{2}\]  
As the following chemical formula K\({}_{1.5}\)Al\({}_{4}\)(Si\({}_{6.5}\)Al\({}_{1.5}\))O\({}_{20}\)(OH)\({}_{4}\) was that generally accepted in the literature for overall illite [76], and in order to simplify our stoichiometric calculations, we decided not to take into account Na and Fe in formulae determination. From the general formulae for illite minerals (Equation (1)), one may conclude that the negative charge in the atomic combination 'Si\({}_{6.5}\)Al\({}_{1.5}\)' was balanced with the positive charge in 'K\({}_{1.5}\)'. On the basis of this, one could obtain an averaged chemical composition of brick illite by comparing to the percentages of all elements given in Equation (2) to that of silicon. By arbitrarily fixing 'Si\({}_{6.5}\)' in the proposed chemical equation, we obtain the following:  
\[\text{(K)}_{1.28\pm 0.14}\ \text{Al}_{4}(\langle Si\rangle_{6.5}\text{Al}_{1.37\pm 0. 53})\text{O}_{20}\text{(OH)}_{3.74\pm 1.17} \tag{3}\]  
This chemical expression was relatively consistent with that generally accepted for illite minerals (like that of Equation (1)). Note that, with the aim of compensating for the atomic deficit, we did not exclude the involvement of other elements, such as Mg and Fe, in complex illitic structures [76].