## 3 Results
### Implication of Fluxing Agent(s) in Thermal Process  
A global ESEM/EDS analysis of several large surfaces (150 \(\upmu\)m \(\times\) 120 \(\upmu\)m) of alkali-brick grains (chosen hazardously) revealed progressive increases of Si content at temperatures \(\geq\) 1000 \({}^{\circ}\)C, while Al and Na underwent a dilution effect (Figure 6). The increase in silicon was due to silicate inputs provided by melted sand. This could be explained by the fact that brick-sand was partially melted at temperatures lower than that expected for pure crystalline silica (quartz) by the involvement of low-melting-point components in contact with silica grains [84; 85]. Indeed, as the structures of illite and zeolites collapsed thermally and transformed progressively into a glassy matrix, the material acquired an important fluxing action owing to large quantity of alkalis (Na and K), enabling the partial melting of silica during brick firing. Under specific thermal conditions, it was possible to generate an additional amorphous glass phase by melting sand-silica. The significant rise of sand-silica glass was revealed by quantitative ESEM/EDS analysis of heated alkali-brick surfaces (see atomic % evolution of silicon in Figure 6) and was confirmed by the gradual increase of a broad XRD diffraction  
Figure 6: ESEM/EDS analysis of several large surfaces (150 \(\upmu\)m \(\times\) 120 \(\upmu\)m) of alkali-brick grains (chosen hazardously) after heating samples separately at 600 \({}^{\circ}\)C, 800 \({}^{\circ}\)C, 1000 \({}^{\circ}\)C, and 1150 \({}^{\circ}\)C for 1 day. hump, detected between 20\({}^{\circ}\): 17-34\({}^{\circ}\) for alkali-brick calcined at temperatures \(\geq\) 1000 \({}^{\circ}\)C; see Figure 1.  
In order to better highlight the implication of fluxing agents in the transformation of alkali-brick into glass-ceramic, it was important to preliminarily examine the chemical composition of alkali-brick in terms of oxides (see Table 1). The relatively high percentage of K\({}_{2}\)O (1.03 w%) in the raw brick reflected the presence of illite, while that of Na\({}_{2}\)O (1.93 w%) indicated the presence of Na-zeolites in the alkali-activated brick. The total percentage of alkaline oxides (K\({}_{2}\)O + Na\({}_{2}\)O) represented about 3 w% of the total brick mass, while that of alkaline-earth oxides (MgO + CaO) was no more than 0.2 w%. Alkaline and alkaline-earth oxides are known to act as flux materials during curing. It is, however, interesting to mention that the thermal behavior of the main fluxing agents in alkali-brick (i.e., K\({}_{2}\)O and Na\({}_{2}\)O) differed somewhat from each other. Indeed, potassium oxide is more capable of generating low-temperature melting eutectics with other minerals, while sodium oxide more effectively reduces viscosity [86]. In order to explain thermodynamically how the sand present in the brick could be melted at temperatures lower than it would be on its own, it was important to apply at best phase diagrams to the thermal events which occurred during alkali-brick firing. Indeed, phase diagrams provided evidence that minerals together could melt at lower temperatures than they would individually. This phenomenon, called \"eutectic melting\", led to the creation of complex eutectic systems. In order to rationalize eutectic systems, we had to conceive more simplified thermodynamic approaches by elaborating eutectic diagrams in which only a limited number of minerals present in the studied material were taken into consideration. Therefore, we attempted to survey the thermal evolution of alkali-brick in the temperature range of 800-1150 \({}^{\circ}\)C. Knowing that the phase diagrams represented thermodynamic models that ought to involve main flux component(s), we decided to limit our thermodynamic investigations of the \"leucite-quartz\" system to 1 atmosphere.