# An Investigation on the Synthesis of Alkali Activated Materials from Thermally Modified Clays
## 3 Results and Discussion  
### Precursor Characterization  
Figure 1a reports the diffraction analysis of the local common clay, before and after calcination at 800 \({}^{\circ}\)C. Considering the intensity of the characteristic diffraction maxima, the content of quartz (PDF#787-0743) appears to be substantial. Accessory mineral phases consisted of feldspar (microcline, PDF#77-0135) and mica (muscovite, PDF#80-0743). Signals consistent with clay minerals such as kaolinite (Al\({}_{2}\)Si\({}_{2}\)O\({}_{5}\)(OH)\({}_{4}\), PDF#89-6538) were visible only in the as-received clays, in good agreement with dehydroxylation and transformation into amorphous metakaolin upon calcination. The formation of metakaolin provided initial evidence in favor of the CRC potential as an alternative precursor in alkali activated systems. The enhanced peak at 2\(\theta\)-23\({}^{\circ}\) is consistent with the formation of anhydrous aluminum silicate (Al\({}_{0.5}\)Si\({}_{0.75}\)O\({}_{2.25}\), PDF#37-1480), likely formed by the same dehydroxylation process. Increases in the intensity of the quartz-related peaks are most likely attributed to the 'concentration' mechanisms, as hydrated minerals undergo obvious mass reductions with dehydroxylation. Thermal decomposition probably did not solely involve clay minerals, but also (partially) muscovite. Similar observations were made by [PERSON] et al. [55], who examined the decomposition of muscovite and reported the formation of additional quartz and leucite. The formation of leucite cannot be proved here considering the overlap with signals from microcline. \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**C1**} & 40 \({}^{\circ}\)C \(\pm\) 95 RH & 20 \({}^{\circ}\)C \(\pm\) 95\% RH & 20 \({}^{\circ}\)C \(\pm\) 65 RH \\ \cline{2-5}  & **1 day** & **6 days** & **21 days** \\ \cline{2-5}  & 20 \({}^{\circ}\)C \(\pm\) 95 RH & 40 \({}^{\circ}\)C \(\pm\) 95 RH & 20 \({}^{\circ}\)C \(\pm\) 95 RH & 20 \({}^{\circ}\)C \(\pm\) 65 RH \\ \cline{2-5}  & **1 day** & **1 day** & **5 days** & **21 days** \\ \cline{2-5}  & 20 \({}^{\circ}\)C \(\pm\) 95 RH & 40 \({}^{\circ}\)C \(\pm\) 95 RH & 20 \({}^{\circ}\)C \(\pm\) 95 RH & 20 \({}^{\circ}\)C \(\pm\) 65 RH \\ \cline{2-5}  & **1 day** & **5 days** & **1 day** & **21 days** \\ \hline \end{tabular}
\end{table} 
Table 2: The examined curing conditions. Figure 1b shows a comparison between the mineralogical assemblage of precursors. MK presents muscovite and quartz as predominant minerals, accompanied by traces of anatase (TiO2, PDF#84-1286). As can be seen, the most significant differences between MK and CRC concern the contents of quartz (much reduced in MK) and amorphous material (much enhanced in MK considering the broad convexity located at 2\(\theta\)-18-32\({}^{\circ}\)). SF was found to be completely amorphous, as already reported in previous works [17; 48]. The high content of highly amorphous Si and the absence of major crystalline peaks justify the selection of SF as a secondary source of reactive silica in activated alkaline systems. The chemical bulk compositions of MK, SF, and CRC are shown in Table 3. The most abundant oxides observed in the MK composition were silicon and aluminum oxides, related to the metakaolin content. These results agreed well with the product data sheet, albeit with a slightly lower content of Al\({}_{2}\)O\({}_{3}\). Some variability in chemical composition can be easily attributed to the MK geological origin, and similar Al\({}_{2}\)O\({}_{3}\) contents have been previously reported elsewhere for this commercial MK [56]. CRC was composed of similar SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) contents whereas major differences were found in alkaline oxides (Na\({}_{2}\)O + K\({}_{2}\)O), TiO\({}_{2}\), and Fe\({}_{2}\)O\({}_{3}\). Such differences are in good agreement with the presence of alkali feldspars and anatase in the CRC mineralogical assemblage. The loss of ignition values in both calcined clays was inferior to 2.0 wt.%, which in part can be attributed to the thermal decomposition of muscovite.