## 3 Results and Discussion  
### Ca\({}_{3}\)Al\({}_{2}\)O\({}_{6}\) Synthesis in Anhydrous and Hydrated Forms  
As shown in Table 1, two preparation methods were attempted to achieve Ca\({}_{3}\)Al\({}_{2}\)O\({}_{6}\): (i) solid-solid reaction at 1100 \({}^{\circ}\)C and (ii) co-precipitation followed by thermal treatment at 1100 \({}^{\circ}\)C. In the case of solid-solid reaction, a Ca\({}^{2+}\)Al\({}^{3+}\) molar ratio 3:2 was used. In the case of co-precipitation, the Ca\({}^{2+}\):Al\({}^{3+}\) molar ratios 3:2 and 1:1 were used. Figure 3 shows the representative diffraction spectra of CA-11 as a function of the calcination temperature (in the range 500-1100 \({}^{\circ}\)C). It is worth noting that the thermal treatment in static air at 1100 \({}^{\circ}\)C allows for Ca\({}_{3}\)Al\({}_{2}\)O\({}_{6}\) formation. Indeed, by evaluating the XRD spectra evolution at increasing calcination time, a clear relationship between thermal treatment and sample crystallinity can be observed. A calcination temperature of 500 \({}^{\circ}\)C leads to a poor crystallinity. The aluminum is mainly present in the form of calcium dialuminate, CaAl\({}_{2}\)O\({}_{4}\) (JCPDS # 01-0888), and calcite, CaCO\({}_{3}\) (JCPDS # 83-1762). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Step n\({}^{\circ}\)** & **Temperature (\({}^{\circ}\)C)** & **Atmosphere** & **Method** & **Time (Min)** \\ \hline
1 & 125\(\rightarrow\)250 & N\({}_{2}\) & Heating & - \\
2 & 250 & N\({}_{2}\) & Isothermal & 120 \\
3 & 250\(\rightarrow\)125 & N\({}_{2}\) & Cooling & - \\
4 & 125 & N\({}_{2}\) & Stabilization & - \\
5 & 125 & N\({}_{2}\)/H\({}_{2}\)O & Hydration & 120 \\
6 & 125 & N\({}_{2}\) & Release of adsorbed water & 30 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Schematization of a single thermogravimetric cycle. After calcination at 700 \({}^{\circ}\)C, the sample still presented amorphous phases; however, some peaks related to crystalline phases were identifiable. Calcium carbonate underwent a partial decomposition into calcium oxide (JCPDS # 70-4068), while the remaining portion was converted into vaterite, CaCO\({}_{3}\) (JCPDS # 19-3758). Aluminum was observable only as boehmite, AlO(OH) (JCPDS # 78-4581). The diffraction pattern continued its evolution after calcination at 900 \({}^{\circ}\)C, where the crystallinity increased and the main phase was mayenite, Ca\({}_{12}\)Al\({}_{14}\)O\({}_{33}\) (JCPDS #09-0413). Additionally, peaks of calcium oxide were also observable, demonstrating a complete decarbonation of the previously detected calcium carbonate phases. As the latest diffraction pattern showed, tricalcium aluminate, Ca\({}_{3}\)Al\({}_{2}\)O\({}_{6}\) (JCPDS # 38-1429), appeared predominantly only after calcination at 1100 \({}^{\circ}\)C; however, a low amount of mayenite was still present. In order to better correlate the phase transformations with the production process, the diffraction patterns of the calcined products CA-32, CA-11 and CA-S at 1100 \({}^{\circ}\)C are compared in Figure 4a. For CA-32, the main crystal phase was tricalcium aluminate, Ca\({}_{3}\)Al\({}_{2}\)O\({}_{6}\) (JCPDS # 38-1429). At the same time, a small presence of calcium oxide, CaO, was confirmed (JCPDS # 70-4068). For CA-11, as described before, the main phase was Ca\({}_{3}\)Al\({}_{2}\)O\({}_{6}\), and a moderate amount of mayenite was also appreciable. CA-S, after calcination, did not show tricalcium aluminate peaks, while it was possible to mainly identify the presence of unreacted CaO and aluminum oxide, Al\({}_{2}\)O\({}_{3}\) (JCPDS # 34-0440). Furthermore, peaks related to mayenite, Ca\({}_{12}\)Al\({}_{14}\)O\({}_{33}\), could be identified. The solid-solid synthesis, therefore, did not lead to the formation of tricalcium aluminate, as observed for the co-precipitation products, at 1100 \({}^{\circ}\)C for 2 h. Probably, temperatures close to melting point (above 1350 \({}^{\circ}\)C) are necessary to obtain this product through this method [37].