## 3 Results and Discussion
### Ca\({}_{3}\)Al\({}_{2}\)O\({}_{6}\) Synthesis in Anhydrous and Hydrated Forms
Figure 3: CA-11 diffraction patterns after 2 h calcination under static air at 500 \({}^{\circ}\)C, 700 \({}^{\circ}\)C, 900 \({}^{\circ}\)C and 1100 \({}^{\circ}\)C. (JCPDS reference cards: CaAl\({}_{2}\)O\({}_{4}\): 01-0888; Ca\({}_{12}\)Al\({}_{14}\)O\({}_{33}\): 09-0413; Ca\({}_{3}\)Al\({}_{2}\)O\({}_{6}\): 38-1429; CaO: 70-4068; CaCO\({}_{3}\): 83-1762/19-3758; AlO(OH): 78-4581). For TES application, a relevant aspect that requires attention to assess the potential suitability of the material in this context is to evaluate the hydrated phases that are formed due a hydration process. In particular, Figure 4b shows the diffraction patterns after the hydration step (as described in Section 2.2). Hydrated samples are named CA-32H, CA-11H and CA-SH. For CA-32H, katoite, Ca\({}_{3}\)Al\({}_{2}\)O\({}_{6}\cdot\)6H\({}_{2}\)O (JCPDS # 24-0217) was detected. A small amount of calcium hydroxide, Ca(OH)\({}_{2}\), was also observed (JCPDS # 44-1481), formed by the hydration of calcium oxide previously observed in CA-32. The diffractogram showed a low peak intensity. In the CA-11H diffraction pattern, only the characteristic peaks of katoite, Ca\({}_{3}\)Al\({}_{2}\)O\({}_{6}\cdot\)6H\({}_{2}\)O, were present, thus indicating that, besides the hydration of the Ca\({}_{3}\)Al\({}_{2}\)O\({}_{6}\) phase, mayenite was also converted into the most thermodynamically stable katoite. Mayenite-to-katoite conversion was also observed for the CA-SH sample. Gibbsite, Al(OH)\({}_{3}\) (JCPDS # 33-0018) and calcium hydroxide, Ca(OH)\({}_{2}\), were also present as hydrated products. Moreover, the characteristic peaks of unreacted Al\({}_{2}\)O\({}_{3}\) were still evident, and calcium oxide was partially converted into calcite, CaCO\({}_{3}\) (JCPDS # 83-1762). The obtained results demonstrated that Ca\({}_{3}\)Al\({}_{2}\)O\({}_{6}\) was almost quantitatively achieved by the co-precipitation method using a Ca\({}^{2+}\)/Al\({}^{3+}\) molar ratio 1:1, which was in defect of the amount of calcium with respect the stoichiometry of Ca\({}_{3}\)Al\({}_{2}\)O\({}_{6}\), and it was fully converted into the hexahydrate form. With regard to the co-precipitation products, it is important to point out the discrepancy between what could be expected from the initial Ca/Al ratio and what is shown by the diffraction patterns of materials CA-32 and CA-11, in terms of phase composition. A rational explanation is given by the acid-base behavior of aluminium cations: amphotericism of Al\({}^{3+}\) makes its solubility increasingly relevant after pH \(\sim\)9, in accordance with the tetrahydroxy aluminate ion formation [38], as described in Equation (3). \[\text{Al(OH)}_{3(5)}+\text{OH}^{-}{}_{(\text{aq})}\rightleftarrows\text{Al(OH )}_{4}{}^{-}{}_{(\text{aq})} \tag{3}\]  
It is therefore understandable that part of the initially employed aluminum is not found in the final product. Consequently, in CA-32, some of Ca\({}^{2+}\) precipitates as Ca(OH)\({}_{2}\) (dehydrated to CaO, subsequently, during calcination), while in CA-11, the initial aluminum excess led to the formation of tricalcium aluminate. The morphology of the samples after calcination, evaluated by SEM micrographs, is shown in Figure 5. A similar morphology is observable for samples CA-32 and CA-11 after calcination (Figure 5a,b), in which rounded particles (red arrows) and flat hexagonal particles of different sizes (black arrows), ascribable to tricalcium aluminate, are present [39]. CA-S shows particles relatively bigger than the co-precipitates (black circle) after calcination (Figure 5c). Furthermore, Figure 6 reports the products viewed with SEM micrographs after hydration. For a better clarity, punctual EDX elemental analysis on the hydrated products was useful to estimate the chemical composition on a specified point by means of the Ca/Al molar ratio (see the attached table in Figure 6). Hexagonal katoite plates can be noted for all the samples (black arrows); these have a more regular shape in CA-11 batch (a), in which katoite XRD peaks were also more intense (Figure 4b).