## 3 Results and Discussion
### Dehydration/Hydration Stability
In order to figure out the heat efficiency of the materials, the amount of released heat per mass unit (\(Q_{rel,m}\)) and volume unit (\(Q_{rel,\
u}\)), referred to by the single cycle (\(n\)), was calculated by Equations (8) and (9):  
\[Q_{rel,m(n)}=Q_{DSC}.\frac{R_{\%,\%,\%,\%,\%}(n)}{100} \tag{8}\]  
\[Q_{rel,V(n)}=Q_{rel,m(n)}.\rho \tag{9}\]  
where \(Q_{DSC}\) is the previously determined dehydration heat by calorimetric analysis, \(R_{\%,\%,\%,\%}(n)\) is the normalized hydration conversion referred to by the n cycle and \(\rho\) is the experimentally determined bulk density for the hydrated materials, corresponding to 745 kg/m\({}^{3}\) for CA-32H, 854 kg/m\({}^{3}\) for CA-11H and 699 kg/m\({}^{3}\) for CA-SH. Figure 10 shows the released heat per mass unit (a) and volume unit (b) as a function of the cycle number (\(n\)). What can be observed in Figure 10a is that CA-11H underwent a notable performance drop during the five cycles, which brought the material to exchange a quantity of heat numerically comparable with the CA-32H one. Furthermore, the trend of CA-11H material suggested that a further small loss in efficiency could be potentially observed in the following cycles. CA-32H, on the other hand, underwent a more moderate performance drop that was greatly softened in the third cycle. In fact, the curve trend between the third and fifth cycles reached an almost constant value. Hence, we expected a stabilization for the following cycles as well. CA-SH showed a progressive decreasing of exchanged heat, reaching the value of 45.12 kJ/kg. Similar considerations regarding the co-precipitated materials can be argued from Figure 10b; however, there is a remarkable difference due to the higher density of CA-11H, which ensures a greater efficiency, despite the lower reaction yield. The materials were structurally and morphologically characterized again by XRD and SEM after cyclic experiment. In Figure 11, the diffraction patterns of the materials are shown. CA-32H, which exhibited the highest conversion and the highest tendency to stabilize (Table 5), showed a similar diffraction pattern to that of the initial material, where katoite, Ca\({}_{3}\)Al\({}_{2}\)O\({}_{6}\cdot\)6H\({}_{2}\)O, and calcium hydroxide, Ca(OH)\({}_{2}\), were recognizable. No additional phases were identified. In CA-11H, originally composed almost exclusively by katoite (98.6%) and still identifiable after five cycles, showed the further presence of characteristic peaks of mayenite, Ca\({}_{12}\)Al\({}_{14}\)O\({}_{33}\), and calcium hydroxide. This can be justified, considering that katoite suffered a partial decomposition, which was reasonably associated  
Figure 10: Released heat per mass unit (**a**) and volume unit (**b**) per cycle. to the formation of mayenite aggregates and the consequent release of calcium oxide, subsequently converted in calcium hydroxide by the water vapor atmosphere. A possible reaction scheme could be (Equations (10) and (11)):  
Under heating:  
\[7\text{Ca}_{3}\text{Al}_{2}\text{O}_{6}\cdot\text{6H}_{2}\text{O}_{(\text{s})} \rightarrow\text{Ca}_{12}\text{Al}_{14}\text{O}_{33}+9\text{Ca}\text{O}_{( \text{s})}+42\text{H}_{2}\text{O}_{(\text{g})} \tag{10}\]  
Under water vapor supply:  
\[\text{Ca}\text{O}_{(\text{s})}+\text{H}_{2}\text{O}_{(\text{g})} \rightarrow\text{Ca}(\text{OH})_{2(\text{s})} \tag{11}\]  
Moreover, broadening and less intensity of the peaks, in comparison to the starting materials, indicate a less crystallinity. CA-SH, characterized by the lowest conversion value, still exhibited typical katoite peaks, although less intense. Similarly to CA-11H, mayenite peaks were visible as a consequence of katoite decomposition. The main crystalline phase was alumina, Al\({}_{2}\)O\({}_{3}\), which prevented the process reversibility. In fact, the high stability of the aluminum oxide inhibited its rehydration step.