## 3 Results and Discussion
### Delydration and Heat of Reaction  
In order to thermally characterize the hydrated materials, thermogravimetric analysis and differential scanning calorimetry measurements were performed. TGA scan of the first dehydration process enabled the evaluation of the operating temperature range and the quantification of the related weight loss. At the same time, DSC scan measured the amount of involved endothermic heat of the observed processes. The comparison of the  
Figure 5: Scanning electron microscopy (SEM) images at magnification = 100 k of calcined products CA-32 (**a**), CA-11 (**b**) and CA-S (**c**). Figure 6: SEM images at magnification = 100 k and punctual energy dispersive X-ray analysis (EDX) estimated Ca/Al molar ratios of hydrated products CA-32H (**a**), CA-11H (**b**) and CA-SH (**c**). thermogravimetric dehydration results of the different tricalcium aluminate hexahydrate samples, shown in Figure 7a, demonstrates that the dehydrated fraction at 375 \({}^{\circ}\)C is significantly influenced by synthesis process. Indeed, CA-11H sample shows the highest weight reduction equal to 21.12%, quite close to the theoretical mass loss due to the katoite partial dehydration (Equation (4)):  
\[\text{Ca}_{3}\text{Al}_{2}\text{O}_{6}\cdot\text{6H}_{2}\text{O}_{\text{(s)} }\rightarrow\text{Ca}_{3}\text{Al}_{2}\text{O}_{6}\cdot\text{1.5H}_{2}\text{O} _{\text{(s)}}+4.5\text{H}_{2}\text{O}_{\text{(g)}} \tag{4}\]  
The mass loss decreases about 19.04% for CA-32H batch and 10.05% for CA-SH one due to a lower amount of hydrated calcium aluminate compound in the latter (see Figure 7a). Consequently, the thermogravimetric curves show a clearly more effective behavior for the co-precipitated specimens compared to the solid-solid one. Furthermore, the heat of dehydration has been determined through integration of the peak obtained by calorimetric analysis (Figure 7b). The results confirmed what was highlighted by TGA measurements. CA-11H revealed a dehydration heat of 807.0 kJ/kg, higher than CA-32H and CA-SH, respectively, 655.8 kJ/kg and 530.2 kJ/kg. Based on these considerations, a preliminary explanation of the dehydration process can be assessed. According to XRD phase identification, the observed chemical process is mainly related to the thermal decomposition of katoite, which leads to 4.5 water molecules loss (See Equation (4)). This process represents the only one observed for co-precipitated products. In contrast, it is still possible to perceive that for CA-SH product there is the presence of an additional process, as a secondary DSC peak shows clearly, ascribed to the decomposition of aluminum hydroxide to beohmite (Equation (5)):  
\[\text{Al}(\text{OH})_{\text{3(s)}}\rightarrow\text{AlO}(\text{OH})_{\text{(s)} }+\text{H}_{2}\text{O}_{\text{(g)}} \tag{5}\]  
This result confirms that this batch is characterized by a relevant microstructural heterogeneity that hinders the hydration/dehydration efficiency. In contrast, the CA-11H and CA-32H samples show a microstructural homogeneity which acts effectively on the vapor phase mass exchange processes. The phenomenon, managed almost exclusively by Equation (3), allows for high thermal capacities in a very narrow temperature range, enhancing the selectivity of the material for a suitable and efficient use of thermal waste. Table 4 summarizes the results obtained from the analysis. Figure 7: CA-32H (red), CA-11H (blue) and CA-SH (orange) thermogravimetric (TG) (**a**) and differential scanning calorimetry (DSC) (**b**) curves, recorded between 150 \({}^{\circ}\)C and 375 \({}^{\circ}\)C, scan rate 10 \({}^{\circ}\)C/min. The last column shows the amount of katoite on the respective material.