# Study of Eco-Friendly Belite-Calcium Sulfoaluminate Cements Obtained from Special Wastes
## 3 Results and Discussion
### Hydration of BCSA Cements  
Figure 2 shows the DT-TG thermograms for the four BCSA cements hydrated for periods between 4 h and 90 days. With DT-TG temperature increase, three different endothermal effects were identified at 93 \(\pm\) 9 \({}^{\circ}\)C, 149 \(\pm\) 9 \({}^{\circ}\)C and 277 \(\pm\) 5 \({}^{\circ}\)C, and were attributed, in this order, to the following hydration products [55]: CSH, C\({}_{6}\)A$\(\sharp\)$\({}_{3}\)H$\({}_{32}\) and AH$\({}_{3}\), respectively. Moreover, no significant effects were found above 500 \({}^{\circ}\)C.  
Figure 2: _Cont._  
On the whole, the DT results reveal the presence of C\({}_{6}\)A5\({}_{3}\)H\({}_{32}\) and AH\({}_{3}\) for all the curing periods, apart from the pastes based on CEM_R and CEM_TW, where aluminum hydroxide materializes after 2 days of hydration; furthermore, both C\({}_{6}\)A5\({}_{3}\)H\({}_{32}\) and AH\({}_{3}\) concentrations increased with aging time. Calcium silicate hydrates were not always distinguished, as their DT effect was overlapped by the ettrinigte (E) peak. Monosulfate could not be clearly detected owing to the overlap of the E signal. TG curves show the mass loss related to dehydration and dehydroxylation phenomena involving CSH, ettrinigte and monosulfate (from room temperature to \(\sim\)200 \({}^{\circ}\)C) as well as aluminum hydroxide (from 200 to \(\sim\)300 \({}^{\circ}\)C), respectively. The higher weight loss values up to about 200 \({}^{\circ}\)C are related to  
Figure 2: Differential thermal (DT, **left**)– thermogravimetric (TG, **right**) thermograms for CEM_R (**a**), CEM_T (**b**), CEM_W (**c**) and CEM_TW (**d**) cured at various ages. the formation of the main hydration products, namely CSH and ettrinigite; moreover, as curing time increases, their amount increases. The BCSA cements' hydration rates were also evaluated using the amount of chemically bound water (Figure 3) calculated from the mass loss values from TG analyses (Figure 2). Figure 3 shows that the four systems followed a similar trend in terms of the evolution of the hydration process. With the exception of the reference system, which displayed the worst hydration behavior, the other binders reached comparable bound water content after 90 days of curing. The hydration process was also investigated using XRD analysis. XRD patterns (for samples hydrated at 4 h, 7 and 28 days) are shown in Figure 4, where only the most significant phases are indicated; they allowed for the detection of: (a) the consumption of reactants, (b) the presence of inert phases, and (c) the development of hydration products (mainly ettrinigite). The absence of CSH and AH\({}_{3}\) among the crystalline phases detected using XRD analysis is associated with their amorphous nature. As for the DT thermograms, it has been found that as curing time increases, the ettrinigite peaks increase at the expense of both the ye'elimite and anhydrite peaks. Weak peaks for monosulfate were detectable in the systems CEM_R, CEM_W and CEM_TW at 7 days of curing, and at 28 days for the CEM_T systems. Furthermore, CEM_TW hydrated more slowly than the other systems. XRD patterns at 28 days still revealed the presence of some unconsumed reactants such as ye'elimite and anhydrite. Figure 4: \(Cont.\)  
Figure 3: Bound water as determined by TG up to 90 days of hydration (normalized to BCSA cement) for CEM_R, CEM_T, CEM_W and CEM_TW plotted against curing time. Figure 5 displays the results of the isothermal calorimetry measurements of the four cements. CEM_TW, namely the system with the highest amount of C4A3$5, showed the highest cumulative heat release over 14 days of hydration. It is worth noting that, compared to the other systems, the main hydration reactions were significantly slower and only started after \(\sim\)24 h, which also agrees closely with the DT-TG data. Another interesting feature was observed for CEM_R, which exhibited a second acceleration of the hydration reactions after about 9 days.