## 3 Results and Discussion
On the whole, the DT-TG results indicate that the hydration behavior of the investigated systems was mainly regulated by the reaction of ye'elimite with calcium sulfate alone (especially at early ages) and also by the formation of C-S-H. The latter formed according to the Reaction (3) and thanks to the pozzolanic  
reaction of reactive SiO\({}_{2}\) (belonging to TTWPSs) with calcium hydroxide (generated as secondary product in (3)). The hydration rates of the three CSA-based systems were also evaluated using the amount of chemically bound water (Figure 4) calculated from the mass loss values from TG analyses (Figure 3) normalized to 100 g dry binder. Figure 4 clearly shows that the three binders followed a similar evolution within the first 24 h; during this period the rate of hydration was very high due to the rapid formation of ettringite, followed by the slower formation of hydration products. From 7 days of curing, the curve of the CSAR exhibits an almost constant value, while the curves of the blended cements show a slight linear increase owing to the pozzolanic reaction of TTWPSs. The hydration process was also examined using QXRD analysis which detected the reactants consumption, the presence of inert phases and the development of new products. Figure 5 displays the patterns of the binders hydrated at 4 h, 28 and 56 days. The evolution of the hydration process, in terms of ettringite, ye'elimite and belite concentration, at different hydration times, is reported in Figure 6. The concentration values were determined according to the Rietveld method and normalized to 100% of the accounted crystalline phases. From and overall examination of both Figures 5 and 6, it can be drawn that XRD results fit well with the indications from DT-TG analyses. Figure 4: Bound water as determined by TG up to 56 days of hydration (w/b = 0.65 and normalized to 100 g dry binder) for CSAR, CSA20 and CSA35 pastes _vs._ curing time. Figure 5: XRD patterns for CSAR (left), CSA20 (middle) and CSA35 (right) pastes hydrated (w/b = 0.65) for 4 h, 28 and 56 days. Legend: A = CaSO\({}_{4}\); B = 2 CaO-SiO\({}_{2}\); E = 3 CaO-Al\({}_{2}\)O\({}_{3}\):3 CaSO\({}_{4}\):32H\({}_{2}\)O; Y = 3 CaO-3 Al\({}_{2}\)O\({}_{3}\)-CaSO\({}_{4}\); M = 3 CaO-Al\({}_{2}\)O\({}_{3}\)-CaSO\({}_{4}\):12H\({}_{2}\)O. In fact, as far as the hydration products were concerned, 3 CaO-3 Al2O3-CaSO4 and CaSO4 quickly reacted, and ettringite concentrations increased up to 56 days of curing. Weak signals relating to the presence of monosulfate appeared from 28 days of curing in the three investigated systems. This phase was not identified using DT-TG apparatus as its endothermic peak was always overlapped by that of ettringite. The absence of AH3 in the crystalline phases detected using XRD analysis is related to its amorphous nature [33]. Figure 7 reports the porosimetric curves for the pastes of the three investigated binders; the left and the right areas in the figure show the cumulative and derivative plots for the intruded Hg volume of CSA-based systems vs. pore radius at various curing times (4 h, 7 and 56 days). It clearly shows that with an increase in curing time both total porosity and threshold pore width reduced. For the CSAR (Figure 7, top), all the differential curves were characterized by a bimodal pores size distribution centered on the threshold width of pore necks connecting a continuous system. Between 4 h and 56 days of curing, the threshold pore radii, respectively, reduced from about 270 nm and 11 nm to 110 nm and 10 nm; in the same period the total cumulative volume decreased from about 258 mm\({}^{3}\)/g to 149 mm\({}^{3}\)/g.