Hydration and interactions between pure and doped C3S and C3A in the presence of different calcium sulfates

This research studied the hydration of C3S-C3A-calcium sulfate systems made of combinations of two C3S (pure triclinic and Al-doped monoclinic), two C3A (pure cubic C3A and Na-doped orthorhombic), and two calcium sulfates (gypsum and hemihydrate). For each system, the hydration of four different SO3 contents (0.25-2.0 wt%) was assessed by calorimetry. The optimum SO3 content was fixed from calorimetry results, and the mixtures were evaluated by in-situ XRD and TGA. The type of C3S was the factor that most affected the sulfate balance of the systems. The mixes with Al-C3S produced a higher amount of ettringite in the first hours, resulting in much earlier sulfate depletions when compared to the mixes with pure C3S. The mixes with ort-C3A also showed faster sulfate depletion due to its higher reactivity compared with cb-C3A. Finally, the replacement of gypsum by hemihydrate resulted in faster sulfate depletion caused by the faster hemihydrate dissolution.