The role of sulfate ions in tricalcium aluminate hydration: New insights

As a major mineral of Portland cement, the hydration of tricalcium aluminate (C3A) has a significant impact on various properties of cement-based materials. However, there is little consensus on its mechanism, especially the interaction between C3A and gypsum which usually co-exist in cement systems. Recent researches have shown that sulfate ions play an important role in the kinetics of C3A hydration, which may provide more promising alternatives on controlling cement hydration besides the present solution on adjusting gypsum contents. In this study, effects of different kinds of solid sulfates (gypsum, Na2SO4, MgSO4) on the heat released during the early age of C3A hydration as well as hydration products were investigated first by isothermal calorimetry, SEM, XRD and thermal analysis. Retardation was observed in all the three systems (C3A-gypsum, C3A-Na2SO4 and C3A-MgSO4). But the duration of induction period varied with cations. Then barium nitrate (Ba(NO3)2) aiming to remove the sulfate ions was innovatively employed to highlight the role of sulfate ions. Interestingly, the renewed hydration of C3A occurred immediately after Ba(NO3)2 solution was added into the three systems with different amount of AFt remained. According to the characterization of hydration products, little monosulphoaluminate (s-AFm) was detected in the induction period of C3A-gypsum hydration and a large amount of h-AFm was found to cover the surface of C3A particles after adding Ba(NO3)2 solution. It is concluded that the coexistence of calcium and sulfate ions absorbed at C3A surface is more likely to dominate the retardation of C3A hydration in the presence of gypsum, rather than the alleged physical barrier mechanism caused by the precipitation of hydration products.