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

As a major mineral of Portland cement , the hydration of tricalcium aluminate (C 3 A) has a significant impact on various properties of cement-based materials. However, there is little consensus on its mechanism, especially the interaction between C 3 A and gypsum which usually co-exist in cement systems. Recent researches have shown that sulfate ions play an important role in the kinetics of C 3 A 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, Na 2 SO 4 , MgSO 4 ) on the heat released during the early age of C 3 A 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 (C 3 A-gypsum, C 3 A-Na 2 SO 4 and C 3 A-MgSO 4 ). But the duration of induction period varied with cations. Then barium nitrate (Ba(NO 3 ) 2 ) aiming to remove the sulfate ions was innovatively employed to highlight the role of sulfate ions. Interestingly, the renewed hydration of C 3 A occurred immediately after Ba(NO 3 ) 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 C 3 A-gypsum hydration and a large amount of h-AFm was found to cover the surface of C 3 A particles after adding Ba(NO 3 ) 2 solution. It is concluded that the coexistence of calcium and sulfate ions absorbed at C 3 A surface is more likely to dominate the retardation of C 3 A hydration in the presence of gypsum, rather than the alleged physical barrier mechanism caused by the precipitation of hydration products.