Competitive roles of sulphate and reactive carbonate in the hydration of tricalcium aluminate

The regulation of tricalcium aluminate (C 3 A) hydration by calcium sulphate is essential for controlling the setting of Portland cement, yet the role of calcium carbonate in the early hydration of C 3 A remains insufficiently characterized due to its low solubility. In this study, the hydration of C 3 A was investigated in gypsum – C 3 A, amorphous calcium carbonate (ACC) – C 3 A and gypsum – ACC – C 3 A systems using ACC as a highly reactive carbonate source. Hydration kinetics, solution chemistry and phase assemblages were monitored by isothermal calorimetry, ion concentration measurements, XRD, TGA/DTG, FTIR, and 27 Al MAS NMR. Gypsum imposed a prolonged induction period by suppressing C 3 A dissolution, even in the presence of ACC. In contrast, ACC promoted rapid formation of carbonate AFm phases in the absence of sulphate, although it rapidly crystallized into CaCO 3 polymorphs. In the ternary system, ACC remained largely unreacted and predominantly amorphous during the gypsum-controlled induction period and reacted rapidly only after sulphate and ettringite depletion. Distinct phase assemblages and pore structures developed in the three systems, reflecting the different roles of sulphate and carbonate in regulating early C 3 A hydration. These results provide a direct basis for distinguishing sulphate and carbonate effects on early C 3 A hydration.