M-value as hydration kinetics governor: Performance development mechanisms in ye'elimite-anhydrite -Portland cement composite system

Blending sulphoaluminate cement (SAC)—known for its rapid hardening, high early strength, and low carbon footprint—with Portland cement (PC) significantly accelerates PC hydration and enhances its mechanical properties. In this study, composite cement is prepared by co-blending ye’elimite and anhydrite with PC. The effect of the molar ratio of calcium sulfate to ye'elimite (M-value) on hydration kinetics, hydration products, pore structure, volume stability, and mechanical properties of the composite cement system is systematically investigated. Results demonstrate that the M-value substantially influences hydration kinetics of composite cement systems. While the fundamental three-stage progression of cement hydration remains consistent, the M-value modulates the duration of each reaction stage. It differentially affects the hydration rates of various mineral phases within the cement system. A positive correlation is observed between the expansion rate of the composite cement systems and the M-value. As hydration progresses, the relationship between the rate of ettringite formation per unit time and the expansion rate becomes more pronounced. The initial 1–3 days of hydration are identified as a critical period for ettringite formation, which plays a significant role in the development of the expansion rate. The formation of ettringite remains positively correlated with the enhancement of compressive strength.