Decoupling Ca ²⁺ and SO ₄ ²⁻ effects on α‐calcium sulfate hemihydrate hydration by thermokinetics and molecular dynamics

Abstract The mechanistic roles of Ca 2+ and SO 4 2− as key ions governing hydration kinetics and gypsum‐based materials performance remain unresolved. Ion‐specific effects of exogenous Ca 2+ /SO 4 2− on α‐calcium sulfate hemihydrate (α‐HH) hydration were systematically examined through thermokinetic analysis, experimental validation, and molecular dynamics simulations. Hydration acceleration was not induced by Ca 2+ , whereas SO 4 2− demonstrated concentration‐dependent enhancement. The Krstulović−Dabić model revealed that the hydration process transitions from nucleation and crystal growth to interphase reactions and diffusion control. While both ions enhanced the thermodynamic driving force, Ca 2+ retarded hydration kinetics by inhibiting the dissolution process via the common ion effect and adsorption onto the HH surface. In contrast, under high supersaturation, SO 4 2− dynamically promoted dissolution by accelerating nucleation and crystal growth, thereby maintaining a high supersaturation level. The elevated SO 4 2− concentration in later stages further alleviated diffusion‐controlled limitations, enhancing overall hydration kinetics. This ion‐specific mechanism led to microstructural differences: exogenous Ca 2+ resulted in coarsened calcium sulfate dihydrate crystals and a 21% decrease in compressive strength, whereas SO 4 2− promoted refined, densely interlocked crystals and a 16% strength increase compared to the control. The elucidated mechanism provides critical guidance for regulating hydration, tailoring microstructure, and optimizing performance in gypsum‐based materials.