Experimental Determination of Solid–Liquid Equilibrium of Calcium in Portland Cement Under Seawater Exposure

Calcium leaching is a key degradation mechanism governing the long-term durability of cement-based materials, particularly in marine environments where multi-ionic interactions significantly alter dissolution behavior. In this study, the solid–liquid equilibrium of calcium in hardened Portland cement exposed to NaCl solution and artificial seawater was experimentally established. A wide range of equilibrium states was achieved by varying the liquid-to-solid ratio, and the corresponding aqueous chemistry and phase assemblage were characterized using ICP-AES, XRD, and SEM-EDS. The results show that both NaCl solution and seawater substantially increase the equilibrium calcium concentration compared to deionized water, with a much stronger effect observed in seawater. In NaCl solution, the equilibrium relationship follows a classical three-stage decalcification process. In contrast, seawater induces coupled dissolution–precipitation reactions due to the presence of Mg2+ and SO42−, leading to rapid and extensive decalcification within a narrow calcium concentration range. The findings provide important experimental evidence for improving thermodynamic models and predicting the long-term degradation of Portland cement concrete in marine environments.