Thermodynamic insights into phase stability of magnesium oxychloride cement under chloride, sulfate, and brine attacks

Abstract Magnesium oxychloride cement (MOC) is recognized for its superior mechanical performance and environmental benefits. Yet, the phase evolution of MOC in aggressive environments, particularly with chloride and sulfate exposure, remains insufficiently understood. A novel thermodynamic database with self‐consistency is constructed in this study to reveal the degradation mechanism of hardened MOC paste exposed to solutions of NaCl, CaCl 2 , Na 2 SO 4 , MgSO 4 , and natural brine environments, accounting for both chemical attack and leaching effects. The predicted phase assemblages are validated through x‐ray diffraction data from published sources and additional experiments. The results reveal that MOC degradation in saline media is governed by three interrelated factors: (i) leaching dominates the degradation process, with chemical attack playing a secondary role; (ii) chloride/sulfate concentrations dictate the thresholds for phase transformation; and (iii) reaction pathways and secondary phase evolution are shaped by the competition between Ca 2+ /Na + and Mg 2+ ions. The present work delivers novel thermodynamic perspectives on MOC degradation and lays a theoretical foundation for its optimization in marine and saline environments.