Role of MgO/Cl2 and H2O/Cl2 Molar Ratios on Microstructure and Strength Degradation of Magnesium Oxychloride Cement When Exposed to Water

This study investigated the role of MgO/Cl 2 ( M ) and H 2 O/Cl 2 ( H ) molar ratios on microstructure and strength degradation of magnesium oxychloride cement (MOC) when exposed to water. The M or H molar ratios were systematically varied ( M of 5, 9, or 13; H from 10 to 30) by adjusting the proportions of raw materials and water. Experimental characterization included isothermal calorimetry, X-ray diffraction, thermogravimetric analysis, mercury intrusion porosimetry, and compressive strength measurements before and after water exposure. The test results indicate that low M molar ratios promote a greater heat of hydration and the formation of Phases 3 and 5 as dominant hydration products, resulting in higher compressive strength and lower porosity, along with reduced strength degradation upon water exposure. Conversely, a high M molar ratio of 13 leads to brucite-dominant phase assemblages exceeding 91 wt.%, substantially reducing compressive strength. Furthermore, increasing H molar ratios contributes to additional brucite formation and a further reduction in compressive strength.