Enhancing water resistance in magnesium oxychloride cement modified with sodium dihydrogen phosphate at low MgO concentrations

Phosphate ions significantly enhance the water resistance of magnesium oxychloride cement (MOC). In this study, MOC was prepared with a MgO/MgCl2 molar ratio of 7 and a H2O/MgCl2 molar ratio of 12, while varying the dosage of sodium dihydrogen phosphate. The aim was to observe the microstructural changes in 5Mg (OH)2.MgCl2.8H2O (Phase 5) crystals and the macroscopic mechanical properties before and after immersion. The research found that phosphate ions increase the content of air-cured Phase 5 and reduce the formation of Mg (OH)2 by lowering the required Mg2+ concentration for Phase 5 production. However, excessive phosphate ions can damage the crystal structure of Phase 5, leading to a decrease in MOC strength. An appropriate amount of phosphate ions increases the output of Phase 5, resulting in a highly dense microstructure, which significantly improves the water resistance of MOC. Additionally, this enhances the pore structure of MOC, matring the overall structure more compact and resistant to water penetration. This research offers valuable theoretical insights into Phase 5 crystal variations during air curing and water-immersion damage under low Mg2+ concentrations.