Study on the phase composition of the MgO–MgSO4–H2O system and mechanisms of chemical additives

Magnesium oxysulfate (MOS) cement is a promising low-carbon and high-performance cement. Basic research on the MgO–MgSO4–H2O system is of great significance for further optimizing the properties of MOS cement. In this study, through the qualitative hydrolysis test of the Mg2+(MgSO4)–OH-–H2O system and the test and characterization of the composition of the hydration products and microstructure of the MgO–MgSO4–H2O system, the relationship between the composition of the hydration products of the MgO–MgSO4–H2O system and the molar ratio of raw materials and the mechanism of chemical additives were investigated. The results show polynuclear complexes [MgP(OH)q(H2O)r]2p-q in the MgO–MgSO4–H2O system. The main group of hydration products of MOS cement without chemical additives, which is the 3·1·8 phase, is formed by the transformation of the unstable 1·1·5 phase through the 5·1·3 phase. However, chemical additives can change the hydration reaction processes of MOS cement through buffering and complexation, which causes the main hydration products of MOS cement to change to the 5·1·7 phase. The 5·1·7 phase is a metastable phase (or stable phase) formed by the transformation of the unstable 1·1·5 phase through the 3·1·8 phase.