Effect of Acid on the Crystal Phase and Water Resistance Properties of Magnesium Oxysulfate Cement

In this paper, the effects of eight different types of acids (oxalic, silicic, acetic, salicylic, citric, phosphoric, tartaric, and malonic acids) on the mechanical properties, water resistance, physical phase composition, and microstructure of magnesium oxysulfate cement (MOS) were investigated. The properties of the samples were characterized by compressive strength tests, X-ray diffraction (XRD) and scanning electron microscope (SEM), and the reasons for acid modification of MOS were analyzed. The results show that the addition of oxalic, silicic, acetic, and salicylic acid does not significantly improve the mechanical and water resistance properties of MOS, and the addition of citric, phosphoric, tartaric, and malonic acid promotes the generation of 5Mg(OH)2·MgSO4·7H2O (5·1·7 phase), which substantially improves the mechanical and water resistance properties of MOS. The mechanism of these four acid modifications is that the corresponding acid radical ions combine with the hydration shell [Mg(OH)(H2O)x]+ generated by the reaction of MgO in water, preventing the [Mg(OH)(H2O)x]+ from combining with OH− to form Mg(OH)2. At the same time, the composite complexes act as nuclei to adsorb the nearby SO42− and Mg2+ to induce the generation of the 5·1·7 phase.