Development of Water-Resistant Magnesium Oxychloride Cement

Research and development of eco-friendly and sustainable cementitious building materials with less carbon dioxide emission have been attracting much research interests in recent decades. Magnesium oxychloride cement (MOC) was invented following ordinary Portland cement (OPC) immediately. MOC is obtained by mixing light-burnt magnesia powder (MgO) and a concentrated solution of magnesium chloride (MgCl2). Compared with conventional cement, MOC has many superior properties, such as air-dried, high early strength, fast setting, low alkalinity, and low thermal conductivity. However, due to the instability of the main hydration product of MOC, the mechanical properties of MOC products decrease sharply after prolonged time in the water. The weak water resistance of MOC restricts its application in civil engineering. Research efforts have been made to improve the water resistance of MOC, and the most common method is adding additives into MOC mixtures, such as high active SiO2, active aluminate, sulphates, and phosphoric acid or soluble phosphates. Changing the curing regime and introducing CO2 in the curing process is also applied to increase the water resistance of MOC. The water resistance of MOC is enhanced to different extent by these methods, but the water resistance in the water at room temperature for a prolonged time and in warm water haven’t been studied systematically. Additionally, the mechanism of the influence of the additives on the properties of MOC also needs to be investigated. This research aims to improve the water resistance of MOC in the water at room temperature and warm water. New additives are added into MOC, and the influences of these additives on the properties of MOC are studied. Furthermore, the mechanism of the impacts on the material properties of MOC is investigated by means of XRD, FTIR, SEM, MIP, TG/DSC and XRM. Based on the comprehensive study, the formula of phosphoric acid 0.5%- sodium monofluorophosphate 0.5%-fly ash 30%-MgO/MgCl2 (molar ratio) 9-H2O/MgCl2 13 is found to be the optimized mixture, of which the compressive strength is 110.07 MPa, flexural strength is 16.8 MPa, and the compressive strength retention coefficients in both room temperature and warm water for 28 days are higher than 0.9