Influence of Magnesium Oxide Activity on Water Resistance of Basic Magnesium Sulfate Cement

Basic magnesium sulfate cement (BMSC) is a new type of gas-hardening magnesium cementitious material. In this study, basic magnesium carbonate was used as the raw material to obtain different active magnesium oxides by calcination at different temperatures to prepare BMSC with MgO:MgSO4:H2O = 5:1:20. The effect of different active magnesium oxides and different mineral admixtures and their content on the compressive strength of BMSC before and after soaking was analyzed, and the influence of MgO activity on the water resistance of BMSC was explored. The experimental results show that the activity of MgO has a significant impact on the water resistance of BMSC, and the water resistance of BMSC prepared from low-activity MgO is better, mainly because the hydration products of BMSC prepared by low-activity MgO are 5•1•7 phases. The MgSO4 in the 5•1•7 phase is insoluble in water; consequently, the free MgSO4 in BMSC is scarce; thus, the phase remains unchanged. After selecting low-activity MgO (T1000) and adding mineral admixtures to study its water resistance, it was found that the BMSC softening coefficient increases with an increase in the mineral admixture content. Both fly ash and silica fume were found to improve the water resistance of BMSC and increase its water resistance.