Effects of inert silica powder on the properties of magnesium oxysulfate cement

The effects of silica powder (SP) with different fineness values on the properties of magnesium oxysulfate cement (MOSC) were compared with the effects of fly ash (FA). X-ray diffraction, mercury porosimetry and scanning electron microscopy were used to analyse the effects of the particle morphology and fineness of FA and SP on the mechanical properties, hydration products and microstructure of MOSC. The results showed that when the α-MgO/MgSO₄.7H₂O/H₂O molar ratio was constant, the concentration of magnesium sulfate and the uniformity of magnesium oxide particles in the slurry were the main factors determining the composition of the hydration products and the microstructure of MOSC. Spherical FA was found to improve the uniformity of magnesium oxide particles in the slurry and exert a nucleation effect, thereby increasing the formation of the 5.1.7 phase of the hydration products. Polyhedral SP with a certain fineness increased the concentration of magnesium sulfate in the slurry and had a filling effect, thus promoting formation of the 5.1.7 phase and optimising the microstructure. Therefore, with a certain magnesium sulfate concentration and a homogeneous slurry, both active FA and inert SP can improve the water resistance of MOSC by optimising the composition of the hydration products and microstructure.