Effects of fly ash and slag on the properties of magnesium oxysulfate cement

In this study, the effects of fly ash (FA) and ground granulated blast-furnace slag (GGBS) on the setting time and mechanical properties of magnesium oxysulfate cement (MOSC) and composition and microstructure of hydration products were investigated. Various specimens were prepared with different proportions of FA or GGBS ranging from 10 to 50% of magnesium oxide (MgO) weight. The hydration products and microstructure were characterized by quantitative X-ray diffraction, scanning electron microscopy and mercury intrusion porosimetry. The results indicated that the incorporation of FA or GGBS at low levels (10–30%) can significantly enhance the hydration reaction of MOSC through the dilution effect to shorten the setting time and increase the production of the 5·1·7 phase and magnesium hydroxide (Mg(OH)₂), thus reducing the amount of the amorphous phase. Meanwhile, FA and GGBS can effectively fill the large pores through the packing effect and nucleation effect of the fine particles, reducing the pore diameter of MOSC matrix. Therefore, the incorporation of FA or GGBS into MOSC at low content can significantly optimize the composition of hydration products and result in a compact microstructure, thus improving the mechanical properties. The contribution of GGBS to the early compressive strength of MOSC is higher than that of FA and that to the late compressive strength is lower than that of FA.