Freezing-thawing resistance of magnesium oxysulfate (MOS) cement with granite powders

One of low-carbon cementitious material is magnesium oxysulfate (MOS) cement with desirable engineering properties, including exceptional fire resistance, lightweight properties, and low thermal conductivity. To encourage the widespread adoption and utilization of MOS cement in various applications, along with the reuse of solid waste granite powders (GPs), GPs were added to MOS cement as a mineral admixture to improve its freezing-thawing (F-T) resistance. Scanning electron microscope, synchronous thermal analyzer, mercury porosimeter, and X-ray diffractometer were employed to detect the microstructure, hydration products, pore characteristics, and phase composition of MOS cement. The compressive strength loss rate and mass loss rate of MOS cement containing GPs were adopted to evaluate its F-T resistance. The results revealed that MOS cement containing 40 wt.% GPs survived 50% more F-T cycles than the MOS cement without GPs. During the F-T cycles, the 5·1·7 phase grew from needle-like crystals to clusters in MOS cement. The matrix deterioration of MOS cement was facilitated by various factors such as the presence of residual MgO in the matrix results in the creation of Mg(OH)₂, the crystallization of pore water, and the decomposing of the 5·1·7 phase. The addition of GPs can improve the packing density and stability of MOS cement matrix, leading to a reduction in the content of air hole of the resulting cement. During the procedure of F-T cycles, a reformation of 5·1·7 phase occurred due to the variation in the pore solution’s pH value in MOS cement, leading to an improvement in the engineering properties.