Mix design optimization of low-grade MgO-based magnesium phosphate cement mortars for cold weather applications

The utilization of low-grade MgO in magnesium phosphate cement (MPC) systems offers significant economic and environmental benefits, yet its performance is often limited by reduced reactivity and sensitivity to curing conditions, particularly at low temperatures. In this study, magnesium ammonium phosphate cement (MAPC) and magnesium potassium phosphate cement (MKPC) mortars were systematically developed using low-grade MgO in combination with ammonium dihydrogen phosphate (ADP) or potassium dihydrogen phosphate (KDP). A sequential optimization strategy was applied to determine the optimal mix design, followed by hydration heat measurements and mechanical performance evaluation after curing under ambient (25 ± 2°C) and sub-zero conditions (up to –20°C). The results demonstrate that both MgO/phosphate (M/P) and water-to-cement r (W/C) ratios exert a strong coupled influence on strength development. While both systems achieved high early strength under ambient curing, their behavior under sub-zero conditions differed, reflecting variations in reaction kinetics and phase evolution. Microstructural characterization of hardened specimens using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) revealed differences in the formation, morphology, and continuity of struvite-type hydration products, which govern the observed mechanical performance. The findings provide a practical framework for designing sustainable MPC mortars and highlight the potential of such systems for cold-weather construction applications.