Setting and strength development of magnesium phosphate cement paste

Magnesium phosphate cement (MPC) paste is prepared by mixing dead-burned magnesia powder and potassium di-hydrogen phosphate with a proper proportion of water. The paste demonstrates superior performance such as fast setting, high early-strength, low shrinkage and good bonding which makes MPC suitable for applications in the rapid repair of infrastructures. However, it has been found that MPC easily sets in a few minutes and thus causes difficulties in manipulation. Therefore, how to retard and control the setting time of MPC becomes a challenging problem in both scientific and engineering fields. The setting time of MPC is greatly influenced by the reactivity of magnesia. A reliable, effective and economic test method has been developed to measure the reactivity of magnesia. In the present study, two kinds of magnesia, namely, high-reactivity magnesia and low-reactivity magnesia were blended in order to adjust the setting process of MPC. The effects of blending ratio and molar ratio of magnesia to phosphate and borax addition on setting time and compressive strength of MPC cement paste were investigated. Finally, the reaction product and reaction mechanism were examined and discussed.