Effects of fly ash, retarder and calcination of magnesia on properties of magnesia–phosphate cement

Magnesia–phosphate cement (MPC) is a new type of cementing material composed of magnesia powder, phosphate and boric acid retarder in a given proportion. In order to study the influence of calcined magnesia, fly ash and boric acid retarder on the performance of MPC under different conditions, magnesia was calcined and mixed with fly ash and retarder in various mixtures to prepare different MPC samples. The compressive strength, hydration mechanism and microstructure of MPC were assessed. The results show that, within a certain range, the compressive strength of MPC increases with increasing calcination temperature and increasing holding time of the calcined magnesia. The addition of fly ash improves the microstructure of MPC and, in a certain range, the compressive strength of MPC increases with an increase in fly ash content. Boric acid reduces the compactness of MPC, thus decreasing the compressive strength.