Investigation on a sustainable magnesium phosphate cement (MPC) with recycled waste MPC powders

Magnesium phosphate cement (MPC) has been used for the structure rapid-repair of damaged buildings. To reduce the cost and resource consumption during MPC preparation, recycled MPC powders were used to prepare an environmental-friendly MPC by replacing the dead burned MgO powders. The workability and early hydration temperature development of MPC paste with recycled powders were investigated. To characterize the effect of recycled powders on the performance of the MPC paste, the mechanical properties, volume stability, phase developments and microstructures were determined. The results showed that due to the small particle size and the large specific surface area, the recycled powders absorbed part of the water during the mixing and reduced the fluidity of the MPC paste. The chemical reaction activity of recycled powders was lower than that of MgO powders, resulting in the reduction of the hydration reaction rate and hydration products of MPC. With the recycled powder content increasing, the mechanical strength and volume stability of MPC decreased. The decreased strengths could be attributed to the poor MPC paste structure with increased pores and microcrack content caused by recycled powders. Nevertheless, when the recycled powder content was 40 %, MPC paste obtained a workability of 20.8 cm, setting time of 24.75 min, 3-h compressive strength of 30.9 and 28-d compressive strength of 50.3 MPa. Therefore, under this replacement rate, the MPC paste still had acceptable properties, which could still meet the rapid-repair requirements.