Properties and Reaction Mechanism of Light-burned Magnesia-based Magnesium Phosphate Cement at Varied Mass Ratio of K<sub>3</sub>PO<sub>4</sub> and KH<sub>2</sub>PO<sub>4</sub>

This paper mainly explores the effects of the relative ratio of tri-potassium phosphate (K3PO4) and potassium dihydrogen phosphate (KH2PO4) on the setting time, fluidity, mechanical properties and microstructure of magnesium phosphate cement (MPC) prepared by lightly burned magnesia. Results show that the composite phosphate with a higher proportion of K3PO4 results in an excellent retarder effect. The longest setting time of 132 min. Comprehensive consideration of setting time and mechanical properties, the superior mass ratio of K3PO4/KH2PO4 is 6/4. Under this condition, the setting time and 28 d compressive strength of MPC are 67 min and 66.4 MPa, respectively. The retarder mechanism of composite phosphate for MPC is that the higher pH value of the liquid phase is acquired with a higher proportion of K3PO4, which can inhibit the dissolution of MgO, leading to a decrease in the hydration heat release rate. Microscopic tests indicate that the increase in the proportion of K3PO4 in composite phosphate reduces the formation of hydrated product MgKPO4·6H2O (MKP), but more K3PO4 promotes the MKP crystal morphology change from prismatic to plate and increases the amount of amorphous K-struvite that fills the gaps between particles, which is conducive to the decrease in porosity and harmful pore proportion.