Properties and Reaction Mechanism of Light-burned Magnesia-based Magnesium Phosphate Cement at Varied Mass Ratio of K₃PO₄ and KH₂PO₄

Abstract

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.