Characterisation of water stability of magnesium phosphate cement blended with steel slag and fly ash

The instability of magnesium phosphate cement (MPC) in moist environments limits its application. In the work reported in this paper, the residual strength and length evolution of MPCs under various curing conditions were investigated to assess the effect of steel slag and fly ash on the stability of MPC. The microstructure of hardened MPC was analysed using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The results indicated that the water stability of MPC is highly dependent on the pre-curing regime. Although increasing the ratio of magnesia to phosphate is detrimental to the strength of MPC in air, it was able to improve the water stability. The strength development of MPCs with steel slag was found to similar to those of the fly ash blends, but showed poor water stability. The shrinkage of MPC paste was found to be more significant with an increase in curing age and the dosage of steel slag and fly ash for specimens cured in air, but an opposite trend was exhibited for specimens immersed in water. The cracks in hydrated MPC with steel slag could be the main reason for its poor water stability.