Potential large-volume beneficial use of low-grade fly ash in magnesia-phosphate cement based materials

•Low-grade fly ash can improve the water resistance of magnesia-phosphate cement paste.•Fly ash is likely to act as an active filler in the MKPC-based paste system.•Struvite-K is the dominant reaction product in both MKPC and FA-MKPC pastes.•The beneficial effect of fly ash in MKPC may be largely attributed to its CaO content.