Properties of magnesium potassium phosphate cement pastes exposed to water curing: A comparison study on the influences of fly ash and metakaolin

Under moist condition the magnesium phosphate cement (MPC) exhibits poor water resistance. In this study, the effects of metakaolin (MK) and fly ash (FA) on the properties of magnesium potassium phosphate cement (MKPC) pastes exposed to water curing were investigated. The compressive strengths, water resistance and microstructures of the MKPC pastes containing 30 wt% and 50 wt% of MK or FA as replacements of MKPC under various water curing regimes for up to 180 d were examined. Results showed that the water curing yielded much slower compressive strength development and lower ultimate compressive strengths of all the MKPC pastes during the curing age in comparison to air curing. Incorporation of MK or FA improved the water resistance of MKPC pastes and the MK contributed more considerably than the FA. This may be attributed to the reaction of MK in the MKPC system, which led to the formation of an intermixed phase of struvite-K with Al and Si and/or other amorphous aluminosilicate-bearing phase and thus was beneficial for the microstructure densification, compressive strength increase, and water stability improvement of the reaction products in the MKPC pastes. In comparison, fewer contents of Al and Si were incorporated in the hydration product of MKPC pastes containing FA probably owing to the lower reactivity of FA in the MKPC pastes.