Differential effects of metakaolin and ground granulated blast furnace slag on the hydration and long-term performance of magnesium potassium phosphate cement

This study systematically compares the effects of low-alkalinity metakaolin (MK) and high-alkalinity ground granulated blast furnace slag (GBFS) on the hydration, K-struvite stability, and long-term mechanical performance of magnesium potassium phosphate cement (MKPC), with the aim of addressing the gap in existing research that lacks a clear correlation between mineral admixture alkalinity and MKPC durability. Specifically, the low-alkalinity MK acts mainly as inert fillers during early hydration, maintaining the pH conditions required for hydration and promoting K-struvite crystallization via nucleation sites from fine particles. At later ages, MK enhances K-struvite stability and refines the microstructure through limited reactivity, leading to improved long-term compressive strength and volume stability of MKPC. In contrast, the high-alkalinity GBFS rapidly raises the system pH during early hydration, hindering MgO dissolution and K-struvite formation. At later ages, its high alkalinity accelerates K-struvite decomposition and induces excessive amorphous phases, ultimately resulting in strength degradation and volume expansion.