Roles of metakaolin in magnesium phosphate cement: Effect of the replacement ratio of magnesia by metakaolin with different particle sizes.

• Metakaolin improves the mechanical properties and durability of MPC. • Metakaolin decreases the early-term hydration heat of MPC and the total porosity. • Mechanisms of metakaolin in MPC include providing nucleation cites, filling effect and pozzolanic effect. Different particle sizes of metakaolin were employed to replace magnesia partially, and it goes MK1, MK2 and MK3 in order of the specific surface area. Experimental results indicate that the presence of metakaolin indeed improves the compressive strength, tensile bonding strength and water resistance of MPC mortar through sacrificing the workability to some extent, and 30% MK2 blended MPC mortar has the best performances except fluidity. Metakaolin decreases the porosity and improves the pore structure of MPC mortar, especially for specimens after water immersion. The dissolution and reaction of metakaolin in MPC are confirmed by BSE analysis. Nevertheless, no new hydration products can be detected in MPC paste with metakaolin through XRD and TG-DTG analysis. The condensation of intermediate products, crystallization and crystal growth occurring on the surface of metakaolin promote the formation of the more compacted and homogeneous microstructure, which is the physical mechanism of metakaolin improving the properties of MPC. [ABSTRACT FROM AUTHOR]