Static and dynamic mechanical properties of magnesium phosphate cement modified by metakaolin after high-temperature treatment

Magnesium phosphate cement (MPC) is widely used in fields of construction and fire-retardant coatings due to its excellent properties. The hydration properties of MPCs at high temperatures are the points of interests. In this work metakaolin was used to improve workability and mechanical properties of MPC, and static and dynamic mechanical properties and hydration products of the MPC after high temperature treatment were investigated. The results showed that the incorporation of metakaolin in MPC allows extending and importantly tuning the setting time of MPC. Notably, 40% replacement of dead burnt magnesia with metakaolin in MPC could extend the setting time of MPC to about 24 min. While the fluidity of MPC significantly decreased with increasing metakaolin content. Typically, increasing the temperature leads the deterioration of mechanical properties of the MPC. However, static and dynamic strength enhancement was also observed at 600℃. The main hydration products of MPC with metakaolin are struvite (MgNH4PO4·6H2O), unreacted MgO, aluminum phosphate (APH) and possibly amorphous substance containing aluminum. Moreover, the impact resistance of MPC at high temperatures were significantly improved with incorporation of metakaolin.