Mechanical properties and microstructure evolution characteristics of fly‐ash/silica‐fume‐modified magnesium phosphate cement after high temperatures

Abstract Magnesium phosphate cement (MPC) is widely used in high‐temperature environments due to its excellent performance. A deep understanding of its properties is considered crucial for further enhancing its stability. Along these lines, in this work, to determine the mechanical properties and microstructure evolution characteristics of fly ash (FA)‐ and silica‐fume (SF)‐modified MPC after high temperatures, the compressive strength of FA‐ and SF‐modified MPC was tested after heating from 105 to 1000°C. The phase change, microstructure, and pore structure evolution characteristics of FA‐ and SF‐modified MPC after the application of high temperatures were systematically investigated by using various microscopy techniques. The compressive strength of MPC first decreased and then increased at 20–1000°C, and 600°C is the critical temperature. FA and SF are beneficial for MPC to improve the compressive strength after high temperature. At 1000°C, in the MPC system with FA and SF added alone, the strength retention rate of MPC modified by 20% FA and 10% SF was the highest. In addition, the proportionally compounded 10% FA and 10% SF exhibited the best effect on improving the compressive strength of MPC. The MPC hydration product MgKPO 4 ·6H 2 O dehydrated under high temperatures, producing MgKPO 4 ·H 2 O at 105°C and MgKPO 4 at 600–1000°C. Although the incorporation of FA and SF did not change the dehydration law of MPC in high‐temperature environments, it induced the generation of MgAl 2 O 4 and Mg 2 SiO 4 at 1000°C. The latter effect could explain the increase in the compressive strength of MPC at that temperature. As the temperature increased, the dimension of the MPC pores increased, and the porosity increased. The established correlation model can better predict the change in porosity as a function of temperature. Our work provides a solid theoretical basis for applications of MPC in high‐temperature environments.