Comparative Study on High-Temperature Performance of MPC and BFPMPC

Because a new magnesium phosphate cement (MPC) developed previously for the rapid repair of cement concrete structures has better mechanical properties and bonding properties, the high-temperature properties were further investigated and compared with ordinary MPC in this study. The mechanical properties of MPC, the hydration products, and the high-temperature mechanism were studied and revealed under the calcination conditions of normal [room temperature (20°C)], 70°C, 300°C, 600°C, and 900°C. As the temperature increased, the compressive strength first decreased and then increased with the lowest being at 300°C. Subsequently, analytical methods such as X-ray powder diffraction (XRD), mercury intrusion porosimeter (MIP), Fourier transform infrared spectrometer (FT-IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and scanning electron microscope/energy dispersive spectrometer (SEM/EDS) were used to comprehensively characterize the changes in MPC hydration products under high-temperature conditions in order to reveal the mechanism of action. The MIP results showed that the detrimental pores gradually increased up to 300°C. When the temperature reached 600°C, the pores were formed and filled by more MgKPO4 crystals, which gradually reduced the harmful pores. XRD, FT-IR, and SEM/EDS results showed that at 300°C, the hydration products did not contain the crystalline struvite but transformed into an amorphous transition state. In particular, basalt fiber–reinforced polymer modified magnesium phosphate cement (BFPMPC) still showed polymer degradation and carbonization. The XRD results were verified by TGA. The hydration products of MPC and BFPMPC at 427°C in the process of amorphous to crystalline transformation were verified by DSC. The proposed MPC materials can be widely applied to the reinforcement and maintenance of key joints of buildings and concrete structures that are prone to fire accidents.