Optimization of Low-Density Hydroceramic Systems for Long-Term Stability at High Temperatures

In this study, various raw materials, including silica sand, silica fume, calcium hydroxide, α-alumina, and nano-activated alumina, were used to produce hydroceramic systems with varying Ca/Si/Al ratios to optimize their high-temperature resistance. The hydroceramic slurries, with a constant density of 1.65 g/cm³, were all designed to have a setting time of more than 4 h at the condition of 240 °C and 50 MPa and then cured at the same condition for 2, 30, and 90 days to evaluate their long-term performances. Subsequently, compressive strength, water permeability, mercury intrusion porosimetry, thermogravimetry, and X-ray diffraction tests were conducted on set samples at various curing times to analyze the hydroceramic systems' long-term stability and the underlying mechanism. The results indicated that the hydration reaction of α-Al₂O₃ was minimal, and its inclusion reduced the incorporation of silica sand in the hydration process. Nano-activated alumina improved the macroscopic properties of the hydroceramic systems and promoted the formation of a significant amount of tobermorite 11 Å. The addition of silica fume can enhance the system's macroscopic properties and the long-term stability, promoting the reaction of silica sand. The long-term stability of slurries with a Ca/Si ratio of 1 was significantly better than that of slurries with a Ca/Si ratio of 0.5. The best-performing slurry can maintain a compressive strength of more than 19 MPa after being cured at 240 °C for 90 days.