Long-term performances of hydroceramic systems as a potential cementing material at 240 °C

The performance of the Ca(OH)₂–Al₂O₃–SiO₂–H₂O hydroceramic system (with and without Al₂O₃) as a potential well cementing material was investigated from both macroscopic and microscopic perspectives. To simulate the harsh conditions typical of deep wells, marked by high temperature and pressure, the material was cured at 240 °C and 50 MPa. Several hydroceramic systems with varying compositions were designed and a retarder was used to optimize the thickening time of all systems. Finally, the optimized hydroceramic systems were cured for different periods ranging from 2 to 90 days to investigate their long-term stabilities. It is found that the raw materials with finer particle sizes and higher reactivity (such as silica fume and nano-activated alumina) could improve the performance of the hydroceramic system, evidenced by increased compressive strengths and decreased permeability. Changing the Ca/Si molar ratio within the range from 2:1 to 1:1 had little effect on the physical and mechanical properties of the hydroceramic system, despite significant variations in the composition of hydration products. The hydroceramic systems exhibited good stability over a curing period from 2 to 30 days, but strength retrogression phenomenon was observed during longer curing periods from 30 to 90 days. Microscopic evaluations revealed that the strength retrogression could be due to the formation of reyerite in high calcium systems, while it was the conversion of amorphous C–S–H to xonotlite that led to the strength retrogression in low calcium systems.