Thermal stability of ettringite in alkaline solutions at 80 °C

The thermal stability of synthetic ettringite was examined in NaOH solutions up to 1 M after 12 h of heat treatment at 80 °C, with or without the coexistence of C₃S in the system. Ettringite was found to convert to the U phase, a sodium-substituted AFm phase, over the heat treatment in the absence of C₃S. The presence of C₃S, leading to C-S-H formation, prevents the U phase formation and results in the conversion of ettringite to monosulfate. Sulfate ions generated from ettringite decomposition mostly remain in the solution, but some is incorporated into C-S-H. During subsequent storage at room temperature, the majority of monosulfate slowly converts back to secondary ettringite under moist conditions, using the supply of sulfate ions from the solution and C-S-H. The observations support the current mechanism of delayed ettringite formation (DEF).