Stability of calcium silicate hydrates produced by alite hydration at high and ultrahigh temperatures

The influence of initial curing temperature on the stability of calcium silicate hydrates (C-S-H) produced by alite hydration in the temperature range of 110 degrees C-240 degrees C was investigated. Test results showed that, alite pastes directly cured under high temperatures yielded dramatically faster crystallization of C-S-H compared to those precured at a low temperature of 80 degrees C before the high temperature curing. The direct high-temperature curing also resulted in simplified crystalline phase evolution with only alpha-C2SH produced below 200 degrees C. Detailed analyses employing XRD, TGA and MAS-NMR techniques revealed that the crystallization of C-S-H is accompanied by the increases of its Ca/Si and H2O/Si ratios as well as the reduction in the number of bridging tetrahedra in silicate chains. The decreased stability of C-S-H formed at direct high-temperature curing was probably induced by the generation of alpha-C2SH nuclei during the initial stages of alite hydration, enabling selective control of C-S-H crystallization.