Phase studies of pozzolanic stabilized calcium silicate hydrates at 180 °C

Abstract Phase studies of calcium silicate hydrates formed at elevated temperature and pressure have been well documented. At 180 °C, the initially formed amorphous calcium silicate gel [C-S-H] transforms into well-defined crystalline phases, the stability of which is primarily dependent on the C/S ratio in the CaO-SiO 2 -H 2 O system and the hydrothermal conditions. Hillebrandite [C 2 SH], α-dicalcium silicate hydrate [α-C 2 SH] and β-tricalcium silicate [β-C 6 S 2 H 3 ] are predominantly the stable phases in the lime-rich part of the CaO-SiO 2 -H 2 O system and are typically associated with high permeability and compressive strength retrogression. Gyrolite [C 2 S 3 H ~2 ], tobermorite [C 5 S 6 H 5 ], truscottite [C 7 S 12 H ~3 ] and xonotlite [C 6 S 6 H] have all been reported to coexist stably in aqueous solution with silica in the silica-rich part of the CaO-SiO 2 -H 2 O system. The addition of excess silica to the CaO-SiO 2 -H 2 O system is usually in the form of silicon dioxide [SiO 2 ], either as microsilica or quartz flour, which, in theory, should not affect the equilibrium chemistry. This has not been found to be the case, and metastable phases formed in the early stages of reaction modify the long-term stability and phase equilibrium. Pozzolanic materials that are predominantly alumino-silicates have also been investigated as a source of excess silica. Partial replacement of aluminum for silicon occurred, but had no apparent influence on the stability of the calcium silicate hydrates formed.