Compatibility of hydrogarnet, Ca3Al2(SiO4)x(OH)4(3 − x), with sulfate and carbonate-bearing cement phases: 5-85 °C

The stable existence of hydrogarnet in Portland cement compositions cured at temperatures below 55 °C has long been predicted from application of equilibrium thermodynamics. However hydrogarnet is not often reported in hydrated commercial Portland cements. The substitutions (SO4-CO3-OH) in AFm have previously been shown to stabilise AFm to higher temperatures and raise the temperature at which AFm converts to Si-free hydrogarnet, C3AH6. But unanswered question remains about the compatibility of AFm and AFm solid solutions with Si-substituted hydrogarnet, Ca3Al2(SiO4)x(OH)4(3 − x). Phase relations of C3AH6 and Ca3Al2(SiO4)x(OH)4(3 − x) at sulfate and carbonate activities conditioned respectively by (gypsum and SO4-AFt) and (calcite and CO3-AFt) have been determined experimentally in the range 5-85 °C. The results confirm the instability of Si-free hydrogarnet with carbonate and sulfate-bearing cement phases, but do indicate that a range of silica-substituted hydrogarnet solid solutions are stable under conditions likely to be encountered in blended cement systems.