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

The stable existence of hydrogamet in Portland cement compositions cured at temperatures below 55 degrees 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 hydrogamet, C(3)AH(6). 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 C(3)AH(6) 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 degrees 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. (C) 2016 Elsevier Ltd. All rights reserved.