Thermal stability and decomposition mechanisms of ettringite at <120°C

Ettringite has been synthesised in phase-pure form and its decomposition and reformation studied as a function of water vapour pressure and temperature. Isobars at P-H2O= 6, 35, 75 and 400 min were determined and combined with data on decomposition at the self-generated water vapour pressure achieved at similar to 120 degreesC. These data follow a well-defined curve although considerable hysteresis occurs in the temperatures for reformation, which are typically 15-20 degreesC less than the corresponding decomposition temperature at each isobar. The dehydrated product is amorphous or nearly so to X-ray powder diffraction, but still gives an electron diffraction pattern: it has a water content that is fixed or variable within narrow limits, 10- 13 H2O per ettiingite formula unit. This product is termed metaettringite. Recommendations are also made for the upper safe service limits for ettringite-based cements. (C) 2001 Elsevier Science Ltd. All rights reserved.