Ettringite has been synthesised in phase-pure form and its decomposition an
d reformation studied as a function of water vapour pressure and temperatur
e. Isobars at P-H2O= 6, 35, 75 and 400 min were determined and combined wit
h data on decomposition at the self-generated water vapour pressure achieve
d at similar to 120 degreesC. These data follow a well-defined curve althou
gh considerable hysteresis occurs in the temperatures for reformation, whic
h are typically 15-20 degreesC less than the corresponding decomposition te
mperature at each isobar. The dehydrated product is amorphous or nearly so
to X-ray powder diffraction, but still gives an electron diffraction patter
n: it has a water content that is fixed or variable within narrow limits, 1
0- 13 H2O per ettiingite formula unit. This product is termed metaettringit
e. Recommendations are also made for the upper safe service limits for ettr
ingite-based cements. (C) 2001 Elsevier Science Ltd. All rights reserved.