Carbonation of ettringite by atmospheric carbon dioxide

Abstract

The mineral name ettringite is used to refer to the calcium trisulphoaluminate hydrate found in several hydrated construction cements. During a programme to study its longer term stability, the extreme sensitivity of synthetic ettringite to attack by atmospheric CO 2 was confirmed. Using a TG/DTG method synthetic ettringite, samples were shown to have decomposed by carbonation at 25°, 50°, 75° and 95°C in moist atmospheres despite attempts to reduce the exposure to atmospheric CO 2. The decomposition products were identified using XRD techniques and quantified by the TG/DTG method. The data suggested a two-stage decomposition mechanism essentially accelerated by Increasing temperature. The two-stage process was found to fit a number of established kinetic equations for solid decomposition and these are discussed in relation to the crystal structure. Elimination of CO 2 from the moist atmosphere effectively prevented similar decomposition although at 95°C ettringite becomes thermally unstable.