Quantitative X-ray diffraction analysis of ettringite,thaumasite and gypsum in concretes and mortars

A method for quantitative Xray diffraction analysis (QXDA) of three sulphate minerals frequently associated with building materials has been devised. Sulphate minerals which form within concretes, mortars and other cementitious-based materials include ettringite, (3CaO.Al₂O₃.3CaSO₄.31H₂O), thaumasite (CaSiO₃.CaCO₃.CaSO₄.15H₂O) and gypsum (CaSO₄.2H₂O). Calibration standards were prepared using pure samples of these minerals and incorporating boehmite as an internal standard. The equations obtained from the standard calibration curves were used to calculate the percentage of ettringite, thaumasite and gypsum in a) samples which contained known percentages of these minerals mixed together and b) laboratory prepared concrete cubes undergoing sulphate attack. The cubes contained 0%, 20% or 40% pulverized fuel ash (pfa) by weigth of cementitious material and had been stored in various sulphate solutions including sea water for one year.Quantitative Xray diffraction analysis of the standard mixtures successfully detected the expected ettringite, thaumasite and gypsum concentrations. The concrete cube results showed that the sulphate mineral concentration within the surface of the cubes decreased when larger amounts of pfa were used in the concrete mixes. This effect was less noticeable in the sea water cubes.