Calibrated quantitative thermogravimetric analysis for the determination of portlandite and calcite content in hydrated cementitious systems

Abstract

Portlandite and calcite are compounds of interest regarding different processes related to microstructural and durability issues of cementitious materials, such as carbonation, pozzolanic action, and hydration degree. The quantification of their contents in cementitious systems is thus frequently required. Thermogravimetry (TG) measures the change in mass of a material (as a function of time) over a temperature range using a predetermined heating rate. The TG method can be applied with certain success to estimate portlandite and calcite contents in the hydrated cement system, considering the temperature range at which each compound decomposes and releases water or carbon dioxide, respectively. However, a mature hydrated cement paste contains hydrated calcium silicate, portlandite and calcite phases. The quantification procedure is therefore complicated because of the concurrent interference among them. The tangential method over the TG signal or the integral method over the derivative TG curve is usually used to discount water loss from hydrated calcium silicates over the temperature range at which portlandite and calcite decompose. However, by the use of TG consistent underestimation of portlandite content in hydrated cementitious systems is still described in the literature. Potential causes for this underestimation are analysed in this paper, and a calibration procedure by means of an internal standard is proposed. Cement paste and aggregate samples are analysed. Differences between TG quantifications of these pure samples and those diluted with low contents of internal standards are compared with the added amounts of internal standard. In that way, a calibration method of the device is applied for correction of the actual portlandite and calcite contents in the samples. For this purpose, both analytic quality calcite and freshly prepared portlandite were used as internal standards. The results show that improved quantifications can be achieved with this calibration. Also, calcite seems to be more suitable as internal standard than portlandite as the best outcome was obtained for the first case.