Interpretation of the thermogravimetric curves of ancient Pozzolanic concretes

Pozzolanic concretes submitted to thermogravimetric (TG) analysis show a continuous weight loss starting from about 400 C-o. In order to reconcile these observations with those from other analytical methods, it is necessary to attribute this weight loss to CO2 removal. It has been proposed, in the literature, that silicates and CaCO3 react at lower temperatures, producing calcium silicates and CO2. In this paper, the FTIR spectra collected on samples submitted to TG analysis, stopped at conveniently selected temperatures, provide direct evidence that the continuous weight loss recorded with thermogravimetric analysis of the pozzolanic concrete in the temperature range 400-900(o) C is to be attributed to the reaction between the silicates and calcite, with the formation of CO2 and of a silicate that is richer in CaO. Therefore it is justifiable that the whole weight loss, in the temperature range 400-900(o) C, should be taken into account in the calculation of the CaCO3 content of the concrete. Moreover, the described procedure--of recovering small samples (2 mg) from the sample-holder of the TG apparatus at various steps of the heating rate and comparing the FFTIR spectra--can help in identifying the pozzolanic nature of a concrete that, in general, is not easily recognizable from the trend of the thermoanalytical curve.