THERMOGRAVIMETRIC EVALUATION OF THE KINETICS OF THE GYPSUM HEMIHYDRATE SOLUBLE ANHYDRITE TRANSITIONS

Study of the gypsum-hemihydrate-soluble anhydrite transitions by thermal, X-ray and IR methods showed differences in the intensity of the approximately 3493 cm-1 IR absorption peak of the gypsum samples and differences in the peak ratios of the DTA curve at the gypsum-hemihydrate transition. There were also differences in the temperature and rate of the gamma - beta anhydrite transition. This suggests that different gypsum species occur, specially among synthetic gypsum. Fifteen equations were tested in order to find models which fitted the gypsum-hemihydrate and the hemihydrate-anhydrite transitions. No model fitted all the samples. The best fit for the gypsum-hemihydrate transition in three samples was an order of reaction equation while for the hemihydrate-anhydrite transition the best fit in four samples was a power law. Differences in crystallite characteristics appear to be one of the main reasons for the differences in kinetics between the samples.