THE DIFFERENTIAL THERMAL ANALYSIS OF NATURAL AND SYNTHETIC HYDRATES OF CALCIUM SULPHATE

A variety of naturally occurring forms of calcium sulphate dihydrate produce substantially identical differential thermograms under conditions of uniform heating rate and particle size. These thermograms closely resemble those for synthetic dihydrate and β-hemihydrate, showing four endothermic effects and one exothermic effect below 500 °C. Two of these endothermic effects, at about 170° and 300 °C, respectively, which have not been reported previously, were found to be easily masked by changes in heating rate or sample concentration. Resolution of the several effects was improved by using different heating rates at different stages of the thermogram. Thermograms of the α-hemihydrate were similar, except that the endothermic effect at 300 °C was not evident and the exothermic effect occurred at a much lower temperature than for the dihydrate. No conspicuous differences were found in the temperatures corresponding to the various endothermic and exothermic effects which might be correlated with the general dehydration behavior of the particular material. The endothermic effect at about 170 °C appears to be associated with part of the hemihydrate to soluble anhydrite transition, possibly arising during the removal of the last traces of water.