[Re-examination of dehydration process of gypsum by micro-DTA (author's transl)]

The results obtained by macro-DTA were not reproducible because they were largely influenced by the packing method, the amount of samples and many other factors related to each apparatus. The dehydration process of gypsum was investigated by micro-DTA in this study. The results were compared with those of macro-DTA studies. Two endothermic peaks were generally observed. These peaks tended to overlap with the increase of particle size. As the water vapor pressure decreased, the two peaks shifted to lower temperature and finally overlapped completely. The effect of the amount of samples was very similar to that of the atmosphere. When the amount was less than about 1 mg, only a single peak was observed. The dehydration process of gypsum was discussed on the basis of a phase diagram of the system of CaSO4-H2O. Moreover, the activation energies of the first and second dehydration reactions were calculated by Kissinger's method. The former was about 25 kcal/mol and the latter was about 24 kcal/mol. They werein good agreement with those obtained by other workers.