A study was made of changes in composition occurring in slabs of cast gypsum when subjected to standard tests of resistance to fire. The temperature of the unfired surfaces of the specimens rose rapidly for ∼55°c and then continued to rise but very much more slowly. Eventually, the temperature again began to rise rapidly and thermal failure occurred. In the cases of the thicker specimens tested, the increase in temperature during the period of slow rise was in the form of two steps separated by a period of faster heating. Observations were made on the composition of specimens heated to various extents. Dehydration was virtually complete at the beginning of the final rapid increase in temperature.
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King et al. (1971) studied this question.