Abstract
The degree of III-CaSO4 to II-CaSO4 transition was estimated on wet specimens, 25mm in diameter and 50mm high, prepared from a gypsum-bonded investment for quick casting. After 30min from mixing the specimens were heated in a 700°C furnace until a specimen temperature of 200°C to 700°C was reached. The estimation was made by measuring mass changes of the specimens before and after heating. The transition reached 39% at 350°C. The same estimation method, when applied to a previous study, showed no transition to this temperature in dry specimens heated slowly (5°C/min). The large difference in transition found between the wet and dry specimens was attributed to the formation of α- and β-hemihydrate, respectively. At 350-450°C, the transition was 3% and 48%, respectively. The pronounced latter transition, generally identified by differential thermal analysis of gypsum, appears as the major investment shrinkage demonstrating its dry dehydration process.
