QUALITY-CONTROL OF COMMERCIAL PLASTERS BY THERMOGRAVIMETRY

Depending on the. manufacturer, some 1 kg bags of commercial plaster o f Paris have little holes in order to decrease their volume for storag e. Because this exposes the plaster to the humidity of the air, there is a partial conversion of the plaster to calcium sulfate dihydrate, c hanging the original quality of the product. This work has, as its obj ective, the development of methods to estimate the contents of calcium sulfate hemihydrate, calcium sulfate dihydrate and inert components o f the plaster by use of dynamic or isothermal thermogravimetry (TG). U sing dynamic TG the different contents can be measured directly by one analysis, otherwise, they can be measured by the combined results of the two isothermal analyses, of the original sample, and of a modified sample obtained after total hydration and a drying step of the origin al one. In isothermal analysis dehydrations of both salts occur simult aneously and there are insufficient data to calculate the contents by only one analysis. The methods and the results obtained for the two ty pes of plasters are shown and discussed, indicating that the analyzed samples have from 82 to 89 mass % of hemihydrate, a content from 5 to 10 mass % of the dihydrate and from 5 to 7 mass % of inerts. (C) 1998 Elsevier Science B.V.