Interrelationship of Thermal Effects on Calcium Sulfate Dihydrate Casts Containing Borax and Kaolin: Differential Thermal Analysis (DTA), Thermogravimetric Analysis (TGA), and Contact Dilatometry

The core composition of gypsum wallboard is calcium sulfate dihydrate (CaSO4·2H2O) with varying impurities, additives, or both. This study compares two commercially calcined sources of hemihydrate (CaSO4·½H2O) from a natural source and a synthetic by-product of flue gas desulfurization and neutralization to reagent grade hemihydrate. Two common ingredients, borax and kaolin, are mixed into a slurry with distilled water. The analysis supports the hypothesis that minor components in the cast have an effect on the high temperature performance of gypsum casts. The analysis is enhanced when the differential thermal analysis, thermogravimetric analysis, and dilatometry data are combined to study the changes in density versus heat flow. Specifically, the thermal performance is affected by (1) the impurities found in hemihydrate sources; (2) during the fluidization phase, the reaction of borax with free Ca++ ions to form new borate salts that melt at lower temperatures; and (3) the intercalation of these and other ions with kaolin, providing thermal stability by reducing the formation of thermally active salts.