Effect of Drying Protocols on Measurement of Sorption Isotherms of Gypsum Building Materials PDF

Sorption isotherms of a number of gypsum building materials have been measured using different drying protocols. The results show that the isotherms are sensitive to the initial drying conditions. The current ASTM test method recommends that gypsum materials be dried at 40°C (104°F), but the drying relative humidity is not specified. The intent of the relatively low drying temperature is to remove the hygroscopically bound water and to prevent the loss of chemically bound water. It was found that drying at 40°C (104°F) and about 0.2% relative humidity in an oven flushed with dry air results in loss of the chemically bound water, while drying at 40°C (104°F) and 11-12% relative humidity in a vented oven does not sufficiently dry the material. Drying at 23°C (73°F) and ~0.5% relative humidity as obtained in an oven or enclosure flushed with dry air or in a desiccator is recommended for these types of materials.