Investigations of surface forces between gypsum microcrystals in air usingatomic force microscopy

This article introduces a new approach to the study of the interactions bet ween gypsum faces in air. We have mounted a gypsum crystal on the end of a microcantilever to measure local forces (van der Waals, capillary forces, e lectrostatic) directly between two crystals. A systematic study with respec t to the orientation of crystalline faces, relative humidity, and the durat ion of contact between crystals improves the understanding of the mechanism involved in the hardening of plaster in air. In dry air, a physicochemical process of matter transfer is added to the van der Waals forces to ensure the cohesion. Others force measurements were performed by varying the shape of the contact (interaction between the various gypsum faces and a standar d silicon tip used in atomic force microscopy). Comparison between capillar y forces permits discussion of the conditions of wetting of each gypsum fac e in wet atmosphere.