REACTIVITY OF GYPSUM FACES ACCORDING TO THE RELATIVE-HUMIDITY BY SCANNING FORCE MICROSCOPY

This article reports the experimental observation of the stability of the different Faces of calcium sulphate dihydrate (gypsum CaSO4.2H(2)O ) according to the relative humidity. Scanning Force Microscopy experi ments were carried out with a view to discerning the topography of the surfaces, the chemical compositional domains, and in an attempt to ev aluate the friction and viscoelastic propel ties of the surface. Our r esults indicate that the (010) Face of gypsum is hydrophilic and very reactive contrary to the less hydrophilic (120) and (<(1)over bar 01>) faces which remain stable depending on the relative humidity. It is c lear from our results that a precipitation like process can be induced by the SFM tip. The dissolution of this precipitate depends on the am ount of water in the meniscus between the probe and the surface. For t he purpose of comparison, the cleaved surfaces of natural anhydrite (C aSO4), calcite (CaCO3), and muscovite mica were also observed. The Lat eral Force Microscopy images provided the undisputed evidence that the process of dehydration of the (010) face actually occurs with a chang e of the elastic constant of the crystal. (C) 1997 Elsevier Science B. V.