Correlating surface forces with surface reactivity of gypsum crystals by atomic force microscopy. Comparison with rheological properties of plaster

Atomic Force Microscopy (AFM) was used to image the surface reactivity as well as to characterize quantitatively the surface forces between two gypsum (CaSO4, 2H2O) crystals. Measurements on different crystal faces, which vary in morphology, structure, hydrophilicity, surface charge, were performed in both air and ionic solutions. In ionic solutions, varying the experimental parameters, the ionic nature and the concentration as well as the duration of the contact leads to the conclusion that the adhesion occurs whatever the orientation of faces. Nevertheless, the magnitude of the adhesion of a physical nature (Van der Waals and ionic correlation) depends on the surface charge density of each face while its increase is related to the growth of the contact area between the two crystals. In air, the systematic determination of surface potential with respect to the orientation of crystalline faces, the relative humidity and the duration and the area of contact between gypsum crystals suggests three types of physical forces: the Van der Waals forces, the electrostatic and capillary forces. In addition, the reactivity study displayed a reactive process of matter transfer, which is added to the surface forces to ensure the adhesion. Correlation between AFM results related to the micrometer scale and rheological measurements, performed on plaster (CaSO4, 0.5H2O) suspensions (pastes) and on macroscopic hardened plaster beams, is also demonstrated.