Disjoining Pressure in Highly Dispersed Systems and Shrinkage

Under accurate climatic conditions viz, temperature and dew point +/- 0,2K over more than 6 months mass change (+/- 10 mu g) and length change (+/- 20nm) were measured simultaneously at small hcp specimens starting at non-dried nascent state for nascent desorption followed by adsorption desorption cycles. A mutual interaction between pore water and solid in the sub-microscopic gel system of hardened cement paste is proved. The attractive, van der Waals part of disjoining pressure is compensated by the repulsive structural and electrostatic terms attributed to gel pore water. With the evaporation of gel water the repulsive terms vanish and the sensitive equilibrium is lost. The system shrinks until balanced by elastic response. When the thickness of the remaining adsorbed layer reduces the surface energy increases additionally now mostly regulated by the change of the van der Waals term. During the metastable desorption a small part of shrinkage can be attributed to capillary pressure vanishing with evaporation. The gel water is highly structured by surface interaction. Following He-pycnometry the density of pore water is at first near to bulk water and increases in later adsorption - desorption cycles to high values between 1.15 g/cm(3) to 1.25 g/cm(3). The heat of fusion is reduced by app. 30%. Results starting with specimen in nascent status show that the first desorption is substantially different from later humidity changes.