Behaviour of crystallised phases of Portland cement upon water attack

The changes occurring in the crystallised phases during degradation of an ordinary Portland cement have been studied. Leaching of cement paste with demineralised water generates a series of dissolution boundaries. Portlandite, then AFm, ettringite and calcite dissolve successively In the zones with no change in mineralogy, local chemical equilibrium and decreasing gradients in calcium and hydroxyls between the core and the surface explain the precipitation of the secondary phases, like AFm, ettringite and calcite. Hydrogarnets in the surface layer in contact with the aggressive solution only dissolved slightly, or not at all. The very low solubility of hydrotalcites at pH values near neutrality explains the precipitation of these magnesium-containing phases in the surface layer.