Water diffusion and microstructure of hydrated cement pastes

Water migration in Portland cement paste follows closely the theoretical diffusion equation. The diffusion coefficient decreases with increasing curing time of the paste specimen at first, but after 14 days the value becomes nearly constant. The temperature dependence of the diffusion coefficient indicates that diffusion involves an activation process, but the characteristic energy depends on the temperature range. The behaviour of aluminous cement is similar, but the value of the diffusion coefficient and its temperature dependence are about double those for Portland cement. Stereoscan electron micrographs of aluminous cement show a temperature sensitive microstructure and this probably causes the change in the diffusion activation energy. No comparable clear change can be seen in Portland cement structure.