Hydrate Grains Research Articles

Electrical double layer, ion transport and reactions in hardened cement paste

In this paper positive ion concentrations in the Gouy-Chapman part of electrical double layers have been estimated both for calcium silicate hydrate and reactive silica grains when each of which is embedded in a hardened cement paste. The estimated positive ion concentrations, especially that of calcium ions, are much higher than that in corresponding pore water. These estimates together with some experimental results indicate that in low water/cement ratio pastes most, if not all, positive ions are transported through these overlapping double layers. This preferential accumulation of calcium ions in the electrical double layers together with their normal mobility can explain a number of unexplained observations, e.g. rapid growth of calcium hydroxide crystals in cement paste, calcium and alkali ion transport to reactive silica grains etc. of cement chemistry. This high concentration of calcium ions in the double layer also explains why calcium ions co-diffuse together with other ions in any diffusion of salts through hardened cement pastes. Attention has also been drawn to the implications of the double layer characteristics to other aspects of cement chemistry. These estimations show that pore water analyses could not be used directly, without further processing, for any interpretive work.