Towards the Understanding of the pH Dependency of the Chloride Binding of Portland Cement Pastes

Hydrated Portland cement paste exposed to a NaCl solution was acidified by adding HCl in small steps, gradually lowering the pH. The chloride binding of the cement paste changed as a function of the pH. For the range of pH from 13.2 to 12.2, decreasing pH resulted in a considerable increase in the chloride binding. At a pH of 11, the cement paste showed almost no chloride binding. In order to explain the changes in chloride binding upon lowering the pH, the phase assemblage was investigated with SEM-EDS, TGA and XRD and compared to a thermodynamic modelling.