Ionic diffusion across an interface between chloride-free and chloride-containing cementitious materials

One end surface of a cylindrical specimen of hardened OPC paste of water/cement ratio 0·5 was cemented to that of a similar specimen which contained 2% chloride ion by weight of cement, added as sodium chloride. The total chloride contents, as well as the concentration profiles across the interface of potassium, sodium, chloride and hydroxyl ions in the pore solution phase, were determined. It was found that the sodium concentration profile could be interpolated by Fick's Second Law to a good approximation. By assuming a Lungmuir-type relationship between the free and bound chloride, the effect of chloride binding on diffusion was incorporated into diffusion equations derived from Fick's First Law and mass conservation. This also yielded a good characterization of the free chloride profile. The concentration profile of hydroxylions in the pore solution was more complex than those for sodium and chlorideions, but it could be modelled by assuming that hydroxyl ions move simply to preserve the charge balance in response to the movements of other difising ions in the system.