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 wat er/cement ratio 0.5 was cemented to that of a similar specimen which c ontained 2 % chloride ion by weight of cement, added as sodium chlorid e. 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 sod ium concentration profile could be interpolated by Fick's Second Law t o a good approximation. By assuming a Langmuir-type relationship betwe en the free and bound chloride, the effect of chloride binding on diff usion was incorporated into diffusion equations derived from Fick's Fi rst Law and mass conservation. This also yielded a good characterizati on of the free chloride profile. The concentration profile of hydroxyl ions in the pore solution was more complex than those for sodium and chloride ions, but it could be modelled by assuming that hydroxyl ions move simply to preserve the charge balance in response to the movemen ts of other diffusing ions in the system.