DIFFUSION OF CHLORIDE AND HYDROXYL IONS IN CEMENTITIOUS MATERIALS EXPOSED TO A SALINE ENVIRONMENT

One end surface of a cylindrical specimen of hardened OPC paste of wat er/cement ratio (w/c) 0.5 was exposed to 1 mol/l NaCl solution saturat ed with Ca(OH)2. The free hydroxyl, free chloride and total chloride c oncentration profiles were determined after 100 days of exposure by an alysis of the paste at various depths from the exposed surface. Attemp ts were made to characterize these concentration profiles by Fick's la ws. It was found that the total and free chloride profiles could be in terpolated by Fick's second law to a fairly good approximation, but th e hydroxyl concentration profile near the exposed surface deviated fro m such an interpolation. It was also confirmed that the relationship b etween free and bound chloride could be approximately represented by t he Langmuir adsorption isotherm. The effect of chloride binding on chl oride diffusion was determined by consideration of mass conservation, and this yielded a good characterisation of the free chloride profile. It is suggested that a reasonably accurate estimation of the experime ntally determined hydroxyl concentration may be obtained from charge b alance consideration after diffusion of all the other ions besides hyd roxyl has been modelled.