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 water/cement ratio (w/c) 0·5 was exposed to 1 mol/l NaCl solution saturated with Ca(OH)2. The free hydroxyl, free chloride and total chloride concentration profiles were determined after 100 days of exposure by analysis of the paste at various depths from the exposed surface. Attempts were made to characterize these concentration profiles by Fick's laws. It was found that the total and free chloride profiles could be interpolated by Fick's second law to a fairly good approximation, but the hydroxyl concentration profile near the exposed surface deviated from such an interpolation. It was also confirmed that the relationship between free and bound chloride could be approximately represented by the Langmuir adsorption isotherm. The effect of chloride binding on chloride 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 experimentally determined hydroxyl concentration may be obtained from charge balance consideration after diffusion of all the other ions besides hydroxyl has been modelled.