Oxygen and chloride diffusion in cement pastes as a validation of chloride diffusion coefficients obtained by steady-state migration tests

Abstract

When chloride ions diffuse through concrete, it has been found that they interact with the surface charge and electrical double layer developed at the cementitious matrix/pore solution interface. As a consequence of this interaction, the diffusion of chloride ions is retarded in comparison with that of dissolved oxygen molecules, although the two species have very similar diffusion coefficients in infinitely aqueous dilute solutions. This is taken to imply that electrostatic, rather than purely steric, factors limit the ability of chloride ions to diffuse through pores below a critical radius. If an electrical field is applied to accelerate chloride transport through concrete, it is uncertain what effect this may have on the size range of pores that are accessible to the migrating ions. At high applied voltages, it might even be that the normal electrostatic interactions that influence ionic diffusion are nullified so that chloride ions can traverse pores of similar radii to those that are accessible to dissolved oxygen. This would be expected to give rise to an apparent increase in the value of ‘diffusion coefficient’ obtained for chloride. To investigate whether, during a steady-state migration test, chloride ions diffuse through the essentially the same range of pores as during a natural steady-state diffusion test, experimental studies (oxygen diffusion, natural diffusion and migration tests) have been carried out with a limited range of cement pastes. The results indicate that, at the recommended applied voltage (12 V) and for cementitious materials of the type studied, steady-state migration tests can be used to determine ‘effective diffusion coefficients’ that are of similar magnitude to those obtainable from natural steady-state diffusion measurements.