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

When chloride ions diffuse through concrete, it has been found that they in teract with the surface charge and electrical double layer developed at the cementitious matrix/pore solution interface. As a consequence of this inte raction, 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 take n 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 p ores that are accessible to the migrating ions. At high applied voltages, i t might even be that the normal electrostatic interactions that influence i onic diffusion are nullified so that chloride ions can traverse pores of si milar 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 c oefficient' obtained for chloride. To investigate whether, during a steady- state migration rest, chloride ions diffuse through the essentially the sam e range of pores as during a natural steady-state diffusion test, experimen tal studies (oxygen diffusion, natural diffusion and migration tests) have been carried out with a limited range of cement pastes. The results indicat e that, at the recommended applied voltage(l? V) and for cementitious mater ials of the type studied, steady-state migration tests can be used to deter mine 'effective diffusion coefficients' that are of similar magnitude to th ose obtainable from natural steady-state diffusion measurements. (C) 2001 E lsevier Science Ltd. All rights reserved.