Mobility of chloride through cement-based materials under partially saturated conditions

This work presents two complementary techniques to study 36Cl and stable chloride diffusion through unsaturated cement-based materials for saturation degrees (Sw) ranging from 0.18 to 1. The first technique is based on steady-state through-diffusion method where suction (up to 9 MPa) is controlled using osmosis process. In the second technique, in-diffusion experiments are performed by putting into contact a spiked-fresh-cement paste with a CEM-V/A-hardened-cement paste (HCP) sample (initial water to cement ratio of 0.4) having a given water saturation imposed by saline solution method. After a controlled diffusion time, the 36Cl activity profiles within the samples are acquired by means of the abrasive peeling method and 36Cl activity is determined by liquid scintillation counting (LSC) technique. The application of the first technique resulted in data showing a sharp decrease of the effective diffusion coefficient (De) (by a factor of ten) between the experiments done on the fully-saturated HCP sample and the HCP sample at Sw = 0.85. Moreover, duplicate in-diffusion experiments carried out on HCP samples partially-saturated at Sw = 0.86 provided 36Cl De range well bracketing the one determined by the first approach at Sw = 0.85. Four other in-diffusion experiments were thus performed on HCP samples with saturation degrees down to 0.18. Interpretation of the experimental results showed a very small decrease of De values for Sw from 0.74 to 0.41 and a sharp decrease for Sw from 0.41 to 0.18. Lastly, comparison with the diffusive behavior of tritiated water (HTO) through unsaturated CEM V/A HCP samples clearly indicated that diffusion of chloride was more affected by desaturation than HTO. Its ionic form would restrict chloride to the liquid phase whom continuity is broken when dehydration is strong enough.