Chloride diffusion in cement materials at different leaching states : an experimental and numerical study

Due to common heavy snowfall in winter, concrete structures in cold countries are regularly in contact with water. Hence, concrete undergoes abnormal leaching of ionic species present inside the liquid pores of the material. The leaching slowly degrades the material, which can have an influence on the diffusive behaviour of the ionic species diffusing inside, like chloride, coming from the de-icing salts and responsible for the corrosion of the steel reinforcements present in the reinforced concrete. In this work, an experimental and numerical study is proposed to better understand the influence of the leaching state of cementitious material on the diffusive behaviour of chloride ions. Natural and accelerated diffusion tests were performed on samples of hydrated cement paste and concrete with lower chloride concentrations (50 mol/m³). The materials were conserved at different leaching state with care not to degrade the microstructure: Safe (S), Partially Leached (PL) and Leached (L). These various tests were then simulated using the PhreeqC reactive transport software to quantify the chloride diffusion. The numerical model allows distinguishing the influence of the geometry of the material (porosity, tortuosity, constrictivity…) to the chemical reactions that take place between chloride ions and the cementitious phases. It then appeared that chloride ions do not diffuse by the same way depending on the leaching state of the material. Same observations are done on hydrated cement paste and concrete samples. In a safe material, the chloride diffusion is lower than in a leached material. This difference observed seems not to provide from the material geometry nor the chemical reactions. The numerical model developed confirms this hypothesis as it allows to both distinguish and quantify their influence on the chloride diffusion. This study, therefore, highlights the importance of considering the leaching effects in the prediction model for concrete structure durability in cold countries