Chloride maximum phenomenon near the surface of cement paste induced by moisture evaporation and carbonation

Abundant studies of cyclic wetting-drying of concrete have suggested a maximum phenomenon by which chloride concentrations first increase with depth to a local maximum and then decrease at greater depths. This phenomenon may arise primarily because of carbonation, but may also be exacerbated by cyclic capillary suction and moisture evaporation during wetting-drying cycles. This paper investigates the mechanism responsible for the maximum phenomenon under conditions that exclude cyclic capillary suction. The results show that different chloride maxima form near the surface of all specimens under conditions of either accelerated carbonation or carbonation in normal air environment. A large amount of bound chloride is released when carbonation causes decomposition of Friedel's salt and C-S-H gel, and the decrease in water content significantly increases the chloride concentration in the pore solution near the specimen surface. The resulting chloride concentration gradient drives free chloride diffusion from the surface to the interior, thereby reducing the actual chloride content near the exposed surface. More severe carbonation conditions increase the severity of the maximum phenomenon.