Impact of calcium leaching on chloride diffusion behavior of cement pastes exposed to ammonium chloride aqueous solution

Calcium leaching is a procedure being able to greatly impact the internal chemical environment and the microstructure, which mainly decide the chloride diffusion behavior of concrete. However, the detailed rules and mechanisms were rarely studied in the current literatures. In this paper, pure cement pastes were casted for the immersion test. Ammonium chloride aqueous solution was employed as the source solution to simulate the coupling interaction of calcium leaching and chloride diffusion. Vickers hardness and chloride profiles were measured to present the effects of calcium leaching on chloride diffusion behavior. Morphology and EDS linear scanning results of degraded pastes were also given to reveal the micro-mechanism. Results show that leaching can greatly change the internal chemical environment and the microstructure of cement pastes, thereby changing the chloride diffusion behavior and degrading the chloride-ingress resistance. In the leached specimens, chloride profiles are remarkably different from the classical curves predicted by Fick's 2nd law. Losing the binding matrix, the total chloride contents of the degraded zones turn smaller and almost approximate to the water-soluble ones. Due to the microstructure degradation, chloride profile fronts in degraded zones move faster towards the deep.