Experimental study on chloride ion diffusion behavior and microstructure in concrete under alternating ambient humidity conditions

The migration of chloride in concrete in atmospheric zones is mainly influenced by climatic conditions, e.g., various dry-wet cycles in marine tidal zones, and ambient humidity is one of the most important influences. Ambient humidity also determines the moisture content in concrete, which is necessary for the diffusion or transport of chloride in concrete and can enhance the migration of chloride in micropores, thus affecting the distribution of chloride in concrete. In this study, the effects of chloride diffusion and its mechanism, and the evolution of moisture in concrete under alternating drying-wetting cycles, were investigated using an environmental moisture simulation chamber. Chloride diffusion tests were also performed to evaluate the effects of different dry-wet ratios and numbers of drying-wetting cycles on the chloride diffusion depth, chloride content and saturation in concrete. The microstructure of concrete was studied and analyzed using XRD, SEM and MIP technology. The findings showed that the saturation of concrete decreased rapidly with increasing number of wet-dry cycles and finally stabilized. Under different alternating moisture conditions, the saturation of concrete was greater, especially when the wetting time was longer. Additionally, in areas near the surface of concrete (depths of 2 mm-6 mm), the chloride content was proportional to the number of wet-dry cycles. In the interior of concrete (depth > 6 mm), when the number of drying-wetting cycles was larger, the chloride concentration was smaller. However, as the number of wet-dry cycles increased, this led to the decalcification of C-S-H gels, which resulted in the reduction in the number of C-S-H reticular gels with good crystallinity. A pore analysis showed that the cumulative pore size and the maximum probability pore size of concrete gradually increased with increasing number of wet-dry cycles, the deterioration of pore structure was more obvious, and the compactness of concrete gradually decreased, which accelerated the migration of chloride in the matrix. Overall, this study can provide reliable and valuable test data for the development and design of high-performance concrete for applications in atmospheric marine tidal zones.