Mesoscale Simulation of Chloride Ion Diffusion in Nanosilica-Modified Recycled Coarse Aggregate Concrete

The inherent defects of recycled coarse aggregate (RCA) cause recycled aggregate concrete’s (RAC) porosity to rise and its durability to fall. To increase RAC’s resilience, nano-SiO2 (NS) modified RAC (NS–RAC) specimens were made by impregnating RCA with a 2% NS dispersion for 48 h. A comparative test of chloride ion diffusion was conducted with RAC and natural aggregate concrete specimens. The test results demonstrated that NS can successfully increase RAC’s resistance to chloride ions, with the chloride ion concentration at a 5-mm depth being 48.46% lower than that of RAC at 30 days. Considering the chloride ion diffusivity coefficient of six phases in NS–RAC, a microscale model was established to predict the diffusion behavior of chloride ions in NS–RAC based on Fick’s second law. The numerical simulation results indicate that as the width of the interface transition zone (ITZ), the volume percentage of RCA, and the substitution rate increased, the diffusion depth of chloride ions also increased to varying degrees. The results of gray correlation analysis show that the greatest impact on the diffusion of chloride ions was caused by the volume percentage of RCA in NS–RAC. The damage caused by adjacent double-sided chloride ion erosion to concrete cannot be ignored. This study helps in comprehending the erosion mechanism of chloride ion diffusion in NS–RAC and provides recommendations for improving the endurance of RAC in chloride-rich areas.