Multi-Species Modeling of Chloride Ingress in Heterogenous Recycled Aggregate Concrete: Bidirectional Effects of Old Mortar

The structural application of Recycled Aggregate Concrete (RAC) in marine and coastal structures remains restricted by its highly variable quality and uncertain durability. Although the adhered old mortar is recognized as the most distinctive feature of RAC, its bidirectional influence on chloride transport, acting as a preferential transport pathway and a chloride-binding reservoir, has not yet been systematically elucidated. This study develops a five-phase mesoscopic numerical framework (natural aggregate, new and old mortars, new and old ITZs) to investigate the bidirectional effects on chloride ingress. The proposed model involves multi-species (K+, Na+, Cl−, OH−, Ca2+, SO42−) coupling and thermodynamic chloride binding on AFm and C-S-H phases, with different binding capacities in old and new mortar. This model was validated against published experimental data, demonstrating high accuracy in predicting effective diffusivity across varying replacement rates. Parametric sensitivity analyses reveal that RAC’s chloride resistance is governed by the competition between the “facilitation effect”, caused by the inherent porosity in attached old mortar, and the “retardation effect”, caused by enhanced binding capacity. This work provides new mechanistic insight into the dual effects of old mortar and establishes a robust theoretical tool for the durability design of RAC structures exposed to chloride environments.