Multiscale chloride resistance of recycled concrete with fly ash and ground granulated blast-furnace slag

Chloride attack is a major cause of durability deterioration in recycled aggregate concrete (RAC). This study investigated the synergistic mechanism of fly ash (FA) and ground granulated blast furnace slag (GGBS) in RAC through mechanical testing, scanning electron microscopy (SEM)-based microstructural analysis and COMSOL-based multiphase diffusion modelling. FA20% and GGBS20% increased the 28-day compressive strength of RAC by 7.1% and 14.0%, respectively, and reduced the effective chloride migration coefficient by 35% and 41%, respectively. The binary FA10%+GGBS20% blend showed the best overall performance, with an 8.7% strength increase and a 52.6% reduction in the chloride migration coefficient. SEM observations revealed a denser calcium silicate hydrate (C–S–H) gel structure and a refined interfacial transition zone in the blended system, consistent with the numerical results. The 730-day simulation confirmed the durability ranking of RAC < FA20% < GGBS20% < FA10%+GGBS20%, demonstrating pronounced synergy against chloride penetration. Compared with conventional concrete NC55, the unit-volume carbon dioxide emissions and direct material cost of RAC with 10% FA and 20% GGBS at a water-to-binder ratio (w/b) = 0.55 (C55-F10S20) were reduced by 29.74% and 27.13%, respectively. Overall, the combined use of FA and GGBS can improve the durability, environmental performance and economic efficiency of RAC.