Durability and service-life performance of recycled-aggregate concrete blended with fly ash and ground-granulated blast-furnace slag under multi-ion environmental exposure

The construction industry consumes vast quantities of natural aggregate while generating enormous volumes of construction and demolition waste, motivating the structural use of recycled coarse aggregate (RCA). However, the porous adhered mortar and weak interfacial transition zone of RCA degrade transport-controlled durability, limiting field adoption. This study reports an integrated experimental and modelling programme on recycled-aggregate concrete (RAC) in which ordinary Portland cement is partially replaced by fly ash (FA) and ground-granulated blast-furnace slag (GGBS). Four binder systems (OPC, 30% FA, 50% FA, 50% GGBS) were cast at RCA replacement ratios of 0, 30, 50 and 100% and characterised for compressive strength, non-steady-state chloride migration, accelerated carbonation, and sulfate wet-dry resistance up to 180 d. A response-surface model linked binder chemistry and water-to-binder ratio to strength, and a Fickian reliability framework translated migration data into probabilistic service-life estimates. GGBS blends recovered most of the strength penalty of full RCA replacement and, together with 50% FA, reduced the 90-day chloride migration coefficient by more than 70% relative to the OPC control, offsetting the durability deficit introduced by RCA. Carbonation depth increased with SCM dosage, revealing a design trade-off between chloride and carbonation resistance. The probabilistic analysis indicates that a 50% GGBS binder at 50% RCA replacement can meet a 100-year design service life at 50 mm cover in a marine splash environment. The results provide a quantitative mix-design basis for durable, low-carbon structural concrete.