Strength and microstructural behaviour of concrete with multi-stage treated recycled aggregates

This study proposes a novel three-stage processing technique to enhance the quality of recycled aggregates (RA) for structural concrete applications. The process consists of (i) soaking raw RA in mild acetic acid to weaken adhered mortar, (ii) controlled mechanical grinding using a Los Angeles abrasion machine to remove loose mortar, and (iii) surface densification through a cement–silica fume slurry coating to seal residual pores. Seven grades of triple-processed recycled aggregates (TPRA), namely, TPRA (0RVNs) to TPRA (600RVNs), were produced and used to replace natural aggregates (NA) at 0%, 20%, 40%, 60%, 80%, and 100% in M40-grade concrete. Experimental results indicated that complete NA replacement led to a 26.27% reduction in flexural strength, along with increased ISAT flow (up to 42%) and decreased ultrasonic pulse velocity values (up to 10%). Despite these reductions, durability performance improved significantly due to reduced permeability and increased electrical resistivity. Among all combinations, concrete incorporating 60% TPRA (500RVNs) exhibited the most balanced performance and is recommended for structural use. Life cycle assessment results showed that TPRA reduced global warming potential by approximately 25%–40% compared with NAs, although processing was associated with additional energy consumption and wastewater generation. Overall, the optimised TPRA system offers a practical compromise between mechanical performance, durability enhancement, and environmental sustainability.