Balancing sustainability and strength in concrete: the combined effects of recycled aggregates, silica fume, and nylon fibers

Concrete, widely used in construction, is a major source of greenhouse gas emissions and resource depletion due to its reliance on cement and NCA. This study evaluates the combined effects of fully replacing NCA with RCA, partially substituting cement with SF, and incorporating NWF on concrete’s engineering performance, durability, and environmental footprint. A three-way ANOVA confirmed highly significant main effects of RCA, SF, and NWF on CS ( p < 10 − 9 ) and STS ( p < 10 − 8 ); 100% RCA reduces mean CS from 76 to 61 MPa, while the synergistic addition of 10% SF and 0.25% NWF recovers 9 MPa of strength. Durability tests reveal that 100% RCA increases water absorption by 25%, but that SF–NWF combinations reduce this penalty by up to 0.48%. CIP doubles under RCA (13–26 mm) yet decreases by 7 mm with SF–NWF synergy, and sulfuric-acid mass loss rises by 3% with RCA but is lowered by 0.6% when additives are applied. A cradle-to-gate life cycle assessment using SimaPro 9.5.0.2 with Ecoinvent 3.10 and the TRACI method shows RCA alone reduces ODP by 18%, GWP by 6.6%, ACP by 8%, and EP by 10%; coupling with 10% SF and 0.1% NWF amplifies these reductions to 25%, 15.7%, 13%, and 16.5%, respectively. Heatmap and sensitivity analyses consistently identify 10SF-100RCA, 10SF0.1N100RCA, and 10SF0.25N100RCA as optimal mixes for balanced performance and sustainability.