Multiscale performance and carbon efficiency of fiber reinforced recycled aggregate concrete with metakaolin and kinetics constrained neural network modeling

Recycled coarse aggregate (RCA) can reduce virgin aggregate demand, but adhered mortar can impair concrete performance, while treatment introduces additional environmental burdens. However, whether RCA treatment and polypropylene-fibre reinforcement provide a net benefit when mechanical performance, environmental burden, and age-dependent strength development are evaluated within a single framework remains insufficiently resolved. This study evaluated 18 concrete mixtures using NCA, RCA, and TRCA, with 0–1.25 vol% polypropylene fiber, 50 kg metakaolin, 500 kg cement, and a water-to-binder ratio of 0.40. A total of 162 cylinders were tested for compressive strength, with three specimens per mixture at 7, 28, and 90 days. Fresh properties, compressive strength, qualitative SEM of selected RCA mixtures, comparative screening cradle-to-grave LCA, efficiency indices, statistical analysis, and a Kinetics-Constrained Neural Network (KCNN) were evaluated. Strength followed NCA > TRCA > RCA. The 1.00% fibre dosage was the highest-performing level, increasing strength by 19.9–37.8% relative to non-fibrous mixtures, whereas 1.25% reduced strength. SEM qualitatively identified an aggregate-paste interfacial region in R-0 and polypropylene-fibre embedment in R-1.00. Series-average GWP ranged from 582.67 to 584.57 kg CO₂-eq/m³ across the experimental aggregate systems, with differences below 0.35%, while the modeled all-cement comparator reached 609.84 kg CO₂-eq/m³. Monte Carlo P2.5-P97.5 endpoints ranged from 530.38 to 637.50 kg CO₂-eq/m³, emphasizing the uncertainty associated with background inventory selection. At 1.00% fibre, NCA showed the greatest binder and carbon efficiency, while TRCA partially recovered RCA efficiency losses. Across 25 seeds under mixture-grouped OOF validation, the proposed KCNN achieved mean R 2 = 0.963 ± 0.002 , MAE = 1.345 ± 0.052 MPa, and RMSE = 1.658 ± 0.049 MPa. Overall, the findings support evaluating RCA treatment and fibre reinforcement through their combined effects on strength recovery, carbon efficiency, and age-dependent performance rather than compressive strength alone.