Multi-Objective Optimization of Low-Carbon Concrete Mix Designs Incorporating Recycled Aggregates and Supplementary Cementitious Materials

The global construction industry faces escalating pressure to reduce its environmental footprint, as conventional concrete production accounts for approximately 8% of anthropogenic greenhouse gas emissions. This research presents a comprehensive multi-objective optimization framework for designing sustainable, structural-grade concrete incorporating recycled concrete aggregates (RCA) and ternary supplementary cementitious materials (SCMs)—specifically ground granulated blast-furnace slag (GGBS) and pulverized fly ash (FA). A response surface methodology (RSM) integrated with an elitist non-dominated sorting genetic algorithm (NSGA-II) was implemented to simultaneously balance three competing objective functions: maximizing 28-day compressive strength, minimizing cradle-to-gate embodied carbon emissions (kg CO₂-eq/m³), and minimizing unit production cost ($/m³). Experimental validation across 30 distinct mix configurations demonstrated that synergistic binder combinations effectively mitigated the mechanical degradation typically caused by the porous interfacial transition zones (ITZ) of RCA. The derived Pareto-optimal solutions identified an optimal mixture containing 45% RCA, 35% GGBS, and 15% fly ash, achieving a 28-day compressive strength of 42.6 MPa while delivering a 47.3% reduction in embodied carbon and an 18.5% cost reduction compared to a benchmark CEM I concrete. The proposed optimization framework provides structural engineers and ready-mix producers with a mathematically rigorous, adaptable tool for formulating economically viable, eco-efficient concrete without compromising structural integrity.