Quantifying the Relationship Between Environmental–Mechanical Parameters of Structural Concrete Incorporating Recycled Materials: A Decision Framework for Optimized Selection

Abstract The integration of recycled materials into structural concrete has been widely adopted to reduce environmental impacts. However, most studies investigate individual waste streams independently and rarely quantify environmental–mechanical connectivity within a unified comparative framework, making it difficult to identify optimal solutions for real-world application. As a result, decision-making regarding the selection of waste type and replacement proportion remains unclear. This study establishes a quantitative environmental–mechanical assessment framework to directly compare single-material replacement scenarios and identify the optimal waste composition and replacement ratio. Four readily available recycled materials in Australia: recycled crushed glass (RCG), recycled concrete aggregate (RCA) 10 mm and 20 mm, and coal bottom ash (BA) were individually incorporated at replacement levels of 10%, 20%, and 30%. Comprehensive material characterization (XRD, XRF, FTIR, and microstructural analysis) was conducted to interpret test data. Mechanical performance was evaluated via compressive strength testing, while a cradle-to-gate life-cycle assessment was performed using Australian databases (AusLCI v1.33 and Ecoinvent v3.12) considering global warming potential, acidification, eutrophication, abiotic depletion, and water scarcity. Results show that although bottom ash achieved the greatest reduction in global warming potential, strength decreased disproportionately beyond 10% replacement. Aggregate-based replacements demonstrated more stable mechanical behavior, with limited environmental variation. Among structurally feasible mixes, RCA 10 mm with 20% replacement achieved the optimal environmental–mechanical balance index.