A Performance-based Eco-efficiency Framework for Green Concrete Incorporating Silica Fume and Recycled Aggregates

This study evaluates the combined use of silica fume (SF) and recycled coarse aggregates (RCA) in green concrete and proposes a CO₂-efficiency-based index to compare mixtures that considers both environmental impact and engineering performance.

Concrete mixtures were prepared with a constant water-to-cement ratio of 0.4. SF was used as a partial cement replacement at 5%, 10%, and 15%, while RCA replaced natural coarse aggregates at 25%, 50%, and 75%. A control mixture without replacement was used for comparison. Slump, compressive strength, splitting tensile strength, porosity, electrical resistivity, and water absorption were measured. Each result was normalized against the control mixture and combined with calculated CO₂ emissions to assess eco-efficiency.

SF improved strength and durability-related properties, mainly through matrix densification and pore refinement, with the best overall performance observed at 10–15% replacement. RCA reduced workability and progressively lowered strength and durability performance, especially at higher replacement levels, due to its higher porosity and weaker aggregate–paste interface. The eco-efficiency analysis showed that SF-containing mixtures achieved a better performance-to-emission balance, whereas RCA-only mixtures did not necessarily improve overall efficiency despite lower emissions.

The findings indicate that lower CO₂ emissions alone are not sufficient to define sustainable concrete. Performance loss must also be considered, particularly when high RCA contents are used.

The proposed index provides a practical basis for comparing green concrete mixtures. The results suggest that SF is the main contributor to improved eco-efficiency, while RCA should be used at controlled levels.