Investigating the Behavior of Self-Compacting Concrete Containing SCMs and Recycled Fine Aggregates

This study addresses the unresolved gap in quantifying how recycled fine aggregates (RFA) and metakaolin (MK) interact synergistically in self-compacting concrete (SCC), a relationship that previous works have studied only in isolation or without mechanistic justification. Twenty-five SCC mixtures containing MK (0–25%) and RFA (0–100%) were evaluated to generate a unified understanding of fresh behaviour, strength development, pore refinement, interfacial transition zone (ITZ) characteristics, and sustainability indicators. Results show that RFA alone increases water demand and reduces strength, while MK compensates for these losses by densifying the matrix and improving ITZ bonding, as confirmed through SEM analysis. An optimum synergy was identified at 15% MK and 50% RFA, producing a 28-day strength of ≈ 47.5 MPa close to conventional SCC (≈ 50.9 MPa). Unlike previous studies focused primarily on mechanical performance, this work integrates life-cycle assessment (LCA) and life-cycle cost (LCC) analysis, demonstrating 12–13% CO2 reduction and 6–10% cost savings. The findings establish a scientific framework that links microstructure–property relationships with sustainability performance, enabling more reliable use of high-volume RFA in SCC.