Sustainable self-compacting concrete with agro-waste and recycled aggregates

Self-compacting concrete (SCC) flows under its own weight without external vibration, making it suitable for complex structural applications. In response to growing global demands for sustainable construction materials and the need to reduce reliance on natural aggregates in line with circular-economy principles and the UN Sustainable Development Goals, this study investigates coconut shell (CS), palm kernel shell (PKS), and recycled coarse aggregate (RCA) as eco-friendly alternatives in SCC production. The materials used included Portland limestone cement, natural sand, granite dust, potable water, a superplasticiser, and locally sourced CS, PKS, and RCA from Nigeria. Thirteen mixes were designed using Central Composite Design (CCD), with water–cement (w/c) ratios between 0.45 and 0.60 and admixture dosages up to 1.5%. Concrete specimens were prepared, cured, and evaluated for rheological properties (slump flow, L-box, V-funnel) and mechanical performance (compressive and split tensile strength) in accordance with EFNARC and BS EN standards. The optimised mixtures were coconut-shell SCC (CSSCC: 0.75% admixture, 0.60 w/c), achieving 22.88 MPa compressive and 2.91 MPa tensile strength; recycled-aggregate SCC (RCASC: 0.53% admixture, 0.60 w/c) with 20.13 MPa and 2.62 MPa; and palm-kernel-shell SCC (PKSSCC: 1.49% admixture, 0.60 w/c) with 18.14 MPa and 2.42 MPa. Results showed that balanced admixture dosage and appropriate w/c ratios enhanced both workability and strength, while excessive admixture or very low w/c ratios reduced performance. The findings provide practical mix design strategies for producing sustainable SCC using agro-waste and recycled aggregates, supporting both environmental conservation and resource-efficient concrete production.