Synergistic effects of DCFRP fibres, granulated blast furnace slag and oyster shell powder on the mechanical performance and failure behaviour of sustainable concrete

Abstract

This study systematically investigates the synergistic incorporation of discarded carbon fibre reinforced polymer (DCFRP) fibres, S105-grade granulated blast furnace slag (GBFS), and oyster shell powder (OSP) in C25 concrete. One control mixture and six modified mixtures were developed, incorporating a constant 1.0 vol% DCFRP fibre dosage, 5–10% GBFS as partial cement replacement, and 5–15% OSP as partial fine aggregate substitution. The fresh and hardened properties were evaluated through slump testing, 28-day compressive strength, axial compressive strength, splitting tensile strength, elastic modulus measurements, and failure mode assessment. 28-day microstructural characteristics were examined using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results demonstrate that moderate substitution levels produce pronounced mechanical enhancement within this multi-waste system. The optimal formulation (10% GBFS combined with 5–10% OSP) achieved a 22.4% increase in compressive strength and a 47.8% increase in splitting tensile strength relative to the control mix. Microstructural analyses indicate that these improvements are associated with secondary hydration induced by GBFS, improved particle packing from OSP, enhanced fibre–matrix interfacial bonding, and refinement of the interfacial transition zone (ITZ). In contrast, excessive OSP incorporation (15%) resulted in dilution effects and the persistence of unreacted calcite phases, leading to microstructural heterogeneity and reduced mechanical gains. These findings support the viability of carefully optimised multi-waste integration as a practical pathway for sustainable structural concrete production.