From waste to worth: Biochar and hemp fiber synergy for carbon-sequestering and durable recycled aggregate concrete

Recycled aggregate concrete is prone to significant drying shrinkage, which hinders its broader adoption and the sustainable recycling of construction and demolition waste. This study presents a novel internal curing strategy using activated coconut biochar (10%–30%) and hemp fiber (10–30 mm) to simultaneously address shrinkage and strength deficits. Experimental findings demonstrate that mix-sized biochar improves both the fresh and hardened properties of the concrete when compared to using small or large biochar particles alone. Notably, while the incorporation of biochar reduces fluidity due to water absorption and increased inter-particle friction, both flexural and compressive strengths were enhanced – by up to 34% and 28%, respectively – with increasing biochar content and fiber length, as a result of matrix densification and fiber-bridging effects. The synergy between biochar and hemp fibers facilitates internal curing by lowering the local water–cement ratio during hardening and promoting cement hydration through both internal curing and nucleation effects. Despite a rapid early-stage increase in drying shrinkage, the combined addition of biochar and hemp fiber ultimately achieves a remarkable reduction of up to 96%. Furthermore, increasing biochar dosage and fiber length enhances matrix compactness, substantially reducing water absorption. This, in turn, improves resistance to chloride penetration by obstructing ion transport pathways. Concrete modified with 30% biochar and 30 mm-long hemp fibers demonstrates significant potential for carbon emission reduction, achieving decreases of 4.4 kg CO2 eq./m3/MPa in compressive scenario and 23.8 kg CO2 eq./m3/MPa in flexural scenario. Overall, this study advances the understanding of shrinkage mitigation mechanisms and offers a scalable pathway for the development of high-performance, low-carbon recycled aggregate concrete.