Dual waste utilization in ultra-high performance concrete using biochar and marine clay

Abstract

Abstract Ultra-high performance concrete (UHPC) contains substantial amount of quartz powder (QP) which is expensive and carcinogenic. Partial replacement of QP with excavation wastes such as marine clays can help curtail its environmental impact. However, clays can degrade the hydration quality and increase the shrinkage in low w/c systems. A possible way to counter these drawbacks may be the use of an internal curing agent. For this purpose, another waste in the form of biochar (a product of pyrolysis of wood biomass) has proven to be an effective candidate. This study explores a novel strategy of dual waste valorisation in UHPC. The synergistic effects of biochar and marine clay towards hydration enhancement and shrinkage control in UHPC were investigated. Samples were prepared by replacing 30% by wt. QP with marine clay calcined at 700\u202f°C and adding biochar at 2% and 5% by wt. of cement, and investigated for changes in hydration, strength, and long-term shrinkage. Compared to the reference, samples with biochar showed up to 10% higher heat evolution at the end of 7th day. Degree of hydration (DOH) at 28 days increased from 51% in reference samples to 60% in samples with biochar. Shrinkage reduction up to 20% was observed in samples with biochar at 100th day. While substitution of QP with marine clay reduced the 28th day compressive strength by 10%, further addition of biochar showed no statistical change in strength. Addition of biochar caused up to 20% reduction in the compression toughness of UHPC compared to reference mix.