Low carbon biochar amended fly ash–cement building blocks: Assessment of metal leaching scenarios

Abstract Demolition cementitious waste poses significant environmental challenges at the end of its lifecycle. To address this, fly ash (FA), a highly leachable material and a supplementary cementitious material, was combined with biochar (BC) to produce eco-friendly mortar bricks with reduced carbon emissions and contaminant leaching. BC was incorporated at 2%, 4%, and 6% by weight, and the resulting blocks achieved compressive strengths of 8–12 MPa after 28 days, meeting Eurocode 6 standards for use in harsh conditions. Leaching tests under synthetic precipitation showed reductions in Al, Se, Ba, and Cr concentrations by 72%, 48%, 58%, and 53%, respectively, with 6% BC. While Al remained above drinking water limits, Cr levels dropped below limits when BC exceeded 4%. Leaching followed typical pH-dependent behaviour: Al exhibited an amphoteric trend, and Cr showed an oxyanionic trend, with minimal leaching at neutral pH. This study highlights the role of BC in reducing leaching potential in cementitious composites and provides critical data for geochemical modelling in sustainable demolition waste management systems. Graphical Abstract