Biochar in cementitious composites: A comprehensive review of properties, compatibility, and prospect of use in sustainable geopolymer concrete

Geopolymer concrete (GPC) finds great potential in reducing global warming emissions from the construction sector. The conventional GPC precursor binders (fly ash, slag, metakaolin, silica fume, etc.) and alkaline activators are expensive, pollution-causing, and insufficient to meet the global requirement of concrete. Biochar (BC), agricultural waste produced through pyrolysis has rapidly been employed in cementitious composites since last decade because of its low cost, low carbon footprint, and ecological advantages. This paper presents a detailed review of the compatibility of BC with GPC, the properties (physical, chemical, and mechanical) in cementitious composites, and the chemical suitability of BC with GPC. The physical and chemical properties of BC can be controlled and highly depend on its production method (temperature, pressure, heating rate, resident time). The BC rich in silica and alumina is a feasible alternative to the solid precursor in geopolymer composites by controlling the particle size and mix design. The formation of a highly alkaline BC with water ensured its compatibility as an alkaline activator solution. They revealed comparable strength to GPC produced using conventional activators. Future studies are needed to investigate BC's experimental and practical applications as a precursor and alkaline activator in geopolymer composites.