A critical review on functionalized biochar from solid waste: Advancing sustainable construction and CO₂ capture

The growing CO₂ emissions from cement production and solid waste present significant challenges to achieving carbon neutrality, demanding sustainable strategies for the carbon-neutral construction sector. Producing biochar-driven concrete via the thermochemical conversion of organic waste offers an eco-friendly approach to mitigating CO₂ emissions from cement production while simultaneously addressing the challenge of large-scale solid waste management. The establishment of biochar as a carbon-negative product makes it a promising alternative to traditional cementitious materials in cement-based products. Additionally, porous biochar can adsorb CO₂, which can react with calcium hydroxide (Ca(OH)₂) in cement and form stable calcium carbonate (CaCO₃) through an internal carbonation process. However, despite these environmental benefits, high doses of unmodified/pristine biochar often compromise concrete's mechanical properties, limiting its practical application. Therefore, this review critically evaluates the development of functionalized biochar, focusing on physical and chemical activation methods to overcome these limitations. Particular emphasis is placed on how functionalization enhances biochar's compatibility with cement and significantly boosts its CO₂ adsorption capacity. To provide a deeper understanding of the literature on this topic, this review first systematically analyzed a comprehensive range of contemporary research on modification methods and the performance of both modified and unmodified biochar, with and without carbonation, in cementitious materials, highlighting their impacts on carbon-capture potential and mechanical properties. Subsequently, various limitations and challenges associated with the application of functionalized biochar in construction materials are highlighted. Finally, the review suggests key areas for future research to optimize biochar applications, aiming to improve mechanical performance, and maximizing carbon capture in waste management systems.