Jaswant Singh, Rajeev Roychand, Al-Muataz Hamood Said Mohammed Al-Aghbari, J. Li, Mohammad Saberian, Shannon Kilmartin-Lynch
This review examines biochar production technologies and their strategic integration into cementitious materials, establishing fundamental relationships between production parameters, biochar characteristics, and construction performance outcomes. Thermochemical conversion processes, including slow pyrolysis, fast pyrolysis, microwave pyrolysis, hydrothermal carbonization, and gasification, exert primary control over biochar functionality through temperature-dependent transformations. Process temperature governs carbon content, specific surface area, pore structure evolution, and alkalinity, thereby determining material suitability for cement applications. Among conversion methods, slow pyrolysis emerges as optimal for construction applications, maximizing biochar yield while developing favourable pore architectures. Feedstock composition introduces secondary modulation of performance characteristics; lignin-rich materials demonstrate superior yield potential, with kraft lignin achieving 50.36% conversion at 450°C compared to lower-lignin agricultural residues. When incorporated into cementitious systems, biochar modifies both fresh-state rheology and hardened-state mechanical properties through multiple concurrent mechanisms. At optimal dosages, appropriately selected biochars enhance compressive strength by 10-40%, flexural strength by 15-107%, and tensile strength by 5-25%, while simultaneously reducing density by 5-20%. These characteristics prove particularly valuable for lightweight structural applications. Performance enhancements arise from five interconnected mechanisms: micropore filling densification, internal curing hydration support, interfacial transition zone refinement, pozzolanic reactivity contributions, and microstructural reinforcement effects. Performance variations between biochar types prove substantial, with rice husk and bamboo-derived biochars consistently demonstrating superior properties, particularly when pyrolyzed above 500°C. However, biochar exhibits threshold-dependent rheological behaviour, necessitating systematic optimization of superplasticizer dosage, particle gradation, and surface treatment protocols to balance workability maintenance with targeted mechanical and durability performance objectives. • Over 180 scientific papers published in last 30 years have been reviewed • Production-to-performance relationships across five conversion technologies explored • Three application modes compared: cement/sand replacement and direct addition • Performance depends on feedstock, pyrolysis conditions, particle size, dosage, and application mode