Bio Char-modified Concrete: A Review of Fresh and Mechanical Properties

Abstract

The construction sector sits at the heart of one of the world's most pressing the construction industry is one of the leading contributors to global carbon emissions, accounting for approximately 40% of energy-related CO₂ output, with cement production alone responsible for nearly 8% of total worldwide emissions. Facing mounting pressure to decarbonize, the sector has turned increasing attention toward supplementary cementitious materials (SCMs) — alternatives that can partially replace ordinary Portland cement (OPC) while preserving, or even improving, structural performance. This review examines bio char as one such alternative — a carbon-rich, highly porous material produced through the thermochemical conversion of biomass under oxygen-limited conditions. What sets bio char apart from conventional SCMs is its potential to be genuinely carbon-negative: rather than merely reducing emissions, it can actively sequester atmospheric carbon within hardened concrete, making it a strategically important material for the global push toward net-zero construction by 2050. Drawing on a broad body of recent experimental literature, the paper systematically traces bio char’s journey from production — through pyrolysis (300–800°C), gasification (above 700°C), or hydrothermal carbonization (180–250°C) — to its performance in cementitious systems. Key physicochemical characteristics examined include particle size, porosity, specific surface area (5–400 m²/g by BET), elemental composition, pH, and surface functional groups, all of which govern how bio char interacts with the cement matrix. The experimental evidence is encouraging. At optimum replacement levels — generally between 1% and 5% by weight of cement — bio char consistently improves compressive strength by up to 18.5% and enhances flexural performance, primarily through internal curing, pore refinement, and accelerated cement hydration. Higher replacement levels reduce fresh concrete workability, but this effect can be effectively managed through superplasticizer optimization. Beyond mechanical performance, bio char’s capacity for CO₂ adsorption and long-term carbon locking distinguishes it from established SCMs such as fly ash and silica fume, and extends its potential applications to thermal insulation, fire-resistant composites, and energy-efficient building envelopes. Bio char properties vary considerably depending on feedstock type and pyrolysis conditions, making cross-study generalization difficult. Long-term durability data are limited, and the field still lacks standardized mix design protocols and quality benchmarks. This review maps these gaps clearly and identifies the research directions most critical for bringing bio char from laboratory promise into mainstream sustainable construction practice.