Biochar’s Promise for Carbon-Neutral Construction Materials
Cement production for concrete is one of the main reasons why the building industry contributes significantly to carbon dioxide emissions. This paper investigates an innovative approach to utilizing CO2 by incorporating mixed biochar in mortar. Various dosages (0%, 3%, 5%, and 10%) of mixed biochar were explored to assess their impact on the structural properties and environmental sustainability.
Biochar, a carbon-enriched sustainable material obtained through the pyrolysis of biomass, has gained increasing popularity among researchers due to its unique properties like large surface area, high porosity, and chemical stability. These characteristics make biochar a versatile material suitable for various applications, including construction materials.
Incorporating biochar into cement-based composites can boost the carbon-capturing ability and enhance the mechanical characteristics of cement by speeding up the hydration process. Biochar’s porous structure and functional groups also contribute to improved water absorption, thermal insulation, and durability of cementitious materials.
In this study, mixed biochar was prepared using the pyrolysis method, combining rice husk and sawdust biomasses. The obtained biochar was then incorporated into a mortar mixture at different dosages (0%, 3%, 5%, and 10%) and cured under a CO2 environment. The performance of the biochar-based mortar was evaluated through various tests, including mechanical strength, porosity, density, water absorption, and CO2 uptake analysis.
Biochar Production and Customization for Construction Materials
Biochar can be produced through various thermochemical conversion processes, including pyrolysis, gasification, and hydrothermal carbonization (HTC). The yield, physicochemical properties, and carbon stability of biochar are influenced by the feedstock selection and production conditions, such as temperature, heating rate, residence time, and the presence or absence of oxygen.
Conventional pyrolysis with a slow heating rate (5-10°C/min) in an oxygen-limited environment is the most widely adopted method to produce biochar. Pyrolysis temperature is a critical factor that affects the porous structure, surface area, and pH of biochar. Increasing the pyrolysis temperature can significantly enhance the pore volume and surface area of biochar due to the conversion of amorphous carbon to graphitic form and the removal of pore-blocking substances.
Gasification with a high reaction temperature (> 700°C) in the presence of gasifying agents produces biochar with abundant pore structures and high aromaticity, which can be beneficial for internal curing and accelerated carbonation in construction materials. For feedstocks with high ash content and alkali/alkaline earth metals, the solid-solid interaction between carbon and these minerals during gasification can improve the reactivity and functionalization of biochar.
HTC, on the other hand, is suitable for processing wet and bulky biomass, producing carbonized solids (hydrochar) without an energy-intensive drying pretreatment. The HTC process involves the depolymerization, dehydration, and repolymerization of biomass components, leading to the formation of hydrochar with different characteristics compared to pyrolytic biochar.
In addition to the production process, various activation or modification methods, such as physical activation (CO2 or steam), chemical activation, and ball milling, can be employed to further enhance the surface functionality, reactivity, and particle size distribution of biochar. These engineered biochars with customized properties can be more effectively incorporated into construction materials to achieve desired performance and environmental benefits.
Biochar for Carbon Sequestration and Decarbonization of Construction Industry
Biochar, as a carbon-negative material, has demonstrated its potential to mitigate greenhouse gas emissions and contribute to carbon neutrality targets. The extent of carbon reduction by biochar depends on its production conditions, application environment, and the utilization of co-products (e.g., pyrolytic gas, bio-oil) for renewable energy generation.
The large-scale deployment of converting waste biomass into biochar has been considered a ready-to-implement negative emission technology (NET) for achieving global carbon neutrality goals. The pyrolytic gas and bio-oil generated during biochar production can be used for electricity, heat generation, and alternative fuels, while the biochar itself can be used in various industries, including agriculture and construction, to enhance environmental performance and sequester carbon.
Compared to conventional cement and concrete production, the incorporation of biochar as a filler or aggregate in construction materials can significantly reduce the carbon footprint. Biochar’s lightweight and brittle nature can alleviate the burden on transportation and energy consumption associated with raw material extraction and processing. Additionally, the porous structure of biochar can promote the diffusion and dissolution of CO2, enhancing the carbonation progress and increasing the lifetime CO2 uptake capacity of cementitious materials.
Life cycle analyses have demonstrated that the integration of biochar in construction materials, such as concrete, can effectively convert them into carbon-negative products, achieving up to 59-65 kg CO2 sequestration per ton of material. This carbon-negative potential of biochar-enhanced construction materials can significantly contribute to the decarbonization of the building and construction industry, fostering the transition towards a sustainable built environment.
Biochar as a Filler and Aggregate in Cement-Based Composites
Biochar can be incorporated into cement-based composites as a filler, partially replacing cement, to enhance various properties, including rheology, hydration, and mechanical performance. The inclusion of biochar in cement paste can improve the compactness and early strength of the composite through the pore-filling effect and internal curing mechanism.
The porous nature of biochar allows it to retain surrounding water, which can be slowly released to contribute to the hydration of the cement matrix, leading to improved long-term strength development. Additionally, biochar can facilitate CO2 diffusion and regulate the moisture content during the accelerated carbonation process, enhancing the carbon sequestration capacity of the cementitious materials.
Compared to conventional fillers, biochar can also mitigate autogenous and drying shrinkage in cement-based composites, improving the durability of the material. The abundant micropores and mesopores of biochar, along with its high specific surface area, can enhance the water adsorption/retention capacity, thermal insulation, and temperature regulation ability of the cement-biochar composites.
The optimal dosage of biochar as a filler in cement-based composites is typically recommended to be within the range of 0.5-2 wt%, considering the balance between mechanical performance enhancement and CO2 sequestration potential. Higher biochar dosages can further reduce the carbon footprint of construction materials, though they may result in some strength loss, which should be carefully evaluated.
Engineered biochars with customized properties, such as hydrophilic functional groups, mineral-rich composition, and tailored particle size distribution, have the potential to provide additional benefits in cement-based composites. These engineered biochars can promote hydration reactions, facilitate pozzolanic activities, and improve the interfacial compatibility with the cement matrix, leading to enhanced mechanical and durability performance.
Biochar as a Lightweight and Porous Aggregate in Concrete
Biochar can also be utilized as a partial replacement for natural aggregates in concrete, contributing to the development of lightweight and porous concrete. The porous structure and low density of biochar can improve the flexibility, impact resistance, and thermal insulation properties of concrete.
Replacing sand with 20% biochar with an average particle size of 26 μm has been shown to enhance the flexural strength by 26% while reducing the bulk density by 10%. Biochar can also provide ductility and strengthen the interfacial transition zones, improving the bending strength and fracture energy of concrete.
Furthermore, the incorporation of 30 wt% biochar as an aggregate can transform conventional concrete into a carbon-negative product, achieving significant environmental benefits and economic advantages. The porous nature of biochar can promote the diffusion and dissolution of CO2, enhancing the carbonation progress and increasing the lifetime CO2 uptake capacity of the concrete.
Biochar in Other Construction Materials
Beyond cement-based composites, biochar has been explored as a filler and modifier in various other construction materials, including asphalt, polymers, and wood-plastic composites.
In asphalt mixtures, biochar can improve the aging resistance, temperature sensitivity, and rutting resistance of the binder, contributing to the durability of asphalt pavements. Biochar can also act as a sorbent for volatile organic compounds (VOCs) generated during asphalt production, mitigating potential health risks to construction workers.
Incorporating biochar into polymer-based construction materials, such as epoxy resins and rubber composites, can enhance the mechanical, electrical, and thermal properties of the composites. Biochar’s porous structure, surface functionality, and carbon content can be tailored to optimize the performance of these biochar-polymer composites.
In wood-plastic composites, biochar can improve the tensile and flexural strength, as well as the flame retardancy, by creating a mechanical and physical interlocking with the polymer matrix. The combination of biochar and other flame retardants, such as magnesium hydroxide, has shown promising results in enhancing the thermal stability of these eco-friendly construction materials.
Emerging Applications of Biochar in Construction
The versatility of biochar has led to the exploration of its applications in various value-added construction materials, including:
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Humidity and Temperature Regulation: The porous structure and hygroscopic nature of biochar can be utilized in pervious concrete and other building materials to regulate humidity and mitigate urban heat island effects.
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Thermal Insulation and Acoustic Insulation: The three-dimensional porous structure and low thermal conductivity of biochar can enhance the thermal and acoustic insulation properties of construction materials.
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Contaminant Immobilization and Indoor Air Quality Improvement: Biochar’s high adsorption capacity can be leveraged to immobilize potentially toxic elements in construction waste and improve indoor air quality by adsorbing volatile organic compounds.
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Electromagnetic Shielding: The electrical conductivity and carbon content of biochar make it a promising additive for enhancing the electromagnetic interference shielding effectiveness of cement-based composites.
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Self-Healing Concrete: Biochar can be used as a carrier for bacteria spores to facilitate the self-healing of cracks in concrete, improving the durability and service life of structures.
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3D Printable Concrete: Biochar’s porous structure and ability to improve the rheological properties of the cement mixture can contribute to the development of sustainable 3D printable concrete.
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Phase Change Materials: The integration of biochar with phase change materials can enhance their thermal energy storage capacity, shape stability, and chemical compatibility, enabling the development of high-performance construction materials for energy-efficient buildings.
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Self-Sensing Cement Composites: The electrical conductivity of biochar makes it a suitable additive for manufacturing self-sensing cement composites, enabling structural health monitoring of civil infrastructure.
These emerging applications of biochar in construction materials showcase its potential to revolutionize the industry, enhancing the sustainability, functionality, and resilience of built environments.
Challenges and Future Prospects
To fully realize the potential of biochar in construction materials, several challenges need to be addressed through further research and development:
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Standardization and Quality Control: Detailed requirements and standardized assessments should be established to ensure the consistent quality and safety of biochar for construction applications.
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Interfacial Reactions and Durability: Advancing the scientific understanding of the interfacial reactions between biochar and cementitious/asphalt/polymer matrices is crucial for optimizing the long-term performance and durability of biochar-enhanced construction materials.
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Tailored Biochar Properties: Continued efforts are needed to develop engineered biochars with customized characteristics (e.g., pore structure, surface functionality, mineral composition) to maximize the technical and environmental benefits in diverse construction applications.
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Techno-Economic and Life Cycle Analyses: Comprehensive techno-economic and life cycle assessments should be conducted to evaluate the sustainability and cost-effectiveness of biochar-based construction materials before large-scale commercialization.
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Synergistic Integration with Other Techniques: Combining biochar incorporation with other emerging technologies, such as accelerated carbonation and internal curing, can further enhance the performance and carbon sequestration potential of construction materials.
By addressing these challenges and continuing to explore the versatile applications of biochar, the construction industry can leverage this carbon-negative material to achieve significant reductions in greenhouse gas emissions and transition towards a more sustainable, durable, and resilient built environment.
The incorporation of biochar derived from waste biomass in construction materials holds great promise for mitigating CO2 emissions, reducing natural resource depletion, and enhancing the mechanical and functional performance of building components. As a carbon-negative additive, biochar can contribute to the decarbonization of the construction industry and the realization of global carbon neutrality targets.
Through the continued development of engineered biochars with tailored properties and the synergistic integration of biochar with other innovative construction techniques, the construction industry can unlock the full potential of this versatile material to revolutionize the way we build and maintain our built environment. By embracing the use of biochar-enhanced construction materials, the industry can lead the way towards a more sustainable and climate-positive future.
