Introduction
The rapid growth of global industrialization and urbanization has led to the excessive use of non-renewable energy sources and the alarming release of greenhouse gases within the construction industry. In response, adopting sustainable and environmentally friendly building materials has emerged as a vital solution for achieving the international sustainable development goals set by the United Nations.
Biochar, a carbon-rich material produced through the pyrolysis of biomass, has garnered significant attention as a potential replacement for cement in construction applications. Its unique properties, such as high porosity, low thermal conductivity, and ability to enhance various mechanical and physical characteristics of cementitious materials, make it an attractive option for sustainable building practices.
This comprehensive review delves into the impact of varying biochar rate and particle size derived from rice husk on the properties of cement mortar. It explores the preparation methods, material properties, emission reduction capabilities, thermal insulation performance, and economic prospects of biochar-based building materials.
Biochar Production Techniques
The production of biochar involves various thermochemical conversion processes, with pyrolysis being the most common method. During pyrolysis, biomass feedstock is subjected to high temperatures, typically ranging from 500°C to 800°C, in an oxygen-depleted environment. This process results in the conversion of the organic matter into a carbon-rich solid material known as biochar.
Pyrolysis
Pyrolysis can be categorized into two main types: fast and slow pyrolysis. Fast pyrolysis occurs at around 500°C for approximately two seconds, with a high heat transfer rate of 300°C/min. This process yields approximately 75% bio-oil, while the char and gas outputs are around 12% and 13%, respectively. Slow pyrolysis, on the other hand, takes place over a longer duration, typically at 300-500°C for 5-30 minutes, with a slower heat transfer rate of 5-20°C/min. Slow pyrolysis results in a lower bio-oil yield of 30-50%, with 25-30% biochar output and 35% gas output.
Hydrothermal Carbonization and Microwave Pyrolysis
In addition to traditional fast and slow pyrolysis methods, newer techniques have emerged, including hydrothermal carbonization and microwave pyrolysis. Hydrothermal carbonization utilizes water at high pressure and temperature to convert biomass into biochar. This method offers benefits such as efficient conversion, decreased operating temperature, and the elimination of supplementary energy for drying.
Microwave pyrolysis is a modern technique that provides advantages like precise process control, cost savings, and reduced raw material preparation requirements. Microwave pyrolysis allows for more uniform temperature distribution during the process, resulting in biochar with enhanced properties, including greater surface area and higher concentrations of functional groups compared to traditional pyrolysis methods.
Biochar in Construction Materials
The integration of biochar into cementitious blends, either as a substitute or an additive, has gained momentum in the engineering community. Incorporating engineered biochar into construction materials offers multiple benefits, including enhanced structural strength, improved permeability, and the potential for carbon capture and storage.
Biochar-Based Bricks
Biochar is not readily suitable for high-temperature firing processes, such as those used in the production of stone or fired clay bricks. Therefore, biochar is typically used as an additive or substitute for concrete bricks. Recent research indicates that bricks made from a mix of 50% biochar and 50% high-density polyethylene display enhanced compressive strength relative to other substances. Additionally, bricks composed of biochar and cement surpass traditional bricks in terms of insulation, rigidity, and water absorption capabilities.
Biochar as a Lightweight Aggregate
Researchers have also investigated using biochar as a lightweight aggregate substitute in concrete formulations. Studies have explored the feasibility of using biochar instead of sand for concrete manufacturing. The substitution of sand by biochar in a 20% volume ratio, with an average granule dimension of 26 µm, results in a decline in bulk density by 10% and a boost in bending strength by 26%.
Environmental Benefits of Biochar
The incorporation of biochar into building materials offers significant potential for reducing greenhouse gas emissions and achieving carbon neutrality goals.
Carbon Sequestration and Emission Reduction
As a product of thermally decomposing biomass waste, biochar offers a sustainable approach to waste reuse and mitigates pollution that typically arises from waste incineration or landfill methods. Pyrolysis is preferred over incineration as it reduces emissions like methane, carbon monoxide, nitrogen oxide, and sulfur oxide.
Various types of biochar have demonstrated adverse greenhouse gas emissions, estimated at approximately −0.90, −0.864, and −0.885 kg of CO2-eq/kg for biochar derived from barley straw, corn stove residue, and yard waste, respectively. Substituting one ton of cement with biochar in brick production can reduce the global warming potential by 1351.2–1504.6 kg of CO2-eq.
Carbon Sequestration in Building Materials
The utilization of biochar as a carbon-negative material in the construction industry offers the potential for buildings to serve as carbon sinks, contributing to the achievement of carbon neutrality goals. Incorporating biochar into building materials can enhance wall insulation and effectively reduce carbon dioxide emissions associated with the walls.
Thermal Insulation Properties of Biochar
The high porosity of biochar, which can vary depending on the raw material and pyrolysis process parameters, contributes to its low thermal conductivity and excellent insulation properties. By incorporating uniformly distributed porous biochar into construction materials, the propagation of heat is disrupted in multiple directions, impeding unidirectional heat transfer and enhancing the thermal insulation capabilities of the composite.
Studies have reported that adding biochar can decrease the thermal conductivity of biochar–cement composites by 25% and biochar–clay composites by up to 67%. Biochar bricks and high-density plastic demonstrate a low thermal conductivity of 0.192 W/(m·K), while replacing 10% of sand with biochar in concrete bricks significantly enhances their insulation performance, reducing the thermal conductivity from 0.64 W/(m·K) to 0.47 W/(m·K).
Water Absorption Characteristics
The water absorption characteristics of biochar-modified cementitious materials are influenced by the porous structure of biochar. Two distinct phases of water absorption can be observed within the first 24 hours and between 24 and 144 hours, categorized as a rapid phase and a slow phase.
The initial rapid phase occurs due to the enlarged capillary absorption facilitated by the capillary and fine gel pores of biochar in the material’s matrix. The subsequent slow absorption phase is driven by the weak capillary forces generated by the air voids and macropores in the biochar-modified cementitious material.
Research has shown that the incorporation of biochar into mortar tends to decrease water absorption, independent of the quantity or nature of the precursor substances involved. However, exceeding a 4% by weight biochar dosage might augment the empty spaces within the mortar, ultimately resulting in increased water absorption.
Flowability and Workability
The integration of biochar into bricks or cement formulations can affect their flowability, which refers to the ease with which these materials can be mixed and shaped during construction activities. Biochar’s pronounced porosity and ability to absorb water can influence the materials’ manageability and ease of handling.
Biochar’s advantageous pore configuration and broad specific surface area facilitate the formation of a layer capable of storing and retaining water, improving the water-holding properties of the composite. However, when the biochar concentration nears or surpasses 10%, the concrete may start to exhibit brittleness as the biochar begins to take up a substantial volume of the material.
Careful management of the water-to-cement ratio or the quantity of biochar included is vital to maintain the desired flowability and workability of biochar-based construction materials.
Compressive Strength
The compressive strength of biochar-based bricks refers to their ability to withstand applied compressive forces without breaking or deforming. Incorporating biochar into brick formulations can influence the compressive strength of the resulting bricks.
Studies have shown that adding pyrolyzed hazelnut shells and coffee grounds to concrete blocks at a low percentage, namely 0.5%, increases compressive strength. This improvement can be attributed to the porous nature of these materials, which serve as sites for the rapid formation of the calcium–silicate–hydrate phase in the cement paste.
However, an increase in the amount of biochar introduced may accentuate inherent shortcomings in the materials, such as diminished strength and increased porosity. Biochar’s compressive strength is inversely related to its particle size, with strength decreasing as particle size enlarges.
The optimal addition of biochar for enhancing the compressive strength of cement mortar has been found to be 1 and 2% by weight. Biochar bricks and high-density plastic exhibit a compressive strength range of 10.34 to 13.79 MPa after 9-20 days of curing, while concrete bricks typically display higher compressive strength, ranging from 16.54 to 17.24 MPa.
Flexural Strength
Flexural strength is an important mechanical property in engineering materials such as cement and bricks. The impact of biochar on flexural strength can produce varying outcomes.
Incorporating a small amount of biochar into concrete bricks can increase their flexural strength. When a minor quantity of biochar is used, it requires more energy than crushed cement and sand particles, resulting in ductile failure and increased energy absorption. However, as the proportion of biochar increases, it can introduce excessive porosity, leading to a decrease in flexural strength.
Studies have reported that the maximum flexural strength is achieved when the biochar concentration is around 0.5-0.8%. Biochar’s ability to form impermeable barriers and alter crack direction has been found to contribute to enhanced fracture energy and toughness of the cement-based composites.
Fire Performance and Thermal Stability
Concrete has the best fire resistance compared to other construction materials, such as steel and wood. Reinforcements can further enhance the thermal stability of concrete, although the reinforcements’ size and shape may not significantly impact thermal stability.
The incorporation of biochar into concrete constructions’ fire behavior has received limited research attention. It is anticipated that adding biochar to concrete could potentially offer benefits due to the formation of strong C–C covalent bonds at high temperatures, which could contribute to the material’s resistance against fire-induced damage.
Biochar exhibits nonflammable properties and can enhance the stiffness and water resistance of cement-based mixtures, particularly at elevated temperatures, improving their ability to withstand fires and provide effective heat insulation.
Economic Viability and Life Cycle Assessment
The economic viability of biochar-based sustainable bricks involves considering various factors, including raw materials, labor, energy consumption, transportation expenses, and potential advantages such as decreased greenhouse gas emissions, enhanced soil quality, and potential income from carbon credit sales.
The price of biochar is generally higher than that of cement, but incorporating a small quantity of biochar, around 0.5%, can greatly enhance the strength of bricks, justifying its cost-effectiveness when used as an additive. Substituting a significant proportion of cementitious materials in concrete bricks with biochar may not increase their compressive strength due to biochar’s relatively lower hardness or tendency to increase the material’s porosity.
Life cycle assessments have demonstrated that the integration of biochar into construction materials can effectively reduce environmental impacts, such as ozone depletion, toxicity, hazardous waste generation, greenhouse gas emissions, and global warming potential. Substituting one ton of cement with biochar in brick production can reduce the global warming potential by 1351.2–1504.6 kg of CO2-eq.
Conclusion
The incorporation of biochar-based materials in the construction industry offers significant potential for reducing carbon emissions and achieving sustainable building practices. Biochar’s unique properties, such as high porosity, low thermal conductivity, and ability to enhance various mechanical and physical characteristics of cementitious materials, make it an attractive option for sustainable construction.
Biochar-based bricks and insulation materials demonstrate improved thermal insulation, water absorption, and compressive strength characteristics compared to traditional building materials. However, the optimal balance between reduced emissions, enhanced insulation, and maintaining structural integrity remains a crucial area for further research.
Ongoing efforts to improve the properties of engineered biochar, address its impact on compressive strength, and conduct thorough life cycle assessments are essential for unlocking the full potential of biochar in the construction sector. By embracing biochar-based solutions, the building industry can contribute significantly to the development of a more sustainable and carbon-neutral future.
