Biochar in construction
MODULE 1 — Introduction to Biochar in Construction
This opening module introduces the role of biochar within the construction sector and explains why this material is increasingly considered a strategic component for low-carbon building materials.
While biochar is widely known for its agricultural and environmental applications, its potential in construction is equally significant.
The module explains how biochar contributes to carbon storage, thermal performance, moisture regulation, indoor environmental quality, and circular resource flows in buildings.
Learners explore the scientific foundations of biochar, its relevance for climate mitigation, and the reasons why it is emerging as a key material for sustainable construction.
The module establishes the conceptual basis for the following modules, which will examine specific applications in building materials, asphalt, and quality standards.
Learning Outcomes
By the end of this module, learners will be able to:
- Explain why biochar is relevant for the decarbonization of the construction sector
- Describe the main construction applications of biochar (concrete, mortars, insulation, asphalt, IAQ, hygro-regulation)
- Understand the link between biochar, carbon storage, and the Circular Carbon Economy
- Identify the key benefits and limitations of using biochar in building materials
- Reflect on how biochar could address local construction or renovation challenges
The construction sector is one of the largest contributors to global greenhouse gas emissions. Buildings require vast quantities of mineral aggregates, cement, steel, and synthetic insulation materials. These materials are energy-intensive to produce, generate significant CO₂ emissions, and rely on finite natural resources. As Europe moves toward climate neutrality, the construction sector must adopt new materials and approaches that reduce embodied carbon while maintaining or improving performance.
Biochar offers a unique opportunity in this context. Produced through the pyrolysis of biomass under low-oxygen conditions, biochar is a stable, carbon-rich material capable of storing atmospheric carbon for centuries. Unlike many construction materials that emit CO₂ during production, biochar removes carbon from the atmosphere and locks it into a solid form. When integrated into building materials, biochar transforms walls, floors, insulation layers, and pavements into long-term carbon sinks.
Beyond carbon storage, biochar provides functional benefits that align with modern construction needs. Its porous structure reduces density, making materials lighter and easier to handle. The air trapped within its pores lowers thermal conductivity, improving insulation performance. Biochar also enhances moisture buffering, helping buildings maintain stable indoor humidity levels. This contributes to healthier indoor environments and reduces the risk of mould or condensation. In addition, biochar can adsorb certain pollutants, supporting better indoor air quality.
Context: Planetary Boundaries (2025)
According to the latest synthesis of Planetary Boundaries (2025), several key environmental thresholds (biosphere integrity, climate change, novel entities, ozone, aerosols, ocean acidification, nitrogen/phosphorus cycles, freshwater, and land use) are under severe pressure. Biochar can help delay the breach of these limits, and in some cases reverse certain trends, through sustainable carbon storage and improvement of degraded soils. Integrating biochar into construction therefore contributes not only to climate mitigation but also to broader planetary health.
Figure 1. Planetary Boundaries in 2025 (Source: Reporterre). The coloured segments indicate the degree of transgression of each boundary. Biochar contributes to improving the climate and biosphere segments.
Biochar vs. Charcoal — Key Distinctions
Biochar and charcoal are both carbon-rich solids produced from biomass, but they differ fundamentally in purpose, production, and properties:
Biochar:
- No PAHs (or within strict limits): reduces contamination risk
- Designed for soil amendment: improves structure and water retention
- Fine particle size: mixes easily into soil or building materials
- Goal: sequester carbon over the long term
Charcoal:
- May contain PAHs: contamination risk
- Primarily for energy use: heat generation
- Coarser particle size: less homogeneous when mixed
- Goal: rapid combustion and energy yield
Biochar is designed to amend soils and store carbon, while charcoal is intended for combustion. This fundamental difference shapes their respective production conditions and end uses.
Biochar Composition
Biochar is a solid residue obtained by pyrolysis of biomass, a thermochemical decomposition process conducted under oxygen-poor conditions (350-1000°C). This process stabilizes carbon in a durable form, limiting its degradation and promoting long-term storage.
- Carbon (C): Main component; provides porosity and stability, representing typically 50-95% of dry mass
- Hydrogen (H) and Oxygen (O): Organic remnants influencing reactivity and pH
- Nitrogen (N) and Sulfur (S): Variable nutrients that support fertility
- Mineral ash: Minerals (Ca, K, Mg, P) contributing to pH and nutrient supply
Origins: Amazonian Terra Preta
Figure 2. Terra Preta soil from the Amazon basin. The dark colour reflects the high stable carbon content. These anthropogenic soils remain fertile after more than 2,000 years.
Terra Preta (‘black soils’) is an anthropogenic soil created more than 2,000 years ago by Indigenous Amazonian peoples. Its stable charcoal content and high carbon levels (up to 10%) improve water and nutrient retention over extraordinarily long time scales. Key characteristics include:
- High carbon content: Better water retention
- High fertility: Less fertilizer reliance
- Stable structure: Reduces erosion
- Active microbiome: Supports nutrient cycling
Ancient Amazonian practices revealed the potential of plant-based charcoal to sustainably improve soils. Today, modern science turns this empirical knowledge into a reproducible, measurable, and optimized technology. Modern biochar is directly inspired by this traditional know-how.
Key Concepts Introduced
- Embodied carbon
- Carbon sequestration
- Circular Carbon Economy (CCE)
- Biochar-based building materials
- Hygro-regulation
Opening Reflections
- What construction materials in your region could benefit from improved insulation or reduced embodied carbon?
- Have you ever encountered bio-based construction materials such as hempcrete, straw, earth, or lime?
- Which construction challenges are most pressing in your context (thermal comfort, moisture, carbon footprint, cost)?
- How familiar are you with carbon-negative building materials (1-5)?
Biochar is one of the few construction materials capable of actively removing carbon from the atmosphere. During pyrolysis, biomass carbon is transformed into a stable aromatic structure that resists decomposition for centuries. Unlike biomass, which decomposes and releases CO₂ within a few years, biochar carbon remains stored for long periods.
When this biochar is incorporated into building materials, the carbon becomes locked into walls, floors, insulation layers, or asphalt pavements. Buildings therefore become long-term carbon reservoirs.
Biochar can store between 2.5 and 3 tonnes of CO₂ equivalent per tonne of material. The IPCC estimates that biochar could contribute around 6% of the global emissions reductions needed to reach carbon neutrality. Integrating biochar into construction materials therefore supports both climate mitigation and material innovation.
Biochar also contributes to circular resource flows. It can be produced from local biomass residues such as forestry waste, agricultural by-products, or organic waste streams. By turning organic waste into a valuable construction material, biochar helps close carbon loops and supports sustainable material cycles
Carbon Reflections
- Why is carbon storage particularly relevant for the construction sector compared to other industries?
- Which building components in your region could integrate biochar without major changes to existing practices?
- Identify one local biomass stream. Could it be used to produce construction-grade biochar?
Biochar can be integrated into a wide range of construction materials. Its porous structure, low density, and chemical stability make it suitable for both structural and non-structural applications.
In concrete and mortars, biochar can partially replace sand or fine aggregates. Research from the National University of Singapore showed that incorporating biochar from 6 tonnes of wood waste into 120 tonnes of concrete produces buildings that are 20% stronger and 50% more watertight. In lightweight insulating materials, biochar-clay, biochar-lime, and biochar-hemp composites offer improved thermal insulation, reduced density, and enhanced moisture buffering.
Figure 3b. The three dominant key biochar applications: agriculture and soil amendment, asphalt additive (durability, strength, lower CO₂ footprint), and construction uses (concrete, mortar, insulation, carbon storage). Source: B4C Project.
Figure 3. Biochar technology in concrete: NUS research demonstrating 20% strength gain and 50% improvement in watertightness (Source: National University of Singapore).
Figure 4. Overview of possible biochar combinations for sustainable construction: clay, lime, hemp, bricks, insulation panels, and repair mortars.
Biochar as a Transversal Component in a Sustainable Building
Biochar can function as a transversal decarbonisation component across all elements of a building, not just in a single material. Consider a sustainable dwelling where biochar is integrated at every level:
Foundation and Floor Slab (concrete):
Biochar partially replaces aggregates in the concrete slab. This permanently sequesters CO₂ while lightening the structure, reducing loads on foundations and improving overall carbon performance.
Walls (biochar-lime or biochar-hemp composite):
At the wall level, a biochar-lime or biochar-hemp mix creates a breathable insulating composite that naturally regulates indoor humidity and purifies indoor air by adsorbing pollutants. The result is a healthy, low-carbon wall system.
Roof and Attic Insulation:
Used on roofs or loosely packed in the attic, biochar reinforces thermal inertia and improves fire resistance. Every construction element becomes an active, long-term carbon sink.
Figure 5. Structure of a sustainable dwelling where biochar (yellow star) is used as a transversal decarbonisation component: concrete slab, insulating walls (biochar-lime/hemp), and roof insulation.
Match each construction challenge with the biochar application that could address it:
Reflection: Which of these applications seems most relevant for your region or professional context?
MODULE 2 — Biochar-Clay Composites and Building Materials
This module explores how biochar can be integrated into mineral and bio-based building materials, with a particular focus on clay-based composites. Clay is one of the oldest and most widely used construction materials in the world. It is abundant, recyclable, breathable, and capable of regulating indoor humidity.
However, traditional clay materials can be heavy, have limited insulation performance, and may require reinforcement to achieve adequate mechanical strength. Integrating biochar into clay mixtures offers a promising pathway to improve these characteristics while reducing the environmental footprint of building materials.
The module examines the thermal, mechanical, and hygrothermal properties of biochar-clay mixtures, explains how biochar influences density and insulation performance, and analyses the trade-offs between strength, weight, and thermal conductivity. It also introduces practical construction applications such as walls, plasters, slabs, and insulating layers.
Learning Outcomes
By the end of this module, learners will be able to:
- Describe how biochar modifies the physical and thermal properties of clay-based materials
- Explain the relationship between biochar content, density, stiffness, and thermal conductivity
- Identify construction applications where biochar-clay composites offer performance advantages
- Analyse thermal resistance and heat flux using real-world exercises
- Evaluate the benefits and limitations of biochar-clay materials in building design
Clay-based materials have been used for centuries in many regions of the world. Traditional earthen buildings in Morocco, Yemen, and France demonstrate the durability and thermal comfort provided by thick clay walls. These walls regulate indoor temperature by storing heat during the day and releasing it at night. However, clay alone has limitations: it is relatively heavy, has moderate thermal conductivity, and may require reinforcement to achieve adequate mechanical performance.
Biochar offers a way to improve these characteristics. Its porous structure significantly reduces the density of clay composites, making walls and panels lighter and easier to handle. The air trapped within biochar pores lowers thermal conductivity, improving insulation performance. Depending on the proportion of biochar, thermal conductivity can decrease substantially, helping buildings maintain stable indoor temperatures and reducing heating and cooling needs.
Mechanically, biochar influences stiffness and compressive strength. At low to moderate percentages, biochar can maintain acceptable structural performance while improving insulation. At higher percentages, stiffness may decrease, requiring careful optimisation. The challenge is to find the right balance between lightness, insulation, and mechanical resistance.
Biochar also influences hygrothermal behaviour. Its porous structure enhances moisture buffering, helping regulate indoor humidity. This reduces the risk of condensation and mould, contributing to healthier indoor environments.
Reflection
- In your region, are clay-based materials commonly used in construction?
- Which performance improvements (insulation, weight reduction, moisture buffering) would be most valuable?
- What percentage of biochar do you think would offer the best compromise between strength and insulation?
Experimental data show clear trends when biochar is added to clay. As the biochar content increases, density decreases, making the material lighter. Thermal conductivity also decreases, improving the material’s insulation performance. Stiffness initially increases slightly due to improved microstructure but eventually decreases at higher biochar percentages.
These relationships help identify optimal formulations for different construction uses. For example, a moderate biochar content may be ideal for insulating walls, while lower percentages may be more suitable for load-bearing components. An intermediate biochar content makes it possible to obtain the best compromise between lightness, rigidity, and thermal insulation.
Figure 6. Normalised material properties (density, stiffness/modulus, thermal conductivity) as a function of biochar content (%). An intermediate content (~15-20%) provides the best compromise between lightness, rigidity, and insulation. Source: EL Ganaoui (2025), LERMAB Longwy.
Application Analysis
Based on the trends described, identify which applications would benefit most from:
Biochar-clay composites can be used in several construction elements. In walls, they provide improved insulation and moisture buffering while maintaining structural stability. In plasters, biochar enhances breathability and indoor air quality by regulating humidity and adsorbing pollutants. In slabs or floor systems, biochar reduces weight and contributes to carbon storage.
Biochar-clay composites are particularly relevant for ecological construction, renovation of historic buildings, and regions seeking to reduce dependence on synthetic insulation materials.
Case Reflection
- Which building element in your home or workplace could benefit from a biochar-clay composite?
- Would you prioritise insulation, weight reduction, or carbon storage? Why?
This unit provides hands-on exercises to apply thermal and mechanical concepts to real-world scenarios. Learners calculate thermal resistance, heat flux, U-value, compressive strength, and carbon footprint.
Exercise 1 — Basic Thermal Resistance and Heat Loss
A clay wall 0.30 m thick with a thermal conductivity of 0.8 W/m·K separates an interior at 20°C from an exterior at 5°C. The wall area is 10 m².
→ Calculate the thermal resistance and heat loss.
Exercise 2 — Material Comparison
A wall with an area of 10 m² and a thickness of 0.25 m is made of wood (λ = 0.12), clay (λ = 0.8), or clay-biochar (λ = 0.25). Indoor temperature 20°C, outdoor temperature 0°C. Surface resistances: R_si = 0.13 and R_se = 0.04 m²·K/W.
→ Calculate the thermal resistance of each material and the heat flux.
→ Rank the materials from most insulating to least insulating.
Exercise 3 — Multilayer Wall
A wall consisting of clay (0.20 m, λ = 0.8 W/m·K) and insulation (0.05 m, λ = 0.04 W/m·K) separates an interior at 20°C from an exterior at 0°C. The wall area is 12 m².
→ Calculate the total thermal resistance and the heat flux through the wall.
Exercise 4 — Multi-Component Wall with U-Value
A wall consists of plaster (2 cm, λ = 1.0 W/m·K), clay (20 cm, λ = 0.8 W/m·K), and insulation (8 cm, λ = 0.04 W/m·K). R_si = 0.13 and R_se = 0.04 m²·K/W. Wall area 15 m². Temperatures: 21°C interior, -2°C exterior.
→ Determine the total thermal resistance, the U-value, and the heat flux.
Exercise 5 — Biochar-Clay Wall with U-Value
A 25 cm clay-biochar wall (λ = 0.25 W/m·K), area 12 m², R_si = 0.13, R_se = 0.04 m²·K/W, separating 20°C interior from 0°C exterior.
→ Calculate the total thermal resistance, the U-value, and the heat flux.
Exercise 6 — Mechanical Strength and Biochar Content
Cylindrical test specimens (Ø50 mm, height 100 mm): 0% biochar → 12 kN; 10% → 9 kN; 20% → 6 kN. A wall (0.3 × 3 m) must withstand 500 kN.
→ Calculate the compressive strength of each mixture.
→ Determine the maximum percentage of biochar for the wall.
→ Draw the strength-biochar curve and discuss the results.
Exercise 7 — Carbon Footprint of a Biochar-Clay Wall
A 25 cm clay-biochar wall (λ = 0.25 W/m·K), area 12 m². Biochar production: 50 kg CO₂-eq/m³; clay: 30 kg CO₂-eq/m³.
→ Calculate the total thermal resistance, the U-value, and the heat flux.
→ Estimate the wall’s CO₂ emissions.
Exercise 8 — Full Building Carbon Assessment
A 120 m² house uses 100 m³ of clay-biochar walls (20% biochar at 50 kg CO₂-eq/m³; 80% clay at 30 kg CO₂-eq/m³). Roofing and finishes: 2,000 kg CO₂-eq. Lifespan: 50 years.
→ Calculate the emissions from the walls and the total carbon footprint.
→ Determine the annual emissions per m². How does biochar influence the carbon footprint?
Exercise 9 — Embodied Energy of a Single-Family Home
A 200 m² home (4 occupants, 50-year lifespan): 80 m³ concrete (350 kWh/m³), 5 t steel (8,000 kWh/t), 15 m³ wood (600 kWh/m³), 200 m² insulation (50 kWh/m²), 30 t bricks (700 kWh/t).
→ Calculate total embodied energy. Deduce annual embodied energy. Calculate kWh/person/day.
Case Reflection
- Which building element in your home or workplace could benefit from a biochar-clay composite?
- Would you prioritise insulation, weight reduction, or carbon storage? Why?
MODULE 3 — Biochar in Asphalt and Road Engineering
This module examines the use of biochar in asphalt and road construction. Road infrastructure represents a major source of energy consumption and greenhouse gas emissions due to the high temperatures required for asphalt production, the transport of heavy materials, and the repeated maintenance cycles needed over the lifespan of a road.
Biochar offers a promising pathway to reduce the environmental footprint of asphalt while improving mechanical performance, durability, and long-term carbon storage.
This module explores how biochar can be integrated into mineral and bio-based building materials, with a particular focus on clay-based composites. Clay is one of the oldest and most widely used construction materials in the world. It is abundant, recyclable, breathable, and capable of regulating indoor humidity.
However, traditional clay materials can be heavy, have limited insulation performance, and may require reinforcement to achieve adequate mechanical strength. Integrating biochar into clay mixtures offers a promising pathway to improve these characteristics while reducing the environmental footprint of building materials.
The module examines the thermal, mechanical, and hygrothermal properties of biochar-clay mixtures, explains how biochar influences density and insulation performance, and analyses the trade-offs between strength, weight, and thermal conductivity. It also introduces practical construction applications such as walls, plasters, slabs, and insulating layers.
Learning Outcomes
By the end of this module, learners will be able to:
- Explain why biochar is relevant for asphalt and road engineering
- Describe how biochar affects stiffness, durability, and energy use
- Understand the circular value chain of biochar-modified asphalt
- Perform basic LCA calculations for asphalt mixtures
- Analyse the environmental benefits of replacing bitumen or aggregates with biochar
- Evaluate the potential of waste heat recovery from biochar production in asphalt plants
Asphalt production is highly energy-intensive. Heating aggregates and bitumen typically requires temperatures between 140°C and 180°C. Transporting aggregates from quarries to asphalt plants, and then to construction sites, consumes significant amounts of fuel. Compaction and paving operations also require heavy machinery, further increasing energy use and emissions.
Biochar offers a way to reduce the environmental footprint of asphalt by partially replacing bitumen or mineral fillers. Its porous structure improves mechanical performance, reduces cracking, enhances stiffness, and increases durability. Biochar can also reduce the viscosity of the asphalt binder, potentially lowering the required mixing temperature.
In addition, biochar stores carbon within the pavement, contributing to long-term climate mitigation. Biochar can store between 2.5 and 3 tonnes of CO₂ equivalent per tonne of material, and this carbon remains locked in the pavement for decades.
Figure 7b. Hot-mix asphalt laying on a road construction site. The high-temperature process (140-180°C) makes the sector energy-intensive and a prime candidate for biochar integration to reduce embodied carbon.
Reflection
- What are the main challenges in road durability in your region (cracking, rutting, heat damage, freeze-thaw cycles)?
- How could biochar help address these challenges?
- Which part of the asphalt mix (bitumen, filler, aggregate) do you think is most suitable for biochar substitution?
Figure 7. Biochar in asphalt: circular value chain from local biomass residues to road construction and long-term carbon storage.
Biochar in asphalt supports a circular value chain by transforming organic residues into a high-value construction material. Instead of relying solely on fossil-based bitumen and mineral fillers, asphalt mixes can incorporate biochar produced from local biomass streams such as forestry residues, agricultural waste, or organic by-products.
Biochar-enriched asphalt helps pavements contribute to climate-neutral mobility by storing carbon and reducing the environmental footprint of road construction.
Differentiated Markets: Rural vs. Urban Applications
Rural and emerging-economy contexts:
In rural roads in emerging countries, biochar can significantly cut costs by using very low-cost organic residues as feedstock. This reduces reliance on expensive, carbon-intensive materials and improves overall project economics, making sustainable road construction accessible in regions with limited budgets.
Urban and advanced-economy contexts: Electric Roads
In urban and high-income regions, biochar-modified asphalt opens the door to next-generation infrastructure. Electric roads can recharge vehicles while driving using conductive rails or inductive coils embedded in the pavement, as demonstrated in Sweden and on France’s A10 motorway, reaching up to 200 kW of dynamic charging power. This technology can reduce battery size, extend vehicle range, and accelerate transport decarbonisation.
This contrast between North African and European markets illustrates that biochar’s value proposition in asphalt adapts to local economic, environmental, and technological contexts.
Mapping Exercise
Identify local biomass sources that could be used to produce biochar for asphalt applications. Consider forestry residues, agricultural by-products, municipal green waste, or industrial organic waste streams.
Life cycle assessment (LCA) helps quantify the environmental impact of asphalt mixtures by evaluating materials, manufacturing, transport, and installation. Biochar reduces embodied energy and enables carbon storage, improving the overall carbon balance of pavements.
Exercise Scenario — LCA Data
The functional unit is 1 km of single-lane road over a 20-year lifespan. The pavement is 3.5 m wide with a 5 cm thick wearing course. Density: 2,300 kg/m³. The wearing course is replaced after 10 years.
Asphalt mix: 95% aggregate and 5% bitumen. Embodied energy: aggregate = 0.15 MJ/kg; bitumen = 48 MJ/kg. Manufacturing: 350 MJ/tonne. On-site placement: 50 MJ/tonne. Transport: 1.4 MJ/tonne/km. Distances: 30 km (quarry to plant); 20 km (plant to site). Carbon factor: 0.074 kg CO₂/MJ.
Biochar variant: 15% of asphalt replaced by biochar. Embodied energy of biochar: 10 MJ/kg. Carbon storage: 2.5 kg CO₂/kg of biochar.
Asphalt LCA Questions
- Q1: Calculate the amount of asphalt needed for the functional unit (1 km of road).
- Q2: Calculate the total mass of asphalt used.
- Q3: Calculate the total embodied energy of the conventional solution (materials, manufacturing, transport, installation).
- Q4: Calculate the cumulative energy over 20 years, including maintenance after 10 years.
- Q5: Recalculate the total embodied energy for the biochar solution.
- Q6: Calculate CO₂ emissions for both solutions and determine the net carbon footprint, accounting for carbon sequestration in biochar.
- Q7: Compare the two solutions: determine energy savings and carbon reduction from biochar.
- Q8: Determine the relative percentage share of each life cycle stage in total embodied energy.
- Q9: Examine the effect of transport distance: recalculate total energy if the distance increases from 20 km to 80 km.
- Q10: Discuss which system parameters have the greatest influence and propose two strategies for reducing energy or carbon footprint.
Biochar production generates significant amounts of waste heat due to the pyrolysis of gases and bio-oils. This heat can be recovered and used in asphalt plants to dry and heat aggregates and bitumen, reducing the need for fossil fuels.
Exercise Scenario — Waste Heat Data
For 1 km of single-lane road (3.5 m wide, 5 cm thick), 3 tonnes of biochar are required. Each tonne of biochar production generates 4 GJ of waste heat. The asphalt mixing plant requires 1.5 GJ of heat per kilometre of road.
Waste Heat Questions
- Q1: What is the total amount of waste heat generated per kilometre of road?
- Q2: What percentage of the heat required by the plant can be supplied by waste heat?
- Q3: If the excess waste heat dries the biomass before pyrolysis, how many GJ remain available for the plant?
- Q4: Estimate the energy savings in MJ per kilometre of road if waste heat completely replaces the plant’s heating energy.
- Q5: Discuss the impact on the carbon footprint when waste heat is recovered, compared to a plant using only fossil fuels.
MODULE 4 — Production, Quality and Standards for Construction Biochar
This module introduces the production processes, quality criteria, and certification standards relevant to construction-grade biochar. Construction applications require stable, safe, and predictable materials with specific physical, chemical, and environmental characteristics. This module explains how biochar is produced, which parameters influence its properties, how quality is assessed, and why certification frameworks such as the European Biochar Certificate (EBC) and the International Biochar Initiative (IBI) are essential for ensuring safe and reliable use in construction.
Learning Outcomes
By the end of this module, learners will be able to:
- Describe the main pyrolysis technologies used to produce biochar
- Identify key quality parameters for construction applications
- Understand EBC and IBI certification criteria
- Evaluate risks and limitations associated with biochar use in buildings
- Assess whether a given biochar is suitable for construction
Biochar is produced through pyrolysis, the thermochemical decomposition of biomass in a low-oxygen environment. Pyrolysis converts organic matter into three separate fractions: biochar (solid), syngas (gas), and bio-oils (liquid).
The process occurs in three main steps: drying (below 200°C), decomposition of hemicellulose, cellulose, and lignin (200-550°C), and carbonisation (above 400°C), where stable aromatic structures are formed.
Figure 8a. Representative biomass feedstocks used in biochar production: wheat straw, rice straw, sugarcane bagasse, and forestry residues. Their varying cellulose, lignin, and ash contents influence final biochar properties.
Figure 8. Industrial pyrolysis facility for biochar production. The process converts biomass into biochar, syngas, and bio-oils under controlled low-oxygen conditions.
Industrial Production Technologies
Figure 9. Industrial biochar production technologies: reactor types (auger, rotary kiln, counter-current), feedstocks, and performance data for the Carbotec-1 reactor.
Industrial biochar production favours slow and intermediate pyrolysis to maximise yield and quality. Three main reactor types are used:
- Auger reactors: Mechanical feed handling with precise residence-time control; well-suited to continuous operation with varied feedstocks
- Rotary kilns: High throughput with uniform indirect heating; widely used for large-scale production
- Counter-current pyrolysis units: Efficient heat use and consistent product quality; favoured where energy recovery is a priority
Industrial Performance — Example of the Carbotec-1 Reactor
- Annual biomass throughput (input): 1,560 t/year
- Biochar output: 187 t/year
- CO₂ removal: 420 t/year
- Thermal energy co-generated: 4.1 GWh/year
Typical Feedstocks by Category:
- Wood residues: pine, maple, spruce, eucalyptus
- Agricultural residues: straw, husks, corn stalks
- Animal manure: chicken manure, cow manure
These industrial systems allow 24/7 continuous operation, energy self-sufficiency, co-generation of electricity from surplus energy, and standardised quality that facilitates certification and market access.
Combustion vs. Carbonisation
Full combustion emits mostly CO₂ and leaves ash behind, releasing all stored carbon. Carbonisation (pyrolysis) retains carbon in the form of a stable solid, namely the biochar. This distinction is fundamental: pyrolysis produces a long-term carbon sink, while combustion simply returns carbon to the atmosphere.
Not all biochars are suitable for construction. The properties of biochar vary considerably depending on the type of biomass used, the production process, and the treatment conditions during pyrolysis. Construction applications require stable, low-toxicity biochars with predictable physical behaviour.
Biomass Feedstock Composition Table
Table 1. Predicted elemental composition of biochar obtained from various biomass feedstocks (Part 1): feedstock type, pyrolyser type, temperature, residence time, and elemental composition of biomass and predicted biochar. Source: Ganesamoorthy et al., Heliyon (2023).
Table 1 (continued). Predicted elemental composition of biochar from various biomass feedstocks (Part 2): Miscanthus, straw, pine dust, rice straw, wood chips, sewage sludge, palm oil sludge, walnut shell.
Key parameters include:
- Carbon content: High carbon content (often above 50%) indicates stability and long-term storage potential
- H/C ratio: A low hydrogen-to-carbon ratio (below 0.7 according to EBC) reflects high aromaticity and resistance to microbial degradation
- pH: Biochar typically has an alkaline pH (7-12), which can influence interactions with binders such as clay or lime
- Ash content: Ash contributes minerals but excessive ash may affect mechanical performance
- Specific surface area: High surface area (can exceed 150 m²/g) enhances adsorption and moisture buffering
- Contaminants: PAHs, heavy metals, PCBs, and dioxins must be below safety thresholds
- Particle size: Influences mixing, porosity, and mechanical behaviour in composites
Functional Groups and Surface Chemistry
The surface chemistry of biochar plays a key role in its interactions with binders and its adsorption capacity. Three main functional groups are relevant:
- Carboxyl groups (-COOH): Promote ion exchange and cation retention, improving interactions with alkaline binders
- Hydroxyl groups (-OH): Support complexation of heavy metals and strengthen contaminant adsorption
- Aldehyde groups (-CHO): Support adsorption and, in some cases, the degradation of certain organic compounds
Key drivers of surface chemistry: feedstock type, pyrolysis temperature, and residence time all shape these functional group distributions.
Quality Assessment
Given is a sample of biochar with the following properties:
Carbon content 65%, H/C ratio 0.6, PAHs 8 mg/kg, pH 9, Ash 15%.
Based on the criteria described, is this biochar suitable for use in construction materials?
Certification frameworks ensure that biochar used in construction is safe, stable, and environmentally responsible. The European Biochar Certificate (EBC) and the International Biochar Initiative (IBI) define quality and safety standards for biochar.
The EBC requires carbon content above 50%, H/C ratio below 0.7, total PAHs below 12 mg/kg for the Basic grade, and strict limits on heavy metals. The EBC distinguishes between Basic and Premium grades, with Premium having stricter requirements.
The IBI requires carbon content above 10%, PAH levels between 6 and 300 mg/kg depending on use, and an O/C ratio below 0.4 for improved stability. It also includes tests for contaminants such as PCBs, dioxins, and heavy metals.
These certifications help ensure that biochar used in construction meets environmental and safety requirements. They also support market development by providing clear quality benchmarks.
Figure 10b. EBC and IBI certification frameworks compared: key criteria, grades, analysed parameters, contaminants monitored, and why certifications matter for quality assurance and market credibility.
Biochar is not a universal solution. Its performance depends on feedstock, production conditions, and integration into building systems.
Potential limitations include:
- Variability in quality: Different feedstocks and pyrolysis conditions produce biochars with different properties
- Contaminants: Poorly controlled pyrolysis can produce biochar with elevated PAHs or heavy metals
- Mechanical performance: High biochar content may reduce stiffness or compressive strength
- Cost: High-quality biochar can be expensive due to production and certification requirements
- Integration challenges: Biochar must be properly mixed with binders to ensure consistent performance
- Regulatory considerations: Construction applications may require compliance with building codes and material standards
Biochar should not be understood as a universal solution that automatically improves all systems. Its effectiveness depends on appropriate system integration, production quality, and adaptation to local conditions.
Reflection
- What risks should be considered when selecting biochar for construction?
- How can certification help reduce these risks?
- Which quality parameters do you think are most important for your region’s construction practices?
Conclusion of the Course
This course has provided a comprehensive introduction to the role of biochar in the construction sector and its potential to support climate-positive, circular, and resilient building practices. Learners have explored how biochar can function not only as a carbon-negative material, but also as a technical component capable of improving thermal performance, moisture regulation, indoor environmental quality, and the durability of construction systems.
The course began by establishing the relevance of biochar for the decarbonisation of the built environment, including its connection to planetary boundaries, the distinction from charcoal, and the inspiring precedent of Terra Preta. Subsequent modules explored clay composites, asphalt engineering, and production quality in increasing technical depth.
The course emphasised that biochar is not a universal solution, but a versatile and promising material whose effectiveness depends on appropriate quality, formulation, and system integration. As the construction sector moves toward climate neutrality, biochar offers a unique opportunity to combine carbon storage, material innovation, and circular resource management.
MODULE 5 — Closing Unit
This final module concludes the course by summarising the key concepts explored throughout the previous modules and helping learners reflect on how biochar can be integrated into construction projects. It also highlights future perspectives, innovation pathways, and opportunities for continued learning within the Circular Carbon Economy.
Learning Outcomes
By the end of this module, learners will be able to:
- Describe the main pyrolysis technologies used to produce biochar
- Identify key quality parameters for construction applications
- Understand EBC and IBI certification criteria
- Evaluate risks and limitations associated with biochar use in buildings
- Assess whether a given biochar is suitable for construction
Biochar offers a unique combination of climate benefits and material performance improvements. Across the modules, learners have explored how biochar can reduce embodied carbon, enhance insulation, regulate moisture, improve indoor air quality, and contribute to durable, low-impact construction systems.
In Module 1, learners discovered why biochar is relevant for the construction sector, including its connection to planetary boundaries, its distinction from charcoal, and its origins in Amazonian Terra Preta. In Module 2, learners examined biochar-clay composites through a comprehensive set of thermal and mechanical exercises. Module 3 focused on asphalt and road engineering, including a full LCA with numerical data and waste heat recovery analysis. Module 4 introduced pyrolysis technologies, surface chemistry, quality criteria, and certification standards.
Innovation in biochar construction materials is accelerating. Several promising directions include:
- Low-carbon concrete: Biochar can partially replace cement or aggregates, reducing embodied carbon and improving certain mechanical properties
- Green roofs: Biochar can be integrated into substrate systems to improve water retention, reduce weight, and support urban biodiversity
- Water filtration: Biochar’s adsorption capacity makes it suitable for filtration systems in buildings or urban infrastructure
- Indoor air quality solutions: Biochar can be used in breathable plasters or wall systems to regulate humidity and adsorb pollutants
- Advanced composites: Biochar can be combined with natural fibres, lime, or hemp to create high-performance, low-impact materials
- Urban climate adaptation: Biochar can support sponge city concepts by improving water retention in soils and underground structures
As production scales up and certification frameworks evolve, biochar is expected to play a growing role in sustainable construction.
Self-Assessment
- Which construction application of biochar do you find most promising (insulation, asphalt, plasters, concrete, IAQ)?
- What barriers exist in your region (cost, supply, regulations, awareness)?
- How could you integrate biochar into a future project in your professional context?
- Which module of the course was most relevant for your needs, and why?
- What further knowledge or training would help you move from learning to implementation?
Personal Action Plan
Write a short action plan (5-8 sentences) describing how you could apply the knowledge gained in this course. Consider: one potential project or application; the stakeholders you would need to involve; the type of biochar required; the expected benefits; the next steps you could take in the coming months.