Biochar in construction

[edit] Introduction

In general, biochar is traditionally associated with agriculture as a material that might be added to soil or compost to improve the health and quality of it as a growing medium. It is also often connected with water filter systems and in relation to charcoal as a fuel. In terms of construction, one might be tempted to relate the traditional charring of timber cladding, which has been carried out in Japan since the 1600s, along with biochar, but there are some significant differences between the nature of these products and biochar.

Biochar is made by heating biomass to a temperature of 400–800°C in the absence of oxygen in a controlled process called pyrolysis. Charred timber cladding is created simply by burning or scorching the surface of timber in air, thus creating a protective charred coating a few millimetresthick. Charcoal is produced at lower temperatures, often less controlled. The result being less stable with fewer pores, it is the number and sizes of the pores created in biochar that are in many ways key to its properties. However, there are many uses that are associated with biochar, as the article 'The 55 uses of biochar' by Hans-Peter Schmidt & Kelpie Wilson explains quite clearly.

The construction industry has been for some time under growing pressure to decarbonise, reduce waste, and deliver healthier, more sustainable buildings. As part of this shift, more and more innovative materials and combinations of materials have been explored not only to minimise environmental impacts but also, ideally, to actively improve the performance of the built environment. Is or could biochar potentially be one such material that covers both grounds?

[edit] What is biochar?

The 'European Biochar Certificate – Guidelines for a Sustainable Production of Biochar' EBC (2012-2022). Carbon Standards International (CSI), Frick, Switzerland. (http://european-biochar.org). Version 10.2 from 8th Dec 2022 defines biochar thus:

"Biochar is a porous, carbonaceous material that is produced by pyrolysis of biomass and is applied in such a way that the contained carbon remains stored as a long-term C sink or replaces fossil carbon in industrial manufacturing. It is not made to be burnt for energy generation. Biochar is produced by biomass pyrolysis, a process whereby organic substances are broken down at temperatures ranging from 350°C to 1000°C in a low-oxygen process. Although torrefaction, hydrothermal carbonisation and coke production are carbonisation processes, the end products cannot, however, be called biochar under the above definition. Biochars are therefore specific pyrolysis chars characterised by their additional environmentally sustainable production, quality and usage features. Gasification is understood as being part of the pyrolysis technology spectrum and can, if optimised for biochar production, be equally certified under the EBC. Biochar is defined by its quality characteristics, by the raw materials used, its sustainable production and its end use. Biochar is a hyper-versatile material with an increasing number of applications in agriculture, environmental engineering, and basic industry. Each application, like the use as a soil amendment, stormwater filter, or additive for building materials, textiles, and plastics, demands specific biochar qualities. Thus, each application requires proper certification parameters that must be specified, controlled, and guaranteed."

In the article from the Biochar Journal, "The use of biochar as building material" by Hans-Peter Schmidt, biochar is described as "made by heating biomass to a temperature of 400 – 800°C in the absence of oxygen." The process used is called pyrolysis. The resultant material is characterised by high specific surfaces of more than 300 m² per gram, distributed over countless nano-, micro- and meso-pores. The ability of these pores to store water makes biochar a very efficient medium for storing moisture. The pores also trap large quantities of practically immobile air, with the result that biochar constitutes one of the best currently known insulation materials. The building of the Ithaka Institute in Switzerland was the first to be restored using biochar plaster."

The article goes on to say that "two of biochar’s properties are its extremely low thermal conductivity and its ability to absorb water up to 5 times its weight. These properties mean that biochar is just the right material for insulating buildings and regulating humidity. In combination with clay, but also with lime and cement mortar, biochar can be used as an additive for plaster or for bricks and concrete elements at a ratio of up to 80%. This blending creates inside walls with excellent insulation and breathing properties, able to maintain humidity levels in a room at 45–70% in both summer and winter. This prevents not only the air inside the rooms from becoming too dry, which is a potential cause of respiratory problems and allergies, but also condensation from forming around thermal bridges and on outside walls, which would lead to the formation of mould."

[edit] The potential benefits of biochar in construction

The most obvious benefit of biochar so far, and in many ways the reason for increased interest in its use, is the potential dual benefit of both locking away carbon from biogenic materials and also being able to do so specifically from waste biogenic materials. In particular, waste biogenic materials that are impure, such as composite materials that may also contain manmade binders or glues, such as LVL, chipboard and so on. Though biochar production can also make use of purer materials such as forestry residues, fire protection cuttings and crop waste. This diverts material from landfill or incineration, as well as open fires in rural areas, creating a circular economy model where both pure natural waste and manufactured waste have the potential to become valuable construction inputs, reduce waste pressures and sequester any contained carbon.

Beyond that biochar can play a unique role in improving indoor air quality due to its exceptional porosity and surface chemistry. Its micro- and nano-pores create a vast internal surface area can be capable of adsorbing volatile organic compounds (VOCs), unpleasant odours, and airborne toxins that typically accumulate inside homes. When used with clay or lime a biochar plaster can help to filter and neutralise contaminants, whilst natural antibacterial and antifungal properties may inhibit the growth of mould and harmful microbes, further reducing exposure to allergens and respiratory irritants.

Furthermore "two of biochar’s properties are its extremely low thermal conductivity and its ability to absorb water up to 5 times its weight. These properties mean that biochar is just the right material for insulating buildings and regulating humidity. In combination with clay, but also with lime and cement mortar, biochar can be used as an additive for plaster or for bricks and concrete elements at a ratio of up to 80%. This blending creates inside walls with excellent insulation and breathing properties, able to maintain humidity levels in a room at 45 – 70% in both summer and winter. This prevents not only that the air inside the rooms become too dry which is a potential cause of respiratory problems and allergies, but prevents also condensation from forming around thermal bridges and on outside walls which would lead to the formation of mold." (The use of biochar as building material by Hans-Peter Schmidt. Biochar Journal)

Dependent on the feed material, biochars ability to absorb up to five times its weight in water, means plasters can keep humidity levels more stable, preventing dry air that can irritate mucous membranes and reduce the risk of condensation that encourages mould growth. This balance not only supports respiratory health but also contributes to a more comfortable and productive indoor environment. It has also been said that it has an ability to absorb electromagnetic radiation and reduce electrostatic charging, creating a cleaner, safer, and more pleasant atmosphere in living spaces.

[edit] Current uses of biochar in construction

[edit] Cementitious Composites

Concrete is the most widely used construction material but also one of the largest sources of global CO emissions. Incorporating biochar into cementitious materials can provide a number of potential benefits. Firstly, because it is effectively an aerated product (it has lots of holes), biochar (and in effect charcoal) can act as a filler, increasing the bulk of a cement mix. In a similar way to the creation of aircrete or the addition of aerated minerals to a mix such as perlite.

When biochar is finely ground and has sufficient amorphous silica (SiO) content as well as suitable carbon content, it can potentially act as pozzolan. That means it is a siliceous or silicoaluminous material that, in finely divided form and with moisture, chemically reacts with the calcium hydroxide (lime) produced during cement hydration to form additional compounds with cementitious (bonding) properties. As such, it can substitute a portion of the cement that is used in concrete mixes, thus reducing the carbon footprint of the production of the end concrete product.

Some research has also shown that biochar can enhance compressive and flexural strength by refining the pore structure of concrete (Study of biochar in cementitious materials for developing green concrete composites and Progress and prospects of biochar as concrete filler: A review). There is also some evidence that biochar can reduce the permeability of cement mortars, which lowers the risk of water ingress and the possibility of chemical attack. (Comparing the influence of inert biochar and silica-rich biochar on cement mortar – Hydration kinetics and durability under chloride and sulphate environments and the effect of biochar on the mechanical and permeability properties of concrete exposed to elevated temperature)

[edit] Lime renders and plasters

[edit] Asphalt composites

Asphalt uses bitumen (a petroleum product) to bind aggregates in a similar way to cement but leading to a softer, more flexible product in some ways and a durable product in other ways because of its flexibility. As such, it is often used in the construction of roads and sometimes as a roofing material. The benefits of using biochar in asphalt mixes have been primarily as a carbon sink, but there has been some indication of beneficial characteristics such as increased viscosity and stiffness of the asphalt binder, leading to enhanced resistance to rutting (permanent deformation) and cracking, perhaps also helping slow down the ageing of asphalt materials. Due to its absorbent properties, biochar has also been investigated for use in protective coatings and sealants to improve resistance to moisture penetration and extend the service life of building surfaces.

[edit] Other composites

[edit] Landscaping and Urban Infrastructure

In urban developments, biochar is used in green roofs, stormwater management systems, and landscaping soils. It helps manage water retention, reduce runoff, and enhance biodiversity around built environments.

[edit] Performance Benefits for Buildings

Biochar’s potentially insulating properties help reduce heating and cooling loads in buildings; combined with its ability to regulate moisture, this can help lower peak differences in temperature, thus helping reduce operational energy use. The porous structure of biochar can absorb and release moisture without degrading, helping regulate indoor humidity and reducing condensation risks. Improving occupant comfort and potentially lowering the risk of mould growth. Because of the cellular nature of biochar, it has the potential to dampen sound transmission. Depending on the feed material, biochar has the potential to be non-toxic but because of its processing is generally fire-resistant. Potentially charring rather than melting (dependent on feed material and correct processing).

[edit] Potential Economic and Social Benefits

In a waste economy where many virgin composite biogenic materials are produced daily, biochar has the potential to introduce circularity that may otherwise be difficult because of the composite nature of the materials. In rural and developing economies biochar has the potential to create useful materials from natural waste products and the potential to estimate carbon savings through sequestration. This in turn can help balance carbon economies, creating job opportunities whilst reducing the need for higher carbon materials. Biochar has the potential to open up new markets for sustainable construction products, firstly in cycling impure waste products from higher carbon emitting countries and secondly in tracking natural waste product use for carbon credits in lower carbon emitting countries.

[edit] Challenges and Considerations

Despite its promise, there are many challenges to address before biochar might become a mainstay in construction. First is standardisation; because of the complex nature of the product, its characteristics and benefits, a lack of consistent product standards can limit wider adoption. The 'European Biochar Certificate – Guidelines for a Sustainable Production of Biochar' goes some way to providing this, but clear international specifications for particle size, strength, purity and the potential for recognised inclusion in carbon calculations are needed.

Large-scale production of biochar remains somewhat limited, and supply chains must be scaled up to meet potential industry demand alongside third-party process checking-based manufacturing and feedstock. This may need to include certification and regulation, integrating such solutions into potentially new regulations that could include embodied carbon as well as operational carbon. This would clearly also need to include standard regulations regarding toxicity and fire resistance. Whilst many successful trials, tests and pilots have been carried out, longer-term field test data is still emerging.

[edit] Where next?

As the UK and global construction sectors move towards net-zero targets, demand for low-carbon, high-performance materials will accelerate. Biochar has the potential to become a cornerstone of sustainable construction in a circular economy, particularly in applications where carbon sequestration, lightweight strength, and thermal performance are critical. Ongoing research into hybrid composites, scalable manufacturing, and lifecycle analysis will strengthen its case for widespread adoption. Coupled both through government regulations to include embodied carbon calculation requirements and potentially incentives for carbon-negative materials, as well as internationally recognising the potential economic value of carbon capture in small-scale initiatives. Biochar could transition from a niche innovation to a mainstream building product within the next decade and help balance financial and carbon deficit models.

Biochar represents a rare opportunity in construction, where a material not only improves performance day to day but also actively reduces environmental impact by storing carbon over the long term. Its versatility, from concrete and asphalt to insulation and wall panels, makes it applicable across the full spectrum of building projects. Meanwhile, beyond its technical benefits, biochar supports circular economy principles whilst having the potential to provide social and economic value via carbon trading to economies most affected by climate change, balancing and strengthening climate resilience. While challenges around supply, standards, and cost remain, the momentum behind biochar reflects a sector urgently seeking sustainable solutions. If scaled effectively, biochar could help transform construction from one of the largest sources of carbon emissions into a proactive driver of climate mitigation and safer, more resilient buildings.

[edit] Related articles on Designing Buildings

• Biofuel
• Biochar
• Charcoal
• Charred timber cladding
• Container laboratory helping slow climate change
• Engineered bamboo.
• Finding the right Sawdust Charcoal Machine
• Modified wood.
• Panelling.
• Petrification
• Plywood.
• Pozzolans.
• Sustainable timber.
• The differences between hardwood and softwood.
• The Art of Pyrography.
• Timber.
• Timber vs wood.
• Types and scale of carbon capture usage and storage
• Types of rapidly renewable content
• Types of timber.

[edit] External links

https://www.ithaka-institut.org/en/

https://www.biochar-journal.org/en

Burn: Using Fire to Cool the Earth by Albert Bates (Author), Kathleen Draper (Author)