Is biochar suitable as a construction material?

Biochar is a porous material made from the pyrolysis of wood and other plant material. Diksha Pandey writes

Woman in kitchen
Biochar performs well on humidity control. It might help avoid interiors becoming too dry, reducing indoor air pollution, attractive in countries like India.

Biochar has applications in soil improvement, waste management, geo-engineering and climate mitigation. It has also received some attention in recent years as a potential building material, with one possibility being its use as an additive or replacement in cementitious composites.

As a construction material it has the advantages of structural strength and permeability, as well as being attractive as a carbon-sequestering additive. It provides great chemical stability, low thermal conductivity, and low flammability

Ongoing population growth and the desire for a better built environment presents a challenge for the construction industry in keeping CO2 emissions within a desirable level. Reducing the use of cement-based building materials is an obvious priority. Engineered biochar has potential in the building industry as a CO2-absorbent material.

Biochar’s has been used as a construction material in projects employing a variety of raw materials and production processes. The structure of the biochar can be modified via variations in parameters such as the pyrolysis temperature, rate, and pressure (the ones with with the most direct bearing on the textural qualities).

In 2013, the Ithaka Institute in Switzerland produced the first structure using this material, which is currently undergoing testing. In construction, the material has been demonstrated to perform well on both insulation and humidity control. There are also opportunities to use char-clay to improve historic buildings which suffer from inadequate insulation, humidity difficulties, or contamination from chemicals such as lead paint.

Some of the principal applications of biochar in construction materials are as follows:
1) Insulation material: Biochar is an exceptionally efficient media for storing moisture given its textural properties and very high porosity. It has low heat conductivity and can absorb up to five times its weight in water.

2) Biochar-based clay and lime plasters: In combination with clay, lime, and cement mortar, biochar can be used as an ingredient for plaster at a ratio of up to 80%. The Ithaka Institute has created biochar-based clay and lime plasters, with black carbon accounting for up to 80% of the material. This high proportion is possible because biochar can totally replace sand, resulting in a plaster that is five times lighter than conventional plaster due to its high porosity.

The biochar-clay plaster also provides outstanding insulation, humidity control, and electromagnetic radiation mitigation, in addition to carbon storage. When used for inside walls such materials can allow humidity levels to be maintained at 45–70% in both summer and winter.

3) Building bricks, tiles and concrete: Biochar can also be used to make building materials such as bricks and tiles. Brick prototypes haveincluded a binder such as cement or lime and have provided a tensile strength of 20 N/mm2, compared to around 3.5 N/mm2 for an ordinary brick. According to the research, bricks made with 50% biochar and 50% high-density polyethylene have the highest compressive strength, and biochar-cement bricks outperform ordinary bricks in terms of insulating value, hardness, and water absorption.

As a geo-engineering material, biochar has only been explored in the lab and on a
small scale. Research is needed to establish its viability for building materials designed with this purpose, and to establish their usability in the field, for things like insulating materials, roof tiles, bricks, tiles, and concrete.

Benefits of biochar for construction materials include its high surface area, porosity, and mineral content.

Future perspective
The Ithaka institute building is undergoing testing. Its performance in relation to insulation and humidity control appears promising – properties owed to the material’s low thermal conductivity and ability to absorb water.

In might be attractive countries like India, where temperatures reach 40°C. Buildings made with biochar will also help reduce indoor pollutants not only by preventing the air inside the rooms from becoming too dry (a potential cause of respiratory problems and allergies) but also by preventing condensation from forming around thermal bridges and on outside walls which would otherwise lead to the formation of mold.

• The author is a PhD candidate in the department of environmental science, GB Pant University of Agriculture and Technology, Uttarakhand, India.