Houses, roads, industrial parks – all are built with concrete. Increasing scarcity of resources and the high share of the concrete industry in global greenhouse gas emissions (11 percent) are challenging businesses worldwide to improve their problematic life cycle assessment. It is not surprising how quickly the good news is spreading that biochar can make a decisive contribution to climate protection in the construction industry – by replacing cement and storing carbon.
Benefits of Concrete with Biochar
- Substitution of cement in concrete
- Improved mechanical properties such as compressive strength and fire resistance1Salim Barbhuiya, Bibhuti Bhusan Das, Fragkoulis Kanavaris (2024) “Biochar-concrete: A comprehensive review of properties, production and sustainability” Case Studies in Construction Materials
- Reduced carbon footprint of buildings and road infrastructure
- When incorporated into roads: pollutant retention, adsorption of de-icing salts and petroleum residues, filtration of rainwater
The use of biochar in the construction industry is comparatively novel. It offers great potential for sustainable development: cement can be proportionally replaced by biochar in concrete, and this has been proven without any loss of properties. In addition, the high C-fix content of biochar additives can bind the carbon. Material loops are closed by converting unused organic residual biomass into the stable state of biochar.
Scientific Evidence
Biochar is an efficient binder that can be used in concrete. In certain mixes, the targeted addition can improve flexural strength and fire resistance2Shailey Singhal (2022) Biochar as a cost-effective and eco-friendly substitute for binder in concrete: a review, European Journal of Environmental and Civil Engineering, DOI: 10.1080/19648189.2022.2068658.
In addition to mechanical strength and thermal conductivity, the electromagnetic performance of hardened biochar-cement composites can also be improved. Furthermore, the addition of biochar can improve the resistance to sulphate attack, chloride-induced corrosion damage, shrinkage and permeability of biochar-cement composites. Biochar also has the potential to reduce the permeability of concrete3Xuqun Lin et.al. (2023) “Biochar-cement concrete toward decarbonisation and sustainability for construction: Characteristic, performance and perspective” Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2023.138219.
In buildings, biochar provides chemical stability, water holding capacity and crack resistance of concrete4Shailey Singhal (2022) Biochar as a cost-effective and eco-friendly substitute for binder in concrete: a review, European Journal of Environmental and Civil Engineering, DOI: 10.1080/19648189.2022.2068658.
In roofs and other lightweight construction applications, the weight of concrete is reduced by the biochar. Silicate aggregates in concrete can basically be replaced by biochar. Biochar concrete also filters surface run-off or infiltrating rainwater and adsorbs salts and petroleum residues washed off the road. Low-quality biochar can also be used for all of the above applications.
Biochar (up to 5% by weight) can also have a positive effect on the durability of concrete. This is attributed to the improved hydration and physical filling, which prevents the infiltration of ions and water5Xuqun Lin et.al. (2023) “Biochar-cement concrete toward decarbonisation and sustainability for construction: Characteristic, performance and perspective” Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2023.138219.
Concrete not only can become carbon neutral in its carbon footprint by adding biochar, but even climate positive, and with improved material properties as well. This is possible by replacing the scarce and conflictual amendments sand and gravel for concrete with locally produced biochar. Just adding one percent by mass of biochar to concrete could sequester about 20% of the annual CO2 emissions caused by the cement industry6Banjo A. Akinyemi, Adeyemi Adesina, Recent advancements in the use of biochar for cementitious applications: A review, Journal of Building Engineering, Volume 32, 2020, 101705, ISSN 2352-7102, https://doi.org/10.1016/j.jobe.2020.101705..
To clarify the terminology, ‘technical carbon’ of biogenic origin or the word ‘biocarbon’ is often used synonymously with biochar
Degree of Market Maturity
Within the last few years, some products have reached market readiness. Concrete containing biochar is already available in the form of precast concrete elements and concrete paving blocks.
Currently, various formulations of concrete containing biochar are being developed in a number of public and private research projects. For example, research is being conducted on the use of biochar in combination with calcium carbonate (CaCO3) in concrete, to create carbon storage systems based on carbon neutral concrete7Winters D, Boakye K, Simske S. Toward Carbon-Neutral Concrete through Biochar–Cement–Calcium Carbonate Composites: A Critical Review. Sustainability. 2022; 14(8):4633. https://doi.org/10.3390/su14084633.
Application
If biochar is used in concrete and cement, it is advisable to comply with the regional laws on harmful substances.
For the production of concrete containing biochar, an addition of 1-3% biochar is considered optimal.8Shailey Singhal (2022) Biochar as a cost-effective and eco-friendly substitute for binder in concrete: a review, European Journal of Environmental and Civil Engineering, DOI: 10.1080/19648189.2022.2068658. Other suppliers add more than 10% fine-grained biochar to concrete.
This is also confirmed by Xuqun Lin et al (2023) in their review, where they state 1-2 wt.% cement replacement by biochar as the optimum for compressive and flexural strength9Xuqun Lin et.al. (2023) “Biochar-cement concrete toward decarbonisation and sustainability for construction: Characteristic, performance and perspective” Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2023.138219. Other suppliers add more than 10 per cent by volume of fine-grained biochar to the concrete.
For targeted use, the biochar must be carefully selected, as each biochar is different due to the source materials as well as the way it is produced. Also biochar with low-quality, low-PAH that has undergone at least one use cycle without significant degradation of the c content shall be favored, such as filter residues. At the moment no degradation rates are assumed for the biochar when used in concrete. However, scientists and practisioners demand research on long-term durability properties under different environmental conditions.
Certification
Biochar intended for usage in a concrete product can be certified according to the European Biochar Certificate (EBC) in the lowest quality class EBC-BasicMaterial, which also complies with the requirements of the EU REACH Regulation10REACH COMMISSION REGULATION (EU) No 1272/2013 of 6 December 2013 amending Annex XVII to Regulation (EC) No 1907/2006 of the European Parliament and of the Council on the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) as regards pol. EU-Regulation 2013, 1272/2013. This ensures basic requirements for the biochar11EBC (2012-2022) ‘European Biochar Certificate – Richtlinien für die Zertifizierung von
Pflanzenkohle’, Ithaka Institute, Arbaz, Switzerland. http://www.european-biochar.org
Version 10.1G vom 10. Januar 2022.
Since 2022, it has been possible in Germany to have a concrete product with a reduced carbon footprint certified according to the Concrete Sustainability Council (CSC) certification for the cement, raw materials, and concrete industry12Concrete Sustanablity Council https://www.csc-zertifizierung.de/. Concrete manufacturers who produce low-CO2 concretes can be certified according to the CSC CO2 module, which can be verified using recognised LCA tools or EPDs. Producers of only biochar cannot be certified.
In concrete, there is little possibility of heavy metals being leached out. For this reason, EBC-BasicMaterial only requires the declaration of heavy metal contents, but does not specify threshold values. Organic pollutants must in part be analytically verified in compliance with specific threshold values. The standard also provides that EBC-BasicMaterial may only be “traded to other companies (B2B) “that fulfil certain handling requirements.
If an concrete additive is to be used outside the European requirements, since September 2023 the World Biochar Certificate (WBC) can be acquired using the WBC material class to guarantee sustainable production.
Another class is EBC-ConsumerMaterials, which, like EBC-BasicMaterial, is intended for non-soil applications, but has a stricter PAH threshold value for use in contact with living organisms and ecosystems13PAH = Polycyclic aromatic hydrocarbons. A third class is EBC-Urban, which is used for the application of biochar in e.g. urban greenery and rainwater filtration and is intended to prevent groundwater and surface water from becoming contaminated. In contrast to EBC-BasicMaterial, EBC-ConsumerMaterial has a stricter threshold value for EFSA PAHs (1 g/t dry matter) and the declaration of a further 16 organic pollutants in accordance with EPA specifications14European Food Safety Aithority, EFSA opinion on suitable indicators for both the occurrence and toxicity of polycyclic aromatic hydrocarbons (PAHs) in food, 4 August 2008https://www.efsa.europa.eu/en/news/efsa-opinion-suitable-indicators-both-occurrence-and-toxicity-polycyclic.
These differences result from the “acceptability of biochar for a specific purpose in relation to applicable laws, regulations and relevant industry standards” regardless of its quality. Further classes are to be introduced in 2022, aligned with demand, e.g. specific to building materials.15EBC (2012-2022) ‘European Biochar Certificate – Guidelines for a Sustainable Production of Biochar.’ European
Biochar Foundation (EBC), Arbaz, Switzerland. (http://european-biochar.org). Version 10.1 from 10th Jan 2022
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