Biochar: from waste vegetation to construction materials
- robball6
- May 30
- 3 min read
This week National Highways revealed that biochar – created from heating waste vegetation at very high temperatures – will be trialled on the A417 Missing Link scheme in Gloucestershire. The biochar will be used to filter microplastics from road runoff and to retain water for plants on the deck of a green bridge over the scheme.
The Missing Link project will see a 3.5-mile section of the A417 in the Cotswolds, which is currently single lane, upgraded to a dual carriageway. Contractor Kier, which is delivering the £460m project, is working with soil specialist TerrAffix on the biochar trials. To date, the project has produced 5 tonnes of biochar on site using vegetation cleared to make way for the scheme, reports National Highways.
Biochar is created through pyrolysis, heating organic materials to a very high temperature in the absence of oxygen. Materials such as wood, agricultural waste – as well as vegetation – can be converted into a carbon-rich charcoal.
Water that runs off roads contains a mixture of pollutants that come from cars, road surfaces and markings and rubbish. These include microplastics, heavy metals and hydrocarbons. Often road runoff is not filtered before it enters water courses.
Microplastics, which can negatively impact on the health of wildlife and humans, can be hard to remove due to their small size. Trials in the US using biochar from wood and sugar cane have shown that it can remove microplastics from agricultural water runoff.
Kier and TerrAffix have already carried out laboratory tests with Swansea University which have shown that biochar can remove microplastics from water. Doses of water containing microplastic were passed through stone containing a mixture of stone and biochar over a period of two weeks to mimic rainwater, with no traces of microplastics in the filtered water.
On the deck of the green bridge over the A417, biochar will replace a manmade gel which would usually be used to help retain water in dry periods, helping to keep the plants on the bridge alive. Biochar will be mixed with soil on the deck rather than with granules of gel.
National Highways wants to investigate how the use of biochar can contribute to carbon saving, since the process of making it locks carbon into the biochar. Lorry movements to carry away waste vegetation are also saved.
Biochar in roads
In a different research project, Aston University is working with Cheshire firm Miles Macadam to look at replacing fly ash used in their road surfacing products, which use cement-based grout and asphalt, with biochar. Since fly ash is a byproduct of coal-fire power stations, of which there are now none in the UK, companies that use it will need a more sustainable solution in the future.
Researchers will make grouts using biochar and then test them for various properties. They will also investigate how they behave over freeze-thaw cycles.
Multiple research projects are underway to look at how biochar, made from a variety of sources, could be used to substitute a small amount of the bitumen used in an asphalt mix. Some studies suggest that adding biochar could improve the performance of a road surface over time, although it is early days yet.
The Ashton University study is due to run until summer 2026.
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