This week (6 June) saw Shell Bitumen officially launch a new type of bitumen called Shell Bitumen CarbonSink. In essence, it is a mixture of traditional bitumen and a similar substance derived from a plant.

Other companies are doing the same. If you are reading about biocomponent binder, biogenic asphalt or bio-derived binder, you are probably reading about the same thing.

With carbon sequestration, and how to do it, a hot topic right now, Shell’s name is well chosen. Using plant-based materials in roads means that the carbon dioxide which the plants have taken in and converted to biomass, becomes part of the road. Shell’s argument is that the carbon taken in – or sequestrated – will be locked in forever because asphalt can be recycled and reused, theoretically ad infinitum.

Shell reckons that by using its CarbonSink binder, the carbon footprint of 1 tonne of asphalt mix can be reduced by 250kg, working on a binder content of 5%. This would equate to 6 tonnes of carbon saved for every kilometre of road, assuming it was used for the surface layer which was 50mm deep and 3.5m wide.

Shell isn’t saying what sort of plant-based material it will use in CarbonSink, but has said that it could vary, depending on the country in which it is produced. A commonly used binder replacement material is tall oil pitch. Tall oil is a by-product of wood pulp manufacture and tall oil pitch is the residue from the distillation of that oil. Other bio-binders include lignin, also a by-product of the wood pulping process, polymerised vegetable oils such as soy or canola and even microalgae.

These types of material were first investigated with a view to rejuvenating recycled asphalt planings (RAP) to make the bitumen in the RAP less brittle. There are quite a few plant-based rejuvenators already on the market which are used relatively widely – but the quantities used are much smaller than would be used in a so-called biogenic asphalt.

By using a biogenic binder, decent proportions of RAP and mixing the asphalt at lower temperatures, it should be possible to create a lower carbon asphalt that still has a long life. In the UK, Aggregate Industries has launched a product which combines SuperLow warm mix range of asphalts with Shell Bitumen’s CarbonSink. It is called SuperLow-Carbon.  Bitumen specialist Nynas has developed a range called Nypol RE which is a polymer modified bitumen (PMB) containing biogenic binder, the aim being to have a lower carbon footprint and a longer life thanks to the PMB – which is vital in lowering whole-life carbon emissions.

Of course, if we are looking at capital cost only, a product such as CarbonSink will be more expensive per tonne than bog standard bitumen. But as carbon pricing regimes kick in, and highway authorities are able to work on whole life cost and whole life carbon, they may become a more attractive option.

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