Transport is the largest source of greenhouse gas emissions in the European Union and has made little overall progress in reducing them sustainably in recent decades. While public and political attention is focused mainly on the electrification of vehicles themselves, the infrastructure – the roads directly beneath their wheels – remains almost entirely dependent on fossil fuels. The main building material for its construction is asphalt mixture, the key binder of which is bitumen, a product of oil refining.

Oil extraction and processing leave a huge carbon footprint. The latest Eurobitume Life Cycle Assessment (LCA 4.0) report warns that oil extraction is responsible for around 70 % of the global warming potential (GWP) associated with bitumen production. The remaining 22 % are emissions from refining and transportation. More accurate monitoring, including the inclusion of leaks, flaring and methane venting directly at oil wells according to the latest AR6 methodology, has pushed the GWP of bitumen up to 530 kg CO2-equivalent per tonne. The transition of transport to true carbon neutrality therefore inevitably requires not only a change in vehicle propulsion, but also an urgent decarbonization of building materials themselves.

Innovative Substitutes: The Rise of Bio-Asphalt and Cement-Free Paving

In order to reduce this dependence on oil, researchers in the field of road engineering are actively exploring sustainable alternatives, with one of the most promising innovations being the use of biomass to produce so-called bio-binders. Biomass of various origins, such as wood waste, agricultural residues, microalgae, used cooking oil (WCO) or even pig manure, is transformed into valuable raw materials through advanced thermochemical conversion. Dry materials typically undergo a fast pyrolysis process, which converts the biomass into liquid bio-oils and a solid component called biochar at temperatures of up to 750 °C in the absence of oxygen. Wet biomass, on the other hand, undergoes so-called hydrothermal liquefaction (HTL).

A great example of the application of these technologies is the pilot section of the N-623 road in northern Spain, created within the framework of the EU-funded LIAISON project. An asphalt carpet was laid on a regular road, in which traditional bitumen was replaced by an innovative binder with a 40% share of biological components. This experimental bio-material was able to reduce the carbon footprint by approximately 1,200 kg CO2-equivalent per tonne of binder, representing an extraordinary 75% reduction in emissions at the level of the entire asphalt mix.. The mixture showed excellent mechanical properties – it resisted permanent deformation (rutting), hardening and material fatigue and tolerated the influence of water well. At the same time, the LIAISON project is also testing other materials, such as prefabricated geopolymer concrete pavements (without the use of emission-intensive Portland cement, replaced by steel slag and coal ash), which can save up to 78 % CO2.

Various bio-components bring specific benefits to asphalt. Adding biochar to conventional asphalt reduces oxidation and aging of the mixture and significantly increases resistance to rutting at high summer temperatures, although it can cause brittleness in extreme cold. In contrast bio-oils act as softeners, exhibit excellent properties in cold climates, and reduce the risk of frost cracks and overall improve the penetration ability of the material.

Forest boundaries: Where does the bioeconomy hit its limits?

Technological innovation is tempting, but it brings with it a serious systemic dilemma regarding resource allocation. As Martin Pigeon, a researcher at the Brussels-based NGO Fern, emphatically points out, the problem with the original fossil fuel was the illusion of infinity. „"The problem with fossil fuels is that they make us think that the possibilities are unlimited. But in the bioeconomy, the limit is the forest,"“ warns Pigeon.

The climate crisis, the collapse of biodiversity and over-harvesting are seriously threatening Europe's forests, whose capacity to absorb carbon has been declining in recent years. For this reason, we cannot rely on bio-materials simply replacing fossil-based infrastructure across the board and without limit. For example, up to half of all wood harvested in Europe is currently burned for energy purposes, which Pigeon criticizes, saying that "we can do better" - but only if the overall harvest does not exceed sustainable limits. If the sharp increase in demand for woody biomass, for example for the construction of bio-asphalt roads, were to lead to further intensification of harvesting or the massive planting of monocultures with poor biodiversity, we would only be shifting the problem rather than truly solving it.

In addition, there are purely technical limits. Martin Junginger, professor of bioeconomics at Utrecht University, points out that lignin (one of the basic building blocks of wood), for example, can only satisfactorily replace petroleum bitumen in the asphalt structure up to a level of about 50 %, not 100 %. Higher concentrations cause problematic crystallization and foaming of the mixture during its production. There simply is not enough biological matter on planet Earth to be able to displace the fossil economy one-for-one without destroying nature.

System solution: Cascade principle and circular economy

How to get out of this impasse? The solution is to combine green chemistry with reducing overall resource consumption. The new EU strategy for the bioeconomy, aimed at unlocking the potential of production using biotechnology, clearly defines the priority of replacing fossil materials, but only if biomass is used sustainably. The emphasis is shifting to the targeted recovery of secondary biomass – i.e. agricultural residues and organic waste – instead of extracting primary forests and arable land for technical raw materials. This approach reflects the so-called cascade principle, i.e. prioritizing the use of materials for products with the highest and long-term added value and postponing energy combustion until the very end of the raw material's life cycle.

The importance of this comprehensive approach is also evident at the local Slovak level. The National Action Plan for the Development of the Bioeconomy in the Slovak Republic, prepared by the BIC Bratislava center, emphasizes the necessity of transforming the domestic industry towards "Effective Forestry" and "Waste Economy". Instead of massive exports of unprocessed logs, the plan proposes the creation of cascading and recycling value chains with high added value. Wood waste and residual agricultural biomass, for which there is no longer any use in the food industry (feed) or in the production of high-quality biocomposites, should ideally be directed to green building modifiers, bio-chemical conversion, or finally to modern energy production from waste in order to reduce landfilling, which in our country, with a share of 66 %, is well above the EU average.

Final and The most rational way to achieve true road sustainability is through the interconnection of bio-connectivity and deep circularity.. Conventional roads are damaged, but their material – recycled asphalt (RAP) – can be reused. The old hardened and oxidized bitumen on it can be intelligently „rejuvenated“ with bio-oils processed from waste, such as ordinary cooking oil. This way we get a double ecological benefit. Professor Junginger sums it up with the following words: „Rejuvenating recycled bitumen with bio-binders makes it possible to simultaneously reduce fossil fuel consumption and achieve full material circularity.“.

Decarbonizing road infrastructure using alternative bio-based materials is technically feasible and highly relevant. However, it will not be fully sustainable if society blindly exchanges fossil fuel extraction for massive deforestation. The future of green roads lies in the combination of smart engineering practices, material recycling and, above all, respecting the ecological boundaries of the planet. JRi&CO2AI