An emerging strategy to remove CO2 from the atmosphere is based on pyrolysis of biomass. In this way, the residual carbon-rich char (biochar) is obtained. Yet, for this strategy to become a proper negative emission technology (NET), the carbon separated in the pyrolysis process needs to be safely stored for very long times so that it does not come back to the atmosphere due to burning or gradual decomposition. The issue of long-term storage of the biochar becomes critical considering the potentially large quantities that may need to be secured in the near future.

The building materials, in particular concrete, could be a promising high-capacity sink for the biochar. In this way, the original emissions of the cement and concrete industry could be compensated, potentially in full. Further, concrete could safely store for centuries the types of biochar that are not appropriate for agricultural use.

Currently, different research projects and first practical solutions worldwide are devoted to integration of biochar into building and construction materials (in particular concrete or asphalt). The biochar in those applications is in most cases used without any significant modification. This leads to a number of practical issues that arise e.g. due to high water demand of the biochar, risk of excessive dusting, potential incompatibility with other components (e.g. plasticizers in concrete).

To address these issues and still benefit from the sequestration of carbon in concrete we propose an alternative solution.

We developed a method to produce biochar-rich lightweight aggregates for concrete.

In our solution, biochar is first processed into lightweight aggregates by pelletization with water and small amount of hydraulic binder. The pellets then harden in a cold-bonding process. In this way, carbon-rich, porous lightweight aggregates (C-LWA) are obtained that are easier to handle than the pure biochar and can be used much like the conventional lightweight aggregates.

When all emissions from the production, pyrolysis and other associate process are included, the pellets are still a high negative emissions material, with about -1 kg CO₂/kg emissions.

Our studies show that it is possible to incorporate about 20 vol-% of C-LWA into concrete (Fig. 1c) while maintaining a good structural performance (strength classes C20/25 – C30/37). With this approach, concrete can be turned into a functional carbon sink - in fact, we demonstrate that concretes with the C-LWA can reach net-zero greenhouse gas emissions.