Exploring the carbon sequestration of an asphalt base course mixture containing novel cold-bonded biochar-rich lightweight aggregates

An emerging strategy to compensate for the greenhouse gas emissions of products is to incorporate carbonaceous materials obtained from removed atmospheric carbon dioxide, mainly obtained through biomass conversion. This approach can turn asphalt pavements into a functional carbon sink. In particular, biochar has been used as a bitumen modifier. However, due to performance limitations, carbonaceous materials were only added in small quantities to asphalt mixtures. An alternative approach is to produce lightweight aggregates to substitute a part of the mineral aggregates of the asphalt mixture. To this end, biochar is pelletised with a hydraulic binder and water in a cold-bonding process, forming spherical pellets labelled as carbon-rich lightweight aggregates (C-LWA). Like other lightweight aggregates, C-LWA showed a reduced mechanical strength compared to conventional mineral aggregates, adversely affecting the asphalt mixture performance. Cracking and rutting resistance almost linearly decreased with C-LWA content. The direct addition of biochar had a similar adverse influence on the mixture performance. Despite a reduced performance, adding biochar and C-LWA reduces the greenhouse gas emissions of asphalt mixtures. Net-zero emissions were estimated for the produced asphalt mixture by adding 5.5 ± 0.4% C-LWA or 3.0 ± 0.2% biochar obtained from the pyrolysis of landscape management wood. A wider range of C-LWA addition (1% to 35.1%) was estimated considering the greenhouse gas emission estimation variability of both asphalt and biochar production.