Balanced asphalt binder performance with hazelnut shell biochar

Decarbonising road materials requires scalable strategies that enhance durability while valorising waste streams. This study investigates the effect of antioxidant biochar derived from European hazelnut shells (BH) on the operational and rheological performance of a PG 64–22 asphalt binder. BH was produced via slow pyrolysis at 300 °C and incorporated at contents of 2.5, 5.0 and 7.5 wt% using two particle size fractions (S < 20 μm and L < 75 μm). Owing to its carbon-rich composition and the presence of antioxidant compounds reported in previous studies, BH was evaluated as a sustainable modifier with the potential to improve the ageing resistance of asphalt binders. Commercial graphite was included as a control modifier for comparison. Operational properties were assessed through rotational viscosity and storage stability, while rheological performance was evaluated using the rutting parameter G*/sin(δ), Bending Beam Rheometer (BBR), Multiple Stress Creep and Recovery (MSCR), |G*| and δ master curves, rheological ageing index (RAI), and time sweep fatigue tests. BH increased rotational viscosity and mixing and compaction temperatures without compromising storage stability. At high temperatures, BH enhanced rutting resistance by increasing G*/sin(δ) and reducing the non-recoverable creep compliance ( Jnr ), particularly for the finer particle size fraction. At low temperatures, BH increased creep stiffness without significantly affecting the stress relaxation capacity compared with control binders. Master curve and RAI analysis indicated lower susceptibility to long-term ageing-induced stiffening at higher BH contents. BH also improved the fatigue response by increasing accumulated dissipated energy and tended to delay the 50% reduction of the initial |G*| compared with the unmodified binder. Based on the combined rheological response, 5.0 wt% BH with a particle size below 20 μm was selected as the final formulation for this type of modifier.