Nano-aggregation of asphaltenes and its influence on the multiscale properties of bitumen recycled through multiple ageing and rejuvenation cycles

With the growing emphasis on the circular economy in road materials, the possibility of continually recycling of bitumen through multiple life cycles has become a subject of increasing interest. A key factor limiting the recyclability of aged bitumen is its deteriorated performance, driven by molecular-level transformations such as the nanoscale aggregation of discretised molecules such as asphaltenes. This study investigated the impact of nano-aggregation of asphaltenes and its impact on the multiscale properties of bitumen during cycles of repeated ageing and rejuvenation. Bitumen was laboratory-aged then rejuvenated using a bio-rejuvenator for four cycles of ageing and rejuvenation. A comprehensive multiscale evaluation was conducted, including examining the nano-aggregation behaviour of asphaltenes using small angle X-ray scattering (SAXS), microstructures and micromechanics using atomic force microscopy (AFM), chemical composition using gas chromatography–mass spectrometry (GC–MS) and thin-layer chromatography–flame ionisation detection (TLC-FID), and rheological properties using a dynamic shear rheometer (DSR). The results indicated that the bio-rejuvenator effectively disaggregated asphaltene clusters, and the polydispersity characteristics of bitumen molecules remained comparable to those of virgin bitumen during multiple recycling. Repeated recycling may not continuously degrade bitumen properties but rather lead to a self-stabilising behaviour as observed in the nanoscale. Moreover, although repeated ageing and rejuvenation cycles altered the microstructures of bitumen and increased the content of resins while keeping the content of asphaltenes dynamically stable, the overall performance—including low-temperature performance, fatigue resistance, and high-temperature stability—remained within acceptable limits. These findings support the feasibility of multiple recycling cycles of bitumen using specific rejuvenators.