Upcycling of pyrolytic char waste as a filler for circular asphalt pavement infrastructure

Incorporating industrial waste into road pavements is a key strategy for advancing asphalt decarbonisation. This manuscript reports the first study into the potential use of a byproduct (defined as PMMA-char) from the industrial pyrolysis of commercial poly(methyl methacrylate) (PMMA) plastic as a sustainable alternative to conventional mineral fillers, namely limestone and hydrated lime, in the asphalt mastic part of an asphalt mixture. Physicochemical characterisation revealed that PMMA-char contained calcite, barium sulfate, and amorphous carbon, exhibiting mineralogical similarities to limestone, which supports its potential as a substitute filler. Various asphalt mastic combinations were prepared by replacing traditional fillers with PMMA-char from 0 to 100% levels, and their rheological properties were evaluated. The overall findings showed that the optimum PMMA-char replacement level depended on the rheological performance metric considered. For limestone-based mastics, the high-temperature Performance Grade (PG) critical temperature peaked at 25–50% replacement, while the fatigue life reached its maximum at 75% replacement. For hydrated lime-based mastics, a 25% replacement level enhanced the high-temperature mastic performance, whereas fatigue life decreased progressively with increasing PMMA-char content. These differences are likely due to variations in microstructure, void ratio, and specific surface area among the fillers, affecting the mastic's effective bitumen fraction. Given that PMMA-char is an industrial byproduct with low economic value, it offers significant promise as a viable low-carbon filler replacement for pavements when compared to current commercial mastic using traditional mineral fillers, which generate significant environmental impact, including high energy consumption, resource depletion, and associated CO2 emissions.