Influence of coal liquefaction residues as fine aggregate substitution on asphalt mixture performance and adsorption characteristics

This study investigates the effects of partially replacing fine limestone aggregates in asphalt mixtures with indirect coal liquefaction residue (ICLR) and direct coal liquefaction residue (DCLR). Through equal-volume substitution based on the Marshall mix design method, the influence of ICLR and DCLR on pavement performance and asphalt-aggregate interfacial behavior was systematically evaluated. Performance assessments included high-temperature rutting resistance, low-temperature bending, freeze-thaw splitting, and immersion Marshall tests. Adsorption characteristics and asphalt film thickness were also analyzed to elucidate the underlying mechanisms. The results indicate that single-size substitution with ICLR (1.18-2.36 mm or 2.36-4.75 mm) significantly enhances rutting resistance, whereas combined (dual or triple) substitutions result in reduced performance. For DCLR, single-size substitution (0.6-1.18 mm) provided the greatest improvement in high-temperature stability. While ICLR had negligible effects on low-temperature flexibility, DCLR slightly reduced the mixture's low-temperature ductility. Both residues markedly improved moisture resistance, with the most substantial benefits observed in single-size substitutions. Limestone aggregates showed the highest total asphalt adsorption capacity, followed by ICLR and DCLR. However, DCLR exhibited faster adsorption kinetics. Increased asphalt film thickness was negatively correlated with rutting resistance and low-temperature strain but had limited influence on moisture stability. Overall, the study recommends using single-size ICLR (1.18-2.36 mm) or DCLR (0.6-1.18 mm) at substitution rates not exceeding 11% by volume to optimally balance high-temperature performance, moisture resistance, and mechanical durability. These findings support the sustainable application of coal liquefaction residues in asphalt pavement engineering.