A review on direct coal liquefaction residue applied in asphalt pavements

Direct coal liquefaction residue (DCLR) is a by-product of coal-to-liquid technology when raw coal is converted into liquid fuel, which is harmful to the ecological environment and human health. DCLR can be developed into a road asphalt modifier via different techniques, that can improve and enhance asphalt performance. In this paper, the global research status of DCLR in asphalt pavement is summarized to promote the further application of DCLR in this field. Firstly, the properties of DCLR are introduced, including physical, chemical and structural properties. Secondly, the comprehensive utilization of DCLR is elaborated, and the research progress of DCLR in asphalt pavement is summarized. Part I introduces the effect of DCLR on asphalt performance, including basic performance, rheological properties, adhesion properties, aging resistance and temperature susceptibility. Part II explores the effect of DCLR on the performance of asphalt mortar, including high- and low-temperature performance, fatigue performance and rheological properties. Part III analyzes the effect of DCLR on asphalt mixture performance, mainly regarding road performance, which focuses on rutting resistance. Finally, the existing problems and deficiencies in the current research of DCLR modified asphalt or modified asphalt mixtures are summarized, and the following relevant suggestions are proposed. The future research direction is to improve the low-temperature and fatigue performance of DCLR modified asphalt. In terms of rheological, adhesion and anti-aging properties, it is necessary to add micro-mechanism analysis, adopt a multi-dimensional evaluation system, and introduce test methods under a multi-factor coupling environment. It is essential to further clarify the constitutive relationship of DCLR modified asphalt mixture and further evaluate its fatigue property and long-term durability. DCLR not only participates in asphalt modification but also plays the role of aggregate filling. It is important to further explore the possible cross composite modification mechanism of DCLR in the mixture while using DCLR as aggregate or filler.