Direct Observation of Crystalline Wax in Asphalt Binders by Variable-Temperature Polarizing Microscope

Crystalline wax in asphalt binder greatly affects the low-temperature performance of the asphalt pavement, and the composition of the phase separation at the microscopic scale has not been completely determined. In this paper, it is demonstrated that of the different observing systems of a microscope, the reflected polarization system is the best method to observe the morphology of the crystalline wax clearly. The components of the phase separation (excluding asphaltene) is also validated. On this basis, base asphalts doped with pure wax and eight asphalts at three aging states were observed. The results showed that the shape of the pure wax changed first from round to polymorphic, and then to an aggregate with an increase in the carbon number of the pure waxes. In addition, aging increased the intermolecular force and solubility of the wax in the asphalt binder. Finally, low temperature reduced the solubility of the wax in the asphalt binder to form a crystalline wax, which can provide an explanation for the physical (thermoreversible) aging of the asphalt binder at low temperature.