Variable-Temperature Raman Spectroscopy Study of the Phase Transition Mechanism in Asphalt Binders

To better understand the mechanism of precipitation and dissolution of the crystalline wax in asphalt, it is necessary to investigate the phase transition process of asphalt at different temperatures. Variable-temperature Raman spectroscopy has been employed to analyze the phase transition properties of eight asphalt binders from the SHRP material reference library. More abundant Raman spectral structure information is obtained by introducing silver nanoparticle colloids (Ag NPs) as a surface-enhanced Raman scattering (SERS) substrate to quench the self-fluorescence. The findings show that Raman spectroscopy can effectively identify different aging states of asphalt according to characteristic peaks. In the temperature range of 20–100 °C, three areas of the Raman spectrum, including 500–1200, 1300–1800, and 2500–3000 cm–1, indicate that the asphalt has transformed from an ordered phase to a disordered phase and to a liquid phase. The order parameters are selected to represent the change of ordered and disordered structures in asphalt. The phase transition occurs mainly from the changes in the intensity and frequency of the Raman crest related to the crystal structure and distorted conformation. The temperature range of phase transitions from an ordered to a disordered range of asphalt has an obvious correlation with the wax melting point of asphalt, and the disorder degree and molecular defects of asphalt are also positively correlated with the wax content in asphalt.