Damage Mechanism of Asphalt Binder Modified with Phosphogypsum Whisker Composite under the Action of Sea Salt Solution

The study of the reaction mechanism of sea salt solution (SSS) on asphalt binder was important for the development of perpetual asphalt pavements in coastal areas. For this, a novel modifier (MPGJ-I) was developed in this study using phosphogypsum whiskers (PSW) synthesized from phosphogypsum (PSP) wastes, then the prepared composite-modified asphalt binders and base asphalt binders were immersed in the SSS with six concentrations. Then, the surface free energy (SFE) test, rheological tests, and Fourier transform infrared spectroscopy (FTIR) test were used to investigate the function mechanism of SSS on the surface properties, rheological properties, and chemical structure. The results showed that the salt in SSS would gradually transfer to the inside of asphalt binder, and with the increase in SSS concentration, the mass accumulation rate of base asphalt binders increased by 0.84% and 0.72%, whereas the modified asphalt binders only increased by 0.64% and 0.61%, respectively. The MPGJ-I effectively alleviated the decline trend of surface properties, with the incorporation of MPGJ-I, the SFE and cohesive work of base asphalt binders increased by 29.14%–32.83% and 5.23%–48.63%, respectively. In addition, with the increase in SSS concentration, the rutting factor (${\mathrm{G}}^{*}/\sin \delta$), and creep recovery rate (R) of base asphalt binders increased by 15.44%–57.69% and 7.58%–22.34%, respectively, and the fatigue life (${\mathrm{N}}_{\mathrm{f}}$) decreased by 6.08%–47.24%. Whereas ${\mathrm{G}}^{*}/\sin \delta$ and R of modified asphalt binders increased by 10.07%–50.77% and 2.15%–19.35%, respectively, and ${\mathrm{N}}_{\mathrm{f}}$ decreased by 1.79%–8.89%, which meant that the incorporation of MPGJ-I effectively restrained the attack action of SSS on base asphalt binders. There was a chemical reaction between SSS and asphalt binder, and the peak intensity of the new characteristic peaks in asphalt binder was positively correlated with SSS concentration, the incorporation of MPGJ-I could reduce the increasing rate of peak intensity to a certain degree. In addition, the principal component analysis (PCA) model showed that the surface properties and fatigue performance were closely related to the SSS concentration, and this correlation was not affected by the modification effect of MPGJ-I.