To solve the nanoparticles (NPs) agglomeration phenomena of nanometer calcium carbonate (nano-CaCO₃) modified asphalt binder, in this paper, solvent-free CaCO₃ nanofluids (NFs) were prepared based on surface-functionalized CaCO₃ NPs to study the effect on asphalt. Microscopic structures, compositions, and thermal stability were characterized by Fourier transform infrared spectrometer (FTIR), X-ray diffractometer (XRD), transmission electron microscope (TEM), and thermogravimetric analyzer (TGA), respectively. Results showed that perfect CaCO₃ NFs were successfully prepared, and were good enough for asphalt mixing due to their excellent thermal stability. Scanning electron microscopy (SEM), conventional tests, dynamic shear rheometry (DSR), and bending beam rheometry (BBR) were conducted to investigate the modifying effect. The SEM results indicated that CaCO₃ NFs had better compatibility with asphalt binder than original CaCO₃ NPs. Conventional and DSR test results demonstrated that CaCO₃ NFs had slight negative effects on high-temperature performance while improving the low-temperature performance of the asphalt binder. The BBR test results confirmed that the modifier addition effectively enhanced asphalt binders’ low-temperature crack resistance performance.

In wet process–modified asphalt, the modifier is easily separated from the asphalt, resulting in poor storage stability. For traditional dry-mixed modified-asphalt mixtures, adding a single modifier to the mixture, it is difficult to improve the comprehensive performance of asphalt mixtures. In this study, rubber–polyethylene composite modified particles were added to a dry-mixed asphalt mixture. The rubber powder was first desulfurized using a twin-screw extruder and then blended and extruded with low-density polyethylene particles to form rubber–plastic composite particles. Then, the high- and low-temperature performance and water stability of the dry-mixed rubber–plastic composite modified asphalt mixture were evaluated. Finally, the asphalt in the rubber–plastic composite modified mixture was extracted. The microscale properties were tested to investigate the mechanism of the composite particles. The results indicated that the rubber–plastic composite modified particles improved the high-temperature deformation resistance of the mixture. Additionally, the low-temperature crack resistance and water stability were slightly improved. Compared with adding desulfurized rubber powder alone, the comprehensive performance of the asphalt mixture can be significantly improved by using a rubber–plastic composite.