# Effect of Aggregate Crystalline Surface Anisotropy on
## 3 Results and Discussion
### Diffusion of Asphalt Compositions on Aggregate Surface
This indicated that the movement of asphalt on the SiO\({}_{2}\) surface was faster than on the CaCO\({}_{3}\) surface. This difference was attributed to the stronger adhesion performance of alkaline aggregates compared to acidic aggregates. The bonding strength between the CaCO\({}_{3}\) surface and asphalt was high, making it difficult for the asphalt to diffuse, resulting in a relatively low diffusion coefficient. Conversely, the bond strength between the SiO\({}_{2}\) surface and asphalt was lower, making it easier for the asphalt to diffuse, resulting in a relatively high diffusion coefficient. Bitumen diffuses more slowly on aggregate surfaces with strong adhesion and faster on surfaces with weaker adhesion. The diffusion properties of asphalt on the aggregate surface were affected by the anisotropy of the aggregate. The greater the diffusion coefficient, the larger the slope of  
Figure 10: Diffusion coefficients of asphalt components at different aggregate crystalline surfaces. Figure 9: MSD of components in the asphalt–aggregate interfacial model: (**a**) asphalt–SiO\({}_{2}\) of (0 0 1) surface; (**b**) asphalt–SiO\({}_{2}\) of (1 0 0) surface; (**c**) asphalt–SiO\({}_{2}\) of (1 0 1) surface; (**d**) asphalt–CaCO\({}_{3}\) of (1 0 4) surface; (**e**) asphalt–CaCO\({}_{3}\) of (0 1 8) surface; (**f**) asphalt–CaCO\({}_{3}\) of (2 1 4) surface. the MSD curve in Figure 9, indicating the faster movement of the asphalt components. As shown in Figure 10, the diffusion coefficient of the asphalt-SiO\({}_{2}\) system followed the order (1 0 0) > (0 0 1) > (1 0 1), with the diffusion coefficient on the (1 0 0) surface being the highest--about 133.55% greater than that on the (1 0 1) surface. This implied that the asphalt moved faster on the (1 0 0) surface, exhibiting lower adhesion performance. Similarly, for the asphalt-CaCO\({}_{3}\) system, the diffusion coefficient followed the order (2 1 4) > (1 0 4) > (0 1 8), with the coefficient on the (2 1 4) surface being about 80.44% higher than on the (0 1 8) surface, indicating faster movement and relatively higher adhesion performance on the (2 1 4) surface. The diffusion pattern of the asphalt components was also affected by aggregate anisotropy. The diffusion rates of the asphalt components were varied, with saturated and aromatic components typically having higher diffusion coefficients and asphaltene and resin having lower diffusion coefficients. This difference was mainly attributed to the varying molecular structures, where molecules with higher relative molecular weights moved more slowly, and those with lower relative molecular weights moved more quickly. However, as shown in Figure 10, the diffusion patterns of the asphalt components differed across aggregate surfaces with different crystalline structures. In the asphalt-SiO\({}_{2}\) system, the diffusion rates of asphaltenes were higher on the (0 0 1) and (1 0 0) surfaces compared to other components, but, on the (1 0 1) surface, the diffusion coefficients of resins were higher. In the asphalt-CaCO\({}_{3}\) system, the asphaltene component with the highest diffusion coefficient varied across the three surfaces. These variations might result from different interactions between the aggregates and asphalt components, which indirectly affected the diffusion coefficients based on the molecular weight.