# Effect of Aggregate Crystalline Surface Anisotropy on
## 4 Conclusions  
This study employed MD simulations to reveal the interaction mechanisms between asphalt and aggregates at the atomic scale. SiO\({}_{2}\) and CaCO\({}_{3}\) were selected as representatives of acidic and basic aggregates, respectively, for modeling. A 12-molecule model was constructed as the asphalt model. By calculating the interaction energy, adhesion energy, MSD, diffusion coefficient, and \(RDF\), the adhesion mechanisms between aggregates and bitumen at different crystalline surfaces, the influence of aggregate crystalline surface anisotropy on the diffusion behaviors of asphalt components in the asphalt-aggregate interfacial system, and the distribution characteristics of asphalt on aggregate surfaces were investigated. The conclusions and recommendations are as follows.  
Figure 13: The adhesion energy of the asphalt–aggregate interface system.  
1. As the distance \(r\) from the aggregate surface increases, the \(RDF\) gradually increases and stabilizes. The spatial distribution of the asphalt components is essentially the same on all exposed surfaces, regardless of whether they are in alkaline or acidic aggregates. The \(RDF\) varies with different crystalline surfaces, and higher \(RDF\) values indicate the better adhesion performance of the system.
2. There are significant differences in the diffusion performance of asphalt molecules under different crystalline surfaces, and the diffusion behaviors of each asphalt component vary markedly. The pattern of the adhesion energy is consistent with the diffusion behavior of asphalt molecules on different aggregates: aggregates with strong adhesion energy exhibit slower diffusion, while those with weaker adhesion energy exhibit faster diffusion. The higher the interaction energy of particles with strong primary adhesion, the greater the attraction between them, resulting in less spreading.
3. The anisotropy of aggregate crystalline surfaces significantly affects the asphalt-aggregate adhesion performance. For the asphalt-SiO\({}_{2}\) system, the interaction energy is dominated by van der Waals energy, with the surface atomic density having a large influence. The smaller the gap, the larger the van der Waals energy. The adhesion energy is ranked as \((1\;0\;1)>(0\;0\;1)>(1\;0\;0)\). For the asphalt-CaCO\({}_{3}\) system, both van der Waals and electrostatic energy's effects are not negligible, and the greater the number of ionic bonds, the stronger the electrostatic attraction of the aggregate surface to the asphalt. The order of the adhesion work is \((0\;1\;8)>(1\;0\;4)>(2\;1\;4)\).  
In this study, the effects of other mineral components in the aggregate on the adhesion properties of asphalt were not considered when constructing the aggregate model. Similarly, only the representative AAA-1 asphalt model was constructed, without considering the adhesion properties between different modified asphalts and different aggregates. Additionally, the properties of asphalt with varying degrees of aging were also not considered.  
Future researchers could consider different mineral compositions and proportions in aggregates when constructing the aggregate model. They could explore the adhesion performance characteristics between aggregates and asphalt under the combined influence of different minerals. Furthermore, it would be beneficial to consider the characteristics of different modified asphalts and their adhesion properties at varying degrees of aging.  
Conceptualization, software, formal analysis, investigation, data curation, writing--original draft preparation, J.L.; methodology, writing--review and editing, supervision, L.H. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
The authors declare no conflicts of interest.