# Effect of Aggregate Crystalline Surface Anisotropy on
## 2 Molecular Models and Simulation Methods
### Molecular Model of Aggregates  
Limestone, basalt, granite, and other materials are widely used in road engineering. \(\alpha\)-quartz (SiO\({}_{2}\)) and CaCO\({}_{3}\) are considered typical representatives of acidic and basic minerals in these aggregates. In nature, aggregate minerals exhibit with various crystal habits, leading to significant differences in the arrangement and density of the exposed surface atoms and active plasmas across different crystallization and disintegration directions. These differences are caused by variations in physicochemical properties such as the surface energy, hydrophobicity, and adsorption, known as the anisotropy of crystalline surfaces. The Miller index (h k l), which is used in this study, is recognized as a widely used notation system in mineral crystallography to describe this anisotropy [22]. Anisotropy influences the growth, morphology, optical properties, and responses of mineral crystals to various physical and chemical actions. For example, certain minerals are much harder along a particular direction than others or exhibit different colors and refractive indices in different directions.  
Before the simulation, the mineral crystals were cut to expose the relevant surfaces. The cutting process of the aggregate surfaces during modeling is shown in Figures 5 and 6. Commonly exposed surfaces of minerals include those that naturally disintegrate and those resulting from mechanical processing. The aggregates used in road projects are mechanically processed from natural rocks, causing fractures along areas where the bonding between the atomic layers is weak. It has been shown that these commonly exposed surfaces are closely related to the interlayer spacing, interlayer strength, surface charge, and free energy of their crystal structures [23].  
The (0 0 1), (1 0 0), and (1 0 1) surfaces of SiO\({}_{2}\) are selected in this work, as shown in Figure 5. The (0 0 1) surface is the most commonly used exposed surface in many studies due to its relatively stable structure [24]. The (1 0 0) surface displays a periodic arrangement of silica-oxygen tetrahedra, features a large contact angle with liquid molecules, and exhibits a certain degree of hydrophobicity, making it widely applicable in optics and electronic devices [25]. The (1 0 1) surface has a specific hydrogen-bonding network and, due to its more complex surface structure, shows significant reactivity and adsorption properties, which are beneficial in mineral flotation [26].  
Figure 4: The relationship between the V specific and temperature of asphalt.  
The (1 0 4), (0 1 8), and (2 1 4) surfaces of CaCO\({}_{3}\) are selected in this study, as shown in Figure 6. Among these, the (1 0 4) surface is one of the most common cleavage surfaces of CaCO\({}_{3}\) in nature, characterized by the largest layer spacing and the lowest surface energy [4]. This is confirmed by relevant simulation studies and surface energy calculations [27]. The (0 1 8) surface is a secondary exposure surface perpendicular to the main cleavage surface, featuring a relatively complex surface structure with a unique hydrogen-bonding network. It has been found to exhibit different adsorption properties for specific elements (e.g., lithium, sodium, magnesium) [28]. Additionally, the (2 1 4) surface also possesses relatively large layer spacing and low surface free energy, although its study is relatively rare [29].