Adhesion Properties Between Rubber Asphalt Mastic and Aggregate: Verification from Surface Free Energy Theory and Molecular Dynamics

Abstract

The adhesive properties between rubber asphalt mastic and aggregate are crucial to rubber asphalt mixtures’ stability and moisture resistance. This paper employs surface free energy (SFE) theory and molecular dynamics (MD) to examine the bond strength and debonding behavior at the rubber asphalt mastic–aggregate interface. The results showed that the dispersion fraction of RC1.0 was 7.12 mJ/m² higher than that of RA, and the limestone mineral powder improved the adhesion properties of rubberized asphalt to aggregate and the anti-stripping properties. SiO₂ and CaCO₃ are contributors to the van der Waals and electrostatic forces between rubber asphalt–aggregate, respectively. The high concentration of mineral powder has a bridging effect in rubber asphalt mastic–aggregate. CaCO₃ filler is more pronounced in enhancing the adhesion properties of rubber asphalt–aggregate. CaCO₃ mineral powder mainly improves the anti-debonding ability of rubber asphalt–aggregate by reducing the thickness of water film between rubber asphalt–aggregate.

1. Introduction

Asphalt pavement undergoes prolonged exposure to environmental factors and traffic loads, leading to various forms of deterioration that significantly reduce its service life [1,2,3]. Consequently, there is a critical demand for road construction materials that combine high-temperature resistance to permanent deformation with favorable fatigue performance at moderate temperatures. To meet these requirements, numerous asphalt modifiers have been developed and applied [4,5,6]. Research demonstrates that incorporating an optimal amount of crumb rubber from recycled tires not only fulfills these functional objectives but also offers an environmentally sustainable solution for tire waste management [7,8,9,10].

The adhesion properties between asphalt and aggregate play a critical role in determining the moisture stability and long-term durability of asphalt mixtures. Recently, the interfacial bonding behavior of rubberized asphalt and aggregates has gained significant research attention. Guo [11] employed a molecular dynamics approach to analyze the influence of rubber content and aggregate type on the adhesive characteristics of the rubberized asphalt–aggregate interface. Xie et al. [12] demonstrated that styrene-butadiene rubber (SBR) enhances van der Waals forces at the interface, thereby improving bond strength. Based on surface free energy theory, Li [13] found that rubber modification increases the asphalt’s surface energy, dispersive component, and adhesion work, further enhancing its bonding performance. Additionally, Zhang [14], Jiao [15], Chen [16], and Mohamed et al. [17] have contributed to the exploration of adhesion mechanisms in rubberized asphalt–aggregate systems.

Asphalt mastic, a blend of asphalt binder and mineral filler, is a vital component in asphalt mixtures. Studies show that mixture failures often initiate at the mastic–aggregate interface, making interfacial adhesion crucial for moisture resistance and overall durability. Xu [18] demonstrated that SiO₂ nanoparticles enhance bonding by strengthening electrostatic interactions at the interface. Tan [19] used surface free energy theory to systematically assess adhesion between mastic and aggregate, while Zhang [20] employed pull-off tests to study how filler composition affects interfacial bonding. However, research on rubber-modified asphalt mastic remains limited, and conventional testing methods often fall short in explaining the complex adhesion mechanisms in rubberized mastic–aggregate systems.

The existing studies mostly focus on the properties of rubber-modified asphalt itself or the ordinary asphalt–aggregate interface, whereas the present study combines the two and focuses on the composite interface system of rubber-modified asphalt mortar (with mineral filler) and aggregate. This study investigates the synergistic effect of two variables, “mineral filler” and “aggregate type”, on the interfacial adhesion of rubber-modified asphalt and reveals the adhesion mechanism of the rubber–filler–aggregate ternary system. Utilizing surface free energy theory, we quantitatively analyze the surface energy components, adhesion work, and debonding energy of mineral fillers in rubberized asphalt systems. Furthermore, molecular dynamics simulations are employed to elucidate the influence of both mineral filler and aggregate types on interfacial adhesion properties, ultimately revealing the fundamental bonding mechanisms at the rubberized mastic–aggregate interface. These findings provide novel insights into the formation of interfacial bond strength, with the complete methodological framework illustrated in Figure 1.

2. Materials and Methods

2.1. Materials

2.1.1. Asphalt Binder

This study utilizes Panjin 90# (penetration grade) as the base material, with its physical properties listed in

Table 1. Physical properties of the base asphalt.

Experiment	Requirements	Measured Value
Penetration (25 °C, 100 g, 5 s)/0.1 mm	80–100	89
Ductility 10 °C, 5 cm/min	≥100	>100
Softening point/°C	43–53	45.4
Dynamic viscosity 135 °C/Pa·s	≤3	2.7

.

2.1.2. Rubber Powder

The physical indices of rubber powder are shown in

Table 2. Physical properties of rubber powder.

Experiment	Requirements	Measured Value
Rubber hydrocarbon content (%)	≥42	58
Carbon black content (%)	≥22	31.58
Acetone extract (%)	6–16	8.35
Ash content (%)	≤8	5.13

.

2.1.3. Mineral Powder

In this work, the mineral powder used is limestone mineral powder, and the relevant test indices are shown in

Table 3. Physical properties of mineral powder.

Experiment	Requirements	Measured Value
Relative density (g/cm3)	≥2.50	2.73
Hydrophilicity coefficient (%)	<1	0.8

.

The chemical composition and mineralogical composition of the limestone were analyzed using X-ray diffraction (XRD) and X-ray fluorescence spectrometry (XRF), and the results are shown in Figure 2 and

Table 4. XRF results of limestone.

Elemental Composition	CaO	SiO2	MgO	Al2O3	K2O	TiO	MnO	Fe2O3	RuO2
Content/%	52.63	26.56	19.36	0.00	0.00	0.00	0.01	1.10	0.34

.

From Table 4, it was found that the CaO content and SiO₂ content of the limestone used were 52.63% and 26.56%, respectively. From Figure 2, it was found that quartzite and calcite are the main mineral components of limestone. To minimize the influence of other components, calcite (CaCO₃) and quartz (SiO₂) were used as the subjects of this work.

2.2. Preparation of Rubber Asphalt (Mastic)

The preparation process of rubber asphalt and rubber asphalt mastic: ① After the base asphalt was heated to a fluid state at 165 °C, 20% (weight of base asphalt) of the rubber powder was weighed and added to the base asphalt. Stir the asphalt at 185 °C and 1500 rpm for 2.5 h to obtain rubber asphalt. ② A certain amount of mineral powder was weighed and added to the rubber asphalt and was stirred at 1500 r/min for 30 min to obtain different powder-to-gum ratios (0.6, 0.8, and 1.0) of rubber asphalt mastic. The specific preparation process is shown in Figure 3.

2.3. Surface Free Energy Theory

According to reference [21], Equation (1) mathematically describes the interaction between the polar and dispersive constituents of a material’s surface energy.

$$\gamma ={\gamma }^{\mathrm{d}}+{\gamma }^p={\gamma }^d+2\sqrt{{\gamma }^{+}{\gamma }^{-}}$$

(1)

where γ^p is the polar component, γ^d is the dispersive component, γ⁺ is the Lewis acid, and γ⁻ is the Lewis alkali.

Young’s equation (Equation (2)) [22] describes the correlation between the surface energy of asphalt–aggregate systems and their contact angle (θ).

$${\gamma }_a\cos \theta ={\gamma }_s-{\gamma }_{as}$$

(2)

where γ_a is the surface energy of the asphalt binder, γ_s is the surface energy of the aggregate, and γ_as is the surface energy of the asphalt mixture.

The interfacial adhesion performance of asphalt and aggregate can be evaluated by the adhesion work (W_as), calculated using Equation (3) [23].

$$W_{as}={\gamma }_a(1+\cos \theta )=2\sqrt{{\gamma }_a^d{\gamma }_s^d}+2\sqrt{{\gamma }_a^{+}{\gamma }_s^{-}}+2\sqrt{{\gamma }_a^{-}{\gamma }_s^{+}}$$

(3)

The adhesion work (W_as) represents asphalt–aggregate bonding in dry conditions, whereas wet conditions typically weaken this interface. The peeling work, defined in Equation (4) [24], quantifies the adhesion under moisture exposure.

$$\begin{array}{ll}{W}_{asw}& =2\sqrt{{\gamma }_a^d{\gamma }_w^d}+2\sqrt{{\gamma }_s^d{\gamma }_w^d}+2\sqrt{{\gamma }_w^{+}}\left(\sqrt{{\gamma }_a^{-}}+\sqrt{{\gamma }_s^{-}}\right)+2\sqrt{{\gamma }_w^{-}}\left(\sqrt{{\gamma }_a^{+}}+\sqrt{{\gamma }_s^{+}}\right)\\ & -2{\gamma }_w^d-2\sqrt{{\gamma }_a^d{\gamma }_s^d}-2\sqrt{{\gamma }_a^{+}{\gamma }_s^{-}}-2\sqrt{{\gamma }_a^{-}{\gamma }_s^{+}}-4\sqrt{{\gamma }_w^{-}{\gamma }_w^{+}}\end{array}$$

(4)

To calculate the surface free energy of asphalt, three solvents: distilled water, glycerin, and formamide are used as known solvents in this paper. The surface energy parameters of the three solvents are shown in

Table 5. Test results of the surface energy parameters of the reagents.

Reagent Type	Surface Energy Parameters (mJ/m2)
	${\mathit{\gamma }}_{\mathit{l}}$	${\mathit{\gamma }}_{\mathit{l}}^{\mathit{d}}$	${\mathit{\gamma }}_{\mathit{l}}^{+}$	${\mathit{\gamma }}_{\mathit{l}}^{-}$
Distilled Water	72.9	21.3	25.8	25.7
Glycerin	65	34	3.96	56.2
Formamide	56	39	2.27	39.5

.

The surface energy parameters and components of the aggregate are shown in

Table 6. Test results of the surface energy parameters of the aggregate.

Aggregate	${\mathit{\gamma }}_{\mathit{s}}$	${\mathit{\gamma }}^{\mathit{d}}$	${\mathit{\gamma }}^{\mathit{p}}$	${\mathit{\gamma }}_{\mathit{s}}^{\mathbf{+}}$	${\mathit{\gamma }}_{\mathit{s}}^{\mathbf{-}}$
Limestone	34.85	22.55	11.04	1.49	20.45

.

3. Molecular Dynamics Simulation

3.1. Construction of the Rubber Asphalt Mastic Molecular Model

3.1.1. Molecular Model of Asphalt

The AAA-1 model proposed by Li and Greenfield has been widely used to represent an asphalt binder [11,25], and its rationality and reliability have long been proven [26,27,28]. Therefore, this paper selects the AAA-1 model to represent the molecular model of the asphalt binder. The 12-component molecular model is shown in Figure 4, and the number of molecules is shown in

Table 7. Molecular numbers of the asphalt 12 components.

Type of Component		Molecular Formula	Number of Molecules
Saturate	SA-Ho	C29H50	5
	SA-Sq	C30H62	4
Aromatic	AR-Do	C30H46	18
	AR-Ph	C35H44	15
Resin	RE-Be	C18H10S2	15
	RE-Pv	C36H57N	4
	RE-Th	C40H60S	4
	RE-Qu	C40H59N	4
	RE-Tr	C29H50O	5
Asphaltene	AS-Ph	C42H54O	3
	AS-Py	C66H81N	2
	AS-Th	C51H62S	3

.

3.1.2. Molecular Model of Asphalt

In general, rubber tires are mainly composed of natural rubber (NR), butadiene rubber (BR), and styrene-butadiene rubber (SBR), with the largest percentage of natural rubber content [11,29,30]. Therefore, NR is used in this paper to represent the rubber molecule. NR is formed by the polymerization of an isoprene monomer with a polymerization degree of 24. Figure 5 illustrates the isoprene monomer molecular model and the NR molecular model.

3.1.3. Molecular Model of Mineral Powder

Mineral powder is an important component of asphalt mastic. In this paper, the SiO₂ cluster structure and CaCO₃ cluster structure are used to represent the most common acidic mineral powders (granite) and alkaline mineral powders (limestone). The crystal parameters of SiO₂ and CaCO₃ are shown in

Table 8. Crystal parameters of SiO₂ and CaCO_3.

Crystal Model	Chemical Formula	Lattice Parameters	Space Group	Group Name
	SiO2	a = b = 4.913 Å c = 5.4052 Å	α = β = 90° γ = 120°	P3121
	CaCO3	a = b = 4.99 Å c = 17.061 Å	α = β= 90° γ = 120°	R-3C

.

The SiO₂ and CaCO₃ cluster structures were constructed using MS based on the SiO₂ and CaCO₃ crystal models in Table 8. The radius of the cluster structure constructed in this paper is 5 Å. Figure 6 shows the construction process of the SiO₂ and CaCO₃ cluster structures.

3.1.4. Molecular Model of Rubber Asphalt Mastic

The molecular model of the rubber-modified asphalt mastic system was constructed using the amorphous cell module of 2021 MS software. The number of NR molecules, SiO₂ cluster structure molecules, and CaCO₃ cluster structure molecules for the molecular model of rubber asphalt mastic are shown in

Table 9. Number of NR, CaCO₃ cluster structures, and SiO₂ cluster structures in the model of rubber asphalt (mastic).

Type of Asphalt Mastic	Number of Molecules
	NR	CaCO3 Cluster	SiO2 Cluster
RA	4	-	-
RS1	4	-	1
RS2	4	-	2
RS3	4	-	3
RC1	4	1	-
RC2	4	2	-
RC3	4	3	-

.

The rubber asphalt mastic was constructed based on the number of molecules from Table 1 and Table 3, as shown in Figure 7.

3.2. Construction of the Rubber Asphalt Mastic-Aggregate Molecular Model

The main components of aggregate for road construction contain quartz (SiO₂) and calcite (CaCO₃); thus, SiO₂ and CaCO₃ are used to represent acidic and alkaline aggregates, respectively. According to the parameters of the SiO₂ and CaCO₃ crystals in Table 8, the SiO₂ and CaCO₃ 3D models were established. Secondly, the SiO₂ and CaCO₃ crystals were cut along the [1, 0, 0] direction. Finally, to make the aggregate layer bonded better with the asphalt mastic layer, the SiO₂ and CaCO₃ 3D crystal models were orthogonalized. Figure 8 shows the construction process of the aggregate layer.

The rubber asphalt mastic–aggregate interface model is obtained by adding the rubber asphalt mastic above the aggregate layer. A vacuum layer of 50 Å is added above the rubber asphalt mastic to avoid cyclicity effects. The 3D interface model of rubber asphalt mastic–aggregate is illustrated in Figure 9.

3.3. Rubber Asphalt Mastic–Water–Aggregate Interface Molecular Model

This study constructed a three-layer interface model of rubber asphalt mastic–water–aggregate to examine the debonding behavior between rubber asphalt mastic and aggregate in the presence of water, as shown in Figure 10.

3.4. Simulation Methods

This paper used Materials Studio (version 2021) throughout to perform molecular dynamics calculations. This study accomplished the molecular dynamics simulation process under 500 ps NVT conditions. It should be noted that the selected force field in this paper is the COMPASS II force field [18,31,32], and the whole simulation is carried out at 298 K. It should be noted that, in order to ensure the accuracy of the results, the simulation is repeated three times for all models in this paper, and the results are averaged.

3.5. Evaluation Indicators

3.5.1. Adhesive Work

The adhesion work represents the energy required to separate the asphalt from the aggregate surface, which is calculated as shown in Equation (5) [33].

$$W_{\mathit{adhesion}}=-{\displaystyle \frac{E_{\mathit{int}}}{A}}=-{\displaystyle \frac{E_{as}+E_{ag}-E_{as+ag}}{A}}$$

(5)

where W_adhesion is the adhesion work, E_int is the interaction energy between asphalt and aggregate, E_as is the potential energy of asphalt, E_ag is the potential energy of aggregate, E_as+ag is the potential energy of the interface between asphalt and aggregate, and A is the area of contact between asphalt and aggregate interface.

3.5.2. Debonding Work

The presence of water at the asphalt–aggregate interface makes it easier for the asphalt to separate from the aggregate interface. This paper uses debonding work to evaluate the bonding characteristics between asphalt and aggregate in the presence of water, as shown in Equation (6) [34].

$$W_{\mathit{debonding}}=-{\displaystyle \frac{E_{as-wa}+E_{ag-wa}-E_{as-ag}}{A}}$$

(6)

where W_debonding is the debonding work, E_as–wa is the interaction energy between asphalt and water, E_ag–wa is the interaction energy between aggregate and water, and E_as–ag is the interaction energy between asphalt and aggregate.

3.5.3. Energy Ratio (ER)

It has been shown that the energy ratio (ER) can represent the ability of the asphalt–aggregate interfacial system to resist moisture damage, as shown in Equation (7) [35].

$$ER=\left|{\displaystyle \frac{W_{\mathit{adhesion}}}{W_{\mathit{debonding}}}}\right|$$

(7)

4. Results and Discussion

4.1. Results Analysis of Surface Free Energy

4.1.1. Surface Energy Component

Figure 11 demonstrates the surface energy of rubber asphalt (mastic) and its components. In Figure 11, the dispersive component of rubber asphalt increases gradually with the increase in mineral powder content. The dispersive components of RA, RC0.6, RC0.8, and RC1.0 are 16.25 mJ·m⁻², 17.51 mJ·m⁻², 21.11 mJ·m⁻², and 23.37 mJ·m⁻², respectively. In contrast, the polar component of rubber asphalt decreases with the increasing mineral powder content. The polar components of RA, RC0.6, RC0.8, and RC1.0 are 4.58 mJ·m⁻², 4.29 mJ·m⁻², 3.97 mJ·m⁻², and 3.79 mJ·m⁻², respectively. The experimental results demonstrate that the limestone mineral filler significantly enhances the surface energy of rubber-modified asphalt. This improvement stems from the calcite (CaCO₃) composition of limestone, where the charged CaCO₃ surface facilitates asphalt molecule adsorption through electrostatic interactions. This interfacial phenomenon substantially increases the dispersive component of the rubberized asphalt system [36].

4.1.2. Adhesion Work

Adhesion work is an indication of the adhesion properties of the asphalt and aggregate system. The greater the work of adhesion, the better the adhesion between the asphalt and aggregate. The adhesion work for RA, RC0.6, RC0.8, and RC1.0 was calculated according to Equation (3), and the results are shown in Figure 12.

Figure 12 shows the adhesion work for RA, RC0.6, RC0.8, and RC1.0, respectively. The adhesion work of RA, RC0.6, RC0.8, and RC1.0 were 52.51 mJ·m⁻², 53.48 mJ·m⁻², 56.88 mJ·m⁻², and 58.85 mJ·m⁻², respectively. This indicates that mineral powder can effectively improve the adhesion work of rubber asphalt. This further confirms Wang and Pasandín’s conjecture [37,38]. After the rubber asphalt mastic is added to the mineral powder, the active sites on the surface of the mineral powder particles come into contact with the asphalt molecules. These sites provide a large number of binding sites, which enhance the physical and chemical bonding between asphalt and mineral powder, thus improving the adhesion work between asphalt and aggregate.

4.1.3. Peeling Work

The peeling work represents the work required to remove the asphalt from the aggregate surface in the presence of water. The greater the peeling work, the more easily the water displaces the asphalt film and the poorer the adhesion of the asphalt and aggregate. The peeling work for RA, RC0.6, RC0.8, and RC1.0 was calculated according to Equation (4), and the results are shown in Figure 13.

4.2. Interaction Energy Between Rubber Asphalt (Mastic)–Aggregate

The interaction energy consists of chemical bonding energy and non-bonded forces. The non-bonding energy consists of van der Waals forces and electrostatic interactions [39,40]. Figure 14 illustrates the asphalt–aggregate interface’s interaction energy, van der Waals forces, and electrostatic energy.

Figure 14a,b show the interaction energy, van der Waals force, and electrostatic energy of rubber asphalt (SiO₂ mineral powder)–SiO₂ aggregate and rubber asphalt (SiO₂ mineral powder)–CaCO₃ aggregate, respectively. The interaction energy of RA–Si, RS1–Si, RS2–Si, and RS3–Si was 365.929 kcal/mol, 378.169 kcal/mol, 345.5 kcal/mol, and 378.522 kcal/mol, respectively. The interaction energy of RA–Ca, RS1–Ca, RS2–Ca, and RS3–Ca was 602.756 kcal/mol, 630.970 kcal/mol, 602.096 kcal/mol, and 685.446 kcal/mol, respectively. The above results show that the low concentration of the SiO₂ cluster structure can improve the interaction energy between rubber asphalt and aggregate. This may be that a small amount of SiO₂ cluster structure not only increases the surface activity of asphalt but also increases the contact area between rubber asphalt and aggregate, which makes the asphalt and aggregate combine more closely, fills up the tiny pores and unevenness on the surface of the aggregate, and thus increases the interaction energy between rubber asphalt and aggregate. As the number of SiO₂ cluster structures increases, the interaction energy between rubber asphalt mastic and aggregate is decreased, as shown in Figure 14a,b. As the concentration of SiO₂ cluster increases, the “H” on the surface of these SiO₂ clusters forms hydrogen bonding with asphaltene and resin molecules to form a larger aggregation structure, which impedes the contact between the rubber asphalt and aggregate and reduces the interaction energy between the asphalt mastic and the aggregate. In addition, the high concentration of the SiO₂ cluster structure rather increases the interaction energy between rubber asphalt and aggregate. This is mainly due to the SiO₂ cluster structure forming a bridging effect between the rubber asphalt and aggregate, increasing the van der Waals force between the asphalt mastic and aggregate [41], and then improving the interaction between the asphalt mastic and the aggregate, as shown in Figure 15. These results indicate that the interaction energy between asphalt mastic and aggregate depends on the type of aggregate [42].

Figure 14c,d show the interaction energy, van der Waals force, and electrostatic energy of rubber asphalt (CaCO₃ mineral powder)–SiO₂ aggregate and rubber asphalt (CaCO₃ mineral powder)–CaCO₃ aggregate, respectively. The interaction energy of RA–Si, RC1–Si, RC2–Si, and RC3–Si was 365.929 kcal/mol, 776.501 kcal/mol, 740.282 kcal/mol, and 850.202 kcal/mol, respectively. The interaction energy of RA–Ca, RC1–Ca, RC2–Ca, and RC3–Ca was 602.756 kcal/mol, 3083.881 kcal/mol, 3209.635 kcal/mol, and 5319.691 kcal/mol, respectively. For example, compared to RS1–Si and RS1–Ca, RC1–Si and RC1–Ca increased by 105% and 389%, respectively. This is mainly attributed to the presence of carbonate ions in the structure of CaCO₃ clusters, which generate electrostatic interactions when they come into contact with asphaltene molecules and resin molecules, enhancing the interaction between the rubberized asphalt and the aggregate.

As for the SiO₂ structure, which is mainly composed of Si-O bonds, the charge distribution is relatively uniform, and its interaction with asphaltene molecules and resin molecules is primarily van der Waals forces, so the interaction between the asphalt mastic and aggregate can be dominated by van der Waals forces, as shown in Figure 14. The van der Waals force between asphalt mastic and aggregate is negative under a high concentration of CaCO₃ filler. The van der Waals force of RC3–Si and RC3–Ca are −35.345 kcal/mol and -228.519 kcal/mol, respectively. This phenomenon may be due to the high concentration of CaCO₃ and asphalt molecules to form a dense network structure, and CaCO₃ acts as a bridge to shorten the contact distance between the asphalt mortar molecules and the aggregate, as shown in Figure 16. In addition, CaCO₃ shows high polarity, resulting in the interaction force between asphalt mastic and aggregate being dominated by electrostatic energy. At this time, the van der Waals force exhibits a repulsive effect, resulting in a negative van der Waals force.

4.3. Adhesion Behavior Between Asphalt and Aggregate Under Dry Conditions

To evaluate the energy required for the separation of rubber asphalt mastic from the aggregate surface under the dry conditions, the adhesion work of rubber asphalt (mastic) was calculated according to Equation (5). The results of the calculation are shown in Figure 17.

As can be seen from Figure 17, the adhesion work between rubber asphalt mastic and aggregate was significantly improved by the addition of mineral powder. For example, the adhesion work of RA–Si, RS1–Si, and RC1–Si was 0.20 kcal·mol⁻¹·Å⁻², 0.21 kcal·mol⁻¹·Å⁻², and 0.42 kcal·mol⁻¹·Å⁻², respectively. The adhesion work of RA–Ca, RS1–Ca, and RC1–Ca was 0.26 kcal·mol⁻¹·Å⁻², 0.27 kcal·mol⁻¹·Å⁻², and 1.34 kcal·mol⁻¹·Å⁻², respectively. However, the improvement effect of SiO₂ filler on the adhesion work between rubber asphalt and aggregate is much lower than that of CaCO₃ filler. This is because the surface of CaCO₃ filler has a positive and negative charge, which can produce a strong electrostatic attraction, significantly enhancing the interfacial bonding strength between asphalt mastic and aggregate. At the same time, the high surface energy of the CaCO₃ surface makes it form stronger adhesion with asphalt, which further enhances the adhesion work between rubber asphalt mastic and the aggregate system. In contrast, SiO₂ filler mainly relies on weak van der Waals forces, and its surface chemical inertness makes it less effective in enhancing the adhesion between the rubber asphalt mastic and the aggregate. Therefore, although SiO₂ filler can also enhance the adhesion work, its effect is inferior to that of CaCO₃ filler. Consistent with previous studies, the adhesion work of rubberized asphalt mastic with CaCO₃ aggregate is much higher than that of rubber asphalt mastic with SiO₂ aggregate [23,43].

4.4. Debonding Work Between Asphalt and Aggregate Under Wet Conditions

To evaluate the debonding behavior of asphalt from the aggregate surface in the presence of moisture, the debonding work of the rubber asphalt (mastic)–water–aggregate system was calculated using Equation (6), the results are shown in Figure 18. The greater the debonding work, the easier it is to be destroyed by moisture.

From Figure 18, it is found that the addition of SiO₂ and CaCO₃ fillers can effectively reduce the debonding work between rubber asphalt–aggregate. For example, the debonding work of RA–W–Si, RS1–W–Si, and RC1–W–Si was 3.66 kcal·mol⁻¹·Å⁻², 3.12 kcal·mol⁻¹·Å⁻², and 2.29 kcal·mol⁻¹·Å⁻², respectively. This indicates that SiO₂ and CaCO₃ fillers can improve the moisture damage resistance of rubber asphalt. However, SiO₂ fillers are very limited in improving the ability of rubberized asphalt to resist moisture damage. This is because the surface of CaCO₃ particles has positive and negative charges. The electronegative atoms (N, S, and O) present in both the crumb rubber and asphalt binder generate substantial electrostatic attraction forces. This molecular-level interaction enables rubber-modified asphalt to maintain strong interfacial bonding with aggregates even under moisture-exposed conditions. Furthermore, CaCO₃ particles exhibit high surface energy and pronounced hydrophilicity, enabling them to establish robust interfacial interactions with both asphalt and water molecules. A portion of water molecules become adsorbed onto the CaCO₃ particle surfaces, forming a structured water film that effectively inhibits further water molecule diffusion (Figure 19). These synergistic effects collectively enhance the interfacial stability, thereby significantly improving the moisture damage resistance of rubber-modified asphalt systems.

However, for SiO₂ particles, the SiO₂ surface relies on van der Waals forces to interact with asphalt and water molecules, and these forces are easily interfered with and weakened by water molecules. In addition, due to the lack of electrostatic forces, SiO₂ is unable to provide sufficiently strong interfacial bonding in the presence of water, making it easier for water molecules to penetrate into the interface, thus reducing the resistance to moisture damage. This is attributed to the fact that the weak electrical properties of SiO₂ cause water molecules to collect between the rubber asphalt mastic and the aggregate, forming a thick water film, as shown in Figure 20. This water film forms an isolation layer between rubber asphalt mastic and aggregate, weakening the bond between rubber asphalt and aggregate.

4.5. Energy Ratio Between Asphalt and Aggregate

The energy ratio (ER) is a comprehensive parameter that characterizes the bonding and debonding behavior between rubberized asphalt mastic and aggregate, and the ER values of rubberized asphalt mastic and aggregate were calculated using Equation (7), and the results are shown in Figure 21.

In Figure 21, the ER values of rubber asphalt mastic–SiO₂ aggregate were lower than those of rubber asphalt mastic–CaCO₃ aggregate. A similar result can be found in previous studies [43]. For example, the ER values for RS1–Si and RS1–Ca are 6.59% and 12.01%. The type of mineral powder greatly affects the ER value of rubber asphalt mastic–aggregate, as shown in Figure 21. The ER values for RS1–Si and RC1–Si were 6.59% and 18.42%, respectively. The ER values for RS1–Ca and RC1–Ca were 12.01% and 77.85%, respectively. This indicates that the debonding of rubberized asphalt–aggregate by CaCO₃ mineral powder is much better than the debonding of rubber asphalt–aggregate by SiO₂ mineral powder. In addition, the rubber asphalt mastic–CaCO₃ aggregate has better anti-debonding properties with the synergistic effect of CaCO₃ mineral powder and CaCO₃ aggregate. This property is one of the factors why limestone is often used as a road building material.

5. Conclusions

This work investigated the adhesion properties and debonding behavior between rubber asphalt (mastic) and aggregate using surface free energy theory and molecular dynamics. Some key conclusions were drawn:

(1) Limestone mineral powder enhances the surface energy and dispersive component of asphalt and reduces the polar component of asphalt. It improves the adhesion work between rubber asphalt and aggregate and inhibits water molecules from displacing the asphalt film between rubber asphalt and aggregate.

(2) The type of mineral powder has a significant effect on the interaction energy between rubber asphalt–aggregate. The interaction energy between rubber asphalt/SiO₂–aggregate is dominated by van der Waals forces, and the interaction energy between rubber asphalt/CaCO₃–aggregate is dominated by electrostatic forces.

(3) Mineral powder plays three important roles between rubber asphalt mastic and aggregate, and they are to (a) promote rubber asphalt mastic to fill the void on the surface of aggregate, (b) form a cluster structure with asphalt molecules in asphalt to increase the direct contact area between rubber asphalt and aggregate, and (c) act as a bridge between rubber asphalt and aggregate.

(4) CaCO₃ clusters form a cluster structure with water molecules and reduce the water film thickness between rubber asphalt–aggregate. On the contrary, SiO₂ clusters do not change the water film thickness between rubber asphalt–aggregate. The synergistic effect of the CaCO₃ cluster structure and CaCO₃ aggregate layer resulted in good adhesion characteristics and an anti-debonding effect of rubber/CaCO₃ asphalt mastic–CaCO₃ aggregate.

(5) While this study elucidates the fundamental mechanisms governing rubber asphalt–aggregate adhesion, several promising directions warrant further investigation: (1) extending the current framework to hybrid filler systems (e.g., CaCO₃–SiO₂ composites) to optimize synergistic effects; (2) evaluating long-term durability under coupled environmental–mechanical aging (UV/oxidation, freeze–thaw cycles, and traffic loading); (3) developing multiscale models bridging molecular interactions to macroscopic performance; and (4) exploring sustainable functionalization of fillers (e.g., nano-CaCO₃ or bio-based modifiers) to enhance interfacial stability. Such advances would accelerate the design of high-performance rubber asphalt mixtures with tailored adhesion properties for diverse climatic and loading conditions.