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Article

Enhancing Interface Performance Through Self-Assembly Mechanisms of APTES on Surface-Modified Tuff Aggregates

by
Mingxin Lai
1,
Xiaoying Gao
2,
Lin Kong
3,4,*,
Lizong Chen
5,
Guoan Gan
3,4,
Haixing Lin
6,
Jiakang Zhang
3,4,
Gen Zhang
1,
Yueling Lin
7,
Hongming Zhu
1 and
Xinping Zhang
1
1
Fujian First Highway Engineering Group Co., Ltd., Quanzhou 362122, China
2
Fujian Expressway Group Co., Ltd., Fuzhou 350001, China
3
School of Civil Engineering, Southwest Jiaotong University, Chengdu 610031, China
4
Key Laboratory of Road Engineering of Sichuan Province, Southwest Jiaotong University, Chengdu 611756, China
5
Dehua County Road and Bridge Construction Co., Ltd., Quanzhou 362599, China
6
Fujian Communications Planning & Design Institute Co., Ltd., Fuzhou 350004, China
7
Quanzhou Road and Bridge Engineering Construction Co., Ltd., Quanzhou 362002, China
*
Author to whom correspondence should be addressed.
Coatings 2024, 14(11), 1422; https://doi.org/10.3390/coatings14111422
Submission received: 13 October 2024 / Revised: 5 November 2024 / Accepted: 6 November 2024 / Published: 8 November 2024
(This article belongs to the Special Issue Surface Engineering and Mechanical Properties of Building Materials)
Browse Figures
Figure 1
<p>Tuff aggregate surface modification procedures.</p> "> Figure 2
<p>The experimental procedure.</p> "> Figure 3
<p>Binder bond strength (BBS) test: (<b>a</b>) schematic diagram BBS test; (<b>b</b>) test sample preparation.</p> "> Figure 4
<p>The Marshall stability test procedure.</p> "> Figure 5
<p>Contact angle test: (<b>a</b>) contact angle test; (<b>b</b>) contact angle diagram; (<b>c</b>) asphalt sample; (<b>d</b>) sliced aggregate pieces.</p> "> Figure 6
<p>X-ray diffraction test results of basalt and tuff.</p> "> Figure 7
<p>Binder bond strength (BBS) test results.</p> "> Figure 8
<p>Immersion residue stability test results.</p> "> Figure 9
<p>Adhesion work and energy ratio.</p> "> Figure 10
<p>SEM micrographs: (<b>a</b>) SEM micrographs (500× magnifcation, 2500× magnifcation) of tuff surface treated with APTES. (<b>b</b>) SEM micrographs (500× magnifcation, 2500× magnifcation) of tuff surface treated without APTES.</p> "> Figure 10 Cont.
<p>SEM micrographs: (<b>a</b>) SEM micrographs (500× magnifcation, 2500× magnifcation) of tuff surface treated with APTES. (<b>b</b>) SEM micrographs (500× magnifcation, 2500× magnifcation) of tuff surface treated without APTES.</p> "> Figure 11
<p>EDS analysis results: (<b>a</b>)untreated specimen; (<b>b</b>)treated specimen.</p> "> Figure 12
<p>Elemental composition.</p> "> Figure 13
<p>Schematic diagram of the enhancement mechanism.</p> ">
Versions Notes

Abstract

:
To enhance the adhesion between tuff and asphalt, this study investigates the efficacy of alkalinization treatment technology using a molecular self-assembly layer derived from the silane-coupling agent γ-aminopropyltriethoxysilane (APTES). APTES hydrolysis solutions at varying concentrations were prepared to assess their impact on the adhesive strength of the aggregate–asphalt interface and water damage resistance. Using surface energy theory, the interface adhesion work of tuff was analyzed, while SEM and EDS were employed to examine changes in surface morphology and composition after treatment. The results demonstrate that an APTES:water:ethanol mass ratio of 5:45:50, along with a curing temperature of 200 °C, significantly improves the bonding strength between tuff and asphalt. The silanol groups on APTES react with hydroxyl groups on the tuff surface to form siloxane bonds (Si-O-Si), anchoring APTES to the tuff. This study elucidates the self-assembly mechanisms of APTES on tuff aggregates and demonstrates the consequent enhancement of interfacial adhesion, providing valuable insights for the application of tuff as tunnel spoil in road engineering.

1. Introduction

The construction of asphalt pavements necessitates substantial quantities of aggregates. Given that conventional aggregates are finite resources, they are unlikely to fulfill the long-term requirements of road construction [1,2,3]. Moreover, the extraction of these materials often involves extensive mining operations, which can lead to geological hazards and environmental degradation [4]. In regions like the Dehua section of the Yongxing Expressway in Fujian Province, China, characterized by mountainous terrain and numerous tunnels, the excavation process generates large quantities of tuff slag, which poses a potential environmental waste if not utilized effectively. Leveraging tuff aggregates from these tunnel excavations aligns with the principles of sustainable construction, reducing both material procurement costs and environmental impact [5,6,7]. This approach not only conserves land and stone resources but also lowers material procurement costs, minimizes mining impacts on vegetation, and yields notable economic and environmental advantages.
The primary tunnel excavation byproduct in this project section is tuff, a material currently underutilized in asphalt pavement construction. Tuff, predominantly formed from volcanic ash and debris through processes such as weathering, erosion, transportation, deposition, and consolidation [8,9,10]. It is rich in silicates and may possess an acidic surface texture [11,12]. Given that asphalt contains acidic components, the adhesion between tuff aggregates and asphalt is inherently poor, necessitating innovative strategies to enhance this interfacial bonding for effective use in road surfaces [13,14]. Improving this bond is crucial for enhancing the durability and performance of asphalt pavements, particularly in demanding environments.
Hot asphalt binders typically exhibit stronger initial adhesion due to the higher mixing and compaction temperatures, which facilitate better binder flow and aggregate wetting. Conversely, cold asphalt mixtures, valued for their ease of use and quick setting at ambient temperatures, often encounter adhesion and durability challenges. Research indicates that the adhesion between aggregate and asphalt in cement-emulsified asphalt mixtures is notably weaker than in hot-mix asphalt concrete, making them more susceptible to cracking, loosening, potholes, and premature rutting [15]. To enhance the adhesion and interface bonding between aggregates and asphalt, thereby extending the lifespan of asphalt pavements, researchers globally have explored various methods [16]. The incorporation of anti-stripping agents into asphalt mixtures has shown promise in improving aggregate–asphalt adhesion [17,18,19]. Hydrated lime, exemplifying the first generation of anti-stripping agents, offers the benefits of low cost and ease of use, enhancing aggregate surface roughness. However, its effectiveness is contingent upon the thoroughness of mixing with the aggregate [20,21,22]. Metal soaps, capable of forming adhesive films at the aggregate–asphalt interface, improve anti-stripping properties but suffer from compatibility issues due to density differences, limiting their application in engineering projects [23]. Surfactants, by improving aggregate surface tension, enhance interfacial bonding, but their thermal instability and susceptibility to decomposition during mixing necessitate further investigation regarding their durability [24,25].
The technology for modifying the surface of aggregates to enhance the surface energy represents an effective strategy for improving the interfacial performance of materials using polymer anti-flaking agents [26]. The silane-coupling agent, γ-aminopropyltriethoxysilane (H2NCH2CH2CH2Si(OC2H5)3, APTES), integrates both organic functional groups and alkoxy groups within a single molecule. Hydrolysis transforms alkoxy groups into silanol groups [27,28]. The organic functional groups engage with the polymer, while the silanol groups establish covalent bonds with the inorganic surface, thereby enhancing adhesion at the inorganic/polymer interface, as confirmed by studies showing improved adhesion in polymer matrices when using APTES [29]. Scholars globally have predominantly applied silane-coupling agents for the surface treatment of inorganic fillers [30,31]. Existing research has demonstrated the effectiveness of silane-coupling agents in enhancing the hydrophobicity and adhesion properties of various inorganic fillers [32,33,34,35]. However, the application of silane-coupling agents specifically to tuff aggregates and the mechanisms through which they improve interfacial adhesion with asphalt have not been systematically studied. This study addresses this gap by investigating the unique application of APTES to enhance the adhesion between tuff aggregates and asphalt.
Silane-coupling agents can create a chemically bonded film on aggregate surfaces, with one end forming a chemical bond with the aggregate and the other end achieving physical or chemical adsorption with asphalt, thus significantly enhancing adhesion [36,37]. Treatment with silane-coupling agents introduces additional basic functional groups, such as hydroxyl groups (-OH), on the aggregate surface. These groups can react with acidic components in asphalt to form stable chemical bonds, thereby boosting adhesion [38,39]. Furthermore, alkalinization treatments can increase the aggregate surface’s microscopic roughness through etching, providing more points for physical interlocking and enhancing mechanical adhesion with asphalt [40]. By improving adhesion to asphalt, alkalinization contributes to the water stability of asphalt mixtures, reducing water damage risks and extending pavement lifespan. Research on the surface modification of tuff using APTES is currently in its preliminary stages, lacking definitive conclusions regarding enhancement effects or modification mechanisms. The self-assembly mechanism of APTES and its impact on improving interfacial adhesion on tuff aggregate surfaces have not been systematically elucidated.
In summary, this study utilized APTES solutions of varying concentrations to create self-assembled monolayers on the surface of tuff. The interaction between APTES and tuff, along with the mechanisms enhancing interfacial performance, was investigated across multiple scales. Initially, the optimal conditions for silane-coupling agents were identified using the binder bond strength test, which determined the most effective modification process. Using unmodified tuff as a control, this study quantified the enhancement effect of silane-coupling agents on the adhesion work at tuff interfaces based on surface energy theory. Lastly, the micromorphology and elemental changes in silane-modified tuff were analyzed at the microscopic level using SEM and EDS tests, elucidating the modification mechanism of the self-assembled silane layer in enhancing adhesion between tuff and asphalt. This study presents a novel application of APTES to enhance the interfacial adhesion of tuff aggregates in asphalt pavements, offering a new perspective on the sustainable use of tunnel spoil in road engineering.

2. Materials and Methods

2.1. Materials

The materials used in this study include tuff aggregates, the silane-coupling agent APTES for surface treatment, and bitumen as the binder for evaluating interfacial adhesion.

2.1.1. Tuff

The tuff used in this study was sourced from tunnel excavation projects managed by the Quanzhou Municipal Government in Fujian Province and the Dehua section of the Yangyuan-Yongding Expressway. Performance testing was conducted in accordance with the JTG E42-2005 “Experimental Procedures for Aggregate in Highway Engineering”, with the results presented in Table 1.

2.1.2. Silane-Coupling Agent

This research introduces γ-aminopropyltriethoxysilane (APTES) as a silane-coupling agent to modify the tuff surface, aiming to enhance interfacial bonding with asphalt through the formation of an APTES self-assembly layer. The APTES used was procured from Dongguan Kangmian New Materials Chemical Reagent Co., Ltd. (Dongguan, China), with its performance metrics detailed in Table 2.

2.1.3. Bitumen

ES70# bitumen, supplied by ExxonMobil Corporation, is characterized within Table 3.

2.2. Tuff Aggregate Surface Modification Procedures

This study prepared APTES hydrolysates at varying concentrations to identify the optimal modification process and experimental conditions, setting unmodified tuff aggregate at different curing temperatures as the control group. The preparation of hydrolysate solutions (“m” refers to the mass ratio) is enumerated in Table 4.
The hydrolysis stage is crucial for activating the coupling agent, enabling effective bonding with various materials, including inorganic fillers and organic polymers. Cleaned and surface-dried tuff aggregates were immersed in the APTES solution for 60 min to ensure thorough interaction with the aggregate surface. After treatment, the tuff was oven-dried for 60 min at a temperature simulating actual production mixing conditions, followed by cooling to room temperature to complete the self-assembly of the APTES coating. The aggregate-processing workflow is illustrated in Figure 1.

2.3. Experimental Plan Design

The experimental plan is outlined in Figure 2. APTES hydrolysis solutions with varying concentrations were prepared, and their impact on the adhesive strength of the aggregate–asphalt interface and its resistance to water damage were assessed to identify optimal testing conditions. Using surface energy theory, the interface adhesion work of tuff was analyzed, while SEM and EDS were employed to examine changes in surface morphology and composition after treatment.

2.4. X-Ray Diffraction

X-ray diffraction (XRD) was employed as the primary method for analyzing the physical phases and crystal structures of the materials, using a Bruker AXS D8 ADVANCE X-ray diffractometer. The apparatus operated with Cu Kα radiation (λ = 1.789 Å), a tube voltage of 40 kV, and a current of 30 mA, scanning from 3° to 80° at a speed of 10°/min.

2.5. Binder Bond Strength (BBS) Test

This study adheres to the methodologies outlined in AASHTO TP-91 (AASHTO 2004) and ASTM D4541 (ASTM 2017), employing an American PosiTest AT-M pull-out adhesion tester, which operates within a range of 0.4–3.5 MPa, to evaluate the adhesion performance of tuff aggregate samples. As illustrated in Figure 3, the testing setup includes a schematic of the binder bond strength (BBS) test procedure and the preparation of test samples. The device features a 20 mm supporting drawing head and a resolution of 0.01 MPa (1 psi) [41]. To maintain the consistency of test data, a controlled spreading amount method is applied, with the spreading amount for all samples uniformly maintained at 0.3 kg/m2.

2.6. Immersion Residue Stability Test

The water stability of an asphalt mixture is a critical metric for assessing its capacity to retain functionality under prolonged water immersion or through freeze–thaw cycles. In this research, a soaked Marshall residual stability test was employed to examine the resistance of tuff asphalt mixtures to water-induced damage, both pre- and post-modification. Figure 4 shows the AC-20 gradation curve and the Marshall stability test procedure. Following the “Testing Procedures for Asphalt and Asphalt Mixtures in Highway Engineering” T0719-2011, a tuff asphalt mixture with an oil-to-stone ratio of 4.3% was prepared, using AC-20 grading. One set of Marshall specimens underwent a conditioning process at 60 ± 1 °C for 30 min, with its stability thereafter denoted as MS2; another set was subjected to the same temperature for 48 h, with its stability measured and labeled as MS1. The residual stability post-water-immersion, MS0, was then calculated utilizing formula 1.
M S 0 = M S 1 M S 2 × 100
where MS0 represents the residual stability after water immersion (%), MS2 denotes the Marshall stability after a 30 min water bath (kN), and MS1 signifies the Marshall stability following a 48 h water bath (kN). To minimize experimental errors, each mixture was subjected to three replicate tests. The average of these three trials was then computed to determine the test result.

2.7. Contact Angle Test

The performance of asphalt mixtures is significantly influenced by the adhesion between the aggregate and asphalt [42]. Evaluating the surface energy of aggregate and asphalt through testing can reveal the efficacy of surface treatment and the interfacial bonding capabilities of the APTES self-assembly layer. These assessments employ the German KRUSS DSA100 contact angle measuring instrument, which offers a contact angle measurement range of 0–180° and an accuracy of ±0.1°, with an image magnification capability of 0.7×.
To prepare the asphalt sample, the procedure involves heating the asphalt to 150 °C in an oven until it reaches a fluid state. The heated asphalt is then carefully titrated onto a transparent glass slide to create a smooth film. This film is allowed to cool to room temperature over a 12 h period. Once cooled, the hanging drop method is used to measure the contact angle.
The method for preparing aggregate samples involves initially using a high-precision cutting machine to achieve a relatively flat test surface on irregular aggregates. Subsequently, the aggregates are cleaned and oven-dried until completely moisture-free. To ensure data precision, testing is conducted on three parallel specimens within each group, with the results being averaged. Figure 5 shows the contact angle test procedure.
The relationship between the contact angle and interfacial tension in the equilibrium state of a solid surface is described via the Young–Dupre equation [43]. At standard atmospheric pressure, the work of interfacial adhesion between solid and liquid phases can be calculated using Equation (2) [44]:
W a = γ l ( 1 + c o s ⁡ θ )
The polar and non-polar components of the surface energy of tuff aggregate and asphalt are determined using Equation (3). This Equation facilitates the calculation of surface energy, distinguishing between its polar and non-polar constituents.
γ l ( 1 + c o s ⁡ θ ) = 2 ( γ s d ⋅ γ l d ) 1 / 2 + 2 ( γ s p ⋅ γ l p ) 1 / 2
where γ l —surface energy, mJ/m2;
γ s d ,   γ l d —dispersive component of solid and liquid, mJ/m2;
γ s p ,   γ l p —nonpolar and polar part of solid and liquid, mJ/m2.
For the experimental investigation, distilled water, ethylene glycol, and glycerol were selected as liquid probes for surface energy analysis. Their surface energies are 72.1 mJ/m2 for distilled water, 48.0 mJ/m2 for ethylene glycol, and 64.6 mJ/m2 for glycerol [45].

2.8. Scanning Electron Microscopy (SEM) and X-Ray Energy Dispersive Spectroscopy (EDS)

The impact of the APTES self-assembly layer on the micromorphology and elemental composition of the tuff aggregates was investigated using scanning-electron microscopy (SEM) and X-ray energy-dispersive spectroscopy (EDS). Comparing the elemental content on the tuff surface before and after treatment with solutions of varying concentrations facilitated the identification of the optimal treatment concentration. To ensure data accuracy, tests were conducted on three parallel specimens per group, with results averaged. The SEM employed was a Zeiss Sigma 300, with specifications including an accelerating voltage of 0.3–30 kV, a magnification range of 8–10 k, and a working distance of 10 mm. EDS data were captured over an area of 280 μm × 210 μm using Aztec Version 5.1, with a resolution of 1024 × 768 pixels.

3. Results and Discussions

3.1. X-Ray Diffraction Test Results

X-ray diffraction (XRD) analysis reveals the diffraction patterns of tuff and basalt, with each peak corresponding to diffraction from specific crystallographic planes within the minerals. This allows for the identification of minerals in the samples by analyzing the position and intensity of these peaks. Figure 6 shows that tuff, a volcanic rock primarily composed of volcanic ash, contains minerals such as volcanic glass, quartz, plagioclase feldspar, and minor clay minerals, depending on the source and weathering processes. A peak at 26.6° in the XRD spectrum suggests the presence of quartz (SiO2) within the tuff. Basalt, an igneous rock rich in iron and magnesium, primarily consists of plagioclase, pyroxene, and olivine. In the XRD spectrum, pyroxene typically shows peaks between 35° and 40° (2θ values). The differences in adhesion performance between tuff and basalt are largely influenced by their mineralogical composition, texture, and surface energy.
These experimental outcomes elucidate the disparities in surface energy between the two rocks from a phase composition perspective, highlighting basalt’s superior adhesion to asphalt, rendering it more apt for use in asphalt mixtures. This underscores the need for surface treatment of tuff with silane-coupling agents to enhance its compatibility with asphalt, facilitating a ‘bridging’ effect that improves asphalt’s wettability on tuff surfaces [46]. Future experiments will investigate the enhancement in adhesion and the modification mechanisms of silane-coupling agents on tuff aggregates at both macroscopic and microscopic levels.

3.2. Binder Bond Strength (BBS) Test Results

Figure 7 demonstrates that the bonding strength between tuff aggregate and asphalt is significantly influenced by both the curing temperature and the concentration of the silane-coupling agent. At a curing temperature of 180 °C, the bonding strengths for tuff and asphalt, with increasing silane-coupling-agent hydrolysate mass fractions from 0% to 2% and 5%, are 0.89 MPa, 0.93 MPa, and 1.11 MPa, respectively. Post hoc analysis revealed that the 5% solution group exhibits a significant increase in bonding strength, with a 24.7% improvement (0.22 MPa) compared to the control (0% solution), indicating optimal performance at this concentration (p < 0.05). However, further increases in the silane-coupling-agent concentration to 10% and 15% resulted in reduced bonding strengths of 0.94 MPa and 0.76 MPa, respectively, suggesting that overly high concentrations may negatively impact adhesion due to excessive film thickness or other limiting factors (p < 0.05). ANOVA results indicated that both curing temperature and silane concentration significantly affect bonding strength (p < 0.01). Additionally, correlation analysis showed a positive relationship between curing temperature and bonding strength, with an average increase of 0.08 MPa for every 10 °C rise in temperature across all concentrations, highlighting the importance of curing temperature in enhancing adhesion.
The analysis indicates that APTES primarily forms covalent bonds, specifically siloxane bonds (Si-O-Si), between the tuff and asphalt, thereby enhancing their interfacial compatibility and bonding properties. Increasing concentrations of the silane-coupling-agent hydrolysate expand the film coverage on tuff surfaces, establishing more chemical bonds and physical adsorption points, thereby augmenting the bond strength by increasing the actual contact area and mechanical engagement points with the asphalt. Post-treatment with the coupling agent modifies the surface energy of the tuff, making it more compatible with asphalt and facilitating stronger bonds. Additionally, the coupling agent forms new interfacial bonds through chemical reactions, further enhancing the bonding properties. Nonetheless, overly high concentrations of the silane-coupling agent may lead to surface saturation and excessive film thickness, impeding effective bonding due to the potential formation of chemical bonds or increased stress concentration at the interface, thus diminishing bond strength. Consequently, the recommended silane-coupling-agent hydrolysis concentration is m (silane-coupling agent)/m (water)/m (ethanol) = 5:45:50, with an optimal curing temperature of 200 °C. Future studies will focus on the effects of the silane-coupling agent’s self-assembled film on the contact angle, surface elemental composition, functional group variations, and micromorphology of tuff.

3.3. Immersion Residue Stability Test Results

The water-immersion Marshall stability test serves as an instrumental method for assessing the stability of asphalt mixtures under conditions of water immersion, thereby indicating the mixture’s capability to withstand water damage in real-world application settings. As depicted in Figure 8, the Marshall stability of untreated tuff changes from 9.14 kN before immersion to 7.13 kN after immersion, with the residual stability recorded at 78%. Treatment with APTES modestly enhances the Marshall stability of tuff from 9.14 kN to 9.46 kN. More notably, the stability after water immersion escalates from 7.13 kN to 8.25 kN, and the residual stability improves from 78% to 89%, indicating a significant enhancement. At an aggregate consolidation temperature of 200 °C, the residual stability of tuff treated with APTES approaches that of basalt under equivalent conditions. Post hoc testing (Tukey’s HSD) showed that the increase in post-immersion stability between untreated tuff (A1) and the 200 °C treated sample (C3) was significant (p < 0.05), supporting the conclusion that higher curing temperatures enhance the efficacy of APTES treatment.
This analysis indicates that immersion in a 60 °C water bath accelerates aging and replicates the effects of hot and humid environments. Water penetration at the fragile bonding interface between asphalt and aggregate leads to the detachment of the asphalt film on the aggregate surface and a reduction in the adhesive force between asphalt and aggregate, consequently diminishing the internal cohesive strength of the asphalt mixture [47]. This improvement is supported by the BBS test results, which demonstrate increased bonding strength at the asphalt–tuff interface, directly correlating with the observed macro-level performance. Unprocessed tuff exhibits weak adhesion to asphalt, rendering the mixture susceptible to delamination in hot and humid conditions and thus decreasing its stability [48]. APTES enhances the bond between asphalt and aggregate by creating a self-assembled layer on the aggregate’s surface, thereby increasing the affinity between the two materials. This self-assembled layer acts to inhibit water penetration at the asphalt-aggregate interface, bolster cohesion within the mixture, preserve structural integrity post-immersion, and mitigate water-induced damage.

3.4. Contact Angle Test Results

Ethylene glycol, glycerol, and distilled water were selected as liquid probes to mitigate experimental errors, with the average of three sets of parallel experiments calculated for each sample. The work of adhesion and peeling are delineated in Equations (4) and (5) [37,49], respectively, with the results illustrated in Figure 9. The modification effect can be more accurately assessed by using the energy ratio (ER), which provides a more comprehensive evaluation of the interaction between the asphalt and aggregate [50].
W adhension   = 2 γ a d γ s d + 2 γ a p γ s p
W tripping   = 2 γ w − 2 ( γ w d γ a d + γ w p γ a p ) + 2 ( γ a d − γ w d γ s d + 2 γ a p − γ w p ) γ s p
E R   = W a | W t |
Figure 9 shows that the adhesion work for unmodified tuff, derived from surface energy components, is 44.34 mJ/m2, with an energy ratio of 1.04. In contrast, for the C3 sample treated with the optimal concentration of silane-coupling agent, the adhesion work and energy ratio increase significantly to 55.28 mJ/m2 and 1.42, respectively.
Post-treatment with an optimal silane-coupling agent concentration notably increases both adhesion works and energy ratio, indicating enhanced resistance of tuff to stripping. This improvement is attributed to the formation of a self-assembled silane film on the tuff surface, increasing its polar components and surface polarity, thereby improving the adhesion between the acidic tuff and asphalt due to the introduction of polar functional groups, such as amino groups.

3.5. SEM Test Results

Scanning-electron microscopy was employed to analyze the microscopic morphology of the tuff aggregate, both under optimal modification conditions and in its untreated state. Observations were conducted at magnifications of 500× and 2500×, with the findings presented in Figure 10.
Scanning-electron microscopy (SEM) analysis at 500× and 2500× magnification of the optimally modified and untreated tuff aggregates revealed significant differences in surface morphology, as shown in Figure 10. SEM images indicate that the self-assembled silane-coupling agent film partially fills or covers the tuff’s scale structure, creating an uniform layer. The silanol groups in APTES react with surface hydroxyl groups on the tuff, forming stable siloxane (Si-O-Si) bonds. This modification enhances surface uniformity, potentially increasing erosion and chemical resistance. The resulting denser surface is crucial for improving the interface with materials like bitumen by filling micropores and strengthening the bond.
Analysis suggests that the silanol groups in APTES can react with hydroxyl groups on tuff surfaces, typically resulting in the formation of stable siloxane bonds (Si-O-Si). This reaction not only establishes stable chemical bonds but also significantly alters the surface properties of tuff. The formation of siloxane bonds alters the tuff surface, transforming it from smooth to a network-like structure of villi “membranes”. This modification is crucial for integrating tuff with materials such as bitumen. Furthermore, the reaction products fill the microporous structure of the tuff surface, thereby strengthening the interface between tuff and asphalt. The SEM analysis further supports these findings by revealing the formation of a uniform and dense APTES layer on the tuff surface.

3.6. EDS Test Results

Quantitative analysis of the EDS spectra (Figure 11) reveals significant changes in the surface elemental composition. The untreated tuff’s EDS spectrum shows characteristic peaks for silicon (Si), oxygen (O), aluminum (Al), and trace elements such as calcium (Ca), sodium (Na), and potassium (K). After APTES modification, there is a noticeable increase in the intensities of the silicon and carbon peaks, with carbon content rising from 3.8% to 10.3%, as shown in Figure 12. These changes confirm the formation of Si-O-Si bonds and a successful silane-bonding process. The increased silicon content, along with shifts in elemental proportions, further validates this interaction. As the concentration of the silane-coupling agent increases, the formation of a self-assembled film on the tuff surface is evident. The EDS analysis supports the expected increase in silicon content, consistent with the chemical reaction where the silanol groups of the silane agent bond with hydroxyl groups on the tuff, leading to the formation of siloxane bonds and an enhanced silicon concentration at the surface.

3.7. Mechanisms of APTES-Induced Interface Enhancement Between Tuff Aggregates and Asphalt

Figure 13 methodically depicts the modification and interface enhancement mechanisms between APTES and tuff aggregates through the formation of an APTES self-assembled molecular layer, involving three key steps: APTES hydrolysis, self-condensation, and condensation with inorganic materials.
Figure 13(1) illustrates the molecular structure of APTES (NH2(CH2)3Si(OC2H5)3) and the stages of its hydrolysis and self-condensation. During hydrolysis, the alkoxy groups on APTES are replaced by hydroxyl groups, producing ethanol as a by-product. These newly formed silanol groups can subsequently undergo self-condensation, forming Si-O-Si bonds that create a cohesive network. This foundational molecular process allows for APTES to chemically bond to the tuff surface in the subsequent stages. Figure 13(2) depicts the environmental stresses experienced by the aggregate–asphalt system, specifically water infiltration and dynamic wheel loading. The mechanism of adhesion failure between aggregate and asphalt involves the dynamic interaction of water molecules at the aggregate–asphalt interface due to the driving load’s “squeezing” and “pumping” effects. This repeated interaction leads to bond failure at the interface, exacerbated by the penetration of water molecules into microscopic defects and voids on the aggregate surface.
Figure 13(3) compares the adhesion performance of untreated and APTES-treated aggregates. In untreated samples, water infiltration leads to adhesion failure due to insufficient bonding at the interface. When APTES bonds to the tuff, the γ-aminopropyl group protrudes from the surface, forming a stronger and more durable bond than physical interlocking alone. This bond reduces the risk of water infiltration at the asphalt–aggregate interface, thereby mitigating moisture-induced damage and enhancing long-term adhesion and pavement durability. The silane-treated tuff creates a hydrophobic barrier, repelling water and preserving the aggregate–asphalt bond even in wet conditions. The final part illustrates the aggregate–APTES–asphalt interface after APTES treatment. The γ-aminopropyl group of APTES is shown as a functional bridge between the inorganic tuff and organic asphalt, providing a robust, moisture-resistant bond. This enhanced interface prevents moisture-induced degradation, effectively preserving long-term adhesion and improving the durability of asphalt pavements.
In summary, while untreated tuff provides some adhesion through physical interlocking and limited chemical compatibility, treating tuff with a silane-coupling agent significantly enhances both chemical and physical bonding mechanisms.

4. Conclusions

This study addresses the challenge of enhancing adhesion between tuff and asphalt by evaluating the impact of an APTES self-assembly layer on tuff interface properties across various scales, encompassing macroscopic bonding, surface energy, and micromorphology. Utilizing the BBS test, Marshall stability test, contact angle measurements, SEM, and EDS analyses, this research explores the bond strength, water damage resistance, adhesion work, and elemental composition changes in tuff before and after surface treatment with the silane-coupling agent. The self-assembly mechanism of APTES and its effect on enhancing interfacial adhesion on tuff aggregate surfaces are systematically elucidated. The conclusions drawn from this work are as follows:
  • A mass ratio of water:ethanol:APTES solution of 5:45:50, combined with a curing temperature of 200 °C, significantly enhances the bond strength between tuff and asphalt.
  • Under optimal modification conditions, the adhesion work and energy ratio are improved to 55.28 mJ/m2 and 1.42, respectively, indicating substantial enhancements in both adhesion strength and resistance to peeling.
  • The APTES treatment results in the formation of a denser silane-coupling agent layer on the tuff surface, which is confirmed by increased silicon content and Si-O-Si bonding, contributing to stronger chemical bonds at the asphalt–aggregate interface.
  • The chemical reaction between APTES and the tuff surface introduces γ-aminopropyl groups, significantly enhancing interfacial adhesion with asphalt. This stronger chemical bond reduces water infiltration and mitigates moisture-induced damage.
The results of this study demonstrate that APTES self-assembly on tuff aggregates significantly enhances interfacial adhesion with asphalt, suggesting that APTES-treated tuff has strong potential for durable applications in road engineering, particularly in utilizing tunnel spoil.

Author Contributions

Conceptualization, Y.L.; data curation, G.G. and J.Z.; formal analysis, H.L. and G.Z.; funding acquisition, X.Z.; investigation, G.G., J.Z. and H.Z.; methodology, M.L.; project administration, G.Z.; software, X.G. and H.Z.; supervision, L.K.; validation, L.C.; visualization, L.C. and Y.L.; writing—original draft, M.L.; writing—review and editing, X.G. All authors have read and agreed to the published version of the manuscript.

Funding

(1) Supported by the National Key R&D Program of China (No. 2022YFB2602602). (2) Supported by the Research on Application Technology of Asphalt Pavement Floor Materials Based on Tunnel Slag Tuff (No. KYL202307-0014). (3) Supported by the National Key R&D Program of China (No. 2022YFB2602604).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

Authors Mingxin Lai, Gen Zhang, Hongming Zhu and Xinping Zhang were employed by Fujian First Highway Engineering Group Co., Ltd. Author Xiaoying Gao was employed by Fujian Expressway Group Co., Ltd. Author Lizong Chen was employed by Dehua County Road and Bridge Construction Co., Ltd. Author Haixing Lin was employed by Fujian Communications Planning & Design Institute CO., Ltd. Author Yueling Lin was employed by Quanzhou Road and Bridge Engineering Construction Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Coatings 14 01422 g001
Figure 1. Tuff aggregate surface modification procedures.
Figure 1. Tuff aggregate surface modification procedures.
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Figure 2. The experimental procedure.
Figure 2. The experimental procedure.
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Figure 3. Binder bond strength (BBS) test: (a) schematic diagram BBS test; (b) test sample preparation.
Figure 3. Binder bond strength (BBS) test: (a) schematic diagram BBS test; (b) test sample preparation.
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Figure 4. The Marshall stability test procedure.
Figure 4. The Marshall stability test procedure.
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Figure 5. Contact angle test: (a) contact angle test; (b) contact angle diagram; (c) asphalt sample; (d) sliced aggregate pieces.
Figure 5. Contact angle test: (a) contact angle test; (b) contact angle diagram; (c) asphalt sample; (d) sliced aggregate pieces.
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Figure 6. X-ray diffraction test results of basalt and tuff.
Figure 6. X-ray diffraction test results of basalt and tuff.
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Figure 7. Binder bond strength (BBS) test results.
Figure 7. Binder bond strength (BBS) test results.
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Figure 8. Immersion residue stability test results.
Figure 8. Immersion residue stability test results.
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Figure 9. Adhesion work and energy ratio.
Figure 9. Adhesion work and energy ratio.
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Figure 10. SEM micrographs: (a) SEM micrographs (500× magnifcation, 2500× magnifcation) of tuff surface treated with APTES. (b) SEM micrographs (500× magnifcation, 2500× magnifcation) of tuff surface treated without APTES.
Figure 10. SEM micrographs: (a) SEM micrographs (500× magnifcation, 2500× magnifcation) of tuff surface treated with APTES. (b) SEM micrographs (500× magnifcation, 2500× magnifcation) of tuff surface treated without APTES.
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Figure 11. EDS analysis results: (a)untreated specimen; (b)treated specimen.
Figure 11. EDS analysis results: (a)untreated specimen; (b)treated specimen.
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Figure 12. Elemental composition.
Figure 12. Elemental composition.
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Figure 13. Schematic diagram of the enhancement mechanism.
Figure 13. Schematic diagram of the enhancement mechanism.
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Table 1. Basic physical characteristics of tuff.
Table 1. Basic physical characteristics of tuff.
Testing IndexesUnitRequirementsTested ParametersTest Methods
Crushing value%≤26%14.4T0316: Determines the crushing value to assess aggregate resistance to compressive loads.
Los Angeles abrasion value%≤28%7.9T0317: Measures the Los Angeles abrasion value to evaluate wear resistance.
Bulk volume relative densityg/cm3≥2.602.684T0304: Tests bulk volume relative density, apparent relative density, and water absorption.
Apparent relative densityg/cm3≥2.702.749
Water absorption%≤2.01.034
Acicular content%≤128.5T0312: Assesses the acicular content, indicating the percentage of elongated and flaky particles.
Table 2. Basic physical characteristics of APTES.
Table 2. Basic physical characteristics of APTES.
Molecular FormulaBoiling Point (°C)Refractive Index (nD25)Flash Point (°C)Purity (%)
C9H23O3NSi2141.41586>99
Table 3. Physical characteristics of ES70# bitumen.
Table 3. Physical characteristics of ES70# bitumen.
PG GradePenetration (25 °C, 0.1 mm)Ductility
(10 °C, cm)		Softening Point (°C)Dynamic Viscosity
(60 °C, Pa·s)		Ignition Point (°C)
58-16664649.3286354
Table 4. Numbering of APTES hydrolysis solutions with different concentrations.
Table 4. Numbering of APTES hydrolysis solutions with different concentrations.
ItemAPTES (m)Water (m)Ethanol (m)Curing Temperature
A1000150
A2000180
A3000200
B124949150
B224949180
B324949200
C154550150
C254550180
C354550200
D1104545150
D2104545180
D3104545200
E1154540150
E2154540180
E3154540200
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MDPI and ACS Style

Lai, M.; Gao, X.; Kong, L.; Chen, L.; Gan, G.; Lin, H.; Zhang, J.; Zhang, G.; Lin, Y.; Zhu, H.; et al. Enhancing Interface Performance Through Self-Assembly Mechanisms of APTES on Surface-Modified Tuff Aggregates. Coatings 2024, 14, 1422. https://doi.org/10.3390/coatings14111422

AMA Style

Lai M, Gao X, Kong L, Chen L, Gan G, Lin H, Zhang J, Zhang G, Lin Y, Zhu H, et al. Enhancing Interface Performance Through Self-Assembly Mechanisms of APTES on Surface-Modified Tuff Aggregates. Coatings. 2024; 14(11):1422. https://doi.org/10.3390/coatings14111422

Chicago/Turabian Style

Lai, Mingxin, Xiaoying Gao, Lin Kong, Lizong Chen, Guoan Gan, Haixing Lin, Jiakang Zhang, Gen Zhang, Yueling Lin, Hongming Zhu, and et al. 2024. "Enhancing Interface Performance Through Self-Assembly Mechanisms of APTES on Surface-Modified Tuff Aggregates" Coatings 14, no. 11: 1422. https://doi.org/10.3390/coatings14111422

APA Style

Lai, M., Gao, X., Kong, L., Chen, L., Gan, G., Lin, H., Zhang, J., Zhang, G., Lin, Y., Zhu, H., & Zhang, X. (2024). Enhancing Interface Performance Through Self-Assembly Mechanisms of APTES on Surface-Modified Tuff Aggregates. Coatings, 14(11), 1422. https://doi.org/10.3390/coatings14111422

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