TiO₂-Modified Emulsified Asphalt Exhibits Enhanced High-Temperature Performance and Photocatalytic Functionality

Abstract

This study developed a nano-TiO₂ modified emulsified asphalt, and the effects of varying dosages of nano-TiO₂ on the performance of emulsified asphalt were systematically investigated. The photocatalytic performance of nano-TiO₂ modified emulsified asphalt was evaluated through NO_x degradation assessment using a self-developed device. Results showed that after adding nano-TiO₂, the high-temperature stability and aging resistance of the modified emulsified asphalt had improved. Specifically, when the TiO₂ content ranged from 4% to 6%, the surface temperature of the photocatalytic fog seal layer decreased by 2.0 °C. At 4% TiO₂ content, the cumulative degradation efficiencies of NO and NO₂ reached 50% and 70%, respectively. When the fog seal layer spraying amount was set at 300 g/m², the pavement performance met the required standards. Overall, nano-TiO₂ photocatalytic materials simultaneously enhance the physical properties of asphalt and its environmental benefits, providing a promising solution for the development of multifunctional pavement materials.

1. Introduction

Based on the demand for environmental protection and green ecological road construction, nanomaterials have been gradually introduced into road engineering and successfully applied in various ecological road constructions, both domestically and internationally [1,2,3,4,5,6]. The advantages and future development directions of binary oxide ceramics in solar cells have been clearly defined [7]. Among these nanomaterials, nano-TiO₂ can absorb and reflect sunlight across different wavelengths, thereby enhancing the physical properties of asphalt and significantly improving asphalt pavement performance.

Ortelli S et al. modified the surface of nano-TiO₂ by coating it with a silica (SiO₂) matrix to achieve the dual goals of reducing the risk of ROS generation and improving the photocatalytic denitrification (NO_x) performance, and verified the feasibility of the “molecular design safetyization (SbD)” strategy [8]. Zhong et al. added varying proportions of nano-TiO₂ to emulsified asphalt to prepare a coating applicable to asphalt pavement. Field tests revealed that higher nano-TiO₂ content resulted in better cooling effects of the coating [9]. Ayar et al. incorporated nano-TiO₂ into asphalt mixtures, finding that its high specific surface area effectively increased asphalt viscosity and reduced rutting and permanent deformation under high-temperature conditions [10]. Another study showed that adding 1%–7% nano-TiO₂ and 0.4%–2.8% nano-SiO₂ to 60/70 asphalt increased asphalt viscosity, reduced elasticity, increased complex shear modulus, decreased phase angle, enhanced permanent deformation resistance, and improved high-temperature performance [11]. Shafabakhsh et al. prepared modified asphalt mixtures with different nano-TiO₂ contents to explore their influence on asphalt mixture performance. Test results indicated that, compared to traditional asphalt mixtures, nano-TiO₂ incorporation significantly improved permanent deformation resistance and fatigue life under high-temperature conditions [12].

Moreover, combining nano-TiO₂ with asphalt pavement materials not only enhances anti-aging and high-temperature performance but also effectively degrades pollutants due to its excellent photocatalytic absorption of ultraviolet light. Tang et al. systematically reviewed the research progress of TiO₂ in photocatalytic treatment of cement and asphalt, focusing on the environmental purification function, and providing a direction for the development of low-carbon and green building materials [13]. Fan et al. developed C-TiO₂ suspensions sprayed on asphalt rutting plates, showing superior NO_x degradation compared to P25 [14]. Jin et al. prepared TiO₂-MMT (T/M) composite photocatalytic materials via sol–gel methods, demonstrating improvements in aging resistance, high-temperature rheology, and tail gas degradation performance [15]. Li et al. prepared TiO₂@SiO₂ composite photocatalytic materials by sol–gel method and made them into photocatalytic coatings to evaluate the degradation efficiency of NO and Rhodamine B. When the content of TiO₂@SiO₂ was 7%, the degradation rates of NO and Rhodamine B by the photocatalytic coating were the highest, reaching 0.17% and 32.94% respectively [16].

In this paper, based on the characteristics of sprayed asphalt pavements, a photocatalytic fog sealing material was prepared by dispersing nano-TiO₂ into emulsified asphalt. This approach enhances the interface bonding force of emulsified asphalt, improves high-temperature resistance and anti-aging performance, and helps maintain road integrity by preventing cracking and rutting. Additionally, the microporous structure formed during emulsification increases the contact between photocatalytic materials and polluted gases, thereby improving photocatalytic degradation efficiency and reducing air pollution [17,18,19,20].

2. Materials and Experiment

2.1. Materials

Nano-TiO₂ was prepared using the sol–gel method in this study [21]. The reagents used, including anhydrous ethanol, tetrabutyl titanate, deionized water, and hydrochloric acid, were all supplied by Sinopath Group Chemical Reagent Co., Ltd. (Shanghai, China). Nano-TiO₂ appears as a white powder. Its main component is a mixed crystal composed of anatase and rutile. The XRD spectrum of it is presented in the Supporting Information (Figure S1), and its physical and chemical properties are shown in

Table 1. Main physical and chemical properties of nano-TiO_2.

Characterization	Units	Typical Value
Specific surface area (BET method)	m2/g	50 ± 15
Mean grain size	nm	21
Compaction density	m/g	about 130
The content of TiO2 (based on the material after calcination)	wt.%	≥99.5

. The dispersant sodium oleate of analytical purity was provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The emulsifier SQ-M3 was produced by Jinan Shengquan Group Co., Ltd. (Jinan, China), and the asphalt used was the 70# matrix asphalt from the Jingbo brand (Binzhou, China). The main technical specifications of these raw materials are summarized in

Table 2. Main technical indicators of sodium oleate.

Technical Index	Units	Typical Values
Molecular weight	—	304.45
Basicity	mmol/100 g	≤.cm/
Heavy metals	-	≤0.001%
Burning residue	%	22.0~25.0

,

Table 3. Basic technical specifications of SQ-M3 emulsifier.

Technical Specifications	Units	Indicator Results
Active ingredient content	%	45~50
Appearance	—	Milky paste
Scent	—	Non-toxic and aromatic
Solubility	—	Soluble in water and certain organic solvents

and

Table 4. The basic technical specifications of the 70# asphalt.

Test Index	Test Results	Technical Requirements
Needle penetration (25 °C, 100 g, 5 s)/(0.1 mm)	65.7	60~80
Ductility (5 cm/min, 15 °C)/cm	100	≥100
Softening point (R&B)/°C	52.2	≥45

.

2.2. Preparation of TiO₂ Modified Emulsified Asphalt

The preparation flow chart of nano-TiO₂ modified emulsified asphalt is shown in Figure 1. The typical preparation process is as follows: (1) The Preparation of Soap Liquid: Weigh the emulsifier SQ-M3 with different mass fractions (0.4%, 0.6%, 0.8%, 1.0%), mix it with water in a 500 mL beaker, heat, and stir until the temperature reaches approximately 70 °C, and maintain this temperature to complete the soap liquid preparation. (2) The Preparation of Asphalt: Place 70# base asphalt into an oven set at 140 °C, heat until fully molten, and store for later use. (3) The Preparation of Emulsified Asphalt: Pour the prepared soap liquid into a colloid mill and shear for 1 min. Slowly add the molten matrix asphalt at 140 °C (the mass ratio of matrix asphalt to soap liquid is 50:50), and continue shearing for an additional 3 min after blending to produce finely dispersed cationic emulsified asphalt. (4) The Preparation of nano-TiO₂ Dispersion Solution: Weigh nano-TiO₂ with different mass fractions, prepare a dispersant solution using sodium oleate at 50% of the TiO₂ mass fraction, and ultrasonicate for 20 min (detailed explanations can be found in Supporting Information Figure S2). (5) The Preparation of Modified Emulsified Asphalt: Incorporate nano-TiO₂ dispersions with different mass fractions (2%, 4%, 6%, 8%) into the prepared emulsified asphalt, stir thoroughly to ensure even distribution, and disperse evenly to prepare the nano-TiO₂ modified emulsified asphalt [22,23].

2.3. Preparation of Fog Sealing Layer of TiO₂ Modified Emulsified Asphalt

The preparation process of the nano-TiO₂ modified emulsified asphalt fog seal layer is as follows: The detailed preparation process comprises the following steps: (1) The Preparation of Asphalt Mixture: Adjust the temperature of the mixing pot to 160 °C. Place the aggregate in the mixing pot and dry mix for 90 s. Add lignin fiber to the mixing pot and dry-mix for an additional 90 s. After ensuring the aggregate and lignin fiber are evenly mixed, add the asphalt and mix for 90 s. Finally, add the mineral powder and mix for another 90 s. Once the asphalt and other minerals are fully mixed, transfer the fully blended mixture into the rutting plate mold and compact it using a rutting compactor. (2) The Preparation of the Modified Emulsified Asphalt Fog Seal Layer: Uniformly apply nano-TiO₂-modified emulsified asphalt (with mass fractions of 0%, 2%, 4%, 6%, and 8%) onto the surface of a 300 mm × 300 mm SMA-13 rutting plate onto the surface of a 300 × 300 mm SMA-13 rutting plate. Allow the coated specimen to cure in a well-ventilated area at ambient temperature until fully dried. The specimens of modified emulsified asphalt fog seal coat after curing are shown in Figure 2.

2.4. Investigation Methods of Modified Emulsified Asphalt

2.4.1. Routine Physical Property Test

The conventional physical performance tests of nano-TiO₂-modified emulsified asphalt were carried out in accordance with the “Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering” (JTG E20-2011) [24]. The specific test procedures followed were T0651-1993 and T0655-1993 to measure the evaporation residue content and storage stability.

The anti-aging performance was evaluated through thermal-oxidative aging tests according to the T0610-2011 test method. Short-term aging experiments were performed on the evaporation residue of emulsified asphalt using an SDY-3061-type asphalt rotary film oven (Cangzhou Changzhi Construction Instrument Co., Ltd., Cangzhou, China). The samples utilized in these tests are illustrated in Figure 3a.

2.4.2. DSR Test

High-temperature rheological properties were measured using a Gemini 200 ADS dynamic shear rheometer (DSR) (Malvern Panalytical, Shanghai, China), following the ASTM D7175 test method to perform a temperature sweep on the modified emulsified asphalt before aging. Figure 3b shows the samples of the evaporated residue of nano-TiO₂-modified emulsified asphalt that were tested.

2.5. Road Performance Evaluation of Fog Sealing Layer

2.5.1. Anti-Skid Performance Test

The sliding resistance of the nano-TiO₂ photocatalytic fog seal layer on asphalt pavement at varying dosages was assessed through friction coefficient measurements and surface structure depth analysis. The surface structure depth was determined using the T0731-2000 method from the “Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering” [24]. The friction coefficient was measured using a pendulum type tribometer according to the T0964-2008 method in the “Field Test Regulations for Highway Subgrade and Pavement” [25].

2.5.2. Water Penetration Performance Test

To investigate the effect of varying nano-TiO₂ content on the impermeability of the photocatalytic fog seal layer, tests were conducted using the T0730-2011 method from the “Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering” [24].

2.5.3. Pavement Cooling Performance Test

A field simulation experiment was designed to validate the temperature-reduction efficacy of the photocatalytic fog seal layer on asphalt pavement. Rut plates coated with the photocatalytic fog seal layer were placed under outdoor sunlight to simulate real-world conditions. The test was conducted on 24 June, with ambient temperatures exceeding 30 °C, and the measurement period was from 12 p.m. to 5 p.m.

The experimental configurations for evaluating the road performance of the modified emulsified asphalt fog seal layer are systematically summarized in Figure 4.

2.5.4. Evaluation of Photocatalytic Performance of Fog Seal Layer

This experiment utilized a self-developed photocatalytic exhaust gas degradation device. The degradation device includes: (1) an ultraviolet lamp, (2) a lamp holder, (3) a negative pressure gauge, (4) sealing cover, (5) an air heating device, (6) a temperature controller, (7) a fan, (8) a tail gas intake valve, (9) a tail gas intake pipe, (10) a tail gas container, (11) a vacuum pump, (12) an air extraction valve, (13) an air extraction pipe, (14) a tail gas analyzer, (15) a tail gas outlet pipe, (16) a tail gas outlet valve, (17) wire, (18) a light source controller, (19) a reaction chamber container, (20) a tail gas reaction chamber, (21) photocatalytic coating, (22) a specimen slot, (23) an elevating bracket, (24) and a fixed bracket. The photocatalytic degradation device has a volume of approximately 40 L, utilizing a UV lamp with a wavelength of 365 nm. The UV irradiation intensity is stabilized at 26 W/m² by voltage adjustment. Testing was conducted at room temperature, with initial NO/NO₂ concentrations around 25 ppm. As illustrated in Figure 5, to evaluate the photocatalytic performance of the TiO₂-modified emulsified asphalt fog seal layer based on the degradation rates of NO and NO₂. The specific evaluation indicators are outlined as follows [26].

(1)

Cumulative degradation rate

$$e={\displaystyle \frac{C_0-C_t}{C_0}}\times 100\%$$

(1)

In the formula, e is the cumulative degradation rate, %; C₀ is the initial gas concentration, ppm; and C_t is the gas concentration at time t, ppm.

(2)

Immediate degradation rate

$$p_t={\displaystyle \frac{C_{\mathrm{Blank},t}-C_{\mathrm{Test},t}}{C_{\mathrm{Blank},t}}}$$

(2)

In the formula, p_t represents the instantaneous degradation rate of the photocatalytic fog seal layer at time t; C_Blank,t is the gas concentration of the blank control group at time t; and C_Test,t is the gas concentration of the experimental group at time t.

(3)

Average value of immediate degradation rate

$$p_{t\text{-}AVE}={\displaystyle \frac{C_k+C_{k+1}+C_{k+2}\dots \dots +C_n}{n-k+1}}$$

(3)

In the formula, P_t-AVE represents the average degradation rate of the photocatalytic fog sealing layer; and C_k + C_k+1 + C_k+2 +……+ C_n represents the instantaneous degradation rates of the photocatalytic fog sealing layer at moments k, k + 1, k + 2, …, n, respectively.

Furthermore, to detect whether nitrate ions are generated during the degradation of NO_x, 20 mL of deionized water was placed in a Petri dish prior to the start of the experiment. Nitrate ions are colorless and soluble in water. If nitrate ions are produced during the experiment, they can be identified using ultraviolet absorption spectroscopy.

3. Results and Discussion

3.1. Performance Analysis of TiO₂ Modified Emulsified Asphalt

3.1.1. Evaporation Residue Content

The evaporation residue content is a critical parameter influencing the performance of emulsified asphalt. According to the specifications, the evaporation residue content for medium split cationic modified emulsified asphalt must not be less than 50%. Figure 6 illustrates the evaporation residue content of modified emulsified asphalt with varying TiO₂ concentrations. As shown in the figure, an increase in nano-TiO₂ content leads to higher evaporation residue content and viscosity, thereby enhancing the storage stability of the emulsified asphalt. However, excessive nano-TiO₂ content results in increased residue on the 1.18 mm sieve, destabilizing the oil-in-water structure of the emulsified asphalt and leading to diminished stability. This is attributed to the nano-scale nature of TiO₂, which has low specific gravity and high specific surface area, causing it to adsorb asphalt particles and increase viscosity. When the concentration exceeds the optimal level, it not only increases the residue on the sieve but also adversely affects performance and increases costs [27].

3.1.2. Three Indexes

Three indices can evaluate the quality and performance of asphalt: penetration, softening point, and ductility. Penetration reflects the consistency of asphalt, the softening point indicates its high-temperature performance, and ductility measures its low-temperature flexibility.

Figure 7 presents the test results for three indicators of the evaporation residue from modified emulsified asphalt with varying TiO₂ contents. As shown in the figure, the needle penetration values are 4% < 2% < 6% < 8% < 0%, the softening points follow the trend 4% > 2% > 6% > 8% > 0%, and the ductility decreases in the order 0% > 2% > 4% > 6% > 8%. Notably, at a TiO₂ content of 4%, there is an inflection point where penetration initially decreases and then increases, while the softening point first increases and then decreases. Ductility consistently decreases throughout, indicating that TiO₂ addition significantly alters asphalt performance.

The aforementioned phenomenon can be attributed to the fact that the addition of nanomaterials alters the viscosity of emulsified asphalt, leading to its increased rigidity. Additionally, nanomaterials possess a large specific surface area and strong surface activity, leading to enhanced adsorption of asphalt components. This reduces light fractions in the asphalt, alters its temperature sensitivity, and consequently improves high-temperature performance while diminishing low-temperature performance [27].

3.1.3. Storage Stability

Figure 8 shows the storage stability of modified emulsified asphalt after 1 day and 5 days. As can be seen from the figure, within a certain range of nano-TiO₂ content, it can improve the storage stability of emulsified asphalt. However, when the content exceeds a certain range, the storage stability will deteriorate. When the content of TiO₂ is 6% and 8%, the storage stability after 1 day is greater than 1%, and the storage stability after 5 days is greater than 5% (the corresponding photos are presented in Supporting Information Figure S3). These do not meet the specification requirements. The reason for the deterioration of the stability effect lies in the fact that TiO₂ is nano-sized, with a light weight and large specific surface area. When the content is small, it will adsorb asphalt particles, increase the viscosity of asphalt, and make the settling speed of asphalt particles slower, thereby improving the storage stability. However, when the content exceeds a certain range, some nano-TiO₂ will adsorb asphalt, causing the asphalt particle size to be larger, resulting in an accelerated settling phenomenon of asphalt particles, while the excess TiO₂ floats on the surface of the emulsified asphalt, leading to the appearance of skinning and subsequently deteriorating the storage stability and affecting the pavement performance of the fog seal layer [28].

3.1.4. Anti-Aging Performance

The anti-aging performance of asphalt is typically evaluated using indicators such as the residual needle penetration ratio, softening point increment, and residual elongation ratio. Higher values for the residual needle penetration ratio and residual elongation ratio, along with a smaller softening point increment, indicate better atmospheric stability, slower aging, and superior anti-aging performance.

Figure 9 presents the test results for the anti-aging performance of the evaporative residue from modified emulsified asphalt with varying TiO₂ dosages. As shown in the figure, the residual needle penetration ratios are 6% > 8% > 4% > 2% > 0%, the softening point increments follow the trend 4% < 6% < 2% < 8% < 0%, and the residual elongation ratios are 4% > 6% > 2% > 8% > 0%. By comprehensive comparison, the anti-aging performance of the evaporative residue of modified emulsified asphalt under different TiO₂ contents ranks as follows: 4% > 6% > 2% > 8% > 0%.

The addition of nano-TiO₂ enhances the anti-aging performance of the evaporative residue of modified emulsified asphalt compared to unmodified asphalt. This improvement can be attributed to two main factors: (1) Nano-TiO₂ has a very high refractive index and strong reflection and scattering capabilities for light, thus having a good shielding function against ultraviolet rays. It can absorb ultraviolet light with wavelengths less than 300 nm, effectively reducing the irradiation of ultraviolet rays on asphalt, and thereby reducing the impact of ultraviolet rays on the performance of asphalt such as penetration index and softening point. (2) Nano-TiO₂ particles have a large specific surface area and high surface activity, and can act as physical cross-linking points to promote the combination of modifiers and asphalt, making the transition of interface bonding between modifiers and asphalt more gradual. This smooth and uniform microstructure provides higher stability and elasticity for asphalt, thereby improving its hardness and high-temperature performance. Therefore, an optimal amount of nano-TiO₂ can significantly improve the anti-aging performance of emulsified asphalt [29,30].

3.1.5. DSR Analysis

Temperature scanning was conducted on the evaporative residue of nano-TiO₂ modified emulsified asphalt with varying dosages to evaluate its high-temperature performance.

The complex modulus G* reflects the asphalt’s resistance to deformation under external pressure. A higher G* indicates stronger resistance to deformation and greater viscoelasticity [31]. The change in phase angle δ can reflect the viscoelastic ratio and fluidity of asphalt. Larger δ signifies a higher viscous component, lower elasticity, and greater fluidity, indicating that the asphalt is gradually becoming a viscous liquid [32,33]. The rutting factor G*/sinδ reflects the high-temperature stability of asphalt. Larger G*/sinδ indicates better resistance to permanent deformation at high temperatures and superior high-temperature stability [34].

Figure 10a illustrates the variation in asphalt complex shear modulus and phase angle with temperature. As shown in the figure, the complex modulus G* of the evaporative residue follows the trend 4% > 2% > 6% > 8% > 0%. This indicates that the shear deformation resistance of modified emulsified asphalt is also 4% > 2% > 6% > 8% > 0%, suggesting that nano-TiO₂ can enhance the anti-deformation ability of asphalt to some extent.

From Figure 10a, it can also be observed that the phase angle δ of the evaporative residue of emulsified asphalt at different TiO₂ contents is positively correlated with temperature; as temperature increases, the phase angle δ also increases. This is because rising temperatures cause the elastic components in asphalt to transform into viscoelastic components, leading to increased viscosity and decreased elasticity, ultimately resulting in a viscoelastic liquid state. Within the temperature range of 46 °C to 82 °C, the modified emulsified asphalt with a TiO₂ content of 4% exhibits the weakest viscous flow, followed by 2%, 6%, and 8%, while the emulsified asphalt with 0% TiO₂ content shows the highest viscous flow. This phenomenon may be attributed to the low specific gravity and large specific surface area of nano-TiO₂, which adsorbs light components of asphalt particles, increasing viscosity, reducing elasticity, and improving anti-deformation ability.

Figure 10b presents the change curve of G*/sinδ for the evaporative residue of modified emulsified asphalt with temperature. As shown in the figure, the rutting factor G*/sinδ follows the trend 4% > 2% > 6% > 8% > 0%. Therefore, the rutting resistance of modified emulsified asphalt at high temperatures is also 4% > 2% > 6% > 8% > 0%, indicating that nano-TiO₂ can improve the high-temperature stability of asphalt to some extent, consistent with the conclusions drawn from the complex modulus G*.

3.2. Road Performance Analysis of Fog Seal Layer

3.2.1. Anti-Skid Performance Analysis

Figure 11 illustrates the relationship between the structure depth and friction coefficient of the fog seal layer, as influenced by nano-TiO₂ content and spray amount. As shown in the figure, when the coating amount is constant, an increase in nano-TiO₂ content results in a slight decrease in surface structure depth and friction coefficient, but this decline is relatively gradual, indicating that nano-TiO₂ has minimal impact on the anti-skid performance of the pavement.

When the nano-TiO₂ content is held constant, both the surface structure depth and friction coefficient of the fog seal layer exhibit a downward trend with increasing spray amount. When the spray amount does not exceed 300 g/m², the friction coefficient remains above 50, meeting the code requirements. However, when the spray amount reaches 400 g/m², the friction coefficient drops below 50, failing to meet the code standards. This indicates that the spray amount significantly affects the anti-skid performance of the road surface.

The observed phenomenon can be attributed to the fact that initially sprayed modified emulsified asphalt fills the cracks in the mixture after demulsification without affecting its anti-skid performance. As the spray amount of modified emulsified asphalt increases, the cracks are gradually filled, altering the surface texture and structure depth of the mixture, which ultimately leads to a deterioration in anti-skid performance.

3.2.2. Analysis of Water Seepage Performance

Figure 12 illustrates the relationship between the water permeability coefficient of the fog seal layer and the nano-TiO₂ content as well as the spray amount. As shown in the figure, the water permeability coefficient of the asphalt pavement meets standard requirements. With increasing nano-TiO₂ content and spray amount, the water permeability coefficient of the fog seal layer decreases, indicating that higher nano-TiO₂ content and spray amount can enhance the water-sealing performance of the asphalt pavement.

This improvement in water-sealing performance is attributed to the modified emulsified asphalt penetrating into the surface voids of the mixture, forming a film on the surface and filling the gaps, thereby preventing water from seeping further. Based on the above test results for anti-skid and water permeability performance, it is recommended that the spray volume of the fog seal layer be approximately 300 g/m² and should not exceed 400 g/m². In practical engineering applications, the optimal spray volume of the fog seal layer can be determined by balancing anti-skid and waterproofing performance.

3.2.3. Analysis of Pavement Cooling Performance

The field test method for evaluating the cooling effect of nano-TiO₂ modified emulsified asphalt fog seal layers is as follows: (1) Weigh the emulsified asphalt and nano-TiO₂ dispersion liquid using an electronic balance, combine the two materials in a beaker, and thoroughly mix them with a stirrer to prepare the fog seal layer. (2) Select five rutting plate specimens, each measuring 30 cm × 30 cm. Apply the fog seal layer with five different TiO₂ concentrations uniformly onto the rutting plates at a coating rate of 300 g/m². Ensure that the fog seal layer completely covers the asphalt surface and allow it to cure naturally at room temperature for 12 h. (3) Choose a placement site where all five rutting plate specimens are fully exposed to sunlight. (4) Measure the temperature every hour from noon until 5 p.m. using a thermometer and record the data.

Figure 13 illustrates the temperature changes over time for the fog seal layer under varying TiO₂ content. As shown in the figure, compared to the fog seal layer without nano-TiO₂, the road temperature of the fog seal layer containing nano-TiO₂ increases more slowly. This indicates that the presence of nano-TiO₂ reduces the surface temperature of the rutting plates. It is widely recognized that nano-TiO₂ exhibits excellent ultraviolet absorption capabilities. When ultraviolet radiation interacts with asphalt pavements containing TiO₂, the particles become excited and undergo photocatalytic reactions, converting absorbed UV energy into chemical energy to degrade pollutants. The photocatalytic mechanism of TiO₂ prevents UV-induced temperature elevation in pavements, thereby achieving the effect of reducing asphalt pavement temperatures.

3.3. Analysis of Photocatalytic Performance of Fog Sealing Layer

3.3.1. Analysis of NO Degradation Performance

During the experiment, the detector will consume some of the gas, and a small amount of gas in the gas reaction chamber may react due to the action of ultraviolet light, resulting in a decrease in the NO_x concentration. Therefore, before the experiment begins, a blank control method should be adopted to calibrate the degradation system, thereby reducing the influence of these factors on the experimental results. The cumulative degradation rate, immediate degradation rate, and average degradation rate were calculated using the stable-state gas concentrations to evaluate the actual degradation of NO and NO₂ in photocatalytic specimens [35].

Figure 14 shows the time-varying curves of NO concentration, cumulative degradation rate, immediate degradation rate, and average degradation rate for the photocatalytic mist seal specimen over a period of 120 min. As shown in the figure, within 120 min, as time increased, the NO concentration in the reaction chamber with different TiO₂ content gradually decreased, while the degradation rates progressively increased. Calculations using Formulas (1)–(3) revealed that the cumulative degradation rates of NO for the photocatalytic mist seal specimens with varying TiO₂ content over 120 min were 36%, 45%, 50%, 42%, and 40%, respectively. The immediate degradation rates were 11%, 20.26%, 5.3%, and 2.5%, and the average degradation rates were 11%, 17%, 8%, and 4%, respectively. Therefore, the degradation effect of NO gas under different nano-TiO₂ dosages was ranked as follows: 4% > 2% > 6% > 8% > 0%.

It was observed that the photocatalytic degradation of NO was accompanied by NO₂ production [36]. Consequently, the photocatalytic fog sealing layer with different TiO₂ content was subsequently used to test the degradation effect on NO₂ gas.

3.3.2. Degradation Performance Analysis of NO₂

Figure 15 shows the time-varying curves of NO₂ concentration, cumulative degradation rate, immediate degradation rate, and average degradation rate for the photocatalytic mist seal specimens over a period of 40 min. As can be seen from the figure, after 40 min of ultraviolet illumination, the photocatalytic fog sealing layers with five different TiO₂ contents exhibited significant degradation effects on NO₂ gas. During this period, the NO₂ gas concentration in the reaction chamber decreased markedly as time progressed. The cumulative degradation rate, immediate degradation rate, and average degradation rate of NO₂ for each photocatalytic mist seal specimen were calculated using Equations (1)–(3). The cumulative and immediate degradation rates of NO₂ gas for each specimen increased continuously with different TiO₂ content and gradually stabilized over time. At the end of the 40 min period, the average degradation rates were 22%, 27%, 15%, and 6%, respectively.

The following conclusions can be drawn from the degradation effects of NO and NO₂ gases: The TiO₂ photocatalytic fog seal with a 4% TiO₂ content exhibits excellent exhaust gas degradation performance. During the test, the degradation conditions for the two primary pollutants, NO and NO₂, differed. The tail gas degradation efficiency of the photocatalytic fog seal is influenced by the nano-TiO₂ content, the effective contact area between the gas and the surface, and the application rate of the fog seal.

The reason for this phenomenon may be that nano-TiO₂ can fully contact and react with the polluted gas when its content is within 4%. However, when the dosage exceeds 4%, the effective contact area between nano-TiO₂ and the gas becomes limited due to the surface area constraints of the specimen. Excessive nano-TiO₂ tends to agglomerate, leading to a saturation effect where the number of effective nano-TiO₂ particles in contact with the gas per unit area does not increase significantly. Consequently, further improvement in degradation efficiency is hindered.

3.3.3. Ultraviolet Absorption Spectrum Analysis of Nitrate Ion

After the NO₂ gas degradation test was completed, the Petri dish containing deionized water was removed, and the presence of nitrate ions was verified using ultraviolet absorption spectroscopy. As shown in Figure 16, there is a characteristic absorption peak of nitrate ions near 205 nm, with the main absorption band ranging from 190 to 230 nm. These findings are consistent with results obtained by other researchers [19,37], indicating the formation of nitrate ions during the NO₂ gas degradation process.

With increasing nano-TiO₂ content, the characteristic absorption peak intensity of nitrate ions follows the order: 4% > 2% > 6% > 8% > 0%. This trend aligns with the positive correlation between the nitrate ion concentration and absorbance, confirming that the nitrate ion concentration also follows the same order. Subsequently, we performed a quantitative analysis of nitrate concentration and established a nitrate standard curve (Figure S4, Table S1). When the TiO₂ content is 4%, the photocatalytic fog sealing layer exhibits the highest NO₂ degradation rate, resulting in the highest characteristic absorption peak for nitrate ions. This is because at 4% TiO₂ content, NO₂ gas in the reaction chamber is more fully contacted, leading to the generation of more nitrate ions in the presence of H₂O and O₂. However, when the TiO₂ content is 6% and 8%, despite the NO₂ gas being fully contacted within the reaction chamber, the excess TiO₂ particles tend to agglomerate due to surface area limitations of the specimen. This aggregation reduces the effective contact area between TiO₂ and the gas, significantly decreasing the degradation efficiency. Consequently, fewer nitrate ions are formed compared to the 4% TiO₂ content condition. In practical engineering applications, the nitrate generated during the degradation process can be removed by rainwater erosion.

4. Conclusions

This study selects appropriate medium-split cationic emulsifiers, uses sodium oleate as the colloidal stabilizer, and laboratory-synthesized nano-TiO₂ as the modifier. By employing an emulsification-followed-by-modification method, it prepares modified emulsified asphalt with varying contents of nano-TiO₂. The effects of different nano-TiO₂ dosages on the aging resistance, high-temperature rheological properties, skid resistance, water permeability, road cooling, and photocatalytic performance of the emulsified asphalt were investigated. The main conclusions are as follows:

Aging Resistance Analysis: Compared to the evaporated residue of unmodified emulsified asphalt, that of the nano-TiO₂-modified emulsified asphalt exhibits increased viscosity and enhanced aging resistance.

High-Temperature Rheological Properties: The viscoelasticity and rutting resistance of the evaporated residue from modified emulsified asphalt improve with elevated temperatures, compared to those of the unmodified emulsified asphalt.

Skid Resistance and Water Seepage Performance: As the spray amount of the fog seal layer increases, the pavement skid resistance decreases while the water sealing performance improves. When the spray amount reaches 400 g/m², the friction coefficient no longer meets the specification requirements. It is recommended that the spray amount of the fog seal layer be approximately 300 g/m² and not exceed 400 g/m².

Pavement Cooling Performance: Due to the ability of nano-TiO₂ to absorb solar ultraviolet rays and enable photocatalysis, the photocatalytic fog seal layer applied to the pavement can reduce surface temperature under high-temperature weather and sunlight irradiation.

We independently developed a photocatalytic tail gas degradation test device, and the NO_x degradation experiment was conducted using an indoor simulation method. The results indicate that incorporating an appropriate amount of nano-TiO₂ into emulsified asphalt can enhance photocatalytic efficiency. Specifically, when the TiO₂ content is 4%, the fog seal layer achieves optimal NO_x degradation efficiency. Additionally, during the NO_x degradation process, nitrate ions are generated, which can be washed away by rainfall, thereby achieving air purification.