Analysis of the Odor Purification Mechanism and Smoke Release of Warm-Mixed Rubber Asphalt

Abstract

This study focuses on the common key technologies of “environmentally friendly and resource-saving” asphalt pavement. Reactive asphalt deodorizers react with volatile chemicals with irritating odors in asphalt under high temperature conditions, converting them into stable and non-volatile macromolecules to remove odors and achieve a deodorizing effect. A goal is to develop clean asphalt pavement materials with the main characteristics of “low consumption, low emissions, low pollution, high efficiency”. In this experimental research, we used gas-emission detection devices and methods to detect and evaluate odor concentration, SO₂, NO, volatile organic compounds, and other gases and volatile substances in the production and construction of clean asphalt and mixtures. By combining rheological experiments, mechanical experiments, and other means, this study investigates the effects of odor enhancers on the penetration, ductility, softening point, high-temperature rheological properties, construction, and workability of warm-mix asphalt and mixtures. Furthermore, infrared spectroscopy experiments are used to conduct in-depth research on the odor-enhancing mechanism of odor enhancers. The results indicate that the addition of odor enhancers has little effect on the penetration and softening point of asphalt and maintains the basic performance stability of asphalt. In terms of high-temperature rheological properties and construction workability, the addition of warm-mix agents has a significant impact on the high-temperature failure temperature and rotational viscosity of asphalt, while the influence of deodorizers is relatively small. At higher temperatures, the rotational viscosity increases with the increase in the amount of deodorant added. Functional group analysis shows that the newly added materials have little effect on the essential properties and chemical composition of asphalt. In addition, during the experimental process, it was found that the coupling effect and other chemical reactions between the deodorizing agent and the warm-mixing agent can effectively improve the degradation effect of harmful gases. After the coupling action of deodorant and the warm-mixing agent, the degradation rate of harmful gas can be increased by 5–20%, ensuring the stable performance of asphalt. The performance of powder deodorizing agent is better than that of liquid deodorizing agent, and an increase in the dosage of deodorizing agent will enhance the degradation effect. This study provides an important basis for a deeper understanding of the performance of warm-mix and odorless modified asphalt.

1. Introduction

In recent years, the number of cars in China has increased sharply, and more and more tires have been scrapped. With the implementation of the national “dual carbon” strategy and the improvement of people’s awareness of environmental protection and health, the efficient comprehensive utilization technology of discarded tires, one of the bulk solid wastes, is increasingly receiving attention. Processing waste tires into rubber powder for asphalt modification can effectively reduce their impact on the environment. Conventional hot-mixture asphalt (HMA) is made of mineral aggregate and asphalt cement at high temperature. In the process, it consumes large amounts of energy and produces a large number of harmful gases and greenhouse gases. Compared with the traditional HMA, the warm-mix asphalt (WMA) method used in this study can produce asphalt mixtures at lower temperatures, which can not only reduce the generation of harmful gases and protect the environment, but can also maintain the stable performance of the asphalt mixture or lead to better performance, improving the compaction, track resistance, fatigue resistance, and durability. Use of WMA can not only reduce the total usage of high-energy consumption asphalt products, but also effectively improve the road performance of asphalt mixtures [1]. However, rubber asphalt can produce toxic smoke with a foul odor during production and construction, which seriously restricts the large-scale promotion and application of rubber asphalt [2]. Therefore, researchers have begun to investigate smoke-suppression and odor-purification technology of rubber asphalt, and the basis for this work is the need to clarify the generation and diffusion mechanism of rubber asphalt smoke [3]. However, further research is needed on how volatile smoke is generated along with the complex chemical reaction of rubber powder in asphalt.

During the production process of rubber asphalt, the rubber powder undergoes complex chemical reactions such as desulfurization and degradation, and at the same time produces a large amount of smoke containing irritating odors [3]. However, under the same temperature conditions, heating rubber powder or asphalt alone does not produce such a phenomenon, indicating that the formation of foul-odor smoke components is accompanied by the desulfurization and degradation of rubber powder, and the desulfurization and degradation process of rubber powder in asphalt is accompanied by the generation reaction of smoke [4]. At present, research on the release of asphalt smoke has achieved some results, but mainly focuses on the volatile components in the matrix asphalt smoke [5]. Research on rubber asphalt smoke also mostly analyzes the smoke components, and has not established a quantitative corresponding relationship between smoke generation and the rubber powder desulfurization and degradation process, resulting in insufficient understanding of the deep principles of smoke generation [6]. In addition, there is no systematic study on the conditional factors and dynamic evolution process of the volatilization of rubber asphalt smoke, and there is a lack of analysis of the entire construction process of the smoke composition and release rate changing with time and spatial scene conditions [7]. Especially during the mixing and paving stages of asphalt mixtures, the specific surface area of rubber asphalt increases after contact with aggregates, providing a more favorable environment for the release of smoke [8]. There is a lack of systematic analysis on the generation and diffusion of smoke from rubber asphalt under high temperature and large specific surface area exposure, and in-depth comparative research has not been conducted on different types of modified asphalt rubber and application scenarios under different conditions, making it difficult to provide targeted measures to suppress the odor and smoke of rubber asphalt [9]. The mechanism and suppression of smoke emissions from rubber asphalt have not received sufficient attention, and the specific mechanism of the desulfurization and degradation process of rubber powder in asphalt on smoke generation is still unclear [10]. There is a lack of systematic analysis on the evolution of smoke emissions with construction space and time conditions. In order to effectively solve the limitations of smoke emissions on the large-scale application of rubber asphalt, it is necessary to carry out basic theoretical research in this area [11].

At present, scholars have conducted research on the composition and influencing factors of asphalt smoke, exploring the mechanism of asphalt smoke generation [12,13]. The smoke of asphalt comes from the physical and chemical reactions during heating. The temperature sensitivity of lightweight components is high, and they are easy to decompose and volatilize at high temperatures [11]. The molecular weight of heavy components is high, and their molecular structure is complex, with low temperature sensitivity. When the heating temperature is high, the side chains break and decompose into small molecules. When asphalt molecules evaporate and decompose at high temperatures, they will react with oxygen to generate new substances and discharge them into the air [14]. There are various types of asphalt smoke, mainly including saturated hydrocarbons, aromatic hydrocarbons, oxygen-containing, nitrogen, and sulfur heteroatom compounds. Asphalt smoke contains harmful and foul-smelling gases, as well as polycyclic aromatic hydrocarbons containing more than two benzene rings, which are carcinogenic. Heating asphalt produces volatile organic compounds (VOCs), which cause changes in its internal composition and properties, especially affecting the light components (saturated and aromatic) significantly [15]. Shafabakhsh et al. collected asphalt smoke and found that its components contained hundreds of organic compounds, including many toxic carcinogens [16]. Research by Zhang et al. revealed that VOCs in asphalt include alkanes, olefins, hydrocarbon derivatives, benzene series compounds, and sulfur-containing substances. Short-chain alkanes and olefins originate from the heating decomposition of asphalt, while benzene series compounds are formed by the volatilization of benzene-containing components in asphalt and the side chain cleavage condensation reaction of asphalt molecules [17]. Zhou et al. studied the smoke emissions of rubber powder modified asphalt and found through gas chromatography–mass spectrometry that the smoke components include alkanes, benzene derivatives, ketones, aldehydes, acids, alcohols, thiophenes, and thiazoles. The smoke emissions of asphalt increased with the increase in rubber powder content [18,19].

Existing research on asphalt smoke indicates that the amount of smoke generated by asphalt heating is relatively large, including volatile substances and foul-smelling substances. The release of smoke causes changes in the internal composition and viscosity of asphalt. The source and temperature are factors that affect the level of smoke release. The study of asphalt smoke is beneficial for further research on asphalt smoke and odor control methods. The source and type of asphalt are among the main factors affecting smoke [20]. Lv et al. selected six different sources of asphalt for research, and the results showed that the asphalt source and heating temperature have a significant impact on the content of polycyclic aromatic hydrocarbons in asphalt flue gas [21]. Gasthauer et al. used powder and liquid deodorizers to mix into rubber asphalt to prepare purified asphalt. The study found that a 0.1% dosage of deodorizers can ensure that the basic performance of asphalt meets requirements and achieves a deodorization effect [22]. Kamarulzaman et al. found that dual-component deodorizers have a more significant smoke suppression effect on hot mix Styrene-Butadiene-Styrene (SBS) mixtures than single-component deodorizers [23].

In order to effectively collect rubber asphalt smoke, scholars have conducted in-depth research. Asphalt contains many chemicals with low boiling points; thus, when asphalt is at high temperatures, these substances will evaporate into the air. Therefore, the asphalt flue gas collection device needs to have heating and sealing devices. The asphalt smoke enrichment device used by Li et al. uses dry air to introduce the asphalt smoke from the sealed system into the collection device. The collection material used is quartz fiber, which can collect the heavier, intermediate, and lighter components in the smoke [24]. In order to more accurately measure the particle content in asphalt smoke, researchers have developed a more advanced asphalt smoke collection device, which can quantitatively evaluate the degree of smoke pollution. The research on asphalt smoke collection devices is extensive, and some scholars have attempted to collect the smoke generated by asphalt mixtures to analyze the smoke generated during the mixing process [25]. Xie et al. designed a smoke collection device in conjunction with an asphalt mixture mixing device, which can directly analyze the type and concentration of smoke generated during the mixing process of the mixture [5]. Some scholars have improved the gas collection device for rubber asphalt, which can collect smoke and statistically analyze its quality while changing the mixing conditions. It is necessary to consider the variation pattern of rubber asphalt smoke during the mixing and paving process of the mixture, in order to further clarify the smoke emissions during the entire process of rubber asphalt construction [26].

Purifiers can effectively remove the foul odor of asphalt, and some scholars have studied the deodorization mechanism of deodorizers [27]. Cui et al. studied the deodorization effect of liquid and powder deodorants on rubber asphalt and found that liquid deodorants have a better deodorization effect than powder deodorants. The deodorization mechanism is that the deodorant adsorbs rubber powder and inhibits its degradation in asphalt [28]. Cao et al. selected an ion chelating material from three deodorizers to prepare purified asphalt. The control of asphalt odor by this material is reflected in the chelation reaction with nitrogen and sulfur heteroatoms in rubber-modified asphalt, thereby inhibiting the volatilization of sulfur elements [2]. Chong et al. studied the inhibitory effect of aldehyde chemical modifiers on toxic gases in coal tar pitch and found that aldehydes can suppress harmful gas emissions by reducing most of the toxic polycyclic aromatic hydrocarbons released during asphalt pyrolysis [29].

For the study of the coupling effect of different temperature mixing agents and deodorizers on the deodorization effect and deodorization mechanism, the combination of temperature mixing agents and deodorizers needs to be carefully considered. Through experimental testing of the harmful component composition of asphalt flue gas, the degradation rate and corresponding change rate of harmful gas concentration should be determined. Exploring the changes in harmful gas components in asphalt smoke can be achieved by adjusting the dosage of deodorizers. At the same time, the orthogonal experimental method is used to demonstrate the coupling effect between the warm-mixing agent and the deodorizing agent, explore whether there is an interaction between the two in the warm-mixing and deodorizing effects, and provide the optimal design scheme for the warm-mixing construction effect and degradation effect. Based on chemical testing methods such as Fourier transform infrared spectroscopy (FTIR) and gas chromatography–mass spectrometry (GC–MS), we can analyze the changes in the composition of warm-mix clean-odor asphalt, explore the chemical reactions caused by clean-odor agents and warm-mix agents, and reveal the mechanism of asphalt clean odor. It is also possible to test the basic performance, anti-aging performance, and high-temperature rheological performance of clean asphalt, select the best clean asphalt, and test the road performance of the best clean asphalt mixture. On the basis of ensuring that the performance of asphalt and its mixture is basically unchanged, we can achieve the goals of reducing the level of flue gas odor and pollution emissions during asphalt heating, improving the olfactory perception of asphalt under high temperature, and reducing the harm of asphalt construction to human beings and the environment.

2. Materials and Methods

2.1. Raw Materials

The base asphalt used in this study is Jingbo70 asphalt, and its basic properties are shown in

Table 1. Basic properties of matrix asphalt.

Experimental Project	JB70	Quality Index	Test Method [16]
Penetration (25 °C, 5 s,100 g) (0.1 mm)	71.2	60–80	T0604
Softening point (°C)	46.3	≥46	T0606
60 °C dynamic viscosity (Pa·s)	192	≥180	T0620
10 °C ductility (cm)	33	≥20	T0605
RTFOT (163 °C, 5 h)
Quality changes (%)	0.18	≤0.6	T0609
Residual penetration ratio (25 °C) (%)	76	≥65	T0604
Residual ductility (10 °C) (cm)	7.6	≥6	T0605

.

We used rubber powder as a modifier to modify the properties of the asphalt matrix and improve the properties of asphalt pavement. According to the experimental requirements, we used 30-mesh rubber powder produced from waste tires by ambient method in the same factory, and its main technical indicators are shown in

Table 2. Main performance indicators of rubber powder.

Technical Indicators	Actual Measurement Results	Technical Standard	Test Method [27]
Residue/%	6.5	<10	GB/T 19208
Metal content/%	0.007	<0.05	GB/T 19208
Fiber content/%	0.06	<1	GB/T 19208
Natural rubber content/%	34	≥30	GB/T 13249-91
Ash content/%	7	≤8	GB 4498-1997
Acetone extract/%	6	≤22	GB/T 3516
Rubber hydrocarbon content/%	55	≥42	GB/T 14837

. We selected a 15% rubber powder dosage to prepare rubber-modified asphalt, as this dosage can balance the high and low temperature performance of the modified asphalt.

The Sasobit warm-mixing agent used in this study is a commercially available product produced in Beijing. It belongs to long-chain aliphatic hydrocarbon compounds and is an artificial hard paraffin. Sasobit has a relatively regular crystal particle shape at the microscopic level, which is conducive to increasing its compatibility with rubber powder and asphalt. The viscosity is high below 98 °C, but lower than that of asphalt above the melting point of 98 °C. It can completely melt in asphalt above 115 °C, thereby reducing the viscosity of asphalt. Its physical properties are shown in

Table 3. Basic physical properties of Sasobit warm-mixing agent.

Testing Items	Unit	Typical Value
Physical state	-	Powdered solid at room temperature
Density	g/cm2	0.94
Melting point	°C	98
Color	-	White or light yellow
Smell	-	Tasteless
pH value	-	Neutral

. For the Sasobit warm-mixing agent product, the supplier recommends a dosage of 1.5% to 3.5% of the mass of crumb rubber modified asphalt (CRMA), and this plan uses a 3% dosage. We added Sasobit in proportion to the rubber-modified asphalt mixing tank in one go, mixing it evenly with the rubber asphalt without deliberately extending the mixing time.

The liquid deodorant used in this study is a colorless, fragrant, and transparent liquid that can remove harmful gases that cause discomfort to the human body, such as the odor and pungent smell generated by hot-mix asphalt mixtures. The physical indicators of this additive are shown in

Table 4. Physical properties of deodorizers.

Nature	Detection Value	Standard Value
Shape	Liquid	Liquid
Exterior	Transparent	Transparent
Ignition temperature (°C)	371	>350 °C
60 °C kinematic viscosity (mm2/s)	15	15 ± 2

. In multiple experiments, we tested dosages of the deodorant of 0%, 0.2%, 0.4%, and 0.6%.

2.2. Preparation Process

Choosing suitable raw materials: choose high-quality crude oil as the raw material to ensure the quality of asphalt. Warm-mixing: heat crude oil to a certain temperature (usually between 150 and 180 °C) to make it liquid. Mixing additives: add a certain amount of additives to asphalt to improve its properties. Uniform mixing: thoroughly mix the asphalt using mixing equipment to ensure that the additives and asphalt are thoroughly mixed. Mixing temperature: determined based on the rotational viscosity of asphalt, where the mixing temperature of base asphalt is 150 °C, that of rubber asphalt is 180 °C, and that of warm-mixed rubber asphalt is 160 °C. Mixing time: 1 h. Mixing speed: 1000 rpm. Asphalt purification: use filtering equipment to purify asphalt by removing impurities and insoluble substances. Asphalt quality testing: conduct quality testing on the processed asphalt, including testing of indicators such as viscosity, softening point, and temperature stability.

According to the preparation plan, we prepared matrix asphalt (NA), rubber asphalt (AR), warm-mix rubber asphalt (WMAR), clean-taste rubber asphalt (DAR), DAR-0.2% liquid deodorant (LD), DAR-0.4% LD, DAR-0.6% LD, DAR-0.2% powder deodorant (PD), DAR-0.4% PD, DAR-0.6% PD, clean-taste warm-mix rubber asphalt DWMAR, DWMAR-0.2% LD, DWMAR-0.4% LD, DWMAR-0.6% LD, DWMAR-0.2% PD, DWMAR-0.4% PD, and DWMAR-0.6% PD, as shown in

Table 5. Summary of test quantity.

Performance	Test Name	Group	Parallel Specimens	Number of Test Pieces
Basic properties of asphalt	Penetration	15	2	30
	Softening point	15	2	30
High-temperature rheological properties of asphalt	Dynamic shear rheological test (DSR)	15	2	30
Asphalt construction and workability	Rotation viscosity (RV)	15	2	30
Functional group analysis of asphalt	Fourier transform infrared spectroscopy (FTIR)	4	3	12
Analysis of asphalt smoke composition	UV–visible spectrophotometer (UV–Vis)	15	4	60

.

2.3. Experimental Methods

The conventional testing methods used in this article are shown in

Table 6. Summary of experimental methods.

Experiment Name	Experimental Equipment	Experimental Methods
Dynamic shear rheological test	Dynamic shear rheometer	The rheological principles of DSR experiments include two main parameters: frequency and strain amplitude. The composite shear modulus G * and loss angle δ are calculated. G * represents the average value of the elastic and viscous properties of asphalt materials, while δ represents the adhesive properties of asphalt materials. According to the standard ASTM D7175-15 [27], the testing temperatures are 52, 58, 64, 70, and 76 °C.
Rotating viscosity experiment	Asphalt rotary viscometer	The asphalt sample is injected into the container of the rotary viscometer, and the container is placed in a hot water bath. The required temperature and speed are set according to the experimental requirements. After the test, the rotor begins to rotate and the viscosity is determined by measuring the required torque of the rotor. According to the test conducted in accordance with ASTM T 382-2020 [28], the test temperatures are 25, 40, and 60 °C.
Infrared spectroscopy experiment	Infrared spectrometer	Infrared spectroscopy is a testing method used to analyze the molecular structure and chemical bonding of substances. It measures the absorption of infrared light by the sample to obtain an infrared spectrum and infers the composition and structure of the substance based on the position and intensity of characteristic peaks in the spectrum. Representative samples of NA, AR, DAR-0.4% LD, and DAR-0.4% PD were selected for the experiment.

. The testing methods for asphalt odor are shown in

Table 7. Summary of smoke components.

Test Items	Gas Characteristics	Analysis Methods	Method Source
Ammonia	Ammonia gas (NH3) is a colorless, toxic inorganic compound with a strong irritating odor, with a boiling point of −33.5 °C. Excessive inhalation by humans can burn the skin, eyes, and mucous membranes of respiratory organs. In severe cases, it can cause lung swelling and even death.	Indophenol blue spectrophotometry	Hygiene inspection methods for public places Part 2: Chemical pollutants (GBT18204)
Nitrogen oxide	Nitrogen oxides (NOx) refer to compounds composed only of nitrogen and oxygen. Nitrogen oxides contain various compounds. Except for nitric oxide and nitrogen dioxide, all other nitrogen oxides are unstable and easily react to produce nitric oxide or nitrogen dioxide, which eventually transforms into nitrogen dioxide. Nitrogen oxides also have varying degrees of toxicity.	Naphthyl ethylenediamine hydrochloride spectrophotometric method	Determination of nitrogen oxides (nitric oxide and nitrogen dioxide) in ambient air—Naphthyl ethylenediamine hydrochloride spectrophotometric method (HJ479 [17])
Sulfur dioxide	Sulfur dioxide (SO2) is a colorless, pungent gas that is one of the main atmospheric pollutants and an important indicator of whether the atmosphere has been polluted. In addition to polluting the environment, sulfur dioxide also poses a great threat to human health. Sulfur dioxide is easily soluble in water and corrosive. When the concentration of sulfur dioxide is 10–15 ppm, it can cause damage to the moist mucosa and stimulate nerve endings when inhaled into the respiratory tract. If the concentration of sulfur dioxide exceeds 20 ppm, it can cause coughing, irritate the eyes, cause discomfort in the bronchi and lungs, and damage lung tissue and liver.	Formaldehyde absorption para rosaniline spectrophotometric method	Determination of sulfur dioxide in ambient air—Formaldehyde absorption para rosaniline spectrophotometric method (HJ482 [29])
Hydrogen sulfide	Hydrogen sulfide (H2S) is a colorless, highly toxic gas with a foul egg odor. The sulfur element in petroleum asphalt produces hydrogen sulfide during the heating and decomposition process. The higher the sulfur content in asphalt, the more hydrogen sulfide is produced. Hydrogen sulfide can dissolve in water. After inhaling sulfur dioxide, people may experience symptoms such as dizziness and nausea, and it can irritate the respiratory mucosa and hinder cellular respiration. When the concentration of hydrogen sulfide is below 0.08 ppm~0.41 ppm, a small amount of free hydrogen sulfide will remain in the body after inhaling hydrogen sulfide gas. However, on-site construction workers who are exposed to environments containing hydrogen sulfide for a long time will inevitably develop chronic respiratory diseases.	Methylene blue spectrophotometry	Air and Waste Gas Monitoring and Analysis Methods

.

3. Results and Discussion

3.1. Analysis of Asphalt Rutting Factors

Asphalt materials play a crucial role in road construction and maintenance, and the stability of their performance is crucial for ensuring normal use and extending the service life of roads. From Figure 1, it can be observed that there are significant differences in the rutting factor data of different asphalt materials at different temperatures. In order to gain a deeper understanding of the significance behind these data, the following is a detailed analysis of the rutting performance of asphalt materials at different temperatures. The rutting factor is an indicator that measures the anti-rutting performance of asphalt materials. The higher the value is, the stronger is the ability of asphalt materials to resist rutting deformation. From the chart, it can be seen that the rutting factors of different asphalt materials exhibit significant differences, which are mainly related to the composition, structure, and temperature conditions of the materials. For the performance of rutting factors at different temperatures, we can observe the following pattern: as the temperature increases, the rutting factors of most asphalt materials show a downward trend, indicating that the anti-rutting performance of asphalt materials under high temperature conditions has decreased. This phenomenon may be related to the changes in the viscoelastic properties of asphalt materials under high temperatures, resulting in a weakened ability to resist deformation. It is worth noting that some asphalt materials exhibit excellent rutting factors at high temperatures, which may be related to their unique composition and structural design. These materials have good stability at high temperatures and can effectively resist rutting deformation, which is of great significance for high-temperature and rainy areas or road sections with high traffic flow.

3.2. Analysis of High-Temperature Rheological Properties of Asphalt

The analysis of Figure 2 reveals significant insights into the high-temperature performance of different asphalt types, particularly those modified with warm-mix agents and deodorants.

Firstly, it is evident that the addition of warm-mix agents significantly enhances the high-temperature resistance of asphalt. The high-temperature failure temperatures of asphalt modified with these agents are notably higher than those of other asphalt types. This suggests that the warm-mix agents effectively improve the asphalt’s ability to withstand high temperatures, potentially due to changes in its chemical structure and physical properties.

At elevated temperatures, asphalt materials undergo chemical and structural transformations that can affect their mechanical properties and durability. The warm-mix agents likely interact with the asphalt in a way that stabilizes its molecular structure and chemical bonds, thereby delaying the onset of high-temperature failure.

Interestingly, the data also indicate that increasing the content of the deodorant has minimal impact on the high-temperature failure temperature of asphalt. This suggests that the chemical composition and reactions associated with the deodorant do not significantly alter the asphalt’s high-temperature performance. This finding could be influenced by various factors, including the asphalt’s initial composition, viscosity, and wax content, which can affect its natural resistance to high temperatures.

From a practical perspective, understanding the high-temperature failure temperatures of different asphalt materials is crucial for road construction and maintenance. In regions with high ambient temperatures, selecting asphalt materials with superior high-temperature resistance can significantly enhance road stability and durability. This, in turn, reduces the likelihood of common road issues such as rutting, oil leakage, and cracking, ultimately extending the lifespan of the roadway and improving overall road safety.

3.3. Asphalt Construction and Workability Analysis

Figure 3 shows the rotational viscosity of experimental asphalt at different temperatures. Analysis of the chart shows that at both low and high temperatures, the rotational viscosity of rubber asphalt with a proposed PD deodorizer content of 0.6% is the highest, reaching 11,800 Pa·s and 2955 Pa·s. At the same temperature, there are differences in the rotational viscosity of different asphalt mixtures. This may be related to the composition and molecular structure of different asphalt types. Asphalt with higher rotational viscosity exhibits greater internal friction resistance, i.e., stronger adhesion, at the same temperature. Therefore, in practical applications, appropriate types and viscosities of asphalt should be selected according to specific needs to meet engineering requirements. In addition, it can be seen from Figure 3 that at lower temperatures, the viscosity of asphalt is higher, while at higher temperatures, the viscosity is lower. This indicates that during the construction process, the influence of different temperatures on asphalt viscosity should be considered and corresponding measures should be taken. The rotational viscosity of different asphalt varies at different temperatures. These data indicate that different types of asphalt have different viscosity temperature characteristics and sensitivity to temperature. In practical applications, understanding the viscosity temperature characteristics of different asphalt is crucial for selecting appropriate asphalt materials and construction conditions. For example, in low-temperature environments, asphalt with higher viscosity should be selected to improve the crack resistance and durability of the road surface. In high-temperature environments, asphalt with lower viscosity should be selected to improve the road’s resistance to rutting. In addition, these data can also provide valuable reference information for further research on the molecular structure and microstructure of asphalt materials.

3.4. Analysis of Asphalt Functional Groups

After obtaining the data from the experiments, the data were imported into Omnic 8.2. After processing data such as absorbance, pass rate, automatic baseline correction, and automatic smoothing, the processed data are saved in scv format and then imported into Origin to create an image. M0, M1, M2, and M3 are matrix asphalt NA, rubber asphalt AR, purified rubber asphalt DAR DAR-0.4% LD, and DAR-0.4% PD, respectively.

From Figure 4, it can be seen that infrared spectroscopy can reveal molecular vibration modes, thereby inferring the structure and chemical bond information of molecules. In the four images of matrix asphalt NA, rubber asphalt AR, pure rubber asphalt DAR DAR-0.4% LD, and DAR-0.4% PD, a large part of the characteristic peaks are similar, which can be attributed to basic and special functional groups. Therefore, the addition of new materials has little effect on the intrinsic properties and chemical composition of asphalt. The main basic characteristic functional groups are:

① Normal alkanes appear at a peak of around 2930 cm⁻¹, while 2930 cm⁻¹ exhibits antisymmetric stretching vibration, 2850 cm⁻¹ exhibits symmetric stretching vibration, 1460 exhibits antisymmetric deformation perturbation, and 1450 cm⁻¹ exhibits shear deformation vibration, overlapping with methyl antisymmetric deformation vibration. In the images obtained from these data, it can be found that there are three downward abrupt starting points related to normal alkanes, which are very consistent with the characteristic peaks of CH₃.

② Aromatic hydrocarbons, at 1600 (centered, benzene not present), have a downward protrusion at 1600, which is not significantly fluctuating, suggesting that it is an aromatic hydrocarbon.

③ Amine: 650–900 nitrogen hydrogen in-plane oscillation, especially at 870 cm⁻¹, 810 cm⁻¹, 846 cm⁻¹, showed three distinct peaks, suggesting that it may be caused by the continuous oscillation of the amino group.

Main differences: DAR-0.4% LD and DAR-0.4% PD of pure rubber asphalt are located at 1700 cm⁻¹, with carboxylic acid and carbon oxygen double bond stretching and perturbation. The other two curves are normal and are characterized by aromatic hydrocarbons, following the curve downwards.

At the same time, at 1080 cm⁻¹, the clean rubber asphalt DAR-0.4% LD and DAR-0.4% PD showed a plateau in two curves, while the other two curves showed a plateau in the matrix asphalt NA and rubber asphalt AR, which continued to return to normal, suggesting a possible disturbance from amino groups or other functional groups.

The results of the four curves indicate that the addition of odor enhancers does not have a significant impact on the chemical composition of asphalt, except for the disturbance that may be caused by carboxylic acids and amino groups. At the same time, there was no significant difference in the two curves M3 and M4 when comparing the addition of two deodorizers.

Figure 4. Infrared spectroscopy analysis.

Figure 4. Infrared spectroscopy analysis.

3.5. Analysis of Asphalt Smoke Composition

3.5.1. Ammonia Gas

As shown in Figure 5a, different types of asphalt will emit harmful gases during heating, which are caused by volatile organic compounds (VOCs) in the asphalt. The content and composition of VOCs in different types of asphalt are different, so harmful gases are emitted at different levels during heating. For matrix asphalt NA, rubber asphalt AR, and warm-mix rubber asphalt WMAR, there are differences in the degradation effect of ammonia. The ammonia concentration generated by the three types of asphalt during the experimental process is 0.383 mg/m³, 0.491 mg/m³, and 0.432 mg/m³, respectively. Among them, the degradation effect of matrix asphalt on ammonia is the most significant, indicating that during the process of asphalt modification, rubber asphalt AR and warm-mix pure-odor asphalt WMAR contain higher volatile organic compounds, so more harmful gases are emitted during heating, and the degradation degree of modified asphalt on ammonia is reduced. When adding new materials or warm-mix agents, modified asphalt may undergo chemical reactions with asphalt, which may lead to an increase in the content of VOCs in the asphalt. The experiment was conducted by adding rubber asphalt AR and warm-mix clean odor asphalt WMAR to LD liquid and PD powder purifiers with proposed concentrations of 0%, 0.2%, 0.4%, and 0.6%. The experimental results showed that the modified asphalt with added purifiers reduced the ammonia concentration produced to varying degrees during the experiment and had better degradation effects.

According to Figure 5b, it can be seen that for liquid deodorant (LD), when the proposed deodorant dosage is 0%, 0.2%, 0.4%, and 0.6%, the ammonia concentration generated by the purified rubber asphalt DAR in the experiment is 0.491 mg/m³, 0.355 mg/m³, 0.292 mg/m³, and 0.271 mg/m³, respectively. The degradation rate is 30.1%. The ammonia concentrations generated by DWMAR, a clean and warm-mixed rubber asphalt, in the experiment are 0.432 mg/m³, 0.362 mg/m³, 0.293 mg/m³, and 0.271 mg/m³, respectively. The degradation rate is 37.2%. For powder deodorizer (PD), when the proposed deodorizer dosage is 0%, 0.2%, 0.4%, and 0.6%, the ammonia concentration generated by the purified rubber asphalt DAR in the experiment is 0.491 mg/m³, 0.345 mg/m³, 0.262 mg/m³, and 0.21 mg/m³, respectively. The degradation rate is 57.2%. The ammonia concentrations generated by DWMAR, a clean and warm-mixed rubber asphalt, in the experiment are 0.432 mg/m³, 0.317 mg/m³, 0.251 mg/m³, and 0.184 mg/m³, respectively. The degradation rate is 57.4%, indicating that for the same type of deodorizer, an increase in the dosage of the deodorizer results in a decrease in ammonia concentration. Among them, when adding the same type and concentration of odor-clarifying agent, the degradation rate of ammonia by DWMAR, a clean and warm-mixed rubber asphalt, is higher, indicating that the coupling effect of temperature mixing agents and odor-purifying agents may increase the degradation rate of ammonia. In addition, adding the same concentration of deodorizing agent to the same type of asphalt and PD powder deodorizing agent has a better degradation effect.

3.5.2. Nitrogen Oxides

According to Figure 6a, it can be seen that there are differences in the degradation effects of nitrogen oxides among the three types of asphalt for matrix asphalt NA, rubber asphalt AR, and warm-mix rubber asphalt WMAR. The nitrogen oxide concentrations generated by the three types of asphalt during the experimental process are 0.681 mg/m³, 0.812 mg/m³, and 0.723 mg/m³, respectively. Among them, the matrix asphalt has the most significant degradation effect on nitrogen oxides, indicating that during the process of asphalt modification, the degradation degree of nitrogen oxides by modified asphalt has decreased. During the process of asphalt modification, rubber asphalt AR and warm-mix pure-odor asphalt WMAR contain higher levels of VOCs, so more harmful gases are emitted during heating, and the degradation degree of nitrogen oxides by modified asphalt is reduced. When adding new materials or warm-mix agents, modified asphalt may undergo chemical reactions, which may lead to an increase in the content of VOCs in the asphalt. The experiment was conducted by adding liquid (LD) and powder (PD) purifiers with proposed concentrations of 0%, 0.2%, 0.4%, and 0.6% to rubber asphalt AR and warm-mix clean-odor asphalt WMAR. The experimental results showed that the modified asphalt with added purifiers reduced the concentration of nitrogen oxides produced to varying degrees in the experiment and had better degradation effects.

In Figure 6b, it can be seen that for liquid deodorant (LD), when the proposed deodorant dosage is 0%, 0.2%, 0.4%, and 0.6%, the nitrogen oxide concentrations generated by the purified rubber asphalt DAR in the experiment are 0.812 mg/m³, 0.452 mg/m³, 0.391 mg/m³, and 0.333 mg/m³, respectively. The degradation rate is 58.9%. The nitrogen oxide concentrations generated by DWMAR, a clean and warm-mixed rubber asphalt, in the experiment are 0.723 mg/m³, 0.397 mg/m³, 0.31 mg/m³, and 0.273 mg/m³, respectively. The degradation rate is 62.2%. For PD odor remover, when the proposed dosage of odor remover is 0%, 0.2%, 0.4%, and 0.6%, the nitrogen oxide concentrations generated by purified rubber asphalt DAR in the experiment are 0.812 mg/m³, 0.425 mg/m³, 0.333 mg/m³, and 0.292 mg/m³, respectively. The degradation rate is 64.0%. The nitrogen oxide concentrations generated by DWMAR, a clean and warm-mixed rubber asphalt, in the experiment are 0.723 mg/m³, 0.366 mg/m³, 0.311 mg/m³, and 0.232 mg/m³, respectively. The degradation rate is 67.9%. This indicates that for the same type of deodorizer, an increase in the dosage of the deodorizer results in a decrease in the concentration of nitrogen oxides and a better degradation effect. When adding the same type and concentration of odor-clarifying agents, the degradation rate of nitrogen oxides in DWMAR with clean and warm-mixed rubber asphalt is higher, indicating that the coupling effect of temperature mixing agents and odor-purifying agents may increase the degradation rate of nitrogen oxides. In addition, the line chart shows that adding the same concentration of odor remover to the same type of asphalt results in a better degradation effect when powder odor remover (PD) is added.

3.5.3. Sulfur Dioxide

From Figure 7a, it can be seen that there are differences in the degradation effects of sulfur dioxide among the three types of asphalt, namely matrix asphalt NA, rubber asphalt AR, and warm-mix rubber asphalt WMAR. The sulfur dioxide concentrations generated by the three types of asphalt during the experimental process are 0.155 mg/m³, 0.284 mg/m³, and 0.238 mg/m³, respectively. The matrix asphalt has the most significant degradation effect on sulfur dioxide, indicating that during the process of asphalt modification, the degradation degree of sulfur dioxide by modified asphalt has decreased. During the process of asphalt modification, rubber asphalt AR and warm-mix pure odor asphalt WMAR contain higher levels of VOCs, so more harmful gases are emitted during heating, and the degradation degree of nitrogen oxides by modified asphalt is reduced. When adding new materials or warm-mix agents, modified asphalt may undergo chemical reactions, which may lead to an increase in the content of VOCs in the asphalt. The experiment was conducted by adding rubber asphalt AR and warm-mix clean-odor asphalt WMAR to liquid odor agent and powder odor agent with proposed concentrations of 0%, 0.2%, 0.4%, and 0.6%. The experimental results showed that the modified asphalt with added odor agent reduced the concentration of sulfur dioxide produced to varying degrees in the experiment, and the degradation effect was better.

In Figure 7b, it can be seen that for liquid deodorant (LD), when the proposed deodorant dosage is 0%, 0.2%, 0.4%, and 0.6%, the sulfur dioxide concentration generated by the purified rubber asphalt DAR in the experiment is 0.284 mg/m³, 0.152 mg/m³, 0.091 mg/m³, and 0.073 mg/m³, respectively. The degradation rate is 74.2%. The sulfur dioxide concentrations generated by DWMAR, a clean and warm-mixed rubber asphalt, in the experiment are 0.238 mg/m³, 0.124 mg/m³, 0.072 mg/m³, and 0.065 mg/m³, respectively. The degradation rate is 72.6%. For PD odor remover, when the proposed dosage of odor remover is 0%, 0.2%, 0.4%, and 0.6%, the sulfur dioxide concentration generated by DAR in the experiment is 0.284 mg/m³, 0.113 mg/m³, 0.071 mg/m³, and 0.057 mg/m³, respectively. The degradation rate is 79.9%. The sulfur dioxide concentrations generated by DWMAR, a clean and warm-mixed rubber asphalt, in the experiment are 0.238 mg/m³, 0.089 mg/m³, 0.063 mg/m³, and 0.052 mg/m³, respectively. The degradation rate is 78.1%, indicating that for the same type of deodorizer, an increase in the dosage of the deodorizer leads to a decrease in the concentration of sulfur dioxide and a better degradation effect. When adding the same type and concentration of deodorizing agents, the degradation rate of sulfur dioxide by DWMAR is lower than that of DAR, indicating that the coupling effect of warm-mixing agents and deodorizing agents may reduce the degradation rate of sulfur dioxide or produce sulfur dioxide in the reaction. In addition, the line chart shows that adding the same concentration of odor remover to the same type of asphalt results in a better degradation effect when powder odor remover is added.

3.5.4. Hydrogen Sulfide

As shown in Figure 8a, there are differences in the degradation efficiency of hydrogen sulfide among the three types of asphalt, namely matrix asphalt NA, rubber asphalt AR, and warm-mix rubber asphalt WMAR. The hydrogen sulfide concentrations generated by the three types of asphalt during the experimental process are 0.153 mg/m³, 0.194 mg/m³, and 0.177 mg/m³, respectively. Among them, the matrix asphalt has the most significant degradation effect on hydrogen sulfide, indicating that during the process of asphalt modification, the degradation degree of modified asphalt on hydrogen sulfide has decreased. During the process of asphalt modification, rubber asphalt AR and warm-mix pure-odor asphalt WMAR contain higher levels of VOCs, so more harmful gases are emitted during heating, and the degradation degree of nitrogen oxides by modified asphalt is reduced. When adding new materials or warm-mix agents, modified asphalt may undergo chemical reactions, which may lead to an increase in the content of VOCs in the asphalt. The experiment was conducted by adding rubber asphalt AR and warm-mix clean odor asphalt WMAR to liquid and powder deodorizers with proposed concentrations of 0%, 0.2%, 0.4%, and 0.6%. The experimental results showed that the modified asphalt with deodorizers reduced the concentration of hydrogen sulfide produced to varying degrees in the experiment, and the degradation effect was better.

In Figure 8b, it can be seen that for liquid deodorant (LD), when the proposed deodorant dosage is 0%, 0.2%, 0.4%, and 0.6%, the hydrogen sulfide concentration generated by the purified rubber asphalt DAR in the experiment is 0.194 mg/m³, 0.142 mg/m³, 0.123 mg/m³, and 0.11 mg/m³, respectively. The degradation rate is 43.2%. The hydrogen sulfide concentrations generated by DWMAR, a clean and warm-mixed rubber asphalt, in the experiment are 0.177 mg/m³, 0.114 mg/m³, 0.107 mg/m³, and 0.095 mg/m³ respectively. The degradation rate is 46.3%. For PD odor remover, when the proposed dosage of odor remover is 0%, 0.2%, 0.4%, and 0.6%, the hydrogen sulfide concentration generated by DAR in the experiment is 0.194 mg/m³, 0.133 mg/m³, 0.119 mg/m³, and 0.107 mg/m³, respectively. The degradation rate is 44.8%. The hydrogen sulfide concentrations generated by DWMAR, a clean and warm-mixed rubber asphalt, in the experiment are 0.177 mg/m³, 0.099 mg/m³, 0.086 mg/m³, and 0.078 mg/m³, respectively. The degradation rate is 55.9%, indicating that for the same type of deodorizer, an increase in the dosage of the deodorizer results in a decrease in the concentration of hydrogen sulfide and sulfur dioxide, and a better degradation effect. When adding the same type and concentration of odor clarifying agents, the degradation rate of hydrogen sulfide in DWMAR is higher, indicating that the coupling effect of temperature mixing agents and odor-purifying agents may increase the degradation rate of hydrogen sulfide. In addition, the line chart shows that adding the same concentration of odor remover to the same type of asphalt results in a better degradation effect when powder odor remover is added.

3.5.5. Analysis of Smoke Types

From Figure 9, it can be seen that the mean and standard deviation of nitrogen oxide emissions are the highest in various asphalt experiments and are greatly affected by the coupling effect of the deodorizing agent and the warm-mixing agent, as well as other chemical reactions. The emissions fluctuate greatly, indicating that the components and chemical reactions of the deodorizing agent and the warm-mixing agent can effectively consume nitrogen oxides or prevent their production. Secondly, ammonia gas is coupled with a purifying agent and a warm-mixing agent, which can effectively reduce emissions. The emissions of sulfur dioxide and hydrogen sulfide have relatively small fluctuations, among which hydrogen sulfide is least affected by the coupling effect of the deodorizing agent and the warm-mixing agent. This may be due to the fact that deodorizing agents usually contain sulfides such as hydrogen sulfide, which produce sulfur-containing gases during chemical reactions, thereby affecting the degradation of hydrogen sulfide and sulfur dioxide.

The comparison of harmful gas data shows that there is a difference in the degradation effect between powder-type and liquid-type deodorizers. Powder-type deodorizers are usually made of porous adsorption materials. The principle of odor purification is mainly based on adsorption, whereby the huge surface area and porous structure of powder-type deodorizers adsorb odor molecules from the air onto the surface of the deodorizer, thereby making the air fresher.

Specifically, when air passes through a powder-type deodorizer, the porous structure of the deodorizer can adsorb odor molecules in the air, including organic and inorganic gas molecules such as sulfides, nitrides, and oxides. After these gas molecules are adsorbed, their concentration decreases, thereby achieving the goal of eliminating or reducing odors. In addition, the adsorbent material in powdered deodorizers can also decompose some odor molecules through physical or chemical means, further purifying the air. The decomposition process mainly depends on the active substances contained in the deodorant, which may include active oxides, metal oxides, etc. They have the ability to undergo redox reactions and can decompose some odor molecules into harmless substances. However, the deodorizing effect of powder-based deodorizers is influenced by factors such as environmental humidity and temperature. In high-humidity environments, the adsorption capacity of deodorants may decrease. In higher-temperature environments, the activity of odor molecules increases, which may reduce the adsorption effect of the deodorant. Therefore, when using powdered deodorizers, it is necessary to choose and use them reasonably based on environmental conditions.

Liquid-type deodorizers are usually made from organic compounds of plant alcohols. The principle of purifying odors is mainly to eliminate odor molecules through chemical reactions. Plant-alcohol organic compounds have strong chemical activity and can react with odor molecules to change their chemical structure, thereby eliminating or reducing odors. For example, certain plant-alcohol organic compounds can react with sulfides to convert them into odorless substances. They can also react with oxides to decompose them into harmless substances. Liquid deodorizers also have certain limitations. Firstly, the stability of plant-alcohol organic compounds is relatively low, and they are easily affected by factors such as light, oxygen, and ultraviolet radiation, leading to oxidation and deterioration.

The purification and degradation principles of powder and liquid deodorizers each have their own characteristics. The powder type mainly purifies air through physical adsorption and chemical decomposition, while the liquid type mainly eliminates odor molecules through chemical reactions. When selecting and using deodorizers, it is necessary to choose according to the actual situation and needs. At the same time, attention should also be paid to safety issues and usage methods to avoid any impact on human health.

3.5.6. Significance Analysis

As shown in the Figure 10, a one-way ANOVA was performed for harmful gas degradation of different asphalt types. The results indicated that in multiple comparisons of the first group with the remaining four groups, the data were statistically significant (p < 0.001). The first group includes three kinds of asphalt without odor agents, and the other four groups are asphalt amended with different concentrations of odor agent, which also shows that the composite treatment of asphalt has a significant effect.

3.6. Influence of Mixing Parameters

As shown in Figure 11, for the same type of asphalt, within a certain time range, as the mixing time increases, the concentration of sulfur dioxide produced by the asphalt during the experimental process increases, and the degradation effect decreases. When the mixing time is 0.5 h, the sulfur dioxide emission is the lowest. After the mixing time reaches 1 h, the trend of sulfur dioxide concentration changes decreases and tends to stabilize. This indicates that increasing the mixing time will lead to more complete reaction of asphalt with air and other gases, increasing the emission of sulfur dioxide.

From Figure 12, it can be seen that for the same type of asphalt, except for WMAR warm-mix rubber asphalt, the concentration of sulfur dioxide produced by the asphalt during the experiment increases with the increase in mixing speed. When the mixing speed is 250 rpm, the emission of sulfur dioxide is the lowest. After the mixing speed reaches 500 rpm, the trend of sulfur dioxide concentration change decreases and tends to stabilize, indicating that the increase in mixing speed may lead to more oxygen supply to the asphalt and accelerate the reaction speed and sulfur dioxide emission. For WMAR warm-mix rubber asphalt, the lowest concentration of sulfur dioxide generated during the experiment is achieved when the mixing speed is 500 rpm.

In summary, reducing mixing time and speed may help reduce the sulfur dioxide content emitted from asphalt, but the specific impact depends on multiple factors and needs to be comprehensively considered.

4. Conclusions

This study conducted experimental analysis on the basic properties, high-temperature rheological properties, workability, functional groups, and smoke composition of asphalt during the modification process of warm-mixing and deodorization. The coupling effect between deodorizers and warm-mixing agents was demonstrated. Based on various experimental data, the following main conclusions can be drawn:

(1)

Infrared spectroscopy revealed that the addition of deodorants primarily induced subtle changes in carboxylic acid functional groups and possible amine functional groups in asphalt, and these changes exhibited no significant differences between the two deodorant-treated asphalt samples.

(2)

The incorporation of deodorants can effectively reduce the ammonia concentration produced during the heating of modified asphalt. Specifically, in deodorized rubber asphalt (DAR) and deodorized warm-mix rubber asphalt (DWMAR), the ammonia concentration decreased significantly with increasing dosages of liquid deodorant and powder deodorant. Powder deodorant exhibited a better degradation effect, with a maximum degradation rate of 57.4%. Furthermore, the combined use of warm-mix agents and deodorants enhances the degradation rate of ammonia.

(3)

With the increase in the dosage of liquid deodorant and powder deodorant, the concentration of nitrogen oxides produced during the heating of DAR and DWMAR decreased significantly. Among them, the use of powder deodorant yielded better results, with a maximum degradation rate of 67.9%. Additionally, the combined use of warm-mix agents and deodorants further improved the degradation rate of nitrogen oxides.

(4)

The addition of liquid deodorant and powder deodorant to asphalt can significantly reduce the sulfur dioxide concentration produced during heating. As the dosage of deodorant increases, the sulfur dioxide concentration gradually decreases. Powder deodorant exhibits a better degradation effect, with a maximum degradation rate of 79.9% in DAR. However, it is worth noting that under the same conditions, the degradation rate of sulfur dioxide in DWMAR is slightly lower than that in DAR, which may suggest that the coupling of warm-mix agents and deodorants may generate additional sulfur dioxide or reduce its degradation efficiency.

(5)

With the increase in the dosage of liquid deodorant and powder deodorant, the concentration of hydrogen sulfide produced during the heating of rubber asphalt (AR) and warm-mix rubber asphalt (WMAR) decreased. Among them, the use of powder deodorant in DWMAR yielded the best degradation effect, with a maximum degradation rate of 55.9%. In addition, experimental results indicate that the combined use of warm-mix agents and deodorants can improve the degradation rate of hydrogen sulfide.

(6)

The mean and standard deviation of nitrogen oxide emissions are the largest, indicating the highest and most fluctuating emissions, which suggests that the coupling effect of deodorants and warm-mix agents has a significant impact on nitrogen oxides. There are differences in the degradation of harmful gases between powder and liquid deodorants. The powder type mainly purifies the air through adsorption and decomposition, while the liquid type eliminates odor molecules mainly through chemical reactions.

(7)

Within a certain time range, for the same type of asphalt, an increase in mixing time and mixing speed will lead to an increase in sulfur dioxide concentration during the asphalt experiment, resulting in a decrease in degradation efficiency. When the mixing time is 0.5 h and the mixing speed is 250 rpm, the sulfur dioxide emissions are the lowest. However, when the mixing time reaches 1 h or the mixing speed reaches 500 rpm, the trend in sulfur dioxide concentration changes decreases and tends to stabilize.