Study on the Properties of Graphene Oxide–Wood Tar-Based Composite Rejuvenated Asphalt

Abstract

This study aims at counteracting the problem of rejuvenated asphalt with poor performance and weak secondary anti-aging ability by improving the existing biomass rejuvenator. In this study, a carbon nanomaterial, graphene oxide (GO) with excellent anti-aging performance, was introduced to the wood tar-based rejuvenator (WR) to prepare a composite rejuvenator. Based on laboratory tests, the effects of the GO–wood tar-based composite rejuvenator (GWCR) on the performance of aged asphalt and on the secondary aging performance were investigated, and its rejuvenation mechanism was analyzed. The results indicate that the GWCR can increase the penetration, ductility, and creep rate (m) of aged asphalt while decreasing its softening point, rutting factor (G*/sinδ), and stiffness modulus (S). This indicates that the high-temperature resistance to the permanent deformation ability of aged asphalt degrades, while the low-temperature performance improves, and all values are slightly greater than those of the 70# original base asphalt. After PAV aging, the S value of the GO–wood tar-based composite rejuvenated asphalt (GWCRA) increased by 83.71%, while the m value decreased by 49.45%. The secondary aging resistance of the GWCRA is better than that of 70# original base asphalt, RA-75 rejuvenated asphalt, and wood tar-based rejuvenated asphalt. When adding the GWCR into aged asphalt, the content of saturates and aromatics in the asphalt increases by 1.08% and 11.1%, respectively. In contrast, the content of asphaltenes and resins decreases by 6.288% and 5.9%, respectively. As a result, the colloidal structure of the aged asphalt transfers from a gel to a sol–gel state. The surface roughness of the GWCRA increases by the synergistic effect of GO and wood tar, making its adhesion better than that of the 70# original base asphalt. Adding GO can improve the performance of wood tar rejuvenated asphalt (WRA) with high-temperature deformation resistance and resistance to secondary aging, and effectively make up for the defects in the performance of WRA rejuvenated asphalt, so as to extend the service life of asphalt pavements, thus increasing the value of wood tar engineering applications, which is of great practical significance.

1. Introduction

Due to its high strength, good stability, high-quality wear resistance, and excellent driving comfort, asphalt pavement is widely used in pavement engineering [1]. However, asphalt pavement is prone to damage under repeated vehicle loads and complex environmental impacts (temperature, ultraviolet light, oxygen, etc.), resulting in cracking, rutting, spalling, potholes, and looseness, affecting its driving quality and driving safety [2,3]. Therefore, the maintenance of asphalt pavement is a necessary measure to ensure pavement safety and service level. By the end of 2022, highway mileage reached 5,354,800 km in China, while the maintenance mileage was 5,350,300 km, accounting for 99.9% of highway mileage [4]. A large amount of reclaimed asphalt pavement (RAP) will be produced during the maintenance of highways, and the storage and stacking of RAP will trigger adverse effects on the environment. On the other hand, asphalt, as a non-renewable resource obtained from crude oil, will be consumed in a large amount in the large-scale reconstruction, repair, or rejuvenation of highways. The background of rising crude oil prices and reduced reserves puts great pressure on the future development of pavement engineering. Therefore, to save the consumption of resources such as asphalt and aggregate and to reduce environmental pollution, mixing RAP with a certain proportion of new asphalt and new aggregate to prepare a rejuvenated asphalt mixture that meets road performance has become the focus of research in the field of road engineering.

Based on the theory of component migration, asphalt undergoes volatilization, oxidation, and condensation reactions during aging [5]. The components with smaller molecular weights volatilize and gradually migrate into heavy components, thus increasing the heavy components such as asphaltene and resins, and reducing the light components such as aromatics and saturates. The macro mechanical properties of asphalt would change by this component migration, which is manifested by the weakened ability of asphalt to resist fatigue damage and low-temperature cracking. From the perspective of asphalt components, it is necessary to reconcile the components in order to restore the performance of aged asphalt in RAP. Therefore, many scholars have used rejuvenators with smaller molecular weights to reconcile the components of aged asphalt in RAP. This method can reduce the modulus and viscosity and improve the ductility of aged asphalt by supplementing its light components [6,7]. At present, petroleum-based (such as extracted oil) products are mainly used as rejuvenators in engineering. However, due to their non-renewable nature and high price, their promotion is restricted to a certain extent [8,9]. Therefore, biomass rejuvenators with green, environmentally friendly, and renewable characteristics, which are prepared from organic biomass-derived bio-binders, have gradually attracted widespread attention [10].

Wood tar is a high-temperature pyrolysis product made from bamboo-based biomass materials, which is mainly derived from waste furniture sawdust and waste wood in production activities. It has significant characteristics with many sources, such as a large output, waste utilization, and environmental protection [11]. As an organic compound, wood tar consists of hydrocarbons, acids, and phenols, with good high-temperature resistance and waterproof qualities. In the paint and shipbuilding industries, it is an excellent material for resistance to corrosion and high temperatures. Research has indicated that the utilization of a wood tar-based rejuvenator (WR) can enhance the penetration of aged asphalt by reducing its viscosity. The rejuvenated asphalt performs better at high temperatures than the original base asphalt and satisfies specified requirements for performance [12]. Unfortunately, the light components of rejuvenated asphalt will quickly be lost following secondary aging, leading to a rapid decline in its performance, since biomass rejuvenators predominantly supply light components for aged asphalt [7]. Therefore, it is essential to strengthen the secondary aging resistance of wood tar-based rejuvenated asphalt (WRA) and to establish the groundwork for its engineering application.

As a carbon-based two-dimensional layered nanomaterial, graphene oxide (GO) is easily dispersed in water or organic solvents to form a suspension [13]. At the same time, the surface of GO is rich in polar oxygen-containing functional groups such as carboxyl, hydroxyl, epoxy, and ester groups, which have high activity and are easily compatible with many polymers, thereby improving its thermal properties, mechanical properties, or tensile properties [14,15,16]. Studies have shown that GO can effectively improve the high-temperature resistance to permanent deformation ability, water stability, and aging performance of asphalt [17,18].

However, most studies merely evaluated the rejuvenation effect of rejuvenators on the performance of aged asphalt based on laboratory tests, and there were fewer studies on the durability of rejuvenated asphalt after short-term aging and long-term aging during construction and service. Luo and Huang studied the secondary aging resistance of asphalt rejuvenated by waste cooking oil, waste bio-oil, and waste engine oil, and the results showed that the waste cooking oil rejuvenated asphalt had the weakest secondary aging resistance [7]. In addition, there was little research on using nanomaterials with excellent anti-aging properties to modify rejuvenators in order to improve the anti-secondary performance of rejuvenated asphalt.

The present study prepared a composite rejuvenator by adding GO to WR to improve the high-temperature stability and secondary aging resistance of WRA. Through a comparison with other rejuvenators, the effects of different rejuvenators on the physical performance and rheological properties of aged asphalt were analyzed, and the changes in the low-temperature properties of rejuvenated asphalt after secondary aging were emphasized. In addition, the effects of the GWCR on the composition, colloidal structure stability, chemical properties, and surface micro-morphology of aged asphalt were investigated by component analysis (SARA), Fourier transform infrared (FTIR), and atomic force microscope (AFM). This comprehensive investigation aims to elucidate the influence of the GWCR on the high- and low-temperature performance of aged asphalt and its resistance to secondary aging, and to reveal its rejuvenation mechanism. The results can provide an idea and method for further improving the secondary aging resistance of rejuvenated asphalt, so as to prolong the service life of recycled asphalt pavement, and then to satisfy the development needs of RAP recycling.

2. Materials and Methods

2.1. Materials

2.1.1. Asphalt

The control asphalt in this study was the 70# original base asphalt (AO), which means 70# base asphalt before aging.

Table 1. Technical properties of 70# original base asphalt.

Technology Index		Technical Specification	70# Original Base Asphalt
Penetration/(25 °C, 0.1 mm)		60~80	63.2
Softening point/°C		≥46	47.5
Ductility/(10 °C, cm)		≥10	25.3
After TFOT * (163 °C, 5 h)	Mass change/%	−0.8~0.8	−0.06
	Residual penetration ratio/(25 °C, %)	≥58	70.6

* Note: thin film oven test.

displays the technical properties of 70# original base asphalt in accordance with Chinese standards Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011) [19] and Technical Specification for Construction of Highway Asphalt Pavements (JTG F40-2004) [20].

2.1.2. Graphene Oxide

In this study, GO is prepared from flake graphite by the Hummers method [21] in the laboratory. The surface of GO contains many oxygen-containing functional groups such as O-H, C-O, and C=O, and has a specific surface area of roughly 2600 m²/g. The Fourier transform infrared (FTIR) spectrum of GO is shown in Figure 1 [22].

2.1.3. Wood Tar

Wood tar is produced by the rapid pyrolysis of bamboo at 550 °C, which is mainly composed of hydrocarbons, acids, phenols, and other organic compounds.

Table 2. Technical properties of wood tar.

Technical Index	Water Content/%	Density/(g/cm3)	pH
Wood tar	5.62	1.18	2.23

shows the technical properties of wood tar.

2.1.4. Plasticizer

To improve the compatibility and thermal storage stability of the GWCRA, dioctyl adipate (DOA) was selected as a plasticizer; its technical properties are shown in

Table 3. Technical requirements of DOA.

Technical Index	Boiling Point/K	Density/(g/cm3)	Flash Point/K
DOA	487.15	0.922	469.15

.

2.1.5. RA-75 Rejuvenator

The RA-75 rejuvenator (its main component is extracted oil) provided by Hunan Yunzhong Regeneration Technology Co., Ltd., Nanyang, China, was used as the control, and its content was 8.0% of the mass of the aged asphalt.

2.2. Material Preparation

2.2.1. Preparation of Aged Asphalt

The long-term aging asphalt samples of 70# original base asphalt were prepared by using a pressurized aging vessel (PAV) after short-term aging (TFOT), following the T0630 test method in JTG E20-2011. The test parameters are 100 °C, 2.1 MPa, and 20 h.

To evaluate the secondary aging performance of rejuvenated asphalt, the short-term and long-term aging tests were also carried out on the rejuvenated asphalt.

2.2.2. Preparation of GWCR

Figure 2 displays the preparation process for the GWCR, which is a composite of GO, wood tar, and DOA. Based on the previous research results [12], the optimal ratios of GO, wood tar, and DOA were 0.06%, 4.64%, and 1.54% of the mass of the aged asphalt, respectively.

To analyze the rejuvenation effect of the GWCR on the performance of aged asphalt, it is compared with a wood tar-based rejuvenator (wood tar + DOA, abbreviated as WR) and a commercial RA-75 rejuvenator.

Table 4. Basic properties of each rejuvenator.

Rejuvenator Type	Viscosity/(60 °C, Pa·s)	Flash Point/°C	Saturates Content/%	Aromatics Content/%	After TFOT
					Ratio of Viscosity	Mass Loss/%
GWCR	5673	305	18.5	59.6	1.1	0.3
WR	5361	276	21.7	64.3	1.6	0.7
RA-75 rejuvenator	5970	241	19.6	62.3	1.8	0.8
Specification requirement	50~60,000	≥220	≤30	--	≤3	−3~3

shows the basic properties of each rejuvenator. It can be seen that the GWCR has better construction safety and thermal stability than the RA-75 rejuvenator and the WR, and its technical indexes met the specification requirements [23].

2.2.3. Preparation of Rejuvenated Asphalt

The rejuvenated asphalt was prepared by using a high-speed shearing instrument to guarantee that the rejuvenator and the aged asphalt were blended evenly. Figure 3 displays the preparation process. The performances of the 70# original base asphalt (AO), 70# PAV aged asphalt (AP), RA-75 rejuvenated asphalt (RAA), wood tar-based rejuvenated asphalt (WRA), and GO–wood tar-based composite rejuvenated asphalt (GWCRA) were compared and analyzed.

2.3. Test Methods

2.3.1. Physical Performance Test

The penetration, ductility, and 10 °C softening point of each asphalt were measured according to the penetration test (T0604), ductility test (T0605), and softening point test (T0606) methods in JTG E20-2011, and the physical properties of the different rejuvenated asphalt binders were evaluated.

2.3.2. Rheological Property Test

Dynamic Shear Rheology (DSR) Test

The complex modulus G* and phase angle δ of each asphalt at 58 °C, 64 °C, 70 °C, and 76 °C were determined by the rheological properties test (T0628) in JTG E20-2011. The rutting factor G*/sinδ was subsequently calculated to assess the impact of each rejuvenator on the high-temperature performance of aged asphalt. The test mode was the temperature scanning mode, and the shear strain was 12%.

Bending Beam Rheology (BBR) Test

The stiffness modulus (S) and the creep rate (m) of the AO, AP, RAA, WRA, and GWCRA were examined at −12 °C and −18 °C based on the bending creep stiffness test (T0627) in JTG E20-2011. The sample size of the asphalt was 127 × 6.35 × 12.7 mm. Before the test, the sample was kept at the test temperature for 1 h. In addition, to evaluate the low-temperature performance of rejuvenated asphalt after secondary aging, the S values and m values of the three rejuvenated asphalt binders after secondary aging were tested at −18 °C.

2.3.3. Component Analysis (SARA) Test

The contents of asphaltene (A_S), resin (R), saturates (S), and aromatics (A_r) in each asphalt were determined to explore the composition distinctions of each asphalt according to the chemical composition test (T0618) in JTG E20-2011. The impacts of various rejuvenators on the chemical composition of the aged asphalt were assessed.

The colloidal instability index I_C of asphalt is calculated by the results of the component composition test, which can reflect its swelling capacity and colloidal structure. Based on the Gaestel theory, the I_C was defined to determine the colloidal structure of asphalt by Siddiqui in 1999 and is shown in Equation (1) [24]. Then, the index was used to justify the compatibility between polymers and asphalt by Mirwald in 2020 [25]. The colloidal structure of asphalt becomes more unstable with a larger I_C value, and its performance is more sensitive to external environmental conditions and vehicle load [26,27].

$$I_{\mathrm{C}}={\displaystyle \frac{\mathrm{Mass}(S+A_{\mathrm{S}})}{\mathrm{Mass}(R+A_{\mathrm{r}})}}$$

(1)

2.3.4. FTIR Test

The effects of the different rejuvenators on the chemical structure of aged asphalt before and after rejuvenation were analyzed by the FTIR test. Before the test, the asphalt was completely dissolved in CS₂ at a mass ratio of 0.05:1. The solvent was fully evaporated by dropping 2 μL of the solution onto the KB_r flakes and heating the mixture to 40 °C in a vacuum oven. The test wavelength range was 400~4000 cm⁻¹, and the resolution was 8 cm⁻¹.

2.3.5. Atomic Force Microscope (AFM) Test

To investigate the impact of the different rejuvenators on the microstructure of AP at the nanometer scale, the microstructure of each asphalt was measured at 20 °C by using Dimension Icon AFM manufactured by Brucker Company (Billerica, MA, USA). The tapping mode was adopted, the scanning speed was 0.7 Hz, and the scanning images with 20 × 20 μm were analyzed by Nano Scope Analysis 1.8.

3. Results and Discussion

3.1. Physical Properties

Table 5. Physical properties of different asphalt binders.

Physical Property	AO	AP	RAA	WRA	GWCRA
Penetration/(25 °C, 0.1 mm)	63.2	20.3	61.2	64.2	63.5
Softening point/°C	47.5	86.1	57.8	46.2	56.3
Ductility/(10 °C, cm)	25.3	2.6	21.6	27.5	26.7

displays the physical properties of the five asphalt binders. The results demonstrate that the penetration and ductility of AP are 67.9% and 89.7% lower than that of AO, respectively. However, the softening point of AP increases by 81.3% compared with AO, indicating that the asphalt has undergone thermo-oxidative aging during PAV aging. The AO becomes hard and brittle, its viscosity increases, and the shear deformation resistance is enhanced, and its ability to resist low-temperature cracking is significantly reduced [28]. The addition of different types of rejuvenators can increase the penetration and ductility of AP and decrease its softening point, indicating that a rejuvenator with a high content of saturates and aromatics can soften the aged asphalt and decrease its high-temperature performance and viscosity, while enhancing its low-temperature crack resistance. There are obvious differences in the softening effect of the different rejuvenators on aged asphalt. The saturates and aromatics content of the RA-75 rejuvenator is relatively low and the viscosity is large, resulting in the worst softening effect on AP. The softening effect of WR on AP is the best, but its viscosity and shear deformation resistance are weaker than that of AO, mainly because the saturates and aromatics content of WR is the highest. Adding the GWCR can increase the penetration and ductility of AP, and decrease the softening point, making the hardness, shear deformation resistance, and low-temperature performance of AP slightly better than that of AO. Yan et al. [29] added 8% waste cooking oil to aged 90# asphalt, but the high-temperature performance and low-temperature performance of the rejuvenated asphalt failed to reach the levels of the 90# asphalt.

3.2. Rheological Properties

3.2.1. High-Temperature Performance

Figure 4 presents the G*/sinδ values of five asphalt binders at different temperatures. The rutting factor G*/sinδ is used to characterize the ability of asphalt to resist permanent deformation under high temperature conditions; this ability is also called high-temperature performance, and the higher the value of G*/sinδ, the better the high-temperature performance of the asphalt [30]. The results suggest that the thermal movement of molecules inside the asphalt intensifies as the temperature increases, and the G*/sinδ value of each asphalt decreases continuously. The G*/sinδ value of AP is 71.3% larger than that of AO at 64 °C, and the G*/sinδ value of AP is also larger than that of AO at other temperatures, indicating that PAV aging can significantly enhance the high-temperature performance of asphalt. The reason is that the light components in asphalt volatilize or convert into macromolecular substances during PAV aging, thus increasing the content of macromolecular asphaltenes and resins in asphalt. Some researchers have characterized the aging-induced stronger molecular interaction forces within asphalt as an increase in cohesive energy density simulated by molecular dynamics [31,32]. As the molecular interaction force increases, the stiffness is increased, and the ability to resist shear deformation at high temperature is enhanced [28]. However, PAV aged asphalt has poor fatigue, moisture sensitivity, and low-temperature resistance, resulting in the fact that it cannot be used to build roads in hot countries [33].

The G*/sinδ values of the five asphalt binders at the same temperature are ranked as follows: AP > RAA > GWCRA > AO > WRA, indicating that adding a rejuvenator decreases the high-temperature performance of aged asphalt, but with different decreased degrees. The results demonstrate that the three rejuvenators can decrease the asphaltene concentration of the blend and soften the aged asphalt; this is consistent with the findings of Caputo et al. [34]. WR has the most significant reduction rate on the high-temperature stability of AP; the G*/sinδ value decreased 46.06% at 58 °C with the addition of WR into AP. As shown in Table 4, WR contains the largest amount of light components (saturates and aromatics), which can effectively supplement the components of aged asphalt and soften it, resulting in its high-temperature resistance to permanent deformation ability being lower than that of AO. The GWCR has the second-largest reduction rate on the high-temperature stability of AP; the G*/sinδ value of the GWCRA decreased 39.16% at 58 °C compared to that of AP, but was still higher than that of AO, and the RA-75 rejuvenator had the worst value. Since GO can raise the viscosity of AP and decrease its content of light components, the degradation of the high-temperature stability of AP by the GWCR is worse than that of WR. However, although the content of saturates and aromatics in the GWCR is less than that of the RA-75 rejuvenator, its reduction in the high-temperature stability of AP is higher than that of the RA-75 rejuvenator. The viscosity of the GWCR is lower than that of the RA-75 rejuvenator, and the DOA contained in the rejuvenator has a lubricating effect on asphalt macromolecules, which effectively increases the fluidity between molecules and reduces the interaction between molecules of aged asphalt. Therefore, the molecules with different molecular weights in the GWCRA are more evenly distributed, and the degree of aggregation of macromolecules is weakened [35]. Wang et al. [9] compared the rheological properties of different bio-oils for the rejuvenation of 70# aged asphalt, and the results showed that 6% gutter oil, vegetable oil, castor oil, straw oil, and soybean oil can reduce the rutting factor of aged asphalt by about 76.11%, 72.24%, 55.39%, 60.05%, and 57.78%, respectively. In summary, the GWCR is more conducive to the resistance of rejuvenated asphalt to high-temperature permanent deformation.

3.2.2. Low-Temperature Cracking Resistance

The BBR test results of each asphalt at −12 °C and −18 °C are shown in Figure 5. As shown in Figure 5, when the temperature decreases from −12 °C to −18 °C, the S values of the five asphalts increase significantly and the m values decrease significantly. This is mainly due to the decrease in temperature which reduces the thermal movement of molecules inside the asphalt, and makes the asphalt brittle and hard, increasing the stiffness, weakening the stress relaxation ability, and decreasing the ability and rate of resistance to low-temperature deformation. At −12 °C, the S value of AP is larger than that of AO by 101.51%, while the m value is smaller than that of AO by 62.54%, indicating that PAV aging can significantly reduce the low-temperature crack resistance of asphalt. This is because the light components of asphalt volatilize and aggregate into macromolecular substances during the aging process, which leads to the enhancement of the molecular interaction force and the weakening of fluidity in asphalt. The asphalt becomes brittle and hard, the modulus increases, and the stress relaxation ability decreases noticeably [36].

Figure 5 shows that adding a rejuvenator into aged asphalt can decrease the S value and increase the m value. This suggests that the rejuvenator can significantly reduce the stiffness modulus of the aged asphalt, effectively enhancing the stress relaxation ability and the low-temperature performance. At −12 °C, the S values of the RAA, WRA, and GWCRA decreased 39.68%, 53.51%, and 52.10% compared to that of AP, respectively. Conversely, the m values of the RAA, WRA, and GWCRA increased 157.63%, 177.97%, and 172.03% compared to that of AP, respectively. These indicate that WR demonstrates the most effective enhancement of the low-temperature performance of aged asphalt, followed by the GWCR, and the RA-75 rejuvenator is the worst. The reasons are as follows: Firstly, the addition of GO can increase the viscosity of wood tar-based rejuvenated asphalt, reduce the content of light components, slightly increase the stiffness modulus, slightly weaken the stress relaxation ability, and slightly decrease the low-temperature cracking resistance [37]. Secondly, the viscosity of the GWCR is lower than that of the RA-75 rejuvenator, and the DOA contained in the GWCR has a lubrication effect on asphalt macromolecules after being added to aged asphalt, which decreases the aggregation degree of the macromolecules in asphalt, effectively improving the fluidity between molecules, weakening the interaction force between molecules, enhancing the elasticity of the asphalt, reducing the modulus, and enhancing the stress relaxation ability [35]. The addition of 6% gutter oil, vegetable oil, castor oil, straw oil, and soybean oil reduced the S value of 70# aged asphalt at −12 °C by about 27.53%, 14.27%, 48.03%, 73.77%, and 29.92%, respectively. On the contrary, the m value of 70# aged asphalt rejuvenated by them significantly increased, indicating that the addition of these rejuvenators can significantly improve the low-temperature cracking resistance of aged asphalt [9]. Considering the high-temperature and low-temperature performance of rejuvenated asphalt, it can be found that the GWCR can effectively improve the low-temperature cracking resistance without significantly reducing the high-temperature performance of aged asphalt, and its comprehensive performance is the best.

3.3. Anti-Secondary Aging Performance

The PAV aging test was performed on the various rejuvenated asphalt binders to assess the anti-secondary aging performance. Figure 6 presents the S value and m value of each rejuvenated asphalt at −18 °C.

The results reveal that the increase in S value for the rejuvenated asphalts before and after secondary aging is ranked from large to small as RAA (116.38%) > AO (110.68%) > WRA (90.99%) > GWCRA (83.71%), while the decrease in m value is ranked from large to small as RAA (79.27%) > AO (64.02%) > WRA (59.27%) > GWCRA (49.45%). The results show that the anti-aging performance of rejuvenated asphalt is greatly influenced by the different rejuvenators. Compared with that of AO, the low-temperature crack resistance of RAA after aging is worse, implying that the RA-75 rejuvenator cannot effectively resist the secondary aging of rejuvenated asphalt after restoring the performance of aged asphalt. The change range of the S value and m value of WRA before and after secondary aging is smaller than that of AO because the DOA contained in WR can effectively improve the anti-secondary aging performance of the WRA [38]. The change range of the S value and m value of the GWCRA before and after secondary aging is the lowest. The reasons are as follows: Firstly, the flake GO contained in the GWCRA can crosslink with asphalt to form a two-dimensional stable structure, which effectively blocks the volatilization of light components and the invasion of free oxygen, and decreases the effect of secondary aging on the low-temperature crack resistance of rejuvenated asphalt [17]. Secondly, wood tar with a high content of light components and DOA with lubrication and flexibility can effectively improve the dispersion uniformity of GO in aged asphalt, promote the crosslinking and fusion between GO and asphalt, further block the volatilization of light components and the invasion of free oxygen, and decrease the effect of aging on the low-temperature crack resistance of rejuvenated asphalt [35]. Pyshyev et al. [39] used humic acids produced by Lignite Processing Products as additives to modify asphalt and found that humic acids can effectively improve the anti-aging properties of asphalt, as they contain functional phenolic and carboxyl groups. In the future, humic acids can be considered to replace GO to prepare composite rejuvenators, and their economic and environmental benefits will be more significant.

3.4. Rejuvenation Mechanism Analysis

3.4.1. SARA

The results of the SARA test of asphalt are shown in Figure 7. The results show that there is a significant difference in the composition between AO and AP. After the aging of AO, the content of asphaltenes and resins increased, the content of aromatics decreased sharply, and the content of saturates with relatively stable properties decreased slightly. Compared with AP, the contents of saturates and aromatics in different rejuvenated asphalt binders increased, while the contents of asphaltenes and resins decreased, indicating that each rejuvenator can effectively soften aged asphalt by decreasing the concentration of asphaltene and resin to achieve the purpose of restoring its performance. However, different rejuvenators have different effects on supplementing the content of light components that are prone to volatilizing or oxidizing into macromolecules in aged asphalt and decreasing the aggregated macromolecule concentration.

The saturates content in aged asphalt increased by adding RA-75 from 13.84% to 14.42%, and the aromatics content increased from 37.32% to 48.02%, while the asphaltenes and resins decreased to 12.12% and 25.44%, respectively. The results show that the addition of the RA-75 rejuvenator can effectively reconcile the components concentration in aged asphalt, so as to achieve the effect of restoring the performance of AP. However, the contents of saturates and aromatics in RAA are lower than those in AO, while the contents of asphaltenes and resins are higher than those in AO. The results suggest that although the RA-75 rejuvenator can reconcile the components of aged asphalt, its rejuvenated asphalt performance is slightly worse than that of AO, which is consistent with the previous research results. The components of AP were reconciled by the WR with the largest content of saturates and aromatics, and the component blending effect of the rejuvenated asphalt is better than that of the RA-75 rejuvenator. The results illustrate that WR has the best recovery effect on the low-temperature crack resistance of aged asphalt, and has a weak negative effect on the high-temperature performance of rejuvenated asphalt. The GWCR has the best blending effect on the components of AP, and the content of each component of rejuvenated asphalt is the closest to that of the AO. Therefore, the GWCR has the best blending effect on the components and colloidal structure of aged asphalt.

3.4.2. Colloidal Structure Analysis

The colloidal instability coefficient I_C of each asphalt is calculated according to Equation (1), and the results are shown in Figure 8. As a colloidal dispersion system, asphaltenes are the core of asphalt, and resins are adsorbed around them to form micelles, which are dispersed in a dispersion medium composed of aromatic and saturated components as a dispersed phase [40]. The colloidal structure of asphalt can be categorized into sol type, sol–gel type, and gel type. This classification relates to the chemical characteristics, rheological properties, and the relative composition of the asphalt components [41]. It can be seen from Figure 8 that the aromatics in asphalt gradually transformed into asphaltenes during the aging process, and increased in I_C value, so the colloidal structure of asphalt changed from sol–gel to gel, and its swelling capacity decreased. Therefore, the temperature sensitivity of aged asphalt is lower, and has good elasticity and high-temperature stability. It is not easy to cause rutting disease, but the fluidity, plasticity, and low-temperature deformation ability are poor, and the pavement cannot heal itself after cracking.

The addition of the rejuvenator decreases the I_C value of aged asphalt, indicating that the aggregated polar macromolecule concentration in the aged asphalt is decreased by the rejuvenator, and the content ratio of each component of the aged asphalt is reconciled, and the internal free volume of the asphalt is increased. Therefore, the asphalt system changes from gel to sol–gel, thereby effectively enhancing the fluidity and ductility of the rejuvenated asphalt [42]. The recovery degrees of the different rejuvenators on the instability coefficient of the aged asphalt colloid are significantly different, and the recovery effect is consistent with the component blending effect. The I_C value of the GWCRA is the same as that of AO, while the I_C values of the other two rejuvenated asphalt binders are greater than that of AO. The results indicate that the GWCR has the best recovery effect on the colloidal structure of aged asphalt and its performance, which is consistent with the previous research results.

3.4.3. Chemical Characteristic Analysis

The FTIR test spectra of each rejuvenator and asphalt are shown in Figure 9. As can be seen from Figure 9a, the common points of the spectra of the three rejuvenators are as follows: Firstly, all of them have C-H and C=C absorption peaks corresponding to the aromatic fraction at 720~885 cm⁻¹ and 1600 cm⁻¹, respectively. Secondly, all of them have sulfonyl S=O absorption peaks at 1030 cm⁻¹. Thirdly, the absorption peaks of the methyl (CH₃) and ethylene groups (CH₂) were present at 1376 cm⁻¹ and 1460 cm⁻¹, respectively. Finally, the aldehyde functional group O=C-H was present at 2870 cm⁻¹ and the alkane C-H absorption peak was present at 2975 cm⁻¹. The results indicate that the three rejuvenators contain compounds such as aromatic fractions, aldehydes, and alkanes [39]. The differences lie in the following: the WR and GWCR have C-H absorption peaks at 3014 cm⁻¹ and carbonyl C=O absorption peaks at 1746 cm⁻¹ and 1700 cm⁻¹, respectively [43]. There is a distinct carbonyl C=O absorption peak at 1160 cm⁻¹. The reason for the difference between the spectra of the GWCR and WR may be that the physical blends of GO and WR during the mixing process result in the difference in the location of the carbonyl (C=O) absorption peak.

As can be seen from Figure 9b, the location of the absorption peak of each asphalt is the same as that of the rejuvenator, which mainly contains more alkanes, olefins, and aldehydes. According to the characteristics of the similar compatibility of materials, it can be inferred that there is better compatibility between the asphalt and the rejuvenator [44]. Compared with AO, the carbonyl C=O absorption peak characterizing the aging degree of asphalt appeared at 1700 cm⁻¹ in the AP, indicating that the oxidation reaction occurred during the PAV aging process. In addition, the sulfoxide S=O absorption peak at 1030 cm⁻¹ was enhanced for AP. Meanwhile, compared with the number of absorption peaks in AP, the addition of the RA-75 rejuvenator to AP did not result in the appearance of new absorption peaks, and there was only a change in the intensity of the functional group peaks, indicating that RA-75 does not react with AP chemically, but only physically co-mingled. In addition, the absorption peak of the C=O stretching vibration appeared at 1760 cm⁻¹ in the WRA and the GWCRA containing wood tar, which corresponded to the absorption peak of the C=O stretching vibration of wood tar at 1760 cm⁻¹. The possible reason was that the wood tar was physically blended with the AP [12].

To quantitatively describe the regeneration effect of the different rejuvenators on aged asphalt, the changes in the sulfoxide index I_S=O (1300 cm⁻¹) and the carbonyl index I_C=O (1700 cm⁻¹), which characterize the degree of asphalt aging, were analyzed. The formulas for the two indexes are shown in Equations (2)–(4) [45]. The two-point method was used to calculate the peak areas [46]. The larger I_S=O and I_C=O values indicate that the aging of the asphalt is more serious. On the contrary, their smaller values with an added rejuvenator indicate that the rejuvenation effect on the aged asphalt is better.

$$I_{\mathrm{S}=\mathrm{O}}={\displaystyle \frac{A_{1030}}{{\displaystyle \sum A}}}$$

(2)

$$I_{\mathrm{C}=\mathrm{O}}={\displaystyle \frac{A_{1700}}{{\displaystyle \sum A}}}$$

(3)

$$\begin{array}{l}{\displaystyle \sum A={\displaystyle \sum A_{1700}}}+{\displaystyle \sum A_{1700}}+{\displaystyle \sum A_{1600}}+{\displaystyle \sum A_{1456}}+{\displaystyle \sum A_{1376}}+{\displaystyle \sum A_{1306}}+{\displaystyle \sum A_{1160}}+{\displaystyle \sum A_{1030}}+{\displaystyle \sum A_{968}}+{\displaystyle \sum A_{861}}\\ +{\displaystyle \sum A_{810}}+{\displaystyle \sum A_{744}}+{\displaystyle \sum A_{722}}\end{array}$$

(4)

The results of I_S=O and I_C=O for each asphalt are shown in Figure 10. As can be seen, the I_S=O of AO after PAV aging increased from 0.0119 to 0.041, and the I_C=O increased from 0 to 0.0162, which indicates that the asphalt undergoes oxidation during the aging process of PAV, manifesting in the viscosity, stiffness fatigue performance, and low-temperature cracking resistance of the asphalt being significantly weakened. Compared to those of AP, the I_S=O and I_C=O values of the RAA, WRA, and GWCRA significantly decreased, indicating that the addition of rejuvenators can reduce the relative content of polar components containing C=O and S=O in aged asphalt, thereby improving its low-temperature cracking performance. However, the impact of the different rejuvenators on the I_S=O and I_C=O values of the aged asphalt varies significantly. Specifically, the addition of RA-75 can reduce the I_S=O and I_C=O values of aged asphalt by 34.88% and 41.36%, respectively; the addition of WR can reduce them by 41.46% and 38.27%, respectively; and the addition of the GWCR reduces them by 21.71% and 82.72%, respectively. Compared to RA-75, WR is more effective in reducing the I_S=O value, but its effect on lowering the I_C=O concentration is slightly weaker. In contrast, the GWCR is less effective in lowering the I_S=O of aged asphalt than the other two rejuvenators. Still, it is the most effective in reducing the formation of C=O during the aging oxidation process. This may explain why the GWCR can provide better low-temperature performance for aged asphalt while not significantly reducing its high-temperature performance.

3.4.4. Microstructure Analysis

Surface Micro-Morphology Analysis

The 2D surface morphologies of each asphalt sample are shown in Figure 11(a1,b1,c1,d1,e1). In general, the three-phase structure can be observed on the surface topography of asphalt, that is, the Catana phase representing the typical “bee-like” structure, the Peri phase surrounding the Catana phase (dispersed phase), and the para phase adjacent to the Peri phase (continuous phase) [36], labeled as I, II, and III, respectively. Meanwhile, to quantitatively analyze the “bee-like” structure changes of asphalt after aging and rejuvenation, the 2D surface morphologies of each asphalt were colorized using ImageJ software (https://imagej.net/ij/) to obtain the binarized graphs of a2, b2, c2, d2, and e2 in Figure 11. The “bee-like” structure area was calculated for the binarized graphs to find the percentage of its area to the total area. The “bee-like” structure area ratios of each asphalt binder are shown in

Table 6. “Bee-like” structure area ratio of each asphalt.

Asphalt Type	AO	AP	RAA	WRA	GWCRA
“Bee-like” structure area ratio of asphalt/%	3.09	4.46	4.29	2.93	2.73

.

From Figure 11, there were “bee-like” structures with random distribution and size differences in all asphalt samples. The formation of “bee-like” structures was caused by asphaltene aggregation, which is a microscopic phase separation behavior of asphalt [47]. By comparing (a1) and (b1) in Figure 11, it can be seen that the distribution of the “bee-like” structure of AO after PAV aging was not uniform enough with an aggregation phenomenon, and as the number increases, it can be seen that the size of the “bee-like” structure of AO after PAV aging was significantly increased, so that the number of “bee-like” structures in the visible area was decreased, and the boundary between the three phases became increasingly blurred. From Table 4, compared with AO, the “bee-like” structure area ratio of AP increased 44.5%. The reason is that the aging process promotes the continuous aggregation of asphaltenes to form asphaltene micelles, resulting in an increase in the average area of the “bee-like” structure during the PAV aging process [48].

Adding a rejuvenator can restructure the right balance between asphaltene/maltene to effectively disperse the asphaltene micelles formed by aggregation in aged asphalt [34]. As shown in Figure 11, the “bee-like” structures of the RAA, WRA, and GWCRA are more uniformly distributed and their maximum sizes are smaller than those of AP. However, there are large differences in the “bee-like” structures of the different rejuvenated asphalt binders. The addition of the RA-75 rejuvenator reduced the maximum size of the “bee-like” structure and increased its number. The “bee-like” structure area ratio of RAA decreased by 3.83% compared to that of AP, but was higher than that of AO by 38.96%. From Figure 11 and Table 6, it can be seen that adding WR resulted in the maximum size and number of the “bee-like” structures in WRA being smaller than those of AP, and the “bee-like” structure area ratio was reduced by 34.42% and 5.24% compared with that of AP and AO, respectively. After adding the GWCR, the size and number of “bee-like” structures in the GWCRA were not much different from those of AO and WRA, and the three-phase structure was clearer. Meanwhile, the GWCRA has the smallest percentage of the “bee-like” structure area ratio. The results indicate that GO could cooperate with WR to restructure the right balance between asphaltene/maltene, and effectively disperse the aggregated asphaltene micelles in the aged asphalt, which is more effective for the performance recovery of aged asphalt.

Quantitative Analysis of Microstructure

According to the analysis of the microstructure characteristics of the above asphalts, it can be seen that the surface of the asphalt at the nanometer scale has significant differences in height between the phases after aging or rejuvenation. Therefore, to quantitatively characterize the evolution of the micro-morphology of each asphalt at the nanoscale, the surface roughness index was calculated [49]. The greater the surface roughness of the asphalt, the better the adhesion and self-healing properties with the aggregate, and furthermore, the higher the energy required to destroy the surface morphology of the asphalt during the aging process [50]. At present, the root mean square roughness (R_q) and arithmetic average roughness (R_a) are commonly used as surface roughness indexes; their calculation methods are shown in Equations (5) and (6), respectively.

$$R_{\mathrm{q}}=\sqrt{Z_i/N}$$

(5)

$$R_{\mathrm{a}}={\displaystyle \frac{1}{N}}{\displaystyle \sum Z_i}$$

(6)

where N is the number of pixels to extract the height data points, Z_i is the height of the sample surface (nm), and i is the pixel number.

In the calculation process, three parallel observation samples were selected for the same asphalt sample, four observation areas were selected for each sample, and 256 × 256 pixels were selected for statistical analysis in each observation area. Figure 12 shows the calculation results of R_q and R_a for each asphalt.

As shown in Figure 12, compared with those of AO, the R_q and R_a values of AP were reduced by 42.16% and 29.32%, respectively, indicating that the surface roughness of the asphalt decreased after aging, the phase height difference at the nanoscale decreased, and the microstructure from multi-phase structure to uniform phase structure was transferred. On the contrary, the R_q and R_a values of rejuvenated asphalt were greater than those of aged asphalt, indicating that the rejuvenator could effectively disperse the aggregated asphaltene micelles in aged asphalt, and the microstructure of the rejuvenated asphalt was restored from a more uniform phase to a multi-phase structure. The adhesion with the aggregates was increased. However, the effects of the different rejuvenators on the microstructure roughness of aged asphalt are different. The R_q and R_a values of the three rejuvenated asphalts were in the same order, and the order from large to small was GWCRA > WRA > RAA. Compared to those of AP, the R_q and R_a values of the GWCRA increased by 79.91% and 45.8%, respectively. At the same time, the R_q and R_a values of the GWCRA were also greater than those of AO, indicating that the GWCR had the most obvious recovery effect on the surface microstructure of aged asphalt, which could be restored to a larger surface roughness than AO, and the adhesion of the rejuvenated asphalt was the strongest.

3.5. Regression Equations Analysis

To describe the relationships between the physical and rheological properties of each asphalt with its composition, chemical characteristics, and microstructure, linear regressions were performed using the least squares method. The regression equations are presented in

Table 7. Linear regression equations for the physical properties, rheological properties, and composition of asphalt.

	X	Asphaltene	Saturate	Aromatic	Resin	IC Value
Y
Penetration		y = 143.2−6.9539x R2 = 0.9942	y = −451.031 + 34.553x R2 = 0.82	y = −125.14 + 3.897x R2 = 0.999	y = 247.712 − 7.287x R2 = 0.999	y = 214.4 − 422.774x R2 = 0.998
Softening point		y = −13.569 + 5.671x R2 = 0.933	y = 503.609 − 30.405x R2 = 0.896	y = 202.918 − 3.127x R2 = 0.907	y = −96.244 + 5.846x R2 = 0.907	y = −69.4 + 338.842x R2 = 0.904
Ductility		y = 68.664−3.756x R2 = 0.984	y = −269.506 + 19.839x R2 = 0.918	y = −75.023 + 2.077x R2 = 0.963	y = 123.711 − 3.883x R2 = 0.962	y = 106.0 − 225.441x R2 = 0.963
58 °C G*/sinδ		y = −0.807 + 0.276x R2 = 0.981	y = 23.511 − 1.422x R2 = 0.87	y = 9.797 − 0.154x R2 = 0.973	y = −4.911 + 0.288x R2 = 0.974	y = −3.584 + 16.652x R2 = 0.97
−18 °C S value		y = −240.5 + 41.944x R2 = 0.996	y = 3437.169 − 214.8x R2 = 0.873	y = 1371.3 − 23.357x R2 = 0.988	y = −863.5 + 43.675x R2 = 0.988	y = −663.7 + 2534.0x R2 = 0.987
−18 °C m value		y = 0.584 − 0.028x R2 = 0.995	y = −1.857 + 0.143x R2 = 0.878	y = −0.482 + 0.015x R2 = 0.986	y = 0.996 − 0.029x R2 = 0.985	y = 0.864 − 1.677x R2 = 0.985

and

Table 8. Linear regression equations for the physical properties, rheological properties, chemical characteristics, and microstructure of asphalt.

	X	IS=O Value	IC=O Value	“Bee-like” Structure Area Ratio	Rq Value	Ra Value
Y
Penetration		y = 89.7 − 1297.06x R2 = 0.527	y = 71.7 − 2239.274x R2 = 0.561	y = 112.16 − 16.477x R2 = 0.492	y = 17.142 + 12.866x R2 = 0.297	y = −2.854 + 11.435x R2 = 0.292
Softening point		y = 24.6 + 1259.649x R2 = 0.701	y = 44.68 + 1831.607x R2 = 0.53	y = 6.949 + 14.806x R2 = 0.56	y = 88.462 − 10.228x R2 = 0.265	y = 103.928 − 9.004x R2 = 0.255
Ductility		y = 39.244 − 681.808x R2 = 0.494	y = 30.2 − 1227.511x R2 = 0.573	y = 56.552 − 10.23x R2 = 0.644	y = −1.414 + 7.634x R2 = 0.355	y = −13.177 + 6.764x R2 = 0.347
58 °C G*/sinδ		y = −1.217 + 55.171x R2 = 0.597	y = 2.046 + 86.72x R2 = 0.527	y = 0.322 + 0.683x R2 = 0.53	y = 4.125 − 0.486x R2 = 0.266	y = 4.869 − 0.43x R2 = 0.258
−18 °C S value		y = 82.4 + 7819.342x R2 = 0.527	y = 190.2 + 13558.69x R2 = 0.567	y = −76.6 + 106.062x R2 = 0.561	y = 527.78 − 80.338x R2 = 0.319	y = 651.95 − 71.262x R2 = 0.312
−18 °C m value		y = 0.368 − 5.085x R2 = 0.509	y = 0.3 − 9.007x R2 = 0.57	y = 0.479 − 0.071x R2 = 0.576	y = 0.073 + 0.054x R2 = 0.33	y = −0.011 + 0.048x R2 = 0.323

, respectively.

As can be seen from Table 7, the physical and rheological properties of each asphalt are linearly correlated with its composition, with the R² values of the linear regression equations all being greater than 0.8. The asphaltenes, resins, and I_C values of the asphalts are negatively correlated with their penetration, ductility, and the m values, while being positively correlated with their softening points, G*/sinδ, and S values. In contrast, the saturates and aromatics in the asphalts are positively correlated with their penetration, ductility, and m values, and negatively correlated with their softening points, G*/sinδ, and S values. The results indicate that the addition of a rejuvenator can reduce the relative concentration of asphaltenes and resins as well as the I_C value in aged asphalt, thereby achieving the effect of softening the asphalt, which weakens its high-temperature performance and significantly enhances its low-temperature cracking resistance.

As shown in Table 8, the physical and rheological properties of each asphalt exhibit a linear correlation with its chemical properties and microstructure; however, the R² values for these linear regression equations are relatively low. Specifically, the R² values for the linear regression equations relating surface microstructure roughness parameters, such as R_q value and R_a value, to asphalt performance are below 0.4, indicating a weak correlation. This aligns with the earlier discussion, as these parameters primarily characterize the adhesion between asphalt and aggregate. The I_S=O value, I_C=O value, and “bee-like” structure area ratios in each asphalt are negatively correlated with the penetration, ductility, and m value, while positively correlated with the softening point, G*/sinδ, and S value. The findings suggest that the addition of a rejuvenator can reduce the I_S=O and I_C=O values and the “bee-like” structure area ratios in aged asphalt, thereby weakening its high-temperature performance while significantly enhancing its low-temperature cracking resistance.

4. Conclusions and Recommendations

In this study, the physical properties, rheological properties, anti-secondary aging performance, composition, chemical properties, and microstructure characteristics were analyzed to evaluate the rejuvenation effects and mechanism of the GWCR on the performance and anti-aging properties of aged asphalt. The following conclusions were obtained:

(1)

GWCR can effectively soften the aged asphalt, increase its penetration and ductility, reduce its softening point, decrease its G*/sinδ and S value, and increase its m value, indicating that the high-temperature performance of the GWCRA is decreased and the low-temperature crack resistance is enhanced, while they are slightly better than that of the 70# original base asphalt.

(2)

The variation range of S value and m value of the GWCRA after PAV aging is smaller than that of AO, WRA, and RAA, indicating that the synergistic effect of GO, wood tar, and DOA could effectively improve the secondary aging resistance of rejuvenated asphalt.

(3)

A rejuvenator can effectively increase the content of saturates and aromatics in aged asphalt, reduce the content of asphaltenes and resins, and decrease the I_C value, so as to restructure the right balance between asphaltene/maltene. Differently, the GWCR has the best blending effect on the components of aged asphalt, and the I_C value is reduced to the same as that of AO, so it has the best recovery effect on the performance of aged asphalt.

(4)

When adding a rejuvenator to aged asphalt, no new absorption peaks are found; the absorption peak of the C=O stretching vibration appeared at 1760 cm⁻¹ in WRA and GWCRA, corresponding to the absorption peak of the C=O stretching vibration of wood tar at 1760 cm⁻¹, indicating that the rejuvenator is physically blended with the AP. The addition of a rejuvenator can decrease the I_S=O and I_C=O values, but the effect of the GWCRA on the reduction in I_C=O value is more pronounced than that of the I_S=O value.

(5)

A rejuvenator can effectively disperse the asphaltene accumulated during PAV aging, reduce the “bee-like” structure area ratio and maximum size, increase the surface roughness, and enhance the adhesion. Compared with the RA-75 rejuvenator and WR, the GWCR can decrease the “bee-like” structure area ratio and increase the surface roughness and adhesion of aged asphalt.

(6)

The asphaltenes, resins, I_C value, I_S=O value, I_C=O value, and “bee-like” structure area ratios in asphalt are negatively correlated with the penetration, ductility, and m value, while positively correlated with the softening point, G*/sinδ, and S value. In contrast, the saturates and aromatics in asphalt are positively correlated with the penetration, ductility, and m value, and negatively correlated with the softening point, G*/sinδ, and S value.

When studying existing rejuvenators, most researchers focus on the rejuvenation effect of the rejuvenators on aged asphalt, and some studies also focus on the problem of the secondary aging of rejuvenated asphalt, but research on the strategy to improve the anti-secondary aging performance of rejuvenated asphalt is relatively limited. Therefore, this paper proposes the preparation of the GWCR by compositing GO with wood tar, aiming to improve the anti-secondary aging performance of rejuvenated asphalt. However, this study is not comprehensive enough and can be improved in the following aspects in the future:

(1)

Explore the anti-secondary aging performance of the GWCRA in depth and in a comprehensive manner, and analyze its mechanism.

(2)

Explore the changes in asphalt performance during secondary rejuvenation using the GWCRA and its secondary rejuvenation mechanism.

(3)

Introduce other more economical and environmentally friendly materials (e.g., humic acids) to improve the performance of rejuvenated asphalt and maximize its economic and environmental benefits.