Preparation and Characterization of Bio-Asphalt Based on Sludge-Derived Heavy Oil

Abstract

To achieve the efficient resource utilization of municipal sludge and promote the sustainability of pavement materials, this study employed liquefaction technology to process municipal sludge. The resulting liquefied-sludge-derived heavy oil was blended with 50^# asphalt to prepare a bio-asphalt that can replace petroleum asphalt. Firstly, orthogonal experiments were conducted to analyze the effects of the solid–liquid ratio (dried sludge:anhydrous ethanol), liquefaction temperature, and reaction time on the yield of the sludge-derived heavy oil. Then, the basic characteristics of the sludge-derived heavy oil were studied using an elemental analyzer, gel permeation chromatography, thermal analysis, gas chromatography–mass spectrometry, and Fourier transform infrared spectroscopy, and the differences between the sludge-derived heavy oil and petroleum asphalt were compared. Finally, to determine the appropriate content range of sludge-derived heavy oil in bio-asphalt, a comprehensive evaluation of the three major indicators, aging resistance, storage stability, low-temperature performance, and high-temperature performance was carried out for the prepared bio-asphalts. The results indicated that the optimal preparation process for liquefied sludge oil involves a liquefaction temperature of 275 °C, a solid–liquid ratio of 1:15, and a reaction time of 1 h, resulting in an oil production rate of 22.36%. The sludge-derived heavy oil demonstrated good thermal stability, with its primary components being aliphatic compounds (carboxylic acids, ketones, aldehydes, alcohols, alkanes, esters, etc.), with esters being the most abundant. Furthermore, the sludge-derived heavy oil was highly compatible with 50^# asphalt, but no chemical reaction occurred between them. When the sludge-derived heavy oil content ranged from 5% to 20%, bio-asphalt showed favorable aging resistance and storage stability. As the content of the sludge-derived heavy oil increased, its low-temperature performance improved, but there was a slight decrease in high-temperature performance. Additionally, correlation analysis highlighted that the influence of sludge-derived heavy oil content on the high-temperature performance of bio-asphalt was notably greater than on other properties. Therefore, the recommended dosage of sludge-derived heavy oil should be between 5% and 10%.

1. Introduction

Asphalt pavements are highly regarded for their superior driving comfort. As the demand for maintaining existing roads and constructing new asphalt pavements continues to rise, the significant consumption of asphalt binder materials becomes unavoidable [1]. Petroleum asphalt binder, a byproduct of crude oil processing, is a nonrenewable resource. Bio-oil can partially replace petroleum asphalt binders in highway engineering, providing significant economic, social, and environmental advantages. Bio-oil is produced from biomass materials such as trees, algae, animal manure, industrial waste, and urban waste through liquefaction or pyrolysis. It is renewable, widely sourced, and environmentally friendly, attracting considerable attention from researchers in recent years [2,3].

Currently, the application of bio-oil in asphalt can be categorized into three domains: (i) as a direct alternative (replacement of asphalt at 100%), (ii) as an asphalt extender (replacement of asphalt between 25%–75%), (iii) as an asphalt modifier (replacement of asphalt < 10%). Lin et al. [4] studied bitumen modified by bio-oil from bamboo charcoal production and found that this bio-oil softens asphalt. They also discovered that bio-asphalt containing 12% bio-oil exhibited improved brittleness resistance and fatigue crack resistance, along with good rutting and aging resistance. Bao et al. [5] compared bio-asphalt derived from corn and waste wood and found that both types achieved the best rutting resistance with 15% bio-oil content. However, the bio-asphalt derived from waste wood exhibited superior durability and storage stability, making it more suitable for bio-asphalt production. Liu et al. [6] investigated castor-oil-based polyurethane-modified bio-asphalt and discovered that incorporating a small amount of castor-oil-based polyurethane enhances the high-temperature performance of bio-asphalt. However, the reaction between the bio-oil and polyurethane diminishes its low-temperature performance. Lorenzo et al. [7] produced bio-oil from residues left over from wood processing into paper and pulp. They incorporated this bio-oil into 50^# and 70^# virgin asphalts at varying mass percentages (0%, 5%, 10%, and 15%). The results demonstrated that this bio-oil can soften asphalt and enhance its low-temperature performance and fatigue resistance, although it does lead to a slight increase in permanent deformation. Gao et al. [8] incorporated bio-oil derived from wood chips into virgin asphalt to produce bio-asphalt. Their study demonstrated that adding bio-oil improved the rutting resistance and deformation resistance of the asphalt mixture. Additionally, aging significantly affected the rheological performance of bio-asphalt, more so than it did for virgin asphalt and SBS-modified asphalt. Wang et al. [9] explored the potential of using bio-asphalt derived from waste cooking oil as a substitute for petroleum asphalt and SBS modifiers. Their study found that bio-oil and petroleum asphalt have similar chemical functional groups. The incorporation of bio-oil significantly decreased the asphalt’s stiffness and viscosity while enhancing its fatigue resistance. Bio-oil can also function as an asphalt rejuvenator by dissolving and dispersing aged asphalt components, thereby enhancing workability, fatigue resistance, and low-temperature performance [10,11].

The performance of asphalt varies significantly depending on the type and processing method of the bio-oil used. Bio-oil derived from materials like willow branches and corn stover can improve the high-temperature performance of asphalt but adversely affect its low-temperature performance [12,13]. Bio-asphalt derived from cooking oil, waste grease, wood chips, waste lubricating oil, castor oil, and pig manure exhibits the opposite trend [14,15,16]. Bio-oil derived from waste wood can enhance the fatigue resistance of asphalt and its mixtures, but it adversely affects high-temperature performance. Waste cooking oil enhances the fatigue resistance of asphalt [17]. Pine-biomass-based heavy oil improves the crack resistance of asphalt to some extent [18]. Municipal sludge, a secondary pollutant generated during the treatment of organic wastewater in sewage plants, has significantly increased in production due to rapid urbanization, industrialization, and the prioritization of wastewater treatment over sludge management in many sewage plants. These municipal sludges can be converted into hydrocarbons through hydrothermal treatment, making them a promising low-cost biomass feedstock for bio-oil production [19,20,21]. At present, the main application of sludge bio-oil is replacing its lighter fractions with fossil fuels in biodiesel production. However, its combustion performance still trails that of fossil fuels. Furthermore, although there has been some research on the application of sludge-derived heavy oil, it has not yet been used in bio-asphalt materials for road engineering.

Therefore, this study uses municipal sludge as a raw material, employs liquefaction technology to produce sludge-derived heavy oil, and blends it with 50^# asphalt to prepare bio-asphalt. The basic characteristics of the sludge-derived heavy oil and the pavement performance of the bio-asphalt are investigated using microscopic characterization methods and macroscopic performance tests.

2. Materials

2.1. Municipal Sludge

Municipal sludge was sourced from a sewage treatment plant in Chongqing, and its industrial analysis results are shown in

Table 1. Industrial analysis results for municipal sludge.

Testing Item	Water	Ash	Volatiles	Fixed Carbon
Content/%	81.63	6.65	10.77	0.95

. If fresh sludge is not promptly treated and preserved, it is highly susceptible to digestion reactions, which lead to a decrease in organic content and can negatively affect the bio-oil yield in subsequent liquefaction experiments. The sludge was dried at 105 °C until it reached a constant weight and then ground into a fine powder and sealed for future use.

2.2. Virgin Asphalts

The conventional properties of virgin asphalts, Donghai 50^#, 70^#, and 90^#, were tested according to the Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011) [22]. The results are presented in

Table 2. Conventional properties of virgin asphalt.

Technical Parameter	50#	Requirements	70#	Requirements	90#	Requirements
Penetration (25 °C, 5 s)/0.1 mm	46.7	40~60	66.7	60~80	83.7	80~100
Softening point/°C	49.5	45~58	47.0	44~57	48.5	42~52
Ductility (15 °C)/cm	>100	≥80	>100	≥100	>100	≥100
TFOT mass loss/%	−0.03	≤±0.08	−0.02	≤±0.08	−0.03	≤±0.08

.

2.3. Preparation of Sludge-Derived Heavy Oil

Dried municipal sludge was used as the raw material to prepare sludge-derived heavy oil via liquefaction. Anhydrous ethanol served as the solvent for liquefaction, with nitrogen used as the protective gas. Liquefaction experiments were carried out in a batch reactor (model CJF-0.5L, Chongqing Dongyue Instrument Co., Ltd., Chongqing, China). The preparation process involved three stages [23,24]. Phase 1: Dried municipal sludge and the appropriate solid–liquid ratio of anhydrous ethanol solvent were added to the reactor vessel. The reactor vessel was immediately sealed and checked for airtightness. After adding the materials, magnetic stirring was performed, and N₂ was introduced into the vessel for approximately 10 min until all internal air was expelled. Phase 2: We turned on the temperature control device and set the reaction temperature and heating rate. The reaction temperature was determined according to specific test requirements, and the heating rate was the same as that of thermogravimetric analysis at 10 °C/min. The reactor vessel was immediately sealed and checked for airtightness. After adding the materials, magnetic stirring was performed, and N₂ was introduced into the vessel for approximately 10 min until all internal air was expelled. Phase 3: The reaction products were separated into solid and liquid phases using suction filtration to isolate the liquid phase. Subsequently, the liquid phase was transferred to a rotary evaporator operating at 20 rpm with a water bath temperature of 85 °C (considering the boiling point of anhydrous ethanol is 79 °C) to reclaim the unreacted solvent for potential reuse in future experiments. After distilling off the necessary fractions, sludge-derived heavy oil was obtained. The specific process for preparing sludge-derived heavy oil is shown in Figure 1. The formula for calculating the thermochemical liquefaction oil production rate of dried municipal sludge is shown in Equation (1).

W_bo (%) = M_bo/M_o × 100

(1)

where W_bo is the thermochemical liquefaction oil production rate of dried municipal sludge (%), M_bo is the mass of sludge-derived heavy oil (g), and M_o is the mass of dried municipal sludge (g).

2.4. Preparation of Bio-Asphalt

Sludge-derived heavy oil was mixed with 50^# base asphalt to produce bio-asphalt. Initially, the sludge-derived heavy oil was heated at 105 °C for 1 h to eliminate absorbed moisture accumulated during storage. It was then blended with 50^# virgin asphalt in varying proportions of 5%, 10%, 15%, and 20% (by mass fraction) at 135 °C. Subsequently, the mixture underwent 30 min of shearing and blending using a high-speed shear mixer (model GS-1, Chongqing Dongyue Instrument Co., Ltd.) at 135 °C and a shear rate of 1000 r/min. Different blends of bio-asphalt with varying proportions of sludge-derived heavy oil were obtained, designated as 5% Bio, 10% Bio, 15% Bio, and 20% Bio. The specific preparation process is shown in Figure 2.

3. Methods

3.1. Elemental Test

An elemental analyzer (model Flash Smart, Thermo Fisher Scientific, Redmond, WT, USA) was utilized to analyze the elemental composition of municipal sludge, sludge-derived heavy oil, and 70^# asphalt, specifically targeting the elements carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sulfur (S).

3.2. Gel Permeation Chromatography Test

The gel permeation chromatography (GPC) of sludge-derived heavy oil and 70^# asphalt was assessed using a gel permeation chromatograph (model Waters 1525, Waters Corporation, Milford, MA, USA). The mobile phase was chromatography-grade tetrahydrofuran in a Shim-pack GPC-80X column, flowing at a rate of 1 mL/min, set at a temperature of 40 °C, with a sample injection volume of 10 μL.

3.3. Thermogravimetric Test

Thermogravimetric analysis (TGA) of the sludge-derived heavy oil was performed using a thermal analyzer (model ATS-STA-1250, Shanghai Aitisen, Shanghai, China). High-purity nitrogen was used as the protective gas at a flow rate of 50 mL/min. The temperature range was from 27 °C to 490 °C with a heating rate of 10 °C/min.

3.4. Gas Chromatography–Mass Spectrometry Test

Gas chromatography–mass spectrometry (GC-MS) was used to analyze the compound composition of the sludge-derived heavy oil. A mass spectrometer (model AISQ, Thermo Fisher Scientific, USA) operated in full-scan acquisition mode, with the ion source temperature controlled up to a maximum of 250 °C. Dichloromethane was used as the solvent and high-purity helium as the carrier gas, and the chromatographic column was HP-5, measuring 30 m in length, 250 μm in diameter, and with a 0.25 μm film thickness [25].

3.5. Fourier Transform Infrared Spectrum Test

The Fourier transform infrared spectrum (FTIR) spectra of sludge-derived heavy oil and various asphalts were analyzed using a Fourier transform infrared spectrometer (model TENSOR II, Bruker, Billerica, MA, USA). The scanning range was 4000 to 400 cm⁻¹, with 16 scans and a spectral resolution of 4 cm⁻¹. All samples were prepared using the KBr plate technique.

3.6. Conventional Properties Test

The softening point, penetration (25 °C), ductility (15 °C), aging resistance, and storage stability of the asphalt binders were determined according to the standard methods: JTG E20T0606, JTG E20 T0604, JTG E20 T0605, JTG E20 T0609, and JTG E20 T0661 [22].

3.7. Low-Temperature Performance Test

The creep stiffness (S) and creep rate (m) of various asphalts were assessed using a Bending Beam Rheometer (model BBR, Shanghai Shenrui, Shanghai, China). The tests were conducted at a temperature of −12 °C with a load application time of 240 s. Both S and m values were measured and analyzed at 60 s.

3.8. High-Temperature Performance Test

The rutting factor and unrecoverable creep compliance (J_nr) of various asphalt samples were evaluated using a dynamic shear rheometer (model Smart Pave 102, Anton Paar GmbH, Graz, Austria). The rutting factor was determined via temperature-scanning tests spanning 52 °C to 82 °C, with a strain control of 12% and a frequency of 10 rad/s. Jnr was assessed using multiple stress creep recovery (MSCR) tests at 65 °C, employing stress levels of 0.1 kPa and 3.2 kPa over 10 cycles. Each cycle consisted of 1 s of creep loading followed by 9 s of stress recovery.

4. Results

4.1. Liquefaction Process

In the experimental design, three factors—the solid–liquid ratio, liquefaction temperature, and reaction time—were considered influencing parameters. The yield of sludge-derived heavy oil served as the evaluation criterion. An orthogonal experiment with three factors and three levels was conducted to optimize the sludge liquefaction oil production process [26,27].

The orthogonal experimental results are presented in

Table 3. Results of orthogonal experiment.

Testing Number	Factors			Results
	A (Solid-to-Liquid Ratio)	B (Liquefaction Temperature)	C (Reaction Time)	Sludge-Derived Heavy Oil Yield/%
1	1:5	250 °C	0.5 h	10.92
2	1:5	275 °C	1.0 h	16.23
3	1:5	300 °C	1.5 h	13.62
4	1:10	250 °C	1.0 h	13.86
5	1:10	275 °C	1.5 h	19.93
6	1:10	300 °C	0.5 h	16.31
7	1:15	250 °C	1.5 h	13.48
8	1:15	275 °C	0.5 h	18.32
9	1:15	300 °C	1.0 h	18.71

. The three ordered degree (OD) values of each factor in the same level, i, were summed, and the value, Ki; the corresponding average value, ki; and the range, R, were calculated as follows:

Ki = ΣODi

(2)

ki = Ki/3

(3)

R = k_max − k_min

(4)

ki represents the impact of level i for each factor on the yield of sludge-derived heavy oil (i = 1, 2, 3). The higher k is, the higher the yield of sludge-derived heavy oil is. k_max is the maximum among the three ki values of each factor, and k_min is the minimum. R reflects the ID of the factors to the yield of sludge-derived heavy oil. A factor with a higher R suggests a stronger impact on the formation of the yield of sludge-derived heavy oil.

Figure 3 shows the R of the three factors, R_b > R_a > R_c, indicating that the degree of influence of each factor on the yield of sludge-derived heavy oil is as follows: liquefaction temperature > solid–liquid ratio > reaction time. This suggests that the liquefaction temperature is the primary factor influencing changes in the yield of sludge-derived heavy oil. The analysis indicates that ethanol reaches a supercritical state between 250 °C and 300 °C, penetrating the sludge structure to accelerate chemical bond cleavage. This process promotes the formation of intermediate products, which subsequently condense or undergo bond cleavage to produce liquid products. Inadequate liquefaction temperatures fail to supply the necessary activation energy for sludge liquefaction, leading to incomplete reactions. Conversely, excessively high temperatures accelerate the secondary decomposition of intermediates and liquid products, impairing heavy oil formation. In addition, the solid–liquid ratio is a secondary factor that affects the yield of sludge-derived heavy oil. A low ratio impedes ethanol diffusion into the sludge, leading to incomplete chemical bond cleavage and lower oil yields. Conversely, excessive ratios saturate the sludge with ethanol needed for liquefaction, limiting further enhancements in diffusion rate and bond-breaking efficiency, thus diminishing the increase in yield of sludge-derived heavy oil.

This study assessed the yield of sludge-derived heavy oil as the performance criterion, determining the optimal parameters for liquefaction: a temperature of 275 °C, a solid–liquid ratio of 1:15, and a reaction time of 1 h. Validation experiments conducted under these conditions demonstrated that the yield of sludge-derived heavy oil is 22.36%.

4.2. Characteristics of Sludge-Derived Heavy Oil

4.2.1. Elemental Composition

Figure 4 shows a significant increase in the carbon (C) and hydrogen (H) content of the sludge-derived heavy oil compared with urban sludge, with a relative increase of 415.6% in C and 305.5% in H. This suggests that during the liquefaction process, a substantial portion of the C and H elements from urban sludge transfer to sludge-derived heavy oil. Although there are changes in elemental composition before and after liquefaction, it predominantly remains composed of carbon compounds. In contrast, 70^# asphalt contains over 80% carbon with relatively low oxygen (O) content, indicating a composition primarily of hydrocarbons compared with sludge-derived heavy oil. Additionally, compared with sludge-derived heavy oil, 70^# asphalt contains higher levels of nitrogen (N) and sulfur (S), leading to increased emissions of nitrogen oxides and sulfur oxides in practical engineering applications. Consequently, partially substituting petroleum asphalt with sludge-derived heavy oil to produce bio-asphalt can help reduce these atmospheric pollutants, thereby enhancing the environmental safety of road construction materials.

4.2.2. Molecular Weight

Figure 5 shows that at elution times of 8 and 9 min, both 70^# asphalt and sludge-derived heavy oil start generating electrical signals. Around 12 min, these signals decrease to a lower level and then stabilize, with the sludge-derived heavy oil signal being slightly higher than that of the asphalt. The number average molecular weight (Mn) of the heavy oil is lower than that of 70^# asphalt, whereas the weight average molecular weight (Mw) is higher. This suggests that 70^# asphalt contains a higher proportion of large molecular compounds, while heavy oil contains more small molecular compounds. The polydispersity index (D = Mw/Mn) of 70^# asphalt is significantly higher than that of heavy oil, indicating that the molecular weight distribution of heavy oil is narrower and more concentrated compared with asphalt.

4.2.3. Thermal Stability

Figure 6 shows that during the initial temperature range (27 °C to 100 °C), there is a mass loss of 3.3%, attributable to the volatilization of small molecules. During the second temperature range (100 °C to 250 °C), there is a mass loss of 5.6%, primarily attributable to the decomposition of certain components. During the third temperature range (250 °C to 490 °C), the mass loss of the heavy oil escalates significantly, reaching 22.9% at 490 °C, attributable to macromolecular cracking. Between 27 °C and 165 °C, the heavy oil exhibits a minimal weight loss of only 5.4%, suggesting excellent thermal stability.

4.2.4. Chemical Composition

Figure 7 shows a total ion chromatogram of sludge-derived heavy oil, detailing its major compounds, which collectively account for 81.65%, as listed in

Table 4. Main chemical components of sludge-derived heavy oil.

Types of Compounds	Chemical Construction	Proportion/%	Molecular Formula
Methyl hexadecanoate		62.11	C17H34O2
Methyl 16-methylheptadecanoate		10.19	C19H38O2
Methyl tetradecanoate		3.64	C15H30O2
Carbazole, 9-phenyl-		2.96	C18H13N
Phenol, 2,2′-methylenebis[6-(1,1-dimethylethyl)-4-methyl-		2.75	C23H32O2

. Table 4 highlights the predominant chemical components of sludge-derived heavy oil, including methyl hexadecylate (C₁₇H₃₄O₂), methyl 16-methylheptadecanoate (C₁₉H₃₈O₂), and methyl tetradecanoate (C₁₅H₃₀O₂), all classified as ester compounds and accounting for a total of 75.94%. These compounds feature long carbon chains, predominantly ranging from C17 to C23. Since the organic matter in sludge undergoes cracking into acid intermediates during high-pressure liquefaction, these acids subsequently undergo esterification with an ethanol solvent, resulting in the production of ester compounds.

4.2.5. Functional Group Structure

The FTIR spectrum of sludge-derived heavy oil, as depicted in Figure 8, shows distinctive absorption peaks: a strong peak at 3431 cm⁻¹ corresponding to the stretching vibration characteristic of alcohols, phenolic O-H, or amine N-H groups. Additionally, peaks at 2924 cm⁻¹ and 2854 cm⁻¹, along with peaks at 1460 cm⁻¹, 1381 cm⁻¹, and 1163 cm⁻¹, represent C-H stretching and bending vibrations of alkanes. The peak at 1739 cm⁻¹ indicates the stretching vibration of C=O in ester compounds, while 1634 cm⁻¹ corresponds to the stretching vibration of olefin C=C. Furthermore, peaks at 1261 cm⁻¹, 1111 cm⁻¹, and 1019 cm⁻¹ can be attributed to the stretching vibrations of C-O bonds in alcohols, ethers, and ester compounds. From this, it can be inferred that sludge-derived heavy oil likely contains various aliphatic compounds such as carboxylic acids, ketones, aldehydes, alcohols, alkanes, esters, and others. Combined with the GC-MS test results, it can be confirmed that the proportion of ester substances is the highest. The functional groups in the four different dosages of bio-asphalt are largely consistent. Upon comparison with sludge-derived heavy oil and 50^# asphalt, no new functional groups were detected, suggesting that there is no chemical reaction between sludge-derived heavy oil and petroleum asphalt in bio-asphalt; rather, it involves only physical blending.

4.3. Performance Evaluation of Bio-Asphalt

4.3.1. Basic Performance

As indicated in

Table 5. Test results for three indexes of asphalt.

Three Indexes	50#	5% Bio	10% Bio	15% Bio	20% Bio	70# Requirements	90# Requirements
Penetration (25 °C, 5 s)/0.1 mm	46.8	64.6	74.3	86.2	99.7	60~80	80~100
Softening point/°C	49.5	47.5	46	46	45.5	44~57	42~52
Ductility (15 °C)/cm	>100	>100	>100	>100	>100	≥100	≥100

, increasing the sludge-derived heavy oil content enhances the penetration of bio-asphalt, surpassing that of standard 50^# base asphalt. This effect arises because sludge-derived heavy oil, with its lower viscosity, softens hard asphalt upon addition. Consequently, this softening raises the viscosity of the base asphalt, thereby increasing penetration. This characteristic suggests that heavy oil can effectively serve as a modifier for hard asphalt. The softening point decreases gradually with increasing dosage, albeit minimally, suggesting that higher heavy oil content has only a slight effect on asphalt’s softening point. Moreover, the ductility of the bio-asphalt exceeded 100 during the experiment, recorded as “>100” according to specifications. When the sludge-derived heavy oil content ranges from 5% to 10%, the basic performance of bio-asphalt meets the specifications for 70^# asphalt. When the sludge-derived heavy oil content ranges from 15% to 20%, the basic performance of bio-asphalt meets the specifications for 90^# asphalt.

4.3.2. Aging Resistance

This study evaluates the aging resistance of bio-asphalt by comparing the mass loss of residual material after aging, the ratio of residual penetration to original penetration, and residual ductility with those of 50^# asphalt, 70^# asphalt, and 90^# asphalt, as shown in

Table 6. Results for asphalt technical index after TFOT.

Testing Indexes	50#	70#	90#	5% Bio	10% Bio	15% Bio	20% Bio
Mass loss/%	−0.03	−0.02	−0.03	−0.008	−0.012	−0.017	−0.023
Penetration ratio/%	68.3	63.2	65.6	77.1	82.5	87.9	89.0
Residual ductility (15 °C)/cm	15.2	18.5	49	51.5	68.7	87.6	98.6

. After TFOT aging, all performance indicators of the four types of bio-asphalt exhibited improvements compared with 50^# asphalt, surpassing both 70^# and 90^# asphalts. This demonstrates that the bio-asphalt performed excellent resistance to aging when the content of sludge-derived heavy oil ranged from 5% to 20%. Moreover, the sludge-derived heavy oil effectively enhanced the aging resistance of 50^# base asphalt.

4.3.3. Storage Stability

Figure 9 shows that as the sludge-derived heavy oil content increases, the difference in softening points between the upper and lower layers of bio-asphalt gradually widens yet remains within specification requirements (≤2.5). This suggests that within the 5% to 20% range, sludge-derived heavy oil exhibits good compatibility with 50^# asphalt, and bio-asphalt demonstrates satisfactory storage stability.

4.3.4. Low-Temperature Performance

Most scholars commonly utilize creep stiffness (S) and creep rate (m) as evaluation indicators for asphalt’s low-temperature performance [28]. Asphalt with superior low-temperature performance typically demonstrates lower S and a higher m under identical experimental conditions [29,30].

According to Figure 10a, the S of the four different doses of bio-asphalt is lower than that of 50^# asphalt and decreased with increasing sludge-derived heavy oil content. At contents of 5% to 10%, the S of the bio-asphalt is comparable to that of 70^# asphalt. At contents of 15% to 20%, the S of the bio-asphalt aligns closely with that of 90^# asphalt. According to Figure 10b, the m of the four different doses of bio-asphalt is higher than that of 50^# asphalt, increasing with the dosage of sludge-derived heavy oil. At dosages of 15% to 20%, the m of the bio-asphalt is comparable to that of 70^# asphalt, suggesting that adding sludge-derived heavy oil can enhance the low-temperature performance of 50^# asphalt, with higher dosages yielding greater improvement.

4.3.5. High-Temperature Performance

The rutting factor was employed to assess the resistance to deformation of the bio-asphalt at elevated temperatures. A higher rutting factor signifies the reduced flow deformation of the asphalt, indicating stronger resistance to rutting. Figure 11 shows that at identical temperatures, the rutting factor of all four types of bio-asphalt is lower than that of 50^# asphalt. Moreover, it decreases with increasing sludge-derived heavy oil content, suggesting that incorporating sludge-derived heavy oil diminishes the rutting resistance of 50^# asphalt. The rate at which the rutting factor of bio-asphalt decreases slows with increasing temperature, suggesting that the impact of sludge-derived heavy oil content on the rutting factor of bio-asphalt diminishes as the temperature rises. When the sludge-derived heavy oil content is 5%, the rutting factor of bio-asphalt is slightly lower than that of 70^# asphalt. With 10% sludge-derived heavy oil, the rutting factor of bio-asphalt is comparable to that of 90^# asphalt. Thus, it is advisable not to exceed higher proportions of sludge-derived heavy oil, with the optimal dosage range identified as 5% to 10%.

To more accurately evaluate the high-temperature performance of bio-asphalt, the multiple stress creep recovery (MSCR) test, based on the repeated creep recovery (RCR) test, has been proposed [31,32,33]. Figure 12a presents the MSCR results for bio-asphalt, 50^# asphalt, 70^# asphalt, and 90^# asphalt. Figure 12b displays the average unrecoverable creep compliance for these asphalts. Specifically, J_nr0.1 and J_nr3.2 denote the average unrecoverable creep compliance at stress levels of 0.1 kPa and 3.2 kPa, respectively.

A smaller value for unrecoverable creep compliance (J_nr) indicates better rutting resistance performance in asphalt. As depicted in Figure 12b, irrespective of stress level, the average irreversible creep compliance of four different doses of bio-asphalt exceeds that of 50^# asphalt. Moreover, these values rise with increasing sludge-derived heavy oil content, suggesting that the addition of sludge-derived heavy oil diminishes the rutting resistance of 50^# asphalt. At sludge-derived heavy oil contents of 5% and 10%, the J_nr0.1 and J_nr3.2 values of bio-asphalt are similar to those of 70^# and 90^# asphalt, respectively. Therefore, to ensure the high-temperature performance of bio-asphalt, the recommended sludge-derived heavy oil content is 5% to 10%.

4.4. Correlation Analysis

The road performance of bio-asphalt depends on various factors, with the quantity of sludge-derived heavy oil being the primary influencing factor. The relationship between these factors can be modeled using a linear regression equation, expressed as Y = a + kx, where k represents the regression coefficient. This equation allows for predicting or estimating the road performance of bio-asphalt based on the quantity of sludge-derived heavy oil.

Regression analysis is employed to assess the correlation between the basic performance (penetration and softening point), aging resistance (mass loss, penetration ratio, and 15 °C residual ductility), storage stability (softening point difference), low-temperature performance (S and m), and high-temperature performance (J_nr0.1 and J_nr3.2) of bio-asphalt and the sludge-derived heavy oil content. Through data analysis, a linear relationship between these bio-asphalt indicators and the sludge-derived heavy oil content can be established. The specific regression equation is depicted in Figure 13. According to Figure 13, the correlation coefficients (R²) between various indicators and the sludge-derived heavy oil content can be ranked as follows: Jnr3.2 (0.999) > Jnr0.1 (0.997) > penetration (0.995) > mass loss (0.992) > 15 °C residual ductility (0.989) > S (0.984) > m (0.972) > penetration ratio (0.938) > softening point difference (0.931) > softening point (0.8).

Except for the softening point, the R² values for all other indicators exceed 0.9, indicating a strong correlation between sludge-derived heavy oil content and various indicators of bio-asphalt. Among these, high-temperature performance (J_nr0.1 and J_nr3.2) has an average R² of 0.998, showing the most significant correlation with sludge-derived heavy oil content. The subsequent correlation ranking is as follows: low-temperature performance (S and m), aging resistance (mass loss, penetration ratio, and 15 °C residual ductility), storage stability (softening point difference), and basic performance (penetration and softening point). The average R² values are 0.978, 0.973, 0.931, and 0.898, respectively.

In summary, the impact of sludge-derived heavy oil content on the high-temperature performance of bio-asphalt is significantly greater than its effect on other performance aspects. Therefore, in practical applications, priority should be given to assessing the influence of sludge-derived heavy oil on the high-temperature performance of bio-asphalt.

5. Conclusions

This study employed liquefaction technology to produce sludge-derived heavy oil. The physicochemical properties of the sludge-derived heavy oil were investigated using elemental composition, GPC, TGA, GC-MS, and FTIR and compared with those of petroleum asphalt. Additionally, the basic performance, aging resistance, storage stability, high-temperature performance, and low-temperature performance of the bio-asphalt were comprehensively evaluated. The main findings were as follows:

(1) The evaluation index for optimizing sludge liquefaction oil production was the yield of heavy oil. An orthogonal experimental design was conducted to ascertain the impact of various factors on oil production rates. The results demonstrated that liquefaction temperature exerts a significant influence, followed by the solid–liquid ratio, with reaction time exerting the least impact. The optimal sludge liquefaction process for oil production was identified as a temperature of 275 °C, a solid–liquid ratio of 1:15, and a reaction time of 1 h, achieving an oil production rate of 22.36%.

(2) Between 27 °C and 165 °C, the sludge-derived heavy oil exhibits a minimal weight loss of only 5.4%, suggesting excellent thermal stability. The sludge-derived heavy oil is primarily composed of aliphatic compounds, including carboxylic acids, ketones, aldehydes, alcohols, alkanes, and esters, among others, with esters being the most abundant. The preparation process for bio-asphalt does not result in the formation of new functional groups, indicating that the sludge-derived heavy oil undergoes physical blending rather than a chemical reaction with 50^# asphalt.

(3) Regarding the three indexes, as the sludge-derived heavy oil content increases, the penetration of bio-asphalt increases while its softening point decreases; additionally, its ductility remains above 100. When the sludge-derived heavy oil content ranges from 5% to 10%, the basic performance of the bio-asphalt meets the specifications for 70^# asphalt. When the sludge-derived heavy oil content ranges from 15% to 20%, the basic performance of the bio-asphalt meets the specifications for 90^# asphalt.

(4) Bio-asphalt exhibits good aging resistance and storage stability when sludge-derived heavy oil content ranges from 5% to 20%. The creep stiffness and creep rate both indicate that the low-temperature performance of bio-asphalt improves as the sludge-derived heavy oil content increases. Both the rutting factor and Jnr indicate that the high-temperature performance of bio-asphalt decreases to some extent as the sludge-derived heavy oil content increases. Correlation analysis reveals that the influence of sludge-derived heavy oil content on the high-temperature performance of bio-asphalt outweighs its effect on other performance parameters. Therefore, in practical applications, priority should be given to considering the impact of sludge-derived heavy oil on the high-temperature performance of bio-asphalt, with the optimal range being 5% to 10%.