Research on Pavement Performance of Steel Slag Asphalt Mastic and Mixtures

Abstract

In order to explore the influence of steel slag on the road performance of asphalt mastic and its mixtures, steel slag powder (SSP) and steel slag aggregate (SSA) were used to replace limestone mineral powder filler (MF) and natural limestone aggregate (LA) respectively to prepare asphalt mastic and mixture. A series of standardized tests including penetration, softening point, ductility, viscosity, pull-off strength, dynamic shear rheometer (DSR), and bending beam rheometer (BBR) were carried out to evaluate the performance of asphalt mastics with SSP. Meanwhile, high- and low-temperature performance, moisture stability, volumetric stability, and fatigue resistance were evaluated by wheel tracking, uniaxial penetration strength, Hamburg, three-point bending, freeze–thaw splitting, immersed Marshall stability, water immersion expansion, and two-point bending trapezoidal beam fatigue tests. The results show that compared to the asphalt mastic with MF, enhanced high-temperature deformation resistance and reduced low-temperature cracking resistance of asphalt mastic with SSP were observed, as well as superior aging resistance. The improvements in high-temperature stability, moisture resistance, and fatigue performance were confirmed for asphalt mixtures with SSP/SSA. Additionally, compromised volumetric stability and low-temperature crack resistance were found when SSP/SSA was used in mixtures. Although asphalt mixtures with SSA exhibited 257.79%–424.60% higher expansion rate after 21-day immersion than those with LA, the 3-day immersion expansion rates complied with specification limits (<1.5% per JTG F40-2004). Critical volume expansion control measures should be adopted for full-component applications of steel slag powder/aggregates due to the hydration potential of free lime (f-CaO) and magnesium oxide (MgO) in steel slag under moisture exposure.

1. Introduction

Steel slag, as a highly promising road construction material, exhibits superior physical and mechanical properties unattainable by natural aggregates, positioning it as an ideal secondary resource. In recent years, extensive research has been conducted globally on its engineering applications, yielding significant advancements. Lin et al. [1] revealed through characterization studies that steel slag aggregates demonstrate an approximately three-fold higher hardness compared to natural limestone aggregates, with crushing values and abrasion loss indices being 25%–40% superior. Bessa et al. [2] conducted an evaluation of steel slag’s abrasion resistance. It revealed that the superior angularity and enhanced hardness compared to conventional granite and limestone aggregates engender steel slag’s exceptional wear resistance. Li et al. [3] found through 3D scanning and texture parameter analysis that the 3D texture parameters (Sq, Vmc) of steel slag aggregates were significantly better than those of conventional aggregates. Its high surface roughness and stable core support volume endow asphalt pavement with excellent anti-skid durability, with wear resistance improved by more than 20%, and the texture attenuation law is controllable. E Guangxun et al. [4] investigated the variation patterns of the skeleton void ratio in steel slag coarse aggregates. Their results revealed a multivariate nonlinear relationship between the skeleton void ratio and the proportion of different particle size fractions within the coarse aggregates. Similar patterns were observed for both basalt coarse aggregates and hybrid coarse aggregates composed of steel slag and basalt, indicating that the interstitial void characteristics of composite aggregate systems are governed by analogous gradation-dependent mechanisms. Asi M [5] et al. investigated the performance of asphalt mixtures prepared by partially replacing coarse limestone aggregates larger than 4.75 mm with steel slag aggregates at varying dosages. The results demonstrated that the asphalt content increased correspondingly with the dosage of steel slag aggregates. Notably, when 30% steel slag was incorporated, the mixture exhibited significantly enhanced skid resistance. Furthermore, at a steel slag replacement level of 75%, comprehensive improvements were observed in multiple performance characteristics of the asphalt mixture, achieving substantial optimization in overall material properties. GOLI H et al. [6] conducted a study on the road performance of steel slag asphalt mixtures with varying substitution ratios. Their findings revealed that the incorporation of steel slag aggregates led to an increase in asphalt content. Notably, the mixture achieved optimal road performance when 25% of the coarse limestone aggregates were replaced by steel slag. Chenet al. [7] found that compared with basalt, the micro-interface phase between steel slag and asphalt was more continuous and uniform. Separately, Liu et al. [8] investigated the enhancement mechanisms of skid resistance in asphalt mixtures by substituting basalt aggregates with steel slag aggregates larger than 2.36 mm. Their mechanistic analysis demonstrated that the application of steel slag aggregates endowed the mixture with enhanced surface textural characteristics, thereby significantly improving its skid resistance properties. Yan et al. [9] prepared steel slag asphalt mixtures by volumetrically replacing basalt aggregates with coarse and fine steel slag aggregates, followed by comparative analysis with basalt aggregate mixtures. Their study revealed that coarse steel slag aggregates effectively enhanced the high-temperature performance and water stability of the mixture; however, this resulted in a deterioration of low-temperature performance. Conversely, the incorporation of fine steel slag aggregates improved the low-temperature performance but adversely affected the high-temperature performance and water stability. Additionally, both types of steel slag aggregates were observed to reduce the volumetric stability of the asphalt mixtures. Shen et al. [10] found that the complex surface texture and pore characteristics of steel slag improved its adhesion to asphalt, thereby enhancing the anti-slip performance of steel slag asphalt mixtures. Wang et al. [11] investigated the effects of steel slag incorporation ratio, asphalt-to-aggregate ratio, and steel slag particle size on the expansion rate of asphalt mixtures. Their findings demonstrated that the expansion behavior could be effectively controlled under the following conditions: steel slag content not exceeding 50%, the actual asphalt-to-aggregate ratio being maintained at 1.5% above the optimum ratio, and the employment of larger-sized steel slag aggregates. Zhang et al. [12] fabricated asphalt mixtures by volumetrically replacing basalt fine aggregates with steel slag fine aggregates. Experimental results revealed that the high-temperature stability and moisture stability of the mixtures initially increased but subsequently decreased with steel slag incorporation. Conversely, both low-temperature crack resistance and volumetric stability progressively declined as the steel slag fine aggregate content increased. Based on systematic testing, the study recommended that the substitution ratio of steel slag fine aggregates should be limited to 36% to achieve balanced performance outcomes. Dong [13] investigated the performance and fatigue damage of steel slag fine aggregates in asphalt mixtures. The study revealed that incorporating steel slag fine aggregates moderately enhances the mixture’s high-temperature stability, low-temperature crack resistance, fatigue life, and skid resistance. However, it concurrently increases the water immersion-induced expansion rate. Based on a comprehensive performance evaluation, the optimal substitution ratio of steel slag fine aggregates in AC-20 asphalt mixtures is recommended as 75%, balancing mechanical enhancements with volumetric stability constraints. Kong D et al. [14] demonstrated through systematic research that steel slag powder particles exhibit richer micro-textural characteristics compared to mineral powder, with an approximately 15% higher angularity index. Additionally, parameters such as the angularity index, sphericity, and surface texture of steel slag powder were found to vary depending on the source and production processes of the steel slag aggregates, highlighting the material’s inherent variability influenced by manufacturing origins and techniques. Li et al. [15] conducted a study on the partial replacement of mineral powder filler with steel slag powder in asphalt mixtures. The findings revealed that the rough surface texture and abundant silicate mineral composition of steel slag powder enhanced its compatibility with asphalt, thereby improving the adhesive bond between asphalt and aggregates. Fu et al. [16] systematically evaluated the performance of steel slag-modified asphalt mastic. A comparative analysis with conventional limestone filler-based asphalt mastic demonstrated that steel slag powder improved the viscosity and high-temperature performance of the asphalt mastic. However, this modification resulted in reduced low-temperature deformation capability and increased temperature susceptibility. The study further identified an optimal filler–bitumen ratio of 1.0, at which the steel slag asphalt mastic achieved balanced comprehensive performance characteristics. Liu [17] suggested through research that the special surface properties of steel slag powder can improve the road performance and durability of asphalt mixtures. Xiao et al. [18] demonstrated that the incorporation of steel slag powder enhances the adhesive bonding between asphalt and aggregates, whereas excessive dosage was observed to compromise the moisture stability of mixtures. Concurrently, Chen et al. [19] conducted a comprehensive rheological characterization of steel slag powder (SSP)-modified bituminous mastic by employing dynamic shear rheometry (DSR). Parallel scanning electron microscopy (SEM) analyses revealed SSP’s distinct microtextural advantages over limestone filler. It is concluded that the adhesion of SSP mastics is better than that of limestone filler asphalt mastic. Li et al. [20] investigated the rheological influence of steel slag powder on asphalt mastics, revealing its capacity to improve viscosity and rutting factor. Notably, the low-temperature performance of steel slag-modified asphalt mastics was found to be comparable to that of conventional mineral filler systems. Sun et al. [21] found that steel slag micro-powder can improve the adhesion between asphalt and aggregates, enhance the water stability and high-temperature stability of asphalt mixture, and increase the rutting resistance of asphalt pavement. Furthermore, researchers have conducted in-depth investigations into the application of steel slag aggregates in asphalt mixtures, encompassing interfacial mechanisms [22,23,24], adhesion characteristics [25,26,27,28,29], and cracking behavior [30,31] at the aggregate–asphalt interface. The pursuit of resource recycling and value-added applications for steel slag materials has remained a persistent research focus globally [32,33,34,35,36,37,38,39], driven by the imperative to reconcile industrial byproduct utilization with sustainable pavement engineering requirements. The resource recycling and value-added applications of steel slag materials have remained a persistent research focus globally. However, current studies and implementations predominantly concentrate on substituting conventional stone aggregates (e.g., limestone/basalt) with steel slag in coarse or fine aggregate forms, with most applications limited to steel slag coarse aggregates in asphalt mixtures. The utilization of steel slag fine aggregates remains underdeveloped in asphalt mix design. Furthermore, research on steel slag powder filler has largely been confined to its rheological effects on asphalt mastics, while minimal attention has been given to full-component steel slag asphalt mixtures where steel slag powder replaces traditional mineral fillers. The feasibility of multi-element, multi-component cascade utilization of steel slag materials (coarse/fine aggregates and powder filler) in asphalt mixtures requires further systematic investigation to establish integrated application frameworks.

This study systematically evaluates and characterizes the performance evolution of steel slag powder-modified asphalt mastics and their corresponding mixtures through a multi-scale experimental framework. At the mastic level, investigations include conventional performance tests (penetration, softening point, ductility) and advanced rheological assessments using dynamic shear rheometry (DSR) and bending beam rheometry (BBR). For full-component mixture validation, comprehensive evaluations were conducted, encompassing high-temperature rutting resistance, low-temperature cracking resistance, moisture stability via Hamburg wheel-tracking and freeze–thaw splitting, fatigue life quantification through four-point bending tests, and volumetric stability monitoring with expansion rate measurements. The integrated experimental approach establishes critical correlations between steel slag composition, interfacial interactions, and macroscopic pavement performance, thereby providing a methodological foundation for achieving the full-component cascade utilization of steel slag materials in sustainable asphalt mixtures.

2. Materials and Experimental

2.1. Materials

2.1.1. Asphalt Binder and Aggregates

In current highway construction practices, the application of steel slag asphalt mixtures is predominantly applied to the surface course of asphalt pavements. This study adopts material specifications from an operational highway project in Shandong Province, utilizing SBS-modified asphalt binder (PG 76-22), with performance evaluation results detailed in

Table 1. Test results of SBS-modified asphalt binder properties.

Test Project			Test Values	Threshold Values
Penetration (100 g, 5 s, 25 °C)/0.1 mm			53	40~60
Softening point (5 °C)/°C			83.5	≥60
Ductility (5 cm/min, 5 °C)			32	≥20
Rotational viscosity at 135 °C (Pa·s)			1.8	≤3
Flash point (°C)			326	≥230
Solubility in trichloroethylene (%)			99.92	≥99
Elastic recovery at 25 °C (%)			93	≥75
Storage stability (phase separation assessed by 48-h softening point difference) (°C)			0.3	≤2.5
Residue after thin film oven test (TFOT, 163 ± 1 °C, 5.5 ± 1 r/min, 5 h)	Mass loss (%)		0.009	±1.0
	Penetration ratio at 25 °C (%)		75	≥65
	Ductility at 5 °C (cm)		19	≥15
Original SBS-modified asphalt	Rutting factor G*/sinδ (kPa)	76 °C	1.995	≥1.0
		70 °C	3.326
Residue after rolling thin film oven test (RTFOT, 163 ± 0.5 °C, 15 ± 0.2 r/min, 85 min)	Mass loss (%)		−0.049	±1.0
	Rutting factor G*/sinδ (kPa)	2.789	2.789	≥2.2
		4.553	4.553
Residue after pressure-aging vessel (PAV, 90~110 °C, 2.1 MPa, 20 h)	Aging temperature (°C)		100 °C
	Fatigue factor G*/sinδ (kPa)	31 °C	643.2	≤5000
		28 °C	990.7
	Bending creep stiffness modulus S (MPa)	−6 °C	85.6	≤300
		−12 °C	132
	Creep rate m-value	−6 °C	0.432	≥0.30
		−12 °C	0.369

. The technical indicators of the bitumen meet the requirements of standard JTG-F40-2004 [40].

The crushed limestone and steel slag (as shown in Figure 1) were used as aggregates, of which the technical indicators meet the requirements of standards JTG-F40-2004 [40] and GBT 25824-2010 [41], respectively, as shown in

Table 2. Test results of coarse aggregates.

Test Project	The Size of Steel Slag Aggregate/mm		The Size of Limestone Aggregate/mm		Threshold Values
	10~15	5~10	10~15	5~10
Apparent specific gravity	3.33	3.34	/	/	≥2.9
	/	/	2.742	2.732	≥2.6
Water absorption (%)	2.25	2.32	/	/	≤3.0
	/	/	0.69	0.54	≤2.0
Aggregate crushing value (%)	10.1	/	20.3	/	≤26
Soundness (%)	2	2	3	4	≤12
Los Angeles abrasion loss (%)	9.5	10.4	/	/	≤26
	/	/	20.2	22.3	≤28
Polished stone value (PSV)	66	/	44	/	≥42
Flakiness and elongation index (≥9.5 mm particles) (%)	4.5		9.9		≤12
Material finer than 0.075 mm by washing (%)	0.3	0.6	0.5	0.6	≤1
Soft particles content (%)	0.3	0.8	1.5	1.9	≤3
Adhesion to asphalt (level)	5		5		≥4
Free calcium oxide (f-CaO) content (%)	2.14	1.48	/	/	≤3
Expansion rate (%)	0.82		/	/	≤1.8

and

Table 3. Test results of fine aggregates.

Test Project	Steel Slag	Limestone	Threshold Values
Apparent specific gravity	3.314	/	≥2.9
	/	2.728	≥2.5
Water absorption (%)	2.38	1.07	/
Free calcium oxide (f-CaO) content (%)	1.14	/	/

.

2.1.2. Filler

The limestone ore powder and steel slag powder were used as the filler of the asphalt mixture. Steel slag powder is obtained by grinding. A predetermined mass of steel slag fine aggregates (0–5 mm particle size range) was weighed and subjected to mechanical grinding in a ball mill. The milled product was subsequently sieved through a series of standard sieves (2.36 mm, 1.18 mm, 0.6 mm, 0.3 mm, 0.15 mm, and 0.075 mm apertures). The residual materials from 0.15 mm and 0.075 mm standard sieves and the materials at the bottom of the sieves were mixed according to the particle size range of the mineral powder. The steel slag powder consistent with the particle size range of the mineral powder was obtained. The processed steel slag powder underwent rigorous performance characterization in compliance with Test Methods of Aggregates for Highway Engineering (JTG 3432-2024) [42], with key test results summarized in

Table 4. Performance test results of steel slag powder/mineral powder filler.

Test Project		Limestone Mineral Powder	Steel Slag Power	Threshold Values
Apparent specific gravity		2.705	3.301	≥2.5
Water content (%)		0.42	0.63	≤1
Size range	<0.6 mm (%)	100	100	100
	<0.15 mm (%)	97.3	98.2	90~100
	<0.075 mm (%)	87.8	91.8	75~100
Appearance		No agglomeration	No agglomeration	No agglomeration
Hydrophilic coefficient (%)		0.53	0.58	<1
Specific surface area (m2/g)		0.78	1.80	-

. The chemical compositions of steel slag powder and limestone mineral powder are compared in

Table 5. Chemical composition of fillers.

Chemical Composition Content (%)	CaO	SiO2	Fe2O3	MgO	MnO	P2O5	Al2O3
Steel slag power	47.61	16.74	20.67	6.25	5.14	1.85	1.74
Limestone mineral powder	73.65	16.14	0.55	2.64	0.49	3.98	2.55

.

2.1.3. The Preparation of Asphalt Mastic

Asphalt mastics were prepared by adding filler (limestone ore powder, steel slag powder) to asphalt. Steel slag powder replacing limestone mineral filler was added into SBS-modified asphalt. The substitution percentage of limestone ore powder filler with steel slag powder by weight was from 0 to 100%. The preparation process of asphalt mastic was as follows (Figure 2):

• Binder Preparation: SBS-modified asphalt was homogenized in a 165 °C oven until achieving fluid state, then poured into a stainless steel mixing tank (volume 500 ± 5 mL) with a certain mass (800 g per piece) and cooled to ambient temperature.
• Filler Treatment: Both the experimental limestone mineral filler and steel slag powder were dried in an oven at 105 °C to constant mass and cooled to room temperature prior to mixing. The two kinds of materials after drying and cooling were individually weighed according to the replacement mass ratio of the test design.
• Mastic Fabrication:

  ○

  The SBS-modified asphalt sample prepared in the first step was reheated to the flowing state in the oven at 165 °C.

  ○

  Then, it was placed in the heating sleeve of the base of the asphalt high-speed mixer (the temperature of the base heating sleeve was set to 175 ± 5 °C).

  ○

  The high-speed mixer (equipped with 20 mm four-blade inclined rotor) was initiated and allowed to stir for a certain time until the temperature of the SBS-modified asphalt in the stainless steel mixing tank was stable.

  ○

  Then, the limestone mineral filler (MF) was added at three times, and the four-blade inclined rotor remained stirring during the addition process.

  ○

  When adding steel slag powder as replacement for mineral filler, the steel slag powder should be added according to the adding process of the limestone mineral filler after it is stirred until no limestone ore powder float is visible to the eye in the asphalt.

  ○

  After the steel slag powder was added, it was stirred for 90 min at the stirring rate of 800 r/min to ensure that the fillers were evenly distributed in the SBS-modified asphalt.

In order to adhere to engineering practice and truly reflect the state of asphalt mastic, the filler-to-binder ratio (F/B) selected was controlled to 1.0 according to engineering experience.

Figure 2. Asphalt mastic preparation process.

Figure 2. Asphalt mastic preparation process.

2.1.4. Mix Design of AC-13

The validation of pavement performance for asphalt mixtures represents a critical phase in the engineering application of steel slag powder filler. Building upon previous investigations, this study fabricated four AC-13 asphalt mixture variants: limestone aggregate–mineral filler (LA-MF), limestone aggregate–steel slag powder filler (LA-SSP), steel slag aggregate–mineral filler (SSA-MF), and steel slag aggregate–steel slag powder (SSA-SSP). The MF in LA-MF and SSA-MF is 100% limestone mineral powder filler, and the SSP in LA-SSP and SSA-SSP is 100% steel slag powder filler. The design of the asphalt mixture adopts the Marshall volume method according to specification JTG-F40-2004 [40]. The gradation composition of the asphalt mixtures of different aggregate types is shown in

Table 6. Percent passing of the composite gradation.

Gradation Type	Sieve Size (mm)
	16	13.2	9.5	4.75	2.36	1.18	0.6	0.3	0.15	0.075
LA-MF	100.0	96.6	74.7	44.1	31.4	23.8	15.8	10.1	7.4	5.8
LA-SSP	100.0	96.6	74.7	44.1	31.4	23.8	15.8	10.1	7.2	5.5
SSA-MF	100	96.7	74.3	43.6	30.2	23.2	15.6	10.3	7.6	5.3
SSA-SSP	100	96.7	74.3	43.6	30.2	23.2	15.6	10.2	7.2	5.4

. The volumetric parameters and optimized asphalt content (OAC) for each asphalt mixture type are summarized in

Table 7. The volume parameters under the optimum asphalt content of different types.

Gradation Type	Optimum Asphalt Content (%)	$\mathbf{Bulk} \mathbf{Specific} \mathbf{Gravity} {\mathit{\gamma }}_{\mathit{f}}$	Percent Air Void VV (%)	Percent Voids in Mineral Aggregate VMA (%)	Percent Voids Filled with Asphalt VFA (%)	Marshall Stability (kN)	Flow Value (mm)
LA-MF	4.8	2.400	4.3	14.9	71.1	14.2	2.2
LA-SSP	4.8	2.416	4.2	14.4	70.8	15.94	3.1
SSA-MF	4.6	2.762	4.4	14.0	68.6	16.12	2.9
SSA-SSP	4.5	2.782	4.5	14.0	67.9	16.3	2.6
Threshold values	/	/	4~6	14~16	65~75	≥8	1.5~4

. As shown, all Marshall volumetric parameters of the designed mixtures comply with the prescribed specification thresholds of JTG-F40-2004 [40]. To ensure comparative validity across mixture groups, comparable percent air voids were systematically maintained during the mix design phase.

2.2. Test Methods

2.2.1. Asphalt Mastic Performance Test Methods

Penetration, Softening Point, Ductility and Viscosity Index

The penetration, softening point, ductility, and viscosity are the basic properties of asphalt mastic. Here, 0, 25%, 50%, 75%, and 100% substitution percentages of mineral filler with steel slag powder by weight were designed to evaluate the effect of steel slag powder on the asphalt mastic. The tests were carried out according to the test specification (JTG E20-2011) [43].

Pull-Off Test

For evaluating the interfacial bond strength between asphalt mastic and aggregate, the pull-off test was carried out according to the standard specification ASTM D4541-22 [44]. Here, 0, 25%, 50%, 75%, and 100% substitution percentages of mineral filler with steel slag powder by weight were designed. A drawing die spindle with a diameter of 20 mm and a limestone slab with a surface polishing degree of 1000 are required for the bond strength test according to ASTM D4541-22 [44].

Dynamic Shear Rheometer (DSR)

The high-temperature rheological behaviors of asphalt mortar before and after the rolling thin film oven test (RTFOT) aging were tested individually by dynamic shear rheometry (DSR), a widely recognized method for assessing pavement performance under thermal loading. The test methods and parameters of RTFOT and DSR were carried out in accordance with the Chinese technical specification JTG E20-2011 [43]. This study employed an automated DSR system to obtain two critical viscoelastic parameters: the dynamic shear modulus (G*) and phase angle (δ) under 5 test temperature, 58 °C, 64 °C, 70 °C, 76 °C, and 82 °C. These metrics, measured under controlled oscillatory shear conditions, serve as fundamental indicators of binder resistance to permanent deformation at elevated service temperatures.

Bending Beam Rheometer (BBR)

The low-temperature rheological characteristics of asphalt materials are commonly assessed through bending beam rheometry (BBR), a standardized methodology for characterizing thermal stress resistance in pavement engineering. In accordance with the Chinese technical specification JTG E20-2011 [43], this investigation utilized an automated BBR system to quantify two critical parameters: creep stiffness (S) and m-value (creep rate). To simulate extreme climatic conditions in cold regions, testing temperatures were established at −6 °C, −12 °C, and −18 °C. Under controlled loading conditions, data acquisition was performed at a standardized duration of 60 s to determine the S- and m-values, which serve as essential predictors of asphalt’s susceptibility to thermal cracking.

2.2.2. Asphalt Mixture Performance Test Methods

A comprehensive performance evaluation protocol was implemented to characterize and compare key functional properties, including high-temperature performance, low-temperature performance, moisture stability, volumetric stability, and fatigue resistance. The test parameters of the asphalt mixture performance are presented in

Table 8. The test parameters of asphalt mixture performance.

Performance Parameter	Test Methods	Test Condition	Specification
High-temperature performance	Wheel-tracking test	60 °C, 1600 times of rolling	JTG E20-2011 [43]
	uniaxial penetration strength test	60 °C	JTG E20-2011 [43]
	Hamburg wheel-tracking test	50 °C, 20,000 times of rolling	AASHTO T324-2014 [45]
Low-temperature performance	Three-point bending test	−10 °C	JTG E20-2011 [43]
Moisture stability	freeze–thaw splitting test	−18 °C, freeze–thaw treatment for 16 h	JTG E20-2011 [43]
	immersed Marshall stability test	60 °C, 48 h immersion treatment	JTG E20-2011 [43]
Volumetric stability	Water immersion expansion test	60 °C, 72 h immersion treatment	JTG E20-2011 [43]
Fatigue resistance	Two-point bending trapezoidal beam fatigue test	10 °C, 25 Hz, 130 με	CSN EN 12697-24 [46]

. The specimen dimensions, test conditions, and testing temperatures for the asphalt mixtures are all carried out in accordance with the requirements of the corresponding standard specifications.

3. Results and Discussions

3.1. Performance of Asphalt Mastic with Steel Slag Powder

3.1.1. Penetration, Softening Point, Ductility, Viscosity, and Bonding Strength

The analysis of the conventional performance metrics of asphalt mastics (Figure 3) reveals a consistent trend: as the proportion of steel slag powder (SSP) replacing mineral powder filler (MF) increases, the penetration and ductility of the mastic decrease, while the softening point and rotational viscosity exhibit progressive enhancements. Specifically, the full replacement of MF with SSP (100% substitution) results in a 10.21% reduction in penetration, 14.13% decline in ductility, and 3.24% and 75.31% increases in softening point and rotational viscosity, respectively. These findings indicate that asphalt mastics with SSP develop enhanced cohesion, rigidity, and shear resistance due to intensified interfacial interactions, albeit at the expense of reduced plastic deformation capacity and low-temperature ductility.

The mechanistic rationale lies in the complex surface texture and elevated surface roughness of SSP particles compared to conventional limestone mineral fillers. These morphological characteristics amplify larger effective adhesion area at asphalt–SSP interfaces, promoting stronger physicochemical bonding [17,24]. Such interactions restrict the mobility of asphalt molecules, thereby increasing the mastic’s viscosity and thermal stability (evidenced by elevated softening points and rotational viscosity) while diminishing its capacity for plastic flow (reflected in reduced penetration and ductility).

Figure 4 demonstrates the progressive enhancement of interfacial bond strength in asphalt mastics with incremental SSP replacement of MF. Quantitative evaluations reveal a linear correlation, where bond strength increases from 1.28 MPa for MF-based mastic (0% SSP) to 1.53 MPa at full SSP replacement (100%), representing a 19.53% improvement. This trend underscores SSP’s efficacy in optimizing asphalt–aggregate interfacial adhesion through enhanced physicochemical interactions. This pronounced improvement in interfacial adhesion is attributed to SSP’s enhanced asphalt adsorption capacity, driven by its unique physicochemical characteristics. The rough, microporous surface texture and larger specific surface area (BET: 1.8 m²/g vs. 0.78 m²/g for MF) of SSP particles promote preferential adsorption of asphalt components [17], facilitating the formation of a thicker structural asphalt layer at filler–asphalt interfaces compared to conventional mineral fillers.

3.1.2. High-Temperature Rheological Property

The complex shear modulus (G*), a critical rheological parameter, quantifies the resistance of asphalt binders and mastics to shear deformation under oscillatory loading, with higher G* values indicating superior deformation resistance. The analysis of the dynamic shear rheometry (DSR) temperature sweep results in Figure 5 demonstrates that all three specimens—steel slag powder-modified asphalt mastic (SSP-MAM), mineral filler-based asphalt mastic (MF-MAM), and SBS-modified asphalt (SBS-MA)—exhibit a consistent decline in G* with increasing temperature. However, at equivalent temperatures, the G* magnitudes follow the hierarchy: SSP-MAM > MF-MAM > SBS-MA. This trend substantiates that SSP-MAM possesses the highest load-deformation resistance, surpassing both conventional filler-modified and polymer-modified systems. The substitution of mineral filler with steel slag powder thereby enhances the high-temperature performance of asphalt composites through amplified viscoelastic stiffness. The incorporation of mineral filler or steel slag powder filler into asphalt induces physicochemical interactions that promote the formation of high-stiffness structural asphalt, thereby enhancing the composite’s shear deformation resistance. Owing to the superior interfacial activity of steel slag powder [17]—attributed to its enhanced surface roughness, elevated specific surface area (BET: 1.8 m²/g vs. mineral filler’s 0.78 m²/g), and alkaline composition (CaO/SiO₂ ratio > 2.5)—the asphalt mastic with SSP exhibits the highest complex shear modulus (G*) among all tested systems.

Figure 6 compares the change in complex shear modulus (ΔG*) for three material systems before and after RTFOT aging. The change in complex shear modulus (ΔG*) before and after aging is an important rheological index to evaluate its anti-aging performance. The greater the value, the more serious the hardening caused by aging and the worse the anti-aging performance [47]. The result reveals a more pronounced increase in ΔG* for the asphalt mastic with limestone powder mineral filler (MF-MAM) compared to the asphalt mastic with steel slag powder (SSP-MAM), with aged MF-MAM exhibiting significantly higher G magnitudes* than its SSP-MAM counterpart. It indicates that SSP-MAM shows superior aging resistance—a critical attribute for long-term pavement durability.

The phase angle (δ), a fundamental parameter in viscoelastic material characterization, quantifies the ratio of viscous to elastic constituents within the material’s mechanical response. Governed by the principles of linear viscoelasticity, δ directly reflects the material’s capacity to resist deformation under cyclic loading. The smaller the δ, the more elastic components there are in the viscoelastic material, and the stronger its ability to resist deformation [48].

Figure 7 delineates the temperature-dependent evolution of the phase angle (δ) for the three material systems. While subtle disparities exist in their δ–temperature trajectories, all specimens exhibit consistent trends before and after aging. Notably, SBS-modified asphalt (SBS-MA) exhibits a monotonic increase in δ with rising temperatures, indicating a progressive dominance of viscous dissipation over elastic recovery. In contrast, both mineral filler mastic (MF-MAM) and steel slag powder mastic (SSP-MAM) demonstrate reduced δ values within the critical 58–70 °C temperature range before and after aging, reflecting the elastic components in the asphalt mastic increase while the viscous components decrease [48]. This elastic predominance enhances the mastics’ recoverable deformation capacity, which is beneficial to the anti-rutting performance of asphalt mixtures. Under isothermal conditions, the phase angle hierarchy follows SSP-MAM < MF-MAM < SBS-MA, indicating that SSP-MAM possesses the highest proportion of elastic constituents and superior deformation resistance. The attenuated δ values of SSP-MAM (e.g., δ = 49.2° vs. SBS-MA’s 65.0° at 64 °C) validate its enhanced improvement efficacy over conventional mastic systems with mineral filler.

The rutting factor (G*/sinδ) serves as a critical indicator of the high-temperature rutting resistance of asphalt materials, with higher values signifying superior performance. As demonstrated in Figure 8, the temperature-dependent evolution of G*/sinδ for SBS-modified asphalt (SBS-MA) and its mastic derivatives follows a trend analogous to that of the complex shear modulus (G*): all systems exhibit a progressive decline in G*/sinδ with increasing temperature. Notably, the rate of reduction is more pronounced below 70 °C due to accelerated thermal softening of the asphalt matrix, whereas the descent plateaus above 70 °C as the binder transitions into a fully softened state. Under isothermal conditions, the G*/sinδ hierarchy is SSP-MAM > MF-MAM > SBS-MA, confirming that: Filler incorporation significantly enhances high-temperature rutting resistance by amplifying the elastic response and delaying viscous flow [49]. Steel slag powder demonstrates superior efficacy over mineral filler, attributed to its enhanced interfacial adhesion.

3.1.3. Low-Temperature Rheological Property

The creep stiffness modulus (S) and creep rate (m-value), derived from bending beam rheometry (BBR) tests under low-temperature conditions, serve as critical indicators of asphalt and asphalt mastic low-temperature performance. These parameters quantify the material’s deformation resistance and stress relaxation capacity, respectively. Specifically, lower creep stiffness (S) correlates with enhanced flexibility, reducing the risk of brittle fracture under thermal contraction. A higher m-value reflects superior stress dissipation capability. The lower the creep stiffness modulus s is and the higher the creep rate m is, the stronger the material’s resistance to deformation and stress relaxation will be, and the lower the probability of low-temperature cracking caused by temperature stress will be [48].

The analysis of the low-temperature bending test results for asphalt and mastic materials in Figure 9 and Figure 10 reveals consistent trends in the temperature-dependent evolution of creep stiffness modulus (S) and creep rate (m-value) for three types of materials. Specifically, creep stiffness (S) increases with decreasing temperature, reflecting reduced material flexibility under cryogenic conditions, while creep rate (m-value) exhibits an inverse correlation, declining as temperatures drop, indicative of attenuated stress relaxation capacity.

The hierarchical performance rankings derived from low-temperature bending tests demonstrate distinct viscoelastic behaviors among the evaluated materials: creep stiffness (S) follows SSP-MAM > MF-MAM > SBS-MA, while the creep rate (m-value) exhibits an inverse hierarchy of SBS-MA > MF-MAM > SSP-MAM. This inverse relationship demonstrates that the incorporation of mineral or steel slag fillers detrimentally impacts the low-temperature crack resistance of asphalt mastics. Notably, steel slag powder exerts a more pronounced adverse effect, attributed to its elevated rigidity (S increased by 18%–22% vs. mineral filler) and restricted stress dissipation capability (m-value reduced by 12%–15% vs. mineral filler) within the composite matrix. The addition of SSP/MF enhances the viscosity and stiffness of the asphalt mortar, reduces its low-temperature deformation resistance and stress relaxation ability, and makes it exhibit strong brittleness in low-temperature environments. Due to its own characteristics, SSP has a stronger interaction with asphalt, which leads to a greater impact of steel slag powder fillers on its low-temperature performance than MF. The elevated rigidity and constrained stress dissipation capability in SSP-MAM relative to MF-MAM highlight the critical trade-off between filler adhesion efficacy and mitigating stress accumulation capability, necessitating compensatory strategies for cold-region pavement applications.

3.2. Performance Evaluation of Steel Slag Powder Asphalt Mixture

3.2.1. High-Temperature Performance

The wheel-tracking test, a standardized method simulating tire-pavement interactions under controlled conditions (AASHTO T 324 [45]), is widely employed to evaluate the high-temperature performance of asphalt mixtures. The analysis of dynamic stability (DS) and relative deformation rate (RDR) in Figure 11 reveals the following performance hierarchy: LA-SSP mixtures exhibit an 11.11% increase in DS compared to LA-MF mixtures, SA-SSP mixtures demonstrate a 6.65% DS enhancement over SSA-MF counterparts, SSA-MF mixtures achieve 20% higher DS than LA-MF mixtures, and SSA-SSP mixtures show a 25.02% DS improvement relative to LA-SSP mixtures.

Concurrently, the RDR decreases by 14.60%, 1.86%, 16.81%, and 4.40% for these respective comparisons, confirming that steel slag powder (SSP) and steel slag aggregate (SSA) significantly enhance the mixtures’ resistance to permanent deformation. This improvement stems from SSP/SSA’s superior angularity, elevated hardness (LA abrasion loss < 11% vs. >20% for limestone aggregates), and enhanced interfacial bonding with asphalt, which collectively restrict shear flow and stabilize the viscoelastic matrix under repetitive loading.

Figure 12 presents the results of the uniaxial penetration strength tests for asphalt mixtures. The comparative analysis of the data reveals the following performance enhancements: LA-SSP mixtures exhibit an 18.02% increase in uniaxial penetration strength compared to LA-MF mixtures, SSA-SSP mixtures demonstrate an 8.39% improvement over SSA-MF mixtures, SSA-MF mixtures achieve a 28.83% higher uniaxial penetration strength than LA-MF mixtures, and SSA-SSP mixtures show an 18.32% enhancement relative to SA-SSP mixtures. These results conclusively validate that the incorporation of steel slag powder (SSP) and steel slag aggregate (SSA) significantly improves the high-temperature shear resistance of asphalt mixtures.

Figure 13 and Figure 14 present the Hamburg wheel-tracking test (HWTT) results for asphalt mixtures under 50 °C water immersion, a method that comprehensively evaluates high-temperature performance (via cumulative deformation) and moisture susceptibility (via stripping inflection point) under coupled thermo-hydro conditions. The stripping inflection point identifies the onset of moisture-induced damage, with values below 10,000 loading cycles indicating poor moisture resistance. It can be seen that cumulative deformation reductions were observed. Notably, LA-SSP mixtures show a 20.46% decrease in cumulative deformation after 20,000 cycles compared to LA-MF. SSA-SSP mixtures exhibit a 9.62% reduction relative to SSA-MF. SSA-MF mixtures achieve a 24.83% improvement over LA-MF. SSA-SSP mixtures demonstrate a 14.58% enhancement compared to LA-SSP. All designed mixtures exhibit stripping inflection points >10,000 cycles, confirming that steel slag powder (SSP) and steel slag aggregate (SSA) significantly enhance both high-temperature rutting resistance and moisture damage resistance.

The application of steel slag powder and steel slag aggregates significantly enhances the high-temperature stability of asphalt mixtures, a conclusion consistent with prior findings that steel slag powder-modified asphalt mastics exhibit higher G*/sinδ values compared to limestone filler-modified mastics, indicating superior high-temperature performance.

The mechanistic enhancement of asphalt mixture performance through steel slag utilization is attributed to enhanced binder-aggregate adhesion resulting from the rough surface texture and high porosity of steel slag, which amplify interfacial contact area and asphalt adsorption capacity [7,17]. Meanwhile, the optimized skeletal structure driven by the angular geometry of steel slag aggregates promotes interparticle interlocking during compaction, creating a densely packed load-bearing matrix resistant to shear deformation [2,3]. The reduction in permanent strain accumulation by 20%–32% under repetitive traffic loading in the Hamburg wheel-tracking test proves shear resistance amplification. These mechanisms synergistically improve the mixture’s resistance to rutting, validating steel slag’s efficacy as a sustainable alternative to conventional aggregates in high-temperature pavement applications.

3.2.2. Low-Temperature Performance

The low-temperature performance of asphalt mixtures, characterized by the maximum flexural tensile strain and flexural strength, was evaluated through bending beam tests at −10 °C (Figure 15). The comparative analysis of the maximum flexural tensile strain at −10 °C reveals systematic performance attenuations across the asphalt mixtures: LA-SSP mixtures exhibit a 3.54% reduction compared to LA-MF mixtures, while SSA-SSP mixtures show a 1.42% decrease relative to SSA-MF counterparts. Notably, SSA-MF mixtures demonstrate a 5.98% decline in maximum flexural strain versus LA-MF mixtures, and SSA-SSP mixtures display a further 3.92% reduction compared to LA-SSP mixtures. These results collectively highlight a progressive deterioration in low-temperature crack resistance with increasing steel slag content. The results align with prior findings on asphalt mastics with 100% SSP, which exhibited inferior stress relaxation capacity (lower m-values) at cryogenic temperatures (<−10 °C) compared to mastics with mineral filler.

The diminished low-temperature performance of steel slag-modified asphalt mixtures is mechanistically attributed to increased rigidity of mixtures with SSP/SSA, where SSP’s elevated creep stiffness (S increased by 18%–22%) synergistically restricts stress relaxation, amplifying stress accumulation. From a microscopic perspective, although the surface pores of steel slag aggregates can enhance the bonding performance of the asphalt–steel slag interface, its angular and porous structure can also induce stress concentration and be prone to crack initiation [50].

3.2.3. Water Stability

The moisture resistance of asphalt mixtures is typically characterized by the freeze–thaw splitting tensile strength ratio (TSR) and immersed residual stability ratio, with higher values indicating superior resistance to moisture-induced damage. Figure 16 and Figure 17 present the freeze–thaw splitting tensile strength ratio (TSR) and immersed residual stability results, respectively. The TSR improvements are as follows: LA-SSP mixtures exhibit a 1.79% increase compared to LA-MF mixtures, SSA-SSP mixtures demonstrate a 2.35% improvement over SSA-MF counterparts, SSA-MF mixtures achieve a 1.65% enhancement versus LA-MF mixtures, and SSA-SSP mixtures show a 2.21% gain relative to LA-SSP mixtures. Similarly, immersed residual stability enhancements include a 2.26% increase for LA-SSP over LA-MF, a 2.27% improvement for SSA-SSP compared to SSA-MF, a 2.36% gain for SSA-MF versus LA-MF, and a 4.68% rise for SSA-SSP relative to LA-SSP. While the observed improvements in TSR and residual stability are modest, they confirm that steel slag powder (SSP) and steel slag aggregate (SSA) moderately enhance the moisture resistance of asphalt mixtures. Additionally, both control and experimental groups exhibited significant increases in splitting strength and Marshall stability during testing, indirectly indicating improved deformation resistance under thermo-hydro-cyclic coupling conditions.

3.2.4. Volume Stability

The presence of unstable components such as free calcium oxide (f-CaO), magnesium oxide (MgO), and the RO phase (CaO-FeO-MnO solid solution) in steel slag powder/aggregates increases the risk of volumetric expansion in asphalt mixtures. Consequently, rigorous evaluation of volumetric stability is imperative to assess expansion characteristics during their engineering application. Figure 18 presents the long-term water immersion test results for various asphalt mixtures, revealing that the expansion rate of steel slag-modified mixtures progressively increases with prolonged immersion time. While the 3-day immersion expansion rates comply with specification limits (<1.5% per JTG F40-2004 [40]), significant differences emerge after 21 days: LA-SSP mixtures exhibit a 19.88% higher expansion rate than LA-MF mixtures, SSA-SSP mixtures show a 7.72% increase compared to SSA-MF mixtures, SSA-MF mixtures demonstrate a 424.60% surge relative to LA-MF mixtures, and SSA-SSP mixtures display a 257.79% rise versus LA-SSP mixtures.

These results confirm that steel slag aggregates exert a far more pronounced influence on volumetric expansion than steel slag powder. Mechanistically, steel slag aggregates—serving as the primary skeletal framework in the mixture—possess greater exposure to water, enabling sustained hydration reactions between f-CaO/MgO/RO phases and moisture. In contrast, steel slag powder, encapsulated within asphalt mastic and confined to interstitial voids, experiences limited water contact, thereby suppressing hydration-driven expansion.

3.2.5. Fatigue Performance

The fatigue resistance of asphalt mixtures represents one of the most comprehensive and stringent performance requirements in pavement engineering. To simulate real-world loading conditions, two-point bending fatigue tests were conducted on the designed mixtures at 10 °C, 25 Hz, and 130 µε, with results presented in Figure 19. Analysis reveals that while the incorporation of steel slag powder (SSP) and steel slag aggregate (SSA) reduces the initial stiffness modulus of the mixtures, it slightly enhances their fatigue life: LA-SSP mixtures exhibit a 2.81% increase in fatigue life compared to LA-MF mixtures, SSA-SSP mixtures show a 0.33% improvement over SSA-MF mixtures, SSA-MF mixtures achieve a 4.09% enhancement versus LA-MF mixtures, and SSA-SSP mixtures demonstrate a 1.58% gain relative to LA-SSP mixtures.

The enhanced fatigue performance of steel slag powder (SSP)- and steel slag aggregate (SSA)-modified asphalt mixtures is attributed to their unique morphological and interfacial characteristics. SSP exhibits a complex surface texture and elevated roughness, providing a greater effective adhesion area compared to conventional mineral fillers. Concurrently, SSA’s irregular porous structure facilitates the adsorption of additional asphalt mastic into voids within the coarse aggregate skeleton. Research indicates that stress concentrations in asphalt mixtures predominantly occur at mastic–coarse aggregate interfaces. The incorporation of SSP/SSA disperses these stress concentration zones, promoting the formation of distributed microcracks that gradually coalesce rather than propagating catastrophically. This microcrack dispersion mechanism reduces the overall stiffness modulus of SSP/SSA mixtures relative to limestone-mineral filler systems.

Under strain-controlled loading, the lower initial stiffness modulus of SSP/SSA mixtures reduces the applied stress required to maintain the target strain level, thereby decelerating crack propagation rates as per Paris’ law (da/dN = C(ΔK)ⁿ). The diminished stress intensity factor (ΔK) at crack tips prolongs fatigue life by extending the cycles required for critical crack growth. Consequently, SSP/SSA-modified mixtures exhibit superior fatigue resistance despite their reduced stiffness, demonstrating the efficacy of steel slag in balancing stiffness–ductility trade-offs for durable pavement design.

4. Conclusions

Based on the limited testing results, the following conclusion can be drawn.

(1) The softening point, 135 °C rotational viscosity and interfacial bonding strength indicators of asphalt mastics increase as the proportion of steel slag powder (SSP) replacing mineral powder filler (MF) increases, while the penetration and ductility indicators gradually decrease with the increase of substitution dosage. When the SSP replacement percentage reaches 100%, the softening point and the rotational viscosity at 135 °C increase by approximately 3.24%, 75.31% and 19.93% respectively, while the penetration and ductility decrease by approximately 10.21% and 14.13% respectively. Asphalt mastics with SSP demonstrate superior high-temperature rheological performance compared to limestone mineral filler mastic systems, albeit with compromised low-temperature rheological properties.

(2) The incorporation of steel slag materials enhances the high-temperature stability, moisture resistance, and fatigue resistance of asphalt mixtures. However, it introduces detrimental effects on low-temperature crack resistance and volumetric stability, primarily due to the hydration potential of free lime (f-CaO) and magnesium oxide (MgO) in steel slag under moisture exposure.

(3) All designed asphalt mixtures comply with the performance criteria outlined in the Technical Specifications for Highway Asphalt Pavement Construction (JTG F40-2004) [40] for Climate Zone 1-3-2 (hot summer, cold winter, humid region), achieving: high-temperature performance (dynamic stability ≥ 2800 cycles/mm), low-temperature flexibility (maximum flexural tensile strain ≥ 2500 µε), moisture resistance (freeze–thaw splitting tensile strength ratio ≥ 80%, immersed residual stability ratio ≥ 85%), and volumetric stability (expansion rate ≤ 1.5%). These results validate the mixtures’ conformity to critical pavement performance thresholds under the specified climatic and mechanical loading conditions.

While these mixtures satisfy conventional pavement performance requirements, full-component utilization of steel slag in engineering applications necessitates rigorous volumetric expansion control measures, such as pre-carbonation of slag or chemical stabilization, to mitigate long-term durability risks.