Studies on the Valorization of Aluminum Production Residues into Bituminous Materials at Different Scales: A Review

Abstract

To conserve natural resources and reduce waste generation, the effective valorization of industrial waste and byproducts in engineering applications is becoming increasingly important. Among these materials, aluminum production residues (APRs) offer a promising and sustainable solution for road pavement applications. Unlike previous reviews, this paper uniquely examines recent research on the use of various APRs in bituminous materials across multiple scales, with particular attention to their roles as additives and fillers. The APRs examined included red mud (RM), aluminum dross (AD), and spent pot lining (SPL) residues, as well as secondary aluminum waste (SAW). These materials have been employed as additives in asphalt binders (microscale), as fillers in asphalt mastics (mesoscale), and as additives or fillers in asphalt mixtures (macroscale). Overall, this review indicates that adopting appropriate treatment approaches for APRs as asphalt modifiers can enhance their dispersion, thermal stability, rheological behavior, and leaching performance. In particular, the use of RM has been shown to improve thermal stability, tensile strength, intermediate-temperature cracking resistance, and rutting resistance, largely due to the increased stiffness it imparts to asphalt mastic and mixture phases. However, there is no clear consensus among researchers regarding other properties, as performance outcomes depend strongly on multiple factors, particularly the physicochemical characteristics of the RM, filler–binder ratios, testing methods, and reference filler types. Other APRs—such as AD, SPL, and SAW—have also shown beneficial effects on the performance of asphalt mixtures. There is still limited research on the influence of APRs physicochemical variability on asphalt–filler interactions and the performance of bituminous materials. For the safe and large-scale adoption of APRs, it is essential to establish standardized characterization procedures, testing methods, and application guidelines while considering diverse climatic conditions. Comprehensive assessments of cost and environmental impacts should also be incorporated to support informed decision-making by engineers and industrial stakeholders.

1. Introduction

Today, rapid urbanization and industrialization generate enormous amounts of waste, leading to significant disposal challenges and harming the environment. Concurrently, the construction industry—particularly road pavement construction—consumes vast quantities of virgin raw materials, contributing to resource depletion and high energy consumption [1]. In response to these challenges, the valorization of waste materials in road pavements has emerged as a highly promising sustainable approach [2]. This strategy not only addresses the issue of waste disposal but also offers an opportunity to conserve natural resources, reduce energy consumption, and potentially lower construction costs. To this end, researchers have incorporated different waste materials—such as reclaimed asphalt pavement (RAP) [3], recycled concrete aggregate (RCA) [4], and crumb rubber (CR) [5]—as replacements for natural aggregates, as well as waste fillers—such as fly ash (FA) [6], ground granulated blast-furnace slag (GGBFS) [7], rice husk ash [8], brick dust (BD) [9], and borogypsum [10].

Driven by global industrial development and urbanization, increased demand for aluminum has led to a rise in the generation of byproducts [11]. Aluminum production residue (APR)—which includes byproducts from bauxite mining and refining operations as well as waste from post-consumer aluminum—has gained significant attention among researchers for valorization in construction materials, particularly road pavements, due to its high volume and potential usage as a sustainable resource [12].

Due to the growing interest in the use of waste materials in asphalt pavements, this research presents a comprehensive review of the incorporation of APRs into bituminous mixtures at different scales, including their application as asphalt binder modifiers, filler replacements, and mixture additives. The review aims to enhance understanding of their interactions with asphalt, highlight standard test methods for characterizing bituminous materials containing APRs, and assess the feasibility of using these residues in large-scale applications as a practical and sustainable approach. To this end, relevant literature was collected from three major bibliographic databases—Web of Science, Scopus, and Google Scholar—through systematic searches based on predefined keywords, as well as inclusion and exclusion criteria. The extracted data covered the type and dosage of APRs, various scales of bituminous materials, test methods, performance-related parameters, and key findings. Both qualitative and quantitative syntheses, with consideration of potential moderators, were employed to analyze the extracted information. This review begins by listing and comparing the physicochemical and mineralogical properties of commonly used APRs in bituminous materials. At the asphalt binder level (microscale), the role of APRs as additives is evaluated based on dispersion stability, rheological behavior, thermal properties, and chemical/nanoscale interactions. In the mastic phase (mesoscale), physicochemical interactions between APRs as fillers are investigated, along with their rheological and thermal effects. At larger scales (macroscale), the influence of APRs—either as additives or partial filler replacements—is assessed in terms of their effects on the volumetric properties, fatigue performance, low-temperature cracking resistance, permanent deformation, and moisture susceptibility of asphalt mixtures. Considering the potential toxicity of APRs, the environmental implications of incorporating these materials into asphalt mixtures are also critically examined.

The different types of APRs studied in this review can be classified as follows.

1.1. Red Mud

Red mud (RM), also known as bauxite residue, is the main waste material generated during aluminum production. It is produced during the alumina (Al₂O₃) refining process, which involves the reaction of bauxite with sodium hydroxide (NaOH) under high temperature and pressure [13]. The type of RM produced depends on the aluminum production method used, including the Bayer process (Figure 1a), the sintering process (Figure 1b), or a combination of both (Bayer-sintering) [14,15]. The Bayer process accounts for over 95% of global alumina production from bauxite, while the remainder comes from other methods [16,17]. For every ton of alumina produced, roughly 2–3 tons of bauxite are consumed, leading to the generation of approximately 0.4–2.0 tons of RM [18]. Approximately 150 million tons of RM are generated globally each year [19,20]. This high rate of generation has led to both environmental and economic challenges. Specifically, the high alkalinity and presence of heavy metals and trace amounts of radioactive elements in RM make its disposal a significant environmental challenge. These factors can lead to serious issues, such as groundwater contamination, often triggered by heavy rainfall or failures in containment structures [21,22]. Additionally, RM’s fine-grained particles are easily dispersed by wind, contributing to air pollution. [23]. As a sustainable solution, significant efforts have been made to identify suitable approaches for the safe and effective utilization of RM, which has been widely employed in the production of various construction materials, including lightweight aggregates [24], bricks [25], low-density foamed products [26], and traditional ceramics [27]. It has also been used as a partial replacement for cement in mortar and concrete formulations [28,29,30]. Moreover, due to its high alkalinity, RM has been valorized in the development of geopolymer composites [31,32,33,34]. In recent years, its utilization has expanded to the pavement sector, where it has been incorporated into road subgrade, base, and surface layer materials [35,36,37,38,39].

1.2. Aluminum Dross

Aluminum dross (AD) (Figure 1c), a byproduct of the aluminum melting process, occurs mainly in two forms, distinguished by their metal and salt content and by their origin in either primary or secondary aluminum production. White dross, which contains little to no salt, is produced during primary aluminum smelting, while black dross, which is rich in salts, is generated during secondary aluminum production [40,41,42]. Compared to black dross, the separation of white dross requires more energy and water and generates a greater volume of waste [43]. Notably, black dross typically contains less metallic aluminum than white dross [44]. Globally, it is estimated that up to 4 million tons of white dross and over 1 million tons of black dross are generated annually, with approximately 95% of this waste disposed of in landfills [45].

AD contains a certain volume fraction of toxic materials, and landfilling these hazardous substances is environmentally unsustainable. If not properly treated, toxic elements can leach into the surroundings and pose significant environmental risks [46]. One practical approach to mitigating the disposal issue is to utilize AD in building applications. This strategy not only reduces the environmental hazards associated with AD but also helps address natural resource scarcity. AD is primarily used in the production of ceramics [47,48] and bricks [49,50], as a partial replacement for cement in cementitious composites [51,52,53,54], and in soil stabilization [55].

1.3. Spent Pot Lining Residues

Spent pot lining (SPL) is hazardous waste generated from the internal lining of aluminum electrolytic pots. It comprises carbon-based materials and refractory bricks and is typically divided into two primary components: (a) spent cathode carbon block (SCCB) residues (Figure 1d) [56], which constitute the carbon-rich lower section of the lining, and (b) spent refractory materials (SRM) (Figure 1e), which include oxide-based bricks and insulation from the side and upper layers [57,58]. SPL is classified as a dangerous waste, primarily due to its high fluoride content of up to 150 kg per ton [59]. If not properly treated, SPL can lead to fluoride accumulation in the environment, causing lesions in plants and animals and posing serious health risks to humans [60,61].

Given the vast scale of the aluminum industry, the annual generation of SPL is substantial, with approximately 25 kg produced per tonne of primary aluminum, amounting to an estimated 1.6 million tonnes annually [62]. In recent years, researchers have increasingly focused on the safe and sustainable valorization of these residues in construction materials [63].

1.4. Secondary Aluminum Waste

Secondary aluminum waste (SAW), which refers to postconsumer aluminum materials that are recycled and reused, reportedly accounts for approximately one-third of total aluminum consumption [64,65]. Valorizing SAW yields monetary and economic benefits, reduces energy consumption, and helps increase landfill capacity. Aluminum shavings (Figure 1f) or chips, as part of SAW, are small, thin fragments of aluminum generated as byproducts during machining processes—such as cutting, drilling, or milling—in various industries. Their physicochemical characteristics—including size, density, shape, and chemical composition—vary depending on the specific machining process used. Due to the technical and environmental benefits offered by aluminum shavings, they have been widely employed in construction materials, primarily as additive fibers in concrete [66,67], cement and geopolymer mortars [68,69,70], and hot mix asphalt (HMA) [71].

This review not only examines various types of APRs but also emphasizes their incorporation into bituminous materials across different scales, distinguishing it from other studies. Different scales of bituminous materials are shown in Figure 2 [72]. The smallest scale is the asphalt binder phase, whose constituents are classified based on their polarity into four fractions—saturates, aromatics, resins, and asphaltenes—commonly referred to as SARA fractions [73]. Resins, saturates, and aromatics—collectively known as maltenes—represent the soluble molecular components and continuous phase of the asphalt binder [74]. The mastic phase incorporates fillers smaller than a specific size threshold (63 μm as per European standards [75], or 75 μm in North America [76]) and contains no air voids. It is regarded as an intermediate phase with relatively consistent properties and is commonly used to assess performance on larger scales. The fine aggregate matrix (FAM) phase bridges the gap between mastic and full asphalt mix; it consists of asphalt binder, fine aggregates typically smaller than 2.00–4.75 mm, filler, and air voids [77]. At the full scale—known as the asphalt mixture phase, which constitutes the road structure—coarse aggregates are incorporated, with the nominal maximum aggregate size defining the characteristic scale.

2. Physicochemical and Mineralogical Properties of Various Aluminum Production Residues

To ensure the effective valorization of APRs, it is essential to understand their physicochemical properties and mineralogical composition, as these characteristics significantly influence the performance of bituminous materials at various phases.

2.1. Chemical Composition of APRs

Figure 3a shows the chemical composition of RM from different processing methods—Bayer, Sintering, and a combination—as determined by X-ray fluorescence (XRF) analysis. As shown, RM is mainly composed of Al₂O₃, Fe₂O₃, and SiO₂, as well as smaller amounts of CaO and Na₂O. Among these, Al₂O₃ is the most abundant component in RM derived from the Bayer process but is less prevalent in the Sintering method. This variance depends on the bauxite source and extraction efficiency. The Fe₂O₃ content, which is generally higher in Bayer RM, can enhance the electrical and mechanical properties of asphalt mixtures [78,79]. Fe₂O₃ can also improve bonding with asphalt by providing active oxide surfaces that promote physical adsorption, hydrogen bonding, and interactions with polar groups in the asphalt binder [80]. Additionally, SiO₂ is more abundant in RM from the Sintering and combination methods. The presence of SiO₂ has been reported to improve interfacial bonding between the asphalt binder and aggregates, thereby increasing the mixture’s stiffness and mechanical strength [81]. A high content of CaO—an alkaline compound found in aggregates and fillers—enhances their bonding with the weakly acidic asphalt binder. This improvement is primarily attributed to the electrostatic potential generated at the interface between asphalt molecules and materials rich in CaO [82,83]. Compared to RM derived from Sintering or combination methods, Bayer RM generally contains lower levels of CaO. Studies have reported that blending Bayer RM with CaO-rich fillers, such as limestone, can improve the bonding performance of asphalt mixtures [84]. Furthermore, the Na₂O content in some Bayer process residues can reach up to 12%, imparting high pH and alkalinity to RM and raising environmental concerns, particularly regarding soil and water contamination [85].

As shown in Figure 3b, AD contains exceptionally high levels of Al₂O₃ (60–80%), formed through the oxidation of molten aluminum. The remaining oxides—such as SiO₂, Fe₂O₃, CaO, and MgO—originate from impurities, alloying agents, or furnace materials [86]. Additionally, the presence of SO₂ suggests possible interactions with sulfur-bearing compounds during processing or storage [87]. Regarding SPL residue materials (

Table 1. Chemical composition of SRM and SCCB.

SRM						Ref.
CaO	SiO2	MgO	Al2O3	Fe2O3	MnO
54.3	0.82	0.62	0.52	0.16	0.15	[88]
SCCB
C	NaF	Fe2O3	CaO	SO3	K2O	[89]
79.6	9.23	3.00	2.03	1.64	1.12

), the main component in SRM is CaO, followed by SiO₂ and Al₂O₃ [88]. In the case of SCCB, the predominant component is carbon (C)—determined by the loss on ignition method—followed by NaF and Fe₂O₃ [89].

Figure 3. (a) XRF analysis results of RM (the ternary phase diagram of the Fe₂O₃–SiO₂–Al₂O₃ system was obtained from normalized values) [30,84,90,91,92,93,94,95,96,97,98,99,100] and (b) chemical composition of AD [101,102,103] (Flash arrows are positioned to facilitate the identification and interpretation of values).

Figure 3. (a) XRF analysis results of RM (the ternary phase diagram of the Fe₂O₃–SiO₂–Al₂O₃ system was obtained from normalized values) [30,84,90,91,92,93,94,95,96,97,98,99,100] and (b) chemical composition of AD [101,102,103] (Flash arrows are positioned to facilitate the identification and interpretation of values).

2.2. Mineralogical Properties

The X-ray diffraction (XRD) patterns of each residue are presented in Figure 4. As can be seen, Sintering RM contains higher amounts of calcite (CaCO₃), whereas Bayer RM is richer in goethite (FeO(OH)) and hematite (Fe₂O₃). These findings are consistent with XRF analysis results [104,105]. It has been reported that AD mainly contains the crystalline compounds Al₂O₃, aluminum nitride (AlN), magnesium aluminate spinel (MgAl₂O₄), sodium hexafluoroaluminate (Na₃AlF₆), and hydrated aluminum oxide (Al₂O₃H₂O), as shown in Figure 4c, which are the key mineral phases present after the extraction of metallic aluminum [106]. Figure 4d shows that CaCO₃ is the most prominent phase, as indicated by its highest diffraction peak, followed by calcium fluoride (CaF₂) and SiO₂. [88]. The XRD pattern of SCCB (Figure 4e) shows that the dominant crystalline phases present are carbon (C) and sodium fluoride (NaF) [89].

2.3. Microstructural Assessment

Scanning electron microscope (SEM) images of various residues are presented in Figure 5. RM exhibits small, rounded, agglomerated particles, mostly less than 10 μm in size, with an irregular granulometric distribution and rough texture. These characteristics may lead to a higher SSA and, consequently, increased asphalt binder absorption [107]. Similar observations have been reported in several other studies [108,109]. As illustrated in Figure 5b, the SEM image of AD reveals smooth, angular particles, along with plate-like fragments and whisker-like surface features [110]. In Figure 5c, the SRM particles exhibit a distinct lamellar morphology, with numerous surface grooves and fine pores. This structural complexity contributes to a higher SSA, which can enhance the absorption of free asphalt during mixing. As a result, it facilitates the formation of an asphalt mastic with stronger cohesive forces, thereby improving high-temperature stability [88]. Figure 5d shows that the surface of SCCB is relatively smooth, with well-defined edges and an absence of visible pore structures. Nonetheless, surface impurities are present, which may be attributed to residual sodium fluoride in the SCCB matrix [89].

Figure 5. SEM images of (a) RM [107], (b) AD [110], (c) SRM [88] (reprinted with permission), and (d) SCCB [89] (reprinted with permission).

Figure 5. SEM images of (a) RM [107], (b) AD [110], (c) SRM [88] (reprinted with permission), and (d) SCCB [89] (reprinted with permission).

Table 2 presents a comparative summary of the key physicochemical properties of RM and AD, based on values reported in the literature. The specific gravity of RM varies considerably, ranging from 2.60 to 3.50 g/cm³, with this variability attributed to differences in chemical composition resulting from various processing methods and the type of bauxite used. The higher specific gravity of RM compared with AD is primarily due to its greater content of high-density oxides, such as Fe₂O₃ and TiO₂.

The average particle size (d₅₀) of RM is typically fine, ranging from 2.75 μm to 7.20 μm, whereas AD consists of coarser particles. Regarding specific surface area (SSA), RM exhibits a wide range, from 8.3 m²/g to 35.4 m²/g, attributed to variations in mineralogy, particle size, and porosity. In contrast, AD had a significantly lower SSA, indicating reduced surface interaction potential. The higher SSA of RM provides a larger contact area with the asphalt binder, which enhances adhesion and improves surface interaction with the binder [111].

The Rigden void (RV) values of both residues indicate relatively porous structures, which depend on particle shape, size, distribution, and surface texture. A higher RV value increases the volume of asphalt that can be retained within the voids, thereby contributing to the greater stiffness of the asphalt mastic phase.

In terms of hydrophilicity, both RM and AD have been reported to exhibit moderate to high hydrophilic behavior. However, it has been reported that a higher hydrophilicity coefficient contributes to improved adhesion between the mineral filler and asphalt [112]. Furthermore, RM is characterized by strong alkalinity, with pH values ranging from 9.0 to 12.5, primarily due to the incorporation of alkaline reagents during the alumina extraction process [113]. Since the asphalt binder is slightly acidic, it tends to form stronger bonds with alkaline materials, offering enhanced resistance to stripping. In other words, a pH above 7 imparts a basic character to the residue surface, rendering it electrically negative, which can positively influence asphalt–filler adhesion.

Table 2. Physical and chemical properties of RM and AD.

APR Type	Specific Gravity (g/cm3)	Average Particle Size, d50 (μm)	Specific Surface Area, SSA (m2/g)	Rigden Void, RV (vol%)	Hydrophilic Coefficient, η	pH Range
RM	3.10 [108,114], 3.50 [115], 2.60 [94], 2.97 [90], 2.80 [116], 2.55, 2.85 [91]	4.50 [108], 3.33 [114], 2.75 [115], 6.50 [94], 7.20 [116]	12.10 [117], 8.30 [116], 30.99, 35.41 [91]	44.05 [114], 47.90 [94], 44.80 [116]	0.85 [108], 0.47 [94], 0.70 [96], 0.62 [97]	9.98 [108], 9.18 [114], 10.25 [109], 11.00 [115], 12.40 [116], 9.90, 10.00 [91]
AD	2.90 [110], 2.79 [118]	24.96 [110], 15.62 [119]	1.03 [110], 1.26 [119]	45.70 [110]	0.67 [110]	-

Table 2. Physical and chemical properties of RM and AD.

APR Type	Specific Gravity (g/cm3)	Average Particle Size, d50 (μm)	Specific Surface Area, SSA (m2/g)	Rigden Void, RV (vol%)	Hydrophilic Coefficient, η	pH Range
RM	3.10 [108,114], 3.50 [115], 2.60 [94], 2.97 [90], 2.80 [116], 2.55, 2.85 [91]	4.50 [108], 3.33 [114], 2.75 [115], 6.50 [94], 7.20 [116]	12.10 [117], 8.30 [116], 30.99, 35.41 [91]	44.05 [114], 47.90 [94], 44.80 [116]	0.85 [108], 0.47 [94], 0.70 [96], 0.62 [97]	9.98 [108], 9.18 [114], 10.25 [109], 11.00 [115], 12.40 [116], 9.90, 10.00 [91]
AD	2.90 [110], 2.79 [118]	24.96 [110], 15.62 [119]	1.03 [110], 1.26 [119]	45.70 [110]	0.67 [110]	-

3. Valorization of APRs in Asphalt Binders (Microscale): Additive Applications

The use of waste additives in asphalt binders offers a sustainable approach to enhancing pavement performance while minimizing environmental impacts. Incorporating these materials—such as waste plastics, CR, and bio-based residues—has been shown to improve key binder properties, including viscosity, fatigue resistance, resistance to permanent deformation, and low-temperature cracking, ultimately contributing to longer-lasting pavements [120,121,122,123]. Among the various waste-derived additives, APRs have emerged as a promising option for asphalt modification due to their distinct physicochemical characteristics.

3.1. Use of RM as an Additive

Tao et al. (2017) [124] investigated the application of RM as an additive in asphalt binders through experimental studies and molecular simulations. They showed that RM reduced the penetration index and ductility while increasing the softening point, temperature sensitivity, and thermal stability of asphalt binders. From a chemical and microstructural perspective, RM’s large SSA enables it to adhere more uniformly to the asphalt binder surface. Furthermore, RM absorbs the oily components of asphalt, creating stronger molecular interactions that improve bonding. Consequently, the asphalt structure becomes thicker, adhesion between asphalt and mineral components is strengthened, and the modified asphalt binder demonstrates better stability and overall technical performance. As illustrated in Figure 6a, the RM-modified asphalt binder exhibits the formation of lumps and micro-blocks, which is attributed to π–π interactions and the accumulation of RM within the asphaltene- and oil-containing components.

In another study, Fu et al. (2020) [125] investigated the microstructural, chemical, and thermal characteristics of RM-modified asphalt under freeze–thaw (F–T) cycles. Their findings revealed that the addition of RM further decreased the heat absorption of the asphalt binder and that F–T cycles significantly reduced the heat absorption capacity of the modified asphalt binder. Microstructural analysis using atomic force microscopy showed that the number of bee structures observed noticeably decreased with the incorporation of RM. However, the remaining bee structures exhibited increased height and length. Furthermore, the F–T cycles had a significant impact on the morphology and uniformity of these structures. Molecular dynamics simulations demonstrated that asphaltenes adsorb onto RM surfaces, with stronger interactions observed with Al₂O₃ than with Fe₂O₃ across various temperatures. Therefore, increasing the Al₂O₃ content or reducing the Fe₂O₃ content in RM can enhance asphaltene adsorption. More specifically, Al₂O₃ surfaces exhibit strong adsorption energies with polar molecules in asphalt, promoting stable interfacial bonding and enhancing binder durability. The polar oxide surface facilitates hydrogen bonding and Lewis acid–base interactions with heteroatoms such as nitrogen and oxygen present in asphalt molecules, thereby improving adhesion and cohesion at the molecular level.

Figure 6b shows an SEM image of aluminum tailing slurry (ATS), categorized as RM, whose coarse surface texture and fine particle size facilitate the formation of a mechanical interlocking structure with the asphalt matrix, thereby enhancing interfacial bonding [126]. It has been shown that using ATS as an asphalt binder additive can reduce the quantity of bee structures and surface roughness, which may contribute to a decrease in the self-healing capacity of asphalt binders. Moreover, increasing the dosage of ATS in asphalt binders improves thermal stability, complex modulus, and rutting resistance. However, when the dosage exceeds 9%, the storage stability of the asphalt binder decreases significantly [127].

Figure 6. SEM image of (a) RM–modified asphalt binder [124] and (b) ATS [127].

Figure 6. SEM image of (a) RM–modified asphalt binder [124] and (b) ATS [127].

Compatibility between the additives and the asphalt binder is a critical factor that should be considered at the outset. For example, incorporating hydrophilic powders—such as RM—into a hydrophobic matrix, such as asphalt binder, can lead to interfacial defects and poor compatibility. Surface treatment of RM can enhance its efficiency as an asphalt modifier [128]. To improve interfacial interaction, Xiao et al. (2023) [129] investigated the use of organically modified RM as an additive in asphalt binders. RM was first treated with oxalic acid and a coupling agent and then analyzed using various microstructural techniques. Their results showed that surface modification successfully transformed the RM from hydrophilic to hydrophobic, yielding a more uniform and smaller particle size distribution. This modification improved both dispersion and interaction within the asphalt binder matrix. The enhanced performance can be attributed to the long organic chains of the modified RM, which intertwine with asphalt molecules, while physical adsorption strengthens the intermolecular forces between the RM particles and the asphalt binder. As a result, the modified asphalt binder exhibited increased resistance to high-temperature deformation and low-temperature cracking. Furthermore, the fine particle size of the modified RM contributed to the improved thermal stability and aging resistance of the asphalt binder. Han et al. (2025) [130] proposed an innovative approach to surface treatment of RM to improve the performance characteristics of RM-modified asphalt binders. They utilized dopamine, hypothesizing that its dual-functional groups could enhance compatibility between asphalt and RM. The catechol group forms stable bonds with metal oxides (e.g., Fe₂O₃, Al₂O₃) on the RM surface, improving dispersion in the asphalt matrix, while the aromatic ring of dopamine interacts with polycyclic aromatic hydrocarbons in asphalt binder via π–π stacking interactions [131,132]. Under mild alkaline conditions, dopamine undergoes self-polymerization into polydopamine, forming a cross-linked network that limits RM aggregation and strengthens cohesion in the asphalt matrix through physical anchoring. Therefore, they used solid waste RM as the base material, which was modified through dopamine self-polymerization and esterification reactions to synthesize a biomimetic composite with a high surface area inspired by the Siberian cocklebur (GA-CDA-RM). The preparation steps and mechanisms are illustrated in Figure 7a. Fluorescence microscopy images showed a more uniform dispersion of GA-CDA-RM compared to untreated RM in the asphalt binder. This treatment also enhanced the binder’s high-temperature rheological properties, rutting resistance, and low-temperature crack resistance while significantly reducing the leaching of heavy metal ions from the RM.

In another study [133], co-hydrothermal carbonization of RM with rice straw was used to produce modified forms of RM (c-HTC). Notably, H-HTC was developed through phosphoric acid–assisted hydrothermal carbonization (Figure 7b). The modified RMs exhibited significant detoxification and enhanced physicochemical properties, such as abundant surface functional groups, a clustered structure with tighter particle bonding (for c-HTC), and a unique flower-cluster structure (for H-HTC), as shown in the SEM images and Fourier transform infrared spectroscopy (FTIR) analysis (Figure 8a–c and Figure 9). The incorporation of this modified RM into asphalt binders substantially improved their high-temperature performance by increasing rutting resistance and enhancing thermal stability, as validated through molecular simulations. In another relevant study, Li et al. (2025) [134] developed a promising method using metallothioneins from plant oil residues to treat RM (Figure 7c). This treatment not only reduces harmful components in RM but also enhances the complex modulus, rutting resistance, and antiaging properties of asphalt binders. From a microstructural perspective (Figure 8d–g), RM modified with canola, soybean, and peanut oil residues forms small spherical structures, likely representing heavy metal ion–protein complexes. This confirms effective heavy metal immobilization and reduced environmental hazards. Additionally, the presence of these organic–inorganic microspheres improves the compatibility between RM and asphalt, enhancing the material’s elasticity and resistance to deformation.

3.2. Utilization of Other APRs as Modifiers

In addition to RM, SPL and SCCB residues have also been used to modify asphalt binders. Fan et al. (2024) [135] developed a novel thermoplastic polyester elastomer–spent refractory composite (TPEE–SRM) as an asphalt binder modifier, synthesized through an emulsion evaporation process, as illustrated in Figure 10a. This composite features a dual-layer structure consisting of a protective layer and a functional layer. Epoxy resins (EPs) incorporated into the protective layer formed a dense film that effectively sequesters fluoride. The organic modification of SRM altered its surface properties, enabling a better and more uniform dispersion of TPEE–SRM within the oily components of asphalt. The TPEE–SRM-modified asphalt binder exhibited enhanced complex modulus and rutting resistance compared to conventional styrene–butadiene–styrene (SBS)-modified asphalt. Furthermore, the TPEE–SRM/SBS composite-modified asphalt demonstrated superior fatigue resistance. In a study with a similar objective, researchers synthesized a bayberry-like hexamethyldisilazane and tannic acid–modified SRM (HMDS–TA–SRM) via a synchronous sedimentation–encapsulation reaction (Figure 10b). This modification enhanced the hydrophobicity of SRM and improved mechanical interlocking with the asphalt binder due to its distinctive nano-rough surface and mesoporous structure. The study demonstrated that asphalt binders modified with HMDS–TA–SRM exhibit excellent long-term performance and significantly reduce fluoride leaching to environmentally safe levels, with a maximum leaching rate of just 0.486% after 1 year. Additionally, the surface-encapsulated SRM showed outstanding fatigue resistance, attributed to the good organic compatibility of tannic acid and the oil-adsorptive surface created through further oil modification. It has also been reported that HMDS–TA–SRM SBS-modified asphalt exhibits significantly improved resistance to permanent deformation and enhanced long-term stability compared to pristine SBS-modified asphalt [136].

In another study, the authors addressed the hazardous nature of SCCB residues—known to contain toxic cyanides and fluorides—by developing a gel-crosslinked composite material, PSB, for asphalt modification. The treatment process involved using hydrogen peroxide to eliminate cyanides and calcium chloride to precipitate fluorides, followed by encapsulation with a polyvinyl alcohol–polyvinyl pyrrolidone (PVA–PVP) gel (Figure 10c). The resulting PSB effectively reduced cyanide concentrations to below environmental thresholds and enabled a slower, controlled release of fluorides, ensuring environmental safety. When incorporated into asphalt, particularly at a 1% PSB/SBS dosage, the modifier enhanced high-temperature rutting resistance, thermal stability, and elastic recovery when compared to conventional SBS-modified asphalt [89].

4. Studies on Asphalt Mastic (Mesoscale) and Binder–Filler Interactions

Several studies have explored the valorization of APRs as full or partial replacements for conventional fillers in asphalt mastics. To evaluate the effectiveness of these residues, a wide range of experimental tests have been conducted in accordance with standards, with the most used listed in

Table 3. Series of tests and simulations for evaluating the performance of APR asphalt mastics.

Property	Test and Simulation Methods	Standard Code
Penetration	Needle penetration	ASTM D5 [137]
Softening point	Ring and ball	ASTM D36 [138]
Ductility	Elongation	ASTM D113 [139]
Viscosity	Rotational viscosity	ASTM D4402 [140]
Short-term aging	Rolling thin-film oven test	ASTM D2872 [141]
Long-term aging	Pressure aging vessel	ASTM D6521 [142]
Linear viscoelastic behavior	Frequency and temperature sweep tests	AASHTO T315 [143]
Permanent deformation resistance and elastic behavior	Multiple stress creep recovery test	AASHTO T350 [144]
Fatigue resistance	Linear amplitude sweep test	AASHTO TP 101 [145]
Low temperature stiffness	Bending beam rheometer	AASHTO T313 [146]
Cracking resistance at low temperature	Direct tension test	ASTM D6732-02 [147]
Adhesion and bonding performance	Binder bond strength	AASHTO TP-91 [148]

.

4.1. Conventional Physical Properties

Conventional physical properties of APR asphalt mastics—including penetration, softening point, and ductility—have been evaluated in various studies. Yao et al. [84] replaced limestone (LS) with RM at 0%, 25%, 50%, 75%, and 100% levels, considering different filler-to-binder (F–B) ratios. They found that the penetration of an asphalt mastic containing RM was lower than that of an LS asphalt mastic. Of the five mixtures, the mastic with 100% RM had the most significant effect on reducing binder penetration. Additionally, the incorporation of RM increased the softening point of the asphalt binder. Zhang et al. (2018) [91] demonstrated that Sintering RM had a significant effect on the softening point, while Zhang et al. (2020) [105] reported that incorporating white mud (WM) into an RM asphalt mastic reduced the softening point and enhanced both penetration and ductility.

4.2. Rotational Viscosity

Many researchers have reported that incorporating RM as a filler in asphalt mastics increases rotational viscosity [84,90,92] and that adding hydrated lime (HL) further enhances this effect [97]. In contrast, introducing WM as a modifier into RM asphalt mastics slightly decreases viscosity [105].

4.3. Linear Viscoelastic Properties

Using temperature and frequency sweep tests, several studies have reported an increase in the complex modulus (G*) with the incorporation of RM into asphalt mastics [84,91,97,105]. This trend is consistent with the observed increase in the dynamic modulus of asphalt mixtures containing RM [149]. Similarly, an increase in G* was reported when conventional filler was replaced with SRM in an asphalt mastic [88].

4.4. Permanent Deformation (Rutting) Resistance

To evaluate the permanent deformation resistance of APR asphalt mastics, two key parameters—rutting index (G*/sin δ) and nonrecoverable creep compliance (Jnr)—were obtained through temperature sweep and multiple stress creep recovery (MSCR) tests, respectively. Studies have reported that incorporating RM as a filler leads to an increase in G*/sin δ [84,91,97] and a reduction in Jnr [92], indicating improved resistance to permanent deformation. Similar improvements have also been observed in asphalt mastics containing SRM [88].

4.5. Fatigue Resistance

Researchers have used G*sin(δ)—obtained from frequency and temperature sweep tests—as an index to evaluate the fatigue resistance of asphalt binders at intermediate temperatures. Lower values of G*sin(δ) typically indicate greater flexibility and improved fatigue resistance. Studies have reported that RM samples demonstrated either a higher [97] or a lower [105] G*sin(δ). Based on parameters obtained from linear amplitude sweep (LAS) tests, RM asphalt mastics exhibited the highest [95], lowest [150], or no clear trend [151] in terms of fatigue resistance performance compared to other samples.

4.6. Resistance to Low-Temperature Cracking

According to the bending beam rheometer (BBR) test, an asphalt mastic with a lower creep stiffness modulus (S) and a higher creep slope (m) indicates better resistance to cracking. Yao et al. (2020) [84] reported that incorporating RM increases the S value and decreases the m value, making mastics more susceptible to low-temperature cracking. However, Zhang et al. (2018) [90] did not reach a definitive conclusion on this effect. Furthermore, Zhang et al. (2020) [105], using a direct tension test, found that RM had a detrimental impact on low-temperature cracking resistance, but this effect could be improved by adding HL and WM.

4.7. Aging Resistance

Based on the carbonyl index obtained from FTIR analysis, the incorporation of diatomite (DT) and RM improved the aging resistance of an asphalt mastic by approximately 37.7% and 44.7%, respectively, compared to an LS asphalt mastic. In contrast, the use of FA resulted in reduced aging resistance. These findings have been supported by aging indices derived from both low- and high-temperature rheological properties [152].

4.8. Bonding, Wettability, and Moisture Susceptibility

Lima et al. (2020) [116] conducted a comparative study on the adhesive and wetting properties of asphalt mastics containing conventional fillers (LS and dolomite (DL)) and alternatives (RM and FA). They found that mastics with 40% RM achieved strong adhesion by passing the boiling water stripping test without chemical additives. Wettability tests showed that DL increased hydrophilicity (lower contact angles) while RM increased hydrophobicity (higher contact angles) (Figure 11a). However, this finding contradicts the observation reported in Section 2, indicating that RM exhibits moderate to high hydrophilic behavior.

In another study, Ou et al. (2024) [95] investigated the wettability and adhesive properties of asphalt mastics containing LS, RM, steel slag, and GGBFS. The contact angle of the mastic surface was measured using various probe liquids. The results showed that the RM asphalt mastic had the lowest surface free energy (SFE) and contact angle (Figure 11b) among the mixes. This may be due to the high SiO₂ content, which affects the electrochemical reactions in the asphalt mastic, resulting in a low SFE. Additionally, the presence of Na₂O can increase the risk of water damage at the bonding interface, as the chemical bonds formed between Na₂O and asphalt are easily dissolved upon exposure to water [153]. This aligns with the findings of Zhang et al. [149], who measured the surface energy parameters of various fillers using an inverse gas chromatography test. They reported that LS exhibited the highest surface energy values while RM exhibited the lowest.

Many researchers have evaluated the moisture susceptibility of asphalt mastics containing RM; however, their findings remain inconclusive. Zhang et al. (2018) [90] reported that asphalt mastics incorporating RM filler exhibited better resistance to moisture-induced damage than LS fillers at the same F–B ratio, based on the binder bond strength (BBS) test. In contrast, Wang et al. (2021) [152] observed opposite results. A separate study [97] investigated the effect of Sintering RM on the moisture sensitivity of aggregate–mastic bonding using an automatic adhesion tester. Specimens were conditioned in a water bath for 1 and 7 days to simulate moisture damage. Bond strength tests showed that substituting LS with RM significantly decreased moisture resistance; however, this reduction was alleviated by incorporating an appropriate amount of HL. Furthermore, Zhang et al. (2020) [105] found that using WM as a modifier could counteract the negative effects of RM on bonding strength at the aggregate–mastic interface after moisture conditioning. In addition, Zhang et al. (2019) [92] demonstrated that the pull-off tensile strength measured with the BBS test—along with the self-healing capacity of asphalt mastics—depends strongly on the type of RM, the F–B ratio, and the testing conditions (dry or wet).

Table 4. Physicochemical findings and rheological properties of asphalt mastics with different types of fillers.

Ref.	Filler Type	F–B Ratios	Physicochemical and Microstructural Assessment	Rheological Characterization
[114]	RM and stone dust (SD)	0.6, 0.9, 1.2, and 1.5 (w/w)	✓ RM consisted of rounded, agglomerated particles with a rough texture, whereas SD contained flaky, irregularly shaped particles. ✓ RM was significantly finer than SD and exhibited higher specific gravity and Rigden voids.	✓ G* was comparable for both fillers, but the SD mastic was stiffer, at a F–B ratio of 1.5. ✓ SD mastics had a lower phase angle, indicating higher elasticity compared to RM mastics. ✓ SD mastics had lower Jnr values, at 3.2 kPa (Jnr 3.2), suggesting higher stiffness than RM mastics. ✓ At lower F–B ratios, SD mastics had a higher recovery. However, at an F–B ratio of 1.5, RM mastics exhibited greater recovery due to a faster increase in recovery with the F–B ratio.
[154]	RM, marble dust (MD), limestone (LS), granite (GR), basalt (BA), and quartz (QZ)	0.5, 1, and 1.5 (w/w)	✓ RM exhibited the lowest fineness modulus, the highest Rigden value (RV), and the highest filler grain coefficient, indicating a well-graded and fine particle distribution.	✓ The RM asphalt mastic exhibited the highest G*LVE of all the samples.
[151]	RM and LS	0.5, 1, and 1.5 (w/w)	✓ Compared to LS, RM showed higher specific gravity and RV but a lower fineness modulus.	✓ There was no consistent superiority between RM and LS fillers in terms of fatigue life, as performance varied depending on test conditions, analysis methods, and filler-to-binder ratios. ✓ The pseudo-strain energy method consistently produced the highest fatigue life estimates for both RM and LS asphalt mastics, followed by dissipated energy and R-based approaches. ✓ Hyperbolic geometry proved more effective in capturing fatigue damage in asphalt mastics for both types of fillers.
[150]	RM and conventional filler	10%, 20%, and 30% (v/v)		✓ Increased RM content resulted in greater fatigue damage. ✓ The newly proposed fatigue parameter (F) proved to be an effective indicator of fatigue damage in both asphalt mastics and binders.
[155]	RM, LS, and MD	10%, 20%, and 30% (v/v)	✓ RM was much finer, with approximately 40% of RM particles smaller than 10 μm, compared to ~12% for the other fillers.	✓ Mastics containing 30% RM filler were excessively stiff and required more time for homogenization, unlike the other mixes. ✓ RM showed the most significant sensitivity of the G* and Glover–Rowe parameters to changes in filler volume fraction. ✓ The fine, porous structure of RM and its strong binder interaction significantly influenced fatigue damage evolution, whereas MD and LS fillers had more moderate effects, with temperature being a more dominant factor in their performance. ✓ The strong contribution of RM was linked to its intensive interaction with asphalt binder, with a critical threshold observed at a 20% volume fraction, likely due to particle interlocking.
[116]	RM, LS, dolomite, and fly ash (FA)	20% and 40% (v/v)	✓ RM exhibited the highest surface area, RV, and pH among the fillers.	✓ RM asphalt mastics demonstrated excellent adhesion to aggregates. Mastics containing 40% RM achieved the required minimum aggregate coverage (≥85%) in the boiling water stripping test without the need for any chemical adhesion improver, indicating robust adhesive properties. ✓ The wettability test results showed that mastics with RM had favorable contact angles, indicating good affinity between the mastics and water, which is important for durability in wet conditions.
[95]	RM, LS, steel slag (SS), and ground granulated blast-furnace slag (GGBFS)	1 (w/w)	✓ RM showed a denser microstructure than the other fillers, with fine, agglomerated particles.	✓ RM asphalt mastic exhibited the lowest G* and the highest phase angle. ✓ RM asphalt mastic showed better fatigue performance than LS asphalt mastic. ✓ Unlike SS, RM showed the lowest recovery and the highest Jnr, indicating the poorest resistance to rutting.
[152]	RM, LS, FA, and diatomite (DT)	33% (v/v)	✓ The RV values of RM and DT were significantly higher than those of LS and FA, indicating a more porous structure.	✓ RM asphalt mastic retained its low-temperature flexibility better than LS mastic after prolonged aging. ✓ The stress sensitivity of the RM asphalt mastic in the fatigue test was higher than that of LS or FA mastics, due to differences in the effect of physical hardening caused by the porous structure of RM. ✓ In unaged samples, RM asphalt mastic exhibited a higher percentage of recovery than LS mastic.
[84]	RM and LS	0.3, 0.6, 0.9, 1.2, 1.5, 1.8, and 2.1 (w/w)		✓ Incorporating and increasing the amount of RM led to a rise in rotational viscosity, G*, rutting index, and creep stiffness, all of which increased further with a higher F–B ratio. ✓ Increasing the substitution of LS with RM increased creep stiffness (S), while the rate of change in stiffness (m-value) decreased.
[90]	LS and RM	0.3, 0.6, and 0.9 (w/w)		✓ Based on the rutting index, the high-temperature performance of RM was superior to that of LM, especially when the F–B ratio was at least 0.6. ✓ Incorporating RM increased percentage recovery and decreased Jnr, indicating improved elasticity and resistance to permanent deformation. ✓ Increasing the F–B ratio reduced low-temperature cracking, and RM did not consistently outperform LS in low-temperature performance.
[91]	Conventional filler, Sintering RM, and Bayer RM	1 (w/w)	✓ Sintering RM contained more aggregated particles than Bayer RM, despite their similar particle sizes. The pore volume of RM was more than five times greater than that of conventional mineral filler.	✓ Sintering RM more effectively enhanced G* and rutting resistance. It also improved elastic recovery, unlike Bayer RM, which had a minimal effect.
[97]	RM, LS, hydrated lime (HL), and FA	1 (w/w)	✓ RM had smaller, rougher, and more porous particles than LS, with fine, near-spherical particles prone to agglomeration, affecting asphalt binder absorption and asphalt mastic rheology.	✓ The viscosity of RM asphalt mastic was over four times higher than that of LS mastic. ✓ The fatigue parameter (G*sin δ) of RM mastic showed the highest values, especially at low frequencies.
[92]	RM, LS, FA, and DT	0.11, 0.22, 0.33, 0.44, and 0.55 (v/v)		✓ Larger RV in fillers such as DT and RM increased asphalt absorption and particle friction, leading to higher viscosity and lower Jnr 3.2 than LS and FA asphalt mastics.
[105]	RM and LS	1 (w/w)	✓ RM consisted of near-spherical, fine particles with coarse, edgeless surfaces, which tended to agglomerate into larger clusters.	✓ RM improved the elastic recovery of asphalt mastic but negatively affected cracking and fatigue resistance, which could be mitigated by adding white mud. ✓ RM significantly increased the viscosity of mastic, and adding HL amplified this effect, while white mud slightly reduced it.
[88]	SRM	0%, 50%, and 100% (v/v)	✓ Both SRM and graphene oxide (GO) featured wrinkled and grooved surfaces that effectively adsorbed free asphalt without undergoing any chemical reaction with it.	✓ The synergistic effect of GO as an asphalt modifier and SRM as a filler improved asphalt mastic stiffness, reduced permanent deformation, and lowered stress sensitivity, mainly due to the CaO and Al2O3 content in SRM and the surface structure of GO.

summarizes studies on the asphalt mastic phase using different types of fillers and F–B ratios, along with key physicochemical, microstructural, and rheological findings.

5. Valorization of APRs in Asphalt Mixtures (Macroscale)

As asphalt mixture is the final product used in road surfacing, its mechanical performance and durability are key criteria for evaluating the feasibility of using APRs. Numerous studies have investigated the effects of these residues on the performance characteristics of asphalt mixtures through various experimental tests, with the most commonly used summarized in

Table 5. Common experimental tests used to evaluate asphalt mixtures containing APRs.

Property	Test	Standard Code
Strength and flow	Marshall stability and flow test	ASTM D6927 [156]
Tensile strength	Indirect tensile strength test	ASTM D6931-12 [157]
Cracking resistance	Semicircular bend test	AASHTO TP124-16 [158]
	Indirect tensile asphalt cracking test	ASTM D8225 [159]
Rutting resistance	Hamburg wheel tracking test	AASHTO T 324–11 [160]
	French wheel tracker test	NF P 98-250-2 [161]
Moisture resistance	Moisture susceptibility test	ASTM D1075-11 [162]
		AASHTO T283 [163]
Adhesion and bonding	Boiling water test	ASTM D3625-12 [164]
Raveling resistance	Cantabro abrasion loss test	AASHTO TP108-14 [165]

.

5.1. Studies on RM Hot Mix Asphalt Samples

5.1.1. Volumetric Properties

Choudhary et al. (2019) [114] reported that the RM mix exhibited a lower optimum binder content (OBC) than the SD mix. Moreover, unlike the SD mix, the OBC values for the RM mix did not vary significantly with an increase in filler content. This behavior was attributed to the finer particle size of RM compared to SD, which enabled it to act as a binder extender and enhanced the workability of the mixture. However, other studies have reported contrasting results, suggesting that the porous structure and high SSA of the RM lead to greater asphalt binder absorption [107,166].

5.1.2. Strength and Flow Measurement

Marshall stability reflects the resistance of asphalt mixes to deformation under load and is influenced by the viscosity of the asphalt mastic, which in turn depends on the interaction between asphalt and filler. Therefore, the type and amount of filler significantly affect the stability. Researchers have reported RM mixes exhibiting higher Marshall stability than OPC [108] and SD [107] mixes, which may be attributed to the higher bearing capacity of RM.

The flow value indicates the plastic behavior of asphalt mixtures and is defined as the total strain or deformation occurring in a specimen between the application of zero and the maximum load during the Marshall test. A higher flow value reflects greater plasticity, while a lower value suggests brittleness [167]. An RM asphalt mix has been shown to exhibit a higher flow value than an SD mix, indicating increased plasticity [107].

5.1.3. Permanent Deformation Resistance

The Marshall quotient (MQ), calculated as the ratio of Marshall stability (kN) to flow (mm), indicates the resistance of an asphalt mix to permanent deformation and shear stress [168]. A higher MQ signifies greater stiffness and enhanced load distribution capacity. Choudhary et al. (2022) [107] found that RM mixes exhibited higher MQ values, likely due to the lower asphalt film thickness of RM and its more integrated structure compared to SD mixes [169]. Lima et al. (2017) [170] conducted a permanent deformation test following French standard NF P 98-253-1. They found that replacing stone powder with RM as a filler improved the deformation resistance of asphalt mixtures, supporting the findings of other research [109]. Due to its lower SSA compared to stone powder, RM can reduce the asphalt binder content required, thereby limiting the thermal susceptibility of the mixture. The superior resistance to permanent deformation of RM mixes compared to conventional mixes has been demonstrated using the Hamburg wheel tracking (HWT) test [90,149]. Furthermore, Tian et al. (2021) [94] reported that porous asphalt mixtures incorporating RM and DT exhibited enhanced rutting resistance—based on dynamic stability and creep slope measurements—compared to those containing LS and FA (Figure 12). This improved performance was attributed to the strong adsorption capacity and hardening effect of these porous fillers on the asphalt binder.

5.1.4. Moisture Susceptibility

Choudhary et al. (2018) [108] revealed that fillers rich in insoluble calcium-based compounds provide superior resistance to moisture damage. Mixes containing carbide lime (CL), ordinary Portland cement (OPC), and limestone slurry dust exhibited higher retained Marshall stability (RMS) values compared to an RM mix, with the CL mix showing the highest RMS value. This was attributed to the inclusion of portlandite and calcite in the CL, which impart antistripping properties. However, Choudhary et al. (2022) [107] reported that the highly alkaline nature of RM, along with its content of adhesion-promoting minerals—such as calcite—contributed to its satisfactory moisture resistance compared to SD mixes [171]. These findings are consistent with those reported by Lima et al. (2020) [109].

Enhanced moisture resistance was also reported by Zhang et al. (2018) [90] and Shanchun et al. [172] (2022) when RM was used either as a replacement for LS filler or as an asphalt binder modifier in the mixture [96]. In contrast, Tian et al. (2021) [94] found that RM porous asphalt mixtures exhibited a lower tensile strength ratio (TSR) than LS mixtures. This reduction was attributed to the presence of Na₂O in the RM, which tends to swell upon water exposure and increase moisture sensitivity—a finding consistent with mastic phase evaluations by Ou et al. (2024) [95]. Zhang et al. [149] also demonstrated that replacing LS filler with RM improved rutting resistance under dry conditions but negatively impacted its performance under moisture conditioning. To mitigate this, the addition of 10% HL or OPC significantly enhanced the moisture resistance of RM mixtures. This improvement was attributed to the presence of Ca(OH)₂ in HL, which formed stable bonds with carboxylic acids and sulfoxides in the binder. Similarly, cement compounds such as 3CaO·SiO₂, 2CaO·SiO₂, and 3CaO·Al₂O₃ contributed to stronger adhesion with the asphalt binder, thereby improving resistance to moisture-induced damage.

5.1.5. Tensile Strength and Cracking Resistance at Intermediate Temperatures

Recent studies have reported that RM mixes exhibited superior indirect tensile strength compared to reference samples [107,108,114]. This improvement can be attributed to the fine particle size of RM, which promotes uniform distribution within a mixture and contributes to the formation of an integrated structure in the bituminous matrix, thereby enhancing stiffness [169]. Tian et al. (2021) [94] performed the semicircular bend (SCB) test and calculated quantitative indicators, including peak load, from load–displacement curves; fracture energy; slope at inflection point; and flexibility index (Figure 13). They concluded that asphalt mixtures containing DT and RM exhibited notable cracking resistance due to the anchoring and hardening effects provided by the fillers. However, RM—in comparison to DT—not only significantly hardened the asphalt binder but also demonstrated a certain toughening effect. The anchoring and toughening effects result from the interaction between RM—primarily composed of interparticle pores—and the crosslinked network structure of the incorporated SBS modifier.

5.1.6. Adhesion and Bonding

Passive adhesion is defined as the ability of bitumen to adhere to an aggregate surface under external influences, such as water and traffic [173]. Mixes with superior passive adhesion generally exhibit higher asphalt coverage in boiling water tests. Researchers have reported that RM exhibits good passive adhesion compared to most other waste fillers, including BD, CL, copper tailings, limestone slurry dust, rice straw ash, and glass powder [108]. This performance was attributed to the stiffening effect of RM’s fine particles on the asphalt binder. However, RM showed lower passive adhesion than mixes containing OPC [108], SD [107,166], and Kota dust (KS) [166]. The superior adhesion observed in the KS and SD mixes compared to RM was attributed to the higher content of calcium-based dolomite and calcite, respectively, in their composition [166].

5.1.7. Raveling Resistance

Based on the Cantabro abrasion test, researchers reported that RM mixes outperformed LS [90,94], DT, and FA [94] but underperformed compared to SD mixes [107].

5.1.8. Aging Resistance

Choudhary et al. [107] assessed the aging resistance of RM asphalt mixes using the mean Marshall stability ratio, defined as the ratio of the average Marshall stability after aging (oven-conditioned at 85 °C for 5 days) to that before aging. Their results indicated that RM mixes exhibited lower aging resistance than conventional mixes, which may be attributed to the lower asphalt film thickness in RM mixes [174]. In contrast, Zhang et al. (2018) [90] reported that porous asphalt mixtures containing RM with an F–B ratio of 0.9 demonstrated better aging resistance than those with LS. These findings align with those of Tian et al. (2021) [94], who observed that the inclusion of DT and RM improved the raveling resistance of aged porous asphalt mixtures by 39.6% and 43.2%, respectively, compared to an LS mix. In summary,

Table 6. Key studies on the valorization of RM in HMA.

Reference Filler	Gradation Type	Type of Asphalt Binder	Effect of RM								Ref.
			OBC%	Stability and Flow	Rutting Resistance	Tensile Strength and Cracking Resistance	Moisture Susceptibility	Adhesion	Raveling Resistance	Aging Resistance
OPC	Dense-graded	60–70 penetration grade	-	↑	↑	↑	↓	↓			[108]
SD	Dense-graded	VG-30	↓			↑			↑		[114]
SD	Dense-graded	VG-30	↑	↑	↑	↑	↑	↓		↓	[107]
SD	Dense-graded	VG-30	↑					↓			[166]
SD	Dense-graded	PG 64-22			↑		↑				[109]
LS	Dense-graded	60–80 penetration grade			↑		↓				[149]
LS	Open-graded	SBS asphalt (PG 76-22)			↑		↑		↑	↑	[90]
LS	Open-graded	SBS asphalt (PG 82-28)			↑	↑	↓		↑	↑	[94]

Note: ↑ indicates an increase, ↓ indicates a decrease, and - indicates no change.

lists important studies on the valorization of RM in HMA.

5.1.9. Valorization of RM in Alternative Asphalt Mixture Types

Few studies have examined the use of RM in other types of asphalt mixtures—such as cold-mix asphalt (CMA) or warm-mix asphalt (WMA)—or its practical application in asphalt plants. Phan et al. (2024) [115] evaluated the performance of a CMA mixture containing RAP, RM as a filler, and recycled waste glass as a fine aggregate. They conducted a comprehensive series of tests, including workability and compactability, IDEAL-CT (Figure 14a), HWT, and dynamic modulus tests. In addition, a 400 m pedestrian road trial section was constructed to assess CMA performance under real outdoor conditions (Figure 14b). However, the study primarily focused on the effects of recycled glass, with the role of RM as a filler not specifically highlighted.

5.1.10. Environmental Aspects of RM Valorization

Due to the presence of certain heavy metals in its composition [175,176] and residual sodium hydroxide, especially from the Bayer process, RM exhibits strong alkaline properties and poses potential environmental concerns. Consequently, many researchers classify RM as hazardous waste; however, some studies have categorized it as noninert waste rather than strictly hazardous [177,178]. Lima et al. (2018) [178] analyzed water from moisture damage tests on RM asphalt mixes using a photocolorimeter to detect potential hazardous leaching. Although the results indicated only minimal leaching of RM, the study recommended more precise and in-depth evaluations using specialized equipment to better assess the risk of water contamination. In another study, Lima et al. (2021) [179] evaluated the environmental potential of asphalt mixtures using alternative fillers (RM and FA) alongside reference fillers (LS and DL) through a cradle-to-gate life cycle assessment. The analysis covered stages from raw material acquisition to asphalt plant production. Three scenarios were modeled: (1) residues with treatment, (2) residues without treatment, and (3) landfilling. The treatment process involved washing, drying, and crushing the RM, while FA underwent Sintering. Environmental indicators included global warming potential, acidification, and cumulative energy demand (CED). The results indicated that residue treatment significantly influenced environmental impacts and played a key role in assessing the feasibility of using RM and FA in asphalt pavements.

Li et al. (2021) [180] investigated the runoff purification performance of porous asphalt with various fillers—including LS, coal fly ash, DT, and RM—using a self-developed rainfall simulation device (Figure 15) introduced by Liu et al. (2021) [181]. The authors reported that the purification mechanism of the asphalt mix involved both physical adsorption and chemical degradation, including chemical precipitation and ion exchange. Alkaline precipitation from fillers and aggregates in porous asphalt leads to the formation of heavy metal precipitates. In terms of ion exchange, a strong interaction occurs between Na⁺ and K⁺ leached from red mud and NH₄⁺ in runoff, enhancing the removal of total nitrogen. Furthermore, RM contains abundant porous structures and a large SSA, significantly improving the micropore characteristics of porous asphalt. Its rougher surface and larger surface area enhance interactions with pollutants, strengthen physical adsorption, and ultimately improve the overall purification performance of asphalt mixes.

In another study, Rashidian et al. (2025) [117] conducted a toxicity characteristic leaching procedure test to assess the level of heavy metal pollution leached from RM microsurfacing asphalt mixtures. The concentrations of heavy metals—As, Pb, Cd, Cr, Ni, Cu, Hg, and Zn—in the RM mixes were well below the allowable limits. This could be attributed to the alkaline nature of RM, which enhances its bonding with the asphalt binder and reduces leaching. Additionally, the stabilizing properties of the asphalt binder helped mitigate the release of harmful metals—a finding also supported by previous research [169,182].

5.2. Utilization of Aluminum Dross in HMA

Some researchers have evaluated the feasibility of incorporating AD as a filler in asphalt mixtures [101,119,183]. Ulga et al. (2024) [184] investigated the moisture susceptibility of HMA samples in which LS filler was partially replaced with AD at replacement levels of 10%, 20%, and 30%. The optimum replacement level was identified as 20%, at which point two key moisture resistance indicators—TSR and the index of retained strength—were found to be 13.42% and 8.73% higher, respectively, than those of the reference mixes. The same results were obtained when incorporating different dosages of RCA and AD in HMA [102].

Ergezer et al. (2025) [118] examined the impact of microwave heating on self-healing and ice melting in asphalt mixtures containing steel fiber (SF) and AD, based on the hypothesis that AD, like other iron-containing materials, absorbs microwaves and is thermally conductive [185]. AD powders were used as a substitute for LS filler at different content levels, and SF was added in different proportions by the weight of the mix. The results showed that increasing the AD and SF content significantly improved the average surface temperature and ice-melting speed by 112.86% and 73.6%, respectively, compared to the reference mix. Indirect tensile tests (IDTs) conducted before and after damage indicated that higher AD and SF contents enhanced healing after 60 s of microwave heating. However, extending the heating to 120 s with a high SF content reduced the healing effectiveness. Soos et al. (2017) [110] performed a dynamic modulus test to evaluate the linear viscoelastic behavior of asphalt mixtures containing LS filler, with AD used as a 50% and 100% replacement for LS. As shown in Figure 16, analysis of the Cole–Cole diagram and the master curve—developed using the sigmoidal model—confirmed the stiffening effect of AD. The results indicate that the increase in stiffness was more pronounced at high temperatures than at low temperatures.

5.3. Incorporation of Aluminum Shavings in HMA

Researchers have incorporated aluminum shavings into HMA due to their favorable reinforcing mechanisms [71]. Serin et al. (2021) [186] evaluated the cracking resistance of mixes containing aluminum chips derived from shavings and iron powders at proportions of 0.5%, 1.0%, and 1.5% by weight of the total mixture using the SCB test (Figure 17a). They reported that incorporating 0.5% aluminum chips improved the fracture energy, peak load, cracking resistance index, and flexibility index compared to the reference mixture. This was supported by SEM images (Figure 17b), which showed no evidence of segregation or loss of adhesion between asphalt binder and aluminum chips; instead, clear wetting and strong bonding were observed. Similarly, Al-Obaidi and Abed (2023) [187] found that adding waste aluminum scrap powder enhanced the cracking resistance of asphalt mixtures. In another study, Atakan et al. (2021) [188] investigated the microwave healing potential of porous asphalt mixes containing waste aluminum and steel shavings, produced with different aggregate types (basalt and limestone), using an IDT at low temperatures. The results showed that aluminum shavings exhibited better healing performance than steel shavings in both aggregate-type groups, consistent with the findings of Pamulapati et al. (2017) [189]. The authors attributed this to aluminum’s lower density, which promotes more uniform dispersion in the mixture and increases microwave absorption efficiency compared to steel [188]. Martinez et al. (2025) [190] showed that incorporating 0.1% aluminum shavings into HMA significantly improved tensile strength, fatigue life, and moisture damage resistance (indicated by higher TSR), reduced mass loss due to abrasion, and maintained a resilient modulus and stiffness comparable to the reference mix, despite a slight decrease in resistance to permanent deformation. However, higher aluminum shaving contents (≥0.5%) resulted in performance deterioration.

6. Challenges and Recommendations for Future Work

Although APRs show promising potential in bituminous materials, several challenges must be addressed before their large-scale adoption in pavement engineering. The physicochemical and mineralogical characteristics of APRs—particularly RM—vary significantly depending on their source, processing method, and treatment history. This variability affects their compatibility with asphalt binders and aggregates, as well as their mechanical and environmental performance. Consequently, inconsistent performance can lead to misinterpretation and confusion among engineers, making it difficult to standardize mixed designs and establish effective quality-control measures.

The hydrophilic nature and strong tendency for particle agglomeration in most APRs—especially RM, which is often reported as a self-agglomerating material—can result in poor dispersion within hydrophobic asphalt matrices, thereby reducing adhesion and workability. Such agglomeration also poses practical challenges during the construction stage, including storage, sieving, and mixing operations. Surface modification techniques—such as organic treatments and coupling agents—show promise for improving compatibility but require further optimization for large-scale field applications.

While researchers generally agree that APRs can enhance certain mixture properties, such as stiffness, stability, and rutting resistance, their effects on other critical indicators—including fatigue performance, low-temperature cracking resistance, and moisture susceptibility—remain inconsistent across studies. Moreover, research on their applicability in cold regions is lacking; in particular, low-temperature cracking behavior, freeze–thaw cycle durability, and fatigue performance at the asphalt mixture scale have not been sufficiently explored.

From an environmental perspective, although several studies have examined the leaching behavior and environmental impact of RM in asphalt mixtures—with promising results—little to no research has addressed these aspects for other APR types. Comprehensive evaluations of leaching potential, long-term environmental stability, and ecotoxicity are necessary to ensure safe and sustainable implementation.

Based on the challenges discussed above, the following recommendations for future research are proposed:

• Investigate the linkage between the physicochemical properties of APRs and the rheological and mechanical performance of bituminous materials using advanced analytical tools, such as machine learning. This will help in developing robust characterization protocols and classification systems for APRs and standardizing their application in mix design, construction, and quality assessment.
• Explore advanced surface modification techniques to enhance APR–asphalt binder compatibility, focusing on scalable, eco-friendly approaches that are feasible for large-scale applications.
• Study the incorporation of APRs at the FAM scale as a more precise approach compared to the mastic phase to simulate the performance of large-scale asphalt mixtures.
• Assess the feasibility of incorporating APRs into various asphalt mixture types—such as CMA and WMA—to achieve a more sustainable approach.
• Focus on the use of APRs in road pavements for cold regions by designing comprehensive experimental testing protocols tailored to these environmental conditions.
• Conduct life cycle assessments and life cycle cost analyses to evaluate the environmental and economic viability of APRs in large-scale industrial applications.

7. Conclusions

The valorization of APRs—including RM, AD, and SPL residues and SAW in bituminous materials—offers a promising pathway to more sustainable pavement engineering. This review presents a comprehensive investigation of various APRs from different stages of aluminum production in bituminous materials, aiming to clarify their reinforcing mechanisms and assess their effects on the performance of asphalt mixtures for large-scale applications. Treating and using different APRs as asphalt modifiers has shown potential; however, scaling up these applications for field use may present economic challenges. Across meso- and macroscale studies, RM has demonstrated the ability to improve thermal stability, tensile strength, cracking resistance at intermediate temperatures, and rutting resistance. Nonetheless, inconsistent results have been reported for other properties, such as fatigue behavior, low-temperature cracking, aging resistance, adhesion, and moisture susceptibility. These variations stem from differences in the physicochemical properties of RM as well as in F–B ratios, reference filler types, mix designs, and other parameters. Despite promising environmental assessment results for RM in asphalt mixtures, mainly due to its stabilizing effect on asphalt binders, which helps mitigate the release of harmful metals, environmental evaluations of other APRs at the macro scale remain largely unexplored. To ensure the safe and widespread adoption of APRs, it is essential to develop standardized characterization methods and application protocols, alongside a deeper understanding of the fundamental interactions between APRs and asphalt binders across various scales.