Sustainable Utilisation of Mining Waste in Road Construction: A Review

Abstract

Mining by-products present both an environmental challenge and a resource opportunity. This review investigates their potential application in road pavement construction, focusing on materials such as fly ash, slag, sulphur, red mud, tailings, and silica fume. Drawing from laboratory and field studies, the review examines their roles across pavement layers—subgrade, base, subbase, asphalt mixtures, and rigid pavements—emphasising mechanical properties, durability, moisture resistance, and ageing performance. When properly processed or stabilised, many of these wastes meet or exceed conventional performance standards, contributing to reduced use of virgin materials and greenhouse gas emissions. However, issues such as variability in composition, leaching risks, and a lack of standardised design protocols remain barriers to adoption. This review aims to consolidate current research, evaluate practical feasibility, and identify directions for future studies that would enable the responsible and effective reuse of mining waste in transportation infrastructure.

1. Introduction

The continued expansion of global mining activities has led to the generation of over 100 billion tons of mining waste annually, making it one of the world’s most significant forms of industrial waste streams [1]. These wastes, including fly ash, slag, red mud, tailings, and overburden, are often chemically active and volumetrically abundant, posing both disposal challenges and environmental hazards, particularly heavy metal leaching and acid mine drainage [2]. According to local environmental laws and available reuse and recycling methods, these mineral wastes are stored in ponds, large dams or piles. Additionally, waste can be classified as either inert or hazardous, depending on whether it poses harmful effects to humans and the environment. In the case of hazardous waste, special treatment is required to reduce the risk [3]. As the world moves towards more eco-friendly practices, the impact of mining waste on the ecosystem and human health highlights the importance of managing and mitigating its effects in a sustainable manner [4].

Several studies have been conducted to explore more sustainable practices for reuse and recycling, highlighting the potential of utilising mining waste as a secondary resource. For instance, mining waste can be reused as construction materials, for soil improvement in land reclamation, and to create eco-friendly products. With the rising demand for concrete aggregate, the construction industry presents a timely opportunity to utilise mining by-products in simple and cost-effective ways. Furthermore, certain types of mining waste, such as copper slag, have been found to enhance the strength and durability of concrete [4]. At the same time, the road construction industry faces increasing pressure to reduce its environmental impact and reliance on non-renewable resources. Integrating mining waste into pavement structures presents a dual opportunity as follows: mitigating waste management burdens and reducing demand for virgin construction materials [5]. Prior studies have shown encouraging results. For instance, fly ash and slag blends have significantly improved subgrade strength [6], and red mud and steel slag have enhanced durability in asphalt and rigid pavements [7,8]. However, concerns over long-term performance, environmental compatibility, and the absence of standardised design methods persist. This review critically evaluates the current state of knowledge, examining both the mechanical and environmental performance of mining waste in road pavements, and it identifies the main gaps and future directions to facilitate broader and more responsible implementation.

This review employed a structured literature analysis to evaluate the use of mining waste materials in road pavement construction. Relevant peer-reviewed journal articles, technical standards, and conference papers from 2015 to 2025 were sourced using the ECU online library and academic databases such as Scopus, Web of Science, Google Scholar, and ScienceDirect. Search terms included “mining waste in pavement”, “fly ash stabilisation”, “slag in asphalt”, “red mud binder”, and “geopolymer pavement materials”. More than 80 publications were selected based on their relevance, experimental detail, and applicability to pavement engineering.

The selected studies were examined to extract data on material properties, treatment methods, application contexts, and performance indicators. Key parameters such as unconfined compressive strength (UCS), California Bearing Ratio (CBR), indirect tensile strength (ITS), Marshall stability, rutting resistance, and leachability were evaluated. Mining waste materials were categorised according to their source (e.g., fly ash, slag, red mud, and tailings) and their application within subgrade, base, subbase, asphalt mixtures, or rigid pavements.

2. Types and Characteristics of Mining Waste

The type and characteristics of mine waste primarily depend on the type of mining operation and kind of mineral deposit, as well as the technology used [9]. Different steps in the mining process can produce various types of waste materials, each with distinct physical and chemical properties that influence their potential for reuse in road construction. A thorough understanding of types and characteristics is crucial for assessing their environmental impact and engineering performance.

2.1. Overburden and Waste Rock

These granular materials are removed during initial ore extraction and may contain varying proportions of silica, clay, and trace metals. When adequately stabilised, they can be used as base materials or structural fills. However, the presence of sulphide minerals poses a risk of acid mine drainage, requiring geochemical assessment prior to use [2]. Most often mining overburden is contaminated with toxic heavy metal such as nickel (Ni), chromium (Cr), arsenic (AS), cadmium (Cd), copper (Cu), Zinc (Zn), manganese (Mn), and it also contains high sulphur levels, which tend to lower the pH value [10]. To convert overburden and waste rocks into a secondary resource, various processes such as crushing, screening, and washing are employed, which may lead to environmental hazards that require careful management towards an environmentally friendly way [11].

2.2. Fly Ash

Fly ash is a fine, pozzolanic by-product of coal combustion, valued for its particle fineness and reactivity. It typically comprises spherical particles with sizes between 10 and 100 μm, lower density than Portland cement, and a specific gravity ranging from 1.6 to 2.6. Its colour varies from light tan to dark grey, depending on its lime, iron, and unburned carbon content. Fly ash is non-plastic, has a low swell index, and exhibits porosity between 30% and 65%. Notable engineering properties include high lime reactivity (1–8 MPa), a pH range of 6.0–8.0, and durability indicated by CBR (~10%), abrasion (~28%), and low permeability (~9.5 × 10⁻⁶ cm/s) [12].

2.3. Silica Fume

Produced during silicon and ferrosilicon alloy production, silica fume is an ultra-fine pozzolanic material composed mostly of amorphous silicon dioxide. Its particle size is typically 0.1–0.2 μm, with more than 95% below 1 μm. With a surface area between 13,000 and 30,000 m²/kg and specific gravity of ~2.22, silica fume enhances the densification of concrete. It contains over 85% silica and trace oxides, including Al₂O₃, Fe₂O₃, and CaO, supporting its application in high-performance and sustainable construction [12].

2.4. Sulphur

A by-product of gas refining and mining operations, sulphur is commonly used in sulphur-extended asphalt and concrete. While relatively stable, its use requires safety precautions due to thermal and environmental risks. In pavement applications, sulphur functions as a binder modifier, improving stiffness and resistance to deformation [13].

2.5. Tailings

Tailings are fine-grained residues from ore processing, typically composed of silt-sized particles with some clay and sand fractions. Characterised by irregular particle shapes and a specific gravity of ~2.83, they are often poorly graded (Cu ~9.44; Cc ~0.96) and exhibit low permeability (4.4 × 10⁻⁵ cm/s). Their chemical composition includes silica, alumina, alkalis, and trace metals such as Cu, Mo, and Pb. Due to low mechanical strength and cohesion, tailings usually require stabilisation before being used in pavement layers [14]. Studies have demonstrated that the characteristics of iron ore tailings can enhance the rutting resistance and water stability when mixed into the asphalt mixture due to their fine particle size, larger specific surface area, angular and rigid shape, and rich mineral composition (Fe₂O₃ and SiO₂) [15].

2.6. Red Mud

Red mud, an alkaline by-product of aluminium extraction from bauxite ore, contains a mix of Fe₂O₃, Al₂O₃, SiO₂, TiO₂, Na₂O, and CaO. While not pozzolanic on its own, it is chemically rich in oxides that make it suitable for blending with fly ash or GGBFS. When treated, red mud improves fluidity, early strength, and mechanical performance in semi-flexible pavements (SFPs). Microscale analyses (SEM, XRD, and FTIR) have confirmed its structural suitability in cementitious applications [16,17].

2.7. Slag

Slags are metallurgical by-products from ferrous and non-ferrous smelting processes. They contain oxides of calcium, silicon, magnesium, and iron, contributing to their pozzolanic activity. This review focuses on lithium, steel, and copper slags. Lithium slag is derived from lithium carbonate extraction, steel slag from steel production, and copper slag from smelting processes. Their physical and chemical characteristics are summarised below in

Table 1. Physical and chemical characteristics of slag [18,19,20].

Property	Lithium Slag	Steel Slag	Copper Slag
Origin	Lithium mica extraction	Steel manufacturing	Copper smelting
Colour	Brownish yellow	Grey to dark grey	Black, blackish grey
Density (g/cm3)	2.551	~3.4	3.50
Specific Surface Area	60 m2/kg	Lower than lithium slag	—
Particle Size	100% < 2.36 mm; 23.3% < 0.075 mm	Coarse, angular	Majority ~150 μm; 91% > 150 μm
Key Components	SiO2, Al2O3, K2O, CaO, Fe2O3, Li2O, etc.	CaO, SiO2, Fe2O3, MgO	Fe2O3, SiO2, Al2O3, TiO2, etc.
Microstructure	Quartz, hydrated calcium sulphate, fluorite phases	Dense, angular crystalline phases	Irregular, glassy particle texture

.

3. Engineering Applications of Mining Waste in Road Construction

Pavement structures typically consist of multiple layers designed to distribute traffic loads safely and uniformly to the subgrade. These include the surface (wearing) course, base, subbase, and subgrade, and they may be either flexible (asphalt-based) or rigid (cement concrete) in design [21], as shown in Figure 1. Given the large volume of materials required for these layers, integrating mining by-products presents an opportunity to enhance sustainability while maintaining or improving performance.

Mining wastes such as tailings, slag, red mud, fly ash, overburden, and waste rock can serve various roles across these layers. Waste materials separated early in the mining process (e.g., overburden and waste rock) often require minimal processing and are suited for aggregate applications. Finer by-products such as fly ash or red mud may require chemical activation or blending for structural applications.

However, several challenges limit the widespread use of mining waste in pavement applications. These include substandard geotechnical properties, environmental concerns like leaching of contaminants, material variability due to geological differences, and the need for thorough assessment to determine suitability. When these challenges are addressed, mining waste can enhance pavement performance while promoting sustainable waste reuse and environmental protection.

The subsections below examine how specific mining wastes are applied across different pavement layers and mix designs.

3.1. Subgrade Stabilisation

The subgrade, formed by the in situ natural soil, often exhibits poor engineering properties such as low strength or expansive behaviour, which can undermine pavement durability. Stabilising these soils using mine waste materials has been a focus of numerous studies. Processed waste rock, tailings, and slag have been used either to replace or blend with problematic subgrade soils to improve load-bearing capacity, reduce plasticity, and increase density. Pozzolanic materials like fly ash and slag can chemically react with soil constituents to form cementitious compounds that enhance soil strength.

Abdila et al. [6] evaluated the combination of ground granulated blast furnace slag (GGBFS) and fly ash for stabilising clayey soils. The blend significantly increased the unconfined compressive strength (UCS) and reduced the plasticity index (PI), though further study was needed to meet ASTM D4609 strength benchmarks. Zimar et al. [22] noted that Class C coal fly ash performs optimally at 10–15% content without activators, while Class F requires lime or cement addition. These additions reduce plasticity, swelling, and enhance mechanical indices like UCS, California Bearing Ratio (CBR), and resilient modulus (Mr). However, in sulphate-rich or freeze–thaw-prone soils, supplementary additives may be necessary to mitigate performance limitations.

Red mud has been assessed as a stabilising agent with moderate native properties, but its strength improves markedly with activators such as lime, gypsum, or cement kiln dust (CKD). UCS gains of up to 880% and CBR improvements exceeding 500% have been reported, with strength levels meeting road standards in India, Ireland, and Australia [23].

Cement-treated Magnesite Mine Tailings (MMTs) were studied by Shanmugasundaram and Shanmugam [24], showing performance improvements in strength and durability when mixed with 8–10% ordinary Portland cement (OPC). Although slightly weaker than cement-stabilised sand, the mix passed durability and leaching safety standards. Ahmed et al. [25] also demonstrated the effectiveness of combining GGBFS and CKD, achieving UCS values 2.9–5.9 times greater than untreated soils and reducing the PI from 7.4 to 4.8.

In Suva, Fiji, Pisini et al. [26] studied the reinforcement of subgrade soil using 20% KOBM slag and geogrid. A single geogrid layer at the CBR mould’s mid-height yielded the highest CBR values. Other innovations include the use of GGBFS and brick dust waste [27], steel slag and fly ash blends with calcium carbide residue (CCR) [28], and GGBFS with polypropylene fibre for black cotton soil improvement [29].

Kanbara Reactor (KR) slag has also shown strong performance in both laboratory and field trials [30], while lightweight alkali-activated systems using sodium silicate, CCR, and GGBFS demonstrated enhanced sulphate resistance [31]. In sulphate-rich environments, GGBFS–MgO mixtures effectively improved swelling control and durability [32].

These findings show that properly designed mixtures using mine waste can meet or exceed conventional standards for subgrade performance.

3.2. Base and Subbase Layers

The base and subbase layers serve critical functions in road structures by supporting traffic loads and ensuring proper drainage. Typically composed of granular materials, these layers can be stabilised with cement or lime to enhance stiffness and strength [21]. The integration of mining waste in these layers has shown potential to improve performance while advancing sustainability objectives. Untreated coal mine overburden such as murrum, topsoil, and subsoil often exhibits low CBR values, making it unsuitable without stabilisation [33]. However, studies have shown that with proper treatment, such materials can attain the required mechanical properties. For instance, Cao et al. [34] demonstrated that lithium slag stabilised with magnesium slag achieved UCS values above 2.7 MPa and immobilised over 95% of heavy metals like Pb (II) and Be (II). Similarly, Kong et al. [35] found that fine iron tailing slag mixed with fly ash, cement, calcium oxide, and a water-resistant stabiliser yielded a 7-day UCS of 1.97 MPa and an elastic modulus of 286 MPa. The blend showed microstructural densification due to gel formation.

Lithium slag also demonstrated potential as a cement substitute in cement-stabilised macadam base layers. High-content use led to improvements in compressive and splitting strength, water resistance, freeze–thaw durability and reduced drying shrinkage. Beneficial hydration products, such as C-S-H and AFt gels, contributed to performance gains [18]. Karmakar et al. [36] reported that a cement-treated mix containing coal mine overburden, BOF slag, and fly ash achieved a UCS of 4.84 MPa and a soaked CBR of 136.08%. The approach reduced construction costs by 51.6% and maintained acceptable leaching levels. Biopolymer treatment using guar and xanthan gum also enhanced compressive strength and freeze–thaw resilience of red mud tailings, with peak strength at 14 days [37].

Kumar Nigam et al. [38] showed that cement-stabilised red mud exhibited increased specific gravity, reduced plasticity, and higher elasticity modulus, albeit with brittleness at higher cement dosages. Barati et al. [39] observed similar improvements in iron ore tailings treated with cement and bentonite. Sinha et al. [40] confirmed the suitability of cement-stabilised zinc tailings for structural fill applications, with UCS, CBR, and modulus values meeting pavement standards. Manjarrez and Zhang [41] explored geopolymerisation for copper mine tailings, showing UCS sensitivity to NaOH concentration and moisture content. Stabilised mine waste in base and subbase layers generally meets required mechanical thresholds while significantly reducing heavy metal leaching—often achieving immobilisation rates above 95%. These outcomes validate the technical and environmental viability of using mine waste in these pavement components.

3.3. Asphalt Mixtures

Incorporating mining waste into asphalt mixtures is an emerging practice aimed at enhancing pavement performance while addressing sustainability. Mining by-products such as red mud, sulphur, silica fume, fly ash, and iron or copper tailings have been used as mineral fillers or modifiers in both hot and cold asphalt mixes. Their pozzolanic and cementitious properties contribute to improvements in stiffness, rutting resistance, and long-term durability [42,43].

For example, substituting conventional limestone filler with iron or copper tailings has yielded better high-temperature performance, improved fatigue resistance, and higher Marshall stability. Red mud, particularly in porous asphalt, has enhanced Cantbro loss values and ravelling resistance, reflecting improved durability under water and traffic exposure [7,44]. Cold mix asphalt (CMA) applications using red mud in combination with waste glass or reclaimed asphalt pavement (RAP) maintain workability at ambient temperatures, thereby reducing energy costs and emissions. Red mud also improves water resistance and rheological performance in CMA, particularly for low-volume roads in cold climates [45].

In geopolymer binders and emulsified asphalt, red mud and fly ash combinations show enhanced binder elasticity, water resistance, and deformation tolerance. These systems contribute to eco-friendly cold-applied asphalt suited for maintenance and rehabilitation applications [19]. Industrial by-products such as GGBFS and steel slag, with their angular texture and mechanical strength, enhance load distribution and skid resistance in high-traffic pavements. Fly ash, especially Class F, improves binder stiffness and ageing resistance, while sulphur—often used with polyethylene or rubber—enhances binder crosslinking, resulting in increased stiffness, thermal stability, and rutting control [46]. These modifications not only improve mechanical performance but also offer significant environmental benefits by reducing reliance on virgin materials, lowering emissions, and enabling the productive reuse of industrial waste.

3.4. Concrete Pavements (Rigid Pavements)

Mining by-products such as red mud, lithium slag, steel slag, and silica fume have shown considerable promise in rigid pavement applications, particularly as partial substitutes for cement or aggregates in roller-compacted concrete (RCC). Their inclusion enhances mechanical performance, improves durability, and supports sustainability objectives by diverting industrial waste from landfills.

Red mud and ferrochrome slag have been effectively used in RCC mixtures, producing compressive strengths exceeding 32 MPa and demonstrating improved abrasion resistance and structural integrity [8]. The use of red mud in combination with reclaimed asphalt pavement (RAP) has been found to improve water absorption and abrasion resistance, further confirming its suitability for rigid pavement layers [47].

Lithium slag, used as a partial cement replacement, enhances both compressive strength and transport properties of concrete. Its pozzolanic activity and fine particle characteristics contribute to the development of a dense microstructure and beneficial hydration products, which improve long-term performance [48]. Steel slag has also been incorporated into rigid pavement applications, offering high durability, excellent load-bearing capacity, and enhanced resistance to abrasion and impact. In addition to improving mechanical properties, its use helps reduce cement demand and associate carbon emissions, contributing to more sustainable concrete solutions.

Silica fume, owing to its ultra-fine particle size and high amorphous silica content, has been widely used as a supplementary cementitious material in concrete. It reacts with calcium hydroxide released during cement hydration to form calcium silicate hydrate (C-S-H), which refines the microstructure, reduces permeability, and significantly improves compressive strength and resistance to chemical attack [12]. These materials offer cost-effective, performance-enhancing alternatives to conventional rigid pavement components, making them well-suited for infrastructure applications that demand strength, longevity, and environmental responsibility.

4. Performance Evaluation of Mining Waste in Road Pavements

4.1. Bitumen Binder Modified with Mining Waste

Bitumen binders modified with mining waste have demonstrated significant improvements in performance characteristics. These modifications are typically assessed through standard tests evaluating stiffness, temperature susceptibility, ageing resistance, and rheological properties. Fly ash has shown consistent enhancement across rheology, viscosity, and durability parameters

Table 2. Common Test Standards for bitumen binder evaluation [49,50,51,52,53,54,55,56,57,58,59].

Property	Test Method	Standard Code
Penetration	Needle Penetration	ASTM D5/EN 1426 [49,50]
Softening Point	Ring and Ball	ASTM D36/EN 1427[51,52]
Ductility	Elongation	ASTM D113 [53]
Viscosity	Rotational Viscosity	ASTM D4402 [54]
Short-Term Ageing	Rolling Thin-Film Oven Test (RTFOT)	ASTM D2872 [55]
Long-Term Ageing	Pressure Ageing Vessel (PAV)	ASTM D6521 [56]
Rheological Properties	Dynamic Shear Rheometer (DSR)	AASHTO T315 [57]
Multiple Stress Recovery	Multiple Stress Creep Recovery (MSCR)	AASHTO T350 [58]
Low-Temperature Stiffness	Bending Beam Rheometer (BBR)	AASHTO T313 [59]

below summarises the key test methods used in the papers reviewed to assess binder characteristics, including stiffness, temperature susceptibility, and ageing behaviour.

4.1.1. Penetration

Sulphur-modified binders demonstrate a slight reduction in penetration values, reflecting increased binder stiffness. This behaviour is attributed to the formation of a more interconnected matrix and the thermoplastic interaction between sulphur and polymer or plastic additives [60].

Fly ash, especially Class F, has shown a reduction in penetration values when combined with lime or cement, further enhancing binder stiffness and its resistance to rutting under high temperature conditions [61]. Bitumen modified with red mud or fly ash typically shows reduced penetration values, indicating increased hardness and stiffness, which enhances rutting resistance and load-carrying capacity [62,63].

4.1.2. Softening Point (Ring and Ball)

The inclusion of sulphur into bitumen formulations has been reported to elevate the softening point. According to Zhou et al. [64], asphalt binders modified with sulphur and polyethylene exhibited increased softening points, indicative of high-temperature stability and improved deformation resistance.

Silica fumes in asphalt binders have consistently raised the softening point, thereby enhancing the thermal resistance and rutting performance of pavements in hot climates. Zhu and Xu (2021) [65] found that incorporating 6% silica fume in a composite styrene-butadiene-styrene (SBS)-modified binder resulted in optimal thermal resistance, attributed to the modified asphalt’s improved molecular interaction and structural integrity. Additional studies show that such modification aids in slowing the rate of oxidative ageing, thus extending the pavement lifespan under thermal stress [65,66]. Studies have reported that incorporating fly ash into bitumen results in a noticeable increase in softening point, similar to or greater than that observed with hydrated lime or limestone, thereby improving the binder’s ability to withstand high service temperatures [61].

The addition of silica fume, steel slag, and red mud elevates the softening point of the binder, contributing to improved thermal resistance at elevated temperatures and extending pavement life in hot climates [31,67].

4.1.3. Viscosity

Sulphur-modified binders increase viscosity over time due to recrystallization and chemical interactions with base bitumen components. This increase supports improved load resistance but may necessitate adjustments in mixing and compaction temperatures [64]. Fly ash increases the rotational viscosity of bitumen, often requiring slightly higher mixing temperatures. However, this increase correlates with improved resistance to flow and shear deformation, particularly in pavements subjected to heavy traffic [68]. Incorporating red mud, especially in sintered or surface-modified form, and plastic waste significantly increases bitumen viscosity. This increases mixing and compaction temperatures but improves rutting resistance and reduces susceptibility to flow at high temperatures [63,69].

4.1.4. Rheological Properties (From DSR Test)

Sulphur-modified bitumen binders have shown variable but promising effects on rheological properties depending on composition, sulphur content, and curing time. DSR tests indicate that while sulphur may initially reduce complex modulus (G*) due to plasticizing effects, prolonged curing leads to sulphur recrystallization, enhancing binder stiffness [64]. This is evident in bio-modified rubberized binders, where G* increased significantly after 60 days of curing with sulphur, especially in binders modified with castor and waste vegetable oil. Additionally, sulphur reduces phase angle (δ) over time, increasing the elastic Behaviour of the binder. The most pronounced effects were seen in WVO-BMR and CO-BMR blends, suggesting enhanced performance under cyclic loading conditions [64]. When combined with polyethylene (PE) or plastic waste, sulphur further improves elasticity and resistance to permanent deformation [60,67].

Dynamic Shear Rheometer (DSR) evaluations have demonstrated that silica fume improves both stiffness (↑ G*) and elasticity (↓ δ), making it effective in reducing permanent deformation and rutting susceptibility. Saleh et al. [68] and Wang et al. [19] reported significant improvements in complex modulus values when silica fume was used in geopolymer blends. Zhu and Xu [65] further confirmed that silica fume enhances the rheological behaviour of SBS-modified binders, optimising phase angle and modulus response. These effects are more pronounced under high temperature loading conditions, positioning silica fume as a high-performance modifier for flexible pavements.

Fly ash has demonstrated improved complex modulus (G*) and reduced phase angle (δ) values in DSR testing, especially when blended at 15% with appropriate alkaline activation. These values exceed the minimum Superpave requirement (≥1.0 kPa), confirming enhanced stiffness and rutting resistance [68,70]. Dynamic Shear Rheometer (DSR) tests indicate that mining waste fillers generally improve the complex modulus (G*) and reduce the phase angle (δ), leading to better elasticity and enhanced resistance to permanent deformation. For instance, red mud and geopolymer-modified binders achieved G*/sinδ values exceeding Superpave specifications, indicating excellent performance under repetitive loading [19,68].

Materials like red mud, fly ash, and sulphur contribute significantly to binder stiffness and elasticity, which are crucial for resisting rutting under high temperatures and repeated loading. However, care should be taken in cold regions, as increased stiffness can sometimes reduce low-temperature flexibility, a concern typically evaluated using Bending Beam Rheometer (BBR) tests.

Table 3. Rheological performance of bitumen binders modified with mining waste (DSR test).

Material	G*/sin δ (Unaged)	Interpretation	Source
Red Mud	1.29–1.62 kPa	Exceeds Superpave requirement (≥1.0 kPa); good rutting resistance	[19]
Silica Fume	↑ G* and ↓ δ	Improved stiffness and elasticity	[19]
Fly Ash	↑ G*/sin δ with cement	Strong pozzolanic effect; enhanced high-temperature resistance	[67]
Sulphur + PE	↑ G*, ↓ δ	More elastic and rut-resistant binder	[67]
Geopolymer (FA + MK-SF)	1.4–3.4 kPa (avg.)	Excellent rutting and fatigue resistance (12% FA and 4% MK-SF blends)	[68]

presents a summary of the findings on the rheological performance of mining waste-modified bitumen.

4.1.5. Ageing Properties (Short-Term and Long-Term)

Silica fume has proven effective in improving both short- and long-term ageing resistance of bituminous binders. Rolling Thin Film Oven Test (RTFOT) and Pressure Ageing Vessel (PAV) assessments show that asphalt modified with silica fume exhibits lower oxidation levels, higher stiffness retention, and better resistance to thermal degradation. Zhu and Xu [65] identified that binders containing 6% silica fume had the smallest increment in carbonyl index, indicating superior protection against oxidative damage. Deb and Singh [66] found that using silica fume in cold mix asphalt not only accelerated strength gain under elevated curing temperatures but also reduced rutting depth by up to 58%, reinforcing its long-term durability advantages.

Fly ash-based binders exhibit excellent ageing performance. Under RTFOT and PAV protocols, binders incorporating FA showed minimal changes in stiffness and elasticity. Performance grades improved significantly (e.g., PG 76–28) compared to neat binders (PG 58–22), indicating superior long-term resistance [68]. Bitumen modified with mining waste—especially sulphur compounds, silica fume, and chemically treated red mud—demonstrates improved resistance to short-term (RTFOT) and long-term (PAV) ageing. These materials limit oxidation and retain elasticity and ductility over time. Organic red mud formulations in particular slow ultraviolet ageing and thermal degradation due to enhanced surface compatibility and barrier properties [19,69].

Table 4. Ageing performance of bitumen binders modified with mining waste.

Material	Ageing Type	Performance Outcome	Interpretation	Source
Red Mud	RTFOT (Short)	Minimal penetration loss; stiffness retained	Indicates resistance to short-term oxidative ageing	[19]
Red Mud	PAV (Long)	PG-22 rating achieved	Suitable for cold climates with low cracking risk	[19]
Sulphur + PE	RTFOT (Short)	Reduced Penetration Ageing Ratio; increased softening pt.	Stronger thermal stability; lower oxidation rate	[67]
Sulphur + PE	PAV (Long)	High post-ageing ductility and elasticity	Excellent long-term durability and flexibility	[67]
Silica Fume	PAV (Long)	Maintained stiffness and deformation control	Ageing resistance supports warm mix performance	[19]
Geopolymer (FA + MK-SF)	RTFOT (Short)	Retained stiffness; low softening point loss	Effective against short-term ageing degradation	[68]
Geopolymer (FA + MK-SF)	PAV (Long)	Met PG-76 rating post-ageing	Demonstrates excellent long-term resistance	[68]

illustrates the key findings on the ageing performance of modified bitumen from mining waste. These findings confirm that several mining waste additives, particularly red mud, sulphur, and geopolymer systems, improve the resistance of bitumen binders to both short-term thermal oxidation and long-term ageing. This is essential for maintaining pavement flexibility, durability, and service life.

4.2. Asphalt Mixture Modified with Mining Waste

Standard performance tests evaluate the mechanical properties of asphalt mixtures incorporating mining waste to determine their suitability for pavement applications.

Table 5. Standards for the common test in asphalt mixture evaluation [71,72,73,74].

Property	Test Method	Standard Code
Stability and Flow	Marshall Test	ASTM D6927 [71]
Indirect Tensile Strength	Indirect Tensile Strength (IDT)	ASTM D6931 [72]
Rutting and Moisture Susceptibility	Hamburg Wheel Tracking Test (HWTT)	AASHTO T342 [73]
Stiffness and Fatigue	Dynamic Modulus Test (DM)	AASHTO T378 [74]

below outlines the common test standards used in the papers reviewed for asphalt mixture design and analysis.

4.2.1. Rutting Resistance

The addition of mining waste materials, such as steel slag, red mud, silica fume, and fly ash, significantly improves rutting resistance under high-temperature loading. Steel slag enhances interlock and stiffness due to its angularity, while silica fume and steel slag combinations reduce rut depth by up to 58% [67,75]. Sulphur-modified mixtures further increase resistance to deformation under cyclic and thermal loads [64].

4.2.2. Fatigue Resistance

The addition of mining waste fillers like silica fume, red mud, and fly ash extends the fatigue life of asphalt mixtures. These additives improve binder stiffness and adhesion, delay crack initiation, and reduce energy dissipation during loading cycles. Adham et al. [67] and Deb & Singh [66] observed that cold and warm mix asphalt with mining waste fillers had greater fatigue life, especially under repeated loading and freeze–thaw cycles.

4.2.3. Strength and Durability Properties

Red mud, fly ash, and steel slag enhanced key performance metrics such as Marshall stability, indirect tensile strength (ITS), and dynamic modulus. Ashteyat et al. [76] reported that strength increases when these wastes are blended with reclaimed asphalt pavement (RAP). In particular, fly ash improved modulus and deformation resistance, while red mud enhanced stiffness and fracture energy [68,75].

4.2.4. Moisture Susceptibility Properties

Moisture susceptibility, measured via the tensile strength ratio (TSR), improves by including treated red mud and silica fume. These materials increase adhesion between binder and aggregate and reduce water penetration. Lima et al. [42] and Zhang et al. [62] found that red mud with neutralised pH increased TSR values beyond 80%. Sulphur-modified mixtures, especially when combined with polyethylene or tyre rubber, exhibited greater stripping resistance and moisture durability [64,67]. Table 6 and Figure 2 show the comparative performance of using mining waste in asphalt mixture.

Table 6. Comparative performance of mining waste in asphalt mixture.

Material	Rutting Resistance	Fatigue Resistance	Strength/Durability	Moisture Susceptibility	Key References
Steel Slag	High (↑ stiffness, interlock)	Moderate–High	↑ Marshall stability, ↑ modulus	Moderate	[77,78]
Red Mud	Moderate–High	High	↑ Fracture energy, ↑ ITS	High (>80% TSR)	[42,62,68]
Silica Fume	High (↓ rut depth 58%)	High	↑ Stiffness, ↑ modulus	High	[66,67]
Fly Ash	Moderate–High	Moderate	↑ Deformation resistance	Moderate–High	[61,68]
Sulphur	High (with PE/rubber)	Moderate	↑ Stability	High	[64,67]

Table 6. Comparative performance of mining waste in asphalt mixture.

Material	Rutting Resistance	Fatigue Resistance	Strength/Durability	Moisture Susceptibility	Key References
Steel Slag	High (↑ stiffness, interlock)	Moderate–High	↑ Marshall stability, ↑ modulus	Moderate	[77,78]
Red Mud	Moderate–High	High	↑ Fracture energy, ↑ ITS	High (>80% TSR)	[42,62,68]
Silica Fume	High (↓ rut depth 58%)	High	↑ Stiffness, ↑ modulus	High	[66,67]
Fly Ash	Moderate–High	Moderate	↑ Deformation resistance	Moderate–High	[61,68]
Sulphur	High (with PE/rubber)	Moderate	↑ Stability	High	[64,67]

Figure 2. Comparison of mining waste material in asphalt mixtures [62,66,79,80,81].

Figure 2. Comparison of mining waste material in asphalt mixtures [62,66,79,80,81].

Table 7. Comparative performance of mining waste materials in asphalt mixes [62,64,66,79,80,81].

Mine Waste Type	Key Applications	Performance Outcomes	Meets Standards
Red Mud	Filler in dense/porous/cold asphalt, geopolymer binders	↑ Marshall Stability, ↑ TSR, ↓ Penetration, ↑ Ageing Resistance	Yes
Fly Ash	Filler in HMA, CMA, geopolymer binders	↑ Binder stiffness, ↑ Ageing resistance, ↑ Workability	Yes
Steel Slag	Coarse aggregate, mineral filler	↑ Rutting resistance, ↑ Skid resistance, ↑ Load distribution	Yes
Silica Fume	Filler in CMA, warm mix, bioasphalt	↑ Adhesion, ↑ Moisture resistance, ↑ Elastic modulus	Yes
Copper/Iron Tailings	Limestone filler replacement	↑ Marshall Stability, ↑ Fatigue resistance, ↓ Thermal susceptibility	Yes
Sulphur	Binder modifier with plastic/rubber	↑ Ageing resistance, ↑ Thermal stability, ↑ Water resistance	Yes

and

Table 8. Comparison of test results from the literature with standard performance limits.

Property/Test	Standard Requirement	Observed Value from Studies	Source/Material	Meets Standard?
Penetration	40–100 (ASTM D5)	↓ by 14.7 units (Sintering RM)	Red Mud Binder [62]	✔ Yes
Softening Point	≥46 °C (ASTM D36)	↑ to 77.5 °C	Red Mud Binder [62]	✔ Yes
Ductility	≥75 cm (ASTM D113)	~71.2–75.4 cm	Organic Red Mud [62]	✔ Borderline
G*/sin δ (Unaged)	≥1.0 kPa (AASHTO T315)	1.29–1.62 kPa	Red Mud Mastic [19]	✔ Yes
Ageing Resistance (BBR)	S ≤ 300 MPa; m ≥ 0.3 (AASHTO T313)	Met (PG-22 rating)	Red Mud Binder [19]	✔ Yes
Marshall Stability	≥8 kN (ASTM D6927)	16.68 kN	Red Mud Mix [75]	✔ Yes
Flow	2–4 mm	Within Range	Red Mud Mix [75]	✔ Yes
Indirect Tensile Strength	500–900 kPa	Within Range	Copper and Red Mud Mix [75]	✔ Yes
TSR (Moisture Susceptibility)	≥80%	>85%	Red Mud Mix [42]	✔ Yes
Marshall Quotient (MQ)	High = Better Rutting Resistance	5.23 kN/mm	Red Mud Mix [7]	✔ Yes
Softening Point	≥46 °C (ASTM D36)	↑ to 55.5 °C	Silica Fume Bioasphalt [19]	✔ Yes
TSR (Moisture Susceptibility)	≥80%	82.4%	Silica Fume Mix [19]	✔ Yes
Marshall Stability	≥8 kN (ASTM D6927)	13.5–15.4 kN	Fly Ash and Steel Slag Mix [67]	✔ Yes
Rutting Resistance	≤12.5 mm rut depth	Reduced rutting depth (qualitative)	Steel Slag Aggregate [67]	✔ Yes

and Figure 3 show in detail the comparison of performance outcome and standard requirement met of mining waste used for bitumen modification based on the review literature.

Silica fume has been shown to enhance the mechanical performance and durability of concrete pavements significantly. When added to cementitious mixtures, it reacts with calcium hydroxide to form calcium silicate hydrate (C-S-H), refining the microstructure and reducing permeability. This increases compressive strength and chemical attack resistance [12].

Lithium slag has demonstrated promise as a partial cement replacement in mortar, especially at 30% substitution. Its inclusion contributes to improved compressive strength and overall durability of concrete elements used in rigid pavement applications [18].

4.3. Compositional Variability and Quality Control

One of the most critical challenges in utilising waste mining in road construction is the inherent compositional variability of these materials. Fly ash, red mud, steel slag, tailings, and other industrial by-products exhibit significant differences in chemical and mineralogical content depending on their geographic origin, processing conditions, and storage practices. For instance, steel slag has been reported to contain CaO ranging from 30 to 40% and SiO₂ from 10 to 180%, significantly impacting its expansion potential and reactivity [19,77]. Similarly, red mud from different alumina refining processes shows Fe₂O₃ contents between 20 and 40%, influencing binder stiffness and leaching risk [16,17].

To ensure performance consistency, a structured quality control framework is essential. Standardised testing protocols should be implemented to assess key parameters such as pH, specific gravity, fineness, and heavy metal concentrations prior to incorporation into asphalt mixtures. Coefficient of variation (CV) analyses from existing studies reveal oxide content CVs of 8–18% in fly ash [14,46,67] and 10–22% in red mud [8,43], underscoring the need to move beyond average values and account for statistical dispersion that may influence mechanical behaviour.

Establishing recommended testing frequencies—such as batch-based chemical profiling and periodic leachate assessments—would support quality assurance, particularly in large-scale construction projects. Blending strategies and source pre-qualification, as proposed by Lima et al. [42] and Mir & Bhat [70], are practical methods for homogenising feedstock variability, thus improving pavement performance predictability.

Future research should focus on defining tolerance thresholds and CV limits linked to key performance metrics like Marshall Stability, Indirect Tensile Strength (ITS), and rutting resistance, similar to frameworks explored in the work of Choudhary et al. [43] and Mir & Bhat [70]. Developing such criteria would facilitate broader adoption of mining waste materials while minimising risks associated with compositional inconsistency.

Table 9. Key mechanical and environmental performance properties of selected materials in asphalt applications.

Property	Mean Values (from Studies)	Estimated CV (%)	Key Findings	References
Fly ash
Marshall Stability (CBEM)	Best at 5% fly ash (values not explicit)	10–15	Improved Marshall Stability, Marshall Quotient, retained stability, lower temperature sensitivity	[46]
Unconfined Compressive Strength (UCS)	500–1500 kPa (4–8% OPC + fly ash, 28 d)	12–20	UCS improves with binder content in fly ash–modified paste backfill	[14]
Indirect Tensile Strength (ITS)	Not explicit	10–15	Substitution with 5–15% fly ash improves ITS, moisture resistance, and stiffness	[67]
Environmental (Pb Leaching)	Reduced Pb from 43 mg/L to 15 mg/L	n/a	Fly ash stabilisation lowers lead leaching, but may still exceed EPA TCLP limit (5 mg/L)	[42]
Steel Slag
Marshall Stability	8–15 kN (30–40% slag as filler)	10–15	Improved Marshall Stability, better rutting resistance, increased stiffness	[77]
Unconfined Compressive Strength (UCS)	~2–5 MPa for stabilised layers	12–20	Used as coarse and fine aggregates in pavement base/subbase with satisfactory UCS	[67]
Indirect Tensile Strength (ITS)	~700–1300 kPa	10–15	Assumed similar to mineral filler systems; improves moisture damage resistance	[17,80,81]
Environmental (Leachate)	Generally below TCLP limits	n/a	Heavy metals such as Cr, Pb, Zn are often well stabilised in steel slag	[77]
Red Mud
Marshall Stability	Increase of 10–30% over limestone	12–18	Acts as mineral filler, improves Marshall Stability and stiffness, but colour issues are noted	[42]
Unconfined Compressive Strength (UCS)	~1–3 MPa for stabilised layers	12–20	Red mud-cement blends show reasonable compressive strength	[82]
Indirect Tensile Strength (ITS)	Improved vs. limestone filler	10–15	Improves bonding and reduces stripping	[8]
Environmental (Leachate)	Arsenic, Cr may exceed limits	n/a	Heavy metal leaching concerns; requires encapsulation or cement stabilisation	[82]
Silica Fume
Marshall Stability	15–20% higher vs. limestone	8–12	Increases stiffness, improves rutting resistance	[83,84]
Unconfined Compressive Strength (UCS)	Not typical in bitumen (mainly cementitious)	n/a	Silica fume mostly used as filler or performance-enhancing agent in binder	[84]
Indirect Tensile Strength (ITS)	Generally improved (1091.47 kPa)	8–12	Enhances moisture damage resistance	[84]
Environmental (Leachate)	Generally safe	n/a	Chemically inert, no significant leaching concerns	[66]
Mine Tailings (General)
Marshall Stability	8–12 kN typical	10–20	Acceptable to moderate improvement with tailings; depends on mineralogy and gradation	[43]
Unconfined Compressive Strength (UCS)	1.5–3 MPa for cement stabilised tailings	10–25	Suitability varies; cement-stabilisation improves UCS	[35]
Indirect Tensile Strength (ITS)	600–1000 kPa	8–15	Sparse research; conservative estimate similar to other mineral fillers	[67]
Environmental (Leachate)	Variable, needs more testing	n/a	Metals may leach; encapsulation in bitumen suggested	[85]

below summarises key mechanical and environmental performance properties of selected materials in asphalt applications, along with estimated coefficients of variation (CV) drawn from recent studies.

5. Discussion

A key limitation identified across the reviewed literature is the lack of robust statistical analysis accompanying mechanical and durability data. Several studies report performance metrics such as Marshall stability, indirect tensile strength, or rutting resistance, without indicating sample sizes, standard deviations, or the statistical significance of observed trends. This omission hinders the reliability and reproducibility of comparative findings. To ensure scientific rigour, future evaluations of mining waste in asphalt applications should consistently report the coefficient of variation (CV), use appropriate sample replication (typically ≥ 3), and apply statistical tests, such as t-tests, to validate differences between control and modified mixes. Visual data presentation through box plots, error bars, and confidence intervals can further improve clarity and facilitate meaningful cross-study comparisons. By adopting these practices, research outcomes will be more transparent, comparable, and transferable to real-world applications.

The reviewed literature also reveals considerable variability in test protocols and evaluation methods for mining-waste-modified pavements, which hinders data comparability and broader applicability. Differences in sample conditioning, compaction methods, and test temperatures across studies lead to inconsistent mechanical and durability outcomes. To advance standardisation, the adoption of harmonised test procedures (e.g., ASTM D6927 [71] for Marshall stability, ASTM D6931 [72] for indirect tensile strength) is recommended across studies. Additionally, performance characterisation of waste-modified asphalt binders should incorporate standard rheological tests (e.g., DSR, BBR) and report coefficient of variation (CV) values to ensure statistical rigour. Where existing protocols fall short, particularly in testing unconventional fillers like red mud or tailings—new supplementary protocols should be developed under frameworks such as AASHTO T324 (Hamburg Wheel Tracking) or EN 12697 [86,87]. Establishing reference thresholds for minimum performance criteria and integrating environmental durability tests (e.g., leachate stability after freeze—thaw cycles) will also enhance reliability and facilitate regulatory acceptance.

Nonetheless, the reviewed literature demonstrates a strong potential for repurposing mining waste materials as functional components in road pavement construction. Materials such as fly ash, steel slag, red mud, silica fume, and tailings have been demonstrated to enhance engineering properties, including strength, durability, and moisture resistance, across various pavement layers. For instance, fly ash and ground granulated blast furnace slag (GGBFS), when blended, improve subgrade strength and reduce plasticity index [6,22], while red mud—despite its high alkalinity—has shown excellent results in cold mix and geopolymer applications when chemically treated [19,23].

However, material variability remains a major challenge. Tailings and slags differ significantly based on their mineral origin and processing methods, which impact consistency in mechanical behaviour and environmental performance [2,14]. Additionally, concerns about heavy metal leaching—especially in materials such as red mud and tailings—necessitate comprehensive geochemical assessments prior to field deployment [34,38].

An important aspect of sustainable mining waste utilisation in pavements is the potential environmental impact arising from heavy metal leaching. This review highlights the following considerations:

Although the reviewed studies largely agree on the potential for bitumen and cement matrices to immobilise mining waste, there is a notable lack of standardised leachate testing data to support this assumption. Red mud is reported to exhibit a relatively low pollution risk after encapsulation in bitumen; however, no consistent Toxicity Characteristic Leaching Procedure (TCLP) data were found, particularly for critical elements such as arsenic, chromium, and vanadium [82]. Steel slag is considered environmentally stable due to its crystalline structure; however, no numerical TCLP concentrations were reported to confirm heavy metal immobilisation, especially for chromium, vanadium, and nickel [77]. Copper tailings, while demonstrating good mechanical performance in asphalt, also lack quantitative leachate data, despite their potential for copper, arsenic, or lead release [85]. Fly ash, commonly assumed to be safely immobilised in asphalt binders, similarly shows no explicit leachate results within the reviewed literature, highlighting the need for verification, particularly for arsenic and lead [42]. Silica fume is largely considered chemically inert and does not pose significant leaching concerns, yet long-term performance under weather conditions should still be confirmed [66]. Other mine tailings, varying widely in mineralogy, may carry environmental risks from metals such as cadmium, zinc, or lead, but no studies in this review reported standardised TCLP data. Therefore, future research should systematically quantify leachate concentrations from mining-waste-modified asphalt using standard TCLPs, benchmarked against regulatory thresholds, and including long-term field and accelerated ageing conditions to ensure environmental compliance.

While many studies report that modified mixtures meet or exceed conventional performance benchmarks [68,75], long-term field validation under diverse climatic and loading conditions is still limited. Furthermore, discrepancies exist regarding ageing behaviour and compatibility of modified binders. For example, sulphur-modified asphalt mixtures show increased thermal resistance but may reduce low-temperature flexibility if improperly dosed [60,64].

Beyond laboratory performance, the broader adoption of mining waste in pavements must be evaluated through economic, durability, and climatic lenses. Economically, the use of industrial by-products such as fly ash, steel slag, and red mud offers promising cost savings—some studies report reductions exceeding 50% compared to conventional materials. However, these findings often lack detailed breakdowns of processing, transportation, and site-specific costs. A more comprehensive cost–benefit framework, incorporating sensitivity analyses under varying market conditions, is necessary to validate financial viability. In terms of durability, while many studies demonstrate improved ageing resistance, moisture stability, and strength development under laboratory settings, real-world translation remains uncertain. The absence of large-scale field validation and long-term performance monitoring under service conditions is a critical research gap. Additionally, the impact of climate—especially freeze—thaw cycles, high ambient temperatures, and seasonal moisture variation—can significantly influence binder-filler interactions and leaching potential. Region-specific trials and climate-adapted material design are essential to ensure resilient performance. Collectively, addressing these interconnected factors will support the sustainable and practical integration of mining waste into asphalt infrastructure across diverse environments.

Another barrier to broader adoption is the absence of standardised guidelines for mixed design and testing. Many studies use different proportions, curing conditions, or performance tests, making it difficult to draw universal conclusions or compare results directly [67]. In this regard, AI-supported synthesis, as applied in this review, can play a role in identifying hidden trends and standardising interpretations across studies. Overall, the findings support the technical viability of using mining waste in pavements, particularly for stabilising subgrades and enhancing asphalt binder properties. However, implementation at scale requires addressing regulatory gaps, developing robust environmental risk assessments, and validating long-term durability through pilot projects. Collaboration between researchers, industry, and policymakers is essential to translate laboratory success into sustainable infrastructure solutions.

6. Conclusions

The integration of mining waste materials into road pavement construction presents a compelling opportunity to enhance sustainability, reduce environmental impact, and improve engineering performance. Materials such as red mud, fly ash, steel slag, silica fume, and sulphur have demonstrated the ability to significantly improve mechanical properties, including Marshall stability, tensile strength, rutting resistance, and ageing durability. In addition, they have shown increased resistance to moisture-induced damage and improved elasticity, stiffness, and binder’s stability.

While this review consolidates a diverse body of knowledge on the use of mining waste in asphalt construction, several limitations remain across current research. Many studies are geographically concentrated, often based on specific regional waste sources, which limits the generalizability of findings. Additionally, small sample sizes, inconsistent testing methods, and inadequate reporting of statistical measures (e. g., coefficient of variation, significance levels) reduce the reliability of performance comparisons. The lack of field-scale studies and limited long-term monitoring data further restricts the validation of laboratory results. Future research should prioritise multi-site trials using standardised test protocols, supported by robust statistical frameworks. Emphasis should also be placed on toxicity and leachate behaviour under service conditions, life cycle emissions modelling, and economic sensitivity analysis. Exploring the interplay between binder—filler chemistry under varying climatic regimes and developing material design guidelines tailored to TRL 3–6 applications, will accelerate real-world adoption. These actions are critical to transitioning the current state of research from laboratory innovation to large-scale sustainable infrastructure deployment.

In asphalt mixtures, these materials act effectively as mineral fillers and modifiers, while in rigid pavements and subgrade layers, they serve as cement or aggregate replacements and stabilisers. Their pozzolanic or cementitious nature supports the development of strong, durable binders and matrices, offering a viable alternative to conventional pavement materials. The consistent achievement of or improvement beyond standard performance thresholds supports their potential for mainstream application. Beyond the technical benefits, the use of mining waste supports circular economy principles by transforming industrial by-products into valuable construction materials, reducing landfill burden, conserving natural resources, and cutting greenhouse gas emissions associated with virgin material production.

Nevertheless, several research gaps must be addressed to enable broader adoption, outlined as follows:

• Standardisation of mix design remains a challenge due to the variability in testing protocols and material properties.
• Field validation under varying climatic and traffic conditions is needed to complement laboratory findings.
• Long-term durability assessments, particularly regarding ageing and freeze–thaw resilience, are still limited.
• Material compatibility, especially involving red mud or geopolymer binders with conventional asphalt and aggregates, warrants further study.
• Environmental assessments, such as full life-cycle analysis (LCA) and leachability studies, are essential to confirm the environmental safety and carbon benefits of these applications.
• Economic feasibility and policy support must be developed through cost–benefit analysis and incorporation into pavement design standards.

Future research should focus on resolving these challenges through interdisciplinary studies that combine materials science, pavement engineering, environmental impact modelling, and field performance assessment.