The Use of Waste Fillers in Asphalt Mixtures: A Comprehensive Review

Abstract

The asphalt industry has long been challenged with finding sustainable solutions to enhance the performance of asphalt mixtures while mitigating their environmental impact. One promising avenue is the incorporation of waste filler materials into asphalt mixtures. This review explores the feasibility and effectiveness of utilizing waste filler in asphalt mixtures, focusing on its effects on the mechanical characteristics, durability, and sustainability of asphalt pavements. Various waste filler materials, such as rice husk ash, fly ash, and construction and demolition wastes, have been examined in terms of their potential as substitutes for traditional filler materials such as limestone and mineral powders. This review synthesizes literature to assess the impact of waste fillers on the performance of asphalt mixtures, including rutting resistance, fatigue behavior, moisture susceptibility, and aging characteristics. This work begins by examining the interaction of the asphalt fillers to provide clarification. The usage of various waste fillers is then examined. With fewer harmful environmental consequences than traditional cement manufacturing has, waste filler materials improve the strength and durability of asphalt mixtures. This research underscores the promising future of waste filler materials as environmentally friendly and innovative materials. To fully capitalize on their benefits, further research, standardization, and widespread use of waste filler-based products are necessary.

1. Introduction

Recently, significant industrial attention has been given to the global energy crisis and its associated environmental issues. Consequently, there has been a growing push to comprehend and enhance the interplay between technology and the environment, as well as the notion of sustainable development [1]. This also applies to the road industry. The International Road Federation estimates that the transportation sector produces more than 20% of the carbon dioxide (CO₂) emissions associated with energy, contributing to approximately 15% of the global emissions of greenhouse gases [2,3]. Furthermore, asphalt mixtures account for more than 90% of pavements in numerous countries; the asphalt binder in these mixtures is often made through the distillation of crude oil [4]. The remaining reserves, according to some estimates, are predicted to run out in approximately 46 years if nonrenewable oil is continuously extracted and used [5]. According to Schipfer et al. [6], the usage and development of resources and renewable energy have become important issues for overcoming the lack of resources and contamination of the environment produced by the processing of petroleum products. Additionally, it presents a chance for the road sector to adopt sustainable and environmentally friendly practices.

Fillers play a highly complicated role in asphalt mixtures [7]. DeSmedt was the first to intentionally utilize filler in asphalt mixtures in the 1870s. Even though, until 1893, experts were unable to determine the specific purpose of the filler added to the asphalt mixture, it was only used to fill the spaces between the stones to increase the density and impermeability of the mixture [8,9]. Early research, such as that of Richardson [8], suggested that the filler in asphalt mixtures served a purpose other than merely filling voids. Additionally, Richardson suggested that the filler–asphalt system (asphalt mastic) might be the site of some physicochemical phenomena. When filler is added to mastic, it stiffens linearly at a rate known as the Einstein coefficient, according to a 1911 study by Einstein [10]. Later, it was discovered that filler has two roles in mixtures. First, it represents a hemorheological simple linear viscoelastic substance that, upon mixing with an asphalt binder, maintains the cohesiveness of the mixture and impacts its behavior under various stresses. Second, filler particles act mainly as inert materials, filling spaces between larger aggregates in the mixture [11]. The inclusion of finer filler particles in asphalt mixtures stiffens the mastic and enhances the mechanical characteristics, such as the strength and density. Smaller filler particles function as bitumen extenders in mastic, suggesting that more binder is present in the mixture [12]. Fat patches, binder leakage, and stability loss are among the issues associated with this behavior. The resistance of the asphalt mastic to fatigue life at intermediate temperatures, cracking resistance at low temperatures, and permanent deformation at high temperatures are all influenced by its stiffness. In addition to changing the binder by increasing its viscosity and creating a stiffer mastic that binds aggregates better, they fill spaces between larger aggregates, increasing the packing density and compaction. This results in a decrease in the material’s vulnerability to deformation and moisture damage and an improvement in its mechanical properties, such as strength, stability, and fatigue resistance. Fillers also improve a material’s resistance to environmental pressures such as oxidation and temperature variations. Fillers enhance the overall strength, workability, and durability of asphalt concrete by maximizing the use of binders and increasing load transfer within the mixture [13]. Because of their interactions with bitumen and aggregates, certain fillers also function as anti-stripping agents, reducing the aging of moist dams [14,15]. The kind and amount of mineral filler used in mastics have considerable effects on their physical and mechanical characteristics, according to Diab and Enieb [16].

Numerous scholars have examined the efficacy of using waste materials in place of traditional mineral filler in asphalt mixtures. Finding substitute materials is crucial due to the scarcity and restrictions of natural resources. In addition to being a financially viable alternative, recycling waste materials is also a green method [17,18]. The incorporation of recycled materials in construction has expanded due to the restrictions and scarcity of natural resources. Research on the substitution of various recycled materials for traditional fillers in asphalt concrete has been conducted on a large scale using waste products [19]. Because they improve the cohesiveness of the asphalt binder and fill voids in the paving mix, mineral fillers, which are crucial components of the aggregate skeleton of pavements, are used in asphalt mixtures [20]. Using waste to build pavement has proven to be an environmentally friendly solution to meet budgetary limits without sacrificing social or technological goals, making it one of the best ways to increase sustainability. Additionally, many recycled waste fillers can enhance the mechanical properties and durability of asphalt concrete, contributing to sustainable pavement solutions without compromising performance. This practice supports environmentally friendly construction and advances the circular economy in infrastructure development. Waste materials from industry and agriculture are among the fillers used recently. Research has been conducted on the properties of these materials, and different findings have been reported regarding the effects of these materials on the strength properties of asphalt mastic.

According to the knowledge of the authors, there is still limited reviews on the use of waste materials as fillers in asphalt. Furthermore, none of the previously published reviews have addressed recently released findings. Hence, this review focuses on newly developing waste materials that haven’t been well-explored in previous review works, such as calcium carbide residue and other novel byproducts. This work incorporates the most recent research, providing up-to-date information on the advantages that waste fillers have in asphalt mixtures, for both the environment and mechanics properties. Furthermore, this review offers a thorough assessment of a larger variety of waste fillers and their effects on the performance of asphalts, emphasizing useful applications and potential future research areas. For this, this review followed a unique approach to resolve the inconsistent findings provided in the current literature. A systematic literature review approach was employed to evaluate and compile a sizable body of work that has been published during the last 20 years (2010–2024). The conclusions made from the critical examination of recent developments highlight the difficulties that exist today and the prospects for environmentally friendly road construction. Consequently, this paper offers an up-to-date evaluation of innovative and recent studies on waste filler integration in pavement domains. This thorough evaluation aims to provide researchers, professionals, and engineers in the construction and materials domains with a thorough grasp of waste filler applications in pavement engineering. Finally, concepts and recommendations for more investigations were also included. This review serves as a critical evaluation of methods and conclusions, directing future research initiatives. It also serves as a foundation for future studies and constitutes an important instructional tool that provides researchers and professionals with a thorough understanding on the subject and advances the body of knowledge in the field.

2. Methodology

Figure 1 shows a flow chart of the adopted procedure. For this purpose, several research databases, including Scopus, Web of Science, and Google Scholar, were used. Information was taken from the research databases that were accessible online. Papers with the terms “asphalt mixture” AND “waste filler” in the title, keywords, or abstract were searched. A combination of keywords, including “recycled filler”, “pavement performance”, “sustainability”, etc., was also used to refine the search queries and ensure comprehensive coverage. More than 1000 articles were found when the search was restricted to those published between 2010 and 2024. A further refinement of the research was made by limiting the results to “search by document type”. Writing, reviews, corrections, meeting abstracts, early access, and proceeding papers were the categories assigned to the documents. Some of the commonly used waste fillers in asphalt are shown in Figure 2. The quantity decreased as the authors concentrated on “articles”. After each publication’s title and abstract were read, the writers chose which papers to consider. For the current review, 125 articles were deemed suitable. Studies not written in English, unrelated to asphalt mixtures, or lacking relevance to waste filler utilization were excluded. To preserve credibility, only peer-reviewed, high-quality publications from reliable sources were selected. To increase the review’s credibility and impact, articles from high impact factor journals, papers in the top quartiles, and articles with a large number of citations were given priority, ensuring that the selected literature is both impactful and widely recognized within the field. The number permits thorough investigation into the subject while still being practical for in-depth analysis given time and resource limitations. Each waste material was subsequently evaluated in terms of its impact on the performance of the asphalt. As a result, the methodology provides a structured approach for conducting a comprehensive review of the use of waste fillers in asphalt mixtures. This review attempts to offer important insights into the possible advantages and difficulties associated with the use of waste fillers in asphalt pavements by combining the literature and research findings.

3. Asphalt–Filler Interaction and the Characteristics of Fillers

On the basis of rheological characteristics, Liu et al. [21] evaluated the interaction between the filler and binder and reported that mastics performed better in terms of interaction ability at high temperatures and low loading frequencies. Specifically, satisfactory performance with adequate viscoelastic behavior and strong adhesion is guaranteed for asphalt mixtures as long as the ratio between the filler and binder is equal to or lower than 1.4%. Mukhtar et al. [22] aimed to redefine the relationship between asphalt and filler by assessing and measuring the disparities in each recognized mastic composite model. The asphalt–filler interaction was further examined via assessment index coefficients on the basis of rheological performance.

Structured asphalt, resulting from the interaction of filler and asphalt, is described as asphalt with ΔR, which refers to the thickness of the asphalt layer. This thickness represents the extent to which the filler influences the asphalt’s properties, creating a more organized structure within the asphalt mixture. After this ΔR thickness, the asphalt is referred to as “free asphalt” since it is in a “free” state and is separated from the filler without interaction. In other words, ΔR marks the boundary between the asphalt that is influenced by the filler and the asphalt that is in a free, unstructured state [23,24,25]. On the basis of these observations, Tunnicliff [26] suggested that there is a gradient for the interaction between asphalt surfaces and fillers such that the degree of interaction gradually decreases as the distance between filler and asphalt surfaces increases. This model can adequately explain the hardening characteristics of asphalt from filler particles that range in surface chemical composition but have similar roughness, shape, and size [27]. The current consensus on the model of the asphalt–filler interaction holds that asphalt wraps around the surfaces of the fillers and interacts with them, and the degree of interaction decreases as the asphalt moves away from the surfaces of the filler. A wide range of theoretical explanations, including surface free energy theory, surface structure theory, molecular orientation theory, chemical reaction theory, electrostatic theory, and mechanical bonding theory, have been presented by researchers to explain the asphalt–filler interaction practically [28,29,30,31,32]. The interaction between asphalt and filler is an extremely intricate process whose principle cannot be adequately explained by a single model or theory. It is challenging to provide a single, broadly applicable theory that can adequately describe the mechanism given the variety of fillers and asphalt in addition to their variable conditions of interaction. In the context of asphalt–filler interactions, the six aforementioned ideas might operate concurrently [33].

It has been suggested that filler particle properties play a significant role in the performance of an asphalt mixture. The specific surface area was the most important factor affecting blend performance at high temperatures [34,35]. Because of its physical properties, the mineral filler in asphalt mixtures reduces the porosity of the granular structure, which makes it more difficult for air and water to enter the mixture and increases its durability in the event of water action [36]. Filler particles interact in a complex and material-dependent way at and above the threshold concentration. These interactions are influenced by various factors, including the filler surface properties, size distribution, and particle shape. The specific concentration at which the addition of filler material starts to noticeably change the system’s physical characteristics—most notably its stiffness and viscosity—is known as the critical concentration. The mixture is still reasonably fluid and workable before this point. However, when the required amount is achieved, the filler particles start interacting more intensely and frequently form a network inside the binder, which causes the binder to become noticeably stiffer and lose its fluidity. This emphasizes how crucial it is to carefully control filler concentrations to maintain beyond this crucial point [37].

Table 1. Chemical composition of waste fillers.

Filler	Oxides										Ref.
	SiO2	Fe2O3	Al2O3	K2O	SO3	CaO	MgO	P2O5	Na2O	TiO2
Rice husk ash	74.89	1.33	1.06	6.09	1.21	2.89	1.96	6.05			[40]
TDF fly ash	25.40	5.59	4.03	0.76		36.40			0.57		[18]
HSP	27.38	0.79	12.55		-	28.12	7.37			21.46	[41]
SSF	15.77	23.179	1.362			46.290	4.170	1.867			[42]
Silica fume	98.212	0.00	0.348	0.812		0.221	0.264			0.161	[43]
CCR	14.08	0.00	0.90	0.20	0.77	81.84	0.77		1.32	0.12	[44]
FA	61.44	6.87	28.31	0.37	0.15	0.46					[45]
GP	74.54	0.26	1.49			8.76			13.93		[45]
Red mud	18.19	17.54	8.03	0.40	0.90	44.64	1.34	0.26	3.21	4.81	[34]

shows the chemical composition of the waste fillers. Earlier studies demonstrated that some of these materials exhibit pozzolan (binding) properties [38,39]. Different materials have varying compositions of oxides, which can influence their properties and potential applications. Rice husk ash has a high percentage of SiO₂ (74.89%) and moderate amounts of other oxides. Silica fume is predominantly composed of SiO₂ (98.21%), with minimal amounts of other oxides. Red mud contains significant amounts of Fe₂O₃ (17.54%) and Al₂O₃ (8.03%), along with other oxides.

4. The Use of Various Waste Fillers

4.1. Rice Husk Ash (RHA)

When rice grains are milled, one of the principal agricultural leftovers that are recovered from their outer covering is the rice husk, which makes up 20% of the 500 million tons of paddy rice produced worldwide. After the RH and RS are burned, rice husk ash (RHA) or rice straw ash (RSA) are produced [46]. RHA has gained attention as a potential filler material in asphalt mixtures because of its abundance, low cost, and pozzolanic characteristics. It contains a high silica content, has pozzolanic properties, and reacts with calcium hydroxide to form cementitious compounds when mixed with water. Some academics have recently attempted to evaluate the feasibility of integrating RHA into asphalt pavements. The use of RHA as a filler in hot mix asphalt was investigated by Sargın et al. [46]. They discovered that adding a filler that was 50% RHA and 50% limestone powder improved Marshall flow and stability in a positive way. The impacts of adding RHA to an asphalt mixture as a filler with a particle size of less than 75 µm were examined by Ramadhansyah et al. [47]. They investigated this phenomenon by combining Marshall and density tests. The results showed that the use of RHA in combination with fine particles enhanced the performance of the asphalt mixture. Tahami et al. [48] used 0–100% RHA to replace limestone filler. The results of the mixture test revealed that increasing the amount of RHA increased the dynamic stability of the asphalt mixture. This suggests that replacing the rice husk ash filler could increase the stability of asphalt at high temperatures. The greatest results were observed when 75% of the total filler was replaced. On the other hand, Al-Hdabi [49] reported an initial increase followed by a decrease in the dynamic stability of an asphalt mixture as the replacement amount of rice husk ash filler increased. It also improved the stability of asphalt at high temperatures, and the maximum impact was obtained at 50% RHA. The effectiveness of fillers and the rheological properties of asphalt mixtures are influenced by the interactions between the binder and filler and the packing density of the particles. This suggests that there is an optimal concentration of filler that maximizes the performance of the asphalt mixture. Helal et al. [50] replaced limestone dust (LSD) in their investigation with the impact of RHA on the mechanical characteristics of asphalt concrete. Adding RHA reduced the values of the indirect tensile strength, rut depth, and Marshall stiffness. Moreover, 50% RHA and 50% LSD were the ideal replacement ratios. Additionally, the flow and dynamic modulus values were greater in RHA-based blends. Depending on the dosage, quality, and mixture design, there can be differences in ITS trends when RHA is employed. Because of its filler effect and pozzolanic qualities, moderately processed RHA tends to increase the tensile strength; excessively processed or badly processed RHA might weaken the mixture because of problems with bonding and mixture uniformity.

The effects of using RHA in place of hydrated lime filler on the characteristics of asphalt mixtures were examined by Ameli et al. [51]. It is possible to enhance Marshall stability by using RHA instead of filler, according to their research. To investigate the impact of filler on the moisture resistance and performance of densely graded asphalt at high temperatures, Mistry et al. [52] employed rice husk ash rather than hydrated lime. For comparison, various percentages of rice husk ash (RHA) and hydrated lime (HL) filler (2%, 4%, 6%, and 8%) were added to the asphalt mixture by following design mixes according to the Marshall method, and the results were compared with those of the control mix prepared with 2% HL. According to the study’s findings, adding RHA and FA to HMA improves its performance and proves to be cost-effective because it reduces the optimal bitumen content of the mix by 7.5% when RHA and FA are added at a filler ratio of 4%.

To enhance their engineering characteristics, Yaro et al. [40] optimized and determined the ideal amount of asphalt binder in asphalt concrete mixtures modified with waste rice straw ash (WRSA) and waste palm oil clinker powder (WPOCP). The filler was replaced by WRSA at 0–100%, while WPOCP was added to 2–8 wt.% asphalt. The asphalt binder percentage of the mixture ranged from 4–6%. Numerical optimization was used to obtain the optimum contents of asphalt, WRSA, WPOCP, and WRSA, which were 5%, 74%, and 8%, respectively (Figure 3). All replies had a mean error of less than 5%, suggesting that the created models accurately reflected the outcomes of the predicted values and fit well with the experimental data.

Response Surface Methodology (RSM), which is a statistical method, was used to model, assess, and improve the process while reducing the required experimental run number [53]. RSM accomplishes this by analysing the distinct impacts and interplay among diverse elements. RSM has been used in recent research on asphalt pavements to investigate the effects of several variables, including modifiers, applied stress, and temperature, on different aspects [54]. In addition to a number of thermal performance indices, Abdelmagid et al. [55] investigated how RH affects the performance of asphalt. Changes in the preparation parameters had a substantial effect on the tensile strength, stability, flow, and volumetric characteristics of the asphalt mixture, according to the RSM analysis. There was a strong association shown by the strong agreement of the suggested models with the empirical results. Furthermore, the numerical optimization results revealed that 7.6% RHA and 5.3% asphalt were the ideal combination proportions for obtaining the intended maximum reactions. The remarkably porous structure is evident in the larger and irregularly shaped RHA particles, as demonstrated by the data shown in Figure 4a. Figure 4b shows that the RHA particles are uniformly dispersed when they are added to the asphalt binder. Its significant reactivity with bitumen and ability to promote the creation of a dense filling structure are due to the activated amorphous SiO₂ found in RHA. The improved asphalt composite has uniformly distributed RHA particles.

4.2. Fly Ash (FA)

According to research by Sobolev et al. [56], FA is a suitable material for use in HMA mixtures because it has been shown to improve asphalt pavement performance while lowering costs and environmental effects. The microscale experiment also revealed that FA particles caused crack-arresting within the binder matrix. The impact of the use of FA in place of regular mineral filler was examined by Mistry and Roy [57]. By adding 2% hydrated lime and 2% to 8% fly ash by total weight of aggregate, samples with varying bitumen contents (3.5% to 6.5% at 0.5% increments) were created. Compared with a conventional mixture and standard specification, the study findings indicated a greater stability value and a lower optimum binder content for a combination containing 4% FA as the ideal filler content. According to this investigation, FA met the standard specification and was a suitable substitute filler for hydrated lime in the asphalt concrete mixture. Yan et al. [58] examined the effectiveness of asphalt mastics with FA with various binder weight ratios f/b (0.6–1.2) that were manufactured at 160 °C. Similar conditions were used to create control asphalt mastics from limestone filler (LF). At 58 °C, 64 °C, 70 °C, 76 °C, and 10 rad/s, the shear modulus was evaluated below the linear viscoelastic zone; bending beam rheometers (BBR) were used to determine the low-temperature creep stiffness and creep rate behavior at −6 °C, −12 °C, −18 °C, and −24 °C. The results of the tests indicated that FA-mastic with 1.2 f/b at 58 °C outperformed conventional mastic at the same ratio in terms of fatigue and creep performance.

Three waste types were investigated by Russo et al. [59] for their potential as fillers: jet grouting (JG), fly ash (FA), and construction and demolition (CD) debris. A total of eight asphalt mixtures were created by blending each type of filler with a 50/70 penetration grade of bitumen using two filler-to-binder weight ratios (f/b) of 0.5 and 1. When a f/b ratio of 1 was used, the asphalt mixtures containing FA and JG fillers exhibited better environmental and mechanical performance than the conventional mixture did. Additionally, these mixtures presented creep compliance values Jnr (averaging 35% less than the traditional asphalt mixture) and higher complex shear modulus values, G* (averaging 50% more than the traditional asphalt mixture), across all tested temperatures. Li and Yang [60] aimed to understand the physicochemical relationship between asphalt binders and filler particles, such as fly ash and limestone. According to the findings, binders using various fillers produced surface morphologies that differed significantly, as shown in Figure 5. On the surface of several asphalt adsorption samples, “flake-like” or round-shaped scattered phases emerged instead of the typical “bee-like” structure. The asphalt–filler interaction effect caused these scattered phases to change in size and quantity as they approached the filler surface. Additionally, the ageing indices (IS=O and IC=O) of each asphalt-adsorbed layer of FA-mastics were lower than those of the equivalent sample of limestone asphalt mastic.

The possibility of substituting conventional mineral filler in bituminous mixtures with municipal solid waste incineration and fly ashes (MSWI-FA) has been explored by several researchers. When comparing MSWI-FA to the conventional filler, there was a negligible detrimental impact on the characteristics at low temperatures and a large favourable impact on the characteristics at high temperatures [58]. The use of MSWI-FA as a filler substitute in HMA mixtures was investigated by Romeo et al. [61] via laboratory experiments. To achieve this goal, two distinct forms of MSWI-FA were used to test a number of HMA combinations with the same gradation. The mechanical characteristics of the samples containing MSWI-FA as a filler are consistent with those of the traditional HMA mixture.

Fly ash was utilized by Al-Hdabi et al. [62] to enhance the mechanical characteristics and chemical resistance of cold-rolled asphalt (CRA) by substituting filler with FA. A considerable increase in uniaxial creep and the stiffness modulus was observed, along with an improvement in resistance to moisture damage. By replacing limestone filler with a binary filler from a fluid catalytic cracking catalyst (FC3R) and FA, Dulaimi et al. [63] developed a novel cold asphalt concrete mixture for a binder course. The use of such materials was shown to significantly increase the resilience of CBEMs to fatigue cracking and rutting. Fly ash was found to enhance the performance of asphalt binders to a degree similar to that of polymer modification in a study conducted by Sobolev et al. [64]. The enhanced resistance to thermal cracking as well as the ability to reduce internal thermal stress build-up during the winter indicated that the inclusion of fly ash increased thermal relaxation. It may be possible to use fly ash to lower the quantity of asphalt binder required to obtain the desired results.

4.3. Bottom Ash (BA)

A portion of municipal solid waste (MSW) is delivered to waste-to-energy facilities so that it can be converted into useful resources. The other portion is landfilled. When operating between 850 °C and 1000 °C, MSWI-BA is the principal residue byproduct of the incineration process. When everything has been completed, FA (approximately 4% of the weight of the burned MSW) is retained and awaits purification before being released into the atmosphere, whereas approximately 18% of MSW becomes BA at the time of combustion. Owing to their recoverability, the BA can nevertheless undergo revaluation procedures that make them ready for reuse [65]. Choudhary et al. [66] conducted a study that examined various waste materials, such as MSWI-BA, that might be utilized as fillers in asphalt mixtures. They reported that MSWI ashes could be utilized as a substitute for fillers due to their exceptional performance, which includes fatigue life and dynamic and Marshall stability, compared with conventional fillers. Santos et al. [67] carried out an experimental study to examine the performance of HMA mixtures utilizing vitrified municipal solid waste bottom ash (VMSW-BA) as a replacement for limestone filler. The process of vitrifying MSW-BA involves high temperatures. A homogenous, inert substance with little to no contamination risk is produced via this technique. The experimental findings demonstrated that replacing mineral filler with VMSWI-BA produces a notably satisfactory level of pavement performance. Because each aggregate binder pair has a different interfacial transition zone thickness and strength, the heterogeneous character of MSWI-BA significantly influences the damage behaviour of HMA [68]. Some publications have assessed the impact of the interfacial zone on the tensile damage behaviour of HMA mixtures with MSWI-BA on the basis of these findings. When assessing the damage process of HMA mixtures, it is important to consider the features of the asphalt mastic zone around the aggregate, which is known as the interfacial transition zone [64]. The stiffness variations between the mastic and aggregate particles cause the damage to start along the interfacial transition zones and spread over time, as per the numerical results. Likewise, the thickness of this zone significantly impacted the tensile strength of the mixtures. Compared with a mixture composed entirely of limestone aggregates, Luo et al. [69] reported an approximately 61.5% increase in the dynamic creep value with the use of 80% BA.

Jattak et al. [70] examined the impacts of adding BA as a filler in warm-mix asphalt, specifically when 20% of the fine aggregate was replaced. When BA was added as a filler to conventional hot mix asphalt, the mixtures created via Marshall method analysis showed that the indirect tensile strength increased, increasing the pavement’s resilience to loads. To investigate both cold and hot mixtures, asphalt mastics composed of bottom ash filler (BAF) were examined by Russo et al. [65]. Weight-based filler–bitumen ratios of 0.2 and 0.7 were investigated; eight mastics were made. The results are shown in Figure 6. When comparing cold asphalt mastics composed of BAF to similar mixtures made of LF, rheological testing revealed a greater ratio between the storage modulus (G′) and the loss modulus (G″) values for all filler-to-binder weight ratios (f/b ratios). There were average increases of 75% above 10 °C and nearly 98% above 30 °C, which led to an 11% decrease in the phase angle. When BAF mastics were compared with equivalent LF mastics, a 90% reduction in nonrecoverable creep compliance was detected at test temperatures greater than 40 °C.

4.4. Waste Slags

Marshall characteristics were utilized to assess the performance of a comparison study using two different types of fillers, namely, cement and ground-granulated blast-furnace slag (GGBFS), each of which is 2% by the total mass of the mixture [71]. Cement was found to have greater density and fewer air holes, although the GGBFS had higher Marshall stability. Compared with those of the CMA and HMA mixes with high cement contents, the durability and mechanical performance of CMA were improved by including 1% less cement with GGBFS and fly ash [72]. It was also discovered that the replacement of GGBFS performed better than the replacement of fly ash [73]. Additionally, the addition of GGBFS makes the features of CMA similar to those of HMA [74]. To create new cold asphalt emulsion mixtures (CAEMs), Dulaimi et al. [75] recently produced a novel cementitious material that contains ground-granulated blast-furnace slag and calcium carbide residue. This material is intended to replace standard mineral limestone filler. The enhancements in both durability and mechanical characteristics can be attributed to the development of cementitious products, including C-S-H gel, portlandite, and ettringite. These factors made early stiffness improvements possible.

A unique blast furnace slag with a TiO₂ content higher than 20% is called high titanium heavy slag (HSP), and it is produced after vanadium–titanium magnetite is smelted. This low reactivity results from its low calcium and high TiO₂ contents. The possibility of using the HSP as a filler was examined for the first time by Wang et al. [41]. To design asphalt mortars, the limestone filler was replaced with HSP at 0–100% with four filler-to-asphalt ratios (F/A ratios of 0.2, 0.3, 0.4, and 0.5). In contrast to limestone powder (Figure 7), HSP had greater specific surface areas, a coarser surface, smaller particle sizes, and more pores, all of which helped to create more “structured asphalt”. Additionally, although HSP has a complicated chemical makeup, it does not leach any harmful components, and its radioactivity, volume stability, and high-temperature stability all match the specifications. Weaker interactions with the asphalt were indicated by its alkalinity being much lower than that of the limestone powder. When the asphalt mortar was made with HSP instead of limestone powder at F/A ratios of 0.2 and 0.3, the behaviour of the materials at low temperatures improved, whereas their high-temperature properties slightly decreased.

Currently, asphalt pavements use increasing amounts of Ladle Furnace Steel (LFS) slag. LFS is typically used as a filler in HMA. Several studies [76,77] have examined the possibility of LFS slags as fillers concentrated on materials that are discharged after steel production. The LFS slag is generated from the molten materials of the furnace with the addition of quicklime during the heated-steel refining process [78]. High porosity, a distinct curve of grading (up to 1–2 mm) [76,79], and heterogeneous chemical composition are the primary features of LFS slags [77]. Both open- and dense-grade HMAs can benefit from the use of LFS as a filler, as shown by Bocci [76] and Skaf et al. [80]. However, workability issues may arise from improperly constructed mixes incorporating LFS. These impacts may also be connected to the chemical and mineral makeup of the LFS. Owing to the presence of particles that are sensitive to water, such as MgO and CaO, the mineral composition can be significantly altered [81]. For example, with the hydration of CaO, more Ca(OH)₂ (Portlandite) is produced, which causes the mastic phase to become stiffer [82]. Roberto et al. [77] noted that because CaO is fully hydrated at the filler level, there is a significant concentration of portlandite. At intermediate and low service temperatures, Pasetto et al. [79] suggested that LFS-based HMAs might be reduced. Because of the physico-chemical interactions between the bitumen and LFS filler, special care needs to be paid to the mastic stiffness [83]. Despite this, compared with materials that are frequently utilized, LFS enhances the resistance to permanent deformation and fatigue in the fabrication of HMAs [81]. Investigating and comprehending the impact of LFS properties on potential asphalt material stiffening effects is the primary goal of the work of Roberto et al. [84]. A pure asphalt binder as well as a binder with a 3.5% cross-linked version of styrene–butadiene–styrene (SBS) were created by combinations of limestone and LFS fillers. While the HMAs’ performance levels and the physical characteristics of the LFS did not significantly correlate, the findings highlight the fact that the LFS content of the filler blend should not be higher than 30%.

Steel slag powder (SSP) is produced by removing impurities and grinding large-particle steel slag. To increase the workability and durability of cement and concrete, SSP is currently commonly used in these materials [85]. The utilization of SSP as a filler in asphalt mixtures has, however, received consideration. In their evaluation of the viability of utilizing SSP in HMA, Wu et al. [86] reported that SSP provided better characteristics than did regular filler, particularly in terms of resistance to rutting and water. This occurred despite a slight volume expansion. In place of traditional mineral filler, Tarbay et al. [87] investigated the use of waste resources (granite and marble) and SSP. Compared with other natural aggregates, steel slag has more iron, which contributes to its superior functional qualities. These qualities include self-healing, microwave heating, and electrical conductivity [88]. In summary, SSP enhances the functional properties or interaction between aggregates and asphalt by serving as a filler to partially replace lime powder in asphalt mixtures. In addition to the interactions between various constituents, the material itself is primarily responsible for the creep and fatigue characteristics of asphalt mastic. For example, the size, shape, and physical characteristics of the inorganic micropowder filler influence how well the asphalt mastic performs. On the basis of the findings of Zhang et al. [89], the fatigue life and resistance to deformation of asphalt mastics improved when SSP was added rather than limestone powder. An increasing trend is observed in the complex shear modulus of the asphalt mastics as the SSP concentration increases. Compared with those of the limestone asphalt mastic, the complex shear moduli were 84.9%, 11.8%, and 256.1% greater with the use of 30%, 50%, and 100% SSP, respectively. Li et al. [90] examined the influence of the use of SSP on the rheological characteristics of asphalt. The surface features, thermal characteristics, chemical compositions, and rheological qualities of steel slag were compared with those of lime as a filler. Because adding steel slag to asphalt mixtures produced a stiffness that made mixtures with steel slag fillers more resistant to deformation at high temperatures than mixtures with lime fillers did, the results revealed adequate chemical bonding between the SSP alkaline components and the asphaltic acid of the bitumen. Because “type B (9.5–13.2 mm)” steel slag filler is manufactured at a temperature balanced for rheological qualities, it can be used in place of lime filler in asphaltic mastic, according to the conclusions drawn. Similar research was conducted by Tao et al. [42], who reported that steel slag fillers have properties that improve deformation resistance and increase the rutting factor as well as the complex shear modulus in asphalt concrete. They also contain chemical components that promote good adhesion with bitumen. Xiao et al. [91] conducted a study that examined the aggregate adhesion and resistance to moisture of asphalt mixtures with various fillers: SSP, slaked line, limestone, and cement. The results indicated that 25% SSP can replace limestone filler. This reduces the susceptibility of asphalt to moisture and, when combined with slaked lime, enhances the cracking resistance and adhesion of the asphalt aggregate. Meng et al. [92] suggested incorporating waste rubber and steel slag into asphalt. The complex modulus and rutting factor of the asphalt mastic were enhanced, and their temperature sensitivity was decreased, according to the experimental results, by linking the modified CR. SSP further improved the mechanical qualities of the asphalt, whereas the coupling-modified CR increased the delay time, relaxation time, viscosity coefficient, and elasticity coefficient of the asphalt mastic by 0.9, 1.3, 3.6, and 2.8 times, respectively. Osuolale et al. [93] examined the potential for recycling quarry dust (QD), a common mineral filler, in favour of SSP and bamboo leaf ash (BLA). The control (QD) had somewhat greater fatigue, resilience, and moisture-resistance moduli (15–20 and 25%, respectively) than did the SSP and BLA. The cost of using SSP and BLA is less than that of using traditional fillers; for every produced tonne of asphalt concrete, the respective costs of SSP and BLA for the binder and wearing courses are lower by 4.13% and 4.35% and 3.03% and 3.17%, respectively, as shown in

Table 2. Cost analysis of 1 ton of asphalt [93].

Materials	Cost/Tons	Binder Course			Wearing Course
		SSP	BLA	QD	SSP	BLA	QD
Fine Aggregates/tons	#6500 ($14.1)	23,510.50 ($51)	23,510.50 ($51)	23,510.50 ($51)	24,596 ($53.35)	24,596 ($53.35)	24,596 ($53.35)
Coarse Aggregates/tons	#8000 ($17.4)	50,576.00 ($109.71)	50,576.00 ($109.71)	50,576.00 ($109.71)	20,896 ($45.33)	20,896 ($45.33)	20,896 ($45.33)
Quarry Dust (Control)/tons	#5800 ($12.6)	0	0	3550 ($7.70)	0	0	3271 ($7.10)
Bitumen/tons	#40,000 ($86.8)	23,160.00 ($50.24)	23,160.00 ($50.24)	23,160.00 ($50.24)	18,400 ($39.91)	18,400 ($39.91)	18,400 ($39.91)
SSP		0	0	0	0	0	0
BLA/tons	#0	0	0	0	0	0	0
0.5% Processing (Naira/dollar per ton)		500 (1.08)	350 ($0.76)	0	500 $(1.08)	350 ($0.76)	0
Cost (Naira/Dollar) per ton		97,247 ($210.95)	97,247 ($210.95)	100,796.10 ($218.65)	63,892 ($138.59	63,892 ($138.59)	67,163 ($145.69)
Total Cost (Naira/Dollar) per ton		97,747 ($212.03)	97,597 ($211.71)	100,796.10 ($218.65)	64,392 ($139.68)	64,242 ($139.35)	67,163 ($145.69)
Cost savings (%)		3.03	3.17	0	4.13	4.35	0

.

4.5. Silica Fume (SF)

A byproduct of the manufacturing of silicon metal or ferrosilicon alloys is silica fume, sometimes referred to as microsilica. This pozzolanic substance has a high degree of reactivity and can be added as a filler to asphalt mixtures. Nassar [94] reported that SF can be added to cold mix asphalt (CMA) mixtures, including binary mixed fillers, to minimize the bitumen emulsion-induced delay in the formation of hydration products. When mixtures containing ground granulated blast slag (GGBS) were compared with those containing FA, the addition of SF considerably increased the stiffness of the former. The inclusion of SF in the CMA was also suggested by Al-Busaltan et al. [95]. When paper sludge ash was combined with 25–40% SF by aggregate weight, the mechanical and durable qualities significantly improved [96].

As an extra filler, SF was also utilized to develop a CMA by Al Nageim et al. [43]. SF greatly increased the ITSM by activating hydration for both mixtures. These findings suggest that when used as surface pavement layers, CMA performs mechanically in a manner similar to that of conventional HMA.

4.6. Calcium Carbide Residue (CCR)

From 64 g of calcium carbide (CaC₂), 74 g of CCR, consisting of 26 g of acetylene gas (C₂H₂) and Ca(OH)₂, can be produced. The landfill disposal of CCR presents numerous ecological challenges because of its high alkalinity. However, CCRs have gained much attention in recent years because of their high percentage of Ca(OH)₂, which allows them to function as activators, similar to Portland cement [97]. A sustainable cold asphalt emulsion mixture was recently created using a novel cementitious filler that substitutes limestone filler with CCR and GGBS [75]. The mechanical properties of that combination are significantly improved when there are notable changes in the microstructure.

Through several ratio substitutions for conventional limestone filler, Dulaimi et al. [44] investigated the potential of using CCR (0–7 wt.%) as a filler in HMA. The results shown in Figure 8 indicate that the use of CCR resulted in greater stiffness than the control mixture with limestone filler. The performance data indicate that the inclusion of the CCR enhances resistance to cracks and permanent deformation as well as the stiffness modulus. The water damage sensitivity of CCR-HMA was also lower than that of regular HMA with limestone filler. On the basis of a toxicity characteristic leaching procedure (TCLP) test, the CCR used in HMA manufacturing has no effect on the environment. Consequently, CCR is a good substitute for traditional fillers in HMA.

4.7. Microwave-Sensitive Additives (MSAs)

Studies conducted in the past few decades have demonstrated that microwave radiation, as opposed to electromagnetic induction heating, is a more effective and ecologically friendly heating technique for improving the healing efficiency of asphalt [98,99]. Therefore, microwave radiation has become commonplace. In general, to create self-healing asphalt, MSAs, including carbon-based materials or metal-based fibres, are mixed directly with the asphalt before being combined with the filler and aggregate [100]. The predispersion procedure is costly and time-consuming, and there is still considerable difficulty in obtaining adequate dispersion of these additives inside the asphalt [101]. Additionally, under microwave radiation, additive aggregation may lead to localized overheating and collapse of the asphalt mixture [102]. Adding steel or iron slag in place of some of the natural aggregates in asphalt mixtures is another viable strategy to increase the effectiveness of microwave-heating therapy [103]. However, concentrated heating and durability issues could arise from this strategy [104]. Furthermore, the practical utilization of solid wastes such as iron or steel slag is limited by their unstable qualities and accompanying transportation costs [105]. With these restrictions in mind, developing precise and efficient methods for the healing of microwave-heated asphalt mixtures is essential.

Recent research has demonstrated the potential of fly ash (FA), ferrite powder (FP), and coal gangue powder (CGP) to enhance the rheological characteristics, resistance to rutting, and resistance to cracking of asphalt materials; these materials are feasible substitutes for limestone powder (LP) [106,107]. In addition, these fillers have good microwave absorption qualities and include a sizable number of metallic oxides [108]. In the work of Lu et al. [109], three MSAs, namely, FA, ferrite powder (FP), and coal gangue powder (CGP), were recycled in place of limestone filler to produce microwave-healing asphalt composites. These findings suggest that the self-healing initiation temperature of ordinary asphalt mastics is 88.2 °C. Conversely, the first self-healing temperatures of FP-asphalt and CGP-asphalt are noticeably lower, with values of 56.2 °C and 65.6 °C, respectively. Remarkably, the FP-modified asphalt mastics continue to exhibit a remarkable healing index of 46.6% even after three cycles of damage-healing damage. The efficiency with which microwave radiation is converted to thermal energy can be improved by mixing asphalt composites with MSA. This enhances the healing characteristics of microwave heating in comparison to traditional asphalt composites. Furthermore, a thorough evaluation of the financial and environmental advantages and disadvantages shows that the use of microwave-heated healing asphalt pavement results in considerable cost savings and a decrease in CO₂ emissions (Figure 9). Thus, it is practically viable to use MSA on the basis of these results, which will increase pavement system sustainability, serviceability, and efficiency during maintenance.

Waste ferrite (FF) and steel slag (SS) were partially used in place of aggregates and fillers by Liu et al. [110]. According to the findings, FF has a higher absorption performance at the same frequency and thickness as SS does, and both have considerable magnetic loss. The FF volume ratio causes its efficiency to be lower, but its homogeneity is better. If the heating time is extended, FF can achieve a better self-healing rate than can SS, which has a higher self-healing rate at a 40 s healing time. One of the most popular and safe adsorbents for eliminating contaminants from gaseous or aqueous environments is activated carbon powder (ACP). It was utilized by Liu et al. [111] as a partial substitute for fillers in asphalt concrete to increase the deicing capacity of pavement via microwave heating, thereby resolving the deicing challenges of asphalt pavement in an environmentally sustainable and effective way. Owing to the porous structure and material properties of ACP, the infiltration impact between ACP and asphalt is greater than that between MP and asphalt. With increasing ACP replacement, the asphalt mixture’s optimal asphalt content and moisture susceptibility somewhat increased. Furthermore, the microwave heating efficiency of the asphalt mastic and mixture might be significantly increased by ACP.

In place of the LP filler, Lu et al. [109] recycled three different kinds of microwave-sensitive fillers: fly ash (FA), ferrite powder (FP), and coal gangue powder (CGP). These fillers were then used to create microwave-heating healing asphalt composites. According to these findings, the self-healing initiation temperature of typical LP-modified asphalt mastics was 88.2 °C. Conversely, the initial self-healing temperatures of the asphalt mastics treated with CGP and FP were considerably lower, measuring 65.6 °C and 56.2 °C, respectively. Comparing standard LP-modified asphalt composites to asphalt composites with microwave-sensitive fillers might improve the efficiency of converting microwave radiation to thermal energy and, consequently, improve the microwave-heating healing properties. Lu et al. [112] revealed in a different investigation that the improved formulation, with a 4% decrease in crack resistance, maintains a significant healing index of 70% after three damage–healing–damage cycles. The practical use of waste ferrite and functional aggregates in asphalt concrete is highly supported by these findings, which also lead to increased maintenance efficiency and pavement system sustainability.

4.8. Construction and Demolition Wastes

The building and demolition sector generates significant amounts of solid waste, and their landfilling can have a negative effect on the environment. These wastes include, among other things, the major components, namely, brick and concrete [113]. Analysing the effectiveness of leftover brick dust as a filler in asphalt mixtures has not been the subject of many studies. In general, brick dust-containing asphalt mixtures have adequate resistance to cracking and rutting [114,115]. However, the findings concerning how brick dust affects the moisture susceptibility of asphalt mixtures are ambiguous, providing a gap in research in this area. To replace limestone filler in asphalt mixtures, Antunes et al. [116] investigated the use of construction and demolition waste, brick powder, and FA. Since FAs provide strong bitumen bonding because of their unique particle characteristics, the results showed that mastics made of these waste materials exhibited significant resistance to water damage, especially when used in place of limestone filler.

Glass dust, which is generated during the polishing of glass slabs and can be utilized as filler, is dumped in an open manner on adjacent grounds, much like limestone. Powdered glass can provide good stability and cracking and rutting resistance to asphalt mixtures, according to recent studies. Stripping and adhesion loss were also observed in these combinations, albeit [66,117].

Choudhary et al. [118] employed a decision-based ranking framework to investigate the effectiveness of four construction wastes as fillers in asphalt concrete mixtures, namely, brick dust (BD), concrete dust (CD), glass powder (GP), and limestone slurry dust (LD) along with conventional stone dust (SD). The results are shown in

Table 3. Performance of asphalt mixtures against various distresses.

Test	SD	LD	GP	CD	BD
Asphalt coverage (%)	97	95	55	95	90
Mixing time (seconds), at 160 ± 5 °C	94	101	154	107	110
Indirect tensile strength (kPa), at 25 °C	3124	3668	3452	3506	2876
Fatigue life (cycles), at 25 °C	6036	7022	6432	6746	6221
Tensile strength ratio (%)	89.26	86.85	17.65	85.27	81.47
Permanent deformation (mm), at 35 °C	0.075	0.045	0.056	0.049	0.065

. In terms of resistance to rutting and cracking, asphalt mixtures that contained GP, CD, and LD performed better. Because minerals are based on calcium, asphalt mixtures combining LD and CD materials have shown superior resistance to moisture. The results revealed that, compared with traditional stone dust mixtures, mixes including BD and LD were more cost-effective and ecologically benign. GP and LD were determined to be the poorest and best fillers, respectively, according to the recommended ranking formula.

To increase the strength, Sadeghnejad et al. [119] investigated the best way to employ waste glass in hot-mixed asphalt mixtures. The main goal was to use ABAQUS software (https://www.3ds.com/products/simulia/abaqus, accessed on 24 May 2024) to examine the impacts of stress and temperature on the rutting behaviour of a glass asphalt combination. The findings of this study demonstrated how effectively the models could predict how glass asphalt mixtures would form under various stress conditions and temperatures. Additionally, model studies have demonstrated that waste glass powder might greatly increase the resistance of asphalt mixtures to permanent deformation. The study by Sangiorgi et al. [17] aimed to address the impact of Rigden Voids (RV) and the nature of the filler on the performance of bituminous mastics. It employs glass powder (GP) and dry mud waste (MW), and the findings indicate that the RV limit for filler waste is lower than that for limestone filler. Additionally, in comparison with those of the limestone filler, the softening temperatures of the GP and MW fillers converge, which may improve the mixture’s workability [8]. Because of the enhancement in the elastic and stiffness performance of bitumen, Al-Khateeb et al. [120] highlighted the reinforcing impact of glass waste against failure under fatigue or rutting. Similarly, Simone et al. [121] reported that the behavior of an asphalt mixture in terms of bearing capacity and resistance to permanent deformation was enhanced by the use of GP as a filler with pure and modified bitumen. Recycled glass powder was found to improve the indirect tensile strength, thermal sensitivity, stiffness modulus, compressive strength, Marshall stability, and rutting performance of asphalt mixtures (excluding the toughness index) in the analysis of rheological and mechanical properties by Ghasemi and Marandi [122]. Paul et al. [45] investigated how different filler materials, such as GP and FA, affect the performance of asphalt mixtures used in road buildings. The best proportions of GP and FA to replace stone dust, according to the Marshall mix design, are 30% and 60%, respectively. Although the GP exhibited decreased moisture damage resistance due to the silica concentration, the use of FA provided superior resistance. Furthermore, both GP and FA increased the stiffness of the asphalt and led to 22.33% and 39.33% decreases in the rut depth, respectively.

Loss of adhesion represents the main cause of water damage due to the smooth surface of the GP. An investigation by Li et al. [123] involved the application of various types and concentrations of silane coupling agent (SCA) to roughen the surface of the GP. The resulting surface modification mechanism was demonstrated by a chemical reaction process. A new chemical bond is formed from the combination of the carboxylic acid of the asphalt with the amino group in SCA; the results indicate that 75 min is the ideal modification time for GP. Furthermore, the greatest improvement in adhesion work was achieved by modifying the GP with 10% KH550 SCA, as shown in Figure 10.

4.9. Marble Waste (MW)

There are major benefits for the environment and the producing company when waste from decorative rock beneficiation is used, increasing profitability and offering the industrial sector a useful alternative. This strategy improves production efficiency by making the most of the resources at hand and strengthening the financial performance of the business, in addition to helping to manage waste in a more sustainable manner [124]. Research was carried out by Tiwari et al. [125] on the incorporation of MW as a filler at 4.5%, 5.5%, 7%, and 8.5% by weight of a mixture of hot mix asphalt. The adjusted mixtures performed satisfactorily within the requirements for abrasion, moisture damage, and Marshall stability tests. A similar study of the use of MW as a filler in hot mix asphalt was conducted by Khan et al. [126], who examined filler levels of 0–6 wt.% of aggregates. Since the Marshall method (by impact) was utilized for dosing, it may not accurately reflect the conditions of field rolling compaction conditions. With the limestone aggregates in the mixtures, 4% MW was found to be the ideal amount. Nevertheless, a decrease in fatigue life was noted when the amount of filler (marble powder) in the asphalt mixture increased.

de Medeiros et al. [124] investigated the viability of replacing filler in hot asphalt mixtures, both partially and completely, with waste from the granite and marble sectors. The typically used filler, hydrated lime, was replaced with residue in quantities of 50% and 100% by weight. The findings showed that in comparison with the reference mixture (containing 0% residue), the asphalt mixture including waste marble and granite performed marginally better mechanically. This was indicated by the lower ratio between the Resilient Modulus (RM) and the Tensile Strength (TS), as shown in Figure 11. There were no discernible changes between the combinations for the tests that were run, according to the statistical test.

4.10. Red Mud (RM)

Red mud waste, a highly alkaline and toxic industrial byproduct of the alumina refining industry, has become a significant environmental concern worldwide. This waste also consumes a large amount of land [127]. On the basis of the above studies, red mud can be utilized as an alternative filler material in asphalt pavements to reduce landfilling issues as well as the cost of producing mixtures [127]. The possibility of using red mud instead of limestone filler can adversely impact moisture resistance. There is a potentially simple way to improve it by adding hydrated lime, but the question remains as to how much should be added [34]. In addition, more testing is required to determine the maximum tensile strain values for asphalt mastics in the presence of red mud after cracking at low temperatures. Consequently, the practical application of asphalt mastic is premature because of the theoretical issues of moisture damage. Lima and Thives [128] assessed the use of red mud as a filler at 3%, 5%, and 7% concentrations. The granulometry test indicated the possibility of the application of red mud filler. Red mud is classified by its chemical composition as hazardous waste since it contains 0.20% V₂O₅ vanadium pentoxide. The moisture-induced damage through diametral compression revealed that the tensile strength of the red mud mixtures was greater than that of the reference mixture. The use of a mixture with 5% red mud and 5% stone powder outperformed the other mixtures in terms of resistance to permanent deformation.

Zhang et al. [129] investigated the possibility of replacing a major widely utilized limestone powder in emulsion-based asphalt mastics. A substitute filler aggregate was developed from residue mud. The introduction of both white mud and hydrated lime improved the durability and cracking performance at low temperatures, strengthening the bond between the aggregate and mastic and helping it resist moisture damage over time. As shown in Figure 12, higher testing temperatures led to decreased viscosity in the asphalt mastics. Specifically, the viscosity of a sample containing limestone powder at a given temperature increased nearly fourfold with the introduction of mud.

5. Conclusions

In conclusion, the utilization of waste filler in asphalt mixtures provides a promising avenue for addressing environmental issues, enhancing sustainability, and optimizing the performance of asphalt pavements. This comprehensive review revealed that the incorporation of waste filler materials offers various benefits, including improved mechanical properties, reduced dependency on virgin resources, and decreased environmental impact through waste diversion and recycling. The incorporation of waste filler materials in asphalt mixtures offers a viable solution for reducing waste disposal and minimizing environmental impact by repurposing industrial byproducts.

Asphalt mixtures are improved by the addition of rice husk ash (RHA), fly ash (FA), silica fume (SF), and calcium carbide residue (CCR). Optimal dosages result in better mechanical and moisture-resistance properties, but excessive use may weaken the mixture due to poor bonding. In addition to improving Marshall’s durability and stability, GGBFS increases the stiffness of cold asphalt mixtures. Studies have shown that steel slag powder increases the fatigue resistance and deformation resistance of asphalt mastics, as well as the resistance to rutting and water damage. Compared with conventional fillers, high-titanium heavy slag (HSP) and municipal solid waste incineration bottom ash (MSWI-BA) provide advantages such as enhanced performance and fewer environmental effects. Compared with conventional fillers, fly ash, ferrite powder, and coal gangue powder are examples of microwave-sensitive fillers that improve the efficiency and self-healing characteristics of asphalt when heated in a microwave. These fillers also have lower costs. While glass dust and brick dust have different impacts on moisture susceptibility, they can increase resistance to rutting and cracking. When marble waste is utilized as a filler, its mechanical performance and production efficiency increase. Red mud can improve resistance to permanent deformation and moisture damage, even if it poses environmental concerns. However, more research is needed to fully understand its application and resolve any potential issues with moisture resistance and tensile strain.

Several research gaps have been identified following a thorough analysis of the previously mentioned studies on waste fillers. While extant waste fillers have been the subject of limited analysis in prior research, the effects of these wastes on the fatigue and rutting resistance of asphalt mastics, as well as their rheological characteristics, have not been sufficiently investigated. Understanding how asphalt mastics perform in comparison to mixtures with varying fillers at their respective optimum asphalt content (OAC) will be interesting to study, as this link has not been fully explored in prior research.

The present methods often involve ranking asphalt binders and mixtures according to how well they perform in a certain area (such as rutting). Nevertheless, an asphalt mixture that performs exceptionally well against rutting could not work as well against other distresses, such as weariness or moisture sensitivity. As a result, evaluating asphalt mixtures according to how well they function in a single area is invalid as a method of assessing their overall applicability. The determination of the overall appropriateness of an asphalt mixture requires the appropriate allocation of weight to each engineering property, such as moisture resistance, fatigue resistance, and rutting resistance.

6. Future Perspectives

Future research could focus on the following:

• The content of waste filler in asphalt mixtures should be optimized to achieve the desired performance characteristics while maximizing cost savings and environmental benefits.
• The use of new types of waste fillers, such as coconut shell powder, textile fibres, palm oil fuel ash, copper slag, sewage sludge ash, recycled plastic, crumb rubber, and wood ash, should be explored to further diversify the range of materials available for asphalt mixtures.
• Sustainable practices for incorporating waste fillers into asphalt mixtures, including the development of guidelines and standards for the use of waste materials in asphalt construction, are needed.
• Conduct comprehensive life cycle assessments to evaluate the environmental impact of using waste fillers in asphalt mixtures compared with traditional materials.
• Develop performance-based specifications for asphalt mixtures containing waste fillers to ensure long-term performance and durability.
• Promote the market acceptance and implementation of asphalt mixtures containing waste fillers through education, outreach, and collaboration with industry stakeholders.

In summary, this review underscores the potential of waste filler utilization as a sustainable and economically viable approach for enhancing the performance and environmental profile of asphalt pavements. Addressing challenges and fostering collaboration will be crucial in realizing the full benefits of incorporating waste fillers in asphalt mixtures. By focusing on these future directions, the asphalt industry can continue to leverage the benefits of waste fillers to create more sustainable and cost-effective pavement solutions.