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Article

Influence of Walnut Shell Ash and Limestone Filler in Hot Mix Asphalt

by
Yasir N. Kadhim
1,2,
Abdulrasool Th. Abdulrasool
1,
Anmar Dulaimi
2,3,*,
Hugo Alexandre Silva Pinto
4 and
Luís Filipe Almeida Bernardo
4,*
1
Civil Engineering Department, College of Engineering, University of Warith Al-Anbiyaa, Karbala 56001, Iraq
2
Department of Civil Engineering, College of Engineering, Kerbala University, Karbala 56001, Iraq
3
School of Civil Engineering and Built Environment, Liverpool John Moores University, Liverpool L3 2ET, UK
4
GeoBioTec, Department of Civil Engineering and Architecture, University of Beira Interior, 6201-001 Covilhã, Portugal
*
Authors to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(1), 22; https://doi.org/10.3390/jcs9010022
Submission received: 11 September 2024 / Revised: 26 December 2024 / Accepted: 2 January 2025 / Published: 6 January 2025
(This article belongs to the Special Issue Sustainable Composite Construction Materials, Volume II)
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Abstract

:
The presence of filler in asphalt concrete may improve the properties of the mixture. This study investigates the mechanical and volumetric properties of such a mixture using walnut shell ash as a filler in various replacement ratios. The mixtures were mixed with various proportions of limestone (0%, 10%, 20%, 30%, 40%, 50%, 60%, 80%, and 100%) in addition to WSA as a replacement filler. Tests were subsequently carried out, including tests of Marshall’s stability and flow, voids in mineral aggregates, air voids, and theoretical maximum specific gravity. The results revealed that increasing the replacement percentage resulted in a considerable improvement in the performance of the asphalt–concrete mixtures. The results revealed that the mixture with a 60% replacement ratio achieved the best Marshall stability, achieving an improvement of 15.02% compared to the conventional sample, alongside good flow properties. This improvement was accompanied by high conformity with the other physical properties of the asphalt mixture, including a 3.55% air void percentage, which is within the permissible limits for the surface layer, as well as a 21.80% increase in the percentage of voids in the mineral aggregate, which is considered an ideal value. These results paved the way for further study and adjustments to other requirements of the asphalt mixture, as there were no issues with the availability or production costs of the filler material, given the abundance of raw materials. However, it is important to note that, as is evident from the results, a complete 100% replacement led to undesirable outcomes, resulting in a 10.68% decrease in Marshall strength compared to that of the conventional sample. This decrease indicates that the mixture was unable to provide its most important property. Although improving the other properties with complete replacement is not beneficial, a detailed investigation into this ineffective percentage revealed that, according to the results, the ideal replacement ratio is 60% walnut shells and 40% limestone, which results in optimal performance.

1. Introduction

The rising traffic loads on the wheels, along with rising traffic volumes, result in increased road stresses and strains, putting the paving layers in danger of deterioration [1]. Fatigue and rutting are two of the most significant types of asphalt layer degradation induced by high traffic loads [2]. To be considered good, roads must have high levels of safety and durability. Furthermore, they must be able to sustain the projected loads which they are designed to bear. The asphalt–concrete mixture is made up of asphalt and aggregates and is widely utilized in flexible pavement roads, especially in the surface layer.
The major function of aggregates in asphalt mixtures is to produce a skeleton that can resist repeated traffic pressures on the lane, while the asphalt acts as an adhesive substance between the aggregates, reflecting the properties of the resultant asphalt mixture [3]. Typically, aggregates are categorized on the basis of their size when combined with particular quantities in an asphalt mixture. Aggregates are typically categorized as coarse when their size is greater than 4.75 mm and as fine when smaller than 4.75 mm. Particles with diameters less than 75 micrometers are referred to as fillers [4].
Aggregates and fillers offer a firm foundation for asphalt mixtures [5,6]. Despite its small size, the filler effect has a significant influence on the performance of asphalt mixtures. Hence, fillers are among the most important components, and depending on their type and quantity, they significantly affect the characteristics of asphalt–concrete mixtures. Mineral small particles such as rock dust, fly ash, hydrated lime, volcanic ash, limestone (LS) dust, Portland cement, mineral sludge, rock flour, silt, reclaimed brick powder, and other materials with particle sizes less than 75 microns are used as fillers in asphalt–concrete mixtures [7].
Increasing the proportion of fillers in the asphalt mixture enhances the strength of the pavement due to the improvement in internal stability and increased cohesion within the mixture [8]. This improvement in cohesion is due to the fillers’ ability to effectively fill the spaces between the blocks, which enhances the stability and cohesion of the mixture. Essentially, fillers improve the mixtures’ mechanical properties by strengthening the bind between the asphalt and all mix components.
Fillers have a significant impact on the properties of asphalt and concrete mixtures due to their ability to interact with the asphalt itself. When asphalt is combined with blocks, a paste consisting of asphalt and filler is formed. This paste helps improve the overall quality of the asphalt mixture, as the fillers fill the microscopic gaps between the block particles, leading to greater stability and cohesion.
Furthermore, the interactions between the filler (which passes through the 200 sieve) and the asphalt mixture lead to specific effects that affect the final properties of the mixture. These interactions affect properties such as strength, durability and abrasion resistance, demonstrating that fillers play a vital role in improving the performance of asphalt pavement and increasing its efficiency over time.
Therefore, the filler not only improves the internal structure of the mixture but also affects the overall properties of the asphalt mixture, enhancing its quality and performance under different conditions. Because the filler has a large surface area, it absorbs and interacts with more asphalt, causing the asphalt mixture to behave differently [9,10]. As a result, the filler can alter the characteristics of the asphalt mixture. Fillers can improve the durability and temperature susceptibility of asphalt and asphalt mixtures [11,12,13,14,15]. The surface area, void content, particle shape, size distribution, mineral composition, surface roughness, and other properties differ across fillers [16].
Recently, several researchers have investigated mineral filler alternatives for asphalt mixtures, including waste cement dust, fly ash (FA), cement bypass dust (CBPD), ferrite particles, brake pad waste (BPW), and alkali-activated binary mixed cementitious filler [17]. The impact of employing CBPD as a filler in asphalt mixtures was explored, and it was reported that a 5% CBPD substitute resulted in the same optimal asphalt binder content (4.5% by weight of aggregate) as the control mixture without affecting the other properties of the asphalt mixture [17]. In [18], it was reported that, compared with conventional specifications and typical blends, the use of 4% FA as a filler leads to greater stability with a lower bitumen concentration.
The authors of [19] studied the mechanical characteristics of a hot asphalt mixture that contained leftover cement (cement dust) as a filler. Following the completion of the trials, the use of cement waste improved the mechanical characteristics of the asphalt mixtures. The Marshall test also revealed a significant improvement in stability as well as a decrease in flow with an increase in the cement dust content, as well as an increase in the number of voids in mineral aggregates and a decrease in the void ratio.
The rheological characteristics of asphalt mastic were examined in [20], where steel slag was employed as a filler in an asphalt mixture. Steel slag has been identified as a filler material that may be utilized in place of LS filler. In terms of mechanical characteristics, three distinct types of recycled fillers were examined and compared to LS fillers [21]. These recycled fillers vary considerably from the traditional fillers used in asphalt mixtures, with voids having the greatest potential influence on the rheology.
In [22], the authors investigated the use of ferrite particles in asphalt to improve the microwave absorption efficiency and therefore accelerate the self-healing process of the compound. The authors reported that a specific fraction of the ferrite filling had a significant influence on the temperature uniformity. It was observed that the enhanced microwave absorption characteristics of ferrite particles promoted asphalt healing. In [23], the authors investigated the feasibility of using BPW as an alternative powder for traditional fillers in asphalt mixtures. They reported that BPW enhanced the temperature performance and viscosity of asphalt mortar.
Based on the above, it is clear that many fillers may be employed in asphalt mixtures; nevertheless, they must have inherent qualities. For replacement, the interaction between the fillers and the binder must be considered [23,24,25,26,27]. The effectiveness of this interaction influences the asphalt mixture and its bonding [28]. A good interaction is not guaranteed. As a result, the replacement fillers with varying characteristics might lead the asphalt mixture to behave inconsistently [4].
Walnut shell ash (WSA) is the residue left after burning walnut shells, a waste product of the walnut processing industry. This ash is fine and light and can vary in composition, but is often rich in silica (SiO2), calcium oxide (CaO), and other minerals. Its properties make it a potential alternative to asphalt mixtures, replacing traditional fillers such as LS dust, clay, or talc [29].
Agricultural waste is a valuable resource for the production of environmentally friendly materials, especially in the construction sector. One such agricultural waste is walnut shells, which hold great potential when reused in construction materials. This review paper [29] aims to explore the various applications of walnut shells in the production of construction materials, highlighting their potential to contribute to sustainability within the industry.
Walnut shells, a byproduct of walnut harvesting, are typically disposed of as waste. However, they have shown promise as a sustainable material when incorporated into the manufacturing process of various construction products. By converting walnut shell waste into usable materials, the construction sector can help reduce environmental pollution while promoting the principles of circular economy and resource efficiency.
In recent years, walnut shell waste has attracted considerable attention from researchers, who are exploring their potential for generating environmentally friendly construction materials. The unique properties of walnut shells, including their low specific gravity (meaning they are lightweight compared to many conventional materials), make them particularly suitable for use in construction materials. Not only does this provide a cost-effective method of agricultural waste disposal, it also provides a sustainable alternative to traditional building materials.
As a cheap and abundant agricultural waste product, walnut shells have emerged as an innovative material for insulation, composites, and even as an additive to concrete mixes. Their incorporation into these materials contributes to enhanced durability, thermal insulation, and even acoustic properties. By using walnut shells as a sustainable material, the construction industry can move towards more environmentally friendly and sustainable building practices [30].
Like other fillers, WSA interacts with the asphalt binder. The fine particles of WSA can absorb some of the binder, affecting the rheologic attributes of the binder (e.g., its viscosity and elasticity). The interactions between the filler and the binder can result in a more cohesive mixture, with improved adhesion among the binder and the solid material [31].
As a mechanical property, the presence of WSA can raise the compressive strength of an asphalt mixture, especially when used in appropriate proportions. This results in roads with structures that are more resistant to heavy loads and deformation. WSA can improve Marshall stability, which refers to the ability of an asphalt mixture to resist deformation under load. This is important for promoting the long-term performance of the pavement. The addition of WSA can help enhance the scaling resistance of asphalt pavement. Scaling is the permanent deformation of the surface under traffic, and fillers help reduce this problem by providing greater internal stability [32].
WSA is a highly durable material, and its addition to hot mix asphalt can improve its resistance to weathering, oxidation and thermal cracking. These properties make it suitable for areas with severe weather conditions [33].
WSAW may also improve the calorific stability of asphalt. Hence, its fabrication is less susceptible to temperature-induced damage. Asphalt containing WSA may perform better in high-temperature environments, reducing the risk of softening and curling. The material may also reduce the moisture sensitivity of the asphalt mixture. In fact, fillers can play an important role in reducing water stripping between asphalt and solid components, improving the mixture’s resistance to moisture damage [34].
Based on the aforementioned literature review, they are still key aspects related to the influence of WSA, as a substitute filler in hot mix asphalt, which need to be addressed, particularly when WSA is used as a substitute for the currently used LS dust. Hence, the purpose of this research is to determine how WSA, which is used as a filler, influences the characteristics of asphalt–concrete mixtures. The performance of the resultant mixtures was evaluated by assessing the mechanical and volumetric characteristics associated with various filler amounts, including at least three samples in each test, and then calculating the average.

2. Methods and Materials

2.1. Raw Materials

For this study, sand and gravel aggregates obtained from the Karbala quarries were utilized. Compliance with standard specifications for roads and bridges was required for the material property criteria. The physical characteristics of the gravel and sand aggregates utilized in this research are shown in Table 1. The sand and gravel aggregates were sieved, sorted, and classified to obtain a harmonious gradient with the surface coarse requirements and the Iraqi specifications (Table 2). The characteristics of the bitumen, shown in Table 3, were obtained from the Alnasseria refinery.

2.2. Limestone

Because of its widespread availability and affordability, LS powder is an alternative filler that has recently attracted the interest of many researchers. To properly use LS, researchers have conducted many studies on the operating mechanism of LS in cement-based materials. The filler effect of LS refines the microstructure and reduces the porosity of the resulting cement-based materials [52,53]. It is well established in the literature that the incorporation of LS into concrete is associated with crystallization, dilution, and chemical reactions, all of which are influenced by factors such as particle size, dosage, dissolution rate, and polymorphic forms of LS, as well as the mineral composition of the cement and any supplementary cementitious materials [52]. The addition of LS to concrete, either as a substitute for cement paste or in the form of fine aggregates, generally enhances its properties. However, when LS is used as a partial substitute for cement, its effect on the performance of concrete is influenced by factors such as particle size and content [53]. In this study, LS was utilized as a filler material, derived from quarries of Karbala, and WSA was used as a substitute. The physical characteristics of the used LS in the hot mix asphalt (HMA) mixture are shown in Table 4.
The finer LS fillers (lower D50) tend to provide better performance, particularly in applications like high-strength mixes, where a high surface area to volume ratio can help reduce porosity and improve the microstructure. They also tend to affect workability and rheology because finer particles help to improve the rheology of fresh mixtures, enhancing flow and reducing segregation. The finer LS can improve the compressive strength and durability of concrete, but an excessively fine filler might lead to issues with shrinkage or excessive water demand, so the D50 should be optimized based on the specific requirements of the project.

2.3. Walnut Shell Ash

In recent years, global population growth, coupled with improved living standards, has led to a significant increase in the consumption of agricultural products, which in turn has led to a rise in agro-industrial waste. To mitigate the environmental impacts of this increase, particularly the depletion of natural resources, over the past decade, there has been a growing focus on the reuse or recycling of such waste. A prime example of this is the valorization of fruit peels and other agricultural waste, which can be reused for various applications.
A prominent example of this trend is the growing production of walnuts. Over the past few years, global walnut production has reached around 4 million tons, with China topping the list of largest producers, contributing around 2.5 million tons, followed by the United States, Chile, and Iran. This rise in walnut cultivation can be attributed not only to the sensory qualities of the nuts, which are becoming increasingly valuable in human diets, but also to the deepening understanding of their potential health benefits.
Walnuts have a range of uses that go far beyond human consumption. In addition to being a nutritious food source, walnut products are also used in the pharmaceutical industry for their medicinal properties, in the production of dyes and alcoholic beverages, and even in the wood processing industry. As a result, the demand for walnuts continues to rise, boosting cultivation rates. The wide range of applications for walnut-derived products highlights the growing recognition of their economic and health value, making walnut cultivation a major agricultural activity with diverse industrial uses [56].
After repeated contact with specialized agricultural agencies, the authors were able to obtain some information about a walnut farm in Iraq, specifically in Diyala Governorate. This farm consists of about 5000 walnut trees, which produce about 55 to 110 tons of walnut shells annually. According to the percentage of converting shells to ash (1–2%), 550 to 1100 kg of WSA can be produced annually. In addition, these quantities are not only related to the size and number of walnut farms but also to the extent of the service provided to care for these trees and their general health, as this will affect the size, shape and weight of the outer shell of the fruits.
The use of WSA, a byproduct of walnut processing, in bitumen highways offers numerous advantages, including less environmental deterioration, lower construction costs, and natural resource conservation. Broadly available bioderived ashes, such as WSA, have the potential to act as alternative supplementary cementitious materials (SCMs) in cementitious composites because their physicochemical properties are similar to those of conventional SCMs. As biomass ash, WSA has abundant silica and calcium dioxide content that plays crucial roles in pozzolanic and hydraulic reactions, respectively [30].
The specific surface area of WSA is 325 m2/g to 663 m2/g, greater than that of conventional cement (approximately 320 m2/kg), which helps promote significant pozzolanic reactions in cementitious composites [57]. These promoted reactions inherently benefit the mechanical and durability properties of hardened cementitious materials [30]. In the construction industry, numerous studies have also reported promising results regarding the use of rice husk ash [58].
In this study, WSA was obtained from a local market and crushed into fine particles via an electric machine. The material was then burned in a very high-temperature oven for 2 h at a temperature of 850 °C (Figure 1). The goal of this high temperature is to convert the walnut shells from active organic materials to inert materials, allowing them to increase the interaction with the components of the HMA. Table 5 and Table 6 provide the chemical composition and physical properties of the used WSA.

2.4. Asphalt Mixtures and Testing Methods

Marshall’s method [60] was applied to create the asphalt–concrete mixtures. Mechanical tests were then performed on the asphalt mixtures, as shown in Table 7. The properties of HMA depend on the parcels of the total admixture of the bitumen, which are, in turn, dependent on the parcels of the individual summations. Marshall’s system, which was designed to help determine the importance of bitumen in terms of whether it is needed to make a blend work, became the most espoused design method in the 1970s. This is because it is simpler, faster, and requires a lower degree of skill than the other used design methods.
For the proposed mix design, a gradation was developed that corresponds to the percentage of LS filler that has been replaced with WAS. Nine different filler content ratios, ranging from 0% to 100%, were prepared as shown in Table 8, following ASTM D 1559 standards with a 75 blow/face compaction protocol. The amounts of WAS and LS were expressed as a percentage by weight of the total mixture. Once the freshly compacted samples cooled to room temperature, their bulk specific gravity was measured according to ASTM D 2726. The stability and flow values of each test sample were then assessed in line with ASTM D 1559. Following these tests, specific gravity and void analyses were conducted to determine the air void percentage in the mineral aggregate, the air void percentage in the compacted mixture, and the voids filled with bitumen. Any values identified as erratic were discarded before the results were averaged [60].
In general, the procedures followed to test the models are routine procedures known to specialists, but at the same time, they require high accuracy and long experience to prevent any malfunction that could cause damage to the test samples or inaccurate results. To avoid this, the test carefully followed the international specifications used worldwide, namely the ASTM and AASHTO specifications. In addition, calibrated devices were also used.
As indicated in Table 8, the asphalt–concrete samples were obtained via nine alternative percentage replacements of LS filler with WSA filler, namely: 0%, 10%, 20%, 30%, 40%, 50%, 60%, 80%, and 100%. For each percentage, three samples were used to be tested.

3. Tests

In this section, the most important tests that were adopted to evaluate the influence of the filler replacement are referred to and briefly discussed. In this study, the asphalt–concrete mixture characteristics, such as Marshall stability, Marshall flow, bulk density, theoretical maximum specific gravity (Gmm), air voids (AVs), voids in mineral aggregates (VMAs), and voids filled with asphalt (VFA), are studied to determine the impact of the filler replacement on the mixture performance. Table 9 shows the outcomes of the tests. The average value was calculated from three samples. For comparative purposes, all asphalt samples were manufactured and analyzed with a 5% asphalt content to ensure consistency throughout the study. This procedure was used by Pasandín et al. (2016) [65], Tahami et al. (2018) [66], Guha and Assaf (2020) [67], and Dulaimi et al. (2020) [68]. Furthermore, it was reported that the variation in optimum binder content with different filler compositions was less than 0.2% [69].

3.1. Marshall Stability Test

Stability is a key indicator of how well an asphalt mixture performs under traffic loads. A higher stability value typically indicates a denser mixture, which resists deformation and erosion more effectively. Stability also reflects the packing efficiency of the mixture and how well the binder is distributed. Higher stability values indicate that the mixture is less likely to crack or erode, resulting in improved pavement lifespan. In short, the Marshall stability test is critical to understanding the mechanical properties of asphalt mixtures, particularly their resistance to deformation under traffic loads. It helps determine the best mix design to ensure long-term performance, resilience to heavy traffic, and pavement durability. By improving stability and flow balance, engineers can produce high-performance asphalt mixtures that result in safer, and more durable roads compared with the traditional value for global specifications, which is at least 8 kN.

3.2. Marshall Flow Test

In addition to the stability test of the asphalt mixture, the flow test provides an indicator which complements the characterization of the asphalt mixture, as it is related to the first one. It can also be stated that the flow test gives an indication of whether the asphalt mixture contains an excessive amount of binder [70]. If we make a comparison with the traditional value for global specifications, the recommendation is that there should be a value of at least 2 mm but no more than 4 mm.

3.3. Bulk Density

Bulk density, also referred to as true bulk density in the Marshall model, is an important parameter that reflects the overall homogeneity of an asphalt mixture. It indicates how evenly the aggregates are distributed within the mixture and how well the binder interacts with them.
A higher bulk density value indicates a greater degree of homogeneity and better interpenetration between the particles. This means that the aggregates are more tightly packed together, with strong contact between the particles, which is facilitated by the binder. The closer the contact and the better the packing, and the denser and more stable the mixture will be under traffic loads.
This parameter is essential for assessing the granular gradation and distribution of materials in the mixture. A high bulk density indicates that the mixture is well compacted, with a uniform distribution of aggregate and binder, resulting in improved durability and resistance to deformation under load. Conversely, a low bulk density may indicate poor packing, increased air voids, and a less stable mixture, which can negatively affect the overall performance of the asphalt.
Thus, bulk density serves as a conclusive indicator of the quality and uniformity of asphalt mixture composition and its performance properties, such as strength, compression, and elasticity under environmental stresses and load.

3.4. Theoretical Maximum Specific Gravity

The theoretical maximum specific gravity (Gmm) of a hot mix asphalt (HMA) is defined as the specific gravity of the mixture when air voids are excluded. Essentially, it represents the density of the combined aggregate and asphalt binder, assuming no air is present. In practice, if all air voids are eliminated from an HMA sample, the resulting specific gravity of the remaining materials (aggregate and binder) will be equal to the theoretical maximum specific gravity. This property is of great importance in the study and design of HMA, as it serves as a key parameter for calculating the percentage of air voids within the compacted mixture. Accurate knowledge of air voids is essential to ensure the quality, durability and performance of the pavement. Therefore, the theoretical maximum specific gravity is an integral part of the Superpave mix design method, which aims to optimize the mixture for performance under different loading and environmental conditions. Furthermore, the Gmm is used on-site to evaluate the actual air void content in the compacted HMA, which helps with monitoring and controlling the quality of the asphalt mixture during construction. This ensures that the pavement meets the required specifications and performs optimally over time.

3.5. Air Voids

The air volume in HMA is important because it profoundly affects the long-term performance of the pavement. The exact nature of this effect is subtle and likely nonlinear. An air void content below a minimum value indicates an unstable mixture that is prone to deformation and fouling. This threshold is often reported at approximately 2–3%. An air void content above a high threshold value results in an expected reduction in the pavement’s service life. This threshold is usually set at 7–8%. An accepted practical rule is that every 1% increase in air voids (above the baseline air void level of 7%) results in an approximately 10% loss in pavement life, which equates to approximately one year [71]. When comparing the results obtained with conventional international specifications, we note that the values should generally fall within a range of at least 3% but should not exceed 5%. This range is widely accepted as a standard in international specifications, ensuring a balance between optimum performance and material efficiency. By adhering to this range, the mixture exhibits desirable properties such as adequate stability, durability, and flexibility. Any deviation from this range can affect the overall quality and long-term performance of the asphalt mixture. Therefore, ensuring that the results conform to this specified range is critical for meeting international standards and achieving consistent performance results.

3.6. Voids in the Mineral Aggregate

Voids in aggregate mix (VTM) and voids in mineral aggregate (VMA) are critical parameters in hot mix asphalt (HMA) design. The minimum requirements for both VMA and VTM are essential to ensure that the asphalt mixture contains sufficient asphalt binder, thereby enhancing the durability of the mixture and preventing problems such as corrosion or erosion. Current VMA standards are primarily derived from research conducted in the 1950s and 1960s. However, a review of various studies in the literature has not provided substantial or conclusive evidence directly linking the durability of HMA pavements to the minimum VMA values prescribed for mixture design.
The term asphalt-filled voids (VFAs) refers to a critical parameter that represents the proportion of voids in mineral aggregates (also known as mineral blocks) that are filled with asphalt binder. It reflects the amount of asphalt content that effectively contributes to the cohesion and performance of the mixture. Essentially, VFAs are an indicator of the amount of void space available in the mixture that is filled with asphalt cement, which enhances the durability and binding properties of the mixture.
VFAs are inversely related to air voids in the mixture. As air voids decrease (meaning the mixture becomes more compact), VFAs increase, as more of the available void space is filled with asphalt binder. This relationship highlights the importance of balancing adequate asphalt content with reduced air voids for optimal mixture performance.
The presence of VFAs is recognized as an important parameter for assessing the following properties:
  • Durability: a higher VFA indicates that a greater portion of voids in the mixture are filled with binder, which improves the overall cohesion and bonding between the aggregates. This enhances the durability of the asphalt mixture by reducing the possibility of water seepage and preventing oxidation of the binder, which can lead to cracking and aging.
  • Binding efficiency: the role of the asphalt binder is to coat and bind the blocks together. A higher VFA ratio ensures that the binder effectively covers the surface of the blocks, providing a stronger bond and reducing the possibility of particles slipping or weak points within the mixture.
  • Optimal mixture performance: the VFA ratio directly affects key performance indicators such as resistance to deformation, fatigue resistance and long-term stability under load. A well-balanced VFA ratio ensures that the asphalt mixture performs optimally under traffic pressure and environmental conditions.
  • Compliance with technical specifications: the VFA ratio must meet specific criteria within the design specifications for asphalt mixtures. If the VFA ratio is too low, this may result in a mixture with insufficient binding capacity, resulting in increased air voids, decreased durability, and an increased likelihood of rutting or cracking. Conversely, if the VFA ratio is too high, this may be an indicator of excessive binder content, which can cause problems such as bleeding or deformation of the pavement surface.
  • Balance with VMAs: VFAs are usually calculated as a percentage of VMA. VMAs represent the total volume of voids in the asphalt that can be filled with asphalt. If the VFAs ratio is too low relative to the VMAs, it may indicate that the asphalt content is insufficient, preventing the mixture from performing well.
It is also important to emphasize that according to widely accepted international specifications, the value should not be less than 14%. This minimum has been unanimously agreed upon in global standards to ensure the safety of infrastructure, durability and overall performance of the asphalt mix. Maintaining a value of at least 14% is critical to meeting these specifications, as it directly affects key properties such as strength, stability and resistance to deformation under load. Any value below this threshold can compromise the ability of the mixture to withstand stresses and real-world application requirements, especially in heavy traffic or harsh environmental conditions. Adherence to this minimum value ensures that the mixture meets international expectations for quality and reliability.

4. Results and Discussion

4.1. Stability Results

Figure 2 shows the Marshall stability of HMA mixtures with varying filler percentages. On the basis of these results, it can be concluded that all mixtures met the requirements for full replacement of LS (total replacement of LS by WSA filler). The results also show that increasing the WSA filler ratio enhanced the stability, which depicts the stability change in the control and modified asphalt mixtures with the replacement of the WSA from the Marshall test. The asphalt mixture shows the maximum stability for 60% WSA and 40% LS. Importantly, all the samples comply with the stability conditions of the surface layer according to the general standard specifications, except for the last ratio, in which the filler was completely replaced with WSA. This means that the gradation and homogeneity of the asphalt mixture at the point of total replacement did not reach their optimal values. Figure 2 shows the values reached the minimum required limit of 8 kN, except for the 100%WSA.0%LS [72].
Compared with those of the control mixture (100% LS), the changes in stability for the 10%, 20%, 30%, 40%, 50%, 60%, and 80% WSA mixtures were 4.56%, 8.17%, 9.49%, 10.45%, 14.90%, 15.02%, and 14.54%, respectively. In addition, for the total replacement ratio, a 10.69% decrease in stability against the control mixture was observed. Owing to this novel filler (WSA)’s capacity to tolerate higher asphalt binder contents, owing to its large surface area, the observed increase in stability can be explained by the enhancement of the interlocking of mixtures and greater filling of gaps in the components of the mixtures. The increasing stability of the mixtures also indicates increased strength.
Based on the stability analysis results, the increase in Marshall stability with the gradual addition of WSA means that the filler material was not completely saturated with asphalt. As is evident from the percentage of voids filled with asphalt shown in Table 9, the angular particles of WSA are able to improve the bonding between the asphalt particles, leading to an increase in the stiffness of the mixture and, thus, an increase in Marshall stability. As for the decrease in the Marshall stability stage, it was found that as the amount of WSA increases, WSA begins to absorb more asphalt, which leads to a decrease in the effective asphalt content. This reduction in asphalt can weaken the cohesion between the asphalt and the filler particles, which leads to the mixture having poor stability under pressure. Moreover, at this stage, the angular particles of WSA may lead to agglomerates (as they fill the voids between the particles), which reduces the ability of the mixture to resist permanent deformations.

4.2. Flow Results

Figure 3 shows the Marshall flow of HMA mixtures and the changes in the Marshall flow value when 30% to 50% of LS was replaced with WSA. For this range, the Marshall flow value decreased. This decrease in flow means that the mixture became stiffer and less flexible. When the proportion of filler, namely WSA, is increased, it fills the spaces between the asphalt particles and increases the cohesion between the particles, which reduces expansion under pressure. As the replacement percentage increased to more than 50 (e.g., 60%, 80%, 100% of WSA), the Marshall flow value gradually increased. This indicates that the mixture became more flexible or susceptible to deformation under loading, which may be due to the increased absorption of asphalt by the WSA or due to decreased interaction between asphalt and the filler when replacing larger amounts of LS. Figure 3 shows the maximum and minimum values for flow according to [72].

4.3. Air Void Results

Figure 4 depicts the air void (VA) ratios inserted between the components of the asphalt mixture as a result of the use of WSA as a substitute filler material. The findings demonstrate that the replacement of LS with WSA does not have a noticeable effect on the air void content, especially at lower percentages of WSA. When 10% WSA was used, the number of air voids increased by only 1.67% compared with the number in the control mixture. Increasing the percentage of WSA by 20%, 30%, 40%, and 50% caused increases of 2.5%, 4.16%, 4.44%, and 5%, respectively, in the air voids compared with the number of air voids in the control mixtures. The increase in air voids was caused by the binder being absorbed by the WSA filler, owing to the increase in the surface area of this mineral filler, which allowed the tiny gaps between coated aggregate particles to be minimized. As a result, the amount of asphalt binder decreased. These findings indicate physical changes in the mixtures, which implies that the thin layer of asphalt binder that coats the aggregate is reduced; therefore, the space within the compacted samples is greater than that within the control mixtures. For percentages of WSA higher than 50%, the opposite trend is observed: the air voids decrease as the percentage of WSA increases. This resulted in two important things happening during the replacement process: the two materials together, under certain proportions, worked in opposite directions. Firstly, when the LS was replaced with the WSA in a proportion of up to 50%, the percentage of air voids increased by an acceptable amount within the limits of international specifications. Secondly, when continuing the process of replacing the LS with WSA, the percentage of air voids gradually decreased to complete replacement and to an acceptable percentage of air voids as well. This means that there was a rearrangement of the filler aggregate particles when changing the replacement proportions between the two materials, which resulted in changes and differences in the structure of the aggregate in the asphalt mixture, thus changing the surface areas coated or filled with asphalt, which in turn directly affected the present of the air voids in the mixture and the final product. These changes widened the range of options when it came to choosing the optimal percentage for replacement of the material based on the availability of materials and the economic aspect, and allowed other physical goals related to the asphalt mixture to be achieved.
Ultimately, it can be said that increasing the replacement ratio of WSA (up to 50%) increases the number of air voids in the mixture (due to absorption of the binder by the WSA), which slightly reduces the cohesion of the mixture. This results in a decrease in the Marshall stability, as shown in Figure 2, and an increase in the Marshall flow, as shown in Figure 3, indicating that the mixture became more elastic but also slightly less resistant to deformation. However, the increase in air voids remains within the acceptable range, and the mixture is still able to meet the performance requirements. In addition, replacing LS with higher WSA (more than 50%) reduces the air voids due to better packing and higher filler content, resulting in a denser mixture, as shown in Table 9. This has an effect on the Marshall stability, which results in a limited increase in the resistance to deformation under load for a replacement ratio of up to 80 percent. It also results in an increase in the Marshall flow because the mixture becomes denser and slightly more elastic, with a higher binder content (with more air voids filled and lower binder absorption). The limit for the air voids is included in Figure 4 [72].

4.4. Theoretical Maximum Specific Gravity Results

Regarding the trend of Gmm as WSA replacement increases, from the results shown in Table 9 and Figure 5, it can be stated that at low WSA ratios (10%), the mixture becomes slightly less dense due to the absorption of the binder by WSA and the low density of WSA compared to LS. At moderate WSA ratios (20–30%), the mixture becomes slightly denser as WSA begins to improve the particle packing and distribution of the binder, resulting in a slight increase in Gmm.
At WSA ratios of 40–50%, Gmm decreases again, most likely due to excessive absorption of the binder, as shown in the results of the voids filled with asphalt; this can also be attributed to a decrease in the packing efficiency resulting in more air voids and a less dense mixture. At very high WSA ratios (over 50%), the mixture becomes denser again, indicating better particle packing and reduced air voids, resulting in an overall increase in Gmm.
Finally, Gmm affects the performance of the asphalt mixture and the changes in Gmm reflect how the overall density and compaction of the mixture are affected by the replacement of LS with WSA. Higher Gmm values (indicating denser mixtures) are typically associated with higher resistance to deformation and better performance under load (as indicated by Marshall stability), while lower Gmm values can indicate poor compression, higher air voids, and lower resistance to deformation, which can negatively impact mixture performance, resulting in lower Marshall stability and increased Marshall flow.
In summary, the behavior of Gmm with increasing WSA substitution shows that the relationship between WSA content and mixture density is complex. At low to medium WSA ratios, the mixture benefits from improved binder distribution and better particle packing, resulting in a denser and more stable mixture. At higher WSA ratios, excessive binder absorption and reduced packing efficiency result in a less dense mixture with more air voids, which can negatively impact the performance. Very high WSA ratios (above 50%) may improve packing efficiency again, resulting in a denser mixture with better performance. Gmm therefore serves as a useful indicator of how compact the mixture is and of its overall potential performance in terms of deformation resistance and stability. Appropriate optimization of WSA substitution in asphalt mixtures is critical to achieve the best balance between performance and density.

4.5. Voids in the Mineral Aggregate Results

The voids in mineral aggregates (VMAs) can be used to evaluate the void structure in an asphalt mixture. Figure 6 shows that VMAs generally increase with the percentage of WSA. The minimum VMAs requirement was achieved with all the percentages of WSA. Even with the slight decrease in the percentage of 10% WSA, the mixture still maintained the minimum requirement. The increases for percentages of 20%, 30%, 40%, 50%, 60%, 70%, 80% and 100% WSA was at 0.55%, 8.20%, 12.67%, 14.76%, 21.80%, 22.41%, and 22.47%, respectively. This means that the incorporation of WSA as a substitute filler provided good percentages of voids for the mineral aggregate. Based on these results, it can be stated that the observed increase in VMAs with increasing WSA replacement is a result of the porous nature and low density of WSA compared to traditional fillers such as LS.
Despite the increase in VMA, the mixture maintains acceptable levels of VMA across all WSA ratios, ensuring that the mixture is still able to meet performance specifications. At higher WSA replacement levels (50% and above), voids in the mineral aggregates continue to increase significantly, which can lead to challenges in compaction and binder distribution. However, these mixtures still meet the minimum VMA requirements, suggesting that appropriate binder content adjustments can mitigate potential problems.
The overall effects of WSA on VMA reflect how the void structure in the mixture changes as the use of lighter and more porous fillers increases, affecting compaction, binder absorption and, ultimately, the performance of the asphalt mixture. These results led us to investigate the suitability of the asphalt mixture at all ratios through the results and comparison shown in Figure 6. All values of VMA were within the minimum required range of 14%.

5. Conclusions

From the results obtained in the present research, the following conclusions can be drawn:
  • The physical and chemical properties of fillers had a profound effect on the properties of asphalt mixtures. Specifically, replacing limestone (LS) with walnut shell ash (WSA) in different proportions caused significant changes in the volumetric properties, durability, load-bearing strengths, and increased serviceability of the mixtures. These changes affected key parameters such as air voids, voids in mineral aggregates, binder content, and aggregate compaction.
  • The results indicated that a wide range of filler replacement ratios can be used effectively while ensuring that the mixtures meet the general standard specifications for asphalt pavement performance. This suggests that WSA can be a viable alternative to LS, providing flexibility in the design and formulation of asphalt mixtures, as long as the correct ratio is selected to maintain the desired mixture stability, durability, and workability.
  • Increasing the percentage of WSA at the expense of LS enhances the stability of the mixtures and reduces the flow to good values. However, very high percentages of WSA cause the opposite trend, which can lead to values that do not comply with the standard specifications.
  • The results obtained in this study indicated that the optimum ratio for replacing LS with WSA in asphalt mixtures is 60% WSA and 40% LS. This ratio was found to provide a balance between maintaining the required volumetric properties (such as air voids and voids in mineral aggregates), while achieving the required performance properties (such as stability and flow) in asphalt mixtures.
  • Using 100% WSA with zero LS has generally led to undesirable results. This observation can result from the fact that, at that percentage, the granules are not in the best state of bonding and interlocking. Hence, based on the obtained results, a 100% ratio should not be used to ensure that the ability of the asphalt mixture complies with the physical specifications, to ensure good performance can be achieved under various conditions and expected loads, and also to ensure that a good performance is maintained for the longest possible period.
  • The results showed that the mixture with a 60% replacement ratio achieved the best Marshall stability, with an improvement of 15.02% compared to the conventional sample, alongside with good flow properties. This improvement was accompanied by high conformity with the other physical properties of the asphalt mixture, including a 3.55% air void percentage, which is within the permissible limits for the surface layer, as well as a 21.80% increase in the percentage of voids in the mineral aggregate, which is considered an ideal value.
  • These results paved the way for further studies and adjustments to other requirements of the asphalt mixture, as there were no issues with the availability or production costs of the filler material, given the abundance of raw materials. However, it is important to note that, as is evident from the obtained results, a complete 100% replacement led to undesirable outcomes, with a 10.68% decrease in Marshall strength compared to the conventional sample. This decrease indicates that the mixture was unable to provide its most important property. Although improving the other properties with complete replacement is not beneficial, a detailed investigation into this ineffective percentage revealed that, according to the results, the ideal replacement ratio is 60% WSA and 40% LS for optimal performance.
  • Although the Marshall method offered insightful information on the characteristics of the asphalt mixtures in this investigation, it is crucial to remember that other experiments are frequently conducted to supplement these results, especially when evaluating durability. While the wheel tracking test is frequently used to assess resistance to permanent deformation, tests like the indirect tensile test can be used to assess resistance to moisture damage. Even though these tests were not carried out in this investigation, it is advised that they be included in subsequent studies to obtain a more thorough grasp of the combinations’ durability and long-term performance.

Author Contributions

Y.N.K.: Conceptualization, Methodology, Investigation, Software, Data curation, Methodology, Visualization, Writing—original draft, Writing—review and editing, Investigation. A.T.A.: Conceptualization, Methodology, Formal analysis, Writing—review and editing. A.D.: Project administration, Supervision, Conceptualization, Methodology, Visualization, Investigation, Formal analysis, Visualization, Writing—original draft, Writing—review and editing. H.A.S.P.: Visualization, Writing—review and editing. L.F.A.B.: Visualization, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partially supported by the GeoBioTec Research Unit, through the strategic projects UIDB/04035/2020 (https://doi.org/10.54499/UIDB/04035/2020) accessed on 10 September 2024. and UIDP/04035/2020 (https://doi.org/10.54499/UIDP/04035/2020) accessed on 10 September 2024, funded by the Fundação para a Ciência e a Tecnologia, IP/MCTES through national funds (PIDDAC).

Data Availability Statement

The original contributions presented in the study are included in the article. Further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors express their sincere gratitude for the support received from Kerbala University and University of Warith Al Anbiyaa in Iraq.

Conflicts of Interest

The authors declare no conflicts of interest.

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Jcs 09 00022 g001
Figure 1. Walnut shells: (a) before and (b) after burning.
Figure 1. Walnut shells: (a) before and (b) after burning.
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Figure 2. Marshall stability against different filler percentages.
Figure 2. Marshall stability against different filler percentages.
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Figure 3. Marshall flow against different filler percentages.
Figure 3. Marshall flow against different filler percentages.
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Figure 4. Air voids resulting from different filler percentages.
Figure 4. Air voids resulting from different filler percentages.
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Figure 5. Theoretical maximum specific gravity against different filler percentages.
Figure 5. Theoretical maximum specific gravity against different filler percentages.
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Figure 6. Voids in the mineral aggregate against different filler percentages.
Figure 6. Voids in the mineral aggregate against different filler percentages.
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Table 1. Characteristics of the aggregates (physical and chemical).
Table 1. Characteristics of the aggregates (physical and chemical).
PropertyStandardResultLimit
Specific weight for coarse aggregate (g/cm3)ASTM C127 [35]2.732
Bulk specific weight for fine aggregate (g/cm3)ASTM C128 [36]2.641
Water absorption of coarse aggregate (%)ASTM C1270.450
Impact value (%)ASTM C131 [37]12.90
Abrasion value (LOS-%)ASTM C13116.70
Sulfur ContentASTM C114 [38]0.040
0.103		Coarse ≤ 0.1% Fine ≤ 0.5%
Clay ContentASTM C142 [39]0.10
0.56		Coarse ≤ 1% Fine ≤ 1%
Water Soluble Sodium and PotassiumASTM C289 [40]0.330
0.301		Coarse ≤ 0.1% Fine ≤ 0.1%
Carbonate ContentASTM C3310 [41]4.50
3.11		Both ≤ 20%
Organic ImpuritiesASTM C40 [42]0.02
0.05		Both ≤ 0.1%
Table 2. Surface coarse gradation.
Table 2. Surface coarse gradation.
Sieve Size¾ Inch½ Inch3/8 InchNO.4NO.8NO.50NO.200
Gradation limits10090–10076–9044–7428–585–214–10
Used gradation10095835943137
Table 3. Physical and chemical characteristics of the bitumen.
Table 3. Physical and chemical characteristics of the bitumen.
TestAverage ValuesStandardLimits
Permeation (25 °C)4 mmASTM D5 [43](4–5) mm
Flash-P235 °CASTM D92 [44]232 °C
Fire-P340 °CASTM D92>400 °C
Softening-P45.5 °CASTM D36 [45](30–157) °C
Ductility (5 cm/min)40 cmASTM D113 [46]>25 cm
Specific gravity1.045ASTM D70 [47]-
Expansion at low temperatures13 °CASTM D7317 [48]≤18 °C
Sulfur content0.101%ASTM D4057 [49]≤0.5%
Mineral content0.217ASTM D2007 [50]≤1%
Asphaltene Content18%ASTM D2007-
Volatile Matter Content0.44%ASTM D2879 [51]≤1.5%
Hydrocarbon Composition57%ASTM D200750–70
pH4.201 5.5–7.0
Table 4. Physical and chemical properties of the limestone.
Table 4. Physical and chemical properties of the limestone.
TestAverage ValuesStandardLimits
Calcium Carbonate (CaCO3)97.6%ASTM C25 [54]---
Silicon Dioxide (SiO2)1.2%ASTM C25---
Aluminum Oxide (Al2O3)1.0%ASTM C25---
Magnesium Oxide (MgO)0.97%ASTM C25---
Iron Oxide (Fe2O3)0.87%ASTM C25---
Other Minor Compounds1.1%ASTM C25---
Specific gravity2.715ASTM C127 [35]2.60–2.80 g/cm3
Saturated specific gravity2.700ASTM C127---
Water absorption (%)0.127ASTM C127≤1.5%
Particle Size (Gradation)51ASTM C136 [55]≤75 microns
D5017.8
Table 5. Chemical composition of WSA.
Table 5. Chemical composition of WSA.
Chemical CompoundPercentage %Description
Silica (SiO2)41%The main component is responsible for improving the strength and bonding of the mixture.
Alumina (Al2O3)7.6%Alumina improves heat resistance and enhances the structural integrity of the mixture.
Iron Oxide (Fe2O3)4%This compound plays a major role in improving the adhesion and structural stability of the mixture.
Calcium Oxide (CaO)9%This compound affects the bond between asphalt and raw materials, contributing to the required stability and durability.
Magnesium Oxide (MgO)2%This compound greatly affects the chemical reaction with asphalt and influences the mechanical properties of the mixture.
Potassium (K2O)2.8%This compound improves thermal stability and can contribute to improved durability of the mixture.
Phosphorus (P2O5)0.76%This compound is a secondary component that may enhance the chemical properties of an asphalt mixture.
Sulfates (SO3)0.08%Sulfur compounds can affect the chemical properties of asphalt if they are used in a high concentration.
Organic Residues0%Organic matter is completely burned away during the process, with little effect on the absorption and bonding with the asphalt.
Table 6. Physical properties of WSA used in the mixture.
Table 6. Physical properties of WSA used in the mixture.
PropertyAverage ValueSpecificationLimits
Distribution56ASTM C136 [55]≤75 µm
Specific Gravity2.23ASTM C127 [35]2.1–2.3
ShapeAngular/IrregularASTM C1252 [59]Angular/Irregular
Abrasion Resistance18%ASTM C131 [37]≤30%
Table 7. Mechanical tests were performed in this study.
Table 7. Mechanical tests were performed in this study.
Mechanical TestsSpecifications [61]
Marshall stability (MS)ASTM D6927-AASHTO T 245 [62]
Marshall flow (MF)ASTM D6927-AASHTO T 245
Voids filled with asphalt (VFA)ASTM D3203-AASHTO T 269 [63]
Voids in the mineral aggregate (VMA)ASTM D3203-AASHTO T 269
Volume of air (VA)ASTM D3203-AASHTO T 269
Theoretical maximum specific gravity (Gmm)ASTM D 2041-AASHTO T 209 [64]
Table 8. Asphalt mixtures were adopted in this research.
Table 8. Asphalt mixtures were adopted in this research.
Mix No.123456789
Sieve No. (Inch)3/4100100100100100100100100100
1/2959595959595959595
3/8838383838383838383
NO.4595959595959595959
NO.8434343434343434343
NO.50131313131313131313
Filler %LS100908070605040200
WSA010203040506080100
Table 9. Marshall test for control mixtures and asphalt mixtures with WSA.
Table 9. Marshall test for control mixtures and asphalt mixtures with WSA.
Mix TypeStability
kN		Flow
mm		Bulk Density
gm/cm3		GmmVA %VMAVFA
WSALS
0%100%8.323.652.3862.4753.616.3368
10%90%8.73.52.3742.4653.6616.3171
20%80%93.12.3832.4783.6916.4272.3
30%70%9.1132.3942.4873.7517.6772
40%60%9.1932.3792.4723.7618.474.71
50%50%9.5632.3622.4553.7818.7475.11
60%40%9.573.42.372.4583.5519.8977
80%20%9.533.52.3792.4593.2619.9977.79
100%0%7.434.152.3932.4672.992080
Asphalt content %5.0
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MDPI and ACS Style

Kadhim, Y.N.; Abdulrasool, A.T.; Dulaimi, A.; Pinto, H.A.S.; Bernardo, L.F.A. Influence of Walnut Shell Ash and Limestone Filler in Hot Mix Asphalt. J. Compos. Sci. 2025, 9, 22. https://doi.org/10.3390/jcs9010022

AMA Style

Kadhim YN, Abdulrasool AT, Dulaimi A, Pinto HAS, Bernardo LFA. Influence of Walnut Shell Ash and Limestone Filler in Hot Mix Asphalt. Journal of Composites Science. 2025; 9(1):22. https://doi.org/10.3390/jcs9010022

Chicago/Turabian Style

Kadhim, Yasir N., Abdulrasool Th. Abdulrasool, Anmar Dulaimi, Hugo Alexandre Silva Pinto, and Luís Filipe Almeida Bernardo. 2025. "Influence of Walnut Shell Ash and Limestone Filler in Hot Mix Asphalt" Journal of Composites Science 9, no. 1: 22. https://doi.org/10.3390/jcs9010022

APA Style

Kadhim, Y. N., Abdulrasool, A. T., Dulaimi, A., Pinto, H. A. S., & Bernardo, L. F. A. (2025). Influence of Walnut Shell Ash and Limestone Filler in Hot Mix Asphalt. Journal of Composites Science, 9(1), 22. https://doi.org/10.3390/jcs9010022

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