Asphalt mixtures remain indispensable in modern road construction due to their exceptional mechanical resistance, durability, and ability to bear significant traffic loads. These materials primarily comprise aggregates ((90% - 95%) by weight) and bitumen (5% - 10%), with bitumen acting as a viscoelastic binder that ensures cohesion and impermeability. The performance and longevity of asphalt mixtures are strongly influenced by the quality of these components, particularly the cohesion provided by bitumen [24] - [27] .

Aggregates, which form the structural backbone of asphalt mixtures, may be natural (gravel, sand, crushed stone) or recycled. Their gradation plays a pivotal role in optimizing performance, as it minimizes voids, enhances compactness, and improves stability [28] [29] . The Fuller-Thompson gradation model is often employed to achieve optimal particle packing, maximizing density and mechanical strength [25] [30] . Bitumen, derived from petroleum, coats the aggregates to ensure viscoelastic behavior essential for resisting mechanical stresses and environmental conditions. Furthermore, its impermeable properties protect underlying pavement layers from water infiltration, a leading cause of structural degradation [13] [31] .

Despite its advantages, the use of bitumen is not without challenges. Its petrochemical origin is associated with significant greenhouse gas emissions, and its sensitivity to temperature extremes can result in plastic deformation under high temperatures and cracking under low temperatures. Advances such as polymer-modified bitumen (PMB), incorporating materials like SBS (styrene-butadiene-styrene) and SBR (styrene-butadiene rubber), have improved its resistance to fatigue, thermal variations, and moisture-induced stripping [32] .

Moisture infiltration is one of the most detrimental factors affecting asphalt mixtures. It compromises adhesion between bitumen and aggregates, leading to stripping, cracking, and deformation over time [2] [32] . The phenomenon of stripping occurs when water displaces bitumen from aggregate surfaces, weakening the cohesive bond and reducing the mixture’s structural integrity. Hydrophilic aggregates, such as those rich in silica, exacerbate this issue, while hydrophobic aggregates, such as limestone, exhibit superior resistance to water infiltration [13] [31] .

Freeze-thaw cycles amplify moisture-induced damage, particularly in climates with significant temperature fluctuations. Water trapped within asphalt voids expands upon freezing, exerting internal pressures that create cracks and allow further water ingress, accelerating degradation and pothole formation [13] . To mitigate these effects, formulations must optimize aggregate gradation for maximum compactness, thereby reducing voids and limiting water pathways [22] .

The rheological and physical properties of bitumen are equally crucial. Modified bitumen, incorporating anti-stripping agents or polymers, enhances adhesion and flexibility, reducing the likelihood of moisture-related damage under thermal and mechanical stress [32] . These innovations underscore the necessity of exploring alternative materials to bolster asphalt mixtures against environmental challenges.

The quest for sustainable and high-performance road materials has prompted extensive research into alternative solutions. Biosourced and recycled materials offer promising avenues to improve asphalt durability while addressing environmental concerns.

Conventional asphalt mixtures are the benchmark for road construction due to their durability, mechanical strength, and adaptability to traffic loads. Their performance is governed by three primary components: aggregates, bitumen, and fillers.

Eggshell powder (ESP) is an agro-industrial byproduct primarily composed of calcium carbonate (CaCO₃). Its chemical and physical properties make it a promising replacement for conventional fillers.

To ensure consistency and reliability, the preparation of ESP involved meticulous cleaning, drying, grinding, and sieving processes ( Figure 1 ).

Three formulations were prepared to evaluate the influence of ESP substitution:

The total filler content was set at 7% of the aggregate weight, following the Fuller and Thompson gradation curve [36] - [38] . This ensured optimal compactness and uniform distribution of filler particles.

Each formulation was mixed in a high-speed laboratory mixer to ensure homogeneity. A controlled temperature environment was maintained to optimize the interaction between the filler, bitumen, and aggregates. The final mixtures were subjected to standardized compaction methods to prepare test specimens ( Figure 3 ).

The performance of the asphalt mixtures was evaluated through a series of standardized tests:

This rigorous experimental methodology aims to validate the potential of ESP as a filler in asphalt mixtures. By comparing mechanical, thermal, and moisture resistance properties, this study highlights the feasibility of ESP in sustainable infrastructure applications.

Objective:

To identify the chemical composition and crystalline phases of ESP.

FTIR Procedure

1) ESP is mixed with potassium bromide (KBr) and pressed into a pellet.

2) Infrared spectra are recorded to detect functional groups and confirm the presence of CaCO₃.

XRD Procedure

1) ESP is analyzed to determine crystalline phases and purity.

2) Peaks corresponding to CaCO₃ are identified, providing insights into the filler’s structural properties.

Objective:

To ensure ESP meets the required specifications for asphalt fillers.

Procedure:

1) Sieving is conducted according to NF EN 933-1, with a sieve stack covering <63 μm and larger sizes. The resulting data verify the filler’s suitability for reducing voids and improving cohesion.

Objective:

To evaluate the compaction potential and moisture resistance of ESP.

Procedure:

1) Density is measured using a pycnometer (NF EN 1097-6).

2) Water absorption is calculated by comparing dry and wet sample weights after soaking, using:

$\text{Water Absorption}\left(\text{\%}\right)=\frac{\text{Wet Mass}-\text{Dry Mass}}{\text{Dry Mass}}\times 100$

Objective:

To measure load-bearing capacity and flow of asphalt mixtures.

Procedure:

Objective:

To assess tensile strength and cracking resistance.

Procedure:

${\sigma }_t=\frac{2P}{\pi Dt}$

where:

- $P$ is the applied load,

- $D$ is the diameter, and

- $t$ is the thickness.

Objective:

To evaluate the filler’s effect on adhesion under wet conditions.

Procedure:

Objective:

To measure tensile strength retention under moisture exposure.

Procedure:

$\text{ITSR}=\frac{\text{Wet Strength}}{\text{Dry Strength}}\times 100$

Objective:

To assess resistance to cyclic freezing and thawing.

Procedure:

Objective:

To simulate long-term aging of asphalt mixtures.

Procedure:

The results of the preliminary tests informed the optimization of the mix formulations. The 7% filler content, determined through the Fuller and Thompson gradation curve [24] , was validated for its effectiveness in balancing compactness, mechanical strength, and moisture resistance.

The systematic integration of ESP into asphalt mixtures demonstrates its potential as a sustainable alternative filler. By replacing limestone partially or entirely, ESP contributes to improved mechanical performance, enhanced moisture resistance, and reduced environmental impact, aligning with the goals of sustainable road construction.

The aggregates used in this study include 0/6 sand and 6/10 gravel sourced from the OKUTA quarry in Zangnanado, Benin.

A series of tests were conducted to evaluate their physical and mechanical properties, focusing on:

Table 1 and Table 2 summarize the test results assessing the physical and mechanical properties of the materials.

Table 1 : Aggregate characteristics, including mechanical properties such as MDE (%), LA (%), and FI (%), follow the relevant standards (e.g., NF EN 1097-1, NF EN 933-3). The results comply with the specified limits, indicating suitable aggregate performance for the intended application.

Table 2 : Aggregate physical characteristics, such as apparent density and water absorption, are evaluated using NF EN 1097-6 and NF EN 1097-3 standards. The results meet the declared specifications and confirm the materials’ adequacy for construction purposes.

The results confirm that the 6/10 gravel is highly suitable for producing 0/10 Semi-Dense Bituminous Mixtures (BBSG) for Category 3 road pavements. Specifically:

In summary, the 6/10 gravel from the OKUTA quarry exhibits the uniformity and quality required for producing stable and durable asphalt mixtures for secondary road surfaces.

The results indicate that the 6/10 gravel complies with the specifications, ensuring durability and suitability for the formulation of semi-dense asphalt concrete (SDAC).

Figure 4 illustrates the particle size distribution curves of the aggregates.

The harmonious gradation of the 0/6 and 6/10 classes ensures optimal compaction and good cohesion in the mixture.

The gradation curves for the 0/6 sand and 6/10 gravel are depicted in Figure 4 , which illustrates the particle size distribution based on sieve analysis.

Key Insights:

The bitumen used in this study is a 35/50-grade binder supplied by CHELL TOGO ( Figure 5 ). Samples were collected from the ADEOTI S.A. facility in Adjura, near Porto-Novo, Benin. The bitumen underwent standard testing to evaluate:

Table 3 summarizes the results of its characterization.

Penetration at 25˚C:

The low penetration value confirms the binder’s hardness, making it suitable for heavily trafficked wearing courses.

Softening Point:

The high softening point ensures resistance to deformation at elevated temperatures, reducing risks of rutting and flow.

Density:

The density aligns with typical values for 35/50-grade bitumen, contributing to the mix’s structural integrity and compaction.

The results comply with NF EN 12591 standards, confirming that the 35/50 bitumen is suitable for wearing courses subjected to heavy loads.

FTIR spectra confirmed the dominance of calcium carbonate in ESP, with characteristic absorption peaks at 1415 cm⁻¹ and 875 cm⁻¹. XRD analysis corroborated this finding, showing calcite as the primary crystalline phase. The bulk density of ESP was 2.62 g/cm³, and its water absorption was lower than that of limestone filler, indicating potential for reduced permeability in asphalt mixtures.

The gradation analysis reveals that powdered eggshells are dominated by fine particles. This distribution is advantageous for their function as a filler in bituminous mixtures. The fines effectively fill voids between aggregates, increasing compaction and reducing water permeability—both critical factors in asphalt pavement durability.

Table 4 presents the cumulative particle size distribution of ESP.

The gradation curve ( Figure 6 ) highlights the predominance of fine particles (<63 μm), making ESP suitable for filling voids between aggregates and improving compactness.

Compaction and Mechanical Strength: The high proportion of fine particles (<63 μm) in EPS enhances the compactness of the mixture, translating into improved mechanical strength and extended durability. These fine particles fill inter-aggregate voids, distributing loads more effectively.

Water Permeability: The predominance of fines significantly reduces water permeability, mitigating the risk of stripping and moisture-induced damage.

Aggregate-Binder Adhesion: The dense gradation provided by EPS improves the adhesion between the binder and aggregates, creating a robust and moisture-resistant matrix. This is particularly critical in climates with high precipitation or freeze-thaw cycles.

Fillers used in asphalt mixtures must meet stringent gradation criteria to ensure optimal performance. According to standards such as NF EN 933-10, an effective filler should exhibit a high proportion of fine particles (≤60 μm) to enhance compaction and reduce moisture sensitivity. The gradation results for EPS align well with these requirements, validating its suitability as a filler.

The gradation analysis demonstrates that EPS can serve as an effective filler in asphalt mixtures, offering both performance and sustainability benefits.

Key advantages include:

The XRD analysis ( Table 5 ) reveals the predominance of calcium carbonate (CaCO₃) in the form of calcite. The results are displayed in Figure 7 .

XRD analysis confirms that powdered eggshells are primarily composed of calcium carbonate (CaCO₃) in its crystalline calcite form. The most intense peaks, observed at 23.1˚C, 29.4˚C, and 36.1˚C, affirm calcite as the dominant phase. Additionally, minor traces of magnesite (MgCO₃) and aragonite (another form of CaCO₃) are detected. These secondary phases, while present in small quantities, do not significantly affect the properties of ESP as a filler.

Implications for Asphalt Mixtures

Mechanical Strength and Cohesion: The calcite-dominant composition improves the mechanical strength and cohesion of asphalt mixtures, ensuring structural integrity under heavy loads.

Durability: The absence of significant impurities ensures high compatibility with bitumen, reducing the likelihood of adverse reactions and enhancing long-term performance.

Sustainability: By utilizing a renewable and biodegradable material like eggshells, ESP offers an eco-friendly alternative to conventional limestone fillers.

The results confirm the chemical purity of ESP and its potential as a substitute for traditional limestone fillers.

The FTIR analysis ( Table 6 ) identifies characteristic carbonate bands, confirming the predominance of calcite. The results are illustrated in Figure 8 .

Table 6 below summarizes key infrared absorption peaks observed in powdered eggshells (ESP), indicating its chemical composition. The results highlight the dominance of calcium carbonate (CaCO₃) and minor traces of organic or residual water content.

Calcium Carbonate Confirmation: Peaks at 712, 875, and 1420 cm⁻¹ confirm the dominance of calcium carbonate, which enhances compaction and cohesion in asphalt mixtures.

Residual Water and Organic Traces: Peaks at 1640 and 2850 - 2950 cm⁻¹ indicate minor water content and residual organic matter. These may necessitate additional drying or treatment to optimize performance in bituminous mixtures.

The FTIR results reinforce the suitability of ESP as a filler in asphalt mixtures, providing both functional and environmental benefits. Key advantages include:

These results validate the favorable properties of ESP for improving asphalt compactness and durability.

The granular composition utilized in this study corresponds to an ongoing project conducted at the ADEOTI SA base. In compliance with NF EN 13108-20 (Article 4), which defines the validity period of a mixture formulation and the conditions requiring reformulation, the proportions used were based on the site’s formulation while ensuring adherence to the reference 0/10 grading envelope. This also aligns with the requirements of NF P 98-130 (Article 6.1), which mandates adherence to the grading curve specifications for semi-dense bituminous mixtures (BBSG).

The selected aggregate percentages are illustrated in Figure 9 .

The pie chart in Figure 9 highlights the distribution of the various aggregate fractions, comprising sand (0/6), gravel (6/10), and filler.

The grading curves of the mixture are depicted in Figure 10 , which includes:

The results demonstrate that the mixture complies with specifications, as its grading curve lies within the required envelope. Its close alignment with the median curve indicates a well-balanced granular structure, optimized for compaction and stability.

Using the Andreasen and Andersen model with a coefficient $q=0.5$, the optimal ESP content was estimated at 7%, ensuring a balance between compactness and stability.

Based on existing literature, the recommended filler proportion for pozzolanic fillers ranges from 1% to 10%, with the most frequently cited range being 1% to 7.5%.

To optimize compaction, stability, and water resistance for the BBSG, the Andreasen and Andersen model was employed, utilizing a coefficient $q=0.5$, as recommended by Fuller and Thompson for achieving optimal compaction. The model is expressed as:

$P\left(d\right)=100\times {\left(\frac{d}{D_{\text{max}}}\right)}^q$

where:

This approach targets a cumulative gradation curve that maximizes the compaction of the granular skeleton. Applying this model to the BBSG 0/10 grading curve and filler diameter ( $d=0.063\text{mm}$), a calculated filler proportion of 7.94% was obtained. While this value yielded excellent compaction, it did not fully satisfy stability requirements.

Referring to NF EN 13108-1, which specifies a grading envelope for the 0.063 mm sieve of 2% to 12%, a filler proportion of 7% was chosen. This value, positioned at the center of the normative range, balances compaction and stability effectively.

To ensure robustness, further tests were conducted by varying filler proportions incrementally by ±0.5% around this value. These variations allowed the identification of an optimal balance between compaction, stability, and water resistance, demonstrating the suitability of the 7% filler proportion.

The Marshall tests determined an optimal bitumen content of 5.9%, which complies with the specifications outlined in NF EN 13108-1. This optimal content plays a critical role in ensuring enhanced stability and long-term durability of the material.

According to NF EN 13108-1, the minimum bitumen content for semi-dense hot bituminous mixtures (BBSG) Type 3 with 0/10 granulometry is 5.1%. Based on this reference, a theoretical bitumen content of 5.5% was initially selected, with variations of ±0.5%.

Marshall Mix Tests were conducted in the laboratory to determine the optimal bitumen content experimentally. The optimal bitumen content was found to be 5.9%, within the admissible variation range (±0.5%) from the theoretical content of 5.5%.

Key Observations:

The experimentally determined bitumen content complies with the specifications and ensures an optimized BBSG formulation ( Figure 11 ). The Marshall Method was employed and validated to determine the optimal composition that meets both technical requirements and performance criteria. The results confirmed that the formulated bituminous mixture complies with standard specifications while delivering the desired mechanical performance.

The inclusion of 7% Eggshell Powder (ESP) was identified as yielding the highest water resistance while maintaining stability. Beyond this threshold, performance declined due to the saturation of fines. The addition of ESP significantly enhances durability in wet conditions, an essential consideration for infrastructure longevity ( Figure 12 ).

Key findings:

This formulation, incorporating 7% ESP as filler, was selected as the focus for subsequent evaluations.

Table 7 below summarizes the results of the indirect tensile strength (ITS) test for various asphalt formulations, including eggshell powder (ESP) as a filler, alongside a reference formulation with 100% limestone filler. The results are expressed in terms of maximum applied force and calculated tensile strength ( Figure 13 ).

Key Observations:

Implications of Results:

Tensile strength increases with the proportion of ESP, indicating improved internal cohesion and durability.

The adhesion test evaluates the ability of bitumen to adhere to aggregates after immersion in water for 24 hours at 60˚C. Table 8 below presents the percentage of bitumen coverage retained after immersion for different asphalt formulations ( Figure 14 ).

Key Observations:

Implications of Results:

Marshall stability tests assess the load-bearing capacity and deformation resistance of asphalt formulations ( Figure 15 & Figure 16 ). The results are summarized below in Table 9 .

Key Observations:

Implications of Results:

The water sensitivity test measures the resistance of asphalt mixtures to moisture-induced damage, expressed as the Indirect Tensile Strength Ratio (ITSR). The results are summarized in Table 10 .

The ITS ratios for RM, BM, and EM were 79.2%, 85.2%, and 91%, respectively, demonstrating that ESP enhances resistance to moisture-induced stripping. The hydrophobic nature of calcium carbonate in ESP likely contributed to improved adhesion between bitumen and aggregates ( Figure 17 ).

Key Observations:

Implications of Results:

The ITSR improvement demonstrates the enhanced moisture resistance provided by ESP.

Under freeze-thaw cycles, EM exhibited the highest retained strength (87%), compared to BM (81%) and RM (75%). Marshall stability tests revealed a 12% increase in stability for EM over RM, highlighting the mechanical benefits of ESP incorporation.

The freeze-thaw cycle test was conducted to evaluate the durability of different asphalt mixtures under repeated freezing and thawing conditions. Formulations incorporating eggshell powder (ESP) as a filler were compared against a reference formulation containing 100% limestone filler. Samples were subjected to 10 freeze-thaw cycles, and the mass loss and tensile strength were measured to assess material degradation and cohesion ( Figure 18 ). The results are summarized in Table 11 .

Analysis of Results:

Mass Loss:

Tensile Strength:

Implications:

Comparison with Limestone Filler:

Applications:

The accelerated aging test was conducted to simulate the long-term performance of asphalt mixtures under high-temperature oxidative conditions. Formulations with varying proportions of eggshell powder (ESP) and limestone filler were exposed to 85˚C for five days. Key parameters, including mass loss, tensile strength, and elongation at break, were measured before and after aging and the key results are summarized in Table 12 and displayed in Figure 19 .

Mixes with ESP exhibit better resistance to thermal aging.

Analysis of Results:

Mass Loss:

Tensile Strength:

Elongation at Break:

Implications:

Comparison with Limestone Filler:

Applications:

The utilization of ESP reduces dependence on mined fillers and diverts waste from landfills, aligning with sustainability goals. Economic analyses suggest cost savings in regions with abundant eggshell waste, further incentivizing its adoption.

The use of eggshell powder (ESP) as a filler in asphalt mixtures offers significant environmental advantages. A life cycle analysis conducted by Xia et al. (2018) demonstrated that different substitution rates of ESP yield substantial reductions in CO₂ emissions:

These findings highlight a direct correlation between the ESP substitution rate and the reduction in greenhouse gas emissions. Higher substitution rates lead to progressively greater reductions, ranging from 5% to 10%, making ESP a viable material for reducing the carbon footprint of asphalt mixtures.

The economic advantages of using ESP have also been documented. For instance, Dhas and Kamaraj (2016) reported cost savings of 7% in asphalt production with an 8% ESP substitution rate. In our study, the economic and environmental impacts of incorporating ESP were evaluated for substitution rates ranging from 6.5% to 8%, with the results summarized in Table 13 .

The use of ESP contributes to a reduction in CO₂ emissions by up to 9%, as shown in Table 13 .

The progressive reduction in CO₂ emissions with increasing ESP substitution rates demonstrates the environmental potential of ESP. Substitution rates of 7.5%-8% are particularly notable, achieving reductions close to 9%. This reduction can be attributed to the replacement of conventional fillers, which are associated with higher carbon footprints, by ESP.

Similar to the trend in CO₂ reductions, cost savings increase with higher ESP substitution rates. The substitution of traditional fillers with ESP reduces production costs without compromising the mechanical and durability properties of asphalt mixtures. Substitution rates of 7.5% - 8% offer the optimal balance between cost-effectiveness and environmental performance.

Dual Benefits

ESP substitution simultaneously reduces CO₂ emissions and production costs, providing a dual advantage. This makes ESP an attractive filler material for achieving sustainability goals in the construction sector.

Optimal Substitution Rates

Substitution rates of 7.5% - 8% ESP are identified as optimal, maximizing environmental benefits and cost reductions. These rates strike the best balance between sustainability and economic viability.

Sustainability in Construction

The integration of ESP in asphalt production promotes sustainable construction practices by:

By replacing traditional mineral fillers with ESP, the asphalt industry can significantly enhance its environmental performance while maintaining or improving the quality and durability of road infrastructure.

These findings confirm ESP’s potential as a sustainable and cost-effective solution for asphalt pavements.

Optimized Formulation: Using the Fuller model, five mixtures with varying ESP contents (0%, 6.5%, 7%, 7.5%, and 8%) were tested. Mixtures with ESP content between 1% and 7.5% demonstrated optimal performance, with 7% identified as the most effective proportion for significantly enhancing stability and moisture durability.

Superior Material Properties:

Enhanced Durability Under Adverse Conditions:

Environmental and Economic Benefits:

The study demonstrates that eggshell powder can be a sustainable and effective solution for enhancing the performance and durability of asphalt mixtures. By leveraging ESP, road construction can achieve significant improvements in moisture resistance, thermal aging, and mechanical properties, while simultaneously reducing the environmental impact of infrastructure projects.

Future research should expand to large-scale field applications to evaluate the long-term performance of ESP-modified asphalt under diverse environmental and traffic conditions. Investigations into further refining ESP processing methods, such as additional drying or purification, could enhance its material properties and broaden its application scope.

The incorporation of eggshell powder into asphalt mixtures emerges as a high-performance and sustainable alternative for road pavement construction. This innovative approach addresses critical environmental and socio-economic challenges by reducing carbon emissions, lowering production costs, and improving pavement longevity.

This study concludes that a 7% substitution of conventional fillers with ESP optimizes asphalt performance, delivering enhanced resistance to moisture, freeze-thaw cycles, and thermal aging. The findings highlight ESP’s potential to support the development of durable and eco-friendly road infrastructure in regions like Benin, where it can leverage locally available resources.

By valorizing agricultural by-products, this research contributes to a broader transition towards eco-responsible civil engineering practices, advancing the goals of sustainability and high-performance transport infrastructure.