Sustainable Asphalt Mixtures Using Agricultural Waste Fillers: A Systematic Review of Rice Husk Ash, Bagasse Ash and Palm Shell Ash

A comprehensive search was conducted in four academic databases: Scopus, ScienceDirect, Google Scholar, and Garuda, covering the publication period 2010 to 2024. The key used included: “asphalt concrete”, “agricultural waste filler”, “rice husk ash”, “bagasse ash”, “palm shell ash”, “Marshall stability” and ‘Marshall parameters” (Ali & Al-Tameemi, 2025).

Studies were initially screened based on their titles and abstracts to exclude unrelated articles. Eligible studies were required to:

• Use RHA, BA, or PSA as partial or full filler replacements.

• Evaluate at least one Marshall parameter (stability, flow, VIM (Void In Mix), VMA (Void in Mineral Aggregate), VFB (Void Filled with Bitumen), and MQ (Marshall Quotient) (Kareem et al., 2024a).

• Report clear filler dosages and experimental procedures.

Selected studies were reviewed in full text. Relevant data were extracted into a structured table, including:

• Type and source of filler.

• Filler dosage (in % by weight).

• Reported Marshall test results.

• Observed threads and limitations.

Extracted studies were grouped by filler type (RHA, BA, PSA) and analysed for performance consistency across dosages. This enabled comparison of trends and identification of optimum content ranges. Where available, data and non-experimental works were excluded.

To ensure review reliability, only peer-reviewed journal articles and conference papers were considered. Duplicate data and non-experimental works were excluded. Methodology ensured transparency, rigour, and actionable insights for infrastructure applications as seen in Table 1.

RHA enhances Marshall stability at 6–8% filler content due to its high silica and angular shape, improving binder interaction and stiffness (Arabani & Tahami, 2017b), (Ragab & Abo El-Naga, 2022). Its pozzolanic activity boosts cohesion and deformation resistance (M. N. N. Khan et al., 2015), (Lu et al., 2020). However, excessive RHA increases brittleness, so optimal dosage is crucial to balance strength and flexibility in asphalt mixes (Fareed et al., 2020), (Rahmouni et al., 2019), (Nava Bravo et al., 2019).

BA enhances asphalt stability through angular texture and silica content (Zainudin et al., 2016),(Sarir et al., 2025), (Jwaida et al., 2024), (F. de A. Silva et al., 2011), (Liu et al., 2020). Performance depends on particle fineness and carbon content; fine, low-carbon BA improves packing and binder adhesion, while coarse or carbon-rich ash reduces homogeneity (Rahmouni et al., 2019), (Nava Bravo et al., 2019), (Zhang et al., 2020). Proper processing ensures consistent improvements in mixture stability and flow (Fjelsted et al., 2020).

PSA offers moderate stability but markedly enhances flow characteristics due to its fine particles and mineralogy (Pelisser et al., 2010), (T.-A. De Silva & Forbes, 2016), (Shi & Reitz, 2010), (Chaudhuri et al., 2018). This promotes flexibility under traffic and thermal stresses (Jwaida et al., 2024), (Amanda et al., 2025). Although PSA may not ensure peak stability, its ductility benefits mitigate cracking risks (Chan et al., 2019). Moreover, PSA supports palm oil waste valorisation, reinforcing sustainability goals in asphalt pavement design (Patel et al., 2017), (Naqvi et al., 2019). A comparative summary of these effects on stability and flow characteristics is presented in Table 2.

Table 2 shows RHA and BA significantly enhance Marshall stability and stiffness when optimally dosed, while PSA improves flow and flexibility, vital for variable loading. Filler selection must balance mechanical performance, local agricultural waste availability, and pavement-specific requirements for sustainable application.

Agro-waste fillers like rice husk ash (RHA), bagasse ash (BA), and palm shell ash (PSA) typically reduce Voids in Mix (VIM) and Voids in Mineral Aggregate (VMA), indicating better particle packing and densification. Finer, angular ashes fill inter-aggregate voids more efficiently, improving mix compactness and durability (Fareed et al., 2020), (Zhang et al., 2020).

RHA’s fine particles and high silica content effectively reduce VIM and VMA at 6–8% filler content (Mohajerani et al., 2017), (Shafabakhsh et al., 2018). Angular and porous, RHA promotes interlock with aggregates, enhancing load transfer and reducing air voids (Raj et al., 2023). However, excessive RHA may cause over-compaction, lowering voids below optimal limits and impairing moisture resistance (Mistry & Kumar Roy, 2021).

BA also reduces voids, provided it's finely ground and low in residual carbon (Yoo et al., 2016), (Kim et al., 2024). Properly processed BA enhances density and reduces permeability (Buritatum et al., 2022). Coarse or carbon-rich BA may limit void reduction and weaken cohesion (Yoo et al., 2016).

PSA provides the greatest reduction in VIM and VMA due to its fine size and mineralogy (Ramdhani et al., 2025), (Oktavia et al., 2022). This densifies mixtures and boosts rutting resistance. Yet, excessive PSA can lower air voids too much, risking bleeding and limiting thermal flexibility (Doğruyol & Durmaz, 2025).

The influence of RHA, BA and PSA on VIM and VMA can be observed in Table 3, which summarises the reported trends across different filler contents.

The use of RHA, BA, and PSA reduces VIM and VMA, increasing VFB and improving cohesion and impermeability (Tiwari et al., 2023), (Liang et al., 2023). Maintaining optimal VFB prevents bleeding and ensures skid resistance (Hermansyah et al., 2022). Proper control of filler content and characteristics enhances density, durability, and moisture resistance in asphalt mixtures. As shown in Table 3, the VFB values tend to increase with filler addition indicating improved compact ability of the mixtures.

Table 3 highlights the importance of filler type, fineness, and dosage in modifying the void structure of asphalt mixes. Each ash type brings distinct benefits and considerations, making its selection critical to achieving optimal asphalt performance.

The Marshall Quotient (MQ), defined as the ratio of Marshall Stability to Flow, is a critical indicator of an asphalt mixture’s stiffness and its resistance to permanent deformation under load.(Taherkhani, 2016) A higher MQ value reflects a stiffer, more rut-resistant mix, while a lower MQ indicates greater flexibility but potentially less resistance to deformation (Taherkhani, 2016).

RHA enhances MQ up to an optimal 6–8% dosage, driven by its high silica and angular, porous particles that improve aggregate-binder interlocking (Damanhuri et al., 2020). This increases stiffness and rutting resistance for traffic-bearing pavements. Excess RHA (>8%) can cause brittleness and cracking (Du et al., 2018). Optimal dosing is key for balancing flexibility and stiffness (Guo et al., 2025).

Finely ground BA at 5–10% improves MQ through angularity and surface roughness that boost interlocking and binder adhesion (Sarir et al., 2025), (Sarir et al., 2022), (Zia & Khan, 2021). Fineness and low residual carbon are crucial, as unburnt matter can reduce stiffness.

PSA's fine particles and minerals improve packing, stability, and rutting resistance via enhanced binder interaction, significantly raising MQ and durability (Omoremiju et al., 2024). In experimentation using palm shell ash fillers at varying content, the addition of PSA notably improved Marshall parameters while offering cost-effective regional agro-waste (Tambunan, 2025). PSA-modified mixes maintain workability, aiding field application, with optimal content at 5–10%. Excess PSA, like RHA and BA, may reduce fatigue life (N. Khan, 2021).

Therefore, within the range of 5–7% PSA content by weight, PSA demonstrates the highest MQ among the three fillers, suggesting it as the most suitable option in terms of mechanical performance, local availability, and economic sustainability. A comprehensive comparison of these effects on Marshall quotient, including optimal dosage ranges and technical implications, is summarised in Table 4.

The use of agricultural waste ashes as filler in asphalt mixture has shown a notable effect on Marshall Quotient. As presented in Table 4 shows RHA and BA boost stability via silica and angularity, while PSA enhances flow. Filler performance depends on particle traits and dosage optimization. Based on comparative MQ values and supported by (Fitra Ramdhani et al., 2024) PSA shows the highest MQ among the reviewed fillers, making it the most effective and sustainable option.

RHA abundant in rice regions is a waste with slow decomposition (During et al., 2018) (Pachchigar et al., 2024). Its asphalt use reduces pollution and landfilling, while silica content improves stiffness and rutting resistance (Raj et al., 2023). However, consistent ash quality requires controlled burning. Standardised production and uniform particle size are crucial for performance reliability (Kartini et al., 2012), (Wagan et al., 2022).

BA, a sugarcane byproduct, is abundant in tropical sugar mills (Kartini et al., 2012), (Raza et al., 2021). Its use in asphalt supports circular economy goals (Raza et al., 2021), (Adeoti et al., 2025). With pozzolanic properties, finely processed BA enhances stability and moisture resistance (Bayapureddy et al., 2024). However, chemical variability and unburnt carbon can impair adhesion and uniformity (Aprianti et al., 2015). Pre-treatment is often needed, increasing implementation complexity and cost (Yadav et al., 2020).

PSA originates from palm oil waste (Ikumapayi & Akinlabi, 2018). Using it as filler mitigates waste issues and carbon emissions. PSA’s fine particles improve mix workability and stiffness for heavy-traffic roads (Inyang et al., 2024). Due to lower reactivity than RHA or BA, optimal PSA use requires blending or dosage control. Consistent quality is vital for broader use (Torres-Ortega et al., 2024). A summary of the environmental and practical implications of using RHA, BA, and PSA as asphalt fillers can be seen in Table 5.

Using RHA, BA, and PSA as asphalt fillers reduces dependence on mineral fillers and diverts agro-waste from landfills, lowering pollution and supporting sustainability goals (Fareed et al., 2020), (Kareem et al., 2024b). These fillers cut material costs near agricultural sources and enhance mechanical properties, extending pavement life (Ali & Al-Tameemi, 2025). However, challenges include inconsistent ash quality, limited field validation, and uncertain long-term performance. Addressing standardisation, moisture resistance, and large-scale logistics is essential for sustainable, durable infrastructure (Jain et al., 2024). A summary of the environmental and practical implications of using RHA, BA, and PSA as asphalt fillers, highlighting the benefits, challenges, and regional suitability based on current literature, can be seen in Table 5.

Table 5 demonstrates that RHA, BA, and PSA reduce landfill waste and asphalt carbon footprints. Regionally sourced RHA/BA benefit rice/sugarcane sectors; PSA suits palm industries. While promoting sustainable infrastructure, standardised processing and field validation require prioritisation for scalability and long-term performance.

The summary of this review research is presented in Table 6.

Table 6 evaluates RHA, BA, and PSA fillers by optimal content and performance. RHA (6–8%) yields the highest stability and stiffness. BA (4–6%) offers moderate gains, while PSA (5–7%) improves flexibility. Performance depends on composition, dosage, and processing. Excess filler impairs dispersion, increasing voids and reducing durability.