The growing emphasis on sustainable construction has prompted interest in repurposing agricultural by-products as alternatives to conventional aggregates in asphalt mixtures. While natural sand is widely available, its use can lead to less stable, more tender mixes, which can limit various specifications. This study explores the potential of palm date seed (PDS) powder as a partial and full replacement for natural sand in asphalt concrete. A series of laboratory tests was conducted to evaluate the effect of PDS incorporation on the mechanical and durability performance of asphalt mixtures, including Marshall properties, resilient modulus, fatigue resistance, moisture susceptibility, and permanent deformation. Additionally, microstructural and mechanistic performance analyses were performed using scanning electron microscopy (SEM) and KENPAVE, respectively. PDS was introduced at five replacement levels by weight of natural sand: 0%, 25%, 50%, 75%, and 100%. Results showed that up to 50% PDS replacement significantly improved Marshall stability, reduced air voids, and enhanced mixture compactness. Moisture susceptibility tests revealed increased indirect tensile strength ratio (TSR), with all mixes surpassing the 80% threshold. Resilient modulus values showed marginal gains, while permanent deformation tests indicated a 38.6% reduction in cumulative strain at 100% PDS. Fatigue performance analysis demonstrated superior service life for mixtures containing 25%–50% PDS at high strain levels, supported by favorable regression parameters. Mechanistic analysis using multilayer elastic theory further confirmed the improved pavement durability achieved with 50% PDS replacement, resulting in a 4.4% increase in design life compared to the control mix. The findings establish PDS as a promising sustainable alternative to natural sand in asphalt mixtures. While 50% replacement emerged as optimal, further field validation is recommended to confirm long-term performance under diverse service conditions.
