This study evaluates the geotechnical performance of LightGeoComposite (LGC), a lightweight composite material composed of fly ash geopolymer-stabilized black cotton soil (BC) and expanded polystyrene (EPS) beads. The research focuses on the combined effects of varying EPS bead densities and mix ratios on critical mechanical properties, including maximum dry density (MDD), unconfined compressive strength (UCS), and California Bearing Ratio (CBR). For the first time, this study explores the influence of EPS density and EPS-to-BC ratios (EPS/BC) on the geotechnical behavior of geopolymer-stabilized BC, contributing significantly to the development of lightweight construction materials. The findings reveal that increasing EPS density enhances MDD, UCS, and CBR, demonstrating improved mechanical performance while retaining the lightweight nature of the composite. However, increasing the EPS/BC ratios at a constant density leads to reductions in these properties due to the higher volume of low-density EPS beads, which reduces the strength of the geopolymer matrix. The study highlights the importance of optimizing both EPS density and EPS/BC ratios to achieve a balance between density reduction and mechanical stability. LGC shows promising potential as a durable, lightweight fill material for ground improvement and construction applications. By addressing the balance between strength and density, this research advances the understanding of lightweight composite geomaterial, providing valuable insights for sustainable infrastructure development. These findings establish LGC as a viable solution for engineering projects requiring efficient, lightweight geomaterials.