This study addresses the growing demand for sustainable fuel alternatives by developing a solid, eco-friendly catalyst for biodiesel production. Waste eggshells and rice husks were repurposed to create a heterogeneous alkaline catalyst, offering a low-cost and efficient solution. Rice husks were alkali-treated to extract silica, which was mixed with eggshells and calcined at 800 °C to produce calcium silicate in varying ratios (10 %–50 %). Among them, the catalyst containing 30 % eggshell (RHA+30 % EH) exhibited the highest calcium (44.58 %) and silica (51.82 %) content and was selected for transesterification of Lluppai ennai oil into biodiesel. Process parameters ethanol-to-oil ratio, catalyst loading, temperature, and time were optimized, yielding a maximum biodiesel conversion of 94.8 % under ideal conditions: 15:1 ethanol-to-oil molar ratio, 2 wt% catalyst, 150 min at 80 °C. The catalyst was characterized using FT-IR, SEM, XRD, XRF, and XPS, while the biodiesel product was validated via FT-IR, GC, and NMR techniques, meeting ASTM standards. The catalyst retained its efficiency over five reuse cycles, confirming its potential for industrial application as a sustainable, reusable, and effective catalyst for biodiesel synthesis. © 2025 Elsevier B.V., All rights reserved.