Montmorillonitic clays are known for their high shrink-swell potential and low shear strength which pose significant challenges in geotechnical engineering. Conventional ground improvement methods using Ordinary Portland Cement (OPC) are energy-intensive and environmentally harmful. This study investigates the use of metakaolin (MK) as a sustainable Supplementary Cementitious Material (SCM) to partially replace cement for enhancing the strength of montmorillonitic clay. Soil samples were prepared with 20% OPC and varying MK proportions (3%-10% by cement weight). Unconfined Compressive Strength (UCS) tests were conducted after 7, 14, and 28 days. Numerical simulations were performed using Mohr-Coulomb (MC) and Drucker-Prager (DP) constitutive models to evaluate the strength behavior of the composite mixture. Microstructural analyses (FE-SEM and FTIR) were conducted to assess hydration products and matrix structure. Results show a 47% strength increase with 7% MK, compared to the traditional soil mix with 20% OPC, achieving peak UCS values of 1.226 N/mm2at 28 days.