The effectiveness of zeolitic tuff (ZT) based geopolymer stabilization as a sustainable alternative to conventional cement stabilization for expansive soils is investigated in this study. Mechanical and geotechnical properties of geopolymer stabilized soil are evaluated in terms of ZT content, sodium silicate to sodium hydroxide (NS:NH) ratio and curing time. Soil improvement was assessed by laboratory tests, unconfined compressive strength (UCS), plasticity, compaction, and free swell tests. The test results show that the geopolymer stabilization increases the UCS significantly, as the NS:NH=2:1 mixture attains the maximum UCS of about 5.0 MPa in 28 days of curing, representing a 40 % increase over 12 % cement-stabilized soil. Furthermore, geopolymer-stabilized soils show a higher swelling reduction with free swell percentages as low as 0.25 %, a 42 % improvement compared to cement. The environmental assessment shows a 19 % lower CO2 emission per MPa of strength for geopolymer stabilization compared to cement-based stabilization, making it an eco-friendly choice. Pavement performance analysis using the Mechanistic-Empirical Pavement Design Guide (MEPDG) indicates that geopolymer-stabilized subbase layers improve structural integrity while reducing overall pavement rutting and fatigue cracking. Scanning Electron Microscopy (SEM) results validate the creation of a dense geopolymer matrix structure that enhances the strength and stability characteristics of soil materials. The study concludes that geopolymer stabilization using ZT with optimized NS:NH ratios delivers effective, high-performing, environmentally sustainable alternatives to traditional cement.