Chemical and polymeric additives can improve the behavior of clayey soils by altering strength characteristics and refining microstructure. In this work, a recycled glass powder–based geopolymer was employed to modify the strength and structural response of an expansive clay. The tested soil was a bentonite whose dominant clay mineral was montmorillonite. Stabilized samples were prepared with varying geopolymer contents by dry weight and cured for 7 and 28 days. Both microstructural and macrostructural analyses were carried out. The findings indicated that the optimal recycled glass powder content was 15% of the dry soil weight. Samples containing 40% geopolymer exhibited the most pronounced formation of amorphous phases and changes in X‑ray diffraction patterns. The results are applicable to the stabilization of expansive or problematic clay soils. Results showed a non-monotonic strength response: UCS increased from 2.1 kg/cm² (untreated) to 23.19 kg/cm² at 15% RGP, dropped to 12.43 kg/cm² at 20%, then surged to 63.22 kg/cm² at 40% RGP (28-day values). Strength gains were consistently greater at 28 days than at 7 days, and samples shifted from ductile to brittle behavior with increasing RGP content. From environmental and economic standpoints, the outcomes are of particular interest due to the use of eco‑friendly materials and waste‑derived components.