In this study, experimental investigation was carried out to enhance the strength of soft clayey soil by incorporating alkali-activated fly ash. The feasibility of the alkali-activated fly ash, also termed fly ash-based geopolymer, was assessed through comparative experimental studies using fly ash–cement mix, fly ash-alkali activator mix, and only alkali activator with the clayey soil collected from the vicinity of the Hooghly River in the Diamond Harbor region of West Bengal. Unconfined compressive strength (UCS) test was conducted on the untreated soil sample. Next, the clayey soil was treated with a cement-fly ash mix and cured for 7 and 28 days, followed by UCS tests on the treated soil samples, revealing a notable enhancement in the strength. In the subsequent UCS tests, fly ash-based geopolymer was employed as a substitute for cement, with the equivalent proportion of liquid alkali in place of cement. The alkali consisted of 70% liquid Sodium Silicate (Na2SiO3) and 30% solution of 8 M Sodium Hydroxide (NaOH). It was found that the UCS value of the fly ash-based geopolymer treated clay sample after 28 days of curing increased around four times that of the untreated clay sample. UCS tests were also conducted on the treated clay samples with only the chemical activator, resulting in clay-based geopolymer, and on samples prepared by blending only fly ash with the clayey soil. The objective was to evaluate the efficacy of fly ash-based geopolymer as a sustainable alternative to cement in the context of soil stabilization. Test results indicated that geopolymers created using fly ash could be a viable and eco-friendly solution for soil stabilization, contributing to the sustainable utilization of solid industrial waste materials.