This study examined the efficacy of Fly Ash Type F-based geopolymer binders in enhancing the impermeability of clayey soils. A clayey soil of the CL type was stabilized using geopolymer mixtures composed of fly ash activated by two different alkaline systems: (1) sodium silicate combined with lime and (2) sodium bicarbonate combined with lime. The FA binders were added at dosages of 10%, 20%, and 30% by weight of dry soil, and FA/AA was 0.2, 0.4, and 0.6. Standard falling head permeability tests were performed to evaluate the efficacy of the therapies. The experimen results indicated a marked improvement in reducing soil permeability with both alkaline activator systems. The greatest reduction was observed at a 30% replacement ratio when the sodium silicate–lime mixture was used. Beyond this level, a slight increase in permeability was recorded, which can be attributed to the excessive alkalinity of the mixture and the potential formation of microcracks. On the other hand, the sodium bicarbonate–lime system showed a consistent trend, where higher replacement levels continued to lower permeability. Overall, the study highlights that fly ash–based geopolymers, when properly optimized in terms of activator type and dosage, provide an effective and sustainable approach for improving the impermeability of clayey soils, particularly in hydraulic and geotechnical engineering applications