<p>Improving the geotechnical properties of problematic soils with high dispersion such as clay is one of the important problems of geotechnical engineering researchers. In this study, a type of stabilizing material called geopolymer was first made by combining fly ash as a pozzolan, potassium hydroxide (KOH), and sodium hydroxide (NaOH). Then, to investigate the effect of this additive on the geo-mechanical parameters of gypsiferous clay soil as a problematic soil, in addition to identification tests, a series of tests including Atterberg limits, compaction, unconfined compressive strength (UCS), and permeability were performed. According to the results, by increasing the amount of gypsum, the optimum water content, UCS, and permeability increased, and maximum dry density and deformation decreased. The test results revealed that for clay soil with 10% gypsum, with the increase of fly ash from 10 to 40%, the UCS increases from 5 to 6.7&#xa0;kg/cm². Evaluating the effects of curing times of 7, 14, and 28 days revealed that in the UCS test, with the increase of curing times, the strain and the deformation of the samples decreased. Results of the permeability test show that by increasing the amount of fly ash, KOH, and NaOH, the paths between the grains are blocked, and a decrease in permeability was seen. Also, it was found the activator of KOH compared to NaOH can cause obvious improvement in geotechnical characteristics. The investigation revealed when soil is stabilized with 10% gypsum, fly ash, and KOH, the maximum stress will reach 6.1&#xa0;kg/cm², and with NaOH, it will reach 5.7&#xa0;kg/cm². Comparing the results of SEM imaging shows that by increasing the amount of gypsum and fly ash, the structure of the soil becomes more uniform and the bonds formed become stronger, which has led to an increase in compressive strength and a decrease in permeability.</p>