The utilization of fly ash (FA) produced from various thermal power plants and other industrial sources is a big challenge and an opportunity for material and civil engineers. This study aims to utilize FA for improving the engineering characteristics of Montmorillonitic Indian clay (BC) soil. A rigorous testing plan is designed and performed using different proportions of FA (5, 15, 25, and 35 %) to investigate the time-dependent deformation behavior of expansive BC soil and also to examine the influence of the wetting–drying cycle on the time-dependent swelling behavior of reconstituted soils in Oedometric free swell condition. It is proposed to evaluate the time-dependent parameters of the FA-BC matrix under wetting–drying cycles using the elastoviscoplastic considering swelling (EVPS) model. The mineralogical characteristics of both natural and reconstituted samples were investigated using the scanning electron microscope and X-ray diffraction. From the experimental result, it is observed that the time-dependent swelling coefficient decreases with the percentage of FA content increase. The FA used in the study results in decreasing not only the swelling potential of BC soil samples but also the swelling–shrinkage behavior of the soil. It is observed that the reconstituted soils are influenced by both the degree of FA content and the wetting–drying cycles. The swelling strain limit decreases exponentially with the increase of the FA content. The EVPS model works very well in predicting the long-term consolidation and swelling behavior of the FA-BC matrix under different stress states.