Soil stabilization using industrial by-products has become a sustainable practice to strengthen weak and expansive soils. Ground granulated blast furnace slag (GGBFS), a largely available industrial waste material, is used in this study as a hydraulic binder in soil stabilization. Bentonite–sand mixtures of different proportions can simulate soils of varying degrees of expansiveness and clay content. In this study, the plasticity and swelling characteristics of bentonite–sand mixtures of different proportions treated with GGBFS are evaluated in terms of consistency limits and modified free swell index of treated mixtures. The swelling/expansive nature of the mixtures is found to be reduced significantly with the addition of GGBFS and the reduction is more predominant with respect to curing time rather than binder content. Furthermore, in an attempt to assess the level of suitability of GGBFS applications in practical conditions, the results are compared with that of various mixtures of bentonite–sand treated with cement. A correlation is developed between the swelling characteristics and influencing parameters such as percentage of sand in the bentonite–sand mixtures, the percentage of binder added, and the curing time for the mixtures treated with GGBFS and cement. The correlation helps to predict the effect of such variable parameters in expansive soils having a wide range of plasticity indices.