Expansive soils are those soils which exhibits volumetric changes depending upon saturation level. These soils might experience alternate swelling and shrinkage during wetting and drying. In order to reduce the susceptibility of expansive soils to environmental changes, soil stabilization is one of the options. Industrial by-products such as Ground Granulated Blast Furnace Slag (GGBS), having relatively high calcium content are usually adopted for stabilization of expansive soils. In this regard, it is preferred to choose a binder material which is available at a low cost and also safe to the geo-environment. Nevertheless, soil stabilization against its expansive nature should be the prime objective of the selected stabilizer/binder. Hence, alkali activators (i.e., chemicals) are generally used along with binders for better modification of expansive soil. In this study, sea water is being used as an alkali activator for GGBS, whose availability is not a concern. Therefore, the alkalinity and saline nature of the sea water is being highlighted to improve the properties of the stabilized soil. A comparative study on highly compressible clay in combination with potable water and sea water in the presence and absence of GGBS has been performed to understand the above-mentioned effect of sea water on stabilized soil properties. Experiments such as Differential free swell index (DFSI) and Unconfined Compressive Strength (UCS) have been performed by adding 0%, 5%, 10%, 15%, 20%, 25%, and 30% GGBS in soil and mixed with tap water and sea water separately. Irrespective percentage of GGBS added to soil, lower DFSI is observed for the samples with sea water compared to tap water and DFSI reached less than 50% for 25% and 30% GGBS for tap and sea water respectively. Also, higher UCS values were obtained for samples with sea water compared to tap water. From the series of tests observed that sea water is more beneficial in modifying the expansive soil compared to tap water.