<p>Soil plasticity plays a vital role in the strength gain of the material; high and low expansivity of soil is more common in nature for any expansive soils. Most of the roads are constructed on high and low plastic soils that can be enhanced by means of fly ash-based geopolymer, “a green cement”. This paper describes the effectiveness of fly ash-based geopolymer for the construction of road subgrade through laboratory experimentation via free swell behavior, shrinkage behavior, unconfined compression test, California bearing ratio, and resilient modulus. The effectiveness and feasibility were shown by the material in terms of reduction in swelling and shrinkage, and improvement in the strength behavior of the soil mass. The optimized fly ash and geopolymer mixes were noted at 25%; this could be observed through the microstructure analysis of soil samples. The soil plasticity plays an important role in governing the strength of soil; low plastic soil has high mechanical strength compared to high plastic soil. Fly ash geopolymer-treated soil material is found to be more suitable for the construction of the subgrade or subbase layer of flexible pavement, and helps to reduce conventional stabilizers and leads to sustainable solutions.</p>