Performance Evaluation of Coal Gangue-GGBS–Based Geopolymer for Treating Black Cotton Soil

Black cotton soils are extremely problematic because they are susceptible to large volume changes with variation in moisture contents. These soils are conventionally stabilized with ordinary portland cement and lime, but the production/utilization of these traditional stabilizers is highly energy intensive, involves quarrying, and emits large quantities of carbon dioxide (CO2) into the atmosphere. Geopolymer is a promising alternative to these stabilizers because it provides high strength, consumes low energy, and emits low CO2 during synthesis and application. In this study, geopolymers synthesized from coal gangue (waste generated during coal mining) and ground granulated blast furnace slag (GGBS; by-product from the iron and steel industry) binders were evaluated for treating black cotton soil. A mixture of sodium silicate and sodium hydroxide is used as an alkaline activator solution for geopolymerization. An attempt is further made to identify the optimal dosages of geopolymer by evaluating the strength and durability characteristics of geopolymer-treated black cotton soil mixtures. X-ray diffraction and scanning electron microscopy were further conducted to distinguish the mineralogical and microstructural changes that occurred because of geopolymerization. From this study, it is found that using coal gangue alone as a precursor in geopolymer is good at improving the strength of black cotton soil but observed to be weak in durability. Hence, an attempt is further made to synthesize a better-performing geopolymer with a combination of GGBS and coal gangue, i.e., effective in both strength and durability aspects. Optimal geopolymer identified in this study can be a sustainable alternative to traditional stabilizers in improving black cotton soils for geotechnical subgrade applications.