Stabilisation of black cotton soil for subgrade application using fly ash geopolymer

It is a common practice around the world to stabilise black cotton soil using lime or cement to improve the strength of stabilised sub-base and subgrade soil. However, production of cement and lime is highly energy intensive. It is also reported to emit large quantity of CO2 into the atmosphere. Moreover, the global warming potential of fly ash (0.00526–0.027 kg CO2eq/kg) being a waste, is very low as compared to that of cement (0.82–0.948 kg CO2eq/kg) and lime (about 0.416 kg CO2eq/kg). Thus, in order to reduce the emission of greenhouse gases associated with lime and cement stabilisation, an experimental investigation is conducted to study the feasibility of using fly ash geopolymer to stabilise black cotton soil. The experimental investigation was carried out by varying fly ash content from 5% to 20% and treating the samples with a lower concentration of 5M NaOH solution. Tests were conducted in the laboratory to obtain the unconfined compressive strength and California bearing ratio and resilient modulus values of the stabilised samples. Moreover, microstructural analysis using X-ray diffraction, scanning electron microscope, energy dispersive spectroscopy and Fourier transformed infrared spectroscopy were conducted to see insight into the material behaviour. It is concluded from the present study that the fly ash-based geopolymer could be used for stabilisation of black cotton soil for highway subgrade and sub-base preparation.