Synthesis and characterization of geopolymers derived from coal gangue, fly ash and red mud

There is a growing need to utilize large stockpiles of the industrial byproducts including coal gangue (CG), fly ash (FA) and red mud (RM) which can pose potential environmental problems. The CG, FA and RM contain aluminosilicates which can be precursors for the synthesis of geopolymers. This research studied the utilization of CG, FA and RM by silicate activation into geopolymers. Binary mixes of CG and RM and ternary mixes of CG, FA and RM were prepared and cured at temperatures ranging from 80 to 800 °C. The unconfined compressive strength tests were performed on the specimens to determine their mechanical strength. Nitrogen adsorption, mercury intrusion porosimetry, Fourier transform infrared spectroscopy and scanning electron microscopy were used to study the textural, structural and morphological characteristics of the end products. At lower curing temperatures, the ternary mixtures of CG, FA and RM showed higher strength gains when compared to the binary mixtures of CG and RM. The textural characteristics revealed that the reduction in pores and specific surface area do not necessarily result in higher compressive strength values. Furthermore, the cementitious gels formed at high temperatures resulted in well-bonded geopolymers with homogenous structures.