Synthesis, Characterization, and Mechanical Properties of Red Mud–Based Geopolymers

Abstract

A pilot study investigates the potential of reusing red mud, an abundant industrial waste produced from alumina refining by the Bayer process, by geopolymerization reactions with another solid waste, fly ash, and sodium silicate. Parameters involved in the synthesis, including red mud to fly ash ratio (values of 80/20, 50/50, and 20/80), presence of sand filler, curing duration (up to 28 days), and sodium silicate solution to solid mixture (consisting of red mud and fly ash) ratio, were examined to understand the extent and degree of geopolymerization. Unconfined compression testing was employed to assess the influence of these synthesis parameters on the mechanical properties of the end products, red mud–based geopolymers. The composition and microstructure were characterized by X-ray diffraction and scanning electron microscopy, respectively, which confirm the geopolymerization reactions. The mechanical properties, including strength, stiffness, and failure strain, were analyzed against the chemical compositions of the red mud geopolymers, such as Si/Al and Na/Si molar ratios. For the studied geopolymers, the unconfined compressive strength, ranging from 7 to 13 MPa, increases with the Si/Al ratio as in some types of portland cement. A higher Na/Si ratio appears to reduce the strength and stiffness but enhance the ductility. The results indicate that red mud geopolymers are a viable cementitious material that can be used in roadway construction. The engineering implications are discussed in terms of waste recycling, environmental benefits, and energy consumption.