An Experimental Investigation on the Hydraulic and Mechanical Properties of Red Mud Amended with Monovalent and Divalent Bentonites

The most suitable method for disposing of municipal solid waste (MSW) is provided by engineered landfills due to the utilisation of extensive liner and cover systems made of natural geomaterials, effectively controlling the movement of leachate. To reduce the utilisation of natural materials in landfill systems, various industrial wastes are being used. This paper presents the possibility of using bauxite residue (red mud (RM)) amended with Na and Ca bentonites as liners in landfills. A series of unconfined compressive strength (UCS) and hydraulic conductivity tests were performed using the falling head permeability method and 1D consolidation tests. Optimal results were obtained with 9% monovalent bentonite addition to the RM, exhibited UCS of 509 kPa after 28 days of curing and a hydraulic conductivity of 6.74 x 10- 10 m/sec by falling head permeability, and 1.63 x 10- 11 m/sec by 1D consolidation tests. The improvement in strength with addition of bentonite was mainly due to the formation of sodium aluminium silicate hydrate. This was validated through mineralogical and microstructure analysis, such as X-ray diffraction and scanning electron microscopy. A decrease in the volume of macropores was revealed by the mercury intrusion porosimetry analysis with an increase in bentonite content. Among the various combinations of mixtures, red mud amended with 9% monovalent bentonite met the United States Environmental Protection Agency (USEPA) guidelines for landfill liners, satisfying both UCS and hydraulic conductivity requirements.