Geoenvironmental Characterization of Bauxite Residue Ameliorated with Different Amendments

Abstract

In bauxite residue (BR), the abatement and rebound of pH when ameliorated with different amendments have been reported as prime concerns. In addition, to the best of the authors’ knowledge, no research has focused on the characterization of BR during the decrease and rebound in pH, which happens over an extended period. This characterization is essential to convert posttreated residue into green construction materials and to affirm that it does not pose a threat to the environment. This study aimed to investigate two important points. The evaluation of the efficacy of commonly and widely employed conventional additives and mineral acids while mitigating the pH of the residue and its characterization included sedimentation and leaching with a change in pH. The uniqueness of this study lay in the exploration of the latter point and pH rebound for ≤180 days. Cement, gypsum (G), fly ash (FA), and ground granulated blast furnace slag (GGBS) as conventional additives, and nitric (HNO3) and hydrochloric acids (HCl) as mineral acids were selected to ameliorate BR. A significant pH rebound with time occurred from 6.59 to 9.51 and 7.54 to 9.78 when treated with 1M HCl and HNO3, which indicated the influence of the curing period and ameliorant on the alkalinity. Conventional additives, except for G, and their combinations proved ineffective when mitigating and maintaining pH within acceptable limits (i.e., 8.5). Settling analysis revealed accelerated particle settling at pH 4.5–7.0, which indicated implications for the safe disposal, washing, or slurry thickening of BR. The extensive leaching studies for potentially toxic elements (PTEs), which were further endorsed with a field case study, demonstrated that the amended residue was environmentally safe to be used as a construction material.