Geotechnical characterization of bauxite residue (red mud)
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Bauxite residue is the iron-oxide rich mine waste from the processing of bauxite ore for the extraction of alumina through the Bayer process. The Bayer process uses caustic soda, high pressure, and high temperature to leach alumina from ferruginous residue slurry (bauxite residue or red mud). This leaching process produces alumina to ferruginous/bauxite residue at ratios of 1:1 to 1:4 with the end result being billions of tons of bauxite residue waste (4 billion tons worldwide by this year (2015)). The residue is associated with being chemically basic (average pH = 12), high in heavy metals, and has shown low level naturally occurring radioactive materials (NORMs) (equivalent to other household building materials like granite, gypsum, and marble). The objectives of this research were to characterize bauxite residue using the traditional methods developed in the field of geotechnical engineering for classifying soils, study possible source and processing effects that may contribute to the behavior of the material, and assess the feasibility of the material for beneficial reuse as a civil engineering building material. Characterization of the material included index properties (grain size distribution, specific gravity, and plasticity), powder x-ray diffraction, compaction behavior, and performance behavior that included compressibility, hydraulic conductivity, shear strength, and erosion resistance. The research focused on the potential beneficial reuse of bauxite residue in the form of fill for a flood protection levee. The research findings are that bauxite residue is a fine-grained, low plasticity material that would be classified under USCS classifications as a ML soil. Bauxite residue's compaction behavior is like a plastic, fined-grained material in compaction. It compacts at higher optimum moisture content and lower maximum dry densities than similar fine-grained soils (CH, CL, and ML) for both standard and modified Proctor efforts. Bauxite residue has a reasonably low hydraulic conductivity that provides low flow rates for a levee application. The material exhibited high drained shear strength in comparison to other fine-grained materials but, if not compacted, does exhibit static liquefaction potential similar to most fine-grained tailings that have low plasticity. Bauxite residue is comparable to a typical compacted clay in terms of compressibility, both in regards to its own compressibility and its total unit weight concerning the applied pressure to the subgrade. Bauxite residue demonstrates a high resistance to erosion. Lastly, bauxite residue has a high pH (pH > 13) that must be addressed before the material could be used in a levee. The issue can be resolved using neutralization or encapsulation by another soil. Overall, in terms of geotechnical properties, bauxite residue shows acceptable characteristics for potential use in a flood protection levee.