Potential of blended byproduct caps for management of bauxite residues: How do texture and weathering impact geochemistry?

• Geochemistry of blended byproduct capping using fine and sand textures was assessed. • Cap pH, salinity and sodicity were lower in blended residues especially after leaching. • Caps based on fine residues had lower pH and higher nutrients than those based on sands. • Eucalypt mulch and fly ash byproducts were best for decreasing pH and adding nutrients. This study evaluated how successive leaching of caustic bauxite residues of different textures, coupled with blending with locally available byproducts, might improve geochemical characteristics of capping materials. Three bauxite residue textures (fines, fines plus 10 % residue sand and residue sand) were blended with three byproducts (fly ash, sewage water and eucalypt mulch) individually or in combination. Treatments were subject to leaching of three wetting/drying cycles over 18 weeks to mimic field weathering. The pH, salinity and Na concentration of blended materials decreased after leaching regardless of texture. Alkaline minerals (sodalite and calcite) content also decreased during weathering due to enhanced dissolution. Combined byproducts improved multiple chemical properties simultaneously more than any single byproduct. Further decreases in pH, sodicity and concentration of toxic elements are needed to improve suitability of blended capping materials for meeting closure targets, including vegetation establishment.