Solidification/Stabilization of Chromium in Red Mud-based Geopolymer

Up to 1.5wt% of Cr(III) salts (CrCl₃, and Cr₂O₃) and Cr(VI) salts (Na₂CrO₄, and CaCr₂O₇) were incorporated into red mud-based geopolymers, respectively. The solidification/stabilization, compressive strength, and durability of the Cr-containing geopolymers were investigated. The experimental results indicate that the red mud-based geopolymer could effectively solidify/stabilize different types of Cr salts with solidification/stabilization rates of above 99.61%. Geopolymers are environmentally safe when the dosage of CaCr₂O₇ is ⩽ 1.0wt%, or the dosage of CrCl₃, Cr₂O₃, and Na₂CrO₄ is ⩽ 1.5wt%, respectively. The effects of Cr salts on the compressive strength varies with the type and content of Cr salts. The freeze-thaw cycle is more destructive to geopolymer properties than sulfate attack or acid rain erosion. The solidification/stabilization of Cr is mainly attributed to the following reasons: a) The chemical binding of Cr is related to the formation of Cr-containing hydrates (eg, magnesiochromite ((Mg, Fe)(Cr, Al)₂O₄)) and doping into N-A-S-H gel and C-A-S-H gel framework; b) The physical effect is related to the encapsulation by the hydration products (e g, N-A-S-H gel and C-A-S-H gel). This study provides a reference for the treatment of hazardous Cr-containing wastes by solid waste-based geopolymers.