| Chromite Ore Processing Residue was produced in large quantities in various countries and was deposited as backfill material in residential and commercial areas. There are two problems associated with COPR: its content in carcinogenic hexavalent chromium and its volume expansion and heave that leads to structural failures. Heave drew attention to two COPR deposition sites, SA7 and DMT; SA7 is currently under court order to remove and dispose all COPR in a hazardous waste landfill. Investigations were thus initiated to understand the mechanism of COPR heave and find a solution to mitigate it while also applying an effective, reductive or other, treatment for Cr(VI).; Three types of COPR with different physical and geotechnical properties were encountered: grey black (GB) granular and hard brown (HB) COPR found in both sites, and fine clayey COPR (C) found only at SA7. The focus of the investigation was the delineation of COPR mineralogy, as it determines Cr(VI) speciation and release, and is also the driver for volume expansion and heave. Quantitative X-ray Powder Diffraction was applied by adjusting the principles of the Rietveld method to the requirements of a multiphase complex cementitious matrix, such as COPR.; The analyses showed that the kinetics of hydration reactions were drastically different in the two types of COPR, GB and HB. While these originated from the same parent material, accelerated hydration reactions resulted in HB formation with only hydration products in the mineral assemblage, while GB remained largely unreacted due to kinetic inhibition of hydration. Temperature emerged as the primary parameter for acceleration of reactions in HB, while passivation of GB COPR was mainly due to the increase of effective particle size by cementation. The presence of sulfate also emerged as a catalyst to accelerate hydration reactions.; Sulfate addition is also an effective mechanism to release Cr(VI) into solution by anion exchange with chromate from the layered COPR minerals that retain Cr(VI): Calcium Aluminum Chromium Oxide Hydrates (CAC), and to a lesser extent, hydrotalcites and hydrogarnets. The acceleration of Cr(VI) release from these phases is necessary for the success of a reductive treatment, along with the use of a slowly dissolving reductant, such as calcium polysulfide.; Sulfate, however, was found to be the driver for volume expansion of COPR due to the formation of the mineral ettringite, a known expansive agent in cements and lime-stabilized clays. The presence of sulfur in COPR is therefore a major concern for its geotechnical stability, which is a requirement for a sustainable remedial solution. |
