Effect of freezing–thawing cycles on the strength and pollutant leachability of stabilized/solidified metallurgical waste

Stabilized/solidified hazardous wastes may lose their structural integrity and stability when exposed to long-term extreme weather conditions, such as repeated wetting–drying and freezing–thawing (F–T). This situation causes wastes to become environmentally risky again even if they are stabilized and solidified. In this study, the effects of F–T cycles on the strength and pollutant leachability of stabilization/solidification (S/S) products of zinc extraction residue (ZER) stabilized/solidified by Portland cement (PC), fly ash (FA), and alkaline phosphate (AP) were comprehensively investigated. S/S efficiency and F–T resistance of the samples were determined by unconfined compressive strength (UCS), and leaching tests widely applied in the USA (TCLP), European Union (EN-12457/1-4 2020), and Turkey (TS-EN 12457/4). In addition, X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses and visual inspection were performed to examine the variations in mineralogical and morphological structures. S/S efficiency and F–T resistance changed depending on the type and amount of S/S reagent used and decreased in the order of PC+AP>PC>>FA. All applied S/S procedures, except for 10% PC addition, were found to be suitable for the S/S of ZER from the viewpoint of the achievement of the minimum required strength and pollutant leachability. F–T events did not change the mineralogical structures of the S/S products, but significantly reduced the UCS value by causing structural deformation. The pollutant concentrations released from the samples that lost their monolithic structure by crumbling after certain cycle increased with the increasing F–T cycle, causing a change in the storage criteria of the samples. Therefore, it is important to consider and apply the F–T cycle as a standard test in addition to the existing leaching and strength tests for S/S products of the wastes in terms of minimizing environmental risks.