Understanding Heterogeneity of a Slag-Derived Weathered Material: The Role of Automated SEM-EDS Analyses

Slag heaps over years may evolve into complexly weathered zones, which are a challenging material for analyses as they contain phases from numerous sources and at different stages of weathering. However, the weathered zones are important parts of slag heaps, because they contain both primary and secondary phases enriched in metal(oid)s that may become soluble under specific conditions. The weathering reactions related to metal release or precipitation may be recorded in a heavy mineral fraction as the fraction contains predominately minerals with elevated toxic elements concentrations. Therefore, an automated SEM analysis on a polished section of included heavy mineral particles was applied in this paper for a rapid recognition of phases in a complex setting and their classification into detrital, primary and secondary phases. The approach was applied to a slag heap in witochowice (Upper Silesia, Poland) and it consisted of analyzing magnetic and non-magnetic heavy mineral fractions from three distinct horizons noted A, B and C. Materials had been previously interpreted as being sourced from the heap itself (lowermost horizon C) and from artificially added materials used later for superficial site remediation (upper horizons A and B). Instead, automated SEM analysis demonstrated that horizon C is derived from the slag heap weathering, horizon B is derived predominately from the artificially added materials, whereas horizon A is a mixture of the B and C horizons. Additionally, when slag particles in horizons A and C are compared, the lowermost horizon C contains more slag-derived secondary phases, whereas horizon A contains more primary slag phases. Therefore, horizon A remains the most prone to releasing toxic elements because, considering its position as the uppermost horizon, it can be submitted to climatic solicitation (fast water circulation).