Mineralogy and glass content of Fe-rich fayalite slag size fractions and their effect on alkali activation and leaching of heavy metals

Abstract

Fayalite slag (FS) is an Fe-rich nonferrous metallurgy (CaO-MgO-) FeOx-SiO2 slag originating from nickel or copper manufacturing processes, which currently is disposed to landfills or used in low-value applications. This study investigates the mineralogy and glass content of certain sized fractions of FS and how it influences the reactivity, mechanical, and microstructural properties of the alkali-activated materials produced. Water-quenched granular FS was sieved into two size fractions: namely, a fine fraction (FF) with a particle size range of 0–0.5 mm and a coarse fraction (CF) with a particle size range of 1.5–2 mm. It was then milled to a similar median particle size of 10 μm to be used as a binder precursor. The reaction kinetics of each fraction was determined via thermal analysis microcalorimeter, and the microstructural evolution and chemical composition of the binder were studied using a scanning electron microscope coupled with an energy dispersive X-ray spectroscopy. The environmental leaching behavior of both fractions before and after alkali activation was assessed according to the EN 12457-2 standard. The results showed that both fractions consisted of fayalite, magnetite crystalline phases, and MgO-SiO2-FeOx (-CaO-Al2O3) glass phase. However, FF had a higher glass content (63 wt.%) in comparison to CF (39 wt.%), and, consequently, FF was more reactive under alkali activation, as evidenced by faster reaction kinetics, faster strength development, and improved microstructural properties. Alkali-activated samples had differences in the chemical compositions of their binder gels at early stages, though later, their binders became increasingly homogenous and consisted of an Na-K-Fe-Si gel with Mg, Ca, and Al as minor constituents in both samples. Additionally, the leaching behavior of potentially toxic metals and substances from precursors and alkali-activated samples prepared was below the limits set for paved structures as specified by Finnish legislation.