Geochemical and mineralogical characterisation of steel-making by-products is essential to understand their long-term behaviour and their potential use as environmental amendments or construction materials. A steel-making by-product generated in Western Australia from iron ore smelting, HIsmelt, has been extensively characterised in terms of its major and trace element geochemistry and mineralogy. More than 95% of the Hlsmelt steel-making by-product is accounted for by the major element oxides CaO (ca. 38%), SiO2 (ca. 31%), Al2O3 (ca. 13-15%), MgO (ca. 7-10%) and FeO (ca. 6%). Compared to other steel-making by-products produced worldwide, the HIsmelt steel-making by-product has a similar major element geochemistry; however, trace element concentrations in the Hlsmelt by-product rarely exceed the mean concentrations reported for steel-making by-products generated elsewhere. Only Ba and V concentrations are sometimes higher in the HIsmelt by-product. There are distinct differences within in the HIsmelt by-product mineralogy based on hand specimen grain size classification and combined scanning electron microscopy/statistical analysis despite the samples having similar geochemical compositions. The majority of mineralogical variation of individual Hlsmelt by-product samples can be explained by a solid-solution series involving the mineral end members monticellite-akermanite-gehlenite-merwinite [CaMgSiO4-Ca2MgSi2O7-Ca2Al2SiO7-Ca3Mg(SiO4)(2)]. These minerals typically constitute 70-90% of the mineralogy in coarse to medium-grained samples and only 37% in fine-grained samples, with a corresponding decline in pore volume from 10.7 to 0.3%. This mineral suite is also consistent with inferred mineral end member compositions present in ternary phase diagrams of major and trace element oxides. The coarser-grained HIsmelt by-product samples may also contain periclase (MgO) and/or enstatite (MgSiO3), the latter also inferred by X-ray diffraction analysis and derived from dolomite (CaMgCO3) decomposition during smelting. Zircon (ZrO2) is a common trace to dominant mineralogical component in the coarser-grained HIsmelt by-product. In contrast, finer-grained HIsmelt by-product generally has more diverse mineralogy and may also contain spinel (MgAl2O4), hercynite (FeAl2O4), ilmenite (FeTiO3), and perovskite (CaTiO3). Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.