Steel slags are unavoidable by-products during the steel making process and currently partly dumped on dump sites. However, due to their beneficial mechanical properties, steel slags have the potential to substitute primary raw materials like gravel or other aggregates, thus contributing to another step towards a complete circular economy. In this thesis, the mineralogical composition as well as the mineral chemical composition of 20 electric arc furnace slag samples was investigated, using reflected light microscopy, X-ray diffraction (XRD) and electron probe micro analysis (EPMA). Additionally, the influence of thermochemical conditioning of four samples on the mineralogical composition as well as the leaching behaviour of the environmentally critical elements Cr, V, Mo and F was examined since elevated leaching of certain elements often limits the substituting application of steel slag. To characterize the leaching behaviour of the slag samples, pH dependent leaching tests as well as hydrogeochemical modelling were performed. The main mineral phases identified in the slag samples were the oxides wuestite (FeO), spinel solid solutions ((Fe,Mg,Mn,Ca)(Fe,Mn,Al,Cr)2O4), and brownmillerite (Ca2(Al,Fe)2O5) and the silicates gehlenite (Ca2Al[AlSiO7]), larnite (Ca2SiO4), and olivine solid solutions ((Ca,Mg,Mn,Fe)2SiO4). In the conditioned samples, an increase in Mg-bearing phases was observed, reflecting higher bulk MgO contents. Larnite was substituted by monticellite ((CaMg)SiO4) and wuestite by a magnesiowuestite solid solution ((Fe,Mg)O). XRD additionally identified the Mg-silicates bredigite (Ca7Mg(SiO4)4) and merwinite (Ca3MgSi2O8). EPMA analysis confirm that spinel solid solution and wuestite are the main Cr-bearing phases. Vanadium was detected in minor amounts in wuestite, spinel solid solution as well as in larnite. Fluorine was partially detected in wuestite and spinel solid solutions but in general the occurrence of F and Mo could not be resolved ultimately by the methods applied. For Cr, the observed leaching behaviour does not correlate with the measured total concentration in the slag samples. To some extent this is also true for V and F. In contrast, molybdenum shows higher leached concentrations in the samples with higher total Mo concentrations. After conditioning, all samples showed reduced leached amounts of the environmentally critical elements.