## 1 Introduction  
Various studies on the influence of the chemical and mineralogical composition on the release of heavy metals from steel slags as well as on surface reactions controlling the leaching behavior of slags are available (e.g., [PERSON] et al. (2015), [PERSON] et al. (2012), [PERSON] et al. (2017), [PERSON] et al. (2016), [PERSON] et al. (2016), [PERSON] et al. (2009), [PERSON] et al. (2012), and [PERSON] et al. (2011) [1; 2; 3; 4; 5; 6; 7; 8]). However, conclusions drawn from these studies are not always applicable for all elements and all slag systems. Therefore, further investigations are required on the leaching control mechanisms of, for example, V in electric arc furnace (EAF) slags. In a previously conducted research [9], a mineralogical and chemical characterization of EAF slags was combined with a hydrogeochemical model and a hypothesis was proposed that the FeO/SiO\({}_{2}\) ratio in the slag is decisive for the amount of V and Cr released. It was assumed that a higher amount of FeO compared to SiO\({}_{2}\) leads to a higher abundance of wuestite (Fe, Mg, Mn)O and impedes the formation of Fe bearing calcium silicate phases such as olivine (A\({}_{2}\)SiO\({}_{4}\), A = Ca, Mg, Mn, Fe) and melilite (Ca\({}_{2}\)M(XSiO\({}_{7}\)), M = Mg, Fe, Al and X = Si, Al) and also indirectly on the formation of spinel (AB\({}_{2}\)O\({}_{4}\), A = Mg, Fe, Mn and B = Cr, Al, Fe, Mn). These previous investigations further showed that V and Cr were bound in spinel and wuestite; additionally, V was incorporated in larnite (\(\beta\)-Ca\({}_{2}\)SiO\({}_{4}\)) and Cr in olivine and/or melilite. It was also shown that olivine, enriched in Fe and Mn, spinel and melilite are less soluble than olivine (with comparable low Fe and Mn amount), wuestite, and larnite. In addition, it is assumed that hydrated Fe bearing calcium silicate phases may provide adsorption sites for V [9; 10]. To minimize the V and Cr release, the formation of spinel must therefore be promoted and the formation of the highly soluble calcium silicate phases impeded. It is further assumed that the distribution and composition of the mineral phases can be tailored toward a reduced V and Cr release by a targeted adjustment of the chemical composition (i.e., the FeO/SiO\({}_{2}\) ratio). A low leaching of potentially environmentally hazardous elements from steel slags is essential for the safe use of EAF slags as a substitution for natural aggregates (e.g., in road construction or concrete). The mechanical properties and the suitability of EAF slags as recycling building material are described by [PERSON] et al. (2016), [PERSON] (2011) and [PERSON] et al. (2015) [11; 12; 13; 14]. Comparable mechanical properties between natural and industrial aggregates (i.e., magmatic rocks and steel slags) originate from similar formation processes with the two main differences being in SiO\({}_{2}\) and CaO concentrations or the basicity (CaO/SiO\({}_{2}\)) as well as the cooling rate. During the steel making process, slag forming components such as lime (CaO) and/or MgO-containing compounds (magnesite (MgCO\({}_{3}\)), dolomite (CaMg(CO\({}_{3}\))\({}_{2}\))) are added to form slag. Additionally, fluxes (e.g., fluorite (CaF\({}_{2}\)) or quartz (SiO\({}_{2}\))) may be added to the slag to influence its viscosity [15; 16]. Therefore, CaO concentrations in natural rocks (except limestone (CaCO\({}_{3}\)) and dolomite) are lower than in slags, whereas the SiO\({}_{2}\) concentrations are higher. Furthermore, while most natural igneous rocks solidify over long periods of time, steel slags cool down within one day, however, changing parameters during cooling such as melt composition (fractional crystallization) and oxygen potential, which also influence the phase composition and distribution in magmatic rocks, can be transferred to slag formation. This is also evident since mineral phases occurring in nature are also found in steel slags [17; 18].