## 2 Materials and Methods
### Preparation of Synthetic Steel Slags  
The three synthetic slag compositions studied in the present investigation were formulated according to alkalanity (C/S mass ratio): 5.5 (S3), 3.5 (S2), and 1.1 (S1). The materials used for synthesis were CaCO\({}_{3}\) (99%, CHEMPUR), SiO\({}_{2}\) (96.5%, POCH), MgO (98%, Acros Organics), Al\({}_{2}\)O\({}_{3}\) (99%, Acros Organics), MnO (99%, Sigma-Aldrich), Fe\({}_{2}\)O\({}_{3}\) (97%, Sigma-Aldrich), and TiO\({}_{2}\) (98%, POCH) as reagents. Dry powders were weighed according to each planned C/S mass ratio and homogenized for 2 h in a vibratory ball mill. The homogenized mixtures were then pressed into cylinders and calcined at 1000 \({}^{\circ}\)C for 10 h. The cylindrical samples were ground in a ball mill to obtain the powder for further corrosion investigation. The three mixes containing the calcined slags and CaF\({}_{2}\) as a VOD flux (0.5 wt.%) were homogenized for 30 min and were left for further research in a desicctor. The intended composition of the slag S1 contained 41.71 wt.% CaO, 37.37 wt.% SiO\({}_{2}\), 11.96 wt.% MgO, 6.68 wt.% Al\({}_{2}\)O\({}_{3}\), 0.60 wt.% MnO, 1.40 wt.% Fe\({}_{2}\)O\({}_{3}\), 0.30 wt.% TiO\({}_{2}\). Slag S2 contained 36.23 wt.% CaO, 10.16 wt.% SiO\({}_{2}\), 16.44 wt.% MgO, 35.37 wt.% Al\({}_{2}\)O\({}_{3}\), 0.20 wt.% MnO, 1.30 wt.% Fe\({}_{2}\)O\({}_{3}\), 0.30 wt.% TiO\({}_{2}\). Slag S3 contained 55.16 wt.% CaO, 9.96 wt.% SiO\({}_{2}\), 4.58 wt.% MgO, 29.69 wt.% Al\({}_{2}\)O\({}_{3}\), 0.20 wt.% MnO, 0.10 wt.% Fe\({}_{2}\)O\({}_{3}\), 0.30 wt.% TiO\({}_{2}\). Thermodynamic Simulation of the Effect of Slag Chemistry on the Corrosion Behavior OfSpinel-Forming Alumina Castables  
Using FactSage thermochemical software (FS 6.4) and databases (FTmisc 6.4, Ftoxid 6.4, FactPS 6.4), thermodynamic simulations of three slags with different C/S mass ratios in the temperature range of 1000-2000 \({}^{\circ}\)C were performed. These resulted in thermodynamic predictions of solid and liquid phases of slags (S1, S2, and S3). In addition, nine reactive mixtures composed of 50 wt.%slag and 50 wt.% catalytic were tested to predict the phase compositions of each reactive mixture as a function of temperature (1250-1650 \({}^{\circ}\)C) fired in the air. The calculations simulating the corrosive reactions in these mixtures consist of finding the minimum Gibbs free energy and determining the composition of the tested mixtures depending on the temperature. The data from Table 1 were used for the calculations. This provided thermodynamic predictions of the solid phases and liquid phase of the slags and reactive mixtures containing components of the cement-free corundum-spinel refractory castable and steel slags when exposed to an oxidizing atmosphere vs. increasing temperature; thermodynamic predictions of the CM\({}_{2}\)A\({}_{8}\) solid phase of the reactive mixtures A_S1, B_S1, C_S1, A_S2, and B_S2 versus increasing temperature. Moreover, both temperatures at the onset (i.e., the temperature at which the first portion of the liquid phase is formed) and liquid content and chemical composition vs. temperature were determined. Laboratory-Scale Investigations of the Mixtures Containing Alumina-Spinel Castablesand Synthetic Slags Using the Heating Microscopy Thermal Analysis (HMTA) and X-Ray Diffraction (XRD)  
Heating microscopy thermal analysis (HMTA) and X-ray diffraction (XRD) were used to analyze the changes in the phase properties of the mixtures containing alumina-spinel castables and synthetic slags vs. an increasing temperature. For this purpose, reactive mixtures of castables and slags in a ratio of 50:50 wt.% were prepared. In order to prepare the mixtures, 50 wt.% castables and 50 wt.%, slag (previously prepared powders) (Sections 2.1 and 2.2) castables (with MgO: 3 wt.%, 5 wt.%, and 7 wt.%) with three types of slag calcinates differing in C/S mass ratio (from 1.1-5.5) were used.