## 3 Results
### An Overview of Reaction Characteristics of Cement-Free Corundum-Spinel Castables  
In this study, corrosion tests were carried out on cement-free alumina spinel castables with a formed in situ spinel. The density in the tested castables was 3.12 g/cm\({}^{3}\); 3.07 g/cm\({}^{3}\); and 3.05 g/cm\({}^{3}\), while the open porosity was 13.72%, 15.34%, and 15.37% for castables A, B, and C, respectively. XRD analysis of the tested castables allowed the determination of the amount of spinel in individual castables, and it was found for castable A = 19.1 wt.%, B = 25.2 wt.%, and C = 37.9 wt.%. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & & \multicolumn{3}{c}{**Samples**} \\ \cline{2-4}  & **A\_S1** & **B\_S1** & **C\_S1** & **A,B,C\_S2 and A,B,C\_S3** \\ \cline{2-4}  & \multicolumn{3}{c}{**Temperature of Characteristic Points [\({}^{\circ}\)C]**} \\ \hline
**Deformation point** & 1350 & 1320 & 1340 \\ \hline
**Softening point** & 1365 & 1370 & 1365 \\ \hline
**Sphere point** & 1377 & 1392 & 1392 \\ \hline
**Half sphere point** & 1408 & 1420 & 1421 \\ \hline
**Total melting point/Flow** & 1411 & 1431 & 1425 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Characteristic transformation points of reactive mixtures containing components of the cement-free corundum-spinel refractory castables and steel slags. Depending on the C/S mass ratio (from 1.1-5.5) in the slag, the division into reaction zones was determined for the tested materials. In molten slags S2 and S3, there are reaction zones in the material (the corroded area of castable below slag-refractory interface) and a slag reaction zone (above the slag-refractory interface), the reaction zone on the slag-refractory interface is the zone separating these two zones. In the case of molten S1, for which the mass ratio of C/S is the smallest, there is a reaction zone only in the material (the corroded area of castable below slag-refractory interface). The division of zones of the tested materials is shown in Figure 10. It can be observed that, along with the decrease in the mass ratio of C/S in the slag, the corroded area of castable below the slag-refractory interface increases. Figure 10: Schematic showing the division into reaction zones in materials. Figure 9: (**a**) The total liquid phase content in castables composition (A, B, and C) with slag (S1) [wt.%] vs. temperature, (**b**) insert figure to a lesser extent. X-Ray Investigations of High-Temperature Relations within the Mixtures Containing Spinel-Forming Alumina Castables and Synthetic Slags  
The course of the reactions was examined in 50 wt.% castables and 50 wt.% slag of mixtures heat-treated at 1350 \({}^{\circ}\)C. Based on XRD and Rietveld's analysis, an assessment was made of what reactions could have occurred. Changes in the phase composition between uncorroded castables and castables corroded by slags (S1, S2, and S3) were analyzed based on Figure 11a-d. The results of the Rietveld analysis for all mixes are presented in Table 4.  
Both qualitative and quantitative (Rietveld) analyses were performed. Castable C was selected for further tests due to the predicted highest corrosion resistance to slags S1, S2, and S3 in the analyzed temperature range. The X-ray diffraction (XRD) method was applied to analyze the non-corroded castable. The analysis of the influence of slags with different mass ratios C/S on this castable was also carried out. Figure 11: (**a–d**) X-ray diffraction of castable C (7 wt.% MgO) and mixtures (50 wt.% slag and 50 wt.% castable) with castable C and slags: S1, S2, S3 after firing at the maximum temperature of 1350 \({}^{\circ}\)C for 2 h.  
Based on the Rietveld analysis (Table 4), it is assumed that a secondary spinel is formed in the mixture of C_S1 and C_S2 during corrosion reactions. This can be assumed because, for these mixes, an increase in wt.% spinel content is visible compared to the wt.% spinel content in the no-corroded castable (C).