## 3 Results
### SEM-EDS Studies of Corroded Castables
\[\text{C}_{2}\text{AS}+\text{3S}+\text{A}\rightarrow\text{2 CAS}_{2} \tag{3}\]  
\[\text{CM}_{2}\text{A}_{8}\rightarrow\text{MA}+\text{C}_{2}\text{M}_{2}\text{ A}_{14}+\text{liq} \tag{4}\]  
\[\text{C}_{2}\text{M}_{2}\text{A}_{14}\rightarrow\text{MA}+\text{CA}_{6}+ \text{liq} \tag{5}\]  
During analysis of the deeper areas of the material, i.e., sub-zone C, significant amounts of CA, CA\({}_{2}\), and CA\({}_{6}\) phases were observed. On the other hand, the CA\({}_{6}\) phase was also detected in sub-zone D, where some amounts of MA phase were also observed. It should be noted that the CA\({}_{6}\) phase could also arise according to Equation (6) [53], and the CA\({}_{2}\) and CA phases present in subzone C and D probably formed according to Equations (7) and (8) [53; 54], respectively. The corrosion progress of the castable in contact with slag S1, compared to slags S2 and S3, was the greatest. This observation may be due to the presence of CAS\({}_{2}\) in sample C_S1 and the presence of the eutectic temperature between C\({}_{2}\)AS and CAS\({}_{2}\). CAS\({}_{2}\) was not calculated for samples C_S2 and C_S3. According to many authors, the temperature eutectic between the C\({}_{2}\)AS and CAS\({}_{2}\) phases is formed around 1400 \({}^{\circ}\)C [38]. \[\text{6A}\xrightarrow{\text{Ca}^{2+}}\text{CA}_{6} \tag{6}\]  
\[\text{CA}_{6}\xrightarrow{\text{2 CA}^{2+}}\text{3 CA}_{2} \tag{7}\]  
\[\text{CA}_{2}\xrightarrow{\text{Ca}^{2+}}\text{2 CA} \tag{8}\]As can be seen in Figure A2, for the castable in contact with the slag S2, the phases were distributed within four sub-zones, i.e., A, B, C sub-zones were above the slag-refractory contact line, while the D sub-zone was formed below the slag-refractory contact line. A characteristic feature of sub-zones A-C is the occurrence of the continuous Ca, Al-rich CMAS phase (the chemical composition was slightly different in the individual A-C sub-zones), where specific phase distribution can be found. Thus, in sub-zone A, we observe grains of MgO (M), spinel (MA), C\({}_{2}\)S, and CA, which are dispersed in the continuous Ca, Al-rich CMAS phase. In sub-zone B, the MgO grains disappear completely, and more secondary spinel is formed. It is predicted that all phases detected in sub-zones A and B were formed as a result of the reaction of the oxide components of the liquid phase, which were exclusively from the slag, according to Equations (9)-(11):  
\[\mathrm{C}_{\mathrm{(liquid)}}+\mathrm{A}_{\mathrm{(liquid)}} \rightarrow \mathrm{CA} \tag{9}\] \[\mathrm{M}_{\mathrm{(liquid)}}+\mathrm{A}_{\mathrm{(liquid)}} \rightarrow \mathrm{MA}\] (10) \[2\mathrm{C}_{\mathrm{(liquid)}}+\mathrm{S}_{\mathrm{(liquid)}} \rightarrow \mathrm{C}_{2}\mathrm{S} \tag{11}\]  
Comparing the EDS results and the theoretical composition, it was noticed that in the C sub-zone, there are phases similar to CA\({}_{6}\), CA\({}_{2}\), C\({}_{2}\)AS, CA, and some secondary MAs were still observed. In sub-zone C, smaller amounts of spinel and CA phase were observed. On the other hand, at the slag-refractory interface (between the reaction subzone C and D), the CA\({}_{2}\) and CA\({}_{6}\) phases were observed, which prevented further corrosion progressing into the material, as a result of which, in sub-zone D located below the slag-refractory interface, the material was only slightly corroded. Many authors have reported that at high temperatures, Ca\({}^{2+}\) ions can easily diffuse into materials containing Al\({}_{2}\)O\({}_{3}\) and segregate at the borders of corundum grains or at the surface [54; 55]. According to [PERSON], this diffusion of Ca\({}^{2+}\) into Al\({}_{2}\)O\({}_{3}\) occurs before CA\({}_{6}\) generation [50]. CA\({}_{6}\) reacts with Ca\({}^{2+}\) ions to form CA\({}_{2}\) followed by CA. Thus, the predicted mechanism of phase formation in the C and D sub-zones follows Equations (6)-(8). It should also be noted that gehlenite is probably formed from the CA\({}_{2}\) or CA\({}_{6}\) phase by reaction with SiO\({}_{2}\), which was from the liquid phase [49].