## 3 Results
### SEM-EDS Studies of Corroded Castables
The significant amounts of CA\({}_{2}\) and CA\({}_{6}\) phase prevent the further diffusion of Ca\({}^{2+}\) ions towards the refractory material, which largely prevents further corrosion of the material. As a result, in zone D, below the contact line of the refractory slag, no significant defects in the refractory material were observed; only some amounts of CA\({}_{6}\) phase were observed. It can be assumed that the reaction components forming the phases above the slag-refractory contact line come mainly from slag reagents, while on the contact line and below the slag-refractory contact line, both slag and material reactants take part in the reaction. In Figure A3, for the castable in contact with the slag S3, the A-C reaction sub-zones were formed above the slag-refractory contact line, while the D sub-zone, they were formed below this line. A characteristic feature of sub-zones A-C is the occurrence of the continuous Ca, Al-rich CMAS phase. Moreover, from microscopic observation, significant amounts of CA phase were observed in sub-zone A, formed from oxides of the liquid phase derived from the components of the slag (Equation (9)). The CA phase formed probably reacts with ions Si\({}^{2+}\) and Ca\({}^{2+}\) of oxide from the liquid phase to form gehlenite, according to Equation (12). Moving towards the slag-refractory contact line, some amounts of MgO grains (sub-zone B) were observed. On this basis, it was found that the secondary spinel was formed from MgO and Al\({}_{2}\)O\({}_{3}\) oxides as in Equation (10). On the other hand, significant amounts of CA\({}_{2}\) phase were formed on the slag-refractory contact line, as well as above this line. The CA\({}_{6}\) phase adjusted to corundum grains prevented further diffusion of Ca\({}^{2+}\) ions into the refractory material, which stopped the corrosion processes of castables. It was found that in phase D subzone CA\({}_{6}\), CA\({}_{2}\) and CA were formed in accordance with Equations (6)-(8), respectively. In contrast, C\({}_{2}\)AS present in the D subzone was probably formed by the reaction of CA\({}_{2}\) or CA\({}_{6}\) with SiO\({}_{2}\) from the liquid phase of the slag components. \[\text{C}_{2}\text{AS}+\text{3S}+\text{A}\rightarrow\text{2 CAS}_{2} \tag{3}\]0  
Microstructure observations of the analyzed samples confirmed that the mass ratio C/S affects the corrosion resistance of castables. As shown in Figure 11, for the C_S1 sample, the smallest mass ratio C/S in the slag caused the entire slag to infiltrate into the material. This was due to the low eutectic temperature between C\({}_{2}\)AS and CAS\({}_{2}\) (around 1400 \({}^{\circ}\)C). However, in the case of an increase in the mass ratio C/S, as shown in Figure 12 for sample C_S2 and Figure 13 for sample C_S3, the slag formed a reaction zone above the slag-refractory contact line and below the slag-refractory contact line, which indicated lower slag aggression. Thus, the presence of SiO\({}_{2}\) in the slag composition had a significant impact on the corrosion processes of the tested castables, while the Ca\({}^{2+}\) ions present in the slag diffused deepest towards the material, as shown by the SEM images. It should also be observed that Ca\({}^{2+}\) ions in contact with Al\({}_{2}\)O\({}_{3}\) coming from the material formed CA\({}_{2}\) and CA\({}_{6}\) phases, which at some distance from the slag-refractory contact line formed a barrier for Ca\({}^{2+}\) ions, which prevented the corrosion processes from progressing. An important observation was the emergence of secondary spinel, which differs in the shape of the grains from the material-derivative spinel. This fact was confirmed in previous studies, both in the FactSage analysis and in the XRD analysis.