## 3 Results and Discussion
### Slags Characterization
#### 3.1.2 Mineralogical Characterization and Microstructure
The environmental conditions during the smelting trials and thus slags tapping (temperature \(<\)0 \({}^{\circ}\)C, rainfall) and formation of microcrystalline material prevented the formation of \(\gamma\)-C\({}_{2}\)S (\(\gamma\)-2 CaO-SiO\({}_{2}\)) and the disintegration of the cooled slag phase due to \(\beta\)\(\rightarrow\)\(\gamma\)-C\({}_{2}\)S transformation even in the air-cooled slags. The later takes place in cases of slow cooling rate and its presence is not desired in cementitious materials, since it exhibits slight hydraulic properties not contributing to strength development. The co-existence of \(\alpha\) and/or \(\alpha^{\prime}\) and \(\beta\)-C\({}_{2}\)S phases in the slag samples indicates that the theoretical thermodynamic transformation was not completed due to rapid temperature drop mainly in the initial phase of the cooling process [30]. The incorporation of Mg in C\({}_{2}\)S could lead to the formation of bredigite, Ca\({}_{14}\)Mg\({}_{2}\)(SiO\({}_{4}\))\({}_{8}\)--once the solubility limits in \(\alpha\) and \(\alpha^{\prime}\)-C\({}_{2}\)S are exceeded. Usually, its composition varies between Ca\({}_{1.75}\)Mg\({}_{0.25}\) and Ca\({}_{1.7}\)Mg\({}_{0.3}\) at \(\sim\)1300 \({}^{\circ}\)C per SiO\({}_{4}\)[31] and its presence has been reported in cases of EAF steelmaking slags [32]. However,  
Figure 4: SEM (SEI) micrographs of calcium aluminates and hydrated calcium sulfo-aluminate phases in 4-2b slag (**a**) rich in calcium aluminates micro-area; (**b**) microstructure of calcium aluminate phases at higher magnification; (**c**) SEM/EDS spectrum of thin hexagonal plate (monosulfate). \(\alpha^{\prime}\)-C\({}_{2}\)S and bredigite have different structures, it has been confirmed [31; 33] that the similarity in their XRD patterns makes distinguishing them very difficult (Figure 1). In this work, the nomenclature of \(\alpha^{\prime}\)-C\({}_{2}\)S has been currently attributed to a probable combination of both phases. The semiquantitative analysis obtained by SEM-EDS verified solubility of Mg in C\({}_{2}\)S, as well as the existence of pure C\({}_{2}\)S micro-areas, while Al, Mn and Fe incorporated in the crystal lattice was also identified (Table 5, Figure 2). The incorporation of foreign ions in the structure of dicalcium silicates, as well as deviation from the stoichiometric composition, favor the stabilization of the \(\alpha^{\prime}\) and \(\beta\) phases [34]. In all examined samples, magnesiowustite phase was identified (Table 5, Figure 3a). Magnesiowustite comprises a solid solution between MgO and FeO and it is commonly found in the mineralogical composition of steelmaking slags [19; 21; 35; 36]. It is reflected in the XRD patterns by displaced peaks located between pure wustite (41.890 20 strongest line) and periclase ones (42.917 20 strongest line), depending on the Mg/Fe\({}^{2+}\) ratio and it has been assigned as Mg\({}_{1-x}\)Fe\({}_{x}\)O (MW). Investigations have shown that the composition of these types of solid solutions in the slag depends  
Figure 5: Ettringite needle like crystals formation in 3-1b water-cooled slag (a) microstructure of ettringite needles development in the slag; (b) magnified micrograph and; (c) SEM/EDS spectrum of ettringite needle-shaped crystals. on slag chemistry and on the cooling rate. Slower cooling rate will promote the enrichment of FeO into the structure, while MgO predominates by increasing the cooling rate [21; 36]. This may explain the difference observed when comparing the microanalysis of MW phase in slags, 3-1a versus 3-1b (Table 5). Nevertheless, it may be noticed that the strongly reduced %FeO in the chemical analysis of 4-2 slag led to rich Mg-MW even in the air-cooled slag sample. Incorporation of Manganese, Calcium and Chromium (most possibly divalent) in magnesium-solid solutions was also detected in the course of SEM-EDS analysis in the slag samples. Chromium was also identified in XRD analysis in chromite phase in two of the air-cooled slag samples (3-1,2a).