# Grain Size Effect in Elution Test of Electric Arc Furnace Slag
## 3 Results  
### Morphological, Chemical and Structural Analysis  
The EAF slags have a black/grey color, with white spots due to a presence of the calcium carbonate phase. Given its density (between 3.3/3.6 g/cm\({}^{3}\)), the high roughness, and porosity, FeO, CaO, SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), MgO in a wide range of percentage, 10%-40%, 22%-60%, 6%-34%, 3%-14% and 13%-14%, respectively [8; 33], and other minor phases. Ca and Si mainly originate from the inert materials added to the steel bath, while the presence of other elements, such as Cr, Ti, and Cu, is due to impurities of the ferrous scraps. Larnite (2 CaO-SiO\({}_{2}\)), brownmillerite (Ca\({}_{2}\)(Al,Fe)\({}_{2}\)O\({}_{5}\)), wustite (FeO), calcium silicate, silicon aluminates, gehlenite (Ca\({}_{2}\)Al[AlSiO\({}_{7}\)]), bedrigite (Ca\({}_{7}\)Mg(SiO\({}_{4}\))\({}_{4}\)), magnetite (Fe\({}_{3}\)O\({}_{4}\)), magnesioferrite (MgFe\({}_{2}\)O\({}_{4}\)) and manganese oxides are the crystalline phases usually identified by XRD. The composition and the phases of the slags are related to the cooling rate, the metal scrap, and the feature of steel plant [34]. A high variability was also observed in slags collected from the steel plant at different times. The slag samples were classified according to the slagging operation, showing that, when SiO\({}_{2}\) is added, the slag appears less porous and dusty, as shown (see Figure 1). Morphological and compositional analyses have been performed to evaluate the differences among the samples. In the SEM micrographs collected in back scattering modes (see Figure 2), the differences in the morphology and contrast can be related to the phases. Grains are formed during the solidification of the liquid slag and correspond to different phases [35]. Indeed, non-uniform solidification occurs in the solid slag because of the temperature gradient in the matrix: high melting point phases--such as silicates--solidify first, giving rise to grain structures with shapes typical of the phase composition [30]. EDXS measurements (Table 2) allowed us to identify the phases in the slags, that were confirmed by XRD analysis (see Supplementary Material Figure S1). Analogies and differences among samples can be associated with the parameters of the solidification process, in particular the cooling rate and the SiO\({}_{2}\) addition. In all the samples, a high percentage of  
Figure 1: Photo of low cooling rate slag (top, sample G) and SiO\({}_{2}\) modified slag (bottom, sample D1). Reproduced in colors. Figure 2: SEM micrographs of samples **A**, **B**, **C**, **D1**, **E1**, **F** and **G**. Numbered purple squares represent the position where Energy Dispersive X-ray Spectroscopy (EDXS) measurements are collected. Reproduced in black and white. a dark matrix is present, with a high content of Si, Ca and also few percentages of Fe and Al. The content of the last two elements is related to the larnite and gehlenite phases percentages, as detected by XRD. In particular, the larnite content is significant in the samples that undergo slow cooling, while the amount of gehlenite is high in the case of fast cooling and SiO\({}_{2}\) addition. The light matrix has a dendritic structure, with rounded and sharp edged grains. Dendritic and rounded grains have a high content of Fe with the presence of Mg and Cr. The composition of the rounded grains is compatible with iron oxides as magnesioferrite, while the phase of the dendritic structures is attributed to the wustite phase. Dendritic structures are present in the samples obtained in the fast cooling process (A, C, F, D1 and E1), and they are in a more significant percentage in the sample obtained by adding silica (D, E). In the latter case, the dendritic structure formation may be related to a reduction in the slag melting temperature induced by the SiO\({}_{2}\)[30]. The slower is the cooling rate, the larger is the size of round shape grains (see samples A, B and G).