# Utilization of Industrial Ferronickel Slags as Recycled Concrete Aggregates
## 3 Results  
### Raw Materials Characterization  
The microscopic observation of the studied slags under polarizing microscope is in accordance with the results of the X-ray diffraction analysis. In general, the studied slags present heterogeneous texture due to uneven aggregate of minerals or usually multi organic fragments. Slags are rich in opaque minerals, mainly wustite and spinels, while microcrystalline to cryptocrystalline, mainly anisotropic calcium-alumino silicate phases, as well as silica minerals complete the overall mineral composition (Figure 3a-c). Alternations of isotropics with anisotropic minerals that form banded structures are also observed. Some grains display different composition around their margins and more specifically they present microcrystalline material possibly alumina-calcium-silicate (anisotropic) in coexistence with an amorphous mass (isotropic). The shape of the opaque minerals and of those of the spinel group usually appears polygonal to spherical with peculiar-sub-dominated plains. The spinel group minerals appear more with dark brown or red to black color and less amber, which indicate their more ferritic-chromit character and less aluminum (Figure 3d). The anisotropic minerals predominate in a prismatic shape, euhedral or subhedral with the exception of silicon dioxide ores which are usually unhedral. Anisotropic minerals usually appear with gray, white and yellow colors, while olivine crystals were observed less frequently (Figure 3a). Figure 2: (**a**) The compressive strength test machine. (**b**) Cylindrical concrete specimen. The carbonate aggregates (limestones) used as components of the produced concrete specimens do not present many cracks or impurities in their structure as can be seen from the petrographic study. More specifically, limestones used display nicotinic texture with numerous of fossils and veinlets of microcrystalline calcite and stylolitic porosity filled with clay minerals and Fe-oxides (Figure 4a,b). Figure 4: (**a**) Stylolitic porosity filled with clay minerals and Fe-oxides, fossils and dense network of veins in a micritic limestone; (**b**) XRD pattern of micritic limestone. Abbreviations: cc: calcite, dol: dolomite. Figure 3: Representative photomicrographs of the studied slags showing: (**a,b**) anhedral olivine, spherical crystals of _w_bsite, spinels and microcrystalline opaque minerals in FeNi slag (XPL and PPL Nicols, respectively); (**c**) grains of spinels, _w_bsite, calc-aluminosilicate and silicon dioxide phases in Electric Arc Furnace slag (XPL Nicols); and (**d**) elongated and curved ferritin in Electric Arc Furnace slag (XPL Nicols). The results of the qualitative and semi-quantitative analysis of the tested slags are presented in Tables 2 and 3 and Figure 5a,b. Similar qualitative phase composition was detected in the studied samples with mainly quantitative variations. In general, wustite, spinel group minerals and lime-alumino silicate phases constitute their modal composition. The highest percentages of the wustite-spinel phases occur in FeNi slag, which reflects to the geochemical effects, as it contains the lowest percentages of calcium, silicon and higher levels of Fe. Indications derived from the mineralogical analysis of the samples for participation of amorphous phase in their mass in percentages up to ~20-30%. Results of the chemical composition analyses of the studied slag samples were performed by X-ray Fluororescence (XRF) are presented in Table 4. The results of XRF analyses shows that the FeNi slag contains higher amount of Fe\({}_{2}\)O\({}_{3}\) (37.25 wt.% compared to the ELECTRIC ARC FURNACE SLAG slag (10.41 wt.%). The ELECTRIC ARC FURNACE SLAG slag displays higher amounts of SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\) and CaO (11.81 wt.%, 7.29 wt.% and 26.85 wt.%, respectively) in contrast to FeNi slag which contains significantly lower amounts (3.77 wt.%, 0.43 wt.% and 10.21 wt.%, respectively). Regarding the alkalies, they are not significant differences between the two types of slags (Table 3). ELECTRIC ARC FURNACE