*3.1. 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 3.** Representative photomicrographs of the studied slags showing: (**a**,**b**) anhedral olivine, spherical crystals of w*υ*¨stite, spinels and microcrystalline opaque minerals in FeNi slag (XPL and PPL Nicols, respectively); (**c**) grains of spinels, w*υ*¨stite, calc-aluminosilicate and silicon dioxide phases in Electric Arc Furnace slag (XPL Nicols); and (**d**) elongated and curved ferritchromit in Electric Arc Furnace slag (XPL Nicols).

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 micritic 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.

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, wüstite, spinel group minerals and lime-alumino silicate phases constitute their modal composition. The highest percentages of the wüstite-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%.

**Table 2.** \*\*\* Mineral composition % of the studied FeNi slag.


\*\*\* In the semi-quantitative analysis, the amorphous phases are not included. It was calculated <30%.



\*\*\* In the semi-quantitative analysis, the amorphous phases are not included. It was calculated <30%.

**Figure 5.** (**a**) XRD pattern of FeNi slag; (**b**) XRD pattern of Electric Arc Furnace slag (1: Gehlenite, 2: C3MS2, 3: C2S, 4: Spinel, 5: W*υ*¨stite, 6: Quartz, 7: Periclase).

Results of the chemical composition analyses of the studied slag samples were performed by X-ray Fluoroscence (XRF) are presented in Table 4. The results of XRF analyses shows that the FeNi slag contains higher amount of Fe2O3 (37.25 wt.% compared to the ELECTRIC ARC FURNACE SLAG slag (10.41 wt.%). The ELECTRIC ARC FURNACE SLAG slag displays higher amounts of SiO2, Al2O3 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

SLAG slag presents LOI (loss on ignition) 1.21, while the FeNi slag does not present any value of LOI. As for the trace elements, FeNi slag is more enriched in Ni, while ELECTRIC ARC FURNACE SLAG slag is more enriched in Cr, Sr, Ba and V (Table 4).


**Table 4.** Results of the chemical composition of the tested samples (<DL: <below detection limit).

The slags have basicity value which range from 2.42 (ELECTRIC ARC FURNACE SLAG) to 3.31 (FeNi) which as calculated according to the following equation:

$$\text{Basicity} = (\% \text{CaO} + \% \text{MgO}) / \% \text{SiO}\_2$$

where %CaO, %MgO and %SiO2 are the contents of those mentioned oxides (wt%) in the studied slags.

Analyses with Scanning Electron Microscope (SEM) and Energy Dispersive X-ray mapping were conducted to determine the distributions of several major elements (Si, Al, Ca, Mg, Fe, Cr, K and Na) in the studied slags. SEM observations are in accordance with the petrographic features and XRD analyses as indicated in Figure 5 where spherical crystals of w*υ*¨stite and curved subhedral spinels are shown. These minerals are surrounded by microcrystalline calc-silicate phases (C2S). FeNi slag presents uniformly distributed porosity of brick type (Figure 6). Regarding ELECTRIC ARC FURNACE SLAG slag, in Figure 7 it is proved that it consists of spherical crystals of w*υ*¨stite, elongated spinels, calc-silicate phase (C2S) and microcrystalline gehlenite (Figure 7a).

**Figure 6.** Back-scattered electron image of spherical crystals of w*υ*¨stite (W), spinels (Sp) and microcrystalline calc-silicate phase (C2S), as well as pores (P, distinguished as black) in FeNi slag (**a**) and mapping of major elements (Al, Si, Ca, Mg, Fe, Cr, K and Na, respectively) (**b**–**i**).

**Figure 7.** Back-scattered electron image of spherical crystals of w*υ*¨stite (W), spinels (sp), calc-silicate phase (C2S) and microcrystalline gehlenite (G) in ELECTRIC ARC FURNACE SLAG slag (**a**) and mapping of major elements (Si, Al and Ca, respectively) (**b**–**d**).

The results of SEM-X-ray mapping are presented in Figures 6 and 7. The SEM-X-ray mapping analysis result in FeNi slag indicates that Fe is uniformly distributed on the entire surface of the slag (Figure 6f). In addition to Mg, components in the slag that are in lower amounts are Si, Cr, Ca, K and Na (Figure 6b–e,g–i). Regarding ELECTRIC ARC FURNACE SLAG slag, the mapping results are in accordance with the chemical compositions and the petrographic results. More specifically, slag composition is relatively uniform, in which Ca and Al are present as the major component of the slag, while Si is in smaller amounts (Figure 7b–d).

The specific gravity of the tested slags (both of two types) and of the natural aggregates (limestone) is identified. As can be seen from Table 5, the specific gravity of FeNi slags presents higher values than of those of ELECTRIC ARC FURNACE SLAG slags in all tested samples. More specifically, as for the SA group, its specific gravity values range from 4.51 to 4.61 T/m3 regarding FeNi slags while from 3.59 to 3.61 T/m3 regarding ELECTRIC ARC FURNACE SLAG slags. Similarly, the values range of SB group is from 4.54 to 4.59 and from 3.52 to 3.70 in FeNi and ELECTRIC ARC FURNACE SLAG slags respectively. As for the SC group, the range of specific gravity values is from 4.59 to 4.72 and from 3.45 to 3.70 in FeNi and ELECTRIC ARC FURNACE SLAG slags, respectively. The lowest value of this property is found in a FeNi slag of SC group, while the highest value in ELECTRIC ARC FURNACE SLAG slags of SB and SC groups. Regarding the measured specific gravity of the tested limestones used as concrete aggregates is 2.70 T/m3, intensively lower than that of the investigated slags.

**Table 5.** Results of the specific gravity test of the tested slags.


#### *3.2. Results of Concrete Specimens*

Regarding the physical properties of investigated concrete specimens such as density, concrete specimens produced by natural aggregates (limestones) show density from 2621 to 2709 kg/m3 (Figure 8a, Table 6). As for the specimens made from various percentages of two different slags (FeNi and ELECTRIC ARC FURNACE SLAG), samples of SA group present values of density range from 3093 to 3125 kg/m3, samples of SB group values from 3095 to 3181 kg/m3 and these of SC group from 3060 to 3169 kg/m3 (Figure 8a). Samples of SA group display water absorption, which ranges from 0.68 to 1.08%, samples of SB group from 1.01 to 1.59% and these of SC group from 1.09 to 1.48% (Figure 8b, Table 6). Furthermore, the uniaxial compressive strength (UCS) of the produced concrete specimens was measured. The compressive strength of the standard concrete specimens (produced by natural aggregates) ranges from 36.00 to 38.00 MPa in 7 days and from 47.00 to 48.50 MPa in 28 days (Figure 8c). In 7 days, the UCS of SA group ranges from 62.24 to 66.35 MPa, while in 28 days ranges from 75.80 to 83.42 MPa simultaneously indicating these samples as those presenting the highest concrete strength (Figure 8c,d, Table 6). As for the SB group, the UCS in 7 days it ranges from 50.02 to 56.91 MPa and from 57.12 to 61.74 MPa in 28 days (Figure 8c,d). The UCS of SC group in 7 days ranges from 45.53 to 55.18 MPa and in 28 days from 52.84 to 66.85 MPa (Figure 8c,d, Table 6).

**Figure 8.** Results of the physical and mechanical properties of the produced concrete specimens. (**a**) density (kg/m3); (**b**) Water absorption (%); (**c**) Uniaxial compressive strength of 7 days (MPa); (**d**) Uniaxial compressive strength of 28 days (MPa).


**Table 6.** Results of the physical and mechanical tests of the produced concrete specimens.

As for the concrete specimens made by slags, the presence of fractures between the slags and the cement paste is partially observed (Figure 8). Moreover, samples SA1-SA6 and SC1-SC6 are characterized by satisfactory bonding with the cement paste (Figure 9a–d,f,h). On the contrary, samples SB1-SB6 presents lower microroughness and unsatisfactory bonding with the cement paste, which indicated from the zone around wüstite grains which acts as detachment zone during the load (Figure 9e,g). During the petrographic study of the concrete specimens, a large number of wüstite and spinel crystals were found to have been detached from the cement paste on a large scale after loading, while in none of the studied slags crystalline microcracks were found penetrating their mass. The general microroughness of the mineral components of the slags does not show any increase which is likely to work adversely within the concrete.

**Figure 9.** Textural characteristics of the studied concrete specimens: (**a**) Photomicrograph of subhedral spinel (Sp), w*υ*¨stite (W) and anhedral calc-silicate phase (C2S) (PPL Nicols, sample SA1). (**b**) Photomicrograph of elogated grain of slag consisting of spinel (Sp) and w*υ*¨stite (W) (PPL Nicols, sample SC2). (**c**) Photomicrograph of subhedral and elogates spinel (Sp) in which scattered cracks are observed which do not penetrate it (PPL Nicols, sample SC5). (**d**) Photomicrograph of an elogated grain of ELECTRIC ARC FURNACE SLAG, which contain calc-silicate phase (C2S) and has a good cohesion with the cement paste (PPL Nicols, sample SA4). (**e**) Back-scattered electron image of a spherical grain of w*υ*¨stite (W) which does not have good cohesion with the cement paste and detachments are observed around it (sample SB6). (**f**) Back-scattered electron image of cracks in the cement paste and around a grain of an ELECTRIC ARC FURNACE SLAG that contains calc-silicate phase (C2S) and spherical w*υ*¨stite (W) (sample SA6). (**g**) Back-scattered electron image of subhedral w*υ*¨stite (W) around which detachments are observed (sample SB2). (**h**) Back-scattered electron image of cracks and detachments between the cement paste and the FeNi slag (sample SC6).

All the above are enhanced either through the processing of microscopic images after the 3D depiction or through the study of the transition zone between the aggregate and the cement paste as it is shown in Figure 10a–d. The grains of C2S phases and spinels (Figure 10 a,c,d) seem to present good cohesion with the cement paste. In addition, it is observed that the grains of w*υ*¨stite have lower microroughness compared to that of the cement paste, where this alternation of the concrete components leads to the better cohesion among them (Figure 10b).

**Figure 10.** 3D depictions of the investigated concrete specimens: (**a**) spinel, w*υ*¨stite and calc-silicate phase (sample SA1); (**b**) spinel and w*υ*¨stite (sample SC2); (**c**) spinel (sample SC5); and (**d**) calc-silicate phase paste (sample SA4).

### **4. Discussion**

Different types of industrial byproducts have been studied in the past few decades in order to find suitable alternatives of natural aggregates in concrete. Blast furnace slag, steel slag, copper slag and foundry slag can be used as concrete aggregates. In this study, different types of slags have been evaluated in various mixtures in order to identify the optimum combination of recycled raw materials such as slags for their use in several construction applications of high demands. For this reason, at the same time, a standard concrete of the same category was prepared containing only natural carbonate aggregates in order to carry out a direct comparative study between these types of concrete specimens so that a wider application of such type of by products could be established. At first, when studying the aggregate properties, significant differences regarding their physical and mechanical properties are identified. More specifically, slags used as aggregates display better properties in terms of their mechanical strength, while limestones present advantages in terms of water absorption and physical properties in order to be used as concrete aggregates. The density of slags used as aggregates appears significantly increased compared to natural aggregates that lead to the characterization of slags as heavy weight aggregates, while the same applies to their density. The water absorption of slags is particularly high compared to that of common limestones used as aggregates. This parameter must be taken into account when studying the composition of concrete, so that no reduced workability can be observed from the absorption of water by the aggregates. Several researchers have studied the influence of the aggregate type on the final concrete strength which is directly depends on their properties [46–48]. During the microscopic study of the raw materials where they

were studied as aggregates, significant differences are identified both between the natural aggregates and the slags as well as between slags of different origins. More specifically, natural aggregates are less compact and contain numerous microcracks, and thus they penetrate their entire mass compared to slags where they are more compact, and they display less intargranular microcracks. However, the porosity in the micro-scale seems to be increased in the whole of the studied slags in relation to the carbonate rocks in which the porosity is more uniformly distributed, something that probably affects in all the properties but also in their final relationship with the cement paste. However, there are significant differences between the slags during their microscopic observation where the FeNi slags show significantly higher percentage of w*υ*¨stite and spinel than those of ELECTRIC ARC FURNACE SLAG, while at the same time, the FeNi slags show lower percentage of calc silicate phases than those of the ELECTRIC ARC FURNACE SLAG significantly affecting to the properties of aggregates. Moreover, a significant difference during the microscopic study is found in the porosity which seems to be greater in the FeNi slags in contrast to the corresponding ones of ELECTRIC ARC FURNACE SLAG, something that is mainly attributed to their special mineral characteristics.

Test results of the compressive strength from different concrete mixtures in 7 and 28 days are plotted in Figure 8c,d. The replacement of limestone aggregates with slags in concrete seems to significantly increase their compressive strength both in 7 and 28 days in all cases. The 28-day compressive strength levels of concrete specimens made by slags as aggregates present range of values from 52 MPa to 83 MPa compared to the corresponding concretes produced only by natural aggregates which do not exceed 40 MPa, i.e., much lower than expected, which is initially attributed to the higher hardness of slags as shown in the comparative table with the properties of the aggregates (natural and recycled).

In general, it seems that when slag is used regardless its source in all the aggregate grain sizes, then the density of the mixture can exceed 3000 kg/m3 in relation to the corresponding concrete specimens which have been produced only by natural raw materials (density values up to 2650 kg/m3). In most cases, however, the use of aggregate slag implies the production of concrete with density greater than 2700 kg/m3, which must be taken into account when designing such type of constructions. The uniaxial compressive strength of concrete specimens produced by slags is better than of those produced by natural aggregates with an increase of 20% in concrete with lower percentage of ELECTRIC ARC FURNACE SLAG slag and with higher percentage of FeNi slag. This increase reaches the percentage of 30% in the cases where it participates in a higher percentage of ELECTRIC ARC FURNACE SLAG slags and a lower percentage of FeNi slags. This is due to all the special characteristics of the slags in relation to their physical and especially to their mechanical characteristics directly dependent on their microscopic characteristics where slags present a compact texture without microcracks and uniformly distributed porosity which seems to affect the smooth crystallization process of the cement and therefore on its stronger bonding with the aggregates in relation to the unevenly distributed porosity of the natural aggregates. During the petrographic study of concrete, when studying the micro scale where the failures are born, significant differences in the microcracks' process are identified after the breaking of the concrete specimens. During the microscopic study of the oriented thin sections during the uniaxial compressive strength test of concrete specimens produced by natural aggregates microcracks are shown to break the carbonate aggregates trangranularly and intragranularly, something that in concrete specimens made by slags was not found in any of the examined mixtures. This fact is attributed to both mechanical of aggregate materials as well as in their mineralogical characteristics. Among the mixtures produced by different types of slags, significant differences in their mechanical strength were observed. The differences in the final compressive strength of the produced concretes are observed as significant even with small variations in the percentage participation of the different types of slag, something that reveals the strong influence of the special mineralogical characteristics on the final quality of the produced concrete specimens.

The superiority of mixtures of SA group in contrast to that of SB group (higher percentage of ELECTRIC ARC FURNACE SLAG slag) and that of SC group (lower percentage of natural sand) is evident both in 7 days and in 28 days of curing. This difference on the final concrete strength where different types of slags are participated is precisely related to their different weights and their different densities as the slag aggregates which are characterized as heavy weight aggregates produce heavy weight type concretes and as their weight increases it seems their durability to be reduced linearly.

The differences in all properties of the produced concretes which are identified between the groups SA, SB, SC can be interpreted by studying and evaluating the particular microscopic characteristics of both raw materials (slags and natural rocks) and the produced concrete specimens. The hydration of ladle slags with a content of hydraulically active mineral and the glass phase is taking place after adding water, without having to supply an alkali-activator. The hydraulic properties of slags are incommensurable compared to Portland cement however hydrating abilities are affected by the chemical composition. Samples SB do not present high compressive strength, while the other samples present better compressive strength after 28 days of hydration. Samples in which compressive strength values are higher than those of SB group contain mineral dicalcium silicate in this group. This mineral occurs in Portland clinker [1] and due to its hydration there are the so [32] called C-S-H phases formed. These phases are bearers of strength in the hydrated cement. Additionally, it is evident that mixtures containing higher amount of ELECTRIC ARC FURNACE SLAG slag and lower amount of FeNi slag (SA and SC group) yield increased mechanical concrete properties compared to the SB group, which seems to be related to the increased concentration of w*υ*¨stite and mervinite in FeNi slags (Tables 1 and 2). The presence of wüstite in the concrete as it was detected through the microscopic study seems to act as a surface of weakness within the structure of the cement paste as the cubic system to which it belongs creates some plan surfaces capable of acting as levels of failure during uniaxial loads. The fact of the negative effect mainly of wüstite on the final mechanical strength of concrete specimens is documented both by the microscope images and by the three-dimensional study of the produced concretes. More specifically, as shown in Figure 9b, an extensive zone of detachment from the adhesive cement is located around the wüstite, which is found throughout the range of concrete. This fact indicates the existence of a point of weakness inside the concrete where during the uniaxial loading they act exactly as detachment points resulting in faster breaking. In addition, as shown in the three-dimensional study, the wüstite crystals do not show a particularly significant microtopography in relation to the rest of the mortar, while at the same time it presents extensive flat areas which may also function as slip levels during loading. The spinel shows similar behavior to wüstite as shown in Figures 8a and 9a. In contrast to wüstite and spinel, the presence of C2S phases seems to have a significant positive effect on the mechanical concrete strength as no significant detachment zones are found perimetrically during the fracture while the three-dimensional microtopography seems to be satisfactory and evenly distributed throughout the surface (Figure 9d) creating formations which seem to mechanically trap the cement mortar. An extra significant parameter for uniaxial compressive strength of concrete with the increased percentage of FeNi slags seems to be the existence of merwinite when is regarded as a low hydraulically active mineral [49,50]. It seems to significantly affect the hydraulic properties of the cement and its smooth hydration reaction, resulting in a significant reduction in its mechanical strength and around these crystals to identify areas of reaction and weakness.

As it is shown through the study of the X-ray diffraction patterns of raw materials it seems that the compressive strength of samples prepared from the slags Electric Arc Furnace Slag were secured primarily by the presence of b-C2S phase. The b-C2 S phase is considered the most important for ensuring the strength of products with water activated ladle slags. The Electric Arc Furnace Slag slags which after adding water have sufficient strength are in accordance with condition C/S. The low ratio C/S shows these slags which is hydraulically inactive. The other type of slags has a satisfactory C/S ratio but its hydraulicity is low. Additionally, in contrast to mervinite and w*υ*¨stite, the presence of gelenite in an increased percentage in the ELECTRIC ARC FURNACE SLAG slag seems to have a significant effect on both the properties of slags and on the behavior of the produced concrete. As can be seen through the microscopic study of the microstructure of concrete, strong adhesion with the cement paste is detected, which is probably due to the partial hydration reaction of these mineral phases and its special morphological characteristics. Additionally, the difference between FeNi and ELECTRIC ARC FURNACE SLAG slag can be observed not only in their mineralogical composition but also in their chemical composition and thus even the study of their chemical composition could potentially be used as a predictor of slag behavior in concrete. More specifically, FeNi slags show intensively lower percentage of free CaO compared to their respective ELECTRIC ARC FURNACE SLAG slag participation and the diversity in their composition confirms the strong heterogeneity of these slags and imbalances in their solidification the hardening of the hydrated slags is also involved in the presence of the amorphous phase, especially in the case of slags containing free CaO as Calcium hydroxide is the activator of the latent hydraulicity for the amorphous phase [51].
