# Utilization of Industrial Ferronickel Slags as Recycled Concrete Aggregates
## 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; 47; 48]. During the microscopic study of the raw materials where they  
Figure 10: 3D depictions of the investigated concrete specimens: (**a**) spinel, w\(\backslash\)stite and calc-silicate phase (sample SA1); (**b**) spinel and w\(\backslash\)stite (sample SC2); (**c**) spinel (sample SC5); and (**d**) calc-silicate phase paste (sample SA4). 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 intragranular 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\(\ddot{v}\)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.