# Utilization of Industrial Ferronickel Slags as Recycled Concrete Aggregates
## 4 Discussion
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/m\({}^{3}\) in relation to the corresponding concrete specimens which have been produced only by natural raw materials (density values up to 2650 kg/m\({}^{3}\)). In most cases, however, the use of aggregate slag implies the production of concrete with density greater than 2700 kg/m\({}^{3}\), 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 transgranularly 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 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.