# Utilization of Industrial Ferronickel Slags as Recycled Concrete Aggregates
## 1 Introduction
Nevertheless, the authors suggest the use of superplasticizers to achieve the appropriate workability as compressive strength shows increased values (up to 45 MPa) in case of 60% replacement most probably due to the pozzolanic effect of the steel slag used. An interesting experimental approach on the production of an eco-friendly concrete was done by [PERSON] et al. [21] aiming on the maximum vaporization of alternative raw materials. Fly ash was incorporated as cement replacement, EAF slag and recycled aggregates from construction demoliti wastes (CDW) as coarse and fine aggregates, respectively. They found that the use of CDW increases the porosity of concrete with subsequent decrease in strength and durability compared to a reference mix, while the synergy of CDW with EAF slag partly overcomes these drawbacks reaching compressive strength values of about 30 MPa at 28 days. In spite the poor bonding between aggregate-binder, the formed concrete might be used in lower-demand applications. Blended concretes have been also studied using FeNi slag aggregates in different grain sizes and by substituting ordinary binding materials as presented for example by [PERSON] et al. [22]. They found that early strength of the test samples was affected, but with time hydration reaction can overcome this issue. According to [PERSON] et al. [23], natural aggregates can feasibly be replaced by FeNi slag as the final concrete mix conforms with type C50 due to the obtained mechanical strength reaching 55 MPa at 28 days of curing. However, best engineering results are observed with the use of blast furnace slag as well. Lately, [PERSON] et al. [24] investigated the combined use of FeNi slag and ground FeNi slag as supplementary binder for enhancing the sustainability of self-compacting concrete. After assessing strength, permeability and microstructure of the produced concrete samples, they came to the conclusion that a replacement of cement up to 30% by ground FeNi slag and, respectively, of sand by 50% replacement of FeNi slag is feasible as they presented high durability. There is still room for further research on the production of eco-friendly concretes from different types of slags and/or other industrial by-products in relation to the technical properties of the new concrete mixes as for sure environmental benefits are anticipated. As the economy and the smelting industry developed rapidly, the quantity of smelter slag hasbeen increasing for a long time. Common smelting furnace slag includes steel smelting slag, red mud, copper smelting slag, lead slag, ferroinckel slag, sulfuric acid slag, etc. Approximately 30 million tons of ferronickel slag are emitted each year, accounting for more than a fifth of the global production [3; 25; 26]. The large stockpiles of nickel slag not only occupy land and pollute the environment, but also bring about severe challenges to the sustainable development of the nickel smelting industry [27; 28]. Furthermore, it is essential to develop and utilize smelter slag through environmental-friendly technologies [29; 30]. Different types of industrial by-products were studied in the past few decades in order to find suitable alternatives of natural sand in concrete. Blast furnace slag, steel slag, copper slag, foundry slag and ferronickel slag are the most common types of by-product slags that can be used as fine aggregate in concrete. [PERSON] et al. [31] showed that the use of blast furnace slag as a replacement of natural sand improved the compressive strength of mortar. The physical and chemical properties of the by-product slag can vary significantly depending on the type of the ore, the smelting process and temperature, and the cooling method. The amount of literature available on the use of ferronickel slag in concrete is very limited. [PERSON] et al. [32] pointed out that freeze-thaw resistance of concrete decreased with the use of 50% ferronickel slag replacing natural coarse aggregate.