# Utilization of Industrial Ferronickel Slags as Recycled Concrete Aggregates
## 1 Introduction
Moreover, the European Waste Framework Directive [8; 9] makes clear the importance of industrial by-products valorization such as iron and steel slag aggregates that contributes to waste minimization and resource conservations. Ferrous containing slags, are silicate melts that are formed during the manufacturing of crude steel and crude iron, mainly due to the combination of slagging agents and fluxes used to remove impurities from iron ore and other metal feeds of the smelting furnaces [10]. The hot slags are tapped from the liquid metal and transported to a slag damp where they are stockpiled, after cooling (open-air or water spraying) and processed to solid material. In Europe, \(\sim\)45 Mt of blast furnace slags and steel slags were produced in 2018 [11], whereas in the U.S., steel slag production is estimated in the range of 8 to 12 Mt [10]. The most common produced metallurgical slags are steel, ferroinckel, copper, lead, zinc, phosphorous and stainless steel slags. An important parameter affecting the physical and chemical properties of the slag by-products is by far the source ore and the metal processing method used by each country. Greece for example is the one of the largest ferroinckel (FeNi) producer in Europe utilizing the domestic laterite ore deposits [12]. The pyrometallurgical process comprises of two main stages: (a) preheating of the feeding materials in rotary kilns and (b) reduction and smelting (1600-1700 \({}^{\circ}\)C) of output calcine in electric arc furnaces (EAFs). This process leads to the production of two co-existing materials: the ferroinckel product and the slag. Given the fact that the EAF slag produced make up about 80-90% of the feeds material's mass [13], it is apparent that large quantities are generated, which concerns steel production industries. During the past decades, extensive work has been published on the utilization of steel slags as alternative aggregates in Portland concretes. Depending on the physical characteristics of the slag aggregates, e.g., density, granulometric size and shape, and the application field of the final product interesting findings are reported by scientists with emphasis on mechanical, physico-chemical and environmental properties of slag-containing concrete. Durability studies of concrete mixes containing electric arc furnace (EAF) slag aggregates include systematic tests of compressive strength, water penetration, freeze-thaw cycles, leaching tests and chemical reactivity of the slag with the surrounding cement components [3; 14; 15; 16; 17]. For example, [PERSON] et al. [18] found that the high porosity of EAF slag used in this work reveals a low-quality concrete in terms of freezing resistance, thus suggesting the additional use of specific admixtures. Leaching tests, on the other hand, showed a beneficial cloistering effect of fluorides and chromium in the concrete mix. In a recent study, conducted by [PERSON] et al. [19], conventional sand and gravel were replaced by EAF and ladle furnace (LD) slags, individually or in combination, resulting in the production of a durable concrete. From the 14 test samples prepared, all quality indexes investigated remained either stable or improved in comparison to the reference concrete. The most efficient mixture was achieved when sand was replaced with 30% by volume of EAF and LD slag sand, respectively, and gravel by 50% of EAF gravel aggregates leading to concretes with reduced carbonate and chloride penetration. However, granulated blast furnace slags (GBFS) attract also special scientific interest as due to their finer grain size they can be utilized instead of natural fine aggregates (e.g., river sand). [PERSON] et al. [20] for examples state that incorporation of GBFS (<5 mm) up to 40% was feasible, whereas higher percentages (up to 60%) lead to reduced workability of the concrete mixes attributed to the higher water absorption of GBFS.