**1. Introduction**

Concrete is the most used and cost-effective construction material in the world with an annual production of about 33 billion tons [1], and in view of the global population growth [2], the usage trend for building new infrastructures is estimated to increase up to 6% [3]. The basic components of concrete are made up of cement—a mixture of clinker and gypsum, aggregates and water, all of whom significantly affect the workability, durability and mechanical performance of the final product. From the environmental point of view, the cement production technology has a strong impact on energy consumption and increased CO2 levels, ~8% of the global anthropogenic emissions [4], mainly associated with the high calcination temperature (~1450 ◦C) of raw materials (limestone and clay minerals) necessary for the cement clinker formation. The latter has the role of the hydraulic binder and is by far the most important ingredient in concrete, constituting about 7% to 15% of

**Citation:** Petrounias, P.; Rogkala, A.; Giannakopoulou, P.P.; Christogerou, A.; Lampropoulou, P.; Liogris, S.; Koutsovitis, P.; Koukouzas, N. Utilization of Industrial Ferronickel Slags as Recycled Concrete Aggregates. *Appl. Sci.* **2022**, *12*, 2231. https://doi.org/10.3390/app12042231

Academic Editors: Laurent Daudeville, Carlos Morón Fernández and Daniel Ferrández Vega

Received: 16 January 2022 Accepted: 17 February 2022 Published: 21 February 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

its total volume. Aggregates, on the other hand, also raise environmental concerns related with their extraction and processing as they make up more than the half of the concrete's volume, approximately 60–75% [5]. Moreover, the quality of the hardened concrete depends upon the suitable use of both fine and coarse aggregates, usually consisting of natural sand and gravel or crushed stone and their efficient bond with the surrounding cement paste. Bearing all this in mind, it is obvious that for the huge concrete volumes produced every year, vast amounts of natural resources are required both for aggregates and cement leading to severe environmental problems.

Based on the ecological footprint caused by the unsustainable linear production model—take, make and dispose—which is clearly impacting climate change, waste management and natural resource depletion, a worldwide challenge have become even more apparent in driving change. Accordingly, a new legally binding agreement was adopted in December 2015 by the United Nations known as the Paris Climate Change Agreement aiming to reduce the greenhouse gas emissions on a global base [6]. The objective of the net-zero CO2 emissions by 2050–2070 applying to heavy industry as well, including the steel, cement, chemicals and other materials sectors. Under this prism, the circular economy initiatives published by the European Commission in March 2020 [7] can certainly offer successful and realistic decarbonization pathways not only in overcoming strict legislation and financial burden, but also in the development of a sustainable future with considerable social and economic benefits in the upcoming years.

One of the targets of the circular economy action plan is waste recycling and industrial by-products utilization, a practice that cement manufacturing industries has already made progress on either through the implementation of different production processes or as well through the adoption of synergy practices.

A great example of the industrial symbiosis is the steel slags valorization that have attracted the construction sector during the last decades as they have the potential to be used as partial replacement of cement clinker due to their pozzolanic characteristics, but as well as aggregates replacement in the formation of concrete. The latter application field though is more favorable as little, or no processing of the aggregates is required against the use as a binder material where grinding of these by-products is prerequisite and associated with high energy consumptions [3]. Thereafter, different types of coarse and/or fine slag aggregates have been investigated for the manufacturing of eco-friendly concrete which may found use in various construction applications such as asphaltic pavement structures, road bases and surfaces or in other concrete shaped applications, e.g., breakwater blocks and dams.

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, ~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, ferronickel, 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 ferronickel (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 ◦C) of output calcine in electric arc furnaces (EAFs). This process leads to the production of two co-existing materials: the ferronickel 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 slagcontaining 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 [3,14–17]. For example, Manso 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 Chatzopoulos 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). Patra 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. 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 Anastasiou et al. [21] aiming on the maximum valorization of alternative raw materials. Fly ash was incorporated as cement replacement, EAF slag and recycled aggregates from construction demolition 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 lowerdemand 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 Kim 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 Sun 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, Nuruzzaman 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 has been increasing for a long time. Common smelting furnace slag includes steel smelting slag, red mud, copper smelting slag, lead slag, ferronickel 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. Rashad 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. Sato et al. [32] pointed out that freeze–thaw resistance of concrete decreased with the use of 50% ferronickel slag replacing natural coarse aggregate. The reason was attributed to the increase of bleeding and the resulting reduction in air content of concrete with the increase of ferronickel slag. Tomosawa et al. [33] pointed out a deleterious expansion due to the alkali-silica reaction of concrete containing 50% ferronickel slag aggregate and recommended to use low alkali cement to mitigate this expansion. Improved frost resistance of concrete was reported by the partial or full replacement of sand by ferronickel slag aggregate [34,35] showed that ferronickel slag could be used as a suitable replacement of natural aggregates in hot mix asphalt and in base and sub-base layers of roads. It was also reported that ferronickel slag could be used to make composites with waste glass. These composites were described as safe to use as any toxic elements in the slag were chemically locked in the composite [36].

It can be observed from the above literature review that utilization of ferronickel slags from different sources was attempted in different applications. The aim of this study is double. From the one hand is the significant improvement of the sustainability of concrete production by reducing the use of natural aggregates and simultaneously by the increase of use of industrial by-products (i.e., slags) enhances the green policy of industries as well as their financial benefits. On the other hand, by the prediction of the concrete behavior through the tools of petrography and mineralogy the human footprint on the environment reduces. A comprehensive study is necessary in order to understand the properties of concrete using these aggregates. The workability of fresh concrete, physic-mechanical properties of hardened concrete using these aggregates as a supplementary cementing material are evaluated in this study. Thus, this work presents an innovative study on a green concrete using two industrial by-products evaluating the engineering properties.

#### **2. Materials and Methods**

The present study emerged after the constructive cooperation of research institutions with industrial companies having as main goal their interconnection and in order to bring significant results both in constructions and in the wider society through the reuse of slags as high-quality aggregates.
