# Utilization of Industrial Ferronickel Slags as Recycled Concrete Aggregates
## 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 CO\({}_{2}\) levels, ~8% of the global anthropogenic emissions [4], mainly associated with the high calcination temperature (~1450 \({}^{\circ}\)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% ofits 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 CO\({}_{2}\) 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.