# Acoustic Characterization of Some Steel Industry Waste Materials
## 1 Introduction  
The steel industry is one of the main global economic sectors providing raw materials for a wide variety of manufacturing processes. During the various activities, a steel plant produces large amounts of waste under different forms. In recent decades, this type of industry is also trying to gradually leave the linear economy model and aim for the global and ambitious \"zero waste\" target [1], focusing its efforts on the development of innovative and sustainable production schemes. The emerging principle of the circular economy supports the reuse and recycling of industrial by-products, creating a symbiosis [2] which encourages collaboration and synergy with different sectors. The final goal is to develop new business opportunities through the conversion of waste into valuable raw materials or secondary materials exploitable in other sectors [3]. These activities allow the steel industry to reduce its environmental impacts: indeed, the reduction in waste materials can be ensured by providing an alternative solution to safe and environmentally friendly disposal of polluting industrial wastes and by avoiding the extraction of new natural resources. In this way, it is possible to achieve both environmental and economic benefits for all the industries involved in the symbiosis. The main waste product of the steel industry is represented by slags. There are different types of slag, depending on the type of furnaces, raw materials and process adopted during production. The slag deriving from melting the scrap iron by an electric arc furnace (EAF--Figure 1a) is generically defined as \"black slag\". Such a type of waste material results from the oxidation of the scrap and includes impurities and compoundsgenerated by the additives used to control the chemical processes. These elements form a layer that floats on top of the molten steel in the furnace, insulating the liquid part from the external environment and helping maintain the temperature inside the furnace at the right set-point. At the end of the process, the floating layer is collected and cooled down, resulting in \"black slag\". The melted steel is then processed and refined in a ladle furnace (LF--Figure 1b). The slag deriving from this process is defined as \"white dress\", or \"white slag\", and has completely different chemical and physical properties if compared to the black slag. The two slag types are kept separate as they have a different chemical composition and must be treated differently. Black slag can be assimilated to natural effusive rocks of volcanic origin, takes on granular characteristics and mainly consists of a ternary mixture of calcium oxide (CaO), silicon dioxide (SiO\({}_{2}\)) and iron oxides (FeO), to which heavy metals and other components, in percentages, are mixed. The white dress chemically differs from the black slag, particularly for the content in iron oxides and calcium; therefore, this dress, after cooling, undergoes a transformation of the crystalline lattice which leads to the formation of a fine and lightweight material. Some studies are actually investigating the physical properties and the environmental compatibility of these materials, to establish the environmental impact of the slags and how to treat them. Depending on the application field, several studies have been carried out to characterize steel slags. One of the first practical applications of steel slags outside the steel production cycle is as sustainable (alternative) aggregates in pavement layers for road construction. They have been used not only for unbound layers, like road bases and sub-bases, but also for bituminous mixtures in surface layers [4]. In [5], the EAF steel slag was preliminarily investigated from chemical, leaching, physical and mechanical points of view. The bituminous conglomerates have also been characterized to verify their potential application in high performance asphalt concretes for road and airport pavements.