# Acoustic Characterization of Some Steel Industry Waste Materials
## 1 Introduction
The comparison with the corresponding traditional natural aggregates shows that using slags as coarse under-pavement material brings both technical and environmental advantages [5; 6]; the mixtures with EAF slag improve the mechanical properties and prevent the depletion of raw materials. Several studies also investigated the suitability of steel slags for civil engineering applications in cement-based materials. In particular, they were used for replacing natural sand [7] in the production of concrete [8; 9; 10; 11; 12; 13], as armor stones for hydraulic engineering constructions [14; 15] (during the restoration of marine environments and stabilization of shores), and finally, as an agricultural fertilizer. In [2], steel slags were investigated as to help  
Figure 1: Schematic drawings of EAF and LF furnaces: (**a**) EAF furnace; and (**b**) ladle furnace. in the removal of harmful elements and wastewater treatment. More recently, slags have been used as green resource in ceramic tile production and for biomedical applications. The development of innovative sustainable solutions, by means of already existing or new technologies, is a goal that the steel industry is willing to pursue in order to further reduce its environmental impacts. Recently, some critical environmental aspects emerged regarding the use of steel slags [2; 14]. The concerns are about volume instability and leaching behavior, the latter being a crucial aspect for environmental considerations, especially in terms of possible water and soil pollution caused by the release of heavy metals. Such aspects must be deeply investigated and solved, depending on the characteristics of the specific chemical composition of the recycled slag and the exposure to atmospheric elements. The study reported in [16] aimed to find an intertization process for the recovery of steelwork slags and granite cutting waste as raw materials for the production of rockwood, which is a good thermal insulator and acoustic absorber for the construction and automotive sectors. It was found that the partial replacement of traditional raw materials does not influence the thermal insulation and fireproof properties of rockwood. In [17; 18], a recent improvement consisting of a new production method was introduced: high-pressure cold air is passed through the molten slag and the result is a material consisting of slag granules characterized by an almost spherical shape. The studies provide a comprehensive experimental characterization, in terms of fundamental and durability properties. The outcomes of both studies confirmed that a fine aggregate of spheric slags is a promising and advantageous alternative to natural sand in concrete pavement, also in terms of workability, water content and cement mechanical requirements. Granular materials are emerging as an interesting alternative to the more popular and conventional sound absorbers. This trend is also encouraged by the large amount of industrial waste or by-products available in granular shape. If properly treated, these materials could become a valuable \"second raw\" resource, instead of using them as waste material for landfill, with all the related costs in terms of money and environmental impact. In this way, these materials can re-enter the production cycle and can be addressed in different application fields. Of course, this depends on how much their properties and potentialities are investigated and optimized. In this perspective, the steel industry is continuing its path towards the \"zero waste\" and circular economy goals by funding studies on the waste reuse and sustainable recycling and developing new technological solutions in an effort to find new fields of application. For instance, the traditional mineral wool production process can be applied to steelwork slag: by means of spin dryers and a high-speed air flow, the white dress molten slag forms long fibers and a sort of wool that could represent a partial or complete substitute to the traditional rockwood.