Evaluation of Electric Arc Furnace Oxidizing Slag Aggregates Quality and Development of Functional Concrete

Introduction

Slag, which is a by-product inevitably generated during steel production, is recycled in an eco-friendly manner by the steel industry. Blast furnace slag is a slag which is generated during manufacturing of steel from magnetite in the blast furnace. Blast furnace slag is utilized as a concrete aggregate for embankments and roads (Faraone et al., 2009) as well as a substitute for raw materials in cement manufacture.

As the amount of slag generated annually increases, the possibility of recycling it as a high value product needs to be studied in order to utilize it efficiently in recycling industry and create economic benefits (Fernandez et al., 2007; Sheen et al., 2015; Sturm et al., 2009).

Blast furnace slag is actively developed and supplied as concrete aggregates and substitute for cement in concrete (Onoue et al., 2014; Motz et al., 2001). On the other hand, steel slag shows the problem of expansion and destruction during the hydration of cement in concrete due to unstable compounds present in it. This expansion and destruction during cement hydration occurs because of free CaO content present in it (Kuo and Shu, 2014; Wang, 2010). However, free CaO can be generated in steel slag during plasticity process. Free CaO content can vary according to temperature for plasticity in steelworks as well as cooling conditions. The melted slag is cooled slowly, aged in storage for approximately 3 months to a year to prevent expansion and destruction and is used as roadbed material.

In a study in Italy by Faleschini et al., 2016, some results of an experimental campaign about the potential use of EAF slag in cement-based materials were reported. The aim was to provide some insights about the important characteristics of the slag and its compatibility in concrete. Particularly, a mechanical characterisation was performed on compressive strength, tensile strength and elastic modulus showing the effects of substituting siliceous aggregates with slag. The performances were significantly enhanced in all cases. From that work, it was suggested that those improvements were gained both by a higher quality of the slag aggregates (in terms of density and strength), and by an improvement of the bond between EAF slag and the cementitious matrix. Similar results were also obtained recently by other authors, who obtained a singular morphology of the ITZ when EAF slag was used, which enhanced concrete mechanical properties.

The positive results obtained in terms of mechanical strength allowed designing concrete mixtures with the desired strength and workability leading to a significantly reduced environmental impact.

This process requires a wide storage yard and additional relevant costs. Electric arc furnace (EAF)-oxidizing slag (EOS) was studied. This slag was evaluated as an aggregate that does not pose problems pertaining to chemical expansion safety based on the Korean Industrial Standard (KS). However, the practical application of EOS aggregates was not implemented due to unstable property of material itself, for the product found in South Korea.

This study examined the physical and chemical properties of EOS and quantitatively evaluated the free CaO content in it and EAF-reducing slag (ERS), according to the period and location of aging using ethylene glycol. Moreover, the EOS aggregates were replaced depending on the EOS content to examine the physical properties of concrete and verify its quality characteristics as construction materials.