## 1 Introduction  
Human influence on the environment has been so strong in the latest centuries that the label for a new geological era, the \"Anthropocene\", has been used to describe the period following the invention of the steam engine in 1784 [1]. More conceptual work followed, refining the idea of the current unsustainability in the manufacturing sector, by calculating the limits to anthropogenic actions, defining nine planetary boundaries: indexes describe the thresholds above which the production of goods is no longer sustainable and can lead to changes in the environment as we know it [2]. Moreover, the European Union identified the generation of waste as a crucial threat to sustainability, which lead policy makers into streamlining a common list of waste management operations, from most to least desirable [3]. This directive is known as the European Waste Hierarchy (EWH), and it holds its foundation on the pioneer work of [PERSON], started in 1979 [4]. According to this set of policies,landfilling is the least desirable option, and it should be avoided as much as possible. The need for proper waste management, for any manufacturing business, calls for immediate solutions in sectors where landfilling is still the primary option. It is with this context in mind that this article was written. The use of slag, to buffer and increase the pH of on-site produced acid wastewater to benefit a circular economy and by-product valorization, was studied. In this study, the overall objective was to investigate whether slag is an effective replacement for slaked lime, since according to the principles of dematerialization this would allow the decoupling from raw materials and reduce also the material flow to landfills [5]. The stainless-steel production processes of two steel mills in Sweden, Outokumpu Stainless (OTK) and Sandvik Materials Technology (SMT), were used as case studies. Both companies produce high-alloyed slags which, in the most part, are landfilled. In this study, it was investigated if it was possible to re-use landfill slag to neutralize acidic wastewater at the respective steel mill. A total of two slag types from each steel mill were selected and evaluated for their ability to neutralize acidic wastewater produced at the steel mills. The results were also compared with the neutralization performed with the lime products currently used at the steel mills. Besides the ability to raise the pH to desired value, the slag should also be able to remove metals from the wastewater. Such a property was tested in additional trials. In Sweden, almost 2000 kt of by-products were produced by iron and steelmaking businesses in 2015 alone [6]. Currently, more than 80% of the total amount of iron and steelmaking residues are valorized. However, specific sectors of steelmaking are struggling more than others to find an adequate solution, as shown in Figure 1. In most of the sectors listed, the handling of these products, either by internal or external use, almost completely matches the produced amount. Despite this fact, there are some critical types of by-products that find little no to application and are currently landfilled. Slag derived from the production of high-alloyed electric arc furnace (EAF) stainless-steel, argon oxygen decarburization (AOD) slag, and ladle slag currently suffer from the incapability of being properly valorized. By comparing Figure 1 with Figure 2, it is clear that the reason why the handling of these products is hindered is their alloying elements content. In fact, the most common use for iron and steelmaking slag is the cement and asphalt industry [7]. However, when the materials present high concentrations of metallic or heavy elements, leaching can occur, which impedes the use of these materials for applications where they are subject to weathering [8]. Moreover, EAF slags present phenomena of swelling and disintegration that further impede their use in these types of applications, e.g., asphalt [9].