## 5 Conclusions  
This study aimed at analyzing the effect of composition of different stainless-steel slags, regarding their ability to increase the pH values of standard acidic solution. The same slag samples used in previous studies conducted by the same authors are also used in the current publication. Although, the methodology developed in the current study is different than the one used in previous experimental trials. The current study used a fixed amount of slag additions (0.5, 1 and 2 g), and track the different pH values obtained, whereas previous experiments focused on reaching the same pH value, with a ranging weight based on the different slags used. Furthermore, industrial acidic waste waters have been replaced with 0.1 M HCl/HNO\({}_{3}\) solutions, and the quantity of solution to neutralize has been reduced from 1 L to 100 mL. Finally, the reacted slags and their leachates were extracted by drying the neutralized solutions. The residues were analyzed with XRD and SEM spectroscopies to determine their composition and infer the solubility of the original slag minerals. Figure 11: SEM-EDS image of the residues extracted after the neutralization trials performed with slag sample S2. Point analysis was used to determine the composition of the different phases present. The tested slags samples successfully rose the pH values to basic values of the 0.1 M monoprotic acid solutions (HCl or HNO\({}_{3}\)) prepared in laboratory trials. The results of this study further strengthen the case for the use of stainless-steel slags for waste waters treatment. However, compared to commonly used materials such CaO, all the slags tested dissolved with a slower rate, showed a lower solubility and reached lower final pH values. Also, slag requires roughly 3 times the weight of lime to reach similar pH values. Nonetheless, for processes characterized by high retention times and where a high basicity is not required, Al-rich slags can be successfully be employed. When the weight and particle size of the slag sample, as well as the acidic environment are controlled, the composition of the slag is shown to heavily influence their performance as reactants. Although, the bulk chemical composition by itself is not able to precisely predict the performance of the material by itself. Similar percentages of the most abundant elements in the slag samples often lead to different results when analyzing the obtained pH values. In these cases, the mineralogical compositions of slags are more important parameters to control. Specifically, the different solubilities of the mineral phases in water media are the parameters controlling the choices of the slag for such use. Minerals with good hydraulic properties such as dicalcium silicate \(\gamma\) or hedigite are favored for such applications. At the same time, the same minerals are often problematic for other applications, because of their tendency to leach. On the contrary, minerals such as akermanite and merwinite are less soluble, which impact the final pH levels obtained by unit of weight. This study also found that high percentages of Al are detrimental to a successful neutralization as they generate hydroxides that dissociate in free protons, which decreases the pH value of the solution. As a result, the same amount of Al-rich slag (S2), that was successfully used for the other samples to rise the pH level, was not sufficient to neutralize the acidic solutions. However, when the quantity of slag is adjusted accordingly, the final pH value is higher (11.0) than the one obtained with the other slags (8.2-9.6). Therefore, when high basicity is needed, rather than weight efficiency, Al slags can be successfully employed for the neutralization of wastewaters. Conceptualization, M.D.C.; methodology, M.D.C., R.K. and A.K. ([PERSON]); formal analysis, M.D.C., R.K. and A.K. ([PERSON]); investigation, M.D.C.; resources, M.D.C.; data curation, M.D.C. and R.K.; writing--original draft preparation, M.D.C.; writing--review and editing, A.K. ([PERSON]) and P.G.J.