## 4 Discussion  
The purpose of this research work is to progress the studies on the use of slag as a reactant for acidic waste waters treatment. This application is of interest especially in cases when the slag composition does not easily allow for a proper recycling in the state-of-the-art applications, namely as part of cement or asphalt mixtures. The same kinetic conditions of previous experiments [15; 16] has been maintained, although the methodology has been improved in order to better study the effect of different mineral compositions on the neutralization performance. The same four slag samples used in the current research were first tested in a first feasibility study [15]. This previous study did not focus on the particle sizes and mineral compositions of the slag samples. In addition, industrial waste waters with unknown compositions with pH values ranging between 1 to 2 were used. Moreover, the study aimed at measuring the amount of slag needed to reach pH 9 \(\pm\) 0.2 by minute 30 by using a method that is highly influenced by the kinetic of the mineral dissolution. The obtained results showed that only slag samples O1 and S1 properly rose the pH to the desired values, and 40 to 50 g of slag per liter of industrial waste waters were needed. The following study by the same authors reduced and homogenized the particle size (\(\leq\)63 \(\upmu\)m) of the slag samples and replicated the same experimental methodology [16]. Contrary to previous findings, in that study even slag O2 (EAF slag) reached the desired pH value, but not slag S2 (AOD slag). The study found that the O1 and S1 slag (landfill slags) samples were the best performing ones, utilizing approximately 20-25 g/L to reach a pH value of 9 in 30 min, whereas slag O2 needed roughly 40 g/L to reach the same target. Thus, the quantity of slag needed was halved due to a decrease in particle size. It should be noted that no possible connections between the pH levels obtained and the mineral composition could be drawn, as the latter was still not assessed. The purpose of the given study is to analyze the differences in mineral compositions between the slag samples and to identify suitable slags for the neutralization of various acidic solutions. To do so, acidic solutions of known composition and controlled pH levels were preferred to industrial acidic waste waters used in previous experiments. Also, the slag particle size distribution was further reduced to diameters between 25 and 50 \(\upmu\)m. Moreover, rather than aiming at the same pH levels between trials, the same weight  
Figure 9: XRD spectra of the residues extracted after the neutralization trials of a 0.1 M HCl solution per-formed with 0.25 g of CaO and 1 g of slag samples S1, O1 and O2. The peaks related with the phase CaCl\({}_{2}\)nH\({}_{2}\)O are highlighted by dots at their respective peaks. values of acidic solutions and slag samples were used in the experimental trials, as it was deemed to be more controllable parameters. Therefore, a reliable comparison between pH levels obtained in this and previous study [15; 16] was not possible to obtain. Nonetheless, although the current study did not aim at obtaining a pH value of 9.0 \(\pm\) 0.2 at minute 30, in the trials in the present research performed with 2 g additions (20 g/L) of slag O1 and S1 the pH levels reached were 9.3 and 9.2 at the 30 th minute, respectively. When 2 g of slag O2 were added, the pH value was 7.8 at minute 30. These results seem to be in line with the ones found in a previous study [16], where around 20-25 g/L of slag samples O1 and S1 were needed to reach the desired pH values. Also, this is an additional confirmation that slag O2 performs worse than the other studied samples, due to its mineral composition. Regarding the slag sample S2, no quantitative comparison with previous experiments can be done [15; 16], since successful neutralizations were never performed with such material before.