## 4 Discussion
### Dissolution of Slag O2  
Slag O2 (EAF slag) is the third best performing slag based on the final pH levels obtained per unit of weight, despite the fact that it has the lowest total of Ca (27.4%) and Mg (4.8%) contents. When 1 g is used, the slag barely surpasses neutral pH values. Furthermore, when a 2 g addition is used, the final pH values are comparable with the pH values obtained using 1 g additions of slags S1 and O1. In fact, the final pH values reached with 1 g additions of slags O1 and S1 are 8.7 and 8.1 respectively. When a 2 g addition of slag O2 is added, a final pH value of 8.3 is obtained. Contrary to the composition obtained by using slags O1 and S1, CaCl\({}_{2}\)-nH\({}_{2}\)O is not the most abundant phase present in residues obtained when using slag O2. In fact, as shown in Figure 9, the peaks associated with CaCl\({}_{2}\)-nH\({}_{2}\)O only partially describe the XRD pattern of the residues obtained in the neutralization trial. Also, according to Figure 8 the abundance of the phase decreases in the 2 g trials in favor of the other phases. This is a common observation for slags S1 and O1 because the amount of Cl\({}^{2+}\) ions dissolved is the same in both trials, as the concentration of the acidic solution is the same. Thus, when the evaporation of the liquid phase occurs,the same amount of CaCl\({}_{2}\):nH\({}_{2}\)O is formed when 1 g and 2 g additions are used. By using 1 g more gram of slag, the CaCl\({}_{2}\):nH\({}_{2}\)O ratio diminishes. At the same time, the ratio of the other phases increases, and they become more visible in the XRD spectrum. In the trials performed with slag O2 it is particularly interesting to notice this phenomenon, because it provides information about what minerals are present when only a partial dissolution takes place. By comparing the results of the XRD analysis performed on the residues with the original slag composition and single mineral XRD patterns, almost all the peaks have been assigned to either merwinite, akermanite or CaCl\({}_{2}\):nH\({}_{2}\)O. In Figure 10, the XRD spectra of both residues obtained after the neutralization with slag O2 are analyzed. Both spectra present the same phases, but the ratio differs as beforementioned. By considering only the intensity of the peaks, and comparing the results to each other, akermanite (4 CaO-1.5 MgO-0.5 Al\({}_{2}\)O\({}_{3}\):3.5 SiO\({}_{2}\), cyan circle mark) is the most abundant phase in both cases. From roughly 47% of the total sample in the 1 g trial it rises to 54% in the 2 g trial. Merwinite (3 CaO-MgO-2 SiO\({}_{2}\), yellow star mark) increases from 9% to 27%. Conversely, the CaCl\({}_{2}\):nH\({}_{2}\)O content (magenta diamond mark) drop from 28% to 10% of the total sample. Therefore, this is evidence that contrary to the results for the other minerals, such as the ones present in slag sample O1, merwinite and akermanite do not fully dissolve in the current experimental conditions. Given the low content % of merwinite in the 1 g trial and its initial % in the slag sample (~40%), merwinite seems to dissolve more than akermanite. This is in line with previous findings [17], where a very different dissolution behaviors were found for the minerals tested and the pH levels of the solvent. Both merwinite and akermanite show high dissolution levels and a fast dissolution rate at a pH value of 4. The dissolution rate at a pH value of 7 drops substantially, especially for akermanite. However, it is still completely dissolved by the end of the trial. At a pH value of 10, the solubility drops significantly. Dicalcium silicate \(\gamma\) instead shows similar dissolution levels as merwinite and akermanite only at a pH value of 4. Contrary to the other two, at a pH value of 7 the mineral dissolves with no visible difference compared to a pH value of 4. However, at a pH 10 the value of the dissolution level decreases. Although, it maintains a fast dissolution rate. In other studies, akermanite is also shown to be less soluble, compared to other minerals like bedigite in a Tri-HCl solution of pH 7.4 [27].