## 4 Discussion
### Dissolution of Slag S2  
Slag S2 has been constantly an outlier in all the experiments performed. Contrary to the results from all the other slag samples, 1 g of slag is not enough to neutralize the acid content of the solution. This is quite surprising, since the slags have high concentrations of Ca (32%) and Mg (\(\sim\)3.8%). Moreover, dicalcium silicate \(\gamma\) makes up roughly 50% of the slag composition. This mineral is also abundant in slag type O1 (46.3%) and a S1 (19.9%). In addition, just like dicalcium silicate \(\gamma\), mserine (12 CaO-7 Al2O3) has been reported to have good hydraulic properties [28; 29] and high solubilities even at high pH values [4]. Therefore, it is expected that the slag should show similar properties, if not better ones, than the previous slags. Instead, only when 2 g of slag are used, the material performs similarly to the other slag samples such as O1 and S1. The final pH value obtained when using a 2 g addition of slag S2 is approximately around 11, whereas additions of 2 g of the other slags could only rise the pH values between 8 and 9.6. Also, the final pH level is not reached gradually, but through a sharp increase around the end of the trial. This phenomenon happened also in the replication trial using the same quantity. Slag S2 is the only one containing mayenite, which is a mineral rich in Al. Al is the second most abundant element in slag sample S2, around 12% in weight, compared to a 2-3% content present in the other slags. As mentioned beforehand, Al is believed to counteract the rise of the pH value provided by elements such as Ca and Mg because its stable phases produce protons when being dissolved. This could explain why the pH values are so low compared to the ones obtained when the same weight of a different slag sample is used. The sharp rise in the pH value also is an interesting phenomenon worth exploring. According to the Purbaix diagram calculated in previous studies [21], after pH value 9 is reached, Al(OH)should further hydrate, which would generate a proton. In theory this should decrease the pH value, but the opposite happens. No explanation of this phenomenon could be found, but it is believed to be connected to the phase transition of the Al species.  
The XRD patterns of the residues obtained with slag S2 are completely different from the ones obtained when using the other three slags. Contrary to slag O2, that partially retained some mineral phases from the original sample, no traces of mayenite or dicalcium silicate \(\gamma\) can be found. However, contrary to slags O1 and S1, there is no formation of CaCl\({}_{2}\)-nH\({}_{2}\)O. Therefore, SEM-EDS imaging was used to understand the composition of the residues. The analyses show an abundant presence of small particles with sizes \(<1\)\(\upmu\)m (points 8,7,10,12 in Figure 11). Given the very small size, these particles are considered to be precipitated compounds during the evaporation of the liquid phase. In fact, the slag samples have all been sieved to reach a size range of 25-50 \(\upmu\)m. Thus, the presence of such a high number of very fine particles is otherwise unjustified. The fine particles contain Ca, O, Al and Cl as the main constituents. This explains why there is no trace of CaCl\({}_{2}\)-nH\({}_{2}\)O: the presence of dissolved Al bounds with Ca, Cl and O during the precipitation. Also, no traces of mayenite suggests that the mineral is the original mineral structure is completely dissolved and all the aluminum is dissolved in the acid.