## 2 Materials and Methods
### pH Buffering Trials  
After assessing both the mineral and chemical compositions of the slag samples, the focus shifted towards testing a reliable method that could be used to compare each sample with respect to its neutralization capacity. All slag samples were used as reactants for the neutralization of different acidic solutions of known pH values. Two HCl and HNO\({}_{3}\) 0.1 molar (M) solutions were prepared, by mixing commercial grade chemicals (Hydrochloric acid 1 M and a solution of 65 wt% Nitric Acid) with distilled water. In every neutralization trial, 100 g of acidic solution were weighted and used as a standard quantity to be neutralized. The quantity chosen for measuring the amount of the solution was its weight, rather than its volume. This is because the former can be measured more precisely than the latter in laboratory conditions. The molar concentration of the acidic solution, corresponding to its pH levels, was chosen in accordance with the pH levels of common industrial acidic waste waters used in previous experiments [15; 16]. Since the pH values of the acidic waste waters varied in a range between 1 and 2, a pH value of 1 was chosen for the HCl and HNO\({}_{3}\) solutions. The experimental procedure was also replicated from previous experiments [15; 16]. The trials were conducted by using a VWR pHenomenal IS 2100 L a pH-meter mounting a sensor with Pt wire junction, and a liquid electrolyte (Phenomenal LS 221). Before each trial, the pH meter was calibrated by using three standard buffers of known pH values of 4, 7 and 10. The neutralization method selected for the experimental trials was performed as follows:  
1. The initial pH value of the acidic solution was measured (pH\({}_{0}\)). 2. The beaker with 100 g of the acidic solution was placed on a magnetic stirrer, a magnet was placed inside the beaker, and the stirring started using a speed of 480 rpm. 3. All the reactant was dropped into the solution at time t = 0. 4. After that, the pH was measured at intervals of 10 min, for 90 min in total (pH\({}_{10}\), pH\({}_{20}\),..., pH\({}_{90}\)). To do so, 30 s before the measurement the stirring was stopped so the suspension could precipitate at the bottom of the beaker. After 30 s, the pH electrode was inserted in the beaker and the pH level was measured. The value was determined once a stable reading was obtained, or after 1 min from the insertion of the electrode. 5. After the reading was taken, the pH electrode was removed from the beaker. Successively, the stirring started again with a speed of 480 rpm. 6. The trial was stopped after 90 min. Thereafter, the beaker was sealed with PARAFILM(r) M until drying occurred. In total 18 trials with this method were conducted using different slag types, slag weights and acidic environments depending on the scope of the investigation. A first tranche of trials tested each slag three times, by using different weights of slag. During these 12 trials a 0.1 M HCl solution was used. To facilitate the comparison between different slags and highlight the effect of composition, for the current study three fixed weight values of 0.5, 1 and 2 g of slag were used. The only exception was applied to slag type S2, where the trial using 0.5 g additions was replaced with a replication of the trial using a 2 g addition of slag. A second tranche of trials tested additions of 1 g of each slag into a 0.1 M HNO\({}_{3}\) solution. The last tranche of two trials used a standard grade CaO powder as a reactant. Specifically, additions of 0.25 g and 0.5 g of CaO were tested against a 0.1 M HCl solution. The weight values of lime were chosen by performing simple chemical calculations. The reactions between the acids and CaO, ideally follow Equations (8) and (9). Since the stoichiometry is identical for both reactions, it is possible to derive that 0.28 g of ideal CaO is needed to rise the pH value of an ideal 0.1 M solution of monoprotic acid (HCl or HNO\({}_{3}\)) up to exactly 7.