## 3 Results
### Effect of Weight and Composition of Added Slag  
The first twelve experiments were aimed at understanding the relationship between the weight of the added slag and its neutralization capacity. For such trials, a 0.1 M HCl solution was used. The results of the trials are grouped by slag type and the results are shown in Figure 3. For three slag samples beside slag S2, a 1 g addition was sufficient to neutralize the acidic content of the solution and to buffer the pH to values \(>\) 7 during a sixty-minutes experiment. Moreover, for all the slags, an addition of 2 g buffered the pH value to the final levels higher than the trials performed with 1 g. The reaction rate during the first 10 min is also faster when 2 g additions are used. This is noticeable, for example, by observing the differences in pH values measured at the beginning and the end of the trials. In fact, the gap in pH levels between the 1 g and 2 g addition curves is the widest at the beginning of the trials, but then it shrinks as the curves start to converge. Specifically, the pH levels obtained with slag sample S1 10 min after the start of the reaction are 5.2 and 8.8 for the curves with 1 g and 2 g addition, respectively. On the other hand, the pH level after 90 min is 8.1 when a 1 g addition is used and 9.5 with a 2 g addition. Thus, at the beginning of the trial the pH values are 64% and 93% of the respective final values. The same happens when slag sample O1 is used: 75% of the final pH value is obtained when a 1 g addition is used, while 91% of the final value is reached when a 2 g addition is used in the first 10 min of the trials. For slag sample O2, the same percentages are 59% and 89%. Doubling the weight of slag increased the final pH value by approximately 0.9 in case of slag O1, 1.5 for slag S1 and 0.78 for slag O2. Finally, the trials using 0.5 g additions confirmed that the quantity chosen is insufficient to neutralize the acidic solution up to a pH value of 7. In fact, the minimum quantities of the given O1, O2 and S1 slags needed to reach pH values \(>\) 7 during 90 min, are between 0.5 and 1 g per 100 g of 0.1 M of HCl solution. Figure 3: Neutralization trials of a 0.1 M HCl solution performed with different weights of the given slag samples. Slag sample S2 represents an interesting outlier that is worth examining as a separate case. In fact, despite its Ca content is comparable to other slag samples, especially to slag sample O1, the trial conducted with a 1 g addition of slag only reached a pH value of 4. This is in contrast with the results for the other three slag samples used. When 2 g of the same material was tested, the reaction profile up to minute 80 resembled the results for the other curves obtained with different reactants. Although, after minute 80 a sudden rise of the pH value up to 11, was detected. A retrial with the same quantity was carried out that confirmed the anomaly. In case of sample S2, the trial with 0.5 g was not performed. This is because the trial with 1 g could not provide a complete neutralization of the acid. The slag samples were also grouped based on the trial weight in Figure 4. For this analysis, slag S2 was excluded given its different behavior. Furthermore, the trials using 0.5 g additions since they do not provide a complete neutralization of the acidic content. When a 1 g addition is used, slag O1 is visibly the best reactant: it reaches higher levels of pH and faster compared to the other two slags. When a 2 g addition is used, there is no visible difference between the O1 and S1 samples, which show overlapping curves. However, slag sample O2 reaches a final pH level of approximately pH 1.3 lower than the other two slags (pH 9.6, 9.5 and 8.2 for samples O1, S1 and O2 respectively). Lastly, in the final tranche of the experiments, which used pure CaO as a reactant, the final pH value obtained during 90 min by using 0.25 g was approximately 1.8, and the solution presented no particulate (i.e., unreacted CaO).