## 3 Results and Discussion
Zn and Pb are found in a high concentration in the raw FA (ranging from 9-11.7 and 92-127.1 mg/L, respectively) with high pH values (about 12), while in the stabilized samples the concentration of these elements is found to be often two orders of magnitude lower. Figure 2 clearly highlights that an increase of the aging time corresponds to a decrease of pH of the solutions [32], in agreement with the already reported evidence of carbonation. This also corresponds to a decrease of the heavy metal leachability, as already observed and discussed [4; 33]. In particular, concerning Pb, three months after stabilization it is sometimes lower than LOD by TXRF spectrometry, as reported in Tables 3 and 4, therefore it cannot be found in Figure 2.  
To better highlight the pH role, the variation of the concentration of Pb and Zn in the leaching solutions of stabilized samples can be considered analyzing data reported in Figure 3. In this Figure, the concentration of Pb and Zn in the solutions of stabilized samples (considering both procedures a) and b)), are plotted versus their values in the leaching solution of raw FA (initial metals concentrations, before the stabilization). Figure 3 allows for the comparison of the results of stabilization, considering that raw FA (corresponding to different combustion lines) had a different amount of leachable Pb and Zn. In particular, it is evident that all samples show a reduction of the Pb and Zn concentration, after the stabilization. Obviously, samples that contained a lower amount of leachable heavy metals in raw FA show a lower concentration of corresponding metals in the solutions of stabilized eco-materials. This means that samples with higher initial concentrations of Pb and Zn need more time (or more stabilizing agents) to reduce the leachable elements concentration. Figure 2: Concentration values of Pb and Zn in the leaching solution of stabilized samples (involving both procedures a) and b)) during the first three months versus the pH. The role of BA in the stabilization can be highlighted by analyzing data reported in Figure 4, that show the concentration of Pb and Zn during the first three months evaluated in the leaching solutions of all stabilized samples (considering both procedures a) and b)), versus the ratio between the total amount of ash (FA, CFA, FGD, and BA) and the sum of CFA and BA amount. It means that the samples that contain the higher CFA + BA amount are better stabilized in comparison to those containing a lower amount of these stabilization agents (the concentration of Pb and Zn are lower). Indeed, it is important to highlight that BA contains amorphous silica [4] and CFA contains aluminosilicate glass [13; 34]. Then, it is possible to suppose that dissolved amorphous silica in the presence of calcium ions (and in a highly alkaline environment) promotes a pozzolanic reaction with FA. Then, it is possible to conclude that BA plays a fundamental role in reducing the Pb and Zn presence in solution of the produced eco-materials. Obviously, it was also shown that the pH role is fundamental, and it must be controlled and possibly adjusted to obtain the best results in terms of stability of the obtained eco-materials. Figure 4: Concentration values of Pb and Zn in the leaching of stabilized samples (involving procedure a)) during the first three months versus the partition of total amount of ash and the sum of CFA and BA quantities. Figure 3: Concentration values of Pb and Zn in the leaching solution of stabilized samples (involving both procedures a) and b)) during the first three months versus the concentration of Pb and Zn in MSWI FA. The obtained stabilized eco-material is a powder that is very similar to the inert material obtained by using similar treatments but using other by-products (such as silica fume) instead of BA for stabilization. This allows for us to suppose that the obtained eco-materials may be used in some applications, already explored for similar products, as a substitute of natural resources [10; 33; 35; 36].