## 3 Results and Discussion  
In order to understand the effects of the addition of different SS amounts to the three combustion lines, all starting ashes (FA and BA) and the obtained stabilized eco-materials were analyzed. Elemental chemical analysis using TXRF was performed to estimate the amount of leachable metals that can be found in the ashes. As reported in the experimental section, as-received FAs were analyzed as well; BA samples were pre-treated before the analysis with a reduction of the grains dimension to 106 \(\upmu\)m. Table 2 reports the results of the leaching tests made on these ashes (BA and FA). It shows the concentration of soluble elements derived from the three combustion lines, as resulted from the TXRF analysis. Furthermore, data about BA leaching are shown. The leaching data of the BA and FA derived from different lines are in agreement with results reported in literature, also considering the alkaline pH of the solutions [23; 24; 25]. The major leachable elements in FA are Cl, Ca, K, S and Br. Relevant quantities of other elements such as Zn, Pb and Rb are also found. The main soluble elements found in the BA solutions are Ca, Cl, K and S. The presence of a high amount of Ca in FA in comparison to BA is expected, due to the addition of lime as a stabilizing agent. Indeed, during the incineration process, acid gases are produced (like HCl and SO2) and lime is normally added to absorb these gases. A higher amount of Pb and Zn are present in FA, in comparison to BA, due to the fact that these metals are moderately volatile elements, then they are generally found in ashes collected at the chimney after a combustion process carried out at a maximum temperature about 1000 \({}^{\circ}\)C [15]. Comparing the leaching data of different combustion lines, it is very interesting to notice that in FA, the concentration of leachable Zn is substantially irrespective of the amount of co-incinerated SS. On the contrary, the amount of soluble Pb increases with the amount of SS, probably due to the presence of this metal in sewage sludge [26]. Concerning P, an element that is well-known to be abundant in SS [27], its concentration appears to decrease in FA with the increase of co-incinated SS. On the contrary, in BA the leachable P shows an inverse behavior. These results are substantially due to the presence of this element in soluble phases (leaching tests only allow to detect soluble elements). Moreover, it means that the increase of SS amount in co-incineration seems to increase the P leachability in BA (see Table 2). Similar consideration can be extended to Cu, for BA. After the stabilization procedure, a significant decrease in heavy metal reachability can be noticed, if samples are analyzed one, two, and three months after the stabilization (see Tables 3 and 4), in comparison to data reported in Table 2. In particular, three months after the stabilization the concentration of Pb is sometimes lower than LOD by TXRF spectrometry, which is 0.002 mg/L, thus demonstrates that the stabilization procedure was effective in reducing the solubility of heavy metals and that the stabilization efficacy increases with time. Moreover, comparing data reported in Tables 3 and 4 (considering the corresponding month of stabilization), it seems that the use of BA is not so fundamental in reducing the heavy metal mobility. Indeed, eco-materials obtained applying the procedures a) and b) (the procedure b) is made without the addition of BA) sometimes show comparable Pb concentrations in their leaching solutions. Another difference concerns the Cu concentration, that is higher in samples treated by BA addition. Even if this metal appears to be stabilized after aging, it is evident that this origin can be attributed to BA due to its higher concentrations in the BA than FA (see Table 2).