## 3 Results
### Leaching Test  
Several studies on heavy metals immobilization suggested thermal treatments based on sintering or vitrification as the best way to obtain an environmentally stable waste [42; 43]. Glass-ceramics, being fine-grained polycrystalline material with a glassy phase, in the recent years, are referred to a metal immobilizer with the advantage of add-value marketable products [28; 44].  
Nowadays, In Italy, there is not a suitable test to verify the leaching onto \"ceramic tiles\" made using alternative raw materials (in this case waste). For this reason, choosing the most unfavorable case, it was selected the methodology following UNI 10802(EN12457, 2002) specific for residues. The results of this test permits to classify the waste as inert, not hazardous, or hazardous following the Italian Normative [45]. Despite this, the test is indicative to understand the danger in using waste in a new process or in a new product. The ceramic sludge (used as secondary raw material) contains small amounts of heavy  
Figure 8: Micrograph and EDS spectra relative to V125 at 1000 \({}^{\circ}\)C, 60 min. W = wollastonite, A = albite, Q = quartz.  
metals such as Pb, Cu, Cr, and Ba (derived from frits and pigments). To evaluate if, after thermal treatment, the materials obtained are inert and suitable for use, an investigation of hazardous substance mobility was performed on the fired samples. Metals leachability has been evaluated in both the eluates coming from received sludge and fired materials containing the sludge. The metals content detected in the eluates was compared. For this analysis V125 and V75 samples after firing at 1000 \({}^{\circ}\)C, 60 min were considered. The results were compared to the eluates concentration limits for acceptability in landfills for inert waste. The data obtained is reported in Table 4.  
Good results were obtained from both pH and specific conductivity measures; pH values for the \"end products\" are within the rule range (5.5-12.0). The increase with respect to the sludge probably is due to the presence of alkaline and alkaline-earth ions release from the glass cullet present in the mixture. On the contrary the conductivity values are lower than the as-received sludge. These data confirmed that the residual glassy phase present into the sintered materials acts immobilizing the ions embedded in the matrix showing a lower release with respect to the sludge's sample.  
Regarding metals eluate, the ceramic sludge shows a high release of some heavy metals, in particular, lead (Pb) value surpasses the regulation limit. The explanation lies in the fact that sludge is a mixture of materials with very different chemical nature as frits, ceramics and other components (pigments, oxides) that are not capable of binding heavy metals that can be released.  
Instead, it is possible to observe that V125 and V75 densified samples show significantly reduced values compared to the as received sludge, indeed, the mobility of metals like Pb, Cu, Cr and anions as chlorine, fluorine and sulphates are strongly reduced (Pb 50-150 times; Cr 20 times,... ) and shows values below Italian regulation limits demonstrating that the obtained materials are inert without provoking further environmental impact.