## 4 Conclusions  
This study suggests the feasibility to obtain eco-compatible building materials for covering floor or wall using a mixture of ceramic sludge and packaging glass cullet.  
As known a huge amount of natural raw materials (90 Gigatonnes/y) is used to produce construction materials worldwide. Considering the high production of ceramic materials and the demonstrated capacity of the ceramic matrix to fix metals, this way permits to use silicate wastes by transformation into useful ceramic tiles products.  
In fact, the leaching tests demonstrated that the sintering process at temperatures around 1000 \({}^{\circ}\)C permits the formation of a glass-ceramic matrix that acts to immobilize the metals ions. All the measured parameters are lower that the Italian regulation limits confirming the obtainment of inert materials.  
The proposed process using sludge that acts as fluxing materials permits to fire at lower temperatures (<200 \({}^{\circ}\)C) than traditional cycles used for commercial ceramic tiles products. Concerning the sintering properties, the end products fired at temperatures around 1000 \({}^{\circ}\)C, comparable WA% and LS% values with respect to traditional wall ceramic  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Industrial** & **Water Absorption** & **Linear Srinkage** & **Apparent Density** & **Young’s Modulus** & **Flexural Strength** \\
**Thermal** & **W.A.\%** & **L.S.\%** & **A.D. (kg/m\({}^{3}\))** & **E(GPa)** & **(MPa)** \\ \hline (960 \({}^{\circ}\)C, 50 min.) & 8.2 \(\div\) 11.0 & 5.55 \(\div\) 6.25 & 1908 \(\div\) 0.006 & 21.68 \(\div\) 31.14 & 26. 23 \(\div\) 37.67 \\ (1020 \({}^{\circ}\)C, 54 min.) & 6.6 \(\div\) 8.6 & 6.72 \(\div\) 7.42 & 2025 \(\pm\) 0.005 & 34.51 \(\div\) 37.54 & 41.76 \(\div\) 45.42 \\ (1087 \({}^{\circ}\)C, 41 min.) & 0.04 \(\div\) 0.09 & 11.34 \(\div\) 11.72 & 2281 \(\pm\) 0.007 & 57.03 \(\div\) 57.63 & 69.00 \(\div\) 69.80 \\ Commercial & Bla \(\leq\) 0.5 & & & & \(\geq\)35 \\ products & Blb \(>\) 0.5 \(\div\) 3 & 7.0 \(\div\) 8.0 & 2000 \(\div\) 2400 & 20 \(\div\) 60 & \(\geq\)30 \\ (1100–1250 \({}^{\circ}\)C) & BIIa \(>\) 3 \(\div\) 6 & & & \(\geq\)22 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Sintering parameters, elastic and mechanical properties for samples fired by industrial cycles.  
tiles. Besides, they showed flexural strength values higher than the minimum required by European Standard norm (ISO 13006) for commercial tiles already on the market.  
Several advantages give to this research an important environmental footprint in the sustainability field: (a) Avoiding the landfilling of ceramic sludge; (b) avoiding the depletion of natural raw materials; (c) saving energy and greenhouse gas emission by reduction of firing temperature; (d) leaching results show that the design of materials from waste that does not cause further pollution.  
Considering that these products contain large amounts of sludge residue and post-consumer glass material, they can be positively accepted by the market, taking into account the certifications as LEED (Leadership in Energy and Environmental Design) and ECOLABEL.  
The following are available online at [[https://www.mdpi.com/article/10.3390/app11083545/s1](https://www.mdpi.com/article/10.3390/app11083545/s1)]([https://www.mdpi.com/article/10.3390/app11083545/s1](https://www.mdpi.com/article/10.3390/app11083545/s1)), Figure S1: EDS Spectrum of V125 fired 1000 \({}^{\circ}\)C, 60 min.  
Conceptualization F.A., L.B., and I.L.; methodology, F.A., L.B., I.L., and P.P.; validation, F.A., L.B., I.L., and P.P.; formal analysis, F.A.; investigation, F.A., L.B., I.L., and P.P.; data curation, F.A.; writing--original draft preparation, F.A.; writing--review and editing, F.A., L.B., and I.L.; visualization, F.A., L.B., and P.P.; supervision, L.B. project administration, L.B. All authors have read and agreed to the published version of the manuscript.  
The authors declare that this research received no external funding.  
Not applicable.  
The authors declare no conflict of interest.