## 2 Materials and Methods
### Samples Formulation and Preparation  
The materials used for ceramic tiles are clays and feldspars of different origins and standard sand. The standard sand was replaced in varied percentages with the foundry sand, as reported in Table 2. For brick materials, different types of clays and basalt were used. The basalt, as for the sand in ceramics tiles, was replaced with the foundry sand, as reported in Table 3.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**Material Composition (\%)** & **STD** & **F10** & **F50** & **F100** \\ \hline Clay (Vicenza) & 33.30 & 33.30 & 33.30 & 33.30 \\ \hline Marl (Possagno) & 23.40 & 23.40 & 23.40 & 23.40 & 23.40 \\ \hline Clay Modena 1 & 16.65 & 16.65 & 16.65 & 16.65 & 16.65 \\ \hline Clay Modena 2 & 16.65 & 16.65 & 16.65 & 16.65 & 16.65 \\ \hline Basalt fine & 10.00 & 9.00 & 5.00 & 0 \\ \hline Foundry sand & 0 & 1.00 & 5.00 & 10.00 \\ \hline \end{tabular}
\end{table}
Table 3: Formulations of brick samples.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Material Composition (\%)** & **STD** & **SF10** & **SF50** & **SF100** \\ \hline Clay 1 & 26 & 26 & 26 & 26 \\ \hline Clay 2 & 14 & 14 & 14 & 14 \\ \hline Feldspar 1 & 20 & 20 & 20 & 20 \\ \hline Feldspar 2 & 28 & 28 & 28 & 28 \\ \hline Sand & 12 & 10.8 & 6 & 0 \\ \hline Foundry sand & 0 & 1.2 & 6 & 12 \\ \hline \end{tabular}
\end{table}
Table 2: Formulations of porcelain stoneware samples.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Material Composition (\%)** & **FSTD** & **F10** & **F50** & **F100** \\ \hline Clay (Vicenza) & 33.30 & 33.30 & 33.30 & 33.30 \\ \hline Marl (Possagno) & 23.40 & 23.40 & 23.40 & 23.40 \\ \hline Clay Modena 1 & 16.65 & 16.65 & 16.65 & 16.65 \\ \hline Clay Modena 2 & 16.65 & 16.65 & 16.65 & 16.65 \\ \hline Basalt fine & 10.00 & 9.00 & 5.00 & 0 \\ \hline Foundry sand & 0 & 1.00 & 5.00 & 10.00 \\ \hline \end{tabular}
\end{table}
Table 1: Chemical composition of sands 17A and 31A.  
For tile samples, the standard mixture shown in Table 2 was used. Raw materials were mixed with water in a 2:1 ratio, and a fluidizer was added to improve the pourability of the dough. Then the dried dough was pulverized and humidified to 6% before being pressed to obtain the samples and cooked up to 1205 \({}^{\circ}\)C.  
For brick mixtures, the raw materials previously dried and sieved under 500 \(\upmu\)m were mixed with around 27% water to obtain a smooth and malleable dough suitable for extrusion. Then, samples were extruded and cut to a size of 5 \(\times\) 2 cm and dried at room temperature and then at 105 \({}^{\circ}\)C. Subsequently, the samples were put in the stove up to 1050 \({}^{\circ}\)C.  
For the geopolymers application, the foundry sand 17A corresponding to a higher ratio of aluminum to silicon, was chosen. As a matrix for geopolymer samples, metakaolin CSC1000-ARGICAL 1000 provided by Bal-Co (Sassuolo, MO, IT) was used. As activating solutions, sodium silicate provided by Ingessil (Montorio, Vr, IT) and NaOH 8 M provided by Merck-Sigma-Aldrich (Milano, IT) were used.  
The fresh pastes were prepared by mixing metakaolin and waste sand with the activating solution (NaOH and Na\({}_{2}\)SiO\({}_{3}\)) into a container with a mechanical mixer for approx. 10 min. When the geopolymer paste was homogenous and achieved suitable workability, it was poured into a mold and covered with a plastic film. The samples were cured at room temperature for 28 days before further characterization.  
A standard mixture of geopolymer was initially prepared using 100% metakaolin and reacted with 30 mL of NaOH 8M and 30 mL of sodium silicate. In the other formulations, part of metakaolin (30%, 50%, and 70% by weight) was replaced with waste sand 17A, and the collected results of chemical stability, durability in water, and mechanical resistance were compared.