## 3 Results and Discussion
### Geopolymer  
The geopolymer formulations are reported in Table 6, with the respective Si/Al molar ratio, which increases by increasing the foundry sand content because the waste is poor in aluminum with respect to metakaolin. The integrity test of samples containing 30%, 50%, and 70% of foundry sand showed that the color of the immersion water became darker by increasing the amount of sand in the formulation, but it remained transparent. The change in color of the immersed water may be an indication of the release of elements from the sample into the water and their subsequent decrease in chemical stability. When samples lose their structural consistency, and the water of immersion becomes cloudy, the formulation is not considered acceptable. This is a qualitative indication of the stability of formulations that will then be quantified by pH and ionic conductivity tests to indirectly evaluate the efficacy of reticulation reactions. Even when the alkaline solution does not react completely with aluminosilicate powders, it may have still leached out during immersion of the sample in water. In this case, almost all samples remain intact after 24 h, so the integrity test is considered passed. In Figure 5 (left), the pH measurements of 30% MK, 50% MK, and 70% MK samples after 24 h are compared with 100% MK. It was noted that the pH value increased over time at short immersion times while remaining constant or even decreasing at longer immersion times. The pH of the eluate after 24 h is in a range between 10 and 11 for all samples, confirming the typical alkalinity of these samples. The standard 100% MK shows pH values between 11.19 at 30 min and 10.73 at 1440 min (24 h) [20]. Figure 5 (right) shows the conductivity measurements carried out on the same samples. The highest ionic conductivity is recorded by the sample with the lowest amount of metakaolin (30% MK), which after 8 h reaches a value of about 500 mS/m but then stabilizes within 24 h at 378 mS/m, similar to the value of 50% MK sample with 372 mS/m. The sample having the highest amount of metakaolin (70% MK) exhibited an overall lower value, approaching the standard 100% MK, with a conductivity of 300 mS/m after 8 h of testing. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Name** & **MK (g)** & **WFS (g)** & **NaOH 8M (mL)** & **Na2 SiO3 (mL)** & **Si/Al** \\ \hline
100\% MK & 100 & 0 & 30 & 30 & 1.4 \\ \hline
70\% MK & 70 & 30 & 30 & 30 & 1.82 \\ \hline
50\% MK & 50 & 50 & 30 & 30 & 2.27 \\ \hline
30\% MK & 30 & 70 & 30 & 30 & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Sample compositions of geopolymer formulation. Figure 5: Results of pH (**left**) and ionic conductivity (**right**) of geopolymer samples cured at room temperature. Zoom of ionic conductivity: sharp rise of conductivity during the first minutes. This behavior follows the typical trend of alkali-activated materials found in literature, and it is related to the lower stability of samples with a very high addition of sand [21]. Figure 6 shows the results of the compression strength tests on samples after the curing time of 28 days. There is a strong dependence of the values on the percentage of MK present in the formulations. In fact, the compressive strength decreases by decreasing the amount of MK. In Figure 6, the values of the Si/Al ratio for each sample are shown as orange points. Mechanical properties improve as the Si/Al ratio decreases to around 1.4-1.8. Acceptable results were, however, achieved for sample 70% MK showing a value around 21 MPa, compared to the standard 100% MK [22], so the addition of a 30% foundry sand is considered feasible. XRD patterns of 70% MK sample and 17A foundry sand are shown in Figure 7. In the sample with 70% MK, the presence of the broadband between 15-35\({}^{\circ}\) 20 can be observed, which is characteristic of the amorphous phase that for pure metakaolin-based geopolymers is positioned between 25-32\({}^{\circ}\) 20 [23].