## 3 Results  
### Technical Assessment on the Suitability of GS Incorporation in Ceramic Paste  
The GS sample studied is characterised by an unusually low moisture content of 23.5% \(w\)/\(w\) for filter-pressed sludge since common values range from 35-45% [45]. According to [6] and to the authors' experience, this moisture content is acceptable for direct incorporation of GS in ceramic pastes without any additional drying step before processing since the production of ceramic paste starts from an aqueous suspension. GS particles are smaller than 60 um, with a mean particle size of 9 um, demonstrating its fineness (Figure S1). This granulometric size is highly desirable for ceramic formulation since it enhances the reactivity of the material upon firing, acting as a fluxing component. Moreover, the fineness of GS allows to save pre-milling or sieving operations commonly associated with natural feldspars processing and thus avoids additional operational costs. Table 2 reports the results of the diametral shrinkage upon firing of the GS sample calculated at 1100 \({}^{\circ}\)C, 1150 \({}^{\circ}\)C, and 1200 \({}^{\circ}\)C.  
These results show that the GS sample suffered the highest shrinkage at 1150 \({}^{\circ}\)C (10.4%). At 1200 \({}^{\circ}\)C, the melting is excessive, and sample expansion is observed, causing the decrease of shrinkage value from 10.4% to 9.65% (Table 2). GS behaviour resembles that of natural feldspars in the same interval of temperature since feldspar is known to increase the fluxing action and particle reactivity, promoting their approach and consequent sample sintering [46]. Figure 2a-e shows the colour change undertaken by GS samples upon firing, whereas Table 3 reports the colour coordinates at 1100 \({}^{\circ}\)C, 1150 \({}^{\circ}\)C, and 1200 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c c} \hline
**Temperature** & **Diameter before Firing** & **Diameter after Firing** & **Shrinkage** \\ \hline \({}^{\circ}\)C & & mm & \% \\ \hline
1100 & & & 22.96 & 8.18 \\
1150 & 25 & & 22.41 & 10.4 \\
1200 & & & 22.59 & 9.65 \\ \hline \end{tabular}
\end{table} 
Table 2: Shrinkage of GS samples from 1000 \({}^{\circ}\)C to 1200 \({}^{\circ}\)C. \begin{table}
\begin{tabular}{c c c c} \hline
**Temperature** & **L\({}^{*}\)** & **a\({}^{*}\)** & **b\({}^{*}\)** \\ \hline \({}^{\circ}\)C & & & \\ \hline
1100 & 50.67 & 10.87 & 12.33 \\
1150 & 38.81 & 6.56 & 5.83 \\
1200 & 40.22 & 4.92 & 8.58 \\ \hline \end{tabular}
\end{table} 
Table 3: Average colour coordinates (L*a*b*) of GS pellets fired at temperatures from 1100 \({}^{\circ}\)C to 1200 \({}^{\circ}\)C.  
The GS sample, after drying and firing at 1000 \({}^{\circ}\)C, acquired a reddish colour (Figure 2b) mainly due to the presence of iron in the wearing scraps of sawing tools [3], which ended up in GS during the sawing process. The presence of iron in the GS composition is confirmed by the XRF analysis lately discussed. Increasing firing temperature from 1100 \({}^{\circ}\)C to 1200 \({}^{\circ}\)C (Figure 2c-e), darker hues are noticed, which are expected from the reduction of iron ions from dominant Fe\({}^{3+}\) to Fe\({}^{2+}\) species. This effect is strongly observed from 1100 \({}^{\circ}\)C to 1150 \({}^{\circ}\)C; then, it tends to stabilise. At 1100 \({}^{\circ}\)C, the sample shows a bright yellowish hue, as revealed by the relatively high L* and by the positive a* and b* coordinates (corresponding to reddish and yellowish hues). With temperature rise, darker and brownish/greyish hues tend to appear. The observed tinting effect, registered with the temperature increase, penalises the use of GS in noble whitish ceramic products and limits its incorporation in selected formulations. Nevertheless, sandstone tiles or tableware products can be produced from reddish pastes, even though the products obtained from white pastes are commonly more appreciated by consumers.