# Exploring Residual Clays for Low-Impact Ceramics: Insights from a Portuguese Ceramic Region
## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**FT** & **DFS** & **TS** & **FS (kgf/cm\({}^{2}\))** & **FS (MPa)** & **WA** & **Ef** & **Color** \\ \hline \multicolumn{8}{c}{_sample A2_} \\
900 \({}^{\circ}\)C & 0.66 \(\pm\) 0.12 & 10.17 \(\pm\) 0.17 & 119.76 \(\pm\) 22.19 & 11.74 \(\pm\) 2.18 & 10.37 \(\pm\) 0.20 & greenish * & orange \\
1000 \({}^{\circ}\)C & 1.21 \(\pm\) 0.09 & 10.40 \(\pm\) 0.18 & 65.90 \(\pm\) 8.27 & 6.46 \(\pm\) 0.81 & 10.06 \(\pm\) 0.10 & - & orange \\
1100 \({}^{\circ}\)C & 0.93 \(\pm\) 0.15 & 9.77 \(\pm\) 0.20 & 34.70 \(\pm\) 8.11 & 3.40 \(\pm\) 0.79 & 19.77 \(\pm\) 1.44 & - & dark red \\ \hline \multicolumn{8}{c}{_sample B1_} \\
900 \({}^{\circ}\)C & 0.70 \(\pm\) 0.13 & 7.50 \(\pm\) 0.10 & 180.72 \(\pm\) 12.76 & 17.73 \(\pm\) 1.25 & 16.17 \(\pm\) 0.08 & - & orange \\
1000 \({}^{\circ}\)C & 3.39 \(\pm\) 0.13 & 9.92 \(\pm\) 0.12 & 230.14 \(\pm\) 18.20 & 22.57 \(\pm\) 1.79 & 11.13 \(\pm\) 0.22 & greenish ** & orange \\
1100 \({}^{\circ}\)C & 5.78 \(\pm\) 0.30 & 11.96 \(\pm\) 0.48 & 261.05 \(\pm\) 30.39 & 25.59 \(\pm\) 2.98 & 6.49 \(\pm\) 0.42 & - & dark red \\ \hline \multicolumn{8}{c}{_sample D2_} \\
900 \({}^{\circ}\)C & 0.082 \(\pm\) 0.079 & 5.56 \(\pm\) 0.52 & 62.46 \(\pm\) 5.36 & 6.12 \(\pm\) 0.53 & 15.30 \(\pm\) 0.16 & greenish * & orange \\
1000 \({}^{\circ}\)C & 0.46 \(\pm\) 0.14 & 5.68 \(\pm\) 0.62 & 61.32 \(\pm\) 8.72 & 6.02 \(\pm\) 0.86 & 15.05 \(\pm\) 0.25 & - & dark orange \\
1100 \({}^{\circ}\)C & 1.38 \(\pm\) 0.28 & 6.62 \(\pm\) 0.83 & 79.34 \(\pm\) 8.79 & 7.78 \(\pm\) 0.86 & 13.84 \(\pm\) 0.25 & - & dark red \\ \hline \multicolumn{8}{c}{_*_ trace; ** clearly visible._} \\ \end{tabular}
\end{table} 
Table 8: Samples’ firing temperature (FT; in \({}^{\circ}\)C), dry-to-fired shrinkage (DFS; in %), total shrinkage (green-to-fired) (TS; in %), flexural strength (FS; in kgf/cm\({}^{2}\) and MPa), water absorption (WA; in %), efflorescence (Ef), and color. atures. K-feldspar was more pronounced at 1100 \({}^{\circ}\)C, indicating enhanced crystallization or phase separation during high-temperature sintering. The increasing intensity of feldspar-related peaks correlates with the observed improvements in mechanical strength and color change (from orange to dark red), which was also affected by Fe oxidation states and phase interactions. Greenish efflorescence was clearly observed at 1000 \({}^{\circ}\)C, more pronounced than in A2, being likely related to soluble vanadates or molybdates formed during intermediate temperature firing. In summary, clay B1 demonstrated higher thermal performance than A2, with higher mechanical strength and lower porosity at increased firing temperatures. However, at intermediate temperatures (~1000 \({}^{\circ}\)C), efflorescence formation may affect the surface aesthetics or durability of the ceramic products. The progressive evolution of mineralogical phases, especially mulilite, feldspar, and spinel-type alumina, contributes to the technological robustness of this clay body. Clay sample D2's total shrinkage was relatively low, with a slight increase with rising firing temperatures, from 900 to 1100 \({}^{\circ}\)C, indicating progressive but limited densification (Table 8). Water absorption showed high values at all temperatures, reflecting a reduction in open porosity due to enhanced sintering and partial vitrification. Mechanical strength was not significant, with a nonsignificant increase from 900 to 1100 \({}^{\circ}\)C, suggesting a less consolidated microstructure, even at higher temperatures, possibly due to a lower content of fluxing agents or poor particle packing (Figure S16). Greenish efflorescence was observed at 900 \({}^{\circ}\)C, also attributed to soluble V and Mo compounds. Mineral phases showed a consistent presence of quartz and hematite across all firing temperatures (Figure S17). Mulilite reflections were identified and showed a growth trend with rising temperature, suggesting progressive solid-state reactions between Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\), enhancing thermal resistance and dimensional stability.