# Exploring Residual Clays for Low-Impact Ceramics: Insights from a Portuguese Ceramic Region
## 3 Results and Discussion
Sample A2 exhibited the highest extrusion moisture, reflecting its high fine fraction and smectite content, associated with good plasticity, but requires higher moisture levels for workable extrusion [7]. Sample A2 showed high green-to-dry shrinkage, consistent with its great clay-bound water content and propensity for shrink-swell movement. Its green flexural strength was the highest, suggesting the strong interparticle cohesion typical of highly plastic clays [51]. Sample B1 required less water for extrusion and moderate shrinkage, reflecting a more balanced kaolinite-illite composition. The green flexural strength was lower than A2 but sufficient for handling and rapid processing. Sample D2, with the lowest EM and GDS, exhibited the greatest dimensional stability during drying, which can be associated with its high sand and carbonate content. However, its green FS was the lowest, suggesting reduced cohesion and potential handling constraints. The ceramic suitability properties of clay samples A2, B1, and D2 after firing at a heating rate of 5 \({}^{\circ}\)C/min with a 60 min soak at the maximum temperature are presented in Table 8. A progressive decrease in flexural strength was observed in sample A2, with increasing firing temperature from 900 to 1100 \({}^{\circ}\)C, despite the formation of higher-temperature crystalline phases. This decline in mechanical strength may be linked to the onset of over-firing effects, identified at 1100 \({}^{\circ}\)C through bloating and fractures, suggesting pyrometric deformation. Total shrinkage stayed relatively constant over the three firing temperatures (Figure S12). Water absorption showed a significant increase, peaking at 1100 \({}^{\circ}\)C, suggesting a rise in porosity. Sample A2 fired specimens exhibited signs of black core formation, a characteristic of over-firing possibly linked to incomplete oxidation of organic matter such as humic acids. The presence of V, a potential contributor to greenish efflorescence, was observed only at 900 \({}^{\circ}\)C, becoming soluble between 800 and 1000 \({}^{\circ}\)C, and returning to being insoluble >1000 \({}^{\circ}\)C, explaining the presence of greenish efflorescence at the lower temperature, likely due to K-vanadates or Na-molybdates [60]. Identified mineral phases (Figure S13) showed the persistence of quartz and hematite phases throughout the firing process. Quartz, although residual at all firing temperatures, showed a decreasing intensity in reflections, suggesting a progressive incorporation into the vitreous phase with temperature increase. Hematite (Fe\({}_{2}\)O\({}_{3}\)) reflection intensity increased with firing temperature, confirming its stability and enhanced crystallinity. Spinel-type (MgAl\({}_{2}\)O\({}_{4}\)) phases were indicative of high-temperature crystalline transformations emerging as firing progressed. These were attributed to the release of Al\({}_{2}\)O\({}_{3}\) from clay minerals. Mul \begin{table}
\begin{tabular}{c c c c c c} \hline
**ID** & **EM** & **GDS** & **FS (kgf/cm\({}^{2}\))** & **FS (MPa)** & **Color** \\ \hline A2 & 26.14 & 9.63 \(\pm\) 0.15 & 133.93 \(\pm\) 4.93 & 13.13 \(\pm\) 0.48 & Brown \\ B1 & 23.60 & 7.08 \(\pm\) 0.10 & 82.76 \(\pm\) 3.15 & 8.12 \(\pm\) 0.31 & Dark orange \\ D2 & 18.26 & 5.32 \(\pm\) 0.43 & 69.76 \(\pm\) 5.24 & 6.84 \(\pm\) 0.51 & Brown \\ \hline \end{tabular}
\end{table} 
Table 7: Samples’ extrusion moisture (EM; in %), green-to-dry shrinkage (GDS; in %), flexural strength (FS; in kgf/cm\({}^{2}\) and MPa), and color. formed by the reaction between free SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\), was also present at all temperatures. Additionally, \(\gamma\)-alumina was identified at 900 \({}^{\circ}\)C and remained detectable at higher temperatures, while K-feldspar exhibited more intense reflections at 1100 \({}^{\circ}\)C, highlighting an increased crystallization at higher temperatures. Muscovite also became more prominent at this temperature. The samples' dark brown color after firing was in agreement with [PERSON] et al.