# Exploring Residual Clays for Low-Impact Ceramics: Insights from a Portuguese Ceramic Region
## 4 Conclusions  
This study characterized residual clays from the Sao Pedro do Corval area and evaluated their suitability for red ceramic applications. The samples showed varied mineralogical and granulometric profiles, reflecting different lithological origins, which significantly influenced their technological behavior. Sample A2, derived from metasedimentary rocks, exhibited a high smectite content and fine particle size, resulting in very high plasticity and high extrusion moisture. These features indicate good workability, but also a need for controlled drying and firing due to shrinkage and efflorescence risk. Sample B1, with a kaolinite-illite composition and balanced granulometry, showed good firing behavior, low porosity, and high mechanical strength, especially at 1100 \({}^{\circ}\)C, making it suitable for ceramics. The chemical alteration degree, assessed through the CIA and CIW indices, revealed a gradient of weathering intensity across the samples. B1 and B2 recorded the highest CIA and CIW values, consistent with their advanced weathering, leaching of base cations, and enrichment in Al\({}_{2}\)O\({}_{3}\). Sample D2, despite its plasticity, revealed a complex thermal profile with higher volatile content and lower strength after firing, suggesting its best use as a plastic additive. This sample showed the lowest CIA and CIW, linked to a coarser texture, exchangeable Ca\({}^{2+}\) and Mg\({}^{2+}\), and Al\({}_{2}\)O\({}_{3}\) content suggesting limited leaching and partial alteration. Samples E1 and D2, with lower clay fraction contents and coarser textures, had reduced plasticity and shrinkage, making them potential temper materials to improve drying behavior in ceramic blends. Intermediate weathering and technological behavior were observed in A1-2 and C1-2. Thermal and firing analyses supported previous findings, confirming the formation of multite and vitrification phases, as well as demonstrating the importance of integrating mineralogical, chemical, and thermal data in determining the ceramic potential of raw clays. Overall, samples A2 and B1 emerged as the most promising for red ceramic applications, while others may complement formulations depending on the product requirements. Future works on Corval residual clays will include the interpretation of potential toxic elements sources and further ceramics tests, such as compression tests. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15158761/s1](https://www.mdpi.com/article/10.3390/app15158761/s1)]([https://www.mdpi.com/article/10.3390/app15158761/s1](https://www.mdpi.com/article/10.3390/app15158761/s1)), Figure S1: Diffractogram of sample B1, fraction < 0.063 mm; Figure S2: Diffractogram of sample C2, fraction < 0.063 mm; Figure S3: Thermogravimetric curves of sample A2; Figure S4: Thermogravimetric curves of sample B1; Figure S5: Thermogravimetric curves of sample D2; Figure S6: Differential thermal analysis curves of sample A2; Figure S7: Differential thermal analysis curves of sample B1; Figure S8: Differential thermal analysis curves of sample D2; Figure S9: Thermodilometric curves of sample A2; Figure S10: Thermodilometric curves of sample B1; Figure S11: Thermodilometric curves of sample D2; Figure S12: Sample A2 specimens after firing at 900 \({}^{\circ}\)C, 1000 \({}^{\circ}\)C, and 1100 \({}^{\circ}\)C; Figure S13: Diffraction patterns of clay A2, fired at 900 (black line), 1000 (blue line), and 1100 \({}^{\circ}\)C (red line). Minerals: Musc--muscovite; Mul--mullie; \(\gamma\)-Al--\(\gamma\)-alumina; K--f-K-feldspar; Qz--quartz; Hem--hematite; Sp--spinel-type phase; Hem--hematite; Figure S14: Sample B1 specimens after firing at 900 \({}^{\circ}\)C, 1000 \({}^{\circ}\)C, and 1100 \({}^{\circ}\)C; Figure S15: Diffraction patterns of clay B1, fired at 900 (black line), 1000 (blue line), and 1100 \({}^{\circ}\)C (red line). Minerals: Musc--muscovite; Mul--mullie; \(\gamma\)-Al--\(\gamma\)-alumina; K-f--K-feldspar; Qz--quartz; Hem--hematite; Sp--spinel-type phase; and Hem--hematite.