# Exploring Residual Clays for Low-Impact Ceramics: Insights from a Portuguese Ceramic Region
## 3 Results and Discussion
The samples' CIA/CIW ratios ranged from 0.88 (E1) to 0.95 (A2, B2), showing moderate variability in K behavior post-weathering. All samples, except E1, presented CIA/CIW ratios from 0.91 (C1) to 0.95 (A2, B2), showing relatively stable K behavior after initial weathering, with little evidence of diagenetic enrichment. Sample E1 revealed a CIA/CIW ratio of 0.88, the lowest in the dataset, suggesting potential K-retention or K-fixation, consistent with illite dominance (61%) and the highest K\({}_{2}\)O content among all samples (3.24%). Overall, the results suggest that most samples underwent moderate-to-advanced primary alteration, with only minor post-weathering K mobility. Thermogravimetric analysis (TGA) allows the assessment of mass changes occurring during controlled heating of raw materials as a result of thermal decomposition or oxidation reactions. The thermogravimetric curves of samples A2, B1, and D2 (Figures S3-S5) were obtained at a heating rate of 10 \({}^{\circ}\)C/min up to 1000 \({}^{\circ}\)C. The samples revealed three main stages of mass loss, usual for natural clayey materials [43]. The first weight loss stage occurred below 150 \({}^{\circ}\)C, corresponding to the release of adsorbed and interlayer water from phyllosilicates, especially smectite in A2 and D2, known for high hygroscopicity [58]. The dehydration step accounted for \(\sim\)4-6% of mass loss, consistent with their expandable clay mineral content. The second mass loss occurred between 200 and 600 \({}^{\circ}\)C, associated with dehydroxylation of structural hydroxyl groups in kaolinite and illite. Sample B1, with a higher kaolinite proportion in the <0.002 mm fraction, was more intensive. This stage overlaps with the oxidation of organic matter and the decomposition of minor sulfides (e.g., pyrite), especially in D2, with 7% pyrite. The third and most intense weight loss occurred between 600 and 800 \({}^{\circ}\)C and can be attributed to the decomposition of carbonate minerals such as calcite and dolomite. This stage released CO2 and resulted in significant mass loss, exceeding 8-10%, depending on carbonate abundance, being more pronounced in D2, with its high pH and carbonate content. Beyond 800 \({}^{\circ}\)C, the TGA curves stabilized, indicating the formation of new mineral phases and the start of sintering processes. Sample A2 exhibited moderate overall mass loss and gradual transitions, suggesting good thermal stability for ceramic firing, while B1 showed sharper transitions due to concentrated kaolinite dehydroxylation, and D2 presented a more complex pattern due to volatile-rich phases  
Figure 4: The chemical index of alteration (CIA), and chemical index of weathering (right \(Y\)-axis) (CIW) vs. oxide molar proportions (left \(Y\)-axis). (carbonates and sulfides), implying a need for controlled firing conditions to avoid bloating or pore development. The differential thermal analysis (DTA), obtained under the same heating conditions as the TGA, identified typical endothermic and exothermic events corresponding to dehydration, dehydroxylation, and phase transitions in clay and accessory minerals (Figures S6-S8). Sample A2 showed a clear endothermic peak around 100-150 \({}^{\circ}\)C, associated with the loss of adsorbed water from smectite and kaolinite surfaces. A more prominent endothermic peak near 520-550 \({}^{\circ}\)C can be attributed to dehydroxylation of kaolinite, a transformation where structural hydroxyls are released to form metakaolinite, consistent with the kaolinite content in the clay fraction. The weaker curve \(\sim\)600 \({}^{\circ}\)C may correspond to additional hydroxyl loss from illite or partial dehydroxylation of smectite. The curve does not present significant exothermic activity, indicating minimal organic matter or pyrite oxidation. Sample B1 showed a more intense endothermic peak at -500 \({}^{\circ}\)C, characteristic of kaolinite dehydroxylation, in agreement with the kaolinite content. A minor endothermic peak around \(\sim\)100 \({}^{\circ}\)C reflected surface-bound water.