# Exploring Residual Clays for Low-Impact Ceramics: Insights from a Portuguese Ceramic Region
## 3 Results and Discussion
No significant exothermic events were detected, suggesting low organic or sulfide content, despite the presence of 5% siderite and 1% pyrite. The thermal behavior confirms a kaolinite clay with stable firing behavior and relatively simple thermal decomposition. Sample D2 showed a curve with an initial endothermic event below 200 \({}^{\circ}\)C, corresponding to water loss, consistent with its high smectite content. A low-intensity endothermic peak at -600 \({}^{\circ}\)C can be attributed to the dehydroxylation of smectite and minor kaolinite. A significant exothermic peak near \(\sim\)400-450 \({}^{\circ}\)C likely corresponds to oxidation of pyrite, producing exothermic energy and sulfur gases. The thermal result suggests a chemically reactive material that may require controlled firing. Thermodilatometric (TD) curves were obtained for the raw (unfired) samples A2 and B1, heated to 1000 \({}^{\circ}\)C, and D2, heated to 1100 \({}^{\circ}\)C (Figures S9-S11), providing a perception of linear dimensional changes during heating, critical to predict firing shrinkage and potential deformation. Sample A2 showed a progressive linear contraction starting \(\sim\)200 \({}^{\circ}\)C, being more pronounced between 550 and 700 \({}^{\circ}\)C, corresponding to the dehydroxylation of kaolinite and sintering. A smooth and continuous shrinkage up to 950-1000 \({}^{\circ}\)C suggests good thermal behavior with limited structural instability. The total shrinkage was moderate (-3-5%), usual in typical red ceramic raw materials [59]. In sample B1, shrinkage started earlier and was more pronounced, with contraction \(\sim\)450-600 \({}^{\circ}\)C, corresponding to the intense kaolinite dehydroxylation peak in DTA, suggesting a structural collapse followed by initial sintering. The overall shrinkage was slightly higher than A2 (-5-7%), consistent with its higher kaolinite content and finer granulometry. No evidence of bloating or expansion was observed, indicating stable densification. Sample D2 showed an initial minor expansion up to \(\sim\)350 \({}^{\circ}\)C, possibly related to pyrite oxidation and gas release. From \(\sim\)600 to 800 \({}^{\circ}\)C, moderate shrinkage was observed, linked to smectite dehydroxylation and carbonate decomposition. The shrinkage was irregular and less linear, suggesting uneven sintering due to the presence of volatile-rich phases. The presence of pyrite and carbonates increases the risk of bloating or porosity if not fired under controlled conditions. Overall, the thermal analysis of the clay samples showed that A2 exhibited a typical kaolinite-smectite thermal profile with moderate water loss, well-defined dehydroxylation, and continuous shrinkage. Its thermal stability, combined with a balanced mineralogy and moderate carbonate content, makes it suitable for red ceramics requiring stable firing behavior and low porosity. B1, dominated by kaolinite (55%) and with lower smectite content, showed a sharper dehydroxylation event and more intense linear shrinkage. The low presence of volatile phases ensures predictable thermal behavior. These features make B1 ideal for applications requiring high plasticity and structural densification without risk of deformation. Sample D2, with high smectite content and volatile-rich accessory phases (e.g., pyrite, carbonates), presented the most complex thermal behavior. Early expansion, exothermic oxidation, and irregular shrinkage reflected a thermally reactive profile. While rich in plasticity, D2's composition suggests a need for controlled firing (slow heating and oxidation stages) or blending with more thermally stable clays to mitigate gas release and sintering irregularities. The forming and drying characteristics of clay bodies vary with water demand, plasticity, and interparticle cohesion [52]. Extrusion moisture (EM), green-to-dry shrinkage (GDS), and green flexural strength (FS) were evaluated for samples A2, B1, and D2 to assess the suitability for extrusion forming and subsequent drying (Table 7).