# Exploring Residual Clays for Low-Impact Ceramics: Insights from a Portuguese Ceramic Region
## 3 Results and Discussion
These materials require more water for workability and are prone to significant drying shrinkage, requiring slow and controlled drying [51; 55]. Samples A1, B2, and C2 also showed high PIs, consistent with their significant clay fractions (48-58%, <0.002 mm) and mixed kaolinite-smectite minerals, suggesting good plasticity for extrusion but a need for tempering to prevent deformation. Samples B1 and D1-2 revealed medium plasticity, with a moderate clay fraction (29-58%, <0.002 mm) and more coarse particles (>40%, >0.063 mm). Their lower PIs can be associated with reduced water demand and shrinkage, improving dimensional stability during drying and firing. Samples C1 and E1 showed the lowest PIs, which can be linked to their lower clay content (<30%, <0.002 mm) and higher coarser fraction. These samples usually have fewer difficulties during drying with minimal shrinkage but may require added plasticizers to form processes [56]. Sample projection in Gippini plasticity chart domains [25] revealed that A1-2, B2, and C2 were above the Gippini A-B boundaries, within the high-to-very-high plasticity domain, indicating excellent workability but significant shrinkage risk. A2 was projected near the top of the chart, reflecting its dominant smectite content, requiring slow and controlled drying. Samples B1 and D1-2 were projected in the medium-high plasticity field, associated with their mixed kaolinite-illite content and moderate clay fraction, balancing plasticity and dimensional stability. In contrast, C1 and E1 were classified in the medium plasticity region of the chart, consistent with their lower clay content (<30%, <0.002 mm) and higher coarser fraction, linked to easier drying and minimal shrink-swell behavior but requiring added plasticizers for formation [6]. The specific surface area (SSA) and further ceramic assays were only performed in three samples: A2 and D2, with higher smectite contents and plasticity indices; and sample B1, with its thinner particle size distribution (D50 = 0.006 mm). The SSA and cation exchange capacity (CEC) are useful tools that reflect the reactivity of clay minerals, particularly their interaction with water and other charged species, being closely linked to the mineral composition, having a direct influence on shaping behavior, drying response, and sintering dynamics in ceramic processes [51]. Sample B1 exhibited the highest SSA (Table 6), consistent with its high smectite content and mixed-layer clays, with high Al\({}_{2}\)O\({}_{3}\) and Fe\({}_{2}\)O\({}_{3}\). This high SSA suggests a strong water retention capacity, contributing to plasticity but also a high shrinkage risk and the need for controlled drying. This sample also showed low swelling, possibly due to its lower exchangeable Ca\({}^{2+}\) and Mg\({}^{2+}\) and a \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**ID** & **LL** & **PL** & **PI** & **Plasticity Classification** \\ \hline A1 & 71.3 & 33.6 & 37.7 & High plasticity clay \\ A2 & 87.2 & 38.6 & 48.6 & Very high plasticity \\ B1 & 65.1 & 37.6 & 27.5 & Medium–high plasticity \\ B2 & 71.2 & 32.6 & 38.6 & High plasticity clay \\ C1 & 50.1 & 27.3 & 22.8 & Medium plasticity \\ C2 & 59.4 & 22.4 & 37.0 & High plasticity clay \\ D1 & 60.6 & 31.5 & 29.2 & Medium–high plasticity \\ D2 & 62.5 & 31.7 & 30.8 & Medium–high plasticity \\ E1 & 49.7 & 26.5 & 23.2 & Medium plasticity \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Samples’ liquid limit (LL), plastic limit (PL), plasticity index (PI) (in %), and plasticity classification based on Gippini chart [25]. lower total CEC, suggesting partial collapse of smectite interlayers or surface blockage by Fe oxides. Sample A2 presented moderate SSA values, but a high swelling capacity and CEC, characteristic of smectite-rich and well-ordered clay assemblages, with highly reactive interlayer surfaces. The high Ca\({}^{2+}\) and Mg\({}^{2+}\) promote interlayer expansion, linked to elevated expansibility.