# Exploring Residual Clays for Low-Impact Ceramics: Insights from a Portuguese Ceramic Region
## 3 Results and Discussion
The A-group clays were highly plastic and reactive, beneficial for forming but requiring care during drying and firing to manage shrinkage and structural integrity. Samples D1-2 displayed high swelling and a relatively high CEC, with a moderate SSA in D2, coupled with significant Ca\({}^{2+}\) and Mg\({}^{2+}\) content, suggesting the presence of expansive smectite phases. However, their high carbonate and plagioclase content reduce plasticity and increase the risk of lime popping or bloating unless firing profiles are adjusted. Group C showed a moderate-to-high CEC, expansibility, and intermediate exchangeable cation levels, aligning with the kaolinite and smectite content and granulometric profiles (zone 3 in Winkler's chart) and suggesting suitability for extrusion-based red ceramics like roofing tiles and light blocks, with good forming properties and manageable shrinkage. Sample E1, with a moderate CEC and high swelling, with 61% illite, suggested that its exchange capacity may be mostly attributed to minor smectite or weathered illite. Its high K\({}^{+}\) content is consistent with its illitic character and may be beneficial for early densification during firing. However, its coarser granulometry and lower clay fraction suggest low plasticity, being more suitable as a structural filler or grog material to control shrinkage in plastic-rich formulations. The chemical weathering degree of the studied residual clays was evaluated using the chemical index of alteration (CIA) [20] and the chemical index of weathering (CIW) [21], based on the molar ratios of major oxides, providing insights into the extent of leaching of labile cations (Ca\({}^{2+}\), Na\({}^{+}\), K\({}^{+}\)) and the relative enrichment in alumina-rich minerals [57]. The CIA and CIW results suggested varying degrees of weathering among the analyzed clays, from poorly altered to highly leached profiles. The highest CIA and CIW values were observed in the B1-2 samples, associated with higher Al\({}_{2}\)O\({}_{3}\) contents (>27%) and lower CaO and Na\({}_{2}\)O (Figure 4), with low concentrations of exchangeable bases. These samples are rich in kaolinite and low in smectite, with minimal carbonates and feldspars. Their fine granulometry and acidic pH (5.5-6.7) were also consistent with intense leaching and advanced weathering. Samples D1, D2, and E1 exhibited the lowest CIA (71.4, 67.9, 69.7, respectively) and CIW (73.7, 72.4, 78.9, respectively) values, suggesting low-to-moderate weathering intensity. These samples contained higher amounts of exchangeable Ca\({}^{2+}\) and Mg\({}^{2+}\), as well as a higher content of coarse material (>0.063 mm, 46.8-59%). Intermediate behavior was found in samples A1-2 and C1-2, with CIA between 77.0 (C2) and 86.8 (A1) and CIW between 84.0 (C2) and 92.8 (A1). These samples showed mixed mineralogical compositions with varying proportions of kaolinite, smectite, and illite, and low-to-moderate contents \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**ID** & **SSA** & **Exp** & **CEC** & **K\({}^{+}\)** & **Na\({}^{+}\)** & **Ca\({}^{2+}\)** & **Mg\({}^{2+}\)** \\ \hline A1 & - & 43.5 & 42.1 & 33 & 19 & 421 & 432 \\ A2 & 22.2 & 52.4 & 38.1 & 23 & 39 & 536 & 505 \\ B1 & 47.9 & 15.0 & 20.2 & 27 & 21 & 286 & 158 \\ B2 & - & 22.0 & 24.1 & 25 & 63 & 654 & 260 \\ C1 & - & 35.4 & 28.0 & 26 & 49 & 569 & 308 \\ C2 & - & 40.3 & 30.6 & 21 & 57 & 618 & 309 \\ D1 & - & 44.0 & 36.0 & 21 & 48 & 978 & 408 \\ D2 & 14.1 & 42.6 & 31.2 & 42 & 55 & 891 & 323 \\ E1 & - & 41.7 & 31.7 & 68 & 43 & 726 & 242 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Samples’ specific surface area (SSA; in m\({}^{2}\)/g), cation exchange capacity (CEC; in meq/100 g), expansibility (Exp; in %), and exchangeable cations K\({}^{+}\), Na\({}^{+}\), Ca\({}^{2+}\), and Mg\({}^{2+}\) (in mg/kg). of feldspars and exchangeable bases. Their granulometric profiles (<2 \(\upmu\)m = 48.7-50.8%) suggested intermediate alteration stages.