Residual and Sedimentary Clays from Santa Maria (azores, Portugal): Preliminary Assessment As Ceramic Raw Materials

Santa Maria Island (Azores, Portugal) hosts diverse clayey materials derived from the intense weathering of basaltic-andesitic volcanic units, volcaniclastic deposits and localized Miocene marine sediments. This study provides a preliminary characterization of 20 clay samples to assess their suitability for ceramics. Textural analyses revealed a clay fraction often of >70%, indicating advanced alteration, and a particle-size distribution favourable for ceramic processing. The clayey materials are dominated by total phyllosilicates, with kaolinite prevailing, along with minor quartz, Fe- and Ti-oxides and scarce feldspars as accessory phases. The materials are characterized by high Al2O3 and Fe2O3 contents and low CaO-Na2O contents, with Chemical Index of Alteration values of 88-98%, consistent with intense leaching under humid weathering conditions. Technological tests revealed low to moderate cation-exchange capacities in kaolinite-rich samples and greater values in smectitic and palygorskitic clays. Abrasivity is variable, controlled by the quartz content, microstructure and grain angularity, while density remained within expected ranges. According to their consistency, most samples are projected within the medium- to high-plasticity field, compatible with requirements for structural ceramics, with plastic index/liquid limit ratios comparable to successful red ceramic formulations. Rheological measurements showed predominantly shear-thinning behaviour, ranging from nearly Newtonian kaolinitic suspensions to highly pseudoplastic smectitic gels, suggesting that blending strategies may be necessary. The SiO2-Al2O3-total flux ternary diagram indicated that most samples fall within compositional domains comparable to those of widely used ceramic clay bodies, with a subset shifted towards greater total flux contents. The Santa Maria Island clays constitute a robust and versatile clay resource base suitable for red structural ceramics and, in specific cases, showing technological compatibility with healing clay/peloid-type applications.