Composition and ceramic properties of Tertiary clays from southern Sardinia (Italy)

The Sardinian ceramic industry produces structural clay products (common and hollow bricks). The raw materials are obtained from Tertiary continental sediments (Cixerri, Ussana and Samassi Formations) found in the south-western part of the island. These clays have different mineralogical, chemical and grain size compositions and a different technological behaviour in the brick and tile production cycle. The Cixerri clays (Eo-Oligocene) are fine-grained and consist of illite, interstratilied I/S, quartz, kaolinite and high percentages of carbonates (19-24%). Overall, they are suitable for the production of high-quality common bricks, while they could also be part of mixes for roofing tiles and wall tiles. The Ussana clays (Oligo-Miocene) consist of illite-chlorite-kaolinite with high contents of quartz and variable percentages of carbonates (3-20%). In terms of grain size, they can be classified as silty clays and clayey silts. The behaviour in ceramic production is suitable for high-quality bricks, including roofing tiles. In addition, they could be the base for bodies to manufacture porous wall tiles. The Samassi clayey silts are rich in quartz and calcite, with a clay fraction consisting mostly of illite, smectite and chlorite. They are characterised by high dry shrinkage and excessive porosity of the fired samples, for which they are suitable only for mixes for common bricks.