Ceramic applications of Middle Ordovician shales from central Spain

This paper describes the mineralogical and ceramic characteristics of three Ordovician shales from the Ciudad Real province (Central Spain). These materials are mainly composed of phyllosilicates (> 85%), quartz, K-feldspar, and hematite. Clay mineral assemblages include illite + kaolinite + paragonite ± chlorite, with traces of mixed-layered muscovite/paragonite. Chemical analyses indicate a Fe2O3 content of less than 8%. Three different types of complementary raw materials (quartzitic sands, quartzo-feldspathic sands, and fly ash) have been used to improve the linear shrinkage and water absorption of clayey raw materials. Firing tests were performed maintaining the sample at Tmax for 2 h on both clay raw materials and improved clay samples and they indicate an optimal firing range between 1100–1150°C. Under these conditions linear shrinkage is less than 8% and water absorption is close to zero. The analytical results therefore indicate that these clays meet the firing specifications required by the ceramic industry for application in the manufacture of red stoneware tiles. Moreover, the addition of fly ash improves the ceramic properties of the clay materials and is an ecological application of an industrial waste.