Firing behaviour of Tertiary, Cretaceous and Permo-Triassic clays from Castellon ceramic cluster (Spain)

This study is focused on the knowledge of clays historically used in manufacturing of ceramic pavement and coatings from the province of Castellon (NE, Spain) with a large ceramic industry. This research began in the early 1980s in the research group led by Professor T. Sanfeliu and continues today. This review paper and the last advances in knowledge have been written in honor of Professor E. Galan, one of the pioneers in the study of ceramic clays and kaolin in Spain. The objective of this paper is to extract conclusions about mineralogy, porosity and bending strength in ceramic test bodies manufactured using Tertiary-age, Cretaceous and Permo-Triassic local clays. Tertiary clays show a high CaO content and their predominant mineral phases are calcite and quartz. Cretaceous clays show a scarce CaO content. These Cretaceous raw materials are clays with high quartz and illite percentages. Permo-Triassic clays present a difference in the K2O and Fe2O3 content. The mineralogical composition is mainly formed by quartz, clay minerals and hematites. Ceramic test bodies were molded by extrusion and undergoing a firing process. Characterization of the fired clays was accomplished by TOM and XRD. Granular texture and porphyry structure, quartz phenocrysts and opaque hematite crystals were observed. Illitic clays rich in carbonates form plagioclases, wollastonite and gehlenite from 950 degrees C. In samples with low content in calcite and dolomite, the predominant mineral phases are quartz and hematite from 1000 degrees C onwards. A mercury porosimeter was used for determining the porous texture of ceramic matrix. A progressive reduction in open porosity and an increase of bending strength are observed with increasing temperature. The essays for bending strength were carried out by an INSTRON. A decline in porosity was observed when temperature rise associated with an increase in bending strength. Establishing relations between bending strength and the seven parameters studied link to the porous behaviour of the ceramic tests it was not possible because empirical equations do not fit experimental results.