The effect of texture on thermal expansion of extruded ceramics

Thermodilatometry of ceramic samples prepared from the mixture of 50 wt.% kaolin, 25 wt.% feldspar and 25 wt.% quartz was studied in the temperature range of 20-1000 degrees C. The samples were cut from the surface layer of the extruded ceramic cylindrical body. The X-ray structural analysis showed that the basal planes of the kaolinite crystals in the samples are predominantly parallel with the direction of extrusion. The shrinkage of the samples which was observed is caused by: 1. evaporating the capillary water (20-200 degrees C), 2. escaping of the OH groups from the octahedrical sheets of the kaolinite crystals (430-750 degrees C), 3. high-temperature changes of metakaolinite (above 850 degrees C). The shrinkage should be bigger in the perpendicular direction to the basal planes. This assumption is confirmed in these temperature ranges except the region of dehydroxylation. To explain this result, the extension of the kaolinite crystals and blocks of these crystals by steam passing along the octahedral sheets and crystal boundaries was supposed.