Researches in the field of composite HCBS and of the refractory materials on their base in Al2O3‒SiO2‒SiC system. Part 4

The effect of long heat treatment (single and double firing in a tunnel kiln) of samples based on a HBCS in Al2O3‒ SiO2‒SiC-system on their indicators was studied. Samples containing 15 % SiC and 1 and 2 % Si are characterized by a significant increase (up to 9 %) an increase in porosity and a decrease in strength compared to the original samples (without SiC) after long heat treatment (120 hours in the range of 1300‒1400 oC). A significant increase in the volume of the samples is due not only to the oxidation of SiC or Si, but also to additional mullite formation. It is assumed that sequentially following the process of oxidation of SiC at a certain stage of heat treatment, the process of mullitization proceeds due to the interaction of the newly formed SiO2 in the form of cristobalite with Al2O3 bauxite. Ill. 6. Ref. 19.

Technological behaviour of some Tunisian clays prepared by dry ceramic processing

Lower Cretaceous (AJO and AJR) and Oligocene clays (AS) from northern Tunisia were analysed to evaluate their possible use in the production of earthenware tiles by dry processing and fast double-firing. The Cretaceous clays are carbonate-rich (AJO = 20%, AJR = 12%) while the Oligocene ones are carbonate-free. This led to noticeable differences in firing behaviour (shrinkage, sintering rate and loss on ignition) and consequently in functional properties (water absorption, mechanical strength, porosity). The AJO firing shrinkage is very small, which makes this clay suitable for rapid firing. The clays are illite-kaolinite-rich but the AS sample is mostly smectitic (44%) and so is used (10 wt.% maximum) only to adjust the consistency of the powder during pressing. The average agglomerate size ranges from 100 to 350 μm and the distribution is suitable for easy pressing of powders without any special need for further adjustments. Characterization of fired products confirms the high potential of these clays since all properties fall within the ceramic International Standards (ISO). For both technical and economic reasons dry processing is recommended for production of earthenware tiles, in particular for countries in sunnier climates, where solar energy can be exploited for clay drying.