Abstract
In the ceramic roofing tile manufacturing, absence of good mechanical properties, such as strength and hardness, leads to reject production of both green and fired tiles. We evaluated the effects of firing temperatures on bending strength and Vickers hardness in fired ceramic roofing tiles from kaolinite and illite-carbonate clay materials. The separately designed ceramics samples were shaped by dry pressing at 25 MPa, and then fired at 960 and 1050 °C following exposures to freezing/thawing cycles. The aim was to compare the bending strength and Vickers hardness, before and after cycling. The results showed that Vickers hardness HV5 and bending strength increased with increasing firing temperatures. For cycling specimens and firing temperature used, the values of hardness and bending strength were always lower. For the tile samples, where the low load hardness was evaluated, we also analysed the indentation size effect (ISE). The results followed the same pattern for samples obtained after freezing and thawing. The best correlation between measured values and available mathematical models was achieved with the modified proportional specimen resistance model. The most reliable results for the roofing tiles examined were achieved for the load independent hardness, H LIH when Vickers hardness measurements were HV0.5 and HV1.
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