Raw kaolinitic–illitic clays as high‑mechanical‑performance hydraulically pressed refractories

The usage possibility of 19 composites of raw refractory clays from Serbia containing approximately 53.29% of SiO2 and 26.73% of Al2O3 is presented. The sum of fluxing oxides was 57.74%, while these materials contained 32% of quartz, 29% of kaolinite and 26% of illite–mica. Dilatometry tests revealed a sudden shrinkage with the peak at approximately 1115 °C, owing to the formation of mullite. The refractoriness was in the range of 1581–1718 °C, which classifies the composites from low- to high-duty refractories. Based on correlation analysis, the refractoriness mostly depended on the content of alumina. The lightness of the fired test pieces was lower after firing when compared to the dry samples, and it decreased with the firing temperature from reddish to grayish. The study presents a novel attempt to define all the necessary properties of raw refractory clays and products fired at the 1100–1300 °C range on a laboratory level. Most of these clays can be used as natural refractory materials for ceramic and glass furnace lining. The organic matter in some of the samples influences negatively the fast-firing process