Clay castables demonstrate robust performance in medium-temperature settings, retaining their strength and effectively resisting spalling. When heated, these castables experience a notable surge in strength between 800℃ and 1000℃, achieving over 30MPa. This is a substantial leap compared to the 10MPa of other high-grade castables. Interestingly, further increases in temperature only lead to a modest boost in strength.
The composition of the raw materials in clay castables dictates their physical and chemical changes. At low temperatures, the free water in the castable is gradually removed, while the bound water is quickly expelled between 450℃ and 600℃. In the medium-temperature range, a significant endothermic reaction takes place, yet the fluctuations in strength remain minimal.
Prior to reaching 800℃, clay castables undergo certain phase transitions, with only a slight increase in compressive and drying strength after firing. As the temperature rises from 900°C to 1050°C, free aluminum and iron begin to decompose. Between 1150°C and 1300°C, the formation of mullite and grain growth continues, causing the clay castable to become dense and more binding. Consequently, the strength also increases.
Due to the higher impurity content in clay compared to high-alumina bauxite, the sintering temperature appears earlier. This early sintering overcomes the challenges faced by castables in the medium-temperature range. In this temperature zone, the castable crystallizes well, with needle-like mullite crystal phases interlocking within the material, providing considerable strength in the medium-temperature range.
Thanks to the presence of clay, under the influence of dispersants and coagulants, the strength of clay castables at room temperature and low temperatures ranges from 800℃ to the liquid phase. As the temperature rises, the amount of liquid phase increases. Simultaneously, the acicular mullite formed in the clay castable enhances its medium-temperature strength. Therefore, clay refractory castables are extremely well-suited for use in medium-temperature areas.
