The effect of microsilica on the thermal conductivity of refractory fireclay-based castable was investigated. The experimental technique employed for thermal conductivity measurements was the hot-wire cross-array method. The temperature of measurements ranged from room temperature up to 1000 °C both for unfired and fired material. In unfired samples, thermal conductivity decreased to 600 ºC due to hydration reactions. The thermal conductivity of fired samples increased with temperature and was higher by up to 8% for castable with microsilica. The results of ultrasonic wave velocity tests allow explaining the changes that occur in the structure of castable under heating.

Keywords: refractory castable, fireclay, thermal conductivity, microsilica, ultrasonic wave velocity