The effect of the pore size on the properties of insulating firebricks was studied when different grain sizes of the expanded polystyrene (EPS) were used as an organic pore-forming agent. In particular, porous firebricks were produced by extruding of EPS and other materials. Due to the different grain sizes of EPS, insulating firebricks with different pore sizes were obtained. The results showed that the bricks with a smaller pore size (EPS 1.4 samples) had higher cold crushing strength. It is due to the higher continuity of the solid phase between the pores of the EPS 1.4 samples regarding EPS 2.8 bricks. Although the percent of reheat change of both EPS 2.8 and EPS 1.4 samples were nearly equal up to 1000 °C, EPS 2.8 bricks shrank faster than EPS 1.4 firebricks above 1000 °C. The thermal conductivity of the EPS 2.8 samples was higher than EPS 1.4 bricks. It can be concluded that in equal volume fraction of voids, the greater the number of pores of smaller size, the lower the thermal conductivity.
