Name two refractories which should not be placed in direct contact with fire clay refractory.
Answers
Refractory materials, like all solids, expand on heating, the degree of expansion being related to the chemical composition of the refractory. Magnesium oxide has the highest coefficient of expansion of the refractory materials commonly used. Silicon carbide has a comparatively low coefficient of expansion and fused or vitreous silica has the lowest expansion of all the refractory materials commonly used. Materials with high coefficients of expansion typically have poor resistance to thermal shock.
Fire clay sand mortar mix dries very slowly, but if you support the side walls by pouring concrete cladding behind them, leave it cure and then form arched ceiling using the 50:50 fireclay : sand mortar is the best option. When I go fast I use a little of Portland cement in the mortar so it’s setting in 2 hours. Portland cement is already a bit refractory but it’s wise to replace one half of the amount with lime. Lime is calcium and it takes over the cement when it burns out in heat. Old ovens were built using only fire clay with lime not cement. These ovens work forever, many of them are being fired daily for decades and never cool down.