Actually, fire clay is not strong. Fireclay are all clinkers with no strength, also known as "blade mud". It is used as the filler with a thickness of not more than 3mm. Therefore, it can not be used for thermal insulation inside the furnace. Inner thermal insulation of the civil boiler can use "alumina cement", also known as refractory cement.
Fire-resistant adhesive material is needed, its effect equals to the cement. Cement will certainly do.
Mix refractory cement (ie aluminate cement CA50) with some refractory aggregate (bauxite, slag from flint clay) and powder (ie, crushed aggregate). Add some water to the mixture and stir, and it can be used. The clinker can be refractory powder.
