Why must dolomite refractories be impregnated with tar or pitch after firing?

Dolomite refractories must be impregnated with tar or pitch after firing to prevent the rehydration of the lime (CaO) contained within the material.1

Because dolomite is composed of calcium and magnesium carbonates, the burning process produces lime, which is highly susceptible to moisture. If left unprotected, this lime would react with water in the atmosphere, causing the refractory to hydrate and disintegrate. Impregnating the bricks with tar or pitch provides a protective barrier that seals the material against moisture. Additionally, the residual carbon left by this treatment enhances the refractory's resistance to the basic slags and iron oxides encountered in industrial processes like steelmaking.1