Refractory Materials, Properties, and Selection
Materials and properties
Monolithic refractories are classified by binder and placement method: castables, plastics, ramming mixes, and gunning mixes. Within castables, cement content drives behavior: conventional castables contain relatively high calcium aluminate cement, low-cement castables (LCC) are formulated with dispersants and minimal cement, and ultra-low-cement or cement-free systems use colloidal silica or hydratable alumina. Aggregates include calcined clay, mullite, bauxite, corundum, spinel, and silicon carbide; each changes density, erosion resistance, and slag chemistry. Key properties are bulk density, apparent porosity, cold crushing strength (CCS), modulus of rupture (MOR), permanent linear change (PLC), thermal expansion, thermal conductivity, and abrasion resistance. Strength must be compared on the same basis: cured, dried, or fired. Traps: assuming LCC means low strength (it often has higher hot strength and better volume stability); selecting on maximum temperature alone while ignoring slag, alkali, or thermal cycling; and treating a high cold CCS as proof of good hot performance. A worked rule: for a burner area with thermal cycling and ash erosion, specify a low-cement castable with a mullite or bauxite aggregate rather than a conventional clay-bonded castable, and require PLC and MOR data at the service temperature. Match the refractory chemistry to the slag: silica-bearing aggregates are poor against basic slags, while high-alumina or spinel systems resist basic attack better.