Refractory cements and raw materials are hard, heat resistant materials that are suitable for applications which require high temperature strength, wear resistance, electrical or thermal insulation, or other specialized characteristics. Refractory cements are used to patch or line furnaces and bond bricks or joints in high temperature applications. Most refractory cements consist of a mixture of a binder or clinker and a coarser aggregate. Clinker is a fused mineral or mineral mixture, such as alumina or bauxite and lime, which is crushed into a fine powder. Aggregates are coarser, granular materials that are added to mortar, grout, and cement to impart special properties such as refractoriness and erosion or oxidation resistance. Refractory raw materials include products such as alumina ceramics, fire clay, bricks, pre-cast shapes, aluminate cement, and monolithics. Monolithic refractories are liners fabricated without joints by ramming, gunning or casting refractory cements in place.
There are many types of refractory cements and raw materials. Mineral aggregates consist of crushed rock, stone, or sand and require thermal processing. Castable products are poured into forms or cavities to fabricate refractory liners. Coatings and sealants can be sprayed or brushed onto surfaces such as furnace walls. Dry refractories are supplied as a dry powder that is applied and fired in place. Granular fill such as vermiculite is loaded into a cavity to provide insulation and remains in a loose, unbonded condition. Gunning mixes are powdered products that are loaded with a gun into a form or onto a furnace wall to create an insulating layer. Investment refractory cements and raw materials consist of a refractory powder with a plaster or phosphate binder. Potting compounds and encapsulants are used to provide electrical insulation. Wet rams are refractory cements with enough plasticity to allow the wet mix to be rammed or formed into place in a furnace or a form. Refractory mortars, rigidizers, and synthetic aggregates are also available.
Selecting refractory cements and raw materials requires an analysis of material types and bonds. Material types include alumina, alumina-zirconia, aluminum silicate or sillimanite, calcium aluminate, calcium silicate, carbon or graphite, chromia or chromite, dolomite, kaolin or fire clay, magnesia or magnesite, magnesium silicate or forsterite, mullite, porcelain, silicon carbide, silica or fused silica, spinel, titania or titanate, yttria, zircon, and zirconia. Cement bond and set types include calcium aluminate, calcium silicate, phosphate, silicate, and sulfur or sulfate. Polycrystalline ceramics or aggregate-based refractories that use chemical, hydraulic, or thermal bonds are also available. Organic or polymer resin binders hold refractories together until firing. Some resins are designed to burn out. Others are converted to carbon.
Refractory cements and raw materials differ in terms of cement properties and material features. Cement properties include maximum use temperature, thermal conductivity, modulus of rupture (MOR) or flexural strength, compressive strength, particle or aggregate size, density, and shrinkage. Material features indicate whether refractory cements and raw materials are acidic, alkaline, or neutral; fiber-reinforced or binder-free; or suitable for corrosive environments or electrical applications. Composite materials consist of a matrix material reinforced with a stronger or higher modulus second phase. The matrix may consist of a ceramic, metal, or polymeric material. The second phase may be in the form of particulates, chopped fibers or continuous fibers.