These materials consist of precision graded coarse and fine refractory grains. They are gelled by means of a binder system in the materials green state. Following the heat-up of the material the binder either transforms or volatilises facilitating the formation of a ceramic bond. The most common binder used in castables is HAC (high alumina cement). Other binders that are often used include hydratable aluminas and colloidal silica. Castables are mixed with water and then installed by either pouring or pumping. Placement of the material then requires vibration.
The cement-containing castables are often classified by the amount of cement they contain. Conventional castables can contain around 15-30% cement binder. As refractory technology evolved chemical additives were included in the package to reduce the amount of cement and water the product required - the impact of this was material with improved strengths and durability. Low cement castables contain between about 3-10% cement by weight. Ultra low cement castables contain less than 3% cement.
A specialised type of refractory castable is the free flow castable, which can be installed without vibration and require much less water than traditional castables. This is due to the fact that they have particle packing and dispersing agents that modify the surface chemistry of the fine particles to improve the flow of the material.
Certain castable formulations may be installed via gunning techniques, which involves spraying the material through a nozzle at a high speed. At the nozzle, cement accelerators are often added to promote rapid hardening of the material. This technique helps workers line applications quickly.