Unshaped (monolithic refractories)
These are without definite form and are only given shape upon application. These types are better known as monolithic refractories. The common examples are plastic masses, Ramming masses, castables, gunning masses, fettling mix, mortars etc.
Dry vibration linings often used in Induction furnace linings are also monolithic, and sold and transported as a dry powder, usually with a magnesia/alumina composition with additions of other chemicals for altering specific properties. They are also finding more applications in blast furnace linings, although this use is still rare.
Types of monolithic refractories
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.
These are monolithic refractory materials, which are tempered with water or added with a binder. They have sufficient plasticity to be pounded or rammed into place.
These materials are very similar to plastic refractories though are much stiffer mixes.
These materials are similar to plastic refractories, but have a soft plasticity, so they can be pounded into place.
This type of product is used to protect refractory linings usually against chemical attack. Coating refractories are normally intended to cover just the working surface of a lining. They tend to be fairly thin layers.
Mortars consist of finely ground refractory materials mixed with water to form a paste. They are used for laying and bonding shaped refractory products such as bricks. They are normally applied by trowelling.
Insulating castables are specialised monolithic refractories that are used on the cold face of applications. They are made from lightweight aggregate materials such as vermiculite, perlite, extend-o-spheres, bubble alumina and expanded clay. Their main function is to provide thermal insulation. They are typically of low density and low thermal conductivity. Insulating refractories have inferior mechanical strength to that of conventional castables.