Background The sole purpose of a refractory binder is to gel materials together in the green state. At elevated temperature binders then either dehydrate, volatise or sinter leaving behind a refractory material.
There are a number of binders which are commonly used for refractory applications, the most common of which is high alumina cement. They are always selected with the environment of the final application in mind.
Types of Bonds Formed in Monolithic RefractoriesTable 1 summarises the types of bonds formed by the various refractory binders.
Table 1. Bonds formed by refractory binders.
Type of binder Nature of bond formed Materials Cement Hydraulic bond Calcium aluminate cement Activated aluminas Hydraulic bond Pseudo boehmite Colloids Hydraulic bond Colloidal alumina, colloidal silica. Phosphate Chemical bond Aluminium phosphate Silicate Chemical bond Sodium silicate or potassium silicate Resin bonded Organic bond Phenolic resin Adhesive Adhesive bond Dextrin, syrup, pitch, polyvinyl alcohol, carboxymethyl cellulose, arabic gum Ceramic Ceramic Bonded All refractory materials after their heat up to high temperatures.
Calcium Aluminate CementsCalcium aluminate cements are the most common types of refractory binders. Following the addition of water they hydrate to form a network of hydrated species which bond the particulate materials together. During heating they dehydrate then undergo sintering, with the aggregate forming a strong ceramic bond.
Activated AluminasActivated aluminas or hydratable aluminas are used in ultra-low and cement-free castables. They consist of high surface area r-alumina species which are formed from partially calcined alumina hydrates. On the addition of water these materials will gel together in a cement-like action by forming alumina hydrates. One advantage of this system is that they do not introduce any calcia into the system, so the fired material will be more refractory. Often the hydration of activated aluminas can be slow and uncontrollable and small amounts of cement additions are required to trigger the reaction.