Shestakov / Dmytro
Gelling materials together in the green state is the main purpose of a refractory binder. At higher temperatures, binders subsequently volatilize, dehydrate, or sinter, leaving behind a refractory material.
There are several binders that are typically used for refractory applications; the most widely used are high alumina cement. They are always chosen keeping in mind the environment of the end application.
Types of Bonds Formed in Monolithic Refractories
The types of bonds formed by the various refractory binders are summarized in Table 1.
Table 1. Bonds formed by refractory binders
|Type of binder
||Nature of bond formed
||Calcium aluminate cement
||Colloidal alumina, colloidal silica.
||Sodium silicate or potassium silicate
||Dextrin, syrup, pitch, polyvinyl alcohol, carboxymethyl cellulose, arabic gum
||All refractory materials after their heat up to high temperatures.
Calcium Aluminate Cements
The most general types of refractory binders are calcium aluminate cements. When water is added, they hydrate to form a system of hydrated species that bond the particulate materials together. While heating, the calcium aluminate cements dehydrate, and subsequently go through sintering, while the aggregate forms a robust ceramic bond.
Activated aluminas, also known as hydratable aluminas, are utilized in ultra-low and cement-free castables. They contain ρ-alumina species that have a high surface area and are developed from partially calcined alumina hydrates. When water is added, these materials gel together in a cement-like action by creating alumina hydrates.
One benefit of this system is that they do not add any calcia to the system, so the fired material tends to be more refractory. Usually, the hydration of activated aluminas can be gradual and uncontrollable and hence small quantities of cement are needed to activate the reaction.