What do you know about the inorganic binders commonly used in refractories?

As we all know, refractory materials are mainly composed of raw materials such as refractory aggregate, refractory powder, binder, admixture and so on. Commonly used binding agents can be divided into four categories, and their binding mechanisms are different.

 

First, calcium aluminate cement

Calcium aluminate cement is a cement whose main mineral component is calcium monoaluminate or calcium dialuminate. It is made by natural bauxite or industrial alumina mixed with calcium carbonate in a certain proportion, and calcined or fused. It is also made by mixing bauxite with limestone and melting. It is mainly used as a binder for refractory castables and spray materials. Medium and low grade refractory castables use ordinary calcium aluminate cement, and high-grade refractory materials (corundum, mullite, chrome alumina, corundum spinel) use pure calcium aluminate cement.

 

Second, phosphoric acid and phosphate

 

Phosphate bonding is thermosetting. In general, if hardeners such as cement are not added, phosphoric acid needs to react with the refractory to form a chemical bond under elevated temperature conditions.

After heating, the phosphate-bound phase undergoes very complicated chemical changes. The specific conditions vary depending on the temperature, the original composition of the binder, and the composition and activity of the refractory. For example, after mixing phosphoric acid and industrial alumina, heat treatment at 1200 ° C will form apatite AIPO4.2H2O; after 200 ° C heat treatment, the binding phase is still aluminite; after heat treatment at 350 ° C, form aluminite AIPO4; After heat treatment above 500 ° C, the formation of AIPO4 in phosphorous quartz and cristobalite began to form.

 

In general, the aluminum phosphate binding phase needs to be heat-treated above 500 ° C to maintain long-term stability in the atmosphere. Because, there are various crystal forms of anhydrous aluminum phosphate AIPO4, in addition to phosphorous quartz type, cristobalite type, and apatite type, there are also metastable crystals such as A, B, C, D, and E. Among them, A, B, C, D, E and other metastable aluminum phosphates will absorb moisture and soften. If the heat treatment temperature is not enough and there are not enough accelerators, the performance and life of phosphate-bonded refractories will be affected.

 

Phosphoric acid is used in phosphoric acid-bound refractory bricks, castables, fire sludge; aluminum phosphate is used in acidic and neutral refractories; sodium hexametaphosphate can be used in alkaline refractory binders.