Almost every high-performance refractory contains alumina, and almost every enquiry we receive names it in one word. But alumina is a family, not a material. Two grades can both assay above 99% Al₂O₃ and behave completely differently in a mix — because what separates them is not chemistry but how they were fired and how fine they are.
Calcined alumina: the starting point
Calcined alumina is produced by heating aluminium hydroxide until it converts to the alpha phase. It is the base material from which the other grades are made. Crystal size and soda content are the two figures worth reading: low soda matters wherever alkali attack or electrical resistivity is a concern, and crystal size governs how the powder behaves during firing.
Reactive alumina: the matrix former
Reactive alumina is calcined alumina ground far finer, typically to a sub-micron particle size. That fineness is the whole point: it fills the space between coarse grains, drives sintering at lower temperature, and lets a castable reach high density with less water. If your castable is short of strength or takes too much water, the matrix fraction is usually where the answer lies.
Tabular alumina: the aggregate
Tabular alumina is sintered above 1800 °C without additives, producing large tablet-shaped crystals and very low porosity. It exists to be the coarse skeleton of a refractory: dimensionally stable, resistant to thermal shock, and largely inert to slag. It is the most expensive of the three, and specifying it where a calcined grade would serve is a common way to overpay.
Reading the three side by side
| Grade | Typical role | Particle size | What you pay for |
|---|---|---|---|
| Calcined | General purpose, ceramics | Coarse to medium powder | Purity and low soda |
| Reactive | Castable matrix, bond phase | Sub-micron | Fineness and sintering activity |
| Tabular | Coarse aggregate, skeleton | Graded grain, 0–6 mm typical | Density and thermal-shock resistance |
| Bauxite | Economical aggregate | Graded grain | Cost per tonne, at lower purity |
How to write the specification
A specification that a supplier can quote against reliably names four things, and none of them is the word “alumina” on its own:
- The grade family — tabular, calcined, reactive, or bauxite-based.
- Minimum Al₂O₃ and maximum Na₂O, because soda is what usually varies between offers.
- The particle size distribution you need, not just a nominal top size.
- Bulk density, where the material is an aggregate rather than a powder.
Those four lines remove most of the ambiguity that makes quotes incomparable, and they let a supplier tell you honestly when a cheaper grade would do the same job.