Alumina, Explained: What the % on the Bag Actually Buys You
The alumina percentage is the headline number on most refractories. It tells you a lot, and hides a few things worth knowing before you spec by it alone.
Pick up almost any fireclay or high-alumina refractory and the first thing printed on it is a number: the alumina (Al₂O₃) content. It is a genuinely useful number. It is also the one people lean on too hard.
What rising alumina actually does
Refractories in this family live on the alumina and silica line. As alumina goes up, the material shifts toward mullite (about 72% Al₂O₃) and then toward corundum (essentially pure alumina). Mullite is the workhorse phase: strong at temperature, low creep, reasonable thermal shock behaviour.
More alumina, broadly, means:
- Higher refractoriness. It resists softening to a higher temperature.
- Better hot strength and creep resistance. It holds load at temperature without sagging.
- Better resistance to many slags, to reducing conditions, and to CO attack.
The reason is mostly about what it does not have. In a lower-alumina body, the glassy silica-rich bond phase is the first thing to soften. Push the alumina up and there is less of that low-melting glass, so the softening point climbs.
Where "high-alumina" starts
By common convention, fireclay refractories run up to roughly 44% Al₂O₃, and "high-alumina" begins around 45%. Above that, standard grades cluster near 50, 60, 70, 80, 85, and 90%, running up toward 99% for tabular-alumina and corundum bodies. The bauxite, andalusite, mullite, and calcined alumina that get you there cost more than fireclay, and the price climbs steeply near the top.
The catch: impurities and the bond phase
Two bricks at the same alumina percentage are not the same brick. What else is in there, such as alkalis (Na₂O, K₂O), iron oxide, and titania, and how the silica and alumina are balanced, can matter more than the last five points of Al₂O₃. A clean 60% brick can outlast a dirty 70% one in the right service, because the impurities are what form the low-melting glass you were trying to avoid.
Alumina content also says nothing directly about bulk density or apparent porosity. A high-alumina brick that is porous will still take slag. Read those lines next to the Al₂O₃ figure, not instead of it.
When more alumina is the wrong answer
- Strong alkaline environments. Alkali vapour and basic slags attack high-alumina materials in some processes. The extra alumina does not save you and can react to form expansive phases.
- Severe thermal cycling. Dense corundum-rich bodies have higher thermal expansion and can be less shock-tolerant than a mullite-rich 60 to 70% material. At furnace doors and roofs, that trade matters.
- Backup layers. The insulating and safety layers behind the hot face are not being attacked. Paying for alumina there is paying for nothing.
Match the alumina to the service temperature and the chemistry, not to the spec sheet. The right answer for the hot face of a ladle is the wrong answer for the roof of a reheat furnace, and both are on the same price list.
The short version
Buying refractory on alumina percentage alone is like buying a car on horsepower alone: informative, incomplete, and a reliable way to end up disappointed. Use the number as a starting filter, then look at purity, density, porosity, and the actual environment the lining has to survive.
If you want help matching a grade to a specific zone, our product range spans fireclay through 90%-plus alumina, and we are happy to talk through where the line should fall for your furnace.