Identify the non–high-alumina refractory: which one of the following is NOT classified as a high-alumina refractory material?

Introduction / Context:
High-alumina refractories are based on alumina-rich phases like mullite (3Al2O3·2SiO2) and corundum (Al2O3), and include processed materials such as calcined bauxite. Recognizing which materials belong to this family helps in selecting linings for high-temperature, corrosive environments. This question asks you to pick the option that does not belong to the high-alumina class.

Given Data / Assumptions:

• Mullite, corundum, bauxite, sillimanite are alumina-bearing systems.
• Dolomite (CaO·MgO) is a basic refractory, unrelated to alumina–silicates.
• Classification is by principal chemistry and phase assemblage.

Concept / Approach:
High-alumina refractories derive strength, refractoriness, and corrosion resistance from alumina-rich phases. Mullite and corundum are canonical; bauxite is a natural alumina source; sillimanite (Al2SiO5) sits within the alumino–silicate family. Dolomite, however, consists primarily of lime and magnesia—typical of the basic class—used where slags are basic and alumino–silicates would be attacked. Therefore, dolomite is the outlier.

Step-by-Step Solution:
List high-alumina candidates: mullite, corundum, bauxite, sillimanite.Identify dolomite as basic (CaO–MgO) rather than alumino–silicate.Select dolomite as NOT high-alumina.

Verification / Alternative check:
Materials manuals classify dolomite alongside magnesia products for basic oxygen furnaces, confirming the non–high-alumina identity.

Why Other Options Are Wrong:
Mullite, corundum, bauxite, sillimanite: all are high-alumina or alumino–silicate refractories.

Common Pitfalls:
Confusing “mullite” with “magnesite”; the latter is basic (MgCO3 → MgO).Assuming bauxite is only an ore; it is also a feed for high-alumina refractories.

Final Answer:
Dolomite