High-alumina refractories: composition limit In commercial practice, the alumina (Al2O3) content of high-alumina refractories can reach up to approximately what maximum percentage by mass?

Introduction / Context:
High-alumina refractories span a wide composition range, from about 45% Al2O3 up to very high alumina contents approaching pure corundum. Knowing the upper limit helps in selecting materials for extreme temperature and corrosive environments.

Given Data / Assumptions:

• We are considering commercial high-alumina refractories, not single-crystal alumina.
• Percentages are approximate mass percentages of Al2O3.

Concept / Approach:
As alumina content increases, refractoriness under load, slag resistance, and hot strength generally improve. Commercial bricks and monolithics are produced with Al2O3 contents in bands (50%, 60–70%, 80–90%). Near the top end, materials may be termed 90% alumina refractories, just below fused or sintered 99% alumina products.

Step-by-Step Solution:
Survey typical product grades: 45–50%, 60–70%, 80%, and ~90% alumina.Recognize that “high-alumina” includes very high Al2O3 contents short of technical alumina (99%).Thus, the practical maximum is about 90% Al2O3.

Verification / Alternative check:
Supplier datasheets and standards (e.g., ASTM classifications) list 90% alumina bricks and castables as common top-tier products in the category.

Why Other Options Are Wrong:
30% and 50% are too low for the “maximum” of the class. 70% is mid-range. 90% matches the upper end before ultra-high-purity alumina ceramics.

Common Pitfalls:

• Confusing “high-alumina” with “pure alumina”; 99% alumina is a different, specialty class.
• Overlooking impurities and glassy phases that limit performance at lower alumina contents.

Final Answer:
90%