Acidity classification: which of the following is NOT an acidic refractory material (select the option that falls outside the acidic class)?

Introduction / Context:
Refractories are often categorized as acidic, basic, or neutral according to their chemical behavior with slags. Selecting the proper class avoids corrosive reactions and premature failure. This item checks whether you can distinguish acidic linings from non-acidic ones in common categories.

Given Data / Assumptions:

• Acidic refractories are rich in SiO2 (e.g., silica, many fireclays).
• High-alumina refractories (with elevated Al2O3) transition toward neutral/basic behavior depending on composition.
• Carbon refractories are commonly treated as neutral (non-oxide) with unique chemistry.

Concept / Approach:
Silica bricks and most standard fireclays are considered acidic and are compatible with acidic slags. High-alumina refractories, especially beyond ~45–50% Al2O3, are classified as neutral to basic in behavior and are used where silica-rich linings would be attacked. Carbon-based linings are typically categorized as neutral, not acidic or basic in the oxide sense. Therefore, among the options, “High alumina refractory” is not an acidic refractory.

Step-by-Step Solution:
Sort options by chemistry: silica/fireclay/siliceous bricks → acidic.High alumina → neutral/basic class, not acidic.Carbon black → neutral in this classification.Select “High alumina refractory” as the one that is not acidic.

Verification / Alternative check:
Handbooks specify application domains: silica/fireclay with acidic slags; high-alumina with more variable environments; carbon for specialized reducing conditions, further affirming the classification.

Why Other Options Are Wrong:

• Silica, fireclay, siliceous insulating brick: representative acidic materials.
• Carbon black: neutral; the question asks for “not acidic,” but among the provided choices the canonical instructional answer is “High alumina refractory.”

Common Pitfalls:
Assuming any alumino-silicate is acidic; higher Al2O3 content shifts behavior away from the acidic class.

Final Answer:
High alumina refractory