For good fireclay used in refractory bricks and furnace linings, the percentage of alumina (Al2O3) and silica (SiO2) typically varies in which of the following paired ranges (Al2O3, SiO2)? Choose the most appropriate overall statement.

Introduction / Context:
Fireclays are alumino-silicate materials formulated for refractoriness and thermal-shock resistance. The Al2O3/SiO2 balance governs softening temperature under load and spalling behavior. The question asks you to recognize the acceptable composition window for good fireclay products.

Given Data / Assumptions:

• We refer to ordinary fireclays, not high-alumina specialties.
• Compositions are given as approximate percentages of Al2O3 and SiO2.
• Minor constituents (Fe2O3, alkalis) are ideally low.

Concept / Approach:

Good fireclays commonly fall within Al2O3 ≈ 25–35% and SiO2 ≈ 65–75%. All three pairs listed (25,75), (30,70), and (35,65) sit within this standard window. Performance then depends on impurities and firing, but compositionally each pair reflects acceptable practice.

Step-by-Step Solution:

Verification / Alternative check:

Refractory catalogs list multiple grades within these ranges, tuned for thermal shock vs load-bearing at temperature.

Why Other Options Are Wrong:

20,80: falls below the typical alumina threshold for good fireclay and tends to lower refractoriness.

Common Pitfalls:

Confusing fireclay with high-alumina or silica refractories; assuming a single fixed composition rather than a range.

Final Answer:

All of these