Natural silica behavior at high temperature: which statements about quartz-based silica used in refractories are correct?

Introduction / Context:
Silica refractories rely on controlled transformations of silica polymorphs. Natural silica (quartz) undergoes phase changes during heating that influence thermal expansion, creep, and spalling resistance. Recognizing these transformations is essential when designing coke-oven walls and glass furnace crowns.

Given Data / Assumptions:

• Natural silica source: high-purity quartzites.
• High-temperature exposure occurs during service and initial bake-out.

Concept / Approach:
On heating, quartz converts to high-temperature silica polymorphs—tridymite and cristobalite—through intermediate steps. These changes are accompanied by substantial volume changes and anisotropic expansion, which must be managed by brick design and expansion joints. Therefore, statements (a), (b), and (c) are all valid.

Step-by-Step Solution:

Verification / Alternative check:
Thermal expansion curves of silica bricks show characteristic transformation plateaus and jumps. Proper firing stabilizes desired phases to improve service stability.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing reversible α↔β quartz transition with high-temperature transformations; neglecting expansion allowance during installation.

Final Answer:
All (a), (b) and (c).