Refractory engineering — Spalling of silica bricks occurs due to abrupt volume changes when cooled below approximately which critical temperature (°C)?

Introduction:
Silica bricks are widely used in high-temperature furnaces because of their high refractoriness and load-bearing capacity. However, they are susceptible to thermal spalling near the quartz polymorphic transformation. This question checks understanding of the critical temperature at which sudden volume changes can damage silica refractories during cooling.

Given Data / Assumptions:

• The brick composition is silica-rich (high SiO2).
• Cooling through the quartz inversion range is the concern.
• Question asks for the approximate temperature at which abrupt volume change causes spalling.

Concept / Approach:
Silica (SiO2) exhibits polymorphism. The alpha–beta quartz inversion occurs around 573°C. When a hot silica brick cools through this temperature, abrupt volume change occurs. If cooling is fast or temperature gradients are high, tensile stresses can exceed the brick’s strength, leading to cracking or layers flaking off (spalling).

Step-by-Step Solution:
Identify the inversion: alpha–beta quartz transformation at roughly 573°C (engineers commonly round to 570°C).Relate inversion to volume change: the structural change causes a significant, sudden specific volume shift.Link to spalling: thermal gradients during passage through ~570°C create internal stress and spall.Hence, the critical threshold is closest to 570°C.

Verification / Alternative check:
Materials handbooks and refractory datasheets consistently warn to slow-cool silica linings through ~600–500°C to avoid spalling—confirming ~570°C as the critical region.

Why Other Options Are Wrong:

• 770°C: Above the critical inversion; not the main spalling trigger.
• 270°C and 70°C: Too low; most inversion-driven volume change has already occurred.

Common Pitfalls:
Confusing the cristobalite or tridymite transformations with the dominant quartz inversion; ignoring the rate of cooling’s influence.

Final Answer:
570