Silica refractories: which of the following statements about silica bricks is correct?

Introduction / Context:
Silica bricks are widely used where high refractoriness and load-bearing at elevated temperatures are required, such as in coke ovens and hot-blast stoves. However, they have well-known limitations, particularly poor thermal-shock resistance. Selecting the correct statement requires recalling both their strengths and weaknesses.

Given Data / Assumptions:

• Silica bricks are rich in SiO2 (often > 93%).
• Service areas include coke oven walls, regenerators, and hot-blast stove crowns.
• Thermal conductivity relative to fireclay depends on temperature but is typically not lower at high temperatures.

Concept / Approach:
Silica bricks possess high refractoriness and good creep resistance at high temperatures, yet they are susceptible to spalling under rapid heating or cooling, i.e., poor thermal-shock resistance. Contrary to option (b), they are indeed used in hot-blast stove domes. Regarding (c), silica's thermal conductivity is not generally lower than that of fireclay bricks at service temperatures; in many data sets, it is higher. Hence only statement (a) is the accurate characterization among the options provided.

Step-by-Step Solution:
Review applications: silica is used in stoves and coke ovens → (b) is incorrect.Compare thermal-shock behavior: silica spalls easily → (a) is correct.Compare conductivity: silica is not lower than fireclay at high T → (c) is incorrect.Therefore, only (a) stands as correct.

Verification / Alternative check:
Property tables list low thermal-shock indices for silica and extensive use in stove crowns, confirming (a) and refuting (b) and (c) for typical service conditions.

Why Other Options Are Wrong:
(b) Misstates a common application; silica bricks are suitable for stove crowns.(c) Misrepresents relative conductivity; silica is not characteristically lower than fireclay at high temperatures.(d) Cannot be true because (b) and (c) are false.(e) Opposite of reality; thermal-shock resistance is a weakness.

Common Pitfalls:
Assuming high refractoriness implies good thermal-shock resistance—properties are independent.Overgeneralizing conductivity at room temperature to high-temperature service.

Final Answer:
They crack when subjected to a sudden change of temperature (poor thermal-shock resistance).