Thermal expansion behavior — Which refractories characteristically show overall expansion upon heating through service temperatures?

Introduction:
Dimensional stability is critical in refractory linings. Silica bricks are known for their unusual positive permanent expansion on heating due to silica polymorphic transformations, which can tighten roof joints in service. Fireclay bricks exhibit more conventional thermal expansion behavior and may not show the same pronounced permanent expansion.

Given Data / Assumptions:

• Silica bricks contain high SiO2; fireclay bricks are alumino-silicate with lower SiO2/higher Al2O3.
• Heating cycles pass through the quartz, tridymite, and cristobalite transformation ranges.
• Question asks which shows expansion on heating as a notable characteristic.

Concept / Approach:
Silica has several high-temperature polymorphs with different specific volumes. As temperature rises, transformations lead to net expansion, some of which may be irreversible (permanent set). This behavior can be advantageous for furnace roofs by tightening joints. Fireclay bricks expand thermally but do not exhibit the pronounced permanent expansion associated with high-silica compositions.

Step-by-Step Solution:
Recognize silica behavior: marked expansion across polymorphic transitions.Recognize fireclay behavior: typical thermal expansion without large permanent growth.Therefore, silica bricks are the clear answer.

Verification / Alternative check:
Refractory property charts show positive permanent expansion for silica bricks; design practices exploit this for roof construction.

Why Other Options Are Wrong:

• Fireclay alone: lacks the same strong permanent expansion trait.
• Both/Neither: conflicts with well-documented silica behavior.

Common Pitfalls:
Confusing reversible thermal expansion with irreversible permanent expansion; ignoring polymorphism in silica.

Final Answer:
Silica bricks