Refractory thermal-shock behavior: the spalling tendency of a brick or castable lining is most effectively reduced by deliberately increasing which material parameter (under controlled, engineered porosity levels)?

Introduction / Context:
Spalling is the tendency of a refractory lining to crack, flake, or shell off when subjected to rapid heating and cooling. It shortens campaign life and increases maintenance in furnaces, kilns, and ladles. Engineers therefore tailor material parameters to resist thermal shock. One practical lever is the controlled introduction of porosity.

Given Data / Assumptions:

• Lining experiences thermal gradients and cyclic temperature swings.
• Microstructure (pores, grain size, bonding) governs crack initiation and growth.
• “Increased porosity” means engineered, well-distributed pores—not random defects.

Concept / Approach:
Thermal spalling is driven by tensile stresses from expansion differentials. Crack arrest and stress relief are enhanced when a material has compliant pathways. Porosity reduces the effective elastic modulus and introduces crack-deflection sites. As a result, stress intensity at flaws is lowered and catastrophic crack propagation is less likely. This is why insulating firebricks and shock-resistant alumino-silicate bricks often contain significant, well-controlled porosity.

Step-by-Step Solution:
Identify failure driver: thermal-stress-induced cracking.Relate stress to stiffness and flaws: lower effective modulus reduces peak stress.Controlled pores provide crack-arrest/deflection and strain accommodation.Hence, increased engineered porosity reduces spalling tendency.

Verification / Alternative check:
Standard refractory design texts show higher thermal-shock resistance indexes for materials with tailored porosity and microcracking. Field practice confirms better cycling endurance for porous shock-resistant grades compared with dense, very stiff bricks.

Why Other Options Are Wrong:
Specific gravity: density by itself does not guarantee stress relief.Thermal conductivity: may smooth gradients but is less decisive than porosity for crack arrest.Strength / Elastic modulus: very high stiffness can increase spalling risk under shock.

Common Pitfalls:
Over-increasing porosity, which can hurt abrasion and slag resistance.Confusing hot strength with shock resistance; they are distinct performance metrics.

Final Answer:
Porosity