Fireclay bricks and thermal spalling behavior: As the open porosity of a fireclay refractory brick increases, how does its resistance to thermal spalling (cracking and flaking due to rapid temperature change) typically change?

Introduction / Context:
Thermal spalling involves crack formation when a refractory is subjected to rapid or repeated temperature swings. Fireclay bricks are widely used liners. Understanding how porosity influences thermal shock resistance is fundamental for choosing the right brick in furnaces and kilns.

Given Data / Assumptions:

• Material: fireclay brick (alumino-silicate composition).
• Property varied: open porosity (void fraction).
• Performance metric: resistance to thermal spalling.

Concept / Approach:
Higher porosity introduces compliant voids that can blunt crack tips and accommodate thermal strain mismatch between grains. This tends to improve thermal shock resistance (i.e., reduce spalling) at the expense of strength, density, and sometimes corrosion resistance. Thus, as porosity increases (within practical ranges), spalling resistance usually increases.

Step-by-Step Solution:
Relate porosity to thermal stress relief: more pores = more strain accommodation.Recognize trade-offs: improved spalling resistance but lower mechanical strength and possibly higher slag penetration.Conclude: resistance to thermal spalling increases with porosity (up to practical limits).

Verification / Alternative check:
Textbook guidance on refractory design notes that insulating, more porous bricks tolerate thermal shock better than dense, low-porosity bricks, which are stronger but more brittle under rapid temperature changes.

Why Other Options Are Wrong:

• Decreases/remains same: contrary to established behavior of porous ceramics under thermal shock.
• May increase or decrease: too vague; the dominant trend is an increase.

Common Pitfalls:

• Assuming “stronger” (denser) always means “more durable” in every mode of failure.
• Ignoring service specifics (chemical attack may still favor denser bricks despite spalling trends).

Final Answer:
increases