Thermal spalling resistance: which of the following actions will NOT increase a refractory’s resistance to spalling under rapid temperature changes?

Introduction / Context:
Spalling is the tendency of a refractory to crack and flake under thermal shock or severe temperature gradients. Design choices in composition and microstructure strongly affect spalling resistance. Knowing which levers help or hurt is vital for reliable furnace operation.

Given Data / Assumptions:

• Spalling resistance improves when thermal stresses are relaxed or deflected.
• Porosity, grain size, and thermal expansion are key parameters.
• Question asks for the action that does NOT improve resistance.

Concept / Approach:
Higher porosity and coarser grog can introduce compliant pathways and crack-arrest features that dissipate thermal stresses. Lowering thermal expansion reduces strain for a given temperature change. Conversely, making a brick extremely dense removes these stress-relief features, often worsening thermal shock performance despite benefits for corrosion resistance. Hence, densification does not increase (and can reduce) spalling resistance.

Step-by-Step Solution:
Identify mechanisms that dissipate thermal stress: pores, microcracks, compliant interfaces.Note that coarser grains and controlled porosity aid crack deflection.Recognize that lower thermal expansion decreases thermal strain directly.Conclude that making the refractory very dense will not increase spalling resistance.

Verification / Alternative check:
Thermal shock testing (quench/ΔT cycling) consistently shows improved performance for materials with controlled porosity/microcracking compared with fully dense analogs, other factors equal.

Why Other Options Are Wrong:
Increased porosity (controlled): provides stress relief → usually improves spalling resistance.Coarser grog: enhances crack deflection and toughness.Lower thermal expansion: directly reduces thermal strain.Improved microcrack networks: dissipate energy and blunt crack growth.

Common Pitfalls:
Confusing corrosion resistance (helped by density) with thermal shock resistance (often helped by porosity and microstructural compliance). Both must be balanced for service.

Final Answer:
Making the refractory denser (very low porosity)