Crushing (cold) strength behavior of refractories: which statement is generally correct for engineering selection?

Introduction / Context:
Cold crushing strength (CCS) is a widely used index for refractory quality control and material selection. While CCS is measured at room temperature, the same microstructural factors that affect CCS (density, porosity, bonding) also influence hot mechanical properties. This question probes a robust, formulation-agnostic trend relevant to design comparisons.

Given Data / Assumptions:

• CCS is a measure of compressive strength at ambient conditions.
• Porosity is a key microstructural variable that can be tailored in manufacturing.
• Temperature effects on strength are complex and depend on phase changes and glass formation.

Concept / Approach:
All else being equal, increasing open porosity lowers the load-bearing cross-sectional area and introduces stress concentrators, reducing CCS. This inverse relationship between porosity and strength is a fundamental materials principle. By contrast, the statement that strength “increases with temperature” is not generally true; most refractories lose strength at high temperature due to softening of glassy bonds or creep of grains. Therefore, the most generally correct statement across brick types is that strength decreases with increased porosity.

Step-by-Step Solution:
Define CCS as room-temperature compressive strength.Note microstructural dependence: higher porosity → fewer load paths → lower CCS.Recognize temperature trends vary; hot strength often drops as glassy phases soften.Select the statement that holds broadly without special caveats: strength decreases with increased porosity.

Verification / Alternative check:
Empirical relations (e.g., exponential or power-law fits) routinely show CCS decreasing with porosity across alumino-silicate, basic, and silica brick families.

Why Other Options Are Wrong:
Increases with temperature: not universal; often the opposite.Unaffected by temperature: contradicted by hot strength and creep data.Increases with slag attack: corrosive attack typically degrades strength.

Common Pitfalls:
Confusing hot modulus of rupture or creep with CCS; trends are system-specific at high T, whereas porosity–strength is robust.

Final Answer:
It decreases with increase in porosity (other factors remaining same).