For high-alumina refractories, how does refractoriness (softening temperature) change as the Al2O3 content is increased, assuming other factors remain comparable?

Introduction / Context:
Refractoriness is the ability of a refractory to withstand high temperatures without softening or deforming. In alumino-silicate systems, changing Al2O3 content has a pronounced effect on softening temperatures and slag resistance. This is a fundamental selection rule in refractory engineering.

Given Data / Assumptions:

• Comparing high-alumina refractories of similar microstructural quality.
• Focus on Al2O3 content variation, not porosity or impurity effects.
• Service involves neutral to mildly basic conditions.

Concept / Approach:

Generally, increasing alumina content raises the softening temperature and improves refractoriness under load. High-alumina phases (e.g., mullite, corundum) possess higher melting/softening points than silica-rich phases. Therefore, as Al2O3 increases (and free silica decreases), refractoriness increases, assuming proper bonding and low impurity levels.

Step-by-Step Solution:

Verification / Alternative check:

Standards and datasheets for fireclay vs high-alumina bricks show progressive increases in refractoriness under load and softening points as Al2O3 rises from ~30% toward 90%+.

Why Other Options Are Wrong:

Decreases/Remains same: Contradict composition–property trends. Unpredictable: While microstructure matters, the dominant trend with higher alumina is increased refractoriness.

Common Pitfalls:

Ignoring impurity and porosity effects; although they modulate values, the core trend with Al2O3 is upward refractoriness.

Final Answer:

Increases