Ceramics and refractories: as the alumina (Al2O3) percentage increases in a high-alumina refractory composition, what is the trend in its fusion (melting/softening) point?

Introduction / Context:
High-alumina refractories are widely used in steel, cement, glass, and non-ferrous industries. Their performance at elevated temperature is strongly linked to the percentage of alumina (Al2O3) in the mix. This question checks your understanding of how increasing Al2O3 content affects the fusion (melting/softening) point and, by implication, hot strength and slag resistance.

Given Data / Assumptions:

• The material system is alumino-silicate refractories with increasing Al2O3 fraction.
• Fusion point refers to the temperature at which the refractory begins to soften/melt under its own weight.
• Other variables (impurities, porosity, mineralogy) are considered secondary for this conceptual trend question.

Concept / Approach:
As alumina content rises, the refractory chemistry shifts from silica-rich phases toward mullite (3Al2O3·2SiO2) and then toward corundum (α-Al2O3). Both mullite and corundum have high melting points and excellent refractoriness. Simultaneously, the share of low-melting, fluxing glassy silicate phases decreases, which raises the overall softening temperature and improves hot load resistance.

Step-by-Step Solution:
Trend recognition: more Al2O3 → more mullite/corundum → higher melting/softening point.Coupled effects: fewer glassy eutectics → less viscous flow at high temperature → improved refractoriness.Conclusion: the fusion point increases with increasing alumina content.

Verification / Alternative check:
Corundum melts near ~2050 °C; mullite shows very high refractoriness and low creep. Standard refractory charts show refractoriness rising as compositions move from fireclay (low Al2O3) to high-alumina bricks and then to fused alumina products.

Why Other Options Are Wrong:
Decreases: contradicts the known phase-equilibria trend.Remains constant: ignores compositional control of liquidus and eutectic content.“May increase or decrease”: while impurities matter, the dominant trend with higher Al2O3 is an increase.

Common Pitfalls:
Confusing porosity (which affects thermal shock/insulation) with chemical refractoriness; assuming all alumino-silicates behave the same regardless of mineralogy or glass phase.

Final Answer:
Increases.