Effect of MgO addition: in alumino-silicate (fireclay) refractories, how does the presence of MgO generally affect refractoriness (softening temperature under load)?

Introduction:
Impurities and deliberate additives can alter the high-temperature behavior of alumino-silicate refractories. MgO is a basic oxide and can react with silica-rich matrices, influencing melting behavior and refractoriness under load (RUL).

Given Data / Assumptions:

• Matrix: alumino-silicate system typical of fireclay refractories.
• MgO added in modest amounts (as impurity or minor additive).
• Assessment concerns general trend in refractoriness.

Concept / Approach:
Basic oxides like MgO can form low-melting silicates with SiO2-rich phases, reducing the softening temperature. Unless engineered as a distinct basic refractory (high MgO content), small MgO in alumino-silicates usually lowers refractoriness due to formation of eutectic compositions or glassy phases at grain boundaries.

Step-by-Step Solution:
Identify chemical interaction: MgO + SiO2 → magnesium silicates with lower melting points.Effect on RUL: presence of low-melting phases promotes earlier deformation.Thus, refractoriness generally decreases.

Verification / Alternative check:
Binary and ternary phase diagrams (MgO-Al2O3-SiO2) show eutectics at lower temperatures compared with pure high-alumina systems, explaining the trend.

Why Other Options Are Wrong:

• Increase/no effect: contradict typical phase equilibria for alumino-silicate matrices with basic impurities.
• Conditional behavior: while level and distribution matter, the general rule for typical fireclay compositions is a reduction in refractoriness.

Common Pitfalls:
Assuming MgO always improves refractoriness because it is used in basic bricks; context (matrix chemistry) is critical.

Final Answer:
Lowers refractoriness