Acidic refractory behavior: adding basic oxides to an acidic refractory generally has what effect on its fusion point (softening temperature)?

Introduction / Context:
Refractory selection involves matching slag chemistry to lining chemistry. Mixing unlike chemistries (acid with basic) tends to form low-melting reaction products or eutectics, which depress the overall softening temperature and promote corrosion or deformation at service temperatures.

Given Data / Assumptions:

• “Acidic” refractory refers to SiO2-rich or other acid oxides.
• Basic oxides include CaO, MgO, etc.
• Fusion point relates to softening/pyrometric cone behavior.

Concept / Approach:
Opposite acid–base oxide interactions commonly produce compounds with melting points lower than either parent, resulting in eutectic mixtures. This is the basis for chemical corrosion of refractories by incompatible slags. Therefore, adding basic oxides to an acidic refractory typically reduces the fusion point and undermines high-temperature stability.

Step-by-Step Solution:
Identify acid vs basic oxide sets.Recall eutectic formation lowers melting/softening temperatures.Therefore, fusion point decreases upon such additions.

Verification / Alternative check:
Phase diagrams (e.g., CaO–SiO2) show deep eutectics; practical linings fail where incompatible slags form low-melting phases at hot faces.

Why Other Options Are Wrong:

• Increase/no effect: contradict typical eutectic behavior.
• “Always above 2500 °C”: unrealistic generalization.
• “Neutral without change”: chemistry does not cancel without creating new phases.

Common Pitfalls:
Assuming mixing refractories averages properties; in reality, deleterious low-melting phases often form.

Final Answer:
Reduces the fusion point