Classification of sillimanite (Al2SiO5) refractories: based on chemical behavior with slags, sillimanite bricks are best categorized as which class of refractory?

Introduction / Context:
Sillimanite (Al2SiO5) and related minerals (kyanite, andalusite) are widely used raw materials in refractories. Upon firing, they convert principally to mullite (3Al2O3·2SiO2) plus silica, producing bodies with good refractoriness and corrosion resistance across a range of slags.

Given Data / Assumptions:

• We classify refractories by their chemical behavior against acidic or basic slags.
• Sillimanite-based bricks form mullite as the main phase after firing.
• Service commonly includes variable slag chemistries.

Concept / Approach:
Neutral refractories resist both acidic and basic slags better than strongly acidic (silica) or strongly basic (magnesia, dolomite) materials. Sillimanite’s mullite-rich structure confers relatively broad chemical compatibility, categorizing it as a neutral refractory. This is distinct from “high alumina” as a composition label; while mullite is alumina-rich, the accepted chemical classification for sillimanite bricks remains neutral.

Step-by-Step Solution:
Recognize sillimanite → mullite formation during firing.Relate mullite chemistry to slag interactions.Assign chemical class: neutral refractory.

Verification / Alternative check:
Handbooks list sillimanite/mullite products under neutral class due to their balanced corrosion resistance profile across slag chemistries compared with silica (acid) and magnesia/dolomite (basic).

Why Other Options Are Wrong:
Basic / Acidic: too one-sided for mullite-rich refractories.High alumina: describes composition, not chemical class.Insulating: refers to thermal property, not chemical classification.

Common Pitfalls:
Conflating compositional labels with chemical classification.Assuming all alumina-bearing bricks are “basic.”

Final Answer:
Neutral