Fire clay used for refractory products contains which principal constituent in a comparatively pure form? Choose the single correct option relevant to refractories and heat resistance.

Introduction / Context:Fire clay is the base material for refractory bricks and shapes. Understanding its chemistry explains thermal stability and suitability for furnaces and kilns.

Given Data / Assumptions:

• We are focusing on the main compound present in fire clay.
• Impurities like iron oxide and lime are kept low because they lower refractoriness.

Concept / Approach:

Fire clay is rich in kaolinitic minerals, essentially hydrated aluminium silicate. This composition provides high melting point, low fluxing, and good thermal shock resistance compared to clays with higher iron/lime content.

Step-by-Step Solution:

Verification / Alternative check:

Refractory standards specify Al2O3 and SiO2 within ranges typical of kaolinitic clays, with low Fe2O3 and CaO.

Why Other Options Are Wrong:

Lime and magnesium act as fluxes reducing refractoriness; iron oxide also lowers softening temperature and can cause deformation under heat.

Common Pitfalls:

Assuming “iron-rich” clay is better—iron reduces refractoriness.

Final Answer:

hydrated aluminium silicate