Refractory engineering: zircon (ZrSiO4) refractories are characterized by which combination of properties in service (consider thermal expansion, refractoriness under load, and resistance to spalling)?

Introduction / Context:
Zircon refractories are based on zircon (ZrSiO4) and are widely used where dimensional stability and resistance to thermal cycling are important, such as glass-contact applications, slide gates, and kiln furniture. This question asks you to recognize the signature trio of properties that make zircon products valuable in demanding thermal environments: low thermal expansion, high refractoriness (including refractoriness under load), and strong resistance to spalling.

Given Data / Assumptions:

• Zircon is the principal phase; minor additives/binders may be present.
• RUL refers to the temperature where a specified creep/deflection occurs under load.
• Spalling resistance relates to thermal shock and sudden temperature changes.

Concept / Approach:
Thermal stress in refractories scales with elastic modulus, temperature gradient, and the coefficient of thermal expansion. Materials with low expansion develop lower thermal stress for the same temperature swing, improving spalling resistance. Zircon also exhibits high refractoriness and a high RUL, meaning it retains shape and load-bearing capability at very elevated temperatures. The combined effect leads to long lining life when temperature fluctuations or mechanical loads are present.

Step-by-Step Solution:
Identify zircon’s thermal expansion: among the lowest for oxide refractories.Correlate low expansion with improved thermal-shock (anti-spalling) behavior.Recall typical RUL for zircon refractories is around 1600 °C, with refractoriness exceeding 2000 °C.Therefore, options (a), (b), and (c) all correctly describe zircon refractories.

Verification / Alternative check:
Service records in glass furnaces and kilns show zircon components resist thermal cycling and maintain tolerances better than many alumino-silicate bricks, confirming the property set.

Why Other Options Are Wrong:

• “None of the above” contradicts well-documented zircon behavior.

Common Pitfalls:
Confusing zircon (ZrSiO4) with zirconia (ZrO2). Zirconia has transformation-toughening phenomena; zircon’s low expansion drives its anti-spalling reputation.

Final Answer:
All (a), (b) and (c)