Effect of zircon addition to silica bricks: the inclusion of zircon (ZrSiO4) in silica refractory bricks primarily improves which properties?

Introduction / Context:
Zircon-bearing additives are used to tailor the performance of silica bricks in aggressive furnace environments. Zircon can react to form zirconia–silica phases and refine the microstructure, leading to better mechanical integrity and improved corrosion resistance. This question asks which benefits are chiefly realized by such additions.

Given Data / Assumptions:

• Silica bricks are valued for high refractoriness but have limitations in corrosion resistance and mechanical strength under certain slags.
• Zircon (ZrSiO4) additions are at levels intended to enhance performance rather than change the base chemistry entirely.
• Service environments may include slags and fluxes typical of metallurgical furnaces.

Concept / Approach:
Zircon contributes hard, refractory phases and can reduce the mobility of the glassy phase at high temperature. This microstructural modification increases the brick’s crushing strength (a proxy for structural integrity) and boosts resistance to slag attack by reducing penetration and reaction rates. Therefore, both targeted properties—strength and slag resistance—are improved, making option (c) correct.

Step-by-Step Solution:
Consider baseline silica brick issues: corrosion and strength limitations.Add zircon: forms ZrO2-containing phases that stabilize the structure.Outcomes: higher crushing strength and better slag corrosion resistance.Select combined improvement: both (a) and (b).

Verification / Alternative check:
Materials data and application notes for zircon–silica composites show improved hot MOR (modulus of rupture) and reduced corrosion depth in simulated slag tests versus plain silica brick.

Why Other Options Are Wrong:
Only (a) or only (b): understate the dual benefits of zircon additions.Neither: contradicts documented performance enhancements.Thermal expansion alone: zircon’s role is broader than expansion control.

Common Pitfalls:
Assuming zircon only changes thermal expansion; it also affects corrosion and strength.Overlooking the influence of additive grain size and distribution on final properties.

Final Answer:
Both (a) and (b)