Manufacturing refractories: to produce refractory bricks with lower permeability (reduced open porosity and better slag penetration resistance), which processing adjustments are effective?

Introduction / Context:
Permeability in refractories governs gas and slag penetration during service. Lower permeability generally improves corrosion resistance and lining life in furnaces, kilns, and ladles. This question focuses on the manufacturing levers that densify the microstructure and close connected pore networks.

Given Data / Assumptions:

• Standard oxide refractories formed by pressing and firing.
• Permeability correlates with pore size, pore connectivity, and total open porosity.
• Processing variables considered: firing temperature, moulding pressure, and grog particle size.

Concept / Approach:
Higher moulding pressure compacts particles, increasing green density. Finer grog helps fill interstices between larger grains (better packing), reducing capillary channels. Higher firing temperature enhances sintering and neck growth, closing pores and strengthening bonds. All three changes act synergistically to lower permeability by reducing both pore volume and the connectivity that allows fluid transport through the brick.

Step-by-Step Solution:
Link moulding pressure ↑ to green density ↑ → fewer, smaller pores post-firing.Use finer grog to improve packing and minimize percolating voids.Increase firing temperature to promote sintering and pore closure.Conclude that all three measures reduce permeability.

Verification / Alternative check:
Permeability tests (e.g., gas permeability) and microstructural images confirm that higher density plus finer grading and stronger sintering reduce open, connected porosity that drives fluid ingress.

Why Other Options Are Wrong:

• Only any two of the variables leaves out a known densification driver; all three are valid and commonly used.

Common Pitfalls:
Over-firing can create excessive glass phases or grain growth causing thermal-shock issues; optimal processing balances permeability against other properties.

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