Why is grog (pre-fired crushed refractory) added to fireclay mixes during brick manufacture? Select the most complete reason.

Introduction / Context:
Grog is a staple addition in refractory manufacturing. As inert, pre-fired granules, it shapes firing behavior and final properties of bricks by controlling shrinkage and microstructure.

Given Data / Assumptions:

• Grog is dimensionally stable during firing.
• It modifies particle packing and minimizes drying/firing shrinkage.
• It can improve thermal-shock resistance and mechanical strength when well graded.

Concept / Approach:
Because grog has already undergone firing, it does not shrink further, interrupting continuous plastic matrix and reducing total shrinkage. It adds crack-deflection sites and tailors pore structure, improving spalling resistance. Proper grading and proportioning also raise fired strength via better packing and reduced defect sizes.

Step-by-Step Solution:
Relate prefired inert grains to shrinkage control.Connect microstructural tailoring to improved shock resistance.Note strength gains from optimized packing and sintering.Choose the comprehensive option: all (a), (b), and (c).

Verification / Alternative check:
Manufacturing guides consistently recommend grog to stabilize dimensions and enhance service performance.

Why Other Options Are Wrong:
Single benefits are incomplete; cost reduction alone is not the main technical reason.

Common Pitfalls:
Overusing coarse grog without fine matrix balance, which can lower strength.

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