Drying of green (unfired) refractories: a faster drying rate tends to increase which characteristics of the moulded refractories?

Introduction / Context:
After shaping, ceramic and refractory products are dried to remove free water before firing. The drying schedule strongly influences microstructure and defect formation. Excessively rapid drying can cause non-uniform moisture gradients, internal stresses, and defect generation that will compromise the final fired properties.

Given Data / Assumptions:

• “Green strength” refers to mechanical strength before firing.
• Voids arise from differential drying, skinning, and trapped vapour paths.
• Shrinkage occurs as particle packing increases when water leaves.

Concept / Approach:
When drying proceeds too fast, the surface densifies (“case-hardens”) while the interior remains wet. As water tries to escape, internal pressure and capillary forces can create microcracks and void networks. Rapid moisture loss also amplifies dimensional change over short times, raising risk of warping and differential shrinkage. Therefore, a carefully staged drying curve is typically used: low temperature with airflow at first, followed by progressively higher temperatures to avoid defects.

Step-by-Step Solution:
Link drying rate to moisture gradient: faster rate → steeper gradients.Steeper gradients → higher stress → microvoids and cracking.Rapid water removal → higher immediate dimensional change → more shrinkage.Conclusion: both voids and shrinkage increase with excessively fast drying.

Verification / Alternative check:
Industrial drying protocols limit ramp rates and use humidity control to minimize defects, showing that slower, controlled drying yields denser, less defective bodies with better fired strength.

Why Other Options Are Wrong:
Green strength generally decreases with defects; fast drying does not inherently increase it.

Common Pitfalls:
Confusing firing shrinkage with drying shrinkage; assuming any reduction in water automatically strengthens the green body without microstructural damage.

Final Answer:
Both (b) and (c)