Creating porosity in insulating refractories: which additive is a classic pore former that decomposes or burns out during firing to leave a fine, insulating pore network?

Introduction / Context:
Insulating refractories rely on controlled porosity to reduce thermal conductivity. Pore formers are combustible or decomposable additives that volatilize during firing, leaving behind a tailored pore structure.

Given Data / Assumptions:

• Pore formers must leave minimal residue and produce uniform pores.
• We compare typical additives used in practice.
• Focus on a well-known, classical choice.

Concept / Approach:
Powdered naphthalene is a textbook pore former. It sublimates/burns out on heating without contaminating the matrix excessively, creating a fine porosity that lowers heat flow. Ammonium salts are sometimes used for gas release in certain mixes, but they can cause processing issues and are not as universally cited as naphthalene, sawdust, rice husk, or polystyrene beads. Calcium phosphate is commonly a bonding or setting agent in phosphate-bonded refractories, not primarily a pore former.

Step-by-Step Solution:
Define the requirement: clean burnout to form pores.Recognize classic choices: naphthalene, sawdust, rice husk.Evaluate alternatives: ammonium salts and calcium phosphate are not standard pore formers across the board.Choose powdered naphthalene.

Verification / Alternative check:
Manufacturing notes for insulating bricks frequently list naphthalene/sawdust as primary pore-forming additives.

Why Other Options Are Wrong:
Ammonium salts: niche use; may cause efflorescence or gas-evolution defects.Calcium phosphate: used for bonding, not porosity creation per se.All of the above/Water glass only: overbroad or incorrect.

Common Pitfalls:
Assuming any decomposable additive is ideal; residue chemistry and pore morphology matter.

Final Answer:
Powdered naphthalene