Insulating refractory bricks — Which of the following is NOT typically used to induce porosity during manufacture (high porosity and low thermal conductivity desired)?

Introduction:
Insulating refractory bricks achieve low thermal conductivity largely through high, controlled porosity. During manufacturing, temporary pore-formers are added that burn out or decompose, leaving behind fine, closed or interconnected pores. This question asks which additive is not used for that purpose.

Given Data / Assumptions:

• Desired properties: high porosity, low density, low thermal conductivity.
• Common pore formers: organic materials or foaming agents that volatilize on firing.
• Candidate materials listed include organic burnouts and inert mineral.

Concept / Approach:
Organic pore formers such as cork and sawdust combust during firing, generating gas and voids. Chemically prepared foams expand within the wet body to create a pore structure. Sand, however, is inert silica grains; it does not volatilize or burn away and thus does not create new pores by removal—rather, it can fill space and may even reduce porosity if added as a solid filler.

Step-by-Step Solution:
Assess cork/sawdust: organic → burn out → leave pores.Assess chemical foam: generates bubbles → pore network upon setting/firing.Assess sand: remains as solid particles; does not disappear → not a porosity inducer.

Verification / Alternative check:
Manufacturing literature on insulating bricks lists sawdust, paper pulp, polystyrene beads, and foaming processes as pore formers; mineral sand is not used for this purpose.

Why Other Options Are Wrong:

• Cork, sawdust, foam: all are classic porosity-inducing agents.

Common Pitfalls:
Assuming any granular addition increases porosity; forgetting that pore formers must be removed by burning or decomposition.

Final Answer:
Sand