Bricks — minimum clay and silt content in good brick earth For a good brick earth mix, what should be the minimum combined percentage of clay and silt in the raw material?

Introduction / Context:
Brick earth composition controls plasticity, drying behavior, firing shrinkage, and final strength. Understanding the minimum clay/silt content ensures adequate plasticity for moulding while avoiding excessive shrinkage and cracking.

Given Data / Assumptions:

• Typical good brick earth ranges: silica (sand) 50–60%, alumina (clay) 20–30%, lime 5–10%, iron oxide 5–7%, and small alkalis.
• Clay and silt contribute plasticity and binding during moulding.
• Excess clay leads to higher shrinkage; too little clay leads to poor cohesion.

Concept / Approach:
A minimum threshold of fine fraction (clay and silt) around 20% is widely cited to secure workable plasticity without excessive drying defects. While optimal mixes often use 20–30% clay, the question asks for the minimum acceptable percentage, which is 20%.

Step-by-Step Solution:
Identify required roles: clay/silt provide plasticity and green strength.Balance with silica to control shrinkage and warping.From standard composition ranges, the minimum combined clay/silt lies at about 20%.

Verification / Alternative check:
Pilot mixes with clay below 20% show poor workability and cracking; specifications for good brick earth list alumina content not less than about 20%.

Why Other Options Are Wrong:
25–50% may be suitable in some cases but are not the minimum threshold; the question asks for the least acceptable value.

Common Pitfalls:
Confusing optimum with minimum; overlooking the role of tempering water and aging; ignoring that silt behaves differently from true clay minerals during firing.

Final Answer:
20%