Pozzolana cement manufacture: For a natural pozzolanic material containing clay up to 80%, what percentage (by mass) is typically used in the finished pozzolana cement?

Introduction / Context:
Pozzolana cement is produced by intergrinding/ blending Portland clinker with pozzolanic material and gypsum. The percentage of the pozzolana component depends on its quality and reactivity. Knowing typical proportions helps with estimating properties and compliance.

Given Data / Assumptions:

• Natural pozzolana containing clay up to about 80% (e.g., calcined clay/surkhi).
• Question asks for typical addition percentage used in manufacturing.
• Standard plant practice and syllabus-oriented values are assumed.

Concept / Approach:
Pozzolanic additions replace a portion of clinker. For materials like calcined clay, a commonly cited addition level in traditional questions is around 30% of the finished cement mass, balancing workability, heat of hydration reduction, and late strength development while meeting strength grades.

Step-by-Step Solution:
Identify the material class: natural/calcined clay pozzolana.Recall customary blend ranges used in teaching examples and older specifications.Select 30% as the representative value.

Verification / Alternative check:
Plant practice varies with standards and SCM reactivity, but 30% has long appeared as the model percentage for such pozzolanas in exam contexts.

Why Other Options Are Wrong:
40–70% would unduly reduce early strength for low-reactivity clays in typical mixes.

Common Pitfalls:
Assuming modern fly ash limits apply identically to all pozzolanas; ignoring that reactivity governs feasible replacement level.

Final Answer:
30%.