Effect of Pozzolana – choose the comprehensive effect on OPC When pozzolana is added to ordinary Portland cement (OPC) to form a blended cement, which of the following effects typically occur?

Introduction / Context:
Pozzolanas (such as fly ash, calcined clay, or silica fume in proportion) react with calcium hydroxide liberated during cement hydration to form additional calcium silicate hydrates. Blended cements alter the fresh and hardened properties of concrete in several predictable ways.

Given Data / Assumptions:

• Moderate replacement levels appropriate to common blended cements.
• Comparable curing conditions.
• Aggregate and mix proportions reasonably controlled.

Concept / Approach:
Early-age strength may decrease because pozzolanic reactions are slower initially, though later strengths catch up or exceed OPC due to secondary C–S–H formation. Pozzolanas typically reduce permeability and the availability of free lime, improving resistance to sulphate attack. Finer pozzolans improve cohesiveness and reduce bleeding. Some blends can show a slight increase in drying shrinkage due to finer pore structure and higher paste volumes if not optimized.

Step-by-Step Solution:
Assess early strength: tends to be lower at 1–7 days compared to neat OPC → (a) true.Assess sulphate interaction: reduced free Ca(OH)2 and denser matrix → (b) true.Evaluate shrinkage: finer pore structure may increase drying shrinkage if curing is inadequate → (c) can increase.Fresh concrete effects: better cohesiveness and lower bleeding with fine pozzolanic particles → (d) true.

Verification / Alternative check:
Performance specifications and durability indexes for blended cements consistently show lower permeability and improved sulphate resistance at equal strength levels.

Why Other Options Are Wrong:

• Each of (a)–(d) reflects a recognized effect; therefore the comprehensive answer is “All of the above”.

Common Pitfalls:
Expecting identical early strengths to OPC without curing or admixture adjustments; ensure adequate curing to realize long-term gains and mitigate shrinkage.

Final Answer:
All of the above