Cementitious by-products — identify the principal constituent of power-plant fly ash commonly utilized in concrete and pozzolanic blends.

Introduction / Context:
Fly ash is a finely divided by-product from coal-fired power stations. In civil engineering, it is valued as a pozzolanic material that improves workability, long-term strength, and durability of concrete mixtures. Recognizing its dominant oxide is helpful for understanding its reactivity and standards compliance (e.g., Class F and Class C fly ashes).

Given Data / Assumptions:

• Typical fly ash oxide composition includes SiO₂, Al₂O₃, Fe₂O₃, and CaO in varying amounts.
• Pozzolanic activity relates strongly to reactive silica and alumina.
• The question asks for the main constituent, not a list of all present oxides.

Concept / Approach:

Most fly ashes (especially low-calcium Class F) have silica as the largest single component by mass, commonly exceeding 50%. Aluminium oxide and iron oxides are also present but at lower proportions. The high glassy silica fraction reacts with calcium hydroxide liberated during cement hydration, forming additional calcium silicate hydrates (C–S–H) for strength and durability gains.

Step-by-Step Solution:

Verification / Alternative check:

Standards and characterization studies (ASTM C618) report silica as the dominant oxide in both Class F and, often, Class C fly ashes, though Class C has higher CaO.

Why Other Options Are Wrong:

Aluminium oxide — Important for pozzolanicity but typically less than silica.
Ferrous oxide — Present, but not the major component.
All of these — Misleading; while multiple oxides exist, the question asks for the main one.

Common Pitfalls:

Confusing chemical presence with dominance; assuming “all of these” whenever multiple species occur. Always read for “main constituent.”

Final Answer:

silica