Which statement best describes the role of fly ash as a supplementary cementitious material?

• A. It reduces durability.
• B. It can improve durability, reduce permeability, and modify heat of hydration.
• C. It increases permeability.
• D. It completely replaces cement.

Answer: (B)

Fly ash acts as a supplementary cementitious material by reacting with the calcium hydroxide produced during cement hydration to form additional cementitious compounds. This pozzolanic reaction refines the pore structure, which lowers permeability and enhances durability, making the concrete more resistant to chemical attack and long-term aging. It also moderates heat of hydration by slowing the overall reaction, which can help manage temperature rise in larger pours. Fly ash is used to partially replace cement, not to completely replace it, so while it changes heat evolution and improves durability, it does not make the concrete less durable or more permeable. This combination of benefits—improved durability, reduced permeability, and modified heat of hydration—best describes its role as an SCM.