Which statement describes a key benefit of fly ash in concrete?

• A. Increases permeability.
• B. Decreases durability.
• C. Increases workability only.
• D. Improves durability, reduces permeability, and modifies heat of hydration.

Answer: (D)

Fly ash acts as a pozzolanic material, reacting with calcium hydroxide from cement hydration to form extra cementitious gel. This fills and refines the pore structure, making the concrete denser and less permeable. The result is improved durability and resistance to chemical attack, as well as a lower heat of hydration than plain Portland cement, which helps reduce thermal cracking in larger pours. While fly ash can also help workability because its spherical particles flow more easily, the most complete description of its benefits is that it improves durability, reduces permeability, and modifies heat of hydration. Permeability would not increase with fly ash, and durability is not decreased; “workability only” misses the durability and heat aspects.