Fly ash effect on permeability?

• A. Reduces permeability, improving durability.
• B. Increases permeability.
• C. No effect on permeability.
• D. Makes surface rough.

Answer: (A)

Fly ash reduces permeability, which helps improve durability. When fly ash is added to concrete, its fine particles participate in pozzolanic reactions with the calcium hydroxide produced during cement hydration. This reaction forms additional C-S-H gel and refines the pore structure, filling many of the capillary pores and reducing the connected pore network. With fewer pathways for water and aggressive ions to travel, the ingress of moisture, chlorides, and other deleterious substances slows down, making the concrete more resistant to freeze-thaw damage and chemical attack. While fly ash can influence workability and the surface finish, its primary durable effect is the reduction of permeability. The other options don’t fit because permeability is indeed reduced, not increased or unchanged, and surface roughness isn’t the main durability mechanism here.