What effect does using silica fume have on concrete properties?

• A. Lowers cost.
• B. Increases strength and durability, reduces permeability, improves abrasion resistance; raises cost.
• C. Lowers strength and increases permeability.
• D. Has no effect on durability.

Answer: (B)

Silica fume acts as a very fine pozzolanic additive that fills tiny voids in the cement paste and reacts with calcium hydroxide to form additional strength-giving gel. This densifies the paste, so the concrete gains higher strength (especially early strength) and much better long-term strength. The tighter pore structure drastically reduces capillary porosity, which lowers permeability and helps resist ingress of water and aggressive chemicals. With less path for penetration, durability improves, including better abrasion resistance on the surface where wear occurs. Silica fume also helps mitigate certain durability issues, like alkali-silica reaction, by consuming alkalis.

Be aware that these benefits come with considerations: silica fume adds material cost and often requires adjustments in mix design, such as the use of a superplasticizer to maintain workability because the surface area is very high and water demand can increase.

So the overall effect is concrete that is stronger and more durable, with reduced permeability and better abrasion resistance, but at a higher cost. The other statements don’t fit: it doesn’t lower cost, it doesn’t increase permeability or ignore durability.