In locations where fine aggregate is expensive or scarce, what can be used as an economical partial replacement?

• A. Air entraining admixtures
• B. Fly ash
• C. Silica fume
• D. Crushed stone

Answer: (A)

The main idea is lowering material cost by substituting part of the fine aggregate with another locally available material when natural fine aggregate (sand) is expensive or scarce.

In practice, crushed stone can be processed to produce sand-sized particles (manufactured sand or crushed stone fines) that can stand in for some of the natural fine aggregate. Using these substitutes requires careful control of grading to match the sand size distribution, as well as adjustments to water and cement content because the shape and roughness of manufactured materials affect workability, compaction, and possibly strength. The goal is to maintain proper slump, strength, and durability while reducing cost.

Air entraining admixtures, fly ash, and silica fume serve other roles: air entrainers create microscopic air voids to improve freeze–thaw durability and workability; fly ash and silica fume modify the cement paste and overall concrete performance, but they do not act as direct replacements for fine aggregate.