What is the definition of an admixture in concrete or mortar?

• A. A material, other than water, aggregates, or cement, added to modify properties such as workability, curing, or durability.
• B. A small amount of water added to improve hydration.
• C. A filler material with no chemical effect.
• D. A cement type added to increase setting time.

Answer: (A)

Admixtures are materials added to concrete or mortar beyond the main ingredients (water, aggregates, and cement) to change how the mix behaves or performs. They’re used in small amounts to modify properties such as workability, curing, or durability, without being part of the primary mix itself. Examples include plasticizers to improve flow without extra water, air-entraining agents for better freeze-thaw resistance, accelerators to speed up early strength, and retarders to slow setting.

The other ideas don’t fit because adding extra water isn’t an admixture activity but a change to the water content of the mix; a filler with no chemical effect doesn’t actively modify properties, and simply using a different cement type to increase setting time changes the cement system rather than adding an external modifying agent.