Which material is commonly used as a pozzolanic additive in cement?

• A. Fly ash
• B. Limestone
• C. Gypsum
• D. Sand

Answer: (A)

Pozzolanic additives react with the calcium hydroxide produced during cement hydration to form additional cementitious compounds, improving strength and durability. Fly ash is a common pozzolanic additive because it contains reactive silica and alumina from coal combustion that, in the presence of water, react with calcium hydroxide to form extra calcium silicate hydrate and related hydrates. This enhances the microstructure, reduces permeability, and can improve long-term strength. Limestone is mainly calcium carbonate and acts more as filler or a limestone cement constituent, not a pozzolanic reaction. Gypsum is added to regulate setting time by influencing ettringite formation, not to provide pozzolanic activity. Sand is an inert aggregate and does not participate in the pozzolanic reaction.