What is the effect of fly ash or slag as supplementary cementitious materials?

• A. They can improve long-term strength, reduce permeability, and lower heat of hydration.
• B. They always increase heat of hydration
• C. They have no effect on strength
• D. They replace cement entirely with no strength gain

Answer: (C)

Fly ash and slag are supplementary cementitious materials that participate in cement hydration by reacting with the calcium hydroxide that cimento releases. This pozzolanic reaction forms additional cementitious compounds, which fills the pore spaces and makes the microstructure more dense. As a result, you get lower permeability and greater durability.

Because these reactions occur more slowly than the hydration of cement itself, they tend to lower the heat of hydration. This is especially beneficial in larger pours where a high heat rise could cause cracking.

They also improve long-term strength. While early strength might be slower if you rely heavily on pozzolanic materials, the continued development of the microstructure leads to higher strength over time. In practice, fly ash or slag partially replaces cement rather than completely replacing it, so you still get bonding and early-age performance plus improved long-term strength and durability.

So, they do affect strength (typically boosting long-term strength) and they reduce heat and permeability—not the opposite, and not a complete replacement with no strength gain.