What is the effect of fly ash and slag on early age strength of concrete?

Fly ash and slag are commonly used as supplementary cementitious materials (SCMs) in concrete mixtures, and their impact on early age strength can be complex. Typically, both fly ash and slag can contribute to the long-term strength and durability of concrete, but they can initially lead to lower early age strength compared to mixtures using only Portland cement.

The reason for the observed early age reduction in strength when using fly ash and slag lies in their slower pozzolanic or hydraulic reaction rates. Fly ash, a byproduct of coal combustion, requires more time to react and effectively bond with calcium hydroxide produced during the cement hydration process. Similarly, slag, a byproduct of steel manufacturing, also hydrates at a slower rate than Portland cement, resulting in less early strength gain.

Due to their lower reactivity at initial stages, concrete containing these materials might show a reduction in early age strength compared to standard mix designs that utilize only cement. However, as the hydration process continues, both fly ash and slag can significantly improve the strength and durability of concrete over time, contributing to a stronger and more resilient final product.