How does the addition of fly ash and silica fume affect bleeding in concrete?

The addition of fly ash and silica fume to concrete significantly reduces the bleeding phenomenon. Bleeding refers to the rise of water to the surface of freshly poured concrete, which occurs when water separates from the cement paste due to the settlement of aggregates. When supplementary cementitious materials like fly ash and silica fume are used, they improve the overall particle packing and reduce the water-cement ratio needed for workability.

Fly ash and silica fume contribute to the formation of a denser and more cohesive paste, which helps to retain water within the mix rather than allowing it to rise to the surface. Additionally, these materials can enhance the pozzolanic reaction, leading to increased strength and reduced porosity over time, further minimizing the potential for bleeding. Consequently, incorporating these materials is beneficial for mitigating bleeding and improving the finishing properties of concrete.