How does fly ash affect the binding properties of concrete?

Fly ash is a byproduct from burning pulverized coal in electric power generating plants, and it plays a significant role in enhancing the properties of concrete, particularly its binding properties. When added to concrete, fly ash serves as a supplementary cementitious material (SCM), which contributes to the strength and durability of concrete.

The correct choice highlights that fly ash increases binding and density of concrete. This occurs because fly ash reacts with calcium hydroxide in the presence of water to form additional cementitious compounds, which in turn, fill voids within the concrete matrix. This reaction not only enhances the overall binding capacity but also improves the density of the resulting concrete mix. A denser concrete can provide better long-term strength and performance, reducing permeability and increasing resistance to environmental factors such as freeze-thaw cycles and chemical attack.

In essence, the inclusion of fly ash helps create a more cohesive and robust concrete structure, which is why this option is the correct choice.