How do fly ashes contribute to concrete performance?

Fly ash is a byproduct of burning pulverized coal in electric power generating plants and has long been utilized as a supplementary cementitious material in concrete. Its inclusion in concrete contributes significantly to performance enhancements.

One crucial aspect is that fly ash can improve the strength of concrete, especially at later curing stages. When fly ash is used, it typically reacts with the calcium hydroxide released during the hydration of cement, forming additional calcium silicate hydrate (C-S-H), which is the primary binder in concrete that provides strength.

Moreover, fly ash reduces permeability, which is vital for creating denser concrete. This attribute allows for better resistance against chemical attacks, moisture infiltration, and damage from freeze-thaw cycles. Lower permeability also enhances the durability of concrete structures.

Furthermore, fly ash improves workability due to its spherical shape and smooth particle surface. This can lead to easier mixing and placing while reducing the water requirement without compromising the necessary performance characteristics of the concrete.

In contrast, increasing the overall weight (as noted in one of the options) is not a benefit of fly ash; instead, incorporation often results in lighter concrete due to the lower density of fly ash compared to traditional aggregates. Providing color to concrete, which is not a primary function of fly ash,