Talk:Fly ash

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This article seems to be based on [1] which appears to be a work of the US government and hence in the public domain.

There are two uses of fly ash in portland cement concretes. One is as a substitute for cement, and its related reactant, lime (calcium hydroxide). Up to 25% of cement can be replaced by fly ash, which results in early strength and improved workability. This decreases the dollar cost of the concrete, but at an often overlooked cost. The fly ash consumes the lime to form the cementitious calcium silicates. This depletes the lime with the result that the pH will begin to decline from 12.3 to some value approaching 8.2, decreasing the field of stability of the cementitious phases. This can and does weaken the strength of the mortar. Thus the statement that fly ash increases the durability of concrete may not be correct.

The second use of fly ash is as the fine aggregate in portland cement concrete. Int his role, fly ash replaces some of the fine sand.