How Fly Ash Works in Concrete

07/14/2020

Most of us in the concrete industry know that fly ash mixed into concrete develops a better concrete, but did you ever wonder about the chemistry behind those benefits? A recent Q&A in Ash at Work, published by the American Coal Ash Association, provides clear answers. Karthik Obla, vice president of technical services for the National Ready Mixed Concrete Association, writes. “Fly ash is an important constituent in concrete that improves concrete performance and makes it more sustainable. We should use as much of it as possible.”

First, Obla explains how fly ash reduces efflorescence in concrete. “When cement reacts with water, it forms calcium silicate hydrate (CSH), which is durable, and calcium hydroxide (lime), which is not.” When concrete cracks, the lime leaches out and creates efflorescence on the surface. But when fly ash is used in the mix, the silica in the fly ash reacts with lime to form more CSH, thus reducing efflorescence.

This same chemical reaction results in the zone that surrounds the aggregates becoming less porous which makes it more difficult for chlorides or sulfates to penetrate the concrete. This reaction reduces the permeability of concrete to help protect the reinforcing steel. Fly ash also binds the alkalis thereby reducing expansion due to alkali-silica reactivity. And finally, “since the pozzolonic reaction is much slower than the cement reaction, fly ash contributes to long-term strength gain…and reduces temperature rise due to the cement reaction, thus reducing thermal cracking in mass concrete elements.”

The remainder of this article deals with the question of how much fly ash Is too much. The ACI 318 building code limits fly ash to 25% replacement of cement in concrete exposed to freeze-thaw conditions. Concrete, however, can be durable with greater than 50% replacement, although at those high dosages, the concrete sets and gains strength slowly and needs to be wet cured for seven days in order for the concrete to realize the durability benefits.

To read the full article, click here and scroll to page 33 in Ash at Work, Issue 1 2020.