Fly Ash Concrete – Part Two: The Good

In Part One of my post on Fly Ash Concrete, I awed and horrified you with the big energy-consuming numbers of what are, at least for the time being, seemingly necessary evils: cement production, and coal-fired power plants. Well, if it seems like finding a way to ‘green’ processes that are fundamentally not, is making the best out of a bad situation, well, that’s because it is. On the other hand, however, these are such massive global industries that even small, incremental improvements can have large and far-reaching positive impact. So, back fly ash.

Not surprisingly, all that burning in the coal-fired power plant creates a lot of soot and by-product that at one time, was either spewed into the air, or collected and tossed into a land fill. As it turns out, the burning process endows some of this waste material with useful qualities. The extreme temperatures alter the chemical states of the coal and embody it with reactive energy. In short, the ash gets cement-like properties.

For concrete, this means that energy intensive cement can be partially replaced by what would otherwise be waste – in some applications, like brick, as much as 50% of the cement has been replaced with fly ash. This alone has huge implications on the carbon footprint of concrete but the benefits don’t end there.

Fly ash concrete is stronger concrete:

Fly ash is an extremely fine particle: as small and smaller than cement particles. This helps it contribute to the overall density of the concrete. It is also pozzolanic, which means that when cement reacts to make concrete, fly ash reacts with some of the cement byproduct, namely calcium hydroxide, to form more hardened material, and reduce the incidence of efflorescence (the chalking or whitening of that sometimes occurs when concrete that is exposed to water). This reaction also contributes to the overall density and reduced permeability of the cured concrete.

Generally speaking, the less mix water that is added to a concrete batch, the stronger the concrete. Fly ash particles are spherical and have a ball-bearing effect when added to concrete. This means that the mix is ‘lubricated’ without the use of water. In other words, it is possible to make workable concrete with less added water and this also results in higher strength concrete.

Fly ash concrete is easier to work with:

The ball-bearing effect and the reduced mix water mean concrete that is more workable with less water. This means that the mix is more stable, less susceptible to common problems like segregation and excessive bleed water. Fly ash concrete is generally easier to pump and to place.

Fly ash concrete is greener concrete:

When you add up all of these factors, fly ash concrete is greener alternative to regular concrete. Not only does it replace a potentially large amount of energy intensive cement with a post-industrial recycled by-product, thereby reducing waste and energy input, it creates concrete that is both easier to work with and ultimately, stronger and denser. This means a longer service life, less maintenance and the potential for labour savings and error reduction. In a world full of concrete, the use of fly ash and other supplementary cementing materials is truly an innovation that can be considered a win-win.