How Fly Ash Can Affect Concrete Color and Performance
Fly ash in concrete is often misunderstood. Because it is a by-product from another industry, many contractors think of fly ash as “filler”. They consider it to be simply a low-cost additive that allows the concrete producer to make higher profits while utilizing inferior materials. There is nothing farther from the truth. Adding fly ash to a concrete mix can provide many benefits. Here I will attempt to explain what fly ash is, as well as the pros and cons associated with using it.
Fly ash is what remains after power plants burn coal to produce energy. This fine ash was originally considered waste material and was therefore discarded. However, it is now considered a “green” product partly because using it in concrete reduces pollution by keeping it out of landfills.
Why did the concrete industry start using fly ash? Because fly ash is a pozzolan, just like portland cement. It was found to have excellent properties when mixed with portland cement (the active ingredient in concrete). The two products are chemically very similar. Adding fly ash to a concrete mix can affect it in the following ways.
Fly ash makes the concrete more workable. Concrete made with fly ash requires less mix water, and bleeds less than portland cement concrete. It also makes the concrete less permeable. This means that outside moisture will not penetrate it as easily. This can come into play during repeated freeze/thaw cycles where excessive moisture in the concrete is unwanted.
Also, by using fly ash the consumer can save some money. Because fly ash is less expensive than cement, mixes containing it are usually a bit less expensive than their straight cement counterparts. Fly ash mixes are often called “performance mixes”, so-called because of how they perform. For example, if a contractor calls for 4,000 psi performance mix, he will get concrete in which a percentage of the portland cement has been removed and replaced with fly ash. The cured concrete will ultimately reach (perform at) 4,000 psi or more. Psi is a measure of compressive strength. In this case, it means that the cured concrete will withstand at least 4,000 pounds of pressure per square inch before becoming crushed.
Conversely, if a contractor calls for a bag mix (or sack mix), he is asking for concrete that will contain a certain number of sacks of portland cement per cubic yard of concrete. For example, he may say he’s pouring a 6 bag mix, which means that his concrete will contain six 94 pound bags of portland cement per cubic yard, but no fly ash.
Contrary to what many opponents of fly ash may think, a bag mix is not sronger than a performance mix. In fact, correctly engineered fly ash mixes usually attain much higher strengths in the long run. They may take a bit longer to attain their designed strengths, but fly ash mixes will ultimately be stronger.
In addition to greater strength, workability, and cost savings, another benefit of fly ash is that it lowers the hydration temperature of concrete and prolongs the set time. Fly ash concrete sets at a considerably slower rate than straight portland cement concrete. Especially in hot weather, this can be very helpful. It gives the concrete crew a longer time in which to complete finishing operations. When stamping concrete, fly ash can buy the time necessary to adequately texture the surface. It should be noted however, that decorative concrete color charts are based upon mixing the color with gray portland cement concrete. Because fly ash concrete is a bit different in color than straight portland cement concrete (fly ash concrete is often beige rather than gray), color charts will not be completely accurate. For this reason, when using color on a job it is always a good idea for the contractor to make a mock-up sample with the same concrete mix he will use on the actual job.
Although fly ash is great in many applications, based upon their particular needs some concrete contractors may view slower set times and longer cure times as a liability. They might want the concrete to set faster because it saves labor costs by reducing man-hours. Perhaps the job schedule is on a fast track and the concrete will be put into use much sooner than normal. Maybe the weather is getting cold and they need the concrete to set more quickly to avoid freezing. Any of these are good reasons to opt for a bag mix instead of a fly ash mix.