2019 American Concrete Website
According to TJ Barton, Certified Concrete Technologist Level 4–National Ready Mixed Concrete Association (NRMCA), Fly Ash is one of the most commonly misunderstood components of ready mixed concrete! He stated, [“… people say that Fly Ash is just a cheap “filler” that ready mix companies use to keep the cost of concrete down … this really couldn’t be further from the truth.”
Fly Ash, known as a “pozzolan”, is generally a byproduct of coal-fired power generation plants. Pozzolanic material will not react with water by itself, but when mixed with water and Portland cement it will form a compound with cementitious properties:
Cement + water = C-S-H + CH + “other phases”
C-S-H = Calcium-Silicate Hydrate—the main “glue” that holds everything together within a concrete mix. The more C-S-H within the mix, will have two main results:
1) Mix will be stronger
2) Concrete will become more impermeable (water resistant)
The other product in the original equation: CH = Calcium Hydroxide. It is good for a concrete mix to have:
A little CH because it helps to maintain a high PH, which is needed to keep steel products from corroding
Too much CH: Start to cause problems as it can easily seep from the concrete and expedite some pretty serious deterioration processes (Alkali Silica Reaction and Sulfate attacks)
This is where Fly Ash really starts to earn it’s reputation
as a valuable and important ingredient in concrete.
Most pozzolans will not react directly with water, however, when these pozzolans are introduced to a source of CH, the reaction will look like this:
Silica (from pozzolan) + CH + water = C-S-H (the good stuff we want lots of!)
In the above equation, you see that the fly ash reacts with CH that was liberated in the hydration process and created more C-S-H, which will ultimately add more strength. It will also, at the same time, leave enough CH present to keep the PH high enough to protect steel from corroding! In simplest terms, fly ash becomes cement! But, fly ash continues to help in so many ways:
- It refines and fills in the pore structure of the concrete “paste”, greatly reducing permeability!
- There are so many efforts to reduce vapor and water migration through concrete (after hydration) and a high fly ash mix will naturally do just that!
- The less permeable concrete is, the less effective outside forces are in penetrating and attacking the mix
- Combats Alkali-Silica Reactivity, Alkali-Carbonate Reactivity and various Sulfate attacks
- Lowers heat of hydration! This is a great benefit to those finishing concrete in hot weather or placing concrete in large volumes (mass pours)
- Lowers carbon footprint
Across the world, engineers and concrete producers are having incredible success with high fly-ash mixes….over 50%-60% by volume and up! The technology is present to pour large structures (bridges, dams etc) with concrete mixes that have MORE FLY ASH THAN CEMENT with results that are nothing short of amazing! Barton said, “I’ve seen 28-56 day breaks of over 6000 psi on some of these high fly ash mixes and these structures are almost impervious to attacks (ASR and sulfate). Fly Ash is not a liability anymore…it is an asset to this industry that can exponentially increase our capabilities in the construction world.”
There is an EccoCrete mix that can be used in flatwork and blockfill, containing over 40% fly ash, which will meet and/or exceed strengths of a standard low fly ash mix! The technology is here….it has been here for quite some time!
For the article titled “Fly Ash is NOT just filler!”, please go to: https://americanconcrete.org/fly-ash-is-not-just-filler/