Fly Ash Is Not Just A Filler!
Fly Ash is one of the most commonly misunderstood components of ready mixed concrete! I routinely hear 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. Let me explain that even a little further:
Cement + water = C-S-H + CH + “other phases”
C-S-H in this equation is Calcium-Silicate Hydrate, and it is the main “glue” that holds everything together within a concrete mix.¬† The more C-S-H that you have within your mix will have two main results: 1) your mix will be stronger 2) the concrete will become more impermeable (water resistant).
You will also notice in the original equation that there is another product that is created, Calcium Hydroxide (CH).¬† 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. However, too much CH can 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.
As I stated earlier in this post, 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!)
What we see in the above equation is 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 we aren’t done here….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.
These are just to name a few!
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! 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 a asset to this industry that can exponentially increase our capabilities in the construction world.
I’ve had great success creating an EccoCrete mix that can be used in flatwork and blockfill, containing over 40% fly ash, that 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!
Unfortunately, the Obama administration led attack against the Fly Ash industry is going strong. They want to label Fly Ash as a hazardous material making it harder and harder to produce. Hopefully they will not succeed, but only time will tell. If the ready mix industry loses access to fly ash, it will only raise concrete prices and prohibit us from using this product to create a better mix!
I would love the opportunity to show you the benefits and performance results of a high fly ash EccoCrete mix!¬† Please feel free to contact me to learn more!
NRMCA Certified Concrete Technologist Level 4
From the Lab
Every gallon of water you add to one cubic yard of concrete of properly designed 3000 psi concrete:
– Increases slump by approximately 1″
– Decreases compressive strength by as much as 200 psi
– Increases shrinkage potential by about 10%
– Wastes the effect of 1/4 bag of cement
– Increases the possibility of seepage through the concrete by up to 50%
– Decreases the freeze-thaw resistance by 20%
– Increases set time by 15 minutes in moderate weather…up to 30 minutes in cold weather