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”
Word did not find any entries for your table of contents. 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.
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, 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!
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 us to learn more!
– TJ Barton
NRMCA Certified Concrete Technologist Level 4
