Fly Ash and Sustainable Concrete

Concrete Green Building Blog (Nov 29 2011)

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   If you’re trying to gauge industry interest in green concrete technology, you actually don’t have to go farther than LinkedIn®. It didn’t take long to find a discussion on the American Concrete Institute’s® page entitled “Difficulties attaining compressive strength using fly ash in concrete mix when 28-day strength is specified?”.  There were a number of interesting contributions made to this discussion that could explain why the 28-day strengths were not achieved, but to take a more holistic approach, let’s look at the three primary reasons why fly ash is used in concrete: for reduced costs, for performance, and the increasingly popular reason, to be sustainable.

    

   Cost reduction:  It is no secret that fly ash is cheaper than cement, and that the price is a major factor for using fly ash. If, for example, cement costs $100/ton and fly ash is $60/ton, a 564-pound mix with 20% fly ash can reduce the cost of the mix by $2.50.  This is the primary reason fly ash use was started over 30 years ago. 

    

   Performance benefits: When cement reacts with water, two main hydration products form: calcium silicate hydrate (CSH – a higher strength material) and calcium hydroxide (CH – a lower strength material).  When fly ash is added, the amorphous (or reactive) silica will convert the CH to CSH, added an important “S”, which results in more strength.  Other beneficial properties, such as a denser matrix, reduce the ingress of aggressive agents like chlorides or sulfates, among others.  There are two things we need to keep in mind however when considering the introduction of fly ash,  the first being that not all fly ashes are created equal.  Only the amorphous silica in fly ash is reactive, and fly ashes from different parts of the country have different amorphous silica contents, making them either more or less reactive.  As a producer, you need to be cautious when selecting fly ash: look at the performance as well as the price.  The other aspect to keep in mind is that fly ash is not cement.  Now I know this seems like an obvious statement, but too often we try to achieve the same performance as cement.  We now know the performance will be different since the cement needs to form CH in order for the fly ash to add the “S” to form CSH, and this takes time. Be prepared to wait a little longer to finish flatwork or strip forms as set times and early strengths may be delayed slightly.  These issues are especially prevalent when fly ash volumes are increased, particularly in cooler weather.  Why then are we seeing increasing volumes of fly ash used in the past 10 years?  It’s largely in part in an effort to be sustainable.

    

   Sustainability: Fly ash is a waste product from coal-fired power plants, and as such, is thought of as a by-product.  Because of this, the carbon footprint is attributed to the power industry and not the concrete industry that uses this waste product.  So, when the fly reaches a ready-mix plant, the primary carbon associated with it is the trucking to get it there.  If we go back to our previously-mentioned 564-pound mix, by incorporating 20% replacement of cement with fly ash, we can reduce the carbon footprint of our concrete from 487 lbs to 410 lbs, a 15% reduction.

   So why are we having problems attaining our targeted 28-day strength?  I think that it’s not a matter of missing the target, but rather that the target itself was off.  We know that we are putting more fly ash in concrete for financial and ecological reasons, but we need to realize that fly ash is not cement, and it will perform differently.  As we push to increase the levels of fly ash to new heights, it will also take more intelligent designs to handle these changes, and fully expose the benefits of using fly ash.

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