Fly Ash in Sustainable Concrete




In my last blog I spoke about the Green Building scenario in India. While writing, I came across a construction practice which involved the use of Fly Ash in Concrete. In this blog I will talk more about some of the benefits of replacing cement by fly ash in concrete. Some of the primary reasons why fly ash is used are the cost benefits, performance and its growing contribution to sustainable construction.
Basically fly ash is nothing but waste product that comes from coal fired plants. On an average over a 100-110 million tons of fly ash is produced annually. This waste is not at all treated properly and rather dumped without paying much attention to it. This has resulted to some severe consequences. Instead of simply letting fly ash harm the environment, it can be put to use in mixing concrete by replacing Portland cement by fly ash.
Costs were one of the vital factors the use of fly ash started over 30 years ago. Fly ash definitely is a lot cheaper than cement and this is one of the compelling factors for using fly ash. E.g. If cement costs $100/ton and fly ash $60/ton then a 564- pound mix with 20% fly ash can save cost of the mix by almost $2.50.  
A major performance benefit of fly ash over cement is strength. 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 are it has a denser matrix, reduces the ingress of aggressive agents like chlorides or sulfates.
How is fly ash sustainable? Concrete is the most widely used man made construction material. Over 100s of millions of cubic yards of concrete is used every year. Cement production releases millions of tons of carbon dioxide into the air every year. In order to produce cement you have to heat limestone up to 1300 degrees and that fire creates carbon dioxide. However introducing fly ash into concrete instead of cement can reduce the carbon footprint. In the example mentioned above, by replacing 20% of cement with fly ash, we can reduce the carbon footprint of the concrete from 487 lbs to 410 lbs which is almost a 15% reduction.
However there have been concerns about the performance of fly ash. That is unlike cement concrete fly ash concrete cannot attain the strength in 28 days. 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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