Mdn Architects Inc.
Article: UNDERSTANDING FLY ASH
Although fly ash has been around for ages, it is often misunderstood, and rightfully so, as the information that is available is highly technical and sometimes intimidating! I set off to educate myself years ago and have come to understand the benefits and implications. First off, what is fly ash, and why is it considered green? This question is easily answered; it is a byproduct of the energy created through the use of coal-fired power plants. Literally, when coal is consumed in the power plants boilers the by product is fly ash, a very fine powder if you will. Currently Texas power plants produce an average of 13 million tons of fly ash each year, of which only 25% is being recycled, making it the state's second largest waste stream. This leads to the answer of the second question, what makes it Green? The fact that it has already been through its first life cycle is one reason; making it's embodied energy zero for its second life, therefore, clearly a recyclable product. Portland cement which is used in concrete is responsible for 9-11% of the total carbon dioxide problem due to its highly energy intensive process, for every ton produced one ton of carbon dioxide is emitted. These carbon emissions are reduced when fly ash is partially substituted in the concrete mix design.
So, what do you need to know about fly ash? First of all, that there are two classification types, F & C. Class C is the most commonly used in concrete mix design due to its higher early yielding strength. Fly ash is a natural retardant when used in concrete, this is important to know and often the culprit when it gets a bad reputation. When it is being considered for concrete one must know when and where it is to be used. For example fly ash in cold climates will prove to be a disaster. It is ideal in hot climates when you often use a retardant to slow the curing process of concrete. The percentage amount is also something you need to know, everything has its place. Based on my experience and research here is what I have come to learn: the slabs top layer which tends to set quicker than the remaining three inches below causes a so called "Jell-O" affect, as a retardant the set time is prolonged more than usual. In slabs I have used 40% fly ash (the limit for slabs in my opinion) which works much like a standard pour and I have not had any issues with setting time, on the contrary I have been surprised at the early strength it has gained. A seven day break has yielded 3,480 PSI strength, in 28 days it has been up to 7,000 PSI. High percentages of fly ash such as 60% are intended for concrete that requires form work for long periods of setting time, such as, piers, columns, flatwork and perimeter beams. Take care to not overwork, or burn the slab as it is typically a product dull blades when floating slab. The microscopic pours are what allows the floor binders to adhere to the slab, this can been an issue when it is not brought up to the subs attention. Another item to note: is if you are planning to use stains on a slab with fly ash you will want to do a test section. Stains commonly created a chemical reaction with the added fly ash and a uniformed look is hard to come by. Fly ash in concrete is a great option when you understand where and when to use it, as it truly is a recycled product that can produce a higher strength slab with less embodied energy.