Through a partnership with Nova Scotia Power Inc., Shaw Resources has marketed class "F" fly ash to the ready-mix concrete industry in Atlantic Canada since 1984. This low calcium fly ash is one of the most researched in the world, with millions of dollars invested to date on the evaluation of its unique engineering properties.
Fly ash is a fine, glassy powder recovered from the gases of burning coal during the production of electricity. These micron-sized earth elements consist primarily of silica, alumina, and iron. When mixed with lime and water it forms a cementitious compound with properties very similar to those of Portland cement. Due to this similarity, fly ash can be used to replace a portion of cement in the concrete, providing some distinct quality advantages:
Fly ash particles are 'spherical' in shape differing significantly from the 'angular' particles of Portland cement. The unique shape allows the fly ash to lubricate the concrete mix by acting as billions of tiny ball bearings. The result is a more workable concrete with enhanced flowability. Concrete made with increasing amounts of fly ash experiences higher slump with no change in water content. Less water results in reduced concrete shrinkage and cracking. Fly ash's lubricating properties allow pump effort to be reduced by up to 30%. Due to the lower pump pressure the need to add water on site is reduced, ensuring high quality concrete is delivered to the form.
Fly ash creates about 20% more paste than an equal weight of Portland cement. This additonal paste allows concrete to place and finish like higher cement mixes. Fly ash causes the concrete to become denser which results in a tighter, smoother surface with less bleeding. Fly ash concrete offers a distinct architectural benefit with improved textural consistency and sharper detail.
Typically, fly ash can be expected to gain strength more slowly in the first few days, although with special considerations, early design strengths can be achieved. When 28 day strengths are equal, the fly ash blend concrete will gain subsequent strength faster than the 100% Portland cement mix, and achieve higher ultimate strengths.
The addition of fly ash produces a denser, less permeable concrete. The 'tighter' concrete reduces infiltration of damaging chemicals, such as salt and sulphate, thereby improving durability. Furthermore, fly ash's unique chemical properties help eliminate alkali aggregate reaction. Fly ash actually uses the available alkalis to produce increased strengths in the concrete, which removes the key link in the destructive alkali aggregate reaction.
Once simply blown into the air as a pollutant, fly ash is now a valuable resource used in the production of quality concrete. Moreover, as a controlled by-product of electrical generation, fly ash is produced without any additional energy consumption. Conversely, the production process for Portland cement requires massive amounts of energy. By replacing a portion of the cement used in concrete with fly ash, substantial levels of energy consumption are therefore avoided.
