When power plants burn coal to generate electricity, about 10% of the inorganic materials in the coal do not burn off. These by-products are called fly ash. U.S. power plants generate approximately 71 million tons of fly ash each year, of which only 32 million tons are put to beneficial use. Most unused flay ash is either placed into landfills or retention ponds. When the fly ash in landfills or retention ponds is not maintained properly toxic materials within the ash leaks out and pollutes the water table.
Construction contractors have included fly ash as a partial replacement for Portland cement in concrete for decades. Even the Hoover Dam was made with concrete that included fly ash. When fly ash is added to concrete it actually strengthens the concrete mix. However, until recently the maximum amount of fly ash that could be added to a concrete design mix was capped at 15% of the total volume. Any fly ash added beyond that point would weaken the concrete mix. Construction contractors that use Reco Cement’s additives can use fly ash to replace up to 60% of the Portland cement they are using in their design mixes without any loss of strengthM. A typical blend might consist of 15% Reco Cement additives, 35% Portland cement and 50% fly ash.
The quality of fly ash is determined by the efficiency with which it is burned. Most modern power plants generate Class C fly ash, which is the fly ash most commonly put to beneficial use. Older power plants generate the less desirable Class F fly ash. However, Reco Cement’s additives will work with either fly ash.
Reco Cement has successfully tested its additives with Off-Spec fly ash, a subcategory of Class C fly ash. It is created when coal-burning power plants burn coal at low temperatures in an effort to lower their CO2 emissions. Off-Spec fly ash has a high carbon and calcium content, which previously eliminated it from use with cementitious materials. The carbon within Off-Spec fly ash absorbs air-entraining admixtures in freshly mixed concrete, making it very difficult to control entrained air. As a result, most Off-Spec fly ash is either placed into road beds or landfills. The reason that the Off-Spec fly ash works so well with Reco Cement’s additives is that the higher strengths produced by the additives offset the negative effects of the high carbon content.
The EPA has stated that the use of fly ash as a partial replacement for Portland cement in the production of fly ash constitutes an appropriate “beneficial use” of the substance. The EPA has defined "beneficial use" “as the reuse of CCRs (coal combustion residuals) in a product that provides a functional benefit; that replaces a product made from virgin raw materials on the market, thus conserving natural resources that would otherwise need to be obtained through practices, such as extraction; and that meets relevant product specifications and regulatory standards. An encapsulated beneficial use is one that binds the CCRs into a solid matrix that minimizes their mobilization into the surrounding environment.”
When fly ash is used (encapsulated) to make concrete the toxic properties of the fly ash are rendered inert.
