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Fly Ash POWER PLANTS Producing More than Energy Electricity is the fuel of the “Information Age” and power plants that burn coal account for more than half of the electricity produced in the United States. These power plants also produce residual materials like fly ash (which is captured from the exhaust of the boiler) and bottom ash (which is heavier and falls to the bottom of the boiler). These and other “coal combustion products” were originally treated as waste and disposed of in landfills. Over the past 30 years, electric utilities and specialized companies like Headwaters Resources have steadily increased the amount of coal combustion products that are recovered for productive uses. Coal combustion products become ingredients in concrete, wallboard, mortars, stuccos, blocks, bricks, shingles, paints and a variety of other building materials. They are also used to stabilize soils or wastes, and can be used as structural fill or road base materials. They’re even used by peanut farmers to improve their crop yields. Because of efforts of environmentally conscious companies, use of coal combustion products has increased more than 50 percent in the past decade. And there are plenty of good reasons to use even more. The difference between fly ash and portland cement is apparent under a microscope. Fly ash particles are smaller and almost totally spherical in shape, allowing them to fill voids, flow easily, and blend freely in mixtures. When water is added to portland cement, it creates two products: a durable binder that glues concrete aggregates together and free lime. Fly ash reacts with this free lime to create more of the desirable binder. Workability. Concrete is easier to place with less effort, responding better to vibration to fill forms more completely. • Ease of Pumping. Pumping requires less energy and longer pumping distances are possible. • Improved Finishing. Sharp, clear architectural definition is easier to achieve, with less worry about in-place integrity. • Reduced Bleeding. Fewer bleed channels decrease permeability and chemical attack. Bleed streaking is reduced for architectural finishes. • Reduced Segregation. Improved cohesiveness of fly ash concrete reduces segregation that can lead to rock pockets and blemishes. Increasing Concrete Performance In its hardened state, fly ash creates additional benefits for concrete, including: • Higher Strength. Fly ash continues to combine with free lime, increasing compressive strength over time. • Decreased Permeability. Increased density and long term pozzolanic action of fly ash, which ties up free lime, results in fewer bleed channels and decreases permeability. • Increased Durability. Dense fly ash concrete helps keep aggressive compounds on the surface, where destructive action is lessened. Fly ash concrete is also more resistant to attack by sulfate, mild acid, soft (lime hungry) water, and seawater. • Reduced Sulfate Attack. Fly ash ties up free lime that can combine with sulfates to create destructive expansion. • Reduced Efflorescence. Fly ash chemically binds free lime and salts that can create efflorescence, and dense concrete holds efflorescence producing compounds on the inside. • Reduced Shrinkage. The largest contributor to drying shrinkage is water content. The lubricating action of fly ash reduces water content and drying shrinkage. • Reduced Heat of Hydration. The pozzolanic reaction between fly ash and lime generates less heat, resulting in reduced thermal cracking when fly ash is used to replace portland cement. • Reduced Alkali Silica Reactivity. Fly ash combines with alkalis from cement that might otherwise combine with silica from aggregates, causing destructive expansion. FLY ASH summary Fly Ash is a mineral admixture used in concrete to enhance finishing characteristics, make the mix more economical, and to improve pumping. Fly ash is finer in consistency than cement, and its particles are round. These fine particles make the mix finish easier, and pump easier. It economizes the mix because it allows up to 30% of the cement volume to be replaced by fly ash. Fly ash is less expensive than cement, therefore the mix costs less. You may have heard that fly ash decreases the compressive strength of concrete, this is only partially true. Fly ash does slow the initial strength gain, however at 28 days the strength of a fly ash mix and a cement mix are the same. Masonry, Excavation and Fence & Barn Services - Remodeling, Rehabbing and Brick Siding |
