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Recycling of fly ash in concrete
 
Electricity - the foundations of modern civilization, by burning coal will inevitably lead to environmental pollution by combustion products. The most dangerous for the environment is the emission of flue gases with sulfur and nitrogen oxides that cause acid rain. In order to reduce emissions undertaken Herculean efforts, for example, the cost of treatment plants - scrubbers is 40% in capital costs and 35 operating, and their share is growing.
Fly ash from burning coal are very different, as are the coals. In addition to depending on the composition of coal ash composition and properties of ash is also determined by the terms of combustion in boilers. At high temperature-fired fly ash consists of fused spherical particles, which are used as pozzolanic additives in cement - generally they are replacing 20% cement, or plasticizer in the concrete and mortar mixes. For example, in Western Europe, there is even a shortage of such evils. One of the current trends in energy - environmental boilers where coal is burned in a fluidized bed with the addition of lime, which absorbs sulfur oxides at the time of expansion and the temperature in the boiler is located within a 780-870oS when nitrogen oxides are not formed (in the conventional boilers T may be higher 1650oS). Such boilers can burn coal of low quality, not exceeding the prescribed limits on the content of nitrogen and sulfur oxides in the exhaust gases. However, during cleaning of exhaust gases from sulfur oxides instead of one problem appears different - solid waste disposal.

For example, the thermal power of 1000 MW, operating on coal, which contains 3% sulfur, produces in a year so much mud that they can cover an area of 1 km2 with a layer thickness of 1 m. More made of solid waste at the stations where the burning of high-carbon. For example, only one metropolitan Novosibirsk produced 1 million tons per year of solid waste - ash and slag, excluding emissions from the chimney fine dust. Solution purification of emissions of oxides of sulfur and nitrogen simply translates the problem into another plane, which also must be addressed. Particularly difficult problem is the solid waste is those plants, where low-quality coal is burned. For example, fly ash in the boilers with low-temperature combustion regime contains non-spherical particles with very high water consumption, as well as the high content of sulfur oxide-related, mostly in the form of sulphate and calcium sulphite. Standard disposal of such ash as an additive in concrete mixtures lead to its destruction because of the formation ettringita - "plague" for concrete. Moreover, there is no reason to believe that the binding of sulfur oxides there forever: calcium sulfite - connection unstable and the formation of acidic or neutral environment can get back into the atmosphere.

Continuation of article in Russian in the file.
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