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Photomicrograph made with a scanning electron microscope and back-scatter detector: cross section of fly ash particles. Fly ash, flue ash, coal ash, or pulverised fuel ash (in the UK)—plurale tantum: coal combustion residuals (CCRs)—is a coal combustion product that is composed of the particulates that are driven out of coal-fired boilers together with the flue gases.
Fly ash bricks. Fly ash brick (FAB) is a building material, specifically masonry units, containing class C or class F fly ash and water. Compressed at 28 MPa (272 atm) and cured for 24 hours in a 66 °C steam bath, then toughened with an air entrainment agent, the bricks can last for more than 100 freeze-thaw cycles.
They are produced at temperatures of 1,500 to 1,750 °C (2,730 to 3,180 °F) through complicated chemical and physical transformation. Their chemical composition and structure varies considerably depending on the composition of coal that generated them. The ceramic particles in fly ash have three types of structures.
Alteration of hardened cement paste composition, with monosulfate aluminates phase converting to ettringite and, in later stages, gypsum formation. The necessary additional calcium is provided by the calcium hydroxide and calcium silicate hydrate in the cement paste; The effect of these changes is an overall loss of concrete strength.
Portland-fly ash cement contains up to 40% fly ash under ASTM standards (ASTM C595), or 35% under EN standards (EN 197–1). The fly ash is pozzolanic , so that ultimate strength is maintained. Because fly ash addition allows a lower concrete water content, early strength can also be maintained.
Coal ash obtained from the combustion of bituminous coal is constituted principally of aluminum oxide (Al2O3), calcium oxide (CaO) and silicon dioxide (SiO2). [3] In the composition of coal, there are many potentially hazardous substances that, if found at elevated concentration in inhaled particles, can cause major health problems in humans.
A coal-fired power plant with ash ponds. Bottom ash is part of the non-combustible residue of combustion in a power plant, boiler, furnace, or incinerator.In an industrial context, it has traditionally referred to coal combustion and comprises traces of combustibles embedded in forming clinkers and sticking to hot side walls of a coal-burning furnace during its operation.
Pozzolanic materials to add to the mix may be, e.g., pozzolan, silica fume, fly ash, or metakaolin. [27] These react preferentially with the cement alkalis without formation of an expansive pressure, because siliceous minerals in fine particles convert to alkali silicate and then to calcium silicate without formation of semipermeable reaction rims.