Corrosion attack on fly ash concrete

Abstract

Fly ash is often used to described as a fine powder resulting from the combustion of coal transported by the flue gases of the boiler and collected in the Electrostatic precipitators (ESP) and also defined that it is a by-product of the coal fired thermal power plant. The characteristics and properties of different fly ashes depend on the nature of the fuel and the size of furnace used. Fly ashes generally fall into two categories, depending on their origin newlineand their chemical and mineralogical composition. Combustion of anthracite or bituminous coal generally produces low-calcium fly ashes; high-calcium fly ashes result from burning lignite or sub-bituminous coal. Fly ash, accounts for 75 to 85% of the total coal ash, and the remaining is collected as bottom ash or boiler slag. Fly ash because of its mineralogical composition, fine particle size and amorphous character is generally pozzolanic and in some newlinecases also self-cementitious. The bottom ash and boiler slag are much coarser and are not pozzolanic in nature. It is thus important to recognize that all the fly ash is not fly ash and the fly ashes produced by different power plants are not equally pozzolanic and, therefore, are not always suitable for use as mineral admixture in concrete. As according to ASTM C-618, two major classes of fly ash are recognized. These two classes are related to the type of coal burned and are designated Class F and Class C. Class F fly ash is normally produced by burning anthracite or bituminous coal while Class C fly ash is generally obtained by burning sub-bituminous or lignite coal. Class F fly ashes with calcium oxide (CaO) content less than 6%, designated as low calcium ashes, and are not self-hardening but generally exhibit pozzolanic properties. Quartz, mullite and hematite are the major crystalline phases identified fly ashes, derived from bituminous coal. Therefore, most research concerning use of fly ash in cement and concrete are dealt with Class F fly ashes. In the presence of water, the fly ash particles produced