Concrete with fly ash
Introduction
Fly ash is a siliceous material that is expelled from chimneys as a by-product of the burning of pulverized coal in thermal electricity generation plants and is made up of pozzolans, which is why it is useful as additives in the manufacture of cement or as raw material in many cement-based products, such as poured concrete, concrete blocks and bricks.[1] These solid wastes are obtained by electrostatic precipitation or by mechanical capture of the dusts that They accompany the combustion gases from the burners of thermoelectric plants powered by pulverized coals.[2].
Characteristics
Fly ash usually occurs in the form of fine grayish powder, as it contains small amounts of unburned carbon as well as other chemical components. Their appearance is also influenced by the degree of spraying, the type of boiler and the type of extractor and the way in which they have been removed from the thermal power plant. Three groups are usually distinguished; those from thermal power plants, those obtained from a fluid bed and those from plants with a desulfurization plant. [3]The size of the raw, untreated particles ranges between 0.2 and 200 microns in diameter, although exceptionally they can reach 500 microns, while the specific weight ranges between 2.0 and 2.3 g/cm (2000 to 2900 kg/m). The color is grayish and may present ocher tones due to the iron oxides present, and may become dark brown when the iron oxides are found in greater quantities.
Chemical composition
The chemical composition depends mainly on the coal of origin from which they have been generated. Depending on its impurities, the proportions can vary significantly, although all mainly contain silica "Silicon(IV) oxide") (SIO) and alumina (AlO) accompanied by iron(III) oxide "Iron(III) oxide") (FeO) and calcium monoxide (CaO), apart from a small fraction of unburned coal. In a smaller proportion, magnesium oxide (MgO) and alkali metal oxides (NaO and KO) and traces of many other metal compounds may be present.[3].
According to its calcium oxide content, fly ash is classified into different types, the most common are:[1].
Currently, more than 50% of the concrete used contains fly ash. Normally, Class F are used in doses of 15 to 25% by mass of cementitious material, while Class C are used in higher doses, up to 40%.[1].