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Traditionally soil; stone aggregates, sand, bitumen, cement etc. are used for road construction. Natural materials being exhaustible in nature, its quantity is declining gradually. Also, cost of extracting good quality of natural material is increasing.

Now-a-days disposal of different wastes produced from different industries is a great problem. These materials pose environmental pollution in the nearby locality because many of them are non-biodegradable.

If these materials can be suitably utilized in highway construction, the pollution and disposal problems may be partly reduced.

The use of these materials in road making is based on technical, economic, and ecological criteria.

Materials such as fly-ash from thermal power plants and other coal fired industries, blast furnace slag from steel industries, cement kiln dust from cement related industries, phosphogypsum from phosphatatic fertilizer industries, and many other solid wastes have already proved to be useful for road construction in many countries.

English: Photomicrograph made with a Scanning ...

Fly ash particles at 2,000x magnification. Fly ash is typically finer than portland cement and lime. Fly ash consists of silt-sized particles which are generally spherical, typically ranging in size between 10 and 100 micron (see Figure). These small glass spheres improve the fluidity and workability of fresh concrete. Fineness is one of the important properties contributing to the pozzolanic reactivity of fly ash. (Photo credit: Wikipedia)

Fly ash is the finely divided residue that results from the combustion of pulverized coal and is transported from the combustion chamber by exhaust gases.

Fly ash is produced by coal-fired electric and steam generating plants. Typically, coal is pulverized and blown with air into the boiler’s combustion chamber where it immediately ignites, generating heat and producing a molten mineral residue.

Boiler tubes extract heat from the boiler, cooling the flue gas and causing the molten mineral residue to harden and form ash.

Coarse ash particles, referred to as bottom ash or slag, fall to the bottom of the combustion chamber, while the lighter fine ash particles, termed fly ash, remain suspended in the flue gas.

Prior to exhausting the flue gas, fly ash is removed by particulate emission control devices, such as electrostatic precipitators or filter fabric bag houses.

Fly ash is most commonly used as a pozzolana in Portland cement concrete applications. Pozzolanas are siliceous or siliceous and aluminous materials, which in a finely divided form and in the presence of water, react with calcium hydroxide at ordinary temperatures to produce cementitious compounds.

The unique spherical shape and particle size distribution of fly ash make it good mineral filler in hot mix asphalt (HMA) applications and improves the fluidity of flowable fill and grout.

The consistency and abundance of fly ash in many areas present unique opportunities for use in structural fills and other highway applications.

Fly ash is used in concrete admixtures to enhance the performance of concrete roads and bridges. Portland cement contains about 65 percent lime. Some of this lime becomes free and available during the hydration process. When fly ash is present with free lime, it reacts chemically to form additional cementitious materials, improving many of the properties of the concrete.

Source : International Journal of Environmental Science and Development, Vol. 1, No. 2, June 2010 and everything-about-concrete.com

(this article written for 1BINA.my)