Wednesday, May 17, 2017

use of fly ash in concrete



Use of Fly Ash in Concrete
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Fly ash is used as a supplementary cementitious material  in the production of portland cement concrete. A supplementary cementitious material, when used in conjunction with portland cement, contributes to the properties of the hardened concrete through hydraulic or pozzolanic activity, or both.


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Fly ash,
a powder resembling cement,
has been used in concrete since the 1930s


Fly ash has been used in concrete at levels ranging from 15% to 25% by mass of the cementitious material component. The actual amount used varies widely depending on the application, the properties of the fly ash, specification limits, and the geographic location and climate. Higher levels (30% to 50%) have been used in massive structures (for example, foundations and dams) to control temperature rise.

Increasing the amount of fly ash in concrete is not without shortcomings. At high levels problems may be encountered with extended set times and slow strength development, leading to low early-age strengths and delays in the rate of construction.

For any given situation there will be an optimum amount of fly ash that can be used in a concrete mixture which will maximize the technical, environmental, and economic benefits of fly ash use without significantly impacting the rate of construction or impairing the longterm performance of the finished product.




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Tuesday, April 25, 2017

Cement hydration

Cement hydration

By the process of hydration (reaction with water) Portland cement mixed with sand, gravel and water produces the synthetic rock we call concrete. Concrete is as essential a part of the modern world as are electricity or computers.

Clinker is anhydrous (without water) having come from a hot kiln. Cement powder is also anhydrous if we ignore the small amount of water in any gypsum added at the clinker grinding stage.

The reaction with water is termed "hydration". This involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the hydration process gradually bond together the individual sand and gravel particles, and other components of the concrete, to form a solid mass.