Fly Ash
From Swikipedia
Introduction
Fly ash is a fine, glass powder recovered from the gases of burning coal during the production of electricity. These micron-sized earth elements consist primarily of silica, alumina and iron.
Brief Description
What makes Fly ash so durable?
Fly ash has a high amount of silica and alumina in a reactive form. These reactive elements complement hydration chemistry of cement.
Hydration chemistry of Cement: When cement reacts with water, the hydration of cement begins. On hydration, cement produces C-S-H Gel. This C-S-H Gel binds the aggregates together and strengthens the concrete. However, one more compound is produced on hydration that is so different in behaviour. It is none other than the Calcium Hydroxide Ca(OH)2. In construction industry, it is generally referred to as Free Lime. Aggressive environmental agents like water, sulphates,CO2 attack this free lime leading to deterioration of the concrete.
It is not only the chemistry provided by fly ash that compliments chemistry of cement, but also the physical properties of fly ash improve the rheology and microstructure of concrete by a great extent. Fly ash, on itself, can not react with water, it needs free lime, produced on hydration of Portland cement, to trigger off its Pozzolanic effect. Once it is triggered, it can go on and on. In simple words, it means a much longer life for concrete structure.
Specific benchmarks have been set up to evaluate the performance of concrete with respect to durability - mainly Strength and Permeability. This means to produce a durable and long lasting concrete, it must possess: High strength and Low permeability.
Fly ash makes concrete denser, and hence less permeable, mainly by:
Reducing water demand in concrete
Improving microstructure of concrete
At the same time, fly ash improves long term strength of concrete due to the continued Pozzolanic reaction as discussed earlier.
Benefits of using Fly ash
It delays the heat of hydration and hence reduces the thermal cracks in concrete.
It improves the workability of concrete.
It makes the mix homogeneous and hence reduces segregation and bleeding.
The concrete finsih is improved due to perfectly spherical fly ash particles.
The concrete permeability is substancially reduced which enhances the life of the structure.
Fly ash contributes to the long term strength in concrete.
Fly ash for a ‘Greener Future’
One of the greatest benefits of utilising fly ash is building a greener future. This is through the use of fly ash bricks in cosntruction. When every single ton of cement will be replaced with fly ash, it would not only eliminate waste from the planet but also reduce the emission of 0.85 tons of CO2 into the atmosphere. Presently, fly Ash is at the forefront amongst the green technologies being now adopted all over the world by leading organisations.
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