Fly Ash Concrete

Benefits

Fly Ash

What is Fly Ash Historical Background Chemical Nature Classification
Chemical Composition Mechanical properties Benefits High Volume
Fly Ash
Need for extending Strength Specifications to beyond 28 days – Restraints on use of Fly Ash in Highway Constructions

Benefits of Fly Ash in Concrete

Benefits/Advantages of use of Fly Ash in concrete are tabulated below:

A. Enhances Concrete Workability:

The “ball-bearing” effect of fly ash particles creates a lubricating action when concrete is in its plastic state. This creates benefits in:

1 Ease of Pumping Pumping requires less energy and longer pumping distances are possible.
2 Improved Finishing Sharp, clear architectural definition is easier to achieve, with less worry about in-place integrity.
3 Reduced Bleeding Fewer bleed channels decrease permeability and chemical attack. Bleed streaking is reduced for architectural finishes.
4 Reduced Segregation Reduced Segregation

B. Increasing Concrete Performance:

In its hardened state, fly ash creates additional benefits for concrete, including:

1 Higher Strength Fly ash continues to combine with free lime, increasing compressive strength over time.
2 Decreased Permeability Increased density and long term pozzolanic action of fly ash, which ties up free lime, results in fewer bleed channels and decreases permeability
3 Increased Durability Dense fly ash concrete helps keep aggressive compounds on the surface, where destructive action is lessened. Fly ash concrete is also more resistant to attack by sulfate, mild acid, soft (lime hungry) water, and seawater.
4 Reduced Sulfate Attack Fly ash ties up free lime that can combine with sulfates to create destructive expansion.
5 Reduced Corrosion By decreasing concrete permeability, fly ash can reduce the rate of ingress of water, corrosive chemicals and oxygen — thus protecting steel reinforcement from corrosion and its subsequent expansive result.
6 Reduced Efflorescence Fly ash chemically binds free lime and salts that can create efflorescence, and dense concrete holds efflorescence producing compounds on the inside.
7 Reduced Shrinkage The largest contributor to drying shrinkage is water content. The lubricating action of fly ash reduces water content and drying shrinkage.
8 Reduced Heat of Hydration The pozzolanic reaction between fly ash and lime generates less heat, resulting in reduced thermal cracking when fly ash is used to reduce portland cement.
9 Reduced Alkali Silica Reactivity Fly ash combines with alkalis from cement that might otherwise combine with silica from aggregates, causing destructive expansion.
10 Increased Resistance to Freezing and Thawing

C. Environmental Benefits:

1 Conserves natural resources Using recovered fly ash conserves natural resources by eliminating the need to produce new raw materials.
2 Conserved Landfill Space Conserving landfill space by utilizing fly ash is an obvious environmental benefit. Just 1 ton of fly ash use avoids landfill requirement corresponding to 455 days of solid waste produced by an average American.
3 Reduces greenhouse gas emission Fly ash use can also significantly decrease greenhouse gas emissions. When fly ash is used to replace cement, it reduces the need for cement production — a highly energy-intensive process that also creates significant amounts of greenhouse gases.

Reducing cement production reduces greenhouse gas emissions on almost a ton for ton basis. Production of one ton of cement emits about one ton of carbon dioxide. In many circumstances, a ton of fly ash can be used to displace a ton of cement.

Experts estimate that cement production accounts for about 5 percent of carbon dioxide emissions from human sources. If all of the fly ash generated in the United States each year were used to replace cement in producing concrete, the reduction in carbon dioxide released because of decreased cement production would be equivalent to eliminating 25 percent of the world’s motor vehicles.

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References:
Concrete-Techgroup thanks Headwaters Resources , USA for granting them permission to use their resources in developing this article.

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