Define
the features of fly ash?
- Spherical shape : Fly ash particles are almost totally
spherical in shape, allowing them to flow and blend freely in
mixtures.
- Ball bearing effect :The "ball-bearing" effect of fly
ash particles creates a lubricating action when concrete is in
its plastic state.
- Higher Strength : Fly ash continues to combine with free
lime, increasing structural strength over time.
- 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
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.
- Reduced Sulfate Attack : Fly ash ties up free lime that
can combine with sulfate to create destructive expansion.
- Reduced Efflorescence : Fly ash chemically binds free
lime and salts that can create efflorescence and dense concrete
holds efflorescence producing compounds on the inside.
- Reduced Shrinkage : The largest contributor to drying
shrinkage is water content. The lubricating action of fly ash
reduces water content and drying shrinkage.
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 replace portland cement.
Reduced Alkali Silica Reactivity : Fly ash combines with
alkalis from cement that might otherwise combine with silica from
aggregates, causing destructive expansion.
- Workability : Concrete is easier to place with less effort,
responding better to vibration to fill forms more completely.
Ease of Pumping. Pumping requires less energy and longer pumping
distances are possible.
- Improved Finishing : Sharp, clear architectural definition
is easier to achieve, with less worry about in-place integrity.
- Reduced Bleeding : Fewer bleed channels decreases porosity
and chemical attack. Bleed streaking is reduced for architectural
finishes. Improved paste to aggregate contact results in enhanced
bond strengths.
- Reduced Segregation : Improved cohesiveness of fly ash
concrete reduces segregation that can lead to rock pockets and
blemishes.
- Reduced Slump Loss : More dependable concrete allows
for greater working time, especially in hot weather.
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