Technical Advantages of Fly Ash in Plastic Concrete
Reduces Water Requirement
Due to the spherical shape of fly ash particles, the concrete mix is not as dependent upon mix water as a vehicle for workability.
Due to the spherical shape of fly ash particles, the workability of concrete making materials is increased. Also, fly ash contains particles finer than Portland cement particles, and a pound of fly ash contains approximately one-third more fines than a pound of Portland cement, due to specific gravity difference. The extreme fines in fly ash will aid in the finishablilty of concrete flatwork.
Since fly ash contains particles finer than Portland cement particles, the fly ash fines and its gradation more easily block off paths of water channels through the concrete.
Other than entrained air in plastic concrete mixes, fines are required to hold a mix together. Fly ash will also substantially improve the pumpability of concrete mix; this is not only due to increased fines that help reduce segregation, but also these same fines increase the workability of the mix. The combination of reduced segregation and increased workability in the fly ash concrete mix will lead to lower concrete pump pressure.
Reduces Heat Of Hydration
By replacing as much as 50% of the Portland cement with fly ash, such as in dam projects, the heat of hydration is greatly reduced. By replacing 15% to 20% of the Portland cement in today’s commercial mixes, the heat of hydration of the fly ash in the mix is approximately 50% of the Portland cement that the fly ash replaces (per a Corps of Engineers study). This lower heat of hydration can be of significant help in reducing concrete quick set in hot weather.
Technical Advantages of Fly Ash in Hardened Concrete
Reduces Drying Shrinkage
Water and Portland cement are the two main contributors to drying shrinkage of ready mixed concrete. By lowering the water demand of concrete-making material and by the removal of Portland cement, drying shrinkage of fly ash concrete is less than a comparable Portland cement mix.
Improves Water Tightness in Concrete
Because the fly ash chemically combines and stabilizes the water soluble calcium hydroxide in concrete, the fly ash concrete is from 5 to 13 times more impermeable to the passage of water than a comparable Portland cement mix. This suggests fly ash should always be specified for containment vessels, water and sewerage treatment plants, basement walls, and slabs on grade.
Reduces Alkali-Silica Reactivity in Concrete
The SEFA Group markets Class F fly ash exclusively. Class F fly ash chemically combines with the alkalis in Portland cement to form stable cementious bonds. Once the alkalis combine with the fly ash, they are no longer available to react with the silica or silicates found in the aggregate. Therefore, the expansive gel that causes Alkali-Silica Reactivity is not formed and the durability is enhanced. When potentially reactive aggregates are used in concrete, Class F fly ash ought to be specified to ensure maximum durability.
Improves Resistance of Sulfate and Acid Attack
Because the fly ash chemically combines and stabilizes the calcium hydroxide available in Portland cement concrete, the calcium hydroxide is not present to be attacked by sulfates and acids. Also, since the water soluble calcium hydroxide is stabilized and not available to leaching action, sulfate and acid waters are unable to penetrate into the concrete as easily. This is important, particularly in the presence of sulfates, since calcium sulfate compounds are expansive and can lead to significant concrete deterioration. In sulfate and acid environments, fly ash is definitely needed as a concrete-making material.
Long Term Strength Gain
Fly ash concrete mixes can be designed to equal the strengths of Portland cement mixes at 7 and 28 days. An advantageous byproduct of design practice is the strength gain after 28 days that is inherent in a pozzolan. A 28 to 56 day strength gain of from 600 PSI to 1200 PSI is typical, with future strength gains for years if curing conditions allow.