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Fly ash added in concrete as a supplementary cementing material achieves one
or more of the following benefits:
• Reduces the cement content.
• Reduces heat of hydration.
• Improves workability of concrete.
• Attains higher levels of strength in concrete especially in the long term.
• Improves durability of concrete.
• Increases the “green” recycled material content of concrete.
• Attains a higher density.
• Lowers porosity and permeability.
The properties of fly ash, whether ASTM C618, Class C or Class F, and the
percentages in which they are used greatly affect the properties of concrete.
Mixture proportioning and trial batches are critical to obtaining concrete with
the desired fresh and hardened properties. Fly ash may be introduced in
concrete as a blended cement containing fly ash or introduced as a separate
component at the mixing stage.
Most of the We Energies fly ash is being used in concrete as a separate
component at the concrete batching and mixing stage. This allows the
flexibility of tailoring mixture proportions to obtain the required concrete
properties for the particular application. Ready-mixed concrete producers
have greater control with respect to the class and amount of fly ash in the
concrete mixture to meet the specified performance requirements.
Fly ash has several other properties, in addition to its cementitious and
pozzolanic properties, that are beneficial to the concrete industry (19). Lowcalcium
fly ash (ASTM C618 Class F) has been used as a replacement for
Portland cement in concrete used for the construction of mass gravity dams.
The primary reason for this application has been the reduced heat of hydration
of Class F fly ash concrete compared to Portland cement concrete. ASTM
C618 Class C fly ash concrete may also have a slightly lower heat of
hydration when compared to Portland cement concrete. However, low calcium
Class F fly ash concrete generates still lower heat of hydration, a desirable
property in massive concrete construction, such as dams and large
foundations.
Studies have also revealed that certain pozzolans increase the life expectancy
of concrete structures. Dunstan reported that as the calcium oxide content of
ash increases above a lower limit of 5% or as the ferric oxide content
decreases, sulfate resistance decreases (20).
Dunstan proposed the use of a resistance factor (R), calculated as follows:
R = (C-5)/F
Where C = percentage of CaO
Where F = percentage of Fe 2 O 3
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Coal Combustion Products
Utilization Handbook