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Concrete & Masonryaka: Class C fly ashaka: Class F fly ashaka: SCMaka: pozzolan

Fly Ash

In Plain English

A recycled industrial byproduct used to replace a portion of cement in concrete, improving its properties and sustainability.

Definition

A byproduct of coal combustion at power plants, used as a supplementary cementitious material (SCM) to partially replace Portland cement in concrete mixes. Class C fly ash has cementitious properties; Class F fly ash is primarily pozzolanic. Using fly ash reduces cement content, lowers heat of hydration, improves workability, and can enhance long-term strength and durability.

Why It Matters in Bidding

Fly ash lets concrete suppliers and estimators lower cement content, which reduces both material cost and the embodied carbon that sustainability-focused owners increasingly require in bids. However, fly ash slows early strength gain, so estimators must weigh mix savings against potential schedule impacts on formwork stripping and post-tensioning in cold or fast-track work.

Example

To hit a green-building target and trim mix cost, an estimator specs a 25 percent Class F fly ash replacement in the foundation concrete, then confirms the slower early strength still meets the formwork-stripping schedule.

Related Terms

Portland CementAdmixtureWater-Cement RatioCompressive Strength

Frequently Asked Questions

Replacing a portion of Portland cement with fly ash, often a lower-cost byproduct, can reduce mix price per cubic yard. The savings depend on local fly-ash availability and the replacement percentage. Estimators verify pricing with the ready-mix supplier, since regional supply has tightened as coal plants close.
Class C fly ash has self-cementing properties and contributes to strength relatively quickly, while Class F is primarily pozzolanic and reacts more slowly, offering strong durability and sulfate resistance. The class affects strength-gain timing, so estimators coordinate the choice with schedule needs and exposure conditions on the project.
Fly ash can delay early strength gain, especially in cold weather, which may push back formwork stripping, shoring removal, or post-tensioning. Estimators should account for this in fast-track work by adjusting cure times or limiting the replacement percentage so schedule milestones are not jeopardized to save on mix cost.

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