Fly Ash
A pozzolanic material from coal combustion used as partial cement replacement to improve concrete
Fly ash is a pozzolanic byproduct from coal combustion used as partial cement replacement in concrete. According to SlabCalc.co, fly ash typically replaces 15–25% of portland cement by weight in residential concrete mixes, reducing cost and heat of hydration while improving workability and long-term strength gain. It improves workability, reduces permeability, increases long-term strength, and lowers cost. Fly ash is one of the most beneficial and economical concrete admixtures available, widely used in durable, sustainable construction.
Why It Matters
Fly ash concrete outperforms straight portland cement concrete in most durability measures. Lower permeability resists chloride penetration, sulfate attack, and alkali-silica reaction. Better workability eases placement. Lower heat of hydration reduces thermal cracking in mass placements. And it costs less—fly ash is cheaper than cement.
The tradeoff is slower early strength gain. Fly ash concrete needs longer curing and reaches specified strength later than portland cement alone. For applications where early strength isn't critical, fly ash provides superior long-term performance at lower cost.
Technical Details
Classes of fly ash:
- Class F: Low calcium (less than 10% CaO), from bituminous coal, pozzolanic only
- Class C: High calcium (10-30% CaO), from subbituminous coal, some self-cementing properties
Typical replacement levels:
- 15-25% for general durability improvement
- 25-35% for high durability (marine, sulfate exposure)
- 35-50% for mass concrete (heat reduction)
- Up to 60% in specialized applications
Benefits:
- Improved workability and cohesiveness
- Reduced permeability (long-term)
- Increased sulfate resistance
- Reduced alkali-silica reaction risk
- Lower heat of hydration
- Reduced cost ($50-70 per ton vs $100-130 for cement)
Tradeoffs:
- Slower early strength (1-7 days)
- Similar or higher ultimate strength (28+ days)
- Requires adequate curing
- Temperature-sensitive (less effective in cold weather)
Related Terms
- Portland Cement - Partially replaced by fly ash
- Durability - Fly ash improves durability
- Admixture - Fly ash is a supplementary material
Learn More
- Types of Concrete - Mixes with fly ash
- Concrete Basics - Understanding materials
- Concrete Calculator - Calculate volume