Effects of fly ash on the durability of concrete

Fly ash serves as an admixture in concrete formulations. The construction industry increasingly uses concrete in harsh environments, particularly in oil and gas operations and nuclear reactors, where it must contain gases and vapours released at extreme temperatures and pressures.

Primary effects of fly ash on concrete durability

• Concrete permeability

• Carbonation processes

• Freeze thaw cycle resistance

• Abrasion and erosion resistance

• Sulfate resistance

• Alkali aggregate reactions

• Steel reinforcement corrosion

• Marine and seawater exposure

Detailed impact analysis

Permeability: the permeability of fly ash concrete was lower than that of controlled concrete after 28 days of curing. After six months, fly ash concrete becomes significantly more impermeable due to pozzolanic activity.

Carbonation: carbon dioxide reacts with calcium compounds in cement, which is a primary cause of steel corrosion concerns. Proper concrete mixture proportions and adequate curing are critical when using high fly ash content.

Freeze thaw durability: properly proportioned fly ash concrete shows good frost resistance. Research supports that fly ash produces no adverse effects on the air void system of hardened concrete.

Abrasion resistance: concrete made with ASTM class F fly ash outperforms concrete made with ASTM class C fly ash or with no fly ash at all.

Sulfate resistance: research confirms that fly ash improves the sulfate resistance of concrete.

Alkali aggregate reactions: low calcium fly ash replacing 25 to 30 percent of cement effectively reduces expansion when alkali content remains below 4 percent.

Reinforcement corrosion: adequate concrete cover protects steel reinforcement from chloride induced corrosion.

Marine applications: concrete with 25 percent fly ash replacement and a water to cementitious ratio below 0.50 performs well in freeze thaw and wet dry marine conditions.