To produce exceptional concrete, it is extremely important to have a smooth gradation of material from rock down to the finest particles (in other words, a good mix of particle sizes, so that the largest practicable rock fills the majority of the volume, while the progressively smaller rock and sand fill the voids left between the larger particles).

Ideally, it is best to have as much volume as possible filled with strong, durable aggregate particles, with enough paste (comprised of as much CSH and as little lime as possible) to coat every particle. Also, voids should not be present in the paste unless they are specifically provided as microscopic entrained air bubbles to provide durability in freeze-thaw environments.

In real life, though, economics and local aggregate sources dictate the quality of materials used. The result is that excess voids often exist between the aggregate particles that must now be filled by paste and air. The challenge becomes producing an appropriate amount of the best possible quality paste, so that the resulting hardened paste will fill the excess voids resulting in concrete of desired strength & high durability.                   

This paste is the key to durable and strong concrete, assuming average quality aggregates are used. At full hydration, concrete made with typical cements produces approximately 1/4 pound of non-durable lime per pound of cement in the mix. Fly ash chemically reacts with this lime to create more CSH, the same “glue” produced by the hydration of cement and water, thereby closing off the capillaries that allow the movement of moisture through the concrete.

 
The result is concrete that is less permeable- an important  property that is one of the main reasons for high durability of Fly ash concrete.  Fly ash concrete is less permeable because fly ash reduces the amount of water needed to produce a given slump, and through pozzolanic activity, creates more durable CSH as it fills capillaries and bleed water channels occupied by water-soluble lime (calcium hydroxide).

Permeability is defined as the coefficient representing “the rate at which water is transmitted through a saturated specimen of concrete under an externally maintained hydraulic gradient”. Permeability is inversely linked to durability in that the lower the permeability, the higher the durability of concrete.

Permeability is most frequently described by the chloride-ion permeability test, which measures the passage of electrical current through a concrete specimen exposed to a batch of sodium chloride. Limits of acceptability are as shown in the table below: 

Chloride permeability based on charge passed

Recent testing has shown that properly proportioned concretes using a combination of fly ash, normal or high-range water reducing admixtures, and air entraining admixtures have the ability to produce the same low levels of permeability as latex modified and silica-fume concretes.

Fly ash increases the cementitious compounds, minimizes water demand, and reduces bleed channels – all of which increase concrete density. These factors yield concrete of low permeability with low internal voids. Durability is increased with regard to freeze-thaw damage and disintegration from attack by acids, salts or sulfates.

Fly ash helps in five ways: Using fly ash in the concrete mix greatly aids permeability (and durability) in five ways:

• Through pozzolanic activity, fly ash chemically combines with water and calcium hydroxide – forming additional cementitious compounds which result in denser, higher strength concrete. The calcium hydroxide chemically combined with fly ash is not subject to leaching, thereby helping to maintain high density.
• The conversion of soluble calcium hydroxide to cementitious compounds decreases bleed channels, capillary channels and void spaces and thereby reduces permeability
• At the same time, the above chemical reaction reduces the amount of calcium hydroxide susceptible to attack by weak acids, salts or other sulfates.
• Concrete density is also increased by the small, finely divided particles of fly ash which act like micro-aggregates to help fill in the tiniest voids in the concrete.
• Fly ash provides a dramatic lubricating effect which greatly reduces water demand (2% to 10%). This water reduction reduces internal voids and bleed channels and keeps harmful compounds out of the concrete.