1. Compressive Strength: Strength gain contributed by Portland cement occurs very rapidly at early ages up to about seven days, after which it slows markedly. Cement normally gains the great majority of its strength within 28 days, thus the reasoning behind specifications normally requiring determination of 28-day strengths as a standard.

Strength development contributed by fly ash occurs through chemical combination of reactive fly ash glass with calcium hydroxide generated by hydration of portland cement.

As lime from cement hydration becomes available (cements tend to vary widely in their reactivity), it reacts with fly ash. As much as 20 pounds of free lime is released during hydration of 100 pounds of cement. This liberated lime forms the necessary ingredient for reaction with fly ash silicates to form strong and durable cementing compounds no different from those formed during hydration of ordinary portland cement.

The Hydration reactions of Ordinary Portland Cement without/with presence of Fly ash are as under:

The main benefit of fly ash in concrete is that it not only reduces the amount of non-durable calcium hydroxide (lime), but in the process converts it into calcium silicate hydrate (CSH), which is the strongest and most durable portion of the paste in concrete.

Typically, concrete made with fly ash will be slightly lower in strength than straight cement concrete up to 28 days, equal strength at 28 days, and substantially higher strength within a year’s time. Thus, fly ash concrete achieves significantly higher ultimate strength than can be achieved with conventional concrete. 

(1 psi = 0.007 N/mm2 )

Conversely, in straight cement concrete, this lime would remain intact and over time it would be susceptible to the effects of weathering and loss of strength and durability.

Concrete made with Class C fly ash (as opposed to Class F) has higher early strengths because it contains its own lime. This allows pozzolanic activity to begin earlier. At later ages, Class C behaves very much like Class F, yielding higher strengths than conventional concrete at 56 and 90 days.

2.  Uniformity: Statistical analyses of compression tests have shown that the use of fly ash often lowers the variability of strengths (lower coefficient of variation). This can result in a reduction in “overdesign”, yielding a direct cost savings to the concrete producer.

3. Flexural Strength: In general, a relationship exists between the compressive and flexural strengths of concrete. Concrete which has a higher compressive strength will have a correspondingly higher flexural strength.

This holds true for fly ash concrete. However, in many cases, fly ash concrete has demonstrated flexural strength exceeding that of conventional concrete when compressive strengths were roughly equal.

4. High Strength Concrete: In instances where high strength concrete has been specified (above 7,000 psi i.e. above M 50), fly ash has consistently proven its usefulness. 

After a certain amount of cement has been added to a mix (usually about 700 pounds i.e. 315 kg), the addition of fly ash usually results in higher strengths than an equal amount of added cement. This is especially true for 56 and 90 day strengths. Production of high strength concrete requires the use of high quality fly ash at a minimum of 15 percent by weight of total cementitious materials.