Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 15

The increasing variability in fly ash may necessitate a relaxation in some
compositioned components of standards. Reliance on performance specifi-
cation is the way to ensure adequate concrete properties.
2.2.3 Effects of fly ash
There are three kinds of effect from the incorporation of fly ash in concrete.
These are physical effects on both fresh and hardened concrete, chemical
effects on setting process and hardened concrete, and physical chemistry
(or surface chemistry) effects on setting process.
2.2.3.1 Physical effects
The fly-ash particles are very similar in size and shape to entrained air
bubbles, and have many very similar effects:
• Water reduction—Perhaps of the order of 5% but varies with different
ashes. A very few ashes (e.g., some Hong Kong ash) slightly increase
water requirement.
• Reduction of bleeding.
• Improved cohesion and plasticity.
• Improved pumpability.
• Reduced slump loss with time.
Fly ash is not compressible and probably does not help frost resistance at all
(and tends to inhibit air entrainment so that a larger dose of air entraining
agents [AEAs] is needed). However, this property (incompressibility) makes
fly ash even more valuable than entrained air for pumpability. Also fly ash
has the benefit that it is present as a clearly defined quantity.
2.2.3.2 Chemical effects
When cement hydrates, it releases free lime. This lime is the softest, weakest,
and most susceptible to chemical attack and leaching of all the constituents
of concrete.
By reacting with the weaker and more porous portlandite, fly ash sub-
stantially reduces permeability in the hardened concrete if properly cured.
The fly ash combines chemically with the free lime to form com-
pounds similar to those produced by the rest of the cement. This reaction
occurs after cement hydration and generates less heat during hydration.
This is generally a valuable property in hot climates and for mass con-
crete, but may be a distinct disadvantage in colder climates.
Fly ash is effectively reactive silica, the very material causing problems in
aggregates through alkali-silica reaction. Actually this is a valuable feature
since there is so much reactive silica that most alkali is used up during an initial
reaction, leaving little to cause problems later, however reactive the aggregate.