Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 18

• Fly ash is particularly useful in marine structures (where curing time
is available before inundation) as otherwise there is the conflict of
requiring high C3A to resist chlorides and low C3A to resist sulfates,
whereas fly-ash concrete resists both.
• Fly-ash concrete reacts extremely well to steam curing with little or
no detrimental effect unlike Portland-cement-only concrete.
2.2.5 Dangers to avoid with fly ash
• Since fly ash is lighter (and usually cheaper) than cement it might be
thought that it would be especially useful in low-strength concrete. In
fact it does produce much better looking concrete, which has greater
segregation resistance and is less prone to bleeding for a given (relatively
high) water to cementitious ratio. However this is sometimes its undo-
ing. Uninformed or thoughtless people tend to overwater it to a greater
extent than plain concrete, yet in fact its strength is more affected by
a given amount of excess water. Thus fly ash should be used with care
and conservatism for low strength requirements. Properly used it is
valuable for such uses but is less resistant to overwatering abuse.
• Because strengths take longer to develop, more efficient and
prolonged curing is necessary for fly-ash concrete. It is true that fly-
ash concrete is substantially less permeable than plain concrete of
similar strength, and therefore may be to some extent “self-curing”
in larger masses (and especially for below ground or on ground
foundations). However, this does not help the exposed cover con-
crete or in thin elements
• Calcium hydroxide is an end product of the reaction of C 3 S and C 2 S with
water. The amount of calcium hydroxide in the hydrated Portland cement
paste can constitute up to 26% of the total volume (Marchand et al.,
2001). Although calcium hydroxide has the disadvantages of being soft,
weak, and easily dissolved by water or chemicals, it is also a source of
the alkalinity, which helps protect steel from corrosion. At high replace-
ment levels, the pozzolanic reaction of fly ash with calcium hydroxide
may reduce alkalinity. This is the reason that rates of carbonation can be
higher in fly ash concrete and therefore the chemical protection available
for the reinforcing steel. The question is whether this is compensated for
by the reduced permeability of the fly-ash concrete. The answer lies in
the curing: yes if well cured, no if not well cured.
• Because fly-ash concrete gains strength more slowly, it is susceptible
to creep if depropped (beams and slabs) too early. The need to prop
longer may be an additional cost. But remember it is the in situ matu-
rity that counts.
• Due to reduced bleeding, plastic shrinkage cracking due to evaporation
can occur more readily without appropriate precautions.