Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 13

spheres known as a cenospheres (as opposed to the extremely jagged particle
shape of cement); and is of lower density (specific gravity [SG] usually 1.9 to
2.4 compared to 3.15 for cement).
Fly ash has a varying “pozzolanicity”, that is, some fly ashes give much
better strength than others. No unmodified fly ash is as good as cement on
a volume for volume substitution basis, but some fly ashes are as good as
cement in terms of 28-day strength and better at later ages when substi-
tuted on a mass for mass basis and when account is taken of their water-
reducing action as well as their strength production at a given w/c.
As fly ash is a by-product of the power industry, it can be affected by
changes in electricity generation due to variation in coal source, partial use of
biomass for fuel, and changes in burning temperature. As a result, fly ash is
becoming a far more variable material and concrete suppliers need to be even
more vigilant in assessing the quality of the material before incorporating it
into their concrete. These changes in the power industry and the strong push
for alternatives to power generation from coal have created an opportunity
for natural pozzolans to play a more significant role in the SCM industry.
There are few materials that do not have some drawbacks and with
fly-ash substitution these include
1. Reduced early strength
2. Increased setting time
3. Reduced heat generation (which is an advantage in hot weather or for
mass concrete, but a disadvantage in cold)
4. Inhibition of air entrainment, if of high carbon content (easily corrected
by higher dosage or specially formulated products for use with fly ash
but may give rise to higher variability if carbon content varies)
5. Added complication—One more factor requiring knowledge and skill
to give best results
Fly-ash concrete does not automatically display all the advantages (or dis-
advantages) of which it is capable. Crude substitution of fly ash for cement can
yield better or worse concrete depending on the circumstances and require-
ments. It could be said that fly ash puts another useful tool in the hands
of competent technologists and presents another trip wire for the uniniti-
ated to fall over. Also there are considerable differences among different fly
ashes and there is not an automatic best buy for all circumstances. There are
examples of troubles exacerbated if not caused by fly ash and, on the other
hand, of the use of fly ash not being permitted through ignorance or blind
prejudice in circumstances where it would have been highly desirable.
The bridge specifications in the state of New South Wales, Australia, specifi-
cally prohibited the use of fly ash in concrete in the early 1990s. Recent surveys
of the bridge stock showed the durability of the bridges built without fly ash
was profoundly inferior to those with fly ash before and after the prohibition.