Civil Engineering Reference
In-Depth Information
calcium hydroxide is a kind of white deposition with layer structure that is adverse
to strength. If the
fly ash is added, the reaction can be continued just like the second
formula and the C
S
H can be produced, playing a role in enhancing the strength
-
-
of concrete further.
It is undoubted that
fly ash can further increase concrete strength. But, it is arguable
howmany Ca(OH)
2
are generated during the hydration of calcium silicate. There is no
accurate statement about that in theoretical studies. From the large amount of infor-
mation, we have read over, some people say 20 %, some say 25 %, and others say
28 %. All in all, it appears to be less than 30 percent. Besides, if the data of less than 30
percent are workable, how many
fly ash is needed in formula (
9.2
)? From what has
been discussed above, there is no theory basis for high-volume
y ash.
The traditional theoretical studies insist that it possess three effects. The
first is the
pozzolanic effect. Fly ash is a kind of pozzolanic material and cannot bind by itself.
At room temperature and in the presence of water,
fly ash secondly reacts with Ca
(OH)
2
produced during the hydration of cement in concrete, generating insoluble
CSH gel, which will make improvement in strength and permeability resistance. The
second is the morphology effect. The main mineral composition of
y ash is vitreous;
this spherical vitreous morphology has smooth surface,
fine particle size, compact
texture,
fine inner speci
c surface area, weak adsorption force for water. As a result,
the addition of
fly ash has decreased the water demand, lowered the early drying
shrinkage and boosted the compactness of concrete significantly. The third is the
filling effect. The
fine granules of
fly ash are equally distributed in cement particles,
which not only
fill the voids among cement particles but also improve the granule
grading of cementitious material and increase compactness of cement paste [
3
].
We believe that the above three effects cannot explain the theoretical foundation
for high-volume
y ash.
Nevertheless, large amounts of experimental results obtained in laboratory by
many experts indicate that there is no negative even positive effect on concrete
strength when the dosage of
fly ash reaches 60 % or even 70 % in high-volume
concrete. This is the question that cannot be explained theoretically.
9.1.2 Unsolved Technological Problems in Engineering
It is well known that the speci
c density of cement is about 3.0 and that of
fly ash is
about 2.0. The difference in their speci
c densities greatly troubles the construction
and seriously impairs the effectiveness of utilization of
fly ash, and the problem is
unsolved yet. The lower density of
fly ash leads to separation of
fly ash from cement
particles, and
floats onto the surface layer, but cement sinks onto the bottom
layer of mortar in construction. Therefore, the strength of the upper part of concrete
will be lowered due to the concentration of
y ash
fly ash, and plastic cracks will appear
easily, doing harm to the concrete, that is to say, the lifetime of concrete will be
remarkably shortened, even the concrete will be destroyed directly, especially for
the structures that the strength of the upper side or the surface are required.
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