the initial strength of concrete but it does
build over time and it is a major contributor to the mature strength of concrete.
Aluminate and Ferrite
Portland cement also contains tricalcium
aluminate and tetracalcium alumina ferrite. These are essential to the formation
of the liquid phase in the calcining process
since they melt at lower temperatures; the
liquid from the melted aluminates hold
the solid belite particles as they are slowly
converted to the more reactive alite. The
calcium aluminate is as soluble as the
C3S and it reacts rapidly to form a mineral called hydrogarnet.
This reaction releases so much heat
that if it were allowed to take place the
concrete would set in minutes, too fast
for good workability. To prevent this, a
small amount of gypsum is added to cement when the clinker is ground. The
gypsum is also highly soluble and the sulfates react with the aluminates to form
the mineral ettringite. If all the sulfate reacts before all the C3A does then the level
of the sulfate ions decreases and the ettringite becomes unstable and undergoes
a second reaction over several hours to
form monosulfate which is lower in sulfate. Because there is not enough gypsum
to fully react with the C3A, there is rarely
ettringite in cured concrete.
The ferrite phase (C4AF) in the cement
has some of the aluminum substituted
with iron. It is less soluble than the aluminate and, as a result, it reacts more slowly.
Both the aluminate and the ferrite phase
contribute to the heat of hydration and affect the rheology of the setting concrete.
In addition, the products of the hydration
of the ferrite phase can contribute to the
compressive strength of the concrete. 7
Induction Period
There is a short period of several hours
after the addition of water during which
the rate of hydration nearly stops; this is
called the induction period. The fact that
this induction period exists is critical to
the successful use of concrete. It means
that the concrete truck can be filled and
sent to a job site and there still be enough
time to apply the wet concrete before it
sets. Concrete chemists disagree on the
cause of the induction period, however
the most common theory is that an un-
stable gel or (in the case of the aluminate
phase) ettringite is formed around the ce-
ment particles C3S and C3A. This unsta-
ble gel prevents the cement particles from
further dissolving in the pore solution
slowing the reaction in the first few hours
after mixing. With time these unstable
gels are converted to other species which
allow the cement particle to continue to
dissolve, speeding the reaction again.
Hydration Reactions
Hydration reactions will continue as long
as there is free water, space in the pores
for the hydrates to grow, and unhydrated
silicates present (the core of the undissolved cement particles). Although most
of the hydration takes place in the first
month, the hydration reactions continue
for months or years. Different components of concrete contribute to cure at
different times, however. After 6-8 hours
the ettringite, calcium hydroxide and
C-S-H gel are being formed and the porosity begins to drop as the gel forms in
the water filled pores. The slower reacting ferrite species only begins to react after several days. After a couple days, the
concentration of the ettringite levels off
then falls as the gypsum is consumed and
monosulfate is produced. 8
If the concrete dries out prematurely,
the hydration reactions will stop and the
concrete cannot build any more strength.
In addition, early drying can lead to stress
cracking since the concrete will shrink
slightly as it dries, which can create
stresses at the surface (since the interior
of the concrete doesn’t dry at the same
rate as the outside). If the concrete can be
kept wet, the strength will build enough
that the concrete can resist the cracking.
As a result, it is common to try to restrict
the loss of humidity and heat from the
concrete in the critical early stages of cure
– usually the first 6 days after the pour.
There are several ways of doing this.
One method of keeping the concrete
surface moist is to mist the surface with wa-
ter or pond the water on the surface. This
is low cost, although it requires a significant
amount of water to mist the surface for sev-
eral days. Misting the surface can also be
difficult on vertical concrete structures and
coatings are often used instead to slow wa-
ter loss on those structures.
The most common method of keeping
the surface moist is to use polyethylene
sheeting over wet burlap, however this
method can lead to several problems.
There is also the chance of staining the
concrete if the sheeting is not evenly applied and, if the burlap is not kept wet, it
can wick water from the concrete, leading
to cracking. Unless the sheeting is well attached, wind can get underneath and lift
the sheeting creating a wind tunnel and
drying the surface even more. As a result, this method is somewhat labor intensive with the possibility of damage to
the concrete.
Coatings for Concrete
There are also several coatings which can
be used to help slow the loss of moisture
and help cure the concrete. These can be
temporary or permanent coatings.
Temporary Coatings
Temporary coatings are designed to help
keep the moisture in the concrete for the
first week or so but then to degrade under
UV exposure, abrasion from traffic, or
removal by using special cleaners. They
have traditionally been coal tar resins,
but are now more commonly hydrocarbon resins and supplied either in solvent
or emulsified in water. The coatings are
usually spray applied once the bleed water has evaporated but before the concrete has had a chance to dry. These
coatings need to be removed if a coating
is to be applied to the concrete after cure
because they can interfere with the adhesion of the topcoat. Since they often do
not degrade evenly, mechanical or chemical removal is often used.
Wax emulsion coatings (typically paraffin emulsions or lower-cost slack wax
emulsions with some resin) are also temporary coatings used to prevent water
loss. While effective at reducing moisture
loss, they can cause discoloration on aging and they also have to be removed if
a coating is to be applied to the concrete
later. If they are not removed they can
interfere with the adhesion of topcoats
which are applied later.
