Gregory Monaghan, Specialty Polymers, Inc., Chester, SC
Abstract
Concrete is a widely used and versatile
construction material but, for some applications, using polymeric coatings or
additives can significantly improve durability, adhesion or appearance. Improper
substrate preparation or mistakes in the
selection of coatings can lead to widespread (and expensive) coating failure.
A background on cement chemistry, uses
for polymeric additives and coatings, and
the potential failure modes of coatings
on concrete are presented in this paper.
Several different coatings for concrete are
discussed along with key advantages and
disadvantages of the different chemistries.
Introduction
Concrete is the most widely used building material because it has many unique
properties which make it ideal for use in
construction jobs. It can be formed into
different shapes on the job site, is very
durable and requires little maintenance to
withstand UV, mold or insects. Concrete is
noncombustible and is relatively inexpensive relative to other building materials. It
is also a particularly strong building material with high compressive strength.
The compressive strength of concrete
is built slowly over a period of time, and
can continue to increase as long as there
is unreacted cement and water present.
Three factors are necessary in develop-
ing the maximum strength – the correct
combination of ingredients, the correct
method of placement of the concrete and
the correct curing. If any of these factors
is not done correctly, then the strength of
the concrete can be as much as 50% lower
than expected. In addition, concrete can
lose strength through chemical attack.
Many of these failures can be eliminated
by changing the concrete formulation to
reduce the porosity of the cured concrete
– or by using polymeric coatings or admixes which can give a better cure and/or
protect from chemical attack.
Concrete Composition
Concrete is composed of cement, water
and aggregate. Aggregate is the sand
and gravel used in concrete – composing 60 to 75% of the concrete. Like the
extender pigments used in paints, the aggregate is a relatively low-cost ingredient (relative to the cement) and it keeps
the cost of the concrete low. Both fine
aggregate (typically sand, but can be
crushed rock with a particle size of less
than 3/8 inch) and coarse aggregate (
typically gravel with a particle size of less
than 1.5 inches) are used. The aggregate
plays an important role in preventing
shrinkage of the paste surrounding the
aggregate during drying which can cause
cracking. Similar to the concept of PVC
in paint, the larger the aggregate size,
and the more rounded the aggregate, the
lower the amount of cement and water
required since more cement paste is required to coat the surface area of smaller
or more angular aggregate.
Cement is a man-made material and
the highest cost component of concrete
but since it is usually only about 15%
of the mix, the overall cost of the con-
crete is low. Cement is formed during a
high-temperature calcining process dur-
ing which minerals like clay, iron ore,
sand and limestone are partially melted
and recombined at up to 2,700°F in large
kilns wide enough to fit a car and as long
as a 40 story building. During a preheat-
ing step at 900°F, the calcium carbonate
is converted to calcium oxide (lime) and
carbon dioxide is driven off. As the tem-
perature is slowly increased to 1500°F,
the aluminate and ferrite phases melt and
some of the calcium oxide reacts with
the silica oxide to form a calcium silicate
mineral called belite. As the temperature
is increased further to 2700°F, much of
the belite is converted to a more reactive
form of calcium silicate called alite which
is key to the early strength of portland ce-
ment. The silicates are cooled rapidly to
form gray softball-size lumps called “clin-
ker” which is ground to form sub-micron
size particles. The high-temperature cal-
cining process results in cement particles
which are soluble and unstable at room
temperature when they become wet. In
the hydration process, the cement par-
ticles form a paste which surrounds the
aggregates. This process involves the dis-
solving of the cement particles in the wa-
ter, followed by the precipitation of the
hydrates (which form the paste) from a
super saturated solution.
The precipitated hydrates are not the
same materials which first dissolved.
There is water associated with the pre-
cipitates which forms hydrates. Some of
these hydrates are larger in volume than
the dry cement particles and they contin-
ue to form and occupy space that was oc-
cupied by the free water in the concrete.
In addition, the precipitates are more en-
ergetically favored than the cement and,
as a result, there is a release of heat – the
heat of hydration – which can accelerate
the hydration reaction.
Concrete Chemistry
and Protective Coatings
