William Heiden
Engl 202C
February 9, 2016
Just take a look around, and most likely you will see something that is made out of concrete.
Many things are made out of concrete, from foundations, to columns and beams, to driveways, to even
counter tops. While it has many uses, it is mainly use for structural purposes. The reason that concrete
is used structurally is due to its strength properties. Concrete is very strong in compression with makes
it a good choice for foundations and columns. Concrete is very weak in tension though, which can make
it a made choice for many things as well. This is all due to how it is made, which we are going to look at
now.
Making concrete is the process of combining a binding material (cement), a granular material
(aggregate), and water. Probably one of the biggest misconceptions when it comes to concrete is that it
is the same thing as cement. Many people use the two words interchangeably. However this is not
true. Cement, which is talked about in the next paragraph, is just one of the ingredients that goes into
making concrete. It is the binding material that holds all of the other ingredients together. Therefore
concrete is comprised of cement, but they are definitely not the same thing.
The main ingredient, and the one that gives concrete its properties is cement. While cement
might just seem like a singular ingredient in concrete, it is actually composed of numerous minerals that
give it the necessary properties to perform. Let’s look at how it is made. It all starts at the limestone
quarry. The rock there contains mainly silica, iron, and aluminum oxide, with varying amounts of
calcium carbonate as you go further down. The ore is taken from here to the cement plant. At the plant
the rock is dumped into the primary crusher, where it is crushed down to a few inches in diameter,
about the size of a
softball. It then goes
through the secondary
crusher, where it is
crushed down to about
the size of golf balls.
Rocks high is calcium
carbonate and rocks low
in calcium carbonate are
crushed separately. The
two are then mixed with
a ratio varying on the
type of cement being
made. This is then
known as the “raw mix.”
This raw mix is then loaded into a grinding machine called the roller mill. Extra minerals may then be
added depending on the amount naturally in the rock. These materials include silica, iron, and
aluminum oxide. The roller uniformly grinds and mixes the ingredients into a dry rock powder call the
“raw meal.” The raw meal now goes into a preheater where it is increased in temperature by ten times.
This starts the bonding process of all the minerals so that they will harden when hydrated with water.
The preheater also removes most of the carbon dioxide in the powder, isolating the lime, the most
important mineral in cement. The powder is then moved into a kiln where the material moves through,
reaching up to 1500 degrees Celsius. When it reaches this temperature it fuses in marble size pieces
called “clinker.” After the clinker is formed it is then quickly cooled back down to about 60-80 degrees
Celsius. The clinker then goes through the final where gypsum is added to delaying the hardening time
of the cement. Here it is crushed down into a fine power, finally ready to mixed and formed into
concrete.
The next ingredient in concrete is aggregate. Aggregate can simply be thought of as stone that
is added to the mixture. This stone is what gives concrete its strength. Depending on the concrete
needed, one might use a more coarse aggregate (bigger rocks), or a finer aggregate (smaller rocks), or
even sand. This is the material that the cement bonds together in the mixture.
The last main ingredient in concrete is water. As crazy as it might sound, adding water to the
cement is what causes it to harden. It does this through the process of hydration. Hydration is a
chemical reaction in which the major compounds in cement form chemical bonds with water molecules
and become hydrates or hydration products. The key to producing the perfect cement for a specific
need is adding the right amount of water to the right amount of cement. This is called the water to
cement ratio. A low water to cement ratio will produce stronger, more durable concrete, however is
will be very difficult to mold and work with. A high water to cement ratio will produce a weaker, more
fragile concrete, however it will be very easy to maneuver and shape.
Now that we have all of the ingredients, all we have to do to make concrete is mix them.
So as was said before, concrete is very strong in compression and weak in tension. This means
that concrete is strong when you push on it but weak when you pull on it. The main reasoning for this is
quite simple. Like we know, concrete is composed of cement, water, and aggregate. It can in very basic
terms be thought of as a bunch of rocks glued together. In order to compress it, one would need to
break the rocks inside, which is very hard to do and would require a lot of force. But to pull it apart, all
one would have to do is break the glue, which is much weaker than any of the rocks. Concrete is made
stronger by adding steel reinforcing bars to places where the concrete is tension. This allows the steel
bars to carry the tensile loads instead of the concrete, and steel is much stronger in tension.