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Chemistry of cement
- PART II
Dr Sirion Robertson
Making concrete is much more than just mixing cement, water and earth
according to Public
science communicator, Dr Sirion Roberston. After some curing of the concrete in PART
ONE of this series on the chemistry of cement, he brings PART TWO on the
science of making concrete. A must for anyone who has ever had to build or will be building
with cement. Don't hear it from the builder - get it from the scientists! Science for the DIY person!
As we've said, the chemically active component of concrete is the cement, and
this is where the strength comes from. The word "cement" refers, in
its widest sense, to anything that holds materials together. Glues are
"cements". The particular type of cement used in concrete, and nearly
all building work, is called "hydraulic cement" because water is
required for the chemical reactions that lead to hardening. There are several
types of hydraulic cement, but the most important and commonly-used is "portland
cement", so-called because its colour, when set, resembles that of a type
of stone found on the Isle of Portland, in the English Channel.
Portland cement is made by crushing, heating, and crushing again a mixture of
rocks and soil-like substances, the main ones being types of limestone, chalk
and clay. These are complex substances, in the sense that they contain several
chemical elements in various combinations. The most important actual chemicals
are calcium, silica, aluminium, iron and oxygen. The purpose of the first
crushing and grinding is to bring the chemicals into sufficiently close
proximity to enable them to react with each other. Heating, to about 1 500
degrees C, provides the necessary energy for these reactions to occur, and new
compounds are formed. The final grinding again produces an extremely fine powder
and close proximity of potentially reactive substances. When water is added, new
compounds are formed, some contraction in volume occurs, heat is given off, and
the individual cement particles fuse into a continuous "matrix" which
locks the sand and stone components into a hard, rigid mass.
Although modern cements are made in factories under carefully controlled
conditions, there are also so-called "natural cements" in many areas
of the world. It was these "natural cements" that our forebears used
in Rome, Greece and elsewhere. The most common are mixtures of limestone and
clays. They are prepared by burning and then crushing into powdered form.
Attaining strength - and measuring it
 The
graph to the right shows increasing strength of a sample of concrete as a
function of curing time. Notice that strengthening is quite rapid at first: the
strength after one week is more than half that attained at the end of four
weeks. Also, although the graph doesn't continue beyond 28 days, the shape of
the curve makes it quite clear that strength continues to increase well beyond a
month. Indeed tests show that, under favourable conditions, concrete is still
"maturing" after 18 months! ("Favourable" here means under
warm and humid conditions.)
The strength unit on the graph is MegaPascals. The Pascal is the basic unit
of pressure, and pressure is defined as force per unit area. The details on the
graph were obtained empirically - in other words, from experiments. In this case
the experiment involves using a hydraulic press to crush specimen cubes of the
concrete, and measuring the pressure at which the cube breaks.
The test we've shown in the graph is for "compressive strength".
Another sort of test is for "tensile strength". You can best visualise
this by imagining a vertical rod of concrete, with a mass suspended from it.
Here the force tends to pull the substance apart, rather than crushing the
components closer together. As we increase the hanging mass, the rod will
eventually break, and the force that finally breaks it will be a measure of the
material's tensile strength.
 The
main "Achilles heel" of concrete is its relatively low tensile
strength: only about one tenth of its compressive strength. This has important
implications for concrete as a mass-bearing structure. Any horizontal beam,
supported at both ends, will tend to sag under its own weight. With additional
loading it will eventually break. Notice the image to the right, that there are
both compressive and tensile stresses acting in the beam.
The tensile force acts along the lower part, where the beam tends to stretch.
And if tensile strength is less than compressive strength, this is where it will
eventually break. Because of their low tensile strength, concrete beams and
slabs aren't good at carrying heavy loads unless they're "helped".
Reinforced concrete: Steel to the rescue
Incorporating steel into concrete produces a composite building material
sometimes called "ferrocrete". (It is this material, or the related
"ferrocement" that is used for boat-building.) In spite of the fact
that concrete has been used since the earliest civilisations, ferrocrete dates
from as recently as the mid-nineteenth century. With the great increase in
strength that it gives, its effect on architectural styles and potentialities
has been enormous.
Steel is the ideal material to embed in concrete as a strengthening agent,
mainly for three reasons. Firstly, steel has extremely high tensile strength;
secondly, by a fortunate coincidence, the extent to which steel and concrete
change their dimensions in response to temperature change (that is, their
coefficients of expansion) are very similar. Thus even with very great
temperature changes, the concrete and the embedded steel more or less keep pace
with each other in their responses, and no severe stresses are set up by one
material "trying" to change its length more than the other. The third
thing that makes steel so appropriate is that - perhaps surprisingly - its
elasticity is less than that of concrete. When a concrete beam or slab bends
(See the image above), the material on the outer part of the curve stretches.
Due to its elasticity, a small amount of stretching can safely occur without
breaking the concrete, and it is this very small amount of bending that puts the
steel rods under tension - the concrete "tries" to stretch them. The
fact that steel has a very much smaller tendency to stretch, under a given
force, allows it to come to the rescue and prevent any further deformation of
the concrete member.
(The tendency of concrete to contract slightly while curing is in one way an
advantage. This "shrinkage" causes the concrete to grip very tightly
onto the steel. As a precaution, to ensure that the concrete-steel interface
doesn't break down under load, the steel members are usually bent in various
ways to give even more effective  bonding.)
From the foregoing you can see that it's extremely important where, in the
concrete beam or slab, the steel reinforcing rods are placed. To put the steel
near the top of the beam in Figure 2 would be a complete waste. The resulting
structure would be no stronger than competely unreinforced concrete. Where would
you place the reinforcing rods in the beam shown in the image to the right.
The amount, type and positioning of steel in reinforced concrete is calculated
very carefully by structural engineers, taking into account the relative tensile
strengths and elasticities of the two materials, so that the resulting structure
will carry any loads that might be placed on it - with a good safety margin. The
size of the "safety margin" built into the design depends on what the
engineers see as a "worst case scenario".
The main weakness of steel, as a structural material, is its tendency to
corrode. Concrete, on the other hand, is highly resistant to corrosion, and
therefore makes an excellent shield for the embedded steel. It's important that
no steel members come closer than about 5cm from the surface of the concrete,
and that the concrete itself is of low permeability. If cracks or incorrect
building procedures do allow the reinforcing steel to corrode, it slowly expands
and may eventually cause parts of the surrounding concrete surface layers to
break off. The condition is known as "spalling". It's clearly a
progressive and potentially dangerous situation, and requires careful attention.
A Final Miscellany: Accelerating; Retarding; Air entrainment; Colouring.
We've covered what are certainly the most important and perhaps the most
interesting aspects of concrete technology. Here are some closing remarks on
other aspects of this extraordinarily versatile medium.
Accelerating of the curing process is achieved either by raising the
temperature, as we've discussed, or by adding substances during the mixing.
Calcium chloride or sodium chloride are the most commonly-used accelerators.
It's inappropriate for large quantities which must be processed at a single
session, because hardening shouldn't begin while work is still in progress. The
final strength of rapidly-cured concrete is usually a little less than
equivalent mixes which haven't been accelerated.
Retarding is achieved by adding calcium sulphate, sodium bicarbonate, or
various other organic compounds.
Air-entrainment is important because this type of concrete, in addition to
being somewhat lighter, is considerably more resistant to damage by very low
temperatures. Regular freezing and thawing of concrete can lead to severe
deterioration. The air-entrained variety is also a very much better insulator
against temperature change. Air entrainment is achieved by mixing in, with the
cement, a compound that react with the cement and water to produce small
bubbles. Animal fats, or various fatty acids are amongst the substances used to
cause bubble formation.
Colouring is done by mixing one or more of several coloured oxides into the
newly-mixed concrete. This adds quite significantly to the cost of the project,
and where appropriate it is more common to "float" the pigment onto
the surface of the wet concrete immediately after placing, so that only the top
few millimetres are pigmented. The colouring is more effective if the concrete
is made with a white or nearly-white variety of cement instead of the ordinary
grey stuff. This too adds substantially to the cost.
Concluding remarks
Concrete remains one of the most commonly-used, durable and versatile
structural materials: a direct descendant of the stone age, that has kept pace
with our most modern and pressing needs.
There is good reason to believe that if life exists elsewhere in the cosmos,
it is based on carbon compounds. For similarly fundamental reasons we may assume
that advanced life-forms elsewhere would have discovered, and used, cement-like
substances - including concrete.
As we said: fascinating stuff.
For Part One of this Article: 
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