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Concrete
Materials
In
order to make
good quality concrete, you need to fully
understand its
different constituents and the roles
they play. So, let's look at concrete's
ingredients more closely.
The
"heart" of concrete is the
cement - the substance that, with water,
does the chemical work, and binds the
sand and stones into an astonishingly
strong, composite material.
Cement
(and
"cement-like substances")
Cementitious
materials for concrete are fine mineral
powders. 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 (a
chemical reaction called hydration
takes place). As
a
result, the cement-water paste gradually
changes from a plastic state to form a
strong rigid
mass that binds the aggregate (sand and
stone) together.
The
word "cement" refers, in its
widest sense, to anything that holds
materials together. Glues are
"cements". 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.
The
rate of strength gain and heat developed
by the reaction between
water and cementitious materials is
governed by the fineness
of the cement particles, and their
constituents. The letter "N"
in a cement strength class designation
indicates normal rate of
strength gain, and "R"
indicates rapid strength gain,
particularly at early ages.
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We
use the word
"cement" to cover
all
branded cements, bearing the SABS
mark,
that comply with SABS
EN 197-1 /SANS 50197-1 strength
class 32,5N
and
are commonly
available from hardware stores
and builder's merchants
in 50
kg
bags.
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(If
considering the use of cements of other
strength classes, refer to the
manufacturer's recommendations
on the bag for mix proportions.) The
SABS mark on cement bags is a legal
requirement, and guarantees that the
correct proportions of constituents are
present in the cement.
The
cement must also comply with other
physical and chemical requirements as
specified in SABS EN 197-1
/SANS
50197-1
Cement
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Part
1: Composition, specifications and conformity
criteria for common cements.
Kindly
note that the term "masonry
cement" is used for cement which is
intended
for use in mortars or plasters and
complies with SABS
ENV 413-1 /SANS 50413-1 strength
class MC
22,5X. Don't
ever
use
masonry cement for
concrete work.
Incidentally,
the difference between
"mortar" and
"concrete" is that mortar has
only fine aggregate. Nothing larger than
about 5mm particle size - see Aggregates
below.
The
Aggregates
The
sand and stones are referred to as
"aggregate": the stone is the
"coarse aggregate" and sand
the "fine aggregate". In
general, stone
and sand for use in concrete should
consist of
particles of hard material of rounded or
roughly cubical
shape, with a fairly smooth surface, and
should
be free from impurities such as earth,
clay, roots,
salt, etc.
Both
types of aggregate should include
particles with widely-varying sizes.
In
a bit more detail:
Stone:
The
stones are usually between about 10 and
20mm in size. Though less important than
the quantity of cement involved, the
sizes and proportions of the aggregate
components are amongst the factors that
determine the final properties of the
concrete. Providing
that stone used for concrete is composed
of clean,
hard, durable particles, the actual
source is less important
than the size of the particles. Single-sized
26,5mm
or 19mm stone is more economical to use,
but
13,2mm stone makes hand mixing easier.
Sand:
The
properties of the sand has a very marked
effect on the quality of the concrete, so
where possible choose your sand
carefully.
The
best sands are "evenly graded"
-
they
contain
particles of a wide range of sizes, the
medium sizes predominating, but with the
coarsest (just passing the 4,75mm sieve)
and the finest being present.
Kindly
note these pointers:
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Sands
containing more than about
10% of shale, or of weathered
basalt or dolomite (all of
which are dark in colour) may
cause excessive shrinkage
on drying
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Sand
should contain a
proportion of very fine material
-
a
lack of fines results in a
harsh concrete
which is hard to compact
and to bring to
a good finish. It can also
lead to excessive "bleeding"
where a layer of water
appears on the
surface of the concrete
after placing
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For
your DIY project, don't use:
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Weak
and porous materials (e.g.
furnace
bottom ash) as
they yield
poor quality concrete of
limited strength
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Soil
dug
out of the garden as organic
material and/ or fertilisers
as
they may retard setting,
and clay may
cause cracking
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Pit
and plaster sands are not
for concrete
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they are too fine
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Wind-blown
(desert)
or
minedump
sands
-
they are generally
fine and single-sized. In
addition, mine
waste may
be
contaminated with chemicals
that
are harmful to the
concrete.
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An
interesting variety
of concrete is
sometimes referred
to as
"cyclopean
concrete",
which is made by
adding massive rocks
to ordinary
concrete. The rocks
form a sort of
"super coarse
aggregate".
This is generally
used for the walls
of large dams and
other massive
structures where
enormous volumes of
concrete are
required.
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Water
Drinking
water is normally suitable for making concrete
and mortar.
Murky
or dirty looking water, e.g. from a borehole
or dam, should be tested by a laboratory
before it is used.
Surprisingly,
seawater can be used for unreinforced
concrete if the appearance of the
concrete is unimportant -
a
white powdery deposit called
efflorescence may develop on the
surface, but doesn't effect the strength
of the concrete.
Note:
Don't
use seawater if the concrete includes
reinforcing steel or metal brackets,
fixings, etc -
the
salt causes
steel
in the concrete to rust.
Please
navigate in the especially provided
table at the top.
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