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Cement
From Wikipedia, the free encyclopedia
In the most general sense of the word, cement is a
binder, a substance which sets and hardens
independently, and can bind other materials together.
The name "cement" goes back to the Romans who used
the term "opus caementitium" to describe masonry
which resembled concrete and was made from crushed
rock with burnt lime as binder. The volcanic ash and
pulverized brick additives which were added to the burnt
lime to obtain a hydraulic binder were later referred to
as cementum, cimentum, cäment and cement. Cements
used in construction are characterized as hydraulic or
non-hydraulic.
The most important use of cement is the production of
mortar and concrete - the bonding of natural or artificial
aggregates to form a strong building material which is
durable in the face of normal environmental effects.
History
Early uses
The earliest construction cements are as old as
construction[1], and were non-hydraulic. Wherever
primitive mud bricks were used, they were bedded
together with a thin layer of clay slurry. Mud-based
materials were also used for rendering on the walls of
timber or wattle and daub structures. Lime was probably
used for the first time as an additive in these renders,
and for stabilizing mud floors. A "daub" consisting of
mud, cow dung and lime produces a tough and water-
proof coating, due to coagulation, by the lime, of
proteins in the cow dung. This simple system was
common in Europe until quite recent times. With the
advent of fired bricks, and their use in larger structures,
various cultures started to experiment with higher-
strength mortars based on bitumen (in Mesopotamia),
gypsum (in Egypt) and lime (in many parts of the
world).
It is uncertain where it was first discovered that a
combination of hydrated non-hydraulic lime and a
pozzolan produces a hydraulic mixture, but concrete
made from such mixtures was first used on a large scale
by the Romans. They used both natural pozzolans (trass
or pumice) and artificial pozzolans (ground brick or
pottery) in these concretes. Many excellent examples of
structures made from these concretes are still standing
, notably the huge monolithic dome of the Pantheon in
Rome. The use of structural concrete disappeared in
medieval Europe, although weak pozzolanic concretes
continued to be used as a core fill in stone walls and
columns.
Hydraulic cements
Hydraulic cements are materials which set and harden
after combining with water, as a result of chemical
creactions with the mixing water and, after hardening,
retain strength and stability even under water. The key
requirement for this is that the hydrates formed on
immediate reaction with water are essentially insoluble
in water. Most construction cements today are
hydraulic, and most of these are based upon portland cement, which is made primarily from limestone, certain
clay minerals, and gypsum, in a high temperature
process that drives off carbon dioxide and chemically
combines the primary ingredients into new compounds.
Non-hydraulic cements include such materials as (non-
hydraulic) lime and gypsum plasters, which must be kept
dry in order to gain strength, and oxychloride cements
which have liquid components. Lime mortars, for
example, "set" only by drying out, and gain strength only
very slowly by absorption of carbon dioxide from the
atmosphere to re-form calcium carbonate.
Setting and hardening of hydraulic cements is caused by
the formation of water-containing compounds, forming
as a result of reactions between cement components and
water. The reaction and the reaction products are
referred to as hydration and hydrates or hydrate phases
, respectively. As a result of the immediately starting
reactions, a stiffening can be observed which is very
small in the beginning, but which increases with time. After reaching a certain level, this point in time is
referred to as the start of setting. The consecutive further
consolidation is called setting, after which the phase of
hardening begins. The compressive strength of the
material then grows steadily, over a period which ranges
from a few days in the case of "ultra-rapid-hardening"
cements, to several years in the case of ordinary
cements.
Modern cement
Modern hydraulic cements began to be developed from
the start of the Industrial Revolution (around 1700),
driven by three main needs:
· Hydraulic renders for finishing brick buildings in wet
·
· climates
· Hydraulic mortars for masonry construction of harbor
·
· works etc, in contact with sea water.
·
· Development of strong concretes.
·
In Britain particularly, good quality building stone
became ever more expensive during a period of rapid
growth, and it became a common practice to construct
prestige buildings from the new industrial bricks, and to
finish them with a stucco to imitate stone. Hydraulic
limes were favored for this, but the need for a fast set
time encouraged the development of new cements. Most
famous among these was Parker's "Roman cement" This
was developed by James Parker in the 1780s, and
finally patented in 1796. It was, in fact, nothing like any
material used by the Romans, but was a "Natural
cement" made by burning septaria - nodules that are
found in certain clay deposits, and that contain both clay
minerals and calcium carbonate. The burnt nodules
were ground to a fine powder. This product, made into a
mortar with sand, set in 5-15 minutes. The success of
"Roman Cement" led other manufacturers to develop
rival products by burning artificial mixtures of clay and
chalk.
John Smeaton made an important contribution to the
development of cements when he was planning the
construction of the third Eddystone Lighthouse (1755-9)
in the English Channel. He needed a hydraulic mortar
that would set and develop some strength in the twelve
hour period between successive high tides. He
performed an exhaustive market research on the
available hydraulic limes, visiting their production sites,
and noted that the "hydraulicity" of the lime was directly
related to the clay content of the limestone from which
it was made. Smeaton was a civil engineer by
profession, and took the idea no further. Apparently
unaware of Smeaton's work, the same principle was
identified by Louis Vicat in the first decade of the
nineteenth century. Vicat went on to devise a method of
combining chalk and clay into an intimate mixture, and,
burning this, produced an "artificial cement" in 1817.
James Frost[3], working in Britain, produced what he
called "British cement" in a similar manner around the
same time, but did not obtain a patent until 1822. In
1824, Joseph Aspdin patented a similar material, which
he called Portland cement, because the render made
from it was in color similar to the prestigious Portland stone.
All the above products could not compete with
lime/pozzolan concretes because of fast-setting (giving
insufficient time for placement) and low early strengths
(requiring a delay of many weeks before formwork
could be removed). Hydraulic limes, "natural" cements
and "artificial" cements all rely upon their belite content
for strength development. Belite develops strength
slowly. Because they were burned at temperatures below
1250 °C, they contained no alite, which is responsible
for early strength in modern cements. The first cement
to consistently contain alite was that made by Joseph
Aspdin's son William in the early 1840s. This was what
we call today "modern" Portland cement. Because of the
air of mystery with which William Aspdin surrounded
his product, others (e.g. Vicat and I C Johnson) have
claimed precedence in this invention, but recent
analysis[4] of both his concrete and raw cement have
shown that William Aspdin's product made at
Northfleet, Kent was a true alite-based cement.
However, Aspdin's methods were "rule-of-thumb": Vicat
is responsible for establishing the chemical basis of
these cements, and Johnson established the importance
of sintering the mix in the kiln.
William Aspdin's innovation was counter-intuitive for
manufacturers of "artificial cements", because they
required more lime in the mix (a problem for his father),
because they required a much higher kiln temperature
(and therefore more fuel) and because the resulting
clinker was very hard and rapidly wore down the
millstones which were the only available grinding
technology of the time. Manufacturing costs were
therefore considerably higher, but the product set
reasonably slowly and developed strength quickly, thus
opening up a market for use in concrete. The use of
concrete in construction grew rapidly from 1850
onwards, and was soon the dominant use for cements.
Thus Portland cement began its predominant role.
Types of modern cement
Blue Circle Southern Cement works near Berrima, New South Wales, Australia.
Portland cement
Portland cement is the most common type of cement in
general usage, as it is a basic ingredient of concrete,
mortar and most non-speciality grout. The most
common use for Portland cement is in the production of
concrete. Concrete is a composite material consisting of
aggregate (gravel and sand), cement, and water. As a construction material, concrete can be cast in almost any
shape desired, and once hardened, can become a
structural (load bearing) element. Portland cement may
be gray or white.
For details of the manufacture of Portland cement, see
the main article.
Portland cement blends
These are often available as inter-ground mixtures from
cement manufacturers, but similar formulations are
often also mixed from the ground components at the
concrete mixing plant.
Portland Blastfurnace Cement contains up to 70%
ground granulated blast furnace slag, with the rest
Portland clinker and a little gypsum. All compositions
produce high ultimate strength, but as slag content is
increased, early strength is reduced, while sulfate
resistance increases and heat evolution diminishes. Used
as an economic alternative to Portland sulfate-resisting
and low-heat cements.
Portland Flyash Cement contains up to 30% fly ash.
The flyash is pozzolanic, so that ultimate strength is
maintained. Because flyash addition allows a lower
concrete water content, early strength can also be
maintained. Where good quality cheap flyash is
available, this can be an economic alternative to ordinary
Portland cement.
Portland Pozzolan Cement includes fly ash cement,
since fly ash is a pozzolan, but also includes cements
made from other natural or artificial pozzolans. In
countries where volcanic ashes are available (e.g. Italy,
Chile, Mexico, the Philippines) these cements are often
the most common form in use.
Portland Silica Fume cement. Addition of silica fume
can yield exceptionally high strengths, and cements
containing 5-20% silica fume are occasionally produced.
However, silica fume is more usually added to Portland
cement at the concrete mixer
Masonry Cements are used for preparing bricklaying
mortars and stuccos, and must not be used in concrete.
They are usually complex proprietary formulations
containing Portland clinker and a number of other
ingredients that may include limestone, hydrated lime,
air entrainers, retarders, waterproofers and coloring
agents. They are formulated to yield workable mortars
that allow rapid and consistent masonry work. Subtle
variations of Masonry cement in the US are Plastic
Cements and Stucco Cements. These are designed to
produce controlled bond with masonry blocks.
Expansive Cements contain, in addition to Portland
clinker, expansive clinkers (usually sulfoaluminate
clinkers), and are designed to offset the effects of drying
shrinkage that is normally encountered with hydraulic
cements. This allows large floor slabs (up to 60 m
square) to be prepared without contraction joints.
White blended cements may be made using white
clinker and white supplementary materials such as high-
purity metakaolin.
Colored cements are used for decorative purposes. In
some standards, the addition of pigments to produce
"colored Portland cement" is allowed. In other standards
(e.g. ASTM), pigments are not allowed constituents of
Portland cement, and colored cements are sold as
"blended hydraulic cements".
Non-Portland hydraulic cements
Pozzolan-lime cements. Mixtures of ground pozzolan
and lime are the cements used by the Romans, and are to
be found in Roman structures still standing (e.g. the
Pantheon in Rome). They develop strength slowly, but
their ultimate strength can be very high. The hydration
products that produce strength are essentially the same
as those produced by Portland cement.
Slag-lime cements. Ground granulated blast furnace slag is not hydraulic on its own, but is “activated” by
addition of alkalis, most economically using lime. They
are similar to pozzolan lime cements in their properties
. Only granulated slag (i.e. water-quenched, glassy slag)
is effective as a cement component.
Supersulfated cements. These contain about 80%
ground granulated blast furnace slag, 15% gypsum or
anhydrite and a little Portland clinker or lime as an
activator. They produce strength by formation of
ettringite, with strength growth similar to a slow
Portland cement. They exhibit good resistance to
aggressive agents, including sulfate.
Calcium aluminate cements are hydraulic cements
made primarily from limestone and bauxite. The active
ingredients are monocalcium aluminate CaAl2O4 (CA in
Cement chemist notation) and Mayenite Ca12Al14O33
(C12A7 in CCN). Strength forms by hydration to calcium
aluminate hydrates. They are well-adapted for use in
refractory (high-temperature resistant) concretes, e.g. for
furnace linings.
Calcium sulfoaluminate cements are made from
clinkers that include ye’elimite (Ca4(AlO2)6SO4 or
C4A3 in Cement chemist’s notation) as a primary phase
.
They are used in expansive cements, in ultra-high early
strength cements, and in "low-energy" cements.
Hydration produces ettringite, and specialized physical
properties (such as expansion or rapid reaction) are
obtained by adjustment of the availability of calcium
and sulfate ions. Their use as a low-energy alternative to
Portland cement has been pioneered in China, where
several million tonnes per year are produced.
Energy requirements are lower because of the lower kiln
temperatures required for reaction, and the lower
amount of limestone (which must be endothermically
decarbonated) in the mix. In addition, the lower
limestone content and lower fuel consumption leads to a
CO2 emission around half that associated with Portland
clinker. However, SO2 emissions are usually
significantly higher.
“Natural” Cements correspond to certain cements of
the pre-Portland era, produced by burning argillaceous
limestones at moderate temperatures. The level of clay
components in the limestone (around 30-35%) is such
that large amounts of belite (the low-early strength,
high-late strength mineral in Portland cement) are
formed without the formation of excessive amounts free
lime. As with any natural material, such cements have
very variable properties.
Geopolymer cements are made from mixtures of water-
soluble alkali metal silicates and aluminosilicate mineral
powders such as fly ash and metakaolin.
Environment
Cement manufacture causes environmental impacts at all
stages of the process. These include emissions of
airborne pollution in the form of dust, gases, noise and
vibration when operating machinery and during blasting
in quarries, and damage to countryside from quarrying
. Equipment to reduce dust emissions during quarrying
and manufacture of cement is widely used, and
equipment to trap and separate exhaust gases are
coming into increased use. Environmental protection
also includes the re-integration of quarries into the
countryside after they have been closed down by
returning them to nature or re-cultivating them.
Due to the large quantities of fuel used during
manufacture and the release of carbon dioxide from the
raw materials, cement production also generates more
carbon emissions than any other industrial process,
accounting for around 4% of the world's
anthropomorphic carbon emissions.
Cement manufacture can provide environmental benefits
by using wastes from certain other industries, including
slag from steel manufacture, fly ash from coal burning,
silica fume from silicon and ferrosilicon manufacturing,
and sometimes recycled concrete from demolition of
older structures.
Fuels
Fuels used in cement manufacture depend on the type of
cement. For information on fuels used in the
manufacture of cement clinker, see Portland cement,
Cement business
In 2002 the world production oF hydraulic cement was
1,800 million metric tons. The top three producers were
China with 704, India with 100, and the United States
with 91 million metric tons for a combined total of
about half the world total by the world's three most
populous states.
"For the past 18 years, China consistently has produced
more cement than any other country in the world.
China's cement export peaked in 1994 with 11 million
tons shipped out and has been in steady decline ever
since. Only 5.18 million tons were exported out of China
in 2002. Offered at $34 a ton, Chinese cement is pricing
itself out of the market as Thailand is asking as little as
$20 for the same quality."
"Demand for cement in China is expected to advance
5.4% annually and exceed 1 billion metric tons in 2008,
driven by slowing but healthy growth in construction
expenditures. Cement consumed in China will amount to
44% of global demand, and China will remain the
world's largest national consumer of cement by a large
margin."
In 2006 is was estimated that China manufactured 1.235
billion metric tons of cement, which is 44% of the world
total cement production
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