A cement is a binder, a substance used for construction that sets,
hardens and adheres to other materials, binding them together. Cement
is seldom used on its own, but rather to bind sand and gravel
Cement is used with fine aggregate to produce
mortar for masonry, or with sand and gravel aggregates to produce
Cements used in construction are usually inorganic, often lime or
calcium silicate based, and can be characterized as being either
hydraulic or non-hydraulic, depending upon the ability of the cement
to set in the presence of water (see hydraulic and non-hydraulic lime
Non-hydraulic cement will not set in wet conditions or under water;
rather, it sets as it dries and reacts with carbon dioxide in the air.
It is resistant to attack by chemicals after setting.
Hydraulic cements (e.g., Portland cement) set and become adhesive due
to a chemical reaction between the dry ingredients and water. The
chemical reaction results in mineral hydrates that are not very
water-soluble and so are quite durable in water and safe from chemical
attack. This allows setting in wet conditions or under water and
further protects the hardened material from chemical attack. The
chemical process for hydraulic cement found by ancient Romans used
volcanic ash (pozzolana) with added lime (calcium oxide).
The word "cement" can be traced back to the Roman term opus
caementicium, used to describe masonry resembling modern concrete that
was made from crushed rock with burnt lime as binder. The volcanic ash
and pulverized brick supplements that were added to the burnt lime, to
obtain a hydraulic binder, were later referred to as cementum,
cimentum, cäment, and cement. In modern times, organic polymers are
sometimes used as cements in concrete.
2.1 Alternatives to cement used in antiquity
2.2 Macedonians and Romans
2.3 Middle Ages
2.4 16th century
2.5 18th century
2.6 19th century
2.7 20th century
3 Modern cements
3.1 Portland cement
Portland cement blends
3.3 Other cements
4 Setting and curing
5 Safety issues
Cement industry in the world
7 Environmental impacts
7.1 CO2 emissions
7.2 Heavy metal emissions in the air
7.3 Heavy metals present in the clinker
7.4 Use of alternative fuels and by-products materials
8 Green cement
9 See also
11 Further reading
12 External links
Non-hydraulic cement, such as slaked lime (calcium oxide mixed with
water), hardens by carbonation in the presence of carbon dioxide which
is naturally present in the air. First calcium oxide (lime) is
produced from calcium carbonate (limestone or chalk) by calcination at
temperatures above 825 °C (1,517 °F) for about 10 hours at
CaCO3 → CaO + CO2
The calcium oxide is then spent (slaked) mixing it with water to make
slaked lime (calcium hydroxide):
CaO + H2O → Ca(OH)2
Once the excess water is completely evaporated (this process is
technically called setting), the carbonation starts:
Ca(OH)2 + CO2 → CaCO3 + H2O
This reaction takes a significant amount of time because the partial
pressure of carbon dioxide in the air is low. The carbonation reaction
requires the dry cement to be exposed to air, and for this reason the
slaked lime is a non-hydraulic cement and cannot be used under water.
This whole process is called the lime cycle.
Conversely, hydraulic cement hardens by hydration when water is added.
Hydraulic cements (such as Portland cement) are made of a mixture of
silicates and oxides, the four main components being:
Tricalcium aluminate (3CaO·Al2O3) (historically, and still
occasionally, called 'celite');
The silicates are responsible for the mechanical properties of the
cement, the tricalcium aluminate and the brownmillerite are essential
to allow the formation of the liquid phase during the kiln sintering
(firing). The chemistry of the above listed reactions is not
completely clear and is still the object of research.
Perhaps the earliest known occurrence of cement is from twelve million
years ago. A deposit of cement was formed after an occurrence of oil
shale located adjacent to a bed of limestone burned due to natural
causes. These ancient deposits were investigated in the 1960s and
Alternatives to cement used in antiquity
Cement, chemically speaking, is a product that includes lime as the
primary curing ingredient, but is far from the first material used for
cementation. The Babylonians and Assyrians used bitumen to bind
together burnt brick or alabaster slabs. In Egypt stone blocks were
cemented together with a mortar made of sand and roughly burnt gypsum
(CaSO4·2H2O), which often contained calcium carbonate (CaCO3).
Macedonians and Romans
Lime (calcium oxide) was used on Crete and by the ancient Greeks.
There is evidence that the
Minoans of Crete used crushed potshards as
an artificial pozzolan for hydraulic cement. It is uncertain where
it was first discovered that a combination of hydrated non-hydraulic
lime and a pozzolan produces a hydraulic mixture (see also: Pozzolanic
reaction), but concrete made from such mixtures was used by the
Ancient Macedonians and three centuries later on a large scale
by Roman engineers.
There is... a kind of powder which from natural causes produces
astonishing results. It is found in the neighborhood of
Baiae and in
the country belonging to the towns round about Mt. Vesuvius. This
substance when mixed with lime and rubble not only lends strength to
buildings of other kinds, but even when piers of it are constructed in
the sea, they set hard under water.
— Marcus Vitruvius Pollio, Liber II, De Architectura, Chapter VI
"Pozzolana" Sec. 1
The Greeks used volcanic tuff from the island of
Thera as their
pozzolan and the Romans used crushed volcanic ash (activated aluminium
silicates) with lime. This mixture was able to set under water
increasing its resistance.[clarification needed] The material was
called pozzolana from the town of Pozzuoli, west of Naples where
volcanic ash was extracted. In the absence of pozzolanic ash, the
Romans used powdered brick or pottery as a substitute and they may
have used crushed tiles for this purpose before discovering natural
sources near Rome. The huge dome of the Pantheon in
Rome and the
Baths of Caracalla
Baths of Caracalla are examples of ancient structures made
from these concretes, many of which are still standing. The vast
system of Roman aqueducts also made extensive use of hydraulic
Although any preservation of this knowledge in literary sources from
Middle Ages is unknown, medieval masons and some military
engineers maintained an active tradition of using hydraulic cement in
structures such as canals, fortresses, harbors, and shipbuilding
Tabby, a building material using oyster-shell lime, sand, and whole
oyster shells to form a concrete, was introduced to the Americas by
the Spanish in the sixteenth century.
The technical knowledge for making hydraulic cement was formalized by
French and British engineers in the 18th century.
John Smeaton made an important contribution to the development of
cements while planning the construction of the third Eddystone
Lighthouse (1755–59) in the
English Channel now known as Smeaton's
Tower. He needed a hydraulic mortar that would set and develop some
strength in the twelve-hour period between successive high tides. He
performed experiments with combinations of different limestones and
additives including trass and pozzolanas and did 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
In the South Atlantic seaboard of the United States, tabby relying
upon the oyster-shell middens of earlier Native American populations
was used in house construction from the 1730s to the 1860s.
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 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 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
hydraulic lime cements of clay and chalk.
Roman cement quickly became
popular but was largely replaced by
Portland cement in the 1850s.
Apparently unaware of Smeaton's work, the same principle was
identified by Frenchman
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 considered the "principal
Portland cement and "...Edgar Dobbs of Southwark
patented a cement of this kind in 1811."
In Russia, Egor Cheliev created a new binder by mixing lime and clay.
His results were published in 1822 in his book A Treatise on the Art
to Prepare a Good Mortar published in St. Petersburg. A few years
later in 1825, he published another book, which described the various
methods of making cement and concrete, as well as the benefits of
cement in the construction of buildings and embankments.
William Aspdin is considered the inventor of "modern" Portland
Portland cement, the most common type of cement in general use around
the world as a basic ingredient of concrete, mortar, stucco, and
non-speciality grout, was developed in
England in the mid 19th
century, and usually originates from limestone. James Frost 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
Portland stone which was quarried on the Isle of Portland,
Dorset, England. However, Aspdins' cement was nothing like modern
Portland cement but was a first step in its development, called a
proto-Portland cement. Joseph Aspdins' son
William Aspdin had left
his fathers company and in his cement manufacturing apparently
accidentally produced calcium silicates in the 1840s, a middle step in
the development of Portland cement. William Aspdin's innovation was
counterintuitive for manufacturers of "artificial cements", because
they required more lime in the mix (a problem for his father), a much
higher kiln temperature (and therefore more fuel), and 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 onward, and was soon the dominant use for cements.
Portland cement began its predominant role. Isaac Charles Johnson
further refined the production of meso-
Portland cement (middle stage
of development) and claimed to be the real father of Portland
Setting time and "early strength" are important characteristics of
cements. 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 1,250 °C (2,280 °F), they contained no alite, which
is responsible for early strength in modern cements. The first cement
to consistently contain alite was made by
William Aspdin 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 Johnson) have claimed precedence in
this invention, but recent analysis 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.
In the US the first large-scale use of cement was Rosendale cement, a
natural cement mined from a massive deposit of a large dolostone rock
deposit discovered in the early 19th century near Rosendale, New York.
Rosendale cement was extremely popular for the foundation of buildings
(e.g., Statue of Liberty, Capitol Building, Brooklyn Bridge) and
lining water pipes.
Sorel cement was patented in 1867 by Frenchman
Stanislas Sorel and was
Portland cement but its poor water resistance and
corrosive qualities limited its use in building construction. The next
development with the manufacture of
Portland cement was the
introduction of the rotary kiln which allowed a stronger, more
homogeneous mixture and a continuous manufacturing process.
Cement Share Company of Ethiopia's new plant in Dire
Calcium aluminate cements
Calcium aluminate cements were patented in 1908 in France by Jules
Bied for better resistance to sulfates.
In the US, the long curing time of at least a month for Rosendale
cement made it unpopular after World War One in the construction of
highways and bridges and many states and construction firms turned to
the use of Portland cement. Because of the switch to Portland cement,
by the end of the 1920s of the 15
Rosendale cement companies, only one
had survived. But in the early 1930s it was discovered that, while
Portland cement had a faster setting time it was not as durable,
especially for highways, to the point that some states stopped
building highways and roads with cement. Bertrain H. Wait, an engineer
whose company had worked on the construction of the New York City's
Catskill Aqueduct, was impressed with the durability of Rosendale
cement, and came up with a blend of both Rosendale and synthetic
cements which had the good attributes of both: it was highly durable
and had a much faster setting time. Wait convinced the New York
Commissioner of Highways to construct an experimental section of
highway near New Paltz, New York, using one sack of Rosendale to six
sacks of synthetic cement. It was proved a success and for decades the
Rosendale-synthetic cement blend became common use in highway and
Modern hydraulic cements began to be developed from the start of the
Industrial Revolution (around 1800), driven by three main needs:
Hydraulic cement render (stucco) for finishing brick buildings in wet
Hydraulic mortars for masonry construction of harbor works, etc., in
contact with sea water.
Development of strong concretes.
Modern cements are often
Portland cement or
Portland cement blends,
but other cements are used in industry.
Components of Cement
Comparison of Chemical and Physical Characteristicsa
(ASTM C618 Class F)
(ASTM C618 Class C)
SiO2 content (%)
Al2O3 content (%)
Fe2O3 content (%)
CaO content (%)
MgO content (%)
SO3 content (%)
aValues shown are approximate: those of a specific material may vary.
bSpecific surface measurements for silica fume by nitrogen adsorption
others by air permeability method (Blaine).
Main article: Portland cement
Portland cement is by far the most common type of cement in general
use around the world. This cement is made by heating limestone
(calcium carbonate) with other materials (such as clay) to
1450 °C in a kiln, in a process known as calcination, whereby a
molecule of carbon dioxide is liberated from the calcium carbonate to
form calcium oxide, or quicklime, which then chemically combines with
the other materials that have been included in the mix to form calcium
silicates and other cementitious compounds. The resulting hard
substance, called 'clinker', is then ground with a small amount of
gypsum into a powder to make 'ordinary Portland cement', the most
commonly used type of cement (often referred to as OPC). Portland
cement is a basic ingredient of concrete, mortar and most
non-specialty 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 grey or white.
Portland cement blends
Portland cement blends are often available as inter-ground mixtures
from cement producers, but similar formulations are often also mixed
from the ground components at the concrete mixing plant.
Portland blast-furnace slag cement, or Blast furnace cement (ASTM C595
and EN 197-1 nomenclature respectively), contains up to 95% 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
Portland-fly ash cement contains up to 40% fly ash under ASTM
standards (ASTM C595), or 35% under EN standards (EN 197-1). The fly
ash is pozzolanic, so that ultimate strength is maintained. Because
fly ash addition allows a lower concrete water content, early strength
can also be maintained. Where good quality cheap fly ash 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. The maximum replacement ratios are
generally defined as for Portland-fly ash cement.
Portland silica fume cement. Addition of silica fume can yield
exceptionally high strengths, and cements containing 5–20% silica
fume are occasionally produced, with 10% being the maximum allowed
addition under EN 197-1. 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 (containing
little or no iron) and white supplementary materials such as
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".
Very finely ground cements are made from mixtures of cement with sand
or with slag or other pozzolan type minerals that are extremely finely
ground together. Such cements can have the same physical
characteristics as normal cement but with 50% less cement particularly
due to their increased surface area for the chemical reaction. Even
with intensive grinding they can use up to 50% less energy to
fabricate than ordinary Portland cements.
Pozzolan-lime cements. Mixtures of ground pozzolan and lime are the
cements used by the Romans, and are present in extant Roman structures
(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
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 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
Calcium aluminate cements are hydraulic cements made primarily from
limestone and bauxite. The active ingredients are monocalcium
aluminate CaAl2O4 (CaO · Al2O3 or CA in
Cement chemist notation, CCN)
and mayenite Ca12Al14O33 (12 CaO · 7 Al2O3, or 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 C4A3S 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
"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 of free lime. As with any
natural material, such cements have highly variable properties.
Geopolymer cements are made from mixtures of water-soluble alkali
metal silicates and aluminosilicate mineral powders such as fly ash
Polymer cements are made from organic chemicals that polymerise. Often
thermoset materials are employed. While they are often significantly
more expensive, they can give a water proof material that has useful
Setting and curing
Cement starts to set when mixed with water which causes a series of
hydration chemical reactions. The constituents slowly hydrate and the
mineral hydrates solidify; the interlocking of the hydrates gives
cement its strength. Contrary to popular perceptions, hydraulic
cements do not set by drying out; proper curing requires maintaining
the appropriate moisture content during the curing process. If
hydraulic cements dry out during curing, the resulting product can be
Bags of cement routinely have health and safety warnings printed on
them because not only is cement highly alkaline, but the setting
process is exothermic. As a result, wet cement is strongly caustic
(water pH = 13.5) and can easily cause severe skin burns if not
promptly washed off with water. Similarly, dry cement powder in
contact with mucous membranes can cause severe eye or respiratory
irritation. Some trace elements, such as chromium, from impurities
naturally present in the raw materials used to produce cement may
cause allergic dermatitis. Reducing agents such as ferrous sulfate
(FeSO4) are often added to cement to convert the carcinogenic
hexavalent chromate (CrO42−) into trivalent chromium (Cr3+), a less
toxic chemical species.
Cement users need also to wear appropriate
gloves and protective clothing.
Cement industry in the world
Cement Production in 2010
Cement Capacity in 2010
See also: List of countries by cement production
In 2010, the world production of hydraulic cement was
3,300 million tonnes (3.2×109 long tons; 3.6×109 short tons).
The top three producers were China with 1,800,
India with 220, and USA
with 63.5 million tonnes for a combined total of over half the world
total by the world's three most populated states.
For the world capacity to produce cement in 2010, the situation was
similar with the top three states (China, India, and USA) accounting
for just under half the world total capacity.
Over 2011 and 2012, global consumption continued to climb, rising to
3585 Mt in 2011 and 3736 Mt in 2012, while annual growth rates eased
to 8.3% and 4.2%, respectively.
China, representing an increasing share of world cement consumption,
continued to be the main engine of global growth. By 2012, Chinese
demand was recorded at 2160 Mt, representing 58% of world consumption.
Annual growth rates, which reached 16% in 2010, appear to have
softened, slowing to 5–6% over 2011 and 2012, as China’s economy
targets a more sustainable growth rate.
Outside of China, worldwide consumption climbed by 4.4% to 1462 Mt in
2010, 5% to 1535 Mt in 2011, and finally 2.7% to 1576 Mt in 2012.
Iran is now the 3rd largest cement producer in the world and has
increased its output by over 10% from 2008 to 2011. Due to
climbing energy costs in Pakistan and other major cement-producing
countries, Iran is a unique position as a trading partner, utilizing
its own surplus petroleum to power clinker plants. Now a top producer
in the Middle-East, Iran is further increasing its dominant position
in local markets and abroad.
The performance in North America and Europe over the 2010–12 period
contrasted strikingly with that of China, as the global financial
crisis evolved into a sovereign debt crisis for many economies in this
region and recession.
Cement consumption levels for this region fell
by 1.9% in 2010 to 445 Mt, recovered by 4.9% in 2011, then dipped
again by 1.1% in 2012.
The performance in the rest of the world, which includes many emerging
economies in Asia, Africa and Latin America and representing some 1020
Mt cement demand in 2010, was positive and more than offset the
declines in North America and Europe. Annual consumption growth was
recorded at 7.4% in 2010, moderating to 5.1% and 4.3% in 2011 and
As at year-end 2012, the global cement industry consisted of 5673
cement production facilities, including both integrated and grinding,
of which 3900 were located in China and 1773 in the rest of the world.
Total cement capacity worldwide was recorded at 5245 Mt in 2012, with
2950 Mt located in China and 2295 Mt in the rest of the world.
Cement industry in China
"For the past 18 years, China consistently has produced more cement
than any other country in the world. [...] (However,) China's cement
export peaked in 1994 with 11 million tonnes shipped out and has been
in steady decline ever since. Only 5.18 million tonnes 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."
In 2006, it was estimated that China manufactured 1.235 billion tonnes
of cement, which was 44% of the world total cement production.
"Demand for cement in China is expected to advance 5.4% annually and
exceed 1 billion tonnes 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 2010, 3.3 billion tonnes of cement was consumed globally. Of this,
China accounted for 1.8 billion tonnes.
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
Global carbon emission by type to 2004. Attribution: Mak Thorpe
Carbon concentration in cement spans from ≈5% in cement structures
to ≈8% in the case of roads in cement.
releases CO2 in the atmosphere both directly when calcium carbonate is
heated, producing lime and carbon dioxide, and also indirectly
through the use of energy if its production involves the emission of
CO2. The cement industry produces about 10% of global man-made CO2
emissions, of which 60% is from the chemical process, and 40% from
Nearly 900 kg of CO2 are emitted for every 1000 kg of
Portland cement produced. In the European Union the specific energy
consumption for the production of cement clinker has been reduced by
approximately 30% since the 1970s. This reduction in primary energy
requirements is equivalent to approximately 11 million tonnes of coal
per year with corresponding benefits in reduction of CO2 emissions.
This accounts for approximately 5% of anthropogenic CO2.
The majority of carbon dioxide emissions in the manufacture of
Portland cement (approximately 60%) are produced from the chemical
decomposition of limestone to lime, an ingredient in Portland cement
clinker. These emissions may be reduced by lowering the clinker
content of cement.
To reduce the transport of heavier raw materials and to minimize the
associated costs, it is more economical for cement plants to be closer
to the limestone quarries rather than to the consumer centers.
In certain applications, lime mortar reabsorbs some of the CO2 as was
released in its manufacture, and has a lower energy requirement in
production than mainstream cement (citation needed). Newly developed
cement types from Novacem and
Eco-cement can absorb carbon dioxide
from ambient air during hardening. Use of the
Kalina cycle during
production can also increase energy efficiency.
Heavy metal emissions in the air
In some circumstances, mainly depending on the origin and the
composition of the raw materials used, the high-temperature
calcination process of limestone and clay minerals can release in the
atmosphere gases and dust rich in volatile heavy metals, a.o,
thallium, cadmium and mercury are the most toxic. Heavy metals
(Tl, Cd, Hg, ...) and also selenium are often found as trace elements
in common metal sulfides (pyrite (FeS2), zinc blende (ZnS), galena
(PbS), ...) present as secondary minerals in most of the raw
materials. Environmental regulations exist in many countries to limit
these emissions. As of 2011 in the United States, cement kilns are
"legally allowed to pump more toxins into the air than are
Heavy metals present in the clinker
The presence of heavy metals in the clinker arises both from the
natural raw materials and from the use of recycled by-products or
alternative fuels. The high pH prevailing in the cement porewater
(12.5 < pH < 13.5) limits the mobility of many heavy metals by
decreasing their solubility and increasing their sorption onto the
cement mineral phases. Nickel, zinc and lead are commonly found in
cement in non-negligible concentrations.
Chromium may also directly
arise as natural impurity from the raw materials or as secondary
contamination from the abrasion of hard chromium steel alloys used in
the ball mills when the clinker is ground. As chromate (CrO42−) is
toxic and may cause severe skin allergies at trace concentration, it
is sometimes reduced into trivalent Cr(III) by addition of ferrous
Use of alternative fuels and by-products materials
A cement plant consumes 3 to 6 GJ of fuel per tonne of clinker
produced, depending on the raw materials and the process used. Most
cement kilns today use coal and petroleum coke as primary fuels, and
to a lesser extent natural gas and fuel oil. Selected waste and
by-products with recoverable calorific value can be used as fuels in a
cement kiln (referred to as co-processing), replacing a portion of
conventional fossil fuels, like coal, if they meet strict
specifications. Selected waste and by-products containing useful
minerals such as calcium, silica, alumina, and iron can be used as raw
materials in the kiln, replacing raw materials such as clay, shale,
and limestone. Because some materials have both useful mineral content
and recoverable calorific value, the distinction between alternative
fuels and raw materials is not always clear. For example, sewage
sludge has a low but significant calorific value, and burns to give
ash containing minerals useful in the clinker matrix.
Normal operation of cement kilns provides combustion conditions which
are more than adequate for the destruction of even the most difficult
to destroy organic substances. This is primarily due to the very high
temperatures of the kiln gases (2000 °C in the combustion gas
from the main burners and 1100 °C in the gas from the burners in
the precalciner). The gas residence time at high temperature in the
rotary kiln is of the order of 5–10 seconds and in the precalciner
more than 3 seconds.
Due to bovine spongiform encephalopathy (BSE) in the European beef
industry, the use of animal-derived products to feed cattle is now
severely restricted. Large quantities of waste animal meat and bone
meal (MBM), also known as animal flour, have to be safely disposed of
or transformed. The production of cement kilns, together with the
incineration, is to date one of the two main ways to treat this solid
effluent of the food industry.
Green cement is a cementitious material that meets or exceeds the
functional performance capabilities of ordinary
Portland cement by
incorporating and optimizing recycled materials, thereby reducing
consumption of natural raw materials, water, and energy, resulting in
a more sustainable construction material.
New manufacturing processes for producing green cement are being
researched with the goal to reduce, or even eliminate, the production
and release of damaging pollutants and greenhouse gasses, particularly
Growing environmental concerns and increasing cost of fuels of fossil
origin have resulted in many countries in sharp reduction of the
resources needed to produce cement and effluents (dust and exhaust
Peter Trimble, a design student at the
University of Edinburgh
University of Edinburgh has
proposed 'DUPE' based on Sporosarcina pasteurii, a bacterium with
binding qualities which, when mixed with sand and urine produces a
concrete said to be 70% as strong as conventional materials.
Cement chemist notation
Energetically modified cement
Energetically modified cement (EMC)
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Media related to
Cement at Wikimedia Commons
"Cement". Encyclopædia Britannica. 5 (11th ed.). 1911.
Ancient Roman architecture
Ground granulated blast furnace slag
Reversing drum mixer
Flow table test
Segregation in concrete
Energetically modified cement
Rosendale cement (natural cement)
Voided biaxial slab
Institution of Structural Engineers
International Federation for Structural Concrete
BNF: cb119757055 (data)