Cement is a binding material used in construction that hardens when mixed with water. Portland cement is the most common type and consists of compounds that hydrate to form crystals or gel. It is made by grinding limestone and clay, blending them precisely, burning the mixture in a kiln at high temperatures, and grinding the resulting clinker with gypsum. When mixed with water or aggregate, cement sets and hardens due to chemical reactions between its compounds and water.

1. CEMENT

2. Cement, in general, adhesive substances of all kinds,
but, in a narrower sense, the binding materials used
in building and civil engineering construction. Cements
of this kind are finely ground powders that, when mixed
with water, set to a hard mass. Setting and hardening
result from hydration, which is a chemical combination
of the cement compounds with water that yields
submicroscopic crystals or a gel-like material with a high
surface area. Because of their hydrating properties,
constructional cements, which will even set and harden
under water, are often called hydraulic cements. The
most important of these is portland cement.

4. Applications of Cement
Cements may be used alone (i.e., “neat,” as grouting materials),
but the normal use is in mortar and concrete in which the
cement is mixed with inert material known as aggregate. Mortar
is cement mixed with sand or crushed stone that must be less
than approximately 5 mm (0.2 inch) in size. Concrete is a
mixture of cement, sand or other fine aggregate, and a coarse
aggregate that for most purposes is up to 19 to 25 mm (0.75 to
1 inch) in size, but the coarse aggregate may also be as large
as 150 mm (6 inches) when concrete is placed in large masses
such as dams. Mortars are used for binding bricks, blocks, and
stone in walls or as surface renderings. Concrete is used for a
large variety of constructional purposes. Mixtures of soil and
portland cement are used as a base for roads. Portland cement
also is used in the manufacture of bricks, tiles, shingles, pipes,
beams, railroad ties, and various extruded products. The
products are prefabricated in factories and supplied ready for
installation.

5. HISTORY OF CEMENT
 The origin of hydraulic cements goes back to ancient
Greece and Rome. The materials used were lime and a volcanic
ash that slowly reacted with it in the presence of water to form a
hard mass. This formed the cementing material of the Roman
mortars and concretes of more than 2,000 years ago and of
subsequent construction work in western Europe. Volcanic ash
mined near what is now the city of Pozzuoli, Italy, was particularly
rich in essential aluminosilicate minerals, giving rise to the
classic pozzolana cement of the Roman era. To this day the
term pozzolana, or pozzolan, refers either to the cement itself or to
any finely divided aluminosilicate that reacts with lime in water to
form cement. (The term cement, meanwhile, derives from the Latin
word caementum, which meant stone chippings such as were used
in Roman mortar—not the binding material itself.)

6.  Portland cement is a successor to a hydraulic lime
that was first developed by John Smeaton in 1756
when he was called in to erect the Eddystone
Lighthouse off the coast of Plymouth, Devon,
England. The next development, taking place about
1800 in England and France, was a material
obtained by burning nodules of clayey limestone.
Soon afterward in the United States, a similar
material was obtained by burning a naturally
occurring substance called “cement rock.” These
materials belong to a class known as natural
cement, allied to portland cement but more lightly
burned and not of controlled composition.

7.  The invention of portland cement usually is attributed
to Joseph Aspdin of Leeds, Yorkshire, England, who in 1824
took out a patent for a material that was produced from
a synthetic mixture of limestone and clay. He called the
product “portland cement” because of a fancied resemblance
of the material, when set, to portland stone,
a limestone used for building in England. Aspdin’s product
may well have been too lightly burned to be a true portland
cement, and the real prototype was perhaps that produced
by Isaac Charles Johnson in southeastern England about
1850. The manufacture of portland cement rapidly spread to
other European countries and North America. During the
20th century, cement manufacture spread worldwide. By
2019 China and India had become the world leaders in
cement production, followed by Vietnam, the United States,
and Egypt.

8. RAW MATERIALS
 Composition
Portland cement consists essentially
of compounds of lime (calcium oxide, CaO) mixed
with silica (silicon dioxide, SiO2)
and alumina (aluminum oxide, Al2O3). The lime is
obtained from a calcareous (lime-containing) raw
material, and the other oxides are derived from an
argillaceous (clayey) material. Additional raw
materials such as silica sand, iron oxide (Fe2O3),
and bauxite—containing hydrated aluminum,
Al(OH)3—may be used in smaller quantities to get
the desired composition.

9.  The commonest calcareous raw materials
are limestone and chalk, but others, such as coral or shell
deposits, also are used. Clays, shales, slates, and estuarine
muds are the common argillaceous raw materials. Marl, a
compact calcareous clay, and cement rock contain both the
calcareous and argillaceous components in proportions that
sometimes approximate cement compositions. Another raw
material is blast-furnace slag, which consists mainly of lime,
silica, and alumina and is mixed with a calcareous material of
high lime content. Kaolin, a white clay that contains little iron
oxide, is used as the argillaceous component for white portland
cement. Industrial wastes, such as fly ash and calcium
carbonate from chemical manufacture, are other possible raw
materials, but their use is small compared with that of the
natural materials.

10.  The magnesia (magnesium oxide, MgO) content of raw
materials must be low because the permissible limit in portland
cement is 4 to 5 percent. Other impurities in raw materials that
must be strictly limited are fluorine compounds, phosphates,
metal oxides and sulfides, and excessive alkalies.
 Another essential raw material is gypsum, some 5 percent of
which is added to the burned cement clinker during grinding to
control the setting time of the cement. Portland cement also
can be made in a combined process with sulfuric acid using
calcium sulfate or anhydrite in place of calcium carbonate.
The sulfur dioxide produced in the flue gases on burning is
converted to sulfuric acid by normal processes.

11. EXTRACTION AND PROCESSING
 Raw materials employed in the manufacture of
cement are extracted by quarrying in the case of
hard rocks such as limestones, slates, and some
shales, with the aid of blasting when necessary.
Some deposits are mined by underground
methods. Softer rocks such as chalk and clay can
be dug directly by excavators.

12.  The excavated materials are transported to the crushing
plant by trucks, railway freight cars, conveyor belts, or
ropeways. They also can be transported in a wet state or
slurry by pipeline. In regions where limestones of
sufficiently high lime content are not available, some
process of beneficiation can be used. Froth flotation will
remove excess silica or alumina and so upgrade
the limestone, but it is a costly process and is used only
when unavoidable.

13. MANUFACTURE OF CEMENT
 There are four stages in the manufacture of portland
cement: (1) crushing and grinding the raw materials, (2)
blending the materials in the correct proportions, (3)
burning the prepared mix in a kiln, and (4) grinding the
burned product, known as “clinker,” together with some 5
percent of gypsum (to control the time of set of the
cement). The three processes of manufacture are known
as the wet, dry, and semidry processes and are so
termed when the raw materials are ground wet and fed to
the kiln as a slurry, ground dry and fed as a dry powder,
or ground dry and then moistened to form nodules that
are fed to the kiln.

14.  It is estimated that around 4–8 percent of the
world’s carbon dioxide (CO2) emissions come from the
manufacture of cement, making it a major contributor
to global warming. Some of the solutions to
these greenhouse gas emissions are common to other
sectors, such as increasing the energy efficiency of
cement plants, replacing fossil fuels with renewable
energy, and capturing and storing the CO2 that is emitted.
In addition, given that a significant portion of the
emissions are an intrinsic part of the production of clinker,
novel cements and alternate formulations that reduce the
need for clinker are an important area of focus.

15. CRUSHING AND GRINDING
 All except soft materials are first crushed, often in
two stages, and then ground, usually in a rotating,
cylindrical ball, or tube mills containing a charge of
steel grinding balls. This grinding is done wet or dry,
depending on the process in use, but for dry
grinding the raw materials first may need to be
dried in cylindrical, rotary dryers. Soft materials are
broken down by vigorous stirring with water in wash
mills, producing a fine slurry, which is passed
through screens to remove oversize particles.

16. BLENDING
 A first approximation of the chemical composition required
for a particular cement is obtained by selective quarrying and
control of the raw material fed to the crushing and grinding
plant. Finer control is obtained by drawing material from two
or more batches containing raw mixes of slightly different
composition. In the dry process these mixes are stored in
silos; slurry tanks are used in the wet process. Thorough
mixing of the dry materials in the silos is ensured by agitation
and vigorous circulation induced by compressed air. In the
wet process the slurry tanks are stirred by mechanical
means or compressed air or both. The slurry, which contains
35 to 45 percent water, is sometimes filtered, reducing the
water content to 20 to 30 percent, and the filter cake is then
fed to the kiln. This reduces the fuel consumption for
burning.

17. BURNING
 The earliest kilns in which cement was burned in
batches were bottle kilns, followed by chamber kilns
and then by continuous shaft kilns. The shaft kiln in
a modernized form is still used in some countries,
but the dominant means of burning is the rotary
kiln. These kilns—up to 200 metres (660 feet) long
and six metres in diameter in wet process plants
but shorter for the dry process—consist of a steel,
cylindrical shell lined with refractory materials. They
rotate slowly on an axis that is inclined a few
degrees to the horizontal. The raw material feed,
introduced at the upper end, moves slowly down
the kiln to the lower, or firing, end.

18.  The fuel for firing may be pulverized coal, oil, or natural
gas injected through a pipe. The temperature at the
firing end ranges from about 1,350 to 1,550 °C (2,460 to
2,820 °F), depending on the raw materials being burned.
Some form of heat exchanger is commonly incorporated
at the back end of the kiln to increase heat transfer to
the incoming raw materials and so reduce the heat lost
in the waste gases. The burned product emerges from
the kiln as small nodules of clinker. These pass into
coolers, where the heat is transferred to incoming air
and the product cooled. The clinker may be immediately
ground to cement or stored in stockpiles for later use.

19.  In the semidry process the raw materials, in the
form of nodules containing 10 to 15 percent water,
are fed onto a traveling chain grate before passing
to the shorter rotary kiln. Hot gases coming from
the kiln are sucked through the raw nodules on the
grate, preheating the nodules.

20.  Dust emission from cement kilns can be a serious
nuisance. In populated areas it is usual and often
compulsory to fit cyclone arrestors, bag-filter
systems, or electrostatic dust precipitators between
the kiln exit and the chimney stack. More than 50
percent of the emissions from cement production
are intrinsically linked to the production of clinker
and are a by-product of the chemical reaction that
drives the current process. There is potential to
blend clinker with alternative materials to reduce
the need for clinker itself and thus help reduce the
climate impacts of the cement-making process.

21.  Modern cement plants are equipped with elaborate
instrumentation for control of the burning process.
Raw materials in some plants are sampled
automatically, and a computer calculates and
controls the raw mix composition. The largest rotary
kilns have outputs exceeding 5,000 tons per day.

22. GRINDING
 The clinker and the required amount of gypsum are
ground to a fine powder in horizontal mills similar to
those used for grinding the raw materials. The material
may pass straight through the mill (open-circuit
grinding), or coarser material may be separated from the
ground product and returned to the mill for further
grinding (closed-circuit grinding). Sometimes a small
amount of a grinding aid is added to the feed material.
For air-entraining cements (discussed in the following
section) the addition of an air-entraining agent is
similarly made.
 Finished cement is pumped pneumatically to storage
silos from which it is drawn for packing in paper bags or
for dispatch in bulk containers.

23. THE MAJOR CEMENTS: COMPOSITION AND
PROPERTIES
 Portland cement
 Chemical composition
 Portland cement is made up of four main
compounds: tricalcium silicate (3CaO · SiO2), dicalcium
silicate (2CaO · SiO2), tricalcium aluminate (3CaO ·
Al2O3), and a tetra-calcium aluminoferrite (4CaO ·
Al2O3Fe2O3). In an abbreviated notation differing from
the normal atomic symbols, these compounds are
designated as C3S, C2S, C3A, and C4AF, where C
stands for calcium oxide (lime), S for silica, A
for alumina, and F for iron oxide. Small amounts of
uncombined lime and magnesia also are present, along
with alkalies and minor amounts of other elements.

24. HYDRATION
 The most important hydraulic constituents are
the calcium silicates, C2S and C3S. Upon mixing
with water, the calcium silicates react with water
molecules to form calcium silicate hydrate (3CaO ·
2SiO2 · 3H2O) and calcium hydroxide (Ca[OH]2). These
compounds are given the shorthand notations C–S–H
(represented by the average formula C3S2H3) and CH,
and the hydration reaction can be crudely

25. Represented by the following reactions:
2C3S + 6H = C3S2H3 + 3CH
2C2S + 4H = C3S2H3 + CH
During the initial stage of hydration, the parent compounds
dissolve, and the dissolution of their chemical bonds generates
a significant amount of heat. Then, for reasons that are not fully
understood, hydration comes to a stop. This quiescent, or
dormant, period is extremely important in the placement
of concrete. Without a dormant period there would be no
cement trucks; pouring would have to be done immediately
upon mixing.

26.  Following the dormant period (which can last
several hours), the cement begins to harden, as CH
and C–S–H are produced. This is the cementitious
material that binds cement and concrete together.
As hydration proceeds, water and cement are
continuously consumed. Fortunately, the C–S–H
and CH products occupy almost the same volume
as the original cement and water; volume is
approximately conserved, and shrinkage is
manageable.

27.  Although the formulas above treat C–S–H as a
specific stoichiometry, with the formula C3S2H3, it
does not at all form an ordered structure of
uniform composition. C–S–H is actually
an amorphous gel with a highly variable
stoichiometry. The ratio of C to S, for example, can
range from 1:1 to 2:1, depending on mix design and
curing conditions.

28. STRUCTURAL PROPERTIES
 The strength developed by portland cement
depends on its composition and the fineness to
which it is ground. The C3S is mainly responsible
for the strength developed in the first week of
hardening and the C2S for the subsequent increase
in strength. The alumina and iron compounds that
are present only in lesser amounts make little direct
contribution to strength.

29.  Set cement and concrete can suffer deterioration from
attack by some natural or artificial chemical agents. The
alumina compound is the most vulnerable to chemical
attack in soils containing sulfate salts or in seawater,
while the iron compound and the two calcium silicates
are more resistant. Calcium hydroxide released during
the hydration of the calcium silicates is also vulnerable to
attack. Because cement liberates heat when it hydrates,
concrete placed in large masses, as in dams, can cause
the temperature inside the mass to rise as much as 40 °C
(70 °F) above the outside temperature. Subsequent
cooling can be a cause of cracking. The highest heat of
hydration is shown by C3A, followed in descending order
by C3S, C4AF, and C2S.

30. TYPES OF PORTLAND CEMENT
 Five types of portland cement are standardized in
the United States by the American Society for
Testing and Materials (ASTM): ordinary (Type
I), modified (Type II), high-early-strength (Type
III), low-heat (Type IV), and sulfate-resistant (Type
V). In other countries Type II is omitted, and Type III
is called rapid-hardening. Type V is known in some
European countries as Ferrari cement.

31.  There also are various other special types of
portland cement. Coloured cements are made by
grinding 5 to 10 percent of suitable pigments with
white or ordinary gray portland cement. Air-
entraining cements are made by the addition on
grinding of a small amount, about 0.05 percent, of
an organic agent that causes the entrainment of
very fine air bubbles in a concrete. This increases
the resistance of the concrete to freeze-thaw
damage in cold climates. The air-entraining agent
can alternatively be added as a separate ingredient
to the mix when making the concrete.

32.  Low-alkali cements are portland cements with a
total content of alkalies not above 0.6 percent.
These are used in concrete made with certain types
of aggregates that contain a form of silica that
reacts with alkalies to cause an expansion that can
disrupt a concrete.

33.  Masonry cements are used primarily for mortar.
They consist of a mixture of portland cement and
ground limestone or other filler together with an air-
entraining agent or a water-repellent
additive. Waterproof cement is the name given to a
portland cement to which a water-repellent agent
has been added. Hydrophobic cement is obtained
by grinding portland cement clinker with a film-
forming substance such as oleic acid in order to
reduce the rate of deterioration when the cement is
stored under unfavourable conditions.

34.  Oil-well cements are used for cementing work in the
drilling of oil wells where they are subject to high
temperatures and pressures. They usually consist
of portland or pozzolanic cement (see below) with
special organic retarders to prevent the cement
from setting too quickly.

35. SLAG CEMENTS
 The granulated slag made by the rapid chilling of suitable
molten slags from blast furnaces forms the basis of another
group of constructional cements. A mixture of portland
cement and granulated slag, containing up to 65 percent slag, is
known in the English-speaking countries as portland blast-
furnace (slag) cement. The
German Eisenportlandzement and Hochofenzement contain up
to 40 and 85 percent slag, respectively. Mixtures in other
proportions are found in French-speaking countries under such
names as ciment portland de fer, ciment métallurgique mixte,
ciment de haut fourneau, and ciment de liatier au
clinker. Properties of these slag cements are broadly similar to
those of portland cement, but they have a lower lime content
and a higher silica and alumina content. Those with the higher
slag content have an increased resistance to chemical attack.

36.  Another type of slag-containing cement is
a supersulfated cement consisting of granulated slag
mixed with 10 to 15 percent hard-burned gypsum or
anhydrite (natural anhydrous calcium sulfate) and a few
percent of portland cement. The strength properties of
supersulfated cement are similar to those of portland
cement, but it has an increased resistance to many
forms of chemical attack. Pozzolanic cements are
mixtures of portland cement and a pozzolanic material
that may be either natural or artificial. The
natural pozzolanas are mainly materials of volcanic
origin but include some diatomaceous earths. Artificial
materials include fly ash, burned clays, and shales.

37.  Pozzolanas are materials that, though not
cementitious in themselves, contain silica (and
alumina) in a reactive form able to combine with
lime in the presence of water to
form compounds with cementitious properties.
Mixtures of lime and pozzolana still find some
application but largely have been superseded by
the modern pozzolanic cement. Hydration of the
portland cement fraction releases the lime required
to combine with the pozzolana.

38. HIGH-ALUMINA CEMENT
 High-alumina cement is a rapid-hardening cement made
by fusing at 1,500 to 1,600 °C (2,730 to 2,910 °F) a
mixture of bauxite and limestone in a reverberatory
or electric furnace or in a rotary kiln. It also can be made
by sintering at about 1,250 °C (2,280 °F). Suitable
bauxites contain 50 to 60 percent alumina, up to 25
percent iron oxide, not more than 5 percent silica, and
10 to 30 percent water of hydration. The limestone must
contain only small amounts of silica and magnesia. The
cement contains 35 to 40 percent lime, 40 to 50 percent
alumina, up to 15 percent iron oxides, and preferably not
more than about 6 percent silica. The principal
cementing compound is calcium aluminate (CaO ·
Al2O3).

39.  High-alumina cement gains a high proportion of its
ultimate strength within 24 hours and has a high
resistance to chemical attack. It also can be used in
refractory linings for furnaces. A white form of the
cement, containing minimal proportions of iron
oxide and silica, has outstanding refractory
properties.

40. EXPANDING AND NONSHRINKING CEMENTS
 Expanding and nonshrinking cements expand
slightly on hydration, thus offsetting the small
contraction that occurs when fresh concrete dries
for the first time. Expanding cements were first
produced in France about 1945. The American type
is a mixture of portland cement and an expansive
agent made by clinkering a mix of chalk, bauxite,
and gypsum

41. GYPSUM PLASTERS
 Gypsum plasters are used for plastering, the
manufacture of plaster boards and slabs, and in
one form of floor-surfacing material.
These gypsum cements are mainly produced by
heating natural gypsum (calcium sulfate dihydrate,
CaSO4 · 2H2O) and dehydrating it to give calcium
sulfate hemihydrate (CaSO4 · 1/2H2O) or anhydrous
(water-free) calcium sulfate. Gypsum
and anhydrite obtained as by-products in chemical
manufacture also are used as raw materials.

42.  The hemihydrate, known as plaster of Paris, sets
within a few minutes on mixing with water; for
building purposes a retarding agent, normally
keratin, a protein, is added. The anhydrous calcium
sulfate plasters are slower-setting, and often
another sulfate salt is added in small amounts as
an accelerator. Flooring plaster, originally known by
its German title of Estrich Gips, is of the anhydrous
type.

43. CEMENT TESTING
 Various tests to which cements must conform are
laid down in national cement specifications to
control the fineness, soundness, setting time, and
strength of the cement. These tests are described
in turn below.

44. FINENESS
 Fineness was long controlled by sieve tests, but
more sophisticated methods are now largely used.
The most common method, used both for control of
the grinding process and for testing the finished
cement, measures the surface area per unit weight
of the cement by a determination of the rate of
passage of air through a bed of the cement. Other
methods depend on measuring the particle size
distribution by the rate of sedimentation of the
cement in kerosene or by elutriation (separation) in
an airstream.

45. SOUNDNESS
 After it has set, a cement must not undergo any
appreciable expansion, which could disrupt
a mortar or concrete. This property of soundness is
tested by subjecting the set cement to boiling
in water or to high-pressure steam. Unsoundness
can arise from the presence in the cement of too
much free magnesia or hard-burned free lime.

46. SETTING TIME
 The setting and hardening of a cement is a continuous
process, but two points are distinguished for test
purposes. The initial setting time is the interval between
the mixing of the cement with water and the time when
the mix has lost plasticity, stiffening to a certain degree.
It marks roughly the end of the period when the wet mix
can be molded into shape. The final setting time is the
point at which the set cement has acquired a sufficient
firmness to resist a certain defined pressure. Most
specifications require an initial minimum setting time at
ordinary temperatures of about 45 minutes and a final
setting time no more than 10 to 12 hours.

47. STRENGTH
 The tests that measure the rate at which a cement
develops strength are usually made on
a mortar commonly composed of one part cement
to three parts sand, by weight, mixed with a defined
quantity of water. Tensile tests on briquettes,
shaped like a figure eight thickened at the centre,
were formerly used but have been replaced or
supplemented by compressive tests on cubical
specimens or transverse tests on prisms.

48.  The American Society for Testing and Materials (ASTM)
specification requires tensile tests on a 1:3 cement-sand mortar
and compressive tests on a 1:2.75 mortar. The British
Standards Institution (BSI) gives as alternatives a compressive
test on a 1:3 mortar or on a concrete specimen. An international
method issued by the International Organization for
Standardization (ISO) requires a transverse test on a 1:3
cement-sand mortar prism, followed by a compressive test on
the two halves of the prism that remain after it has been broken
in bending. Many European countries have adopted this
method. In all these tests the size grading of the sand, and
usually its source, is specified.

49.  In the testing of most cements, a minimum strength
at 3 and 7 days and sometimes 28 days is
specified, but for rapid-hardening portland cement a
test at 1 day also is sometimes required. For high-
alumina cement, tests are required at 1 and 3 days.
 Strength requirements laid down in different
countries are not directly comparable because of
the differences in test methods. In actual
construction, to check the strength of a concrete,
compressive tests are made on cylinders or cubes
made from the concrete being placed.