1. Cement is the mixture of calcareous, siliceous, argillaceous and other substances.
2. Cement is a hydraulic binder and is defined as a finely ground inorganic material which,
when mixed with water, forms a paste which sets and hardens by means of hydration
reactions and processes which, after hardening retains its strength and stability even
under water.
The term cement is commonly used to refer to powdered materials which develop strong
adhesive qualities when combined with water. When this paste is mixed with sand and gravel
and allowed to dry it is called concrete.
 Cement was first invented by the Egyptians and later reinvented by the Greeks.
 About ninety-nine percent of all cement used today is Portland cement. The name
Portland cement is not a brand name. This name was given to the cement by Joseph
Aspdin, England who obtained a patent for his product in 1824.
 The first cement manufactured in the United States was produced in 1871 by David
Saylor.
There are two types of raw materials which are combined to make cement:
 Lime-containing materials, such as limestone, marble, oyster shells, marl, chalk, etc.
 Clay and clay-like materials, such as shale, slag from blast furnaces, bauxite, iron ore, silica,
sand, etc.
It takes approximately 3,400 lbs. of raw materials to make one ton (2,000 lbs.) of Portland
cement. The mixture of materials is finely ground in a raw mill. The resultant raw mix is burned
in a rotary kiln at high temperature to form clinker. The clinkers are then ground with gypsum
to produce cement.
Cement used in construction is characterized as
Hydraulic Cement (e.g., Portland cement) is a product used to stop water and leaks in concrete
and masonry structures. It is a type of cement that sets extremely fast and hardens after is has
been mixed with water.
Hydraulic cement is used widely in the construction industry and in situations where structures
can be affected or submerged in water. This makes it ideal for brick buildings in wet climates,
structures that are in contact with sea water, and many other applications.
One common application for hydraulic cement blends is repairing cracks and leaks. Certain
types of cement contain an ingredient which causes the cement to expand as it sets filling in
leaks and holes in concrete or other structural materials. There is often a need for this type of
repair work because of the fact that ordinary Portland cement shrinks slightly as it dries. Places
like basements in areas with wet climates are especially prone to leaks, because of this cement
is used.
PORTLAND CEMENT
Portland cement is made by mixing substances containing CaCO3 (Lime stone) with substances
containing SiO2, Al2O3, and Fe2O3 (Clay) and heating them to a clinker which is subsequently
ground to powder and mixed with gypsum.
Limestone (CaCO3) is mainly providing calcium in the form of calcium oxide (CaO)
CaCO3 → CaO + CO2
Clay is mainly providing silicates (SiO2) together with small amounts of Al2O3 + Fe2O3
Clay → SiO2 + Al2O3 + Fe2O3 + H2O
RAW MATERIALS
Calcareous Materials: Supplies Lime to cement
Lime Stone (CaCO3), Marl, Chalk, Shale, Calcite, Alkali waste.
Argillaceous Materials: Supplies Silica, Alumina and Iron Oxide. Clay, Marl, Shale, Blast Furnace
Slag, sand etc. Gypsum: increases setting time. Powdered Coal and Fuel Oil: For generating
required temperatures.
Lime (CaO)
60-66%
Excess reduces strength and make
cement disintegrate and in less
amount reduces strength and makes
it quick setting.
Silica (SiO2)
17-25%
Provides strength to cement
Alumina (Al2O3)
3-8%
Helps in quick setting
Calcium Sulphate
(Gypsum)
2-6%
Enhances setting of cement
Iron Oxide
2-6%
Gives color, Strength and hardness.
1-3%
Provides soundness
0.5-1.5%
in excess makes cement efflorescent
Sulphur
(SO3)
trioxide
Alkali
Oxides
(Na2O and K2O)
PRODUCTION PROCESSES
The cement manufacturing process involves four distinct stages, and these are outlined below.
Step 1 - Quarrying
The raw material for cement manufacture is a rock mixture which is about 80% limestone
(which is rich in CaCO3) and 20% clay or shale (a source of silica, alumina and Fe2O3). These are
quarried and stored separately.
Step 2 - Raw material preparation
The steps involved here depend on the process used. There are two main cement
manufacturing processes currently used:
1. Dry process
2. Wet process
The two processes are discussed separately below.
The dry process
 In this process calcareous material such as lime stone (calcium carbonate) and
argillaceous material such as clay are ground separately to fine powder in the absence
of water and then are mixed together in the desired proportions.
 This process is usually used when raw materials are very strong and hard.
 In this process, the raw materials are changed to powdered form in the absence of
water.
 Dehydration zone requires a somewhat shorter distance than wet process.
 74% of cement produced.
 kilns less fuel requirements
The wet process
 In this process, the raw materials are changed to powdered form in the presence of
water.
 In this process, raw materials are pulverized by using a Ball mill, which is a rotary steel
cylinder with hardened steel balls. When the mill rotates, steel balls pulverize the raw
materials which form slurry (liquid mixture). The slurry is then passed into storage tanks,
where correct proportioning is done. Proper composition of raw materials can be
ensured by using wet process than dry process. Corrected slurry is then fed into rotary
kiln for burning.
 This process is generally used when raw materials are soft because complete mixing is
not possible unless water is added.
 Actually the purpose of both processes is to change the raw materials to fine powder.
 dehydration zone would require up to half the length of the kiln
 easiest to control chemistry & better for moist raw materials
 high fuel requirements - fuel needed to evaporate slurry water 30+%
Step 3 - Clinkering
This is the step which is characteristic of Portland cement.
The finely ground material is dried, heated and then cooled down again. While it is being
heated various chemical reactions take place to form the major mineral constituents of
Portland cement.
The powder from the dry process doesn't contain much moisture, so can be dried in a
preheated tower. As it falls through the tower (which takes 30 seconds) it is heated from 70 to
800oC. The moisture evaporates and decarbonation (loss of CO2) occurs and some intermediate
phases such as CaO•Al2O3 begin to appear. The mixture is then fed into the kiln.
The slurry from the wet process contains too much moisture to be successfully dried in a
preheated tower. Instead, the slurry is fed directly into the kiln where it is formed into dry balls
by the heat and rotation of the kiln.
The kiln
 The kiln is a continuous stream process vessel in which feed and fuel are held in dynamic
balance.
 The kilns used is inclined on a shallow angle and lined with heat-resistant bricks.
 The kiln shell is steel, 60m long and inclined and rotates slowly. The shell is supported
on rollers and weighs in at over 1100 T.
 The kiln is heated by injecting pulverized coal dust into the discharge end where it
spontaneously ignites due to the very high temperatures.
The reaction processes occurring within the kiln are not easily understood due to the wide
variations in raw-mix chemistry, raw-mix physical properties and kiln operating conditions, and
the physical difficulties of extracting hot materials from the process for investigation before
they cool.
Breaking the reaction processes into a number of simple zones means we can make some
approximations about the cement formation process.
Zone 1: 0 - 35 min, 800 - 1100oC
Decarbonation Formation of CaO and CO2.
CaCO3 → CaO + CO2
Zone 2: 35 - 40 min, 1100 - 1300oC
Exothermic reactions and the formation of silicate phases as follows:
2CaO + SiO2 → 2CaO•SiO2
 Zone 3: 40 - 50 min, 1300 – 1450 oC
 This reaction form ternary silicates and tetra calcium aluminoferrates:
 2CaO•SiO2 + CaO → 3CaO•SiO2
 3CaO•Al2O3 + CaO + Fe2O3 → 4CaO•Al2O3•Fe2O3

 Zone 4: 50 - 60 min, 1300 - 1000oC
 Cooling and crystallization of the various mineral phases formed in the kiln.

 The cooler
 Immediately following the kiln is a large cooler designed to drop the temperature of the
clinker (as the fused material is now called) from 1000oC to 150oC. This is achieved by
forcing air through the bed of clinker via perforated plates in the base of the cooler. The
plates within the cooler slide back and forth, shuffling the clinker down the cooler to the
discharge point and transport to a storage area.
 At this point in the process the materials have been formed into all the required
minerals to make cement. Like cement, the clinker will react with water and harden, but
because it is composed of 1-3 cm diameter fragments it is too coarse to be used.

 Step 4 - Cement milling
 To produce the final product the clinker is mixed with gypsum (CaSO4•2H2O), and
ground for approximately 30 minutes in large tube mills. The cement flows from the
inlet to the outlet of the mill (a rotating chamber), being first ground with 60 mm then
30 mm diameter steel balls. The first grinding breaks up the material and the second
grind it to a fine powder.
 The amount of grinding is governed by the volume of cement fed into the mill: the
greater the volume the coarser the grind. This has practical limits, with too much
cement clogging up the mill and not enough actually increasing the particle size. The
particle size is measured by laser diffraction analysis, and the quantity of material
entering the mill adjusted accordingly. Over time the charge (steel grinding balls) wear
out, so when they reach a certain size they fall through a sieve and then are replaced.
The cement grinding process is highly energy intensive. The largest mill at Golden Bay
Cement is some 11 m in length, weighs over 230 T, is driven by a 2100 kW motor and
can produce over 60 T hr-1. The rotating mill generates significant quantities of energy
and water is added to both the inlet and outlet ends of the mill to cool the product and
the mill itself.

 The fine powdered cement is now ready for packing and dispatching.

CHEMISRTY OF SETTING OF CEMENT
When water is mixed with cement to form a paste, reaction starts. In its pure form, the finely
ground cement is extremely sensitive to water. Out of the three main compounds, viz. C3A, C3S
and C2S, reacts quickly with water to produce a jelly-like compound which starts solidifying. The
action of changing from a fluid state to a solid state is called ‘setting’ and should not be
confused with ‘hardening’.
CEMENT MODULUS
Origin of cement modulus
Even though Portland cement was manufactured by practical experience and process, when
you see the chemical analysis, there exists a relation between the percentages of lime with the
percentages
of
combination
of
silica,
alumina
and
iron
oxides.
 The proportions of the 4 components are controlled by the continuous sampling and
testing of this raw meal.
 The raw meal chemical composition is determined by the use of an x-ray fluorescence
analyzer. This is linked to the computer which will automatically adjust the weighfeeders, so that the resultant raw meal stored in the blending silo meets the preset
parameters. After blending this material is then discharged into the storage silos ready
for the next phase of production.
 The parameters used in the control of the raw meal are lime saturation factor, silica
modulus and iron modulus. These are actually proportions of the various chemical
components which are desired in the resultant clinker.
The oxide ratios were later developed into formulas called cement modulus.
Silica ratio:
It’s the ratio of silica to the flux (iron oxide + alumina).
SiO2
SM = -------------------Al2O3 + Fe2O3
 2.3 to 3.5 (desired at least 3.0)
 Slow
reaction
if
SM
is
high
Also increase in silica modulus causes cement to have a property of increase in
resistance to chemical and atmospheric attack and a higher strength.
 A decrease in silica ratio causes increase in liquid content in raw mix. Thus a low silica
ratio raw mix burns easily and quickly in shorter time resulting good nodulisation of
clinker.
Alumina modulus
It shows the factor or ratio or proportion of iron oxide and alumina. It was also called alumina
ratio.
Al2O3
AM = -------------Fe2O3
 Generally the alumina ratio for Portland cements to be maintained between 1.5 to 2.5
to have a good liquid phase composition in the clinker.
 High alumina ratio cement has fast setting of cement. So more gypsum was required for
such
cements
to
delay
the
setting
time.
Hydraulic modulus or lime saturation factor:
It’s the ratio between percentage of lime to percentages of combination of alumina, iron oxide
and silica.
CaO
LSF = ----------------------------SiO2 + Al2O3 + Fe2O3
LSF: 0.92-0.96
Or
CaO
LSF = --------------------Al2O3 + Fe2O3
The hydraulic modulus to be maintained between 1.7 to 2.3.
THE ROLE OF THE LABORATORY
The laboratory forms an integral part of the control systems on site with testing from raw
materials to finished product. The laboratory operates a 24 hour facility in line with a
continuous manufacturing facility responsible for the following aspects:
 Testing raw materials prior to blasting in the quarry and assisting with development of
quarrying strategies forms the first step in the process.
 Analyzing rock samples from the raw mill at regular intervals during the day and night
and fine tuning the process to ensure chemical control is maintained.
 Analyzing clinker at the end of the cooler (before grinding) to ensure that the
manufactured process meets specification.
 Checking that cement mills are undertaking grinding correctly and that customers
receive the right product.
 Checking dispatched materials for quality and compliance with Standards requirements.
Certificates of conformance are issued to customers based on these analyses.
 Product development.
ENVIRONMENTAL IMPACTS
Many of the aspects of the cement making process are potentially environmentally damaging,
although these risks can be minimized. The areas of potential concern are listed below.
Dust emissions
The manufacturing of cement generates large quantities of dust. These must be prevented
(both on environmental and economic grounds) from escaping to the atmosphere.
CO2
Cement manufacture is an energy intensive process. One of the most significant challenges
facing the industry into the 21st century is a requirement to reduce CO2 emissions. CO2 is
produced during the calcination phase of the manufacturing process and also as a result of
burning fossil fuels.
Quarry and plant water runoff
Treatment of waste water from quarries is a problem for almost all quarry operations. Usually
this is trapped in wetland areas where the water is treated in a controlled manner. Within the
factory runoff can be contaminated by oils and lubricants, but the runoff is monitored and
training programmers are regularly undertaken to ensure this does not happen.
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, thallium,
cadmium and mercury are the most toxic. Heavy metals 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.