Cement

Cement.
• Cement is a very important materials for

building and bridges. It is dirty grayish powder
containing lime stone along with aluminates
and silicates e.g.
• Tricalcium silicates(3CaO.SiO2),
• Dicalcium silicates(2CaO.SiO2),
• Calcium aluminoferrite(4CaO.Al2O3.Fe2O3).
Raw materials for cement production.
1. Lime stone, CaCO3.
• It provides lime, CaO. Chalk, marble and alkali waste also be used, since this supply CaO.
2. Clay.
• The composition of clay is Al2O3.SiO2.Fe2O3.2H2O. Clay supplies silica (SiO2) and
alumina(Al2O3) to the cement. Some of the clay do not contain Fe2O3 and cement obtained in this
case is white and hard to burn.
3. Gypsum, CaSO4.2H2O.
• The addition of gypsum increases the setting time of cement.
4. Coal.
• Fine powder coal can be used as fuel which provides thermal energy.
5. Cement clinker.
• It is the backbone of cement production. It is essentially a mixture of limestone and
aluminosilicate materials such as clay heated in a rotary kiln. It occurs as lumps or nodules usually
3-25 mm in diameter.
6. Iron powder.
• Iron powder(Fe2O3) is used in cement production to increase the hardness of cement stone and
decrease the compressive strength.
Main steps in cement production:
1. Crushing and grinding.
• It is done for the preparation of raw meal or slurry.
The raw materials lime stone and clay are mixed in
proper portion by any of the following methods.
a. Dry process:
• This process is employed when the raw materials lime
stone and clay are hard. In this process the lime stone
is first broken into small pieces. It is then mixed with
clay in the proper proportions and finally pulverized to
such a fineness that 90-95% passes through 100 mesh
sieve. The mixture is made homogenous to produce
what is known as raw meal.
b. Wet process:
• This process is used when raw materials lime stone
and clay are soft, the climate is fairly damp and the
fuel is cheap. In this process, the lime stone is crushed
to suitable size and the clay is washed with water in
wash mill to remove foreign materials like
flint(quartz). The powdered limestone is then mixed
with clay paste in the proper proportion and the
mixture is made homogenous by means of compressed
air mixing arrangement. The resulting paste is known
as slurry. The slurry contains about 40% water.
2. Heating process:
• Raw meal or slurry prepared as above is introduced into
the rotary kiln. The rotary kiln consists of a long cylinder 6
to 8 feet in diameter and 100 to 250 feet in length. Strong
heating process is done for 2-3 hours by applying heat of
about 800-1500 ⁰C. Reaction taking place in the rotary kiln
can be divided into following three parts:
a. Reaction taking place in moderate temperature zone.
• In this zone temperature is upto 800 ⁰C. In this zone free
moisture is removed and clay is broken into Al2O3, SiO2
and Fe2O3
Al2O3.2SiO2.Fe2O3.2H2O → Al2O3+2SiO2+Fe2O3+2H2O
b. Reaction taking place in average temperature zone.
• Here the temperature is 800-1000 ⁰C. In this zone lime stone
(CaCO3) decomposes into lime(CaO) and CO2.
CaCO3 ⎯ 
• ⎯→ CaO + CO2↑
c. Reaction taking place in maximum temperature zone.
• In this zone the temperature is 1000-1500 ⁰C. The oxides CaO,
SiO2, Al2O3 and Fe2O3 combine together and form calcium silicate
2CaO.SiO2, 3CaO.SiO2, Calcium aluminate 2CaO.Al2O3,
3CaO.Al2O3 and Calcium aluminoferrite 4CaO.Al2O3.Fe2O3.
• The resulting product is known as cement clinker as it comes out
into the cooler, it has the appearance of small greenish black or
grey coloured balls varying in size from nuts to pea.
• The total reaction occurred in this zone:
2CaO + SiO 2 → 2CaO.SiO 2

Dicalcium silicate
3CaO + SiO 2 → 3CaO.SiO 2
Tricalcium silicate
2CaO + Al2 O 3 → 2CaO.Al2 O 3
Dicalcium aluminate
4CaO + Al2 O 3 + Fe 2 O 3 → 4CaO.Al2 O 3 .Fe 2 O 3
Tetracalcium alumino -
ferrite
Final grinding:
• In this process cement clinker is mixed with gypsum. The cement
clinker is cooled in the cooler by cold air. The cooled clinker is
mixed with 2-3% of its weight of gypsum (CaSO4.2H2O).
3CaO.Al2O3 which is fast setting constituents of the clinker reacts
with gypsum to form the crystals of calcium sulpho-aluminate,
3CaO.Al2O3.3CaSO4.2H2O.
3CaO.Al2 O 3 .3(CaSO 4 .2H 2 O) + 2H 2 O
 Gypsum
3CaO.Al2 O 3 .3CaSO 4 .2H 2 O + 6H 2 O
Calcium sulpho - aluminate
• Thus we see that addition of gypsum removes the fast setting and
hence the process of setting cement gets retarded and this result in
better strength of the mass which sets.
Types of cement:
1. OPC cement.
• Ordinary Portland cement
2. PPC cement
• Portland pozzolana cement
Portland cement process with
flow-sheet diagram.
Difference between OPC and PPC cement.
OPC PPC
1. Definition: A mixture of limestone and PPC is prepared by adding
other raw materials like pozzolanic materials to
argilaceous, calcareous, OPC so the main
gypsum is then grinded to components are OPC
prepare OPC cliker, gypsum and
pozzolanic materials.
Which includes cacinated
clay, volcanic ash, silica
fumes.
2. Strength: Initial strength is higher PPC has higher strength
than PPC than OPC over longer
period of time.
3. Heat of hydration: Generates more heat than It has slow hydration
PPC in hydration reaction process and thus generates
which make it less suitable less heat than OPC.
for mass casting.
4. Durability: Less durable in aggressive More durable in aggressive
weather. weather.
5. Cost: Costlier than PPC Cheaper than OPC
6. Environmental impact: Emits CO2 during the It constitutes industrial

manufacturing process and natural waste which
makes it eco-friendly.
7. Setting time: Lower than PPC. Its initial Setting time of PPC is
setting time is 30 minutes higher than OPC. Its
and final setting time is initial setting time is 30
280 minutes. Its faster minutes and final setting
setting time helps faster time is 600 minutes. Its
construction. slower setting time helps
to get better finishing.
8. Grades available: 33 grade, 43 grade and 53 No specified grade of PPC

grade OPC cement are cement is available.
available.
9. Workability: Lower than PPC. Higher than OPC
10. Resistance against It has lower resistance It has higher resistance

Cement Industry in Nepal:
• Public sector:
1. Himal cement Company
2. Hetauda Cement Industry Ltd
3. Udayapur Cement Industry Ltd
• Private sector:
1. Maruti Cement Udhyog
2. Pancha Ratna Cement Udhog
3. National Cement pvt.Ltd
4. Shivam Cement Pvt.Ltd
5. Budha Cement Udhog
6. Balagi Cement Udhog
7. Triveni Cement Udhog
8. Butwal Cement
9. Om Cement Pvt.Ltd
10. Koshi Cement Pvt.Ltd

Cement

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Cement.

• Cement is a very important materials for

2CaO + SiO 2 → 2CaO.SiO 2

6. Environmental impact: Emits CO2 during the It constitutes industrial

8. Grades available: 33 grade, 43 grade and 53 No specified grade of PPC

9. Workability: Lower than PPC. Higher than OPC

10. Resistance against It has lower resistance It has higher resistance

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