Cement is produced through a process involving crushing and grinding raw materials such as limestone and clay, heating the materials in a kiln to form clinker, cooling and grinding the clinker, and adding gypsum. The main raw materials used are limestone, clay, iron oxide, and aluminum oxide. During the heating process in a rotary kiln, the raw materials undergo chemical reactions to form calcium silicates and calcium aluminates which fuse together to form clinker. Gypsum is added to the ground clinker to regulate the setting time of cement.

1. Cement

2. General introduction
❖ Cement was first discovered in England by Joseph Aspdin.
❖ It is essentially a mixture of limestone and clay.
❖ It is a powdered material which has initially plastic flow when mixed with water
or other liquid. Then it forms a solid structure in several hours with varying degree
of strength and bonding properties which continue with age.
❖ Specially, Portland cement is defined as finely grinded calcium aluminates and
silicates of varying compositions which hydrate when mixed with water to form
rigid continuous structure with good compressive strength.

3. Raw Materials for Cement Production
Cement is essentially a mixture of calcium silicates and calcium aluminates with small
amounts of added gypsum. The essential constituents are lime, silica, aluminium oxide,
magnesium oxide and ferric oxide. The approximate composition is
The essential materials are used are lime stone and clay. Limestone provides calcium oxide
and clay provides silica, alumina and ferric oxide.
S.N. Name of constituent Formula Content
1. Calcium oxide CaO 50-60%
2. Silica SiO2 20-25%
3. Alumina Al2O3 5-10%
4. Magnesium oxide MgO 1-2%
5. Ferric Oxide Fe2O3 1-2%
6. Sulphur trioxide SO3 1-2%

4. Main Steps in Cement Production
❖ Crushing and grinding of raw materials (mixing the raw materials)
❖ Strong heating or burning ( formation of clinker)
❖ Cooling clinker and mixing additives
❖ Final grinding
1.Crushing and grinding of raw materials (Mixing of the raw materials)
This is carried out either by dry process or by wet process. In the dry process, the raw
materials (lime stone and clay ) are crushed to powder and mixed in calculated proportions.
This mixture which is ready to be fed into rotary kiln is called ‘raw-mix’.
In the wet process, calcareous materials (CaO) is crushed, powdered and stored in big
tanks. Argillaceous materials (Al2O3 and SiO2) is washed with water to remove impurities
and stored. These raw materials are mixed in right proportions and made into paste in a
grinding mill. This paste is known as slurry. The slurry is then pumped into rotary kiln
where it is burnt.

5. 2.Strong heating or burning ( formation of clinker)
Depending on the process used in the first step, the dry powder or wet slurry
is burnt in a rotatory kiln. The rotary kiln is made up of steel tubes or drums.
The inner side of the kiln is lined with refractory bricks. The rotatory kiln is
slightly inclined downwards towards the exit end. It is mounted on rollers
and can be rotated at any desired speed. The burning process is carried out in
the rotary kiln while the raw materials are rotated at 1-2rpm at its longitudinal
axis. As the kiln position is inclined and it rotates slowly, the material charged
from upper end moves towards lower end. The kiln is heated with the help of
powdered coal or oil or hot gases from the lower end of the kiln so that the
long hot flames is produced. The maximum temperature attained inside the
kiln is 1750°C.

8. i. Drying Zone: The upper part Zone of kiln known as drying Zone and maintained at, at
400°C. water or moisture in the material is evaporated.
ii. Calcination Zone : The central part of the kiln is called calcination zone and the
temperature is around 1000°C, where decomposition of lime stone takes place. Powdered
limestone present in the raw-mix or slurry on decomposition to give quick lime.
CaCO3 → CaO + CO2
iii. Clinkering zone: The lower part (clinkering zone) have temperature in between 1500-
1700°C where lime and clay are reacts to yielding calcium aluminates and calcium
silicates. This products of aluminates and silicates of calcium fuses together to form hard and
small stones known as clinkers. The size of the small and hard clinkers varies from 5 to
10mm.
2CaO + SiO2 →Ca2SiO4 (dicalcium silicate (C2S))
3CaO + SiO2 → Ca3SiO5 (tricalcium silicate (C3S))
3CaO + Al2O3 → Ca3Al2O6 (dicalcium aluminate (C2A))
4CaO + Al2O3 + Fe2O3 → Ca4Al2Fe2O10 (tetracalcium aluminoferrite(C4AF))
The clinker coming from the burning zone are very hot. To bring down the temperature of
clinkers, air is admitted in counter current direction at the base of the rotary kiln. The cooled
clinkers are collected in small trolleys.

9. 3. Final grinding
The cooled clinkers are received from the cooling pans and sent into mills. The clinkers
are grinded finely into powder in ball mill or tube mill. Powdered gypsum
(CaSO4.2H2O) is added around 2-3% as retarding agent during final grinding. The final
product (mixture of clinker and gypsum) is known as cement. This final product
Portland cement is now ready for packing.
4. Storage and packaging
The grinded cement is stored in silos, from which it is marketed either in container load
or 50kg bags.

10. Depending on hardnening and setting behavoiur, cement is mainly classified in to
two types.
Hydraulic cement : The cement which sets very quickly and hardens with addition of water
to finely grinded cement is called as hydraulic cement. It is composed of mainly limestone
and gypsum clay. It is a non-corrosive,non-rusting and non-shrinkable substances widely
used in construction.
Non-Hydraulic cement: The cement which sets very slowly the by absorption of carbon dioxide
from the atmosphere and can not harden while in contact with water is called non-hydraulic
cement. It is composed of lime,gypsum,plaster and oxychloride. For example gypsum and
magnesium cement are referred as non-hydraulic cement. Due to a long drying period for its
setting, use of this type of cement is less common. It’s still used for making brick and mortar.
• 2079 SLC : Give a difference between hydraulic and non-hydraulic cement

11. What is PPC Cement & OPC Cement?
PPC Cement: Pozzolana is a natural or artificial material which contains silica in the
reactive form. Portland Pozzolana Cement is cement manufactured by combining
Pozzolanic materials. This cement comprises of OPC clinker, gypsum and pozzolanic
materials in certain proportions. The Pozzolanic materials include fly ash, volcanic ash,
calcined clay or silica fumes. These materials are added within a range of 15% to 35%
by cement weight.
OPC Cement or Ordinary Portland Cement (OPC) is manufactured by grinding a
mixture of limestone and other raw materials like argillaceous, calcareous, gypsum to a
powder. This cement is available in three types of grades, such as OPC 33 grade, OPC
43 grade and OPC 53 grade. OPC is the most commonly used cement in the world. This
type of cement is preferred where fast pace of construction is done.

12. Difference Between OPC and PPC Cement
The main differences in their characteristics and uses are
•Portland Pozzolana Cement is a variation of Ordinary Portland Cement. Pozzolana
materials namely fly ash, volcanic ash, are added to the OPC so that it becomes PPC.
Pozzolana materials are added to the cement in the ratio of 15% to 35% by weight.
•Both are ecofriendly materials but Pozzolana Cement uses natural and industrial waste
thus reducing the environmental pollution.

13. OPC PPC
It has higher strength than PPC in the initial stage.
The strength of PPC is good than OPC in long
terms.
It has high heat of hydration making it
unfavorable for mass concreting.
The hydration process is slower than OPC
resulting low heat of hydration. Therefore, it is
suitable for mass concreting.
The presence of sulphates, alkalies, chlorides, etc.
is higher and less resistant than PPC.
It has low percentage of sulphate alkalis,
chlorides, magnesia and free lime in its
composition, which makes the concrete durable.
OPC is not favorable in aggressive weather. Show greater resistance to aggressive weather.
OPC cement are available in three grades, such as
33 Grade, 43 Grade, 53 Grade
PPC is available in any specific grades.
It is slightly costlier than PPC. Cheaper than OPC.

14. Conclusion: Both OPC and PPC are commonly used cements in construction.
These days, PPC is used as a substitute of OPC. PPC is a variation of OPC which
adds a mixture of a pozzolanic material that helps to enhance the strength of the
concrete. PPC also brings down the amount of OPC requirement in making
concrete.

15. Cement Industries in Nepal
The use of cement in Nepal begins from 1950’s but supply was diversified in 1965. in 1975, the first cement plant,
Himal cement a state owned company started manufacturing cement 160 tons per day was production capacity.
Then hetauda cement industry and udayapur cement industry limited were established.
Name of Industry
Public sector
1. Himal cement company limited
2. Hetauda cement industries limited
3. Udayapur cement limited.
Private sectors
1. Honsi cement
2. Sarbottam cement
3. Hongshi-Shivam cement,a Nepal-China joint venture company is the country’s largest cement factory.

16. NEB Question
1. Define clinker
2. What is Portland cement?
3. Name the major component present in Portland cement
4. Why is gypsum is used in clinker during cement production process?
5. Give any two instruments used for the quality control of cement .

17. ANS:
1. Clinker : The mixture of limestone and minerals which have heated in a klin and is the
backbone of cement production is called Clinker in cement industry.
2. Portland cement : It is a fine powder, produced by heating limestone and clay minerals
in a kiln to form clinker, grinding the clinker, and adding 2 to 3 percent of gypsum.
3. 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.

18. 4. Gypsum prevents Flash Setting of cement during manufacturing. It retards the setting time
of cement. Allows a longer working time for mixing, transporting and placing.
5. Two instruments used for the quality control of cement are Flame Photometer and X-ray
fluorescence spectrometer.

20. • What are the major challenges in establishing cement industries in the countries like
Nepal?
• Why is gypsum used in clinker for the quality control of cement? Give any two
instruments used for quality control of cement.
• Give a difference between hydraulic and non-hydraulic cement
• List the composition of Portland cement.
• Which is the first established state owned cement industry in Nepal?[Ans. Himal cement
company a state owned manufacturing cement 160 tons per day was production
capacity.
• Define the term clinker.
Practice question
❖ Cement is important materials for the constructions of building,road,bridge etc.[2+1+1+1]
I. Give a difference between hydraulic and non-hydraulic cement
II. List the compositions of Portland cement.
III. What is the purpose of adding gypsum during cement production.
IV. Name any two instruments to check the quality of cement.