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Portland Cement
Manufacturing and Uses of Portland
Cement
By: Haseeb Jamal
Manufacturing
of Portland Cement
Definition
of OPC
Cement
can be defined as the bonding material having cohesive & adhesive
properties which makes it capable to unite the different construction materials
and form the compacted assembly.
Ordinary/Normal
Portland cement is one of the most widely used type of Portland Cement.
The
name Portland cement was given by Joseph Aspdin in 1824 due to its similarity
in colour and its quality when it hardens like Portland stone.
Portland
stone is white grey limestone in island of Portland, Dorset.
Production
& Manufacturing:
Today,
Ordinary Portland cement is the most widely used building material in the world
with about 1.56 billion tones produced each year.
Annual
global production of Portland cement concrete is around 3.8 million cubic
meters per year.
In Pakistan; cement production will go beyond
45 million tons per year in the next two years
Manufacturing
Raw
Materials
Calcareous
(material having content of lime)
Argillaceous
(material having contents of silica & alumina)
Gypsum
Process
Cement
is usually manufactured by two processes:
Wet
process
Dry
process
These
two processes differ in operation but fundamentals of both these processes are
same.
In
Pakistan, most of the factories use Wet Process for the production of cement.
There are five stages in manufacturing of cement by wet process:
Crushing
and grinding of raw material
Mixing
the material in proportion
Heating
the prepared mixture in rotary kiln
Grinding
the heated product known as clinker
Mixing
and grinding of cement clinker with gypsum
Crushing
and Grinding:
In
this phase, soft raw materials are first crushed into suitable size. This is
done usually in cylindrical ball or tube mills containing the charge of steel
balls
Mixing
the Material:
In
this part, the powdered limestone is mixed with the clay paste in proper
proportion (75%=lime stone; clay=25%)
The
mixture is then grounded and made homogeneous by mean of compressed gas. The
resulting material is known as slurry having 35-40% water.
Heating
the slurry in rotary kiln:
Slurry
is then introduced in rotary kiln with help of conveyor. The rotary kiln
consists of large cylinders 8 to 15 feet in diameter & height of 300-500
feet. It is made with steel & is usually lined inside with firebricks.
Rotary
Kiln
Kiln
rotates at the rate of 1-2 revolution per minute. In rotary kiln, slurry is
passed through different zones of temperature. This whole process in kiln
usually covers 2 to 3 hours. Different temperature zones are as under:
Preheating
Zone
In
this zone, temperature is kept at 500 degree Celsius & usually the moisture
is removed & clay is broken into silica, aluminum oxide, iron oxide.
Decomposition
Zone
Temperature
is raised up to 800 degree Celsius. In this zone lime stone decomposes into
lime and CO2.
Burning
Zone
In
this zone temperature is maintained up to 1500 degree Celsius and the oxides
formed in above zones combine together and form respective silicate, aluminates
& ferrite.
Cooling
Zone
This
is last stage where the whole assembly cooled is up to 150 to 200 degree
Celsius.
Clinker
Formation
The
product which is obtained from the rotary kiln is known as the cement Clinker.
Clinker is usually in the form of greenish black or grey colored balls.
Grinding
the Clinker with Gypsum
The
Cement Clinker is then air cooled. The required amount of Gypsum (5 %) is
ground to the fine powder, and then mixed with the Clinker. Finally. cement is
packed in bags and then transported to the required site.
Setting
and Hardening:
When
ordinary Portland cement is mixed with water its chemical compound constituents
undergo a series of chemical reactions that cause it to set.
These
chemical reactions all involve the addition of water to the basic chemical
compounds. This chemical reaction with water is called "hydration".
Each
one of these reactions occurs at a different time and with different rates.
Addition
of all these reactions gives the knowledge about how Ordinary Portland cement
hardens and gains strength. Those compounds and their role in hardening of
cement are as under:
Tricalcium
silicate (C3S): Hydrates and hardens rapidly and is largely responsible for
initial set and early strength. Ordinary Portland cements with higher
percentages of C3S will exhibit higher early strength.
Dicalcium
silicate (C2S): Hydrates and hardens slowly and is largely responsible for
strength increases beyond one week.
Tricalcium
aluminate (C3A): Hydrates and hardens the quickest. It liberates a large amount
of heat almost immediately and contributes somewhat to early strength.
Gypsum
is added to Ordinary Portland cement to retard C3A hydration. Without gypsum,
C3A hydration would cause ordinary Portland cement to set almost immediately
after adding water.
Tetracalcium
aluminoferrite (C4AF): Hydrates rapidly but contributes very little to
strength. Most ordinary Portland cement color effects are due to C4AF.
Uses
of OPC (Ordinary Portland Cement):
It
is used for general construction purposes where special properties are not
required. It is normally used for the reinforced concrete buildings, bridges,
pavements, and where soil conditions are normal.
It
is also used for most of concrete masonry units and for all uses where the
concrete is not subject to special sulfate hazard or where the heat generated
by the hydration of cement is not objectionable.
It
has great resistance to cracking and shrinkage but has less resistance to
chemical attacks.
Tests On Ordinary Portland Cement
1.
Fineness test
2.
Soundness test
3.
Setting time test
4.
Strength tests
·
Tensile strength
test
·
Flexural strength
test
5.
Specific gravity
test
6.
Consistency test
7.
Heat of hydration
test
8.
Loss of ignition
test
Heat
of hydration test
Loss
of ignition test
Haseeb
Jamal. I am a Civil Engineer,
graduated from University
of Engineering and Technology, Peshawar, Pakistan in 2010. I also
have a PG-Diploma in Disaster Management and
MS in Urban
Infrastructure Engineering (In Progress). My expertise include civil
related softwares like AutoCAD,
SAP2000, MS Project, Primavera, MS Office and GIS. My technical skills include
project management, monitoring and evaluation, structural assessment, disaster
risk management, Quantity survey, land survey, material testing, site
management and technical writing. I am trained in writing project progress
reports as well as proposals and concept papers. I have also received advanced
training on surveying, proposal writing, Monitoring and Evaluation of projects
as well as organizations.
I have worked as Project
Engineer at National Research and Development Foundation, Peshawar and
CENCON Associates. I also worked with Spectra Engineering Solutions as Senior
Civil Engineer in monitoring of World Bank and UNDP funded projects all
over Khyber Pakhtunkhwa and FATA. Currently, I am working as Deputy
Manager Development at NayaTel, Peshawar.
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