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CEMENT
Cement:

Definition,

Composition,

basic

constituents

and

their

significance,

Manufacturing of Portland cement by Rotary Kiln Technology, Chemistry of setting and
hardening of cement and role of gypsum
...
Any change in the composition can seriously affect the strength of the material,
which in turn can affect the strength of the building
...
Cementing materials are materials that exhibit characteristic
properties of setting and hardening, when mixed to a paste with water
...
These materials
are classified as
a) Hydraulic cements: These materials set and harden on the addition of water
...
After
hardening it retains its strength and stability even under water
...
Eg: Portland cement
...
They harden slowly by absorbing carbon
dioxide naturally present in the air forming carbonates
...

Type of

Preparation

Properties

Uses

cement
It

is

made

by

calcining

a ∑

Possess

naturally occurring argillaceous

hydraulic

Natural

lime stone at high temperature

qualities
...


product
...


aluminates are formed by the

∑

Used for laying
bricks

∑ Used for laying
foundations

combination
alumina

of

with

silica

calcium

and
oxide

during calcinations
...
It

hydraulic

Portland cement

cement

is

qualities
...

Prepared from hydrated lime ∑

Low setting
...


of

calcium

and

aluminum

∑

Used for making
concrete in bulk
construction
...


PORTLAND CEMENT
In 1824, Joseph Aspdin, a British (Leeds) stone mason, obtained a patent for a cement
he produced in his kitchen
...
He
took patent under the name “Portland Cement” because the concrete made from it
looked like Portland stone, a widely-used building stone in England
...

MANUFACTURE OF PORTLAND CEMENT
There are two methods for the manufacture of Portland cement
...
These processes differ only in the treatment of raw material
...

The manufacture of Portland cement requires following raw materials
i)

Calcareous materials, CaO [such as limestone, chalk, marble etc]

ii)

Argillaceous materials, Al2O3 and SiO2 [such as clay, shale, slate etc]

iii)

Powdered coal or fuel oil

iv)

Gypsum (CaSO4
...
No

Raw material

Function
Principal constituent of the cement
...
Excess lime reduces the strength of the

1
...
Less
lime reduces the strength and makes it quick setting
...


Silica

3
...

Alumina makes the cement quick setting
...


4
...
It actually enhances
the intial setting time of the cement
...


Iron oxide

Imparts color, strength and hardness to the cement
...

In dry process, the calcarious materials and argillaceous materials are crushed
separately in crushers and finally ground into a fine powder
...

In wet process, the calcarious materials (lime) are crushed and powdered
...
The powdered
lime and clay slurry are mixed in required proportions and then fed to a rotary kiln
...

(ii)

Burning:

Burning is done in rotary kiln which is lined inside with refractory bricks
...
The upper part of kiln temperature is
400∞ C but the temperature gradually increases up to 1750∞ C at the bottom
...
The following reactions take place in the rotary kiln
...


b)

Calcination zone: It is the central part of the kiln where the temperature is
around 1000∞C
...

CaCO3 → CaO+ CO2

c) Clinkering zone: In the lower part of the rotary kiln, the temperature is between
1500∞C to 1700∞C
...


The aluminates and silicates of calcium fuse together to form hard grayish stones called
clinkers
...
The cooled clinkers is collected in trolleys
...
1
(i)

Rotary kiln for the manufacture of cement

Grinding:

Cooled clinkers are ground to a fine powder in ball mills
...
Gypsum, thus act as
retarding agent for early setting of cement
...
Each bag usually contains 50 kg of cement
...
SiO2

C2S

27%

Tricalcium silicate

3CaO
...
Al2O3

C3A

10%

4CaO
...
Fe2O3

C4AF

8%

Tetracalcium alumino
ferrite

SETTING AND HARDENING OF CEMENT

When cement is mixed with water and allowed to stand it solidifies to a hard rigid mass
by a series of complex reactions
...
Setting is defined as the stiffening of the original cement paste, due to initial
gel formation
...
First
setting occurs within 24 hours and subsequent hardening occurs in 15-20 days
...

The process of setting of cement is due to partly chemical change and partly physical
change
...
Physical change is gel formation followed by separation of
crystalline products
...


Also dicalcium silicate starts hydrolyzing to a tobermonite gel (which possesses a very
high surface area and thus a very high adhesive property) which also contributes to
initial setting
...


The total process of setting and hardening occurs in about 28 days and it is explained
below
...
2H2O) in cement to an extend of 2-3% is to
prevent the early setting of cement at a rapid rate
...
This situation is not preferred
...
Al2O3
...
7H2O



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