Portland Cement

What Is Cement????

Material with adhesive and cohesive properties or any material that binds or unites – essentially like glue

Function Of Cement are

to bind the sand and coarse aggregate together
to fill voids in between sand and coarse aggregate particle
to form a compact mass

Types of Cement

2 types of cement normally used in building industry are as follows:
a) Hydraulic Cement
b) Nonhydraulic Cement

Hydraulic Cement

Hydraulic Cement sets and hardens by action of water. Such as Portland Cement
In other words it means that hydraulic cement are:
“ Any cements that turns into a solid product in the presence of water (as well as air) resulting in a material that does not disintegrate in water.”

Nonhydraulic Cement

Any cement that does not require water to transform it into a solid product.
2 common Non-hydraulic Cement are
a) Lime- derived from limestone / chalk
b) Gypsum

Portland Cement

Chemical composition of Portland Cement:
a) Tricalcium Silicate (50%)

b) Dicalcium Silicate (25%)

c) Tricalcium Aluminate (10%)

d) Tetracalcium Aluminoferrite (10%)

e) Gypsum (5%)

Function :Tricalcium Silicate

Hardens rapidly and largely responsible for initial set & early strength
The increase in percentage of this compound will cause the early strength of Portland Cement to be higher.
A bigger percentage of this compound will produces higher heat of hydration and accounts for faster gain in strength.

Function : Dicalcium Silicate

Hardens slowly
It effects on strength increases occurs at ages beyond one week .
Responsible for long term strength

Function :Tricalcium Aluminate

Contributes to strength development in the first few days because it is the first compound to hydrate .
It turns out higher heat of hydration and contributes to faster gain in strength.
But it results in poor sulfate resitance and increases the volumetric shrinkage upon drying.
Cements with low Tricalcium Aluminate contents usually generate less heat, develop higher strengths and show greater resistance to sulfate attacks.
It has high heat generation and reactive with soils and water containing moderate to high sulfate concentrations so it’s least desirable.

Function : Tetracalcium Aluminoferrite

Assist in the manufacture of Portland Cement by allowing lower clinkering temperature.
Also act as a filler
Contributes very little strength of concrete eventhough it hydrates very rapidly.
Also responsible for grey colour of Ordinary Portland Cement

Manufacturing Of Portland Cement

The 3 primary constituents of the raw materials used in the manufacture of Portland Cement are:
a) Lime
b) Silica
c) Alumina
Lime is derived from limestone or chalk
Silica & Alumina from clay, shale or bauxite

There are 2 chief aspects of the manufacturing process:

First
To produce a finely divided mixture of raw materials – chalk / limestone and clay / shale

Second

To heat this mixture to produce chemical composition
There 2 main process that can be used in manufacturing of Portland Cement that is
i) wet process
ii) dry process

Wet Process

Raw materials are homogenized by crushing, grinding and blending so that approximately 80% of the raw material pass a No.200 sieve.
The mix will be turned into form of slurry by adding 30 – 40% of water.
It is then heated to about 2750ºF (1510ºC) in horizontal revolving kilns (76-153m length and 3.6-4.8m in diameter.
Natural gas, petroluem or coal are used for burning. High fuel requirement may make it uneconomical compared to dry process.

Dry Process

Raw materials are homogenized by crushing, grinding and blending so that approximately 80% of the raw material pass a No.200 sieve.
Mixture is fed into kiln & burned in a dry state
This process provides considerable savings in fuel consumption and water usage but the process is dustier compared to wet process that is more efficient than grinding.

Dry Proces & Wet Process

In the kiln, water from the raw material is driven off and limestone is decomposed into lime and Carbon Dioxide.

limestone ——> lime + Carbon Dioxide

In the burning zone, portion of the kiln, silica and alumina from the clay undergo a solid state chemical reaction with lime to produce calcium aluminate.

silica & alumina + lime ——> calcium aluminate

The rotation and shape of kiln allow the blend to flow down the kiln, submitting it to gradually increasing temperature.
As the material moves through hotter regions in the kiln, calcium silicates are formed
These products, that are black or greenish black in color are in the form of small pellets, called cement clinkers
Cement clinkers are hard, irregular and ball shaped particles about 18mm in diameter.
The cement clinkers are cooled to about 150ºF (51ºC) and stored in clinker silos.
When needed, clinker are mixed with 2-5% gypsum to retard the setting time of cement when it is mixed with water.
Then, it is grounded to a fine powder and then the cement is stored in storage bins or cement silos or bagged.
Cement bags should be stored on pallets in a dry place.