What is Cement?

 

CEMENT:

Cement is one of the most important building materials which is a binding agent that sets and hardens to building units such as stones, bricks, tiles, etc. Cement generally belongs to a very fine powdery substance mainly made up of limestone (calcium), sand or clay (silicon), bauxite (aluminum) and iron ore, and may include shells, chalk, marl, shale, clay, blast furnace slag, slate. The raw ingredients are processed in cement manufacturing plants and heated to form a rock-hard substance, which is then ground into a fine powder to be sold. Cement mixed with water causes a chemical reaction and forms a paste that sets and hardens to bind individual structures of building materials.

Composition of Cement

There are eight major ingredients in cement. The general percentage of these ingredients of which cement is made of is given below:

Ingredient	Percentage in cement
Lime	60-65
Silica	17-25
Alumina	3-8
Magnesia	1-3
Iron oxide	0.5-6
Calcium Sulfate	0.1-0.5
Sulfur Trioxide	1-3
Alkaline	0-1

Functions of Cement Ingredients

The main features of these cement ingredients along with their functions and usefulness or harmfulness are given below:

1. Lime: Lime is calcium oxide or calcium hydroxide.
• The presence of lime in a sufficient quantity is required to form silicates and aluminates of calcium.
• Deficiency in lime reduces the strength of the property to the cement.
• Deficiency in lime causes the cement to set quickly.
• Excess lime makes cement unsound.
• The excessive presence of lime causes the cement to expand and disintegrate.

 

 

2. Silica: Silicon dioxide is known as silica, chemical formula SiO₂.
• A sufficient quantity of silica should be present in cement to dicalcium and tricalcium silicate.
• Silica imparts strength to cement.
• Silica usually presents to the extent of about 30 percent cement.

 

3. Alumina: Alumina is Aluminium oxide. The chemical formula is Al₂O₃.
• Alumina imparts quick setting property to the cement.
• Clinkering temperature is lowered by the presence of the requisite quantity of alumina.
• Excess alumina weakens the cement.

 

4. Magnesia: Magnesium Oxide. The chemical formula is MgO.
• Magnesia should not be present more than 2% in cement.
• Excess magnesia will reduce the strength of the cement.

 

5. Iron oxide: Chemical formula is Fe₂O₃.
• Iron oxide imparts color to cement.
• It acts as a flux.
• At a very high temperature, it imparts into the chemical reaction with calcium and aluminum to form tricalcium alumino-ferrite.
• Tricalcium alumino-ferrite imparts hardness and strength to cement.

 

6. Calcium Sulfate: Chemical formula is CaSO₄
• This is present in cement in the form of gypsum(CaSO₄.2H₂O)
• It slows down or retards the setting action of cement.

 

7. Sulfur Trioxide: Chemical formula is SO₃
• It should not be present for more than 2%.
• Excess Sulfur Trioxide causes the cement to unsound.

 

8. Alkaline:
   
   
• It should not be present more than 1%.
• Excess Alkaline matter causes efflorescence.

TYPES OF CEMENT:

Cement is a binding material which makes a bond between aggregates and reinforcing materials together. With the development of technology, quality and types of cement have also developed. So there are different types of cement for different construction works.

Cement is mainly classified into two categories depending on the hardening and setting mechanism. These are- 

1. Hydraulic Cement
2. Non-hydraulic Cement

Apart from these main types, depending on the composition and characteristics there are many types of cement.  Followings are the other cement types:

1. Ordinary Portland Cement (OPC)
2. Portland Pozzolana Cement (PPC)
3. Rapid Hardening Cement
4. Quick Setting Cement
5. Low Heat Cement
6. Sulphate Resisting Cement
7. Blast Furnace Cement
8. High Alumina Cement
9. White Cement

Brief descriptions of these cement types with their uses are given below.

Hydraulic Cement

As the name indicates, hydraulic cement is those which harden by hydration in the presence of water. Limestone, clay, and gypsum are the main raw material to produce non-hydraulic cement. This raw material is burned at a very high temperature to manufacture Hydraulic Cement.

Hydraulic cement (cement that not only hardens by reacting with water but also forms a water-resistant product) produced by pulverizing clinkers which consist essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulfate as an inter ground addition.

Non-Hydraulic Cement

The non-hydraulic cement doesn't require water to get harden. It gets with the help of carbon dioxide (CO₂) from the air. This type of cement needs dry conditions to harden. Lime, gypsum plasters, and oxychloride are the required raw material to produce non-hydraulic cement.  Example: slaked lime is a non-hydraulic cement.

1. Ordinary Portland Cement (OPC)

Ordinary Portland cement is the most widely used type of cement, which is suitable for all general concrete construction. It is the most commonly produced and used type of cement around the world, with annual global production of around 3.8 million cubic meters per year.  This cement is suitable for all kinds of concrete construction.

he composition of Ordinary Portland Cement:

• Argillaceous or silicates of alumina (clay and shale)
• Calcareous or calcium carbonate (limestone, chalk, and marl)

Uses of Ordinary Portland Cement

• It is used for general construction purposes.
• It is also used in most of the masonry works.

2. Portland Pozzolana Cement (PPC)

To prepared PPC or Portland Pozzolana cement, you need to grind pozzolanic clinker with Portland cement.

PPC has a high resistance to different chemical assaults on concrete. It is widely used in construction such as:

·         Marine structures

·         Sewage works

·         Bridges

·         Piers

·         Dams

·         Mass concrete works

3. Rapid Hardening cement:

Rapid hardening cement is very similar to ordinary Portland cement (OPC). The early strength is achieved by adding excessive C₃S in the mix and by lowering the C₂S content in the cement. As the name itself resembling that, this type of cement is used where there is a need for high early strength.Ex.Pavements,busiestroadways
The strength of Rapid Hardening cement at age of 3 days is almost same as the 7 days strength of Ordinary Portland cement. It requires same water-cement ratio as OPC. This type of cement is not used for massive concrete constructions.

4.Quick Setting Cement

Quick setting cement is the cement which sets in a very short time. The initial setting time is 5 minutes and the final setting time is 30 minutes. The composition of Quick Setting Cement:

• Clinker
• Aluminum sulfate (1% to 3% by weight of clinker)
• The aluminum sulfate increases the hydration rate of silicate.

Uses of Quick Setting Cement

• It is used in underwater construction.
• It is also used in rainy & cold weather conditions.
• It is used a higher temperature where water evaporates easily.
• Used for anchoring or rock bolt mining and tunneling

5.Low Heat Cement

It is a spatial type of cement which produces low heat of hydration during the setting. Some chemical composition of Ordinary Portland Cement is modified to reduce the heat of hydration. The chemical composition of low heat cement:

• A low percentage (5%) of tricalcium aluminate (C3A)
• A higher percentage (46%) of declaiming silicate (C2S).

Uses of Low Heat Cement

• It is used for the construction of dam’s large footing, large raft slabs, and wind turbine plinths.
• It is also used for the construction of chemical plants.

6. Sulfates Resisting Cement

This type of cement is manufactured to resist sulfate attack in concrete. It has a lower percentage of Tricalcium aluminate. 

Sulfates resisting cement is used for constructions in contact with soil or groundwater having more than 0.2% or 0.3% g/l sulfate salts respectively.

It can also be used in concrete surfaces subjected to alternate wetting and drying like bridge piers. 

7. Blast Furnace Cement

Portland cement clinker and granulated blast furnace slag are intergraded to make blast furnace cement. A maximum of 65 percent of the mixture could be comprised of blast furnace slag.

Uses of Blast Furnace Cement

• It is highly sulfate resistant
• Frequently used in seawater construction.

8.High Alumina Cement

High Alumina cement is obtained by mixing calcining bauxite (it’s an aluminum ore) and ordinary lime with clinker during the manufacture of OPC. In which the total amount of alumina content should not be lesser than 32% and it should maintain the ratio by weight of alumina to the lime between 0.85 to 1.30.

Uses of High Alumina Cement

• It is used where concrete structures are subjected to high temperatures like workshops, refractory, foundries, etc.
• It also used where the concrete is subjected to frost and acidic action.

9.White Cement

White cement is quite similar to Ordinary Portland Cement except for color. Amounts of iron oxide and manganese oxide are low in White Cement. It is expensive then OPC so not economical for ordinary work.

Uses of White Cement

• It is usually used in decorative work.
• It can also use for traffic barriers, tile grouts, swimming pools, roof tiles patching materials, and terrazzo surfaces.

Coloured Cement:-

• This type of cement is also known as Colcrete. It is manufactured by adding coloring pigment to the OPC. it is used in joining tiles and different decorative work.