cement

cement

cements are the binding materials. its bind one to the other materials. so its known as the cement.

 composition of the cement

Introduction
Portland cement gets its strength from chemical reactions between the cement and water.this process known as hydresion.

this is a complex process that is best understood by first understanding the chemical composition of cement.
Manufacture of cement
Portland cement is manufactured by crushing, milling and proportioning the following materials:

• Lime or calcium oxide, CaO: from limestone, chalk, shells, shale or calcareous rock
• Silica, SiO₂: from sand, old bottles, clay or argillaceous rock
• Alumina, Al₂O₃: from bauxite, recycled aluminum, clay
• Iron, Fe₂O₃: from from clay, iron ore, scrap iron and fly ash
• Gypsum, CaSO₄.2H₂0: found together with limestone

The materials, without the gypsum, are proportioned to produce a mixture with the desired chemical composition and then ground and blended by one of two processes - dry process or wet process. The materials are then fed through a kiln at 2,600º F to produce grayish-black pellets known as clinker. The alumina and iron act as fluxing agents which lower the melting point of silica from 3,000 to 2600º F. After this stage, the clinker is cooled, pulverized and gypsum added to regulate setting time. It is then ground extremely fine to produce cement.Chemical shorthand
Because of the complex chemical nature of cement, a shorthand form is used to denote the chemical compounds. The shorthand for the basic compounds is:

Compound	Formula	Shorthand form
Calcium oxide (lime)	Ca0	C
Silicon dioxide (silica)	SiO2	S
Aluminum oxide (alumina)	Al2O3	A
Iron oxide	Fe2O3	F
Water	H2O	H
Sulfate	SO3	S

 Chemical composition of clinker
The cement clinker formed has the following typical composition:
 

Compound	Formula	Shorthand form	% by weight1
Tricalcium aluminate	Ca3Al2O6	C3A	10
Tetracalcium aluminoferrite	Ca4Al2Fe2O10	C4AF	8
Belite or dicalcium silicate	Ca2SiO5	C2S	20
Alite or tricalcium silicate	Ca3SiO4	C3S	55
Sodium oxide	Na2O	N	) )Up to 2
Potassium oxide	K2O	K
Gypsum	CaSO4.2H2O	CSH2	5

Representative weights only. Actual weight varies with type of cement.
Source: Mindess & YoungProperties of cement compounds
These compounds contribute to the properties of cement in different ways

Tricalcium aluminate, C₃A:-

It liberates a lot of heat during the early stages of hydration, but has little strength contribution. Gypsum slows down the hydration rate of C₃A. Cement low in C₃A is sulfate resistant.
 

Tricalcium silicate, C₃S:-

This compound hydrates and hardens rapidly. It is largely responsible for portland cement’s initial set and early strength gain.
 

Dicalcium silicate, C₂S:

C₂S hydrates and hardens slowly. It is largely responsible for strength gain after one week.
 

Ferrite, C₄AF:

This is a fluxing agent which reduces the melting temperature of the raw materials in the kiln (from 3,000^o F to 2,600^o F). It hydrates rapidly, but does not contribute much to strength of the cement paste.By mixing these compounds appropriately, manufacturers can produce different types of cement to suit several construction environments.

Type of cement 

13 Types of Cement and their Uses

1. Ordinary Portland Cement (OPC)
2. Portland Pozzolana Cement (PPC)
3. Rapid Hardening Cement
4. Quick setting cement
5. Low Heat Cement
6. Sulphates resisting cement
7. Blast Furnace Slag Cement
8. High Alumina Cement
9. White Cement
10. Coloured cement
11. Air Entraining Cement
12. Expansive cement
13. Hydrographic 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 most widely 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 type of concrete construction.

2. Portland Pozzolana Cement (PPC)

Portland pozzolana cement is prepared by grinding pozzolanic clinker with Portland cement. It is also produced by adding pozzolana with the addition of gypsum or calcium sulfate or by intimately and uniformly blending portland cement and fine pozzolana.

This cement has high resistance to various chemical attacks on concrete compared with ordinary portland cement and thus it is widely used. It is used in marine structures, sewage works, sewage works and for laying concrete under water such as bridges, piers, dams and mass concrete works etc.

3. Rapid Hardening Cement

Rapid hardening cement attains high strength in early days it is used in concrete where formworks are removed at an early stage and is similar to ordinary portland cement (OPC). This cement has increased lime content and contains higher c3s content and finer grinding which gives greater strength development than OPC at an early stage.

The strength of rapid hardening cement at the 3 days is similar to 7 days strength of OPC with the same water-cement ratio. Thus, advantage of this cement is that formwork can be removed earlier which increases the rate of construction and decreases cost of construction by saving formwork cost .

Rapid hardening cement is used in prefabricated concrete construction, road works, etc.

4. Quick setting cement

The difference between the quick setting cement and rapid hardening cement is that quick setting cement sets earlier while rate of gain of strength is similar to Ordinary Portland Cement, while rapid hardening cement gains strength quickly. Formworks in both cases can be removed earlier.

Quick setting cement is used where works is to be completed in very short period and for concreting in static or running water.

5. Low Heat Cement

Low heat cement is prepared by maintaining the percentage of tricalcium aluminate below 6% by increasing the proportion of C2S. This makes the concrete to produce low heat of hydration and thus is used in mass concrete construction like gravity dams, as the low heat of hydration prevents the cracking of concrete due to heat.

This cement has increased power against sulphates and is less reactive and initial setting time is greater than OPC.

6. Sulphates Resisting Cement

Sulfate resisting cement is used to reduce the risk of sulphate attack on concrete and thus is used in construction of foundations where soil has high sulphate content. This cement has reduced contents of C3A and C4AF.

Sulfate resisting cement is used in construction exposed to severe sulphate action by water and soil in places like canals linings, culverts, retaining walls, siphons etc.

7. Blast Furnace Slag Cement

Blast furnace slag cement is obtained by grinding the clinkers with about 60% slag and resembles more or less in properties of Portland cement. It can be used for works economic considerations is predominant.

8. High Alumina Cement

High alumina cement is obtained by melting mixture of bauxite and lime and grinding with the clinker. It is a rapid hardening cement with initial and final setting time of about 3.5 and 5 hours respectively.

The compressive strength of this cement is very high and more workable than ordinary portland cement and is used in works where concrete is subjected to high temperatures, frost, and acidic action.

9. White Cement

It is prepared from raw materials free from Iron oxide and is a type of ordinary portland cement which is white in color. It is costlier and is used for architectural purposes such as precast curtain wall and facing panels, terrazzo surface etc. and for interior and exterior decorative work like external renderings of buildings, facing slabs, floorings, ornamental concrete products, paths of gardens, swimming pools etc.

10. Colored cement

It is produced by mixing 5- 10% mineral pigments with ordinary cement. They are widely used for decorative works in floors.

11. Air Entraining Cement

Air entraining cement is produced by adding indigenous air entraining agents such as resins, glues, sodium salts of sulphates etc. during the grinding of clinker.

This type of cement is especially suited to improve the workability with smaller water cement ratio and to improve frost resistance of concrete.

12. Expansive Cement

Expansive cement expands slightly with time and does not shrink during and after the time of hardening . This  cement is mainly used for grouting anchor bolts and prestressed concrete ducts.

13. Hydrographic cement

Hydrographic cement is prepared by mixing water repelling chemicals and has high workability and strength. It has the property of repelling water and is unaffected during monsoon or rains. Hydrophobic cement is mainly used for the construction of water structures such dams, water tanks, spillways, water retaining structures.