Cement: Ingredients of cement, Properties of good Cement and Bogue compounds

Cement

A cement is a binding material, used for construction that sets, hardens, and adheres( जुड़ जाना) to other materials to bind them together.
Cements in a general sense are adhesive and cohesive materials which are capable of bonding together particles of solid matter into a compact durable mass.
Cement is an extremely fine material having adhesive and cohesive properties which provide a binding medium for the discrete ingredients.
Cement is the mixture of calcareous, siliceous, argillaceous and other substances.
The cement is a product obtained by pulverizing (to make into a powder form, पीसना) clinker formed by calcinating the raw material preliminary(प्रारंभिक) consisting of Lime (CaO), Silicate (SiO2), Alumina (Al₂O) and Iron oxide (FeO3).
Cement is seldom(कभी-कभी) used on its own, but rather to bind fine aggregate (sand) and coarse aggregate (gravel) together. Cement mixed with fine aggregate produces mortar for masonry, or with fine aggregate and coarse aggregate, produces concrete.
Cements used in construction are usually inorganic, often lime or calcium silicate based, which can be characterized as hydraulic or the less common non-hydraulic, depending on the ability of the cement to set in the presence of water (see hydraulic and non-hydraulic lime plaster).

Hydraulic cements (e.g., Portland cement) set and become adhesive through a chemical reaction between the dry ingredients and water. The chemical reaction results in mineral hydrates that are not very water-soluble and so are quite durable in water and safe from chemical attack. This allows setting in wet conditions or under water and further protects the hardened material from chemical attack. The chemical process for hydraulic cement was found by ancient Romans who used volcanic ash (pozzolana) with added lime (calcium oxide).
Non-hydraulic cement (less common) does not set in wet conditions or under water. Rather, it sets as it dries and reacts with carbon dioxide in the air. It is resistant to attack by chemicals after setting.
Cement can be manufactured either from natural cement stones or artificially by using calcareous and argillaceous materials. The examples of natural cements are Roman cement, Puzzolana cement and Medina cement and those of artificial cement are Portland cement and special cements.
The natural cement is obtained by burning and crushing the stones containing clay, carbonate of lime and some amount of carbonate of magnesia. The clay content in such stones is about 20 to 40 per cent. The natural cement is brown in colour and its best variety is known as the Roman Cement. The natural cement resembles very closely eminent hydraulic lime. It sets very quickly after addition of water. It is not so strong as artificial cement and hence it has limited use in practice.
The artificial cement in obtained by burning at a very high temperature a mixture of calcoreous and argillaceous materials. The mixture of ingredients should be intimate and they should be in correct proportion. The calcined product is known as the clinker. A small quantity of gypsum is added to the clinker and it is then pulverised into very fine powder which is known as the cement.

Ingredients of cement and their Functions
OR
Constituents of cement / Chemical Composition of Cement

The main ingredients in the manufacturing of cement are lime, silica, alumina and other oxides.

S.No.	Ingredients	Range of percentage
1.	Lime(CaO)	60–65
2.	Silica(SiO₂)	17–25
3.	Alumina(Al₂O₃)	3.5–9.0
4.	Iron oxide(Fe₂O₃)	0.5–6.0
5.	Magnesium oxide(MgO)	0.5–4.0
6.	Sulphur trioxide(SO₃)	1–2.0
7.	Alkalis (soda and or potash), K₂O and Na₂O	0.4–1.3

The ingredients of ordinary cement perform the following functions:

1. Lime -

It is the main constituent which contributes to the strength. Its proportion has to be properly maintained so as to make the cement sound and strong.

Excess of lime makes the cement unsound and causes the cement to expand and disintegrate. On the other hand, if lime is deficient, the strength of the cement is reduced and the cement sets quickly.

2. Silica -

It contributes to the strength of cement due to the formation of dicalcium and tricalcium silicates.

The excess presence of silica prolongs(को बढाता है) the setting time of cement but increases its strength.

3. Alumina -

This ingredient imparts quick-setting property to the cement.

Excess alumina reduces the strength of cement.

4. Iron Oxide -

It provides colour, hardness and strength to the cement.

5. Magnesium Oxide -

The correct quantity of magnesium oxide imparts hardness and colour to the cement.

Soundness of cement is seriously affected, if it is present in excess quantity.

6. Sulphur Trioxide -

It makes the cement sound when present in small quantities. Excess of sulphur trioxide causes the cement to become unsound.

7. Alkalis -

Alkalis should be present only in small quantities. Excess quantities of alkalis cause efflorescence and straining when used in concrete or mortar.

Properties of good Cement -

1. Cement should have a uniform color.

2. Cement should be free from lumps.

3. It should be smooth and uniform. As a test, if a small quantity of cement is thrown in water, it should sink completely.

4. The ratio of percentage of alumina to that of iron oxide should not be less than 0.66 in cement.

5. The total Sulphur content in cement should not be greater than 2.75%.

6. Magnesia content in cement should not exceed 5% by weight.

7. Insoluble residue in cement should not be greater than 1.5% by weight.

8. Cement should not lose more than 4% of its weight when heated.

9. The specific surface of cement as found from the fineness test should not be less than 2,250 mm2/gm.

10. The initial setting of cement should be about 30 minutes, and the final setting Time should be about 10 hours.

11. Expansion of cement should not be greater than 10 mm in a soundness test.

Setting action of cement -

When water is added to the cement, the ingredients of cement react chemically with water and form various complicated chemical compounds. The formation of these compounds is not simultaneous. But setting action of cement continues for a long time. The mixing of cement and water results in a sticky cement paste and it goes on gradually thickening till it achieves a rock like states.

It is found that ordinary cement achieves about 70% of its final strength in 28 days and about 90% of its final strength in one year or so.

When water is added to cement, it react with the ingredients of the cement chemically and results in the formation of complex chemical compounds terms as BOGUES compounds.

The compounds formed in the burning process have the properties of setting and hardening in the presence of water.

They are known as Bogue compounds. Le-Chatelier and Tornebohm have referred these compounds as Alite (C3S), Belite (C2S), Celite (C3A) and Felite (C4AF).

S.no.	Name of Compound	Formula	Name	Symbol
1.	Tricalcium silicate	3CaO.SiO₂	Alite	C₃S
2.	Dicalcium silicate	2CaO.SiO₂	Belite	C₂S
3.	Tricalcium aluminate	3CaO.Al₂O₃	Celite	C₃A
4.	Tetracalcium alumino ferrite	4CaO.Al₂O₃.Fe₂O₃	Felite	C₄AF

The abbreviated notations stand for

C for CaO

S for SiO₂

A for Al₂O₃

F for Fe₂O₃

Tricalcium silicate -

In ordinary cement, the C₃S content is about 30-50 %. The hydrolysis of C₃S is mainly responsible for 7 day strength and hardness. The rate of hydrolysis of C3S and the character of gel developed are the main causes of the hardness and early strength of cement paste. The heat of hydration is 500 J/g.

Dicalcium silicate -

It is about 25-40% (normally about 32 per cent) of cement. It hydrates and hardens slowly and takes long time to add to the strength (after a year or more). It imparts resistance to chemical attack.

The hydrolysis of C₂S proceeds slowly. At early ages, less than a month, C₂S has little influence on strength and hardness. While after one year, its contribution to the strength and hardness is proportionately almost equal to C₃S. The heat of hydration is 260 J/g.

Tricalcium aluminate -

It is about 5-11% (normally about 10.5 per cent) of cement. It rapidly reacts with water. The rapidity of action is regulated by the addition of 2-3% of gypsum at the time of grinding cement. Tricalcium aluminate is responsible for the initial set, high heat of hydration and has greater tendency to volume changes causing cracking. The heat of hydration of 865 J/g.

Tetracalcium alumino ferrite -

It is about 8–14% (normally about 9 per cent) of cement. It is

responsible for flash set but generates less heat. It has poorest cementing value. Raising the C₄AF content reduces the strength slightly. C₄AF does not provide strength but is stable than C₃A. The heat of hydration is 420 J/g.

Types of Cement: Uses and their properties

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