Main Physical and Chemical Properties of (OPC) cement

Cement

The cement is highly used in construction, without cement the construction is not may be possible. The cement is extremely fine material rather than the other type of construction materials its having some physical and some chemical properties, which provides a binding medium for the discrete ingredients. The important physical properties of OPC (Ordinary Portland Cement) can be mainly two types given below.

1. Physical properties of cement
2. Chemical properties of cement

1. Physical properties of cement

Based upon the physical properties of cement, it is possible to draw a comparison for the quality of cement from different sources. The mainly physical properties some key parameters control the quality of cement. The main physical properties of good cement are discussed below. 

1. Fineness of cement
2. Initial and final setting time of cement
3. Soundness of cement
4. Compressive strength of cement
5. The specific gravity of cement
6. The heat of hydration of cement
7. Bulk density of cement

1. The fineness of cement

The fineness of cement is a measure of the particle size of cement expressed as the specific surface of the cement.

It offers a greater surface area for hydration and hence faster is the rate of reaction which results in faster development of strength. Too much fineness is also undesirable as it generates greater he which results in cracking of concrete.

2. Initial and final setting time of cement

An arbitrary division has made for the setting time of cement as initial setting time and final setting time. Cement when also mainly mixed with water forms paste which gradually becomes less plastic, and at last a tough mass is obtained. During this process of setting, a stage is reached when the freshly prepared cement paste is enough rigid to withstand a definite amount of pressure. The time to achieve this stage is known as setting time. 

The time at which cement paste loses its plasticity is called mainly initial setting time and therefore the time at which the paste becomes solid hard is termed as final setting time. 

The initial setting time is required for mixing, transportation, placing, and compacting of concrete. During the time of the initial setting, the temperature of the combination rises and final setting time is achieved. The values of setting time as specified for various kinds of cement initial setting time is 30 minutes and the final setting time is also 10 hours.

3. The soundness of cement

It is extremely important that the cement after setting shall not undergo any appreciable change in volume. Some types of cement have been found to undergo a large expansion after setting resulting in disruption of the set and hardened mass. This will cause serious problems for the In such cement is used. The testing for soundness of cement is performed to ensure that the cement does not show any appreciable subsequent expansion. 

The unsoundness in cement is due to the presence of an excess of lime even after the final grinding. This free lime hydrates very slowly. The free lime is covered with a thin film of cement. This prevents direct contact of water and lime. After the cement matrix has set, moisture penetrates deep into the free lime, and hydration starts thus resulting in cracks after some time. The unsoundness is generally indicated by the expansion of cement paste by the Le-Chatelier method. The unsoundness in cement can be controlled by limiting the MgO content to less than 0.5%. Fine grinding of raw ingredients. Thorough mixing. Allowing the cement to aerate for several days. 

Soundness test of cement - The soundness test is very important for cement. Le-Chatelier apparatus is used to perform this test. The value of soundness is not exceeding 10 mm for ordinary, rapid hardening, and low heat portland cement.

4. Compressive strength of cement

This is mainly one of the foremost important properties of cement. Therefore, the cement is always subjected to a compressive strength test before it is used in important works. The strength tests are mainly not made also on neat cement paste because of excessive shrinkage and subsequent cracking. The strength of cement is directly found on cement sand mortar in specific proportions (generally taken as 1 : 3). 

The compressive strength of hardened cement is the most important of all the properties. Therefore, cement is often tested for its strength at the laboratory before the cement is used in important construction works always. A compression testing machine is employed to perform this test. The compressive strength when standard sand is used.

After 3 days - (33 grade) cement is 16 N/mm²  (43 grade) is 23 N/mm²  (53 grade) is 27 N/mm²

After 7 days - (33 grade) cement is 22 N/mm²  (43 grade) is 33 N/mm²  (53 grade) is 37 N/mm²

After 28days - (33 grade) cement is 33 N/mm²  (43 grade) is 43 N/mm²  (53 grade) is 53 N/mm²

5. The specific gravity of cement

The specific gravity of cement is generally used in calculation proportions (i.e. Design Mix). The specific gravity of portland cement is about 3.15, but cement manufactured from materials aside from limestone and clay, the value may fluctuate. 

6. The heat of hydration of cement

The chemical reaction between cement and water is termed hydration of cement. The reaction of cement and water is exothermic i.e., considerable quantity heat is generated during hydration of cement. This heat is understood because of the heat of hydration. 

The heat of hydration may be defined as the quantity of heat, in calories per gram of hydrated cement, generated during complete hydration at a particular temperature. It is estimated that about 120 calories of heat are generated in the hydration of Ig of cement. The heat of hydration is an important property, and much attention has been paid to the heat evolved during the hydration of cement in the interior of mass concrete works (such as dams, retaining walls, etc.) 

The heat of hydration increases with the temperature at which hydration takes place. The rate of heat generation and the quantity of heat depends on the chemical composition of cement.

7. Bulk density

This is not very important. When cement is mixed with water, the water replaces areas where there would normally be air. Cement has a varying range of density depending on the cement composition percentage. The density of cement may be anywhere from 28 to 35 kilograms per cubic foot.

2. Chemical Properties of Cement

The difference within the various forms of ordinary portland cement is due to the variations in relative proportions of these always chemical compounds in the cement. The properties of chemical compounds and the function are discussed below the following points.

1. Tetracalcium Alumino ferrite (C₄AF)

• It is the compound that is formed first in the kiln.
• It contributes a little to the strength of cement.
• It hydrates rapidly.
• It generates less heat of hydration.
• It is very weak against sulphate attack.
• It has poor cementing value and hence does not contribute to developing ultimate strength.

2. Tricalcium Aluminate (C₃A)

• It is the compound that is formed second in the kiln.
• It reacts fast with water.
• It does not contribute to developing the ultimate strength.
• It causes the initial setting of cement and helps in early strength development.
• It is weak against sulphate attack.
• It generates a large amount of heat of .hydration.

3. Dicalcium Silicate (C₂S) 

• Dicalcium silicate is third to form in the kiln.
• It hydrates slowly.
• It generates less amount of heat of hydration.
• It hardens slowly and hence contributes very little in early strength development of cement.

4. Lime (CaO)

Lime is one of the major constituents of cement and is added in a range of 60 - 65%. the important function of lime is it providing strength and soundness to cement.

• In excess - it makes the cement unsound and causes the cement to expand and finally disintegrate.
• If deficient - it reduces the strength of cement and also reduces the setting time of cement (I.e., quick setting time of cement).

5. Silica (SiO₂) - it is another important ingredient of cement and generally varies between 17 - 25%. the function of silica is that it may be the help of providing the proper strength of cement.

• In excess - It causes the cement set slowly.

6. Alumina (Al₂O₃) - The main function of alumina is it lowering the temperature of clinker formation. It is more helpful in quick setting time of cement.

• In excess - It lowers the strength of cement.

7. Iron Oxide (Fe₂O₃) - The main function of Iron Oxide is its help in the fusion of raw materials during the manufacturing of cement. It may provide the colour to cement. It gives hardness property. It provides the strength of cement.

8. Magnesium oxide (MgO) - The main function of magnesium oxide are maybe provide the colour to cement. It provides the hardness of cement.

• In excess - the provide in excess makes the cement unsound.

9. Sulphur trioxide (SO₃) - the function of trioxide are 

• In excess - It makes the cement is mainly unsound.
• If deficient - If present in small quantity, it makes the cement sound and durable.

10. Gypsum (CaSO₄) - it is added to the cement at the time of grinding. It is generally added up to 3% in cement manufacturing. It helps to be increasing the initial setting time of cement. It makes cement sound.

11. Alkalies (e.g., Soda and potash) - Through they are desirable in very small quantity but are very much required for the manufacturing of cement.

• In excess - it may cause efflorescence in cement concrete.

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