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Rural Technology

Cement is a fine, soft, powdery-type substance. It is prepared from a mixture of elements that are natural materials such as limestone, clay, sand and/or shale. When cement is mixed with water, it can bind sand and gravel into a hard, solid mass called concrete.

Manufacturing of cement

Cement is prepared from calcium oxide, aluminum oxide, silica, and iron oxide. Calcium (which is the main ingredient) can be obtained from limestone, whereas silicon can be obtained from sand and/or clay. Aluminum and iron are required in small amount and can be extracted from bauxite and iron ore. Following are the steps in cement manufacturing: 1) Mixture of limestone, sand, aluminum oxide and iron oxide is powered and is heated in the furnace. 2) Water present in these materials gets evaporated and the material comes to the melting stage. This partially molten material combines together. 3) The material that comes out of the furnace is large, glassy, red-hot cinders called “clinker”. 4) After cooling the clinker, its powder is made. This powder is called Portland cement.

How cement gets it strength?

When cement is mixed with water, its molecules form bond with each other with the help of water. Because of this chemical bond, the cement gets hardened. Cement is always used to join two things. Concrete is made by mixing sand and gravels in the cement. Cement holds together gravels tightly and the block become a like a stone, though the quantity of cement is less, the concrete become like a stone. Cement must be stored in dry place. If it gets wet due to water or humidity in storage then we cannot used it again by drying it.

Mortar and concrete

Cement is used to join the small stones together. In this process the gaps between this mixtures is filled using cement. To reduce the cost of cement different small sized stones (sand, grovel) are used. The mixture of cement and sand is called mortar. The mixture of cement, sand and gravel is called concrete. Concrete becomes strong as the stones are joined to each other.

Characteristics of cement

1. Once chemical reaction is complete then cement is durable in water. 2. Cement never gets rusted or rotten in normal environment. 3. Cement concrete is strong in compression but weak in tension. Therefore, steel is used in the concrete in places with ten- sion. This is called as R.C.C (Reinforced Cement Concrete). For e.g. columns and slabs of multistoried buildings. 4. There is no effect of normal heat on cement concrete or it never catches fire. But it gets cracked in big fire. 5. Cement is bad conductor of electricity or heat. 6. If water contains in cement mixture is more then after curing, hollows spaces are formed and cement becomes weak. 7. Strength of the cement is more if its density is more.

Curing and Drying

Cement powder solidifies due to the reaction of water on it. Therefore, if sufficient water is not available while using the cement, this process will not get completed and it will not get solidified. Therefore, for first 20 days cement should be kept wet. This is called curing. If excess water is used immediately after applying the cement then small cement particles will washed away. Therefore, on the first day of the construction, care should be taken that the cement will remain wet. Further during the process of solidification, cement needs water but outside air will dry it quickly. Therefore, it is necessary to give sufficient water. Curing takes place quickly if temperature is more. Strength increases with curing: Following table show how strength of concrete increases with time:- The ratio of Mortar/Concrete required for variable work Following is the ratio of cement: sand: gravel for construction: Concrete code and concrete ratio:

Intext Questions

Fill in the blanks: i) Cement is strong in but in tension. ii) Concrete is mixture of . iii) The mixture of cement and sand is called . iv) Cement power solidifies with its reaction with water. This process is called .

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