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Note for CONCRETE TECHNOLOGY - CT By Vinod Shikhare

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Vinod B Shikhare
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1. CEMENT 1.1 OBJECTIVES After studying this unit, you should be able to: 1. Describe the manufacturing process of cement, 2. Explain the chemical composition of cement, 3. Describe the physical properties of cement, 4. Classify the various types of cement explaining their uses, 5. Verify the quality of cement by the field test, and 6. Get acquainted with the laboratory testing of Portland cement. 1.2 DEFINITION OF CEMENT Cement is defined as the product manufactured by burning and crushing to powder an intimate and well-proportioned mixture of calcareous and argillaceous materials. Calcareous Materials The materials which contain calcium or lime as their major constituent are known as calcareous materials. The various calcareous materials used in the manufacture of cement are lime stone, marl, chalk, shells, etc. These materials provide the required proportion of lime to the cement. Argillaceous Materials The argillaceous materials contain alumina as their major constituent. The various argillaceous materials used in the manufacture of cement are shale, clay, laterite, etc. These materials provide the required proportion of silica, alumina, oxide of iron, etc. to the cement. Cement is manufactured in a variety of forms these days. The cement, which is generally used for preparing concrete, is the Ordinary Portland Cement. But for special purposes other qualities of cement such as Low Heat Cement, Rapid Hardening Cement, High Alumina Cement, White Cement, Blast Furnace Slag Cement, Sulphate Resisting Cement, etc. are also used. The selection of a particular type of cement to be used for manufacturing of concrete depends upon the following factors: Concrete Technology/ Cement 1

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(a) The required strength of the concrete structure. (b) The type of structure. (c) The conditions under which the construction of structure is to take place. 1.3 MANUFACTURING PROCESS OF CEMENT The process of manufacture of cement consists of grinding the raw materials (calcareous and argillaceous materials) mixing them intimately in certain proportions and burning them in a kiln at a temperature of about 1500°C at which the material sinters and fuses to form nodular shaped clinker. The clinker is cooled and ground to a fine powder with addition of about 2 to 3 % of gypsum. The product obtained by this procedure is called as Portland cement. There are two processes known as Wet and Dry processes depending upon whether the mixing and grinding of raw materials is done in wet or dry condition. The wet process requires more fuel as slurry contains about 35-50 % water. The dry process requires less fuel as materials are already in dry state. A. Wet Process In this process, the limestone is crushed to smaller fragments and then it is taken to a ball or tube mill where clay or shale is mixed with it and ground to a fine consistency of slurry with the addition of water. The slurry is pumped to slurry tanks where it is kept in an agitated condition by means of rotating arms with chains to prevent setting of limestone and clay particles. At this stage, the chemical composition of slurry is adjusted as necessary. The corrected slurry is stored in storage tanks and kept in homogeneous condition by the agitation of slurry. The corrected slurry is injected at the upper end of a rotary kiln. Rotary kiln is formed of steel tubes. The diameter of rotary kiln varies from 3 m to 8 m and length varies from 30 m to 200 m. The kiln is supported at intervals by columns of masonry or concrete. It is laid at a gradient of about 1 in 25 to 1 in 30. The refractory lining is provided on the inside surface of rotary kiln. It is so arranged that the kiln rotates at about one to three revolutions per minute about its longitudinal axis. The burning is carried out in this rotary kiln. The hot gases or flames are forced through the lower end of the kiln. The portion of the kiln near its upper end is known as dry zone and in this zone; the water of slurry is evaporated. As the slurry gradually descends, there is rise in temperature and in the next section of kiln; the Concrete Technology/ Cement 2

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carbon dioxide from slurry is evaporated. The small lumps, called as nodules, are formed at this stage. These nodules then gradually roll down passing through zones of rising temperature and ultimately reach to the burning zone, where temperature is about 1500°C. In burning zone, the calcined product is formed and nodules are converted into small hard dark greenish blue balls, which are known as clinkers. The size of clinkers varies from 3 mm to 20 mm and they are very hot when they come out of burning zone of kiln. The temperature of clinker at the outlet of kiln is nearly 1000°C. The clinker drops into a rotary cooler where it is cooled under controlled conditions. Figure A: Flow Diagram of Wet Process Concrete Technology/ Cement 3

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The clinkers as obtained from the rotary kiln are finely grounded in ball mills and tube mills. During grinding a small quantity, about 2 to 3% of gypsum is added. If gypsum is not added, the cement would set as soon as water is added. The gypsum controls the initial setting time of cement. The gypsum acts as a retarder and delays the setting action of cement. Thus, gypsum permits cement to be mixed with the aggregates and to be placed in position. A ball mill consists of several compartments charged with progressively smaller hardened steel balls. The particles crushed to the required fineness are separated by currents of air and taken to storage silos from where the cement is bagged or filled into barrels for bulk supply to dams, bridges or other large work sites. B. Dry Process In the dry process, the raw materials are crushed dry and fed in correct proportions into a grinding mill. The raw materials are dried and crushed into a very fine powder. The dry powder is called raw meal. The dry powder is further blended and corrected for its right composition and mixed by using compressed air. The aerated powder tends to behave almost like liquid and in about one hour of aeration a uniform mixture is obtained. The sieved blended meal fed into a rotating disc called as granulator. The pellets of blended meal are formed by adding water approximately 12% to permit air flow for exchange of heat for chemical reactions and conversion of the same into clinker. The dry process is economical. In this method, equipment used is comparatively smaller. The consumption of coal in this method is very low as compared to wet process. In case of mixing of raw materials by dry process, the raw mix is formed and in case of mixing of raw materials by wet process, the slurry is formed. The remaining operations, e.g. burning and grinding are same as that of the wet process. The flow diagram of dry process of cement manufacturing is given in Figure B. Concrete Technology/ Cement 4

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