Cement: Cement is the mixture of calcareous, siliceous, argillaceous and other substances. Cement is used as a binding material in mortar, concrete, etc.
Chemical Composition of cement is:Lime 63%Silica 22%Alumina 06%Iron oxide 03%Gypsum 01 to 04%
MANUFACTURING OF CEMENT: • (1) Mixing and crushing of raw materials • Dry process • Wet process • (2) Burning • (3) Grinding
(a) Dry process: In this process calcareous material such as lime stone (calcium carbonate) and argillaceous material such as clay are ground separately to fine powder in the absence of water and then are mixed together in the desired proportions. Water is then added to it for getting thick paste and then its cakes are formed, dried and burnt in kilns. This process is usually used when raw materials are very strong and hard. In this process, the raw materials are changed to powdered form in the absence of water.
(b) Wet process: In this process, the raw materials are changed to powdered form in the presence of water. In this process, raw materials are pulverized by using a Ball mill, which is a rotary steel cylinder with hardened steel balls. When the mill rotates, steel balls pulverize the raw materials which form slurry (liquid mixture). The slurry is then passed into storage tanks, where correct proportioning is done. Proper composition of raw materials can be ensured by using wet process than dry process. Corrected slurry is then fed into rotary kiln for burning.
(b) Wet Process:Continued------ This process is generally used when raw materials are soft because complete mixing is not possible unless water is added. Actually the purpose of both processes is to change the raw materials to fine powder.
(2) Burning:Corrected slurry is feed to rotary kiln, which is a 150-500 feet long, 8-16 feet in diameter and temperature arrangement is up to 1500-1650 degree C.At this temperature slurry losses moisture and forms into small lumps, after that changes to clinkers. Clinkers are cooled in another inclined tube similar to kiln but of lesser length.
(3) Grinding: Now the final process is applied which is grinding of clinker, it is first cooled down to atmospheric temperature. Grinding of clinker is done in large tube mills. After proper grinding gypsum (Calcium sulphate Ca SO4) in the ratio of 01-04 % is added for controlling the setting time of cement. Finally, fine ground cement is stored in storage tanks from where it is drawn for packing.
TYPES OF CEMENT: • Ordinary Portland Cement • Sulphate Resisting Cement • Rapid Hardening Cement (or) High Early Strength cement • Quick Setting Cement • Low Heat Cement • High Alumina Cement • Air Entraining Cement • White Cement
(1) ORDINARY PORTLAND CEMENT: It is the variety of artificial cement. It is called Portland cement because on hardening (setting) its colour resembles to rocks near Portland in England. It was first of all introduced in 1824 by Joseph Asp din, a bricklayer of Leeds, England.
Chemical Composition of O.P.Cement: • O.P.C has the following approximate chemical composition: • The major constituents are: • Lime (CaO) 60- 63% • Silica (SiO2) 17- 25% • Alumina (Al2O3) 03- 08%
Chemical Composition of O.P.Cement:Continued------- The auxiliary constituents are: • Iron oxide (Fe2O3) 0.5- 06% • Magnesia (MgO) 1.5- 03% • Sulphur Tri Oxide (SO3) 01- 02% • Gypsum 01 to 04%
Functions of Cement Manufacturing Constituents
(i) Lime (CaO):Lime forms nearly two-third (2/3) of the cement. Therefore sufficient quantity of the lime must be in the raw materials for the manufacturing of cement. Its proportion has an important effect on the cement. Sufficient quantity of lime forms di-calcium silicate (C2SiO2) and tri-calcium silicate in the manufacturing of cement.Lime in excess, causes the cement to expand and disintegrate.
(ii) Silica (SiO2): The quantity of silica should be enough to form di-calcium silicate (C2SiO2) and tri-calcium silicate in the manufacturing of cement. Silica gives strength to the cement. Silica in excess causes the cement to set slowly.
(iii) Alumina (Al2O3): Alumina supports to set quickly to the cement. It also lowers the clinkering temperature. Alumina in excess, reduces the strength of the cement.
(v) Magnesia (MgO): • It also helps in giving colour to the cement. Magnesium in excess makes the cement unsound.
(vi) Calcium Sulphate (or) Gypsum (Ca SO4) : • At the final stage of manufacturing, gypsum is added to increase the setting of cement.
(2) SULPHATE RESISTING CEMENT: It is modified form of O.P.C and is specially manufactured to resist the sulphates. In certain regions/areas where water and soil may have alkaline contents and O.P.C is liable to disintegrate, because of unfavourable chemical reaction between cement and water, S.R.C is used. This cement contains a low %age of C3A not more than 05%. This cement requires longer period of curing. This cement is used for hydraulic structures in alkaline water and for canal and water courses lining. It develops strength slowly, but ultimately it is as strong as O.P.C.
(3) RAPID HARDENING CEMENT: This cement contains more %age of C3S and less %age of C2S. This is infact high early strength cement. The high strength at early stage is due to finer grinding, burning at higher temperature and increased lime content. The strength obtained by this cement in 04 days is same as obtained by O.P.C in 14 days. This cement is used in highway slabs which are to be opened for traffic quickly. This is also suitable for use in cold weather areas. One type of this cement is manufactured by adding calcium chloride (CaCl2) to the O.P.C in small proportions. Calcium chloride (CaCl2) should not be more than 02%. When this type of cement is used, shuttering material can be removed earlier.
(4) QUICK SETTING CEMENT: When concrete is to be laid under water, quick setting cement is to used. This cement is manufactured by adding small %age of aluminum sulphate (Al2SO4) which accelerates the setting action. The setting action of such cement starts with in 05 minutes after addition of water and it becomes stone hard in less than half an hour.
(5) LOW HEAT CEMENT: • In this cement the heat of hydration is reduced by tri calcium aluminate (C3 A ) content. It contains less %age of lime than ordinary port land cement. It is used for mass concrete works such as dams etc.
(6) HIGH ALUMINA CEMENT: • This cement contains high aluminate %age usually between 35-55%. It gains strength very rapidly with in 24 hours. It is also used for construction of dams and other heavy structures. It has resistance to sulphates and action of frost also.
(7) AIR ENTRAINING CEMENT: This type of cement was first of all developed in U.S.A to produce such concrete which would have resistance to weathering actions and particularly to the action of frost. It is found that entrainment of air or gas bubbles while applying cement, increases resistance to frost action. Air entraining cement is produced by grinding minute air entraining materials with clinker or the materials are also added separately while making concrete. Entrainment of air also improves workability and durability. It is recommended that air contents should be 03-04 % by volume. Natural resins, fats, oils are used as air entraining agents.
(8) WHITE CEMENT: This cement is called snowcrete. As iron oxide gives the grey colour to cement, it is therefore necessary for white cement to keep the content of iron oxide as low as possible. Lime stone and china clay free from iron oxide are suitable for its manufacturing. This cement is costlier than O.P.C. It is mainly used for architectural finishing in the buildings.
TO CHECK THE QUALITY OF CEMENT IN THE FILED: • Colour greenish grey. • One feels cool by thrusting one’s hand in the cement bag. • It is smooth when rubbed in between fingers. • A handful of cement thrown in a bucket of water should float.
QUALITY TESTS OF CEMENT:(1) Fineness Test,(2) Consistency test / setting time test(3) Setting Time Test(4) Compressive strength test
(1) Fineness Test: Finer cements react quicker with water and develop early strength, though the ultimate strength is not affected. However finer cements increase the shrinkage and cracking of concrete. The fineness is tested by: By Sieve analysis: Break with hands any lumps present in 100 grams of cement placed in IS sieve No.9 and sieve it by gentle motion of the wrist for 15 minutes continuously. The residue when weighed should not exceed 10 percent by weight of the cement sample.
(2) Consistency Test /Setting Time Test : This test is performed to determine the quantity of water required to produce a cement paste of standard or normal consistency. Standard consistency of cement paste may be defined as the consistency which permits the Vicate’s plunger (10 mm, 40 to 50 mm in length) to penetrate to a point 5 mm to 7 mm from the bottom ( or 35 mm to 33 mm from top) of Vicat mould. When the cement paste is tested within the gauging time ( 3 to 5 minutes) after the cement is thoroughly mixed with water. Vicat apparatus is used for performing this test.
(3) Setting Time Test: In cement hardening process, two instants are very important, i.e. initial setting and final setting.
Initial Setting Time: The process elapsing between the time when water is added to the cement and the time at which the needle ( 1 mm square or 1.13 mm dia., 50 mm in length) fails to pierce the test block ( 80 mm dia. and 40 mm high) by about 5 mm, is known as Initial Setting Time of Cement.
Final Setting Time: The process elapsing between the time when water is added to the cement and the time at which a needle used for testing final setting upon applying gently to the surface of the test block, makes an impression thereon, while the attachment of the needle fails to do so, is known as final Setting Time of Cement.
(4) Compressive Strength test of Cement: This test is very important. In this test, three moulds of (face area 50 cm2) are prepared and cured under standard temperature conditions and each cube tested by placing it between movable jaws of the compressive strength testing machine. The rate of increasing load is zero in the beginning and varies at 350 kg/cm2 per minute. The load at which the cube gets fractured divided by the cross sectional area of the cube, is the compressive strength of the cube. The average of the compressive strengths of three cubes is the required compressive strength of the cement sample.