LESSON 7

• Cement is a binder, a substance that sets and hardens and can bind other materials together.
• Cement is the mixture of calcareous, siliceous, argillaceous and other substances.
• Cement is the principal binding material of modern time.
• Portland cement is the most common type of cement in general use around the world, used as a basic ingredient of concrete and mortar
• Lime forms nearly two-third (2/3) of 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.
• The quantity of silica should be enough to form di-calcium silicate and tri-calcium silicate in the manufacturing of cement.
• Silica gives strength to the cement.
• Silica in excess causes the cement to set slowly
• Alumina supports to set quickly to the cement.
• Alumina also lowers the clinkering temperature.
• Alumina in excess, reduces the strength of the cement.
• Iron oxide gives color to the cement.
• Iron Oxide gives hardness and strength to the cement.
• Magnesia also helps in giving colour to the cement.
• Magnesium in excess makes the cement unsound.
• At the final stage of manufacturing, gypsum is added to increase the setting of cement.
• (MINERAL COMPOUNDS FOUND IN PORTLAND CEMENT) Four main phases present in the clinker or non-hydrated Portland cement.

  C3S, C2S, C3A, C4AF
• HYDRATED CEMENT PASTE:


  HYDRATION PRODUCTS FORMED IN HARDENED CEMENT PASTES (HCP) ARE MORE COMPLICATED, BECAUSE MANY OF THESE PRODUCTS HAVE NEARLY THE SAME FORMULA AND SOME ARE SOLID-SOLUTIONS WITH OVERLAPPING FORMULA.
• Quarrying of limestone and shale is accomplished by using explosives to blast the rocks from the ground.
• The ocean floor is dredged to obtain the shells, while clay and marl are dug out of the ground with power shovels.
• After blasting, Excavators (or power shovels) are used to load dump trucks or small railroad cars for transportation to the cement plant, which is usually nearby.
• Grinding


  After the raw materials have been transported to the plant, the limestone and shale which have been blasted out of the quarry must be crushed into smaller pieces. Some of the pieces, when blasted out, are quite large. The pieces are then dumped into primary crushers which reduce them to the size of a softball.
• Grinding


  After that the pieces are carried by conveyors to secondary crushers which crush the rocks into fragments usually no larger than 3/4 inch across.
• WET PROCESS - In this process, the raw materials are changed to powdered form in the presence of water.
• WET PROCESS - 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).
• WET PROCESS - This process is generally used when raw materials are soft because complete mixing is not possible unless water is added.
• WET PROCESS - Actually the purpose of both processes is to change the raw materials to fine powder.
• The dry process of fine grinding is accomplished with a similar set of ball or tube mills; however, water is not added during the grinding. The dry materials are stored in silos where additional mixing and blending may be done.
• 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.
• DRY PROCESS - In this process, the raw materials are changed to powdered form in the absence of water.
• The dry materials exiting either type of mill are called "kiln feed"
• In wet process, water is added to ingredients before mixing them and in dry process, water is added at the time of mixing