CEMENTS ARE CLASSIFIED BY THEIR PROPERTIES AND CHEMICAL COMPOSITION.

The Principal chemical constituent of cement are 60 to 67 percent lime ,17 to 25 percent silica and 3to 8 percent alumina , which are intimately mixed together with water to form into a slurry ,which is subsequently heated ,dried ,calcined and ground to a very fine powder .A small proportion of gypsum is added before grinding in order to control rate of setting. Portland cement may be made quick or slow setting.If there are special reasons which make quick setting desirable, normal setting cement should be used as quick setting cement require exceptional care and skill in handling. There are mainly five kinds of cement and cement are classified by their properties and chemical composition. Ordinary Portland cement is the most commonly used cement and used for general engineering works .Other cements with different properties are used for particular purposes .Rapid hardening cement is in fact, a high early strength cement and has the same composition as common cement but is ground more finally and is used where high early strength is required.It sets and hardens in a much shorter time than the common cement and develops higher strength in the early stages ,but the ultimate strength is the same as of the normal setting cement The advantage of this cement is that over the other common cement, that form work can be removed earlier and the structure can be loaded earlier It has in 4 days the same compressive strength as common cement in 28 days .It is manufactured in India in limited quantities only and it's use is also very limited however it is costlier than that of the common cement.The setting time is about the same as of the ordinary cement and is useful for the repair work and is also used for the road work where it is imperative to open the road to traffic with minimum of delay. Quick setting cement sets initially after about five minutes and sets finally in about 30 minutes .It's use are generally restricted to works in running water.The quick setting action of this cement allows very little time for mixing, placing and compacting of the concrete.