PORTLAND CEMENT

DEFINITION The hydraulic cement is obtained by burning a mixture of calcareous and argillaceous materials at a high temperature in a kiln, the calcined product is known as clinker. A small quantity of gypsum is added to clinker and it is then pulverised into very fine powder which is known as ordinary portland cement. Portland cement= clinker(pulverised)+gypsum

Portland cement clinker is a hydraulic material which shall consist of at least 2/3 by mass of calcium silicates(C3S ALITE and C2S BELITE) and remainder consisting of aluminium and iron containing clinker phases like C3A, C4AF . Clinkers make up more than 90% of the cement along with a limited amount of gypsum(which control the set time) an up to 5% minor contituents(filler).

Clinkers are nodules having diameter (5-25mm) of a sintered material that is produced when a raw material of predetermined composition is heated to high temperature.

CHRACTERISTICS The colour of cement should be uniformly grey with greenish shade. It gives an indication of excess of lime or clay and the degree of burning. It should feel smooth when touched or rubbed in between fingers. If hand is inserted in a bag of cement, it should feel cool and not warm.

COMPONENTS OF OPC

CONSTITUENTS OF PORTLAND CLINKER CCN MASS% Tricalcium silicate C3S 45-65 Dicalcium silicate C2S 15-35 Dicalcium aluminate C3A 4-14 Tetracalcium aluminoferrite C4AF 10-18 Gypsum 2-10

APPLICATION Portland cement is the most common type of cement in general use around the world, used as a basic ingredient of concrete, mortar, and most non-speciality grout(cement/water mixes squeezed into gaps to consolidate foundations).

MANUFACTURING OF CEMENT

The manufacturing process in a modern plant is now controlled through a microprocessor based programmable logic control system to maintain a constantly uniform quality of cement and a high rate of production. Three distinct operations are involved in the manufacture of portland cement. Mixing of raw materials Burning grinding

RAW MIXING DRY PROCESS: In this process, firstly the dumper tranports boulders of limestone into the hopper of the crusher. The hammer mill crusher crushed the limestone to 75mm size which is then moved by a series of conveyors for stacking along with the argillaceous materials. The raw materials are fed to the raw mill by means of conveyor and proportioned by use of weigh feeders.

The materials are ground to the desired fineness in the raw mill and then pumped into the preheater in which the hot gas at 1000°c is blown against the falling gradient. The material from the bottom of the preheater is fed to the rotary kiln.

ROTARY KILN

The burning is carried out in rotary kiln and is made of steel tubes with internal lining of fire bricks. Its diameter varies from 2.50-3 m and length varie from 90- 120 m . It has a gradient and rotates about its longitudinal axis. The slurry or raw mix is injected at upper end of kiln known as dry zone. As the slurry descends, the temp increases and co2 is evaporated, small lump are formed called nodules.

These nodules then gradually roll down passing through zones of rising temperature and ultimately reach to the burning zone, where temp. is about 1450°c. In burning zone, the calcined product is formed and nodules are converted into small hard dark greenish blue balls known as clinkers. These clinkers are cooled and at temp. of 95°c are stored in container of suitable sizes.

GRINDING Clinkers obtained from the rotary kiln are finely ground in ball or tube mill with addition of small quantity of gypsum. Gypsum controls the intial setting time of cement. The grinding of clinkers are also done in cement mill which contains chromium steel balls of various sizes. These balls roll within the mill and grind the mixture which is collected in hopper and then taken in silos. The cement from silos is fed to the packer machines.

PACKING The cement is packed in multi-walled polypropylene paper laminated bags. These bags replace the conventional jute bags. They are light in weight and leak proof. They protect cement from moisture. It fulfills the desired quality and quanity of product for the consumers.