PORTLAND CEMENT PRODUCTION PROCESS

PORTLAND CEMENT PRODUCTION PROCESS

Production of Portland cement starts with two basic raw ingredients: a calcareous material and an argillaceous material. The calcareous material is a calcium oxide, such as limestone, chalk, or oyster shells. The argillaceous material is a combination of silica and alumina that can be obtained from clay, shale, and blast furnace slag. As shown in Figure 1, these materials are crushed and then stored in silos. The raw materials, in the desired proportions, are passed through a grinding mill, using either a wet or dry process.

Fig 1. Steps in the manufacture of Portland cement

The ground material is stored until it can be sent to the kiln. Modern dry process cement plants use a heat recovery cycle to preheat the ground material, or feed stock, with the exhaust gas from the kiln. In addition, some plants use a flash furnace to further heat the feed stock. Both the pre heater and flash furnace improve the energy efficiency of cement production. In the kiln, the raw materials are melted at temperatures of 1400°C to 1650°C (2500°F to 3000°F), changing the raw materials into cement clinker. The clinker is cooled and stored. The final process involves grinding the clinker into a fine powder. During grinding, a small amount of gypsum is added to regulate the setting time of the cement in the concrete.

The finished product may be stored and transported in either bulk or sacks. In the United States, a standard sack of cement is 94 lb, which is approximately equal to of loose cement when freshly packed. The cement can be stored for long periods of time, provided it is kept dry.