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Cement was first used in the construction of a tunnel in the Thames river in the year 1828. Cement is essential to provide binding properties to concrete. It allows the concrete to harden into a strong and durable material. It is a greenish-grey-colored powder made of a calcined mixture of clay and limestone.
Concrete is the amalgamation of cement and other aggregates like sand, gravel, and water. The mixture forms a paste that hardens over time.
Cement is manufactured from several raw materials after going through a manufacturing process. The essential components/minerals used to make cement are several major raw materials as outline below:
Limestone:
Limestones like chalk and Jurassic are primarily used to make cement. Chalk is porous and has a high moisture content used in the semi-dry or wet manufacturing process of making cement. The limestone selected for manufacturing cement should contain magnesium carbonate, free silica, and iron sulfides.
Shale:
Shale is a sedimentary rock made of fine clay, silt, and quartz and falls in the category of mudstones.
Gypsum:
Gypsum is a soft sulfate mineral composed of calcium sulfate dehydrate and is used in cement production.
Cement manufacturing process:
The cement manufacturing process is complex and involves several important stages. Each stage plays an integral role in the quality of finished products. The process involves the use of various raw materials and their processing.
There are several stages in cement manufacturing. The end product i.e. cement is obtained at the end of these processes. The processes in cement manufacturing are outlined below.
Extraction of raw material or Quarry:
The raw materials needed for cement production are limestone (calcium), sand and clay (silicon, aluminum, and iron), shale, fly ash, mill scale, and bauxite. These are quarried and then crushed into smaller pieces of about 6 inches. Further hammer mills and secondary crushers are used to reduce the size to 3 inches. Now the ingredients are prepared for pyro-processing.
Mixing of raw material:
Two methods are used to mix the ingredients – dry process and wet process
Dry process:
The raw materials (argillaceous and calcareous materials) are crushed in the gyratory crusher. The crushed materials are dried by heating at a high temperature. These are further grounded to get fine particles by using ball mills or tube mills to reduce the size of materials to a fine powder. The finely grounded materials are stored in a hopper. The powdered minerals are mixed and then the dry raw mix is finally stored in silos and made ready to transport to the rotary kiln.
Wet process:
A wet process is applied when the raw material is soft. In this process, argillaceous and calcareous materials are used. In the beginning, the calcareous materials are crushed using a crusher, and the argillaceous material is washed with water in the container.
After crushing, limestone is stored in silos, and the washed clay is stored in basins. The crushed materials from silos and basins are drawn in the correct proportion in a channel known as the wet grinding mill. Both materials are mixed in the presence of water to form a paste called slurry.
Heating of raw materials:
The material is burned in a large rotary kiln at a temperature of around 1500 degrees Celsius. The raw materials are turned into oxides to be burned in the kiln. CO2 is driven from the limestone when the heating starts the decarbonization. This results in the formation of clinker.
Cooling and grinding:
The clinker is cooled and ground to a fine powder in a tube or ball mill. Gypsum is added during the grinding process to control the setting of the cement.
Storage and packaging:
The grounded cement is stored in silos and marketed in container loads or 50kg bags.
Wrapping up:
Portland is the most widely produced cement. From houses to bridges to sidewalks to sewer systems, cement is the primary ingredient to make concrete used for building and construction purposes.