CEMENT, THE BUILDING MATERIAL
Cement is a finely milled mineral powder, adhesive substances of all kinds. It is usually gray in color and acts as the binding materials. Mostly cement is used in building and civil engineering construction. Limestone (calcium), sand or clay (silicon), and marl are the raw materials for the production of cement. Cement serves as an adhesive to bind sand, gravel, and hard rock in concrete when mixed with water, set to a hard mass.
Top Cement is an integral part of urban infrastructure and development. To secure the infrastructure by binding the building blocks, it is used to make concrete as well as mortar. Mortar consists of cement, water, and lime aggregate whereas concrete is made of cement, water, sand, and gravel mixed in definite proportions. Both are used to bind rocks, bricks, stones, and other building units, seal or fill gaps, and make decorative patterns as per wish. Cement is also used for water-proofing when mixed with water silicates and aluminates and forms a water repellent hardened mass.
Cement mixed with water causes a chemical reaction called Hydration Process and forms a paste that sets and hardens to bind individual structures of building materials. Due to the chemical combination of the cement compounds with water the submicroscopic crystals or a gel-like material yields a high surface area. Now the hydrating properties harden under water.The cement is used in building and civil engineering construction.
Cement can be considered one of the most important building materials. Cement generally refers to a very fine powdery substance chiefly made up of limestone (calcium), bauxite (aluminum) and iron ore, sand or clay (silicon), and may include marl, shale, shells, chalk, clay, blast furnace slag and slate. These raw ingredients are processed in cement manufacturing plants or factories. These ingredients are now heated to form a rock-hard substance, which is then ground into a fine powder. These final products ready to be sold are often called hydraulic cements. The most important cement among the hydraulic cements is Portland Cement.
HISTORY OF CEMENT INDUSTRY
Nowadays we find the cement is a refined product but it has been used in many forms since the onset of human civilization. The origin of cement goes back to ancient Rome and Greece. The first hydraulic cement used by the Romans in the middle ages from volcanic ashes, crushed pottery, burnt gypsum and hydrated lime. Until the 18th century the development of cement continued. At that time James Parker patented Roman cement, which gained popularity. Later it was replaced by Portland cement in the 1850s.
In the 19th century, in Russia Louis Vicat a Frenchman laid the foundation for the chemical composition of Portland cement. In England Joseph Aspdin brought Portland cement to the market. Later on his son, William Aspdin, developed the Modern Portland cement, which was in high demand very soon. But Isaac Charles Johnson, the real father of Portland cement contributed immensely by publishing the process of developing Meso-Portland cement.
Afterward, Rosendale cement was discovered in New York. Its rigidity made it quite popular initially but the market demand soon declined because of its long curing time. Portland cement was again the favorite. Thereafter, a new blend of Rosendale-Portland cement came into light, which is both highly durable and needs less curing time. This was synthesized by Catskill Aqueduct which is now often used for bridge construction or highway.
The cement used today has undergone quite an experimentation, and testing. Significant improvements noticed to meet the needs of today’s world. Cements are used in constructions like stucco for wet climates, developing strong concrete for roads and highways and hydraulic mortars that endure sea water. There are different kinds of modern cement present to fulfill different types of constructions and developed various structures. Among them Portland cement or blends, blast furnace cement, Portland fly-ash cement, Portland Pozzolana cement, slag-lime cement, Pozzolan-lime cement etc.
Cements can be used alone. But normally it is used in mortar and concrete where the cement is mixed with inert material called aggregate. Mortar is a mixture where cement is mixed with sand or crushed stone (less than approximately 5 mm (0.2 inch) in size). Concrete is also a mixture of cement, sand or other fine aggregate. Whereas the coarse aggregate used for most purposes is up to 19 to 25 mm (0.75 to 1 inch) in size. As per the need sometimes the coarse aggregate may be as large as 150 mm (6 inches). This type of coarse mixture is placed in large masses such as dams. Mortars are used for stone in walls or as surface renderings, binding bricks and blocks. For a large variety of construction purposes, Concrete is used.
Base for roads are made up of mixtures of soil and Portland cement. Portland cement also is used for multiple purposes like in the manufacture of tiles, bricks, pipes, shingles, railroad ties, beams and various extruded products. The products are manufactured in factories and supplied ready for different uses. Since concrete is the most widely used of all construction materials in the world today, manufacture of cement is extremely widespread.
Cement is mainly of two kinds depending on the way it is set and hardened. Hydraulic cement hardens due to the addition of water, while non-hydraulic cement hardens by carbonation with the carbon present in the air. This is the reason that the non-hydraulic cement cannot be used underwater.
The following steps (lime cycle) are followed to produce Non-hydraulic cement:
- Calcinations: Lime is produced from limestone at over 825°C for about 10 hours. (CaCO3 → CaO + CO2)
- Slaking: Calcium oxide is mixed with water to make slaked lime. (CaO + H2O → Ca(OH)2)
- Setting: Water is completely evaporated.
- The cement is exposed to dry air and it hardens after time-consuming reactions. (Ca(OH)2 + CO2 → CaCO3 + H2O)
On the other hand, hydraulic cement is mainly made up of only silicates and oxides:
- Belite (2CaO·SiO2);
- Alite (3CaO·SiO2);
- Tricalcium aluminate/ Celite (3CaO·Al2O3)
- Brownmillerite (4CaO·Al2O3·Fe2O3)
The ingredients are processed in cement plants. The chemistry of the reactions is still a subject of research.
EXTRACTION AND PROCESSING
Raw materials are extracted with the aid of blasting when necessary. Extraction done of hard rock’s such as limestone, slates, and some shale for the manufacture of cement. Some minerals are mined by underground methods too. Softer rocks such as clay and chalk can dig directly by excavators. Now these materials are transported to the crushing plant by trucks, conveyor belts, railway freight cars, or ropeways. They also can be transferred in a wet state or semi-liquid state by pipeline
MANUFACTURING OF CEMENT
There are four stages in the manufacture of mostly used Portland cement:
(1) Crushing and grinding the raw materials,
(2) Blending the materials in the correct proportions,
(3) Burning the prepared mix in a kiln, and
(4) Grinding the burned product, known as “clinker”. This clinker is added with some 5% of gypsum (this helps to control the time of set of the cement).
CRUSHING AND GRINDING
All materials except soft ones are first crushed, mostly in two stages, and then ground, usually in a cylindrical ball, rotating or tube mills containing a charge of steel grinding balls. This grinding process is done either wet or dry, depending on the process. For dry grinding the raw materials first need to be dried in cylindrical, rotary dryers. Soft materials are broken down by intense stirring with water in wash mills, resulting in fine slurry, which is passed through screens to discard oversize particles.
After the crushing and grinding, the chemical composition required for particular cement is obtained by selective quarrying. In this process the control of the raw material is essential. Finer control is attained by collecting material from two or more batches. In the dry process these mixtures are stored in silos. Slurry tanks are used for the wet processes. Vigorous circulation induced by compressed air for thorough mixing of the dry materials in the silos. In case of the wet process, mechanical means or compressed air or both can be used for stirring in the slurry tanks.
The Initial kilns where cement was burned in batches were bottle kilns. Secondly it is followed by chamber kilns and at last by continuous shaft kilns. The shaft kiln is still used in some countries in a modernized form. Predominantly the burning has been done by the rotary kiln. For the wet process plants these kilns measure up to 200 meters (660 feet) long and six meters in diameter. But the kiln is shorter for the dry process.
These kilns are made up of steel, cylindrical shells lined with refractory materials. The raw material feeds, or poured at the upper end, which moves slowly down the lower end of the kiln. Pulverized coal, oil, or natural gas is used as the fuel which is injected through a pipe. The temperature ranges from about 1,350 to 1,550 °C (2,460 to 2,820 °F) at the firing end. Temperature depends on the raw materials which are being burned. The burned product emerges as small nodules of clinker from the kiln. These clinkers pass into coolers to be cooled. The clinkers are then instantly ground to cement or stored in stockpiles for later use.
For control of the burning process now-a-days modern cement plants are well equipped with elaborate instrumentation. Raw materials in some plants are sampled automatically, and a computer calculates and controls the raw materials mix composition. The largest rotary kilns have output more than 5,000 tons per day.
Grinding done in horizontal mills to a fine powder of the clinker and the required amount of gypsum. The materials pass straight through the mill (open-circuit grinding). The coarser material separated from the ground product and returned to the grinding mill for further grinding (closed-circuit grinding).
In national cement specifications, various tests are laid down to control the fineness, soundness, setting time, and strength of the cement. These tests are described below.
Previously the fineness was controlled by sieve tests. But now more sophisticated methods are broadly used on the Blaine Air permeability apparatus. The fineness test of the cement is done to determine the particle size of the cement. As the rule of thumb, the finer the cement particle is the best quality cement and on the other hand the coarser the particle is not considered to be that good.
Soundness of cement means the ability to resist volume expansion. The testing of the soundness of cement is very important to ensure that the cement should not show any subsequent expansion, which determines the quality of the cement. The unsoundness of the cement occurs mainly due to adding an excess lime with acidic oxide at the kiln. The soundness of cement is determined by two methods. These two methods are the Le-Chatelier method and the autoclave method. In the soundness test a specimen of hardened cement paste is boiled for a fixed time so that if there is any tendency to expand, it can be detected. Once it sets, cement must not undergo any appreciable expansion, which could disturb a mortar or concrete.
Setting time is essential for cement to determine its quality. It should not set too rapidly or too slowly. The setting and hardening of cement is a continuous process, but two setting points are noted for test purposes The Initial setting time (the interval between the mixing of the cement with water) of cement is when the cement paste starts losing its plasticity and final setting time (the set cement has acquired a sufficient firmness to resist a certain defined pressure) of cement is when the cement paste completely loses its plasticity. For OPC, usually the initial minimum setting time value at ordinary temperatures is 30 to 45 minutes and for final setting time is no more than 10 to 12 hours. The device which is used to determine the initial setting time of cement is called a Vicat apparatus.
The tests to measure the strength of the cement are usually made on a mortar common mixture of one part cement to three parts sand with a defined quantity of water. For masonry cement, compressive strength is ranging between 33 – 53MPa (megapascals) at 28 days, 22- 27 MPa (megapascals) at 7days and 16 -27MPa (megapascals) at 3days of curing after casting. Compressive strength of ordinary Portland cement (OPC) represented as OPC33, OPC43 & OPC53. Compressive strength test, measures the maximum amount of compressive load a material can bear before fracturing. The test piece, usually in the form of a prism, cube or cylinder is compressed between the platens of a compression-testing machine by a gradually applied load. In this type of testing of cements, a minimum strength at 3 and 7 days and sometimes 28 days is specified. In case of a rapid-hardening Portland cement test 1 day is required. For high-alumina cement, tests 1 and 3 days are required.
Cement Manufacturing Industries in the World
The USA, China, and India are among the top three cement producers of the world as recorded in 2010. Among these three countries, China alone manufactures about 45% of the total worldwide need. Since cement is a non-recyclable product, every new construction or repair needs new cement. So the global consumption of cement automatically rises. Cement production is an important element of progress chiefly in the economies of Asia and Eastern Europe.
Today the market is crowded with various cement manufacturing companies. Among them, few are considered as the most popular cement manufacturing companies.
Here are the top 10 best cement companies in India.
- Ultratech Cement
- Ambuja Cement
- ACC Cement
- Shree Cement
- Dalmia Bharat
- Rain Industries
- JK Cement
- The Ramco Cement
- Birla Corporation
- JK Lakshmi Cement