History of Cement
Throughout history, cementing materials have played a vital role. They were used widely in the ancient world. The Egyptians used calcined gypsum as a cement. The Greeks and Romans used lime made by heating limestone and added sand to make mortar, with coarser stones for concrete.The Romans found that cement could be made which set under water and this was used for the construction of harbors. The cement was made by adding crushed volcanic ash to lime and was later called a ‘pozzolanic’ cement, named after the village of Pozzuoli near Vesuvius. In places such as Britain, where volcanic ash was scarce, crushed brick or tile was used instead. The Romans were therefore the first to manipulate the properties of cementations materials for specific applications and situations. After the Romans, there was a general loss in building skills in Europe, particularly with regard to cement. Mortars hardened mainly by carbonation of lime, a slow process. The use of pozzolana was rediscovered in the late Middle Ages.The great mediaeval cathedrals, such as Durham, Lincoln and Rochester in England and Chartres and Rheims in France, were clearly built by highly skilled masons. Despite this, it would probably be fair to say they did not have the technology to manipulate the properties of cementitious materials in the way the Romans had done a thousand years earlier.The Renaissance and Age of Enlightenment brought new ways of thinking, which for better or worse, led to the industrial revolution. In eighteenth century Britain, the interests of industry and empire coincided, with the need to build lighthouses on exposed rocks to prevent shipping losses. The constant loss of merchant ships and warships drove cement technology forwards.Smeaton, building the third Eddystone lighthouse (1759) off the coast of Cornwall in Southwestern England, found that a mix of lime, clay and crushed slag from iron-making produced a mortar which hardened under water. Joseph Aspdin took out a patent in 1824 for "Portland Cement," a material he produced by firing finely-ground clay and limestone until the limestone was calcined. He called it Portland Cement because the concrete made from it looked like Portland stone, a widely-used building stone in England.
While Aspdin is usually regarded as the inventor of Portland cement, Aspdin's cement was not produced at a high-enough temperature to be the real forerunner of modern Portland Cement. Nevertheless, his was a major innovation and subsequent progress could be viewed as mere development.A ship carrying barrels of Aspdin's cement sank off the Isle of Sheppey in Kent, England, and the barrels of set cement, minus the wooden staves, were later incorporated into a pub in Sheerness and are still there now.A few years later, in 1845, Isaac Johnson made the first modern Portland Cement by firing a mixture of chalk and clay at much higher temperatures, similar to those used today. At these temperatures (1400C-1500C), clinkering occurs and minerals form which are very reactive and more strongly cementitious.While Johnson used the same materials to make Portland cement as we use now, three important developments in the manufacturing process lead to modern Portland cement:
- Development of rotary kilns
- Addition of gypsum to control setting
- Use of ball mills to grind clinker and raw materials
Rotary kilns gradually replaced the original vertical shaft kilns used for making lime from the 1890s. Rotary kilns heat the clinker mainly by radiative heat transfer and this is more efficient at higher temperatures, enabling higher burning temperatures to be achieved. Also, because the clinker is constantly moving within the kiln, a fairly uniform clinkering temperature is achieved in the hottest part of the kiln, the burning zone.
The two other principal technical developments, gypsum addition to control setting and the use of ball mills to grind the clinker, were also introduced at around the end of the 19th century.
Cement manufacturing: components of a cement plant
View of a cement kiln (the long nearly-horizontal cylinder) and preheater tower.
Summary of manufacturing process
Cement is typically made from limestone and clay or shale. These raw materials are extracted from the quarry crushed to a very fine powder and then blended in the correct proportions.
This blended raw material is called the 'raw feed' or 'kiln feed' and is heated in a rotary kiln where it reaches a temperature of about 1400 C to 1500 C. In its simplest form, the rotary kiln is a tube up to 200 metres long and perhaps 6 metres in diameter, with a long flame at one end. The raw feed enters the kiln at the cool end and gradually passes down to the hot end, then falls out of the kiln and cools down.
The material formed in the kiln is described as 'clinker' and is typically composed of rounded nodules between 1mm and 25mm across.
After cooling, the clinker may be stored temporarily in a clinker store, or it may pass directly to the cement mill.
The cement mill grinds the clinker to a fine powder. A small amount of gypsum - a form of calcium sulfate - is normally ground up with the clinker. The gypsum controls the setting properties of the cement when water is added.
The basic components of the cement manufacturing process.
The most common raw rock types used in cement production are:
- Limestone (supplies the bulk of the lime)
- Clay, marl or shale (supplies the bulk of the silica, alumina and ferric oxide)
- Other supplementary materials such as sand, pulverised fuel ash (PFA), or ironstone to achieve the desired bulk composition
Limestone blocks being taken away for crushing.
Raw materials are extracted from the quarry, then crushed and ground as necessary to provide a fine material for blending. Most of the material is usually ground finer than 90µm - the fineness is often expressed in terms of the percentage retained on a 90µm sieve.
Once the the raw materials are ground fine enough, they are blended in the proportions required to produce clinker of the desired composition.
The blended raw materials are stored in a silo before being fed into the kiln. The silo stores several days' supply of material to provide a buffer against any glitches in the supply of raw material from the quarry.
Technically, a cement producer can have almost complete control over clinker composition by blending raw materials of different compositions to produce the desired result. In practice, however, clinker composition is largely determined by the compositions of the locally-available raw materials which make up the bulk of the raw meal.
Supplementary materials are used to adjust the composition of the raw meal but cost and availability are likely to determine the extent to which they are used. Transport costs in particular become significant in view of the large quantities of materials used in making cement.
Rotary kilns were introduced in 1890s and became widespread in the early part of the 20th century. They were a great improvement on the earlier shaft kilns, giving continuous production and a more uniform product in larger quantities.
A rotary kiln is basically a long cylinder rotating about its axis. Cement kilns rotate once every minute or two. The kiln is inclined at a slight angle, the end with the burner being lower. The rotation of the kiln causes the raw meal to gradually pass along from where it enters at the cool end, to the hot end where it eventually drops out and cools.
Wet process kilns
Principle of a basic wet-process kiln.
The original rotary cement kilns were called 'wet process' kilns. In their basic form they were relatively simple compared with modern kilns. The raw meal was fed into the kiln at ambient temperature in the form of a slurry.
A wet process kiln may be up to 200m long and 6m in diameter. It has to be long because a lot of water has to be evaporated and the process of heat transfer in a wet process kiln is not very efficient.
The slurry may contain about 40% water. This takes a lot of energy to evaporate and various developments of the wet process were aimed at reducing the water content of the raw meal. An example of this is the 'filter press' (imagine a musical accordion 10-20 metres long and several metres across) - such adaptions were described as 'semi-wet' processes.
The wet process has survived for over a century because many raw materials are suited to blending as a slurry. Also, for many years, it was technically difficult to get dry powders to blend adequately.
Quite a few wet process kilns are still in operation, usually now with higher-tech bits bolted on. However, new cement kilns are of the 'dry process' type.
Dry process kilns
In a modern works, the blended raw material enters the kiln via the pre-heater tower. Here, hot gases from the kiln, and probably the cooled clinker at the far end of the kiln, are used to heat the raw meal. As a result, the raw meal is already hot before it enters the kiln.
A dry process kiln is much more thermally efficient than a wet process kiln.
Firstly, and most obviously, this is because the meal is a dry powder and there is little or no water that has to be evaporated.
Secondly, and less obviously, the process of transferring heat is much more efficient in a dry process kiln.
An integral part of the process is a heat exchanger called a ‘suspension preheater.’ This is a tower with a series of cyclones in which fast-moving hot gases keep the meal powder suspended in air. All the time, the meal gets hotter and the gas gets cooler until the meal is at almost the same temperature as the gas.
The basic dry process system consists of the kiln and a suspension preheater. The raw materials, limestone and shale for example, are ground finely and blended to produce the raw meal. The raw meal is fed in at the top of the preheater tower and passes through the series of cyclones in the tower. Hot gas from the kiln and, often, hot air from the clinker cooler are blown through the cyclones. Heat is transferred efficiently from the hot gases to the raw meal.
The heating process is efficient because the meal particles have a very high surface area in relation to their size and because of the large difference in temperature between the hot gas and the cooler meal. Typically, 30%-40% of the meal is decarbonated before entering the kiln.
A development of this process is the ‘precalciner’ kiln. Most new cement plant is of this type. The principle is similar to that of the dry process preheater kiln but with the major addition of another burner, or precalciner. With the additional heat, about 85%-95% of the meal is decarbonated before it enters the kiln.
Basic principle of precalciner kiln.
Since meal enters the kiln at about 900 C, rather than 20 C in a wet process kiln, the kiln can be shorter and of smaller diameter for the same output. This reduces the capital costs of a new cement plant. A dry process kiln might be only 70m long and 6m wide but produce a similar quantity of clinker (usually measured in tonnes per day) as a wet process kiln of the same diameter but 200m in length. For the same output, a dry process kiln without a precalciner would be shorter than a wet process kiln but longer than a dry process kiln with a precalciner.
Kiln and preheater tower: raw meal passes down the tower while hot gases rise up, heating the raw meal. At 'A,' the raw meal largely decarbonates; at 'B,' the temperature is 1000 C - 1200 C and intermediate compounds are forming and at 'C,' the burning zone, clinker nodules and the final clinker minerals form.
The kiln is made of a steel casing lined with refractory bricks. There are many different types of refractory brick and they have to withstand not only the high temperatures in the kiln but reactions with the meal and gases in the kiln, abrasion and mechanical stresses induced by deformation of the kiln shell as it rotates.
Bricks in the burning zone are in a more aggressive environment compared with those at the cooler end of the kiln (the 'back end'), so different parts of the kiln are lined with different types of brick.
Periodically, the brick lining, or part of it, has to be replaced. Refractory life is reduced by severe changes in temperature, such as occur if the kiln has to be stopped. As the cost of refractories is a major expense in operating a cement plant, kiln stoppages are avoided as far as possible.
As the meal passes through the burning zone, it reaches clinkering temperatures of about 1400 C - 1500 C. Nodules form as the burning zone is approached. When the clinker has passed the burning zone, it starts to cool, slowly at first, then much more quickly as it passes over the 'nose ring' at the end of the kiln and drops out into the cooler.
The clinker cooler
There are many types of clinker coolers.
Cooler - red-hot clinker falls onto the grate, cooled by air blown from beneath.
The purpose of a cooler is, obviously, to cool the clinker. This is important for a several reasons:
· From an engineering viewpoint, cooling is necessary to prevent damage to clinker handling equipment such as conveyors.
· From both a process and chemical viewpoint, it is beneficial to minimize clinker temperature as it enters the clinker mill. The clinker gets hot in the mill and excessive mill temperatures are undesirable. It is clearly helpful, therefore, if the clinker is cool as it enters the mill.
· From an environmental and a cost viewpoint, the cooler reduces energy consumption by extracting heat from the clinker, enabling it to be used to heat the raw materials.
· From a cement performance viewpoint, faster cooling of the clinker enhances silicate reactivity.
The cooled clinker is then conveyed either to the clinker store or directly to the clinker mill. The clinker store is usually capable of holding several weeks' supply of clinker, so that deliveries to customers can be maintained when the kiln is not operating.
Cement clinker is usually ground using a ball mill. This is essentially a large rotating drum containing grinding media - normally steel balls. As the drum rotates, the motion of the balls crushes the clinker. The drum rotates approximately once every couple of seconds.
The drum is generally divided into two or three chambers, with different size grinding media. As the clinker particles are ground down, smaller media are more efficient at reducing the particle size still further.
Grinding systems are either 'open circuit' or 'closed circuit.' In an open circuit system, the feed rate of incoming clinker is adjusted to achieve the desired fineness of the product. In a closed circuit system, coarse particles are separated from the finer product and returned for further grinding.
Gypsum is interground with the clinker in order to control the setting properties of the cement. Clinker grinding uses a lot of energy and the cement becomes hot - this can result in the gypsum becoming dehydrated, with potentially undesirable results - see the link at the bottom of this page for more information.
Cement Milling
Cement milling is usually – if not always – carried out using ball mills with two or more separate chambers containing different sizes of balls. Diaphragms perforated with holes or slots control the passage of particles from one chamber to the next.
Grinding of clinker requires a lot of energy. How easy a particular clinker is to grind - ‘grindability’ - is difficult to predict, but rapid cooling of the clinker is thought to improve grindability due to the presence of microcracks in alite and to the finer crystal size of the flux phases. It is frequently observed that belite crystals, which have a characteristic round shape, tend to separate and form single crystal grains during grinding.
As part of the grinding process, calcium sulfate is added as a set regulator, usually in the form of gypsum (CaSO4.2H2O).Natural anhydrite may also be added to discourage lumpiness of the gypsum due to its water content.
Since the clinker gets hot in the mill due to the heat generated by grinding, gypsum can be partly dehydrated. It then forms hemihydrate, or plaster of Paris - 2CaSO4.H2O. On further heating, hemihydrate dehydrates further to a form of calcium sulfate known as soluble anhydrite (~CaSO4). This has a similar solubility in water to hemihydrate, which in turn has a higher solubility than either gypsum or natural anhydrite.
Cement mills need to be cooled to limit the temperature rise of the cement. This is done by a mixture of both air-cooling and water-cooling, including spraying water inside the mill.
The relative proportions and different solubilities of these various types of calcium sulfate are of importance in controlling the rate the rate of C3A hydration and consequently of cement set retardation. Problems associated with setting and strength characteristics of concrete can often be traced to changes in the quantity of gypsum and hemihydrate, or with variations in cooling rate of the clinker in the kiln and subsequent changes in the proportions or size of the C3A crystals.
For set regulation, the most important feature of aluminate is not necessarily the absolute amount present, but the amount of surface which is available to water for reaction. This will be governed by many factors, such as the surface area of the cement, the grinding characteristics of the different phases and also the size of the aluminate crystals. Over-large crystals can lead to erratic setting characteristics.
TYPES OF CEMENT
The types of cement have increased over the years with the advancement in research, development, and technology. The Indian cement industry is witnessing a boom as a result of which the production of different kinds of cement in India has also increased.
By a fair estimate, there are around 11 different types of cement that are being produced in India. The production of all these cement varieties is according to the specifications of the BIS.
Some of the various types of cement produced in India are:
· Clinker Cement
· Ordinary Portland Cement
· Portland Blast Furnace Slag Cement
· Portland Pozzolana Cement
· Rapid Hardening Portland Cement
· Oil Well Cement
· White Cement
· Sulphate Resisting Portland Cement
In India, the different types of cement are manufactured using dry, semi-dry, and wet processes. In the production of Clinker Cement, a lot of energy is required. It is produced by using materials such as limestone, iron oxides, aluminum, and silicon oxides. Among the different kinds of cement produced in India, Portland Pozzolana Cement, Ordinary Portland Cement, and Portland Blast Furnace Slag Cement are the most important because they account for around 99% of the total cement production in India.
The Portland variety of cement is the most common one among the types of cement in India and is produced from gypsum and clinker. The Ordinary Portland cement and Portland Blast Furnace Slag Cement are used mostly in the construction of airports and bridges. The production of white cement in the country is very less for it is very expensive in comparison to grey cement. In India, while cement is usually utilized for decorative purposes, marble foundation work, and to fill up the gaps between tiles of ceramic and marble.
Ordinary Portland Cement
Ordinary Portland Cement (OPC) is manufactured in the form of different grades, the most common in India being Grade-53, Grade-43, and Grade-33. OPC is manufactured by burning siliceous materials like limestone at 1400 degree Celsius and thereafter grinding it with gypsum.
Tata Chemicals Limited is a major producer of OPC Grade 43 and 53. The value of each of these grades of cement has been briefly mentioned below:
· Ordinary Portland Cement-Grade 43: Having been certified with IS 8112:1989 standards, Grade 43 is in high demand in India and is largely used for residential, commercial, and other building construction purposes. It has a compressive strength of 560 kg per square cm. Today OPC 43 is most widely available in Gujarat through an extensive distribution network.
· Ordinary Portland Cement-Grade 53: Having been certified with IS12269:1987 standards, Grade 53 is known for its rich quality and is highly durable. Hence it is used for constructing bigger structures like building foundations, bridges, tall buildings, and structures designed to withstand heavy pressure. Expert opinions and directions from technicians and engineers are a must in this regard. With a good distribution network this cement is available most abundantly in Gujarat.
As such, Ordinary Portland Cement is used for quite a wide range of applications. Some of the Ordinary Portland applications are in pre-stressed concrete, dry-lean mixes, durable pre-cast concrete, and ready mixes for general purposes. The chemical components of Ordinary Portland Cement are Magnesium (MgO), Alumina (AL2O3), Silica (SiO2), Iron (Fe2O3), and Sulphur trioxide (SO3).
Some of the big names involved in OPC manufacture are Tata Chemicals, Ultratech Cement, and ACC cement. Ordinary Portland Cement is in great demand in India and will continue to be used in Indian infrastructural upgradation and other constructions.
Portland Pozzolana Cement
Portland Pozzolana Cement is manufactured by blending pozzolanic materials, OPC clinker, and gypsum either grinding them together or separately. Today Portland Pozzolana Cement is widely in demand for industrial and residential buildings, roads, dams, and machine foundations.
Pozzolana is an important ingredient in PPC which is commonly used in the form of:
· Fly ash
· Volcanic ash
· Silica fumes
· Calcined clay
PPC is resistant to harsh water attacks and prevents the formation of calcium hydroxide at the time of cement setting and hydration. It withstands aggressive gases, thermal cracks, wet cracking, etc.
The BIS quality specifications for Pozzolana materials used in PPC have been mentioned below:
· Fly ash - IS 3812:1981
· Calcined clay - IS 1344:1981
PPC is used in heavy load infrastructure and constructions such as marine structures, hydraulic structures, mass concreting works, plastering, masonry mortars, and all applications of ordinary Portland cement. One of the top Indian brands of Portland Pozzolana is 'Shudh Cement' manufactured by Tata Chemicals Limited.
Shudh cement has 5 percent of the market share and is available abundantly in Gujarat, penetrating all 3 - primary, secondary, and tertiary markets. Some of the other big names in the Portland Pozzolana manufacture are Ultratech, Ambuja, ACC cements, Star Cement, and Birla group.
Portland Pozzolana Cement is highly popular in India and with many cement plants setting up jetties for transportation, initial costs would gradually decrease as well.
Portland Blast Furnace Slag Cement
In recent years, there has been a significant growth in the production of Portland Blast Furnace Slag Cement and its sales have also increased considerably over the last few years. This has given a major boost to the Indian cement industry.
The Slag Cement of the Portland Blast Furnace is a type of cement that is hydraulic and is manufactured in a blast furnace where iron ore is reduced to iron. The molten slag which is tapped is quickly drenched with water, dried, and then grounded to a fine powder. This fine powder that is produced is commonly known as the Portland Blast Furnace Slag Cement.
The manufacture of Portland Blast Furnace Slag Cement requires 75% less energy than that needed for the production of the Portland cement. The low cost of production of Portland Blast Furnace Slag Cement makes it cheaper than Portland cement. It is for this reason that in recent years, the sales of Portland Blast Furnace Slag Cement have increased.
Portland Blast Furnace Slag Cement has a typical light color and an easier 'finish' ability. Its concrete workability is better and it has a higher flexural and compressive strength. It is resistant to chemicals and also has more hardened consistency. This is the reason that Portland Blast Furnace Slag Cement is used in the construction of dams, bridges, building complexes, and pipes.
The various raw materials required for the production of Portland Blast Furnace Slag Cement are:
· Limestone
· Iron Ore
· Iron Scrap
· Coke
The major companies producing Portland Blast Furnace Slag Cement in India are:
· J K Cement
· Grasim Industries and Ultra Tech
· ACC
· India Cement Ltd
· Gujarat Ambuja Cement Ltd
The major countries where Portland Blast Furnace Slag Cement is exported from India are:
· South Africa
· UAE
· Sri Lanka
· Nepal
· Bangladesh
· Australia
· Doha-Qatar
The production and use of Portland Blast Furnace Slag Cement have increased over the years. The Indian government has undertaken several investments in the production of the Portland Blast Furnace Slag Cement so that its quality and durability can be improved.
Oil Well Cement
Oil Well Cement as the name suggests, is used for the grouting of the oil wells, also known as the cementing of the oil wells. This is done for both, the off-shore and on-shore oil wells.
As the number of oil wells in India is increasing steadily, the sales of Oil Well Cement have also increased. This has boosted the Indian cement industry to a large extent.
Oil Well Cement is manufactured from the clinker of Portland cement and also from cements that have been hydraulically blended. Oil Well Cement can resist high pressure as well as very high temperatures. Oil Well Cement sets very slowly because it has organic 'retarders' which prevent it from setting too fast. It is due to all these characteristics that it is used in the building of the oil wells where the pressure is around 20,000 PSI and the temperature is around 500 degrees Fahrenheit.
There are 3 grades of Oil Well Cements. Grades O is ordinary and is used commonly; HSR is high sulphate resistant; and MSR is moderate sulphate resistant. Each grade is used where it is applicable at a particular range of oil well sulphate environments, temperatures, pressures, and depths. Oil Well Cement has proved to be very beneficial for the petroleum industry due to its characteristics. For it is due to the Oil Well Cement that the oil wells function properly.
The various raw materials required for the production of Oil Well Cement are:
· Limestone
· Iron Ore
· Coke
· Iron Scrap
The major companies manufacturing Oil Well Cement in India are:
· ACC
· Gujarat Ambuja
· India Cement Ltd.
· Grasim Industries and Ultra Tech
· J K Cement
Rapid Hardening Portland Cement
Rapid Hardening Portland Cement (RHPC) is a type of cement that is used for special purposes when a faster rate of early high strength is required. RHPC has a higher rate of strength development than the Normal Portland Cement (NPC).
The Rapid Hardening Portland Cement's better strength performance is achieved by increasing the refinement of the product. This is the reason that its use is increasing in India.
Rapid Hardening Portland Cement is manufactured by fusing together limestone (which has been finely grounded) and shale, at extremely high temperatures to produce cement clinker. To this cement clinker, gypsum is added in small quantities and then finely grounded to produce Rapid Hardening Portland Cement. It is usually manufactured using the dry process technology.
Rapid Hardening Portland Cement is used in concrete masonry manufacture, repair work which is urgent, concreting in cold weather, and in pre-cast production of concrete. Rapid Hardening Portland Cement has proved to be a boon in the places where quick repairs are required such as airfield and highway pavements, marine structures, and bridge decks.
The Rapid Hardening Portland Cement should be stored in a dry place, or else its quality deteriorates due to premature carbonation and hydration. As the Indian cement industry produces Rapid Hardening Portland Cement in large quantities, it is able to meet the domestic demand and also export to other countries. The cement industry in India exports cement mainly to the West Asian countries.
The raw materials required for the manufacture of Rapid Hardening Portland Cement are:
· Limestone
· Shale
· Gypsum
· Coke
The major companies producing Rapid Hardening Portland Cement in India are:
· ACC
· Gujarat Ambuja
· J K Cement
· Grasim Industries
· Indian Cement Ltd.
Sulphate Resisting Portland Cement
Sulphate Resisting Portland Cement (SRC) is a type of Portland cement in which the quantity of tricalcium alumiante is less than 5%. It can be used for purposes wherever Portland Pozzolana Cement, Slag Cement, and Ordinary Portland Cement are used.
The use of Portland Sulphate Resisting Cement has proved beneficial, particularly in conditions where there is a risk of damage to the concrete from sulphate attack. The use of Sulphate Resisting Portland Cement is recommended in places where the concrete is in contact with the soil, ground water, exposed to seacoast, and sea water. In all these conditions, the concrete is exposed to attack from sulphates that are present in excessive amounts, which damage the structure. This is the reason that the use of the Sulphate Resisting Portland Cement have increased in India.
The Sulphate Resisting Portland Cement should be kept in a place which is dry otherwise through premature hydration and carbonation the quality of cement deteriorates. The cement industry in India manufactures Sulphate Resisting Portland Cement in large quantities so that it is able to meet the domestic demand and also export to other countries as well. The Indian cement industry exports cement chiefly to the West Asian countries.
The various uses of Sulphate Resisting Portland Cement are:
· Underground and basements structures
· Works in coastal areas
· Piles and foundations
· Water and sewage treatment plants
· Sugar, chemical, and fertilizers factories
· Petrochemical and food processing industries
The raw materials required for the production of Sulphate Resisting Portland Cement are:
· Coke
· Limestone
· Iron Ore
· Iron Scrap
The major companies manufacturing Sulphate Resisting Portland Cement in India are:
· ACC
· J K Cement
· Indian Cement Ltd
· Grasim Industries
· Gujarat Ambuja
Sulphate Resisting Portland Cement has proved beneficial for construction purposes in India due to its climatic conditions. The cement industry in India must take steps in order to ensure that its quality is improved and to ensure that it is readily available in the market.
The Sulphate Resisting Portland Cement should be stored in a dry place, or else its quality deteriorates due to premature carbonation and hydration. As the Indian cement industry produces Sulphate Resisting Portland Cement in large quantities, it is able to meet the domestic demand and also export to other countries. The cement industry in India exports cement mainly to the West Asian countries.
White Cement
White Cement has registered growth in production and sale in India in the last few years. The White Cement sector has been growing at the rate of 11% per year. This has given the Indian cement industry a major boost.
White Cement is much like the ordinary grey cement except that it is white in color. In order to get this color of the White Cement, its method of production is different from that of the ordinary cement. However, this modification in its production method makes White Cement far more expensive then the ordinary cement.
The production of White Cement requires exact standards and so it is a product which is used for specialized purposes. White Cement is produced at temperatures that hover around 1450-1500 degrees Celsius. This temperature is more than what is required by the ordinary grey cement. As more energy is required during the manufacture of White Cement, it goes to make it more expensive than the ordinary grey cement.
White Cement is used in architectural projects the use of white cement has been specified. It is used in decorative works and also wherever vibrant colors are desired. White Cement is used to fill up the gaps between marble and ceramic tiles for a smoother and more beautiful finish.
The various raw materials required for the production of White Cement are:
· Limestone
· Sand
· Iron Ore
· Nickel
· Titanium
· Chromium
· Vanadium
The major companies producing White Cement in India are:
· ACC
· J K Cement
· Gujarat Ambuja Cement Ltd.
· India Cement Ltd.
· Grasim Industries and Ultra Tech
The major countries where White Cement is exported from India are:
· UAE
· Australia
· South Africa
· Sri Lanka
· Doha- Qatar
· Bangladesh
· Nepal
Clinker Cement
Clinker Cement has registered a growth over the last few years in India. The Indian cement industry is growing at a rapid pace and this has given a major boost to the production and sale of Clinker Cement in India.
The cement industry in India is highly technologically intensive and as a result, the quality of clinker cement that is produced in India is of a very high grade and is often considered among the best in the world. The production of Clinker Cement requires a lot of energy because it needs to be manufactured at the temperature of around 1400-1450 degree Celsius.
The various raw materials required for the production of Clinker Cement are:
· Iron Ore
· Bauxite
· Clay
· Limestone
· Quartz
Clinker Cement in India is produced in such large quantities that it is able to meet the domestic demand and is also exported. In 2001- 2002, 1.76 million tons of clinker cement were exported. In 2002- 2003, that figure stood at 3.45 million tons, and in 2003- 2004 5.64 million tons of clinker cement was exported from India. This shows that the export of clinker cement from India has been increasing gradually but steadily.
Clinker Cement is usually ground with calcium sulphate so that it becomes Portland cement. It is also ground with other ingredients to produce Pozzolanic Cement, Blast Furnace Slag Cement, and Silica Fume Cement. If Clinker Cement is kept in a dry condition, it can be stored for a long period of time without any loss of its quality. It is for this reason that Clinker Cement is preferred in the construction of houses, bridges, and complexes.
The major companies producing Clinker Cement in India are:
· ACC
· Gujarat Ambuja Cement Ltd.
· JK Cements
· Grasim Industries and Ultra Tech
· India Cements Ltd.
Cement hydration
By the process of hydration - reaction with water - Portland cement, mixed with sand gravel and water, produces the synthetic rock we call concrete. Concrete is as essential a part of the modern world as are electricity or computers.
Cement clinker is anhydrous - without water - having come from a hot kiln. Cement powder is also anhydrous if we ignore the small amount of water in any gypsum added at the clinker grinding stage.
The process by which cement reacts with water is termed 'hydration.' In cement, this involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles, and other components of the concrete, to form a solid mass.
The hydration process – reactions
Portland cement is composed largely of four types of minerals: alite, belite, aluminate (C3A) and a ferrite phase (C4AF). For more information on the composition of cement clinker, see the clinker pages. Also present are small amounts of clinker sulfate (sulfates of sodium, potassium and calcium) and also gypsum, which was added when the clinker was ground up to produce cement powder.
When cement and water are mixed together, the reactions which occur are mostly exothermic – heat is produced. We can get an indication of the rate at which the minerals are reacting by monitoring the rate at which heat is evolved using a technique called conduction calorimetry. An illustrative example of the heat evolution curve produced by cement is shown below.
Three principal reactions occur:
Almost immediately on adding water, some of the clinker sulphates and gypsum dissolve, producing an alkaline, sulfate-rich solution.
Soon after mixing, the (C3A) phase - the most reactive of the clinker minerals - reacts with the water to form an aluminate-rich gel (Stage I on the heat evolution curve above). The gel reacts with sulfate in solution to form small rod-like crystals of ettringite. (C3A) hydration is a strongly exothermic reaction but it does not last long, typically only a few minutes and is followed by a period of a few hours of relatively low heat evolution. This is called the dormant, or induction period (Stage II).
The first part of the dormant period – up to perhaps half-way through - corresponds to when concrete can be placed. As the dormant period progresses, the paste becomes too stiff to be workable.
At the end of the dormant period, the alite and belite in the cement start to hydrate, with the formation of calcium silicate hydrate and calcium hydroxide. This corresponds to the main period of cement hydration (Stage III), during which time concrete strengths increase. The cement grains react from the surface inwards, and the anhydrous particles become smaller. (C3A) hydration also continues, as fresh crystals become accessible to water.
The period of maximum heat evolution occurs typically between about 10 and 20 hours after mixing and then gradually tails off. In a mix containing Portland cement as the only cementitious material, most of the strength gain has occurred within about a month. Where the cement has been partly-replaced by other materials, such as fly ash, strength growth may occur more slowly and continue for several months or even a year. Final strengths may exceed those from Portland-cement-only mixes.
Ferrite hydration also starts quickly as water is added, but then slows down, probably because a layer of iron hydroxide gel forms, coating the ferrite and acting as a barrier, preventing further reaction.
Cement hydration products
The products of the reaction between cement and water are termed 'hydration products.' In concrete (or mortar or other cementitious materials) made using Portland cement only as the cementitious material there are four main types of hydration product:
Calcium silicate hydrate: this is the main hydration product and is the main source of concrete strength. It is often abbreviated, using cement chemists' notation, to 'C-S-H,' the dashes indicating that no strict ratio of SiO2 to CaO is inferred. The Si/Ca ratio is somewhat variable but typically approximately 0.45-0.50.
Calcium hydroxide - Ca(OH)2: often abbreviated, using cement chemists' notation, to 'CH.' CH is formed mainly from alite hydration. Alite has a Ca:Si ratio of 3:1 and C-S-H has a Ca/Si ratio of approximately 2:1, so excess lime is available from alite hydration and this produces CH.
Ettringite: ettringite is present as rod-like crystals in the early stages of cement hydration. The chemical formula for ettringite is [Ca3Al(OH)6.12H2O]2.2H2O] or, mixing cement notation and normal chemistry notation, C3A.3CaSO4.32H2O.
Monosulfate: monosulfate tends to occur in the later stages of hydration, after a few days. Usually, it replaces ettringite, either fully or partly. The chemical formula for monosulfate is C3A.CaSO4.12H2O. Both ettringite and monosulfate are compounds of C3A, CaSO4 (anhydrite) and water, in different proportions.
AFm and AFt phases: monosulfate is one of a group of minerals called ‘AFm’ phases. Ettringite is a member of a group known as AFt phases. The general definitions of these phases are somewhat technical, but ettringite is an AFt phase because it contains three (t-tri) molecules of anhydrite when written as C3A.3CaSO4.32H2O and monosulfate is an AFm phase because it contains one (m-mono) molecule of anhydrite when written as C3A.CaSO4.12H2O.
Important points to note about AFm and AFt phases are that:
· They contain a lot of water, especially the AFt phases.
· They contain different ratios of sulfur to aluminium.
· The aluminium can be partly-replaced by iron in both AFm and AFt phases.
· The sulfate ion in AFm phases can be replaced by other anions; a one-for-one substitution if the anion is doubly-charged(eg: carbonate, CO22-) or one-for-two if the substituent anion is singly-charged (eg: hydroxyl, OH- or chloride, Cl-). The sulfate in ettringite can be replaced by carbonate or, probably, partly replaced by two hydroxyl ions.
Monosulfate gradually replaces ettringite in many concretes because the ratio of available alumina to sulfate increases with continued cement hydration. On mixing cement with water, most of the sulfate is readily available to dissolve, but much of the C3A is contained inside cement grains with no initial access to water. Continued hydration gradually releases alumina and the proportion of ettringite decreases as that of monosulfate increases.
If there is eventually more alumina than sulfate available, all the sulfate will be as monosulfate, with the additional alumina present as hydroxyl-substituted AFm phase. If there is an excess of sulfate, the cement paste will contain a mixture of monosulfate and ettringite. Near the concrete surface, carbonation will release sulfate as carbonate ions replace sulfate in the ettringite and monosulfate phases.
Hydrogarnet: hydrogarnet forms mainly as the result of ferrite or C3A hydration. Hydrogarnets have a range of compositions, of which C3AH6 is the main phase forming from normal cement hydration and then only in small amounts. A wider range of hydrogarnet compositions can be found in autoclaved cement products.
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