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Portland cement is the most common type of , Shale , Sand , Iron Ore , Bauxite , Fly Ash and Slag . When a Cement Kiln is fired by coal, the ash of the coal acts as a secondary raw material. HISTORY Portland cement was developed from cements (or correctly hydraulic limes) made in Britain in the early part of the nineteenth century, and its name is derived from its similarity to Portland Stone , a type of building stone that was quarried on the Isle Of Portland in Dorset , England . Joseph Aspdin , a British bricklayer, in 1824 was granted a patent for a process of making a cement which he called Portland cement. His cement was an artificial hydraulic lime similar in properties to the material known as " Roman Cement " (patented in 1796 by James Parker) and his process was similar to that patented in 1822 and used since 1811 by James Frost who called his cement "British Cement". The name "Portland cement" is also recorded in a directory published in 1823 being associated with a William Lockwood and possibly others. Aspdin's son William in 1843 made an improved version of this cement and he initially called it "Patent Portland cement" although he had no patent. In 1848 William Aspdin further improved his cement and in 1853 moved to Germany where he was involved in cement making."The Cement Industry 1796-1914: A History," by A. J. Francis, 1977 Many people have claimed to have made the first Portland cement in the modern sense, but it is generally accepted that it was first manufactured by William Aspdin at Northfleet , England in about 1842 P. C. Hewlett (Ed)''Lea's Chemistry of Cement and Concrete: 4th Ed'', Arnold, 1998, ISBN 0-340-56589-6, Chapter 1. The German Government issued a standard on Portland cement in 1878. PRODUCTION cement plant, Midlothian, Texas ]] There are three fundamental stages in the production of Portland cement: #Preparation of the raw mixture #Production of the Clinker #Preparation of the cement The chemistry of cement is very complex, so Cement Chemist Notation was invented to simplify the formula of common oxides found in cement. This reflects the fact that most of the elements are present in their highest oxidation state, and chemical analyses of cement are expressed as mass percent of these notional oxides. See Also: Rawmill The raw materials for Portland cement production are a mixture (as fine powder in the 'Dry process' or in the form of a Slurry in the 'Wet process') of minerals containing Calcium Oxide , Silicon Oxide , Aluminium Oxide , Ferric Oxide , and Magnesium Oxide . The raw materials are usually quarried from local rock, which in some places is already practically the desired composition and in other places requires the addition of Clay and Limestone , as well as Iron Ore , Bauxite or recycled materials. The individual raw materials are first crushed, typically to below 50 mm. In many plants, some or all of the raw materials are then roughly blended in a "prehomogenization pile". The raw materials are next ground together in a Rawmill . Silos of individual raw materials are arranged over the feed conveyor belt. Accurately controlled proportions of each material are delivered onto the belt by weigh-feeders. Passing into the rawmill, the mixture is ground to rawmix. The fineness of rawmix is specified in terms of the size of the largest particles, and is usually controlled so that there are less than 5-15% by mass of particles exceeding 90 μm in diameter. It is important that the rawmix contains no large particles in order to complete the chemical reactions in the kiln, and to ensure the mix is chemically homogenous. In the case of a dry process, the rawmill also dries the raw materials, usually by passing hot exhaust gases from the kiln through the mill, so that the rawmix emerges as a fine powder. This is conveyed to the blending system by conveyor belt or by a powder pump. In the case of wet process, water is added to the rawmill feed, and the mill product is a slurry with moisture content usually in the range 25-45% by mass. This slurry is conveyed to the blending system by conventional liquid pumps. Rawmix blending The rawmix is formulated to a very tight chemical specification. Typically, the content of individual components in the rawmix must be controlled within 0.1% or better. Calcium and silicon are present in order to form the strength-producing calcium silicates. Aluminium and iron are used in order to produce liquid ("flux") in the kiln burning zone. The liquid acts as a solvent for the silicate-forming reactions, and allows these to occur at an economically low temperature. Insufficient aluminium and iron lead to difficult burning of the clinker, while excessive amounts lead to low strength due to dilution of the silicates by aluminates and ferrites. Very small changes in calcium content lead to large changes in the ratio of alite to belite in the clinker, and to corresponding changes in the cement's strength-growth characteristics. The relative amounts of each oxide are therefore kept constant in order to maintain steady conditions in the kiln, and to maintain constant product properties. In practice, the rawmix is controlled by frequent chemical analysis (hourly by X-Ray Fluorescence analysis, or every 3 minutes by prompt gamma Neutron Activation Analysis ). The analysis data is used to make automatic adjustments to raw material feed rates. Remaining chemical variation is minimized by passing the raw mix through a blending system that homogenizes up to a day's supply of rawmix (15,000 tonnes in the case of a large kiln). See Also: Cement kiln The raw mixture is heated in a Cement Kiln , a slowly rotating and sloped cylinder, with temperatures increasing over the length of the cylinder up to a peak temperature of 1400-1450 °C . A complex succession of chemical reactions take place (see Cement Kiln ) as the temperature rises. The peak temperature is regulated so that the product contains Sintered but not fused lumps. Sintering consists of the melting of 25-30% of the mass of the material. The resulting liquid draws the remaining solid particles together by surface tension, and acts as a solvent for the final chemical reaction in which Alite is formed. Too low a temperature causes insufficient sintering and incomplete reaction, but too high a temperature results in a molten mass or glass, destruction of the kiln lining, and waste of fuel. The resulting material is Clinker . On cooling, it is conveyed to storage. Some effort is usually made to blend the clinker, because although the chemistry of the rawmix may have been tightly controlled, the kiln process potentially introduces new sources of chemical variability. The clinker can be stored for a number of years before use. Prolonged exposure to water decreases the Reactivity of cement produced from weathered clinker. The enthalpy of formation of clinker from calcium carbonate and clay minerals is ~1700 kJ/kg. However, because of heat loss during production, actual values can be much higher. The high energy requirements and the release of significant amounts of carbon dioxide makes cement production a concern for Global Warming . See "Environmental effects" below. See Also: Cement mill In order to achieve the desired setting qualities in the finished product, a quantity (2-8%, but typically 5%) of calcium sulfate (usually Gypsum or Anhydrite ) is added to the clinker and the mixture is finely ground to form the finished cement powder. This is achieved in a Cement Mill . The grinding process is controlled to obtain a powder with a broad Particle Size range, in which typically 15% by mass consists of particles below 5 μm diameter, and 5% of particles above 45 μm. The measure of fineness usually used is the " Specific Surface ", which is the total particle surface area of a unit mass of cement. The rate of initial reaction (up to 24 hours) of the cement on addition of water is directly proportional to the specific surface. Typical values are 320-380 m&2.kg-1 for general purpose cements, and 450-650 m&2.kg-1 for "rapid hardening" cements. The cement is conveyed by belt or powder pump to a silo for storage. Cement plants normally have sufficient silo space for 1-20 weeks production, depending upon local demand cycles. The cement is delivered to end-users either in bags or as bulk powder blown from a pressure vehicle into the customer's silo. In developed countries, 80% or more of cement is delivered in bulk, and many cement plants have no bag-packing facility. In developing countries, bags are the normal mode of delivery. USE Constituents that are permitted in Portland-composite cements are blastfurnace slag, Silica Fume , natural and industrial Pozzolan s, silicious and calcareous Fly Ash , burnt shale and limestone. See Also: White Portland cement White Portland cement differs physically from the gray form only in its color, and as such can fall into many of the above categories (e.g. ASTM Type I, II and/or III). However, its manufacture is significantly different from that of the gray product, and is treated separately. SAFETY AND ENVIRONMENTAL EFFECTS Safety When cement is mixed with water a highly Alkali ne solution ( PH ~13) is produced by the dissolution of Calcium , Sodium and Potassium Hydroxide s. Glove s, Goggle s and a Filter Mask should be used for protection. Hands should be washed after contact. Cement can cause serious burns if contact is prolonged or if skin is not washed promptly. Once the cement Hydrates , the hardened mass can be safely touched without gloves. In Scandinavia , France and the UK , the level of Chromium(VI) , which is thought to be toxic and a major skin irritant, may not exceed 2 Ppm (parts per million). Environmental effects Portland cement manufacture can cause environmental impacts at all stages of the process. These include Emissions of airborne pollution in the form of dust, gases, noise and vibration when operating machinery and during blasting in quarries, consumption of large quantities of fuel during manufacture, release of CO2 from the raw materials during manufacture, and damage to countryside from quarrying. Equipment to reduce dust emissions during quarrying and manufacture of cement is widely used, and equipment to trap and separate exhaust gases are coming into increased use. Environmental protection also includes the re-integration of quarries into the countryside after they have been closed down by returning them to nature or re-cultivating them. ''Epidemiologic Notes and Reports Sulfur Dioxide Exposure in Portland Cement Plants'', from the Centers for Disease Control states "Workers at Portland cement facilities, particularly those burning fuel containing sulfur, should be aware of the acute and chronic effects of exposure to SO2 dioxide , and peak and full-shift concentrations of SO2 should be periodically measured." http://www.cdc.gov/mmwr/preview/mmwrhtml/00000317.htm An independent research effort of AEA Technology to identify critical issues for the cement industry today concluded the most important environment, health and safety performance issues facing the cement industry are atmospheric releases (including greenhouse gas emissions, dioxin, NOx, SO2, and particulates), accidents and worker exposure to dust. http://www.wbcsd.ch/web/projects/cement/tf3/final_report10.pdf The CO2 associated with Portland cement manufacture falls into 3 categories: (1) CO2 derived from decarbonation of Limestone , (2) CO2 from kiln fuel combustion, (3) CO2 produced by vehicles in cement plants and distribution. Source 1 is fairly constant: minimum around 0.47 kg CO2 per kg of cement, maximum 0.54, typical value around 0.50 world-wide. Source 2 varies with plant efficiency: efficient precalciner plant 0.24 kg CO2 per kg cement, low-efficiency wet process as high as 0.65, typical modern proactice (e.g UK) averaging around 0.30. Source 3 is almost insignificant at 0.002-0.005. So typical total CO2 is around 0.80 kg CO2 per kg finished cement. This leaves aside the CO2 associated with electric power consumption, since this varies according to the local generation type and efficiency. Typical electrical energy consumption is of the order of 90-150 kWh per tonne cement, equivalent to 0.09-0.15 kg CO2 per kg finished cement if the electricity is coal-generated. Overall, with nuclear- or hydroelectric power and efficient manufacturing, CO2 generation can be as little as 0.7 kg per kg cement, but can be as high as twice this amount. The thrust of innovation for the future is to reduce sources 1 and 2 by modification of the chemistry of cement, by the use of wastes, and by adopting more efficient processes. Although cement manufacturing is clearly a very large CO2 emitter, Concrete (of which cement makes up about 15%) compares quite favorably with other building systems in this regard. See also Cement Kiln Emissions . CEMENT PLANTS AS ALTERNATIVES TO CONVENTIONAL WASTE DISPOSAL OR PROCESSING s]] Due to the high temperatures inside Cement Kiln s, combined with the oxidizing (oxygen-rich) atmosphere and long residence times, cement kilns have been used as a processing option for various types of waste streams. The waste streams often contain combustible material which allows the substitution of part of the fossil fuel normally used in the process. Waste materials used in cement kilns as a fuel supplement: {Link without Title} # Car and truck tires; steel belts are easily tolerated in the kilns # Waste solvents and lubricants. # Hazardous waste; cement kilns completely destroy hazardous organic compounds # Bone meal; slaughter house waste due to Bovine Spongiform Encephalopathy contamination concerns # Waste plastics # Sewage sludge # Rice shells # Sugar cane waste Portland cement manufacture also has the potential to remove industrial byproducts from the waste-stream, effectively sequestering some environmentally damaging wastes."As a generalization, probably 50% of all industrial byproducts have potential as raw materials for the manufacture of Portland cement." 1 These include: # Slag # Fly Ash (from power plants) # Silica Fume (from steel mills) # Synthetic Gypsum (from desulphurisation) SEE ALSO REFERENCES EXTERNAL LINKS
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