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Cement is manufactured through a closely controlled chemical combination of calcium, silicon, aluminum, iron and other ingredients.Common materials used to manufacture cement include limestone, shells, and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand, and iron ore. These ingredients, when heated at high temperatures form a rock-like substance that is ground into the fine powder that we commonly think of as cement. The most common way to manufacture cement is through a dry method. The first step is to quarry the principal raw materials, mainly limestone, clay, and other materials. After quarrying the rock is crushed. This involves several stages. The first crushing reduces the rock to a maximum size of about 6 inches. The rock then goes to secondary crushers or hammer mills for reduction to about 3 inches or smaller. The crushed rock is combined with other ingredients such as iron ore or fly ash and ground, mixed, and fed to a cement kiln. The cement kiln heats all the ingredients to about 2,700 degrees Fahrenheit in huge cylindrical steel rotary kilns lined with special firebrick. Kilns are frequently as much as 12 feet in diameter—large enough to accommodate an automobile and longer in many instances than the height of a 40-story building. The large kilns are mounted with the axis inclined slightly from the horizontal. The finely ground raw material or the slurry is fed into the higher end. At the lower end is a roaring blast of flame, produced by precisely controlled burning of powdered coal, oil, alternative fuels, or gas under forced draft. As the material moves through the kiln, certain elements are driven off in the form of gases. The remaining elements unite to form a new substance called clinker. Clinker is manufactured by heating raw materials inside the main burner of a kiln to a temperature of 1450 °C. The flame reaches temperatures of 1800 °C. The material remains at 1200 °C for 12–15 seconds at 1800 °C for 5–8 seconds (also referred to as residence time). These characteristics of a clinker kiln offer numerous benefits and they ensure a complete destruction of organic compounds, a total neutralization of acid gases, sulphur oxides and hydrogen chloride. Furthermore, heavy metal traces are embedded in the clinker structure and no by-products, such as ash of residues, are producedClinker comes out of the kiln as grey balls, about the size of marbles. Clinker is discharged red-hot from the lower end of the kiln and generally is brought down to handling temperature in various types of coolers. The heated air from the coolers is returned to the kilns, a process that saves fuel and increases burning efficiency.After the clinker is cooled, cement plants grind it and mix it with small amounts of gypsum and limestone. Cement is so fine that 1 pound of cement contains 150 billion grains. The cement is now ready for transport to ready-mix concrete companies to be used in a variety of construction projects. Cement plant laboratories check each step in the manufacture of cement by frequent chemical and physical tests. The labs also analyze and test the finished product to ensure that it complies with all industry specifications. Although the dry process is the most modern and popular way to manufacture cement, some kilns use a wet process. The two processes are essentially alike except in the wet process, the raw materials are ground with water before being fed into the kiln. Use of alternative fuels and by-products materials – A cement plant consumes 3 to 6 GJ of fuel per tonne of clinker produced, depending on the raw materials and the process used. Most cement kilns today use coal and petroleum coke as primary fuels, and to a lesser extent natural gas and fuel oil. Selected waste and by-products with recoverable calorific value can be used as fuels in a cement kiln (referred to as co-processing), replacing a portion of conventional fossil fuels, like coal, if they meet strict specifications. Selected waste and by-products containing useful minerals such as calcium, silica, alumina, and iron can be used as raw materials in the kiln, replacing raw materials such as clay, shale, and limestone. Because some materials have both useful mineral content and recoverable calorific value, the distinction between alternative fuels and raw materials is not always clear. For example, sewage sludge has a low but significant calorific value, and burns to give ash containing minerals useful in the clinker matrix. Scrap automobile and truck tires are useful in cement manufacturing as they have high calorific value and the iron embedded in tires is useful as a feed stock. Use of alternative fuels provides benefits for both society and the company: CO2-emissions are lower than with fossil fuels, waste can be co-processed in an efficient and sustainable manner and the demand for certain virgin materials can be reduced. Green cement is a cementitious material that meets or exceeds the functional performance capabilities of ordinary Portland cement by incorporating and optimizing recycled materials, thereby reducing consumption of natural raw materials, water, and energy, resulting in a more sustainable construction material. New manufacturing processes for producing green cement are being researched with the goal to reduce, or even eliminate, the production and release of damaging pollutants and greenhouse gasses, particularly CO2. Growing environmental concerns and the increasing cost of fuels of fossil origin have resulted in many countries in a sharp reduction of the resources needed to produce cement and effluents (dust and exhaust gases). |
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Cement Consumption
Cement consumption is dependent on the time of year and prevalent weather conditions. The seasonal nature of the industry can result in large swings in cement and clinker (unfinished raw material) inventories at cement plants over the course of a year. Cement producers will typically build up inventories during the winter and ship them during the summer. The domestic cement industry is regional in nature. The cost of shipping cement prohibits profitable distribution over long distances. As a result customers traditionally purchase cement from local sources. Nearly 98 percent of U.S. cement is shipped to its customers by truck. Barge and rail modes account for the remaining distribution modes.The majority of all cement shipments, approximately 70 percent, are sent to ready-mix concrete operators. The rest are shipped to manufacturers of concrete related products, contractors, materials dealers, oil well/mining/drilling companies, as well as government entities. With emerging markets showing positive trends, the global cement industry is expected to experience a compound annual growth rate of 9 percent by 2020. Increases in purchasing capacity, urban populations and residential construction will spur the demand for cement and drive growth. |
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Cement Types
Construction documents often specify a cement type based on the required performance of the concrete or the placement conditions. Certain cement manufacturing plants only produce certain types of portland cement. What are the differences in these cement types and how are they tested, produced, and identified in practice? In the most general sense, portland cement is produced by heating sources of lime, iron, silica, and alumina to clinkering temperature (2,500 to 2,800 degrees Fahrenheit) in a rotating kiln, then grinding the clinker to a fine powder. The heating that occurs in the kiln transforms the raw materials into new chemical compounds. Therefore, the chemical composition of the cement is defined by the mass percentages and composition of the raw sources of lime, iron, silica, and alumina as well as the temperature and duration of heating. It is this variation in raw materials source and the plant-specific characteristics, as well as the finishing processes (i.e. grinding and possible blending with gypsum, limestone, or supplementary cementing materials), that define the cement produced. Standards – To ensure a level of consistency between cement-producing plants, certain chemical and physical limits are placed on cements. These chemical limits are defined by a variety of standards and specifications. For instance, portland cements and blended hydraulic cements for concrete in the U.S. conform to the American Society for Testing and Materials (ASTM) C150 (Standard Specification for Portland Cement), C595 (Standard Specification for Blended Hydraulic Cement) or C1157 (Performance Specification for Hydraulic Cements). Some state agencies refer to very similar specifications: AASHTO M 85 for portland cement and M 240 for blended cements. These specifications refer to standard test methods to assure that the testing is performed in the same manner. For example, ASTM C109 (Standard Test Method for Compressive Strength for Hydraulic Cement Mortars using 2-inch Cube Specimens), describes in detail how to fabricate and test mortar cubes for compressive strength testing in a standardized fashion. Nomenclature Differences – In the US, three separate standards may apply depending on the category of cement. For portland cement types, ASTM C150 describes: Cement Type Description Type I Normal Type II Moderate Sulfate Resistance Type II (MH) Moderate Heat of Hydration (and Moderate Sulfate Resistance) Type III High Early Strength Type IV Low Heat Hydration Type V High Sulfate Resistance For blended hydraulic cements – specified by ASTM C595 – the following nomenclature is used: Cement Type Description Type IL Portland-Limestone Cement Type IS Portland-Slag Cement Type IP Portland-Pozzonlan Cement Type IT Ternary Blended Cement In addition, some blended cements have special performance properties verified by additional testing. These are designated by letters in parentheses following the cement type. For example Type IP(MS) is a portland-pozzolan cement with moderate sulfate resistance properties. Other special properties are designated by (HS), for high sulfate resistance; (A), for air-entraining cements; (MH) for moderate heat of hydration; and (LH) for low heat of hydration. Refer to ASTM C595 for more detail. However, with an interest in the industry for performance-based specifications, ASTM C1157 describes cements by their performance attributes: Cement Type Description Type GU General Use Type HE High Early-Strength Type MS Moderate Sulfate Resistance Type HS High Sulfate Resistance Type MH Moderate Heat of Hydration Type LH Low Heat of Hydration Note: For a thorough review of US cement types and their characteristics see PCA’s Design and Control of Concrete Mixtures, EB001 or Effect of Cement Characteristics on Concrete Properties, EB226. Physical and Chemical Performance Requirements – Chemical tests verify the content and composition of cement,while physical testing demonstrates physical criteria. In C150/M 85 and C595/M 240, both chemical and physical properties are limited. In C1157, the limits are almost entirely physical requirements. Chemical testing includes oxide analyses (SiO2, CaO, Al2O3, Fe2O3, etc.) to allow the cement phase composition to be calculated. Type II cements are limited in C150/M 85 to a maximum of 8 percent by mass of tricalcium aluminate (a cement phase, often abbreviated C3A), which impacts a cement’s sulfate resistance. Certain oxides are also themselves limited by specifications: For example, the magnesia (MgO) content which is limited to 6 percent maximum by weight for portland cements, because it can impact soundness at higher levels. Typical physical requirements for cements are: air content, fineness, expansion, strength, heat of hydration, and setting time. Most of these physical tests are carried out using mortar or paste created from the cement. This testing confirms that a cement has the ability to perform well in concrete; however, the performance of concrete in the field is determined by all of the concrete ingredients, their quantity, as well as the environment, and the handling and placing procedures used. Although the process for cement manufacture is relatively similar across North America and much of the globe, the reference to cement specifications can be different depending on the jurisdiction. In addition, test methods can vary as well, so that compressive strength requirements (for example) in Europe don’t ‘translate’ directly to those in North America. When ordering concrete for construction projects, work with a local concrete producer to verify that cement meeting the requirements for the project environment and application is used, and one that meets the appropriate cement specification. |
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