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Lightweight Aggregates

Lightweight Aggregates can be divided into two major categories: 1.Those occurring naturally and are ready to use only with mechanical treatment, i.e., crushing and sieving. 2.Those produced by thermal treatment from either naturally occurring materials or from industrial byproducts, waste materials, etc. The industrial byproducts are pulverized fly ash PFA, blast furnace slag, industrial waste, sludge, etc. These are produced either by expansion, agglomeration or sintering. The heat treatment is carried out in different types of industrial furnaces, such as rotary kilns. Lightweight aggregate is a type of coarse aggregate that is used in the production of lightweight concrete products such as over deck waterproofing application, concrete block, structural concrete, and pavement. Most lightweight aggregate is produced from materials such as clay, shale, or slate. Blast furnace slag, natural pumice, vermiculite, and perlite can be used as substitutes, however. To produce lightweight aggregate, the raw material (excluding pumice) is expanded to about twice the original volume of the raw material. The expanded material has properties similar to natural aggregate, but is less dense and therefore yields a lighter concrete product. The production of lightweight aggregate begins with mining or quarrying the raw material. The material is crushed with cone crushers, jaw crushers, hammermills, or pugmills and is screened for size. Oversized material is returned to the crushers, and the material that passes through the screens is transferred to hoppers. From the hoppers, the material is fed to a rotary kiln, which is fired with coal, coke, natural gas, or fuel oil, to temperatures of about 1200°C (2200°F). As the material is heated, it liquefies and carbonaceous compounds in the material form gas bubbles, which expand the material; in the process, volatile organic compounds (VOC) are released. From the kiln, the expanded product (clinker) is transferred by conveyor into the clinker cooler where it is cooled by air, forming a porous material. After cooling, the lightweight aggregate is screened for size, crushed if necessary, stockpiled, and transported. In addition, a few plants process naturally occurring lightweight aggregate such as pumice, nodullar, vermiculite, etc.