Portland Cement Essay -- essays research papers

PORTLAND CEMENTChemical composition.Portland Cement is made up of four chief(prenominal) compounds tri atomic number 20 silicate (3CaOSiO2), dicalcium silicate (2CaOSiO2), tricalcium aluminate (3CaOAl2O3), and a tetra-calcium aluminoferrite (4CaO Al2O3Fe2O3). In an abbreviated notation differing from the normal atomic symbols, these compounds atomic number 18 designated as C3S, C2S, C3A, and C4AF, where C stands for calcium oxide (lime), S for silica, A for alumina, and F for weigh oxide. Small amounts of uncombined lime and magnesia also are present, on with alkalies and minor amounts of other elements Hydration.The most important hydraulic constituents are the calcium silicates, C2S and C3S. Upon mixtureing with water, the calcium silicates react with water molecules to recoil calcium silicate supply (3CaO 2SiO23H2O) and calcium hydroxide (Ca OH 2). These compounds are given the shorthand notations C-S-H (represented by the just formula C3S2H3) and CH, and the Hydration re action posterior be crudely represented by the following reactions 2C3S + 6H = C3S2H3 + 3CH2C2S + 4H = C3S2H3 + CHDuring the initial stage of hydration, the parent compounds dissolve, and the dissolution of their chemical bonds generates a significant amount of heat. Then, for reasons that are not fully understood, hydration comes to a stop. This quiescent, or abeyant, period is extremely important in the placementumumum of concrete. Without a dormant period there would be no cement trucks, pouring would assimilate to be done immediately upon mixing. Following the dormant period (which can last several hours), the cement begins to harden, as CH and C-S-H are produced. This is the cementitious tangible that binds cement and concrete together. As hydration proceeds, water and cement are unceasingly consumed. Fortunately, the C-S-H and CH products occupy almost the same volume as the original cement and water volume is approximately conserved, and shrinkage is manageable. Although the formulas above treat C-S-H as a specific stoichiometry, with the formula C3S2H3, it does not at all form an ordered structure of uniform composition. C-S-H is actually an amorphous gel with a highly variable stoichiometry. The ratio of C to S, for example, can range from 11 to 21, depending on mix design and curing conditions. geomorphological properties.The strength developed by portland cement depends on its composition and the leanness to whi... ...s. Dust emission from cement kilns can be a unsafe nuisance. In populated areas it is usual and often compulsory to fit cyclone arrestors, bag-filter systems, or electrostatic dust precipitators between the kiln exit and the chimney stack. Modern cement plants are equipped with elaborate instrumentation for control of the burning process. affectionate strongs in some plants are sampled automatically, and a computer calculates and controls the raw mix composition. The largest rotary kilns have outputs exceeding 5,000 tons per day. GrindingThe clinker and the ask amount of gypsum are ground to a fine powder in horizontal mills similar to those used for cranch the raw squares. The material may pass straight through the mill (open-circuit grinding), or coarser material may be separated from the ground product and returned to the mill for come along grinding (closed-circuit grinding). Sometimes a small amount of a grinding aid is added to the feed material. For air-entraining cements the addition of an air-entraining agent is similarly made. Finished cement is pumped pneumatically to storage silos from which it is drawn for packing in report card bags or for dispatch in bulk containers.