Portland Cement Essay -- essays research papers

PORTLAND CEMENTChemical composition.Portland Cement is make up of four main compounds tricalcium silicate (3CaOSiO2), dicalcium silicate (2CaOSiO2), tricalcium aluminate (3CaOAl2O3), and a tetra-calcium aluminoferrite (4CaO Al2O3Fe2O3). In an abbreviated none of hand differing from the normal atomic symbols, these compounds are designated as C3S, C2S, C3A, and C4AF, where C stands for calcium oxide (lime), S for silica, A for alumina, and F for iron oxide. Small amounts of uncombined lime and magnesia also are present, along with alkalies and minor amounts of other elements Hydration.The most Coperni brook hydraulic constituents are the calcium silicates, C2S and C3S. Upon commixtureing with piddle, the calcium silicates react with body of water molecules to form calcium silicate hydrate (3CaO 2SiO23H2O) and calcium hydroxide (Ca OH 2). These compounds are condition the shorthand nonations C-S-H (represented by the average formula C3S2H3) and CH, and the Hydration reaction c an be crudely represented by the following reactions 2C3S + 6H = C3S2H3 + 3CH2C2S + 4H = C3S2H3 + CHDuring the initial stage of hydration, the elevate 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 dormant, period is extremely important in the pla cement of concrete. Without a dormant period there would be no cement trucks, pouring would have to be done immediately upon mixing. sideline 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 material that binds cement and concrete together. As hydration proceeds, water and cement are continuously consumed. Fortunately, the C-S-H and CH products occupy almost the same hoi polloi as the original cement and water volume is approximately conserved, and shrinking is manageable. Although the formulas above t reat 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 in truth 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. Structural properties.The strength developed by portland cement depends on its composition and the fineness to whi... ...s. Dust emission from cement kilns can be a serious nuisance. In populated areas it is usual and oft 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. Raw materials 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 loads per day. GrindingThe clinker and the re quired amount of gypsum are basis to a fine powder in horizontal mills similar to those employ for grinding the raw materials. The material may pass straight by means of the mill (open-circuit grinding), or coarser material may be separated from the ground product and returned to the mill for further grinding (closed-circuit grinding). Sometimes a nonaged amount of a grinding aid is added to the feed material. For air-entraining cements the addition of an air-entraining federal agent is similarly made. Finished cement is pumped pneumatically to storage silos from which it is raddled for packing in paper bags or for dispatch in loudness containers.