Portland Cement Essay -- essays research papers

PORTLAND CEMENTChemical composition.Portland Cement is made up of four main compounds tricalcium 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 ar 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 in any case are present, along with alkalies and minor amounts of other elements Hydration.The most important hydraulic constituents are the calcium silicates, C2S and C3S. Upon mixing with water, the calcium silicates react with water molecules to form calcium silicate hydrate (3CaO 2SiO23H2O) and calcium hydroxide (Ca OH 2). These compounds are given the shorthand notations C-S-H (represented by the average formula C3S2H3) and CH, and the Hydration reaction can be crudely represented b y 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 dormant, check is extremely important in the placement of concrete. Without a dormant period there would be no cement trucks, pouring would sustain to be make 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 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 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 stoic hiometry, with the formula C3S2H3, it does not at all form an ordered structure of uniform composition. C-S-H is actually an unstructured 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 circle 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 heavy 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. 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 tons per day. GrindingThe clinker and the required amount of gypsum are estate to a fine powder in horizontal mills alike to those used for grinding the raw materials. 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 further 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 withal made. Finished cement is pumped pneumatically to storage silos from which it is drawn for packing in paper bags or for dispatch in bulk containers.