PORTLAND CEMENTChemical composition.Portland cement is composed of four main compounds: tricalcium silicate (3CaOSiO2), dicalcium silicate (2CaOSiO2), tricalcium aluminate (3CaOAl2O3) and tetracalcium aluminoferrite (4CaO Al2O3Fe2O3). ). In an abbreviated notation different from normal atomic symbols, these compounds are designated C3S, C2S, C3A, and C4AF, where C represents calcium oxide (lime), S represents silica, A represents alumina, and F represents iron oxide. . Small amounts of uncombined lime and magnesia are also present, as well as alkalis and minor amounts of other hydrating elements. The most important hydraulic constituents are calcium silicates, C2S and C3S. When mixed with water, calcium silicates react with water molecules to form hydrated calcium silicate (3CaO 2SiO23H2O) and calcium hydroxide (Ca[OH]2). These compounds are given the abbreviated notations CSH (represented by the average formula C3S2H3) and CH, and the hydration reaction can be roughly represented by the following reactions: 2C3S + 6H = C3S2H3 + 3CH2C2S + 4H = C3S2H3 + CHD during the phase initial 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 stops. This period of rest or dormancy is extremely important in the placement of concrete. Without a dormant period there would be no cement trucks, pouring would have to be done immediately after mixing. After the dormant period (which can last several hours), the cement begins to harden, as CH and CSH are produced. It is the cementitious material that binds cement and concrete together. As hydration occurs, water and cement are continually consumed. Fortunately, CSH and CH products occupy almost the same volume as the original cement and water; volume is approximately preserved and shrinkage is manageable. Although the above formulas treat CSH as a specific stoichiometry, with the formula C3S2H3 it does not form an ordered structure of uniform composition at all. CSH is in reality an amorphous gel whose stoichiometry is very variable. The C/S ratio, for example, can vary from 1:1 to 2:1, depending on mix design and curing conditions. Structural properties. The strength developed by Portland cement depends on its composition and the fineness of the cement. ... middle of paper ......s.Dust emissions from cement kilns can be a serious nuisance. In populated areas, it is customary and often mandatory to install cyclone arresters, bag filter systems or electrostatic precipitators between the furnace outlet and the chimney. Modern cement plants are equipped with elaborate instruments to control the combustion process. Raw materials from some plants are sampled automatically and a computer calculates and controls the composition of the raw mixture. The largest rotary kilns have a production of more than 5,000 tonnes per day. Grinding The clinker and the required amount of gypsum are ground to a fine powder in horizontal mills similar to those used for grinding raw materials. The material may pass directly 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 grinding aid is added to the raw material. For air-entraining cements, the addition of an air-entraining agent is carried out in the same way. The finished cement is pumped pneumatically to storage silos from where it is suctioned..