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1Chemical Composition of Cement The raw materials used for the manufacture of cement consist mainly of lime, silica,alumina and iron oxide. These oxides interact with one another in the kiln at high temperatureto form more complex compounds. The relative proportions of these oxide compositions areresponsible for influencing the various properties of cement; in addition to rate of cooling andfineness of grinding. Table 1.4 shows theapproximate oxide composition limits of ordinaryPortland cement.Table 1.4. Approximate Oxide Composition Limits of Ordinary Portland CementOxidePer cent contentCaO60–67SiO217–25Al2O33.0–8.0Fe2O30.5–6.0MgO0.1–4.0Alkalies ( K2O, Na2O)0.4–1.3SO31.3–3.0Theidentification of the major compoundsof cementis largely based on Bogue’s equations and hence it iscalled “Bogue’s Compounds”. The four compounds usually regarded as major compounds arelisted in table 1.5.Table 1.5. Major compounds of cementName of CompoundFormulaAbbreviated FormulaTricalcium silicate3 CaO.SiO2C3SDicalcium silicate2 CaO.SiO2C2STricalcium aluminate3 CaO.Al2O3C3ATetracalcium aluminoferrite4 CaO.Al2O3.Fe2O3C4AFIt is to be noted that for simplicity’s sake abbreviated notations are used. C stands for CaO,S stands for SiO2, A for Al2O3, F for Fe2O3and H for H2O.The equations suggested by Bogue for calculating the percentages of major compoundsare given below.C3S = 4.07 (CaO) –7.60 (SiO2) –6.72 (Al2O3) –1.43 (Fe2O3) –2.85 (SO3)C2S = 2.87 (SiO2) –0.754 (3CaO.SiO2)C3A = 2.65 (Al2O3) –1.69 (Fe2O3)
2C4AF= 3.04 (Fe2O3)In addition to the four major compounds, there are many minor compounds formed inthe kiln. Two of the minor oxides namely K2O and Na2O referred to asalkalis in cement are of someimportanceand Expressed in terms ofNa2O. These alkalis basically react with active silica in aggregate and producewhat is calledalkali-silica gel of unlimitedswelling type underfavourable conditions ofmoisture and temperature in voids and cracks and further it causes disruption and pattern cracking.Tricalcium silicate and dicalcium silicate are the most important compounds responsiblefor strength. Together they constitute 70 to 80 per cent of cement. The average C3S contentin modern cement is about 45 per cent and that of C2S is about 25 per cent.The calculated quantityof the compounds in cement varies greatly even for a relatively small change in the oxidecomposition of the raw materials. To manufacture a cement of stipulated compoundcomposition, it becomes absolutely necessary to closely control the oxide composition of theraw materials.SO3 also appear in cement analysis which comesfrom adding gypsum (4-6) %during clinker gridding. The Iraqi and British specification for normal high rapid Portland cement pointed that SO3content must be between ( 3-2.5 )% according to type of cement and C3A content.The percentage of MgO in cement which is come from Magnesia compoundsin raw material. is about ( 4-1)% and 5% as maximum range to control expansion from hydration of this oxide in hard concrete.An increase in limeCaOcontent beyond a certain value makes it difficult to combinewith other compounds and free lime will exist in the clinker which causes unsoundness incement. Insoluble residue is that part of the Cement non-soluble in hydrochloric acidHCl and arise mainly from non active silica to form cement compounds dissolved in thisacid therefore it express the completeness of the chemical reactions inside the rotary kiln.Loss on Ignition Loss on ignition is calculated by heating up a cement sample to 900 –1000°C until a constant weight is obtained. The weight loss of the sample due to heating is then determined.A high loss on ignition can indicate prehydration and carbonation, which may be caused byimproper and prolonged storage. Iraqi specification limits is 4% as maximum for normal and rapid hardening Portland cement.