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Tobermorite gel is the main cementing component of cement paste. Natural tobermorite is <br />Ca5$16O16(~H)? ' 4(HZO). <br />The average diameter of a grain of ponland cement as ground from the clinker is about <br />10 Nm (microns). The particles of the hydration product, tobermorite gel, aze on the order of a <br />thousandth that size. The enormous surface area of the gel (about 3 million cm'`/g) results in <br />very large attractive forces, or cementation. <br />It is through water addition and cement hydration that curing and hardening occur. Concrete <br />does not "dry out" to harden, as is commonly thought. If concrete actually dries, or loses water, <br />it stops getting stronger. The reaction of water with cement in concrete may contimse for many <br />years after the concrete is poured, and the strength of the concrete wi{I continue to increase. <br />Each of the basic components of ponland cement contribute to its behavior. Upon the addition <br />of water to cement, tricalcium silicate rapidly reacts to release calcium ions, hydroxide ions, and <br />a large amount of heat. The pH quickly exceeds 12 due to the release of hydroxide (OH~) ions <br />(which are alkaline). This reaction is primazily responsible for the high eazly strength of <br />hydrated portland cement. Hydrated tricalcium silicate compound attains most of its strength in <br />7 days. <br />Dicalcium silicate takes several days to set. It is primarily responsible for the later- <br />developing strength of Portland cement paste. Since the hydration reaction proceeds slowly, the <br />heat of hydration is low. Hydrated dicalcium silicate compound produces little stre:~gth until <br />after 28 days. Tricalcium aluminate exhibits an instantaneous or flash set when hydrated. Ir is <br />primarily responsible for the initial set of Portland cement and gives off large amounts of heat <br />upon hydration. Gypsum added to Portland cement during grinding of the clinker combines with <br />tricalcium aluminate to control the time to set. Hydrated tricalcium aluminate compound <br />develops very tittle strength, and shows little strength increase after one day, but is useful in <br />varying concentrations and in combinatioa with gypsum to control set times. Fast setting <br />cement, with high concentrations of tricalcium aluminate, is less resistant to sulfate attack. <br />Tetracalcium aluminoferrite also hydrates rapidly and develops only a low strength. but it does <br />not exhibit a flash set. <br />CEMENT KILN DUST <br />Cement kiln dust (GIGO) is a by-product of cement manufacturing. CKD is an inorganic <br />material collected by air pollution control devices in Portland cement manufacturing plants and is <br />the finely-divided particulate matter carried from the cement kiln by exhaust gases. The dust is <br />composed of variable mixtures of calcined and uncalcined feed materials, fuel combustion <br />byproducts, condensed alkali compounds, and fine cement clinker formed during the high <br />temperature processing. Alkalis may be concentrated in the dust through volatilization in the <br />high temperature zones in the kiln then condensed in the exhaust gases as they pass through [he <br />dust collection system. The composition of CKD varies depending on production conditions and <br />the nature of the raw material and fuel. The actual form of the components may tyliically be: <br />Calcium Carbonate (CaCO3) 10 percent <br />Available Lime (as CaO) 30-50 percent <br />Potash (as Na and K salts) 6-]0 percent <br />Chloride 4 percent <br />