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From the above discussion, it is clear that it is through [he addition of water and hydration of <br />cement that curing and hardening may occur. Concrete does not dry out to harden, as is <br />commonly thought. When concrete dries, it actually stops getting stronger. The reaction of <br />water with cement in concrete may continue for many years after the concrete is poured, and the <br />strength of the concrete will continue to increase. Each of the basic components of portland <br />cement contribute to its behavior. Upon the addition of water to cement, tricalcium silicate <br />rapidly reacts to release calcium ions, hydroxide ions, and a large amount of heat. The pH <br />quickly rises to over 12 because of the release of alkaline hydroxide (OH) ions. This reaction is <br />primarily responsible for the high early strength of hydrated portland cement. Hydrated <br />tricalcium silicate compound attains most of its strength in 7 days. <br />Dicalcium silicate takes several days to set. It is primarily responsible for the later-developing <br />strength of portland cement paste. Since the hydration reaction proceeds slowly, the heat of <br />hydration is low. Hydrated dicalcium silicate compound produces little strength until after 28 <br />days. Tricalcium aluminate exhibits an instantaneous or flash set when hydrated. It is primarily <br />responsible for the initial set of portland cement and gives off large amounts of heat upon <br />hydration. Gypsum added to portland cement during grinding of [he clinker combines with <br />tricalcium aluminate to control the time to set. Hydrated tricalcium aluminate compound <br />develops very little strength, and shows little strength increase after one day, but is useful in <br />varying concentrations and in combination 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 />In addition to varying the composition of the four major components of cement discussed above, <br />speed of hydration is also affected by: <br />• Fineness of grinding. To achieve faster hydration, cements are ground finer. <br />• Amount of water added. Presence of a sufficient amount of water will speed the reaction rate <br />and enhance workability of concrete. All concrete is mixed with more water than is needed <br />for the hydration reactions. This is done to produce flowing concrete that will develop <br />adequate 7 and 28 day strength. However, water not consumed in the hydration reaction will <br />remain in the microstructure pore space. These pores make the concrete weaker due to the <br />lack of strength forming calcium silicate hydrate bonds. Thus a higher water:cement ratio <br />yields a lower strength concrete, and workability and reaction rate must be balanced against <br />ultimate strength. <br />• Higher temperatures of the constituents (cement, water, aggregate) at the time of mixing will <br />also speed the hydration reaction rates. <br />Certain admixtures may be added to concrete that will modify its characteristics. Use of <br />admixtures must be evaluated carefully since improvement of one characteristic often results in <br />an adverse effect on another characteristic. In addition to admixtures, concrete properties may be <br />varied by using the different grades of portland cement that are available and by adjusting the <br />basic ratios of the concrete mixture of water, cement and aggregate. <br />4 <br />