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PRODUCTION, CHEMISTRY, AND PROPERTIES OF CEMENT <br />Cemen[ is produced by burning limestone and clay at around 2700EF in a horizontal, <br />inclined rotazy kiln. It can take up to 2 hours for the raw materials to pass through the kiln <br />depending on its length. Moving down the cylinder, the mixture progresses through four stages <br />of transformation. Initially, free water is driven off. Next, calcination occurs as bound water and <br />carbon dioxide are liberated. After calcination, the limestone has been converted to lime (Ca0). <br />In the third or clinkering stage, lime and decrepitated clay combine to form calcium silicates and <br />calcium aluminates (see equations following). The fourth stage involves cooling of the clinker. <br />In some cement plants, the first three steps all occur in the same kiln; in other plants, the process <br />occurs in separate calciners and cement kilns. <br />CaC03 + [SiOz + A1z03 + Fe203 + H20(bound)] + A~ <br />{limestone) (clay) (heat) <br />~ 3Ca0•Si02 + 2Ca0•Si02 + 3Ca0•A120; + 4Ca0•A1203~Fez03 <br />(tricalcium silicate) + (dicalcium silicate) + Qricalcium aluminate) + (tetrncalcium aluminoferrite) <br />Compounds on the product side of the above equation comprise about 90 percent of portland <br />cement. The two calcium silicates form approximately 75 percent of cement by weight. When <br />water reacts with the two calcium silicates, tobermorite gel and calcium hydroxide are produced. <br />Tobermorite gel is the main cementing component of cement paste. Natural tobermorite is <br />CaSSibOi6(OH)z' 4(H20). <br />The average diameter of a grain of portland cement as ground from the clinker is about <br />10µm (microns). The particles of the hydration product, tobermorite gel, are on the order of a <br />thousandth that size. The enormous surface area of the gel (about 3 million cmz/g) results in <br />very lazge 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 continue for many <br />years after the concrete is poured, and the strength of the concrete will continue to increase. <br />Each of the basic components of portland 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 primarily responsible for the high early 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 primazily 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 strength until <br />after 28 days. Tricalcium aluminate exhibits an instantaneous or flash set when hydrated. It 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 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 />