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• Air-entraining agents, usually natural or synthetic soaps, create microscopic bubbles within <br />the paste to relieve pressure and prevent cracking during freezing conditions. <br />• Calcium chloride is used to accelerate the set and development of strength of concrete. It <br />improves workability, reduces bleeding, and creates a more durable surface. Possible <br />problems include increased drying shrinkage, increase in the rate of heat liberation, and <br />corrosion of reinforcing steel. <br />• Pozzolanic materials are siliceous substances (e.g., Fly ash or pumice) that can be used in <br />combination with or for partial replacement of ponland cement. Pozzolan enhances <br />workability with less total water, provides cost reduction through cement savings, reduces the <br />heat of hydration, and increases resistance to sulfates. Disadvantages of pozzolans include <br />slow development of final strength, increased drying shrinkage, and impaired durability. <br />• Set retarders may increase setting times for concrete by several hours. These agents <br />generally improve workability so the amount of mixing water may be reduced with an <br />increase in ultimate compressive strength. <br />• Superplasticizers increase ultimate concrete strength by decreasing the amount of water <br />needed to prepare workable concrete. <br />• Five types of Portland cement have been classified by ASTM specification C150. Types I <br />and II are general purpose with Type II having lower heat of hydration and enhanced sulfate <br />resistance by limiting tricalcium silicate and tricalcium aluminate content. Type III is a high <br />early strength cement prepared by increasing tricalcium silicate and tricalcium aluminate and <br />by finer grinding. Type IV is aloes-heat-of-hydration cement prepared by limiting tricalcium <br />aluminate and tricalcium silicate. This limitation also results in lower early strength. Low <br />heat of hydration is critical in mass-concrete applications because heat can expand the <br />volume of a large pour while the concrete is plastic, then differential cooling after hardening <br />causes shrinkage cracking. Type V is a sulfate resistant cement prepared by reducing sulfate <br />susceptible tricalcium aluminate content to a minimum. Note that the pozzolan admixture <br />can enhance Type IV and V cements, while at the same time reducing cost. <br />• The water cement ratio is the prime factor affecting the strength of concrete. In applications <br />where high strength is critical, the mixing water should be reduced to the minimum needed to <br />attain necessary workability. <br />• Cement content in the concrete mix is also critical to ultimate compressive strength, with <br />strength decreasing as cement content is decreased. However, cement is by far the most <br />expensive component of concrete, so an economic advantage is gained by using as little as <br />possible while obtaining the minimum specified strength. <br />• Contrazy to popular belief, concrete does not harden by drying, but rather by hydrating. <br />Curing conditions are vital to the development of ultimate compressive strength, and <br />moisture must be maintained in the concrete during the curing period. Concrete strength is <br />detrimentally affected by curing in a dry atmosphere. <br />• Mixing water needs to be pure in order to prevent side reactions from occurring that may <br />weaken the concrete or otherwise interfere with the hydration process. Potable water is often <br />specified. <br />CEMENT KILN DUST <br />