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  Portland cements are hydraulic cements composed prima-rily of hydraulic calcium silicates (Fig. 2-1). Hydrauliccements set and harden by reacting chemically with water.During this reaction, called hydration, cement combineswith water to form a stonelike mass, called paste. When thepaste (cement and water) is added to aggregates (sand andgravel, crushed stone, or other granular material) it acts asan adhesive and binds the aggregates together to formconcrete, the world’s most versatile and most widely usedconstruction material.Hydration begins as soon as cement comes in contactwith water. Each cement particle forms a fibrous growth onits surface that gradually spreads until it links up with thegrowth from other cement particles or adheres to adjacentsubstances. This fibrous build up results in progressivestiffening, hardening, and strength development. The stiff-ening of concrete can be recognized by a loss of workabil-ity that usually occurs within three hours of mixing, but isdependent upon the composition and fineness of thecement, any admixtures used, mixture proportions, andtemperature conditions. Subsequently, the concrete setsand becomes hard. Hydration continues as long as favorable moisture andtemperature conditions exist (curing) and space for hydra-tion products is available. As hydration continues, concrete CHAPTER 2 Portland, Blended, and Other Hydraulic Cements  becomes harder and stronger. Most of the hydration andstrength development take place within the first month, but then continues, though more slowly, for a long timewith adequate moisture and temperature. Continuousstrength increases exceeding 30 years have been recorded(Washa and Wendt 1975 and Wood 1992). THE BEGINNING OF AN INDUSTRY Early builders used clay to bind stones together into a solidstructure for shelter and protection. The oldest concrete dis-covered so far dates from around 7000 BC and was found in1985 when a concrete floor was uncovered during the con-struction of a road at Yiftah El in Galilee, Israel. It consistedof a lime concrete, made from burning limestone to producequicklime, which when mixed with water and stone, hard-ened to form concrete (Brown 1996 and Auburn 2000). Acementing material was used between the stone blocks in the construction of the Great Pyramid at GizainAncient Egypt around 2500 BC. Some reports sayit wasalime mortar while others say the cementing material wasmade from burnt gypsum. By 500 BC, the art ofmaking lime- based mortar arrived in Ancient Greece. The Greeks usedlime-based mate-rials as a binder between stone and brick and as arendering materialover porous lime-stones commonlyused in the con-struction of theirtemples and pal-aces.Examples of early Roman con-crete have beenfound dating backto 300 BC. Thevery word con- Fig. 2-1. Portland cement is a fine powder that when mixedwith water becomes the glue that holds aggregates togetherin concrete. (58420)Fig. 2-2. Isle of Portland quarry stone(after which portland cement wasnamed) next to a cylinder of modernconcrete. (68976) 21 HOME PAGE  22 Design and Control of Concrete Mixtures   EB001 Fig. 2-3. Steps in the traditional manufacture of portland cement.  23 Chapter 2  Portland, Blended, and Other Hydraulic Cements Fig. 2-4. Steps in the modern dry-process manufacture of portland cement.  Aspdin was the first to prescribe a formula for port-land cement and the first to have his product patented.However, in 1845, I. C. Johnson, of White and Sons, Swans-combe, England, claimed to have “burned the cement rawmaterials with unusually strong heat until the mass wasnearly vitrified,” producing a portland cement as we nowknow it. This cement became the popular choice during themiddle of the 19th century and was exported from Englandto various parts of the world. Production also began inBelgium, France, and Germany about the same time andexport of these products from Europe to North America began about 1865. The first recorded shipment of portlandcement to the United States was in 1868. The first portlandcement manufactured in the United States was produced ata plant in Coplay, Pennsylvania, in 1871. MANUFACTURE OF PORTLAND CEMENT Portland cement is produced by pulverizing clinker whichconsists primarily of hydraulic calcium silicates. Clinkeralso contains some calcium aluminates and calcium alu-minoferrites and one or more forms of calcium sulfate(gypsum) is interground with the clinker to make thefinished product.Materials used in the manufacture of portland cementmust contain appropriate amounts of calcium, silica,alumina, and iron components. During manufacture,chemical analyses of all materials are made frequently toensure a uniformly high quality cement. Steps in the manufacture of cement are illustrated inthe flow charts in Figs. 2-3 and 2-4. While the operations of all cement plants are basically the same, no flow diagramcan adequately illustrate all plants. There is no typical port-land cement manufacturing facility; every plant has signif-icant differences in layout, equipment, or general ap-pearance (Fig. 2-5). Selected raw materials (Table 2-1) are transported fromthe quarry (Fig. 2-6), crushed (Fig. 2-7), milled, and propor-tioned so that the resulting mixture has the desired chemi-cal composition. The raw materials are generally a mixturecrete is derived from the Latin word “concretus” meaninggrown together or compounded. The Romans perfected theuse of pozzolan as a cementing material. Sometime duringthe second century BC the Romans quarried a volcanic ashnear Pozzuoli; thinking it was sand, they mixed it with limeand found the mixture to be much stronger than they hadproduced previously. This discovery was to have a signifi-cant effect on construction. The material was not sand, buta fine volcanic ash containing silica and alumina, whichwhen combined chemically with lime, produced what became known as pozzolanic cement. This material wasused by builders of the famous Roman walls, aqueductsand other historic structures including the Theatre atPompeii (seating 20,000 spectators), and the Colosseumand Pantheon in Rome. Pozzolan seems to have beenignored during the Middle Ages when building practiceswere much less refined than earlier and the quality of cementing materials deteriorated. The practice of burninglime and the use of pozzolan was not introduced againuntil the 1300s.Efforts to determine why some limes possess hy-draulic properties while others (those made from essen-tially pure limestones) do not were not made until the 18thcentury. John Smeaton, often referred to as the “father of civil engineering in England,” concentrated his work inthis field. He found that an impure, soft limestone, contain-ing clay minerals made the best hydraulic cement. Thiscombined with a pozzolan, imported from Italy, was usedin his project to rebuild the Eddystone Lighthouse in theEnglish Channel, southwest of Plymouth England. Theproject took three years to complete and began operation in1759; it was recognized as a significant accomplishment inthe development of the cement industry. Anumber of discoveries followed as efforts within a growing naturalcement industry were now directed to the production of aconsistent quality material.The difference between a hydraulic lime and naturalcement is a function of the temperature attained duringcalcination. Furthermore, a hydraulic lime can hydrate in a“lump” form, whereas natural cements must be crushedand finely ground before hydration can take place. Naturalcement is stronger than hydraulic lime but weaker thanportland cement. Natural cement was manufactured inRosendale, New York in the early 1800s (White 1820) and was first used to build the Erie Canal in 1818 (Snell andSnell 2000).The development of portland cement was the result of persistent investigation by science and industry to producea superior quality natural cement. The invention of port-land cement is generally credited to Joseph Aspdin, anEnglish mason. In 1824, he obtained a patent for his prod-uct, which he named portland cement because when set, itresembled the color of the natural limestone quarried onthe Isle of Portland in the English Channel (Fig. 2-2)(Aspdin 1824). The name has endured and is used through- out the world, with many manufacturers adding their owntrade or brand names. 24 Design and Control of Concrete Mixtures   EB001 Fig. 2-5. Aerial view of a cement plant. (70000)
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