Portland cement, 1.25

Cement is made by heating limestone (ealeium earbonate) with small quantities of other materials (sueh as clay) to 1450°C in a kiln, in a process known as calcination, whereby a molecule of carbon dioxide is liberated from the calcium carbonate to form calcium oxide, or quicklime, which is then blended with the other materials that have been included in the mix. The resulting hard substance, called clinker , is then ground with a small amount of gypsum into a powder to make Ordinary Portland Cement , the most eommonly used type of cement (often referred to as OPC). 

Portland cement is a basic ingredient of concrete, mortar and most nonspeciality grout. The most common use for Portland cement is in the production of concrete. Conerete is a composite material consisting of aggregate avel and sand), cement, and water. As a construction material, concrete can be cast in almost any shape desired, and once hardened, can become a structural (load bearing) element. Portland cement may be grey or white. 

Portland cement, which was discovered in 1824, sets from within. It is made by firing limestone ( 75%) with clays at about 1450 °C, causing partial fusion and the formation of a clinker, which is then powdered. This material typically contains the equivalents of about 67% CaO, 22% Si02, 5% AI2O3, 3% Fe20s, and the balance other materials, and the four chief components are the following anhydrous phases  

High ferrite contents give the cement a dark color. 

Conversely, coarsely ground clinkers with lower alite content are preferred for situations where cement emplacement is slow or temperatures can be high, as in the lining of oil wells. In such circumstances, organic 

Rapid cooling of the clinker is preferred for many reasons, notably to prevent the reversion of alite to belite and lime in the 1100-1250 °C regime and also the crystallization of periclase (MgO) at temperatures just below 1450 °C. The magnesium content of the cement should not exceed about 5% MgO equivalent because most of the Mg will be in the form of periclase, which has the NaCl structure, and this hydrates slowly to Mg(OH)2 (brucite), which has the Cdl2 layer structure (Section 4.6). Incorporation of further water between the OH layers in the Mg(OH)2 causes an expansion that can break up the cement. Accordingly, only limestone of low Mg content can be used in cement making dolomite, for example, cannot be used. Excessive amounts of alkali metal ions, sulfates (whether from components of the cement or from percolating solutions), and indeed of free lime itself should also be avoided for similar reasons. 

Many cements used today are composites of Portland cement and industrial waste materials that can enter into the hydration reactions and contribute to the strength of the hardened product. These substances include pulverized fuel ash (PFA) from burning of pulverized coal in thermal power stations, crushed blast-furnace slag (Section 17.7), and natural or artificial pozzolanas—that is, volcanic ash and similar finely particulate siliceous or aluminosilicate materials that can react with the Ca(OH)2 in Portland cement to form hydrated calcium silicates and aluminates. As noted earlier, the solubility of Ca(OH)2 is such that the pH of pore water in Portland cements will be about 12.7, at which the Si-O-Si or Si-O-Al links in the solid pozzolanas will be attacked slowly by OH to form discrete silicate and aluminate ions and thence hydrated calcium silicate or aluminate gels. 

While these attempts to optimize the strength and durability of cement were more or less unsystematic and empirical, the exact details of the chemistry of cement were first elucidated by Le ChateHer (1904). Later developments included the invention of reinforced concrete by Wilkinson and Lambot in 1855, and of blast furnace cement by Emil Langen in 1862. Thereafter, the twentieth century witnessed the invention and optimization of sulfate-resistant alumina cements (1908), the addition of plasticizers such as Hgnosulfonic acid or hydroxylated polysaccharides and superplasticizers such as sulfonated naphthalene-formaldehyde condensate, and the advent of macro-defect-free (MDF) and polymer fiber-reinforced cements, to name only a few. 

Although cement represents a construction material with a long and colorful tradition, research into the product still provides a rich field of endeavor, with modern cements about to enter the realm of high-tech materials, including applications for lightweight and durable motor blocks, barges, counter tops, and many more. Further details on the history and future prospects of cement and concrete can be found in the reports of Gani (1997), Aitkin (2000), Bensted and Barnes (2002), Hewlett (2004), and Locher (2006). 

Portland cement is fabricated by heating limestone with small quantities of materials carrying silica and alumina (such as clay) to about 1500°C in a rotary kiln this process is known as calcination. The resultant hard, sintered clinker is then ground together with a small amount of gypsum (ca. 3%) into a fine powder to produce so-called Ordinary Portland Cement (OPC), the most commonly used type of cement. 

Material Density (tm- ) Elastic modulus (CPa) Tensile strength (MPa) Energy per meter (CJ) Energy per meter/unit tensile strength (CJ MPa ) 

The cement type manufactured in the largest quantities and used most is Portland cement . It consists of so-called Portland cement clinkers admixed with gypsum or anhydrite. 

Lime- and clay-containing raw materials such as limestone, lime, clay, marl, lime marl, or clay marl (marls are naturally occurring mixtures of limestone and clay) are utilized as raw materials in the manufacture of Portland cement clinkers. 

According to the standard ASTM C125 from the American Society for Testing and Materials and the Portland Cement Association (PGA) an hydraulic cement is an inorganic material or 

Pozzuoli is an Italian town near Naples where volcanic ash was mined. 

There are numerous hydrated silicates of the alkali and alkaline-earth metals. Of the sodium compounds, with empirical formulae Na20. Si02. 5, 6,8, and 9 H2O, the first and last have been studied. Their structural formulae are Na2 Si02(0H)2]. 4 and 8 H20. They apparently contain ions [Si02(0H)2] in which Si-0 = 1-59 A and Si-OH = 1-67 A. 

In the following crystals we may regard the Si207 groups as part of more extensive tetrahedral systems in which the larger cations are 8-coordinated (melilite, danburite)  

Hemimorphite, an important zinc ore, is of interest as an example of the way in which X-ray studies lead to the assignment of correct structural formulae. It was formerly written H2Zn2Si05, but one-half of the H in this empirical formula is in the form of H2O molecules which may be removed by moderate heating of the crystal, whereby its form and transparency are not destroyed. The removal of further H2O, at temperatures above 500°C, results in destruction of the crystal. These facts are consistent with the crystal structure, which shows that the composition is Zn4(OH)2Si207. H20. The structure may be regarded as built from Zn03(0H) and Si04 tetrahedra these share three vertices to form layers of 

CAS [65997-15-1] gray powder containing less than 1% crystalline silica composed of dicalcium silicate, tricalcium silicate, and small amounts of alumina, iron oxide, and tricalcium aluminate insoluble in water. 

Portland cement is classified as a nuisance particulate. It does not cause fibrosis or lung damage. Exposure to its dusts may cause irritation of the eyes and nose, coughing, wheezing, bronchitis, and dermatitis. TLV-TWA 10 mg/m (ACGIH), 50 mppcf (OSHA and MSHA). 

Documentation of the Threshold Limit Values and Biological Exposure Indices, 5th ed. Cincinnati, OH American Conference of Governmental Industrial Hygienists. 

Hodgson, E., R. B. Mailman, and J. E. Chambers. 1988. Dictionary of Toxicology. New York Van Nostrand Reinhold. 

Occupational Exposure Limits for Airborne Toxic Substances. Geneva International Labor Office. 

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Portland cement A hydraulic cement made from CaC03 and aluminium silicates. 

Mixed with sand it hardens as mortar and plaster by taking up carbon dioxide from the air. Calcium from limestone is an important element in Portland cement. 

Although materials such as Portland cement (see Cement), solder (see Solder and brazing alloys), and siUcates can be considered to be adhesives, this discussion only includes organic materials such as those that form the materials presented in Table 1. 

Combinations of lignite flyash from North Dakota and hydrated lime can increase the strength and durabiHty of soils. The lime content varies from 2—7% and 1ime flyash ratio from 1 1 to 1 7 (61). Lignite flyash can also be used as a partial replacement for Portland cement to produce strong, durable concrete (62). 

T. R. Dobie md N. E. Henning, Eignite Flyash as a Partial Replacementfor Portland Cement in Concrete, final report PB 247414, Twin City Testing md Engineering Lab., Inc., St. Paul, Minn., 1975. 

Portland Cement Manufacture. The second greatest use of limestone is as raw material in the manufacture of Pordand cement (10). The average limestone factor per ton of Pordand cement is 1.0—1.1 t of pulverized limestone. The limestone, calcined to lime in the cement rotary kiln, combines with siUca and alumina to form tricalcium siUcate and tricalcium aluminate and other lesser cementing compounds (see Cement). Most cement companies operate captive limestone quarries. 

Several industries are highly dependent on cheap electric power. These include the aluminum industry, the Portland cement industry, electrochemical industries such as plating and chlorine production, the glass industry, and the pulp and paper industry. Other industries such as the petrochemical industry, which is highly competitive, depend on low priced power. About two-thirds of the cost of producing ammonia is electrical cost. 

In concrete, triethanolamine accelerates set time and increases early set strength (41—43). These ate often formulated as admixtures (44), for later addition to the concrete mixtures. Compared to calcium chloride, another common set accelerator, triethanolamine is less corrosive to steel-reinforcing materials, and gives a concrete that is more resistant to creep under stress (45). Triethanolamine can also neutralize any acid in the concrete and forms a salt with chlorides. Improvement of mechanical properties, whiteness, and more even distribution of iron impurities in the mixture of portland cements, can be effected by addition of 2% triethanolamine (46). Triethanolamine bottoms and alkanolamine soaps can also be used in these type appUcations. Waterproofing or sealing concrete can be accompUshed by using formulations containing triethanolamine (47,48). 

The Shoe grouting system is considered nonhazardous and nonpolluting. Sodium silicate is essentially nontoxic. Formamide is toxic and corrosive, but does not present a serious hazard if normal safety precautions are followed. Shoe chemical grout materials are two to five times more expensive than Portland cement, depending on the sodium silicate to formamide concentration ratios. Installed costs are generally more similar to those for cement grouts. 

Water-Repellent. Three techniques used for water repeUency are modification of cement by the addition of waterproofers, use of repellent additives to the concrete mix, and surface treatment of concrete stmctures with repellents. The modification of portland cement by intergrinding with stearate salts or other water-repellent material can reduce the water permeabiUty of mortar. Considerable controversy exists, however, as to whether these cements produce concrete that is superior to carefully mixed concrete without such additives (79). 

Portland cement is the most widely used constmction material in the world (see Cement), especially in Third World nations, because of its availabiHty, ease of use, and versatiHty. Estimated 1989 worldwide production is almost 1.12 biUion metric tons. The United States represented 71.2 million metric tons, ie, fourth, behind China (207 million metric tons), the former USSR (140 million metric tons), and Japan (82 million metric tons). Spain is tenth with 27 million tons. The top 10 world producers of Portland cement account for just under 43% of the total production. 

Portland cement is classified as a hydrauHc cement, ie, it sets or cures in the presence of water. The term Portland comes from its inventor, Joseph Aspdin, who in 1824 obtained a patent for the combination of materials referred to today as Portland cement. He named it after a grayish colored, natural limestone quarried on the Isle of Portland, which his cured mixture resembled. Other types of hydrauHc cements based on calcium materials were known for many centuries before this, going back to Roman times. Portland cement is not an exact composition but rather a range of compositions, which obtain the desired final properties. The compounds that make up Portland cements are calcium siHcates, calcium aluminates, and calcium aluminoferrites (see ). 

Portland cement is manufactured by two basic processes, the wet process and the dry process. The dry process uses approximately 25% less energy per ton of Portland cement and is used to produce about 68% of the U.S. Portland cement. Both processes start by mixing selected raw materials, cmshed and/or milled to approximately s in. (1.9 cm) diameter, in the correct ratios to give the final desired chemical composition. 

The manufacture of Portland cement is very energy intensive. It has been identified by the Department of Commerce as one of the six most energy... 

Other Types of Portland Cements. White Portland cementis standard Type I or III Pordand cement with raw materials selected and controUed to have negligible amounts of Hon and manganese oxides, which impart the gray color. The white Pordand cement is used in decorative and architectural appHcations like precast curtain waUs, terra22o surfaces, stucco, tile grout, and decorative concrete. 

Portland Cements, IS004.10T, Portland Cement Association, Skokie, lU, 1988. 

Portland Cement. Portland cement is obtained by calcining a mixture of substances to produce an appropriate ratio of the oxides CaO, MgO, AI2O2, Fe202, and Si02 (9) (see Cement). 

Table 2. Properties of the More Common Phases in Portland Cement Clinker ...

Calcium Silicates. Cements aie hydiated at elevated tempeiatuies foi the commercial manufacture of concrete products. Using low pressure steam curing or hydrothermal treatment above 100°C at pressures above atmospheric, the products formed from calcium siUcates are often the same as the hydrates formed from their oxide constituents. Hence lime and siUca ate ftequendy used in various proportions with or without Portland cement in the manufacture of calcium siUcate hydrate products. Some of these compounds are Hsted in Table 6. 

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