1121 CONSTRUCTION USES INTRODUCTION This chapter uses the term cement in it is historical meaning of gray and white portland cement, a crystalline compound of cal- cium silicates and other calcium compounds having hydraulic properties. Several other hydraulic cements of similar chemical form exist and are produced in similar manner, but they have lim- ited and specialized markets. Much of this chapter applies to these cements as well, although they are not directly discussed. Portland cement is produced by intergrinding cement clinker and gypsum in an approximate ratio of 95:5. Cement clinker is a fused product consisting of principally dicalcium silicate, tricalcium silicate, tri- calcium aluminate, and tetracalcium alumino ferrite formed by high-temperature reaction in a rotary kiln of carefully proportioned and blended ratios of lime, silica, alumina, and iron oxide. The lime fraction is derived from limestone in the calcining phase of production. White clinker is extremely low in iron oxide, which requires purer raw materials. Table 1 summarizes typical percent- ages of clinker compounds for the most common portland cement, Type I. MARKET A few large companies dominate the marketing and production of cement. Widely distributed plants minimize transportation costs to customers. Although the volume of cement to be sold in the market- place can be considered inelastic in the short run, a company’s sales effort makes a difference. In any given market, although cement sold by competitors must meet certain basic specifications to be consid- ered, other factors dominate, including delivered cost, quality, prod- uct consistency, technical assistance, and sales relationships with the user companies. Sales relationships ensure the cement company of consideration in the first place, they allow the settlement of questions surrounding the sale, and they offer the selling company a last “look” before sales commitments are made. Sales relationships are based on positive past relations, trust, and friendship. In the long run, several factors drive the consumption of cement in the marketplace, including economic growth, private and governmental capital investment, and population growth. Influenc- ing decision makers by promoting the architectural, economic, and durability advantages of concrete also drives the market, as does financial or other assistance to concrete suppliers. Cement companies often evaluate sales success as their con- tinued penetration of the local or regional market expressed as mar- ket share. Sometimes this devotion to market share evades the real corporate target of profitability. In the 1960s the federal highway program was a chief reason for cement plant expansion, along with the nation’s continued eco- nomic and population growth. At the same time, a new trend estab- lished distribution terminals to sell cement to greater geographical markets accessible via water transportation or favorable rail rates. This trend increased competition and held down profits. From 1957 to 1969 the inflation-adjusted price of cement declined significantly. In the 1970s, real prices began to increase but profits were diverted to meet new environmental requirements rather than creating greater efficiency and capital. With a surge in building in coastal markets in the late 1970s, imports began to be a factor. When cement companies did not make the investment to increase capacity or import cement to Table 1. Probable clinker compounds in Type I portland cement Compound Abbreviation Common Abbreviation Oxide Composition Stoichiometric Composition Approximate Content in Type I Portland Cement, % * Tricalcium silicate (alite) 3CS C 3 S (CaO) 3 SiO 2 Ca 3 SiO 5 45 Dicalcium silicate (belite) 2CS C 2 S (CaO) 2 SiO 2 Ca 2 SiO 4 27 Tricalcium aluminate 3CA C 3 A (CaO) 3 Al 2 O 3 Ca 3 Al 2 O 6 11 Tetracalcium-aluminoferrite † 4CAF C 4 AF (CaO) 4 (Al 2 O 3 )(Fe 2 O 3 ) Ca 4 Al 2 Fe 2 O 10 08 Adapted from Clausen 1960. * Commercial cements contain 4%–6% gypsum or anhydrite (for regulation of the “setting time” of the concrete), approximately 0.5% each of alkali oxides (Na 2 O and K 2 O) and uncombined CaO, plus a few percent impurities, largely MgO. † This composition of the iron-containing phase is an approximation and may range from (CaO) 2 Fe 2 O 3 to (CaO) 6 (Al 2 O 3 ) 2 (Fe 2 O 3 ). Cement and Cement Raw Materials John D. Macfadyen
Transcript
C O N S T R U C T I O N U S E S
Cement and CementRaw Materials
John D. Macfadyen
INTRODUCTIONThis chapter uses the term cement in it is historical meaning ofgray and white portland cement, a crystalline compound of cal-cium silicates and other calcium compounds having hydraulicproperties. Several other hydraulic cements of similar chemicalform exist and are produced in similar manner, but they have lim-ited and specialized markets. Much of this chapter applies to thesecements as well, although they are not directly discussed. Portlandcement is produced by intergrinding cement clinker and gypsum inan approximate ratio of 95:5. Cement clinker is a fused productconsisting of principally dicalcium silicate, tricalcium silicate, tri-calcium aluminate, and tetracalcium alumino ferrite formed byhigh-temperature reaction in a rotary kiln of carefully proportionedand blended ratios of lime, silica, alumina, and iron oxide. Thelime fraction is derived from limestone in the calcining phase ofproduction. White clinker is extremely low in iron oxide, whichrequires purer raw materials. Table 1 summarizes typical percent-ages of clinker compounds for the most common portland cement,Type I.
MARKETA few large companies dominate the marketing and production ofcement. Widely distributed plants minimize transportation costs tocustomers. Although the volume of cement to be sold in the market-place can be considered inelastic in the short run, a company’s saleseffort makes a difference. In any given market, although cement soldby competitors must meet certain basic specifications to be consid-ered, other factors dominate, including delivered cost, quality, prod-
11
uct consistency, technical assistance, and sales relationships with theuser companies. Sales relationships ensure the cement company ofconsideration in the first place, they allow the settlement of questionssurrounding the sale, and they offer the selling company a last “look”before sales commitments are made. Sales relationships are based onpositive past relations, trust, and friendship.
In the long run, several factors drive the consumption ofcement in the marketplace, including economic growth, private andgovernmental capital investment, and population growth. Influenc-ing decision makers by promoting the architectural, economic, anddurability advantages of concrete also drives the market, as doesfinancial or other assistance to concrete suppliers.
Cement companies often evaluate sales success as their con-tinued penetration of the local or regional market expressed as mar-ket share. Sometimes this devotion to market share evades the realcorporate target of profitability.
In the 1960s the federal highway program was a chief reasonfor cement plant expansion, along with the nation’s continued eco-nomic and population growth. At the same time, a new trend estab-lished distribution terminals to sell cement to greater geographicalmarkets accessible via water transportation or favorable rail rates.This trend increased competition and held down profits. From 1957to 1969 the inflation-adjusted price of cement declined significantly.In the 1970s, real prices began to increase but profits were diverted tomeet new environmental requirements rather than creating greaterefficiency and capital. With a surge in building in coastal markets inthe late 1970s, imports began to be a factor. When cement companiesdid not make the investment to increase capacity or import cement to
Table 1. Probable clinker compounds in Type I portland cement
Adapted from Clausen 1960.* Commercial cements contain 4%–6% gypsum or anhydrite (for regulation of the “setting time” of the concrete), approximately 0.5% each of alkali oxides (Na2O
and K2O) and uncombined CaO, plus a few percent impurities, largely MgO.† This composition of the iron-containing phase is an approximation and may range from (CaO)2Fe2O3 to (CaO)6(Al2O3)2(Fe2O3).
21
1122 Industrial Minerals and Rocks
meet construction needs, outside entrepreneurs did. These importerswere not needed during the recession of the early 1980s, and whensales went down, prices often went down and several left the market.Soon local cement producers took over most of the import terminalsowned by those “outsiders.” In the surge of the late 1980s andthrough today, most imported cement has been sold by the producingcompanies and basically used to meet peak demands of the marketand allow U.S. producers to maintain higher utilization of productionfacilities even in low-demand times.
Imports of clinker and finished cement peaked at 29.351 Mt in1999, and 24.756 Mt of that was finished cement. Although clinkerimports dropped in 2001 as new capacity came on stream, finishedcement imports remained between 24 and 25 Mt. The drop in clin-ker imports reflected new capacity of domestic U.S. cement pro-ducers. Countries with the greatest exports to the United States inthe peak year of 1999 were Canada, Thailand, China, Greece, andVenezuela. In 1999, New Orleans was the largest port of entry forcement and clinker, followed by Houston and Detroit. The UnitedStates is not a significant exporter of cement. Over the next fewyears, as new capacity comes on stream aided by the higher cost ofimported cement, imports are expected to be flat or drop slightly,but not because of the increased production capacity. This anomalyis due to the regional nature of the market, the high cost of truckand rail transportation, and imbalance between production capacityand demand in any given market. In 1997, with the Asian financialcrisis, imports from Asian sources began to increase dramatically.In 2003, with Asian markets improving and ocean freight ratesescalating rapidly, import sources to the U.S. market began to shiftto South America and the Mediterranean region as ocean freightrates became a greater cost component. U.S.-levied “dumpingduties” curtailed imports from Japan, Mexico, and Venezuela.
Within the United States, California, Texas, and Pennsylvaniahave the greatest cement grinding capacity, and Texas, California,and Florida are the top consuming states. Compound growth ratesfor cement consumption between 1971 and 2001 show Nevada tobe the greatest growth state at 5.6% per year, followed by Utah at3.6% and Arizona at 3.3%.
The ready-mixed concrete industry dominates U.S. cementconsumption, accounting for nearly 75% of all cement consumed.Concrete products account for another 12% to 13%, followed bydirect sales of 6% to 8% to contractors on major projects. Buildingmaterials firms, which are the key sellers of sack cement, accountfor another 3% to 4%. Sales to governments, mining companies,waste handlers, and oil-well cementers normally fall in a range of1% to 2% of all cement consumed. Types I and II account for 88%to 90% of cement sold with a rising amount of Type V of 5% beingsold in the last 3 years as Type V becomes required on many Cali-fornia projects. Blended cements run approximately 1% to 2% ofsales in the United States. Table 2 lists the American Society forTesting and Materials (ASTM) and the American Association ofState Highway and Transportation Officials (AASHTO) standardsfor the various types of cement.
Table 3 summarizes production, shipments, and imports from1991 to 2003. The U.S. Geological Survey (USGS) maintains themonthly shipments and import statistics (arithmetic mean) of theindustry and also publishes an annual review. Not all importersreport their shipments to customers to the USGS; therefore, theUSGS monthly production and shipments reports and the importstatistics leave a gap of about 2%.
PRODUCTS AND STANDARDSTable 2 lists the five types of portland cement and their standardsfor use in the United States. Although the principal standards are
the ASTM values, portland cement for highway construction is heldto the AASHTO standards. Masonry cement and various market-driven specialized forms of masonry cement produced in certainmarkets are typically not held to any standard specifications. Poz-zolanic cements and blended cements also must meet ASTM stan-dards, as must blast furnace slag as a portland cement substitute.Blast furnace slag producers are able demonstrate good 28-daystrengths comparable to portland cement standards in addition toalkali reactivity benefits. Where consumers do not stipulate con-crete strength requirements, ready-mix producers sometimes sub-stitute fly ash for some of the portland cement.
RAW MATERIALSMarket competition, ongoing pressure on cement prices, and theresultant lower costs of production have changed the framework forlocating new cement plants and the types of raw materials for pro-ducing cements at new and existing plants. Although the fundamen-tal chemical constituents to produce portland cement have notchanged, the choices of raw materials as sources of these chemicalcompounds have changed and are expected to continue changing.Abundant limestone and argillaceous reserves exist throughoutmost of North America and many parts of the world, but accessibil-ity, quality, cost of extraction, and transportation increasingly affectcement production economics and therefore the ability to extractthese minerals. The increasing demand by governments to recyclematerials and the changed economic structure of recycling are hav-ing a major impact on the cement industry. Additionally, the effectof more stringent environmental regulations and market forces onquality of cement, and modern preheater and pre-calciner produc-tion processes, have forced changes in what raw materials arerequired for producing cement clinker and cement. The productionof cement, however, is a chemical process requiring an accurate andconsistent blend of the four key inorganic oxides and the limitationof several undesirable oxides and elements.
Minerals RequiredThe fundamental chemical compounds for producing cement clin-ker—lime (CaO), silica (SiO2), alumina (Al2O3), and iron oxide(Fe2O3)—are now joined by the increasing need to reduce undesir-able chemical compounds such as sodium oxide (Na2O), potassiumoxide (K2O), sulfur, chlorine, carbon, and organics. Additionally,the increasing use of additives with cement clinker to produce dif-ferent types of cement has affected the use of other materials suchas fly ash, gypsum, synthetic gypsum, anhydrite, blast furnace andmetallurgical slags, silica fume, limestone, volcanic ash, etc. Limeis not found in nature; it is created from calcium carbonate (CaCO3)in the production process or obtained from recycled materials. Theother key chemical compounds are found naturally and are alsopresent in many recycled and waste materials. Table 4 summarizesthe raw materials used in the production of cement clinker.
Calcium Carbonate Resources and Mining PracticesLimestone continues to be the principal source of CaCO3. A com-mon mineral on the earth’s surface, CaCO3 is principally found innature as sedimentary deposits of marine origin. It is also found asmarble (a metamorphosed form of limestone), chalk, marl, coral,aragonite, oyster and clam shells, travertine, tuff, and some otherforms. In the United States, environmental regulations have essen-tially eliminated the use of aragonite and oyster and clam shells,and travertine and tuff deposits are rare sources of CaCO3. Low-grade sources of CaCO3 such as calcareous feldspars and argilla-ceous limestone, which are upgraded by flotation, are also rare andnot processed by North American plants.
Cement and Cem
ent Raw M
aterials1123
Type IV Type V
ASTM AASHTO ASTM AASHTO
— — — —
— — — —
6.5 6.5 — —
6.0 6.0 6.0 6.0
2.3 2.3 2.3 2.3
NA NA NA NA
2.5 2.5 3.0 3.0
0.75 0.75 0.75 0.75
35 35 — —
40 40 — —
7 7 5 5
— — 25§ 25§
— — — —
— — — —
— — — —
0.60 0.60 0.60 0.60
12 12 12 12
— — — —
160 160 160 160
— 150 — 150
— 220 — 220
— 230 — 230
280 280 280 280
— 260 — 260
— 400 — 400
— 420 — 420
0.8 0.8 0.8 0.8
60 60 60 60
600 600 600 600
(Table continued next page)
Table 2. Comparison of ASTM and AASHTO requirements*
Type of Cement
Type I Type IA Type II Type IIA Type III Type IIIA
* AASHTO 2004; ASTM 2004. Dashes indicate that no standard applies.† ASTM specifications state that there are cases where optimum SO3 for a particular cement exceeds the limit in this specification. Where it has been demonstrated by Method C563 that this cond
case more than 0.5 by weight percent of cement, is permissible, provided that when the cement with the additional calcium sulfate is tested by Method C265, the calcium sulfate in the hydrated exceed 0.5 g/L. When the manufacturer supplies cement under this provision, supporting data will be provided, on request, to the purchaser.
‡ Expressing chemical limitations by calculated assumed compounds does not necessarily mean that the oxides are actually or entirely present as such compounds.§ Does not apply when the sulfate expansion limit under Optional Physical Requirements is specified.
** This limit applies when moderate heat of hydration is required and tests for heat of hydration are not requested.†† This limit may be specified when the cement is to be used in concrete with aggregates that may be deleteriously reactive. Reference should be made to ASTM Specification C33 for suitable crite‡‡ Compliance with the requirement of this specification does not necessarily ensure that the desired air content will be obtained in concrete.§§ Either of two alternative fineness methods may be used at the option of the testing laboratory. When the sample fails to meet the requirements of the air permeability test, however, the turbidimete
turbidimetric method shall govern.*** These values were revised in ASTM specifications because of the change in unit from cm2/g to m2/kg.††† The purchaser should specify the type of setting time test required. If not specified, the requirements of the Vicat test only shall govern.‡‡‡ The strength at any specified age shall not be less than that attained at any previous test age.§§§ When the optional heat of hydration or the chemical limit on the sum of the tricalcium silicate and tricalcium aluminate is specified.
**** When the heat of hydration requirements are specified, the sum of the tricalcium silicate and tricalcium aluminate shall not be specified. These strength requirements apply when either heat of hydrais specified.
†††† When the sulfate expansion is specified, it shall be instead of the limits of C3A and C4AF + 2C3A listed under Standard Chemical Requirements.
Table 2. Comparison of ASTM and AASHTO requirements* (continued)
Type of Cement
Type I Type IA Type II Type IIA Type III Type IIIA
Table 3. Annual production, shipments, and capacity of the cement industry of the United States and Puerto Rico, and average prices for cement sold in the United States, kt
Adapted from USGS Mineral Surveys (to adjust for unreported shipments) and PCA 2000–2002.* Portland and masonry cement, and cement produced from imported clinker, for the 50 states and Puerto Rico.† Includes portland cement shipped by domestic producers and distributors.‡ Includes clinker.§ Annual prices are based on an average per-ton value of cement sales, f.o.b. plant, reported by producers to the USGS.
In the cement industry, limestone is a generic term thatincludes calcium carbonate, magnesium carbonate (MgCO3), anddolomite (CaMg(CO3)2). The geological definition of limestone isa sedimentary rock mostly containing calcite (CaCO3). Dolostoneis a sedimentary rock mostly consisting of dolomite. Magnesia(MgO), derived in the clinker production process from MgCO3, ishighly undesirable in the formation of cement clinker compounds.Many limestone deposits are unusable for portland cement clinkerbecause of MgCO3 content. Normally, the upper limit in the lime-stone is 3%, although this can be less depending on magnesia con-tent in the other required raw materials. The many classifications oflimestone are of little concern to a cement producer because theprincipal factor in selecting a suitable limestone is the CaCO3 con-tent and the minimization of the undesirable minerals. The ideal“cement rock”—77% to 78% CaCO3, 14 % SiO2, 2.5% Al2O3, and1.75% Fe2O3, coupled with <3% MgCO3, <0.4% Na2O, and 0.3%K2O—is rarely found but highly desirable. Limestone with lowerlevels of CaCO3 and higher levels of alkalis and magnesia requiresblending with high-grade limestone. Limestone normally has to beblended with argillaceous material to adjust the chemistry of silica,alumina, and iron oxide. Limestone that is high in CaCO3 presentscost and operating challenges to the cement producer because itrequires large quantities of argillaceous minerals such as clay,which are usually more difficult to convey, store, and feed.
Sedimentary limestone deposits, the preponderant source ofCaCO3 for cement, are principally extracted by bench mining, inwhich 150-mm to 300-mm holes are drilled in 3- to 4-m patterns todepths of 10 to 25 m, charged with ammonium nitrate and fuel oil(ANFO) explosive and blasted. Typical drill patterns and blastingtechniques will fracture the rock to <300-mm size. The rock is exca-vated with front-end loaders, often of 10-m3 capacity, and loadedinto 70- to 90-t haul trucks and transported to the primary crusher.Marl and chalk deposits are increasingly extracted with modernmining machines because they normally do not require drilling andblasting. The typical modern mining machine includes the excava-tor, primary crusher, and a feeder that deposits the excavated and
crushed marl or chalk onto moveable belt conveyors for transport tostorage and preblending facilities. A trend in the extraction processis to use in-pit, moveable, impactor-type primary crushers and beltconveyors to transport the rock to fixed secondary crusher andscreening plants, thereby reducing the number of trucks and haulagedistance. Load-haul-dump practices with front-end loaders are alsoemployed with these in-pit moveable crushers.
The increased emphasis on chemical uniformity and quality inthe subsequent processing of the raw materials into cement clinkerhas caused the proliferation of various techniques of quality controlin extracting the limestone and other raw materials. A commonpractice is sampling and x-ray fluorescence (XRF) analysis ofblast-hole drill cuttings to establish rock grade, quality, and alkalilevels. Increasingly, plants are installing online prompt gamma neu-tron activation analyzers (PGNAAs) immediately after the crushingcircuits to analyze the quarry run material and control subsequentblending operations to produce kiln feed. There is also an increas-ing use of x-ray diffraction (XRD) in quality control of kiln feedand the produced clinker. XRF analysis has been the dominantmethod for the past 40 years.
Underground mining of limestone deposits is not typical of thecement industry, although in coming years this is expected toincrease as economically accessible surface or near-surface sedimen-tary deposits play out, particularly at established plants constricted byurban encroachment. Currently, one U.S. plant obtains its limestone
Table 4. Summary of raw material used in the manufacture of portland cement clinker in the United States
Types of Raw Materials
Sources of calcium carbonate Limestone, marl, chalk
Ferrous: iron ore, pyrites, millscale, other 1,310 NR 1,500 NR 1,260 NRSiliceous
Sand and calcium silicate 3,142 NR 3,500 NR 2,960 NRSandstone, quartzite soils, other 925 NR 500 NR 692 NRFly ash 1,679 88 1,600 70 1,960 64Other ash, including bottom ash 930 NR 800 NR 990 NRGranulated blast furnace slag NR 303 NR 300 60 369Other blast furnace slag 43 NR 200 NR 162 NRSteel slag 805 NR 500 NR 481 NROther slag 12 10 50 5 67 4Natural rock pozzolans NR 40 NR 50 NR 28Other pozzolans 38 8 100 9 165 7Other
Gypsum and anhydrate NR 4,655 NR 4,800 NR 4,740Clinker imported NR 4,573 NR 5,030 NR 5,230Other, not elsewhere classified NR 46 40 50 21 52
Total 133,400 11,600 135,400 12,200 141,400 12,100
Adapted from USGS Mineral Surveys (to adjust for unreported shipments) and PCA 2000–2002.* Includes Puerto Rico.† NR = not reported.
from underground operations using the room-and-pillar miningmethod. Those plants located on navigable waterways may have theoption of extracting limestone from surface deposits some distancefrom the plant and transporting the limestone by barge or ship. Cur-rently one U.S. cement plant obtains its limestone from the Domini-can Republic and two others receive limestone from Canada. SeveralU.S. and Canadian plants receive their limestone and other raw mate-rials by barge or ship from quarries separated from the plant. Anincreasing number of plants have installed overland belt conveyors totransport limestone to the plant over distances of more than 5 km. Inseveral plants, such as those in Great Britain and Trinidad (but nonein the United States), marl or chalk or limestone is ground and slur-ried and then pumped to the distant plant. Several wet-process plantshave been replaced with flash-dryer-equipped, preheater calciner sys-tems that accept the slurried raw material transported by pipeline.This method has allowed extraction of limestone deposits heretoforeinaccessible or allowed continuation of production at plants that oth-erwise would have shut down.
Argillaceous Mineral Resources and Mining PracticesThe typical practice is to use clay or shale and sand for the silicaand alumina, and a source of iron oxide such as mill scale or iron
ore to adjust the chemistry to the desired composition, in additionto the ash in the coal used to fire the kilns. Other natural sources ofsilica and alumina, such as loess, silt, sandstone, volcanic ash,diaspore, diatomite, fuller’s earth and bauxite, are used based oneconomics and cement quality. Many plants are using waste orrecycled materials for this purpose and sometimes are paid to takethis material. Although the industry has used power plant fly ashand bottom ash for decades, alkali levels often limit the use of thesematerials. The variety of recycled and waste materials containingsilica, alumina, and iron oxide precludes a complete listing. Theprincipal factor in the selection of the argillaceous component iseconomic. Often plants have to invest in additional materials han-dling, feeding, and proportioning facilities to be able to use wastematerials, thus reducing the economic benefits. As a result of theeconomic pressures on the North American cement industry, manyplants are using some waste materials in place of natural minerals.Table 5 summarizes the quantities of raw materials used in the pro-duction of cement.
The typical methods of extracting clays, shales, and sand havechanged little. These raw materials are typically surface deposits,often overburden of the limestone beds. Clay and shale are nor-mally extracted using front-end loaders and loaded into haul trucks.
Cement and Cement Raw Materials 1127
In some cases, the deposits are first ripped with large bulldozers.When these raw materials are present as overburden, a commonpractice is to contract for the removal and stockpiling of the over-burden to expose sufficient limestone to run the plant for severalmonths. The clay or shale is then reclaimed as required for the pro-cess over ensuing months. Increasingly, older plants must removelarger amounts of overburden to expose the underlying limestone.The clays and shales not used in the process are often reused as filland reclamation material in mined-out areas of the quarry becauseplants are required to return the mined-out land to a reusable state.
Because most cement production facilities use the dry process,slurrying clay in wash mills at the plant is rarely practiced. Moretypical is the conventional crushing of the clay or shale in crushersdesigned for these materials and their characteristics. In some plantsthe clay is dried in a rotary dryer, a crusher-dryer, or a semi-autoge-nous mill before mixing it with limestone for further processing.
Waste Materials as SubstitutesBeginning in the early 1980s, environmental regulations and politi-cal factors in the United States combined with economics of cementproduction have created a significant incentive to use wastes con-taining silica, alumina, and iron oxides as raw materials. Some ofthese wastes contain minor fuel values, such as carbon in fly ashand coal in tailings, which potentially make them more valuable.Stack emission limits in some cases prevent or limit their use, butmany plants now use these wastes. Only those plants permitted touse so-called hazardous wastes can use soils contaminated withorganic wastes or other hazardous materials as raw materials andthen only as part of the fuel to the kiln. The subject of alternatefuels is discussed in the section on Fuel Firing Systems in thischapter. Substituting waste containing lime, silica, alumina, andiron oxide is a practice that is expected to grow and become preva-lent throughout the cement industry as environmental regulationsspread and economic factors make such use attractive.
Geology of Calcium Carbonates and Argillaceous MineralsCalcium carbonate originates from the biological deposition ofshells and skeletons of plants and animals to form beds of limestone.Some of the beds or formations within a bed were formed by naturalprecipitation of calcium carbonate taken into solution by carbondioxide in water, forming calcium bicarbonate. The precipitationprocess occurs as a result of evaporation or temperature changesresulting in a saturated solution; this genesis, however, is rare. Mostindustrial quality limestone is of biological origin. Massive beds oflimestone accumulated over the millions of years of geologic time.Much of the North American continent in Paleozoic time was underwater, allowing the formation of limestone deposits. Typically, lime-stone resources in the Midwest and the eastern United States are ofOrdovician, Devonian, and Mississippian ages. Westward they areoften of the younger Cretaceous age.
Most deposits are relatively unaltered with little faulting andfolding except those in the mountainous West and the Appala-chians where alteration is typical. Steeply sloping or near-verticalbeds with noticeable offsets along faults are the rule in theseregions. Along the Andes Mountains in South America, extensivealteration of limestone beds is the rule. The prevalence of volcanicaction, often evident in the sulfur content of the limestone and theinterbedding of volcanic deposits, limits the economic use of theselimestones.
Deposits of the argillaceous minerals are also of sedimentaryorigin. Shales, mudstones, and sandstones are often interbeddedwith the limestone. Shales, typically beds of clay altered by pres-sure, and sandstone, beds of sand often cemented by precipitated
silica or calcium carbonate, were deposited as the vast inlandwaters and oceans covered the land masses. Clays are typically sur-face deposits of more recent times; in some cases from recedingglaciers that covered much of the northern hemisphere.
PRODUCTION PROCESSESThe production of cement begins at the quarry with chemicalassessment of the limestone and sources of argillaceous materials.The mining plan to extract these raw materials in today’s plants iscarefully designed to meet the chemical quality objectives of thevarious types of clinker to be produced. The height of benches andthe extraction plan of the various beds of limestone are dictated bytheir chemistry and how best to blend these beds to achieve thechemical targets and maximize recovery of reserves. Subsequentprocess steps continually focus on chemical uniformity of the rawmaterials and of the feed to the kiln. Greater emphasis is nowplaced on quick analysis and frequent sampling of material streamsincluding the practice of automated (robotic) sample preparation.
Fundamental Processing StepsModern plants, which are diagrammed in Figures 1 and 2, use theprocess steps described in the subsections that follow.
Crushing and Screening
Rigid rotor impactor-type crushers are favored in plants that use avertical roller mill to grind the raw materials. Scalping ahead ofthe crusher reduces the fines and potential for clogging thecrusher. Plants that use ball mills to grind the raw materials favora hammermill-type secondary crusher. Screening the rock fromthe secondary crusher is the preferred process, with the oversizeconveyed back to the crusher in closed circuit. The practice ofusing gyratory or jaw crushers at the primary stage has given wayto use of impactor crushers for cost reasons. Similarly, the use ofcone or roll crushers as secondary crushers and a tertiary crushingstage has disappeared except in unusual conditions.
Preblending
Depending on limestone variability and process considerations,preblending is now a common practice. It takes two basic forms;either (1) the limestone is laid down in longitudinal or circular bedsin a chevron pattern and reclaimed by mechanical rakes that cutacross the layers to produce a more chemically uniform limestoneor (2) the various raw material components are stockpiled andreclaimed with weigh feeders in set proportions and laid down inlongitudinal or circular beds in a chevron pattern and reclaimed bymechanical rakes that cut across the layers to produce a reasonablychemically uniform feed to the subsequent milling stage. Anincreasing number of plants use PGNAAs to adjust these weighfeeders to achieve superior blending and uniformity of mill feed.The method selected is dictated by factors such as the chemical andphysical variability of the limestone, the moisture content of theraw materials, the physical characteristics (stickiness, particle size,etc.), the space available, number of raw material components, andcapital cost.
Raw Material Storage and Milling
Storing raw materials ahead of the raw mill is usually limited to thepreblend piles with minimal bin or silo storage. The capital cost ofstoring materials in silos is significant, and the pressure to mini-mize capital cost usually dictates only a few hours of mill feed heldin bins. Weigh feeders withdraw the raw materials from theirrespective bins in the desired proportion to achieve the requiredchemical composition and deposit them on a belt conveyor that
1128 Industrial Minerals and Rocks
FromBlending
Silos
Limestone Iron Ore Sand Clay
Raw Materials AreProportioned
PGNAA
Hot Gases fromPreheater Tower
Roller Mill
Cyclone
DustCollector
FanStack
Air Air
DryMixing
andBlending
Silos
To PreheaterTower
2. Proportioning and fine grinding of materials.
DrillingRig
Overburden
Shale
Limestone
ToCrusher
Crusherto 125 mmsize or less
To Vibr
ating
Scree
n
Oversize
PGNAA
ReclaimingScaperM
M
MM
MMStacker—Reclaiming SystemStorage and Blending
Raw Materials Conveyedto Proportioning Bins
1. Quarrying and blending of raw materials.
3. Kiln system. Preheating, cooling and clinker storage.
ConditioningTower
To RawMill
I.D. Fan
4-Stage SuspensionPreheater
Rotary Kiln
Coal, Oil, orGas Fuel
Air
Clinker Cooler
Clinker
MaterialsAre StoredSeparately
Gypsum
Clinker andGypsum Conveyed
to Grinding Mills
Figure 1. Steps in the manufacture of portland cement by the dry processes using a four-stage preheater
4. Finish grinding and shipping.
Clinker Gypsum
Materials Are Proportioned
Oversize
Grinding Mill
Air
High EfficiencyAir Separator
Fines
DustCollector
CementPump
BulkStorage
BulkTruck
BulkCar
Packing Machine
Cement and Cement Raw Materials 1129
Figure 2. Kiln system with five-stage preheater and calciner
Air AtomizedWater Spray
ConditioningTower
To RawMill
I.D. Fan
FromBlendingSilos
5-Stage SuspensionPreheater
Calciner
FuelTertiary Air Duct
Rotary Kiln
Coal, Oil, orGas Fuel
Air
Clinker Cooler
To Clinker Storage
Fuel
feeds the raw mill. A significant improvement in uniformity of millfeed and subsequent kiln feed is achieved by installing a PGNAAon either the mill feed conveyors or on the mill product stream.Although ball mills are occasionally selected to dry grind the rawmaterials, the preferred dry-grinding mill in the modern plant is thevertical roller mill. This type mill is superior in drying the rawmaterials using the hot waste off-gases from the kiln system andrequires less power. For high raw material moistures (14% to 18%),the vertical roller mill uses a combination of waste kiln systemgases and waste hot air from the clinker cooler to achieve a 0.5% to1.0% product moisture, which a ball mill is unable to do. As plantcapacities have increased, the vertical roller mill and its relatedstructure and foundations have gained a capital cost edge. Thesemills operate at significant pressure drop, necessitating large driveson the exhaust fan and careful attention to the mill feed airlock.None of the types of airlock is problem free, each having their limi-tations, compromises, and cost consequences. Product finenessfrom the raw mill has increased in recent years from the 70% to75% passing a 200-mesh Tyler screen (74 µm) acceptable 25 yearsago to 82% to 86% in new plants. Wet- or semi-wet-process plantscontinue to use ball mills as the method of grinding the raw materi-als into a slurry. Either of two methods of classifying the slurry is inuse: DSM screens or multiple cyclones. Each of these methods hasits limitations, compromises, and consequences. Product finenessfor the semi-wet-process applications has increased to the samelevel as that for dry-grinding systems. Slurry moisture contentobjectives for the older wet process and the new semi-dry processeshave been reduced in continuing efforts to reduce fuel consump-tion. Filtration to lower water content of slurry either with plate fil-ter presses or belt-type filters is encountered in very few olderplants (none in the United States).
Blending and Kiln Feed
Blending systems for dry-process plants continue to evolve. Therealization that large (50,000-t capacity) silos with a random multi-
ple withdrawal system are costly and do not provide commensurateimprovement in blending has focused attention on smaller silos(5,000- to 10,000-t capacity) with inverted cone and random orsequenced multiple withdrawal systems. The older small-capacitysilos agitated by compressed air (2,000 to 2,500 t) are rarely appliedin modern plants, although they are still used in older plants. Sepa-rate kiln feed silos or bins are typically not applied in new plants orfound in older plants. Kiln feed systems have undergone significantchanges in recent years. Belt-type weigh feeders, always havingproblems because of the characteristics of kiln feed, have beenreplaced with flow meters coupled to weigh bins on load cells orthe Pfister feeder. These new methods are totally enclosed and spillfree, and provide lower operating cost and improved accuracy andreliability without capital cost penalties. Wet-process blending andkiln feed systems are unchanged with two or more slurry tanks,traveling rakes, and compressed air agitation. The number of slurrybasins has been reduced and significant improvement in kiln feeduniformity has been achieved by using PGNAAs on the mill prod-uct stream to control weigh feeders. Flow and density meters con-tinue to be used on the kiln feed system; using adjustable-speeddrives on the kiln feed pumps, however, is the preferred method ofadjusting flow rates in place of orifice-type control valves.
Preheater and Precalciner Systems
New plants typically adopt the newest precalciner preheater sys-tem. The preheaters are the classic cyclones but now typically oflow-pressure-drop (100-mm WC [water column]) design. If kilnfeed moisture is a factor, the preheaters will have three or fourstages; otherwise the norm is a five-stage preheater. Few newplants are fitted with six stages because the incremental efficiencydoes not justify the added capital and operating cost. A number ofplants in the United States and Canada installed in the 1970s haveonly preheaters, and typically four stages. Sixty percent to 70% ofthe calcining is in the rotary kiln portion of the kiln system, and thebalance is in the preheaters or in the duct between the rotary kiln
1130 Industrial Minerals and Rocks
and the lower stage preheater cyclone (riser duct). In most suchsystems, 15% to 20% of the fuel is introduced in the riser duct.Kiln manufacturers have achieved considerable development inprecalciner technology in recent years. Depending on the rawmaterial characteristics and emissions limits (especially NOx),two-stage calciners and very tall calciners are used. Gas and kilnfeed retention times are greater (8 to 10 sec), and various forms ofstaged combustion are the norm. The evolution of precalciners inthe last 15 years has been significant and calcining rates of 90% to95% are an accepted norm. The typical fuel consumption rate inthese calciners is 55% to 60% of the total kiln system fuel. Com-bustion air for this fuel (tertiary air) is drawn from the clinker cool-ers through separate ducts at 700°C to 800°C. The termprecalciner continues to confuse because effectively the industryhas introduced a distinct step in the process to calcine the kiln feedbefore introduction into the rotary kiln. The rotary kiln’s sole pur-pose is to complete the calcining and cause the chemical reactionthat forms the clinker compounds. The result is smaller diameterand shorter rotary kilns turning at higher rates (240 revolutions perhour) to achieve any given capacity. A recent development hasbeen the use of two support rotary kilns with these more efficientprecalciners. This results in lower kiln shell stresses and, when off-set by thicker kiln shells, less ovality and greater refractory life. Itis not uncommon for these properly designed two-support rotarykilns to operate for more than 12 months without refractory failureor replacement, a significant productivity improvement. Fuel effi-ciency improvements have also been achieved, although the gapbetween normal operating fuel consumption and that demonstratedduring initial performance acceptance tests remains. The kiln man-ufacturers continue to use net fuel calorific values in their perfor-mance guarantees, but producers must continue to deal in the realworld in which fuel calorific values and pricing are quoted in grossvalues and the unwary encounter the 4% to 9% cost discrepancy inaddition to the difference between performance guarantees andreal-world operating fuel consumption.
Semidry Kiln Systems
A development of the last 10 years has been the commercializationof the flash dryer to use preheater gases to dry kiln feed slurry.These systems are applied in special circumstances such as convert-ing wet-process kilns to achieve much higher production rates froman existing kiln and clinker cooler or transporting limestone to theplant as a slurry via overland pipeline. Typically a semidry systemwill double output and lower fuel consumption by about 15% to20% of any given wet-process kiln. The preheaters are typically onestage and in some cases two stages. The flash dryer is a tall duct,often 100 m tall, in which preheater gases are fed into the bottomaround a single-shaft, rigid-rotor crusher and the kiln feed slurry isinjected into the hot gas stream. The mass of gas and kiln feed isdrawn up the flash-dryer duct and separated in a cyclone. The kilnfeed, now at very low moisture levels, is fed into the preheatercyclone for final drying. The kiln feed drops out from the preheatercyclone into either a calciner or a second-stage preheater cycloneand then the calciner. In these systems, the fuel consumption isgreater in the calciner than in a typical dry-process precalciner sys-tem. Calcining rates typically are greater than 90% and overall fuelconsumption is 35% to 50% greater than a five-stage dry-processprecalciner system, but still less than a typical wet-process system.It is interesting to note that the most efficient wet-process systems,such as the Holcim plant in Clarksville, Missouri, have almost thesame fuel consumption rate as a semidry system but not the rotarykiln refractory life and productivity.
Wet and Long Dry Kiln ProcessesThe wet and long dry kiln processes are treated together becausethey are essentially obsolete. They are diagrammed in Figure 3. Asignificant number of plants in the United States and Canada con-tinue to operate with these types of kilns, their retirement beingdelayed by favorable economics of their use of waste fuels, relativenewness, competitive location, difficult environmental situationpreventing new plants from locating in their market area, or otherfactors. Fuel efficiencies have not changed in the last 20 yearsbecause there has been no significant improvement in chain sys-tems or other means. These kiln systems have benefited from theimprovement in kiln feed uniformity achieved by using PGNAAsand from introducing car and truck tires as fuel into the kiln down-stream of the chain section (mid-kiln firing).
Clinker CoolingA significant development of the grate cooler in the last 10 years hasbeen the compact cooler with a deep clinker bed depth (800 mm to1,000 mm) and reduced undergrate airflow rates (<3 kg of air per kgclinker) at significantly higher pressure (1,000 mm WC). Thesecoolers are normally fitted with seven to nine static rows of gratesand cooling air introduced to the fixed and movable grates throughchambers, not undergrate compartments. The smaller grate areasand lower airflow rates result in a compromise in clinker tempera-ture. The accepted norm of 80°C 25 years ago has given way to 120°to 125°C performance, a figure common 50 years ago but consid-ered unacceptable in ensuing years. Newer plants are now dealingwith the consequent problems in cement temperature and storing hotclinker. A perceived benefit of these modern clinker coolers isreduced exhaust airflow rates and higher tertiary air temperatures tothe precalciner.
Fuel Firing SystemsRotary kilns and precalciners are almost exclusively fired by coal ora combination of coal and petroleum coke. Kiln startup and warm-ing is typically either with natural gas or No. 2 fuel oil. The degreeof substitution of coal with petroleum coke is dictated by the abilityof the grinding mill to reduce the pet coke to a high fineness (90%to 96% –200-mesh Tyler, 74 µm) and by sulfur dioxide permitemissions rates, because the sulfur content of petroleum coke is sig-nificantly greater (5% to 7%) than typical rates in coal (1% to 3%).Outside the United States and Canada, allowable sulfur dioxideemission rates enable producers to use 100% petroleum coke to firethe rotary kiln when stable operating conditions prevail with thehigh fineness. The combustion temperatures in the precalciner arerelatively low (<1,100°C) compared to the rotary kiln, restrictingthe use of petroleum coke. Direct grinding and firing of coal andpetroleum coke has given way to indirect grinding and firing as aresult of the need to fire two separate systems, the rotary kiln andthe precalciner. The requirement to reduce NOx emissions fromexisting kiln systems has also resulted in direct firing beingreplaced with semidirect firing and indirect grinding and firing toreduce primary air and allow the use of specially designed burners.A greater variety of coal (and petroleum coke) grinding mills isnow found in U.S. and Canadian plants. The vertical roller millwith its high-efficiency internal classifier is increasingly used.Replacing the static classifiers with dynamic classifiers in olderring-roller mills has gained acceptance, particularly with the needto grind petroleum coke and coal to higher fineness. Many pre-heater kilns are now burning tires, often aided by a small tippingfee. The normal practice is to introduce whole tires into the kilnthrough airlock ports in the riser duct of preheater kilns or into the
Cement and Cement Raw Materials 1131
Figure 3. Steps in the manufacture of portland cement by the old processes
Each Raw Material isStored Separatey
Raw Materials Conveyedto Grinding Mills
To V
ibra
ting
Scre
en
SecondaryCrusher
Dust CollectorCyclone
ToCrusher
PrimaryCrusher
Limestone
To Air Separator
Drilling RigOverburden
Shale
1. Stone is first reduced to 5 in. in size, then 3/4 in. and stored.
Raw materials are ground, mixed with water to form slurry, and blended.
2. Raw materials are ground to powder and blended, or
3. Burning changes raw mix chemically into cement.
4. Clinker with gypsum is ground into portland cement and shipping.
Limestone Iron Ore Sand Clay
Raw Materials AreProportioned
Dry
Pro
cess
Wet
Pro
cess
OR
AirSeparator
Air
GrindingMill
FinesOversize
PneumaticPump
Raw
Mix
Dry Mixing andBlending Silos
Ground RawMaterial Storage
To Kiln
To KilnLimestone CementRock
Clay Iron Ore
Raw Materials AreProportioned
WaterAddedHere
Oversize
Dust Collector To Kiln
Rotary Kiln
Grinding Mill
Coal, Oil, orGas Fuel
DSM Screen
Fine
s
SlurryPump
Clinker
Slurry is Mixedand Blended Slurry
Pump
Materials AreStored Separately
Gypsum
Storage Basins
Clinker and Gypsum Conveyedto Grinding Mills
PackingMachine
BulkTruck
BullkCar
Air
Clinker Cooler
DustCollector
BulkStorage
CementPump
Fines
Clinker Gypsum
Materials AreProportioned
AirSeparator
Oversize
Grinding Mill
Air
DustBin
Fan
1132 Industrial Minerals and Rocks
back end of precalciner kilns. Tires are introduced into long wetand dry kilns through openings in the shell downstream of thechain systems, a process covered by a mid-kiln firing patent issuedto Benoit, Hansen, and Reese (1989). A few plants introduce tirechips; the economics, however, are not as attractive. Tires containsignificant levels of sulfur and iron. Sulfur content limits substitu-tion rates to typically < 20% of total kiln fuel. The iron contentresults in the need to adjust kiln feed chemistry and adds the com-plexity of a change in clinker chemistry when tire feed rateschange abruptly before kiln feed chemistry can be changed. Usingwaste fuels is increasing, although using hazardous wastes as afuel is static because of the high cost of permitting and ongoingpermit compliance. Plants are seeking various combustible materi-als that ordinarily are wastes as substitutes for the normal coal orpetroleum coke.
Clinker Storage
Environmental emission restrictions have made storing clinker inenclosed structures the norm. The choice of silos or domed or lon-gitudinal storage buildings is typically dictated by economics. Silos(some as large as 50,000 t) offer 100% reclaim without mechanicalreclaimers but typically cost more than US$150/t, including theconveying systems. Domed or longitudinal structures requiremechanical reclaimers or clinker must be reclaimed by a combina-tion of gravity to conveyors in tunnels and front-end loaders or bull-dozers. The capital cost of these systems ranges from US$100 toUS$120/t of capacity but results in greater maintenance and higheroperating costs. A significant trend in the industry is using pan- orbucket-type conveyors for conveying hot clinker. These units arefavored when clinker must be elevated to the top of storage silos orcircular or longitudinal storage buildings. Although relativelyexpensive, their low maintenance cost, reliability, low dust emis-sion rates, and ability to convey red-hot clinker favor their use.
Cement Grinding
The industry continues to use the two-compartment ball mill inclosed circuit with the air separator type classifier as the principalmeans to grind clinker and gypsum into cement. Almost all grind-ing circuits also use heat exchangers to cool the cement before con-veying it to storage. There has been extensive application of thehigh-efficiency air-swept separator in place of the older-technologyair separators. In addition to some cooling of the cement, these sys-tems provide a better particle-size distribution, slight improvementin grinding efficiency, and typically improved cement strengths at agiven fineness. The competitive conditions in the cement industryhave resulted in marketing cements with much greater 28-daystrengths in the last 10 years. To achieve these strengths, tricalciumsilicate levels have increased and fineness of grind has increased byabout 500 Blaine points.
There is considerable interest in using vertical roller millsrather than ball mills for grinding clinker and gypsum. Althoughthe trend is still not established, one plant came on line in 2002 inthe United States with this type of mill, and one new plant is stillin the permitting stage plans to install four of these types ofmills. There are several applications of this type of mill outsideNorth America. Several plants using balls mills have installedhigh-pressure roll presses to prefracture the clinker before grindingin the ball mill. This technology, which Germany introduced intoNorth America in the mid-1980s, results in a significant total powerreduction and improved productivity of the ball mill circuit. Themajor drawback to its application is the need to maintain an inven-tory of “pressed” clinker ahead of the ball mill to not compromiseball mill production, because a change in the ball charge of the first
compartment of the ball mill is required. The preferred method ofconveying cement to storage is still the screw-type pump pneumaticconveyor.
Cement Storage and Shipping
Cement shipments vary with the season, declining significantlyduring cold winter months and for short periods during severe hotspells or hurricane-type storms. Consequently, plants must maintainsignificant cement storage capacity either at the plant or in combi-nation with distribution terminals and distant grinding plants.Those plants on the Great Lakes must also contend with closure ofshipping because of ice, typically from mid-December to March.The typical practice of storing cement in silos has undergone aninteresting change. The practice for many years was to carefullybalance clinker storage with cement grinding capacity and cementstorage. Until the mandated storage of clinker in enclosed struc-tures for environmental reasons, clinker storage was less costly.Providing excess cement grinding capacity was economicallysound in conjunction with limited cement silos. The recent trend isinstalling large dome structures to store greater quantities of cementbecause the cost is about the same as clinker storage. In new plants,cement grinding capacity is more closely matched to plant capacity.Storage silos are still needed to allow efficient loading of bulktrucks and railcars and to maintain an inventory of low-demandcements such as blended cements (Type IP), high early strength(Type III), and sulfate resistant (Type V). Very few plants operatebagging systems. In some cases, bagging cement has been con-tracted out; in other cases, bagging has been consolidated at oneplant in a region. Southern Florida and southern California producethe highest levels of bagged cement.
Special Plant ProcessesThe production of cement is predominantly of gray cement usingconventional fuels to fire the kilns. Three plants in the United Statesand one in Canada, however, produce white cement—althoughimports of white cement have a significant market share. One smallplant in Juarez, Mexico, and one in Virginia produce special propri-etary cements for the U.S. market. Colored cement is produced fordecorative architectural purposes at several plants by carefully mix-ing dyes with gray or white portland or masonry cement.
Automation and Process ControlConsiderable advances have been made in the last few years in auto-mation of process control and in motor control. Distributed controlsystems (DCSs) typically control modern plants using fiber-opticcables as the data highway. Many older plants have been converted toDCSs but more commonly they are connected to programmable logiccontrol (PLC) systems. Cost considerations and the unavailability ofreplacement parts have necessitated these conversions. The algo-rithms for process control have not changed significantly in the last10 years, but the ability to provide quick response and finite controlhas improved kiln and mill operations. Additionally, the powerfuldata gathering and diagnostic capacity built into both the DCSs andthe PLC systems has greatly improved the understanding of the pro-cesses, and has driven improvements in process control and improvedcompliance with emission limits. Virtually all plants now have a cen-tral control room from which the plant is operated, although a fewplants still have two control rooms, one for the mills and one for thekilns and remainder of the plant. Commonly a single operator con-trols all processes from this room with an array of monitors using atrackball or touch screen for motor start and stop and a keyboard forprocess set-point adjustments. The large modern plants also have aprocess engineer at a nearby workstation to provide guidance to the
Cement and Cement Raw Materials 1133
operator and assistance when operating parameters are exceeded orsystems malfunction. Mimic boards have been replaced with graph-ics on the monitors that offer more information and the flexibility tomake changes. The emission data gathered at some plants are nowsent electronically to environmental agencies at periodic intervals, afeature unavailable just a few years ago.
CAPITAL COST OF MODERN PROCESS PLANTSThe trend to construct large plants has continued as older plants areshut down. A typical modern plant has a capacity of 1.5 to 2.0 Mtpy,although a few plants have come on stream in the last 10 years withcapacities in the 0.65- to1.0-Mtpy range. These smaller-capacity pro-duction lines have replaced older, lower-capacity, less-efficient pro-cesses. The capital cost for construction of a modern 1.5- to 2.0-Mtpyplant at the site of an older smaller-capacity plant is US$125 toUS$175 per annual ton of capacity; and for the 0.65- to 1.0-Mtpyplant, the cost is US$150 to US$200 per annual ton.
Construction cost of a “greenfield” plant is very difficult togauge because of the number of variables and the length of time ittakes to permit and construct a new plant. Costs range from US$200per annual ton for a supersized plant of more than 3.0 Mtpy toUS$225 per annual ton for one of half that capacity.
PRODUCTION COSTSProduction costs in the industry vary depending on age of the plant,type of process, capacity, and unique features. Cash costs rangefrom US$27.50/t for a modern large-capacity plant to US$50/t forolder wet-process plants. The elements of cash cost are supervisionand labor, purchased raw materials, fuel, power, maintenance andparts, operating consumables, local taxes, and miscellaneous costs.Capital recovery costs for a modern low-cost plant add US$15/t tocash costs. When corporate overhead and sales costs are added, theindustry is left with very narrow margins over recent selling prices.
Production costs can be classified as fixed or variable. Modernplants require significantly fewer people to operate and maintain, solabor and management are essentially fixed costs. Purchased rawmaterials, fuel, power, maintenance and operating supplies, andmiscellaneous costs are essentially variable costs although somehave a small fixed-cost element. Local property taxes, insurance,and capital recovery are fixed costs. Most kilns are fired with coal,but petroleum coke is substituted when pricing is favorable. A mod-ern plant of 1.5-Mtpy capacity will typically operate at <0.2worker-hours per ton of cement, 3.2 MJ of fuel per ton of clinker,and 140 kW-hr of electrical power per ton of cement.
EMISSION STANDARDSU.S. cement plants must meet restrictive emission standards.Although federal regulations stipulate only a limit for particulatesfrom the kiln and clinker cooler stacks, 0.3 lb/st of kiln feed(0.15 kg/t) and 0.1 lb/st (0.05 kg/t) of kiln feed, the reality is thatplants have to meet much more stringent regulations, and these aremultilayered and not necessarily codified as a uniform standard.The principal emission limits are set by the requirement to meetNational Ambient Air Quality Standards (NAAQS) for particulates,sulfur dioxide, volatile organic compounds (VOCs), and carbonmonoxide. Building a new plant or replacing an existing processsystem requires demonstrating that the plant will not “consume”more than the allowable increment of these pollutants in the area ofimpact. Additionally, these standards may be trumped by the needto comply with a state implementation plan (SIP) that mandates alower emission level. Many plants are required to meet NOx emis-sion limits in order for the state to meet SIP requirements. Addi-tionally, plants are now required to meet best available control
technology standards for certain pollutants such as particulates. TheClean Air Act Amendments of 1990 also imposed on cement plantsthe need to achieve the emission standards set to be the “Best 12%”of the plants. More recent emission standards require that cementplants meet dioxin and furan limits of 0.2 ng per dry standard cubicmeter of stack gas. These multiple layers of regulations and emis-sion limits have made permitting new greenfield plants a costly andtime-consuming process. Even obtaining permits for new produc-tion systems at existing plants, where there are “offsets” created bythe new process emitting fewer pollutants than the existing process,takes considerable time despite typically avoiding the more time-consuming permitting process of demonstrating compliance withPrevention of Significant Deterioration of Ambient Air QualityStandards (PSD Review) and the New Source Performance Stan-dards (NSPS Review).
Table 6 presents the NAAQS that new cement plants mustdemonstrate they meet. This is a complex process that requiresmathematical modeling of the emissions from the various parts ofthe process. Table 7 presents the emission limits of certain hazard-ous pollutants that new cement plants must demonstrate they meet.
The cement industry has also had to contend with more restric-tive emission limits, especially of NOx and SO2 but also VOCs andtrace amounts of metals. By definition, cement kilns are also majoremitters of carbon dioxide, which is an unregulated gas but is a polit-ical issue.
Those plants that burn hazardous wastes as kiln fuel mustcomply with federal regulations under the Resource Conservationand Recovery Act of 1976 (RCRA). These regulations are in addi-tion to those just discussed. Compliance with RCRA regulationsimposes a significant cost, partly offsetting the economic benefitsof this cost-saving measure.
The U.S. cement industry has developed interesting emissioncontrol techniques to comply with permit requirements. To controlSO2 emissions, lime slurry is added to the spray water in gas cool-ing towers. Although this technique works better with fabric filterair pollution control systems, it is applied on kilns with electrostaticprecipitators. To meet NOx emission limits, the cement machinerymanufacturers developed staged combustion systems in the calcin-ers that reverse the NOx formed in the rotary kiln. Modern calcinerkilns emit <25% of the NOx of older long dry or wet kilns. Kilnburner manufacturers also developed burners that reduce NOx by upto 66%. To meet the dioxin-furan emission limits, kiln gases arenow cooled to <200°C in gas cooling towers or in specially config-ured duct systems. Modern compressed air atomizing spray nozzletechnology combined with gas cooling towers and duct systemsdesigned using computational fluid dynamics allows gas cooling to120°C with no consequences such as wetting the dust, a remarkableachievement not possible 10 years ago.
The need for reliability in meeting emission limits from clin-ker coolers has forced plants to install gas cooling systems ahead ofthe fabric filters which are the preferred particulate removaldevices. Very few plants use electrostatic precipitators or gravel bedfilters for this function. The preferred gas cooling system is an air-to-air heat exchanger, although these devices are not without theirproblems due to abrasion, thermal expansion, and contractionresulting in fugitive emissions.
OUTLOOK, FUTURE TRENDS, AND DEVELOPMENTS IN THE INDUSTRYThe U.S. cement industry continues to install single large-capacitykiln and mill systems, a trend that began 50 years ago. In 1960, a verylarge kiln was 2,000 tpd of clinker. In 1967, the then-largest kiln inthe world—4,000 tpd—was installed in Clarksville, Missouri. By
1134 Industrial Minerals and Rocks
Table 6. National Ambient Air Quality Standards
Pollutant Primary Standards Averaging Times Secondary Standards
Carbon monoxide 9 ppm (10 mg/m3) 8 hr* None
35 ppm (40 mg/m3) 1 hr* None
Lead 1.5 µg/m3 Quarterly average Same as primary
Nitrogen dioxide 0.053 ppm (100 µg/m3) Annual (arithmetic mean) Same as primary
Particulate matter (PM10) 50 µg/m3 Annual† (arithmetic mean) Same as primary
150 µg /m3 24 hr*
Particulate matter (PM2.5) 15 µg/m3 Annual‡ (arithmetic mean) Same as primary
65 µg /m3 24 hr§
Ozone 0.08 ppm 8 hr** Same as primary
0.12 ppm 1 hr†† Same as primary
Sulfur oxides 0.03 ppm (78.5 µg/m3) Annual (arithmetic mean) No standard applies
0.14 ppm (366 µg/m3) 24 hr* No standard applies
No standard applies 3 hr* 0.5 ppm (1,300 µg /m3)
Source: CFR 2004a.* Not to be exceeded more than once per year.† To attain this standard, the expected annual arithmetic mean PM10 concentration at each monitor within an area must not exceed 50 µg/m3.‡ To attain this standard, the 3-year average of the annual arithmetic mean PM2.5 concentrations from single or multiple community-oriented monitors must not
exceed 15 µg/m3.§ To attain this standard, the 3-year average of the 98th percentile of 24-hr concentrations at each population-oriented monitor within an area must not exceed
65 µg/m3.** To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hr average ozone concentrations measured at each monitor within an area
over each year must not exceed 0.08 ppm.†† The standard is attained when the expected number of days per calendar year with maximum hourly average concentrations above 0.12 ppm is ≤1. The 1-hr
NAAQS will no longer apply to an area 1 year after the effective date of the designation of that area for the 8-hr ozone NAAQS. The effective designation date for most areas is June 15, 2004.
Table 7. Emission limits of hazardous pollutants and operating limits
Affected SourcePollutant or
Opacity Emission and Operating Limit
All kilns and in-line kiln/raw mills at major sources(including alkali bypass)
PM 0.15 kg/Mg of feed (dry basis)
Opacity 20%
All kilns and in-line kiln/raw mills at major and area sources (including alkali bypass)
Dioxins/furans 0.20 ng TEQ/dscm or 0.40 ng TEQ/dscm when the average of the performance test-run average particulate matter control device (PMCD) inlet temperature is 204°C or less. [Corrected to 7% oxygen]. Operate such that the 3-hr rolling average PMCD inlet temperature established at performance test.
If activated carbon injection is used: Operate such that the 3-hr rolling average activated carbon injection rate is no less than rate established at performance test. Operate such that either the carrier gas flow rate or carrier gas pressure drop exceeds the value established at performance test. Inject carbon of equivalent specifications to that used at performance test.
New greenfield kilns and in-line kiln/raw mills at major and area sources
THC 50 ppmvd, as propane, corrected to 7% oxygen
All clinker coolers at major sources PM 0.050 kg/Mg of feed (dry basis)
Opacity 10%
All raw mills and finish mills at major sources Opacity 10%
New greenfield raw material dryers at major and area sources
THC 50 ppmvd, as propane, corrected to 7% oxygen
All raw material dryers and material handling points at major sources
Opacity 10%
Source: CFR 2004b.
1990, a large kiln in the United States and Canada was considered tobe 5,000 tpd, although larger ones were being installed in southeastAsia. Holcim is in the permitting stage of a 12,000-tpd kiln to beinstalled at a greenfield plant near Bloomsdale, Missouri, on the Mis-sissippi River. Mills have followed a similar trend. Raw materials fora 6,000-tpd kiln system are ground in a single vertical roller mill.
Cement is ground in 6,000-kW ball mills, although typically twomills of this size are required to complement a 6,000-tpd kiln.
This trend to increasingly larger production units is driven byeconomics. The labor force requirements for a 12,000-tpd plant are20% greater than those of one half that capacity. The most efficientmodern large plants (5,000 to 6,000 tpd of clinker) operate with
Cement and Cement Raw Materials 1135
100 hourly employees and 30 salaried personnel. The number ofitems of equipment dictates maintenance personnel requirements.The number of operating personnel is essentially static because ofthe modern central control systems, the computer controlled pro-cess, and the stability of preheater precalciner kilns. Quarryingoperations require no more personnel because of the very largesizes of haulage and loading equipment, use of in-pit crushers, andbelt conveyor transport systems.
Perhaps the future trend in the industry is described not interms of major developments but in terms of continued develop-ment of large kiln and mill systems and more efficient grinding sys-tems. The 10,000–12,000 kW ball mills are on the way. Verticalroller mills of 12,000 kW are also just a matter of time.
Considerable improvements in automation of process controlhave evolved over the past 15 years using the modern powerful PCsin tandem with PLCs for motor control and rapid data gathering. Amodern plant uses a fiber-optic data highway for rapid data trans-mission which, with DCS and “smart PLCs,” maintains far moreaccurate control of the process. This trend is bound to continue asimprovements in PCs and PLCs and the process control softwarecontinue.
Continued development of emission control systems, princi-pally for SO2 and NOx reduction, can also be predicted. Some pro-ducers have declared the intention to reduce carbon dioxideemissions from kiln systems. To meet this objective, greater use ofpozzolanic materials, both natural and synthetic, is likely. Alterna-tive sources of calcium oxide are also objectives, reducing theamount of limestone required.
There has been considerable consolidation in ownership andnumber of plants in the last few years. This trend is likely to con-tinue as company managements see the need to lower operating andadministrative costs and build fewer, larger-capacity plants. Olderwet- and dry-process plants will continue to be replaced with largerplants. The North American cement industry is dominated by majormultinationals. About 62% of the production capacity is concen-trated in six major international cement companies—Holcim,Lafarge, CEMEX, Ital Cimenti, Buzzi Unicem, and HeidelbergCement. All foreign companies combined control 80% of the clin-ker capacity and a greater percentage of cement sales.
ACKNOWLEDGMENTSThe author thanks John A. Ames and William E. Cutcliffe, authors ofthis chapter in the 5th edition of Industrial Minerals and Rocks, whoset a standard to be followed with their thorough and well-developedwork. The author acknowledges use of Table 2 from their work,updated with a recent change in standards. The author thanks JohnRohrer for his contribution to the section on Markets, Eric Hansen forhis guidance on environmental standards, F. MacGregor Miller forhis guidance on quality control, and Polina Levin for her work on thefigures and tables.
ADDITIONAL RESOURCESA significant body of literature and information on portland cementand its producers exists, not only in the United States but alsoaround the world. Numerous government and Portland CementAssociation (PCA) publications cover all aspects of the industryand its vital contribution to raising standards of living. This sectionlists just a few of the publications readers may wish to consult formore details.
Select Organizations, Web Sites, Trade Journals, and Other PublicationsAmerican Association of State Highway and Transportation Offi-
cials (AASHTO), http://www.aashto.org.American Ceramic Society, Journal of the American Ceramic Soci-
ety, http://www.ceramics.org.American Concrete Institute (ACI), Journal of the American Con-
crete Institute, published monthly; ACI Manual of ConcretePractice (3 parts), current edition, ACI Special Publications,http://www.aci-int.org.
Cement and Concrete Research, published bimonthly, http://www.elsevier.com.
Cement International, published bimonthly, http://www.verlagbt. de.Concrete Construction, published monthly, http://www.concrete-
construction.net.Concrete Products, published monthly, http://www.concreteprod-
ucts.com.Engineering News Record, published weekly, http://www.enr.com.International Cement Review, published monthly, http://www.cem-
net.co.uk.PCA, various publications, http://www.cement.org.Pit & Quarry, published monthly, http://www.pitandquarry.com.Rock Products and Cement Americas, published monthly, http://
www.rockproducts.com.SME, Mining Engineering, published monthly, http://www.
smenet.org.U.S. Bureau of Reclamation, various publications; e.g., Concrete
Manual (current edition), usually 600 to 700 pp., http://www.usbr.gov.
U.S. Department of Defense, Army Corps of Engineers, variouspublications (e.g., Handbook for Concrete and Cement, 1949,with loose-leaf revisions, 928 pp.), http://www.usace.army.mil.
U.S. Geological Survey, Minerals Industry Survey (CementMonthly); Minerals Yearbook (Annual Cement Chapter), http://www.usgs.gov.
World Cement, published monthly, http://www.worldcement.com.Zement Kalk Gips International, published monthly, http://www.
baudialog.de.
BIBLIOGRAPHYAASHTO. 2004. Standard Specifications for Transportation Mate-
rials and Methods of Sampling and Listing. 24th edition. Sec-tion M85-04. Washington, DC: AASHTO.
ASTM (American Society for Testing and Materials). 2004.Annual Book of ASTM Standards—Concrete and MineralAggregates. Part 14, ASTM C150. West Conshohocken, PA:ASTM International.
Benoit, M.R., E.R. Hansen, and T.J. Reese. 1989. Method forenergy recovery from solid hazardous waste. U.S. Patent4,850,290. July 25.
Blanks, R.F., and H.L. Kennedy. 1955. The Technology of Cementand Concrete. New York: John Wiley & Sons.
Bogue, R.H. 1955. The Chemistry of Portland Cement. 2nd edition.New York: Reinhold Publishing.
Bransome, M. 1985. Draft Summary Report on Hazardous WasteCombustion in Calcining Kilns. Cincinnati, OH: U.S. Environ-mental Protection Agency.
CFR (Code of Federal Regulations). 2004a. Title 40—Protection ofEnvironment, Part 50.9. Final Rule to Implement the 8-HourOzone National Ambient Air Quality Standard—Phase 1. Avail-able from http://www.access.gpo.gov/su_docs/fedreg/a040430c.html.
1136 Industrial Minerals and Rocks
———. 2004b. Title 40—Protection of Environment; Part 63.National Emission Standards for Hazardous Air Pollutants forSource Categories; Subpart LLL. National Emission Standardsfor Hazardous Air Pollutants from the Portland Cement Manu-facturing Industry. Available from http://www.epa.gov/fedrgstr/EPA-AIR/2002/April/Day-05/a8161.htm.
Clausen, C.F. 1960. Cement materials. Pages 203–231 in IndustrialMinerals and Rocks. 3rd edition. Edited by J.L. Gillson. NewYork: AIME.
Duda, D.I., and W.H. Duda. 1985. Cement Data Book. Volume 1.Wiesbaden and Berlin, Germany: Bauverlag GmbH.
Hansen, W.C., and J.S. Offutt. 1969. Gypsum and Anhydrite inPortland Cement. 2nd edition. Chicago, IL: United StatesGypsum.
Harben, P. 1979. The clamor for cement. Industrial Minerals143:52–53.
Helming, B. 1980. Die Zement-Herstellung. Uni Siegen: Krupp-Polysius. (In German)
Labahn, O., and W.A. Kaminsky. 1971. Cement Engineers Hand-book. 3rd edition. Wiesbaden and Berlin: Bauverlag GMBH.
Lea, F.M. 1971. The Chemistry of Cement and Concrete. 3rd edi-tion. New York: Chemical Publishing.
LeLonde, W.S., and M. Janes. 1961. Concrete Engineering Hand-book. New York: McGraw-Hill.
PCA (Portland Cement Association). 2003. U.S. and CanadianPortland Cement Industry: Plant Information Summary. Skokie,IL: PCA.
Peray, K.E., and J.J. Waddell. 1972. The Rotary Cement Kiln. NewYork: Chemical Publishing.
Queen, D.M. 1971. The US Cement Industry—A Memorandum.New York: Evans & Co.
Randall, R. 1961. A Reference for Manufacturing Process Depart-ment Literature. MP-100. Skokie, IL: PCA.
Taylor, H.F.W., editor. 1964. The Chemistry of Cements. 2 volumes.New York: Academic Press.
Troxell, G.E., H.E. Davis, and J.W. Kelly. 1968. Composition andProperties of Concrete. 2nd edition. Civil Engineering Series.Berkeley: University of California.
