Research and Development Bulletin RDIOST
Role of Minor
lements in
Cement
anufacture and Use
ABSTRACT: In this review, the effects of almost all the stable minor and trace elements on the production and performance of portland cement have been reported. Emphasis has been given to elements that occur in natural and by product materials used for cement manufacturing. The elements for which detailed information has been obtained are dealt with in an order based on the periodic classification of elements. The volatilities of the elements have also been discussed where ever necessary. Elements reviewed include: hydrogen, sodium, potassium, lithium, rubidium, cesium, barium, beryllium, strontium, magnesium, boron, gallium, iridium, thallium, carbon, germanium, tin, lead, nitrogen, phosphorus, arsenic, antimony, bismuth, oxygen, sulfur, selenium, tellurium, fluorine, chlorine, bromine, iodine, helium, neon, argon, krypton, xenon, yttrium, titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, cobalt, nickel, copper, silver, zinc, cadmium mercury, and the lanthanides.
REFERENCE: Bhatty, J. I.,
Role of Minor Elements in Cement Manufacture and Use,
Research and Development Bulletin RD109T, Portland Cement Association, Skokie, Illinois, U.S.A., 1995.MOTS CL ES: ciment portland, 616ments mineurs, 616ments trace, fabrication, mati$res premi?u-es
RESUME: Cedocument rapporte les effets de presque tous les Mrnents mineurs stables et Uments trace sur la /
production et la performance du ciment portland. L’accent a W mis sur les dldments qui se trouvent ~ I’dtat naturel clans les mat6riaux aussi bien que sur ceux des rclsidus utilisds lors de la fabrication du ciment. Les 416ments pour lesquels de l’information ddtaillde a W obtenue sent abord4s aans un ordre basal sur la classifica-tion p(%iodique des Wments. La volatility des Wrnents est aussi traitde lorsque n6cessaire. Parmi les d~ments couverts, on retrouve: l’hydrog~ne, le sodium, le potassium, le lithium, le rubidium, le c&sium, le barium,
b&yllium, le strontium, le magn6sium, le bore, le gallium, I’indium, le thallium, le carbone, le germanium, l’6tain, le plomb, l’azote, le phosphore, I’arsenic, l’antimoine, le bismuth, l’oxygtme, le soufre, le sdh%ium, le tenure, le fluore, le chlore, le brome, I’iode, l’h61ium, le neon, l’argon, le krypton, le xdnon, l’yttrium, le titane, le zirconium, le vanadium, le niobium, le tantalum, le chrome, le molybdbne, le tungst$ne, le manganbse, le cobalt, le nickel, le t
cuivre, l’argent, le zinc, le cadmium, le mercure et Ies lanthanides.
REFERENCE: Bhatty, J. I.,
Role of Minor Elements in Cement Manufacture and Use,
Research and Development Bulletin RD109T, Portland Cement Association [R61e et utilitd des Wirnents mineurs clans la fabrication du ciment, Bulletin de Recherche et D6veloppement RD109T, Association du Ciment Portland], Skokie, Illinois, U. S. A., 1995.Role of Minor Elements in Cement
Manufacture and Use
Contents
Page
INTRODUCTION ... 1
ALTERNATIVE MATERIALS AS PARTIAL RAW FEED OR FUEL IN CEMENT MAKING ...1
DEFINITloNs ...l...i ...m...2
Major Elements ... 2
Lesser Elements ... , 3
Minor Elements ... 4
Trace Elements ... 4
SOURCES OF MINOR ELEMENTS ... 4
MINOR ELEMENTS IN CEMENT MAKING ... 6
ELEMENTS IN GROUP I (Hydrogen, Lithium, Sodium, Potassium, Rubidium, Cesium)., ... 7
Hydrogen ... 7
Lithium ... 8
Sodium and Potassium ... 8
Rubidium and Cesium ... 11
ELEMENTS IN GROUP II (Beryllium, Magnesium, Calcium, Strontium, Barium) ... 11
Beryllium ... 11
Magnesium ... 11
Calcium ... 11
Strontium ...c...i ... 11
Barium ... 11
ELEMENTS IN GROUP Ill (Boron, Aluminum, Gallium, Iridium, Thallium) ...12
Boron. ... 12
Aluminum ... 12
Gallium, Iridium, and Thallium ... 12
ELEMENTS IN GROUP IV (Carbon, Silicon, Germanium, Tin, Lead) ... 13
Carbon ... 13
Silicon ... 13
Germanium ... 13
Tin ... 13
Lead ... 13
ELEMENTS IN GROUP V (Nitrogen, Phosphorous, Arsenic, Antimony, Bismuth) ... 13
Nitrogen ... 13
Phosphorus ... 14
Arsenic ... 14
Antimony ... 15
Contents
Page
ELEMENTS IN GROUP VI (Oxygen, Sulfur, Selenium, Tellurium) ... 15
Oxygen ... 15
Sulfur ... Selenium ... 1;
Tellurium ...<. 17 ELEMENTS IN GROUP Vll (Fluorine, Chlorine, Bromine, lodine) .c... 17
Fluorine ... 17
Chlodne ... 19
A/ir7iteCements ...j...c.
19 Bromine ... 19lodine ...2O ELEMENTS IN GROUP Vlll (Helium, Neon, Argon, Krypton, Xenon) ... 20
TRANSITION ELEMENTS ...2O Yttrium.. ... 20 Titanium ...2O Zirconium ...d... 21 Vanadium ... 21 Niobium ... 22 Tantalum ... 22 Chromium,... ... 22 Molybdenum ...!.. ... 23 Tungsten ... 23 Manganese ... 23 Cobalt ... 24 Nickel ... 24 Copper ... 24 Silver ... 24 Zinc ... 24 Cadmium ... 25 Mercury ... 25
THE RARE EARTHS ... 25
CONCLUSIONS ... 26
ACKNOWLEDGEMENTS ... 26
REFERENCES ... 27
APPENDlx ...!c...c...35
Role of Minor Elements in Cement
Manufacture and Use
by
Javed 1.Bhatty*
INTRODUCTION
The purpose of this review is to col-Iect pertinent information on the be-havior of minor and trace elements on the manufacture and use of cement. Attempts have been made to identify gaps, if any, in the information thus far available and suggest work for further investigations.
As cement manufacturers continually strive to conserve re-sources, use of alternative raw feeds and seconda~fuels derived from con-tinuously generated industrial by-products isgaininginterest. The likely concerns from alternative or new natu-ral sources are the incorporation of trace elements into clinker and their effects on the performance of cement. These effects are, to a large extent, dependent upon the type of trace ele-ments contained in the raw feed, their concentration levels, and the operat-ing conditions of the kiln.
The effects of minor elements on the production of clinker and perfor-mance of cement are summarized in the appendix.
ALTERNATIVE
MATERIALS
AS PARTIAL RAW FEED OR
FUEL IN CEMENT MAKING
There has been an increasing move toward using a variety of alternative
materials in cement manufacturing, with the multiple aims of reducing by-product accumulation to address environmental problems and to achieve technical advantages during clinker processing without sacrific-ing the quality of cement.
Chlorideby-products, and wastes from the soda ash industry, when mixed with fly ash and limestone, are reported to have produced low tem-perature clinkers (1200°C) with com-parable compressive strength (Patel, 1989). Phosphogypsumhasbeen used as a source of limeinkilnfeed. Though the clinker attained a different micro-structure, the cement compared fa-vorably with the conventional type (Toit, 1988). In separate studies, spent clays from lubricating oil refining, have also been tested as raw feed components for clinker production (Midlam, 1985).
Sewage sludge as a partial kiln fuel was reported by Obrist (1987). Heavy metals in the sludge were per-manently withdrawn from the bio-sphere with little toxic emissions. Or-ganic pollutants were reliably de-stroyed without leaving any toxic by-products. The only exception maybe mercury which must be controlled adequately.
Ostrovlyanchik et al. (1986) re-ported that the use of fly ash from coal power plants as raw material, in place of argillaceous material, was effective
for improving the wet process kiln output with savings in the fuel con-sumption.
Bhatty et al. (1985) produced an ASTM C150 Type I cement from the copper-nickel and taconite tailings used as a partial substitute for ce-ment raw feed. The resulting ce-ment had a microstructure and strength properties comparable to that produced with conventional feed.
Weatherhead and Blumenthal (1992), of the Scrap Tire Manage-ment Council, concluded from a re-cent field stud y that tires can be used successfully as an alternative fuel in cement kilns. The fuel cost is signifi-cantly lowered, and the production rate is enhanced without adversely affecting the quality of cement. No significant change in the environ-ment quality, due to emissions, was noted.**
*
u’!+
Senior Scientist, ConstructionTechnol-ogy Laboratories, inc., Skokie, lllinois 60077,U.S.A.Tel: (708)965-7500 A distinctionbetween the terms “tire-derived fuel” and “whole tires” may be made here. The tire derived fuel (TDF) uses shredded tires in combination with other conventional fuels (coal, coke, oil, gas, etc.) usually in the burner end, tire chips are also fed to calciners, whereas the whole tires are fed into the feed end of aprecalciner or a preheater kiln or into the calcining zone of a long kiln.
Use of scrap tires and lubricating oil, as alternative fuel in cement manu-facturing, has also been reported in a number of test burns in Australia and Canada (McGrath, 1993; and Heron, 1993). The tests suggested substantial reduction in manufacturing costs, im-provement in waste minimization, ad-vances in resource recovery, extended life of existing landfills, and better en-vironmental control.
De Zorzi (1988) reported Italian experience on the use of municipal solid waste as a partial fuel in cement manufacturing. The chemical and physical characteristics of clinker and cement were comparable to those pro-duced conventionally. No significant change inorganic or inorganic pollut-ants, such as dioxins, furans, SOX and NOX, were detected in the stack emis-sions. There were no material han-dlingproblems,but storage of the solid waste, especially the refuse derived fuel (RDF), was expensive, because it needed to be contained in totally en-closed compartments for technical and environmental reasons.
Regarding Norwegian experi-ence, Ingebrigtsen and Haugom (1988) demonstrated that using hazardous liquid wastes containing PCBS” as a partial kiln fuel offered an efficient way to solve a difficult environmental problem. For waste containing chlo-rine levels in excess of().()ls~o, the use
of a by-pass was recommended. Krogbeumker (1988) also reported that waste oils containing PCBS were tested as effective kiln fuel with adequate atomization of the oils into the gas stream. The levels of polychlorinated dibenzodioxin and dibenzofuran in emissions were very low and close to the limits of detection. Although these tests were very successful, PCBS are not burned in U.S. cement kilns.
Huhta (1990) surveyed some twenty North American cementplants operating with wastes as supplement fuel, and noted that the predominant waste being used was waste oil fol-lowed by solvent derived fuel and scrap auto tires; wood chips and fluid
coke were also mentioned. However, it is projected that the tire derived fuel (TDF) will become the most advanta-geous fuel in the near future, because of its availability and easy handling.
Kelly (1992), and Mantus et al. (1992) reported that the use of wastes as supplemental fuel in well designed and properly operated kilns results in metal emissions too negligible to cause any adverse health effects. It was also demonstrated that the cements and kiln dusts thus produced were not sub-stantially different from those conven-tionallyproduced. The effects of wastes on the emissions of organic compounds and metals from kilns were also stud-ied by von Seebach et al. earlier in 1990. It was reported that a virtually complete destruction and removal of hazardous organic compounds occurs in the kiln. A destruction and removal efficiency (DRE) of hazardous com-pounds was recorded at 99.9996Y0. DREs in access of 99.97 are routinely achieved.
Siemering, Parsons, and Loch-brunner (1991) have also reported on their experiences of burning wastes as kiln fuel and have reported both tech-nical and economical advantages with minimal adverse environmental im-pact.
In a recent article, Hansen (1993) has strongly advocated the use of solid wastes in cement manufacturing, em-phasizing potential environmental and political advantages. It was suggested that using wastes as fuel has two-fold environmental benefits; it would not only avoid fossil fuel extraction and transportation, but would also mini-mize emissions that would have oc-curred by disposal of these wastes through treatment or by landfilling. Politically, the liability of landfills and demands for waste minimization in an environmentally sensitive society, such as the United States, can be substan-tially reduced by waste utilization in cement making.
Gossman (1988) pointed out some of the risks and liabilities associated with the use of hazardous
waste-de-rived fuels, and proposed certain ana-lytical means to minimize liabilities in order to achieve full economical and quality control advantages.
Although there are opportuni-ties to beneficially use wastes in ce-ment production, their total substitu-tion in the industry is still in the ex-perimental stages. One recommen-dation has been to limit the use of waste to 5°/Obyweight of the raw feed (Vogelet al.,1987). Huhta (1990), and von Seebach et al. (1990) have re-ported a potential of 20-307. or even more for waste as a fuel replacement in cement kilns; some plants have already used 50-10O% replacement. Nonetheless, the level of application and degree of success largely depends upon the waste composition in terms of the type and concentration of mi-nor or trace elements.
In summary, under favorable practical conditions, wastes can have the following combined benefits: a. b. c. d. e. f. g.
Respond to commercial and en-vironmental pressure to use al-ternate raw materials and waste by-products.
Recover potential energy value from the wastes.
Conserve nonrenewable raw materials and fossil fuels. Enhance process efficiency. Produce more reactive raw mixes.
Produce cement of improved quality.
Reduce COZ emissions.
DEFINITIONS
Major Elements
According to Rompps Chemie-Lexikon (1987), the elements that are more abundantly present (>.5’XO)in
cement clinker are the major elements. These are calcium (Ca), silicon (Si), aluminum (Al), iron (Fe), and oxy-gen (0). Carbon (C) and nitrogen
●
Table 1. Typical Compositions and Physical Properties of Portland Cements
Compound Composition (O/.) Type I Type II Type Ill Type IV Type V
C,s 58 49 60 25 40 C2S 15 26 15 50 40 C3A 8 6 10 5 4 C,AF 10 10 8 12 10 Gypsum 5 5 5 4 4 Loss on Ignition 1.7 1.5 0.9 0.9 0.9 Blaine (m’/kg) 350 350 450 300 350 1-day Strength (psi) 1000 900 2000 450 900 7-day Heat of Hydration (J/g) 330 250 500 210 250
(N), because of their abundance in the raw material and the earth atmo-sphere respectively, can also be re-garded as major elements.
In clinker and cement analyses, Ca, Si, Al, and Fe are expressed as the oxide form (CaO, SiOz, A120~, and FezOJ. However, they eventually ex-ist as more complex compounds. The approximate formulae of these com-pounds, alsoknownasclinker phases, are tricalcium silicate 3CaO*SiOz or C#*; dicalcium silicate 2CaO*SiOz or CZS; tricalcium aluminate 3CaO*A110~ or C~A, and tetracalcium aluminoferrite 4CaO*AlzO~*FezO~ or CgAF. Since the role of major ele-ments in cement manufacturing has been fairly well understood, only a brief summary on the presence of major compounds in cement is given here.
Calcium is an essential compo-nent of cement, which comes from the decomposition of the primary raw material such as limestone, chalk, marl or cement rock depending upon the geological location of the cement manufacturing plant. Silicon in ce-ment is derived from silica sand, or from clay, shale, or slate, which are also sources of aluminum andiron in the raw material. Iron is sometimes derived from iron ores, or mill scale, and added separately if the raw mix is deficient in iron. Aluminum may be added with bauxite or other sources. Auxiliary materials such as fly ash and blast furnace slag are also often added as raw feed substitutes.
Aground mixture of the raw ma-terial containing major components in a required proportion is burned in a rotary kiln at about 14500C, where the constituents become fully oxi-dized and form stable solid solutions or the phases as described above. Impure CJS is also frequently known as alite, and C$ as belite”’. After cooling, the clinker is interground with approximate y 5% of gypsum to about 350 m2/kg Blaine fineness, to obtain portland cement.
A typical composition of ASTM Type I cement, the most commonly used cement in general construction, is normally 58’70 CaS, 15°/0C2S, 80/0 C~A, 107. CdAF, and 5?0 gypsum. Other ASTM cement types are Type II, III, IV, and V, which vary in com-position and are used where special properties are required. Typical corn-position and physical properties of various cement types are given in Table 1 (adapted from CTL, 1993; Mindess and Young, 1981).
Type III cement is a high heat of hydration cement with high C.$ con-tent and a finer particle distribution and is used where rapid hardening is required for early strength develop-ment. Type IV is a low heat of hydra-tion, slow setting cement because of low C~S and high C2S contents. It is intended for mass concrete in order to avoid thermal cracking, but is now rarely produced. Since the strength development of Type IV cement is low, Type II cement, which can be specified as a moderate heat of
hydra-tion cement, is generally recom-mended due to its higher strength and market availability. For even lower heat of hydration, Type II ce-ment with fly ash is used. Type V cement is also a low heat of hydration cement because of low C~S and low C~Acontents; it isnormallyused when high sulfate resistance is required. Type II is primarily used as a moder-ate sulfmoder-ate resistant cement.
A knowledge of the compound composition can reasonably be used to predict the properties of cement. One of the known methods for calcu-lating compound compositions from the oxide analysis are the Bogue for-mulae (1955). Although a number of sophisticated techniques are now available for Bogue calculations, the simplest Bogue formulation that has been found suitable for most applica-tions is given in the ASTM C 150 specifications.
Lesser Elements
Fourlesserelements, i.e., sodium (Na), potassium (K), magnesium (Mg), and sulfur (S), which appear in virtually all commercial clinkers at l-5y0
con-* In cement chemist’s notation S=Si02,
C=CaO, A=A1203,F=Fe203and S.S03
** Alites and Mites are never pure forms
of C3S and C2Srespectively. Due to the geological source of the raw materials, alite and belites will always have small quantitiesof impuritiesor traceelements.
centration, are represented in chemi-cal analyses as oxide forms: NazO, KZO, MgO, and SOS. Rompps Chemie-Lexikon (1987) has termed these ele-ments as the secondary eleele-ments. In cement chemist’s notation Na20=N, K20=K, MgO=M, and S0,=3.
Minor Elements
According to Miller (1976) and Gartner (1980), elements other than the major and the lesser constituents (i.e. Ca, Si, Al, Fe, O, Na, K, Mg, S) may be consid-ered as minor elements with regard to cement manufacturing. The concen-tration levels of minor elements in the clinker are almost always less than 17. and are generally categorized on the basis of the frequency with which they occur in the raw material mix.
Trace Elements
Blaine et al. (1965) regarded the ele-ments occurring at less than 0.02% each as the “trace” elements. Accord-ing to Sprung (1988), elements present at levels less than 100 ppm are classi-fied as trace elements. Because of their extremely small concentration levels, it seems unlikely that the presence of trace elements will have any signifi-cant effects on cement manufacturing. However, their effects on clinker can significantly change if concentrations are increased beyond certain levels.
For the sake of convenience, the terminology “minor elements” has been used throughout the text to cover both minor and trace elements, as de-fined by Blaine et al. (1965) and Rompps chemie - Lexikon (1987) re-spectively, unless mentioned other-wise.
Rompps Chemie-Lexikon (1987) has exemplified the classification of several major, secondary, and trace elements in cement clinker in Figure 1.
Emphasis in this report is given to the minor and trace elements because of their likely presence not only in the wastes but also in the conventional raw materials, and their potential
in-1 ppq 1 ppt 1 ppb 1 ppm
0.0019’0 1
?40Figure 1. Concentration ranges (by mass) of main, secondary, and trace elements in cement clinker (Sprung, 1988).
fluence on cement manufacturing and use. It maybe pointed out that trace elements in a raw feed at one cement plant could significantly dif-fer from another. As an extreme, lead content in one plant maybe 100-500 ppm compared to only 1 ppm in another plant (Chadbourne, 1990).
SOURCES
OF MINOR
ELEMENTS
Minor elements in cement primarily come from the raw materials and fuel used in cement making. Ex-amples of these are limestone, clay/ shale, and coal. They also come from the widely used auxiliary materials such as blast furnace slag, fly ash, silica sand, iron oxide, bauxite, and spent catalysts. A secondary but im-portant source of minor elements comes from the wide range of indus-trial by-products which are partially or totally being substituted for the primary fuel. These include petro-!eum coke, used tires, impregnated sawdust, waste oils, lubricants, sew-age sludge, metal cutting fluids, and waste solvents, as listed in Table 2.
Minor compounds found in sev-eral raw feeds for cement manufac-turing as quoted by Bucchi (1980) are shown in Table 3. Similar data on
major components of raw materials are shown in Table 4 and 5. They are limestone and shale/clay; widely used auxiliary raw minerals, i.e. blast furnace slag (used up to 307. by weight of raw material), and coal fly ash (used up to 15”/.by weight).
Minor elements found in con-ventional kiln fuel (coal), along with two secondary fuels (used oil and petroleum coke) are shown in Table 6. Average values of minor compo-nents found in typical clinkers (Moir and Glasser, 1992) are given in Table 7.
Although blast furnace slag can be used up to 307. by weight, the level of use maybe reduced due to its magnesium oxide (MgO) content, particularly if the MgO level is al-ready high in the other raw materi-als. Bauxite is reported to contain 2-87. titanium oxide (TiOz) and 0.04-0.4% chromium oxide (CrzO,). Iron ores frequently contain chro-mium, arsenic, cadchro-mium, and thal-lium, and may have adverse envi-ronmental consequences because of their toxicity characteristics. A list of metals having regulatory and envi-ronmental concerns has been speci-fied by waste characterization regu-lations under the Resources Conser-vation and Recovery Act (RCRA) and the Boiler and Industrial Furnace (BIF)
Table 2. Sources of Minor Elements in Cement Manufacturing
:Iements (as per group) Sroup I .ithium Group II 3eryllium Strontium 3arium Group HI 3oron
Gallium, Iridium, Thallium Group IV Germanium Tin Lead Group V Nitrogen Phosphorus Arsenic Antimony Bismuth Group VI Sulfur Selenium, Tellurium Group WI Fluorine Bromine Chlorine lodine Transition Elements Titanium Zirconium Vanadium Chromium Molybdenum Manganese Cobalt Nickel Copper Zinc Cadmium Mercury Sources
Waste lubricating oil
Fly ash
Limestone, aragonite, slag, waste lubricating oil Waste lubricating oil, refuse derived fuel (RDF)
Raw material, iron ore
Raw material, fly ash, coal, secondary fuel, waste derived fuel (WDF)
Raw material, coal Fly ash, RDF, fuel
Raw material, tires, RDF, WDF, copper shale, fly ash
Coal, air
Raw material, slag, sewage sludge, sandstone, RDF Fly ash, secondary fuel, coal, used oils
Petroleum, coke Fuel
Coal, slag, lubricating oil, petroleum coke, pyrite, tires Fly ash, coal, RDF, coke
Limestone, fuel Fly ash
Coal, slag, fly ash, waste lubricating oil, chlorinated hydrocarbons, RDF, chlorine-rich fuel Coal
Raw material, clay, shale, iron ore, bauxite, slag, RDF Raw material, silicon ores
Petroleum coke, crude oil, black shale, substitute fuel, coke, fly ash Bauxite, slag, recycled refractories, copper shale, tires, WDF, coal Waste lubricating oil
Raw material, limestone, clay, shale, bauxite, slag, fly ash Waste oil, fly ash
Fly ash, black shale, copper shale, waste oil, tires, RDF, WDF, coal, petroleum coke Fly ash, black shale, copper shale, lubricating oil, tires
Used oil, tires, metallurgical slags, filter cake, furnace dust, RDF, WDF Fly ash, black shale, copper shale, WDF, paint
WDF, paint fungicides
regulations that control treatment of hazardous waste in cement kilns. These are shown in Table 8 (Klemm, 1993). Both RCRA and BIF regula-tions apply to wastes and require the use of the TCLP (toxicity characteris-tic leaching procedure) tests.
The levels of sulfur and chlorine I in bituminous coal, the main fuel for
Cement kilns, varies frOIn 0..5to 4y0 and 0.007 to 0.39Y0, respectively. In some Illinois coals, sulfur is present up to 60/0by weight. Petroleum coke, u~ed as an-auxilia~ fuel, contains up to 5~0 sulfur and 0.6% vanadium
OX-ide, and can contribute certain levels of S and V to clinker when supple-menting for coal. Tires have a zinc content of 1.2-2.6 Yo. However, if tires replace 10% of the primary fuel, the resulting zinc oxide (ZnO) contents in clinker are increased by only ().W70 (Sprung, 1985),
Additional sources of minor com-ponents could be the refractories, chains, and the grinding media such as liners and grinding balls. A dam-aged chrome refractory lining can en-ter into the incoming raw mix and incorporate a detectable amount of chromium into the clinker. Partly for this reason, and mostly because of problems with their safe disposal, the use of chrome bricks is being phased out in most parts of the world (Moir and Glasser, 1992).
MINOR ELEMENTS
IN
CEMENT MAKING
The role of minor and trace elements in the formation of clinker and their effect on cement properties are dis-cussed in this report as per their oc-currences in raw mixes. The elements chosen for discussion are categorized according to the periodic table, as highlighted in Figure 2. They are discussed in their increasing order of atomic number. The presence of any information gaps are identified and referred for further investigation.
Table 3. Average Concentrations (%) of Some Minor Com~ounds in Raw M-eals Used in European Cement Plants (Adopted from Sprung et al., 1984; and Bucchi, 1980) - “
Minor Compounds MgO K20 so, Na20 TiO, Mn,O, P20, SrO Cr,O, AS,O, BeO NiO
V*05
cl
F Raw Meals 1,05 0.57 0.31 0.17 0,16 0.12 0.09 0.07 0.01 0.002 0.0005 0.003 0.024 0.02 0.06Table 4. Concentrations (ppm) of Some Minor Elements in Limestone and Clay/Shale (Sprung, 1985)
Minor Elements As Be Cd Cr Pb Hg Ni Se Ag TI v Zn cl F Br I Limestone 0.2-12 0.5 0.035-0.1 1.2-16 0.4-13 0.03 1.5-7.5 0.19 n.a. ” 0.05-0.5 10-80 22-24 50-240 100-940 5.9 0.25-0,75 Clay/Shale 13-23 3 0.016-0.3 90-109 13-22 0.45 67-71 0.5 0.07 0.7-1.6 98-170 59-115 15-450 300-990 1-58 0.2-2.2
Table 5. Average Concentrations (%) of Some Minor Compounds in Major Auxiliary Raw Materials, i.e. Blast Furnace (B. F.) Slag and Fly Ash (Moir and Glasser, 1992: and Smith et al.. 1979} Minor Compounds MgO K,O so, Na,O TiO Cr,&, MnzO, P20, SrO V206 B.F. Slag 7.2 0.57 3.00 0.44 0.66 n,a.* 0.64 0.03 0.06 n.a. AS20, n.a,
“n.a,= informationnot available
Fly Ash 5.28 4.05 2.25 1.99 1.21 0.03 0.14 <3.66 0.17 0.09 0.02
ELEMENTS
IN GROUP I
(Hydrogen, Lithium, Sodium, Potassium, Rubidium, Cesium)
Hydrogen
The role of hydrogen (H) in cement manufacturing has not been docu-mented in detail, because hydrogen per se does not exist for long in the kiln as hydrogen is highly combustible. It is present in the kiln as water vapor, which results from the evaporation of physically bound moisture from the raw material, and from the evapora-tion of water sprayed on the raw feed to control dust during processing. Water vapor can also be present in the kiln gases from combustion of fuel such as CHA + 20Z ~ COZ + 2HZ0 (Hawkins, 1994; Miller, 1994). A por-tion of water may also come from the dehydration of raw materials such as clays, where it can be present in signifi-cant quantities depending upon their mineralogical nature. In a wet plant, water comes from slurry.
The water vapor present in the kiln might have an indirect effect on the volatility of alkalies which can in-crease with vapor pressure at higher temperatures, for example:
2H,0 + 21$S04 -’ 4KOH + 2S02 + 0, less more very volatile volatile volatile
Apart from that, hydrogen may not have any significant effect on the an-hydrous nature of clinker.
As a point of information, it may be mentioned that early cement kilns sometimes used “producer gas” as fuel. The gas was generated (as a mixture of CO and Hz) by the action of steam on hot coal or charcoal as follows:
H20+C~CO+H,
Currently, no kilns in the USA Canada use this technology.
or
Groups
1
2
6
7 1=” !3s ‘AC Unq Unp Unh Uns
Lanthan~de Series Act inide Series
. L? d
Pr’
Nd@ml”am”
EU ‘ Kid lb’ DY” Ho” Er6 Tm6 ~ . “ Th Pa’ , Ua NP93 Pu” Am’ Cm Bk v Cf98Figure 2. Elements from the periodic table selected for studies.
Table 6. Average Concentrations (ppm) of Some Minor Elements in Coal aid Used Oil (Sprung; 1985; and Weisweiler and Kr6mar, 1989)
Minor Elements Coal Used Oil Petroleum Coke
Sb 1.19 n.a.* 0.0429 .4s 9-50 <0.01-100 0.6 Ba 24.5 0-3,906 8.4 Be 2.27 n.a. n.a. Cd 0.1-10 4 n.a. Cr 5-80 <5-50 11.0 Pb 11-270 10-21,700 8.7 Hg 0.24 n.a. n.a. Ni 20-80 3-30 208.0 Se 3.56 n.a. 0,1 Ag 0.06 n.a. n.a. TI 0.2-4 <0.02 0.1 v 30-50 n.a. 778.0 Zn 16-220 240-3,000 n.a, Sr n.a. n.a. 4.3 cl 100-2,800 10-2,200 n.a. F 50-370 n.a. n.a. Br 7-11 n.a. n.a. I 0.8-11.2 n,a. n.a.
Lithium
Lithium (Li) is found in some waste materials such as used lubricants, but occurs only in traces in the kiln raw , feeds and common fuels.
Lithium might behave somewhat differently from sodium or potassium in that it would tend to form a rela-tively nonvolatile oxide (LizO) at el-evated kiln temperatures, Gouda (1980) reported that LizO is most reac-tive in lowering the temperature of the initial liquid phase; the effectiveness has been shown as LizO>NazO>KzO, The presence of L<O also disturbs the course of the burning process during which lime dissolves in the liquid phase and results in higher reactivity. As a negative effect, Li10 also inhibits the conversion of CZS to C,S. The effects are more pronounced with Li.O comuared to Na.O and K.O.
Ra~garao’(1977) poin~ed out t~at up to l% of Li10 in the raw mix im-parts a mineralizing effect, but lime fixation isimpaired afterwards. At a lower LizO addition (0.1-0.3% by weight), limestone dissociation acti-vation energy is reduced, and mineral formation becomes more intensive.
If present in adequate amounts as an admixture, Li can have beneficial effects on cement properties, since it is known to greatly reduce the alkali-silica reaction (ASR) in concrete. Re-cent studies by Stark et al. (1993) have demonstrated that Li salts like LiOH and Li carbonates (if added in appro-priate amounts) reduce the ASR sig-nificantly. It is conceivable that Li in clinkers could also have the same af-fect on ASR.
Sodium and Potassium
Since both sodium (Na) and potas-sium (K) occur together in raw feed, and by virtue of similarities in their behavior in cement manufacture, it is appropriate to discuss them together,
Sodium and potassium are mainly derived from the raw materials; their
Table 7, Average Concentrations of Some Minor Compounds Foundin Conventional Clinkers (Moir and Glasser, 1992)
Minor Compounds MgO K,O so, Na20 TiO, Mn20, P*05 SrO Minor Compounds ZnO Cr,O, V*05
cl
As,O, Cuo PbO CdO T120 Mean Value (%) 1,48 0,73 0.80 0.16 0.27 0.06 0.10 0.09 Mean Value (ppm) 120 103 100 90 56 55 16 0.5 0.3Table 8. Elements of Regulatory and Environmental Concern (Klemm, 1993) Elements Antimony Arsenic Barium Beryllium Cadmium Chromium (Total) Chromium (Vi) Lead Mercury Nickel Selenium Silver Thallium ‘RCRA Metals Yes Yes Yes Yes Yes Yes Yes Yes Yes RCRA Limit ‘* Using TCLP 1.0 mg/L 5.0 mg/L 100 mg/L 0.007 mg/L 1.0 mg/L 5.0 mg/L not defined 5.0 mg/L 0.2 mg/L 70 mg/L 1.0 mg/L 5.0 mg/L 7.0 mg/L “ BIF Metals Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
● RCRA . Resource Conservation and Recovery Act
“’ TCLP = Toxicity Characteristic Leaching Procedure “’* BIF = Boiler and Industrial Furnace
BIF Carcinogen Yes Yes Yes Yes
main carrier is clayey rock. Sedimen-tary rocks, including the carbonate ores, sometimes contain soluble alkali salts. Lea (1971) has quoted the occur-rence of Na and K (by weight O/.)in different components of raw materi-als used for cement manufacturing as shown in Table 9.
Table 9. Presence of Sodium and Potasium in Different Raw Materials (Lea, 1971 )
7. by wt. Na20 K20
Typical raw mix 0.13 0.52
Limestone 0.26 0.11
Chalk 0.09 0.04
Marl 0.12 0.66
Clay 0.74 2.61
Shale 0.82 4.56
Alkalies frequently occur in auxil-iarv raw materi~ls su~h as blast fur-na~e slag and fly ash as shown in Table 5. NazO and KZO in European fuels range between 0.05 -O.6?40and ().5-2%, respectively (Bucchi, 1980); for typical raw feed, their average concentration Ievek are 0.177. and ().57Y0, respec-tively, as shown in Table 3.
Jawed and Skalny(1977, 1978) and Skalny and Klemm (1981) have re-viewed in detail the effects of alkalies in cement manufacture and use. NazO and KZOare volatile in nature, giving rise to a cycle inside the kiln. The extent of alkali volatilization varies with raw material composition; for instance, volatilization of alkalies in clays is higher than those found in feldspar. About half of the total alka-lies by weight in the feed are volatil-ized between 8OO-1OOO’Cas the mix nears the burning zone, but condenses at cooler parts of the system, such as in suspension preheater riser ducts or in chain systems in dry kilns. The forma-tion of rings and coatings on kiln lin-ing resultlin-ing from this heatlin-ing-cool- heating-cool-ing cycle are generally attributed to alkali condensation and reaction with refractories or incoming material. To
%’
800 – 600-400 – ‘“\ 200 – NaCl 30 20 10 1000 1“100 1200 1300 1400 1500 Temperature, ‘CFigure 3. Vapor pressure of Na and K chlorides and sulfates (Bucchi, 1980).
avoid excessive buildups in the kiln or preheater vessels, a percentage of gases may be bled through a by-pass, so that the alkali sulfates and chlorides maybe continuously removed and end up in the cement kiln dust (CKD). Thus,CKDs collected from by-pass dust collector are typically high in alkali contents. Usually potassium compounds are more volatile than the sodium com-pounds.
According to Bucchi (1981), the in-tensity of the alkali cycle depends upon the nature of their presence in raw ma-terial, on operating practices, and on type of kiln. The retention of alkalies in clinker is generally higher for high effi-ciency kiln systems (Lea, 1971). With gas and oil as fuels, the alkalies tend to volatilize more as compared to coal used as a fuel. This may be due to the high intensity of the flame with oil and gas compared to coal.
In the presence of chlorides and sulfate, the volatilization behavior of both Na and K is modified greatly, as shown by the vapor pressure-kiln tem-perature relationship in Figure 3. The vapor pressures of alkali carbonates resemble those of sulfates and they ex-hibit similar effects. In the presence of sulfur, alkalies preferentially form sul-fates. If their amount is more than the required stoichiometric balance, the excess will be dissolved in the silicates, aluminates, and ferrites. The common alkali sulfate phases formed are K2S01,
E’
also known as arcanite, sodium potassium sulfate, also known as aphthitalite of a general solid solution composition (K,Na)2S01*, and NazSO1, also known as thenardite (Taylor, 1990).
According to Skalny et al. (1981), the ratio of Na to K in cement raw materials in the North America and Europe varies. There is usually a sub-stantial excess of KZO over NazO. Therefore, in the presence of suffi-cient amount of SO~ a range of double alkali sulfates, as described later in this report, is formed depending upon the KIO to Na20 ratio. If KZO is in excess of that required to produce aphthitalite, it forms arcanite.
Burning conditions also signifi-cantly influence the formation of sul-fate so that oxidatizingconditions pro-duce calcium-potassium sulfate and a reducing condition produces sodium-potassium sulfate. Potassium is twice as likely to produce soluble sul-fates as sodium. According to Pollitt and Brown (1968), the calcium potas-sium salt, calcium langbeinite 2CaSOA”KzSOq**, is also sometimes found.
Introducing SO, jointly with K20 and NazO into clinker melt leads to phase separation. Since alkalies
re-. Commonly written as K3N54in cement chemist’s notation
2CS.KS-duce the melt temperature, and the rate of C$ formation is proportional to the amount of liquid phase, a posi-tive effect on C$ formation could be expected. However, Johansen (1977) reported that C~S,with or without the presence of alkali, has the same amount of free lime after firing at 1400-15000C. Alkali sulfate melt and clinker liquid are immiscible phases. Alkalies inhibit the formation of C,S from CzSand lime bystabilizinglower energy CZSin the absence of sulfates.
After allocating for the sulfates, the remaining alkalies are distributed between silicates, aluminates, and aluminoferrite. Lea (1971) has re-ported ranges of alkalies in the major clinker phases shown in Table 10.
The values are in general agree-ment with those quoted by Taylor (1990) tin the distribution of alkalies in different clinker phases.
Gartner (1980) and Gies et al. (1986) reported that in the absence of SO~, Na20 is preferentially incorpo-rated in CqA by replacing CaO and forms “alkali-aluminate” of an approximate composition NaO*8CaO*3AlzO~*, thereby reducing its reactivity. This results in clinkers rich in free lime and aluminate, and can reduce the burnability. K is sub-stituted in C$ as a compound with an approximate composition of KzO*23CaO*12SiOz,** and the overall reactivity of clinker is decreased due to the slower reaction with CaO to form C~S in the burning process. In the presence of sulfate, KZOincreases the C,Areactivity(Strungeet al., 1986). Richartz (1986) reported that SOS re-duces the extent of alkali solid solu-tion in C~A and hence the reactivity, but improves the cement properties. The mineralizing effectiveness of alkalies (in terms of decreasing melt viscosity, and free lime contents in clinker) also appears to be a function of their cation size, electronegativity, or the ionic potential. Such relation-ships for K, Na, Li and other relevant cations are given in Table 11 (Grachian et al.), and in Figure 4 (Teoreanu and
Table 10. Range of Alkali Distribution in Clinker Phases (Lea, 1971)
I
Clinker PhaseL
C3S
C*S
C,A C,AF Na,O (wt.%) 0.1-0.3 0.2-1.0 0.3-1.7 0.0-0.5 K,O (wt.%) 0.1-0.3 0.3-1.0 0.4-1.1 0,0-0.1 Table 11. Effect of Ionic Potential of Minor Elements on theMelt Viscosity (Grachian et al., 1971)
Effect of Different Ions Ionic Potential of Elements
on Melt Viscosity (ratio of number of iogic
(in decreasing order) charge/cationic, Rvl in Al) Be+2 Mg+2 .942 Li+l Ba+2 Na+l K+l 5.71 2.50 1.65 1.22 1.39 0.91 0.68 7 I
[Cao]o= Free CEO in the Absence of Mineralizer (%) Mg+2 [@O], ❑Free Cso in the Presence of Mineralizer (%) ●
6 5 ~+2 ● 4-0 0.1 0.2 0.3 0.4 0.5 Field Strength, m-2
Figure 4. Mineralizing effectiveness of cations for clinker with LSF=O.96, SM=2.2, AF=2.O at 1350°C (Teoteanu and Tran von Huynh, 1970).
Tran van Huynh, 1970) respectively. It might be mentioned that although alkalies, NazO in particular, may act as fluxes, they are technically less desirable compounds than many of the other available minor compounds
@ucchi, 1980).
If present in excess, alkalies often lead to higher pH and better early strength, but lower later strengths. They are not desirable because of
* Commonly written as NC8A3
their deleterious alkali-silica reaction (ASR) with reactive aggregates that leads to expansive reactions and can cause serious cracking in concrete. ASR can be prevented with proper use of pozzolans.
Butt et al. (1971) reported that the deleterious effects of alkalies on the mechanical properties of cement may be reduced by gypsum addition to the raw feed. They considered this be-cause of the possible elimination of solid solutions of alkalies with clinker minerals. One possible speculation derived from the microscopic studies by Prout (1985), is that gypsum would either increase the volatilization, or eliminate NaO”8Ca0-3Al,0~ or K,0*23CaO”12SiOz formation. According to Lokot et al. (1969), the addition of gypsum to raw feed produces cement of high 28-day strength, enhancing kiln output and fuel savings.
Rubidium and Cesium
The remaining Group I elements, ru-bidium (Rb) andcesium (Cs),arefound only as traces in cement raw mix or in the fuel. Rubidium generally occurs in cement at 0.017. or less (Blaine, 1965). Both Rb and Cs are expected to behave similarly to Na and K, in that they would both form stable sulfates and volatile chlorides in the kiln (Gartner, 1980). On the other hand, their concentrations may be too low to effectively influence the clinker forma-tion or cement properties.
ELEMENTS
IN GROUP Ii
(Beryllium, Magnesium, Calcium, Strontium, Barium)
Beryllium
Beryllium (Be) would be present only in trace amounts in the raw feed and fuel (see Tables 4 and 6). It is found only occasionally in the fine fractions of fly ash, an auxiliary material fre-quently used as substitute raw
mate-rial. Beryllium is found at a 55 ppm levelin=4 pm fraction compared to 12 ppm in >45 p,m fraction of fly ash (Davison et al., 1974).
It maybe suggested that because of its low volatile oxides, beryllium would stay in the clinker, Nonethe-less, beryllium has not been measured in significant amounts in clinkers to have any measurable effects on clin-ker formation or cement use. Beryl-lium in todays cements has occured up to 3 ppm (PCA, 1992).
Magnesium
Magnesium (Mg) in portland cement is mainly derived from magnesium carbonates present in the lime-stone in the form of dolomite CaCO,*MgCO,, while smaller amounts coming from clay and shale (Lea, 1971), ordiopside(Fundal, 1980). If present in small quantities, mag-nesium improves the burnability of clinker (Christensen, 1978). Accord-ing to Long (1983), the behavior of MgO in clinker formation primarily depends upon the cooling rate. When clinker is burnt at high temperature (>15Cr0°C) and rapidly cooled, it re-tains the bulk of the MgO mostly in aluminate and ferrite phases, with a lesser amount in alite. Under condi-tions of slow cooling, ody 1..57. of MgO is retained in solid solution and the rest is crystallized as large periclase crystals. MgO in cement is usually limited to under 5’Yo,because MgO content in excess of 2?40can occur as periclase (Taylor, 1990), The presence of larger crystals of periclase in ce-ment slowly reacts with water to form expansive Mg(OH)z and can lead to destructive expansion of concrete. ASTM C150 specifications allow MgO contents up to 6% in portland cements.
Magnesium salt solutions (sulfate and chloride) are aggressive towards concrete and react with the calcium hydroxide phase to form basic salts. The reactions are expansive and may lead to deterioration of concrete.
Calcium
The role of calcium (Ca) in cement manufacturing has alread ybeen dealt with in the section of major elements.
Strontium
A major portion of strontium (Sr) found in clinker as SrO comes from limestone and aragonite.
Strontium as SrO, frequently oc-curs in clinkers. The mean value quoted by Moir and Glasser (1992) in Table 7 is O.09%. Brisi et al, (1965) and Gilioli et al. (1972, 1973), demonstrated that small amounts of S@ favor alite formation, but at 4-5y0 addition, Sr preferentially distributes in belite rather than alite, Sr in belite inhibits alite formation. Phase equilibrium studies indicate that Sr in raw feed also favors free lime formation, with SrO preferably going into solid solu-tion and displacing CaO from other compounds, The tendency of free CaO release during clinkering makes SrCOq more labile than SrSO~, and the clinkers having a high lime saturation factor (LSF) maybe more vulnerable to free lime expansion during hydra-tion.
Butt et al, (1968) reported that the hydraulicity and strengths developed by Sr-doped alites are significantly lower than the normal alites. This may be attributed to the smaller sizes of the lattice voids in strontium-in-corporated alite.
Kantro (1975) reported a slight set acceleration effect withS~lz~ 6HZ0 used as an admixture in CaS paste.
Barium
Barium (Ba) occurs in varying amounts in limestones, mostly as bar-ite (BaSOd). It can also occur in clayey sediments in appreciable amounts. The average amount of barium in ce-ment is 280 mg/kg. The average for CKD is 172 mg/kg (PCA, 1992).
Timashev et al. (1974) reported a decrease in clinkerization temperature from 1450 to 1400”C and increase in the clinker production rate from 8.2 to 9 tonnes/hr, when using raw mixes con-taininghigheramounts of barium. They also noted an improvement in the min-eralogical composition of the resulting clinker. However, Kurdowski (1974) reported only a marginal usefulness of BaO when added in small amounts, stating that it did not significantly af-fect either the properties of the liquid phase or the rate of lime assimilation; Ba replaced Ca in all the clinker phases, except for the ferrite phase. The opti-mum BaO concentration was between
0.3 to 0.57., preferably for clinker con-taining less flux (silica modulus >30) and high CIS levels.
According to a number of studies, Ba also appears to be an effective activa-tor of hydraulicity and strength. The strength obtained from Ba incorporated clinkers is 1O-2O’7Ohigher than that of regular clinker of all ages tested under identical conditions (Kurdowski, 1974; Butt et al., 1968; Kurdowski et al,, 1968; Kruvchenko, 1970; Peukert, 1974).
Barium can be present in used oils. Excessive amounts in raw mix can in-crease the free lime content of clinker due to CaO displacement and can cause expansion in concrete under certain cir-cumstances. It can also lead to paste shrinkage.
ELEMENTS
IN GROUP Ill
(Boron, Aluminum, Gallium, Iridium, Thallium)
Boron
Boron (B) is generally found in traces (3 ppm) in most cement raw materials, particularly those containing iron ore. Fromearlystudiesby Mircea (1965), it appears that BzO~ reacts with CJS to form CZS, C~BS*, and free lime. Upon further addition of BzO~,C~Scompletely disappears. Timashev (1980) established a relationship between the electronega-tivity of boron and the melt viscosity,
0
CaZn VBe NiKCr AsPb S
957
Cd Cl TI
Figure 5. Relative volatilities of elements in clinker burning in a cyclone preheater kiln (Sprung, 1988).
‘Restive volatility as a percentage of ratio between the total
external and internal balance for a given element.
and noted a similarity between be-rates, phosphates, and sulfates. Boron inhibited the formation of C$ and af-fected the stability of the other major clinker phases. In the presence of bo-ron C$ is decomposed to a stabilized C2S as follows:
C,S ~ C2S + Cao
It was also pointed out that although B20q may not be a useful addition for regular alite clinker required for early strength development, it might be use-ful as a mineralizer for clinkers rich in belite. Gartner (1980) has reported on the effectiveness of BzO~ to stabilize /1-C2Sand to improve its hydraulicity. According to Miller (1976), boron can also stabilize ~-CzSin alumina andiron-poor systems.
However, Miller (1976) has cau-tioned that the indiscriminate addi-tion of boron can produce unpredict-able hydration results. Gartner (1980) explained that this behavior of boron is probably sensitive to the presence of other trace elements. Bozhenov et al. (1962) reported that even small addi-tions of BzO~(-0.040/.), as an
admix-ture, to cements can have adverse
ef-fects on setting properties. These ob-servations indicate that B20~is a strong retarder of cement hydration.
Aluminum
Role of aluminum (Al) in cement manufacturing has been dealt with in the section of major elements.
Gallium,
Iridium, and
Thallium
Gallium (Ga), iridium (In), and thal-lium (Tl) are found only in traces in raw material; their typical concentra-tions in coal are 5-10 ppm, 0.07 ppm, and 1.1 ppm respectively. Thallium and gallium are also found sometimes in the coal fly ashes. Thallium may also be found in some pyritic minerals used as an iron source for raw feed. The average concentration of thallium in cement is 1.08 mg/kg, ranging from nondetectable to 2.68 mg/kg. The av-erage concentration of thallium in CKD is 43.24 mg/kg (PCA, 1992). * Bis B103 in C5BS
Although thallium occurs in traces in the raw feed, it is the most volatile element* after mercury in the kiln (melt-ing point=30~C), and is most likely to concentrate in the kiln dust. The volatil-ity of T1relative to other elements in the kiln is shown in Figure 5 (Sprung et al., 1984). Sprung et al. determined the volatility on the basis of the difference between the external and internal bal-ances of individual elements during the clinker burning in a cyclone preheater kiln. Iridium is also volatile and largely ends up in the kiln dust. Since thallium may concentrate in the fly ash from the coal firing power plants, in the cement kiln operation it tends to build up in extremely large internal cycles if no dust is discarded.
ELEMENTS
IN GROUP IV
(Carbon, Silicon, Germanium, Tin, Lead)
Carbon
Carbon (C) is a major component of fuel, It is also present as carbonate in the limestone. A significant amount of car-bon can also present in flyashas unburnt coal.
Carbon as C02 is extensively present in cement kiln systems, but is not present in any significant levels in clinker.
Be-cause of the limestone and fuel that are used in the kiln, the gases emitted from the kiln system are constituted mainly of COz,~O, and N2. Limestone (CaCO~) decomposes to CaO and C02 at about 9000C. Roughly for every ton of clinker, one ton of COZ is generated in the kiln, which essentially is released through stack emissions.
Silicon
Role of silicon (Si) in cement manufac-turing has already been discussed in the section on major elements.
Germanium
Germanium (Ge) is a trace element found in raw material and coal.
Germanium oxide (GeOz), is not volatile (Gartner, 1980), and is likely to concentrate in clinker. When present in larger amounts, GeOz can form C~G**, tricalcium germanate with CaCO~ at 1500”C and isstablebetween 1335”C-1880”C. At temperatures be-low 1335°C, C,G decomposes to CZG and free lime (Hahn et al., 1970; Boikova et al., 1974). These forms of calcium germinates are similar to C~S and CZSrespectively. COGis hydrau-lic and produces calcium germanate hydrate (C-G-H) and calcium hydrox-ide (CH) with water, whereas ~G is assumed to be non-hydraulic. Ac-cording to Gartner (1980), it is un-likely that the trace amounts of Ge would seriously affect the formation of clinker and the properties of the resulting cement.
Tin
Tin (Sri) is a trace element in both the raw feed and fuel.
Tin is reasonably nonvolatile (boiling point=2265°C). Tin oxide (SnO) or natural cassiterite melts at 1630”C and sublimes between 1800 and 1900°C. It is very likely that tin will stay in the clinker. The presence of trace amounts of tin in clinker should not affect cement properties, although not much is known about the effect of tin in clinker manufac-ture.
Lead
Lead (Pb) can be present in trace amounts in raw material mainly in clay and shale. It would be present at appreciable levels in coals, used oils, lubricating oils, and scrap tires. In fly ash, lead tends to concentrate in the fine fractions (Coles, 1979), Lead lev-els in coal, used oil, and petroleum coke are shown in Table 6. Another source of lead could also be the lead shot from shot gun shells used to shoot out rings.
The effect of lead in cement manufacturing and properties has
been studied in some detail, Lead compounds are fairly volatile. They tend to vaporize in the kiln, and exit the kiln as fines and are collected in the kiln dust.
There is also evidence that de-spite the partitioning of lead into the CKD, some lead can still be retained in the clinker (Davison et al. (1974), and Berry et al. (1975)). However, Pb has been shown to have no adverse effect on cement properties if present below 70 ppm. The effect of lead levels higher than that in clinker is uncertain (Sprung et al., 1978). Ac-cording to a recent PCA study (PCA 1992) the average lead levels in the CKDS and cements produced in North America are 434 ppm and 12 ppm respectively.
Some research on the effect of lead compound additions on hydrat-ing cement properties has recently been studied, where Bhattyand West (1992) have noted that additions ei-ther as a soluble compound (PbNO~: 7,300 ppm level ) or insoluble oxide (PbO:38,000ppm level) substantially retards the hydration of pastes, but enhances the workability, The retar-dation effects are more pronounced with oxides. The initial setting time is increased with a consequent loss in early strength, but the 28- and 90-day strengths are comparable to or higher than those of the control.
ELEMENTS
IN GROUP V
(Nitrogen, Phosphorus, Arsenic, Antimony, Bismuth)
Nitrogen
Nitrogen (N) can be present up to 0.01’% by weight in the raw materials, but in coal and other fuels nitrogen can be as high as 1-2Y0, often as hetrocyclic nitrogen compounds.
Clinker made under reducing conditions tend to have up to 0.057. N
* Nonvolatile elements are often called
refractory elements. ““ G= GeO,
as nitrides. Under normal oxidizing conditions, nitrogen in clinker is present only at a few ppm.
High concentration of nitrogen, higher residence temperatures, par-tial pressure in the flame zone, and the subsequent oxidation of nitrogen leads to the formation of several ox-ides of nitrogen (NO and NOZ and NZO1) in the kiln emissions collec-tively known as NOX. Total NOX re-sults from fuel nitrogen NO, thermal NO, and prompt NO. In the cement manufacturing, fuel nitrogen NO and thermal NO play a significant role. The prompt NO which is formed by the participation of CH in the oxida-tion of nitrogen in air, plays a less significant role (Bretrup, 1991).
The quantity of thermal NO formed is closely related to the burn-ing zone temperature (BZT). Accord-ing to Lowes et. al (1989), a reduction in BZT from 15000C to 1300”C can reduce the NO, levelsby200-400 ppm. Nitrogen in coal orotherfuels,present at about the 1-2% level, is considered significant in producing NO emis-sions from cement plants. However, it is not known to the degree in which the nitrogen in the kiln raw feed also contributes to NO, emissions (Gartner, 1980). In precalciners, fuel nitrogen may play a role, but in the burning zone the temperature is so high that thermal NO is virtually in equilib-rium.
Phosphorus
Phosphorus (P) as phosphates is present in limestone and shale (Moir et al. 1992); they are also present in sandstones, sands, and in detritalclays (Bucchi, 1980). Phosphorus also oc-curs in the blast furnace slags, electric furnace slags, convectorslags, and fly ash which are often used as substitute raw feed for cement manufacturing. Phosphate is found in sewage sludge which is a potential partial kiln fuel. Cement clinkers contain typically around 0.2% PzO~(Lea, 1971). A high PzO~concentration decomposes C~S
to CZS and excess lime. If PZ05 is present in excess of 2..57. by weight, the formation of free lime occurs (Nurse, 1952). However, by correct proportioning and proper burning, sound clinker can be produced, but cement hardening becomes slower. Matkovich et al. (1986) reported higher hydraulic actively for (x’CzS stabilized by PZ05 than for the &CzS.
Odler et al. (1980-1) reported the addition of hydroxyapatite Ca~(PO1)~OOHleads to an increased formation of free lime at 1300”C, being directly proportional to the PzO~content. This was attributed to the preferential stabilization of CZS solid solution and formation of free lime at increasing P205 additions. However, Halicz et al. (1983) dem-onstrated that a satisfactory C~S phase in clinker was formed by add-ing PzO~in the raw feed and main-taining lime salmation factor (LSF) and silica ratio (SR) at 1.0 and 2.75 respectively.
In a CaO-CzS-C~P* system at 1500°C, raw mix with more than a few percent P,O, does not yield C,S. However, in the presence of fluo-rine, the tolerance to PzO~is some-what improved. It is very likely that the thermodynamics of the system favor the fluoride-aluminum-CIS solid solution rather than P-C$ solid solution (Gurevich et al., 1977) and apparently form a fluoroapatite phase (10Ca0.3Pz05*CaFz) which is dissolved in C,’S. Gartner (1980) suggested that chlorides may also help stabilize PzO~ in C~S by forming a stable chloroapatite (10CaO*3P,0,*CaC12) which also forms a stable solid solution with fluoroapatite.
Coleman (1992) reported that an appropriate level of PzO~in clin-ker reduces the negative effects of alkali on the strength properties of cements. He reported that in ce-ment clinkers with “normal” NazO contents of 0.8Y0, the maximum 28-day strength was achieved at 1.07. PZ05 level.
Arsenic
Arsenic (As) bearing mineral arseno-lite or claudite AszO~ (or AslOG), oc-curs only in small amounts in coal and used oils, and are unlikely to influence cement manufacturing in any way. Smith et al. (1979) have indicated that in coal-fired power plants, As tends to concentrate in the fly ash, but its concentration level, as detected by the XRF method, is ex-tremely low. It tends to concentrate in the fine fractions of fly ash where the levels can go up to 70 ppm. Weisweiler et al. (1989) has reported up to 5 ppm of As in raw material and only 0.6 ppm in petroleum coke. Ar-senic levels found in various materi-als are shown in Tables 4-6. The aver-age concentration of As in cement and CKD is 19 mg/kg and 18 mg/kg respectively (PCA, 1992).
Although AszO~ is volatile (sub-limes at 1930C) and should be ex-pected to condense on kiln dust par-ticles, Weisweiler et al. (1989) ob-served that a substantial amount of As is incorporated in the clinkers, and only a negligible portion of As ends up in the dust. The cause of As entering into clinker was attributed to the excess CaO, oxidizing condi-tions in the kiln, and high kiln tem-perature. Under oxidation condi-tions, As is primarily oxidized to AszO~and forms a series of low vola-tile calcium arsenates, among which Ca,(AsO,), is more stable at 13000C. Czamarska (1966) found that 0.157. AS+5significantly decreased the rate of C~S formation at 1450”C.
As a metalloid occurring in dif-ferent oxidation states, arsenic can have complex effects on the hydration properties of cement (Conners, 1990). Tashiroet al. (1977) reported that AszO~ only slightly retards the paste hydra-tion when added up to 5’Yo. It was found that the As leaching rate from hardend cement mortars using either ordinaray water or sea water, although measurable, was very low.
Antimony
Antimony (Sb) occurs as traces in cement raw materials. It has been reported to occur at 0.08 ppm in the raw feed and 0.0429 ppm in petro-leum coke (Weisweiler et al., 1989), Sprunge (1985) has quoted 1.19ppm Sb in coal. According to measure-ments in BIF certification of compli-ance (C.O. C.) and other authors, the Sb levels in raw materials are higher.
Like arsenic, a considerable por-tion of antimony is incorporated in clinker in the form of low volatile calcium antimonates under oxidiz-ing kiln conditions at high tempera-tures (Weisweiler et al., 1989), The mechanics of stable calcium antimonate is more likely the same as for arsenate formation. The oxides, SbzOj, natural seranmontite, and valentinite, are not very volatile at kiln temperatures; they sublime at 1550”C. Although usually not de-tected in cement and CKD, Sb levels as high as 4.0 and 3.4 mg/kg have been reported for cement and CKD, respectively (PCA, 1992).
Bismuth
Bismuth (Bi) occurs as a trace element in the raw feed and fuel. The stable oxide BizOqis not volatile at clinker-ing temperature (boiling point =186WC). Little is available on the influence of Bi in cement manufac-turing and cement hydration, but, owing to trace concentration, it is conceivable that the effects will be practically insignificant.
ELEMENTS
IN GROUP VI
(Oxygen, Sulfur, Selenium, Tellurium)
Oxygen
The role of oxygen (0)
per se
on the manufacture and use of cement has not been studied. Nonetheless a con-siderable portion of raw material and clinker phases incorporate oxygenPossible carry-through of complex calcium sulfides in clinker
S-2 present as organic and inorganic forms in fuel, etc
ReducedS Species
S02 prominent Molten sulfates in vapor pressure
Sulfites, SO; in solids Sulfate solids which become S03vapors increasingly unstable
with rising temperature
IntermediateS Species Oxidized S Species
Increasing Oxygen Pressure ~
Figure 6. Formation of different sulfur species in cement clinkering (C~oi and Glasser, 1988).
in one form or the other. Raw mate-rial is primarily composed of CaCO~ (-75%),Si0, (-20%), and A1,O, (-2%). CaCO~ in the raw mix is derived from limestone; SiOz and AlzOqfrom clays, shales, sandstones, and bauxite, and FezO~from iron oxides andiron ores. The clinker is formed by heating a powdered raw material of an ap-propriate proportion to 1400-1550”C in a kiln having a z-s~. oxygen level. As stated previously, the final four phases in clinker are in the fully oxi-dized forms. They are: tricalcium silicate 3CaO”SiOz, known as alite; dicalcium silicate 2Ca”SiOz, known as belite; tricalcium aluminate 3CaO”A110q, known as aluminate, and tetracalcium aluminoferrite, 4CaO”A120~”FezO~,known as ferrite.
The importance of oxygen levels is also related to the effect on the environment of the kiln and the kind of reactions that are favored. Thus, the presence of oxydizing or reduc-ing atmosphere greatly influence the reaction into which the various ele-ments will enter. Clinker made un-der oxidizing conditions tends to in-corporate trace metals of higher oxi-dation states than clinker prepared under reducing conditions. Ex-amples of chromium and sulfur can be cited here. Cr+s would tend to form under oxidizing conditions, in-stead of Cr+3, which results under
reducing conditions. Cr has also been reported to occur as CrA, Cr~s, and Cr+5(Johansen, 1972), but eventually they disproportionate to more stable Cr+3or Cr+swhen mixed with water. Alkali sulfates formed in the kiln are preferably decomposed under re-ducing conditions. Kilns having strongly oxidizing conditions and low burning zone temperature tend to retain more sulfur in clinker than those produced under reducing con-ditions and for high burning zone temperature. Thus, the oxidation or reducing conditions in the kiln can lead to significant phase modifica-tions in clinker.
Clinker produced under reduc-ing conditions are brownish as com-pared to darker gray clinkers made under oxidation conditions, most probably because of the oxidation state of iron. Burning conditions ma y also have an effect on the crystallinity of major phases. The effects can be pronounced if trace metals are also present.
Sulfur
SUIfur (S) is frequently present in coals and some fuel oils; sulfates and sul-fides are also often present in the limestones. Clayey sediments, marls, also contain both sulfides and sul-fates. Lecher et al. (1972) have
re-ported occasional use of gypsum and anhydrite as mineralizers and modifi-ers of the alkali cycle in the kiln.
Sulfides and sulfur from raw mate-rials and fuel are oxidized and are in-corporated into the solid phases as sul-fates in the clinker, though some sulfur as SOZ will almost always escape with the exiting gases.
Sulfur forms volatile compounds and its behavior in a kiln is a complex one. Depending upon the burning con-ditions in the kiln, both oxidized and reduced species may occur in solid, molten and vapor phases, as explained by Choi et al. (1988) in Figure 6. Under oxidizing conditions at high tempera-ture, the formation of SOzis most likely. In the presence of lime, S02 is partly removed to form CaSO1 by the follow-ing mechanism:
S02 +
CaO ~ CaSO,CaSOz + l/20z ~ CaSO1
In the presence of alkali, alkali sulfates are formed which are later condensed at the lower temperature regions. These
condensates, from liquids and solids, contribute to build up problems in various kiln systems. Intermediate compounds such as “sulfospurrite”, 2C2SOC~,and the ternary compound “sulfoaluminate” C1@ also condense at lower temperatures.
Another well known problem of sulfur being volatile is its cycle of vaporization and condensation with alkalies. They are volatilized at high temperatures and subsequently con-dense on the relatively cooler incom-ing raw feed resultincom-ing in high sulfur and alkali levels in the middle zone of kiln, especially with preheater. The use of an alkali by-pass is often effective to break this cycle and lead to the reduction of sulfur and alkalies in the incoming kiln feed. However, alkali sulfate levels are significantly increased in the by-pass dust, which is captured by the dust-collector and generally discarded.
Sulfates preferably combine with alkalies to give alkali sulfates in clinker as (K, Na)#O1, known as aphthitalite, or K2S01 known as
arcanite. If sulfate is ptesent in ex-cess, the balance between alkali is achieved by forming calcium lang-beinite, Caz~(SO,)Y which is stable up to 10110C in a CaW1-KzWi sys-tem. However, this phase is known to evaporate inconWuently at high temperatures, and vaporizes K and S (Arceo et al., 1990).
Major alkali salts formed with sulfates and their approximate melt-ing temperatures according to Gartner et al. (1987) and Skalny and Klemm(1981) are shown in Table 12. Strungeet al. (1985) reported that increasing sulfate contents distinctly decreases alite, increases belite; the aluminates and ferrite contents are unchanged in clinkers irrespective of their silica modulus (SM) values. On the other hand with increasing SM, irrespective of the sulfate, the alite contents are higher, belite are un-changed, and aluminates and ferrite are somewhat lower. Relationships between clinker phases and sulfate content in the clinker are shown in Figure 7. With increasing sulfate
Table 12. Major Alkali Sulfates Formed During Clinkering and their Approximate Melting Temperatures (Adopted from Skalny and Klemm, ~981; and Gartner et aL~1987)
Alkali Compounds Chemical Formulae Melting Temperature ‘C
Potassium Sulfate (arcanite) K,SO, 1074
Sodium Sulfate (thenardite) NapSO, 884
Calcium Sulfate (anhydrite) CaSO, 1450
(Decomposes toCaO + S03
and 02at about 1200”C)
Sodium Potassium Sulfate (aphthitalite) K, S0,”Na2S0,
or 968
(K, Na)2S0, Calcium Potassium Sulfate (calcium Iangbeinite) 2CaS0,+K2S0,
or 1o11
Ca,K2(SO&
Calcium Potassium Sulfate (syngenite) K2SO~CaSO;H20
or 1004
Ca, K2(S0,)ZOHZ0
(Partial decomposition at lower temperature)
contents, the alite crystals in clinker grow larger, and the tendency ofbelite inclusion in alite is progressively re-duced. The crystal size of aluminate and ferrite phases are also signifi-cant y reduced.
Gies et al. (1986, 1987) reported the development of a belite-rich ce-ment by using increased sulfate con-tents in alkali free raw materials; this clinker showed reasonable hydraulic activity which was attributed to the presence of 0.6-0 .8% sulfate in belite. The rate of clinker cooling did not have any significant effect on the strength properties of resulting ce-ment pastes. To the contrary, Gartner (1980) suggested that sulfate in clin-ker is rather unreactive and does not necessarily contribute to set control or to the hardening of paste. So, even a high sulfate clinker may require additional sulfate, which generally comes from gypsum interground with clinker to achieve adequate set con-trol. This, however, depends upon the C~A content, and sulfate should not exceed the maximum limit speci-fied by ASTM C150 without the sul-fate expansion test. It might be noted that excessive sulfate in cement can lead to expansion problems in con-crete. Clinkers might also contain certain amounts of unreactive sulfate, which unfortunately can lead to other problems due to insufficient avail-able sulfate for reaction with the alu-minate phase.
Another related concern is the level of SOz in the kiln exhaust area. Very frequently, 15-40% of pyritic (sul-fide) sulfur in raw material is con-verted to SOZ in the emissions (Neilson, 1991).
It should be pointed out that in the preheater system much of the SOZ in the kiln is taken up by the incoming raw material. This reaction is also observed in plants which use kiln exhaust to provide heat to the raw milling system. Significant amounts of SOZ may still escape if its original concentration is high, or if reducing conditions are generated locally.
SM=l .6 Belite d Aluminate Ferrite
I
SM=2.4 Belite /I
Ferrite SM=3.2 Alite / Belite Aluminate Ferrite 0123 0123 0123SOS Content, % mass
Figure 7. Different phases of clinker as a function of S03 content and different values of silica modulus (Strunge et al., 1985).
-Selenium
Selenium (Se) could be associated with sulfur in coal, but only in traces. It is also present in fly ash where it tends to concentrate in the fine fractions (Coles, 1979). Selenium is usually not detect-able in cement but is detected in CKD in small amounts (PCA, 1992).
Selenium is volatile (boiling point=684°C) and expected to end up in kiln dust or in the emissions. Sele-nium could form less stable selenates (SeO,), which are unlikely to stay in clinker (Gartner, 1980). Since their con-centration is extremely low in the kiln feed, it is very unlikely that they will have any significant effect onthemanu-facture or properties of cement.
Tellurium
Like selenium, traces of tellurium (Te) are generally associated with sulfur in coal.
At optimum kiln temperature tel-lurium could be somewhat volatile de-pending upon the form in which it is present (amorphous form boiling
point= 990°C; rhombohedral form boiling point =1390°C). Gartner (1980) suggests that tellurium might form un-stable tellurates in clinkers and end up in the kiln dust or the emissions.
ELEMENTS
IN GROUP WI
(Fluorine, Chlorine, Bromine, lodine)
The halogens fluorine, chlorine, bro-mine, and iodine, are frequently found in kiln raw feed and primary as well as alternative fuels, and therefore play an important role in cement manufactur-ing. Some halides such as fluorides are also frequently used as mineralizers in clinker production andinlow-tempera-ture manufacturing of belite-rich ce-ments. Mishulovich (1994) addresses halides as catalysts for calcination. Con-centration of halogens found in raw materials and fuels is given in Tables 3, 4 and 6.
Fluorine
Fluorine (F) is commonly present in limestone, clay/ shale, and coal (Sprung