Role of Minor Elements in Cement Manuf and Use

Portland

Cement Association

Research and Development Bulletin

RDIOST

Role of Minor lements in Cement anufacture and Use

by Javed I. Bhatty

/

t

KEYWORDS:

manufacturing,

minor elements, portland cement, raw materials, trace elements

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: Ce document 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 classification 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 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.

I

PCA R&D Serial No. 1990

PCA Research and Development Bulletin RD109T

Role of Minor Elements in Cement Manufacture and Use by Javed 1. Bhatty

ISBN 0-89312-131-2 @ Portland Cement Association 1995

Role of Minor Elements in Cement Manufacture and Use

PCA Research and Development Bulletin RD109T

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) ....................................... Boron. ................................................................................................................................... Aluminum ............................................................................................................................. Gallium, Iridium, and Thallium ..............................................................................................

12 12 12 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 Bismuth ................................................................................................................................ 15

,,.

Ill

Role of Minor Elements in Cement Manufacture and Use

Page

Contents

ELEMENTS IN GROUP VI (Oxygen, Sulfur, Selenium, Tellurium) ................................................... 15 Oxygen ................................................................................................................................. 15 Sulfur .................................................................................................................................... Selenium .............................................................................................................................. 1; Tellurium .............................................................................................................................5’XO)in cement clinker are the major elements. These are calcium (Ca), silicon (Si), aluminum (Al), iron (Fe), and oxygen (0). Carbon (C) and nitrogen

●

PCBs=polychlorinated biphenyls

PCA Research and Development Bulletin RD109T

Table 1. Typical Compositions

and Physical Properties

of Portland Cements

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

Compound Composition (O/.)

Gypsum Loss on Ignition

5

5

5

4

4

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

330

250

500

210

250

7-day Heat of Hydration (J/g)

(N), because of their abundance in the raw material and the earth atmosphere respectively, can also be regarded 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 exist as more complex compounds. The approximate formulae of these compounds, 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 elements 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 component 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 cement 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 material containing major components in a required proportion is burned in a rotary kiln at about 14500C, where the constituents become fully oxidized 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 composition and are used where special properties are required. Typical cornposition 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.$ content and a finer particle distribution and is used where rapid hardening is required for early strength development. Type IV is a low heat of hydration, 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 recommended due to its higher strength and market availability. For even lower heat of hydration, Type II cement 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 moderate sulfate resistant cement. A knowledge of the compound composition can reasonably be used to predict the properties of cement. One of the known methods for calculating compound compositions from the oxide analysis are the Bogue formulae (1955). Although a number of sophisticated techniques are now available for Bogue calculations, the simplest Bogue formulation that has been found suitable for most applications 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.

3

Role of Minor Elements in Cement Manufacture and Use

centration, are represented in chemical analyses as oxide forms: NazO, KZO, MgO, and SOS. Rompps ChemieLexikon (1987) has termed these elements as the secondary elements. 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 considered as minor elements with regard to cement manufacturing. The concentration 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 elements occurring at less than 0.02% each as the “trace” elements. According to Sprung (1988), elements present at levels less than 100 ppm are classified as trace elements. Because of their extremely small concentration levels, it seems unlikely that the presence of trace elements will have any significant 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 defined by Blaine et al. (1965) and Rompps chemie - Lexikon (1987) respectively, unless mentioned otherwise. 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-

4

1 ppq

1 ppt

1 ppb

1 ppm

0.0019’0 1?40

Figure 1. Concentration ranges (by mass) of main, secondary, 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 differ from another. As an extreme, lead content in one plant maybe 100500 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. Examples 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 important source of minor elements comes from the wide range of industrial 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, sewage sludge, metal cutting fluids, and waste solvents, as listed in Table 2. Minor compounds found in several raw feeds for cement manufacturing as quoted by Bucchi (1980) are shown in Table 3. Similar data on

and

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 conventional kiln fuel (coal), along with two secondary fuels (used oil and petroleum coke) are shown in Table 6. Average values of minor components 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 already high in the other raw materials. Bauxite is reported to contain 2-87. titanium oxide (TiOz) and 0.04-0.4% chromium oxide (CrzO,). Iron ores frequently contain chromium, arsenic, cadmium, and thallium, and may have adverse environmental consequences because of their toxicity characteristics. A list of metals having regulatory and environmental concerns has been specified by waste characterization regulations under the Resources Conservation and Recovery Act (RCRA) and the Boiler and Industrial Furnace (BIF)

PCA Research and Development Bulletin RD109T

Table 2. Sources of Minor Elements in Cement Manufacturing :Iements

(as per group)

Sources

Sroup I .ithium

Waste lubricating oil

Group II 3eryllium Strontium 3arium

Fly ash Limestone, aragonite, slag, waste lubricating oil Waste lubricating oil, refuse derived fuel (RDF)

Group HI 3oron Gallium, Iridium, Thallium

Raw material, iron ore Raw material, fly ash, coal, secondary fuel, waste derived fuel (WDF)

Group IV Germanium Tin Lead

Raw material, coal Fly ash, RDF, fuel Raw material, tires, RDF, WDF, copper shale, fly ash

Group V Nitrogen Phosphorus Arsenic Antimony Bismuth

Coal, air Raw material, slag, sewage sludge, sandstone, Fly ash, secondary fuel, coal, used oils Petroleum, coke Fuel

Group VI Sulfur Selenium, Tellurium

Coal, slag, lubricating oil, petroleum coke, pyrite, tires Fly ash, coal, RDF, coke

Group WI Fluorine Bromine Chlorine lodine

Limestone, fuel Fly ash Coal, slag, fly ash, waste lubricating oil, chlorinated hydrocarbons, Coal

Transition Elements Titanium Zirconium Vanadium Chromium Molybdenum Manganese Cobalt Nickel Copper Zinc Cadmium Mercury

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

RDF

RDF, chlorine-rich fuel

“ Raw material includes natural materials such as limestone, clay, shale, sand, etc.

5

Role of Minor Elements in Cement Manufacture and Use ,

I

regulations that control treatment of hazardous waste in cement kilns. These are shown in Table 8 (Klemm, 1993). Both RCRA and BIF regulations apply to wastes and require the use of the TCLP (toxicity characteristic leaching procedure) tests. The levels of sulfur and chlorine in bituminous coal, the main fuel for Cement kilns, varies frOIn 0..5 to 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 OXide, and can contribute certain levels of S and V to clinker when supplementing 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 components could be the refractories, chains, and the grinding media such as liners and grinding balls. A damaged chrome refractory lining can enter 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 discussed in this report as per their occurrences 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) “

1,05

MgO K20 so, Na20 TiO, Mn,O, P20, SrO Cr,O, AS,O, BeO NiO

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 0.06

V*05 cl F

Table 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

B.F. Slag

Fly Ash

MgO

7.2

K,O so, Na,O TiO Cr,&, MnzO, P20, SrO V206

0.57 3.00 0.44 0.66 n,a.* 0.64 0.03 0.06 n.a. n.a,

5.28 4.05 2.25 1.99 1.21 0.03 0.14 30) and high CIS levels. According to a number of studies, Ba also appears to be an effective activator of hydraulicity and strength. The strength obtained from Ba incorporated clinkers is 1O-2O’7O higher 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 increase the free lime content of clinker due to CaO displacement and can cause expansion in concrete under certain circumstances. It can also lead to paste shrinkage.

ELEMENTS IN GROUP Ill (Boron, Aluminum, Thallium)

Gallium, Iridium,

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 electronegativity of boron and the melt viscosity,

12

957

0 CaZn VBe NiKCr

AsPb S

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 berates, phosphates, and sulfates. Boron inhibited the formation of C$ and affected the stability of the other major clinker phases. In the presence of boron 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 useful 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 andironpoor systems. However, Miller (1976) has cautioned that the indiscriminate addition of boron can produce unpredictable 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 additions of BzO~ (-0.040/.), as an admixture, to cements can have adverse ef-

fects on setting properties. These observations 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 thallium (Tl) are found only in traces in raw material; their typical concentrations 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 average concentration of thallium in CKD is 43.24 mg/kg (PCA, 1992). *

B is B103 in C5BS

PCA Research and Development Bulletin RD209T

Although thallium occurs in traces in the raw feed, it is the most volatile element* after mercury in the kiln (melting point=30~C), and is most likely to concentrate in the kiln dust. The volatility 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 balances 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 carbon 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. Because 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 manufacturing 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 below 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 hydraulic and produces calcium germanate hydrate (C-G-H) and calcium hydroxide (CH) with water, whereas ~G is assumed to be non-hydraulic. According to Gartner (1980), it is unlikely 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 manufacture.

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 despite 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). According 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 hydrating cement properties has recently been studied, where Bhattyand West (1992) have noted that additions either 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 retardation 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 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 levels 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

IN GROUP V

(Nitrogen, Phosphorus, Antimony, Bismuth)

Arsenic,

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,

13

Role of Minor Elements in Cement Manufacture and Use

as nitrides. Under normal oxidizing conditions, nitrogen in clinker is present only at a few ppm. High concentration of nitrogen, higher residence temperatures, partial pressure in the flame zone, and the subsequent oxidation of nitrogen leads to the formation of several oxides of nitrogen (NO and NOZ and NZO1) in the kiln emissions collectively known as NOX. Total NOX results 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 oxidation of nitrogen in air, plays a less significant role (Bretrup, 1991). The quantity of thermal NO formed is closely related to the burning zone temperature (BZT). According 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 emissions 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 equilibrium.

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 occurs 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

14

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) demonstrated that a satisfactory C~S phase in clinker was formed by adding PzO~in the raw feed and maintaining 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 fluorine, the tolerance to PzO~is somewhat 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 clinker reduces the negative effects of alkali on the strength properties of cements. He reported that in cement clinkers with “normal” NazO contents of 0.8Y0, the maximum 28day strength was achieved at 1.07. PZ05 level.

Arsenic Arsenic (As) bearing mineral arsenolite or claudite AszO~ (or AslOG), occurs 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 extremely 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. Arsenic levels found in various materials are shown in Tables 4-6. The average concentration of As in cement and CKD is 19 mg/kg and 18 mg/kg respectively (PCA, 1992). Although AszO~ is volatile (sublimes at 1930C) and should be expected to condense on kiln dust particles, Weisweiler et al. (1989) observed 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 conditions in the kiln, and high kiln temperature. Under oxidation conditions, As is primarily oxidized to AszO~and forms a series of low volatile 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 different 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 hydration 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. *

P=P20,

PCA Research and Development Bulletin RD109T

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 petroleum coke (Weisweiler et al., 1989), Sprunge (1985) has quoted 1.19ppm Sb in coal. According to measurements in BIF certification of compliance (C.O. C.) and other authors, the Sb levels in raw materials are higher. Like arsenic, a considerable portion of antimony is incorporated in clinker in the form of low volatile calcium antimonates under oxidizing kiln conditions at high temperatures (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 detected 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 clinkering temperature (boiling point =186WC). Little is available on the influence of Bi in cement manufacturing 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 considerable portion of raw material and clinker phases incorporate oxygen

Possible carry-through of complex calcium sulfides in clinker

S02 prominent in vapor pressure

Molten sulfates

S-2 present as organic and inorganic forms in fuel, etc

Sulfites, SO; in solids which become increasingly unstable with rising temperature

Sulfate solids S03vapors

ReducedS Species

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 material 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 appropriate 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 oxidized 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 reducing atmosphere greatly influence the reaction into which the various elements will enter. Clinker made under oxidizing conditions tends to incorporate trace metals of higher oxidation states than clinker prepared Exunder reducing conditions. amples of chromium and sulfur can be cited here. Cr+s would tend to form under oxidizing conditions, instead 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 reducing conditions. Kilns having strongly oxidizing conditions and low burning zone temperature tend to retain more sulfur in clinker than those produced under reducing conditions and for high burning zone temperature. Thus, the oxidation or reducing conditions in the kiln can lead to significant phase modifications in clinker. Clinker produced under reducing conditions are brownish as compared 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 sulfides are also often present in the limestones. Clayey sediments, marls, also contain both sulfides and sulfates. Lecher et al. (1972) have re-

15

Role of Minor Elements in Cement Manufacture and Use

ported occasional use of gypsum and anhydrite as mineralizers and modifiers of the alkali cycle in the kiln. Sulfides and sulfur from raw materials and fuel are oxidized and are incorporated into the solid phases as sulfates 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 conditions 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 temperature, the formation of SOzis most likely. In the presence of lime, S02 is partly removed to form CaSO1 by the following mechanism:

S02 + CaO ~ CaSOz + l/20z ~

CaSO, 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 condense on the relatively cooler incoming raw feed resulting 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 excess, the balance between alkali is achieved by forming calcium langbeinite, Caz~(SO,)Y which is stable up to 10110C in a CaW1-KzWi system. 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 melting 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 unchanged, 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 (Adopted from Skalny and Klemm, ~981; and Gartner et aL~1987) Alkali Compounds Potassium

Chemical

Sulfate (arcanite)

Formulae

Melting Temperatures

Melting Temperature

K,SO,

1074

Sodium Sulfate (thenardite)

NapSO,

884

Calcium Sulfate (anhydrite)

CaSO,

‘C

1450 to CaO + S03 and 02at about 1200”C)

(Decomposes

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)

16

PCA Research and Development Bulletin RD109T

contents, the alite crystals in clinker grow larger, and the tendency ofbelite inclusion in alite is progressively reduced. The crystal size of aluminate and ferrite phases are also significant y reduced. Gies et al. (1986, 1987) reported the development of a belite-rich cement by using increased sulfate contents 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 cement pastes. To the contrary, Gartner (1980) suggested that sulfate in clinker 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 control. This, however, depends upon the C~A content, and sulfate should not exceed the maximum limit specified by ASTM C150 without the sulfate expansion test. It might be noted that excessive sulfate in cement can lead to expansion problems in concrete. Clinkers might also contain certain amounts of unreactive sulfate, which unfortunately can lead to other problems due to insufficient available sulfate for reaction with the aluminate phase. Another related concern is the level of SOz in the kiln exhaust area. Very frequently, 15-40% of pyritic (sulfide) sulfur in raw material is converted 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

I

SM=3.2

SM=2.4

Alite

Belite

Belite

Belite

Aluminate /

d

/ Ferrite

0123

I

Ferrite

Aluminate Ferrite

0123

0123 SOS 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 detectable 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. Selenium could form less stable selenates (SeO,), which are unlikely to stay in clinker (Gartner, 1980). Since their concentration is extremely low in the kiln feed, it is very unlikely that they will have any significant effect onthemanufacture or properties of cement.

Tellurium Like selenium, traces of tellurium (Te) are generally associated with sulfur in coal. At optimum kiln temperature tellurium could be somewhat volatile depending 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 unstable 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, bromine, and iodine, are frequently found in kiln raw feed and primary as well as alternative fuels, and therefore play an important role in cement manufacturing. Some halides such as fluorides are also frequently used as mineralizers in clinker production andinlow-temperature manufacturing of belite-rich cements. Mishulovich (1994) addresses halides as catalysts for calcination. Concentration 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

17

Role of Minor Elements in Cement Manufacture and Use

et al., 1968, 1985) as a minor element and plays an important role in cement making. In raw feed, fluorine could be up to 0.06%by weight (see Table 3), whereas in limestone and clay/shale it can go upto 940 and 990 ppm respectively (Table 4). Calcium fluoride (CaF2) isalsofrequently added to raw meal as a mineralizer and flux to lower the burning temperature and accelerate the formation of C,S (Klemm et al., 1976, 1979). Miller (1976) has, however, cautioned not to use fluoride beyond 0.2570 to avoid adverse effects on clinker behavior by selectively incorporating it into the aluminates or silicate phases at certain burning temperatures. At lower temperatures, fluoroaluminates (C1lA+CaF,) are formed, which are decomposed at high temperatures to CqA and fluorides. These fluorides are then incorporated into silicates at higher temperatures to form often stable fluorosilicates but their excessive amounts can cause decomposition of alite. Gartner (1980) also reported the formation of alkali fluorides as NaF and KF at higher alkali presence; these fluorides being somewhat volatile (boiling points 1700° and 1500”C respectively) are expected to end up in the fine kiln dust. However, Sprung et al. (1968) reported that between 88’7. to 987. of fluorides are incorporated in the clinker and only a small fraction end up in the kiln dust, probably as CaFz Fluoride emissions were reported low (0.009-1 .42 mg F/Nm3) depending not necessarily on the magnitude of fluoride balance but on the efficiency of the precipitators. Akstinat et al. (1988), reported that fluorides have no adverse effects on the cement production process, and the fluoride cycle does not cause any operational problems like coating, because of their presence in small amounts. However, recent experiences have shown that use of fluoride based compounds can occasionally cause plugging. Gartner (1980) reported that the presence of fluorides

18

600

3oo-

0

40

3-day {r

1

~

20–

0 0

I 1.0

I 0.5

Fluoride

I

1.5 (Yo

2.0

mass)

Figure 8. Effect of fluoride on strength and setting time of high alite cements (Moir, 1983). beyond ()..5~o can cause both operational and quality control problems, which, under certain situations, can be controlled by PzO~addition. Goswami et al. (1991), Bolio-Arceo et al. (1990), and Gilioli et al. (1979), have reported the formation of spurrite (2Cz9CaCO~), and fluor-ellestadite that cause kiln deposits, but the resulting low burning temperatures control the alkali cycle and reduce the alkali-sulfate deposits. Palomo et al. (1985) suggested that 0.2?4. fluoride promotes low temperature formation of aluminates such as fluorinated CIZA,, C,A, and CZAS (gehlenite); however, the final aluminate mineralogy was not significantly affected, as both ferrite and CqA were present at 1250°C and above. Perez Mendez et al. (1986) reported that with the addition of 0.5-1.50/. fluorides, as

CaFz, clinkering reactions were completed in 0.5 hr at 13540C; the clinkers had much of C$ developed, with P-C,S, C,AF, and C,A also present therein. Imlach (1974) observed that fluoraluminate CllAToCaFz, forms at fluorine levels of about 0.5% in clinkers fired below 1320°C or slowly cooled from 1340”C to 12650C. Fluoroaluminate imparts rapid setting to cement pastes compared to the normal cements. According to Aldous (1983), and Shame et al. (1987), the presence of F and Al beyond the threshold level renders C,S a rhombohedral symmetry, which is associated with improved hydraulic properties. Moir (1983) demonstrated that by optimizing the levels of F, alumina, alkalies, and sulfates, the C3S in clinker could be maximized to enhance the setting properties of

PCA Research and Development Bulletin RD109T

cement. Figure 8 shows the relationship between the fluoride addition and compressive strength of cement pastes at various curing ages. An optimum fluoride addition for maximum strength at early ages (24 hours) was 0.27., for later strengths (7 days and 28 days) addition of ().75Y0 were acceptable.

Chlorine As mentioned above, chlorine (Cl) as chlorides is frequently found in limestone, clays and in some cases in both the primary and secondary fuels. In limestone and clay the predominant chloride is sodium chloride (Akstinat et al., 1988). Some coals can contain up to 0.28°/oCl, mainly as rock salt (NaCl). The formation of stable yet volatile alkali chlorides NaCl (boiling temperature= 1413°C) and KC1 (subliming temperature= 1500°C) at clinkering temperature is well known. Both the chlorides volatilize in the burning zone and condense in the cooler parts to form kiln rings or preheater build-ups which impair plant performance. Bhatty (1985) also concluded that agglomeration due to the presence of molten alkali chlorides was one of the major reasons for the build-ups. Cl also enhances the formation of spurrite and sulfospurrite (2C,S.CaSO,). In cases of plants without preheater, the volatile chlorides end up in the kiln dust. In preheater kilns, up to 9%J’ochlorides are recaptured by the incoming feed in the calcining zone (Ritzmann, 1971); the concentration of chloride at that point could be extremely high (>lYo) compared to that of raw feed (-0.017.). Relative volatility of Cl, and other elements in the kiln system is already shown in Figure 5 (Sprung et al., 1984). The wet processing plant and grate preheater may tolerate raw feed with higher chlorides, but the limits primarily depend upon the efficiency of dust collecting and the level of kiln dust recycling. With the advent of the alkali by-pass, the chlorine cycle can be

broken at the most intense point of kiln and the alkali chloride can be conveniently directed to the dust collectors. Otherwise, as reported by Norbom (1973), a total chloride intake of 0.015% (in both raw material and fuel) can result in build-ups in a preheater without a by-pass. Since most of the chlorides are volatile, the amount retained in clinker is extremely small (d.03~0). VOlatile chlorides react readily with alkalies, so that the alkali level in the clinker is often reduced when chloride is present. The combined influence of alkali chloride on cement properties is therefore regarded as insignificant. In some cases, calcium chloride is added to the kiln for the express purpose of increasing alkali volatilization and removal, and result in the production of a low alkali clinker. According to Mishulovich (1994), the addition of calcium chloride and chlorine-containingorganiccompounds at the clinkering stage, accelerated both lime reaction and alkali volatilization. In a preheater klin, the addition of calcium chloride in the burning zone, resulted in 2070 increased production with corresponding fuel saving. In waste-derived fuels such as waste-oils contaminated with chlorides, chlorinated hydrocarbons and scrap tires, the chlorides would occur indifferent compounds at much higher concentrations (Akstinat et al., 1988), and cause serious operational problems even in kilns equipped with bypass. In order to make their use feasible, a larger portion of by-pass dust would have to be discarded to prevent building up a large chloride cycle High chlorides in the raw feed have also been reported to form condensation plumes in the emission stacks in long wet or dry kilns which are difficult to remove at times. Such detached plumes are generally the result of NHAC1 formation. Excessive chlorides can also have a deleterious effect on kiln basic brick lining. Chlorides, particularly CaClz, accelerate the hydration and hardening

of cement paste and increase the very early strength but, at the same time, chloride ions are also known to promote corrosion of steel reinforcing bars in concrete. AliniteCements: The development of less energy intensive “alinite cements” from the CaClz incorporated raw material has generated great interest (Nudelman, 1980). The formula ascribed to the alinite phase is close to 21 CaO”6SiOz”A110q* CaClz with some MgO inclusion (Lecher, 1986). The burning temperature for alinite clinker is between 1000-11 OO”C. The raw mix is composed of 6-2370 CaClz by weight. MgO is added to stabilize the alinite phase at 60-80?40, belite at l&30~o, calcium aluminoat 5-1OYO, and calcium chloride aluminoferrite 2-107. (Bikbaou, 1980). Ftikos et al. (1991) reported that the strength development of alinite cement was comparable to that of regular portland cement.

Bromine With some exceptions, bromine (Br) plays a minor role in cement manufacturing. Bromine occurs only as a minor element in raw materials, i.e. limestone (6 ppm), clay (10-58 ppm), and coal (7-1 lppm). (Akstinat et al., 1988) and Sprung et al. (1985). Bromine has also been detected at measurable levels in some of the fly ashes generated at coal operated power plants. Bromine is volatile and expected to end up instackemissions (Akstinat, 1988). Under oxidation conditions, bromine gas (Br2) would form and end up in emissions. Retention of bromine in clinker is negligible. Alkali bromides can also be found in cement kiln dust. Between 5–10 ppm of bromine was reported in one CKD sample using fly ash as a partial raw feed (Klemm 1995). At higher levels of bromides, the formulation of bromine-alinites analogous to chlorinealinites, as mentioned above, has also been reported by Kurdowski et al.

19

Role of Minor Elements in Cement Mmzufactureand Use

(1987, 1989). Bromine-alinites are much more reactive than the alites (I@rdowski et al, (1989). Kantro (1975) reported that at equivalent concentrations CaBrz is a stronger accelerator for C~Spastes than the chlorides or iodides.

Iodine

11.2 ppm

Ti

Aluminate

r

v Cr Mn co Ni Cu

The presence of iodine(I) in limestone and clay is negligible. Up to 0,75 ppm in limestone, 2.2ppm in clay and shales (Mantus

Belite

Aluminoferrite

et al.), and between is found

in coal

I

I

I

I

of the low levels

of io-

t

I

5

01234

Weight

0.8 to (Sprung

1985). Because

Zn

Figure 9. Distribution (Hornain, 1971).

;oo15i

0 0.5 Y.

of transition elements in clinker phases

dine in the feed, the effect on the burn-

ing process is negligible. Iodine salts are volatile in nature and mostly end up in emissions. Conversion of io-

Table 13. Relative Ratios of Ti02 in Different Clinkers Phases, (After Different Workers)

dine gas (IJ from iodides is easier than bromides. Their concentration

in clinker is detected at very low levels. There is no literature report on iodine presence in the CKD. The concentration of iodine in CKD is expected to be extreamly low, maybe in the ppb, because of its’ presence in small amounts in the raw materials. CaIz is reported to accelerate C~S pastes though not as effective as bromides or chlorides (Kantro, 1975).

ELEMENTS IN GROUP Vlll (Helium, Neon, Argon, Krypton, Xenon) Helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xc), being inert gases, are not known to impart any noticeable effect on clinker manufacturing or cement hydration properties.

TRANSITION

ELEMENTS

(Yttrium, Titanium, Zirconium, Vanadium, Niobium, Tantalum, Chromium, Molybdenum, Tungsten, Manganese, Cobalt, Nickel, Copper, Silver, Zinc, Cadmium, Mercury)

20

Clinker Phases Alite Belite Aluminate Ferrite TiOp in Clinker (wt.”A)

Hornain (1971)

Regourd et. al. (1974)

Knofel (1977)

1 2

1 1.7

1 2

0.8

3.3

3

6

10.8

7

0.78

(not reprinted)

1.0

The elements 21-30,39-48, and 57-80 in the periodic table are known as the transition elements. Not all of these elements have been studied in cement manufacturing, but the ones that have been studied in some detail are dealt with in this section. Some of these transition elements are introduced into the clinkering process through the use of spent catalysts as an alumina source.

Yttrium Isomorphismbetweenyttrium (Y) and calcium frequently occurs in natural materials; for instance fluoroapatite, Ca2Ca~(P01)F,can contain up to 10.6% YzO~(Povarennykh, 1966). But presumably in cement raw materials, yttrium occurs only in traces. Yttrium substitutes for Ca in both C~Sand CZS(Boikova, 1986). It yields

both triclinic and monoclinic forms of C~S. In a CZS-Yg(SiOi)~system, the region of homogeneity can exist up to 35% YA(SiOA)~ by weight (Toropov et al., 1962-2). Yttrium chloride is reported to have an accelerating effect on CqS paste when added as an admixture (Kantro, 1975). Since yttrium is unlikely to volatilize at kiln temperature(melting point= 1522°C), it can hardly be expected to concentrate in the kiln dust. It should preferentially become incorporated in clinker.

Titanium Titanium (Ti) as oxide could be present in typical cement raw material at the ().()2-O.4~0level by weight (Bucchi, 1980). Gartner (1980) reported a higher concentration of O.1-

PCA Research and Development Bulletin RDI09T

1.0’?4.Ti02 in many raw mixes. Concentrations of TiOz in some of the auxiliary raw materials is even higher. Blast-furnace slag for instance, can contain 1.!7~0TiOz, and the bauxites may have between 28% TiOz by weight. TiOzisarefractory material (boiling point= 2500-3000°C) and is essentially incorporated in clinker. At low levels the effects of Ti on the manufacturing of cement is insignificant (Miller, 1976), higher levels of up to 2% may improve the compressive strength of clinker (Knofel, 1979). Hornain (1971), and Marinho et al. (1984), reported that TiOz is preferentially distributed in ferrite phase. Distribution of other selected transition elements indifferent clinker phases, as determined by Hornain (1971), is also shown in Figure 9. Relative ratios of TiOz distribution in clinker phases as reported by different workers is also given in Table 13 for comparison. Titanium from ilmenite (FeTiOJ additions to kiln feed has been used to produce a patented buff-colored cement. Knofel (1977) observed a sharp reduction in alite with equal gain in the belite phase when Ti02 was increased in the raw mix; the variation in ferrite and aluminate was not significant. Calcium titanate (CaTiOJ is apparently the major phase present in clinker. It was also reported that about 1’ZOTiOz addition in the raw mix reduces the melt temperature by 5O-1OO”C,probably because of a favorable relationship between ionic potentials and the melt viscosities as shown in Figure 10, This relationship was developed by Timashev (1980). It shows that increasing the ionic potential of transition elements in groups of elements with equivalent atomic radii decreases the clinker melt viscosity. Although TiOz enhances the early hydraulicity of alite (Kondo, 1968), the clinkers have shown slow initial setting. However, 1% TiOz clinker have roughly 207. higher

-600

- 400

- 200

-o

0.115

0.130

0.145

0.160

Viscosity, Ps sec Figure 10. Relationship between viscosity and ionic potential to radius ratio, and cation-oxygen bond of transition elements at 1450”C (Timachev, 1980). 3-and 90-day strengths (Knofel, 1977, 1979).

increased the early strength of cement.

Zirconium

Vanadium

Zirconium (Zr)isconcentrated mostly in siliceous ores which can be used as a raw feed component (Miller, 1976). Blaine (1965) reported about 0.5% zirconium, probably in the fully oxidized form of ZrOz, in US. clinkers. Kakali et al. (1990) found no significant change in the burning and cooling conditions for clinker prepared with 0.73-1 .45% ZrzOJ; the principal phases, alite, belite, aluminate, and ferrite, were satisfactorily crystallized. However, ZrzO~ changed the size and shape of alite, while the type of belite crystal was modified. Zr20~ also imparted a noticeable color change in clinker (Kakali, 1988). A significant retarding effect and a subsequent delay in strength for cements prepared with ZrO containing raw mixes was also reported (Kakali et al. , 1989). However, earlier studies by Blaine et al. (1966) indicated that smaller ZrO additions

Vanadium (V) occurs at a measurable level in cement raw material (10-80 ppm inlimestone,98-170 ppm in clay/ shale, and 30-50ppm in coal) (Sprung, 1985). It is also present in fly ash where it tends to concentrate in the finer fractions (Coles, 1979). Fairly high levels of vanadium are also reported in crude oils (Gartner, 1980). In one study, Weisweiler et al, (1990) has reported nearly 800 ppm vanadium in petroleum coke used in cement manufacturing. Ash from petroleum coke also contains very high levels of VzO~ (up to 607.). Because the petroleum coke has a low overall ash content, Moir et al. (1992) found no more than 0.08% VzO~ in clinker produced in modern cement plants that use 50% petroleum coke as a substitute fuel. Use of vanadium is known to decrease the melt viscosity primarily because of its higher ionic potential as

compressive

21

Role of Minor Elements in Cement Manufacture and Use

shown in Figure 10. Vanadium is present as VzO~in cement clinker. It concentrates in alite and forms larger crystals. However, according to Hornain (1971), vanadium preferably concentrates in belite rather than alite, as shown in Figure 9. VzO~is unlikely to vaporize at normal kiln temperatures. On the other hand, vanadium present in fuel may not have adequate contact with the reacting mass in the kiln and largely ends up in the kiln dust as suggested by data from Weisweiler et al. (1990). Odler et al. (1980-1) reported that l% V20~ can significantly reduce the free lime in clinker when fired at 1200°C. Xinji et al. (1986) used V20~ for stabilizing B-CZSin clinker apparently by substituting V01-3 for Si014. A concentration of 1.57. V20~ is reported to increase hydraulicity of alite; however, higher concentrations adversely affect the grindability of resulting clinkers. High VzO~levels as found in some crude oils could also deteriorate kiln lining in some cases (Gartner, 1980). V,O, in clinker can also increase sulfate expansion under certain circumstances (Blaine et al., 1966).

Niobium Niobium (Nb) is another element to be found in traces in cement raw materials. Weisweiler et al. (1990) has reported more than 30 ppm niobium in the raw feed of a German plant. Because of the low level presence in the raw mix, niobium would have very little effect either on the clinker formatitm or on the cement hydration properties. Kakali et al. (1990) reported a very feeble effect of Nb+5addition (up to 1.5% by weight) on the mineralogical texture and the viscosity of clinker melts because of its low ionic charge to atomic radius ratio. Cementpastesprepared from these clinkers did not show any noticeable change in their setting or strength properties when compared to regular cement pastes (Kakali et al., 1989).

22

Table 14. Chromium Distribution in Typical Clinker* Phases Containing 0.55YI0 Cr,O, (Hornain, 1971 ) Phases

Cr(%)

Belite

0,87

Ferrite

0.55

Alite

0.39

Aluminate

0.04

*The clinker contained C,S=76.3Y0,C,S=9. 1“A, C~A=5.3°/’,and C,AF=8°/.

Because niobium is a high temperature metal (melting point =24680C), it would unlikely concentrate in the kiln dust or in the stack emissions.

Tantalum Tantalum (Ta) is only a trace element in cement raw material. It reported to be present at less than 9 ppm in raw material and 0.3 ppm in the oil coke used as fuel in cement manufacturing (Weisweiler et al., 1990). Since tantalum is present as trace in both the raw feed and fuel, it is unlikely to impart any noticeable effect on the clinker formation and cement use. Weisweiler et al. (1990) have reported 14.3 ppm and 3.3 ppm tantalum respectively in clinker and kiln dust prepared from a raw material containing 8.9 ppm tantalum.

Chromium Chromium (Cr) can be present in raw feed immeasurable quantities. Sprung (1985) has reported up to 16 ppm in limestone, nearly 100 ppm in clay and shales. Coals and used oils may contain up to 80 ppm and 50 ppm Cr respectively. Some of the auxiliary raw materials, such as bauxites, which are used up to 4°/0 in cement manufacturing, may contain between 0.04-0.40/. CrzO~. In addition to that, a proportion of Crcanalsoentercement from the grinding media during raw meal preparation and finished cement grinding, and refractory linings. The presence of Cr in raw materials is known to reduce the viscosity of

clinker melt due to its high ionic charge as is shown in Figure 10. Miller (1976) has reported improved clinker burnability at 1°/0CrzO~addition. Chromium can exist in a number of oxidation states in clinker, the most stable being Cr+3and Cr+b. Their formation is sensitive to the oxygen level in kiln. High oxygen tends to form Cr%compounds as chromates which are readily soluble in water and markedly affect the hydration characteristics of the paste. Reducing conditions favor the formation of Cr+3compounds which are less soluble in mix water. Under oxidation conditions, Cr can also exist as Cr~ and Cr+5in C2S, which can then disproportionate to the more stable Cr+3 and Cr% upon mixing with water (Feng Xiuji, 1988). Johansen (1972) has reported Cr”, Cr-b, and Cr+5 in alite substituting for Si+4.Hornain (1971) reported that Cr preferentially resides in belite followed by ferrite, alite, and aluminates, as shown in Table 14 (see also Figure 9), Although Cr+b can be present in both alite and belite, it is reported to be stabilizing the ~-CzS form (Hornain, 1971, and Kondo, 1963). Subarao et al. (1987) developed an active belite-rich cement from raw feed containing 4-5% CrzOJ by weight. Imlach (1975) used O.11I.qzy. Cr203 in the raw feed as a flux. The resulting cement exhibited improved 8- and 24-hour strengths, but 28-day strengths always decreased. A significant portion of Cr can also enter the finished cement from chrome-rich grinding media (Klemm, 1994). It is reported that the level of

PCA Research and Development Bulletin RD109T

Cr4in ground cement is almost doubled by the use of high-chromium alloy balls during grinding. A number of patents report the use of inorganic reductants to control the Cr+Aleaching from cement. Most of these patents are of European origin and use ferrous sulfate heptahydrate, ammonium-ferrous sulfate, and manganese sulfate during intergrinding to convert Crk to Cr+3. Chromium is known to accelerate the hydration of paste and improve the early strength, and has thus been used to develop high strength cements. Recent studies (Bhatty et al., 1993) have shown that 0.7570 addition of chromium as chromium chloride and nitrate, accelerate paste hydration and result in high initial hydration peaks. The workability and the initial setting times are reduced, but the early strengths (3 da ys) are significant yimproved over the control. The 28- and 90-day strengths are, however, close to those of the control. The addition of insoluble chromium oxide (Cr20~), even up to 1.37., did not significantly affect the hydration or the strength behavior of the pastes. The degree of Cr stabilization in cement matrices as determined by leachability in both these cases was almost 100Yo. Chromium may also contribute to high sulfate expansion, increased 24-hour shrinkage, and reduced autoclave expansions. Although a major portion of chromium is incorporated in clinker, usually tied up in belite, ferrite, or sulfate phases, chromium can be found in the CKD. Between 100-1000 ppm of Cr have been reported in CKD (Lee et al., 1973; Howes et al., 1975), although recent studies (PCA, 1992) have shown only between