VDZ-Concrete.pdf

Concrete

Hardas rock

to the environment

StrongFair

Environmental compatibility of cement and concrete.Manufacture, application and use of alternative materials.

Information from the German cement industry.

on performance

Elements such as lead, cadmi-um, and zinc are present ininsoluble form in the freshconcrete phase and are there-fore not released. Chromium,although readily soluble in the beginning, is fixed in thehydration phases as hydration

progresses, i.e. as the con-crete gradually hardens. After-wards it is present in insolubleform.

Trace elements in concretecomponents are fixed in thepaste matrix and thus immo-bile. Hence, there is littlerelease of heavy metals fromconcrete. No adverse effectscaused by the liberation ofgases from concrete aredetected during service life.The radioactivity of concreteis of marginal importance aswell. The average radon exha-lation of concrete falls shortof that of the natural soil bytwo orders of magnitude.

To make an ecological assess-ment of cement and concrete,all their life phases extendingfrom production and process-ing, via service life, and as faras disposal must be taken intoconsideration. Results fromcomprehensive investigations

on the environmental compati-bility during all these phasesare available. The emissionsreleased in cement manufac-ture result in ambient pollu-tion concentrations at produc-tion sites that amount to less than one per cent of theapplicable limit values. Thecontribution to ambient pollu-tion by organic substancesfalls significantly below thepermissible values. The inputof substances, e.g. heavymetals, into the soil in thevicinity of cement plants is ofno environmental relevanceeither.

3

Summary0

Nowadays many of the initialmaterials required in cementmanufacture are substitutedfor by suitable alternativematerials. This affords severaladvantages: The use of alter-native materials in cementmanufacture allows the

improvement of the economicefficiency of the productionprocess, simultaneouslyenabling cement plants tocontribute to environmentallycompatible utilization. Froman overall ecological point of view, the recovery of alter-native materials in cementplants outperforms othermethods of recovery ordisposal.

The investigations carried outshow that the concentrationsof heavy metals in the rotarykiln exhaust gas are not envi-ronmentally relevant if alterna-tive materials are utilized. Thedioxin and furan contents fallsignificantly below the limit

value of 0.1 ng TE/m3, regard-less of whether standard fuelsor alternative fuels are used.All the organic compoundscontained in the fuel aredestroyed in the rotary kiln.The utilization of alternativematerials in the clinker burn-ing process permits completematerial and energy recoverywithout any product-specificresiduals being produced. Allthe investigations have sub-stantiated that the environ-ment is not polluted by heavymetals being released fromconcrete components.

Even if cement is made fromalternative materials, theconcrete produced from it isfully recyclable.

As a consequence, the use of alternative materials incement manufacture neither

impairs the environmentalcompatibility of the cementproduction process, nor thatof cement and concrete asproducts. Thus, concrete is anecological building materialthat is environmentally com-patible both in production andin use and can be fully recy-cled.

Table of contents

1

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2

3

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Figures

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0 Summary 2

1 Manufacture and application of cement 8

1.1 Properties and initial materials 8

1.2 Portland cement clinker production: The clinker burning process 12

2 Environmental compatibility of cement 16

2.1 Cement manufacture 16

2.2 Heavy metal behaviour in concrete 18

2.3 Release of gases and radioactivity 24

2.4 Life cycle assessment 25

3 Use of alternative fuels in cement clinker production 26

3.1 Input control 26

3.2 Composition of alternative materials 27

3.3 Legal requirements for alternative material use 30

3.3.1 17th Ordinance under the Federal Pollution Control Act 30

3.3.2 Waste Management and Recycling Act 31

3.4 Emissions 32

3.5 Preservation of resources 36

3.6 Energy utilization 38

3.7 Effects on the product 39

3.8 Concrete recycling 41

3.9 Integrated assessment 42

Imprint 46

Fig. 1: Location of cement plants and their raw material depositsin Germany 9

Fig. 2: The cement production process 11

Fig. 3: Rotary kiln plant with cyclone preheater (preheater kiln) 13 and precalciner

Fig. 4: Rotary kiln plant with grate preheater (Lepol kiln) 14

Fig. 5: Fuels used in the German cement industry, referenceyear 1994 15

Fig. 6: Life cycle of cement and concrete 16

Fig. 7: Structure of concrete: hardened cement paste, aggregates, capillary pores 22

Fig. 8: Comparison of the heavy metal quantities leached from concretes after 200 days 23

Fig. 9: Ternary diagram for CaO, SiO2, and Al2O3 plus Fe2O3

with cement clinker and ash constituents of different raw materials and fuels 27

Fig. 10: Comparison between permissible emission concentrations of individual heavy metals and their average contents in the exhaust gas of a particular kiln plant taken as an example 33

Fig. 11: List of dioxin and furan values measured by the Research Institute of the Cement Industry 35

Fig. 12: Utilization of alternative materials in the clinker burning process, based on the example of used tyres and carpet remanent 36

Fig. 13: Fuel energy distribution in the clinker burning process for the determination of combustion efficiency 38

Fig. 14: Recycling of concrete: Behaviour of trace elements upon repeated recycling 41

Fig. 15: Integrated assessment of plastic use: Change in potential environmental impact for various methods of recovery 42

Fig. 16: Integrated comparison between utilization in cement plants and disposal in dedicated plants 43

7

Tables

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Table 1: Environmental relevance of the ambient pollution caused by relevant airborne pollutants 18

Table 2: Environmental relevance of heavy metals emitted by a rotary kiln in a cement works at maximum permissible concentrations (limit value) 19

Table 3: Band widths of heavy metal contents in limestone, lime marl and clay/argillaceous rock, given in ppm (g/t) 20

Table 4: Trace element contents in fuels (standard fuels: maximum contents according to Winkler) 20

Table 5: Band widths of heavy metal contents in granulated blastfurnace slag, fly ash, trass, and oilshale as the main cement constituents, given in ppm (g/t) 20

Table 6: Band widths of heavy metal contents in gypsum and anhydrite, given in ppm (g/t) 21

Table 7: Maximum values for heavy metal contents in concrete and initial materials for concrete, given in ppm (g/t) 21

Table 8: Mean heavy metal contents in natural rock, given in ppm (g/t) 21

Table 9: Results of a leaching test (DEV-S4) carried out on crushed concrete 23

Table 10: Primary energy consumption and potential effects of selected building materials 24

Table 11: Group classification of alternative raw materials with examples for individual materials 27

Table 12: Trace element levels in alternative fuels: Typical band widths for German cement plants 29

Table 13: Levels in alternative fuels for cement clinker production, using the elements chromium and cadmium as examples 29

Table 14: Factors influencing the emission behaviour of different exhaust gas constituents in the clean gas of rotary kiln systems in the cement industry 32

Table 15: Effects of alternative material use on the release of heavy metals from concrete, based on the example of selected alternative fuels 40

Cements used in Germanymust comply with the require-ments and definitions ofcement standard DIN 1164.Factory production controland third party inspectiontherefore serve to ensurecement quality as early as inthe production stage.

The product is an hydraulicbinder, i.e. an inorganic, finelyground substance that auto-matically sets with waterwhether it is exposed to air orsubmerged in water. Underthese conditions it retains itsstrength permanently.

Last year, 41 companies withtheir 70 plants producedabout 31 million tonnes ofcement in Germany. The ce-ment plants are located in theimmediate vicinity of lime-stone deposits. Cement is ahomogeneous bulk commoditythat, given the cost of trans-port, is almost exclusivelydelivered to local markets. Forthat reason, production facili-ties are spread evenly all overthe Federal Republic, as canbe seen from Fig. 1.

Page 8 9

Fig. 1Location of cementplants and their raw material depositsin Germany

Manufactureandapplication of cement

1

Properties and initialmaterials

1.1

New Tertiary

Old Tertiary

Upper Cretaceous

Jurassic

Upper Muschelkalk

Lower Muschelkalk

Devonian

Cement Plants

Cement is produced by inter-grinding Portland cementclinker and calcium sulfate(natural gypsum, anhydrite orgypsum from flue gas desulfur-ization plants). In addition tothat, cement can contain oth-er main constituents, such asgranulated blastfurnace slag,natural pozzolan (e.g. trass),fly ash, burnt oil shale, or lime-stone. Fig. 2 schematicallyillustrates the cement produc-tion process.

11

Upon addition of calciumsulfate and, in case of need,other main constituents theclinker burnt in the rotary kiln is subsequently ground to cement in finish mills.

The calcium sulfate serves toadjust the setting behaviour ofthe cement in order to obtainoptimum workability of theproduct during concrete pro-duction. Apart from cementclinker, materials of silicate oraluminate nature or lime-con-taining substances representthe main constituents. Theycontribute to the hydraulicsetting of cement, or havebeneficial effects on the physi-cal properties of concrete.

The twelve cement typespresently approved in Ger-many cover the wide range ofcement-bound buildingmaterials.

For quality requirements to be met, the properties anddosage of clinker and gypsumas well as of the intergroundadditives used in cementgrinding must be such as toensure a defined and homoge-neous composition of thecements produced.


1exclusively consists of silicondioxide. The iron oxide alsorequired for melt formation ispresent in the clay either as aconstituent of the clay miner-als or as ferrous hydroxide, orit is added in the form of ironore.

For the cement to conform tothe DIN standard, the rawmaterial composition must beaccurately complied with.Only a small margin of devia-tion can be tolerated.

For that reason, the raw mate-rials are subject to strict quali-ty control. Only if the require-ments for the preparation andhomogenization of the rawmaterial mix are strict can itbe ensured that a defined andconstant composition of thestarting materials is fed to thekilns. The raw material mix isheated up to a temperature ofmore than 1400°centigrade in a rotary kiln until it startssintering. This results in the starting materials formingnew compounds known asclinker phases. These arecertain calcium silicates andcalcium aluminates whichconfer on the cement itscharacteristic features ofhydraulic setting.

What is known as Portlandcement clinker is made from amix of raw materials primarilyconsisting of calcium oxide(CaO), silicon dioxide (silica(SiO2)), aluminium oxide(alumina (Al2O3)), and ironoxide (Fe2O3).

These chemical constituentsare derived from limestone,chalk and clay, or their naturalblend, lime marl. Limestoneand chalk are composed ofcalcium carbonate (CaCO3).The main constituents of clayare fine-grained mica-like clayminerals and smaller quanti-ties of quartz and felspar. Clayminerals and felspar are com-pounds of silicon dioxide withaluminium oxide and the alkalioxides Na2O and K2O. Quartz

Properties and initialmaterials

1.1

ElectrostaticprecipitatorBlending bed

QuarryRotary kiln

Raw mill

Raw mealRaw material Clinker CementStorage Storage LoadingFinish grinding mill

Fig. 2The cementproductionprocess

13

In Germany most of the Port-land cement clinker – or inshort cement clinker – isnowadays burned in rotarykilns with so-called cyclonepreheaters (preheater kilns)by using the dry process. Partof the clinker is also producedaccording to what is known asthe semi-dry process, whichconsists of heating the rawmaterial in a grate preheaterprior to burning it to clinker inthe rotary kiln (Lepol kiln).

Limestone and clay or theirnatural mix, lime marl, whichare the starting materials forcement clinker, are comminut-ed, dried, and heated to1450°C. This procedure trig-gers the chemical reactionsthat result in the formation ofcement clinker. It later conferson the cement its hydraulicproperties (setting whether itis exposed to air or submergedin water).

In rotary kiln plants compris-ing cyclone preheaters the rawmaterial is fed to the preheateras finely ground meal and pre-heated to about 750°C by thecounter-current flow of the kilnexhaust gas.

The limestone contained in theraw meal is hardly calcined bythat time. Calcination, i.e. theseparation of CO2, mainlytakes place in the calciningzone of the rotary kiln. Cyclonepreheater plants comprisingprecalcining devices areequipped with a so-called cal-ciner. The hot meal dischargedfrom the second cyclone stagefrom the bottom is swept awayby the hot gas flowing upwardfrom the rotary kiln and con-veyed to the calciner. In thisprocess, the kiln exhaust gas is suddenly cooled frombetween about 1000 and1100°C to the calcinationtemperature of approximately850°C. For the endothermiccalcination reaction to bemaintained, fuels that may – depending on the kiln plant –account for a thermal input ofup to 60% of the aggregatefuel energy requirement, aresupplied in the calciner (Fig. 3).

Fig. 3 Rotary kiln plant withcyclone preheater(preheater kiln) andprecalciner


1

Portland cement clinkerproduktion: The clinkerburning process

1.2 With grate preheater plants,the raw material mix, which isdry at the beginning, is formedinto pellets by adding water. In the grate preheater, thesepellets, which are disposed on

a travelling grate, are passedthrough a tunnel subdividedinto a hot chamber and a dry-ing chamber. Not until after-wards do they enter the actualrotary kiln (Fig. 4).

Cyclonepreheater

to raw mill and electrostatic precipitator 300-350°C

Calciner

850°C

Rotary kiln

1050-1150°C

Tertiary air duct700-1000°C

Exhaust air fromclinker cooler 350°C

2000°C

Air inflow to coolerClinker

Clinker cooler

Raw meal

Fuel

Fuel

15

The rotary tube of the kilnplant is lined with refractorymaterial. The rotation and theinclination of the kiln axiscause the kiln feed to beconveyed from the kiln inlettowards the burner, which isinstalled at the kiln outlet. In the so-called sintering zonethe kiln feed reaches tempera-tures of up to 1450°C. Thepassage time of the materialthrough the kiln ranges be-tween 20 and 30 minutes.

From the rotary tube, theclinker is conveyed to theclinker cooler, where it is

Fig. 5 Fuels used in the Germancement industry, referenceyear 1994, all indications aregiven as a percentage of theaggregate heat quantity of103 .106 GJ per year

Fig. 4 Rotary kiln plant

with grate preheater(Lepol kiln)


1

Portland cement clinkerproduktion: The clinkerburning process

1.2

49.9

Coal

Petroleum coke

Natural gas andother gases

Heavy/light fuel oil

Other fossil fuels

10.41.9

0.5

5.8

1.9

32.5

Flue gas to electric precipitator120-150° C

Raw meal feed

Auxiliary chimney

Grate preheater

Rotary kiln

Exhaust air from clinker cooler

Clinker cooler

Burn

er1050-1150° C

2000° C

cooled by air injection. Thethermal energy is recuperatedin this process, heating up thesecondary air (combustionair). In rotary kiln plants withprecalcination and tertiary air duct, part of the heatedcooling air is conducted pastthe rotary kiln to the calciner(tertiary air). It serves ascombustion air for the fuelssupplied in the calciner.

Since energy is reclaimed inthe clinker cooler, and theheat of the kiln exhaust gas ismoreover used for drying andheating up the raw material,the clinker burning process

Alternative fuels

Lignite

boasts a high degree of effi-ciency of 80 to 90%.

Fuel energy is utilized for burning the cement clinker incement manufacture. The hightemperatures prevailing in therotary kiln are indispensablefor the formation of the clinkermineral phases that are cru-cial for the cement properties.

The predominant energysources used in the clinkerprocess in Germany are coaland lignite. Fig. 5 gives acomplete summary of the fuelquantities consumed by theGerman cement industry in1994. In comparison with1994, about 10 to 13 per centof the aggregate fuel energyrequirement of the German

cement industry is currentlybeing met by the use of alter-native fuels.

All the ashes resulting fromfuel combustion form neces-sary mineral constituents ofthe clinker which are takeninto account when the compo-sition of the raw materialconstituents is chosen. A corresponding proportion of raw material can thus beconserved. For that reason,fuels rich in inerts, i.e. havinga high content of ash, areparticularly suitable for use in the clinker burning process.In addition to that, they do not generate any residues.

17

Environ-mentalcompati-bility of cement

2

Cement manufacture

2.1

Fig. 6Life cycle of cement andconcrete

Raw materials Concrete production Use Disposal/Re-use

LeachingLeachingAggregate

Additions

Admixtures

Fuels

Emissions

Sulfate agentsOther main

constituents

Clinker burningprocess

Cement grinding

Clinker

Cement plant

For the ecological assessmentof cement, all its life phaseshave to be taken into consid-eration. This includes both themanufacture of cement and itsuse in concrete (Fig. 6). Ambi-ent pollution levels in thevicinity of production facilitiesare an environmentally rele-vant aspect of cement manu-facture. Throughout the ser-vice life of the component itmust be investigated whetherany environmentally relevantconstituents are releasedfrom concrete components. Ifso, their quantity and the timeperiod of their release are tobe determined. Ecobalancessummarize all the environmen-tally relevant input and outputfactors throughout the lifecycle of a component. Onlythe evaluation and the com-parison of such inventoriesmake it possible to comparedifferent building materialswith regard to the extent oftheir environmental impact.

The environmental compatibil-ity of the cement productionprocess can be measuredusing ambient pollution levelsin the vicinity of productionfacilities. Cement plants areno longer the main source ofambient pollution in the vicini-ty of works these days. Forexample, there is just as littledust deposition in rural areaswhere cement is produced asin purely rural areas withoutthat kind of industry. It be-comes apparent from the fig-ures listed in Table 1 that thesame applies to the contribu-tion to ambient pollution lev-els by other air-borne pollu-tants. The additional impactcaused by individual ambientpollution components rangeswithin one per cent of recog-nized action thresholds at themost and usually lies signifi-cantly below this value.

The contribution of organicsubstances to ambient pollu-tion levels even falls short ofthe permissible values byseveral orders of magnitude.

Based on the example of heavymetals, Table 2 illustrates themaximum calculated input

into the soil in the vicinity of acement works resulting fromthe emissions of the rotarykiln plant. The input figuresare compared with guidevalues for unrestricted use. As can be gathered from thetable, the values of the heavymetals listed lie significantlybelow the respective soil

guide values. If the calcula-tions were not based on theemission limit values, but onactual emission concentra-tions instead, input into thesoil would be even less.

19


Heavy metalbehaviour in concrete

2

2.2

Component Guide values*) Forecast Portion

30 yearsmg/kg µg/kg %

Arsenic 20 11 0.06

Lead 100 182 0.18

Cadmium 1 4 0.40

Chromium 50 56 0.11

Cobalt 30 11 0.04

Copper 50 34 0.07

Nickel 40 56 0.14

Thallium 0.5 0.7 0.15

Mercury 0.5 14 2.7

Zinc 150 20 0.01

Tin 50 11 0.02

Table 2 Environmental relevance of heavy metals emitted by a rotary kiln system in a cement works at maximum permissible concentrations (limit value) (total accumulation over 30 years)

Table 1 Environmental relevance of the ambient pollution caused by relevant airborne pollutants (emission concentration corresponds to limit value)

*) without fugitive sources

Pollutant Contribution to ambient pollutionin % of recognized action thresholds

Suspended dust*) 0.3Mercury 0.04Cadmium 0.05 - 1Thallium 0.002Lead ~ 1Sulfur dioxide 0.5 - 1Nitrogen dioxide 1.1Dioxins/Furans 0.32, 3, 7, 8 TCDD < 0.001Dust deposition*) ~ 1Components in the dustdeposition 0.001 - 0.8

■ Fresh concretephase

In concrete production, ce-ment, aggregates, and – ifnecessary – additions andadmixtures are mixed withwater. The fresh concreteready for use has a pH value of 12.5 to 13.5 owing to thealkalis of the cement. Thishigh natural alkalinity maycause eye irritation upondirect contact, as well as skinirritation following fairly longcontact. For that reason,

appropriate gloves and protec-tive clothing should be worn iffresh concrete is handled man-ually.

Investigations carried out bythe Research Institute of the Cement Industry haverevealed that for example theelements lead, cadmium, andzinc are present in insolubleform and can therefore not bereleased. Tests in which theseheavy metals were deliberate-ly added to the mixing waterhave shown that, in the

presence of cement, they areintegrated into insoluble com-pounds and fixed in the hard-ened cement paste matrix.Less than one per cent of thequantity of thallium input ispresent in soluble form. Onlychromium is highly solubleinitially. As hydration pro-gresses, i.e. as the concretehardens, the dissolvedchromate inevitably generated in cement manufacture is,however, fixed in the hydrationphases. In the hardenedconcrete or mortar it is

insoluble and completely harm-less.

The content of water-solublechromate can trigger a sensiti-zation of the skin in case offrequent contact. Dependingon a person’s physical consti-tution and on the duration and intensity of exposure, anallergy to chromate may bethe consequence (cement ec-zema). Owing to largely auto-mated and machine-controlledproduction, however, this is aminor hazard nowadays.

Frequent and intensive skincontact can, however, notalways be avoided when han-dling bagged cement. Due tothe allergeneous effects ofthe chromate and the alkalini-ty of the mixed cement im-proper handling without takingprotective measures cancause skin problems. For thatreason, German cement manu-facturers have started to addi-tionally offer low-chromatestandard cement in bags. Thiscement is marked by the im-print ”Low-chromate accord-ing to TRGS 613” and con-forms to DIN 1164 quality. As fresh concrete and mortarcontinue to be alkaline, how-ever, persons handling it mustwear suitable protectivegloves and take precautionaryskin care measures all thesame. This combination ofmeasures is the only way ofeffectively combating aller-gies to chromate.

*) Guide values for soils subjected to multifunctional use according to Eikmann-Kloke

21

them, primary and secondaryconstituents account for more than 99% by mass. Theconcentration of heavy metalspresent in traces is usually < 100 ppm, or 0.1 kg/t.

By way of example, the Tablebelow summarizes bandwidths of heavy metal con-tents of lead, cadmium,chromium, nickel, thallium,mercury, and zinc for the rawmaterials limestone, limemarl, and clay/argillaceous

rock. It becomes apparentthat the heavy metal contentsof the raw materials are lowand grow higher from lime-stone via lime marl to clay/argillaceous rock. The subse-quent table features the maxi-mum trace element contentsin standard fuels, such as coaland lignite, as compiled by theState Environmental Ministryof North Rhine-Westphalia inEssen. Two additional tablessummarize the contents foundin other possible main con-



2

2.2

Limestone Lime marl Clay/argillaceous rockElements

from-to from-to from-toLead 0.27 - 21 1.3 - 8.5 9.7 - 40

Cadmium 0.02 - 0.50 0.04 - 0.35 0.05 - 0.21

Chromium 0.70 - 12.3 4.6 - 35 20 - 90

Nickel 1.4 - 12.9 5.9 - 21 11 - 70

Mercury 0.005 - 0.10 0.009 - 0.13 0.02 - 0.15

Thallium 0.06 - 1.8 0.07 - 0.68 0.60 - 0.90

Zinc 1.0 - 57 24 - 55 55 - 110

Table 3 Band widths of heavy metal contents in limestone, lime marl and clay/argillaceous rock, given in ppm (g/ t)

Constituent mg/MJ Constituent mg/MJBeryllium 0.13 Tellurium 0.04

Cadmium 0.3 Antimony 0.07

Mercury 0.06 Lead 10

Thallium 0.15 Chromium 3.7

Arsenic 1.9 Copper 3.7

Cobalt 1.2 Vanadium 6.7

Nickel 3.5 Tin 0.4

Selenium 0.2 Zinc 8

Table 5 Band widths of heavy metal contents in granulated blastfurnace slag, fly ash, trass, and oilshale as the main cement constituents, given in ppm (g/ t)

Initial materials for concrete

ElementesAddition Gravel

SandConcrete Aggregate Fly ash from

Cement bituminous coal

Lead 260 20 800 100

Cadmium 6 1 4 3

Chromium 130 70 330 100

Nickel 100 10 300 50

Mercury 0.2 0.1 1 0.2

Thallium 4 1 4 2

Zinc 680 50 910 200

Table 7 Maximum values for heavy metal contents in concrete and initialmaterials for concrete, given in ppm (g/t)

Grey- Lime-Granite

GneissBasalt Granulite

Continen-Elements wacke stone shale tal crust

Lead 22 14 5 32 16 4 10 15

Cadmium 0.13 0.09 0.16 0.09 0.10 0.10 0.10 0.10

Chromium 90 50 11 12 76 168 88 88

Nickel 68 40 15 7 26 134 33 45

Mercury 0.45 0.11 0.03 0.03 0.02 0.02 0.02 0.02

Thallium 0.68 0.20 0.05 1.10 0.65 0.08 0.28 0.49

Zinc 95 105 23 50 65 100 65 65

ElementsGranulated Fly ash from

Oilshale Trassblastfurnace slag bituminous coal

Lead 1 - 10 58 - 800 10 - 50 10 - 70

Cadmium 0.01 - 0.5 0.2 - 4 0.5 - 3.0 0.1 - 1.0

Chromium 1 - 75 71 - 330 20 - 40 2 - 90

Nickel 1 - 10 92 - 250 unknown 1 - 5

Mercury <0.01 - 0.2 0.04 - 2.4 0.05 - 0.3 <0.01 - 0.1

Thallium <0.2 - 0.5 0.7 - 5.1 1.0 - 3.0 <0.1 - 1.0

Zinc 1 - 20 67 - 910 160 - 250 60 - 190

Table 8 Mean heavy metal contents in natural rock,given in ppm (g/t)

■ Working life

Heavy metals are present invarying concentrations in allthe initial materials for theproduction of cement andconcrete. The respective con-tents and the input quantitiesdetermine the trace elementcontent in the finished con-crete component.

All the initial materials con-tain primary, secondary, andtrace elements. Between

Elements Gypsum AnhydriteLead 0.3 - 20 0.4 - 15

Cadmium <0.2 - 3 <0.2 - 0.3

Chromium 2.8 - 33 1.0 - 9.0

Nickel unknown unknown

Mercury <0.01 - 1.3 <0.01 - 0.02

Thallium <0.2 - 0.6 <0.2

Zinc 1.0 - 61 5.0 - 22

Table 4 Trace element contents in fuels(standard fuels: maximum contents according to Winkler)

Table 6 Band widths of heavy metal contents in gypsum and anhydrite,given in ppm (g/t)

stituents of cement and in thecalcium sulfates anhydrite andgypsum.

Trace elements are input intoconcrete via aggregates,cement, and additions. Thequantity input via admixturesis negligible. The same appliesto the input via mixing water.Gravel and sand are the princi-pal aggregates used; depend-ing on their local availability,limestone and basalt chip-pings, among others, may beapplied as well. Fly ash frombituminous coal is almost theonly concrete addition used.The table below summarizesthe maximum heavy metalcontents for the aggregatesgravel and sand and for theaddition fly ash from bitumi-nous coal, and their maximumcontents in concrete. Thecomparison of maxiumumheavy metal contents in con-cretes shows that thesecontents are comparable tothose in natural rocks (Tables7 and 8).

Argilla-ceousrock

23

The fact that only a small per-centage of heavy metals isreleased from concrete isattributed to the excellentbinding capacity of the hard-ened cement paste. In addi-tion to that, the dense porestructure that concrete hasmakes it highly resistant todiffusion. This prevents therelease of heavy metalsdissolved in the pore water ofthe hardened cement paste(see Fig. 7).

All available investigationscorroborate that the release ofheavy metals from concretecomponents is extremelysmall. Research projects atthe Research Institute of theCement Industry have shownfor the elements mercury,thallium and chromium asexamples that the valuesmeasured in leaching tests lieconsiderably below those setby the Drinking Water Ordi-nance even when extremelylong leaching times werechosen.

The Figure below shows theinvestigation results as blockdiagrams. In one test seriesthe test pieces (4x4x16cm3)were stored in drinking water.In another test series drinkingwater was enriched with lime-dissolving carbonic acidin order to investigate theeffects of a severe chemicalattack. In a further test se-ries, the test material wasdoped in order to be able toillustrate leaching with in-creased heavy metal quanti-ties as well. The leached quan-tities of the elements underinvestigation were low bothwith pure drinking water,chemical attack, and in-creased heavy metal content.

Also in these cases, values laysignificantly below the limitvalues of the Drinking WaterOrdinance.

The leaching behaviour ofheavy metals from used con-cretes was investigated aswell. The recycling of concreteresults in its specific surfacearea getting larger by com-minution. The leaching behav-iour thus changes with regardto that of solid components.

The table below depicts theheavy metal quantity releasedfrom crushed concrete. This isbased on a shaking test ac-cording to DEV-S4, duringwhich the crushed material isshaken in water for 24 hours.The interpretation leads to theconclusion that the heavymetals are solidly fixed in thehardened cement paste ma-trix and that the quantities

released are tiny. In most cas-es the concentration found inthe leaching water was belowthe detection limit of theanalysis method.


2.2

Content inContent in

Content inContent in

Element cementleaching

cementleaching

water Element water[mg/kg] [mg/l] [mg/kg] [mg/l]

As 5 < 0.0002 Mn 500 0.002

Be 1 < 0.0002 Mo 1 0.0004

Cd < 0.5 < 0.0001 Ni 28 0.002

Co 10 < 0.0002 Pb 17 0.001

Cr 58 0.003 Sb 1 < 0.0002

Cu 22 0.0008 Sn 3 < 0.0002

Hg < 0.05 < 0.0002 V 66 < 0.0002

Tl < 0.5 < 0.0002 Zn 310 0.001

Table 9 Results of a leaching test (DEV-S4) carried out on crushed concrete

Fig. 8Comparison ofthe heavy metalquantitiesleached fromconcretes after200 days

Fig. 7Stucture of concrete:hardened cementpaste, aggregates,capillary pores


2 Aggregate

Hardenedcement

paste

Capillarypore Diffusion

Air pore

Doped concretes

Ordinary concretes

0.6

Cr Tl Hg

D r i n k i n g w a t e r

Tota

l of h

eavy

met

als

elut

ed a

fter

abo

ut 2

00 d

ays,

in m

g

A g g r e s s i v e c a r b o n d i o x i d e

Cr Tl Hg

0.5

0.4

0.3

0.2

0.1

0

25

Life cycle assessment consti-tutes a common tool foranalysing environmental im-pact from an overall ecologi-cal point of view. It forinstance consists of compilingand assessing the input andoutput flows of raw materialand energy consumption aris-ing when a structure is erect-ed. In addition to that, thepotential environmental ef-fects of a product system inthe course of its life cycle are

evaluated. Globally, the reper-cussions on global warmingand ozone depletion in theatmosphere have to beanalysed. From a regionalpoint of view, the effects onsoil acidification, the nutrientbuild-up in water systemsknown as eutrophication, andthe formation of photo-oxi-dants causing summer smogare investigated. Comprehen-sive data on concrete hasbeen compiled by now. Table10 shows the primary energyconsumption and other poten-tial effects associated withthe production of 1m3 con-crete. Corresponding indica-tions for lime sand bricks,foam concrete, and clay ma-sonry bricks serve as compar-ative values. The data onthese building materials is notavailable in as much detail asthat for concrete.

Mineral building materials likeconcrete do not cause anygaseous emissions. Organicconstituents that might beliberated in gaseous form canonly be introduced into con-crete via concrete admixturesor aggregates with organicloads. Various investigationshave shown that the release of small concentrations ofammonia or formaldehyde, for example, can only be ob-served during or shortly follow-ing production, if at all. Forthat reason, application inhousing construction is utterlyharmless.

Numerous measurementshave indicated that the radio-active radiation of concrete isnegligible. It is lower than thatof natural rocks such as gran-ite and basalt. Differences inthe radioactivity of differentconcrete types solely dependon the aggregate used. Theinfluence that cement andadditions such as fly ash exerton the radioactivity of con-crete is marginal.

Radon exhalation from con-crete is slight as well. It islower than the average radonexhalation of the natural soil


Release of gases andradioactivity

2

2.3Life cycleassessment

2.4

Table 10 Primary energy consumption and potential effects of selected building materials

Lime Clay masonry

Building material Concrete sand Foam bricks

bricksconcrete

Full Light

Primary energy MJ/m3 1750 1410 1610 1370 4180 1760Potential effects:

Global warming kg CO2 eq 251 227 320 n.i. n.i. n.i.

Ozone depletion kg CCI3 Feq n.i. n.i. n.i. n.i. n.i.

Acidification kg SO2 eq 0.76 0.84 0.92 n.i. n.i. n.i.

Eutrophication kg PO3-4 eq 0.15 n.i. n.i. n.i. n.i. n.i.

Photo-oxidants kg C2 H4 eq < 0.01 n.i. n.i. n.i. n.i. n.i.

nocontribu-

tion

by two orders of magnitude.As a consequence, concretedoes not contribute to interiorcontamination by radon. Quiteon the contrary: a concretefloor slab forestalls radoninflow from the soil. By takingcorresponding constructionmeasures, residential build-ings contaminated by radonthat are situated in miningregions of the new federalstates have recently beenremediated.

materials, such as iron oxideagents or substances provid-ing sulfur and fluorine, respec-tively.

The table below groups to-gether possible alternativeraw materials according totheir constituents.

Used tyres (currently about250,000 t per year) and waste

oil (currently about 170,000 tper year) constitute theprincipal alternative fuelsutilized by the Germancement industry. To asmaller extent, alsofuller earths, wood,plastic materials,and light frac-tions of do-mestic andindustrial

Page 26 27

Use ofalternativefuels incementclinkerproduction

3However, the cement industryonly purposefully uses wasteas alternative raw materialsand alternative fuels. Thematerial constituents of thewaste must comply with thestrict quality requirements tobe met by the product. It isimportant that environmental-ly relevant trace elements donot have any adverse effectson emissions during the ce-ment production process andon the environmental compati-bility of the product. For thatreason, trace element levelsin waste materials have tomeet tough requirements. Tothat end, it might becomenecessary to strictly limittrace element levels in individ-ual cases.

Quality assurance schemesensure that all the alternativematerials used comply withthe requirements for originand/or constituents. The sam-pling and testing schedule forinput control is determinedwith respect to eachindividualalternative material takinginto account its compositionand bulk size when deliveredto the plant. It is jointly laiddown by the works and licenc-ing authorities. Testing takesplace according to DIN proce-dures, VDI (Association ofGerman Engineers) codes ofpractice, and approved wasteand residue inspection meth-ods.

Many cement works in theFederal Republic of Germanyuse, or are planning to use,alternative fuels in cementmanufacture. By utilizingalternative materials thecement plants improve thecost-effectiveness of theproduction process. The useof alternative materials in thecement industry simultane-ously contributes to theenvironmentally compatibledisposal of a variety of wastematerials.

Input control3.1

Fig. 9Ternary diagram for CaO,SiO2, and AI2O3 plus Fe2O3

with cement clinker andash constituents ofdifferent raw materialsand fuels

Raw materials applicable incement manufacture are main-ly substances found in nature,such as limestone, lime marl,clay, sand, or calcium sulfate.Alternative raw materialshaving SiO2, Al2O3, Fe2O3, and /or CaO as their mainconstituents can be combinedwith natural raw materials insuch a way as to ensure com-pliance with the requirements

both for clinker quality andenvironmental protection andoperational reliability if theirdistribution is homogeneous.The demands placed on analternative material primarilydepend on the raw materialsituation prevailing at a cer-tain works. The decisivefactoris the quality of the extract-able deposits of limestone and lime marl, respectively.The materials used includeseveral substances of thecalcium group and corrective

waste are used. The energy content of alterna-tive fuels is recovered. Justlike all fuels, however, theyalso make a material contribu-tion. Their ashes contribute tothe mineral constituents ofthe cement clinker. The Figurebelow depicts the ternarydiagram for CaO, SiO2, andFe2O3 plus Al2O3. It principallycovers the clinker, the compo-sition of which must beprecisely adjusted via the feedmaterials for the cement toobtain its characteristichydraulic properties.

Composition of alternativematerials

3.2

Table 11 Group classification of alternative raw materials with examples for individual materials

Group classificationof alternative raw materials (examples)

• Ca-Group - Industrial lime- Lime sludge

• Si-Group - Used foundry sand• Fe-Group - Roasted pyrite

- Synthetic haematite- Red mud

• Si-Al-Ca-Group - Fly ashes- Slags- Crushed sand

• S-Group - Gypsum from flue gas desulfurization

- Chemical gypsum• F-Group - CaF2 filter mud

Usedfoundry

sand

Fullerearth

Plastics,rubber

Coal

Cementclinker

Lignite

Shreddedtyres Roasted

pyrite

0

20

40

60

80

100

0 20 40 60 80 100

Al203 + Fe203 in % ➞

Ca0

in %

➞ Si02 in %

➞

100

80

60

40

20

0

29

can thus be compared witheach other. It becomes appar-ent that heavy metal concen-trations can occur in certainband widths both in alterna-tive fuels and in coal.

A comparison between alter-native fuels and so-calledstandard fuels leads to theconclusion that alternativefuels can have both higher andlower contents depending onthe particular circumstances(cf. Table 4).

In addition the ashes of differ-ent fuels are depicted. Justlike with natural raw materi-als, their contribution to theraw mix varies depending ontheir composition and thequantity utilized. In contrastto the procedure in pure com-bustion systems, the ashesgenerated are integrated intothe product. Otherwise, theseconstituents would have to beprovided via raw materials.

Trace elements are introducedinto the clinker burningprocess via both raw materialsand fuels. The use of alterna-tive materials substitutes foran equivalent proportion ofnatural constituents, whichalso contain trace elements.The table below summarizesthe band widths of elementsto be found in the alternativefuels currently utilized in Ger-man cement works. Some ofthe maximum levels (ceiling ofthe band width) represent

voluntary limits self-imposedby operators. Actual values liesignificantly below these val-ues in individual cases.

Another Table shows the pos-sible levels, here again for theelements chromium and cad-mium, in used tyres, waste oil,plastic materials from the DSDsystem, scrap wood andrefuse-derived fuel (RDF).

The concentrations listedhave been normalized on therespective calorific values and


3Constituent mg/MJ Constituent mg/MJBeryllium < 0.01 - 0.1 Tellurium < 0.01 - 0.01

Cadmium 0.01 - 0.7 Antimony 0.03 - 0.05

Mercury 0.01 - 0.1 Lead 0.09 - 25

Thallium < 0.01 - 0.1 Chromium 0.09 - 21

Arsenic 0.003 - 1 Copper < 0.01 - 67

Cobalt 0.02 - 8 Vanadium 0.03 - 16

Nickel 0.1 - 25 Tin 0.03 - 0.7

Selenium 0.03 - 0.4 Zinc 0.5 - 625

Table 12 Trace element levels in alternative fuels:typical band widths for German cement plantsComposition

of alternativematerials

3.2

Table 13 Levels in alternative fuels for cement clinker production, using the elements chromium and cadmium as examples

Levels in alternative fuels[mg/MJ]

Chromium Cadmium

Used tyres 0.1 - 4 0.1 - 0.35Waste oil 0.1 - 0.4 0.02 - 0.12Plastics (DSD) 1 - 8 1 - 2Scrap wood 0.1 - 6 0.06 - 0.6RDF 0.6 - 23 0.06 - 0.9

31

The requirements for combus-tion conditions apply both tocement works and to wastedisposal plants. To indirectlyensure the burn-out of fuels,organic exhaust gascompounds and carbonmonoxide are limited in dedi-cated combustion plants(waste incineration plants andpower plants). In cementworks, however, organic ex-haust gas compounds andcarbon monoxide attributableto the raw materials for clink-er production are not subjectto these requirements. Thisdoes not permit any conclu-sions about combustion condi-tions and the toxic organictrace gases that might occur,nor does it imply that rotarykiln plants in the cement in-dustry are exempt from therequirements 17. BImSchVplaces on combustion condi-tions. Much rather, the quanti-ty of organic compounds emit-ted in relation to the quantityof waste used is smaller in theclinker burning process thanfor example in waste incinera-tion plants.

Cement works are plants thatare required to be licenced.Their operation is subject tothe provisions of the FederalPollution Control Act (BImSchG). A licence must be obtained for each individualfeed material used, includingalternative materials.

The use of alternative fuels is regulated by the 17th Ordi-nance under the FederalPollution Control Act (17.BImSchV). The require-ments to be met by individualwaste materials and theirenvironmentally compatibleutilization are laid down in theWaste Management andRecycling Act (KrW/AbfG).

Rotary kiln plants in the ce-ment industry are governed bythe 17th Ordinance under theFederal Pollution Control Act(17. BImSchV) if the energycontent of waste is recoveredin cement clinker production.

If waste is used, emissionlimit values are considerablylower than laid down in theClean Air Guide (TA Luft). Thelimit values depend on theproportion of alternative fuelsin the overall fuel energy ex-pended by a kiln system. Thehigher the proportion ofwaste, the lower the emissionlimit values (weighted averagecalculation). This ensures thatthe emissions caused bywaste incineration do notexceed those stipulated forwaste incineration plants.

As cement works have to com-ply with limit values that maysometimes be higher thanthose applicable for wasteincineration plants, criticsallege that the use of alterna-tive fuels in the clinker burn-ing process serves to ”fill up”permissible emission concen-trations and to ”dilute” pollu-tant levels. In fact, however,emission concentrations, e.g.of heavy metals, are independ-ent of the feed materialschosen and lie on a very low

level. Thus, there is no ”fillingup” and ”diluting”. If alterna-tive fuels are used, the actualemission concentrations ofheavy metals, for example, areoften lower than the concen-trations permissible for wasteincineration plants. For thatreason, the projection of emis-sion loads on the basis of limitvalues gives figures that donot actually occur in practice.The projection suggests pollu-tion that will, according to the calculation, only diminishif the waste is disposed of inwaste incineration plants.This way of proceeding isimproper as it deliberatelyignores actual facts.

The German Federal Commit-tee for Air Pollution Control(LAI) has specified therequirements of 17. BImSchVfor cement plants. For nitro-gen oxides, for example, theprogressive limit values laiddown in the Clean Air Guidehave to be complied with asNOx emission concentrationsdo not depend on the type offuels used. Cement workshave to comply with emissionconcentrations of 0.50 g/m3

(new plants) and 0.80 g/m3

(existing plants), respectively.Before, the limit values of theClean Air Guide were almosttwice as high.


3

Legalrequirements for alternativematerial use

3.317th Ordinance underthe Federal Pollution ControlAct

3.3.1WasteManagement and Recycling Act

3.3.2

The German Waste Manage-ment and Recycling Act (KrW-/AbfG) adopts the Euro-pean concept of waste. Adistinction is made between”product” and ”waste”.According to the terms of theWaste Management and Recy-cling Act, the alternativematerials utilized in the rotarykilns of the cement industryare basically waste. This per-ception changes if they areproduced for the very purposeof being used in cement works– in this case, they constituteproducts. With waste, a dis-tinction is made betweenwaste materials for recoveryand waste materials for dis-posal. Recovery can pertainboth to the material and ener-gy contents. The type of re-covery that is more environ-mentally compatible is givenpreference.

In cement plants both theenergy and the material con-tent of waste is recovered. Inaddition to that, rotary kilnplants in the cement industrycomply with the requirementsfor combustion efficiency. Theprinciple governing wastedisposal is that recovery ofwaste must take precedenceunless disposal represents themore environmentally compat-ible solution. In this context,the following factors must betaken into account:– potential emissions,– preservation of natural

resources,– consumption and generation

of energy,– pollutant enrichment in

products, waste for recovery,or products made from them.

33

Individual emission compo-nents showing similar behav-iour in a rotary kiln plantduring the clinker burningprocess can be classedtogether. The table lists theindividual exhaust gas con-stituents opposite the deci-sive factors influencing thecontent of the respectiveconstituents in the exhaustgas. Dust as an emission con-stituent is influenced by theprecipitation behaviour of theexhaust gas cleaning device.The content of particulateheavy metal compositions inthe exhaust gas is additionallydetermined by the inputsituation in the kiln plant.

The generation of nitrogenoxides is inherent to theprocess. It is not influenced by the use of alternative mate-rials. The raw material compo-sition is decisive for the content of total carbon andcarbon monoxide in the ex-haust gas. Moreover, depend-ing on the respective loca-tions, the raw materialsituation can effect the emis-sions of sulfur dioxide andammonia and ammonium com-pounds. Toxic organic exhaustgas constituents such as

dioxins and furans are unaf-fected by the type of alterna-tive fuels used. Their contentin the clean gas of rotary kilnplants is extremely low.

The behaviour of heavy metalson the one hand and dioxinsand furans on the other iselaborated on separately be-low.

■ Heavy metalemissions

The conditions prevailing dur-ing the clinker burning pro-cess, which, in contrast todedicated combustion plants,constitutes a material conver-sion process, ensure low con-centrations of trace elementsin the clean gas. This is alsotrue if alternative materialsare used. As can be gatheredfrom Fig. 10, emission concen-trations produced during theuse of alternative fuels for

example fall below those ac-cording to 17. BImSchV. TheFigure illustrates the compari-son between the concentra-tions of trace elements in theclean gas and the respectivelimit values calculated as so-called ”weighted averagelimit values” according to 17. BImSchV. The alternativematerial picked was a light-weight fraction of domesticwaste. Its share in the firingheat capacity amounted to 25 per cent.

Emission concentrations arebased on an input calculationand also allow for maximumcontents of elements in thealternative fuels utilized.

The Figure shows that giventhis input scenario, which isconsidered pessimum, thevalues were considerably low-er than the emission limitvalues. This statement alsoapplies to other alternativefuels utilized in cement works.The heavy metal input inducedby alternative fuels does notbecome relevant until levelsare significantly higher. Mer-cury is the only element thatmay require input to be limitedin individual cases.

Table 14 Factors influencing the emission behaviour of different exhaust gas constituents in the clean gas of rotary kiln systems in the cement industry

Exhaust gas constituent Influencing factorDust Exhaust gas purificationHeavy metals Exhaust gas purification, inputN0x ProcessTotal carbon, CO, SO2, NH3 Raw materialDioxins and furans Process

Fig. 10Comparsion between permissibleemission concentrations of individualheavy metals and their averagecontents in the exhaust gas of a particular kiln plant taken as anexample


3

Emissions3.4

Weighted average limit value

Emission concentration

Cd, Hg, Tl As, Co, Ni, Se, Te Sb, Pb, Cr, Cu, V, Sn

1.0

0.5

0.0

3,8 mg/m3

Conc

entr

atio

n in

mg/

m3

contribution of a cementworks to ambient pollutionlevels lies within one per centof the concentrations that can be tolerated (Chapter1.3). This is especially thecase for waste utilized incement plants for energyrecovery, as it is subject tothe more stringent limit values of 17. BImSchV.

The content of groups of or-ganic substances, such aspolychlorinated biphenyls(PCB), polycyclical aromatichydrocarbon (PAH), polychlo-rinated naphthalenes (PCN),hexachlorobenzene, or pen-tachlorophenol, in the cleangas of rotary kiln plants in thecement industry is so low thatthe contribution to ambientpollution levels falls below thepermissible ambient pollutionconcentrations by severalorders of magnitude.

■ Effects ofemissions with alternative material use

Critics accuse the cementindustry of increasing emis-sion concentrations to suchan extent by utilizing alter-native materials that thepermissible limit values are

almost completely exhausted.There is no such ”filling up” of limit values. In addition,investigations carried out bythe Research Institute of the Cement Industry haveshown that the ambient pollu-tion level in the vicinity of aworks would lie below thepermissible values even if limitvalues were fully exhausted as purported. Studies on thebasis of the limit values laiddown in the Clean Air Guide(TA Luft) indicate that the

Page 34 35

Fig. 11List of dioxin and furanvalues measured by the Research Institute of the Cement Industryat rotary kiln plants in Germany up to thesummer of 1996

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

0.29


3

Emissions3.4

Standard fuels with substitute fuels with substitute raw meterials

Emis

sion

s in

ng

I-TEQ

/m3

0 20 40 60 80 100 120 140 160Measurement No.

■ Dioxin and furanemissions

Dioxin and furan emissions areoften discussed in conjunctionwith the use of alternativefuels. All the emission datacompiled by the ResearchInstitute of the CementIndustry and other approvedmeasuring bodies indicate,however, that dioxin and furanemission concentrations arelow regardless of the type offuel used. The Figure belowgives all the results the mea-surements carried out by theResearch Institute of theCement Industry have yielded.The emission concentrationsmeasured are plotted on theordinate, and about 160 indi-vidual measurements areplotted on the abscissa. Thealternative use of standard oralternative fuels did not haveany effect on the measure-ment results. All the organic

compounds are completelydestroyed in the rotary kiln. In addition to that, the use ofsubstitute raw materials didnot bring about any alterationin emission concentrationseither. All the emission con-centrations but one were be-low the value of 0.1 ng TE/m3

according to 17. BImSchV.

Since the organic compoundscontained in the fuels are de-stroyed completely, the dioxinand furan emissions of rotarykiln plants in the cementindustry are very low. For thatreason the chlorine content of fuels need not be limited inorder to restrict emissions. Incontrast to waste incinerationplants, moreover, hardly anydioxins and furans are gener-ated in the exhaust gas ofrotary kiln plants. No HCl is tobe found in the the exhaustgas of rotary kiln plants in thecement industry either, andthe concentrations of catalyt-ic heavy metals present arelow. Furthermore, the resi-dence time of particulates andgases within the temperaturerange critical for reformationis very short. In contrast towaste incineration plants,rotary kiln systems in thecement industry therefore donot have to be equipped with a special dioxin filter.

37

Fig. 12Utilization of alternative materials in the clinker burning process based on the exampleof used tyres and carpet remanent


3

Preservationof resources

3.5

Alkalis, Zn: <2%

Fe content: 16%Si02, Ca0, S, Al203: 3%

Organics:Calorific value: 32900 kJ/kg

Materialrecovery - 19%

Energy recovery - 79%

Used tyres

N, Cl, F, Others: 4.5%

Ca content: 15.1%S content: 0.9%

Organics:Calorific value: 19500 kJ/kg

MaterialRecovery - 16.0%

Energyrecovery - 79.5%

Carpet remanent

■ Recovery ofmaterial and energy contents

The material conversion pro-cess taking place in cementclinker production ensuresthat both the material and theenergy content of waste isrecovered in the rotary kiln.The energy content of a wastematerial is used as combustionheat (energy recovery). Theashes from combustion arenecessary mineral consti-

tuents of the cement clinker(material recovery). The ex-ample of used tyres and carpetremanent demonstrates thatthe material and energy con-tent of almost all the rawmaterial and fuel constituentsis recovered in the clinkerburning process. The mineralfuel constituents form clinkerminerals. The calorific value of the alternative fuel derivesfrom the remaining organicconstituents.

Relative to the organic frac-tion, the specific calorificvalue is higher than that of theinitial material, since the frac-tion for material recoverymust not be taken into consid-eration for energy recovery.This is of importance insofaras the Waste Managementand Recycling Act stipulates aminimum calorific value of11000 kJ/kg for the energycontent of alternative fuels tobe deemed recoverable.

The simultaneous recovery ofthe energy and material con-tent in alternative materialsdistinguishes the clinker burn-ing process from combustionprocesses in waste incinera-tion and combustion plants.The latter require the dis-charge of the ashes produced.They cannot be put to furtheruse until they have undergonelaborious reprocessing.

Depending on the type ofwaste, the main focus of re-

covery in the rotary kiln plantsof the cement industry isplaced either on the materialor the energy aspect, as canbe seen from the followingexamples:– Waste oil utilization is pri-

marily concerned with recov-ering the energy content.

– The utilization of used tyresinvolves recovering both theenergy and material content.As iron accounts for about15% of the steel carcass oftyres, they constitute amaterial component of theclinker burning process.Conventionally, correspond-ing quantities of iron orehave to be added to the rawmaterial mix.

– In case of low-clay raw mate-rial deposits, fuel ashes fromindustrial combustion plants(fly ashes, grate ashes) fur-nish the SiO2 and Al2O3 con-stituents important for theclinker phases. Materialrecovery is paramount eventhough the small content ofresidual carbon in the ashescontributes to the fuelrequirement in clinker pro-duction.

To be able to assess the ef-fects the use of alternativematerials in cement manufac-ture has on the product, thebehaviour of trace elements in the concrete has to beanalysed.

Alternative materials can influ-ence the content of trace ele-ments in cement. Their levelsmay be higher, but also lower

than those found if standardmaterials are used. In practicealternative materials are as-sumed to behave like standardmaterials, thus not substan-tially affecting the traceelement content in cementand concrete. As can be seenfrom 1.3, the heavy metals are fixed in the concrete andthus immobile. All the investi-gations corroborate that onlyvery small quantities of heavymetals are released fromconcrete components.

In this context, the effect ofalternative material use on therelease of heavy metals fromfinished concrete componentswas investigated as well. The inputs into a rotary kilnplant were determined forused tyres, waste oil, wood,plastics, and refuse-derivedfuels, which alternative mate-rials were selected by way ofexample. Assuming average

heavy metal contents for thealternative fuels, the releasefrom the concrete was meas-ured on crushed material(DEV-S4 method). This presup-poses that all the heavymetals dissolved in the porewater are released to a largeextent.

Page 38 39

Fig. 13Fuel energy distribution

in the clinker burning process

for the determination of combustion

efficiency


3

Energyutilization

3.6Effects on the product

3.7

Combustion efficiency is cal-culated in compliance withthe preamble to the WasteManagement and RecyclingAct. It is calculated from theenergy expended and the ener-gy lost via the exhaust gas.The fractions used for dryingand heating up the initial ma-terials must not be consideredlosses. This definition appliesto all kinds of industrial firingsystems. The so-called ”ex-tended combustion efficien-cies” cannot be applied tomaterial conversion processessuch as cement clinker pro-duction. This is an aspectcritics of alternative fuel usein cement works like to pointto in discussions in order toraise doubts about the com-bustion conditions prevailingduring the clinker burningprocess.

■ Combustion efficiency

The rotary kiln plants in thecement industry comply withthe requirements set up forenergy recovery in the WasteManagement and RecyclingAct. At 80 to 90%, the com-bustion efficiency of rotarykiln plants in the cement in-dustry is higher than the figureof 75% demanded. The Figurebelow depicts the variousfractions of heat quantity tak-en into account in calculatingcombustion efficiency. In theexample given, the combus-tion efficiency amounts to83%.

16.6 %

10.0%

23.5%

49.9%

Clean gas losses

Drying of raw material

Heating up of raw material

Calcination of raw material andformation of clinker phases

Combustion efficiency here: 83%

■ CO2 emissions

Owing to its process technolo-gy the clinker burning processensures that the energy con-tent of fuels is utilized to ahigh degree. The process-technology optimization ofrotary kiln plants in the pasthas resulted in a significantreduction in the specific fuelenergy requirement for ce-ment manufacture. In additionto that, the German cementindustry contributes its shareto the reduction of climaticallyrelevant gases by taking vari-ous measures aimed at furtherdiminishing its specific CO2

emissions by the year 2005.The use of alternative fuelscan make a vital contributionto reaching this target in thefuture.

and the concrete made fromit. This result is transferrableto other alternative materialsas well.

The use of alternative materi-als in cement manufacturedoes not impair the environ-mental compatibility of ce-ment and the concrete madefrom it. Just as a cement

The results the investigationsyielded are illustrated in theTable below. Values turned outto be lower than the limit val-ues laid down in the DrinkingWater Ordinance. The use inthe clinker burning process ofthese alternative fuels investi-gated by way of example doesnot have any environmentallyrelevant effects on cement

Page 40 41

works' contribution to ambi-ent air pollution falls belowpermissible ambient pollutionvalues, the trace elementsreleasable from concretecomponents do not reachenvironmentally relevantquantities. As a consequence,concrete is an ecologicalbuilding material. It is environ-mentally compatible in pro-duction and use and can berecycled completely.

Trace element content in the fuel (ppm)

Table 15 Effects of alternative material use on the release of heavy metals from concrete bases on the example of selected alternative fuels

Used tyres Waste oil Scrap wood Plastics RDF

25% 50% 50% 50% 50%

Hg 0.17 0.05 0.2 1.3 1Cd 8 0.4 3.4 72 1Tl 0.25 0.1 < 0,1 0.3 0.3Cr 97 10 50 48 40Pb 410 250 1000 390 160Zn 15000 500 1500 550 400

Trace element content in the concrete (ppm)

Hg < 0.01 < 0.01 < 0.01 < 0.01 < 0.01Cd 0.1 0.1 0.1 0.4 0.1Tl 0.1 0.1 0.1 0.1 0.1Cr 3.4 3.1 3.6 3.4 3.6Pb 6.1 5.9 15.5 6.5 6.5Zn 71.7 21.4 35.2 21.7 23.7

Trace element content in the eluate (mg/l)

Hg < 0.000001 < 0.000001 < 0.000001 < 0.000001 < 0.000001Cd 0.000002 0.000001 0.000002 0.000005 0.000001TI < 0.000001 < 0.000001 < 0.000001 < 0.000001 < 0.000001Cr 0.0021 0.0020 0.0023 0.0021 0.0022Pb 0.0004 0.0004 0.001 0.0004 0.0004Zn 0.0045 0.0013 0.0022 0.0014 0.0015

Concrete aggregate can beobtained from old concrete,which serves to recover thematerial content from concretecomponents and to recyclethe latter into new concrete.The substantial precondition,which is not always easily metat present, is the diligent sep-aration of the concrete fromother building materials whena structure is demolished.Only old concrete in its pureform fulfils the prerequisitesfor high-grade aggregate suit-able for re-use.

Concrete made using recycledaggregate also complies withthe requirements for an envi-ronmentally compatible build-ing material. Recycling doesnot lead to an enrichment intrace elements that wouldchallenge environmental com-patibility. For that reason,surveillance of standard con-cretes as called for in the setof rules annexed to the WasteManagement and RecyclingAct is not necessary.


3

Effects on the product

3.7Concreterecycling

3.8

0 2 4 6 8 10

60

50

40

30

Recycling stages

Chromium content in concrete in ppm

Chromiumcontent in thecement: 25 ppm

in the aggregate:45 ppm

Chromiumcontent in thecement: 100 ppm

in the aggregate:45 ppm

Firingheatcapacity

Fig. 14uses the element chromiumas an example to show theinfluence of repeatedrecycling on the overallcontent in concrete(proportion of recycledaggregate in this case:50%). What is decisive isthe contents in the cementand in the natural aggre-gate used. It becomesevident that the traceelement content can bothincrease and decrease byre-use. In both cases thecontents obtained rangewithin the order of magni-tude of concrete madeusing natural aggregateexclusively.

43

The use of alternative materi-als in the clinker burningprocess considerably lessensthe impact to the environ-ment. Integrated assessmentsshow that this allows thereduction of energy consump-tion and thus CO2 emissions.In addition to that, the quanti-ty of waste is diminishedsignificantly.

Life cycle analyses serve toinvestigate the paths wastematerials follow in differentmethods of utilization anddisposal. All the constituentsteps are analysed, and theenvironmentally relevant inputand output variables are deter-mined. Initial integrated as-sessments are available forthe use of plastics and wasteoil.

■ Example plastics

Different alternatives for plas-tics utilization have beenstudied. If plastics are recy-cled into chemical products by recovering their materialcontent, their thermal contentcannot be recovered incement works. Standard fuelshave to be utilized instead. If plastics are used in theclinker burning process, thechemical products are madefrom the corresponding con-stituents of crude oil. If thetwo methods of recovery arecompared, it must be takeninto consideration that bothcement and the correspondingchemical goods need to beproduced (basket of goods).The life cycle analysis deter-mines the most ecologicalway of producing this basketof goods. There are twooptions: plastics can either be utilized in a cement plantor, as in the present example,be used in the production ofchemical goods.

The Figure below gives theresults of a comparative inte-grated balance based on theexamples of CO2 emissions,energy consumption, and haz-ardous waste. The comparisoncovers the use of plastics in acement works, in a wastedisposal plant, in a blastfur-nace and for single-materialcombustion of plastics. Meth-ods for raw material recoveryinclude hydrogenation and theconversion into gaseous orliquid synthesis products.

It becomes evident from theFigure that, as compared tolandfilling of the plastic mate-rials, which served as thereference scenario, the use of1 kg of plastics in a cementplant reduces (i.e. negativefigure) the emissions of green-house gases, such as CO2,by about 1kg (in CO2 equiva-lents). This value includes thesubstitution of coal.


Integratedassessment

3.9

Fig. 16Emissions and energyconsumption during theclinker burning processare largely independent of the type of fuel used. If waste is thereforedisposed of in plantsinstalled for that verypurpose, or reprocessedin special plants at highenergy expenditure,additional emissionsoccur, causing a deterio-ration in the life cycleanalysis of waste disposal.

Cement works

Waste incineration

Blastfurnace

Hydrogenation

Conversion

Single-materialcombustion

-1,5 -1 -0,5 0 0,5 1

Change in the potential environmental impactin kg CO2/kg plastics

-40 -35 -30 -25 -20 -15 -10 -5 0 5 10

Saving as a result of utilization of the energy content recoverable from resources

in MJ/kg plastics

-0.05 0 0.05 0.1 0.15 0.2

Change in the amount of hazardous waste produced in kg/kg plastics

-1.13

0.87

-0.35

-0.21

-0.18

-0.29

-28.1

-15.9

-29.3

-25.9

-28.6

-26.4

0

0.029

0

0

0

0.141

Pure waste incineration emits0.87 kg CO2, as compared tolandfilling of the plastic mate-rials. Single-material combus-tion and methods focused onmaterial recovery and utiliza-tion in blastfurnaces only mar-ginally reduce CO2 emissions.

Single-material combustionoutperforms pure waste incin-eration, reducing CO2 emis-sions by 0.29 kg CO2 per kg ofplastics incinerated. The ex-ample of CO2 emissions under-scores that plastics utilizationin cement plants is clearlysuperior to the other methods.

In terms of the quantity ofwaste produced and energyconsumed, the cement pro-duction process also outper-forms all the other methods bycomparison. It utilizes plasticwaste to the extent of 100 percent. Thus, municipal waste

no longer needs to be dis-posed of. Moreover, no haz-ardous waste is produced that would have to be tippedat great expense, as is the case with waste incinerationplants.

CO2SO2NOx

CO2SO2NOx

CO2SO2NOx+

Waste WasteFossil Fuels Fossil Fuels

Waste incineration plant+

cement works

Recovery of thermaland material contents

in a cement works

Fig. 15Integrated assessment of plastic use: Change in potential environmental impact for various methods of recovery

45

■ Example waste oil

Waste oil is either utilizedenergetically in cementworks, or recycled into petro-leum products in waste oilrefineries. Integrated assess-ment takes into account thatit thus substitutes for coal asa fuel in cement works. Recy-cled waste oil must complywith the specifications applic-able for high-grade engine oil.

The Figure depicts the use ofwaste oil as an energy medi-um in cement plants in com-parison to the reprocessing ofwaste oil into base oil for lubri-cant production. This balancetakes the entire previous his-tory into account, thus creat-ing the basis for integratedassessment. From the exam-ples of energy consumptionand greenhouse gas emissiongiven it becomes evident thatwaste oil use in cement plantsclearly outperforms reprocess-ing.


3

Integratedassessment

3.9

C O A L

Waste oil into REPROCESSING PROCEDURES Waste oil into CEMENT PLANTS

WASTE OIL WASTE OIL

C R U D E O I L

R E S U L T

plants and reprocessing waste oil into lubrication oil products waste oil)

Energy used for obtaining and processing fuel for cement (MJ/t

M J p e r t w a s t e o i l

plants and repocessing waste oil into lubrication fuel for oil productswaste oil)

Greenhouse gas emission from obtaining and processing fuel for cement (kg CO2/t

Calorific value Calorific value+ base oil+ flux oil+ lubricating oil

Crude oil: extraction and transport 1434Primary refining 2676Waste oil in cement plants 17Total 4121

Coal: mining and transport 551Waste oil refining 149Coal in cement plants 4023Total 4723

Crude oil: extraction and transport 124Primary refining 246Waste oil in cement plants 2536Total 2905

+ base oil+ flux oil+ lubricating oil

8000

6000

4000

2000

0

40005000

3000

2000

10000

K g C O2 p e r t w a s t e o i l

ECOBA LANCE

Coal: mining and transport 4300Waste oil refining 2115Coal in cement plants 462Total 6877

47

Copyright:

Verein Deutscher Zementwerke e.V.

All rights reserved

Summary of the '93 to '96 activity

report of the Research Institute of

the Cement Industry, April 1996,

and of the status reports (partly

unpublished) of the VDZ/BDZ Com-

mission „Umweltverträgliche Verwer-

tung von Sekundärstoffen“, the

„Umweltverträglichkeit zementge-

bundener Baustoffe“ working group,

and the „Recyclingstoffe“ working

group.

Published by the German Cement

Works' Association e.V.

Research Institute of the

Cement Industry

Tannenstraße 2 - 4

D-40410 Düsseldorf

Conception and design:

NFP/Werner Fuchs, Creative

Consultant, Stresemannstr. 21,

40210 Düsseldorf

Composition/Production:

EGG Glake Design & Schrift/ART,

Düsseldorf

Imprint

VDZ-Concrete.pdf

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