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Life-cycle analysisCarbon emissions

antbalf ce

the thover tl clima

world carbon emission of 0.81 kg CO2 per kg cement produced.On average, approximately 1 t of concrete is produced each year

for every human being in the world [19]. Therefore, concrete(i.e. cement) is one of the Worlds most signicant manufacturedmaterials. Because of its abundance in the world market, un-derstanding the environmental implications of concrete and

milled into a ne powder before entering a preheater and even-tually a large rotary kiln where materials reach temperaturesgreater than 1400 C [17]. The clinker or kiln product is cooled andthe excess heat is typically routed back to the preheater units. Priorto packaging, gypsum is added to the clinker to regulate the settingtime. The end product is a very ne-grainedmixture (90%y 10 mm)known as portland cement [17]. Fig. 1 provides a process owdiagram of the general cement manufacturing process and theassociated inputs and emissions during various steps of the

Contents lists availab

Journal of Clean

.e ls

ARTICLE IN PRESS

Journal of Cleaner Production xxx (2008) 18* Corresponding author. Tel.: 1 906 487 1756; fax: 1 906 487 3371.carbon dioxide (CO2) in the atmosphere is approaching 380 ppm[8,29]. Without drastic market, technological, and societal changesCO2 concentrations are projected to increase to over 800 ppm bythe end of the century [8]. Approximately 5% of global carbonemissions originate from the manufacturing of cement. The calci-nation process (driving off CO2 from CaCO3 to form CaO) accountsfor roughly half of the CO2 emitted, while the remaining carbonresults from energy usage during the production process [13,34].According to the International Energy Agencys (IEA) GreenhouseGas R&D Programme [13], cement production generates an average

optimize the manufacturing process by reducing adverse environ-mental impacts.

2. Background

2.1. Cement manufacturing and examined processes

Traditional portland cement is composed primarily of calciumsilicate minerals (Table 1). The raw materials are quarried or minedand transferred to the manufacturing facility to be crushed and1. Introduction

Increased public awareness ofwarming has led to greater concerngenic carbon emissions on the globaE-mail address: [email protected] (D.N. Huntz

0959-6526/$ see front matter 2008 Elsevier Ltd.doi:10.1016/j.jclepro.2008.04.007

Please cite this article in press as: Deborahcomparing the traditional process with alte(1) the production of traditional portland cement, (2) blended cement (natural pozzolans), (3) cementwhere 100% of waste cement kiln dust is recycled into the kiln process, and (4) portland cement pro-duced when cement kiln dust (CKD) is used to sequester a portion of the process related CO2 emissions.To reduce uncertainty, this manuscript presents a cradle-to-gate life-cycle assessment of several cementproducts. Analysis using SimaPro 6.0 software shows that blended cements provide the greatest envi-ronmental savings followed by utilization of CKD for sequestration. The recycling of CKD was found tohave little environmental savings over the traditional process.

2008 Elsevier Ltd. All rights reserved.

reats posed by globalhe impact of anthropo-te. The current level of

cement manufacturing is becoming increasingly important[19,20,35,36,38]. For globally signicant products such as cement,environmental life-cycle assessment (LCA) is a valuable tool forimproving our understanding of the environmental hazards posedby a products life stages. In addition, it allows cement producers toKeywords:Cement manufacturingthis paper, LCA is used to evaluate the environmental impact of four cement manufacturing processes:addition to the generation of CO2 the cement manufacturing process produces millions of tons of thewaste product cement kiln dust (CKD) each year contributing to respiratory and pollution health risks. InA life-cycle assessment of portland cemthe traditional process with alternative

Deborah N. Huntzinger a,*, Thomas D. Eatmon b

aDepartment of Civil and Environmental Engineering, University of Michigan, 1351 BeabNelson Mandela School of Public Policy, Southern University, Baton Rouge, LA 70813,

a r t i c l e i n f o

Article history:Received 26 October 2006Accepted 13 April 2008Available online xxx

a b s t r a c t

Concern over the impact ofyears due to growth in glofrom the manufacturing o

journal homepage: wwwinger).

All rights reserved.

N. Huntzinger, Thomas D. Ernative technologies, J Cleanhropogenic carbon emissions on the global climate has increased in recentwarming awareness. Approximately 5% of global CO2 emissions originatement, the third largest source of carbon emission in the United States. Int manufacturing: comparingchnologies

nue, Ann Arbor, MI 48109, United States

le at ScienceDirect

er Production

evier .com/locate/ jc leproproduction process.

atmon, A life-cycle assessment of portland cement manufacturing:Prod (2008), doi:10.1016/j.jclepro.2008.04.007

Within the preheater and kiln systems, particulate controldevices are used to capture ne particulates of unburned andpartially burned raw material that become entrained in thecombustion gases. This collected particulate matter is referred to ascement kiln dust (CKD). A large portion of CKD is disposed of inlandlls or stored in stockpiles on site. Van Oss and Padovani [38]estimate that CKD generation rates approximate 1520% (by mass)of clinker production. Most plants in the United States producebetween 0.2 and 2 Mt of clinker each year [38,39]. Somemanufacturing facilities recycle all or a portion of their CKD into theraw material line entering the kiln. However the degree to whichCKD can be recycled depends on its composition (trace metal andcontaminants) and regional alkali standards (i.e. potential for alkalisilica reactions (ASR) with aggregates), which varies widely withinand between plants [1,12,38,39].

According to the EPA [30,32,33] CKD is a potential hazardouswaste, in part because of the caustic nature and its potential to bea skin, eye, and respiratory irritant. In addition, contaminants fromthe raw materials and fuels tend to concentrate in CKD [38]. Boththe environmental and health risks associated with CKD canpotentially be reduced by reaction with CO through a process

reduces health risks and the generation of harmful leachate (e.g.,[3,6,14]). Because of its close composition to the nished product(cement), disposal of CKD in landlls represents a loss in potentialrevenue.

The amount of clinker needed to produce a given amount ofcement can be reduced by the use of supplementary cementitiousmaterials such as coal y ash, slag, and natural pozzolans (e.g., ricehusk ash and volcanic ashes; [22]). The addition of these materialsinto concrete not only reduces the amount of material landlled (incase of industrials byproducts), but also reduces the amount ofclinker required per ton of cement produced. Therefore cementsubstitutes may offer reduction in environmental impacts andmaterial costs of construction. The use of natural pozzolans couldsave contractors up to 25% per bag of cement (if the cement isblended and concrete mixed onsite), which can provide aneconomic benet to the building of new infrastructure [21]. Indeveloping countries such as the Philippines, where previousresearch has mapped natural pozzolans with socioeconomic andindustrial indicators [10], pozzolanic (blended) cement could be animportant technology for sustainable development.

Based on work done by Dodson [5], Lippiatt and Ahmad [19],Mihelcic et al. [21], and summarized in Helmuth [11] the strength,durability, and life of certain blended cements such as thoseutilizing industrial or natural pozzolans is equivalent to traditionalgeneral use portland cements within a substitution range of2560%. As the extent of substitution (pozzolan for portlandcement) increases, differences emerge in the strength anddurability characteristics of the blended versus traditional cements.

sio

ion

ry a

Ble

E

Table 1Rawmaterial composition of clinker, the primary component of portland cement [7]

Raw materials Sources Mass percent

Lime Limestone, shells, chalk 6067Silica Sand, y ash 1725Alumina Clay, shale, y ash 28Iron oxide Iron ore 06

D.N. Huntzinger, T.D. Eatmon / Journal of Cleaner Production xxx (2008) 182

ARTICLE IN PRESS2

called mineral carbonation. As observed in the carbonation of otherindustrial wastes, sequestering of carbon in CKD may yield addi-tional benets by stabilizing the waste (reducing the pH) which

Quarrying RawMaterials

1 E

Processing Rawmaterials (crushing)

1

1

2

E

H

Particulate Emis

Gaseous EmissEnergy

Heat

Raw materialPreparation(grinding)

1

D

EEPackagingShipping

11

EE

Fig. 1. Process ow diagram for the cement manufacturing process, showing energy and he[31]). Note that emissions at the preheating and kiln stages include both fugitive emission

Please cite this article in press as: Deborah N. Huntzinger, Thomas D. Ecomparing the traditional process with alternative technologies, J Clean2.2. Life cycle assessment

Life cycle assessment (LCA) is a method of evaluation used toassess the environmental impacts of technologies from cradle to

2

n

Mixingnd

nding

1

Preheater RotaryKiln

11

2

ClinkerCooler

FinishGrinding

ProductStorage

21

1Gypsum

H

H

Eat consumption or inputs, as well as gaseous and particulate emissions (adapted froms and cement kiln dust or those particulates captured with controlled devices.


al of

ARTICLE IN PRESSgrave and may be performed on both products and processes.There are several benets of LCA including the ability to evaluatethe material and energy efciency of a system, identifying pollutionshifts between operations, and providing benchmarks forimprovement [23,26]. The production of cement involves theconsumption of large quantities of raw materials, energy, and heat.Cement production also results in the release of a signicantamount of solid waste materials and gaseous emissions. Themanufacturing process is very complex, involving a large number ofmaterials (with varying material properties), pyroprocessing tech-niques (e.g., wet and dry kiln, preheating, recirculation; [35,36]),and fuel sources (e.g., coal, fuel oil, natural gas, tires, hazardouswastes, petroleum coke [35,36]). Therefore inventory analyses andcomplete LCAs can be quite complicated. The Portland CementAssociation (PCA) has conducted life-cycle assessments of productssuch as insulated foam concrete forms and blocks, comparing theassociated environmental impact of the concrete forms to thematerials used in traditional wood frame construction (PCA web-site2 [25]). Although these types of analysis can be useful whencomparing different construction materials (e.g., concrete formsversus wood frame), they do not provide information for reducingthe environmental impact of cement manufacturing itself. Lippiattand Ahmad [19] conducted a combined environmental andeconomic life-cycle assessment of concrete using a Building forEnvironmental and Economic Sustainability (BEES) approach. Intheir analysis they examined the environmental life-cycle andeconomic performance data of ve (5) different concrete products(100% portland cement, 20% limestone cement, Lafarge silica fume,Lafarge NewChem (50%), and 35% y ash cement). The functionalunit for their analysis was a concrete slab with a thickness requiredto create a 25-ft span with a compressive strength of 21 MPa(life 50 years). Their results indicate that while the limestoneblend concrete examined might be more environmentally friendlyon a mass by mass basis, more of the blended limestone cementwas required to make an equivalent strength slab. The increasedmass requirements of the limestone blend resulted in an overallperformance score equal to the traditional or generic portlandcement concrete. The other blended concretes examined hadequivalent strength characteristics as generic concrete and there-fore earned better environmental and economic performancescores than the traditional concrete product. In another study,Masanet et al. [20] examined the life-cycle emissions of cement andconcrete manufacturing by comparing the total estimated green-house gas (GHG) emissions from the manufacturing, use andend-of-life stages to the annual operation of an equivalent numberof automobiles. Their results indicate that the most signicantenvironmental burden and GHG emissions result from themanufacturing of cement (comparing complete life cycle of con-crete). Their study also suggest that the combined use of wastefuels, blended cements and the implementation of improvedenergy efcient technology can reduce GHG emissions fromcement manufacturing by up to 11% (in California). Unlike the studyby Lippiatt and Ahmad [19], Masanet et al. [20] estimate that theaddition of limestone to portland cement (during the production ofconcrete) can reduce GHG emissions by up to 4%. Improvements tothe transportation and end-of-life processes had relatively littleimpact on the overall GHG emissions of the cement and concretemanufacturing processes (less than 2% combined).

While several life-cycle assessment studies have been con-ducted to examine the performance of different concrete products(e.g., [19,20]), the life span, performance, and strength of theseproducts greatly depend on their applications and end-uses.Because of thewide range of strength requirements (early and late),setting and curing times, and concrete applications (e.g., volume

D.N. Huntzinger, T.D. Eatmon / Journrequirements, steel reinforcement requirements), comparativeLCA between different concrete products is difcult and the

Please cite this article in press as: Deborah N. Huntzinger, Thomas D. Ecomparing the traditional process with alternative technologies, J Clean3.1. Scope of the study

The scope of the project focuses on the rawmaterial acquisition,processing, and product manufacturing stages (Fig. 1). Each of theproducts examined has a life cycle, beginning with raw materialextraction to the packaging and shipment of the nished product.Complete life-cycle assessments also include the use and disposalstages of products. However, since the four products outlined abovehave relatively equal use and disposal impacts, these stages of theLCA are not examined in this study.

The overall scope of this study includes the stages outlined inFigs. 13. Several of the processing stages have been combined tosimplify the analysis and the calculation of inventory elements(refer to Life-Cycle Inventory section). The functional unit of analysisin this study is the production of the equivalent of 20 bags (each bagw100 lb) of general use Class I portland cement. Two hundred bagsis approximately 1 t of concrete, which is a convenient quantityoften used in reporting of energy andmaterial consumption as wellas emissions.

3.2. Life cycle inventory

The United States cement manufacturing industry producesapproximately 79,500 kMt of clinker each year. On average 1.58 t ofraw materials are required to produce 0.95 t of clinker or 1.0 t ofnished cement (refer to Fig. 2 for a material ow diagram of themanufacturing process). Non-fuel rawmaterial consumption can bebroken down into ve major components that provide: (1) calciumoxides, (2) aluminum oxides, (3) silica, (4) ferrous oxides, and (5)calcium sulfate [35,36]. In this assessment the raw materials andcorresponding quantities outlined in Fig. 2 are assumed to provideextrapolation of these results to a variety of application types islimited. Therefore in an attempt to reduce uncertainty this studypresents a cradle-to-gate life-cycle assessment of several cementproducts. Since cement manufacturing is the most energy andemission intensive process in the production of concrete, sucha reduced scope analysis is reasonable.

The goal of the study is to determine the environmentalimpacts, specically global warming potential, from the productionof approximately 1 t of cement by examining four products/processes:

(1) Traditional portland cement;(2) Blended cement (natural pozzolans, y ash);(3) Portland cement when a portion of the process related

emissions are captured back using sequestration in wastematerials (CKD); and

(4) Portland cement when CKD is recycled back into the kiln.

3. Methods

The LCA methodology used in this study follows the stagesoutlined by International Organization for Standards (ISO) 14040[17], as well as those described by Allen and Shonnard [2], Owens[23], Curran [4], and Hunt et al. [15]. The four major stages of theLCA applied in this study include:

(1) Determination of the assessment scope and boundaries;(2) Selection of inventory of outputs and inputs;(3) Assessment of environmental impact data compiled in the

inventory; and(4) Interpretation of results and suggestions for improvement.

Cleaner Production xxx (2008) 18 3the necessary chemical balance for the kiln feed. Figs. 13 and Table2 show the material acquisition, processing, and manufacturing


gg

yps

g

hngng

al of

ARTICLE IN PRESSTraditional Portland Cement

Blended Cement

RawMaterial

ExtractionKiln

FinishGrindinBlendin

GCementKiln Dust

RawMaterial

Extraction Kiln

FinishGrindinBlending

CementKiln Dust

Portland Cement: Recycling

RawMaterial

ExtractionKiln

FinisGrindiBlendi

CementKiln Dust

D.N. Huntzinger, T.D. Eatmon / Journ4steps considered alongwith the associated life-cycle inventory (LCI)data for the four cement manufacturing processes/products ofinterest. The life-cycle assessment software SimaPro 6.0was used toevaluate the environmental impacts of inventory elements and tocreate product assemblies and life cycles for the four different ce-ment products. Inventory data for rawmaterial acquisition (miningof limestone, sand, iron ore, and clay), along with electricity pro-duction and heat generation by fuel type for the various processingsteps were obtained from the SimaPro libraries and databases. Al-though SimaPro has several eco-proles for portland cement whichaccount for all cradle-to-gate impacts of cement manufacturing,a separate prole was created in order to compare the alternatemanufacturing processes with a higher level of precision.

3.2.1. Energy and heat input dataThere is a wide range of energy related information available in

SimaPro. A majority of cement kiln operations in the United Statesare powered (heated) by coal, coal and petroleum coke, and wastefuels/materials [18,3537,38]. However the specic blend of fuelsources depends heavily on the manufacturing facility and caninclude a unique ratio of fuel types including natural gas, fuel oil,tires, hazardous materials [35,36,38], as well as the morecommonly used fuels listed above (refer to Table 3). Based onrelative consumption (by fuel type) and information availability inSimaPro, threemajor fuel sources were selected for inclusion in thisstudy along with an assumed consumption by mass percent: coal(70%), fuel oil (15%), and natural gas (15%). In addition, published

Portland Cement: Sequester CO2 in CKD

RawMaterial

ExtractionKiln

FinishGrindinBlendin

CementKiln Dust

CO2St

Fig. 2. Scope of comparative LCA for cement manufacturing process. T

Please cite this article in press as: Deborah N. Huntzinger, Thomas D. Ecomparing the traditional process with alternative technologies, J CleanGypsum

Packagingand

Shipping

um Pozzolan

Packagingand

Shipping

Packagingand

Shipping

GypsumUse Disposal

Cleaner Production xxx (2008) 18values for energy usage by process step [37] were applied to theselected fuel mixture to calculate the energy and electricityallocations for each manufacturing step (Table 2).

3.2.2. Blended cement dataIn this study pozzolans are generically addressed and can refer

to natural pozzolans such as volcanic ash or rice husk ash or toindustrial wastes such as coal y ash. Since these materials areeither naturally occurring or an existing byproduct (i.e. waste) ofanother manufacturing process, they are not given an environ-mental benet or penalty in this study. In other words they areconsidered benign inputs that have no associated environmentalimpact. This is a reasonable assumption since these products (e.g.,y ash, volcanic ash, rice husk ash) would be produced (naturally orotherwise) regardless of their inclusion in cement products. A moredetailed study might include the environmental impact of acquir-ing the material (collection procedures, transportation, etc.);however such steps are most likely insignicant compared to thecement manufacturing steps considered in this study. In order toensure an equivalent performing cement product (i.e. strength,durability, life) a substitution percentage of 25% (by mass) isassumed. Therefore, the amount of clinker need to produce 1 t ofcement is reduced from 0.95 t to 0.71 t (Table 2).

3.2.3. Carbon sequestration in cement kiln dust dataBased on stoichiometry and material composition, CKD gener-

ated within the U.S. has the potential to capture up to

gg

Packagingand

Shipping

GypsumabilizedWaste

he dashed line signies the boundaries of the system examined.


Limestone(CaO)*

1.41 Tons

Sand(SiO2)*

0.034 Tons 0.015 Tons

Clay(Al2O3)*

0.139 Tons

Blended Mix1.598 Tons

Clinker0.95 Tons

Materials and Products

PortlandCement1.0 Tons

Gypsum(CaSO4)*0.05 Tons

*Major component/constituent of the raw ma

Finish Grindingand Mixing

Calcination(Pyroprocessing)

Process

Fig. 3. Material ow diagram for the production of 1 t o

D.N. Huntzinger, T.D. Eatmon / Journal of Cleaner Production xxx (2008) 18 5

ARTICLE IN PRESSapproximately 0.4 t of CO2 per ton of CKD [16]. In recent batch andcolumn studies, CKD readily sequesters CO2 at ambient tempera-tures and pressures, with greater than 80% of the theoreticalcapacity for carbonation being achieved without amendments ormodication to the waste [16]. As previously stated, the carbonintensity of cement manufacturing is approximately 1 t of CO2 perton of clinker produced [13,35,36,38, 20]. For this analysis, it isassumed that CKD can capture 0.4 t of CO2 per ton of CKD [16]. If theaverage rate of CKD production is 0.15 t CKD per ton of clinker, andit is assumed that 100% of the waste is utilized for sequestration,then CKD has the potential to capture approximately 0.06 t of CO2for each functional unit (1 t of nished product) in this study.Table 2Energy and emission inventory elements for the traditional cement manufacturingprocess

Units Input oremission

Crushing, grinding, and blendingEnergy (electricity)

Coal GJ 0.224Fuel oil GJ 0.048Natural gas GJ 0.048

EmissionsParticulate matter lbs 0.011

Preheating and kilnEnergy (heat)


EmissionsParticulate matter kg 0.02Carbon dioxide (process related) ton 0.51Cement kiln dust ton 0.10

Finish grinding and blendingEnergy (electricity)


EmissionsParticulate matter lbs 0.012

The energy inputs and emissions are divided among the three major processingsteps.

Please cite this article in press as: Deborah N. Huntzinger, Thomas D. Ecomparing the traditional process with alternative technologies, J Clean3.2.4. Recycling cement kiln dust dataA majority of cement manufacturing facilities recycle a portion

of their cement kiln dust [35,36,38,38]. However, as mentionedabove, the degree to which CKD is recycled is limited by thechemical composition of the waste. For comparison purposes thislimitation is overlooked in this study and 100% recycling of CKD isassumed. Therefore, all CKD generated (approximately 0.15 t perton of clinker) is recycled into the kiln feed, reducing the amount ofraw materials required. The CKD has a comparable composition toclinker. Therefore, all input amounts of raw material weredecreased proportionally (each by 3.75%) to account for theaddition of 0.15 t of CKD in the raw material feed.

terialf traditional portland cement (adapted from [28]).Iron Ore(Fe2O3)*

Rawmaterial

Acquisition

CrushingGrinding and

Blending3.2.5. Packaging and transportation dataInformation on the environmental impact from cement pack-

aging and transportation steps (on a mass basis) was available inSimaPro and therefore utilized in this assessment.

4. Results

4.1. Life cycle impact assessment

The environmental impacts of cement manufacturing can belocal, regional, or global in scale. Local effects include noise, airquality, and natural disturbance (e.g., change in landscape, impactsto local ecosystem) of mining raw materials such as limestone, iron

Table 3Energy consumption by fuel type in 2000 by the United States cement industry(from [18])

Fuel consumption Amount/year Percentage of total

Coal (kMt*) 10,095 67Coke (from coal) (kMt) 442 3Petroleum coke (kMt) 1351 11Fuel oil (ML**) 124 1Natural gas (Mm3) 338 3Tires (kMt) 374 3Other solid wastes (kMt) 1016 4Liquid waste (ML) 929 8

* kMt: thousand metric tons** Ml: million liters


ore, and clay. Emissions such as sulfur dioxide (SO2) and nitrogenoxides (NOx) contribute to acid rain on a regional scale. Carbonemissions originating from the calcining process and combustion offossil fuels (e.g., coal, natural gas, fuel oil) contribute to globalclimate change. The focus of this analysis is global environmentalimpacts, particularly global warming, and how alternative cementmixes and or processing technologies impact the overall globalwarming potential of cement production.

The Eco-indicator95 in SimaPro was used to assess the envi-ronmental impact of the four cement products/processes. Theproduction of clinker (pyroprocessing step) is the most energyintensive and emission intensive process in the manufacturing ofcement (Fig. 4), accounting for more than 8085% of the overallenvironmental impact score. In addition, coal and fuel oil accountfor a majority of the fuel related environmental impacts.

4.2. Classication and characterization

The environmental impact score by impact categories for thefour different cement products is shown below in Table 4. Ofprimary interest is the global warming potential of the traditionalcement product versus the three (3) process/product alternatives.The main greenhouse gas generated from the manufacturingcement is carbon dioxide (CO2). As mentioned previously,approximately half of the carbon released during the manufactur-ing of cement originates from the calcining process (driving off CO2

be expected. As discussed above the kiln or pyroprocessing step isthe most energy intensive stage of the manufacturing process andtherefore has the most signicant environmental impact. Thisresults both from the energy requirements to heat the kiln (toabove 1400 C) and the carbon released during the calcining step.By reducing the demand for clinker (through substitution), theenvironmental impacts of the end cement product are reducedproportionately. What is not considered, however, is the currentnational and global supply and demand relationship for cementproducts. High demand and low imports of cement have led toshortages within the United States [9]. Because many kilns areoperating at or above their effective capacity [26], the use ofblended cements is not likely to reduce net emissions of CO2.

nt

Gypsum0.087%

Heat Gas0.981%

Heat Oil14.7%

Limestone0.743%

Iron Ore0.354%

nt

Table 4Classication of process inputs and outputs for portland cement manufacturing intoenvironmental impact categories

Environmental impactcategory

Traditional Blended RecycledCKD

CO2sequestration

Greenhouse 0.088 0.069 0.088 0.084Acidication 0.043 0.034 0.043 0.043Eutrophication 0.006 0.005 0.006 0.006Heavy metals 0.204 0.161 0.204 0.204Carcinogens 0.003 0.003 0.002 0.003Winter smog 0.039 0.031 0.039 0.039Summer smog 0.009 0.007 0.009 0.009Energy resources 0.050 0.040 0.050 0.050


ARTICLE IN PRESSElectricity Coal,Gas, Oil 24.6%

Clinker84.4%

Heat Coal52.5%

Kiln Feed11.2%

Sand0.0145%

Clay0.0481%from limestone to form clinker) and the remaining carbon resultsfrom the burning of fuels to re the kiln and power the othermanufacturing processes. Of the cement products examined, theblended cement has the lowest global warming potential followedby cement produced when a portion of the process related carbonemissions are captured back by sequestration in CKD. The recyclingof CKD back into the kiln feed has little to no effect on reducingcarbon emissions compared to traditional cement.

In the context of this LCA, the improvement of blended cementson global warming potential over traditional portland cement is to

Portland Ceme99%

TraditionalPortland Ceme

100%Fig. 4. Life-cycle network (cradle to gate) showing the allocation of environment im

Please cite this article in press as: Deborah N. Huntzinger, Thomas D. Ecomparing the traditional process with alternative technologies, J CleanInstead, blended cements could allow for an increase in concreteproduction without any real modication to the existing amountsof clinker or portland cement produced within the United States.

The high impact scores for heavy metals and acidication arebelieved to be an artifact of the types of fuels selected and the in-complete representation of chemical interactions occurring in thekiln. For instance the actual sulfur dioxide emissions (a majorcontributor to acid rain) are much lower than those predicted inSimaPro. The clinker and CKD serve as partial scrubbers for SO2,removing a signicant portion of the sulfur oxides (greater than

Packing1.02%pact for each step of the traditional portland cement manufacturing process.


[26] Portland Cement Association (PCA). Cement shortage assessment, the moni-tor, ash report: breaking analysis of the economy, construction, and cement

al of

ARTICLE IN PRESS70%) in the combustion gases before they are released to the at-mosphere [38]. Actual fugitive emissions of SO2 equal approxi-mately 0.27 and 0.54 kg t1 of clinker produced [18],[38], comparedto the 4.86 kg estimated by SimaPro. As for heavy metals, the lead(Pb) content of the fuels selected (i.e. fuel oil) is assumed to be thesource of the high heavy metal impact score. While heavy metalsare released during the burning of many of the fuels typically usedduring the manufacturing of cement, as with sulfur dioxide (SO2),metals tend to concentrate in both the cement kiln dust (primarily)and clinker [38]. Since the focus of this assessment was globalwarming potential or greenhouse impact, corrections for heavymetals and sulfur dioxide were not made. Such corrections shouldbe included in the future for a more detailed analysis.

4.3. Valuation and recommendation

The impact scores from the categories outlined in Table 4 can beweighted and combined in a single environmental impact score.The blended cement process has approximately three quarters ofthe environmental impact as the other three processes, includingtraditional portland cement (Impact Score 2.0 pts). Based on thecomparison conducted in this LCA, such a reduction is expectedsince the addition of natural pozzolans to the nal mix replaces onequarter of the portland cement with an environmentally benignsubstitute (in terms of the system boundaries selected for thestudy). Substitution of pozzolans for clinker effectively reduces theheaviest environment impact process (the kiln or pyroprocessingstep) by 21.6%. However, as stated previously, this reductionassumes that clinker production would decrease due to the use ofpozzolanic cements. Based on the current demand for concrete andcement productions, such a reduction in cement production is notlikely and therefore no real net reduction in CO2 emissions will beachieved (given the current capacity of U.S. cement manufacturingfacilities).

The overall impact score for the cement manufacturingprocesses utilizing carbon sequestration in the waste product CKDis also lower than the traditional process. Although sequestrationonly captures approximately 7% of carbon emissions (translating toapproximately a 5% reduction in impact score over traditionalportland cement), unlike pozzolanic cements sequestration mayoffer a real reduction in net CO2 emissions. Recycling CKD into theprocess line has little to no effect on the overall impact score. Sincethe CKD is recycled into the kiln feed there is no real reduction inemissions or energy required in pyroprocessing, the mostenvironmentally damaging step.

5. Conclusion

Mitigating sources of anthropogenic carbon emission will helpto lower greenhouse gas levels globally. This study has addressedthe environmental impacts associated with three alternativetechnologies for the cementmanufacturing process. Environmentallife-cycle assessment (LCA) is a valuable tool for understanding theenvironmental hazards of products and for optimizing themanufacturing process to reduce adverse environmental impacts.However, there are limitations to the study and it should be notedthat data aggregation problems associated with secondaryinformation sources may cause variation in impact values forreplication studies. Much of the absolute environmental impactwill depend on energy and heat input amounts that vary betweenprocesses. Although the results of this LCA show that blendedcements provide the greatest environmental savings, the reductionin GHG potential in blended over traditional cements might be anillusion given the current demand for cement and concrete prod-

D.N. Huntzinger, T.D. Eatmon / Journucts. The utilization of CKD for sequestration, however, appears tooffer a means to reduce carbon emissions and a reduction in

Please cite this article in press as: Deborah N. Huntzinger, Thomas D. Ecomparing the traditional process with alternative technologies, J Cleanindustries May 13, 2004.[28] Rundman K. Chapter 5: material ow in industry, draft chapter for materials

book, Prentice Hall, never published, http://www.me.mtu.edu/wjwsuther/erdm/materials.pdf.

[29] Sabine CL, Feely RA, Gruber N, Key RM, Lee K, Bullister JL, et al. The oceaniccements environmental impact score of approximately 5% over thetraditional portland cement. The CKD recycle process was found tohave little environmental savings over the traditional process.

References

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ARTICLE IN PRESSPlease cite this article in press as: Deborah N. Huntzinger, Thomas D. Ecomparing the traditional process with alternative technologies, J Cleanatmon, A life-cycle assessment of portland cement manufacturing:Prod (2008), doi:10.1016/j.jclepro.2008.04.007

A life-cycle assessment of portland cement manufacturing: comparing the traditional process with alternative technologiesIntroductionBackgroundCement manufacturing and examined processesLife cycle assessment

MethodsScope of the studyLife cycle inventoryEnergy and heat input dataBlended cement dataCarbon sequestration in cement kiln dust dataRecycling cement kiln dust dataPackaging and transportation data

ResultsLife cycle impact assessmentClassification and characterizationValuation and recommendation

ConclusionReferences

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