3.1 alternative materials

Portland cement concrete is themost widely used material intoday's construction industrybecause of its structural proper-

ties, durability and economy. More than100Mt of cement is consumed annually inthe United States, and about 45Mt, inCentral America. This places the cementindustry among the largest process indus-tries in terms of the output.

There are several factors that make theindustry’s environmental impact quite sig-nificant (Tresouthick, Mishulovich, 1990).Considerable energy consumption and rawmaterial requirements of approximately1.6t per tonne of product are the most sig-nificant, along with the attendant CO2

emissions. Other emissions, such as sulphurand nitrogen oxides, particulates (dust)etc, are controllable within the regulationlimits but nevertheless not negligible.

The conservation efforts of the cementindustry have been directed toward the useof alternative fuels and raw materialsderived from industrial byproducts.Alternative developments have been con-cerned with the wider introduction of sup-plementary cementitious materials partiallyreplacing cement in concretes therebyreducing the cement consumption.

Portland cement compositionPortland cement is produced by pulverisingclinker consisting essentially of hydrauliccalcium silicates, usually containing one ormore of the forms of calcium sulphate asan addition (ASTM, 1995). Typical chemicalcomposition of portland cement is as fol-lows (Kosmatka, 1996):

SiO2 19 – 22 per centAl2O3 3.5 – 6.5 per centFe2O3 2.0 – 5.0 per centCaO 60 – 66 per centIt is interesting to note that the five

chemical elements comprising about 99 percent of the clinker chemical composition

(O, Si, Al, Fe, Ca) are the most abundantelements of the Earth’s crust. It means thatthe cement industry depends only on themost common mineral resources. Moreover,these elements are found in the materialstreams of other high-tonnage industries.

As we know, the principal crystallinephases of Portland cement clinker are: tri-calcium silicate (C3S), dicalcium silicate(C2S), tricalcium aluminate (C3A), andtetracalcium alumoferrite (C4AF). They arepresent in clinker in the following propor-tions:

C3S 37 – 68 per centC2S 9 - 32 per centC3A 4 – 12 per centC4AF 6 – 13 per centA source of calcium is represented in

the greatest proportion in the cement rawmix. Typically, it is one of calcareous rocks(limestone, chalk, etc). Calcium carbonateis the only calcium compound that can befound in natural deposits large enough toprovide cement plants with long-termresources. Not only does calcium carbonateemit CO2 during calcination, but this reac-tion requires about 1850kJ/kg (800Btu/lb)

from the combustion of fuel. This is whyeven a partial replacement of carbonateswith calcium-containing industrial byprod-ucts may be beneficial.

Alternative raw materialsThe clinker compounds are produced as aresult of chemical reactions during high-temperature treatment in a rotary kiln.Calculations known as mix design take intoaccount only the chemical (oxide) compo-sition of the ingredients, not their physicalor mineralogical form. Therefore, the rockssuch as limestone are usually chosen as theprincipal source of lime only because oftheir wide availability. Any other sourcescan be used in this capacity, provided theyhave a sufficiently high-calcium contentand are competitive with limestone interms of costs and sufficient supply.

Examples of such byproducts are metal-lurgical slags. Not unlike Portland cementclinker, slags are products of high tempera-ture processing of mixes containing naturalcalcium carbonates. Their further use,therefore, would not involve calcination,which consumes energy and generates CO2.Typical slag compositions (percentages) arepresented in Table 1 (Mishulovich, 1994).

Comparison of this data with thePortland cement compositions demon-strates that many slags are close tocements and therefore can be easily fittedinto cement raw mixes. Blast-furnace slagsare often added into raw mixes with othermix ingredients. Steel-making slags areeven closer chemically to cement clinkers.

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Alternativematerials

In the foreseeable future Portland cement or its derivatives will remain thebasis of structural concrete. Therefore, finding the ways of the energy andmaterial conservation in the cement industry attain the first-rate importance, both from the economic and ecological standpoints. In thispaper, presented at APCAC XIX Technical Conference, Alex Mishulovichlooks at alternative materials for the cement industry.

by Alex Mishulovich, Construction Technology Laboratories Inc

Table 1: composition of metallurgical slags

Blast furnace slag Steel slag Arc furnace slagSiO2 33-42 16-19 23-26Al2O3 10-16 2-3 7-9Fe2O3 0.5-2.0 10-23 2-7CaO 36-45 40-55 38-40MgO 3-12 6-15 11-13

Material p59 62 27/12/02 12:28 pm Paul Pauls Hard Drive:Desktop Folder:Paul s share file:ICR January 2003:Article or

However, steel slags often containa significant amount of finely dis-persed metallic iron, which makesgrinding more difficult. Thisobstacle has been overcome in apatented process by whichcoarsely crushed steel slag isadded to the kiln feed separatelyfrom the rest of raw meal (Young,1995).

Slags are an example ofbyproducts that can be utilisedwith major benefits for thecement industry. Another group ofwaste materials is not so muchessential for cement manufactur-ing, but their utilisation relieves the generating industriesfrom the expense of disposal.These byproducts contain some ofthe oxides that can be incorpo-rated in the clinker composition.Table 2 shows some of suchbyproducts tested by CTL in therecent years.

The significance of these materials inthe cement production process varieswidely depending on the chemical compo-sition. For example, alumina byproduct (1)contained a relatively high amount of sil-ica, which is required in some raw mixdesigns. This silica was present in chemi-cally reactive microcrystalline form, whichmade it especially valuable. The presenceof fluoride in the product gives it somemineralising, or catalytic, properties. As aresult, raw mixes containing this productcould be burned at much lower (by 50-80ºC) temperatures than conventionalmixes. Another alumina byproduct con-tained the three major oxides in the ratioclose to that in normal clinker. It neededonly a relatively small addition of lime-stone to form the required composition,with the attendant advantages.

Silica, alumina, and iron oxide are pre-sent in many byproducts that can beutilised in cement mix designs to balancethe mix composition. Addition of theseingredients in the range of 1-10 per centmay be both environmentally friendly andcost effective. Such additions containsometimes a considerable amount of car-bon or combustible organic matter thatcontribute to the general fuel input. Anexample is high-carbon fly ash, which wassuccessfully tested as an addition at one ofthe Illinois cement plants (Bhatty et al,1998). Another example is spent pot linergenerated in large amounts by the alu-minum industry. It contains carbon in theform of graphite and fluorides (Tresouthick,1986).

Organic substances, such as woodfibres and poly-hydrocarbon fillers, were

contained in paper sludge andfood containers, respectively.The mineral portion of thosematerials was readily accommo-dated by the mixes, whereas theenergy contribution by burningthe organic portion was consid-erable.It can be seen that the cementindustry offers an economicaland environmentally sound wayof waste management. However,certain restrictions apply to theuse of byproducts by the indus-try. First is the available amountof a byproduct. Even an averagecement plant uses daily about3500t of raw materials, and thewaste stream should be largeenough to be considered for util-isation. Its composition andother properties should be rea-sonably stable. The material cancontain small amounts of ele-ments that may be detrimental

to the product quality (Hewlett, 1998),and extensive testing is recommended priorto its introduction into the industry.Contamination by toxic or otherwise dele-terious substances always complicates theprocess. Finally, transportation costs canmake the use of certain products prohibi-tively expensive.

Waste-derived fuelsProduction of cement clinker in existingrotary kilns is an energy-intensive processrequiring 0.1 to 0.2t/t of fuel of the prod-uct. Fuel cost, being the largest itemamong all cement production costs, hasbeen the focus of attention of cement pro-ducers ever since the beginning of thecommercial cement production. In manycases, utilisation of combustible and oftentoxic waste is not only economicallyadvantageous, but in many cases presentsan environmentally sound alternative toincineration.

According to the 2000 reports, eightcement plants in the US used WDF as pri-mary fuels exclusively (two plants) or withother fuels (coal, coke, oil). In the sameperiod, 22 plants reported the use of WDFas alternate fuels, with 26 more plantsusing WDF as alternate mixed with otherfuels. In Canada, seven plants used WDF asalternative fuels. The quantity of WDFactually used is difficult to estimate.

Three groups of waste materials areused as kiln fuels:

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Table 2: examples of byproducts

SiO2 Al2O3 Fe2O3 CaO F Organic Heating valuematter BTU/lb

Deinking sludge 15 25 50 5000Shredder fines 22 6 12 7 12 2100Alumina byproduct (1) 44 12 18 13Alumina byproduct (2) 23 15 57Spent pot liner 11 10 60 7500Oily residue 21 19 9 28 86Coal fly ash 50 18 16 5 10 1200Food containers 30 52 7300Pigment byproduct 55 20

O

Si

Al

Fe

Ca

Others

O

Si

Al

Fe

Ca

Others

Elements in Earth crust

Elements in cement clinker


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1. liquid fuels (spent solvents, oils,etc)2. used automotive tyres3. other solid wastes.

Substitution of byproducts formore conventional fuels in thecement kiln may affect the productionprocess and the environmental perfor-mance. Since the flame temperatureneeds to be sufficient, it is desirableto use fuels whose adiabatic flametemperature (AFT) is high, and thosewhich generate of ‘black body radiat-ing’ species. On balance, higher car-bon fuels tend to give higher AFT val-ues. By contrast, higher hydrogen fuelstend to be volatile, with low carbon con-tents, and often are poor ‘black body radia-tors’. If we apply these considerations towaste fuel firing, several factors becomeapparent:1. High-hydrogen, high volatile fuels aresuitable for the front end of the kiln, inthe sintering zone. Very high moisture con-tents are deleterious to obtaining andmaintaining the required high flame tem-perature.2. Some carbon and/or ash is desirable inthe sintering zone fuel, to carry out thedesirable heat transfer via the ‘black bodyradiation’ mechanism.3. It is not desirable to fire low energyfuels in the sintering zone, as the heat willnot be transferred at a high enough tem-perature to ensure acceptable free limevalues in the clinker.

The balance of thermal processes in therest of the kiln require relatively low-potential heat. Many fuels not suitable foruse in the sintering zone are potentiallyvery useful when added to the calciningzone of the kiln. Here, the key issue isensuring complete burnout of the fuel.The firing of fuel in the calcining zone hasa number of advantages:

It reduces the thermal load in the sin-tering zone, potentially improving refrac-tory life.

The reduced thermal load results inreductions in NOx generated in the sinter-ing zone. The vast majority of dust emis-sions from cement kilns are related to rawmaterials, not fuels. For this reason, stackemissions of particulates are only veryslightly influenced by the use of wastefuels. Of all the regulated metals, onlylead, cadmium, thallium, and mercury arevolatile enough to be of any concern.However, the practical experience hasshown that lead is retained in the kiln

system to an extent of about 99.85 percent, and emissions from cement kilnsaveraged four per cent of the allowablelimit, with none greater than 22 per cent,even when maximising lead input.Cadmium is retained to an extent of about99.4 per cent, and the average emissionsduring these tests were again less thanfive per cent of the allowable, with nonegreater than 40 per cent of the allowable.Thallium emissions were at least threeorders of magnitude lower than any regula-tory limits, as were mercury emissions withone exception. In US waste fuels, thalliumcontents are often lower than in the fossilfuel they are replacing – the same is alsooften true of arsenic, mercury, beryllium,and silver.

Testing on dioxin and furan emissionshas generally shown that levels are funda-mentally unaffected by substitution ofwaste fuels for fossil fuels. Emissions ofSO2 are usually significantly reduced whenwaste is burned, unless the sulphur stemsprincipally from the kiln feed. NOx emis-sions are generally unchanged when fuel isburned only in the firing zone of the kiln.When mid-kiln firing is practiced, reducedNOx levels are often observed, since theoverall thermal load in the sintering zoneis reduced (Miller, 2002).

In conclusion, the use of waste fuelsas a partial replacement for fossil fuels incement manufacture has a number ofpotential benefits, economic and technical.From the technical perspective, productquality may be improved, sulphur andnitrogen oxide emissions may decrease,and added flexibility may permit increasedproduction capacity. Recognising therequirements of each zone of the kiln withrespect to the separate issues of heat andtemperature will aid in selecting the properfiring location. Product quality mayimprove when these decisions are made

properly, and in no case is there anemissions risk that threatens tocause any harm to human health andthe environment.

Supplementary cementitious materialsAn obvious way to reduce the powerconsumption and CO2 emission incement manufacturing is partialreplacement of clinker with hydrauli-cally reactive materials not requiringpyroprocessing. In the US practice,supplementary cementitious materialsare used in by cement manufacturers

in blended cements and by ready-mix producers in concretes.

In general, active mineral additions orsupplementary cementitious materials foruse in blended cements or in concretesmay be subdivided into categories of pozzolanic materials and latent hydrauliccements. Pozzolans are siliceous orsiliceous and aluminous materials which initself possess no cementitious value butwhich will chemically react with calciumhydroxide at ordinary temperatures to formcementitious compounds. Typical pozzolansare some natural rocks (volcanic ashes,tripoli) or industrial byproducts, the mostwidely used being fly ash.

Latent hydraulic cements act as cemen-titious materials if mixed with water and aminimal amount of certain activatingagents. Granulated blast-furnace slag is thewell-known example of a material that isactivated by alkali environment (Schroeder,1968). Slag is produced in the ratio of280-340kg/t of pig iron (USGS, 2002), andits utilisation serves both cement manufac-turing and steel making.

Hydraulic cements form the basis ofmodern concrete construction, andstrength is their most essential property.Ability of supplementary cementitiousmaterials to contribute toward the con-crete performance is based primarily ontheir chemical reactivity. Most pozzolanscontain amorphous or microcrystalline sil-ica either as a natural ingredient, or as aproduct of thermal decomposition ofhydrosilicates. Water-granulated moltenslag, as well as fuel ash, contains a highamount of a vitreous phase (mineral glass).The latter, being a supercooled liquid, isthermo-dynamically unstable and, there-fore, chemically reactive (Keil, 1958). Inmixes with Portland cement it is activatedby the solution of calcium hydroxide whichforms as a product of cement hydration.

0

500

1000

1500

2000

2500

3000

3500

4000

1 2 3

Cement Fly ash Slag Aggregates

Concrete ingredients


As a result, the properly formulatedand manufactured supplementary cementi-tious materials are fully competitive withPortland cement in terms of strength.Another important advantage of blendedcements containing slags and pozzolans istheir ability to improve concrete durability(Roy, 1985).

Aside from mechanical and physicalfactors leading to degradation of concretein service, concrete durability can beadversely affected by chemical reactionsinvolving a combination of some agentspresent in the environment or in concreteitself. The most common are reactionsbetween alkalies and concrete aggregates,and reactions of the hardened cementmatrix with sulphate-bearing water(Stark,1991). In both cases, reactive silicapresent in the cements or concretes containing supplementary cementitiousmaterials neutralises the aggressive ionscausing the concrete deterioration.

ConclusionThe cement industry by its sheer sizeinevitably impacts the environment byextraction of the natural raw materials andgenerating solid and gaseous emissions.

On the other hand, due to the chemicalcomposition of its product, the industry isable to absorb a substantial portion ofbyproducts generated by other industries.This not only minimises the industry’simpact on the environment but helps toreduce the waste streams of the industrialproduction in general.

ReferencesAmerican Society for Testing and Materials,1995, Standard Specification for PortlandCement C150 Bhatty, J, Detwiler, RJ, Miller, FM,Mishulovich, A, 1998, Use of High CarbonFly Ash as a Component of Raw Mix, Report#WO 566101, EPRI, Palo Alto, CAPeter C Hewlett (Ed), 1998, Lea's Chemistryof Cement and Concrete, Arnold Publishers,London, UKKeil, F and Locher, FW, 1958, HydräulischeEigenschaften von Glässen, Zement-Kalk-Gips, Nr 6, s 245-253Kosmatka, S, 1996, Portland Cement: Pastand Present Characteristics, ConcreteTechnology Today, Vol17, No2, pp1-4, PCA,Skokie, Illinois, USAMiller, FM, 2002, Private communicationMishulovich, A, 1994, Reduction of CO2

Emissions, Portland Cement Association,Skokie, Illinois, USARoy, Della M, Luke, K, Diamond, S, 1985,Characterisation of Fly Ash and its Reactionsin Concrete, Fly Ash and Coal Conversion By-Products: Characterisation, Utilisation,and Disposal – I, pp 3-20, MaterialsResearch Society, Pittsburgh, Pennsylvania,USASchroeder F, Blast Furnace Slags and SlagCements, The Fifth Int Symposium on theChemistry of Cements, Part 4, Tokyo, 1968.Stark D, 1991, Handbook for theIdentification of Alkali-Silica Reactivity inHighway Structures, SHRP-C-315, NationalResearch Council, Washington DC, USATresouthick SW, 1986, Spent Pot Liner as aSupplementary Fuel in Cement Production,23d International Cement Seminar,Chicago, Illinois, USATresouthick SW, Mishulovich A, 1990,Energy and Environmental Considerations for the Cement Industry, Proceedings of the Energy and the Environment in the21st Century Conference, Cambridge, MassYoung, Rom D, Method and Apparatus for Using Steel Slag in Cement ClinkerProduction, US Patent 5,421,880 (1995)._________________________________❒

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3.1 alternative materials

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