an interesting alternative raw material for a sustainable portland

Construction and Building Materials 36 (2012) 391–403

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Construction and Building Materials

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Waste fibrecement: An interesting alternative raw material for a sustainablePortland clinker production

Joris Schoon a,b, Luc Van der Heyden c, Pierre Eloy d, Eric M. Gaigneux d, Klaartje De Buysser e,Isabel Van Driessche e, Nele De Belie b,⇑a S.A. Sagrex N.V., Heidelberg Cement Benelux, Heidelberg Cement Group, Terhulpsesteenweg 185, B-1170 Brussels, Belgiumb Magnel Laboratory for Concrete Research, Department of Structural Engineering, Faculty of Engineering and Architecture, Ghent University, Technologiepark Zwijnaarde 904,B-9052 Ghent, Belgiumc Redco N.V., Etex Group, Kuiermansstraat 1, B-1880 Kapelle-op-den-Bos, Belgiumd Institute of Condensed Matter and Nanosciences (IMCN), Division MOlecules, Solids and reactiviTy (MOST), Université catholique de Louvain, Croix du Sud 2/17,1348 Louvain-la-Neuve, Belgiume Sol–gel Centre for Research on inorganic Powder and Thin films Synthesis (SCRiPTS), Department of Inorganic and Physical Chemistry, Faculty of Sciences, Ghent University,Krijgslaan 281-S3, B-9000 Gent, Belgium

h i g h l i g h t s

" Quantified inorganic CO2 emission reduction and decarbonation energy gain." No significant chemical and mineralogical influence on the final clinker." Introduction at a hot point in the process is a necessity." Quantified energy gain by degradation of organic compounds." Estimated energy consumption for the liberation of chemically bound H2O.

a r t i c l e i n f o

Article history:Received 31 January 2012Received in revised form 17 April 2012

Keywords:ClinkerFibrecementCO2

EnergyCementCellulosePolyvinyl alcoholPolypropylene

0950-0618/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.conbuildmat.2012.04.095

Abbreviations: Ant, Antoing; IC, Iron Carrier (Fe2ORaw Material; LSF, Lime Saturation Factor; ARM/CS, CLiqSimple, Liquid Simple; ARM/HD, autoclaved HighLOI, loss on ignition; ARM/MD, autoclaved Medium DLoam (SiO2-source); ARM/RS, Fibrecement Roofing SClinker Meal; Maa, Maastricht; Cl, Clinker; Ma, MarlCRM, Classic Raw Material; PL, Poor Limestone; DecaRef, Reference; FA, Fly Ash (Al2O3-source); RL, Rich LimSabulous Clay (SiO2-source); HCM, Hot Clinker Meal;limestone).⇑ Corresponding author. Tel.: +32 9 264 55 22; fax:

E-mail address: [email protected] (N. De Beli

a b s t r a c t

This paper aims to examine the use of fibrecement waste as an alternative raw material for Portland clin-ker kilns with enumeration of possible limitations. Numerical simulations were carried out to maximisethe use of fibrecement waste in clinker kilns taking into account the compositional variation of the waste,its behaviour within a clinker kiln, the impact on the energy consumption and finally the influence on thecarbon footprint calculated from the decarbonation. Furthermore, experimental clinkers were producedcorresponding with waste dosages that were esteemed as realistic by the numerical simulations. Theseclinkers were fully analysed and evaluated according to the above described evaluation concept.

� 2012 Elsevier Ltd. All rights reserved.

ll rights reserved.

3-source); ARM, Alternativeorrugated fibrecement Sheet;

Density fibrecement board;ensity fibrecement board; Lo,late; Lxh, Lixhe; CCM, Cold(specific type of limestone);rb E, Decarbonation Energy;estone; FC, Fibrecement; SC,Tu, Tuffeau (specific type of

+32 9 264 58 45.e).

1. Introduction

In June 2005, the World Business Council for Sustainable Devel-opment defined five Key Performance Indicators within the Ce-ment Sustainability Initiative [1]. These indicators cover varioussubjects such as energy reduction and an increasing alternativeraw material use as well as the reduction of CO2-emissions askey issues in the sustainable development of cement industry[2]. The high temperatures needed to obtain optimal reactivity inclinker kilns, are highly energy-consuming. In current processes,

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Table 1Average chemical analysis of the limestones of CBR Antoing, CBR Lixhe and ENCIMaastricht.

CRM (wt%) CRM/Ant/PL CRM/Ant/RL CRM/Lxh/Tu CRM/Maa/Ma

CaO 42.9 50.1 51.8 50.8SiO2 15.1 6.4 4.7 7.1Al2O3 2.2 0.9 0.4 0.8Fe2O3 0.9 0.4 0.3 0.4K2O 0.68 0.21 0.07 0.13Na2O 0.25 0.25 0.02 0.20SO3 0.90 0.57 0.09 0.21MgO 1.1 0.9 0.7 0.8Cl – – 0.011 –LOI 975 �C (O2) 35.04 40.18 42.03 40.18

392 J. Schoon et al. / Construction and Building Materials 36 (2012) 391–403

this energy is gained by burning both traditional and alternativehydrocarbon fuels [2]. In many countries, already a threshold ofmore than 50% replacement of traditional in favour of alternativefuels is already achieved. Despite the significant CO2-emissions re-lated to the combustion of these fuel materials, CO2 emissions aremainly generated by the use of limestone as the main raw materialfor clinker. Limestone, primarily consisting of CaCO3, will releaseCO2 during decarbonation, delivering CaO as main constituent ofalite (3CaO�SiO2), belite (2CaO�SiO2), aluminate (3CaO�Al2O3) andferrite (4CaO�Al2O3�Fe2O3) which are the major clinker phases [3].The use of alternative fuels based on recovered materials or non-marketable by-products is already well established in the cementindustry [4]. In contrast, the use of alternative raw materials basedon recovered materials as replacement for limestone is less fre-quently implemented. Nevertheless, the replacement of limestonecould reduce the CO2 emissions, by acting as non-carbonate CaOsource, minimise the effects of quarrying as well as improve theenvironmental impact by energy reduction. This publication docu-ments the effects of using recuperated fibrecement materials asalternative raw material for Portland clinker production with enu-meration of possible limitations. Therefore a strategy was chosento make the simulations and tests as realistic as possible by usingthree reference clinker factories considered to be representative forclinker factories worldwide. Taking into account that in Europefibrecement has been gradually introduced as an alternative forasbestos cement since the mid eighties of last century, it can be ex-pected, that the combined volumes of fibrecement demolitionwaste will start to increase as of 2020 to measurable volumes.Once a steady supply of fibrecement demolition waste will havecome on stream, some millions of tonnes per year could be ex-pected in Europe each year. By this investigation, it will be shownthat fibrecement materials are interesting candidates for recyclingand that their use as alternative raw material for clinker produc-tion should be investigated further at industrial scale. The expectedreduction of CO2 emissions as well as the potential energy gainsshould have a positive effect on the sustainability of clinkerproduction.

2. Materials and methods

2.1. Classic Raw Materials (CRMs)

As representative CRM, materials are selected that are used at a daily base in thethree reference clinker factories. These factories are CBR Antoing (CRM/Ant) andCBR Lixhe (CRM/Lxh) in Belgium and ENCI Maastricht (CRM/Maa) in the Nether-lands, all belonging to the Heidelberg Benelux group. From each of these three greyPortland clinker factories, four of their most important CRM were collected, morespecifically those that act as the main sources of the four critical metal oxides inclinker [5]: CaO, SiO2, Al2O3 and Fe2O3. CBR Antoing uses two kinds of limestones:Rich (CRM/Ant/RL) and Poor (CRM/Ant/PL). These act as CaO and SiO2 sources. CBRLixhe uses Tufa (CFM/Lxh/Tu) and Loam (CRM/Lxh/Lo), ENCI Maastricht a typicalMarl (CFM/Lxh/Ma) and sabulous clay (CRM/Lxh/SC) as CaO and SiO2 source respec-tively. All three factories use Fly Ash (CRM/Ant,Lxh,Maa/FA) as Al2O3 source and anartificially produced Fe2O3 source (CRM/Ant,Lxh,Maa/IC). All four of the CRM fromeach clinker factory were sampled at three different instances. To generate repre-sentative samples, the separate samples from each factory were homogenised.These representative samples were grinded for 2 min in a Siebtechnic disc mill toobtain sufficient fineness to perform analytical testing. The average chemical anal-ysis of the limestones, as measured by XRF is presented in Table 1.

2.2. Alternative Raw Material (ARM): fibrecement

Fibrecement is the generic term given to a wide variety of composite materialsconsisting of Portland cement, inert and/or reactive mineral fillers and a mixture ofseveral types of organic fibres [6,7]. Within the context of the present paper, onlyfibrecement products produced by Hatschek technology [8–10] are considered.Fig. 1 illustrates the sheet formation principle used in this technology. The productsharden by normal hydration of Portland cement at ambient pressure, hereafter indi-cated by ‘‘air-curing’’ or by means of hydrothermal reactions, further indicated by‘‘autoclaving’’. Except for crystalline portlandite and ettringite, the products formedby normal hydration mainly concern calcium silicate hydrates with predominantly

amorphous or cryptocrystalline nature [5]. In the case of autoclaving, the reactionsoccurring in an atmosphere of saturated steam at 7–10 bar also lead to calcium sil-icate hydrates but now it contains mainly crystalline phases next to some amor-phous or cryptocrystalline phases formed by the reaction of the available CaOand SiO2 [11,12]. Most commonly used CaO sources are Portland cement and hy-drated lime. Quartz is the preferred SiO2 source, but the silica present in the Port-land cement also participates in the reactions. The fibrecement materials used forthis investigation were taken from four different production lines, correspondingwith two roofing products, Corrugated Fibrecement Sheets (ARM/CSs) and Fibrece-ment Roofing Slates (ARM/RSs), and two façade products, Medium Density Fibrece-ment Boards (ARM/MDs) resp. High Density Fibrecement Boards (ARM/HDs). Theroofing products concern air-cured products whereas the façade products are auto-claved. The air-cured products have very similar formulations and contain syntheticreinforcement fibres (polyvinyl alcohol and polypropylene), process fibre (cellu-lose), Portland cement, pozzolanic and inert filler. The autoclaved products mainlycontain cellulose reinforcement and cellulose process fibres, Portland cement,quartz flour and some stabilizing agent. For each ARM source, material was recov-ered at 24 different points in time spread over 6 months after which they were 2 by2 homogenised. After crushing in a Retsch cutting mill those samples were groundfor 1 min in a Siebtechnic disc mill to obtain sufficient fineness. For each of the fourtypes of fibrecement product, the average chemical analysis determined by XRF(Section 2.3) of the 12 samples is presented in Table 2. The mutual relationship be-tween the CaO and the SiO2 content is graphically illustrated in Figs. 2 and 3 for theair-cured and autoclaved products respectively. Fig. 4 illustrates how the compoundcomposition of the roofing slates (air-cured) applied at the fibrecement plant fromwhich the samples for this study were taken, has evolved through the years sincethe change-over to non-asbestos products. Based on this figure, we can concludethat fibrecement product-selective recycling will generate an alternative raw mate-rial with a low chemical variation.

2.3. Testing of raw materials, clinker and cement properties

To prepare the different CCM compositions, all raw materials were crushed inadvance in a Siebtechnic Disc mill. The calculated dosage to achieve 500 g of CCMwere brought together in a vessel used for the analysis of the micro-Deval abrasionresistance. This procedure [13] was used to homogenise as good as possible the rawmeal before it was thermally treated in a kiln. Before sintering, the different CCMcompositions were first granulated on granulation plates (5 mm holes). Sinteringwas performed in an electric high temperature static kiln (Carbolite BLF1800) to1450 �C at a constant heating rate (10 �C/min). The Hot Clinker Meals (HCMs) weremaintained for 1 h at 1450 �C after which they were immediately air-cooled toroom temperature by open air to form the final clinker. XRF analyses were per-formed on a Philips PW2404 in accordance with EN 197-2 [14] to chemically char-acterise the classic (CRM) and Alternative Raw Materials (ARMs), all Cold ClinkerMeals (CCMs) and final clinkers. For the analysis of the total C and S, a LecoCS230 was used. A Netzsch STA 449F3 was used to evaluate by TGA/DTA measure-ment all the CRM and selected ARM as well as all the CCM compositions on theirthermal decomposition mechanism as also on the energy required during theirheating up to 1450 �C and were performed in an atmosphere of 5 vol% O2 and95 vol% N2 which corresponds with the typical vol% O2 in the atmosphere of a clin-ker kiln. A Mettler Toledo Star TGA/DTA was used to evaluate the same materials ina pure N2 gas atmosphere, as also to evaluate by TGA/DTA the cement pastes de-scribed in Section 3.2 in an atmosphere of 5 vol% O2 and 95 vol% N2. As heating rate,10 �C/min was chosen for both devices which gave the best resolution for evalua-tion of the graphs. XRD analyses were performed on a Bruker D8 ADVANCE witha PSD-type detector (LynxEye) refined by Rietveld method using a March Dollasecorrection model and simultaneous refinement of preferred orientation to mineral-ogically characterise all the final clinkers. An IKA Calorimeter system C7000 wasused to measure the gross calorific values of the CCM and the selected fibrecementARM in accordance with ISO 1928 [15]. A Mettler Toledo Star DSC was used todetermine the required energy for the liberation of chemically bound H2O on thecement pastes described in Section 3.2. Four pastes were made out of 450 g CEM I

Fig. 1. Hatscheck machine.

Table 2Average chemical analysis of the fibrecement materials.

ARM (wt%) Corrugatedfibrecementsheets

Fibrecementroofingslates

Autoclavedhigh densityfibrecementboards

Autoclavedmedium densityfibrecementboards

ARM/CS ARM/RS ARM/HD ARM/MD

CaO 51.24 52.14 28.55 25.01SiO2 18.85 18.42 44.2 51.06Al2O3 2.80 3.55 5.95 4.95Fe2O3 2.52 3.15 2.37 1.74K2O 0.37 0.28 0.22 0.15Na2O 0.20 0.17 0.06 0.12SO3 1.87 1.35 0.69 0.61MgO 1.38 1.57 0.59 0.71TiO2 0.25 0.43 0.34 0.38P2O5 0.44 0.31 0.16 0.18Cl 0.03 0.01 0.01 0.01LOI 975 �C (O2) 19.65 18.11 16.13 14.83Ctotal 3.94 4.10 3.66 3.44Stotal 0.86 0.66 0.33 0.31

Autoclaved Alternative Raw Material, L.O.I. exclusive

50515253545556575859606162

28 29 30 31 32 33 34 35 36 37CaO (m%)

SiO

2 (m

%)

Autoclaved Medium Density Fiber Cement BoardsAutoclaved High Density Fiber cement Boards

Fig. 3. CaO [wt%] in function of SiO2 [wt%] without Loss of ignition (950 �C) ofautoclaved fibrecement materials. The arrows mark the selected autoclavedfibrecement samples ARM/HD/S8 and ARM/MD/S5.

Air Cured Alternative Raw Material, L.O.I. exclusive

21

22

23

24

25

26

61 62 63 64 65 66CaO (m%)

SiO

2 (m

%)

Corrugated Fibre Cement SheetsFiber cement Roofing Slates

Fig. 2. CaO [wt%] in function of SiO2 [wt%] without Loss of Ignition (950 �C) of air-cured fibrecement materials. The arrow marks the selected air-cured fibrecementsample ARM/RS/S7.

J. Schoon et al. / Construction and Building Materials 36 (2012) 391–403 393

type cement and W/C ratios of 0.25, 0.30, and 0.35. To obtain the paste with thelowest W/C ratio (0.18), a paste with a W/C ratio of 0.21 was made, which was sub-sequently filtered off in view of obtaining a still lower W/C ratio. The pastes werestored for 28 days at 20 ± 1 �C hermetically wrapped in plastic and were afterwardsdried at 105 �C for the respective curing times that had been selected. For the anal-ysis of the formed volatiles after thermal degradation of the organic fibres in fibre-cement ARM, a Varian GC (3900) – MS (Saturn 2100T) was used and for the formedgases, a MS Balzers Quadstar 422.

3. Theory/calculation

3.1. Chemical and mineralogical limitations of each reference clinkerand clinker kiln

For each clinker factory, the chemical and mineralogical data ofthe clinkers produced in the first 6 months of 2011, were used todefine the reference clinker of each clinker factory (Table 3). Toprevent undesirable effects on both the clinker production processas well as on the clinker quality and to remain within the frame-work of the cement standards [14,16], chemical and mineralogicallimits specific for each clinker factory were defined and used toevaluate the feasibility of applying fibrecement as an alternative

0,00

10,00

20,00

30,00

40,00

50,00

60,00

70,00

80,00

90,00

100,00

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011Year

[wt%

]

OthersPigmentPPPVASilicaCelluloseFly AshChalkCement

Fig. 4. Evolution of the composition of air-cured fibrecement roofing slates produced in Eternit Kapelle o/d Bos.

Table 4Chemical and mineralogical limitations on the final clinker.

Clinker (wt%) Antoing Lixhe Maastricht

Cl x < 0.08 x < 0.08 x < 0.08SO3 x < 1.4 x < 1.2 x < 1.1Na2Oeq x < 1.2 x < 1.2 x < 1.2MgO x < 4.0 x < 4.0 x < 4.0DoS-level 80.0 < x < 120.0 80.0 < x < 120.0 80.0 < x < 120.0LSF_MgO 98.5 < x < 98.5 98.5 < x < 98.5 98.5 < x < 98.5C3A 7.4 < x < 7.4 6.7 < x < 6.7 7.3 < x < 7.3LiqSimple 19.2 < x < 19.2 22.7 < x < 22.7 23.0 < x < 23.0


raw material (Table 4). One of possible problems in a clinker kilnconcerns clogging of material on the sides of the kiln, by whichthe smooth operation of the clinker kiln can be disturbed. This ex-plains why the SO3, Na2Oeq and Cl contents of the clinkers pro-duced in the three reference clinker factories have to meet evenstricter limits (Table 4) than those required in order to be in linewith the cement standards [14,16]. SO3 [wt%] is the general formto describe all SO2�

4 [wt%] present in clinker or cement. SO2�4 can

occur mineralogically in Portland clinker in different forms withtheir own specific solubility and will be particularly present inthe alite (C3S) and belite (C2S) phases [17]. In an alkali-rich envi-ronment, mainly installed by Na and K from the raw materials,these sulphates can be found particularly in the form of alkali sul-phates. If no sufficient SO3 content is available to combine all Naand K into sulphates, the majority of the free alkali will be builtinto the belite phase but also into the Aluminate phase mainly asNa favoring the orthorhombic crystal structure [5]. The free alkaliwill increase viscosity of the melt and decrease the formation ofalite [5]. Also an excess of alkali emitted to the gas phase of a clin-ker kiln by its high volatility [18], will make that clogging phenom-ena appear faster. However, when the amount of SO3 stronglyexceeds the amount that can be bound by the alkali, CaSO4 willbe formed, whereas in the case of a moderate excess of SO3 severalother forms of alkali sulphates will dominate like arcanite, aphih-italite, thenardite and calcium langbeinite. In addition an imbal-ance between alkali and SO3 could damage the furnace process

Table 3The average chemical composition of the reference clinker of each clinker factory.

Clinker (wt%) Average

Antoing Lixhe Maastricht

CaO 65.90 65.87 64.92SiO2 21.42 21.42 20.40Al2O3 4.44 4.84 5.01Fe2O3 2.61 3.65 3.52K2O 0.74 0.57 0.47Na2O 0.21 0.30 0.34SO3 1.21 0.51 0.92MgO 1.70 2.01 2.08Cl 0.068 0.017 0.020CaO free 1.46 0.55 2.33

LSF_MgO 98.24 98.19 98.20C3A 7.35 6.65 7.33LiqSimple 19.18 22.73 22.97

and have negative influences on the clinker quality and by thaton the final cement hydration. Like described above, a stoichiome-tric balance is imposed, expressed as the so called Degree of Sul-fatisation (DoS) value calculated by Eq. (1) using the chemicalanalysis of the final clinker.

DoS ¼ 77:41 � SO3=ðNa2Oþ K2O � 0:658Þ ð1Þ

DoS levels between 80 and 120 wt% are recommended andtherefore implemented in the three factories. For this reason theseDoS levels will be retained in all simulations presented in thiswork. Increasing levels of Cl as well as SO3 in the gas phase of aclinker kiln increase clogging and therefore disturb the process.As shown in Fig. 5, based on the Cl [wt%] and SO3 [wt%] presentin the Hot Clinker Meal (HCM) the risk for kiln clogging/coatingcan be evaluated, and classified (Little Coating, Frequent CleaningRequired, Bypass Advisable). Each factory is different with regardto its clinker raw meal materials and technical installations. Howthese differences work out on the Cl and SO3 (and alkalis) contentof the Hot Clinker Meal (HCM) and finally on the clinker is de-scribed by the so-called enrichment factor (e) specific for Cl, SO3

and alkalis which calculates their return [wt%] from the gas phaseto the HCM. Furthermore, levels of Cl and SO3 (and alkalis) can belowered by the presence of a bypass installation which allows for ahigher content Cl and SO3 (and alkalis) content in the gas phasewithout compromising the process and the final clinker quality.The Bypass factor (b) specific for each clinker kiln with bypassinstallation calculates the amount of Cl, SO3 (and alkalis) that willbe captured of the gas phase lowering the amount of Cl, SO3 andalkalis that could return to the HCM. These considerations are de-scribed in Fig. 6 and by the linear Eq. (2) which will be used in thesimulation program described in Section 3.3.


½X�HCM¼½X�CCMþeX � ½X�HCM�bX �eX � ½X�HCM¼½X�CCM=½1�eX � ð1�bXÞ�ðX¼Cl;SO3;K2O or Na2OÞ

ð2Þ

Based on this assumption, limits on Cl and SO3 can be set for thefinal clinker. The introduction of fibrecement materials will notmarkedly change the Cl content of the kiln feed because the Cl con-tent of fibrecement ARM is very low and in line with the otherCRM. Actually, Cl is especially brought in by fuel materials suchas waste oils, tires, plastics and animal meals. In the present inves-tigation the fuel mix is not adapted. So instead of putting a limita-tion on a parameter in which SO3 and Cl are taken into account, therequirement by which ‘‘safe’’ combinations of SO3 and Cl contentsare guaranteed can be simplified to a SO3 limitation (max) on theraw meal as listed for each clinker factory in Table 4. These SO3

limitations were derived from Fig. 5 by using Eq. (2) for SO3 andCl with the specific e and b factors of each clinker kiln and the aver-age Cl of each reference clinker (Table 2). With respect to the min-eralogy of the final clinker, limits are defined for the followingthree parameters: Lime Saturation Factor (LSF), C3A and the liquidphase (LiqSimple) [5]. The LSF_MgO governs the ratio of alite andbelite and concerns a refined version of the normal lime saturationfactor, by the fact that it holds a correction for the incorporation ofMgO in C3S.

LSF MgO ¼ 100 � ðCaOþ 0:75 �MgOÞ=ð2:8 � SiO2 þ 1:18

� Al2O3 þ 0:65 � Fe2O3Þ ð3Þ

The approximative calculation of the C3A content is done by acommonly known Bogue calculation described in the followingequation:

C3A ¼ ð2:650 � Al2O3 � 1:692 � Fe2O3Þ ð4Þ

Fig. 6. Circulation diagram of volatiles by gas and HCM stream (X = [SO3], [Cl],[Na2O], [K2O] of the HCM).

0

1

2

3

4

30 1 2 4 5 6 7

SO3 [wt%] in Hot Clinker Meal (HCM)

Cl [

wt%

] in

Hot

Clin

ker

I Little Coating / II Frequent Cleaning Required / III Bypass Advisable

Mea

l (H

CM

)

Fig. 5. Increasing clogging levels in function of SO3 [wt%] and Cl [wt%] of hot clinkermeal.

The composition of the liquid at 1450 �C is to a high extent gov-erned by the ratio of Al2O3 to Fe2O3. In the simulations of the pres-ent study the combined influence of these two components isexpressed by means of a parameter defined by the following sim-plified equation:

LiqSimple ¼ ð3 � Al2O3 þ 2:25 � Fe2O3Þ ð5Þ

By limiting the three above parameters for the final clinker, thefour metal oxides are sufficiently anchored within a safe zone foreach factory. Actually, in the present study, the three parameterswere each given fixed, but factory dependent, values as displayedin Table 4. Also MgO has to be monitored because of its limitationwithin the cement standards [14,16]. Though, this parameter wasnot critical in this investigation as levels were not greatly influ-enced by the use of fibrecement ARM and were not near the limitsdescribed by the cement standards.

3.2. Emissions and energy consumption

A variety of organic materials are commonly used in the clinkerprocess to generate energy [2–4]. They are brought in the clinkerprocess at specific places where a complete thermal decompositionis made possible. Raw materials are usually introduced at the coldside of the kiln. Therefore thermal decomposition products fromcellulose, PVA and PP might not be completely degraded when theyreach volatility at low temperature and will be picked up by thegas stream coming from the opposite end of the cement kiln. Thiscould cause blockage of the cyclone tower and generate an excessof the Total Organic Carbon emission limits at the chimney. A solu-tion to this problem could be to introduce the raw materialthrough a pre-calciner [2] or through a pre-heater system thatfeeds the cyclone tower with heat, leading to a complete thermaldegradation. The decomposition reaction mechanism of Broido–Shafizadeh for cellulose (Fig. 7) is generally accepted as the mostlikely mechanism [19]. The formation of each of the four compo-nents is influenced by the decomposition atmosphere, the heatingrate, presence of alkali, etc. [19–23]. Under inert atmosphere, Poly-VinylAlcohol (PVA) is characterised by two distinctly differenttemperatures, 380 �C and 420 �C at which thermal decompositionoccurs during heating [24]. The decomposition mechanism of Poly-Propylene (PP) is very complex and varies greatly depending on thedegree of polymerisation, the decomposition atmosphere, theheating rate, the polymer structure (isotactic, syndiotactic, atactic),etc. Various volatile degradation products can already be formedbelow a temperature of 150 �C [25]. The diversity of compoundsthat the organic materials of fibrecement can liberate by heatingthem up to 1450 �C makes it almost impossible to quantify thesecompounds completely. With thermogravimetric analysis (TGA)/differential thermal analysis (DTA), gas chromatography–massspectrometry (GC–MS) and mass spectrometry (MS) measure-ments, an effort is made to analyse the organic compounds formedafter thermal degradation at specific temperatures. A specific testsetup was designed to heat up the selected fibrecement samplesas fast as possible to a specific temperature. The selection of thetemperatures considered for these tests resulted from the evalua-tion of the TGA/DTA analyses and the literature study on thedecomposition of the organic fibres [19–26] present in the fibrece-ment ARM. In this specific test, a quartz reactor was loaded with400 mg of sample under a gas flow of 95 vol% He and 5 vol% O2.Preheated tubular furnace was then quickly raised up around thereactor resulting that the final temperature was reached within5 min. All the organic volatiles generated over this period wereanalysed by GC–MS while the lower molecular weight gases weremonitored by MS. The heating rate as well as the vol% O2 in the sur-rounding gas flow was set to be comparable with those present in areal clinker kiln. The knowledge of the amount as well as the

Fig. 7. Reaction mechanism of Broido–Shafizadeh.


nature of these volatiles and gases is important in view of thedetermination of the ideal point of introduction of the fibrecementARM in the clinker kiln. Because organic compounds are present inthe fibrecement ARM and an additional energetic benefit is ex-pected from the reduced endothermic decarbonation part, it isinteresting to comparatively evaluate the energy necessary to heatthe classic cold clinker meal up to 1450 �C, with that necessary fordoing the same but with alternative cold clinker meal. TGA/DTAand calorimetric measurements were performed to give an ideaof this possible energetic gain. On the other hand, the presenceof chemically bound H2O in the fibrecement ARM also implies lossof energy while heating. The energy required to perform dehydra-tation reactions is not so easy to quantify because of a variety ofhydration products that are generated upon the hydration of ce-ment. By performing DSC analyses on cement pastes with differentW/C ratios like those described in Section 2.3, an effort is made toestimate the total dehydratation energy. The pastes are made withthe CEM I cement presently used at the fibrecement plant fromwhich the fibrecement production waste for this study was taken.

3.3. Different clinker feed calculations and preparations

A simulation program based on linear equations calculated rawmeal compositions for each factory (CCM/Ant,Lxh,Maa/Ref). Theseraw meal compositions were in line with the chemical and miner-alogical requirements listed in Table 4. These compositions indi-cated as classic Cold Clinker Meals (CCMs) are very close to thecompositions actually used in these factories. The composition ofthe raw materials was merely adjusted to obtain the mineralogicalsettings, but without taking into account the ashes of the fuels thatwill actually be used to heat up the clinker meal. The compositionsof these classic cold clinker meals are presented in Table 5. Fur-thermore alternative compositions were calculated with the sameprogram and limits, aiming at the maximisation of the use of thefibrecement ARM. These alternative Cold Clinker Meals (CCMs)are also presented in Table 5. The chemical analyses of the lime-

Table 5Compositions of the different clinkers made to be fed to the static kiln.

CRM + ARM Quantity (wt%) CRM + ARM

CCM/Ant/Ref CCM/Lxh/RefCRM/Ant/CP 55.25 CRM/Lxh/TuCRM/Ant/CR 37.50 CRM/Lxh/LiCRM/Ant/FA 6.38 CRM/Lxg/FACRM/Ant/IC 0.87 CRM/Lxh/ICARM/HD/S8 0.00 ARM/RS/S7

Sum 100.00 Sum

CCM/Ant/FC CCM/Lxh/FCCRM/Ant/CP 0.00 CRM/Lxh/TuCRM/Ant/CR 79.08 CRM/Lxh/LiCRM/Ant/FA 7.03 CRM/Lxh/FACRM/Ant/IC 0.61 CRM/Lxh/ICARM/HD/S8 13.28 ARM/RS/S7

Sum 100.00 Sum

stones in Table 1, show that the limestones of Antoing have a highSO3 content, by which that kiln already operates close to his SO3-limit (Table 4) in the routine condition. After simulation, it is clearthat the introduction of air-cured ARM increases the SO3-contentof the Hot Clinker Meal (HCM) because of its higher level of SO3

compared with that of the CRM of CBR Antoing. Unlike air-curedARM, the use of Autoclaved ARM lowers the SO3 in the HCM sinceits SO3 content is lower than that of the two limestones used byCBR Antoing. Clogging phenomena should logically decrease bythe lower SO3 content of the HCM. Therefore, it was chosen to max-imise the Autoclaved ARM with the highest CaO [wt%] namelyAutoclaved High Density Fibrecement Boards (ARM/HDs) in theCCM/Ant/FC as presented in Table 5. The limestone used by CBRLixhe has an S content that is much lower than the limestones ofAntoing. Also the S feed by the fuel mix is limited. Therefore theSO3-content of the HCM is not as critical as the one of CBR Antoing.An increase of the SO3 [wt%] could therefore be accepted. By max-imisation of the air-cured ARM in the CCM/Lxh/FC, the SO3 [wt%] ofthe simulated final clinker was increased to 0.73 wt% which wasstill below the maximum of 1.2 wt% (Table 4) that is allowed forand in line with the limits set for the DoS factor. Performed simu-lations with the typical fuel mix of CBR Lixhe showed an SO3 in-crease of about 0.25 wt% which still is sufficiently below themaximum limit. The limestone or marl used by ENCI Maastrichtcontains a higher S content than the limestone or tufa used byCBR Lixhe. Also the used fuel mix brings more S to the HCM com-pared to the fuel mix of CBR Lixhe. Also the maximum allowed SO3

[wt%] is lower than that of CBR Antoing. The SO3 [wt%] could there-fore not be changed significantly what automatically suggests theuse of Autoclaved ARM. In the case of ENCI Maastricht, the Auto-claved ARM are still quite high in SO3 [wt%] compared to the usedlimestone or marl. This differentiates the use of autoclaved ARM inthis plant from that in CBR Antoing. Because the ARM from Auto-claved Medium Density Fibrecement Boards (ARM/MDs) were thelowest in SO3 [wt%], they were maximised in the CCM/Maa/FC likepresented in Table 5.

Quantity (wt%) CRM + ARM Quantity (wt%)

CCM/Maa/Ref79.44 CRM/Maa/Ma 84.38

6.63 CRM/Maa/SC 2.9012.34 CRM/Maa/FA 11.02

1.59 CRM/Maa/IC 1.700.00 ARM/MD/S5 0.00

100.00 Sum 100.00

CCM/Maa/FC38.52 CRM/Maa/Ma 81.89

0.00 CRM/Maa/SC 0.009.32 CRM/Maa/FA 10.330.53 CRM/Maa/IC 1.77

51.63 ARM/MD/S5 6.01

100.00 Sum 100.00


4. Results and discussion

4.1. Chemical analysis

The Chemical analyses of the CRM, ARM and CCM are presentedin Tables 1, 2 and 6, that of the final clinkers in Table 10. The aver-age analyses of the CRM were as expected and were therefore di-rectly used in the simulation program. The analyses of the ARMwere evaluated by sorting them by source and plotting them inFigs. 2 and 3 by their respective CaO and SiO2 without LOI sincethat is the way how the ARM will be fed to the HCM. From Figs. 2and 3, it is clear that air-cured ARM are very similar regardless oftheir source whereas the different Autoclaved ARM were chemi-cally totally different. From each source, three samples were se-lected based on their LOI namely the one with the lowest andhighest LOI and the one closest to the average LOI of the specificsource. For each source, the chemical analysis of the ARM closestto the average LOI was used in the simulation program as alsothese samples were used as source for the experimental produc-tion of the different alternative clinker feeds (Table 5). Further-more the GC–MS and MS analysis were performed on thesesamples. The chemical analysis of the CCM in Table 6 and that ofthe final clinker in Table 10 shows that the mass percentages ofthe used ARM and CRM were properly assessed by the simulationprogram. The calculated CCM which are presented in Table 5, gen-erate specific mass percentages for the four critical metal oxidesthat after decarbonation and sintering at 1450 �C, give a chemicalanalysis similar to that of the targeted reference clinker as pre-sented in Table 3. The chemical influence on possible mineralogicaldifferences between the clinker based on the CRM and the clinkerpartly produced with the ARM should therefore be minimal.

Table 7TGA/DTA evaluation of all the used Classic Raw Materials (CRMs) of CBR Antoing, CBR Lix

CRM Anorg CO2 Organic + H2O CaCO3 Total CaOTGA Meas(wt%)

TGA Meas(wt%)

TGA Der(wt%)

XRF Meas(wt%)

CRM/Ant/RL 39.9 1.0 90.8 50.09CRM/Ant/PL 34.9 1.2 79.4 42.87CRM/Ant/IC 4.8 4.0 11.0 6.20CRM/Ant/FA 0.0 4.7 0.0 6.20CRM/Lxh/Tu 39.5 3.4 89.8 51.75CRM/Lxh/Li 4.2 3.1 9.6 5.63CRM/Lxh/IC 6.2 3.1 14.2 10.70CRM/Lxh/FA 0.0 6.0 0.0 10.65CRM/Maa/Ma 40.0 1.1 91.0 50.78CRM/Maa/SC 1.1 1.99 2.6 2.70CRM/Maa/IC 4.1 1.1 9.4 18.40CRM/Maa/FA 0.0 4.0 0.0 18.38

Table 6Chemical analysis of the Cold Clinker Meals (CCMs) fed to the static kiln.

CCM (wt%) CCM/Ant/Ref CCM/Ant/FC CCM/Lxh

CaO 43.48 44.63 45.26SiO2 14.00 13.56 12.01Al2O3 2.89 2.72 2.90Fe2O3 1.84 1.80 2.72K2O 0.59 0.33 0.41Na2O 0.11 0.10 0.13SO3 0.48 0.41 0.24MgO 1.10 0.94 0.88TiO2 0.16 0.26 0.21P2O5 0.12 0.12 0.15Cl 0.02 0.02 0.04LOI 975 �C(O2) 34.89 34.79 34.75Ctotal 8.98 8.89 8.82Stotal 0.36 0.25 0.09

4.2. TGA/DTA analysis

TGA/DTA analyses of the CRM are quite straightforward. Thelimestones, tufa and marl have an endothermal decarbonation areabetween 700 and 900 �C which is quantified in Table 7. A goodmatch is found between XRF analysis and the CO2 loss by TGA/DTA by comparing the CaCO3 [wt%] derived from TGA analysisand the intrinsic [Ca,Mg]CO3 [wt%] derived from XRF analysis. Thisindicates that Ca is almost completely present as CaCO3. The Decar-bonation Energy (Decarb E) of CaCO3 can be derived from theknown reaction enthalpy of 1782 kJ/kg for CaCO3 stated by Taylor[5] and CaCO3 [wt%] derived from TGA analysis. As can be noted inTable 7, the calculated decarbonation energies derived from theTGA relate in the same way as the decarbonation energies (lVs/mg) measured by DTA. Loam and sabulous clay also show a smallbut distinct quantifiable decarbonation area between 650 and800 �C (Table 7). Here, a deviation between the CaCO3 [wt%] de-rived from TGA analysis and [Ca,Mg]CO3 [wt%] derived from XRFanalysis is seen, indicating that not all Ca and Mg are present ina carbonated form. Whereas the Decarb E of CaCO3 can be derivedfrom the TGA/DTA analysis rather easily, the quantification of theloss of H2O (i.e. the amount of chemically bound water) is less easy.Assuming that there is no or only negligible amount of organicmaterial present in loam or sabulous clay, the changes registeredin the TGA which are normally attributed to the combined loss oforganics and H2O, may be considered to be mainly due to the evap-oration of chemically bound H2O. The Fe2O3 sources are more dif-ficult to evaluate due to the fact that they mostly concernartificially made raw materials, derived from waste. In inert atmo-sphere, the loss of mass is two times bigger than in the presence of5 vol% O2 what indicates that the Fe2O3 sources perform oxidation

he and ENCI Maastricht.

Total MgO Intrinsic [Ca,Mg]CO3 Decarb E CaCO3 Decarb E CaCO3

XRF Meas(wt%)

XRF Der(wt%)

lVs/mgMat DTA

J/g Mat TGA Der

0.9 90.8 226.2 16181.1 78.9 219.4 14150.3 11.7 28.5 1961.6 13.5 0.0 00.7 92.2 251.8 16010.8 11.6 37.8 1711.1 20.7 31.2 2521.4 20.0 0.0 00.8 91.7 255.1 16220.3 5.3 12.2 461.4 32.0 6.6 1671.8 31.4 0.0 0

/Ref CCM/Lxh/FC CCM/Maa/Ref CCM/Maa/FC

47.92 44.79 44.8514.03 12.83 12.71

3.35 3.09 3.052.65 2.42 2.400.32 0.39 0.340.12 0.13 0.130.81 0.28 0.291.18 0.97 0.960.29 0.20 0.210.23 0.09 0.100.04 – –

28.61 34.51 34.686.66 8.79 8.760.36 0.15 0.16

Table 8TGA/DTA evaluation of all the used Alternative Raw Materials (ARMs) coming from fibrecement.

ARM Anorg CO2 Organic + H2O CaCO3 Total CaO Total MgO Intrinsic [Ca,Mg]CO3 Decarb E CaCO3 Decarb E CaCO3

TGA Meas(wt%)

TGA Meas(wt%)

TGA Der(wt%)

XRF Meas(wt%)

XRF Meas(wt%)

XRF Der(wt%)

lVs/mg Mat DTA J/g Mat TGA Der

ARM/CS/S2 7.1 11.1 16.1 52.09 1.54 70.99 14.6 287ARM/CS/S3 9.6 12.7 21.9 49.99 1.36 70.22 22.5 389ARM/CS/S5 10.0 13.5 22.7 50.86 1.64 71.53 41.5 405ARM/RS/S3 7.7 9.8 17.5 52.55 1.62 71.81 26.9 312ARM/RS/S7 8.2 11.0 18.7 51.89 1.52 71.41 33.6 334ARM/RS/S9 7.9 11.6 17.9 52.20 1.56 71.53 32.9 319ARM/HDB/S8 0.6 16.1 1.4 28.20 0.67 42.32 5.3 25ARM/HDB/S9 0.5 16.8 1.1 28.62 0.59 42.67 9.7 20ARM/HDB/S11 0.5 15.8 1.2 28.53 0.67 42.68 8.6 21ARM/MDB/S5 0.3 14.3 0.6 24.83 0.79 38.28 4.8 11ARM/MDB/S6 0.3 14.5 0.6 25.24 0.75 38.73 5.2 10ARM/MDB/S11 0.2 16.6 0.6 25.12 0.59 38.35 4.2 10

Table 9TGA/DTA evaluation of the reference cold clinker meals of CBR Antoing, CBR Lixhe and ENCI Maastricht.

CCM Anorg CO2 Organic + H2O CaCO3 AnorgCO2 Der

CaCO3 Der DecarbE CaCO3

DecarbE CaCO3

DecarbE CaCO3 Der

DecarbE CaCO3 Der

Mat TGA Meas(wt%)

TGA Meas(wt%)

TGA Der(wt%)

TGA Meas TGA Meas(wt%)

lVs/mgMat DTA

lVs/mgMat DTA RM

J/g Mat TGA J/g Mat TGA RM

CCM/Ant/Ref 34.0 0.6 77.3 34.3 78.0 213.4 206.3 1378 1390CCM/Ant/FC 29.7 4.8 67.6 31.7 72.1 190.5 179.7 1205 1284CCM/Lxh/Ref 32.5 0.5 73.9 31.8 72.2 189.3 203.0 1318 1287CCM/Lxh/FC 16.9 9.3 38.4 19.5 44.3 105.9 115.3 685 800CCM/Maa/Ref 34.2 0.0 77.7 33.9 77.0 219.3 215.7 1385 1373CCM/Maa/FC 33.3 1.2 75.8 32.9 74.7 199.7 209.3 1350 1332

Table 10Chemical analysis and Bogue calculations of the final clinkers produced in a static kiln.

Clinker (wt%) Cl/Ant/Ref Cl/Ant/FC Cl/Lxh/Ref Cl/Lxh/FC Cl/Maa/Ref Cl/Maa/FC

CaO (wt%) 65.90 66.71 66.28 65.61 66.18 66.44SiO2 (wt%) 22.27 22.72 21.93 21.53 21.39 21.56Al2O3 (wt%) 4.14 3.97 4.40 4.77 4.54 4.45Fe2O3 (wt%) 3.02 2.79 4.21 3.81 3.98 3.99K2O (wt%) 0.59 0.24 0.21 0.25 0.33 0.29Na2O (wt%) 0.17 0.14 0.20 0.20 0.21 0.19SO3 (wt%) 0.89 0.44 0.12 0.58 0.36 0.29MgO (wt%) 1.73 1.48 1.28 1.7 1.52 1.44TiO2 (wt%) 0.25 0.39 0.30 0.40 0.30 0.3P2O5 (wt%) 0.21 0.20 0.24 0.33 0.17 0.16Cl (wt%) n.a. n.a. n.a. n.a. n.a. n.a.LOI (wt%)975 �C (O2) 0.48 0.39 0.39 0.33 0.48 0.45

DoS-factor 123.42 114.33 27.47 123.18 65.24 58.95Alite (C3S) 66.84 68.19 67.52 65.92 70.61 70.97Belite (C2S) 13.44 13.71 11.95 12.01 8.07 8.29Aluminate (C3A) 5.86 5.80 4.54 6.20 5.30 5.04Ferrite (C4AF) 9.19 8.49 12.81 11.59 12.11 12.14


reactions while heating. TGA/DTA curves of the CRM/Lxh/IC andCRM/Maa/IC show small decarbonation reactions that can be quan-tified in the same way as for the other CRM (Table 7). The fly asheshave an opposite behaviour in the two atmospheres, with a biggerloss of mass in oxidative than in inert atmosphere indicating thepresence of organic material. The mass loss in inert atmosphereindicates the formation of organic volatiles in absence of O2. Be-cause fly ash is a material which has had already a thermal treat-ment at high temperatures, no decarbonation-related mass lossesare detected (Table 7). Because there is a loss of organic volatilesand CO2 visible in the whole temperature range between 400 �Cand 1450 �C under inert and oxidative atmosphere, the presenceof char is indicated. TGA/DTA analyses of the fibrecement materials(Table 8) give a lot of information on their raw materials and how

they will behave when thermally degraded up to 1450 �C. The TGApeaks of the autoclaved ARM show the absence of limestone fillerwhich is present for maximum 15 wt% in the air-cured materials.This makes it possible to quantify the CO2 part coming from thelimestone filler of the air-cured products. Nicely visible is alsothe two-step degradation starting at 480 �C of PVA, present atabout 1.7–1.9 wt% in the air-cured materials in Fig. 8 and the ab-sence of PVA in the hydro-thermally hardened materials like plot-ted in Fig. 9. This indicates that the reaction products of PVAsuggested by Peng and Kong [24], will be formed when heated.Also perfectly quantifiable is the mass loss of the char that isgenerated by the thermal degradation of Cellulose between800 �C and 1000 �C (Figs. 9 and 10). The presence of char in theTGA/DTA analysis of the ARM, confirms the Broido–Shafizadeh


reaction mechanism (Fig. 8) and therefore indirectly, the presenceof levoglucosan. Furthermore, TGA/DTA coupled with XRF analysismakes it possible to calculate the CO2 fraction originating from[Ca,Mg](CO3)2 and the associated energy consumption of eachraw material presented in Tables 7 and 8. Unfortunately the gainin energy by the decomposition of organic components and theloss of energy coming from the decomposition of chemically boundH2O is not quantifiable by the performed analysis by TGA/DTA. Bycomparing the TGA/DTA analyses of the raw materials and the ColdClinker Meals (CCMs), the reduction of CO2 emission as well as theassociated energy gain by the lowered endothermal decarbonationcan be quantified like presented in Table 9. This is done by calcu-lating the decarbonation energies of CaCO3 out of the TGA/DTA

75

80

85

90

95

100

105

30 100 170 240 310 380 450 520 590 660 730

T (°C)

TGA

(wt%

)

Fig. 8. TGA/DTA analysis of the three selected a

75

80

85

90

95

100

105

30 100 170 240 310 380 450 520 590 660 730 8

T (°C

TGA

(wt%

)

Fig. 9. TGA/DTA analysis of the three selected autoc

analysis of the CCM (J/g Mat TGA) and calculating the decarbon-ation energies derived of the TGA/DTA analysis of the raw materi-als (J/g Mat TGA RM) taking in account the calculated compositionsof Table 5. The fact that these two totally separated calculationsgave a comparable result, proofs that the interpretation of theTGA/DTA analyses were objective as well as correct and couldtherefore be used to evaluate the CO2 reduction as well as the gainin Decarb E by the use of the ARM.

4.3. XRD analysis

The XRD analyses of the final clinkers presented in Table 11,show different weight percentages than those calculated by Bogue

800 870 940 1010 1080 1150 1220 1290 1360 1430-0,20

0,00

0,20

0,40

0,60

0,80

1,00

DTA

(µV/

mg)

EXO

ir-cured Fibrecement Roofing Slates (RSs).

00 870 940 1010 1080 1150 1220 1290 1360 1430

)

-0,20

0,00

0,20

0,40

0,60

0,80

1,00

DTA

(µV/

mg)

EXO

laved high density fibrecement materials (HD).

0102030405060708090

100

Reference (CCM/Lxh/Ref) Alternative (CCM/Lxh/FC) Fibrecement (ARM/RS/S7)

Com

posi

tion

wt%

(bar

s)

-1500-1250-1000-750-500-2500250500750100012501500

Ener

gy J

/g (l

ines

)

Tufa (Limestone) Loam Fly Ash Iron CarrierFibrecement (RS/S7) Chem bound H2O GCV (Total) NCV CelluloseNCV PVA NCV PP Decarb E CaCO3 (Total) Chem Dehydr Energy (Total)

Fig. 10. CBR Lixhe (bars: composition + total chemical bound H2O wt%), (full lines: Gross Calorific Values (GCVs) + decarbonation energy + chemical dehydratation energy),(dotted lines: individual Net Calorific Values (NCVs) of the organic fibres).


equations out of the chemical analysis of the final clinker pre-sented in Table 10. Like stated in Section 4.1, the chemical analyseswere as expected and in line with the average values of the clinkersproduced in the first 6 months of 2011 on the three factory siteslike presented in Table 3. The difference between the C3A andthe C4AF measured with XRD and the corresponding values calcu-lated with the Bogue equations stems from the method itself. TheBogue equations indeed are merely estimations which describe thephases formed when perfect clinker production conditions couldbe achieved. They are still widely used in routine as steeringparameters within the clinker production process. There accuracycan be improved by using the so called refined Bogue calculations[5]. The XRD analysis indicates the actual mineralogical composi-tion as it appears in the real clinker itself. Though, in view of theevaluation of the possible influence of the ARM on the mineralogyof the clinkers, a reliable and objective comparison of the ClassicClinkers with the Alternative Clinkers can be done by any of thesemethods. For the kilns of CBR Antoing and ENCI Maastricht, it canbe noted that the 13.28 wt% dosed autoclaved high density fibrece-ment board ARM (Antoing) or the 6.01 wt% dosed autoclaved med-ium density fibrecement board ARM (Maastricht) had nosignificant influence on the mineralogy of the clinker (Tables 10and 11). Although a difference of 1–2 wt% could be interpreted tobe within the error range, it can be noted that in case of the CBRLixhe kiln, the alite [wt%] of the alternative clinker (Cl/Lxh/FC) islower compared to the classic clinker (Cl/Lxh/Ref) for XRD analysesand Bogue calculations. Because the difference in alite [wt%] be-tween Cl/Lxh/Ref and Cl/Lxh/FC is bigger by XRD analysis(�2.32 wt%) than by Bogue calculation (�1.6 wt%), it may be con-cluded that the difference in alite [wt%] does not originate from asmall difference in the chemical composition of both clinkers only.In fact, this difference indicates that most probably, for equalchemistry, there still would be an influence of the air-cured ARM

Table 11Mineralogical analysis by XRD of the final clinkers produced in a static kiln.

Clinker (wt%) Cl/Ant/Ref Cl/Ant/FC Cl/Lx

Alite (C3S) 64.52 64.50 65.04Belite (C2S) 19.73 20.61 14.93Aluminate (C3A) 1.79 2.58 3.68Ferrite (C4AF) 12.86 11.23 15.87Free Lime (CaO) 0.23 0.51 0.23Periclase (MgO) 0.39 0.19 0.18Arcanite (K2SO4) 0.32 0.20 0.07

on the mineralogy of the alternative clinker. From Table 10, itcan be noted that the alternative clinkers of CBR Lixhe were quitehigh in SO3 compared to the reference clinker, but lower than cal-culated with the simulation program (Table 4). The same can be re-marked for the clinkers of CBR Antoing where both the reference aswell as the alternative clinkers were high in SO3 but lower than cal-culated. Also the alkali contents of all clinkers were lower thansimulated. It can be noted that some of the DoS-factors (1) wereout of equilibrium: both clinkers of CBR Antoing were still betweenthe limits as also the alternative clinkers of CBR Lixhe but all clink-ers of ENCI Maastricht and especially the classic clinker of CBR Lix-he had a DoS-factor that was too low. Like described in Section 3.1,an excess of alkali will decrease the viscosity of the melt by whichthe alite formation is increased [27]. The reason for these devia-tions lies in the fact that the static kiln has other enrichment fac-tors (2) than the real clinker kilns that were simulated by thesimulation program like described in Section 3.1. In fact, theenrichment factors of a static kiln will be much lower than thoseof the real clinker kilns. There is no gas stream flowing in the oppo-site direction of the Hot Clinker Meal (HCM) stream towards colderregions in the clinker kiln allowing alkali and SO3 to return to theHCM flow like described in Fig. 6. Because the alkali have a highervolatility at 1450 �C than SO3, they will be much lower in contentin a static kiln and also SO3 [wt%] will be lowered significantly. Thedecreased alite formation of the alternative clinker of Lixhe by theuse of air-cured ARM, could therefore be explained by a differencein DoS-factor influence by the higher SO3 [wt%], brought in by theair-cured ARM and at the other hand the influence of the static kilnon the volatility of SO3 and alkali. The conclusion after the evalua-tion of the XRD analysis is therefore that the use of fibrecementARM does not influence greatly the chemical and mineralogicalquality of the produced alternative clinker if compared with theuse of classic raw materials.

h/Ref Cl/Lxh/FC Cl/Maa/Ref Cl/Maa/FC

62.72 71.33 70.5817.61 8.56 9.79

1.56 4.64 4.2217.61 14.89 14.53

0.28 0.35 0.750.22 0.23 0.13– – –


4.4. Estimation of the influence on energy consumption: calorimetricevaluation

As indicated in Section 4.2, the decarbonation energies werecalculated and presented in Tables 8 and 9. These decarbonationenergies together with the Gross Calorific Values (GCVs) of the dif-ferent CCM (Section 2.3) and the selected fibrecement ARM deter-mined by calorimetric analysis, were plotted in Figs. 10 and 11. Theaverage dry starting compositions of the four types of ARM give usthe ratio between the three types of organic fibres [6,7] in the air-cured ARM: cellulose, PVA and PP. In the autoclaved ARM, only cel-lulose is used as organic fibre. Unfortunately, these dry startingcompositions could not be revealed. The TGA/DTA analysis of theselected ARM, made it possible to divide the loss of ignition be-tween 100 �C and 1000 �C in a decarbonation part and an organ-ic/chemically bound H2O part. Because the decarbonation partconcerns CO2, the part of the total C (Table 6) related to CO2 canbe calculated, revealing the C [wt%] related to the organic fibre(s).By the molecular mass of each organic fibre and the average ratiobetween the three types of organic fibres in the air-cured ARM,the wt% of the organic fibres in the ARM can be calculated. The to-tal wt% of these organic fibres reveals, by using the organic/chem-ically bound H2O part measured by TGA/DTA, the chemicallybound H2O. The wt% of the organic fibres and chemically boundH2O in the alternative CCM can be deduced from the used wt% ofARM (Table 5) and were plotted in Figs. 10 and 11. Cellulose hasa Net Calorific Value (NCV) of about 17,520 J/g, PolyVinylAlcohol(PVA) of about 21,310 J/g, and PolyPropylene (PP) of about42,660 J/g [28]. The specific NCV of the organic fibres present inthe CCM and the selected ARM were generated out of the calcu-lated wt% and the individual NCV of the organic fibres. They wereplotted in Figs. 10 and 11. It could be noted that the GCV of theCCM and the used fibrecement samples are similar to the sum ofthe specific NCV of the organic fibres. This proofs that the calcula-tions that were performed to determine the wt% of the organic fi-bres and therefore also the chemically bound H2O in the ARM andCCM, are representative for their compositions. We could estimatethat the corrugated fibrecement sheets on average contain 7.8 wt%chemically bound H2O, the fibrecement roofing slates 6.5 wt%, themedium density fibrecement boards 8.4 wt% and the high densityfibrecement board 9.5 wt%. The Portland cement [wt%] in theARM could be calculated by using the average dry starting compo-sitions and the chemically bound H2O [wt%] of the ARM which re-vealed the total paste [wt%] present in the ARM. The ratiochemically bound H2O/paste [wt%] present in the selected ARM

0102030405060708090

100

Reference (CCM/Ant/Ref) Alternative (CCM

Com

posi

tion

wt%

(bar

s)

Poor limestone Rich limestoneIron Carrier Fibrecement (HGCV (Total) NCV Cellulose

Fig. 11. CBR Antoing (bars: composition + total chemical bound H2O wt%), (full lines: Gro(dotted lines: individual Net Calorific Values (NCVs) of cellulose).

are 8.7 wt% for the roofing slate sample (ARM/RS/S7), 23.8 wt%for the high density fibrecement plate sample (ARM/HD/S8) and20.4 wt% for the medium density fibrecement plate sample(ARM/MD/S5). Like presented in Section 2.2, the autoclaving pro-duction process will generate other calcium silicate hydratesexplaining the high chemically bound H2O [wt]. These circum-stances are not present in the air-cured production process whereonly cement will hydrate which results in much lower chemicallybound H2O [wt%]. Like stated by Alarcon-Ruiz et al. [29], the chem-ically bound H2O of cement hydrates will be liberated between100 �C and 550 �C. Three major decomposition reactions will takeplace liberating H2O: the decomposition of gypsum, ettringiteand a part of the calcium aluminate hydrates between 110 �Cand 170 �C, the loss of chemically bound H2O from the decomposi-tion of the C–S–H and the other part of calciumaluminate hydratesbetween 180 �C and 300 �C and finally the dehydroxylation of port-landite between 450 �C and 550 �C. Two distinct endothermalenergies could be distinguished out of the TGA/DTA analysis mea-sured between 30 �C and 600 �C on cement pastes, like described inSection 2.3. The first between 150 �C and 250 �C and the second be-tween 400 �C and 500 �C. Also by TGA/DTA, the cement [wt%] andthe chemically bound H2O [wt%] could be determined for each ce-ment paste. The endothermal energies required to perform thedehydratation reactions were estimated by performing DSC analy-ses on these same cement pastes. These energies were subtractedwith the calculated heat capacities of the available Portland ce-ment [wt%] in the cement pastes and were plotted in Fig. 12 infunction of the ratios chemically bound H2O/paste. The specificheat constant for Portland cement was defined as 0.84 kJ/(kg�K)[30]. A good linearity was found what endorses the findings ofAbdelrazig et al. [31]. By using the ratio chemically bound H2O/paste [wt%] and the paste [wt%] determined for the selected aircured ARM, the energies necessary to perform these decompositionreactions in the selected air cured ARM and alternative CCM werecalculated by regression out of Fig. 12. This is not possible for theautoclaved ARM because of the presence of calcium silicate hy-drates which are not formed by classic cement hydratation. Nicelyvisible in Figs. 10 and 11 is the significant energy gain coming fromthe lowered endothermal decarbonation energy. It can safely bestated that this energy will be fully recuperated. For the air curedARM, there will also be a balance, in favour of the first, betweenthe energy coming from the exothermal decomposition energiesof the organic fibres and the energy loss coming from the dehydra-tation of the cement. And relying on the measured chemicallybound H2O of the autoclaved ARM, a partial recovery of the

/Ant/FC) Fibrecement (ARM/HD/S8)

-1500-1250-1000-750-500-2500250500750100012501500

Ener

gy J

/g (l

ines

)

Fly Ash D/S8) Chem bound H2O

Decarb E CaCO3 (Total)

ss Calorific Values (GCVs) + decarbonation energy + chemical dehydratation energy),

W/C 0.18W/C 0.25

W/C 0.30W/C 0.35

y = -36,725 x - 128,76R2 = 0,9897

-800

-700

-600

-500

-400

-300

-200

8 9 10 11 12 13 14 15 16

Chem bound H2O (wt%)

J/g

Past

e

Fig. 12. Endothermal dehydration energies for Portland based cement pastes.

MC

ount

s

150

125

100

75

25

0

50


exothermal decomposition energy of the organic fibres is certainlyrealistic.

Minutes17.515.012.510.07.55.02.5

Fig. 14. GC–MS measurement of air cured ARM from bottom to top at 390 �C,500 �C and 800 �C.

MC

ount

s

60

50

40

30

10

0

17.515.012.510.07.55.02.5

20

4.5. GC–MS and MS analyses

Two of the selected fibrecement ARM, ARM/RS/S7 and ARM/HD/S8 were heated up in three separated tests from room temperatureto 390 �C, 500 �C and 800 �C. By comparing the integral calculationof the CO and CO2 measurement graphs coming from the MS pre-sented in Fig. 13, it is clear that the formation of the low molecularCO and CO2 gases by thermal degradation of the air-cured andautoclaved ARM increases by higher maximum degradation tem-peratures like also stated for thermal decomposition of celluloseby Shen and Gu [21]. Although these calculations could be seenas arbitrary, they can objectively be used to compare the formationof CO and CO2 gases of the three separated tests (390 �C, 500 �C and800 �C). Also the formation as well as the variety of the high molec-ular volatiles, measured by GC–MS, will increase when the maxi-mum degradation temperature of autoclaved ARM is increased to800 �C like demonstrated in Figs. 14 and 15. Only the total ion cur-rent chromatograms are presented due to the impossibility at thisstate of identifying so much of compounds. Similar on what wasfound in literature on the thermal decomposition of cellulose([19–23]), the total formation of high molecular volatiles increaseswhen the maximum degradation temperature was increased from390 �C to 500 �C. According to the Broido–Shafizadeh mechanism(Fig. 7), levoglucosan should be generated in high quantities.Unfortunately, the spectrum of levoglucosan was not clearly de-tected in any of the six GC–MS measurements. Although the glasspiping was isolated, there was a brown deposition at the exit of the

02468

1012141618

ARM/RS/S7(390°C)

ARM/RS/S7(500°C)

ARM/RS/S7(800°C)

ARM/HD/S8(390°C)

ARM/HD/S8(500°C)

ARM/HD/S8(800°C)

CO (AMU m/z= 28) CO2 (AMU m/z=44)

Fig. 13. Integral calculation of the CO and CO2 measurement graphs of air-curedand autoclaved samples measured by MS.

Minutes

Fig. 15. GC–MS measurement of autoclaved ARM from bottom to top at 390 �C,500 �C and 800 �C.

reactor. We suspect that this deposition descends from the levo-glucosan [32]. Like stated by Hosoya et al. [33], the pyrolyticbehaviour of levoglucosan at 400 �C results in a vapour phaseand a liquid/solid phase. In the vapour phase, levoglucosan willbe converted to non-condensable gases (mainly CO and CO2), whilechar and other condensable low molecular weight products wereformed in the liquid/solid phase. This could explain the absenceof levoglucosan in the GS–MS analysis. The GC–MS measurementsof the air-cured ARM samples differ from those of the autoclavedARM samples due to the presence of PVA and PP and the lowerwt% of cellulose. Like described by Peng and Kong [24], acetalde-hyde should be generated above 420 �C by thermal degradation


of PVA but the big variety of generated volatiles prevents its iden-tification in the GC–MS measurements of the air-cured ARM.Although it was not possible to distinguish in a qualitative andquantitative way the different high molecular volatiles by GC–MS, this specific test proofs that introducing the fibrecementARM in large quantities by the cold side of the clinker kiln is abad idea. Even when the clinker kiln is quite modern an has a cy-clone tower to heat up the CCM within minutes to 800 �C, a lot ofgenerated volatiles will return by the gas stream to colder regionsof the cyclone tower causing clogging if returned to their solidstate or worse be emitted by the chimney causing an exceedingof the emission limits. The deposition in the glass tubing is a per-fect example to endorse this statement. Therefore large quantitiesof fibrecement ARM should be introduced at a hot point like a pre-calciner or pre-heater where it will be heated up in seconds to900 �C which will ensure a complete thermal degradation of the or-ganic fibres. Further research has to be performed to distinguishthe different high molecular volatiles by GC–MS and to heat upfibrecement ARM samples on lab scale in seconds to 900 �C forexample by pyrolysis chromatography.

5. Conclusions

Like could be noted in the different sections, fibrecement recy-cled materials could be an interesting alternative raw material forPortland clinker production. It was shown that compared to a sit-uation where pure limestone is used in the cold clinker meal, aninorganic CO2 emission reduction as well as a decarbonation en-ergy gain is possible. Furthermore, it was demonstrated that thechemistry and mineralogy of the final clinkers were not influencedsignificantly by the use of the fibrecement materials. Because of itscompositional constancy, and thus its chemical stability, fibrece-ment is a valid raw material. However, when used in practice,these materials should be introduced at a hot point in the process,e.g. a pre-calciner or a pre-heater, to ensure full thermal degrada-tion of the organic fibres for physical and chemical reasons, like toavoid clogging of the filter system and exceeding organic volatileemissions levels. Therefore the use of fibrecement for clinker pro-duction should be investigated further at an industrial scale. Thepotential energy loss by skipping the cyclone tower as energeti-cally most profitable way of heating, should be minimal becausethe biggest energy consuming reaction in these cyclones, thedecarbonation of CaCO3, will barely be present in fibrecementARM. The possible energy gain for use of fibrecement versus lime-stone, coming from the exothermal degradation of organic com-pounds, as also from the lowered decarbonation energy, werequantified. Together with the estimated energy consumptionneeded for the liberation of chemically bound H2O, it was shownthat using fibrecement as raw material for clinker production low-ers the total required energy compared to the use of classic rawmaterials, without compromising neither on physical, chemicalnor mineralogical properties. It should, together with the alreadyavailable alternative fuels and raw materials, be considered as away to get in line with the Cement Sustainability Initiative.

Acknowledgements

The authors wish to thank the labs of Redco N.V. and HC Bene-lux for their support, Els Bruneel for her aid during the practicalexecution of the tests and Ruben Snellings for critically readingthe manuscript.

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an interesting alternative raw material for a sustainable portland

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