Sludge recycling within cemented paste backfill: Study of the mechanical and leachability properties

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Minerals Engineering 19 (2006) 420–432

Sludge recycling within cemented paste backfill: Study of themechanical and leachability properties

Mostafa Benzaazoua a,*, Jean-Francois Fiset b,1, Bruno Bussiere a,Mathieu Villeneuve a, Benoıt Plante a

a University of Quebec in Abitibi-Temiscamingue, Canada Research Chair on ‘‘Integrated Management of Sulphidic Tailings using Fill

Technology’’, Industrial NSERC Chair Poly-UQAT in ‘‘Mining environment’’, 445 boul. De l’Universite, Rouyn-Noranda (Quebec), Canada J9X 5E4b Natural Resources (CANMET-MMSL), 555, Booth Street, Ottawa (Ontario), Canada K1A 0G1

Received 30 June 2005; accepted 27 September 2005Available online 11 November 2005

Abstract

Cemented paste backfill (CPB) technology has become widely used in Canada and around the world. While most of the studies relatedto CPB have concentrated on the mechanical, physical, rheological and economical aspects, very few studies have been conducted onmetal leachability. The objective of this study was to investigate the mechanical and environmental behaviour of a novel CPB techniqueconsisting of incorporating various treatment sludges within the conventional paste mixture. For the CPB studied, the binder PC10-5050/50 offers higher mechanical resistance than the slag-based cement. While PC-based binders are more or less penalized by the additionof sludge, slag-based cement benefits from this additive. In terms of leachability, the results show that PC and slag-based binders wereefficient in minimizing the release of Zn and Mn even at a low binder proportion. The addition of 0.15% and 0.30% of three differentsludges to the tailings paste backfill did not have any negative impact on metal leachability.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Acid rock drainage; Tailings disposal; Paste backfill; Sludges; Leaching

1. Introduction

Daily, the mining industry produces different types ofwaste such as mill tailings, waste rock, and water treatmentsludge (Aubertin et al., 2002). Various techniques are com-monly used in the mining industry to dispose of its waste.Waste rocks are usually stored at the surface in stackscalled waste rock piles; they can also be used as under-ground backfill material (cemented or uncemented rockfill). Tailings can be stored in a surface tailings pond orcan be used as backfill material (as hydraulic fill or pastebackfill) to fill underground open stopes. Tailings and

0892-6875/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.mineng.2005.09.055

* Corresponding author. Tel.: +1 819 762 0971x2404; fax: +1 819 7976672/4727.

E-mail addresses: [email protected] (M. Benzaazoua),[email protected] (J.-F. Fiset).1 Tel.: +1 613 995 4641; fax: +1 613 996 9041.

waste rocks can also be stored in old mine shafts, workingsand abandoned open pits with subsequent flooding (Ledinand Pedersen, 1996). The main concerns with tailings andwaste rock disposal are the physical stability of the surfacestorage area and the reactivity of the sulphide mineralsoften contained in the mine waste.

The acid mine drainage (AMD) generated by sulphideoxidation due to the action of oxygen and water is treatedby active treatment using generally lime addition. This pro-cess allows meeting the regulations in terms of dischargedwater quality but producing a certain amount of sludgescontaining toxic elements. Water treatment sludge frommine operations contains usually between 1% and 30% sol-ids depending on the treatment method (Aubertin et al.,2002). In Canada, it is estimated that as much as 6.7 mil-lion cubic meters of sludge are produced annually (Zinckand Aube, 2000). Canadian mining companies usually dis-pose of their sludge in ponds or lagoons, or they combine

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M. Benzaazoua et al. / Minerals Engineering 19 (2006) 420–432 421

them with tailings or other waste. Obviously, sludge dis-posal is a concern due to the large amount of waste thatmust be disposed of. The chemical stability of sludgedepends on the metal hydroxide solubility product (Ks)which varies according to temperature, metal species andionic strength. Other factors also control the solubility suchas the complexing agents, the co-precipitation and theadsorption processes. Recently, several studies have beenundertaken to study and evaluate the degree of metalmobility from water treatment sludge and its long-termchemical stability (Zinck, 1999, 2000; Zinck et al., 1996,1997). A recent study using a stabilization–solidificationtechnique similar to the paste backfill method, transformedwater treatment sludge into an inert material by the addi-tion of hydraulic binders (Fiset et al., 2003). Red mudhas also been investigated as an additive within backfillmaterial (Zhou, 2004). Red mud is created when bauxiteundergoes the Bayer process, in which caustic soda isadded to bauxite, producing alumina and residual miner-als. This process creates large amounts of waste red mud.The red mud is extremely alkaline owing to the caustic sodaused in the Bayer process. Different approaches have beenproposed in the literature to reuse red mud as a stabilizingagent for waste material. Red mud can be used as animmobilization agent for heavy metal ions in aqueous solu-tion, and then solidified by a cement-based process(Kilinckale et al., 1997). Lotze and Wargalla (1985) alsoused red mud as a sealant material after stabilization withlime.

Tailings paste backfill technologies are extensively usedin Canada and in several other countries (Hassani andArchibald, 1998; Benzaazoua et al., 2002; Fall and Benzaa-zoua, 2003; Benzaazoua et al., 2005). Paste backfill is amixture of water (fresh water and/or mine process water),tailings material (often a percentage of solids between70% and 85%) and hydraulic binders. Usually, ordinary

Tawith

Ore Pro

Flotation. c

Polyme

Extr

Acidicwaters

Water treatement

Sludges

Note: PAG: Potential of acid generating

Fig. 1. Schematic flow sheet focusing on t

Portland cement is used alone or blended with variousbinders such as sulphate-resistant Portland cement, blastfurnace slag, fly ash, etc. (Benzaazoua et al., 2004a,b).While most of the studies related to tailings paste backfillhave concentrated on the mechanical, physical, rheologicaland economical aspects, very few studies have been con-ducted on metal leachability and innovative waste disposaltechnologies. The addition of hydraulic binders to minebackfills increases their strength, but also their durabilityand overall acid neutralization potential. Cement hydra-tion can also ensure contaminant stabilization within thematrix of paste backfill (Benzaazoua, 1996; Benzaazouaet al., 2004a,c; Kwong, 2004). Many researchers haveinvestigated some of the environmental impacts of backfillmaterial when used in underground mines, emphasizingvarious aspects including metal release and fixation, neu-tralization and acidification mechanisms, influence of bin-der type, water retention and hydraulic conductivity,oxygen consumption, etc. (e.g. Thomson et al., 1986;Levens et al., 1996; Ouellet et al., 2003; Benzaazouaet al., 2004a,d). However, despite these interesting results,there are still significant uncertainties regarding the envi-ronmental impacts of paste backfill.

Fig. 1 presents a flow sheet diagram focusing on themine waste management purposes. The objective of thisstudy was to evaluate the behaviour of a conventional tail-ings paste backfill mixture when treatment sludge (two dif-ferent samples coming from high density sludge limetreatment) or red mud (aluminium processing) are incorpo-rated into it as illustrated in Fig. 1. The study is based onthe characterization of the mechanical behaviour as well asthe leachability potential of various paste cured mixturescontaining two types of binders. The paste samples werealso studied in terms of their microstructural and mineral-ogical characteristics. Eventually, the addition of watertreatment sludge and red mud into cemented paste backfill

Underground backfill Surface disposal

Pastefill preparation

ilings PAG

cessing

yanidation ...

tallic ores

actionWaste rocks

with PAG

Cyanide destruction

he mine waste management purposes.

Table 2Chemical analyses of LaRonde tailings

Parameter (wt.%) Tailings #1 Tailings #2

Al 3.99 3.97B 0.009 0.015Ba 0.015 0.015Ca 1.05 1.06Cd 0.001 0.001Co 0.002 0.002Cr 0.009 0.009Cu 0.066 0.067Fe 25.3 25.5Mg 0.339 0.345Mn 0.021 0.021Na 1.15 1.16Ni 0.006 0.005Pb 0.111 0.111Stotal 22.7 23.1SSO�

40.338 0.337

Zn 0.343 0.344

Note: Total digestion HNO3/Br2/HF/HCl with QA–QC controls exceptSO�

4 (40% HCl extraction).

422 M. Benzaazoua et al. / Minerals Engineering 19 (2006) 420–432

could become a new management strategy for these twotypes of waste.

2. Material and methods

2.1. Tailings samples

A representative sample was collected directly fromLaRonde Mine�s backfill plant (Quebec) after filtrationprocess. Particle size analyses (Fig. 2) were conducted onthe tailings sample using a laser particle size analyser (Mal-vern Mastersizer). Table 1 shows the main particle sizecharacteristics and relative density (Dr) of the tailings stud-ied. The diameter corresponding to 90% passing was118.59 lm, 50% of the particles were inferior to 26.12 lmand 10% were smaller than 3.70 lm. The relative densitywas 3.83 which reflects the high proportion of sulphidesin the LaRonde tailings.

Table 2 presents the main chemical characteristics of theLaRonde tailings used in this study. Results are duplicatedensure of the homogeneity of the whole sample. One canobserve a high sulphur content which would indicate a cer-tain amount of sulphides (about 40%) in the tailings. Thetailings are composed mainly of pyrite, aluminosilicateand quartz.

2.2. Mixing water

Additional water is used during the preparation of thepaste backfill (fresh municipal water) to reach the targetedslump test value (about 7 in.) which corresponds to a per-centage of solids between 77% and 80% depending on the

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 10 100 1000

Particle size (µm)

Cu

mul

ated

vo

lum

e(%

)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Vo

lum

e(%

)

1

Fig. 2. Particle size distribution (cumulative and histogram) of LaRondetailings.

Table 1Physical characteristics of LaRonde tailings

Parameters Values

D10 3.70 lmD50 26.12 lmD90 118.6 lmRelative density, Dr 3.83

mixture type. It is worth mentioning that the residual waterremaining in the tailings after filtration was highly sulph-ated. Concentrations between 1700 and 3774 mg/L weremeasured in LaRonde pore–water which could compro-mise backfill durability (Benzaazoua et al., 2004b). Sul-phates could come from the SO2 air process, which isused for cyanide destruction before tailings are mixed withthe backfill material, as well as from pyrite oxidation dur-ing the previous processes due to the high sulphide contentin the tailings.

2.3. Binders

Three types of cement were used in this study. Theirphysical characteristics are summarized in Table 3. The fol-lowing cements were blended (see Table 5 for the propor-tion) to make two types of binders widely used by mineoperators:

• Type 10 Portland cement produced by Ciment St-Lau-rent (PC10);

• Type 50 Portland cement produced by Ciment St-Lau-rent (PC50);

• Slag produced by Ciment Lafarge (Slag).

Three binder percentages were tested: 3%, 5% and 7% ofthe total dry solid weight. These are typical binder dosagesused by the mine operation respecting technical and eco-nomical requirements (Hassani and Archibald, 1998).

2.4. Mixture preparation and slump tests

Mixing: The mixing was conducted using a mortarmixer (Equip. Mathieu Inc. 3HP) which produces a goodhomogenized mixture (binder, tailings, and water). The lat-ter was blended for about 7 min.

Table 3Specific surface area of the binder and sludge samples

Parameter Portland cement T10 Portland cement T50 Slag cement Sludge A HDS Sludge B HDS Red mud C

Specific area (m2/g) 1.32 0.97 2.16 59.44 172.24 14.02Relative density, Dr 3.12 3.21 3.27 2.42 2.55 3.10


Moulding: The moulding was done according to theB.N.Q. 2622-913 norms and using non-drained plasticmoulds with tight caps.

Conditioning: Caped moulds from the testing series wereplaced in a room where the humidity and temperature werecontrolled. The minimal relative humidity was fixed at 85%and the temperature was maintained at 23 �C.

2.5. Sludge samples

Three different types of sludge were used in this study.Their chemistry is summarized in Table 4. Sludge A con-sisted of a high density sludge (HDS) produced by a tetraHDS process and it was collected from a pond. Sludge Acontained 12.1% iron, 27.6% sulphate, 3.5% aluminiumand a relatively low content of heavy metals. Sludge B, alsoproduced by a conventional HDS process, had a higher con-tent in Fe (28.0%), Zn (2.82%) and Al (4.59%). The red mud(Mat C) was a totally different material. The red mud stud-ied had a higher content of Al (11.7%) and Fe (29.0%) thanthe water treatment sludges used in this study, while theother metals (Ni, Cu, Zn, Cd, Pb) were present at low levels.

2.6. Paste preparation

Table 5 presents the proportions of the various mixtures(total of 36) that were prepared and tested for their com-

Table 4Chemical analysis of the sludge samples

Element (wt.%) Sludge HDS A Sludge HDS B Red mud C

Al 3.46 4.59 11.7Si 1 3 5K 0.031 0.122 0.088B 0.710 n/a 0.554Ba 0.001 0.002 0.004Ca 13.8 3.83 0.92Cd <0.001 0.001 0.001Co 0.018 0.029 0.004Cr 0.003 0.003 0.123Cu 0.093 0.761 0.001Fe 12.1 28 29.2Mg 0.781 2.73 0.025Mn 0.308 0.911 0.028Na 1.3 1.16 5.08Ni 0.013 0.013 0.068Pb <0.003 0.003 0.003Stotal 9.23 1.83 0.189SSO�

410.7 1.95 0.15

Zn 0.048 2.82 0.008

Note: Total digestion using HNO3/Br2/HF/HCl with QA–QC controlsexcept SO�

4 (40% HCl extraction) Si (Na2O2 fusion) and K (external labanalysis).

pressive strength. Four different curing times (7, 28, 56and 120 days) have been chosen. For a given mixture, ablend of two cements was used at three proportions (2, 5and 7) calculated by the total solid dry weight. The sludgematerials were added at two different proportions (0.15%and 0.30% by total solid dry weight). Control samples thatcontained only the binder but no sludge were also preparedin order to independently understand the effect of sludgeaddition to paste backfill. Samples which were too plasticat short-term curing (7 days) due to poor cohesion werepostponed for long-term testing (180 days). A total of144 samples were prepared in triplicate in this study fora total of 432 moulds according to the proportions of Table5.

2.7. Mechanical behaviour: Unconfined compressivestrength (UCS)

Before the trials, the samples (Table 5) were rectified toobtain a length to diameter ratio (L/D) of 2 with an accu-racy of 1 mm. The unconfined compressive strength wasmeasured according to the B.N.Q. 2622-912 norms. Trialswere conducted using a MTS 10/GL mechanical press witha capacity of 50 kN and a minimal charging rate of 1 mm/min.

2.8. Leachability: SPLP normalized tests

Several leaching protocols have been developed to testwaste leachability. However, none of these leaching testshave been specifically designed for evaluating AMD watertreatment sludge leachability (Zinck, 1999). The syntheticprecipitation leaching procedure (SPLP) was selected sincethe extraction fluid is more representative of mine wastedisposal conditions. The SPLP is designed to determinethe mobility of both organic and inorganic compounds inthe waste using a mixture of sulphuric and nitric acid(60/40 wt.) as the extraction fluid. The particle size of thepaste backfill material was reduced; the sample wascrushed and sieved. The 2.36–9.5 mm granulometric frac-tion was subjected to the 24 h leaching tests. After theextraction, the samples were filtrated (0.45 lm) and acidi-fied using 0.4% (v/v) nitric acid. Total dissolved metal con-tents (Al, As, Ca, Cd, Cr, Cu, Mn, Ni, Pb, S, Se, Zn) weredetermined by inductively coupled plasma mass spectrom-etry, using an ELAN 6100 DRC (PerkinElmer, Wellesley,MA). Metals analysis for Fe and Mg were conductedthrough Inductively Coupled plasma atomic emission spec-trometry, using a Varian Vista RL ICP-AES (Varian Inc.,Palo Alto, CA).

Table 5Proportion and cement types used for the paste backfill study

Names ProportionPC10

ProportionPC50

Proportionslag

Total binder (%)/dry weight

Sludge (%)/dry weight

Sludge type

Mix #1 50 50 0 2 0 No sludgeMix #2 50 50 0 5 0 No sludgeMix #3 50 50 0 7 0 No sludgeMix #4 50 50 0 2 0.15 Sludge AMix #5 50 50 0 2 0.3 Sludge AMix #6 50 50 0 2 0.15 Sludge BMix #7 50 50 0 2 0.3 Sludge BMix #8 50 50 0 2 0.15 Red mud CMix #9 50 50 0 2 0.3 Red mud CMix #10 50 50 0 5 0.3 Sludge AMix #11 50 50 0 5 0.3 Sludge BMix #12 50 50 0 5 0.3 Red mud CMix #13 50 50 0 7 0.15 Sludge AMix #14 50 50 0 7 0.15 Sludge BMix #15 50 50 0 7 0.15 Red mud CMix #16 50 50 0 7 0.3 Sludge AMix #17 50 50 0 7 0.3 Sludge BMix #18 50 50 0 7 0.3 Red mud C

Mix #19 20 0 80 2 0 No sludgeMix #20 20 0 80 5 0 No sludgeMix #21 20 0 80 7 0 No sludgeMix #22 20 0 80 2 0.15 Sludge AMix #23 20 0 80 2 0.3 Sludge AMix #24 20 0 80 2 0.15 Sludge BMix #25 20 0 80 2 0.3 Sludge BMix #26 20 0 80 2 0.15 Red mud CMix #27 20 0 80 2 0.3 Red mud CMix #28 20 0 80 5 0.3 Sludge AMix #29 20 0 80 5 0.3 Sludge BMix #30 20 0 80 5 0.3 Red mud CMix #31 20 0 80 7 0.15 Sludge AMix #32 20 0 80 7 0.3 Sludge AMix #33 20 0 80 7 0.15 Sludge BMix #34 20 0 80 7 0.3 Sludge BMix #35 20 0 80 7 0.15 Red mud CMix #36 20 0 80 7 0.3 Red mud C


2.9. Methods for microstructural and mineralogical

characterization

A Hitachi� 3500-N scanning electron microscope(SEM) was used to characterize the microstructure and tex-ture of the various backfill samples. The backscattered elec-tron (BSE) imaging mode was chosen because it producesimages with a chemical contrast superimposed upon therelief contrast. Energy dispersive spectrometry (EDS) ofan X-ray probe coupled to the SEM was used to character-ize some hydrated phases.

X-ray diffraction (XRD) analyses were carried out on thetailings and cemented paste samples to investigate the min-eralogical effect of the cement and sludge additions. Theanalyses were conducted on a Bruker A.X.S. D8 advanceinstrument equipped with a Co anticathode and scintilla-tion counter. Data treatment was done using BrukerA.X.S. EVA and TOPAS software bundles. Diffractogramswere obtained by a total of five sequential integrations for2h ranges of 5–70� with a step size of 0.005� and 0.5 s inte-

gration time per step. These sequential integrations wereused to improve the signal-to-noise ratio and to better iden-tify trace phases. In order to reduce the effect of patternshifts, the diffractograms obtained were then corrected forvertical sample displacement using the correction toolincluded in the TOPAS software using the Quartz patternas a reference. The corrected diffractograms for the control(no sludge) cemented pastes PC10-50 and PC10-Slag werethen subtracted from the diffractograms of the correspond-ing cemented pastes containing HDS and red mud using theEVA software. This operation is illustrated in Fig. 3 whichcontains the spectra of PC10-50 and PC10-50-red mud C,the phase identification showing the position of all peaksexcept those of the tailings minerals (not shown for clarity),and finally the result of the subtraction.

Fig. 3 highlights discrepancies between cemented pastescontaining red mud and those free of red mud. Quantifica-tion of identified mineral phases was obtained by using theRietveld method (Rietveld, 1993; Taylor and Hinczak,2001) which is integrated into the TOPAS software.

01-083-2384 (C) - Boehmite - AlO(OH) - Y: 20.00 %

01-074-1119 (C) - Bayerite - Al(OH)3 - Y: 20.00 %

01-081-0464 (C) - Goethite, syn - FeO(OH) - Y: 20.00 %

01-076-1782 (C) - Gibbsite - Al(OH)3 - Y: 20.00 %

00-021-1276 (*) - Rutile, syn - TiO2 - Y: 20.00 %

00-005-0586 (*) - Calcite, syn - CaCO3 - Y: 20.00 %

00-033-0311 (*) - Gypsum, syn - CaSO4·2H2O - Y: 20.00 %

00-041-1451 (*) - Ettringite, syn - Ca6Al2(SO4)3(OH)12·26H2O - Y: 20.00 %

00-044-1481 (*) - Portlandite, syn - Ca(OH)2 - Y: 20.00 %

7% CP10CP50 (Alcan - Temoin) - File: 7%CP10CP50 Alcan qual.raw -Type: 2Th/Th locked - Start

7% CP10CP50(temoin) qual - File: 7%CP10CP50(temoin) qual.raw - Type: 2Th/Th locked - Start:

7% CP10CP50 Alcan qual -File: 7%CP10CP50 Alcan qual.raw - Type:2Th/Th locked - Start: 4.89

Lin

(Cou

nts)

0

100

200

300

2-Theta - Scale

5 10 20 30 40 50 60 70

-190

-100

0

100

5 10 20 30 40 50 60 70

Amorphous CSH zone

Feroxyhyte (delta-FeO(OH)) (?)

1: 7%CP10-50 Red mud

2: 7%CP10-50

Subtraction 1-2

Fig. 3. XRD data treatment: case of PC10-50-Red mud C (red) and PC10-50 (blue) XRD spectra subtraction (result in black). (For interpretation of thereferences in colour in this figure legend, the reader is referred to the web version of this article.)

600

800

1000

1200

1400

1600

Str

eng

th (

kPa)

5% T10-T50

5% T10-T50, Mat A 0.30%

5% T10-T50, Mat B 0.30%

5% T10-T50, Mat C 0.30%

5% T10-Slag

5% T10-Slag, Mat A 0.30%

5% T10-Slag, Mat B 0.30%

5% T10-Slag, Mat C 0.30%


3. Results and discussion

3.1. Mechanical behaviour

The UCS values obtained for the various trials are pre-sented in Figs. 4–6. A good degree of reproducibility wasobserved between the triplicates for each mix and a givencuring time, which indicates good sample homogeneity.

0

50

100

150

200

250

300

350

0 28 56 84 112 140 168 196

Curing time (days)

Str

eng

th (

kPa)

2% T10-T50

2% T10-T50, Mat A 0.15%2% T10-T50, Mat A 0.30%

2% T10-T50, Mat B 0.15%2% T10-T50, Mat B 0.30%

2% T10-T50, Mat C 0.15%2% T10-T50, Mat C 0.30%

2% T10-Slag2% T10-Slag, Mat A 0.15%

2% T10-Slag, Mat A 0.30%2% T10-Slag, Mat B 0.15%

2%T10-Slag, Mat B 0.30%2% T10-Slag, Mat C 0.15%

2% T10-Slag, Mat C 0.30%

Fig. 4. UCS evolution during curing time for pastefill mixtures containing2% binder.

0

200

400

0 28 56 84 112 140 168 196

Curing time (days)

Fig. 5. UCS evolution during curing time for pastefill mixtures containing5% binder.

Results can be analyzed in terms of the effect of the var-ious binders and sludge composition at various curingtimes. Figs. 4–6 present the UCS versus curing time forthe LaRonde tailings prepared with 2%, 5%, and 7% bind-ers, respectively. The strength acquisition slowed downafter approximately 56 days of curing and reached a pla-teau in terms of UCS values. This type of behaviour due

0

500

1000

1500

2000

2500

3000

0 28 56 84 112 140 168 196

Curing time (days)

Str

eng

th (

kPa)

7% T10-T50

7% T10-T50,Mat A 0.15%

7% T10-T50,Mat A 0.30%

7% T10-T50,Mat B 0.15%7% T10-T50,Mat B 0.30%

7% T10-T50,Mat C 0.15%

7% T10-T50,Mat C 0.30%

7% T10-S lag

7% T10-S lag,Mat A 0.15%

7% T10-S lag,Mat A 0.30%

7% T10-S lag,Mat B 0.15%7% T10-S lag,Mat B 0.30%

7% T10-S lag,Mat C 0.15%

7% T10-S lag,Mat C 0.30%

Fig. 6. UCS evolution during curing time for paste backfill mixturescontaining 7% binder.


to the hardening limitation of backfill materials has beenreported in many studies (e.g. Benzaazoua et al., 2002,2004b, etc.). This behaviour was observed for all of thebinders studied with or without sludge addition and inde-pendently of their proportions.

From these results, it is also clear that whatever the bin-der concentration, the Portland cement (PC10-50, 50/50)appears to produce a stronger backfill material for theLaRonde tailings. At 56 days, a UCS value of 200 kPawas reached with 2% binder, 1200 kPa with 5% binderand 2200 kPa with 7% binder. The addition of sludgeappears to be not suitable for the studied tailings. Thisbehaviour has been already observed in the case of sul-phide-rich tailings where the slag is often characterizedby a very slow strength acquisition (Benzaazoua et al.,2004b). However, the sludge incorporating within slagbased paste backfill did not improve the mechanical resis-tance of the backfill material except for the red mud withthe higher concentration (0.3%). The paste backfill usingslag additive to the Portland cement offered poor resultsindependently of cement proportion. This can be explainedmainly by the sulphate effect that inhibits the hydrationprocesses (Benzaazoua et al., 2004b; Fall and Benzaazoua,2005).

Also, with higher binder concentrations (5–7%), addi-tion of any of the sludge types used in this study to thePortland cement and slag combination was beneficial,making it possible to achieve a mechanical resistance atleast comparable to Portland cement (PC10-50) after alonger curing time (after 100 days). However, the Portland(PC10-50) cement appeared to be penalized by the additionof sludge. If the trend of hardening is confirmed, the slagbased pastefill containing sludges or red mud became moreresistant.

These phenomena imply some assumptions to explainthe mechanism of strength acquisition of paste materialwith and without sludge addition for the two types of usedbinders. First, the sludge phases may co-precipitate withsulphate species contained by the pore water leading to bet-ter hydration behaviour of the slag cements. The sulphateprecipitation in this case contributes to the paste harden-

ing. Contrarily, it is well known that the Portland cementhydration often use sulphates to ensure good strength.However, if there is an excess of sulphate availability, sul-phate attack can occurs and lead to an inhibition of thehardening process. This is in accordance with the highgrade of ettringite and gypsum in these sludge-bearing sam-ples in comparison with sludge-free samples.

For higher binder content (5–7%), it is difficult to notedifferences in the overall mechanical behaviour dependingon the type of sludge incorporated into the backfill. Forthe given sludge proportion range that was studied (0–0.3%), sludges seem to behave in the same manner indepen-dently of the sludge amount mixed into the LaRondebackfill material.

3.2. Leachability

This section presents the batch leaching results for thevarious materials studied and paste backfill mixtures. Table6 presents the tailings, sludges A and B as well as the redmud C leaching tests, while Table 7 shows the results forpaste backfill samples leaching tests. The SPLP results werecompared to the regulatory discharge limits for indicativepurpose even if they are not fully comparable.

The addition of hydraulic binders significantly decreasedthe tailings metal mobility in regards to zinc and manga-nese. The zinc concentration leached from the tailingswas not in compliance with the Quebec governmentDirective 019 on mining effluents with a value of0.99 mg L�1 (Table 6). Addition of hydraulic binders sig-nificantly decreased the Zn mobility to values typicallybelow 0.05 mg L�1 which is in conformity with theQuebec-regulated discharge limit. The manganese leach-ability decreased from 0.75 mg L�1 to values below detec-tion limit. Compared to the mixture of Portland cementand slag, the Portland cement alone was more efficientin stabilizing manganese and zinc. As a result of the addi-tion of Portland cement (PC10-50 50/50) and slag-based cement (PC10-Slag 20/80), an increased mobility ofaluminium, calcium, sulphates, iron and silica wasobserved. However, the increased metal concentrationwas not a concern because several parameters (Al, Ca,SO2�

4 and Si) are not regulated and the Fe concentrationremained below the Quebec-regulated discharge limit.The addition of only 2% hydraulic binders (PC10-50 50/50 or PC10-Slag 20/80) is sufficient to minimize the Znand Mn leachability.

When sludge B was added to the tailings, Zn mobility inthe tailings decreased (Table 6) because of the sludge�salkalinity. Like sludge A, the addition of binders to the tail-ings and sludge B significantly decreased the Zn and Mnconcentrations. The sludge addition did not have anyimpact on metal mobility for this disposal scenario. How-ever, the addition of both hydraulic binders, PC10-50 50/50 and PC10-Slag 20/80, slightly increased the metal mobil-ity for Al, Ca, SO2�

4 , Fe and Si. The Portland cement bin-der seems to be more efficient than the slag-based cement

Tab

le6

DetailedSPLPresultsobtained

forthematerialstudied

Mixture

Final

pH

Al

(ppm)

As

(ppm)

Ca

(ppm)

Cd

(ppm)

Cr

(ppm)

Cu

(ppm)

Fe

(ppm)

Mg

(ppm)

Mn

(ppm)

Ni

(ppm)

Pb

(ppm)

Si

(ppm)

SO

4

(ppm)

Zn

(ppm)

Tailings

Residues

100%

7.50

0.007

<0.04

199

0.009

<0.002

0.004

0.005

1.77

0.754

0.02

<0.03

0.548

547

0.991

Sludges

Mat

B0.15

%7.22

0.00

8<0.04

207

0.00

7<0.00

2<0.00

40.00

42.13

0.84

9<0.02

<0.03

0.53

758

80.69

1Mat

B0.30

%7.35

0.00

7<0.04

206

0.00

6<0.00

2<0.00

40.00

62.34

0.83

0<0.02

<0.03

0.50

761

10.37

4Mat

C0.15

%7.34

0.00

9<0.04

200

0.00

8<0.00

2<0.00

40.00

71.69

0.69

0<0.02

0.04

0.65

559

0.80

9Mat

C0.30

%7.26

0.00

9<0.04

191

0.00

6<0.00

2<0.00

40.00

41.65

0.62

2<0.02

0.04

0.81

655

50.65

4Mat

A0.15

%7.38

0.01

1<0.04

196

0.00

5<0.00

2<0.00

40.00

72.00

0.65

9<0.02

<0.03

0.84

557

60.71

5Mat

A0.30

%7.25

0.00

8<0.04

206

0.00

6<0.00

2<0.00

40.00

62.49

0.86

5<0.02

<0.03

0.85

662

60.95

0Mat

A0.30

%a

7.25

0.00

8<0.04

212

0.00

7<0.00

2<0.00

40.00

82.53

0.92

1<0.02

<0.03

0.85

160

60.99

9

Paste

without

sludge

PC10

-502%

8.11

0.04

9<0.04

393

<0.00

3<0.00

2<0.00

40.02

05.28

0.02

3<0.02

<0.03

2.00

1203

0.03

7PC10

-502%

a8.96

0.22

0<0.04

387

<0.00

20.00

57<0.00

40.01

13.67

<0.00

7<0.02

<0.06

1.25

1098

<0.02

PC10

-502%

a8.95

0.21

1<0.04

397

<0.00

20.00

49<0.00

40.01

23.87

0.00

8<0.02

<0.06

1.29

1106

<0.02

PC10

-505%

7.87

0.43

<0.04

418

<0.00

30.00

8<0.00

40.00

71.62

<0.00

7<0.02

<0.03

1.19

1599

0.01

5PC10

-507%

10.15

0.25

9<0.04

443

<0.00

30.00

620.00

40.00

80.64

5<0.00

7<0.02

<0.03

1.88

1817

0.01

0PC10

-Slag2%

8.66

0.97

4<0.04

213

<0.00

20.00

10.00

40.00

41.09

0.00

7<0.02

<0.06

0.77

476

6<0.02

PC10

-Slag2%

a8.85

1.36

<0.04

169

<0.00

20.00

150.00

40.00

50.81

7<0.00

7<0.02

<0.06

0.69

662

7<0.02

PC10

-Slag5%

8.79

0.48

3<0.04

398

<0.00

20.00

140.00

90.00

93.38

0.00

9<0.02

<0.06

1.2

1457

<0.02

PC10

-Slag7%

8.82

0.29

2<0.04

541

<0.00

20.00

220.00

50.01

04.03

0.01

0<0.02

<0.06

1.78

1912

<0.02

aTestduplicate.


and a higher concentration of hydraulic binders did notsignificantly minimize the Zn and Mn leachability.

The addition of red mud C to the tailings did not haveany significant impact on metal leachability; the Al, Ca,Fe, SO2�

4 and Mn remained relatively at the same level.Similar to sludges A and B, the leached metal concentra-tion from red mud mixtures decreased considerably for Znand Mn with the addition of the binders. Also, no majordifferences in terms of zinc and manganese leachabilitywere observed with an increasing amount of hydraulicbinders.

3.3. Effects of binder and sludge additions on the

microstructures and mineralogy of the pastefill materials

Among the samples broken for the UCS testing, eightsamples (cured at 56 days then oven-dried at 50 �C) wereselected for further microstructural and mineralogicalinvestigation. This investigation concerned only samplescontaining 7% binder. The two binders (PC10-50 andPC10-Slag) have been characterized with and withoutthe three sludge additions. A limitation of this character-ization must be mentioned here: the samples were all driedprior to SEM and XRD analysis. This unavoidable stepled to the formation of some new phases which contrib-uted to the densification of the cementitious matrix inthe samples. These phenomena reflect, to a certain degree,an artificial curing acceleration, and the sample obtainedcan be considered as well-cured backfill.

The SEM investigation was conducted to point out thedifferences between the microstructures of the paste back-fill samples. The most important parameter in the under-ground backfill technology is the strength acquisition thatconditions the mining operation sequences. The strengthis linked to the microstructure (Ouellet et al., 2003). Thelatter is formed by the cementitious phase network ofthe paste material. The more these phases are abun-dant and dense, the more the microstructure is charac-terized by a finer porosity and the material is stronger.BSE-SEM images were taken at different magnificationsfor the eight samples concerned and are presented inFig. 7.

The main observations are that very few differencesexist between the samples studied. Some cementitiousphases between the tailings grains are easily observed.No evident relationship has been detected that couldexplain the observed differences in strength behaviour ofthe samples. This is probably due to necessary pre-treat-ment (drying) before the SEM investigation.

EDS analyses were done on some phases to get an ideaof the chemical composition of the cementitious phasesobserved by SEM imaging. This operation faced the prob-lem of the probe size that does not allow the analysis ofthese phases without interference from the grains aroundthe targeted particle. This problem occurs when one hasto deal with very finely consolidated material usingX-ray spectrometry based on an electron primary source

Table 7Detailed SPLP results obtained for the paste backfill leachates

Mixture FinalpH

Al(ppm)

As(ppm)

Ca(ppm)

Cd(ppm)

Cr(ppm)

Cu(ppm)

Fe(ppm)

Mg(ppm)

Mn(ppm)

Ni(ppm)

Pb(ppm)

Si(ppm)

SO4

(ppm)Zn(ppm)

Paste PC and slagwith sludge

PC10-Slag 2% Mat A 0.15% 8.32 0.277 <0.04 315 <0.002 <0.0008 0.007 0.004 1.80 0.033 <0.02 <0.06 0.931 886 0.090PC10-Slag 2% Mat A 0.30% 8.64 0.624 <0.04 232 <0.002 0.0008 0.006 <0.005 1.05 0.013 <0.02 <0.06 0.544 671 0.033PC10-Slag 5% Mat A 0.3% 8.94 0.631 <0.04 377 <0.002 0.0012 0.006 0.007 2.48 0.008 <0.02 <0.06 1.01 1214 0.012PC10-Slag 7% Mat A 0.15% 8.76 0.371 <0.04 566 <0.002 0.0012 0.004 0.010 4.62 0.012 <0.02 <0.06 1.37 1794 0.014PC10-Slag 7% Mat A 0.30% 8.31 0.241 <0.04 631 <0.002 0.0009 0.005 0.009 4.77 0.021 <0.02 <0.06 1.64 1896 0.032PC10-Slag 2% Mat B 0.15% 8.63 0.630 <0.04 247 <0.002 <0.0008 0.006 <0.005 1.30 0.010 <0.02 <0.06 0.661 777 0.033PC10-Slag 2% Mat B 0.30% 8.69 1.17 <0.04 211 <0.002 0.0009 0.005 <0.005 1.02 0.008 <0.02 <0.06 0.65 640 0.024PC10-Slag 5% Mat B 0.30% 8.30 0.229 <0.04 507 <0.002 0.0012 0.004 0.008 3.94 0.022 <0.02 <0.06 1.68 1550 0.045PC10-Slag 7% Mat B 0.15% 8.45 0.292 <0.04 601 <0.002 <0.0008 <0.004 0.0099 4.72 0.015 <0.02 <0.06 1.54 1815 0.020PC10-Slag 7% Mat B 0.30% 8.47 0.301 <0.04 602 <0.002 0.0011 0.005 0.0091 4.78 0.018 <0.02 <0.06 1.51 1912 0.022PC10-Slag 7% Mat B 0.30%a 8.45 0.314 <0.04 610 <0.002 <0.0008 0.005 0.0098 4.68 0.018 <0.02 <0.06 1.51 1918 0.017PC10-Slag 2% Mat C 0.15% 9.00 1.06 <0.04 186 <0.002 0.001 0.004 <0.005 0.857 0.009 <0.02 <0.06 0.633 583 0.024PC10-Slag 2% Mat C 0.30% 8.95 1.18 <0.04 196 <0.002 0.0013 0.004 <0.005 0.81 0.007 <0.02 <0.06 0.569 597 0.023PC10-Slag 5% Mat C 0.30% 6.25 0.444 <0.04 400 <0.002 <0.006 0.006 <0.005 3.02 0.002 <0.02 <0.06 1.02 1240 0.018PC10-Slag 7% Mat C 0.15% 8.75 0.386 <0.04 526 <0.002 0.0014 0.007 0.010 4.53 0.010 <0.02 <0.06 1.46 1746 0.013PC10-Slag 7% Mat C 0.30% 8.70 0.286 <0.04 518 <0.002 0.0015 0.013 0.010 4.04 0.011 <0.02 <0.06 1.88 2005 <0.02

Paste PC withsludge

PC10-50 2% Mat A 0.15% 9.30 0.748 <0.06 256 <0.003 0.0106 <0.002 0.004 1.04 <0.009 <0.02 <0.06 0.715 750 0.022PC10-50 2% Mat A 0.30% 9.00 0.437 <0.06 306 <0.003 0.0047 <0.002 0.005 1.82 <0.009 <0.02 <0.06 0.804 904 0.013PC10-50 5% Mat A 0.30% 10.00 0.409 <0.06 345 <0.003 0.0081 0.003 0.007 0.585 <0.009 <0.02 <0.06 1.61 1403 0.008PC10-50 7% Mat A 0.15% 10.00 0.346 <0.06 617 <0.003 0.0060 0.008 0.006 0.661 <0.009 <0.02 <0.06 1.83 2905 0.006PC10-50 7% Mat A 0.3% 9.95 0.345 <0.06 586 <0.003 0.0058 0.004 0.007 0.908 <0.009 <0.02 <0.06 1.57 2256 0.007PC10-50 7% Mat A 0.3%a 10.00 0.410 <0.06 353 <0.003 0.0075 0.003 0.006 0.596 <0.009 <0.02 <0.06 1.58 1428 0.007PC10-50 2% Mat B 0.15% 9.35 1.04 <0.06 268 <0.003 0.0119 0.003 0.009 1.01 <0.009 <0.02 <0.06 0.741 789 0.018PC10-50 2% Mat B .3% 9.23 0.949 <0.06 287 <0.003 0.0085 0.003 0.005 1.08 <0.009 <0.02 <0.06 0.642 919 0.017PC10-50 5% Mat B 0.3% 8.73 0.157 <0.06 608 <0.003 <0.004 0.007 0.009 4.42 0.011 <0.02 <0.06 1.27 1944 0.015PC10-50 7% Mat B 0.15% 10.00 0.293 <0.06 503 <0.003 0.0052 0.007 0.008 0.50 0.008 <0.02 <0.06 2.11 2424 0.006PC10-50 7% Mat B 0.30% 10.00 0.354 <0.06 534 <0.003 0.0053 0.012 0.012 0.603 <0.009 <0.02 <0.06 1.9 2427 0.007PC10-50 2% Mat C 0.15% 8.82 0.199 <0.04 407 <0.002 0.0042 <0.004 0.005 2.42 0.010 <0.02 <0.06 1.05 1111 0.025PC10-50 2% Mat C 0.30% 9.41 0.828 <0.04 345 <0.002 0.0104 <0.004 <0.005 1.26 <0.007 <0.02 <0.06 0.692 936 0.015PC10-50 5% Mat C 0.3% 9.67 0.537 <0.04 522 <0.002 0.006 <0.004 0.006 1.42 <0.007 <0.02 <0.06 1.03 1424 <0.02PC10-50 7% Mat C 0.15% 10.17 0.252 <0.04 521 <0.002 0.0075 0.007 0.009 0.563 <0.007 <0.02 <0.06 2.18 1744 <0.02PC10-50 7% Mat C 0.30% 10.07 0.318 <0.04 585 <0.002 0.0065 0.006 0.008 0.559 <0.007 <0.02 <0.06 1.8 1931 <0.02PC10-50 7% Mat C 0.30%a 10.05 0.326 <0.04 596 <0.002 0.0067 0.007 0.008 0.547 <0.007 <0.02 <0.06 1.87 2045 <0.02

a Test duplicate.

428M.Benzaazouaet

al./Minera

lsEngineerin

g19(2006)420–432

Fig. 7. SEM backscattered images on different paste backfill (7% binder) with and without of sludge addition.


signal (Benzaazoua, 1996). However, it provides a generalidea of the chemical composition of the binding matriceswhich are composed of silicon, calcium, and aluminium

Table 8Mean EDS analyses of the two spots on each of the 2000X SEM-BSE images

Element Element wt.%

CP10-SLAG

Control HDS-A HDS-B RM-C

Na 0.7 <0.5 1.5 1.0Mg <0.5 <0.5 <0.5 <0.5Al 4.0 2.9 5.3 4.1Si 9.6 8.7 9.7 10.4S 10.3 11.4 9.8 8.5K <0.5 0.7 1.2 0.5Ca 4.0 11.3 2.3 3.0Ti <0.5 <0.5 <0.5 <0.5Fe 26.2 17.1 23.5 28.0Zn 1.0 4.0 3.0 1.5O 43.5 43.1 43.2 42.6

as major constituents, as well as iron and sulphur. Table 8shows the mean EDS analyses of the 2 spots of cementedphases identified on each of the 2000X BSE images

CP10-CP50

Control HDS-A HDS-B RM-C

<0.5 <0.5 0.5 1.3<0.5 <0.5 0.5 0.32.5 2.7 5.3 4.65.7 7.0 10.6 10.86.1 7.0 11.5 12.2

<0.5 1.0 0.7 <0.54.7 14.9 10.2 5.0

<0.5 <0.5 <0.5 2.537.4 26.2 14.0 14.35.5 2.0 1.4 2.8

37.4 39.0 45.2 45.8


presented in Fig. 7. It should be noted that the EDS anal-yses were normalized to 100% and that the oxygen was

Table 9Detailed XRD data analyses and quantification results obtained for the paste

Mineral Formula

Silicates Quartz SiO2

Muscovite KAl3Si3O10(OH)2Paragonite NaAl3Si3O10(OH)2Chlorite (Mg,Al)6(Si,Al)4O10(OH)8Oligoclase (An25) Na0.75Ca0.25Al1.2Si2.8O8

Sulphides Chalcopyrite CuFeS2Pyrite FeS2Pyrrhotite Fe(1�x)S (0 < x < 0.17)Sphalerite (ferrous) Zn0.59Fe0.41S

Cementitious phases Calcite CaCO3

Ettringite Ca6Al2(SO4)3(OH)12 Æ26(H2O)Gypsum CaSO4 Æ2(H2O)Portlandite Ca(OH)2

Red mud contribution Bayerite Al(OH)3Boehmite AlO(OH)Gibbsite Al(OH)3Goethite FeO(OH)Rutile TiO2

Others gamma-FeO(OH) FeO(OH)Rodochrosite MnCO3

Table 10Detailed XRD data analyses and quantification results obtained for the paste

Mineral Formula

Silicates Albite NaAlSi3O8

Chlorite (Mg,Al)6(Si,Al)4O10(OHMuscovite KAl3Si3O10(OH)2Paragonite NaAl3Si3O10(OH)2Quartz SiO2

Sulphides Chalcopyrite CuFeS2Pyrite FeS2Pyrrhotite Fe(1�x)S (0 < x < 0.17)Sphalerite (ferrous) Zn0.59Fe0.41S

Cementitious phases Calcite CaCO3

Ettringite Ca6Al2(SO4)3(OH)12 Æ26Gypsum CaSO4 Æ2(H2O)Portlandite Ca(OH)2

Steel slag contribution Akermatine Ca2Mg(Si2O7)b-C2S Ca2SiO4

Bredigite Ca7Mg(SiO4)4C2F Ca2Fe2O5

C3S Ca3SiO5

C4AF Ca4Al2Fe2O10

Fayalite (magnesian) (Fe,Mg)2SiO4

Forsterite (iron) (Mg,Fe)2SiO4

Gehlenite (C2AS) Ca2Al2SiO7

Red mud contribution Bayerite Al(OH)3Boehmite AlO(OH)Gibbsite Al(OH)3Goethite FeO(OH)Rutile TiO2

quantified by stoichiometry using the oxides of the otheranalysed elements.

backfill PC10-50

Sample

PC10-50 control PC10-50 Mat C PC10-50 Mat A PC10-50 Mat B

23.43 23.56 13.78 26.427.65 6.33 1.70 3.453.40 6.98 6.60 3.574.30 1.53 5.12 3.456.94 8.06 7.00 8.05

0.55 0.67 0.29 1.3942.63 36.83 49.46 40.430.98 1.05 1.62 3.411.47 2.96 2.40 0.76

1.68 0.99 1.80 0.103.19 3.58 3.36 4.041.29 2.80 5.38 4.900.09 0.19 0.17 0.01

0.721.610.88

0.25 0.610.64

2.18 0.02 0.12 0.021.19

backfill PC10-Slag

Sample

PC10-SlagControl

PC10-SlagMat C

PC10-SlagMat A

PC10-SlagMat B

8.48 9.30 5.69 5.06)8 3.86 3.03 2.82 0.24

6.72 7.66 11.53 10.136.97 6.29 5.65 4.6725.23 19.65 15.65 16.21

0.80 0.89 1.24 2.0033.98 30.89 40.70 38.860.15 0.11 0.42 0.270.75 0.00 1.96 4.79

0.72 0.33 0.91 0.67(H2O) 0.00 0.00 0.72 1.20

0.77 2.92 4.76 4.060.01 0.01 0.00 0.32

2.02 0.87 0.50 0.861.25 0.49 0.94 2.390.49 2.30 0.66 0.321.31 2.01 0.00 0.000.76 0.56 1.45 1.591.07 2.41 2.17 2.050.53 1.03 0.00 0.000.98 0.79 0.00 2.791.15 0.81 0.63 0.63

0.391.99 1.56 1.58 0.89

1.532.541.64


In addition to the EDS investigation, which faced theproblem of the precision of the punctual probe analysesmentioned above, XRD analysis only allows the character-ization of crystallized phases. Thus, all amorphous cemen-titious phases cannot be well identified and quantified usingXRD. It was observed that the materials did not containhigh amount of amorphous minerals.

Literature on steel slag outlines a variable compositioncontaining olivine (fayalite and forsterite), gehlenite,akermanite, bredigite, b-C2S, C2F, C3S and C4AF (e.g.Shi, 2001; Benard, 2003; Shen et al., 2004). Red mud isreported to contain hematite, calcite, diaspore, gibbsite,bayerite, rutile (or anatase), goethite, boehmite, sodalite,katoite and gypsum (e.g. Doye and Duchesne, 2002; Col-lazo et al., 2004; Tsakiridis et al., 2004). Only the mineralswhich were positively observed on the diffractograms weretaken into account in this study.

Within the samples studied, it was found that XRD wasmore effective than SEM to identify the cementitiousphases. The main results are summarized in Tables 9 and10 which contain the identified phases and their quantifi-cation using the Rietveld method for the PC10-50 andPC10-Slag paste samples. In addition to the main tailingsminerals (gangue and sulphides), the main phases wereettringite, gypsum, iron oxi-hydroxides and other phasescoming from the paste additives. Differences between thesamples studied were obtained after the Rietveld spectraquantification using TOPAS software. Thus, PC10-50pastes seem to contain more hydrated sulphates (ettringiteand gypsum) than the PC10-Slag pastes which containhigher amounts of un-hydrated cement phases. This mayexplain the higher performances of the PC10-50 bindercompared to the PC-Slag binder which is characterizedby a slower strength acquisition. Differences can also beobserved between the different additives (sludges and redmud) for a given binder (See Tables 9 and 10).

4. Conclusions

As conclusion of the study, it could be stated that for theLaRonde tailings, Portland cement-based (PC10-50 50/50)binder is more suitable than the slag-based cement (Slag-PC10 80/20). The addition of red mud seems to improvepaste backfill strength in comparison with the two HDSsludges which, in many cases, led to a small loss of strengthfor the Portland cement-based materials at low binder con-tent. In the case of the slag-based binder, the sludge or redmud additions led to increase strength at relatively highbinder content (5% and 7%) but had a negative impacton the strength at low binder content (2%). The additionof sludge or red mud to slag-based binder allows to reachhigher strength at long term curing time.

In term of metal leachability, Portland cement and Port-land cement/slag mixtures were efficient in minimizing Znand Mn release in batch tests. Comparable leachabilityresults for Zn and Mn were obtained using a lower binderdosage (2%) of Portland cement and Portland cement/slag

binders compared to higher dosages (5% and 7%). Theaddition of 0.15% and 0.30% of the three different sludgesto the tailings paste backfill did not have any negativeimpact on metal leachability.

The SEM–EDS characterization was not successful inthe examination of mineralogical and microstructural dif-ferences and in relating these parameters to strength behav-iour. Finally, the XRD analyses allow the identification ofsome specific cementitious phases that can explain the dif-ferences observed in mechanical behaviour.

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Sludge recycling within cemented paste backfill: Study of the mechanical and leachability properties

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