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Preliminary geotechnical assessment of the potential use of

mixtures of soil and acid mine drainage neutralisation sludge as material for the moisture retention layer of covers

with capillary barrier effects

Journal: Canadian Geotechnical Journal

Manuscript ID cgj-2014-0319.R2

Manuscript Type: Article

Date Submitted by the Author: 23-Jul-2015

Complete List of Authors: Mbonimpa, Mamert; Université du Québec en Abitbi-Témiscamingue, Bouda, Médard; UQAT, RIME Demers, Isabelle; UQAT, RIME Benzaazoua, Mostafa; UQAT, RIME Bois, Denis; UQAT, RIME Gagnon, Mario; IMAGOLD,

Keyword: Acid mine drainage, Soil-sludge mixtures, Hydrogeotechnical properties, Mine site reclamation, Neutralisation sludge

https://mc06.manuscriptcentral.com/cgj-pubs

Canadian Geotechnical Journal

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Preliminary geotechnical assessment of the potential use of mixtures of soil and acid mine

drainage neutralisation sludge as material for the moisture retention layer of covers with

capillary barrier effects

Mamert Mbonimpa1,a, Médard Bouda1,b, Isabelle Demers1,c, Mostafa Benzaazoua1,d, Denis Bois1,e

and Mario Gagnon2,f

1Research Institute on Mines and Environment (RIME) – Université du Québec en Abitibi-

Témiscamingue, 445 Boulevard de l’Université, Rouyn-Noranda, Québec, J9X 5E8, Canada

a [email protected]

b [email protected]

c [email protected]

d [email protected]

e [email protected]

2IAMGOLD, Division Doyon - Mouska –Westwood; Chemin Arthur-Doyon, Preissac, Québec,

J0Y 2E0, Canada

f [email protected]

Corresponding author: Mamert Mbonimpa ([email protected])

Submitted in August 2014 for publication in the Canadian Geotechnical journal

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ABSTRACT

Lime treatment of acid mine drainage (AMD) generates sludge that is commonly stored in ponds

for dewatering. The use of soil-aged sludge-based mixtures for mine site rehabilitation can allow

emptying existing basins, extending their storage capacity, reducing the volume of borrow soil pit

required for mine site rehabilitation, and consequently reducing the mine footprint. The authors

investigated the geotechnical properties of silty soil–sludge mixtures (SSM) as possible component

of covers with capillary barrier effects (CCBE) to prevent AMD generation from mine waste. SSM

with 10, 15, 20, and 25% sludge (β= wet sludge mass/wet soil mass) were studied. Two water

contents were considered for each of the mixture components: 175 and 200% for the sludge and 7.5

and 12.5% for the soil. Results indicated that saturated hydraulic conductivity values ksat were in the

range of 10-5 cm/s for the soil and SSM mixtures at void ratios ranging from 0.28 to 0.53, with

values slightly decreasing when β was increased from 0 % to 25%. The air entry value (AEV)

increased from 20 kPa for the soil alone to 35 kPa for the SSM with β= 25%. These values of ksat

and AEV are comparable to those of materials used in the moisture retention layers of existing

efficient CCBE. However, the volumetric shrinkage increased from about 2% for the soil alone to

values ranging between 24 and 32% for the SSM with β = 25%, depending on the initial water

contents of the components. Tools are provided to estimate to which extent the use of sludge in

SSM can reduce the volume of borrow natural soil required for a moisture retention layer of a

CCBE.

Keywords: Acid mine drainage, Neutralisation, Soil-sludge mixtures, Hydrogeotechnical

properties, Moisture retention layer, CCBE, Mine site reclamation

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RÉSUMÉ

Le traitement du drainage minier acide (DMA) à l'aide de la chaux génère une boue généralement

stockée dans des bassins pour la décantation. L'utilisation de mélanges sol-boue (MSB) pour la

réhabilitation des sites miniers peut permettre la vidange des bassins existants, l’extension de leur

capacité de stockage et la réduction de l'empreinte de la mine. Pour cela, les auteurs ont étudié si

des mélanges d’un sol silteux et de la boue (MSB) peuvent présenter des propriétés géotechniques

appropriées pour être utilisés dans des couvertures avec effets de barrière capillaire (CEBC) visant

empêcher la génération du DMA. Des MSB avec des teneurs en boue β (= masse de boue humide /

de masse de sol humide) de 10, 15, 20 et 25% ont été étudiés. Deux teneurs en eau ont été

considérées pour chacun des constituants des mélanges: 175 et 200% pour les boues et 7,5 et 12,5%

pour le sol. Les résultats indiquent des valeurs de conductivité hydraulique saturés ksat dans la

gamme de 10-5 cm/s pour le sol et les MSB, avec des indices de vides compris entre 0,28 et 0,53.

Les valeurs ksat baissent légèrement lorsque β augmente de 0% à 25%. La pression d'entrée d'air

(AEV) passe de 20 kPa pour le sol seul à 35 kPa pour le MSB avec β = 25%. Ces valeurs de ksat et

AEV sont comparables à celles des matériaux utilisés dans les couches de rétention d'eau de CEBC

existantes et efficaces. Néanmoins, le retrait volumique est passé d'environ 2% pour le seul sol à des

valeurs comprises entre 24 et 32% pour le MSB avec β = 25% selon les teneurs en eau initiales des

composantes. Des outils sont fournis pour estimer dans quelle mesure l'utilisation de la boue dans

des MSB peut réduire le volume de matériaux naturels d'emprunt requis pour une CEBC

Mots clés : Drainage minier acide, neutralisation, mélanges sol-boues, propriétés

hydrogéotechniques, couche de rétention d’eau, CEBC, réhabilitation de sites miniers.

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1 INTRODUCTION

Sulphide minerals such as pyrite, pyrrhotite, chalcopyrite, and others contained in tailings and waste

rocks can oxidize in the presence of oxygen and water and generate acid mine drainage (AMD).

Once triggered, AMD is one of the major environmental challenges for the mining industry. A

discharge treatment plant must usually be implemented to meet regulatory effluent loading limits.

The most common method is active AMD treatment, which involves neutralization using different

alkaline chemicals, such as NaOH, Na2CO3, CaO, or CaCO3 (Brown et al. 2002; Younger et al.

2002; Zinck and Griffith 2012a, 2013), and produces large quantities of sludge. As the majority of

metals dissolved in the AMD water precipitate at alkaline pH values, sludge typically consists of

metal hydroxides, sulfates (ettringite, gypsum,..), unreacted lime, and other less abundant phases

(Zinck et al. 1997). Sludge from AMD neutralization requires responsible and effective

management. Sludge is usually stored in ponds for dewatering and permanent disposal. Sometimes

it is stored in the tailings ponds. Alkaline to neutral pH can be sustained within the sludge ponds for

decades, even centuries, due to excess alkalinity induced by lime in the sludge (Zinck et al. 1997).

This alkalinity helps to achieve chemical stability of the sludge (no sludge dissolution and no metal

remobilization) over time. Exposure of the sludge to acidic conditions can allow the re-dissolution

of metals, which constitutes the main environmental disadvantage that should be evaluated for each

case of sludge reuse. Thus, the long term conditions in which the sludge will be placed should be

evaluated to reduce the risk of metal remobilization.

Efficient sludge reuse can provide an environmental advantage because it can allow the extension

of the storage capacity of emptied ponds, which can then be used for ongoing temporary sludge

disposal. The use of soil covers incorporating sludge-based mixtures for mine site rehabilitation can

provide additional environmental and economic advantages: i) reducing the soil borrow pit volume

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and the transport distances of the borrow materials (sludge ponds are usually near waste storage

facilities), ii) potential use as substrate for vegetation. The assumption underlying this study is that

silty soil–sludge mixtures (SSM) have the potential for use as earthen cover material for mine site

rehabilitation. To verify this assumption, a laboratory testing program was conducted in this study

to assess the relevant hydrogeotechnical properties of SSM when used as a CCBE component in a

reclamation scenario (see also Bouda et al. 2012). In this paper, material (sludge and soil) sampling,

SSM preparation, and characterization of the soil, sludge and SSM are described. Results are

presented and discussed with respect to the possibility of using SSM as material for the moisture

retention layer in CCBE to prevent AMD generation from mine wastes.

2. BACKGROUND

AMD neutralisation sludge must be disposed of in an environmentally acceptable manner. Besides

the classical method of sludge deposition in ponds, Zinck (2006), Zinck et al. (2010), and Zinck and

Griffith (2012a,b; 2013) reviewed several sludge disposal alternatives and their effectiveness. These

alternatives include sludge mixed with tailings and waste rock, sludge covers over tailings, sludge

disposal in tailings ponds, with waste rock, in mine workings, in pits, under water covers, and as

hazardous waste, and sludge integration in cemented mine backfills and landfilling. Various

stabilization and solidification techniques have also been reported, including lime-based, cement-

based, and thermoplastic polymeric encapsulation (Zinck 2005; 2006; Zinck and Griffith 2012a,b).

However, sludge disposal along with tailings and waste rock is effective only when the mine wastes

are non-acid generating and/or oxidation is prevented. If the wastes oxidize and generate acid,

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sludge dissolution and metal remobilization become a serious concern. Zinck et al. (2010) showed

that a sludge cover placed over tailings does not control tailings oxidation.

Benzaazoua et al. (2006) investigated the option of adding a small quantity of sludge (0.15 and 0.3

% by total solid dry weight) to cemented tailings paste backfill and showed that no contaminants

were subsequently released. The contaminants are immobilized in the backfill material matrix

without reducing its strength. Tsang et al. (2013) observed that AMD sludge can be used

successfully for in situ soil stabilisation to minimize the environmental risk of As and Cu release at

a timber treatment site. Some AMD neutralisation sludge required adding compost or lignite to

simultaneously reduce the Cu and As leachability.

Demers et al. (2015a) performed column leaching tests in the laboratory to evaluate the efficiency

of the tailings-sludge mixture and waste rock-sludge mixtures to control AMD produced by tailings

and waste rocks. Results indicated that waste rock–sludge mixtures placed over waste rock (which

represented the worst case scenario because this contact must be avoided) reduces the metal loads in

the column effluent, which however remained acidic, while tailings-sludge mixtures deposited over

tailings reduced sulphide oxidation from tailings. Indeed sludge placed in acidic conditions (such as

in contact with waste rock) were more likely to dissolve and eventually would release metals. These

results were confirmed by results gathered by Demers et al. (2015b) from instrumented test cells

constructed on site to evaluate the performance of these waste-sludge mixtures to control AMD

under natural field conditions.

Water and earthen covers are effective technologies for mine site rehabilitation to control AMD

production. Multilayer earthen covers are widely used as conventional infiltration-limiting cover

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systems that typically consist of a low saturated hydraulic conductivity (ksat) layer (clay or

geosynthetic membrane, or a combination of the two) combined with other layers to provide lateral

drainage, resist erosion, prevent moisture loss and desiccation cracking. For this low ksat layer to

effectively control infiltration, the ksat value must be less than 10-7 cm/s to satisfy the regulatory

requirements.

Earthen covers that reduce the oxygen flux towards the tailings appear to be one of the most

efficient ways to prevent AMD generation in humid climates like in Canada. Single- and multi-

layered covers can be used for this purpose. Single-layer covers can consist of non-acid generating

tailings (such as desulfurized tailings) that are slightly reactive with oxygen (Bussière et al. 2004;

Demers et al. 2008, 2009, 2010; Sjoberg Dobchuck 2013). Such materials can contribute to reduce

the oxygen flux through the covers. Natural soils can also be used in single-layer covers (e.g. Pabst

et al. 2011, 2014). The long-term efficiency of single-layer covers as an oxygen barrier is improved

when the covers are used in combination with an elevated water table (MEND 1996; Ouangrawa et

al. 2010; Pabst et al. 2011, 2014). Water table located in the tailings and higher air-entry values of

the cover materials can help to maintain water saturation and to reduce oxygen fluxes. An

additional top layer made of coarse-grained materials can prevent moisture loss by evaporation and

evapotranspiration.

Multi-layer covers such as covers with capillary barrier effects, CCBE, are more efficient than

single-layer soil covers when the water table cannot be raised. Capillary barrier effects are created

under unsaturated conditions when a fine-textured material layer (the moisture retention layer

MRL) is placed over a coarse-textured material layer (the capillary break layer) (e.g., Nicholson et

al. 1989; Aubertin et al. 1994; Woyshner and Yanful 1995; O’Kane et al. 1998; Bussière et al.

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2006, 2007). When the underlying coarse material is drained, its unsaturated hydraulic conductivity

becomes much lower than that of the overlying fine-textured material. Downward movement of soil

moisture from the upper fine-grained layer is then prevented. The material used in the moisture

retention layer generally has high water retention potential, with an air entry value (AEV) ≥ 20 to

50 kPa (e.g., Aubertin and Chapuis 1991). An additional coarse-grained top layer provides drainage

and erosion control, and limits revegetation. Keeping the fine-grained layer near saturation reduces

the influx of atmospheric oxygen to reactive wastes and subsequent production of acidic drainage

(e.g., Aubertin et al. 1994; Yanful et al. 1999; Mbonimpa et al. 2003; Bussière et al. 2004; Dagenais

et al. 2012).

The hypothesis underlying this study is that a mixture of soil and sludge (both non-acid generating)

can be a good MRL material in CCBE for the reclamation of mine sites that generate AMD. This

mixture should have appropriate hydrogeotechnical properties and long-term physical and chemical

stability. This paper focuses on the laboratory evaluation of basic hydrogeotechnical properties of

SSM, including compaction behavior, saturated hydraulic conductivity and its evolution under

freeze-thaw cycles, free shrinkage behavior, and water retention curve. The potential use of soil

sludge mixtures (SSM) as MRL material is then evaluated in a preliminary manner by a comparison

between the saturated hydraulic conductivity and air entry values of the mixtures and of soils used

in efficient CCBE.

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3. MATERIALS AND METHODS

3.1. Sampling and SSM preparation

The sludge used in this study was sampled from the Doyon mine site operated by IAMGOLD,

located in northwestern Quebec, Canada. At this site, lime is used for AMD neutralization and a

flocculent is added prior to thickening. Two sludge ponds (pond B and North pond) containing a

volume of nearly 1 million cubic meters of sludge were used for sludge disposal over a period of

approximately 15 years (personal communication), and have been inactive for many years. The

sludge layer at the surface was relatively stable, but the sludge appeared wet and plastic at a very

shallow depth. Diverse and dense natural vegetation has grown on the sludge ponds. A survey to

evaluate the risk of metal bioaccumulation by the vegetation was a part of neutralizing lime sludge

valorization assessment (Smirnova et al. 2013).

In August 2010, eight sampling cores were drilled in pond B and seven cores in the North pond at

different accessible locations. At the drilling locations, the sludge thickness varied from 2.5 to 6.0

m in pond B and from 2.5 to 7.5 m in North pond. For each boring, undisturbed sludge was sampled

at intervals of 1 m from the surface to the natural soil substratum. These samples were stored in

tight plastic bags under 4° C until they were tested as described below. Furthermore, disturbed

sludge was sampled using a shovel close to each boring location at a depth of about 1.5 m and

stored in a barrel hermetically sealed after filling. For tests that required dry sludge samples, oven

drying was used in this study. Vertical water content profiles were determined for each boring

location. Sludge water content typically ranged from 100 to 250% (w/w) for the cores in pond B

and from 150 to 300% for the cores in North pond.

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The spatial distribution of the chemical composition of the sludge was determined by analyzing a

composite of the sludge cores collected at each sampling point in the two ponds. The elemental

analysis of all the sludge samples (results not presented here) revealed significant homogeneity of

the chemical composition for each boring location. Therefore, the disturbed sludge samples

collected from the different locations in the North pond were mixed to obtain a representative

composite sludge sample (called “sludge” hereafter) for further laboratory investigation.

Homogeneous soil-sludge mixtures (SSM) were prepared in the laboratory using a stand mixer with

10%, 15%, 20%, and 25% wet sludge content (β) with respect to the soil wet mass (i.e., β = wet

sludge mass/wet soil mass). According to the estimated sludge quantity available for potential use

over the tailings ponds at the Doyon site, 25% sludge mixture was identified as the maximum

sludge content. Samples of soil alone (β = 0) and sludge alone (β = ∞) were also studied as

reference materials. The corresponding proportions (%) of wet soil [(=100/(β +100)] and of wet

sludge [( = β /(β +100)] in the wet mixtures are given in Table 1. For the mixture preparation, initial

water content was set at a wi-sl of 175 and 200% for sludge and wi-so of 7.5 and 12.5% for soil.

Indices “sl” and “so” will be used in the following for sludge and soil, respectively. The sludge

contents and wet soil and sludge proportions in the mixtures given in Table 1 can also be expressed

in terms of dry soil, sludge and mixture weight (using β, wi-sl and wi-so). For example, the sludge

content β* based on dry mass mixture ratios (β* = dry sludge mass/dry soil mass) can be expressed

as follows: β*=.(1+ wi-so)β/(1+ wi-sl). With wi-so = 12.5%, wi-sl = 200% and β =25%, β* = 9.37%. In

this case, the corresponding proportions (%) of dry soil [(=100/(β* +100)] and of dry sludge [( =

100×β* /(β* +100)] in the dry mixtures are 91.4% and 8.6 %.

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3.2. Soil, sludge and SSM characterisation

The X-ray diffraction (XRD) data for the mineralogical characterization of the soil and sludge were

determined using the quantitative Rietveld method (relative precision of 0.5%) using TOPAS

software (Rietveld 1993; Young 1995). It should be mentioned that only crystalline materials were

identified by XRD while sludge generally contains amorphous or poorly-crystalline phases (e.g.,

MEND 2013). Chemical characterization were performed on the mixture components separately

(i.e., soil and sludge) using Inductively Coupled Plasma and Atomic Emission Spectrometry (ICP-

AES).

Mostly geotechnical properties, including specific gravity of solid grains, grain-size distribution,

dry density/water content relationship using the Proctor test, saturated hydraulic conductivity under

freeze–thaw cycles, free shrinkage, and water retention curves were determined for the soil alone

and for mixtures. The specific gravity of solid grains DR was determined using a Micromeritics

Accupyc 1330 helium pycnometer according to ASTM D5550-06. Grain-size distribution was

obtained by dry or wet sieving combined with a Malvern Mastersizer laser grain-size analyzer,

which provides a volume size distribution for diameters from 0.05 to 900 µm. Modified Proctor

tests were performed on the soil and SSM according to ASTM D1557.

Saturated hydraulic conductivity ksat was determined with a rigid wall permeameter using the

variable head method according to ASTM D5856. Samples were compacted at water contents close

to the Proctor optimum water content in the permeameter (except for the mixture at β =25% for

which a water content close to the natural value was targeted). Samples were then saturated by

subjecting them to an upward flow of distilled and de-aired water for at least 24 h using a vacuum

pump. Degrees of saturation of at least 98% were reached. The effect freeze–thaw cycles on ksat was

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also investigated. The rigid permeameter containing the sample was insulated laterally with glass

wool so that the sample underwent one-dimensional freezing from top to bottom. Freezing occurred

in a temperature controlled refrigerator. After 72 h of freezing at a temperature of approximately -

15° C, sample height hj (for each cycle j) was measured at different positions using a caliper and an

average value was calculated. The sample was then allowed to thaw for 48 hours at laboratory

temperature (about 20 °C) before determination of the saturated hydraulic conductivity according to

the ASTM D5856 procedure. For each of the 15 cycles applied, the saturated hydraulic conductivity

(ksat) and swelling ∆hj were determined, with ∆hj = hj - h0, where h0 is the height of the sample at

cycle 0 (before the first freezing step).

The sludge was expected to shrink due to its high water content and fine-grained particle size

distribution. The effect of adding sludge to the soil (assuming negligible shrinkage in the soil alone)

on SSM shrinkage was investigated. Free shrinkage behavior of the soil, sludge, and SSM was

examined in terms of the relationship e(w) between the void ratio e and the water content w. An

initially saturated sample was placed into a mold with smooth walls coated with a thin layer of

grease to prevent cracking. The volume of the mold, the water content and the specific gravity of

solid grains of the material were used to estimate the mass of the wet material to be placed in the

mold under saturated conditions at given initial water content. Air bubbles were removed using

small vibrations. The sample was allowed to dry naturally under laboratory conditions, and changes

in the sample water content and volume with time were monitored. The volume of soft material

that could not be removed from the mold in the early shrinkage stages was determined by

measuring the sample height with a caliper and determining the sample surface area from a photo of

the sample using ImageJ software (Girish and Vijayalakshmi 2004; Saleh-Mbemba et al. 2010). The

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ImageJ method consists in digitizing the picture, and converting the number of pixels into an area.

For samples sufficiently stiff for removal from the mold, the volume was determined by underwater

weighing. For this purpose, the sample was first coated with liquid wax of known density (0.84

g/cm3) to guarantee the sample integrity during immersion. Because the wax-coated sample was not

reusable, many molds filled with the same material were used to obtain the shrinkage curve e(w). At

the end of the natural shrinkage process, the last sample was oven dried (at a temperature of 105°C)

to obtain the solid mass and the sample volume was determined to obtain the final void ratio (at w =

0). Most shrinkage tests were performed with molds having a diameter and height of 4.84 cm and

1.52 cm, respectively (called hereafter molds M-I). The mold scale effect on e(w) was also

examined using molds with a diameter of 11.61 cm and heights of 1.52 cm (molds M-II) and 3.02

cm (molds M-III).

Water retention curves (WRC) were determined under drying conditions for the soil and SSM with

the highest sludge content β = 25%. For the soil alone, which was expected to undergo negligible

shrinkage, the WRC was obtained using a Tempe cell (ASTM D6836-02e2) with suctions up to 620

kPa. Given the expected shrinkage under increasing suction for the SSM sample with β = 25%, the

drying WRC test was performed using a 100 bar pressure membrane extractor (Model 1020, Soil

Moisture Equipment Corp.). Twelve saturated samples were prepared into circular flexible retaining

rings with a height of 1 cm and an internal diameter of 5.5 cm. The samples were placed directly in

contact with a cellulose membrane disc previously saturated in the pressure extractor vessel filled

with distilled and de-aired water. This membrane has an air entry value (AEV) > 104 kPa or 100

bars. With this AEV, the pressure in the cell may reach 100 bars, maintaining the cellulose

membrane saturated. Pressure was applied (using a nitrogen cylinder) and maintained constant over

time, and the water expelled from the samples was periodically monitored in a graduated burette.

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When no liquid drained out of the cell for 24 h, one sample was removed from the cell and a higher

pressure was subsequently applied to continue drying of the remaining samples. The removed

sample was used to determine the actual water content and volume of the sample. The volume,

required for estimating the actual porosity, n, and void ratio, e, was obtained using the water

immersion method. The change in porosity or void ratio with suction ψ, (i.e., n(ψ) or e(ψ)) is

required for a material that is deformable under suction. In this case, expressing the WRC in terms

of degree of saturation versus suction becomes more appropriate than in terms of gravimetric and

volumetric water contents versus suction (Mbonimpa et al. 2006). The function e(ψ) also represents

a shrinkage curve (Mbonimpa et al. 2006).

Most hydrogeotechnical tests were conducted with SSM prepared with soil and sludge at initial

water contents of 7.5% and 175%, respectively. For the shrinkage tests, initial water content was set

at 7.5% and 12.5% for soil and 175% and 200% for sludge. The initial water content of the mixture

wm was measured for a few SSM samples. Theoretically, wm can be estimated from the initial water

contents of the components as follows:

(1) )w1(w1

)w1(w)w1(ww

soisli

soislislisoim

−−

−−−−

+++

+++=

β

β

In equation (1), wi-sl and wi-so are the initial water contents of sludge and soil, respectively. Because

the wm values calculated with Eq. (1) were in good agreement with values measured on a few

mixtures (results not shown here), Eq. (1) was used for calculations for further mixtures. Table 2

presents the calculated wm values corresponding to the different SSM studied.

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4. RESULTS

4.1.Chemical and mineralogical properties

Table 3 presents the main elements of environmental interest according to the Directive 019

(Gouvernement du Québec, 2012) for soil and sludge alone. For all materials, both duplicate

samples had similar chemistry. The soil contains mainly aluminum (6%), iron (4%), and calcium

(2%). The sludge contains mainly calcium (18%), iron (9%), and sulphur (7%).

The mineralogical compositions determined by XRD for the soil and the crystalline materials in the

sludge are summarized in Figure 1. The soil contained mainly albite Na(AlSi3O8) (28%), quartz

SiO2 (22%), hornblende Ca2(Fe2+4Al)(Si7Al)O22(OH)2 (11%), microcline K(AlSi3O8) (11%),

anorthite Ca(Al2Si2O8) (10%), muscovite KAl2(Si3Al)O10(OH)2 (7%), phlogopite

K(Mg,Ti,Fe)3[(Si,Al)4O10](O,F)2 (4%), gypsum Ca(SO4)·2H2O (4%), and talc Mg3Si4O10(OH)2

(4%). The crystalline phases in the sludge contained 80 % of calcium sulphates, mainly ettringite

Ca6Al2(SO4)3(OH)12·26H2O (50%) and gypsum (30%). The remaining 20% consisted of silicates:

quartz (4%), muscovite (7%) and pyrophyllite Al2Si4O10(OH)2 (9%). Although the chemical

analysis indicated that the sludge contained 9.2% Fe, it should be mentioned that amorphous

minerals such as iron-oxyhydroxides and clay minerals may be difficult to analyze by XRD due to

their weak crystallinity. In particular, the clay minerals are difficult to quantify accurately by XRD

due to their compositional variation, variable degrees of structural order/disorder, and their

tendency towards preferred orientation in the whole sample.

According to the chemical and mineralogical properties, neither soil nor sludge contained sulphide

minerals that can oxidize and generate acid. All the sulfur was present in sludge as sulfates and the

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sludge possessed a strong neutralization potential (above the limit of the method of Sobek modified

by Lawrence and Scheske; 1997) (Demers et al. 2015a). As already mentioned, excess alkalinity

induced by lime in the sludge can sustain alkaline to neutral pH within the sludge ponds for

decades, even centuries (Zinck et al. 1997).

4.2. Specific gravity of solid grains

The average specific gravity of solid grains DR was 2.10 and 2.74 for the sludge and soil,

respectively. The low specific gravity of the sludge can be explained by the presence of hydrated

phrases such as ettringite (with DR≈ 1.8) and gypsum (with DR ≈ 2.3).

Based on the relative densities of the sludge DR-sl and soil DR-so, the specific gravity DR-m of a given

SSM could be estimated using the following equation:

(2) [ ]

soRsoislRsli

soislislRsoRmR

D)w1(D)w1(

)w1()w1(DDD

−−−−

−−−−−

+++

+++=

β

β

Table 4 compares typical measured and calculated specific gravity of solid grains for different

SSM. The initial water contents of the soil (wi-so) and sludge (wi-sl) used in these mixtures were

7.5% and 175%, respectively. Because the measured and calculated values were in good agreement,

DR-m for the other SSM was estimated using Eq. (2). The specific gravity of the different SSM

decreased with increasing β due to the low value DR-sl compared to DR-so.

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4.3. Grain-size distribution

Figure 2 shows the grain-size distributions for the sludge, soil, and SSM with β = 25%, considering

initial water contents of 7.5% for the soil and 175% for the sludge. The grain-size distribution

curves for the mixtures with β = 10, 15, and 20% lie between the curves for the soil and the SSM

with β = 25%, indicating that the grain-size distribution of the mixtures, even up to β = 25%,

remained very close to that of the soil alone. The effective diameter D10, corresponding to 10%

passing on the cumulative grain-size distribution curve, is 0.0026 mm for the sludge alone, 0.009

mm for soil alone and 0.007 mm for the SSM with β =25%. The coefficient of uniformity CU is 17.7

for the sludge alone, 66.7 for soil alone and 68.6 for the SSM with β =25%.

According to the Unified Soil Classification System (USCS) (ASTM D2487-06e1), the soil and

SSM are coarse-grained and can be classified as SM. The sludge is fine-grained and can be

classified as MH considering that the liquid and plastic limits of the sludge were approximately

88% and 62%, respectively. Since the water content of the sludge in the North Pond ranged in

general between 150 and 300%, the water contents are higher than the liquid limit, so the sludge

was extremely soft and in a viscous-liquid state.

4.4. Modified Proctor tests

Modified Proctor tests were performed on the soil and the SSM containing 10, 15, 20, and 25%

sludge. The initial water contents of the soil and sludge in the mixture were 7.5% and 175%,

respectively. Figure 3 shows typical modified Proctor curves (relationship of dry density ρd to water

content w) for the soil and for the different SSM. For the four tested mixtures, when β was

increased from 10% to 25%, maximum dry density ρdmax increased from 2025 kg/m3 to 2052 kg/m3.

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The optimum water content wopt ranged from 8.5% to 10.5%. Compared to the results obtained on

the soil alone (ρdmax ≈ 1988 kg/m3 and wopt ≈ 10%), addition of up to 25% sludge to the soil appears

to improve the compactibility because the fine sludge fills the voids between the soil particles. The

porosity (n), void ratio (e), and degree of saturation (Sr) reached at the optimum Proctor for the soil

were 0.27, 0.38, and 0.72, respectively. For the SSM mixtures, 0.23 ≤ n ≤ 0.25, 0.30 ≤ e ≤ 0.33, and

0.79 ≤ Sr ≤ 90 were obtained at the optimum Proctor. In general, n decreased and Sr increased with

increasing sludge content (for β ≤ 25%) at the optimum Proctor.

The change in dry density ρd and void ratio e with respect to water content w for each mixture

sample at a given degree of saturation can be calculated theoretically using the following equations:

(3)

r

mR

wmRd

S

Dw1

D

−

−

+

=ρ

ρ

(4) r

mR

S

Dwe −=

where ρw is water density. The curves for the maximum dry density and minimum void ratio at

saturation can be obtained by setting Sr = 1 in Eqs. (3) and (4). Because the values of DR-m are very

close for the SSM tested (see Table 4), a mean value of DR-m of 2.69 was used in eqs (3) and (4).

The curves for maximum dry density and minimum void ratio are presented in Figure 3. Figure 3

also shows the natural water contents of the different mixtures at sludge contents 10, 15, 20, and

25%, i.e.13.8, 16.8, 19.6 and 22.4 %, respectively (see Table 2). These water contents are higher

than wopt. If these SSMs could be compacted up to saturation at their natural water contents, the

expected maximum dry density and minimum void ratio would be about 1972 kg/m3 and 0.37 for

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the mixture with β = 10%, 1857 kg/m3 and 0.45 for β = 15%, 1761 kg/m3 and 0.53 for β = 20%, and

1667 kg/m3 and 0.59 for β = 25%, respectively.

4.5. Saturated hydraulic conductivity ksat

Saturated hydraulic conductivity (ksat), determined on SSM prepared with soil and sludge at water

content wi-so = 7.5% and wi-sl = 175%, is shown in Table 5. It was difficult to compare the values

due to the different sample void ratios, even for duplicates. The soil and SSM mixtures showed a

ksat in the range of 10-5 cm/s at void ratios e ranging from 0.28 to 0.53. The impact of adding sludge

by up to 25% wet weight on ksat seems limited. The studied SSM cannot be used alone as

conventional hydraulic barriers, because the saturated hydraulic conductivity values are higher than

the value required to meet regulatory requirements, i.e., 10-7 cm/s.

The effect of freeze–thaw cycles was examined in terms of swelling ∆hj for each cycle j (with ∆hj =

hj - h0) and change in saturated hydraulic conductivity. To compare swelling between the mixtures,

the swelling ratio ∆hj/h0 was used. Average swelling ratios are presented in Figure 4 for the SSM.

The initial sample height h0 varied from 10.5 to 11.8 cm. As a results, swelling increased generally

rapidly during the three first cycles, followed by a slight stabilization from cycles 3 to 5. Peak

swelling ratios increased with increasing sludge content (from 1.5% for β = 10% to 3.7% for β =

25%). Sample heights subsequently decreased starting from cycle 5, followed by stabilization from

cycle 8 to 9. This decrease could be due to a change in the morphologic structure of the samples, as

described by Othman and Benson (1993) and Wang et al. (2007) in their study on the freezing–

thawing effect in compacted clay membranes. In fact, during the freeze–thaw process, the soil

passes from an unstable state into a stable state. Thus repeated freeze–thaw cycles will change the

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soil structure towards a new dynamic stable balance state. This morphologic structure change can

be explained by the formation of ice lenses in the sample. During the first cycles, increasing of the

ice lens size leads to the increase in hydraulic conductivity. However, it appears that from the 7th

freeze-thaw cycle the hydraulic conductivity and swelling stabilized. The swelling ratio after

stabilization increased from approximately 1.1% for β = 10% to 2.5% for β = 25%. Although

freeze–thaw behavior was not studied for the soil alone, the addition of sludge to the mixtures

induced limited swelling under freeze–thaw effects (for the conditions tested).

The change in void ratio ∆ej/e0 associated with the swelling ratio ∆hj/h0 can be estimated with the

following equation:

(5) 0

0

0

j

0

j

e

)e1(

h

h

e

e +=∆∆

The maximum swelling ratio, ∆hj/h0 = 3.7%, was reached for β = 25% (with e0 = 0.53), ∆ej/e0 =

10.7%, or ej= 0.59.

The effect of freeze–thaw cycles was also examined in terms of changes in the saturated hydraulic

conductivity. The ratio of hydraulic conductivity at the end of cycle (i) ksat-j to hydraulic

conductivity at cycle (0) ksat-0 (i.e ksat-j/ksat-0, where ksat-0 is given in Table 5) is used for a comparison

purpose. Figure 5 shows typical results (average values) for the SSM with β = 25%. In this case, the

ratio ksat-j/ksat-0 is around 1.52. This freeze–thaw effect can be considered negligible (less than one

order of magnitude) for the studied SSM.

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4.6. Free shrinkage

Free shrinkage tests were performed on the soil alone at initial water contents of 7.5%, 12.5%, 20%

and 25%, and on the sludge alone at initial water contents of 100%, 150%, 175 % and 200%. Free

shrinkage tests were also performed on the SSM with sludge content at β = 25%. The mixtures were

prepared using soil samples with initial water contents wi-so of 7.5% and 12.5% and sludge with wi-sl

of 175% and 200%.

Figure 6 shows photographs of typical shapes of the soil, sludge, and SSM (with β = 25%) samples

at the end of shrinkage. Qualitatively, the sludge samples shrunk considerably compared to the

SSM mixtures, which in turn shrunk slightly more than the soil. Adding up to 25% sludge to the

soil affects shrinkage behavior.

Because the shrinkage curves (i.e., plots of the void ratio e with respect to water content w)

obtained for the triplicate samples were almost similar, average values are presented. As mentioned

previously, the mold scale effect on e(w) was investigated at the beginning of the testing program

using molds M-I (with a diameter of 4.82 cm and a height of 1.52 cm), molds M-II (with a diameter

of 11.61 cm and a height of 1.52 cm) and molds M-III (with a diameter of 11.61 cm and a height of

3.02 cm). Figure 7 shows typical results comparing the shrinkage curves obtained using the three

mold types. These results indicate that the shrinkage curves were almost independent from the mold

dimensions. Accordingly, the small molds (M-I) were mostly used. With these molds, shrinkage

was nearly completed after 4 days for the laboratory testing conditions: during the testing period,

the relative humidity varied between 40 % and 70 % and the temperature between 18°C and 25°C.

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Figure 8 shows the influence of the initial water contents on the shrinkage curves for sludge alone.

Also shown is the curve Sr = 1 representing the void ratio at a given water content w when the

material is saturated (e = w×DR for Sr = 1; see Eq. (4)). The sludge shrinkage curves show a wide

saturated shrinkage (or normal) phase where the soil remains saturated (shrinkage curves follow the

saturation line) because the volume decrease was equal to the volume of water lost. This phase

extended from the initial value to about w ≈ 50%. The residual phase with combined desaturation

and shrinkage occurred at water contents from 50% to 15%. The quasi-constant volume, or no-

shrinkage, phase occurred from w ≈ 15% to 0%. These shrinkage phases are described in the

literature (Marshall et al. 1996; Braudeau et al. 1999; Mbonimpa et al. 2006). For the sludge tested

at initial water contents wi-sl of 175 and 200% with ei = 3.7 and 4.2, the final void ratios ef are 0.54

and 0.65, respectively (see Figure 8). The shrinkage limit (ws) defined on the saturation line, as

typically used in geotechnical engineering, is approximately 30% for the sludge.

Figure 9 shows the influence of the initial water contents on the shrinkage curves for the soil alone.

The saturated (normal) and residual shrinkage phases are inexistent for the soil with wi-so ≈ 7.5%

(initial void ratio ei-so = 0.21 and final void ratio ef = 0.21) indicating that the soil dried without

volume change. The shrinkage curve for the soil with wi-so ≈ 12.5% was characterised by very short

saturated shrinkage (or normal) and residual phases (from 12.5 to 10%). However, the quasi-

constant volume or no-shrinkage phase is large: from w ≈ 10% to 0. With increasing initial water

contents, all shrinkage phases became larger. The final void ratio (ef) at full dryness (w = 0)

increased with the initial water content of the soil samples: ef = 0.21 for wi-so = 7.5% (ei = 0.21), ef =

0.32 for wi-so ≈ 12.5% (ei = 0.34), ef = 0.34 for wi-so ≈ 20% (ei =0.55) and ef = 0.42 for wi-so ≈ 25% (ei

= 0.69).

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Figure 10 shows the SSM shrinkage curves with β = 25% at different initial sludge water contents

(wi-sl ≈ 175 and 200%) with the soil at wi-so ≈ 7.5% and wi-so ≈ 12.5%. The specific gravity of the

solid grains DR-m in these mixtures varied from 2.665 to 2.673. The resulting line e(w) at Sr = 1

almost merged so that only one saturation line is presented in the figure for a mean value of DR-m.

Some data in the normal and residual shrinkage phases are missing due to factors beyond our

control. The final void ratios increased from around 0.20 to 0.36 when the initial water content of

the soil used in the mixtures was increased from 7.5% to 12.5%. Changing the water content of the

sludge used in the mixtures from 175% to 200% had the lowest impact on the shrinkage path and

final void ratio ef at full dryness (w = 0).

For comparison purposes, the volumetric shrinkage (VS) at full dryness (w = 0) was introduced and

calculated using the initial (Vi) and final (Vf) volumes or the initial (ei) and final (ef) void ratios of

each sample, as follows:

(6) i

fi

i

fi

e1

)ee(100

V

)VV(100(%)VS

+

−=

−=

The calculated VS are given in Table 6. The VS were 0, 2.3, 11.8 and 15.4% for the soil tested at

initial water contents wi-so of 7.5, 12.5, 20 and 25%, respectively. For the sludge material, the VS

were 67.2% and 68.3% at initial water contents wi-b of 175% and 200%, respectively. The VS

obtained for SSM with soil at wi-so ≈ 7.5% including 25% sludge at wi-sl of 175% and 200% were

24.3% and 30.45%, respectively. The VS obtained for SSM with soil at wi-so ≈ 12.5% including 25%

sludge at wi-sl of 175% and 200% were 25.7% and 32.5%, respectively. The volumetric shrinkage

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depends on the initial sludge and soil water content. Note that VS increased with the initial sludge

water content for a given initial soil water content. Consequently, adding up to 25% sludge to the

soil at 12.5% water content increased the VS from 2.3% to 25.7% or 32.5%, depending on the

sludge water content (175% or 200%, respectively). All these conclusions are valid for initially

saturated materials.

4.7. Water retention curves

Water retention curves (WRC) were determined for the soil and for the SSM mixtures with β = 25%

and initial water contents of 12.5 % and 175% for the soil and sludge, respectively. Two soil

samples were tested at initial porosities of 0.40 and 0.42 using Tempe Cells. Porosity was assumed

to remain constant during the drainage process, which is acceptable given the very low VS of the

soil. For the SSM, the 100 bar pressure membrane extractor was used due to expected volume

change with increasing suction. Mixing water was added up to a water content of 40.4% to make

sure that the SSM was homogeneous. The initial porosity (n) of the SSM was 0.52, or a void ratio e

= 1.08.

Figure 11 shows the shrinkage curve e(ψ) observed during WRC testing in the 100 bar membrane

extractor. The void ratio of the SSM decreased with increasing suction. Figure 12 presents the

WRC, expressed here in terms of degree of saturation Sr vs suction ψ, for the soil and SSM tested.

The air entry value (AEV) determined using the tangent method as described by Fredlund and Xing

(1994) is 35 kPa for the SSM (with initial water content of 40.4%) versus 20 kPa for soil alone.

Mixing sludge with soil increased the AEV (even for a mixture with a very high initial water

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content), which is beneficial in the context of a material to be used in the MRL of a CCBE to limit

oxygen migration towards underlying mine waste.

5. DISCUSSION

In this paper, various hydrogeotechnical properties of soil–sludge mixtures (SSM) were determined

in the laboratory to determine their potential for use as material for the moisture retention layer

(MRL) of CCBE. The mixture with β = 25% presented ksat values in the order of 10-5 cm/s and an

AEV value of 30 kPa, which would be appropriate as a component of a CCBE acting as oxygen

barrier, based on a comparison with materials used in efficient CCBE. For example, non-acid

generating tailings were used in the MRL of the CCBE at the LTA site, located in Abitibi-

Témiscamingue (Québec, Canada). The void ratio, ksat and AEV of this layer was 0.78 (or porosity

n = 0.44), 5×10-5 cm/s and 28 kPa, respectively (Bussière et al. 2003). In the case of the CCBE used

for the reclamation of the Lorraine site, also located in Abitibi-Témiscamingue (Québec, Canada),

laboratory tests on the silt used for the MRL revealed ksat between 2.15×10-6 cm/s and 1.06×10-5

cm/s for e ranging from 0.619 to 0.923, and AEV ranging from 29 kPa to 50 kPa for e from to 0.85

to 0.617 (Dagenais et al. 2012). The void ratio of the moisture retention layer as built was

approximately 0.61 (or a porosity n = 0.38).

Volumetric shrinkage (VS) at full dryness (w = 0) was calculated from free shrinkage tests to assess

the shrinkage susceptibility of the mixtures. In all cases, adding sludge to soil increased VS.

However, the calculated values of VS at complete dryness represent the worst case scenario. In

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practice, protective soil layers are required to minimize the impact of desiccation (shrinkage) due to

evaporation, frost action, erosion, animal burrowing, and/or plant rooting. If the initial water

content of the sludge could be lowered before mixing with the soil, the SSM could be easily

compacted to reach relatively low initial water content and void ratio, which may reduce the

susceptibility of the mixture to shrinkage.

One more interesting aspect is to estimate the volume of sludge that can be recovered from an

existing sludge pond for use in soil mixture to obtain the desired properties for mine site

rehabilitation. The total mass of wet sludge Msl that can be used in a MRL with a thickness H and

an area A compacted up to a void ratio em can be estimated with the following equation, where all

parameters have been previously defined (see Eqs. (1), (2) and (3)).

(7) �� = ��

��

��

If the bulk density the sludge in the sedimentation pond is known, the sludge volume reused Vsl can

then be calculated. Combining equations (1), (2) and (7) can allow choosing optimal water contents

of the sludge and soils to maximize the mass of sludge to be reused (assuming that these initial

water contents can be controlled).

The sludge mass Msl obtained from Eq. (7) can be used to calculate the borrow soil mass Mundist that

does not need to be disturbed or excavated and transported (possibly over longer distance than for

the sludge). Assuming that the void ratio of the MRL made exclusively with the soil is eso, one

obtains:

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(8) �� = �� − ��

��

In the case of the soil and sludge studied, for a 1 ha (104 m2) MRL with a thickness H of 1 m, and

SSM placed at a void ratio em = 0.6, Msl, = 4243 tonnes for the SSM with β = 25% considering

initial water contents wi-so of 12.5 % for the soil and wi-sl of 175% for the sludge (i.e. wm = 27.6 %,

DR-m = 2.66). Assuming an in situ sludge density of 1.5 tonnes/m3 (which is a conservative value),

the sludge mass Msl, given above corresponds to a sludge volume Vsl = 2828 m3. Assuming another

void ratio em = 0.8, Msl = 3771 tonnes and Vsl = 2514 m3 per ha of MRL. In these two cases, the

volume Vsl is significant for a surface of 1 ha. Compared to a MRL made of soil only with eso = 0.8,

wi-so = 12.5 % and DR-so = 2.74, the use of a SSM with β = 25% considering initial water content wi-sl

= 175% for the sludge (i.e. wm = 27.6 %, DR-m = 2.66) will lead to a soil mass Mundist of 2039 tonnes

per hectare that does not need to be disturbed or excavated.

The investigation performed in this study was based on one silty soil and neutralisation sludge from

one pond at the Doyon mine site. It should be kept in mind that the testing procedures presented

here should be repeated for other types of soils and sludge involved in a mixture. Additional work is

required to evaluate the use of different granular soil types and sludge in SSM. Furthermore, an

assessment of the long-term effect of potentially damaging environmental forces (cracking due to

desiccation, swelling and structure changes due to freezing, precipitates remobilization, sludge

dissolution through acidic rain, etc.) should be performed before any use of SSM at a large scale.

Kinetics tests conducted at laboratory scale (with leaching columns tests) and at intermediate scale

(with in situ pilot cells) can be used to evaluate the chemical stability of soil-sludge mixture under

realistic conditions. Based on results presented by Zinck et al. (1997), aged rather than fresh sludge,

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sludge with a high neutralizing potential, low initial metal concentrations and stable amorphous

mass has lower risk for metal leaching. Such sludge with a relative chemical stability is preferred.

6. CONCLUSION

Large volumes of sludge are generated by the active treatment of acid mine drainage at different

mine sites in Canada and worldwide. The objective of the paper was to assess the

hydrogeotechnical potential use of silty soil–sludge mixtures as earthen cover material for mine site

rehabilitation using AMD neutralisation sludge from the Doyon mine site –IAMGOLD (Quebec,

Canada). Both the soil and sludge were non-acid generating. For that purpose, the impact of the

sludge content on the hydrogeotechnical properties of soil–sludge mixtures (SSM) was studied, i.e.

grain-size distribution, specific gravity, Proctor curve, freeze-thaw effects on the saturated

hydraulic conductivity, desiccation shrinkage and water retention curves. The results indicated that

adding up to 25% sludge to a silty soil can provide a mixture with appropriate saturated hydraulic

conductivity (about 10-5 cm/s) and water retention properties (AEV about 30 kPa) for the mixture to

be used in the moisture retention layer (MRL) of a cover with capillary barrier effects (CCBE),

based on comparison with existing efficient CCBE. The impact of sludge addition to the silty soil

on freeze–thaw behavior was relatively limited. Volumetric shrinkage at complete drying of the

mixtures (worst case scenario) increased with the sludge content, but shrinkage can be reduced by

covering the mixture with a layer of coarse material (the drainage and protection layer of the

CCBE) to control evaporation. It has been shown that the sludge mass that can be reused in the

MRL of CCBE could be significant, consequently reducing the mass (or volume) of borrow soil

required as well as the footprint of mine sites rehabilitation.

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7. ACKNOWLEDGEMENTS

The authors would like to acknowledge the contribution of their partners, the Ministère du

Développement Économique, Innovation et Exportation (MDEIE) du Québec, IAMGOLD,

Division Doyon – Mouska – Westwood, the MISA group, URSTM-UQAT, Golder & Associates,

and ACCORD Abitibi-Témiscamingue.

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Table captions

Table 1. Proportions of wet components in the wet SSM

Table 2. Calculated water content wm (%) of SSM.

Table 3. Content (%) of the main elements of environmental interest in sludge and soil samples.

Table 4. Typical measured and calculated specific gravity of solids for the soil, sludge, and SSM

prepared with soil and sludge at 7.5% and 175% water content, respectively.

Table 5. Saturated hydraulic conductivity ksat (cm/s) for the soil and SSM, with void ratio e in

brackets (mixtures prepared with wi-so = 7.5% and wi-sl = 175%)

Table 6. Calculated volumetric shrinkage values

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Figure captions

Figure 1. Mineralogical semi-quantitative composition (wt.%) of a) the soil and b) the crystalline

materials in sludge (amorphous or poorly-crystalline phases were not quantified).

Figure 2. Grain-size distribution curves for the sludge, soil, and SSM with β =25% (mixture

prepared with wi-so = 7.5% and wi-sl = 175%)

Figure 3. Modified Proctor curves, minimum void ratio and maximum dry density curves at

saturation for SSM (mixtures prepared with wi-so = 7.5% and wi-sl = 175%)

Figure 4. Swelling ratio curves for SSM mixtures versus freeze–thaw cycles (mixtures prepared

with wi-so = 7.5% and wi-sl = 175%).

Figure 5. Variation of the ksat /ksat-0 ratio versus the freeze–thaw cycle (SSM with β =25% prepared


Figure 6. Typical photographs showing views of the samples (in triplicate) at the end of free

shrinkage in molds M-I : a) soil samples (wi-so at 12.5%), b) sludge samples (wi-sl at 175%) and

c) SSM samples with β=25% ( wi-sl = 200% and wi-so = 12.5%; wi-m = 28.6; see Table 2)

Figure 7. Typical results for the effect of mold scale on the shrinkage curves of SSM (initial water

contents wi-sl = 200% and wi-so = 12.5%; with wi-m = 28.6%; see Table 2).

Figure 8. Shrinkage curves for sludge at initial water contents of 175% and 200%.

Figure 9. Soil shrinkage curves (for initial water contents of 12.5 %, 20%, and 25%).

Figure 10. SSM shrinkage curves with β = 25% at different initial sludge water contents

Figure 11. Shrinkage curves e(ψ) observed during the drying process in the 100 bar membrane

extractor for SSM with β = 25% and wi-so = 12.5% and wi-sl = 175%.

Figure 12. Water retention curves of the soil and SSM with β = 25%, initial water contents of 12.5

% for the soil and 175% for the sludge

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Figure 1. Mineralogical semi-quantitative composition (wt.%) of a) the soil and b) the crystalline

materials in sludge (amorphous or poorly-crystalline phases were not quantified).

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Figure 2. Grain-size distribution curves for the sludge, soil, and SSM with β =25% (mixture

prepared with wi-so = 7.5% and wi-sl = 175%)

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Figure 3. Modified Proctor curves, minimum void ratio and maximum dry density curves at

saturation for SSM (mixtures prepared with wi-so = 7.5% and wi-sl = 175%)

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Figure 4. Swelling ratio curves for SSM mixtures versus freeze–thaw cycles (mixtures prepared


Figure 5. Variation of the ksat-j /ksat-0 ratio versus the freeze–thaw cycles (SSM with β =25%

prepared with wi-so = 7.5% and wi-sl = 175%).

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a)

b)

c)

Figure 6. Typical photographs showing views of the samples (in triplicate) at the end of free

shrinkage in molds M-I : a) soil samples (wi-so at 12.5%), b) sludge samples (wi-sl at 175%) and c)

SSM samples with β=25% ( wi-sl = 200% and wi-so = 12.5%; wi-m = 28.6; see Table 2)

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Figure 7. Typical results for the effect of mold scale on the shrinkage curves of SSM (initial water

contents wi-sl = 200% and wi-so = 12.5%; with wi-m = 28.6%; see Table 2).

Figure 8. Shrinkage curves for sludge at initial water contents of 175% and 200%.

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Figure 9. Soil shrinkage curves (for initial water contents of 12.5 %, 20%, and 25%).

Figure 10. SSM shrinkage curves with β = 25% at different initial sludge water contents

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Figure 11. Shrinkage curves e(ψ) observed during the drying process in the 100 bar membrane

extractor for SSM with β = 25% and wi-so = 12.5% and wi-sl = 175%.

Figure 12. Water retention curves of the soil and SSM with β = 25%, initial water contents of 12.5

% for the soil and 175% for the sludge

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Table 1. Proportions of wet components in the wet SSM

β

(%)

Wet sludge

(%)

Wet soil

(%)

0 (soil) 0 100

10 9.1 90.9

15 13.0 87.0

20 16.7 83.3

25 20.0 80.0

∞ (sludge) 100 0

Table 2. Calculated water content wm (%) of SSM.

wi-so (%) 7.5 12.5

wi-sl (%) 175 200 175 200

β = 10% 13.8 14.2 18.9 19.3

β = 15% 16.8 17.3 21.9 22.5

β = 20% 19.6 20.4 24.8 25.6

β = 25% 22.4 23.3 27.6 28.6

Table 3. Content (%) of the main elements of environmental interest in sludge and soil samples.

Element Sludge Soil

Al 2.1 5.940

Ca 18.6 2.31

Mg 1.590 1.155

Fe 9.210 4.135

S 7.370 0.341

Ti 0.014 0.300

As <0.003 <0.003

Cu 0.03 0.030

Ni 0.006 0.018

Pb 0.001 0.000

Zn 0.010 0.004

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Table 4. Typical measured and calculated specific gravity DR of solids for the soil, sludge, and SSM

prepared with soil and sludge at 7.5% and 175% water content, respectively.

Material Measured

(-)

Calculated

(-)

β = 0% (Soil) 2.74 -

β = 10 % 2.71 2.71

β = 15 % 2.70 2.69

β = 20 % 2.69 2.68

β = 25 % 2.66 2.67

β = ∞ ( Sludge) 2.10 -

Table 5. Saturated hydraulic conductivity ksat (cm/s) for the soil and SSM, with void ratio e in

brackets (mixtures prepared with wi-so = 7.5% and wi-sl = 175%)

Sample Average w before

saturation (%)

ksat (cm/s)

Sample 1 Sample 2 Average

Soil - β = 0% 9.0 8.4×10-5

(e = 0.46) 3.1×10-5

(e = 0.42) 5.7×10-5

(e = 0.44)

SSM - β = 10% 10.5 4.9×10-5

(e = 0.37) 6.3×10-5

(e = 0.41) 5.6×10-5

(e = 0.39)

SSM - β = 15% 11.9 2.5×10-5

(e = 0.34) 6.2×10-5

(e = 0.38) 4.4×10-5

(e = 0.36)

SSM - β = 20% 10.8 2.3×10-5

(e = 0.32) 1.4×10-5

(e = 0.25) 1.8×10-5

(e = 0.29)

SSM - β = 25% 18.6 3.4×10-5

(e = 0.52) 3.2×10-5

(e = 0.53) 3.3×10-5

(e = 0.52)

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Table 6. Calculated volumetric shrinkage values

Material VS (%)

Soil (wi-so ≈ 7.5%) 0

Soil (wi-so ≈ 12.5%) 2.3

Soil (wi-so ≈ 20%) 11.8

Soil (wi-so ≈ 25%) 15.4

Sludge (wi-sl = 175%) 67.2

Sludge (wi-sl =200%) 68.3

SSM (β = 25%)

wi-sl ≈ 175% and wi-so = 7.5% 24.3

wi-sl ≈ 175% and wi-so= 12.5% 25.7

wi-sl ≈ 200% and wi-so= 7.5% 30.4

wi-sl ≈ 200% and wi-so= 12.5% 32.5

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