hydro-mechanical evaluation of stabilized mine tailings

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Hydro-mechanical evaluation of stabilized mine tailings A.M.O. Mohamed M. Hossein F.P. Hassani Abstract In this study, mine tailings waste was stabilized using a combination of lime, fly ash type ‘‘C’’, and aluminum. Treated samples were subjected to mineral identification for evaluating the forma- tion of ettringite and gypsum. Also, unconfined compression, hydraulic conductivity, and cyclic freeze and thaw tests were performed to evaluate the hydro-mechanical properties of the stabilized sam- ples. Experimental results have shown that the ap- plication of lime and fly ash type ‘‘C’’ to high sulfate content tailings has improved its plasticity, work- ability, and volume stability. Moreover, upon addi- tion of aluminum to lime and fly ash in a sulfate-rich environment, ettringite and calcium sulfo-aluminate hydrate are formed in these samples. Application of 5% lime, 10% fly ash type ‘‘C’’, in combination with 110 ppm aluminum, resulted in the formation of a solid monolith capable of producing more than 1,000 kPa of unconfined compressive strength, and reduced tailings permeability to 1.96·10 –6 cm s –1 , which is less than the recommended permeability of 10 –5 cm s –1 by most environmental protection agencies for reusability of solidified/stabilized sam- ples. The permeability of the treated tailings samples remained below the recommended permeability, even after exposing the treated samples to 12 freeze and thaw cycles. Therefore, based on the experimental results, it is concluded that treatment of high sulfate-content tailings with lime and fly ash, combined with the availability of aluminum for re- actions, is a successful method of solidifying highly reactive mine tailings. Keywords Aluminum Fly ash Lime Reactive tailings Solidification Introduction Numerous investigators have performed research in the area of acid mine drainage (AMD) and heavy metal sta- bilization (Hossein 1991; Hedin and Watzlaf 1994; Kepler and McCleary 1994; Kuyucak and others 1995; Mohamed and others 1995; Mohamed and Antia 1998; Hossein and others 1999) by means of chemical admixtures, but the bulk of the studies performed seem to concentrate on the use of lime as the chemical additive. Moreover, previous research does not consider the reuse potential of the stabilized tailings, as it concentrates more on the reduced leachability of heavy metals as a result of treatment. Therefore, development of a treatment proce- dure capable of reducing the leachability of heavy metal and other environmental concerns, as well as producing an inert material with reuse potential, is of great interest to the mining industry. In the context of the present study, it is proposed to use lime and fly ash, as well as aluminum, in the presence of sulfate compounds to enhance and promote the formation of mineral ettringite as a means of solidification/stabili- zation of reactive tailings. However, it has been reported that ettringite is an expansive mineral with deteriorating consequences, which could cause physical and chemical instability (Mitchell and Dermatas 1992; Mohamed and Antia 1998; Mohamed 2000). Occurrence of expansion in ettringite not only could result in structural instability, but could also lead to release of absorbed and/or adsorbed metals from its structure. Moreover, disintegration of ettringite could result in the release of excess sulfate ions into the surrounding environment and lower the pH. In a low pH condition, the solubility of heavy metals, which were otherwise precipitated, will increase with potentially harmful effect to the surrounding environment. Therefore, studying the factors that could induce or prevent ettringite Received: 9 July 2001 / Accepted: 24 September 2001 Published online: 11 December 2001 ª Springer-Verlag 2001 A.M.O. Mohamed (&) Civil Engineering Department, UAE University, P.O. Box 17555, Al Ain, United Arab Emirates E-mail: [email protected] Tel.: +971-3-7051698 Fax: +971-3-7623154 M. Hossein Hart Crowser Inc., New Jersey, USA F.P. Hassani Department of Mining and Metallurgical Engineering, McGill University, 3450 University Street, Montreal, Quebec, H3A 2A7 Canada DOI 10.1007/s00254-001-0458-y Environmental Geology (2002) 41:749–759 749 Original article

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Page 1: Hydro-mechanical evaluation of stabilized mine tailings

Hydro-mechanical evaluationof stabilized mine tailingsA.M.O. Mohamed Æ M. Hossein Æ F.P. Hassani

Abstract In this study, mine tailings waste wasstabilized using a combination of lime, fly ash type‘‘C’’, and aluminum. Treated samples were subjectedto mineral identification for evaluating the forma-tion of ettringite and gypsum. Also, unconfinedcompression, hydraulic conductivity, and cyclicfreeze and thaw tests were performed to evaluate thehydro-mechanical properties of the stabilized sam-ples. Experimental results have shown that the ap-plication of lime and fly ash type ‘‘C’’ to high sulfatecontent tailings has improved its plasticity, work-ability, and volume stability. Moreover, upon addi-tion of aluminum to lime and fly ash in a sulfate-richenvironment, ettringite and calcium sulfo-aluminatehydrate are formed in these samples. Application of5% lime, 10% fly ash type ‘‘C’’, in combination with110 ppm aluminum, resulted in the formation of asolid monolith capable of producing more than1,000 kPa of unconfined compressive strength, andreduced tailings permeability to 1.96·10–6 cm s–1,which is less than the recommended permeability of10–5 cm s–1 by most environmental protectionagencies for reusability of solidified/stabilized sam-ples. The permeability of the treated tailings samplesremained below the recommended permeability,even after exposing the treated samples to 12 freezeand thaw cycles. Therefore, based on the

experimental results, it is concluded that treatmentof high sulfate-content tailings with lime and fly ash,combined with the availability of aluminum for re-actions, is a successful method of solidifying highlyreactive mine tailings.

Keywords Aluminum Æ Fly ash Æ Lime Æ Reactivetailings Æ Solidification

Introduction

Numerous investigators have performed research in thearea of acid mine drainage (AMD) and heavy metal sta-bilization (Hossein 1991; Hedin and Watzlaf 1994; Keplerand McCleary 1994; Kuyucak and others 1995; Mohamedand others 1995; Mohamed and Antia 1998; Hossein andothers 1999) by means of chemical admixtures, but thebulk of the studies performed seem to concentrate on theuse of lime as the chemical additive.Moreover, previous research does not consider the reusepotential of the stabilized tailings, as it concentrates moreon the reduced leachability of heavy metals as a result oftreatment. Therefore, development of a treatment proce-dure capable of reducing the leachability of heavy metaland other environmental concerns, as well as producing aninert material with reuse potential, is of great interest tothe mining industry.In the context of the present study, it is proposed to uselime and fly ash, as well as aluminum, in the presence ofsulfate compounds to enhance and promote the formationof mineral ettringite as a means of solidification/stabili-zation of reactive tailings. However, it has been reportedthat ettringite is an expansive mineral with deterioratingconsequences, which could cause physical and chemicalinstability (Mitchell and Dermatas 1992; Mohamed andAntia 1998; Mohamed 2000). Occurrence of expansion inettringite not only could result in structural instability, butcould also lead to release of absorbed and/or adsorbedmetals from its structure. Moreover, disintegration ofettringite could result in the release of excess sulfate ionsinto the surrounding environment and lower the pH. In alow pH condition, the solubility of heavy metals, whichwere otherwise precipitated, will increase with potentiallyharmful effect to the surrounding environment. Therefore,studying the factors that could induce or prevent ettringite

Received: 9 July 2001 / Accepted: 24 September 2001Published online: 11 December 2001ª Springer-Verlag 2001

A.M.O. Mohamed (&)Civil Engineering Department, UAE University,P.O. Box 17555, Al Ain, United Arab EmiratesE-mail: [email protected].: +971-3-7051698Fax: +971-3-7623154

M. HosseinHart Crowser Inc., New Jersey, USA

F.P. HassaniDepartment of Mining and Metallurgical Engineering,McGill University, 3450 University Street, Montreal,Quebec, H3A 2A7 Canada

DOI 10.1007/s00254-001-0458-y Environmental Geology (2002) 41:749–759 749

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expansion is of great importance for the assessment ofstabilized mine tailings waste.The phenomenon of expansion in cement-based mortars isknown as sulfate attack in the concrete industry. Expansionand loss of structural integrity can take place in two forms;expansion caused by the formation of ettringite, and/orloss of strength through softening that has been attributedto gypsum formation. Depending on sulfate concentration,the corrosive mechanism of concrete is divided into twoparts (Cohen and Mather 1991). At low concentrations(approximately 830 mg/l of SO4 – this limit changes withchange in aluminum availability in the sample) mecha-nisms causing change are controlled by the ettringite for-mation. When SO4 concentration attains a higher value, themechanism is dominated by gypsum formation.Gypsum formation is not generally believed to cause ex-pansion, but to softening and mushiness in the sample,whereas expansion and subsequent microcracking is gen-erally attributed to ettringite formation (Mitchell and Der-matas 1992; Mohamed and Antia 1998). Moreover, additionof natural pozzolans such as fly ash can further stabilize thephysical and mechanical structure of the treated mortars.According to Mehta (1986a), the technical significance of theuse of fly ash is derived mainly from three features of thepozzolanic reaction. First, the reaction is slow; second, thereaction is lime consuming instead of lime producing; andthird, the reaction products are very efficient in filling uplarge capillary space. The second and third features ofpozzolans are responsible for the capacity of fly ash toimprove the sulfate-attack resistance of mortars. In the caseof internal sulfate attack, addition of fly ash to cement wasstudied by Ouyang and others (1988). After conductingresearch on expansion and subsequent loss of strengthcaused by ettringite formation in the cement mortar, theauthors concluded that substitution of cement with fly ashgreatly reduced expansion. Mehta (1986b) considered boththe chemistry and mineralogy of fly ash to explain the effectof fly ash to sulfate resistance of cement paste. The sulfateresistance of the class ‘‘C’’ fly ash/cement mixture has beenattributed to its tendency to form ettringite prior to excesssulfate exposure; ettringite remains stable in the presence ofadditional sulfate ions.In the event of reusing the stabilized tailings, the strength ofcompacted treated specimens is an important property toassess its recycling potential. Special emphasis should alsobe given to controlling any possible material deteriorationcaused by freezing–thawing or wetting–drying processes.This deterioration results in the loss of the specimen’sstructural integrity, or visual cracking caused by its highswelling potential. Furthermore, hydraulic conductivity ofthe treated samples should be evaluated to examine theeffect of the proposed treatment procedure on seepagepotential through the stabilized mine tailings waste.

Materials and methods

Mine tailings samplesSulfate-rich mine tailings samples were obtained frommine sites in Quebec, Canada. Samples were received in

sealed plastic containers and during the duration of thetests were kept in a humid room. Portions of the sampleswere air dried to conduct the initial physical and chemicalanalysis needed for characterization of the samples. Theair-dried samples were gently crushed with a mortar andpestle and were kept in sealed glass container for furtheranalysis.The following experiments were conducted according toAmerican Society of Testing Materials (ASTM) standardstesting procedures. The selected analyses were conductedin triplicate and their average values are reported. Theexperimental results indicate that tailings samples have amoisture content of 10.29%, a specific gravity of 3.60, aspecific surface area of 10.62 m2 g–1, a liquid limit of18.23%, and a plastic limit of 18.0%.The results of tailings samples grain size analysis indicatethat samples contained only 3.5% of clay-sized particles.Most of the particle-size distribution fell in medium to finesand, with very limited amounts of silt- and clay-sizedparticles. pH measurement of 1:10 tailings–water extractwas conducted according to ASTM method D4972 (Stan-dard Test Method for pH of Soils). The experimental re-sults indicate that tailings samples had a pH of 2.49, aredox potential of 410 mV, and a cation exchange capacityof 12.26 meq 100 g–1.Tailings samples elemental composition was determinedusing X-ray fluorescence analysis. The XRF is a usefultechnique in determining the total composition of asample. The results of XRF analysis are given in Table 1.The X-ray diffraction analyses of tailings samples, shownin Fig. 1, indicate that the mineralogical composition ismainly composed of pyrite, graphite, quartz, illite,clinochlore, thulium tellurate, nimite, copper sulfide,nickel sulfide, tin sulfide, strontium oxide, and cobaltoxide.

Fly ash compositionThe chemical composition of the fly ash used in this studyis given in Table 2. The fly ash is type ‘‘C’’, and was sup-plied by CANMET research laboratory in Ottawa, Canada.The chemical composition of fly ash type ‘‘C’’ has beencompared with that of ordinary Portland cement type 10,as is shown in Table 2.

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Table 1XRF analysis of mine tailings samples

Element Content

Al2O3 (%) 6.6Fe2O3 (%) 41.07MgO (%) 1.31CaO (%) 0As (ppm) 93Cd (ppm) 25Cr (ppm) 0Cu (ppm) 768Pb (ppm) 115Zn (ppm) 336

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Experimental procedure

Unconfined compression strength testAddition of lime and fly ash type ‘‘C’’ to sulfate contenttailings is expected to enhance its cohesive properties, and,thus, increase its compressive strength. The developmentand maintenance of high strength and stiffness is achievedby elimination of large pores, by bonding particles andaggregates together, by maintenance of flocculent particlearrangements, and by the prevention of swelling. More-over, upon addition of aluminum, ettringite will form inthe treated mine tailings (Mohamed and others 1995;Hossein and others 1999; Mohamed and others 2001),which will further enhance the strength properties of thetreated samples. Based on this analogy, the unconfinedcompression test was used to evaluate the changes in thestrength of the stabilized mine tailings and to assess itsreusability.The unconfined compressive strength (UCS) test refers tothe sample’s resistance to compression with increasingstrain, measured under unconsolidated, undrained con-ditions. In this study, the unconfined compression testswere performed in accordance with the ASTM D2166

procedure, a test method for unconfined compressivestrength of cohesive soils. The US Environmental Protec-tion Agency (EPA) generally considers a stabilized mate-rial as satisfactory if it has a UCS of at least 345 kPa;however, the minimum required strength should be de-termined from the design loads to which the material maybe subjected.To measure the compressive strength of the treated sam-ples, cylindrical specimens (with a diameter of 30 mm anda height of 60 mm) were prepared at solid to water ratio of0.5, as recommended by Hossein and others (1999).Samples were dry mixed and water was added and allowedto mellow and cure for 24 h at 90% relative humidity and25 �C before being compacted in specifically designedmolds. The compacted samples were wrapped in a plasticsheet and subsequently stored in a humid room untiltesting. Samples were tested after 1, 28, 140, and 360 daysof curing. The strain rate was 0.83% per min. The exper-iments were conducted in triplicate to ensure the reli-ability of the test results. A total of 57 cylinders were madeand tested to determine the unconfined compressivestrength of the samples and to establish a general trend forthe gain in strength with different treatment additives.

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Table 2Chemical composition of cementand fly ash type ‘‘C’’

Element Portland cement Fly ash(% w/w) (% w/w)Lafarge type 10 Type ‘‘C’’

SiO2 21.36 53.3Al2O3 3.93 23.63TiO2 0.19 0.71P2O5 0.21 0.12Fe2O3 3.15 4.4CaO 62.41 15.81SrO 0.27 0.22MgO 2.57 1.15Na2O 0.2 3.03K2O 0.8 0.42SO3 3.43 0.2LOI 1.72 0.71

Fig. 1X-ray diffraction pattern of tailing sample. FA Fly ash

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Hydraulic conductivity testThe permeability of a stabilized waste is an importantfactor because it indicates the ability of a material topermit the passage of water and to limit the loss ofpollutants from the stabilized waste to the environment.Addition of lime and fly ash to mine tailings waste willimprove its plasticity, workability, and volume changecharacteristics. Furthermore, most of these materials willalso exhibit improved strength, stress–strain, and fatiguecharacteristics, and develop lower values of hydraulicconductivity. The reduction in hydraulic conductivity isachieved through modification of pore-size geometry andits distribution as a result of pozzolanic activity.Samples were premixed with water and were allowed tomellow for 24 h before being compacted into a cell with adiameter of 30 mm and a length of 60 mm. The treatedsamples were cured at 90% relative humidity at 25 �C for28 days before being tested for their permeability using thefalling head test method (ASTM 1992). Prior to the test, thecell was placed in a sink in which the water was about50.8 mm above the cover of the cell while the outlet wasopened so that the water could back up through thespecimen. This procedure was done to insure saturation ofthe sample and to eliminate entrapped air. When the waterin the plastic inlet tube on the top of the mold reachedequilibrium with the water in the sink (allowing for cap-illary rise in the tube), it was assumed that the sample wassaturated. The permeability was corrected by multiplyingits measured value by a factor that relates the viscosity ofwater at the test temperature to that at 20 �C. The exper-iment was conducted in duplicate to increase the reliabilityof the test results.

Freeze and thaw testThe durability of a treated sample is usually measured byevaluating its resistance to repeated freeze–thaw cycles. Itis suggested that freezing and thawing breaks down thenatural bonds of the hydration products by pore-ice for-mation (Leroueil and others 1991; Yong and Mohamed1992; Mohamed and others 1993; Mohamed 1997). Labo-ratory investigations have also shown that freeze–thawcycles can increase the hydraulic conductivity of com-pacted fine-grained soils by one or two orders of magni-tude (Chamberlain and others 1990; Othman 1992; Yongand Mohamed 1992; Mohamed and Antia 1998). Thesestudies have shown that ice lenses formed during freeze–thaw result in a network of cracks. When the soil tem-perature drops below 0 �C, ice crystals nucleate in thecenter of large pores. When water changes to ice, its vol-ume increases by about 9% because of the opening of thelattice of its hexagonal crystal structure. As the ice crystalsgrow, they interfere with each other and the soil particles.Thus, ice exerts pressure on the surrounding soil andcauses it to move, rearrange, and consolidate (Alkire andMorrison 1982). Moreover, the cyclic freezing and thawingduring ice-crystal formation causes mechanical stresses,which result in the formation of planes of weakness in theaggregates and, as a consequence, damages the structuralintegrity of the treated samples. However, addition of flyash type ‘‘C’’ to the mine tailings is expected to improve

tailings plasticity and volume change characteristics. Fur-thermore, addition of fly ash will improve the flocculationof the tailings particles’ arrangement and will reduce itshydraulic conductivity and porosity. Therefore, the overallresult of the proposed treatment procedure is to lower themagnitude of the hydraulic conductivity and prevent orreduce the penetration of moisture into the treated mortar.To perform the freeze and thaw tests on the treated tailingssamples, eight compacted and 28-day cured specimenswere subjected to repeated freeze and thaw conditions.Each cycle included a 24-h freeze at –10 �C and a 24-hthaw at 25 �C. Samples were thawed at a constant tem-perature and relative humidity room. Overall, four speci-mens were subjected to six sets of freeze/thaw cycles andthe other four to 12 sets of freeze–thaw cycles before thesamples were tested for their unconfined compressivestrength, as well as permeability measurements accordingto the procedures described previously.

Results and discussion

Unconfined compressive strength test resultsThe strength of compacted specimens is an importantproperty for assessing the success of stabilization andsolidification, and the potential reusability of the treatedtailings. Improvement in physical and mechanical char-acteristics of the treated tailings will enhance its handlingand disposal capabilities in a tailings pond, or as aggre-gates in related applications. Development of sufficientstrength in treated tailings will allow its use (as an inertmaterial) for embankments, retaining walls, or otherapplications that require a certain level of shear strength.To examine the gain and/or loss of strength in thesesamples, control specimens were made and compactedwithout the addition of any treatment agent. These spec-imens were identified as S3L0A0F0, where ‘‘S3’’ refers tothe site number of the tested mine tailings; ‘‘L0’’ refers tothe lime and its added percent by weight (in this case, it is0%); ‘‘A0’’ refers to aluminum and its added aluminumconcentration in ppm (in this case, it is 0 ppm), and ‘‘F0’’refers to the fly ash and its added percent by weight (inthis case, it is 0%).Twelve cylinders were compacted with the addition of 5%lime as the treatment agent and were identified asS3L5A0F0. Furthermore, to examine the effect of fly ash onthe strength of the samples, another 12 cylinders werecompacted with the addition of 5% lime plus 10% fly ashtype ‘‘C’’ and were identified as S3L5A0F10. In addition, toexamine the effect of aluminum addition on the strength ofthe treated samples, an additional 21 cylinders were madewith the addition of 5% lime plus 10% fly ash type ‘‘C’’, aswell as 110 ppm of aluminum, and were identified asS3L5A110F10. The aluminum was added to promote theformation of ettringite in the samples (Hossein and others1999) and to study its effect on the strength of the treatedspecimens. Most of the specimens were tested for curingperiods of 1, 7, 14, and 28 days. However, to study thelong-term effect of ettringite on the strength of the treated

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samples, the aluminum-treated specimens were furthercured and tested after 90, 140, and 360 days of hydration.

No treatmentThe results for the unconfined compressive strength testperformed on all the samples up to 28 days of curing areshown in Fig. 2. The control samples (S3L0A0F0), whichwere made without any additives, failed to gain anystrength during the hydration period.

Treatment with limeAddition of 5% lime to the tailings sample (S3L5A0F0),which is the threshold of lime needed to stabilize the pH ofthe sample to near 11 (Hossein and others 1999), resultedin a gain of moderate strength after 7 days of hydration.These samples reached an unconfined compressivestrength of 137 kPa between 7 and 14 days of hydration.However, further curing led to disintegration and loss ofstrength in these samples.

To explain these results, one has to discuss the interactionbetween mine tailings waste and lime. The major strengthgain of lime-treated mine tailings waste is mainly derivedfrom two reactions (Mohamed and Antia 1998): namely,hydration and pozzolanic reaction. The calcium hydroxidethat results from the hydration of lime dissociates in thewater. As a result, calcium concentration and pH of thepore fluid will increase. The hydrous alumina present inthe waste will then gradually react with the calcium ionsliberated from the hydrolysis of lime to form insolublecompounds of calcium alumina hydrate (CAH; secondarycement-type products). This secondary reaction is knownas the pozzolanic reaction, which contributed to the in-crease of strength. In addition, the disintegration and lossof strength is attributed to the formation of gypsum asshown from the X-ray pattern (Fig. 3). Therefore, it ispostulated that the addition of lime alone could not in-stigate enough pozzolanic activity to generate a substantialamount of strength in the samples.

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Fig. 2Stress variations with curing times for un-treated and treated samples

Fig. 3X-ray diffraction pattern of tailing sample treatedwith 5% lime

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Treatment with lime and fly ashApplication of 5% lime plus 10% fly ash type ‘‘C’’(S3L5A0F10) generated higher strength in the samplesthan those that were treated with lime alone (Fig. 2).Within the first day of treatment, 90 kPa of strength wasmeasured in these samples. Generation of high earlystrength in these samples is attributed to the activation oflime’s pozzolanic properties in the presence of fly ash type‘‘C’’. The strength of these samples increased to 300 kPaafter 7 days of hydration and almost remained the sameuntil 28 days of curing.The strength increase is attributed to the (1) formation ofcement-type products, (2) formation of ettringite, (3) re-duction in gypsum content, and (4) formation of a floccu-lated structure. The formation of cement-type products canbe explained based on the interaction between mine tailingswaste, lime, and fly ash. Upon the addition of these materialsand mixing, hydration of both lime and fly ash occurs rap-idly. The major hydration products are hydrated calciumsilicates (C3S2H3), hydrated calcium aluminate (C3AH6),and hydrated lime [Ca (OH)2]. The first two of these are themain cement-type products formed. The third, hydratedlime is deposited as a separate crystalline solid phase. Thesenew products bind together and form a hardened skeletonmatrix. The silicate and aluminate phases are internallymixed, so it is likely that none is completely crystalline. Partof the Ca(OH)2 may also be mixed with other hydratedphases because it is partially crystalline.Ettringite formation is observed from the X-ray diffractionanalysis of lime and fly-ash-treated samples as shown inFig. 4. Moreover, there is a slight reduction in gypsumcontent for samples treated with the combination of limeand fly ash (Fig. 4) compared with those treated with limealone (Fig. 3).As stated before, application of lime and fly ash resulted inthe elimination of large pores, the creation of bondingbetween particles and aggregates, and the maintenance offlocculent particle arrangements, which contributes to thehigher strength of the samples.

Treatment with lime, fly ash, and aluminumSamples treated with an application of aluminum, lime,and fly ash (S3L5A110F10) gained more than 700 kPa after28 days of hydration (Fig. 2). It should be noted that theminimum recommended unconfined compressionstrength for reusability of treated waste material is 345 kPa(US EPA 1989). Because aluminum has a retardation effecton the lime and fly ash pozzolanic reactions, samplestreated with aluminum did not gain any significantstrength until 7 days of hydration. However, after 7 daysof curing, about 460 kPa of strength was generated in thesesamples and they continued to gain strength up to 28 daysof hydration. The X-ray diffraction analysis of 28-daycured samples, shown in Fig. 5, indicates the strong for-mation of ettringite in the sample because of aluminumaddition. Therefore, the high strength generated is attrib-uted to the formation of ettringite as well as otherpozzolanic products such as hydrated calcium silicates(C3S2H3), hydrated calcium aluminate (C3AH6), andhydrated lime [Ca (OH)2], as discussed previously.The long-term strength performance of the aluminum-,lime-, and fly-ash-treated sample is shown in Fig. 6. Theunconfined compression reached nearly 1,000 kPa after90 days of hydration and continued to gain strength until140 days of curing. The rate of strength gain in the sam-ples became almost flat after 90 days of hydration becausemore than 90% of the samples strength was reached withinthe first 90 days of hydration. The strength of the sampleremained at more than 1,000 kPa after 360 days ofhydration.From the compressive strength viewpoint, one can con-clude that application of fly ash is responsible for thegeneration of latent strength and slow pozzolanic reactionsin these samples. In addition, the formation of ettringite asa stable mineral prevents the acidification of thespecimen – as a result of excess free sulfate availability– and provides high strength in the sample. Maintainingsuch a high strength is a good indication of physical sta-bility and the success of the proposed treatment technique.

754 Environmental Geology (2002) 41:749–759

Fig. 4X-ray diffraction pattern of tailing sample treated with 5%lime and 10% fly ash (FA) class ‘‘C’’

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Moreover, because of this high strength, treated tailingscan potentially be used in a variety of mining structural fillapplications. The formation of a monolithic solid alsoresults in a significant reduction of the contact area be-tween the solid and any leachant, and, therefore, reducesthe inherent leachability of the stabilized tailings. Fur-thermore, the treated tailings can be potentially used in alandfill liner or as top-cover applications because of its lowhydraulic conductivity, as discussed below.

Hydraulic conductivity test resultsThe hydraulic conductivity of solidified and stabilizedwaste (tailings) to leaching fluid in a disposal facility is oneof the most important properties of the solidified materialin governing its two interrelated properties, namely,durability and leachability. Durability addresses the

long-term stability of the treated material and leachabilityrefers to the amount of pollutant released over time. Hy-draulic conductivity has been defined as the measure offluids that pass through the tortuous pore structure of thewaste, or the treated waste (Means and others 1995). Asstated before, the permeability of a stabilized waste is animportant factor because it indicates the ability of a ma-terial to permit the passage of water and to limit the loss ofpollutants from the stabilized waste to the environment.Hydraulic conductivity is examined in conjunction withleach test results to evaluate the potential of the stabilizedwaste to release pollutants into the environment.The relevance of hydraulic conductivity measurements canbe understood by comparing the hydraulic conductivity ofthe stabilized mine tailings with natural materials. Sand, ahighly permeable material, has a hydraulic conductivity on

Environmental Geology (2002) 41:749–759 755

Fig. 6Unconfined compression stress (UCS) variations withcuring times for tailing samples treated with 5% lime (L),10% fly ash (FA) class ‘‘C’’, and 110 ppm aluminum (A)

Fig. 5X-ray diffraction pattern of tailing sampletreated with 5% lime and 10% fly ash (FA)class ‘‘C’’, and 110 ppm of aluminum (Al)

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the order of 10–2 cm s–1. Clay, a material that is used toline lagoons and surface impoundments, can have hy-draulic conductivity on the order of 10–6 cm s–1 or lessand is considered to be relatively impermeable. Thus, it isdesirable to have stabilized tailings with a hydraulic con-ductivity similar to clay because it will not permit the freepassage of water through the stabilized tailings. By slowingthe contact of fluid with the tailings, it reduces the possibletransport of pollutants out of the stabilized tailings. Typ-ical hydraulic conductivities for stabilized waste rangefrom 10–4 to 10–8 cm s–1. Hydraulic conductivities of lessthan 10–5 cm s–1 are recommended for stabilized wastedestined for land disposal by the US EPA (1989). Reduc-tion of hydraulic conductivity of the aluminum-, lime-,and fly-ash-treated tailings to 10–7 m s–1 is considered thesuccess criteria for the proposed treatment procedure.Falling head hydraulic conductivity tests have been con-ducted on the aluminum-, lime-, and fly-ash-treated tail-ings. A moisture (water) to solid ratio of 0.5 was added tothe treated specimens, and then the specimens were al-lowed to mellow for 24 h at 95% relative humidity and25 �C. The 24-h cured samples were compacted in modi-fied molds (30 mm in diameter and 60 mm in length) andwere cured in controlled humidity and temperature con-ditions for 28 days before being subjected to hydraulicconductivity testing. Prior to conducting the experiment,samples were fully saturated by the back-suction tech-nique. Experiments were conducted in duplicate to ensurethe reliability of test results.

No treatmentThe results of the hydraulic conductivity test are depictedin Fig. 7 and point to several changes concerning the hy-draulic conductivity of the treated specimens. In general,the untreated sample (S3L0A0F0) has a magnitude of hy-draulic conductivity of 3.5·10–4 cm s–1 after 28 days ofcuring in a humid room and at constant temperature(25 �C).

Treatment with limeAddition of 5% lime (S3L5A0F0) to this sample resulted inlowering its permeability to 2.5·10–4 cm s–1 after 28 daysof curing. Therefore, the hydraulic conductivity of thelime-treated sample is still more than the minimum hy-draulic conductivity of 10–5 cm s–1 recommended for thesuccess of the treatment procedure.

Treatment with lime and fly ashApplication of 10% fly ash type ‘‘C’’ in conjunction with5% lime (S3L5A0F10) reduced the hydraulic conductivityof the treated specimens to 3.2·10–5 cm s–1 after 28 daysof curing. Such a low hydraulic conductivity is the result ofmodification of pore size and pore size distribution in thesamples that have the characteristics of the fly-ash-treatedtailings. Moreover, addition of lime and fly ash to thetailings sample resulted in the elimination of large pores,which increased the bonding particles and maintained theparticle arrangements. However, it still did not meet thestandard of 10–5 cm s–1.

Treatment with lime, fly ash, and aluminumAddition of aluminum to lime- and fly-ash-treated sam-ples (S3L5A110F10) resulted in the formation of ettringiteand a further reduction in hydraulic conductivity of thetested specimens. The hydraulic conductivity of the alu-minum-treated specimens was reduced to 1.96·10–6 cms–1 after 28 days of curing, which is below the standardof 10–5 cm s–1. Therefore, in addition to the beneficialproperties of treatment with lime and fly ash type ‘‘C’’,formation of ettringite in these samples helped to producean inert material with much lower hydraulic conductivity.Hence, it is postulated that ettringite crystals formed andfilled the capillary pores of the cylindrical specimens andfurther reduced their hydraulic conductivity. Therefore,the results clearly indicate that mine tailings samplesstabilized with aluminum, lime, and fly ash reached anacceptable hydraulic conductivity for safe disposal in theenvironment.

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Fig. 7Permeability variations for untreated and treated samplesafter 28 days of curing. * Untreated

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To better understand and delineate the relationship be-tween hydraulic conductivity and the mechanical proper-ties of the tested specimens, a relationship was developedbetween the strength of the material and the correspond-ing hydraulic conductivity, as shown in Fig. 8. It can beseen from Fig. 8 that, as the strength increases, thehydraulic conductivity decreases. Therefore, formation ofettringite in the aluminum-, lime-, and fly-ash-treatedsamples not only increased its unconfined compressivestrength properties, but also reduced its hydraulicconductivity.

Freeze and thaw test resultsIn cold climates, repeated cycles of freezing and thawingcan cause physical deterioration of an exposed solidifiedwaste product, thereby increasing its contact withgroundwater. According to US EPA (1989), durabilitytesting evaluates the resistance of a stabilized/solidifiedwaste mixture to degradation as a result of external envi-ronmental stresses. These tests were designed to mimicnatural conditions by stressing the sample through freez-ing–thawing and wetting–drying cycles. The stabilized/solidified specimens undergo repeated cycles during test-ing. Unconfined compressive strength, permeability, orother performance-based tests may be conducted on thesolidified/stabilized samples after each cycle to determinehow the physical properties of the solidified/stabilizedwaste changes as a result of simulated climatic stresses.The number of cycles a material can withstand withoutfailing can be used to judge the mechanical integrity of thematerial.Stabilized and solidified samples, treated in combinationwith lime, fly ash, and aluminum (S3L5A110F10), as wellas untreated controls (S3L0A0F0), were subjected to re-peated cycles of freezing and thawing. Samples were curedfor 28 days at 95% humidity and 25 �C before beingsubjected to freeze and thaw cycles. Six samples weresubjected to six cycles of freeze and thaw and a similarnumber of samples to 12 cycles before being tested forunconfined compressive strength as well as hydraulicconductivity measurement. The unconfined compressive

strength and permeability results are shown in Figs. 9 and10 and are discussed below.

Unconfined compressive strength variationsThe 28-day cured and treated samples (S3L5A110F10)hydrated under normal conditions reached a magnitude of730 kPa without being subjected to the harsh and deteri-orating effect of freeze and thaw cycles. As shown in Fig. 9,subjecting the samples to six cycles of freezing andthawing resulted in about 7% loss in its strength. However,specimens did not show any visual sign of deterioration. Inother words, the treatment procedure used to solidify andstabilize the tailings sample was capable of stabilizing thespecimens and resisting physical and mechanical disinte-gration after six sets of freeze/thaw cycles. However, theuntreated specimens showed significant deterioration afterthe second set of freeze–thaw cycles and finally disinte-grated.Samples that were subjected to 12 cycles of freezing andthawing showed almost 28% reduction in the magnitude oftheir unconfined compressive strength, while they main-tained their structural integrity during the repeated freeze–thaw cycles. The reduction in the strength of these samplesis attributed to high tailings moisture suction developed inthe freezing zone. This will cause an increase in the ef-fective stress in the region immediately below the freezingfront and the formation of segregated ice lenses. Suchbehavior could result in the shrinkage of the tailingsstructure, formation of cracks, and a change in the pore-size characteristics of the unfrozen section immediatelybelow the ice lenses.

Hydraulic conductivity variationsThe results of permeability studies conducted on thesesamples are shown in Fig. 10. It was expected that freezingand thawing cycles would result in an increase in thepermeability of samples because of the formation ofaggregated structures in the solidified matrix (Mohamedand Antia 1998). However, the results indicate that aftersix freeze–thaw cycles, the permeability increased by aboutfourfold whereas, after 12 cycles, it had increased by about

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Fig. 8Permeability variations with unconfined compressionstrength (UCS) for untreated and treated samples

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eightfold compared with the controls, as shown in Fig. 10.This means that, after 12 cycles of freeze–thaw, the finalpermeability had increased from its normal value of2.0·10–6 to 8.6·10–6 cm s–1, which indicates that it is stilllower than the standard permeability of 10–5 cm s–1.The low permeability change in the samples, even after 12cycles of freeze–thaw, is mainly attributed to the pozzol-anic properties of lime and fly ash, which resulted in theparticles’ arrangement and the lower void ratio in thesamples. Moreover, formation of ettringite, as a result ofaluminum addition, resulted in a reduction of capillaryspaces in the samples. The fine, needle-shaped ettringitecrystals prevented the penetration of water into the cap-illary zones, which otherwise could have become frozenand increased in volume to cause internal stress duringfreezing.The preceding discussion clearly indicates that theproposed treatment procedure is an effective method to

maintain the physical integrity of the stabilized tailingsunder extreme environmental conditions.

Conclusion

Application of lime, aluminum, and fly ash type ‘‘C’’ tohigh sulfate content tailings resulted in the formation of asolid monolith capable of producing more than 1,000 kPaunconfined compressive strength in the samples. Thetreated monolith was exposed to 12 freeze and thaw cyclesand preserved its structural integrity. Development andmaintenance of high strength and stiffness in these sam-ples are attributed to the pozzolanic properties of lime andfly ash in an environment rich in aluminum and sulfidecompounds.Application of lime and fly ash type ‘‘C’’ to high sulfatetailings improved its plasticity, workability, and volume

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Fig. 10Unconfined compression strength (UCS) variations withfreeze and thaw (F&T) cycles for samples treated with 5%lime (L5), 110 ppm of aluminum (A110), and 10% fly ash(F10) and cured for 28 days

Fig. 9Unconfined compression strength (UCS) variations withfreeze and thaw (F&T) cycles for samples treated with 5%lime (L5), 110 ppm of aluminum (A110), and 10% fly ash(F10) and cured for 28 days

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stability. Moreover, upon addition of aluminum to limeand fly ash in a sulfate-rich environment, ettringite, cal-cium sulfo-aluminate hydrate also formed in these sam-ples. The formation of ettringite further contributed to thehigh strength and stiffness of the treated tailings. Fur-thermore, the needle-type ettringite crystals that formed asa result of excess aluminum addition to the samplesblocked the capillary spaces of the tailings and, therefore,reduced the permeability of the treated samples. Applica-tion of 5% lime, 10% fly ash type ‘‘C’’ in combination with110 ppm aluminum reduced tailings permeability to1.96·10–6 cm s–1, which is lower than the maximum per-meability allowed (10·10–5 cm s–1) by most environmen-tal protection agencies to render a waste stabilized andimpermeable. The permeability of the treated tailingssamples remained below the recommended permeability,even after exposure of the treated samples to 12 freeze andthaw cycles.Overall, based on the results of the durability analysis,treatment of high sulfate content tailings with lime and flyash, combined with the availability of aluminum for re-actions, is a successful method for solidifying these highlyreactive tailings. The pozzolanic properties of lime and flyash, coupled with the possibilities of ettringite formationin the samples, will lead to the generation of a solidifiedmonolith capable of withstanding harsh environmentalconditions.

Acknowledgements The authors would like to acknowledge theassistance provided by CANMET, Canada, and the financialsupport provided by the Natural Sciences and EngineeringResearch Council of Canada.

References

Alkire BD, Morrison JM (1982) Changes in soil structure due tofreeze-thaw and repeated loading. Transportation ResearchRecord 918, Transportation Research Board, Washington, DC,pp 15–22

ASTM (American Society for Testing and Materials) (1992)Annual book of ASTM standards. ASTM, Philadelphia

Chamberlain EJ, Iskander I, Hunsiker SE (1990). Effect of freeze–thaw on the permeability and macrostructure of soils. Pro-ceedings of International Symposium on Frozen Soil Impacts onAgriculture, Range, and Forest Lands. Spokane, Washington,pp 145–155

Cohen M, Mather B (1991) Sulfate attack on concrete – researchneeds. Am Concrete Inst Mater J 88(1):62–69

Hedin RS, Watzlaf GR (1994) The effects of anoxic limestonedrains on mine water chemistry. International Land Reclama-tion and Mine Drainage Conference and Third InternationalConference on the Abatement of Acidic Drainage, Pittsburgh,PA, 24–29 April , vol 1. US Dept of the Interior. Bureau of MinesSpecial Publication SP 06A-94, pp 185–194

Hossein M (1991) Environmental and technical feasibility andcost analysis of surface disposal of metal mines tailings. CivilEngineering and Applied Mechanics Dept, McGill University

Hossein M, Mohamed AMO, Hassani FP, Elbadri H (1999)Ettringite formation in lime-remediated mine tailings: II.

experimental study. Canadian Institute of Mining, Metallurgy,and Petroleum (CIM). Bull Can Rock Mech Div 92(1029):75–80

Kepler DA, McCleary EC (1994) Successive alkalinity-producingsystems (SAPS) for the treatment of acidic mine drainage.International Land Reclamation and Mine Drainage Conferenceand Third International Conference on the Abatement of AcidicDrainage, Pittsburgh, PA, 24–29 April , vol 1, US Dept of theInterior. Bureau of Mines, SP 06A-94, pp 95–204

Kuyucak N, Payant S, Sheremata T (1995) Improved lime neu-tralization process. Mining and the Environment, ConferenceProceedings, An Integrated Approach to Planning and Reha-bilitation for the 21st Century, 28 May–1 June, Sudbury,Ontario, pp 129–138

Leroueil S, Tardif J, Roy M, La Rochelle P, Konrad JM (1991)Effects of frost on the mechanical behavior of Champlain seaclays. Can Geotech J 28(5):690–697

Means JL, Smith LA, Nehring KW, Brauning SE, Gavaskar AR,Sass BM, Wiles CC, Mashni CI (1995) The application of so-lidification/stabilization to waste materials. Lewis Publishers,New York

Mehta PK (1986a) Concrete: structure, properties and materials.Prentice-Hall, Englewood Cliffs

Mehta PK (1986b) Effect of fly ash composition on sulfate resis-tance of cement. Am Concrete Inst J 83(6):994–1000

Mitchell JK, Dermatas D (1992) Clay soil heave caused by lime-sulphate reactions. American Society for Testing and Materials,Philadelphia, ASTM Standard Testing Procedures 1135, pp 41–64

Mohamed AMO (1997) Effect of cyclic freeze–thaw on thedrainage characteristics of a multi-layer soil cover for minetailings storage sites. International Conference of EngineeringMaterials, 8–11 June, Ottawa, pp 159–172

Mohamed AMO (2000) The role of clay minerals in marly soils onits stability. Eng Geol 57:193–203

Mohamed AMO, Antia HE (1998) Geoenvironmental engineering.Elsevier, Amsterdam

Mohamed AMO, Yong RN, Caporuscio F, Yanful EK, Bienvenu L(1993) Chemical interaction and cyclic freeze–thaw effects onthe integrity of the soil cover for Waite Amulet tailings. In: YongRN, Hadjinicolaou J, Mohamed AMO (eds) Proceedings of the1993 Joint CSCE-ASCE National Conference on EnvironmentalEngineering. Montreal, pp 259–272

Mohamed AMO, Boily JF, Hossein M, Hassani FP (1995)Ettringite formation in lime remediated mine tailings: I. ther-modynamic modeling. Canadian Institute of Mining, Metallur-gy, and Petroleum (CIM). Bull Can Miner Process Div 88:69–75

Mohamed AMO, Hossein M, Hassani FP (2001) Ettringite for-mation in lime remediated mine tailings: III. rule of fly ashaddition. Can Inst Mining Metall Petrol Bull (in press)

Othman MA (1992) Effect of freeze–thaw on the structure andhydraulic conductivity of compacted clays. PhD Thesis, Uni-versity of Wisconsin-Madison, Madison

Ouyang C, Nanni A, Chang WF (1988) Internal and externalsources of sulfate ions in Portland cement mortar: two types ofchemical attack. Cement Concrete Res 18:699–709

US EPA (US Environmental Protection Agency) (1989) Stabili-zation/solidification of CERCLA and RCRA wastes: physicaltests, chemical testing procedures, technology screening, andfield activities. EPA/625/6-89/022. Risk Reduction EngineeringLaboratory, Cincinnati

Yong RN, Mohamed AMO (1992) Cyclic freeze–thaw consider-ation in design of engineered soil covers for reactive tailings,vol 3. Proceedings of the Canadian Society of Civil Engineering2nd Environmental Special Conference, pp 173–182

Environmental Geology (2002) 41:749–759 759

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