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U N I V E R S I TAT I S O U L U E N S I SACTAC

TECHNICA

U N I V E R S I TAT I S O U L U E N S I SACTAC

TECHNICA

OULU 2019

C 721

Jenni Kiventerä

STABILIZATION OF SULPHIDIC MINE TAILINGS BY DIFFERENT TREATMENT METHODSHEAVY METALS AND SULPHATE IMMOBILIZATION

UNIVERSITY OF OULU GRADUATE SCHOOL;UNIVERSITY OF OULU,FACULTY OF TECHNOLOGY

C 721

AC

TAJenni K

iventerä

C721etukansi.fm Page 1 Thursday, October 10, 2019 8:55 AM

ACTA UNIVERS ITAT I S OULUENS I SC Te c h n i c a 7 2 1

JENNI KIVENTERÄ

STABILIZATION OF SULPHIDIC MINE TAILINGS BY DIFFERENT TREATMENT METHODSHeavy metals and sulphate immobilization

Academic dissertation to be presented with the assent ofthe Doctoral Training Committee of Technology andNatural Sciences of the University of Oulu for publicdefence in the OP-Pohjola auditorium (L6), Linnanmaa, on1 November 2019, at 12 noon

UNIVERSITY OF OULU, OULU 2019

Copyright © 2019Acta Univ. Oul. C 721, 2019

Supervised byProfessor Mirja Illikainen

Reviewed byProfessor Konstantinos KomnitsasProfessor Valérie Cappuyns

ISBN 978-952-62-2395-7 (Paperback)ISBN 978-952-62-2396-4 (PDF)

ISSN 0355-3213 (Printed)ISSN 1796-2226 (Online)

Cover DesignRaimo Ahonen

JUVENES PRINTTAMPERE 2019

OpponentProfessor João P. de Castro-Gomes

Kiventerä, Jenni, Stabilization of sulphidic mine tailings by different treatmentmethods. Heavy metals and sulphate immobilizationUniversity of Oulu Graduate School; University of Oulu, Faculty of TechnologyActa Univ. Oul. C 721, 2019University of Oulu, P.O. Box 8000, FI-90014 University of Oulu, Finland

Abstract

Millions of tons of mine tailings are generated worldwide annually. Since many valuable metalssuch as Ag, Cu, Pb, Zn, Au and Ni are usually incorporated into sulphidic minerals, a largeproportion of the tailings generated contain high amounts of sulphates and heavy metals. Some ofthese tailings are used as paste backfill material at mining sites, but large amounts are still beingdeposited into the tailings dams under water coverage. Sulphidic minerals are stable undergroundbut after mining of the ore and several processing steps these minerals can be oxidized when theycome into contact with water and air. This oxidation generates acid and thus reduces the pH of thesurrounding environment. Furthermore, the heavy metals present in the mine tailings can beleached into the environment. This phenomenon, called Acid Mine Drainage (AMD), is one of themost critical environmental issues related to the management of sulphidic-rich tailings. SinceAMD generation can still occur hundreds of years after closure of the mine, the mine tailings needstable, sustainable and economically viable management methods in order to prevent AMDproduction in the long term.

The aim of this PhD thesis was to study various solidification/stabilization (S/S) methods forthe immobilization of sulphidic mine tailings. The main focus was to develop a suitable chemicalenvironment for achieving effective heavy metal (mainly arsenic) and sulphate immobilizationwhile simultaneously ensuring good mechanical properties. Three treatment methods were tested:alkali activation, stabilization using hydrated lime (Ca(OH)2) and blast furnace slag (GBFS), andcalcium sulphoaluminate-belite (CSAB) cement stabilization.

The mine tailings used in this study contained large amounts of sulphates and heavy metalssuch as Cr, Cu, Ni, Mn, Zn, V and As. The leaching of arsenic and sulphates from powderedtailings exceeded the legal limits for regular and inert waste. All treatment methods were found togenerate a hardened matrix that was suitable for use as a backfilling or construction material, butthe calcium-based binding system was the most suitable for effective immobilization of all theheavy metals (including arsenic) and the sulphates. Precipitation in the form of calcium sulphates/calcium arsenate and the formation of ettringite are the main stabilization methods employed incalcium-based stabilization/solidification (S/S) systems. Some evidence of physical encapsulationoccurring simultaneously with chemical stabilization was noted. These results can be exploitedfurther to develop more sustainable mine tailing management systems for use in the future. Thetailings could be stored in a dry landfill area instead of in tailing dams, and in this way a long-termdecrease in AMD generation could be achieved, together with a high potential for recycling.

Keywords: mine tailing management, alkali-activation, CSAB cement, ettringiteformation, heavy metal immobilization, sulfate stabilization, sulfidic mine tailings

Kiventerä, Jenni, Sulfidisten rikastushiekkojen stabilointi erilaisin menetelmin.Raskasmetallien ja sulfaattien sitoutuminenOulun yliopiston tutkijakoulu; Oulun yliopisto, Teknillinen tiedekuntaActa Univ. Oul. C 721, 2019Oulun yliopisto, PL 8000, 90014 Oulun yliopisto

Tiivistelmä

Monet arvometallit kuten kulta, kupari ja nikkeli ovat sitoutuneena sulfidipitoisiin mineraalei-hin. Louhittaessa ja rikastettaessa näitä sulfidimineraaleja syntyy miljoonia tonneja sulfidipitoi-sia rikastushiekkoja vuosittain. Rikastushiekat voivat sisältää myös runsaasti erilaisia raskasme-talleja. Osa rikastushiekoista hyödynnetään kaivostäytössä, mutta suurin osa rikastushiekoistaläjitetään edelleen ympäristöön rikastushiekka-altaisiin veden alle. Kun sulfidipitoinen malmikaivetaan ja käsitellään, sulfidiset mineraalit hapettuvat ollessaan kosketuksissa veden ja hapenkanssa. Hapettuessaan ne muodostavat rikkihappoa, laskien ympäristön pH:ta jolloin useimmatraskasmetallit liukenevat ympäristöön. Muodostuvia happamia kaivosvesiä voi syntyä vielä pit-kään kaivoksen sulkemisen jälkeen ja ovat näin ollen yksi suurimmista kaivosteollisuuteen liitty-vistä ympäristöongelmista. Lisäksi suuret rikastushiekka-altaat voivat aiheuttaa vaaraa myösihmisille, mikäli altaan rakenteet pettävät. Rikastushiekkojen kestäviä ja ympäristöystävällisiävarastointimenetelmiä täytyy kehittää, jotta näitä ongelmia voidaan tulevaisuudessa ehkäistä.

Tässä työssä tutkittiin menetelmiä, joilla kultakaivoksella syntyvät sulfidipitoiset vaarallisek-si jätteeksi luokitellut rikastushiekat saataisiin stabiloitua tehokkaasti. Työssä keskityttiin kol-meen erilaiseen menetelmään: alkali-aktivointiin, stabilointiin kalsiumhydroksidin ja masuu-nikuonan avulla ja stabilointiin CSAB sementin avulla. Valmistettujen materiaalien mekaanisiaja kemiallisia ominaisuuksia arvioitiin. Tavoitteena oli ymmärtää, miten eri menetelmät soveltu-vat raskasmetallien (erityisesti arseenin) ja sulfaattien sitoutumiseen ja mikä on eri komponentti-en rooli reaktioissa.

Alkali-aktivoimalla rikastushiekkaa sopivan sidosaineen kanssa saavutettiin hyvät mekaani-set ominaisuudet ja useimmat haitta-aineet sitoutuivat materiaaliin. Ongelmia aiheuttivat edel-leen sulfaatit ja arseeni. Kalsiumpohjaiset menetelmät sitoivat raskasmetallit (myös arseenin) jasulfaatit tehokkaimmin. Sulfaatit ja arseeni saostuivat muodostaen niukkaliukoisia komponentte-ja kalsiumin kanssa. Samanaikaisesti rakenteeseen muodostui ettringiittiä, jolla on tutkitustihyvä kyky sitoa erilaisia raskasmetalleja rakenteeseensa. Raskasmetallit myös kapseloituivatrakenteen sisään.

Työn tuloksia voidaan hyödyntää, kehitettäessä rikastushiekkojen turvallista varastointia.Kun materiaalille saavutetaan riittävän hyvä lujuus ja kemiallinen stabiilius, rikastushiekat voi-taisiin läjittää tulevaisuudessa kuivalle maalle altaan sijaan. Näin vältyttäisiin rikastushiekka-altaiden rakentamiselta ja voitaisiin vähentää happamien kaivosvesien muodostumista pitkälläajanjaksolla. Saavutettujen tulosten perusteella rikastushiekkoja voidaan mahdollisesti tulevai-suudessa hyödyntää myös erilaisissa betonin tapaisissa rakennusmateriaaleissa.

Asiasanat: alkali-aktivointi, CSAB sementti, ettringiitti, raskasmetallien stabilointi,sulfaatin stabilointi, sulfidiset rikastushiekat

Nothing worth having comes easy Theodore Roosevelt

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9

Acknowledgements

The work for this thesis was carried out in the Fibre and Particle Engineering Unit

at the University of Oulu during the years 2014-2019. Funding was received from

TEKES, as the research was part of the international ERA-MIN project GEOSULF,

carried out in collaboration with AGH Technical University of Krakow, the

University of Aveiro and companies (Agnico Eagle, Outotec, First Quantum

Minerals). I would also like to thank the Renlund Foundation, Tekniikan

edistämissäätiö (TES), the Jenny and Antti Wihuri Foundation and the Tauno

Tönning Foundation for financial support.

I would like to express my deepest gratitude to my supervisor, Prof. Mirja

Illikainen, for the opportunity to perform this interesting research in her group. I

highly appreciate all her support and encouragement throughout this work and her

guidance related to my future career. Secondly, I am grateful to Prof. Cristina

Leonelli and Isabella Lancellotti, who made it possible for me to spend four months

at the University of Modena under their supervision. I would like to thank both of

them for the discussions related to my research topic that we had during my visit. I

would also like to thank the official reviewers of my thesis Valérie Cappuyns and

Konstantinos Komnitsas and my follow-up group, Prof. Ulla Lassi and Minna

Patanen, for their valuable comments.

I would like to acknowledge the support given to me by my colleagues at the

Fibre and Particle Engineering Research Unit, and the laboratory personnel.

Especially, I would like to thank Juho for all his support and guidance since I started

my work. I wish to thank all my co-authors in the research project for their

contribution to the original articles. I am grateful for Päivö and Katja for helping

me to carry my research further. I would like to thank Jonna for all the peer support

she has given me during this project and Katri for all lunch discussions we have

had.

All my friends are warmly thanked for their support in balancing my life

between work and free time, especially thank you Jenni for listening me whenever

I have it needed throughout this project. Special thanks go to my friend and brother-

in-law Sami, who has been encouraging me to trust myself. I am grateful for the

many inspirational discussions we have had related to my work. Thanks are due to

all the members of my family who has given the support and help to balance the

work and family life. Mother and father, thank you for all your support, you have

showed me how to be a survivor and given me help whenever I have it needed. I

am grateful to my husband, Toni, since without his support this project would never

10

have been possible. He trusted me and pushed me to reach my goals, for which I

am immensely grateful. Finally, I would like to thank the three stars in my life, my

daughters Julia, Josefina and Jadessa, who together make my life very special.

Oulu, 2019 Jenni Kiventerä

11

Abbreviations

AFm monosulphate

𝐴𝐻 Aluminum hydroxide

AMD Acid mine drainage

Ca(OH)2 Calcium hydroxide, hydrated lime

CaSO4, Calcium sulfate, Gypsum

CSAB Calcium sulphoaluminate-belite cement

C4A3�̅� Ye’elimite

C4AF Ferrite

C2S Belite

C12A7 Mayenite

C�̅� Anhydrite

C2A • 3C�̅� • 32H Ettringite

C3A • C�̅� • 12H Monosulphate

C-A-S-H Calcium-aluminum-silicate-hydrate

C-S-H Calcium-silicate-hydrate

CuFeS2 Chalcopyrite

FeAsS Arsenopyrite

FeS2 Pyrite

Fe1-xS Pyrrhotite

GBFS Granulated blast furnace slag

l/s liquid/solid

KOH Kaliumhydroxide

MK Metakaolin

MT Mine tailing

NaOH Sodiumhydroxide

Na-Sil Sodiumsilicate

NiAsS Gersdorffite

OPC Ordinary Portland cement

PbS Galena

S/S Stabilization/solidification

ZnS Sphalerite

12

13

Original publications

This thesis is based on the following publications, which are referred throughout

the text by their Roman numerals:

I Kiventerä, J., Golek, L., Yliniemi, J., Ferreira. V., Deja, J., & Illikainen, M. (2016). Utilization of sulphidic tailings from gold mine as a raw material in geopolymerization. International Journal of Mineral Processing 149, 104–110.

II Kiventerä, J., Lancellotti, I., Catauro, M., Dal Poggetto, F., Leonelli, C., & Illikainen, M. (2018) Alkali activation as new option for gold mine tailings inertization. Journal of Cleaner Production 187, 76-84.

III Kiventerä, J., Sreenivasan, H., Cheeseman, C., Kinnunen, P., & Illikainen, M. (2018). Immobilization of sulfates and heavy metals in gold mine tailings by sodium silicate and hydrated lime. Journal of Environmental Chemical Engineering 6, 6530–6536.

IV Kiventerä, J., Piekkari, K., Isteri, V., Ohenoja K., Tanskanen P., & Illikainen M. (2019). Solidification/stabilization of gold mine tailings using calcium sulfoaluminate-belite cement. Journal of Cleaner Production 239, 118008 .

The author’s responsibilities have included the experimental design, analysis of the

data, reporting of the results and writing of the first draft of all four publications.

The co-authors participated in designing the experiments, helping with the

analytical methods and analysing the results. They also participated in writing the

papers.

14

15

Contents

Abstract

Tiivistelmä

Acknowledgements 7 

Abbreviations 11 

Original publications 13 

Contents 15 

1  Introduction and aims 17 

1.1  Background ............................................................................................. 17 

1.2  Approach and aims of the thesis ............................................................. 19 

1.3  Outline of the thesis ................................................................................ 20 

2  State of the art 21 

2.1  Key figures depicting the mining industry .............................................. 21 

2.2  Mine tailings ........................................................................................... 21 

2.2.1  Sulphidic mine tailings ................................................................. 23 

2.2.2  Treatment technologies for AMD ................................................. 24 

2.2.3  Paste backfilling of mining waste ................................................. 25 

2.3  Alkali-activated materials ....................................................................... 26 

2.3.1  Synthesis of alkali-activated materials ......................................... 27 

2.3.2  Heavy metal stabilization by alkali activation .............................. 28 

2.3.3  Alkali activation of mine tailings ................................................. 29 

2.4  Cement based stabilization ...................................................................... 31 

2.4.1  Solidification/stabilization using CSAB cement .......................... 32 

2.4.2  Heavy metal immobilization by ettringite formation ................... 33 

2.5  Environmental analysis of stabilized materials ....................................... 35 

3  Materials and Methods 37 

3.1  Materials ................................................................................................. 37 

3.1.1  Mine tailings ................................................................................. 37 

3.1.2  Co-binders .................................................................................... 40 

3.1.3  Alkaline solutions ......................................................................... 40 

3.1.4  CSAB cement ............................................................................... 41 

3.2  Methodology ........................................................................................... 41 

3.2.1  Alkali-activated materials and tailings stabilized with

Ca(OH)2 solution .......................................................................... 42 

3.2.2  Samples stabilized using CSAB cement ....................................... 44 

3.2.3  Physical characterization of the specimens .................................. 44 

16

3.2.4  Leaching test ................................................................................. 45 

3.2.5  Phase identification....................................................................... 45 

3.2.6  Microstructural analysis ............................................................... 45 

4  Results and discussion 47 

4.1  Characterization of the mine tailings ...................................................... 47 

4.1.1  Mineralogical analyses ................................................................. 48 

4.2  Characterization of the physical properties of the stabilized mine

tailing specimens ..................................................................................... 49 

4.2.1  Comparison of the mechanical strengths of S/S specimens.......... 49 

4.3  Environmental stability of the S/S specimens ......................................... 52 

4.3.1  Immobilization of heavy metals with various S/S methods .......... 52 

4.3.2  Immobilization of sulfates by different S/S methods ................... 57 

4.4  Microstructure analysis of the specimens ............................................... 59 

5  Conclusions 67 

List of references 69 

Original publications 77 

17

1 Introduction and aims

1.1 Background

Millions of tons of mine tailings are generated in the mining industry worldwide

each year. Mine tailings are produced after the extraction and subsequent

processing of the minerals. Most of the generated mine tailings have currently no

economic value, and they end up in landfill areas at the mining site being dumped

in tailings dams or in impounding lakes. The landfill material consumes a huge area

of land and causes notable short and long-term environmental concerns. The

increasing generation of mining waste and related environmental issues have

increased the researcher’s interest in investigating the utilization and recycling

potential of various mining wastes.

Many valuable metals are extracted from sulphidic minerals, generating large

quantities of sulphidic-rich tailings. Sulphidic minerals can be oxidized in the

presence of air and moisture, thereby reducing the pH of the surrounding waters.

Mine tailings themselves typically consist of high amounts of heavy metals (such

as arsenic in gold mine tailings), which can be dissolved into the environment

because of low pH. The phenomenon is known as Acid Mine Drainage (AMD),

which can cause both short and long-term environmental problems, while sulphidic

oxidation can occur within mine tailings for several decades after closure of the

mine. Usually, the mine tailings consist of very fine particles making the safe

management even more challenging and requiring careful maintenance and

constructions as well as a large surface landfill area [1].

Sulphidic tailings are typically neutralized by adding neutralizing agents

usually limestone or hydrated lime to prevent AMD production. The lime increases

the pH to the required level for precipitating sulphates and some of the heavy metals

have lower solubility. The neutralized mine tailings are then deposited under water

in tailings dams. The method is simple but needs continuous monitoring even after

closure of the mining site, increasing management costs. Besides, the large-scale

use of pure limestone is not economically sustainable, as limestone is classified as

a resource and not a residue [2]. Some of the tailings are mixed with proper binder

material (such as cement or fly ash) and water and the resulting paste is used in the

mine as a backfilling material. Even though the paste backfilling is commonly used

at mining sites, all generated mine tailings cannot be utilized for this purpose.

18

Further investigations are needed to find alternative economically sustainable

methods for stabilizing and recycling mine tailings.

Stabilization/solidification (S/S) prior to landfilling has been used to cope with

many kinds of industrial waste. The main target is to mix the waste with water and

a suitable binder material in order to form a solidified material which can

immobilize harmful components inside its matrix [3]. When the chemical

composition of the waste is complex, the immobilization mechanism can be highly

challenging. The binding system should be modified to suit each waste stream,

because heavy metals can affect the properties of the resulting material. In addition,

a high sulphate content and acid generation can have a harmful effect on the

mechanical strength and chemical properties of the stabilized matrix.

Stabilization/solidification of mine tailings, employing various treatment

method has gained much interest among researchers and could provide

economically viable and sustainable long-term method for mine tailing

management in the future [4]. Three binding systems were studied in this research

for stabilization of sulphidic gold mine tailings: alkali activation, stabilization with

Ca(OH)2 and blast furnace slag (GBFS), and CSAB cement stabilization. The

results could be used for understanding the chemical environment required for the

sulfidic rich tailings/wastes stabilization including high heavy metal

immobilization. In addition, the results could be used for developing dry landfill

technologies for use with mine tailings instead of disposal in tailings dams with a

water coverage.

Alkali activation as an S/S method has shown good immobilization capacity

with respect to several heavy metals. In alkali activation an aluminosilicate raw

material is activated by a highly alkaline solution, so that aluminium and silicon

units will dissolve to form new hardening phases with a relatively high mechanical

strength [5]. Besides mechanical strength, the material may also have good

durability, chemical and acid resistance, high permeability and low CO2 emissions

compared with OPC production [6]. This process of synthesis also creates matrices

for the encapsulation or chemical interaction of hazardous components. The alkali

activation of mine tailings has shown a potential for producing materials having a

matrix with good mechanical properties, including the immobilization of certain

heavy metals. Even though the alkali activation of various mine waste materials

has been widely investigated in recent years, sulphidic gold mine tailings as a raw

material have not been studied very much. In addition, gold mine tailings are a

challenging but interesting raw material to provide a better understanding of the

immobilization capacity of alkali activation, and it was studied in Papers I and II.

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Calcium-based binder materials have shown a high capability for the effective

immobilization of arsenic and sulphates. Sulphates can be precipitated as gypsum

and arsenic as calcium arsenate/arsenite, having low solubility properties. In

addition, calcium can form ettringite together with aluminium and sulphates in a

suitable chemical environment, and ettringite has shown a high potential for the

immobilization of heavy metals, including oxyanions such as arsenic [7], [8].

Ettringite formation is therefore an attractive method for stabilization of sulphidic

tailings. Calcium-based binding systems were studied in Papers III and IV. In Paper

III the alkaline reagent Ca(OH)2 and blast furnace slag (GBFS) were mixed with

sulphidic tailings, Ca(OH)2 being used to increase the pH of the system and blast

furnace slag as a calcium-rich precursor for yielding calcium and aluminium for

the reaction. Sulphidic mine tailings were used as a source of sulphate. The target

was to precipitate the sulphates and arsenate as calcium-containing components and

also to form ettringite in the system. Leaching of the various elements was studied

by means of an environmental leaching test.

In Paper IV the ettringite immobilization system was studied in more detail by

using calcium sulphoaluminate-belite (CSAB) cement as a binder material.

Ordinary Portland cement (OPC) has usually been used as a binder in cement-based

S/S methods, but the high sulphidic content can have a deleterious effect on the

resulting matrix. The hydration of CSAB cement can produce concrete with high

early strength, quick setting and excellent chemical resistance, having good

dimensional stability, low alkalinity, low permeability and comparable strength to

OPC but with lower CO2 emissions [9]. Among its good mechanical properties, the

hydration of CSAB cement can yield large amounts of ettringite into the matrix if

a suitable amount of gypsum is present. Gypsum can be added by using pure

reagents or other residues. The possibility for using sulphidic mine tailings as a

sulfate source in ettringite formation during CSAB hydration process is

investigated in Paper IV. The other hypothesis was to achieve high immobilization

of heavy metals, especially arsenic, together with a final material having high

mechanical strength.

1.2 Approach and aims of the thesis

The main aim of the thesis was to develop a proper binding system for

stabilizing/solidifying sulphidic gold mine tailings by testing three S/S methods:

alkali-activation, stabilization using Ca(OH)2 and GBFS, and stabilization using

CSAB cement. The research was mainly focused on the immobilization of harmful

20

substances, especially arsenic and sulphates, in a hardened matrix. The general

target for this research was to generate new knowledge of sustainable sulphidic

mine tailing stabilization prior to landfilling. Also the possibility to utilize the

solidified materials further, e.g. as construction materials at mining sites was

explored.

The main objectives of the research were the following:

1. To develop a suitable chemical environment for the effective immobilization

of sulphates and heavy metals from sulphidic mine tailings.

2. To gain an understanding of the immobilization mechanisms of different heavy

metals and sulphates by different treatments.

3. To research is it possible to obtain good mechanical properties simultaneously

with high immobilization efficiency, to be able to utilize the materials produced

in paste backfilling or as construction material in mining sites.

The alkali activation of sulphidic mine tailings was studied in Papers I and II, the

emphasis in Paper I being on the reactivity of the mine tailings and the role of the

co-binder material from a mechanical point of view, while the immobilization

efficiency of the various harmful components was studied in Paper II. Calcium-

based binding systems were studied in Papers III and IV, including the

immobilization of heavy metals and sulphates by means of Ca(OH)2 and blast

furnace slag in Paper III and stabilization with CSAB cement in Paper IV.

1.3 Outline of the thesis

This thesis consists of 5 chapters. Chapter 1 provides an introduction to the topic,

Chapter 2 describes the state of the art, key figures describing the mining industry

and the nature of mining waste and explanations of what the sulphidic mine tailings

are, how they are disposed of and what are currently the main challenges associated

with them. Various stabilization methods are described (notably alkali activation

and calcium-based binding systems) and the analyses used to evaluate the

environmental performance of the materials are discussed. The materials and

methods used in this work are introduced in Chapter 3 and the results and

discussion are presented in Chapter 4. The conclusions to be drawn are set out in

Chapter 5.

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2 State of the art

2.1 Key figures depicting the mining industry

Mining is one of the oldest human industrial endeavours, and the earliest extractive

activities have been dated to the Stone Age. With growing populations and greater

demands for raw materials, mining activities have adopted a central role in

technological, economic and societal development, and the mining industry has

grown rapidly during recent decades [10].

The extractive industries include the production of fossil fuels, metalliferous

ores as well as industrial and construction minerals. The EU, for example, had more

than 32 000 registered extraction sites in 2012, representing total investments of

over 220 billion euros in industrial and construction minerals, fossil fuels and

metalliferous ores. The worldwide estimated ‘run-of-mine capacity’ in 2013 was

more than 60 000 Mt [10]. Mining activities in Finland have also grown remarkably

in the last few years. The first known mining activities in Finland took place in

1540, when iron started to be mined, and by 2017 the total number of registered

quarries was 44, including 9 mines producing metallic ores, having a total output

of more than 120 Mt/year [11].

In the case of metallic ores, the valuable metal is recovered by different

processing steps and what is left over is disposed of in a landfill area as mining

waste. Thus the mining industry produces billions of tons of mining waste of

different kinds, including solid, liquid and gaseous waste [12]. Mining activities in

the EU are known to have produced 500-700 Mt of such wastes in 2012 [10] and

the worldwide annual solid mining waste production is more than 20 000 Mt [12].

2.2 Mine tailings

The mining industry generates billions of tons of solid waste, of which mine tailings,

as a fine-grain residue, is the largest single waste stream. Mine tailings are produced

after the valuable minerals have been separated from the mined ore by various

processing steps. A simplified representation of a mining process is shown in Fig.

1. First, the ore is crushed under dry conditions and ground in water. It is then

separated from the gangue mineral by gravimetric, magnetic, electrical or surface-

based methods [12]. Once the valuable metal has been separated out, the original

excavated material ends up as mine tailings and is usually dumped in tailings dams

22

under water coverage. The total volume of produced mine tailings in different

mining sites varies in the range 97 – 99% (even 99.9% in connection with gold

mining) of mined ore implying a total annual production of mine tailings over 10

000 Mt globally [12]–[14].

Fig. 1. A simplified flow diagram of mineral processing and mine tailing production

(Modified from [12]).

The properties of mine tailings depend on the mineralogy of the ore and the process

used for treating it. Usually the mine tailings consist of components having particle

sizes between those of sand and clay (2 µm – 2 mm). It is the particle size of the

tailings that affects their behaviour in response to wind, water erosion and settling

when deposited in a landfill area. In addition, the chemical composition of the

tailings can vary greatly, while ore mined for the recovery of valuable metals can

contain large quantities of heavy metals such as Fe, Cu, Pb, Zn, Sn, Ni, Co, Mo,

Hg, Cd and Ti together with metalloids like As, Sb, Se and V [12]. The particle size

distribution of the resultant tailings, together with their chemical and mineralogical

properties, needs to be analysed carefully when planning safe procedures for

depositing them in landfills at mining sites.

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2.2.1 Sulphidic mine tailings

Valuable metals such as gold, copper and nickel usually appear in sulphidic-rich

minerals, so that significant quantities of sulphidic-rich tailings are generated

during metal mining. The typical sulphidic minerals found in nature are pyrite (FeS),

pyrrhotite (Fe1-xS), arsenopyrite (FeAsS), chalcopyrite (CuFeS2), galena (PbS),

gersdorffite (NiAsS) and sphalerite (ZnS) [4]. Pyrite (FeS2), the most common

naturally occurring sulphidic mineral, is stable underground under acidic and

alkaline conditions but after excavation and in the presence of oxygen, water and

microorganisms is highly soluble, releasing ferrous ions, sulphates and H+ into the

environment [15]. The oxidation of pyrite (in an aqueous environment) is shown in

equations (1) and (2) [2], [16].

𝐹𝑒𝑆 3.5𝑂 𝑎𝑞 𝐻 𝑂 → 𝐹𝑒 2𝑆𝑂 2𝐻 (1) 𝐹𝑒𝑆 14𝐹𝑒 8𝐻 𝑂 → 15𝐹𝑒 𝑆𝑂 16𝐻 (2)

The oxidation of pyrite generates sulphuric acid, thus reducing the pH of the water

with which is in contact. If other sulphidic minerals are present, they can be

oxidized and also the heavy metals presence in tailings can be leached into the

solution under acidic conditions [17]. This phenomenon is called acid mine

drainage (AMD) and is one of the most critical environmental issues related to

sulphidic –rich mine tailings [4], [18]. Also, the pH can change the composition of

the tailings with time and certain naturally occurring bacteria can accelerate AMD

production by breaking down the sulphidic-rich minerals further [1]. Acid mine

drainage can continue to contamination of surface water and groundwater in mining

areas hundreds of years after the closure of mining, and is therefore a challenging

prospect to handle in the long term [4].

Many gold-bearing ores also contain large quantities of arsenic minerals such

as arsenopyrite. Arsenic can be released during the sulphide oxidation process (by

the mechanism shown in equation (3) and the leachable arsenic need to be

effectively removed from the effluent [19]. Arsenic can be present as As (III) or As

(V) and the solubility of the compound containing arsenic is highly dependent on

the pH of the environment [20]. Since arsenic can have several oxidation states, its

chemistry is challenging from the management point of view [21].

24

𝐹𝑒𝐴𝑠𝑆 1.5𝐻 𝑂 2.75𝑂 → 𝐹𝑒 𝑆𝑂 𝐻 𝐴𝑠𝑂 (3)

Mine tailings are usually deposited in tailings dams in the form of a slurry having

20-40 w-% solid particles [12]. The geology and topography of the landfill area,

the available construction material, the configuration of the dam, the amount and

properties of the tailings to be disposed of and the need for protection from erosion

require careful analysis [19]. If the deposition of tailings is not designed properly,

it can cause disasters and have expensive consequences, as in the case of dam

failure, for instance. Over 200 accidents related to mine tailings have been reported

globally, and 17 of them have occurred in the present century [13], [22].

Techniques for the proper management of sulphidic tailings should be carefully

designed to take into account the most cost-effective and sustainable storage

facilities in both the short and long term, and also the potential for reusing mining

waste [14], [23]. The physical and chemical properties of the tailings pose

challenges for the development of safe management methods. The slimes and fines

in tailings can detract from the stability of impoundment sites; fine particles can

pollute the air and soil and cause a threat to human health long after the cessation

of mining. In addition, high sulphidic content, as discussed previously, including

the use of cyanide in the gold separation process, for example, make the

development of safe management technologies for sulphidic gold mine tailings

more challenging [14], [19]. The cost of management technologies can be high,

including design, construction, maintenance and closure as well as rehabilitation of

the disposal facilities [12].

2.2.2 Treatment technologies for AMD

Various prevention/remediation techniques have been developed to minimize the

harmful effects of the AMD generation in the case of sulphidic mine tailings. These

can be divided roughly into active and passive methods. The addition of limestone

is a passive, neutralizing treatment method that has been in use for decades because

of its simplicity [4]. By adding limestone, the sulphates can be precipitated as

gypsum and some heavy metals as hydroxides, whereupon the neutralized mine

tailings can be deposited in tailings dams. The stability of the precipitated and co-

precipitated metals is highly dependent on the pH of the surroundings [20]. Arsenic,

if present as As(V), can be precipitated by controlling the pH. For some metals such

as Mn, Ni and Zn, however, the addition of limestone is not suitable as a

25

stabilization method because the pH achieved is too low for precipitation. Instead,

by using hydrated lime, Ca(OH)2 (as an active-based method), the Mn2+ ions have

been successfully precipitated, as the pH has risen higher [4]. On the other hand,

there are some metals such as As(III) which cannot be stabilized by pH control, so

that additional treatment steps are required. Even with a high neutralization

capacity, achieved by adding a neutralizing agent, the oxidation of sulphidic

minerals can still occur within a longer time span and the precipitated heavy metals

can start to solubilize and leach into the surrounding environment again. The

method using alkaline additives to increase the pH also needs continuous control

after the closure of the mine, which will again increase the overall management

costs. In addition, an effective method for treating the heavy metal sludge generated

in this way still needs to be found [4], [19].

Other alternative method reported to treat mine tailings is to prevent the AMD

production by excluding any driven force such as oxygen, water or microorganisms

from the deposited tailings. The methods currently used in mine tailing

management include dry or water cover, to prevent the access of oxygen into the

impounding basin, sub-aqueous disposal or bactericide methods [4].

Stabilization/solidification (S/S) is used to treat hazardous waste before it is

disposed of or recycled [24], this being a suitable alternative for treating mine

tailings. In the S/S method the industrial waste is incorporated into a solid matrix

which provides a suitable chemistry for fixation of the harmful components in a

less toxic form. The harmful components can also be physically encapsulated inside

the resulting matrix. The different S/S methods for mine tailing management to be

studied here will be introduced later. The target is to find effective S/S method that

the sulphidic mine tailings could be transformed into a solid matrix for deposition,

either in a dry landfill area or underground. The material could also be recycled as

a construction material if good consolidation can be achieved [20].

2.2.3 Paste backfilling of mining waste

Some mine tailings have been used as a paste backfilling material at mining sites over

the last 100 years. Backfilling is essential for continuing activities in a mining area and

by using some of the tailings generated by the mine for this purpose the consumption

of natural raw materials for backfilling can be reduced [25]. At the same time the total

amount of mine tailings to be deposited in landfills will be lower and the potential

AMD production in tailings dams can be reduced [12].

26

In the paste backfill method the mine tailings (around 80 w-%) are mixed with a

binder material (cement or an alternative binder such as fly ash or slag, 3-7 w-%)

and water [26]. The final properties depend on the physical, chemical and

mineralogical properties of the mine tailings, the co-binder material used and the

amount of water and its properties. The target is to achieve the required mechanical

and rheological properties of the paste so that it can be pumped underground easily,

without focusing mainly on the immobilization of harmful components. Ordinary

Portland Cement (OPC) has been the most commonly used binder material for this

purpose. When the mine tailings have a high sulphidic content, the co-binder needs

careful evaluation. Sulphates can be oxidized, producing acid, which can further

react with OPC hydration products such as Ca(OH)2 and cause damage to the

stability of the backfill material [27], [28]. To improve the acid resistance of paste

backfill material, the other cementitious materials such as fly ash or blast furnace

slag have been proposed for use as a co-binder. These ‘pozzolanic products’ have

shown higher acid resistance when used as binders in sulphidic-rich tailing paste

backfilling. This is due to their capability for consuming unstable hydration

products, yielding more phases in the mixture that remain stable against acids. In

addition, by using these industrial residues as a binder material the overall costs of

paste backfill management can be greatly decreased [26]. Based on this chemistry

the pozzolanic products could be suitable binder materials used in different S/S

methods and were used as co-binders in this research.

2.3 Alkali-activated materials

Alkali-activation, also known as geopolymerization, is one effective

stabilization/solidification method for the management of waste materials or

industrial residues such as fly ash or metallurgical slags [29]. Alkali-activated

materials can possess similar performance to that of materials stabilized with OPC.

Besides the good mechanical and chemical properties of alkali activated materials,

the process yields lower CO2 emissions than OPC production, as it does not use

high temperatures [6], [30].

The first alkali-activated materials were prepared and the method patented in

the early 1900s by German cement scientists [31], and a number of alkali-activated

blast furnace slag materials were tested between 1940 and 1970 [32]. The term

‘geopolymer’ was first introduced in 1970 by Davidovits, who investigated the

geopolymerization of metakaolin [33]. Since the 1990s, the alkali-activated

27

materials have attracted a substantial interest among researchers. There were more

than 100 active research centres worldwide operating in this field in 2013 [32].

2.3.1 Synthesis of alkali-activated materials

Alkali-activated materials can be produced from raw materials that are rich in

silicon and aluminium and are reactive under alkaline conditions. A highly alkaline

solution such as KOH or NaOH is introduced into the aluminium and silicon-rich

raw material, dissolving the aluminium and silicon into the solution. In the

following hours the neighbouring ions are linked together by sharing their oxygen

atoms forming new three-dimensional amorphous Si-O-Si or Si-O-Al bonds and

loss of water [34]. A negative charge occurs in the structure in the presence of a IV-

fold coordination of Al3+ions which needs to be balanced by sufficient cations (such

as Na, K or Ca) coming from the raw materials or from the alkaline solution [5]. A

simplified scheme for the synthesis of alkali activation is shown in Fig. 2.

Fig. 2. Simplified scheme for the synthesis of alkali activation (Reprinted by permission

from [35] © 2017 Acta Universitatis Ouluensis).

The reaction takes place at or slightly above room temperature, and the final

material can have a high compressive strength, low density, micro- and

nanoporosity, low shrinkage, high resistance to chemicals, fire and sulphates and

good durability [36], [37]. The nature of the raw material, the water/binder ratio

and the process conditions such as mixing and curing temperatures affect the

properties of the final matrix [38]. Also the other components such as Ca, Fe, Mg

and heavy metals can be involved in the reaction, affecting the final properties of

the resulting material [32], [39]–[41].

Metakaolin, aluminium and silicon-rich highly amorphous raw material has

been used for generating alkali-activated materials since 1970 [33]. Metakaolin is

28

produced by the calcination of kaolinite clays at temperatures between 500 and

800 °C to reduce the amount of water bound in the material [42]. The final matrix

of alkali-activated metakaolin is highly amorphous and can acquire advanced

mechanical properties with the proper recipe design. Besides good strength

properties the matrix has proved to be resistant to chemicals [43]. Metakaolin is

often blended with other precursors, since the aluminium content of the industrial

waste is usually low and for this reason metakaolin was used as a binder material

in alkali-activation of gold mine tailings in Paper II.

Blast furnace slag is also a highly amorphous material that is reactive under

alkaline conditions. The main reaction product in its activation is an aluminium-

substituted C-A-S-H gel together with a C-S-H structure [44], [45]. Other

additional phases such as hydrogarnet, strätlingite, ettringite or C-(N)-A-S-H gel)

are produced, depending on the other components available and the alkaline

solution used [32], [46], [47]. Blast furnace slag was used as a binder material in

Paper I, II and III. Based on these findings related to alkali-activated materials the

alkali activation of mine tailings has also gained much interest among researchers.

The main results of the recent research studies have been described in section 2.3.3.

2.3.2 Heavy metal stabilization by alkali activation

Alkali activation can be used to immobilize solid wastes [5], [48]. Stabilization can

be achieved within an alkali-activated matrix by either chemical immobilization or

physical encapsulation, and the methods can occur simultaneously [5], [49]. In

chemical immobilization, heavy metal cations coming from the waste material, can

be bonded to the alkali-activated structure to balance the negative charge of the

matrix. On the other hand, the pH of alkali activation is relatively high and some

heavy metals can be precipitated resulting in low solubility products. Alongside the

chemical bonding method and precipitation, heavy metals can be immobilized by

physical encapsulation within the geopolymer matrix, whereupon the particles can

be entrapped in the geopolymer structure. This physical encapsulation occurs on a

micron scale, while chemical stabilization operates on a nano-scale [5], [49], [50].

Cationic species such as Co, Ni, Mn, Cu, Zn, Cd, Cr and Pb have been

stabilized effectively by alkali activation [40], [51]–[53] and some anionic such as

Mo, while higher leachability has been recorded for oxyanions such as As and Sb

and Mo,after alkaline treatment compared to leaching rates recorded in starting

waste material [52]. Oxyanions are present in a highly soluble form under high pH

conditions and cannot be precipitated, nor can they balance the negative charge of

29

an alkali-activated structure. Some researchers have shown that the presence of

calcium can reduce the leachability of oxyanions, since it is possible that they can

be precipitated as calcium-bearing components having lower solubility [52].

Bankowski, Zou and Hodges (2004) [54] achieved good stabilization of As, Ba, Se

and Sr, but only for limited amounts of waste. When the amount of waste increased,

the efficiency of immobilization decreased.

Parameters such as the concentration and nature of the alkaline solution, the co-

binder material and the curing time can affect the efficiency of immobilization of

various elements [55]. The immobilization efficiency of heavy metals depends on

the mineralogy, morphology, silica and alumina content, fineness and reactive

surface area. Some researchers have also suggested that the Si/Al and Na/Al ratios

play a central role in immobilization and the development of mechanical properties

during alkali activation. According to Davidovits (1999) [56], the Si/Al ratio should

be <3 for a material to be suitable for waste encapsulation. Rowles & O’Connor

(2003) [57] obtained the highest strength of the geopolymer matrix with Si/Al and

Na/Al ratios of 1.5-2.5 and 1.0-1.29, respectively, while Aly et al. (2008) [58]

deemed the proper Si/Al ratio to be near 2 and the Na/Al ratio near 1 for nuclear

waste stabilization. Ponzoni et al. (2015) [59] showed good stabilization of heavy

metal cations with a Si/Al ratio of 3.

Even though a number of heavy metals have been successfully stabilized by

alkali activation, the characterization of such materials is challenging due to the

resulting complicated and disordered matrix [60]. When several heavy metal

species are present simultaneously the matrix will be even more difficult to

characterize. This possesses challenges for the investigation of the immobilization

mechanism within the produced alkali-activated matrix.

2.3.3 Alkali activation of mine tailings

Mine tailings are interesting raw materials for alkali activation because of their fine

particle size and in some cases the high Si and Al content. A variety of mining

wastes have been studied as materials for alkali activation since the beginning of

the present century. Davidovits (1987) stabilized mine tailings derived from base

metal, potash, coal and uranium mining by geopolymerization obtaining high

immobilization of harmful components and long-term stability of the matrix

produced [61]. Jaarsveld et al. (2000) alkali-activated mine tailings with good

mechanical and durability properties and the used mine tailing content was 60-75

w-% of the mixture [62]. Pachego-Torgal, Castro-Gomes and Jalali et al. (2007,

30

2008) used tungsten mine tailings as a raw material in alkali-activation. They

obtained high early stage strength when using thermal treatment before the alkali

activation [63], [64].

Ahmari & Zhang (2011) studied the alkali activation of copper mine tailings

by mixing them with different concentrations of a NaOH solution [65]. Improved

mechanical properties were obtained with higher concentrations of the NaOH

solution, and similar results were observed with regard to water content. By

optimizing the curing temperature and pressure, excellent mechanical properties

can be achieved in the final material. They also exhibited high immobilization of

heavy metals such as Cu, Fe and Zn under different environmental conditions, as

analysed after 105 days of curing [66]. Similarly, Zhang, Ahmari, and J. Zhang

(2011) [67] and Giannopolou and Panias (2006) [68] studied the alkali activation

of copper mine tailings by adjusting the high Si/Al ratio of the tailings in the

mixture by means of additions of fly ash. They observed high mechanical

performance of the resulting material and identified as the main parameters

affecting the mechanical properties the fly ash content of the mixture and the

concentration of the alkali solution. Effective immobilization of several heavy

metals (notably Cu, Zn, Pb, Ni, Mn and Al) was also achieved. When Jiao, Zhang

and Chen (2013) [69] studied the thermal stability of alkali-activated vanadium

mine tailings mixed with fly ash, they obtained geopolymers with high fire

resistance, while the mechanical properties were stable upon heating up to 900 ˚C.

Much interest in the alkali activation of mine tailings has been shown by

several researchers in 2018 and 2019. Jiang et al. (2019) [70] studied alkali-

activated slag as a binding material for use in gold tailing’s paste backfilling

technology, and achieved better workability and strength properties than using OPC

as a binding material. Xing et al. (2019) [71] also studied the alkali activation of

mine tailings as a method for paste backfilling, and found poor reactivity of the

studied mine tailings. They studied alkaline fusion as a pre-treatment method for

increasing the reactivity of the tailings. Manjarrez and Zhang (2019) [72] studied

the alkali-activation of copper tailings mixed with low-calcium slag and an NaOH

solution, and obtained a matrix that was suitable for use as a road construction

material. The immobilization of lead in mine tailings by using metakaolin as a co-

binder was studied by Wan et al. (2018, 2019) [73], [74], who observed a reduction

in the strength properties of the matrix when the lead content was too high.

Sulphidic gold mine tailings have not been examined widely as a raw material

for alkali activation. When Barrie et al. (2015) [75] investigated the alkali

activation of gold mine tailings by using volcanic glass and calcinated halloysitic

31

clay as co-binders, they obtained relatively good mechanical properties of the final

material and good immobilization of Pb and Zn within the matrix. On the other

hand, there was poor immobilization of oxyanions such as As and Cu and the mine

tailing content of the mixture was relatively low. Sulphidic-rich mine tailings with

poor reactivity under alkaline conditions have been studied as a fine aggregate in

geopolymerization by Paiva et al. (2019) [76], who were able to use them as

aggregate in construction materials with good chemical resistance under varying

test conditions. Sulphidic gold mine tailings are a challenging raw material for

alkali activation as they include large amounts of sulphides and a lot of heavy

metals. On the other hand, most of the generated mine tailings are sulphidic

containing tailings while they are a good material for studying the immobilization

mechanisms of various elements, including the challenging elements such as

arsenic. The potential for using sulphidic gold tailings in alkali-activation is studied

in Papers I and II from both a mechanical and chemical point of view.

2.4 Cement based stabilization

Cement based stabilization/solidification has been used for hazardous wastes with

good mechanical and immobilization results, and it can also be used for arsenic-

bearing waste materials [77]. OPC has been popular as a binder material on the

grounds of its consistent chemical composition between different sources and the

clarity of its setting and hardening process. The hydration of OPC can be divided into

three main parts: 1) dissolving, 2) precipitation of C-S-H, portlandite and other

hydration products, and 3) equilibriation of the hydration products. If there are any

heavy metals present in the hydration reaction they can accelerate the process and

reduce the pH [78]. Heavy metals can be precipitated, adsorbed or incorporated into

the hydrate lattice but the characterization of these phases is often difficult due to

amorphous or poorly crystalline compositions occurring on a nanoscale [79]. In

addition, a large quantity of sulphates can have deleterious effect on the properties of

the final matrix as discussed already in section 2.2.3. The poor strength and stability

performance of sulphidic-rich tailings stabilized with OPC has been shown to be

attributable to the generation of acid through oxidation of the sulphidic mineral, pyrite,

which was observed with longer curing times [80]. The possibility for using industrial

residues such as fly ash or slags as a binder material instead of OPC, has been discussed

in section 2.2.3.

Limestone and hydrated lime have already been used for mine tailing neutralization

in the mining industry, as mentioned in connection with the remediation of mine tailings

32

in section 2.2.2. This method is based on the precipitation of sulphates as gypsum and

other metals such as arsenic as calcium arsenate components. In addition, some other

hydration products can be formed. A simplified scheme for the hydration of lime is

shown in equations (4)–(6) [81].

𝐶𝑎 𝑂𝐻 → 𝐶𝑎 2 𝑂𝐻 (4) 𝐶𝑎 2 𝑂𝐻 𝑆𝑖𝑂 → 𝐶𝑎𝑙𝑐𝑖𝑢𝑚 𝑆𝑖𝑙𝑖𝑐𝑎𝑡𝑒 𝐻𝑦𝑑𝑟𝑎𝑡𝑒 𝐶 𝑆 𝐻 (5) 𝐶𝑎 2 𝑂𝐻 𝐴𝑙 𝑂 → 𝐶𝑎𝑙𝑐𝑖𝑢𝑚 𝐴𝑙𝑢𝑚𝑖𝑛𝑎 𝐻𝑦𝑑𝑟𝑎𝑡𝑒 𝐶 𝐴 𝐻 (6)

In addition to its precipitation as calcium-containing components, calcium can also

react with sulphates and aluminium under alkaline conditions to form a calcium

sulphoaluminate hydroxide hydrate, an ettringite phase, which is capable of

immobilizing large quantities of several heavy metal ions [81]. The immobilizing

mechanisms of ettringite phase is discussed in section 2.4.2.

Calcium-based stabilization/solidification was the second method studied for

the immobilization of sulphidic-rich gold mine tailings, as reported in Papers III

and IV. In Paper III the Ca(OH)2 was used to adjust the pH and blast furnace slag

was used as a co-binder to form a hardened matrix. The target was to obtain both

precipitation and ettringite stabilization simultaneously, while still achieving good

mechanical strength. The calcium-based binding system using CSAB cement that

was reported in Paper IV will be introduced in the next section.

2.4.1 Solidification/stabilization using CSAB cement

The high CO2 emissions associated with OPC production have attracted researchers to

look for new alternative sustainable cement materials. One alternative cementitious

binder that was developed in China over 30 years ago is calcium sulphoaluminate-belite

(CSAB) cement [9], [82], [83]. The production of CSAB involves lower CO2

emissions on account of its lower energy consumption compared with OPC

production. The CSAB cement can be produced from natural materials: limestone,

bauxite or aluminious clay and gypsum. Alternatively, it can be produced from

industrial residues: scrubber sludge [84], fly ash [85], Al-rich sludge [86], marble

dust waste [87], ceramic dust waste [87] and red mud [9]. The final properties of

CSAB clinker include high early strength, quick setting and excellent chemical

resistance, with good dimensional stability, low alkalinity, low permeability and

comparable strength properties to those of OPC [9].  In addition to its good

33

mechanical properties and durability, the possibility of stabilizing harmful

components in the hardened matrix is attractive from a waste management point of

view.

CSAB cement clinker can vary in its chemical composition, containing

different amounts of ye’elimite (C4A3�̅� ), ferrite (C4AF) and belite (C2S) phases

together with other elements such as anhydrite (C�̅�) or mayenite (C12A7) [88]. The

typical amount of ye’elimite is 30 – 70 w-%, making it the major component in

CSAB cement [89]. The ye’elimite phase reacts with a source of sulphate (gypsum,

anhydrite or basanite) to form ettringite ( 𝐶 𝐴 ∙ 3𝐶�̅� ∙ 32𝐻 ) and aluminum

hydroxide 𝐴𝐻 ) phases (equation (7)). If the gypsum is running out, a

monosulphate (𝐶 𝐴 ∙ 𝐶�̅� ∙ 12𝐻 phase start to form (equation (8)). The other phases

of CSAB cement, such as belite (C2S), can form amorphous C-S-H or AFm phases

during hydration and the ferrite phase can form C-(A,F)-H hydrates and also an

amorphous FH3 gel [90], [91].

𝐶 𝐴 2𝐶�̅�𝐻 34𝐻 → 𝐶 𝐴 ∙ 3𝐶�̅� ∙ 32𝐻 2𝐴𝐻 (7) 𝐶 𝐴 �̅� 18𝐻 → 𝐶 𝐴 ∙ 𝐶�̅� ∙ 12𝐻 2𝐴𝐻 (8)

Since ettringite should be formed during the early stage of curing in order to

achieve good mechanical strength, its late production can have a deleterious effect

on the hardened structure by expanding and cracking it [83], [92]. The amount of

additional sulphate is thus an important parameter to design properly, since too

much sulphate present can cause expansion of the material. The other parameters

that affect the final properties of the material produced are the water/binder ratio,

the particle size of the material and the ye’elimite content of the clinker [83], [92].

Since the source of sulphate for mixing with CSAB cement can be either a

natural material such as pure gypsum or an industrial residue such as

phosphogypsum [83], [92], [93], this cement is an interesting option for the

immobilization of sulphidic tailings. The possibilities for replacing the additional

sulphate (gypsum) with sulphidic mine tailings are explored in Paper IV.

2.4.2 Heavy metal immobilization by ettringite formation

Ettringite, a phase produced in calcium-based binding systems, as studied in Papers

III and IV, has a high capacity for effective heavy metal immobilization. Ettringite

can be produced by a reaction between calcium, aluminium and sulphates in a

34

suitable chemical environment, and it is also the most important hydration product

when mixing CSAB cement together with a sulphate source and water.

Ettringite was first introduced by Moore and Taylor in 1970 [94], and its

structure consists of columns {Ca6[Al(OH)6]2·24H2O} and channels

{(SO4)3·2H2O}6- [95]. Ettringite has a high capacity for immobilizing several heavy

metals, including oxyanions. Immobilization of the ettringite crystal structure is

based on the substitution of the elements, as depicted in Fig. 3. The Ca2+, Al3+ or

SO of the ettringite structure can be replaced by ions such as Fe3+ ,Cr3+, Sr3+ ,

Mg2+ , Zn2+ ,Co2+ and oxyanions (such as As and Cr) [7], [96].

Fig. 3. Replacement of parts of the ettringite structure by other cations and anions.

It has been shown that the substitution of various elements is based on the ion

charge and size [96]. When several ions are present in the system the dominant one

will be replacing the others most easily. Heavy metals can also be adsorbed into the

negatively charged ettringite structure, or precipitated under highly alkaline

conditions [81]. In addition, physical encapsulation can occur simultaneously.

Immobilization of arsenic in the ettringite structure has been detected under

alkaline conditions being a suitable method for arsenic containing waste

stabilization. The immobilization of sulphidic gold mine tailings by ettringite

formation was studied in Papers III and IV.

The stability of the ettringite phase needs careful scrutiny in order to

understand the durability of the materials produced. Some studies have revealed

that temperature, pH and carbonation can affect the formation and stability of the

ettringite phase. When there are several heavy metals incorporated into the matrix,

the stable environment required can vary a lot. Some researchers have shown the

35

stable pH range for the ettringite phase to extend from 10.5 to 13, and it has also

been seen that when substitution has occurred the pH required for good stability

may also change [97]–[100]. When designing the large scale applications, the

variability of the materials and environmental conditions need to be taken account

carefully.

2.5 Environmental analysis of stabilized materials

The regulations relating to the deposition of waste in landfills have become stricter in

recent years, and this trend will be emphasized further in the future. The release of

heavy metals into the environment from landfill waste will be more strictly controlled

by law and more careful disposal will increase the cost of treatment technologies.

Any substance or object, which is to be discarded, can be called waste, and all

waste residues have their own classifications and definitions in law at both the national

and EU level. Finland’s waste legislation is based on the EU regulations, with some

exceptions and restrictions [101]. Landfilling is the last choice if the waste has no reuse

or recycling potential. If a certain form of waste ends up in a landfill its chemical and

physical properties will need to be evaluated and the landfill area will be classified

based on the Government Decree on Landfilling (331/2013) [102]. The waste to be

disposed of in this way will be classified as either inert, regular or hazardous, where the

last-mentioned implies that it may cause harm to health or the environment. Gold mine

tailings, which contain large quantities of heavy metals and leachable sulphates, are

classified as hazardous waste.

When solid materials ending up as waste containing heavy metals, the possible

release of these when exposed to interactions with the environment needs to be

assessed. Leaching tests are the most effective way to evaluate the efficiency of the

immobilization of heavy metals in solidified materials [48]. Leaching is defined as a

process in which inorganic, organic or radioactive substances are released from solid

material when this is in contact with water. The dissolving of minerals,

sorption/desorption equilibria, complexation, pH, biological activity and dissolved

organic matter can all influence the leaching behaviour of a solid material. Components

can leach out from the surface or from the interior of the material, the leaching

behaviour being dependent on the porosity of the material [60].

There are several leaching test procedures, each of which has its own advantages

in the context of immobilization research. All the components involved have their own

forms of behaviour under different environmental conditions, so that the leaching of a

particular element is not directly related to its total concentration in the starting

36

material. The real environmental conditions should be simulated during the leaching

test when studying the durability of a material [60].

The European standard SFS-EN 12457-2; “Characterization of waste- Leaching-

Compliance test for leaching of granular waste materials and sludge” is used to

characterize the release of soluble components from waste in contact with water, being

the main potential mechanism for the environmental risk attached to waste disposal.

The main parameters affecting the leaching of harmful components are the liquid/solid

ratio, the leachant composition, pH, redox potential, complexing capacity and physical

parameters. The amounts of the various components leached out of stabilized material

have been compared with the waste disposal law limits in Paper II, III and IV.

37

3 Materials and Methods

3.1 Materials

3.1.1 Mine tailings

The gold mine tailing sample was received in slurry form from the gold mining

industry from Northern Finland. The overall process as it took place at the mining

site is presented in Fig. 4. The mine tailings were sampled after the flotation and

neutralization process. In the neutralization step, the flotation tailings and acid

water are mixed with Ca(OH)2 solution to increase the pH of the slurry in order to

precipitate the sulphates as gypsum in accordance with the chemical reactions noted

in section 2.4 (Management of mine tailings). The total quantity of mine tailings

neutralized annually at this gold mining site is around 1.5 Mt, of which 20% are

recycled underground as paste backfilling material. The remaining tailings are

disposed of in tailings dams located nearby.

Fig. 4. Mineral processing plant flowsheet of gold mining site (Reprinted by permission

from [103] © Agnico Eagle Mines Ltd.).

The sample tailings were dried to a constant weight at 105 ˚C before being used in

powdered form in the experiments. Their particle size distribution in this form was

38

analysed using a Beckman Coulter LS 13320 device, and the specific surface area,

based on the physical adsorption of gas molecules to a solid surface, was

determined with an ASAP 2020 (Micrometrics) physisorption analyser. The results

were reported as BET isotherms. Loss on ignition was studied at 525 °C and 950 °C

using thermo-gravimetric analysis (PrepAsh, Precisa). An Omnian Panalytics

Axios X-ray fluorescence (XRF) spectrometer was used to identify the chemical

composition of all materials, and the trace element content was analysed by

microwave-assisted wet digestion of the mine tailings with a 3:1 mixture of HNO3

and HCl and determination of the elements using an inductively coupled plasma-

optical emission spectrometer (ICP-OES).

Data on chemical composition, loss on ignition (LOI), mean particle size and

specific surface area are presented in Table 1, and the minor components present in

the powdered mine tailings are listed in Table 2. Three different batches of tailings

were analysed and each batch was well mixed before analysis. The mine tailings

consisted of a fine-grained fraction having a mean particle size of around 49 µm

and representing in terms of its chemistry mainly SiO2, Al2O3 and CaO in addition

to Fe2O3. The total SO3 content was around 5 w-% and the presence of a large

quantity of arsenic was confirmed by ICP-OES. The iron, sulphur and arsenic were

derived from the pyrite and arsenopyrite that constituted the main minerals at this

site. The other heavy metals present were Cr, Cu, Ni, Mn, Pb, Zn, B, Co, Sb, V and

Ba. The results were comparable with the chemical composition analyses made by

the mining site during longer period.

39

Table 1. Chemical composition, mean particle size, LOI and BET surface area of the

sample of mine tailings.

Component Mine tailing (w-%)

SiO2 48.1 ± 3.1

Al2O3 10.5 ± 0.2

Fe2O3 10.2 ± 1.5

CaO 11.8 ± 0.8

SO3 5.5 ± 1.7

MgO 6.4 ± 0.4

K2O 1.4 ± 0.1

Na2O 2.9 ± 0.3

TiO2 1.3 ± 0.07

pH 9.6

LOI 525 °C 1.9 ± 2.3

LOI 950 °C 13.5 ± 1.1

Particle size d50 (µm) 49

BET surface area (m2/kg) 7.2

Table 2. Minor components of the powdered mine tailings.

Component Mine tailing (mg/kg)

As 1323 ± 195

Cd <0.3

Cr 89.3 ± 26.6

Cu 190 ± 104.4

Ni 110 ± 10.0

Mn 1503 ± 5.8

Pb 5.9 ± 2.7

Zn 97 ± 30.1

B 8.6 ± 1.5

Be <1.0

Co 24.7 ± 3.0

Mo 1.3 ± 0.2

Sb 32.7 ± 20

Se <3.0

Sn <3.0

V 75.7 ± 14.5

Ba 16.7± 28.8

Hg 0.2 ± 0.03

40

3.1.2 Co-binders

Commercial ground granulated blast furnace slag (GBFS, KJ 400, Finnsementti)

having sum of a SiO2, Al2O3, and CaO content > 80 w-% was used as a co-binder

in Papers I, II and III, and commercial metakaolin (MK, Argical 1000, Imerys

Minerals Ltd., UK) was used as an aluminosilicate precursor in Paper II. The

chemical composition calculated in terms of the oxides of these co-binder materials

is presented in Table 3.

Table 3. Chemical composition of the co-binder materials: blast furnace slag (GBFS)

and metakaolin (MK).

Component Blast furnace slag

(w-%)

Metakaolin

(w-%)

Paper(s) I, II, III II

SiO2 32.3 58.9

Al2O3 9.6 34.7

Fe2O3 1.2 1.4

CaO 38.5 0.1

MgO 10.2 0.1

Na2O 0.5 0.1

K2O 0.5 0.7

TiO2 2.2 1.3

3.1.3 Alkaline solutions

Sodium hydroxide (NaOH), sodium silicate (Na-Sil) and calcium hydroxide

(Ca(OH)2) were used as alkaline solutions for sample preparation in Papers I, II

and III (Table 4). The sodium hydroxide solution was prepared by mixing solid

NaOH pellets (> 99 %, Merck, Germany) with distilled water to achieve the desired

concentration and was then used as an alkaline activator in Papers I, II and III.

Commercial sodium silicate solutions with Na2O/SiO2 ratios of 3 and 3.3 were used

as activators in Papers II and III, respectively. The 5 w-% Ca(OH)2 solution used

as an alkaline reagent in Paper III was formed by mixing Ca(OH)2 pellets (> 99 %,

Merck, Germany) with distilled water.

41

Table 4. The list of the alkaline solutions used in this research.

Alkaline solution NaOH

(5 M)

Na-Sil

(Na2O/SiO2 3)

Na-Sil

(Na2O/SiO2 3.3)

Ca(OH)2

(5 w-%)

Paper(s) I II III III

3.1.4 CSAB cement

The CSAB cement used as a binder in Paper IV was synthesized from pure reagent-

grade chemicals (RGC): CaSO4, calcium oxide (CaO), aluminium oxide (Al2O3),

silicon dioxide (SiO2) and iron (III) oxide (Fe2O3). The chemical and the phase

composition of the resulting clinker used in Paper IV are shown in Table 5 and

Table 6 respectively. The ye’elimite (C4A3�̅� content of CSAB cement clinker is

usually in the range 30-70 w-% [89], and was also the main component of the

CSAB cement used here. The other main phases identified were belite/larnite (C2S)

ferrite phases (andradite and ferrite), anhydrite and mayenite.

Table 5. Chemical composition of the CSAB clinker, as obtained by XRF (Formulated

from Paper IV).

Oxides CSAB (w-%)

Al2O3 24.2

CaO 45.5

Fe2O3 10.3

SiO2 12.5

SO3 7.6

Table 6. Phase composition of the CSAB cement clinker, as obtained by XRD

(Formulated from Paper IV).

Phases Formula CSAB (w-%)

Ye'elimite Ca4Al6(SO4)O12 50.9 ± 0.1

Larnite Ca2SiO4 29.7 ± 0.8

Andradite Ca3Fe2Si3O12 4.0 ± 0.3

Ferrite Ca2Fe2O5 11.1 ± 0.5

Anhydrite CaSO4 1.78 ± 0.1

Mayenite Ca12Al14O33 2.3 ± 0.2

42

3.2 Methodology

3.2.1 Alkali-activated materials and tailings stabilized with Ca(OH)2

solution

The alkali-activated formulations studied in Papers I and II and the tailings

stabilized using an alkaline Ca(OH)2 solution in Paper III were prepared by

following steps:

1. Powdered mine tailings and co-binder materials were weighed and then

carefully mixed together.

2. The alkaline solution (NaOH and Ca(OH)2) was prepared by mixing a weighed

amount of NaOH or CaOH2 pellets with distilled water to obtain a solution of

the desired concentration. The commercial Na-SIl solution was mixed with the

NaOH solution (in Paper II) and the resulting mixture was used as an alkaline

solution for sample preparation.

3. The powdered mixture of mine tailings and co-binders was mixed with the

liquid alkaline solution and water for 5 min with intensive stirring until a

homogeneous paste was achieved.

4. The paste was placed in moulds of various shapes and sizes and cured at room

temperature with an approximate moisture level of 50-60%. The specimens

were removed undamaged from the moulds after 3 days and allowed to cure

further for 7, 28 or 90 days or 18 months.

The formulations of the mixtures prepared in Paper I are presented in Table 7. The

main focus was on analysing, from a mechanical point of view, the possibilities for

using sulphidic mine tailings as a raw material in alkali activation.

Table 7. Formulations of the alkali-activated mine tailings for mechanical strength

analysis as mixtures of blast furnace slag (GBFS) and mine tailings (MT) activated with

NaOH solutions of different concentrations (Modified from Paper I).

Sample name MT (w-%) GBFS (w-%) NaOH conc. (M) l/s (ml/g)

MT/GBFS_100/0 100 0 5, 10 0.25

MT/GBFS_95/5 95 5 5, 10 0.25

MT/GBFS_90/10 90 10 5, 10 0.25

MT/GBFS_75/25 75 25 5, 10 0.25

MT/GBFS_50/50 50 50 5, 10 0.25

MT/GBFS_25/75 75 25 5, 10 0.27

MT/GBFS_0/100 0 100 5, 10 0.30

43

The focus in Paper II was on effective immobilization of the heavy metals by alkali-

activation, and the formulations of the samples prepared for this are presented in

Table 8. All the mixtures were designed bearing in mind that the Si/Al and Na/Al

molar ratios can affect the physical properties and inertization capacity of an alkali-

activated matrix. The Si/Al molar ratio was designed to be < 3 and Na/Al near 1,

in accordance with previous results showing the best mechanical and

immobilization properties with these molar ratios [37], [56]–[58]. When blast

furnace slag was used as a co-binder the calculated Si/Al ratio of the mixture was

3.5 and the Na/Al ratio 1.5. Metakaolin was used to adjust the aluminium content,

which was low in the other two precursors. As these calculated ratios take into

account the overall chemical composition rather than that of the reactive amorphous

fraction, they must be regarded simply as indicative estimates.

Table 8. Formulations of the alkali-activated mine tailing (MT) samples for heavy metal

immobilization studies using blast furnace slag (GBFS) and metakaolin (MK) as co-

binders (Under CC BY-NC-ND Paper II © 2018 Authors).

Sample name NaOH (conc. M) Na-Sil Si/Al Na/Al

MT/GBFS/MK_50/50/0 30 ml (8M) 0 ml 3.5 1.5

MT//GBFS/MK_50/20/30 20 ml (8M) 25 ml 2.6 0.9

MT/GBFS/MK_40/30/30 20 ml (8M) 25 ml 2.5 0.8

MT/GBFS/MK_50/20/30 20 ml (9M) 25 ml 2.6 0.9

MT/GBFS/MK_40/30/30 20 ml (9M) 25 ml 2.5 0.9

The formulations of the samples prepared for Paper III using Ca(OH)2 as an

alkaline additive and blast furnace slag as a co-binder are presented in Table 9. The

sulphate stabilization results were compared with those for the alkali-activated

mine tailing samples.

Table 9. Chemical formulations of the mine tailing samples stabilized using Ca(OH)2 or

NaOH/Na-Sil as the alkaline solution and blast furnace slag (GBFS) as a co-binder

(Formulated fro Paper III).

Sample name CaOH2 (5 w-%) NaOH Na-Sil

MT/GBFS_90/10 30 ml 0 0

MT/GBFS_75/25 30 ml 0 0

MT/GBFS_75/25 0 30 ml 50 ml

44

3.2.2 Samples stabilized using CSAB cement

The samples stabilized with CSAB cement were prepared by the following steps:

1. Pure gypsum, mine tailings and CSAB cement were first mixed together.

2. The powdered mixture was fully mixed with bidistilled water and with standard

sand for 2.5 minutes in order to keep the workability constant and thereby to

achieve a homogeneous paste.

3. The paste was placed in cubic moulds with 25 mm of diameter cured at room

temperature with 65% humidity. The specimens were removed undamaged

from the moulds after only 3 days of curing and were allowed to cure for 7, 28

and 90 days before further characterization.

The formulations of the specimens prepared with CSAB cement (Paper IV) are

presented in Table 10. The target was to explore the possibility for replacing the

cement/gypsum mixture with varying amounts of sulphidic mine tailings and

thereby achieving effective immobilization of the heavy metals and sulphates.

Another aim was to achieve specimens with high strength, thus standard sand was

used as an aggregate in the formulations.

Table 10. The chemical formulations of stabilized mine tailings with CSAB cement

(Modified from Paper IV).

Sample name CSAB cement

(w-%) Gypsum (VWR)

(w-%) Mine tailing

(w-%) CEN sand

(w-%) w/b ratio

MT/CSAB_0/100 22.50 2.50 0.00 75 0.50

MT/CSAB_25/75 16.90 1.80 6.30 75 0.50

MT/CSAB_50/50 11.25 1.25 12.50 75 0.50

MT/CSAB_75/25 5.60 0.65 18.75 75 0.50

MT/CSAB_90/10 2.25 0.25 22.50 75 0.50

3.2.3 Physical characterization of the specimens

The mechanical strength of the specimens was determined using a Zwick 100

device at a loading rate of 1.2 kN/s. Three to five samples of each composition were

tested and the average of the results was calculated. The samples were tested after

7, 28 or 90 days of curing.

45

3.2.4 Leaching test

The European standard EN 12457 “Characterization of waste – Leaching –

Compliance test for leaching of granular waste materials and sludge” was used to

determine leaching of heavy metals and other components from the materials

produced [104]. The specimens were crushed and sieved to a particle size under 4

mm after the given curing times (7, 28, 90 days or 18 months). The crushed

specimen immersed and shaken by an end-over-end shaker in bidistilled water with

a liquid/solid weight ratio of 10 l/kg. The specimens were shaken in water for 24 h

and the concentrations of the components analysed by ICP-OES and IC (for sulfates)

after extraction and filtration followed by acidification with HNO3 in order to

achieve pH < 2.

3.2.5 Phase identification

The crystalline phases of the powdered mine tailings and the resulting specimens

were identified using a Siemens 5000 X-ray diffractometer. In Paper I and II the

analysis were made using a step interval of 0.04°, an integration time of 4 s/step,

and an angle interval of 10-50° (2Θ), while for Paper IV the step interval was 0.02°

and the scan rate 4°/min for a scan range of 5-120° (2θ). The ICDD database (PDF-

2, 2006) was used to identify the crystalline phases in the material, employing an

internal standard method that involved co-grinding the sample, to be analyzed with

20 w-% of rutile (TiO2) and calculating the results for the phases on the basis of

this standard.

3.2.6 Microstructural analysis

The Environmental Scanning Electron Microscopy (ESEM) for the chemical

analysis of the alkali-activated specimens in Paper II was performed with a

Quanta200 FEI instrument from Thermo Fischer Scientific, Waltham, MA, USA)

combined with Energy Dispersion Spectroscopy (EDS) with an INCA-350 from

Oxford Instruments Plc, Tubney Woods, Abingdon, UK). The microstructural

analysis of the specimens in Paper III was carried out using a scanning transmission

electron microscope (STEM) with a LEO 912 OMEGA EFTEM, with an energy-

dispersive X-ray spectroscopy (EDS) detector (Oxford Instruments, X-Max 80).

An Ultra Plus Field Emission Scanning Electron Microscope was employed to

analyze the microstructure of the powdered mine tailings and the specimens

46

produced for Paper IV, which were impregnated in an epoxy resin under a vacuum,

allowed to harden, polished and coated with carbon before the FESEM analysis.

AZtec software (Oxford Instrument) was used to analyse the back-scattered

electron (BSE) images and the elemental chemical compositions of selected EDS

points. The BSE technique was used for imaging of the specimen and Energy

Dispersion Spectroscopy (EDS) for the chemical analysis. Scanning electron

images were taken under an acceleration voltage of 15 kV with the working

distance around 5.5 or 8.5 mm.

47

4 Results and discussion

4.1 Characterization of the mine tailings

The total concentration of heavy metals and sulphur in the powdered mine tailing,

the concentration of leached components from tailings and the legal limits for the

elements in inert and regular waste are presented in Table 11. Even though the total

concentration of the heavy metals is high, the leaching of these from the powdered

mine tailings was found to be low, except in the leaching case of sulphates and

arsenic, which were both over the inert waste limits. Based on the leaching results

the studied tailing is classified as a hazardous waste according to waste law limits.

Table 11. Total concentration of heavy metals and sulphates in the studied mine tailings,

leaching from fresh mine tailings and legal limits for inert and regular waste (Formulated

from Paper I, II, III and IV).

Component Total concentration

in MT powder

(mg/kg)

Leaching from fresh

MT powder

(mg/kg)

Legal limit for inert

waste

(mg/kg)

Legal limit for regular

waste

(mg/kg)

SO4 46 800±1400 28 267 ± 6 800 1000 20 000

As 1323 ± 195 1.5 ± 0.88 0.5 2

Cd <0.30 0.02 ± 0.01 0.04 1

Cr 89.3 ± 26.6 0.17 ± 0.10 0.5 10

Cu 190 ± 104.4 0.10 ± 0.08 2.0 50

Ni 110 ± 10.0 0.08 ± 0.03 0.4 10

Mn 1503 ± 5.8 < 0.30

Pb 5.9 ± 2.7 0.23 ± 0.14 0.5 10

Zn 97 ± 30.1 0.10 ± 0.20 4.0 50

B 8.6 ± 1.5

Be <1.0 < 0.02

Co 24.7 ± 3.0 0.03

Mo 1.3 ± 0.2 0.10 ± 0.01 0.5 10

Sb 32.7 ± 20.0 0.20 ± 0.09 0.06 0.7

Se <3.0 < 0.15 0.1 0.5

Sn <3.0 < 0.10

V 75.7 ± 14.5 0.05 ± 0.10

Ba 16.7± 28.8 0.18 ± 0.10 20 100

Hg 0.2 ± 0.03 <0.20

48

4.1.1 Mineralogical analyses

The powdered gold mine tailings studied were shown in XRD analysis to be a

crystalline material containing a large quantity of silica in the form of quartz (Fig.

5). The other main phases were dolomite/ankerite, muscovite, albite and

clinochlore. In FESEM analysis the tailings were seen to consist of fine particles

of irregular shapes and sizes, as illustrated in Fig. 6. The main elements found by

EDS analysis, in addition to the high iron content which had already been detected

in the analysis of chemical composition, were quartz, dolomite and gypsum.

Fig. 5. XRD pattern for the powdered mine tailings (c=chlinoclore, m=muscovite,

a=albite, q=quartz, d=dolomite and r=rutile derived from the internal standard).

49

Fig. 6. Secondary electron images of the powdered mine tailings.

It is important to analyze the mineralogy of mine tailings when designing proper

technologies for their management. As discussed earlier, the environmental

problems associated with sulphidic mine tailings are related to the possible

generation of acidity. The primary ore minerals can easily be oxidized and the

elements can be induced to form secondary minerals by co-precipitation, ion

exchange, or sorption, or they can be released into the environment as harmful

components under different temperature, pH and humidity conditions [105]. The

oxidation of sulphidic minerals can lower the pH of the environment and can

release large amounts of heavy metals, including the carcinogen arsenic, over long

periods of time [106], which is why it is important to develop long-term

stabilization technologies for their management.

4.2 Characterization of the physical properties of the stabilized

mine tailing specimens

4.2.1 Comparison of the mechanical strengths of S/S specimens

Powdered mine tailings are an unreactive material under alkaline conditions and

need a proper co-binder in order to produce a hardened matrix. This was observed,

as the alkali-activated specimen of pure mine tailings without any co-binder was

weak and easily broken when in contact with water, while only 5 w-% of GBFS as

a co-binder increased its mechanical strength to 4 MPa (Fig. 7). Although the

mechanical strength was adequate for use as a paste backfill material, for example,

the matrix was broken when immersed in water for 6 days. A greater strength was

obtained by increasing the GBFS content. Similar results have been observed by

50

Cihangir et al. (2012)[80], who studied the use of alkali-activated blast furnace slag

as a binder material in paste backfilling with sulphidic-rich mine tailings. Greater

mechanical strength was obtained with a higher slag content, and this was

accompanied by higher acid resistance. Similarly, Giannopoulou and Panias, (2006)

[68] obtained a weak matrix by the alkali activation of flotation mine tailings

without any co-binder, and found that fly ash as a co-binder improved the

mechanical strength.

Molarity and the liquid/solid ratio both affect the final strength of an alkali-

activated material [107]. If the liquid/solid ratio is too high, polycondensation of

the alkali-activated material may be hindered and the excess water can form pores,

resulting in a material of lower mechanical strength. Conversely, a liquid/solid ratio

that is too low will detract from both the strength of the material and its workability

[108], [109]. The concentration of the alkaline solution had a slight effect on the

mechanical strength of the specimen produced in this study (Fig. 7), but a higher

NaOH concentration notably reduced the workability of the mixture and the

resulting paste was difficult to mould. When the NaOH solution was replaced with

a Ca(OH)2 solution, the mechanical strength was the same or just slightly decreased.

Fig. 7. Strength of alkali-activated mine tailing specimens obtained after alkali

activation with NaOH solutions and stabilized with a Ca(OH)2 solution (two

formulations), after 28 days of curing (Modified from Paper II and III).

51

The stabilized mine tailing specimens showed the highest strength when CSAB

cement was used as a co-binder (Fig. 8). The mechanical strength of the specimens

produced with a 50/50 CSAB/mine tailings ratio was equivalent to 70% of those of

the reference specimen (without any mine tailings). This result indicates some

interaction between the mine tailings and the CSAB cement during the hydration

process, most probably some degree of ettringite formation, while the mine tailings

consisted of large amounts of sulphides and gypsum after neutralization, which can

react with CSAB cement. The strength improved greatly during longer curing times

and decreased with increasing mine tailings content.

The slow reaction rate of the larnite/belite phase has been detected previously

as the reason for the improvement in mechanical strength with longer curing time

[93]. In addition, Martin et al. (2017) [89] observed lower mechanical strength if

the amount of fly ash involved in the CSAB hydration process was too high. In

addition, Chen, Hargis and Juenger (2012) [83] have shown that the properties of

the additional sulphate (gypsum, anhydrite, basanite) and the water/cement ratio

are important parameters affecting the final mechanical properties of CSAB

cement-based concretes. The sulphate in the mine tailings used was partly obtained

as a basanite phase (Paper III). The fastest known initial setting time, the highest

viscosity and the lower mechanical strength have all been obtained when using

basanite as a sulphate source in CSAB cement hydration [110] supporting the poor

strength properties and workability obtained with higher mine tailing content in

these tests.

52

Fig. 8. Mechanical strength of the CSAB-stabilized mine tailing specimens as a function

of curing time (Modified from Paper IV).

4.3 Environmental stability of the S/S specimens

The leaching of heavy metals from the pure powdered mine tailings during a single-

stage leaching test shaking in water was low, even though the total concentrations

of certain elements were high. The leaching of sulphates and arsenic was slightly

above the legal limits. If AMD generation occurs within the mine tailings, large

amounts of heavy metals can be leached out in the course of time. The

immobilization of various elements by various S/S treatment methods was studied

in Papers II, III and IV.

4.3.1 Immobilization of heavy metals with various S/S methods

Alkali activation has a high capacity for immobilizing heavy metals, mostly

cationic elements, as demonstrated in Paper II. The leaching levels of various heavy

metals from powdered mine tailings and alkali-activated materials are shown in

Table 12. Pronounced immobilization of Cr, Cu, Ni, Zn, Mn, Co and Pb was noted

after 7 days of curing, and the results improved after longer curing times. As, B and

V were the most problematic elements to immobilize effectively, as higher leaching

53

of these elements was observed after alkali activation compared to the initial mine

tailing powder. The most important parameter leading to an increase in the

immobilization efficiency of these challenging components in alkali-activated

specimens was curing time, so that the leaching of these elements was greatly reduced

after 18 months and all the results were below the legal limits. This is an important

result as it indicates improved consolidation and immobilization of the alkali-activated

matrix with a longer curing time.

Table 12. Total concentrations of Cr, Cu, Ni, Zn, V, As, Sb, Mn and B in powdered mine

tailings (MT), leaching of elements from studied mine tailings, legal limits for inert and

regular waste and leaching of elements from alkali-activated specimens (Under CC BY-

NC Paper II © 2018 Authors).

Cr

mg/kg

Cu

mg/kg

Ni

mg/kg

Zn

mg/kg

V

mg/kg

As

mg/kg

Sb

mg/kg

Mn

mg/kg

B

mg/kg

Co

mg/kg

Pb

mg/kg

pH

Total concentration in MT powder  74 120 100 71 59 1520 32 1500 9.6 22 4.3

 

Leaching from MT powder 0.3 0.2 0.1 0.5 1.3 68 0.1 4.2 7.0 0.4 0.1 9.6

Legal limit for inert waste 0.5 2 0.4 4

0.5 0.06

Legal limit for regular waste 10 50 10 50

2 0.7

MT/GBFS/MK _50/50/0 (8M NaOH)

7d 0.2 1.7 0.1 0.4 59 113 0.4 0.3 10 0.3 0.4 11.2

28d 0.5 0.1 0.2 0.1 5.2 52 0.1 3.3 5.5 0.1 0.2 12.8

MT/GBFS/MK _50/20/30 (8M NaOH)

7d 0.6 2.3 0.3 0.9 50 173 0.2 3.3 5.5 0.1 0.1 10.7

28d 0.6 0.1 0.2 0.1 6.0 43 1.1 3.9 8.3 0.1 0.3 10.7

MT/GBFS/MK _40/30/30 (8M NaOH)

7d 0.7 1.7 0.3 0.6 63 133 0.2 1.4 8.3 0.3 0.2 12.1

28d 0.6 0.1 0.3 0.1 5.0 24 0.7 3.5 8.7 0.1 0.2 11.3

18mos 0.1 0.05 < QL <QL 1.3 0.1 < QL < QL 7.4 <QL 0.1

MT/GBFS/MK_50/20/30 (9M NaOH)

7d 0.4 3.3 1.6 1.0 59 160 0.4 11.6 9.9 2.5 0.2 11.2

28d 0.3 0.1 0.3 0.1 5.9 32 0.5 3.6 9.7 0.1 0.5 11.1

MT/GBFS/MK_40/30/30 (9M NaOH)

7d 0.4 1.3 0.1 0.8 52 105 0.2 4.9 9.6 0.1 0.1 11.5

28d 0.5 0.1 0.2 0.1 4.2 35 1.0 3.8 9.1 0.1 0.2 11.2

18 mos < QL < QL < QL < QL 1.7 0.15 < QL < QL 7.3 <QL <QL

54

Several researchers have found more pronounced leaching of As, Sb, V, Mo and Se

from alkali-activated materials than from the starting material [51], [66], [111]–

[113], and it is known that these elements can form highly soluble species due to the

high pH of the alkali activation process. The degrees of inertization for certain elements

under different pH conditions are summarized in Fig. 9 [48]. Arsenic, for example, can

be highly soluble over a wide pH range from acid to alkaline and is not effectively

immobilized during the alkali activation process. Cd, Pb, Sn, Zn, Ni, Mn, Cr and Cu,

on the other hand, could theoretically be immobilized effectively under conditions of

high pH.

Fig. 9. Influence of pH on the inertization efficiency of certain elements (Reprinted by

permission from [48] © 2015 Elsevier Ltd).

The calcium-based binding system

Calcium-based binding systems were the most effective methods for immobilizing

complex oxyanions, especially arsenic and vanadium. This was observed in Papers

III and IV. The rates of leaching of various heavy metals from powdered mine

tailings and materials stabilized using calcium-based binding systems are shown in

Table 13. When mixing Ca(OH)2 and GBFS (in Paper III) together with powder

mine tailings, the immobilization of the heavy metals, including the challenging

components As, V, reached a high level after only 7 days of curing and remained

constant, or even slightly improved, with longer curing times. The total amount of

55

mine tailings in the mixture with high immobilization efficiency was 90 w-% and

the degree of leaching of the various components were still below the legal limits

for inert waste. The leaching of Sb from the S/S specimens was slightly above the

legal limit for inert waste, but the total amount of Sb leached was already below

the limit for regular waste after 7 days of curing. Sb is one of the elements which

has higher solubility at higher pH values.

Table 13. Total concentration of heavy metals in powdered mine tailing (MT), leaching

of elements from studied mine tailings, legal limits for inert and regular waste and

leaching of elements from S/S specimens stabilized with Ca(OH)2 and GBFS (Modified

from Paper III).

As

(mg/kg)

V

(mg/kg)

Cr

(mg/kg)

Cu

(mg/kg)

Ni

(mg/kg)

Zn

(mg/kg)

Sb

(mg/kg)

Pb

(mg/kg)

Ba

(mg/kg)

pH

Total concentration in concentration in MT powder

1520 59 74 120 100 71 32 4.3 29

Leaching from MT powder 1.5 0.05 0.1 0.05 0.5 0.1 0.3 0.1 0.09 8.0

Legal limit for inert waste

0.5

0.5 2.0 0.4 4.0 0.06 0.5 20

Legal limit for regular waste

2.0

10 50 1.0 50 0.7 10 100

MT/GGBFS_90/10

7d 0.40 0.50 0.19 0.19 0.20 0.19 0.18 0.19 1.00 11.7

28d 0.30 0.30 0.19 0.20 0.20 0.23 0.20 0.20 0.70 10.4

MT/GGBFS_75/25

7d 0.60 0.50 0.20 0.20 0.22 0.21 0.20 0.20 0.20 11.9

28d 1.00 0.40 0.20 0.20 0.20 0.23 0.19 0.19 0.70 10.4

56

The most effective immobilization of heavy metal cations and oxyanions such as

As, V and Sb was achieved with CSAB stabilization, as observed in Paper IV. Apart

from Cr, the degree of leaching of all the elements remained below the legal limits

for regular and inert waste after 7 days of curing and the immobilization remained

constant with longer curing times (Table 14). The maximum mine tailings content

of the mixtures with good immobilization results was 90 w-%.

Table 14. Total concentration of heavy metals in powdered mine tailings (MT), leaching

from studied mine tailing powder, limits for inert and regular waste and leaching of

elements from CSAB-stabilized specimens (Under CC BY-NC-ND Paper IV © 2019

Authors).

As

(mg/kg)

V

(mg/kg)

Cr

(mg/kg)

Cu

(mg/kg)

Ni

(mg/kg)

Zn

(mg/kg)

Sb

(mg/kg)

Pb

(mg/kg)

Ba

(mg/kg)

pH

Total concentration in MT powder 1520 85 74 310 120 71 32 4,3 29

Leaching from MT powder 0.18 0.05 0.5 0.05 0.5 0.1 0.3 0.1 0.09 8.0 Legal limit for inert waste

0.5 0.5 2.0 0.4 4.0 0.06 0.5 20

Legal limit for regular waste 2.0 10 50 10 50 0.7 10 100

MT/CSAB_0/100

7d < 0.15 <0.05 0.26 <0.05 <0.05 <0.10 <0.15 <0.15 0.10 11.3

28d < 0.15 <0.05 0.43 <0.05 <0.05 <0.10 <0.15 <0.15 0.16 10.9

90d < 0.15 <0.05 0.37 <0.05 <0.05 <0.10 <0.15 <0.15 0.25 11.8

MT/CSAB_25/75 7d <0.15 <0.05 0.65 0.06 <0.05 <0.10 <0.15 <0.15 0.09 11.5

28d <0.15 <0.05 1.30 <0.05 0.21 <0.10 <0.15 0.28 0.06 11.4

90d <0.15 <0.05 0.94 <0.05 <0.05 <0.10 <0.15 <0.15 <0.05 11.7

MT/CSAB_50/50 7d <0.15 <0.05 0.65 <0.05 <0.05 0.14 <0.15 <0.15 <0.05 11.5

28d <0.15 <0.05 1.10 <0.05 <0.05 <0.10 <0.15 <0.15 0.05 11.2

90d <0.15 <0.05 1.40 <0.05 <0.05 <0.10 <0.15 <0.15 <0.05 11.4

MT/CSAB_75/25

7d <0.15 <0.05 0.50 <0.05 <0.05 <0.10 <0.15 <0.15 0.10 11.2

28d <0.15 <0.05 0.58 <0.05 <0.05 <0.10 <0.15 <0.15 0.06 11.0

90d <0.15 <0.05 0.41 <0.05 <0.05 0.53 <0.15 <0.15 <0.05 10.6

MT/CSAB_90/10

7d 0.52 <0.05 0.16 <0.05 <0.05 <0.10 <0.15 <0.15 0.10 10.7

28d 0.46 <0.05 <0.10 <0.05 <0.05 <0.10 <0.15 <0.15 0.08 9.9

90d <0.15 <0.05 0.16 <0.05 <0.05 0.26 <0.15 <0.15 0.09 9.5

57

Good immobilization of arsenic has been found by using cementitious binding

materials. Ca and As can form calcium arsenate components which are stable above

pH 10 [20], [114]. The reactions between arsenic and other hydration products of

cement such as ettringite and calcium silicate hydrate have also been observed to

stabilize arsenic in the matrix when using cement-based binding systems [106]. It

is important, however, to remember that the binding system also needs to be

designed to be resistant to acid attack if the sulphidic minerals are oxidized within

the matrix.

Sb leaching was slightly higher when using GBFS as a binder material than

with CSAB cement, coinciding with the observations of Salihoglu (2014) [115],

who observed higher leaching of Sb when using blast furnace slag as binding

material instead of cement. The most effective method for Sb immobilization has

been achieved by using a cement-based binding system or cement together with fly

ash and gypsum.

Cr was leached out from CSAB stabilized material. Although Cr was already

leached out of the CSAB cement reference specimen (without the addition of mine

tailings) part of the soluble Cr must have come as contamination from the crushing

and milling equipment used during CSAB clinker production, while it should not

be present in CSAB cement clinker. It is also significant that the leaching of Cr

decreased as the amount of mine tailings increased, even though theoretically the

total amount of Cr would increase with a higher mine tailing content. Cr

immobilization is mainly due to the reduction of its toxic Cr(VI) form to the more

stable Cr(III) form [116]. According to Zhang et al. (2008) [117], a higher sulphide

content could enhance the transformation of Cr(VI) to Cr (III), thus reducing the

amount of leachable Cr. Additionally, if Cr is presence as Cr(VI) it can be highly

soluble in high pH. When CSAB stabilized specimen included higher amount of

mine tailings the pH decreased which can decrease the solubility of Cr(VI) [118].

4.3.2 Immobilization of sulfates by different S/S methods

Calcium also plays an important role in the immobilization of sulphates, as can be

seen in Table 16. Sulphates were leached out in abundance from the alkali-activated

specimens, but the immobilization efficiency improved greatly when calcium-

based binder materials were used. The main reason for the poor immobilization of

sulphates during alkali activation is related to the negative charge of sulphate ions

when present in an alkaline solution. When an alkaline solution yields positive ions

such as Na, the sulphates can form highly soluble species such as sodium sulphate.

58

When Ca(OH)2 was used as an alkaline source and GBFS as a binder material a

considerably greater immobilization efficiency was achieved, as also with a higher

slag content.

When using CSAB cement the sulphates in a 50/50 mixture of CSAB/mine

tailings were readily immobilized during the early stage of curing, as can be seen

in Table 15. Sulphates most probably react with calcium and aluminium producing

ettringite, being a suitable sulphate source for CSAB hydration. The interaction

between mine tailings and CSAB cement was already seen in the characterization

of mechanical properties of mine tailings specimens stabilized by using CSAB

cement. Low amounts of sulphates were already leached out from the reference

specimen (without any mine tailings), which means that there is excess amount of

sulphates in the system. In futures studies, the the optimum level of added gypsum

should be determined, and the acid resistance of the resulting materials needs to be

studied further, given that the oxidation of sulphidic minerals can occur during the

time for which they are inside the stabilized matrix. In addition, a variety of

environmental conditions would need to be studied in order clearly assess the

durability of the resulting matrix.

59

Table 15. Total concentration of leached sulphates from the powdered mine tailings

(MT), leaching from different S/S specimens and the immobilization efficiency recorded

for specimens stabilized by various treatment methods.

Sample Alkaline

solution

SO4

(mg/kg)

Immobilization

efficiency (%)

Leaching from MT powder

28 267 ± 6 800

Legal limit for inert waste 1 000

Legal limit for regular waste 20 000

Alkali-activated mine

tailings (Paper III)

MT/GGBFS_75/25 NaOH/Na-SIl > 20 000 0

Calcium-stabilized

using Ca(OH)2 and

GBFS (Paper III)

MT/GGBFS_90/10 (7 days) Ca(OH)2 8 700 52

MT/GGBFS_90/10 (28 days) Ca(OH)2 5 500 70

MT/GGBFS_75/25 (7 days) Ca(OH)2 2 800 81

MT/GGBFS_75/25 (28 days) Ca(OH)2 1 500 90

CSAB-stabilized

mine tailings

(Paper IV)

MT/CSAB_25/75 (7 days)

- 1 280 100

MT/CSAB_25/75 (28 days) - 1 284 100

MT/CSAB_25/75 (90 days - 1 564 100

MT/CSAB_50/50 (7 days) - 3 580 100

MT/CSAB_50/50 (28 days) - 4 600 100

MT/CSAB_50/50 (90 days) - 5 360 99

MT/CSAB_90/10 (7 days) - > 20 000 0

MT/CSAB_90/10(28 days) - 15 960 12

MT/CSAB_90/10 (90 days) - >20 000 0

4.4 Microstructure analysis of the specimens

The addition of metakaolin as an aluminium-rich precursor favours alkali activation

rather than a formulation containing only mine tailings and slag, as was observed

in Paper II. The calculated Si/Al and Na/Al ratios were compared with the values

detected in the SEM analysis and the images of the specimens are shown in Fig. 10.

The Si/Al ratio calculated by EDS analysis was 3.9 (theoretical value 3,5) when the

mine tailings were mixed only with blast furnace slag and Si, Al, Ca and Na

quantity were 11.8%, 3%, 15% and 10.2% respectively. When metakaolin was

added to the mixture the Si/Al ratio determined by EDS was 2.3 (theoretical value

2.6) and the Si, Al, Ca and Na values 19.3%, 9%, 6% and 8%, respectively.

Reducing the mine tailings content the Si/Al ratio (obtained by EDS) was 2.6 and

60

Ca content of 11%, indicating that the mine tailings and slag were reacting under

alkaline conditions. These are important results, since they show the importance of

balancing the aluminosilicate co-binder in order to obtain a stable matrix. No results

of heavy metal immobilization could be seen in the SEM analysis.

Fig. 10. SEM images of the MT/GBFS/MK specimens: 50_50_0 (a), 50_20_30 (b) and

40_30_30 (c) when activated with an 8M NaOH and sodium silicate solution (Under CC

BY-NC-ND Paper II © 2018 Authors).

The TEM analysis supported the theory that the main sulphate immobilization

mechanism when using Ca(OH)2 and GBFS as binder materials was gypsum

precipitation (Fig. 11). The main phases in the powdered mine tailings, such as

albite, dolomite and quartz, were detected and iron sulphate was also found in the

immobilized mine tailing.

61

Fig. 11. Dark-field STEM images of two areas (a and b) after 28 days of curing (Reprinted

by permission from Paper III © 2018 Elsevier Ltd.).

No clear evidences of ettringite formation could be obtained by TEM analysis, but

some evidence of an ettringite phase was seen in the XRD analysis (Fig 12).

Ettringite was formed from calcium and aluminium contained in the blast furnace

slag and partly also from the Ca(OH)2 solution and sulphates derived from the mine

tailings. The Ca(OH)2 used increased the pH to the required level for forming the

ettringite structure.

62

Fig. 12. XRD analysis of the mine tailing stabilized with Ca(OH)2 and GBFS (Modified

from Paper III).

The CSAB hydration products identified by XRD analysis are shown in Table 16.

Ettringite, the main product of CSAB cement hydration, was formed during the

early stage of curing, which is good, since later ettringite formation can cause

expansion of the matrix [83]. Part of the ettringite can be reduced to an amorphous

Al(OH)3 phase during longer periods of curing due to the relatively large amount

of aluminium in the starting material [83]. XRD analysis also indicated the presence

of some unreacted ye’elimite (C4A3�̅� and gypsum after 7 days of curing, which

can mean that the stabilized specimens have a higher leachable sulphate content. A

slow reaction of the larnite phase was observed by XRD analysis, and a reduction

in larnite was seen after 90 days of curing as compared with the level after 28 days,

supporting the observation of a slow development of mechanical strength.

63

Table 16. Phase composition of CSAB-hydrated material without any mine tailings as a

function of curing time (Paper IV).

Specimen Phase composition

C2S C4A3�̅� C4AF Gypsum Ettringite Amorphous/

unidentified

MT/CSAB_0/100 (7 d) 19.8 4.8 1.9 2.0 31.3 44.1

MT/CSAB_0/100 (28 d) 16.0 4.2 0.8 2.5 31.3 46.9

MT/CSAB_0/100 (90 d) 8.8 2.2 0.3 1.4 15.9 71.8

Ettringite formation was also detected in stabilized specimens by XRD and TGA

analysis (Fig. 13 and 14), and a reduction in the ettringite peak was clearly observed

when mine tailings content was > 50 w-%, supporting the previous results

regarding leaching and mechanical properties. The TGA analysis supported the

findings of ettringite formation in various specimens, while the endothermic peak

between 165 and 180 ˚C illustrates the formation of the ettringite phase [119].

Fig. 13. XRD pattern of the CSAB-stabilized mine tailings with different ratios of mine

tailings (CE24) after 28 days of curing (e=ettringite, a=albite, g=gypsum, l=larnite,

q=quartz, d=dolomite and r=rutile from the internal standard) (Under CC BY-NC-ND

Paper IV © 2019 Authors).

64

Fig. 14. TGA analysis of the CSAB-stabilized mine tailings after 28 days of curing.

(Under CC BY-NC-ND Paper IV © 2019 Authors).

The ettringite needles in the CSAB-stabilized system with a 50 w-% mine tailings

content were identified by FESEM analysis (Fig. 15), which also enabled the

physical encapsulation to be detected (Fig. 16). Some particles containing arsenic

(probably arsenopyrite) were surrounded by ettringite and other hydration products

such as C-S-H, proving that physical encapsulation had occurred in this

stabilization system simultaneously with chemical immobilization. No clear

evidence of any reaction between these arsenopyrite particles and ettringite/other

hydration products could be seen at the level of resolution of the FESEM analysis.

Phenrat et al. (2005) [120], studying the stabilization of arsenic-iron hydroxide

sludge, noted that stabilization occurred through sorption to a C-S-H structure,

replacing SO in the ettringite phase or arsenic was solidified as a calcium-arsenic

component, all of which can occur simultaneously, supporting findings. Further

analyses should be made in order to gain a better understanding of the stability of

the matrix. Advanced characterization methods such as XPS or EPMA could be

used for further experiments.

65

Fig. 15. Ettringite needles identified in an MT/CSAB 50/50 specimen.

Fig. 16. Microstructural analysis of an MT/CSAB 50/50 specimen (Under CC BY-NC-ND

Paper IV © 2019 Authors).

66

67

5 Conclusions

Millions of tons of mine tailings are produced every year and a large proportion

end up as a slurry in impounding lakes because of their lack of economic value.

Depending on the leachability of the harmful components the mine tailings can be

classified as hazardous waste. Sulphidic-rich mine tailings can contain several

sulphidic minerals, such as pyrite, pyrrhotite or arsenopyrite, and numerous

harmful heavy metals and metalloids. When this hazardous waste is deposited in

the environment the sulphidic-bearing minerals are oxidized under atmospheric

conditions to yield acids, thus lowering the pH of the surroundings and potentially

causing the leaching of harmful components into the environment. This

phenomenon, called acid mine drainage (AMD), can occur hundreds of years after

the cessation of mining and needs careful control during and after the mine’s

lifetime. Suitable stabilization technologies to replace landfill methods need to be

evaluated in order to develop more sustainable mining in the future.

The studied gold mine tailing received from gold mining site from Northern

Finland, is classified as hazardous waste based on high leaching of sulfates and

arsenic. In this study, three stabilization/solidification treatment methods for

immobilization of sulphidic components and heavy metals present in mine tailings

are analysed: alkali activation, stabilization with Ca(OH)2 and GBFS as binders and

stabilization with CSAB cement. The main focus was on the effective

immobilization of heavy metals and sulphates, but a further aim was to achieve

good mechanical properties in the resulting matrix.

Poor reactivity during alkali activation was found in the mine tailings studied,

and a proper co-binder material was required in the mixture in order to obtain a

hardened matrix. Relatively high mechanical strength could be obtained by adding

blast furnace slag, and the strength was further increased by increasing the GBFS

content. Alkali activation provided a suitable method for the immobilization of

cationic species, but the immobilization of elements such as As and V proved to be

a challenge because of the high pH, as these elements are highly soluble under such

conditions. Furthermore, large quantities of sulphates were leached out after alkali

activation, because of the incorporation of sulphates into highly soluble sodium

sulphate compounds with the available sodium ions. A longer curing time was the

most important parameter for improving the immobilization of hazardous elements,

including oxyanions, during alkali activation, indicating a higher consolidation of

the matrix with longer curing times.

68

Calcium-based stabilization resulted in good immobilization of sulphates and

heavy metals, and also of As and V. Arsenic is most probably immobilized by

precipitation as calcium arsenate or calcium arsenite. Similar observations were

made for sulphate stabilization, as sulphate was precipitated as calcium sulphate

(gypsum). The maximum mine tailings content was 90 w-%, having an

immobilization efficiency with respect to sulphates of 70%. The immobilization

efficiency could be increased further with a higher slag content.

An ettringite structure was found in the Ca(OH)2/GBFS-stabilized matrix,

being another possible immobilization method. Since CSAB cement hydration

leads to high ettringite formation, ettringite stabilization was the main focus of

interest when studying stabilization with CSAB cement. While CSAB hydration

need sulphate source in order to form ettringite, the sulphidic mine tailings could

be an ideal material to use for this purpose. CSAB cement provided the highest

mechanical strength and highest efficiency in the immobilization of harmful

components. The maximum amount of mine tailings with good mechanical and

immobilization properties in the final material was 50 w-%, since both the chemical

and mechanical properties declined sharply with a higher mine tailings content.

Ettringite was highly formed in this system, and this may be the reason for the good

immobilization properties. Physical encapsulation was also observed by FESEM

analysis, as some solid particles containing arsenic were found to be surrounded by

ettringite and other hydration products in the matrix (such as C-S-H gel). No clear

evidence could be seen of any interaction between the heavy metals containing

solid particles and the ettringite/other hydration gel.

When there are several heavy metals in addition to oxyanions and sulphates

present, the binding system needs to be designed carefully. The most effective

immobilization method for treating sulphidic-rich mine tailings is to introduce

calcium and aluminium into the powdered tailings, whereupon ettringite formation

will become possible. High immobilization of heavy metals, including oxyanions,

was achieved. Several methods can be used for this purpose, as was seen in this

thesis. Ettringite precipitation requires a relatively high pH, which can be adjusted

by adding an alkaline solution such as Ca(OH)2. The other approach is to use binder

materials (such as CSAB cement) in which ettringite is the main hydration product.

If the material is either stable or properly stabilized, it will be possible in the future

to store it in a dry landfill area instead of submerging it in a tailings dam. In addition,

stabilization could increase the recycling potential of mine tailings, so that both the

total volume of tailings disposed of in landfills and the possible AMD generation

of sulphidic tailings could be reduced.

69

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Original publications

I Kiventerä, J., Golek, L., Yliniemi, J., Ferreira. V., Deja, J., & Illikainen, M. (2016). Utilization of sulphidic tailings from gold mine as a raw material in geopolymerization. International Journal of Mineral Processing 149, 104–110.

II Kiventerä, J., Lancellotti, I., Catauro, M., Dal Poggetto, F., Leonelli, C., & Illikainen, M. (2018) Alkali activation as new option for gold mine tailings inertization. Journal of Cleaner Production 187, 76-84.Kiventerä, J., Sreenivasan, H., Cheeseman, C.,

III Kiventerä, J., Sreenivasan, H., Cheeseman, C., Kinnunen, P., & Illikainen, M. (2018). Immobilization of sulfates and heavy metals in gold mine tailings by sodium silicate and hydrated lime. Journal of Environmental Chemical Engineering 6, 6530–6536.

IV Kiventerä, J., Piekkari, K., Isteri, V., Ohenoja K., Tanskanen P., & Illikainen M. (2019). Solidification/stabilization of gold mine tailings using calcium sulfoaluminate-belite cement. Journal of Cleaner Production 239, 118008

Reprinted with permission from from Elsevier Ltd. (I and III) and under Creative

Commons CC BY-NC-ND license (Papers II and IV).

Original publications are not included in the electronic version of the dissertation.

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