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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 2017

C 616

Juho Yliniemi

ALKALI ACTIVATION-GRANULATION OF FLUIDIZED BED COMBUSTION FLY ASHES

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

C 616

AC

TAJuho Yliniem

i

C616etukansi.kesken.fm Page 1 Tuesday, May 2, 2017 3:54 PM

ACTA UNIVERS ITAT I S OULUENS I SC Te c h n i c a 6 1 6

JUHO YLINIEMI

ALKALI ACTIVATION-GRANULATION OF FLUIDIZEDBED COMBUSTION FLY ASHES

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 Arina auditorium (TA105), Linnanmaa, on16 June 2017, at 12 noon

UNIVERSITY OF OULU, OULU 2017

Copyright © 2017Acta Univ. Oul. C 616, 2017

Supervised byProfessor Mirja IllikainenDocent Minna Tiainen

Reviewed byProfessor Konstantinos KomnitsasProfessor Raffaele Cioffi

ISBN 978-952-62-1561-7 (Paperback)ISBN 978-952-62-1562-4 (PDF)

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

Cover DesignRaimo Ahonen

JUVENES PRINTTAMPERE 2017

OpponentProfessor Cristina Leonelli

Yliniemi, Juho, Alkali activation-granulation of fluidized bed combustion fly ashes. University of Oulu Graduate School; University of Oulu, Faculty of TechnologyActa Univ. Oul. C 616, 2017University of Oulu, P.O. Box 8000, FI-90014 University of Oulu, Finland

Abstract

Biomass, such as wood, binds CO2 as it grows, and is thus considered an environmentally friendlyalternative fuel to replace coal. In Finland, biomass is typically co-combusted with peat, and alsomunicipal waste is becoming more common as a fuel for power plants. Wood, peat and waste-based fuels are typically burned in fluidized bed combustion (FBC) boilers.

Ash is the inorganic, incombustible residue resulting from combustion. The annual productionof biomass and peat ash in Finland is 600 000 tonnes, and this amount is likely to increase in thefuture, since the use of coal for energy production will be discontinued during the 2020s.Unfortunately, FBC ash is still largely unutilized at the moment and is mainly dumped in landfills.

The general aim of this thesis was to generate information which could potentially improve theutilization of FBC ash by alkali activation. The specific objective was to produce geopolymeraggregates by means of a simultaneous alkali activation-granulation process.

It was shown that geopolymer aggregates with physical properties comparable to commerciallightweight expanded clay aggregates (LECAs) can be produced from FBC fly ash containingheavy metals. Although the ashes were largely unreactive and no new crystalline phases wereformed by alkali activation, a new amorphous phase was observed in the XRD patterns, possiblyrepresenting micron-sized calcium aluminate silicate hydrate-type gels.

The heavy metal immobilization efficiency of alkali activation varied with the type of fly ash.Good stabilization was generally obtained for cationic metals such as Ba, Pb and Zn, but incommon with the results obtained with alkali activation of coal fly ash, anionic metals becameleachable after alkali activation. The efficiency of immobilization depended on the physical andchemical properties of the fly ash and was not related to the total content of the element.

All the geopolymer aggregates met the criteria for a lightweight aggregate (LWA) as definedby EN standard 13055-1. Their strength depended on the reactivity and particle size distributionof the fly ash. Mortars and concretes prepared with such geopolymer aggregates had highermechanical strength, higher dynamic modulus of elasticity and higher density than concreteproduced with commercial LECA, while exhibiting similar rheology and workability.

Keywords: fluidized bed combustion, fly ash, geopolymers, granulation, heavy metals,immobilization, stabilization

Yliniemi, Juho, Leijupetipolton lentotuhkien alkali-aktivointi ja rakeistus. Oulun yliopiston tutkijakoulu; Oulun yliopisto, Teknillinen tiedekuntaActa Univ. Oul. C 616, 2017Oulun yliopisto, PL 8000, 90014 Oulun yliopisto

Tiivistelmä

Biopolttoaineet, esimerkiksi puu, ovat ympäristöystävällinen vaihtoehto kivihiilelle, koska nesitovat hiilidioksidia kasvaessaan. Suomessa biopolttoaineita poltetaan tyypillisesti turpeenkanssa, ja nykyään myös jätteen hyödyntäminen polttoaineena on yleistynyt. Puu, turve ja jäte-polttoaineet poltetaan tyypillisesti leijupetipoltto-tekniikalla.

Tuhka on polton epäorgaaninen, palamaton jäännös. Puun ja turpeen tuhkaa tuotetaan Suo-messa 600 000 tonnia vuodessa ja määrän odotetaan kasvavan, sillä kivihiilen poltto lopetetaan2020-luvulla. Leijupetipolton tuhkaa ei tällä hetkellä juurikaan hyödynnetä ja tuhka päätyykinpääasiassa kaatopaikoille.

Tämän tutkielman päämääränä oli tuottaa tietoa, joka parantaisi leijupetipolton tuhkien hyö-dyntämistä alkali-aktivaatiolla. Erityisesti tavoitteena oli valmistaa geopolymeeriaggregaattejayhtäaikaisella alkali-aktivaatiolla ja rakeistuksella.

Tutkielmassa osoitettiin, että raskasmetalleja sisältävistä tuhkista valmistettujen geopolymee-riaggregaattien fysikaaliset ominaisuudet ovat vertailukelpoiset kaupallisten kevytsora-aggre-gaattien (LECA) kanssa. Vaikka tuhkien reaktiivisuus oli matala, ja uusia kidefaaseja ei muodos-tunut alkaliaktivaatiolla, uusi amorfinen faasi havaittiin XRD-mittauksissa. Uusi amorfinen faa-si oli mahdollisesti mikrometrikokoluokan kalsium-aluminaatti-silikaatti-hydraatti-tyyppinenrakenne.

Raskasmetallien stabiloinnin tehokkuus vaihteli tuhkien välillä. Kationiset metallit, kutenbarium, lyijy ja sinkki, stabiloituivat pääasiassa hyvin, mutta anionisten metallin liukoisuus kas-voi alkali-aktivoinnin myötä. Stabiloinnin tehokkuus riippui tuhkien fysikaalisista ja kemiallisis-ta ominaisuuksista, mutta raskasmetallin kokonaispitoisuudella ei ollu vaikutusta.

Kaikki geopolymeeriaggregaatit olivat kevytsora-aggregaatteja standardin EN 13055-1mukaisesti. Aggregaattien lujuus riippui tuhkan reaktiivisuudesta ja partikkelikokojakaumasta.Geopolymeeriaggregaateilla valmistettujen laastien ja betonien mekaaninen lujuus, Younginmoduuli ja tiheys olivat korkeampia kuin kaupallisella kevytsora-aggregaateilla valmistetut,vaikka niiden reologia ja työstettävyys olivat samanlaisia.

Asiasanat: geopolymeerit, immobilisaatio, leijupetipoltto, lentotuhka, rakeistus,raskasmetallit, stabilointi

Nothing lasts but nothing is lost

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Acknowledgements

The research work reported in this thesis was carried out in the Fibre and Particle Engineering Research Unit at the University of Oulu during the years 2013-2016. The work formed part of the GEOPO Project, supported by the Finnish Funding Agency for Technology and Innovation (Tekes) and various companies (Boliden Harjavalta, Ekokem, Fortum, Helen Oy, Jyvaskylan Energia, Kemira, Valmet Power, Metsa Group, Rovaniemen Energia, Stora Enso and UPM) and of the GEOSULF ERA-MIN Project, which is supported by Tekes, the Portuguese National Funding Agency for Science, Research and Technology (FCT), the National Centre for Research and Development (BR), and companies (Outotec, Agnico Eagle, and First Quantum Minerals). The author would like to thank the Renlund Foundation, the Tauno Tönning Foundation and the Riitta and Jorma J. Takanen Foundation for their financial support.

I would like to thank my supervisors, Rector Jouko Niinimäki, Professor Mirja Illikainen, and Docent Minna Tiainen, for their guidance throughout the course of this work. Rector Jouko Niinimäki was the original main supervisor of the thesis and is greatly gratituded for his guidance and for giving the opportunity to work in the field of circular economy. Professor Mirja Illikainen continued as main supervisor and always offered the best possible help and guidance during the thesis project. Docent Minna Tiainen was co-supervisor and the person who first introduced me to the topic of fly ashes and geopolymers during my master’s degree studies.

Dr. Henk Nugteren is gratituted for his help and support during my visit at the Delft University of Technology. Professor Victor Ferreira and Dr. Helena Paiva are gratituted for their guidance with mortars and concretes during the visit at the University of Aveiro. Professor Konstantinos Komnitsas and Professor Raffaele Cioffi are acknowledged for their efforts in reviewing this thesis.

I would like to thank all the people at the Fibre and Particle Engineering Research Unit. It is truly a pleasure to work with you and the amount of support and encouragement has always been immense in and outside of this work. Special thanks for Dr. Paivo Kinnunen for numerous interesting discussions related to geopolymers. I wish to thank all my co-authors and collegues in the research projects, especially Janne Pesonen and Pekka Tanskanen.

Finally, my deepest and warmest gratitude go to my family for their support and encouragement over the years. Most of all, this thesis is devoted to Elisa for her endless love, support and understanding.

18.4.2017 Juho Yliniemi

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Abbreviations

BET Braun-Emmett-Teller

BSE Back-Scattered Electron

CASH Calcium Aluminate Silicate Hydrate

CSH Calcium Silicate Hydrate

EDS Energy Dispersive Spectrometer

ESP Electrostatic Precipitators

BFS Ground Granulated Blast Furnace Slag

FA Fly Ash

FBC Fluidized Bed Combustion

FESEM Field Emission Scanning Electron Microscope

ICP-OES Inductively Coupled Plasma Optical Emission Spectrometry

ITZ Interfacial Transition zone

K-Sil Potassium Silicate

LECA Lightweight Expanded Clay Aggregate

LOI Loss-on-Ignition

L/S Liquid-to-Solid ratio

LWA Lightweight Aggregate

NASH Sodium Aluminate Silicate Hydrate

Na-Sil Sodium Silicate

PCC Pulverized Coal Combustion

PUNDIT Portable Ultrasonic Nondestructive Digital Indicating Tester

REF Recovered Fuel

SE Secondary Electron

SRF Solid Recovered Fuel

S/S Stabilization/Solidification

XRD X-Ray Diffraction

XRF X-Ray Fluorescence

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Original Papers

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

the text by their Roman numerals:

I Yliniemi J, Nugteren H, Illikainen M, Tiainen M, Weststrate R, Niinimäki J (2016) Lightweight aggregates produced by granulation of peat-wood fly ash with alkali activator. International Journal of Mineral Processing. 149: 42-49

II Yliniemi J, Tiainen M, Illikainen M. (2016) Microstructure and physical properties of lightweight aggregates produced by alkali activation-high shear granulation of FBC recovered fuel-biofuel fly ash. Waste and Biomass Valorization. 7:1235–1244

III Yliniemi J, Paiva H, Ferreira V, Tiainen M, Illikainen M. (2017) Development and incorporation of lightweight waste-based geopolymer aggregates in mortars and concretes. Construction and Building Materials 131: 784-792.

IV Yliniemi J, Pesonen J, Tiainen M, Illikainen M (2015) Alkali activation of recovered fuel–biofuel fly ash from fluidised-bed combustion: Stabilisation/solidification of heavy metals. Waste Management. 43: 273-282.

V Yliniemi J, Pesonen J, Tanskanen P, Peltosaari O, Tiainen M, Nugteren H, Illikainen M (2016) Alkali activation–granulation of hazardous fluidized bed combustion fly ashes. Waste and Biomass Valorization.8: 339-348.

The author of this thesis was the primary author of papers I-V and wrote the first

draft of all the publications, was principally responsible for designing and

conducting all the related experiments and analysing the results, with help from the

other authors. Henk Nugteren and Rick Weststrate helped in designing and

conducting the granulation experiments in Paper I, Helena Paiva and Victor

Ferreira contributed to the designing and preparing of the mortars and concretes for

Paper III, Janne Pesonen contributed to conducting the leaching and sequential

leaching experiments in Papers IV and V, and Pekka Tanskanen and Olli Peltosaari

conducted the XRD analysis in Paper V.

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Contents

Abstract

Tiivistelmä

Acknowledgements 9 Abbreviations 11 Original Papers 13 Contents 15 1 Introduction and aims 17

1.1 Background ............................................................................................. 17 1.2 Approach and aims of the thesis ............................................................. 18 1.3 Outline of the thesis ................................................................................ 19

2 Review of the literature 21 2.1 Fluidized bed combustion technology..................................................... 21

2.1.1 Properties of FBC fly ash ............................................................. 22 2.2 Geopolymers and alkali-activated materials ........................................... 25

2.2.1 Geopolymer synthesis .................................................................. 25 2.2.2 Geopolymer raw materials ........................................................... 26 2.2.3 Heavy metal stabilization in geopolymers .................................... 27 2.2.4 Leaching tests ............................................................................... 29

2.3 Granulation ............................................................................................. 30 2.3.1 Factors affecting granulation ........................................................ 30 2.3.2 The high-shear granulator ............................................................. 32

2.4 Lightweight aggregates ........................................................................... 33 2.5 Lightweight mortar and concrete ............................................................ 33

3 Materials and methods 35 3.1 Materials ................................................................................................. 35

3.1.1 FBC FA samples ........................................................................... 35 3.1.2 Co-binders .................................................................................... 36 3.1.3 Activators ..................................................................................... 37

3.2 Methods ................................................................................................... 37 3.2.1 Characterisation of the materials .................................................. 37 3.2.2 Leaching tests ............................................................................... 38 3.2.3 Granulation ................................................................................... 39 3.2.4 Physical characterisation of the geopolymer LWAs ..................... 40 3.2.5 Preparation of lightweight mortars and concrete .......................... 41 3.2.6 Mortar and concrete properties ..................................................... 42

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4 Results and discussion 43 4.1 Chemical and microstructural properties of the fly ash samples ............. 43 4.2 Granulation .............................................................................................. 46

4.2.1 Liquid-solid-ratio .......................................................................... 46 4.2.2 Granule growth ............................................................................. 48 4.2.3 Microstructure of the geopolymer aggregates .............................. 51

4.3 Geopolymer aggregates ........................................................................... 52 4.3.1 Geopolymer matrix of the aggregates ........................................... 53 4.3.2 Physical properties of geopolymer aggregates ............................. 57

4.4 Lightweight mortars and concretes ......................................................... 63 4.4.1 Physical properties of geopolymer LWA mortars and

concretes ....................................................................................... 63 4.4.2 Microstructure of the geopolymer LWA mortars .......................... 65

4.5 Stabilization-solidification of heavy metals ............................................ 66 4.5.1 Leachable hazardous components ................................................ 67 4.5.2 Conclusions regarding stabilization-solidification ....................... 81

5 Summary and concluding remarks 85 References 87 Appendix 99 Original Papers 101

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1 Introduction and aims

1.1 Background

The European Union has set targets of 20% reduction in greenhouse gas emissions,

20% increase in the use of renewable energy sources and 20% improvement in

energy efficiency by 2020 relative to the levels in 1990 [1]. In Finland the

renewable energy source target has been set as high as 38% and the government

has recently set a new target of becoming carbon neutral by 2030 [2]. This means

that the use of coal for energy production will be discontinued during the 2020s.

Biomass, such as wood, binds CO2 as it grows, and is thus considered an

environmentally friendly alternative fuel to replace coal, while municipal waste is

becoming more common as a fuel for power plants. Fluidized bed combustion

(FBC) is the most competent technique to burning these fuels, which may fluctuate

greatly in quality, composition and moisture content.

Ash is the inorganic, incombustible residue resulting from combustion. As

enormous amounts of solid fuels are consumed, an enormous amount of ash is

produced. The annual production of peat-wood ash alone in Finland is 600 000

tonnes [3].

FBC ash can be divided into two categories. Bottom ash remains in the bottom

of the FBC boiler and consists of large, dense ash particles and sand bed particles,

while the smaller and lighter ash particles that are driven out of the boiler with the

flue gases are referred to as fly ash (FA). Electrostatic precipitators (ESPs) are

typically used to separate FA from the flue gases so that it is not released into the

environment. Bottom ash and FA differ significantly in their chemical, physical and

mineralogical composition. This thesis focused entirely on FA.

Several targets for the utilization of peat and wood-based FA have been

proposed. The use of FBC FA as a cement additive, in the same manner as coal FA,

would consume large quantities of the material, but the strict regulations[4] for the

chemical composition of concrete [4] largely inhibits its use. The most critical

factor is the typical high CaO content of biomass FA, while the reactivity of

biomass ash is lower than that of coal FA and its irregular particle shapes and

organic matter content can affect the workability of the cement [5].

One interesting option would be to use biomass FA as a forest fertilizer in view

of its nutrient content in terms of Mg, K and P [6–8]. The main problem with this

approach is the varying chemical composition of FA batches and the lack of

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nitrogen (a critical fertilizing component) in ash. Also, the heavy metal content of

FBC FA means that only a small proportion of it could be used in this way.

Briefly, the heterogeneous chemical and physical nature and heavy metal

content of FBC FA, together with the lack of relevant research, has led to a situation

in which most of it is still dumped in landfills, causing expenses for the industry

and entailing potential environmental hazards. New solutions are needed to

overcome these problems.

Geopolymerization, which involves alkali activation, is a promising possibility

for utilizing a wide variety of waste materials [9]. In alkali activation the silicates

and aluminates of the raw materials are dissolved in a high pH solution, whereupon

they reorganize and harden, forming solid material. Geopolymers can be described

as concrete or ceramic-type materials and could partially replace concrete as a

building material.

Coal FA has been under intensive research as a raw material for geopolymers

for decades, with excellent results [9], but unfortunately, less attention has been

paid to the geopolymerization of FBC FA.

Additionally, it has been found that geopolymers can immobilize heavy metals

[10], but although promising results have been obtained with metakaolin and coal

FA for certain heavy metals, it is not known how well the heavy metals present in

FBC FA can be immobilized in geopolymers.

A new possible option for utilizing FBC FA would thus be to prepare

geopolymer lightweight aggregates (LWAs), which is the main aim of the present

thesis. Strength is a less important factor for this kind of material, as LWAs are

commonly used in earthworks and as aggregates in mortars and concretes. Since

some forms of FBC fly ash contain heavy metals, it is important to study their

stabilisation in the structure of geopolymer aggregates.

1.2 Approach and aims of the thesis

The general aim of this thesis was to generate information that would improve the

utilization potential of FBC ash by means of alkali activation, leading to the

production of geopolymer granules that could be used as lightweight aggregates.

The specific objectives were the following:

1. to find suitable operative parameters for producing geopolymer granules from

various FBC ashes,

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2. to understand the physical and chemical properties of FBC ashes in relation to

the physical properties of geopolymer granules,

3. to evaluate the potential for using geopolymer aggregates in mortars and

concretes, and

4. to gain an understanding of the mechanisms by which heavy metals are

immobilized inside geopolymer aggregates.

The granulation and alkali activation processes were studied in Paper I, and the

same methodology was then used in the subsequent publications. Paper II

examined the physical properties and microstructure of geopolymer LWAs, while

Paper III considered their utilization in mortars and concretes. The potential

immobilization of heavy metals by means of alkali activation was studied in Papers

IV and V.

1.3 Outline of the thesis

This thesis is organized into five chapters. Chapter 1 gives an introduction to the

topic and states the aims of the research. Chapter 2 describes the formation of FBC

fly ash, the synthesis process involved in alkali activation, the stabilization of heavy

metals in geopolymers, the granulation process and the use of lightweight

aggregates in mortars and concretes. Chapter 3 presents the materials, experimental

setups and analytical methods used in the research, while Chapter 4 presents and

discusses the results. The conclusions to be drawn from these results are given in

Chapter 5.

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2 Review of the literature

2.1 Fluidized bed combustion technology

Fluidized bed combustion (FBC) is a commonly used method for producing energy

in heat and power plants, as it allows the combustion of fuels of fluctuating quality,

composition and moisture content such as biomass and waste at reasonably low

capital and operating costs [11–13].

Inside the FBC boiler is a sand bed floating on a forced high velocity air flow

(Fig. 1). This fluidized bed ensures a balanced combustion of the varying fuels at a

relatively low operating temperature of 700-900°C [14]. By contrast, the fuel in

typical pulverized coal combustion (PCC) boilers must be ground to a fine powder

and fired at a temperature of 1300-1700°C. A third common combustion

technology is grate firing, which is used mainly for municipal waste incineration.

The combustion temperature is the most critical factor determining the properties

of the ash, as discussed below.

Fig. 1. Schematic illustration of a fluidized bed boiler.

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2.1.1 Properties of FBC fly ash

The chemical and physical properties of FBC FA are determined by the fuel

properties and combustion conditions. Due to the high variation in the chemical

composition of the fuel in particular, the properties of the ash can vary significantly.

Chemical composition of FBC fly ash

Biomass combustion transforms plant tissues such as cellulose, hemicellulose,

lignin etc. into CO and CO2 gases in an exothermic reaction, while all the non-

volatile components that are left after the combustion process may be regarded as

ash. The main ash-forming elements are Ca, Na, K, Si, Al, Mg, Fe, P and Ti. The

content of S, N and Cl in biomass typically remains below 1 wt% [15].

Every fuel type has a unique chemical composition, which has an effect on the

formation of ash during combustion. Wood typically has a high alkali and alkaline

earth metal content, whereas peat has more silicon, aluminium, calcium and iron.

Wood and peat are nevertheless highly heterogeneous materials, so that factors such

as growth location, harvesting time and parts of the wood (pure wood, leaves,

branches or bark) yield ashes of quite different elemental compositions [16]. Bark,

for example, contains a lot more alkali metals than inner wood material.

As for other biomass ashes, rice husk ash typically contains up to 90 wt% of

SiO2 and few percent of K2O, but negligible amounts of other elements [17,18],

while corn cob ash typically has 60-70 wt% of SiO2, 3-10 wt% of Al2O3, ~10 wt%

of CaO and less than 5 wt% of iron, magnesium and potassium [19,20].

PCC FA is less heterogeneous than biomass ash and may be divided into classes

F and C according to the ASTM standard [21]. Class F ash contains more than 70

wt% of SiO2+Al2O3+Fe2O3 and typically has less than 10 wt% of CaO, whereas the

combined amount of SiO2+Al2O3+Fe2O3 in Class C ash is 50-70 wt% but there may

be 20-40 wt% of CaO. The amount of sulphur is typically higher in coal ash than

in biomass ash.

The heavy metals present in a fuel become enriched in the ash. This is because

heavy metals are vaporized during combustion [22,23] and as the partly melted and

partly solid ash particles flow through the air the gas-phase heavy metals adsorb to

the surfaces of the particles [24,25]. The resulting heavy metal concentrations in

the ash depend on the chemical characteristics and vaporization points of the metal

compounds, and also on the operating conditions of the boilers (fuel properties, fuel

feeding rate, temperature) [22,26,27].

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Sometimes, biomass is incinerated together with waste fuels produced from

municipal or industrial waste, such as recovered fuel (REF) and Solid Recovered

Fuel (SRF) [28]. One general feature of waste-based fuels is that their heavy metal

content is often high [29,30], since heavy metals often originate from plastics,

paints and other materials present within the waste.

Physical properties and mineralogy of FBC fly ash

It is not just the chemical composition that can vary between the types of ash, but

the microstructure and mineralogy of the ash can also be affected. The high

combustion temperature of the PCC process as compared with FBC, combined with

fast cooling, results in a higher glassy phase, i.e. a higher amorphous content in

PCC ash than in FBC ash [31]. The ash may partly melt at a high temperature, and

upon rapid cooling it cannot form an organized crystalline structure but instead

solidifies with an amorphous structure. As FBC ash is rarely above its melting point

it cannot form amorphous material to the same extent as PCC ash, although

sintering is possible even at low combustion temperatures [32–35]. The amorphous

material in ash has a high chemical reactivity, i.e. pozzolanic reactivity [36].

Typical XRD patterns of PCC ash and FBC wood-peat ash are shown in Fig.

2. Coal ash has less crystalline signals and more amorphous material than peat-

wood ash. Amorphous material can be seen as a “hump” in the XRD pattern.

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Fig. 2. X-Ray diffractograms of coal FA and peat-wood ash. The crystalline nature of

peat-wood FA relative to coal ash can be seen as a lower amorphous “hump” between

20° and 30° 2 θ. The high proportion of crystalline phases in the FBC ash arises from

the low combustion temperature.

A clear difference in microstructure can also be observed between biomass FBC

ash and PCC ash (Fig. 3). Coal FA particles are spherical in shape, whereas wood-

peat FA consists of very irregular particles.

Fig. 3. Scanning electron microscope images of biomass FBC FA (left) and coal FA (right)

particles. The coal FA image is courtesy of Fortum Power and Heat Oy.

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2.2 Geopolymers and alkali-activated materials

Geopolymers, or alkali-activated materials, are inorganic polymeric-type materials

with a chemical structure similar to zeolites but possessing an amorphous physical

structure. Geopolymers can be described as concrete or ceramic-type materials and

can partly replace cement as a construction material.

Alkali activation has a long history. The first geopolymer patent was issued in

USA in 1895, to Whiting [37], while Glukhovsky investigated alkali-activated slag

binders in Ukraine in the 1960’s and 1970’s [38]. In Finland, Bob Talling invented

several formulae in the 1980’s using slags as precursors [39]. The term

‘geopolymer’ was introduced by Davidovits in the late 1970’s, in studies in which

he used metakaolin as a raw material [40].

It was only in the early 2000’s, however, that Shi et al. [41], van Deventer et al. [42], and Palomo et al. [43], to mention a just few, began to publish widely in

scientific journals, and since then a wide variety of raw materials have been studied

and many details of the reaction mechanisms have been revealed. Very valuable

books on geopolymers that illustrate the current state of the art have been compiled

by Provis et al. [44] and Pacheco-Torgal et al. [9].

The reason why geopolymers are such an interesting research topic is the

possibility for using aluminosilicate waste as raw material. In addition,

geopolymers have valuable properties such as high compressive strength, fire

resistance, light weight and the ability to stabilize heavy metals [44]. Geopolymers

can be produced in a more energy-efficient and environmentally friendly way than

can cement or ceramics, and it has been calculated that CO2 emissions could be

reduced by 40-80% if Ordinary Portland Cement (OPC) was replaced by

geopolymers [45].

2.2.1 Geopolymer synthesis

In alkali activation the silicate and aluminate units are dissolved from

aluminosilicate raw materials such as FA, slags, calcined clays or other natural

minerals by an alkaline solution, typically of sodium/potassium hydroxide or

sodium silicate. The reaction is carried out under mild temperature (<85°C). A

simplified model of the reaction was presented by Duxson et al. [46] (Fig. 4). It

involves the following steps:

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1. Dissolution: Aluminosilicate raw materials are dissolved by alkaline

hydrolysis and aluminate and silicate species are produced.

2. Speciation equilibrium: the formation of a supersaturated aluminosilicate

solution.

3. Gelation: oligomeric species start to form networks by condensation.

4. Reorganization and hardening: the connectivity of the gel network increases

and the three-dimensional structure starts to form.

5. Polymerization and hardening: This final hardening step can vary in time from

a few hours to several weeks depending on the raw material and the synthesis

parameters.

Fig. 4. Conceptual model of geopolymerization.

The reaction results in a 3-dimensional zeolite-type structure in which the negative

charge of the aluminate units is balanced by the alkali metal, Na+ or K+. Water

which is released during the gelation process plays the role of a reaction medium

and resides in the pores of the gel [46].

Calcium, magnesium and iron can also participate in the reaction if they are

present [47–50], which makes the overall reaction very complicated when

chemically heterogeneous raw materials are used. In addition, the temperature, the

amount of water, the mixing method and the physical properties of the raw

materials (particle size, shape and surface area) can all significantly affect the

synthesis.

2.2.2 Geopolymer raw materials

Probably the most intensively studied raw material for geopolymers synthesis is

metakaolin, which is produced by the calcination of kaolin at 750°C. The combined

proportion of silicate and aluminate in metakaolin, which is completely amorphous

and thus an ideal precursor for geopolymers, is usually more than 90 wt%.

27

Although calcination as such is an energy-intensive process, which increases the

cost of the end products, metakaolin is still a more energy-efficient and

environmentally friendly raw material than cement, because cement is

manufactured at around 1400°C and its production generates high CO2 emissions

[51,52].

Another widely studied geopolymer raw material is ground granulated blast

furnace slag (BFS), which is a by-product of the steel industry. The main

components of BFS are calcium and silicon, with lower amounts of Al and Mg.

Like metakaolin, BFS is also completely X-ray amorphous and thus highly reactive

in alkali activation. Incidentally, BFS is used also as a cement additive. The phases

formed by the alkali activation of BFS are known as calcium-aluminium-silicon

hydrates, or CASH phases [48].

Another industrial by-product that is used for geopolymers is coal FA, some

750 million tonnes of which is produced globally per year [53]. The mineralogy of

coal FA is mostly X-ray amorphous and often the only crystalline phase found in it

is mullite (Fig. 2). Coal FA has been under intense research as a geopolymer raw

material, especially in Australia and Spain, and there is an abundance of scientific

literature on it.

Several authors [54–57] have conducted geopolymerization studies with

waste-based ashes from municipal solid waste incineration (MSWI) plants. MSWI

ash is typically mixed with a co-binder (metakaolin or BFS, for example) and then

alkali-activated. The results have been variable and research has been concentrated

mostly on the heavy metal stabilization potential of the process (see below).

Much less research has been devoted to FBC FA, and in most cases the fuel

has been coal [58–61]. Some authors [62,63] have used FBC rice husk FA as a

geopolymer raw material with promising results (although they also used OPC, coal

FA or red mud as a co-binder).

As explained in Section 2.1.1, the chemical composition of ash varies greatly,

so that each ash type must be studied separately in order to prepare durable

geopolymers. To the best of my knowledge, the only studies prior to the present

work (Papers I-V) that have used biomass or peat-based FBC FA as a geopolymer

precursor have been those conducted by Tyni et al. [60,64] and Pesonen et al. [65].

2.2.3 Heavy metal stabilization in geopolymers

One of the most interesting aspects of geopolymers is their ability to immobilize,

i.e. stabilize/solidify, heavy metals. In general, stabilization/solidification (S/S)

28

involves the mixing of waste with a binder to convert it into a monolithic solid and

reduce the likelihood of hazardous components being released into the environment

[66].

S/S mechanisms can be divided into two categories, physical encapsulation and

chemical stabilization, although a clear-cut separation between these mechanisms

would not be meaningful, as they can work in conjunction [67] and it is often

difficult to say into which category the immobilization belongs.

Physical encapsulation occurs on a micron scale. The zeolite-type cage

structures can trap some metals if they are of suitable size, but another type of

physical encapsulation can take place when a physical barrier prevents the leaching

medium (water) from contacting the metals, thus preventing leaching.

Chemical stabilization occurs on a nano-scale. The heavy metal can react with

other reactive compounds in the mixture and become part of the aluminosilicate

structure, e.g. by replacing the silicon atoms [68]. The removal of metals from

wastewater by sorption has also been shown to be effective [69,70], since it is based

on the ion-exchange capacity of a geopolymer. Heavy metals could also be

precipitated as non-soluble compounds (e.g. hydroxides) [66] and become trapped

inside the geopolymer.

The stabilization of waste by geopolymerization can be regarded as a technique

whereby the waste particle itself is part of the formatting binder. The surface

reactivity of a waste particle is the key parameter determining the success of S/S,

as it controls binding to the aluminosilicate matrix and the release of hazardous

components. Chemical properties, mineralogy, particle size, particle shape and the

surface area of waste particles have a significant effect on their surface reactivity

[71].

A considerable amount of research into the immobilization of heavy metals in

geopolymers [54,55,72–81] has been done with materials such as metakaolin,

cement, MSWI FA, coal FA and BFS, but prior to this thesis (Papers IV and V)

there had been no studies in which the heavy metals in wood, peat or waste-based

FBC FA had been immobilized by geopolymerization.

One of the most frequently used raw materials mentioned in the literature is

metakaolin, to which heavy metals are added as pure chemical reagents. These

modelling studies have made possible to define immobilization efficiency

accurately and to investigate the mechanism by which immobilization takes place.

In real-life situations, however, there is more than one heavy metal present and

several species can exist simultaneously. This makes determination of the actual

chemistry lying behind immobilization very challenging.

29

In general, cationic species in geopolymers are more efficiently immobilized

than anionic species [66], while transition metals which form oxyanionic species at

a high pH are particularly difficult to immobilize. Some of the results are

contradictory, but it is reasonable to assume that different raw materials lead to

different outcomes, as the metals can be present in different forms. On the other

hand, the experimental setup can also have a major impact on the results, especially

the question of how the efficiency of immobilization is determined.

2.2.4 Leaching tests

Leaching tests are the most effective way of evaluating the efficiency of S/S. Abora

et al. [82] defined leaching as “a process by which inorganic, organic contaminants

or radionuclides are released from a solid phase into surrounding water under the

influence of mineral dissolution, sorption/desorption equilibria, complexation, the

pH redox environment, dissolved organic matter and/or biological activity.” The

definition itself displays the complex nature of the leaching process, since water

will leach elements from its surface or its interior of any material that it is in contact

with if that material is porous.

Leaching tests can yield different S/S results [83], since the pH of the leaching

solution in particular affects the solubility of the components and their precipitation

as hydroxides. Also, the L/S-ratio, particle size and surface area can have major

impact on leaching.

There are several protocols for modelling real-life conditions in order to

evaluate the environmental risks associated with the use of waste materials. The

leaching tests that are commonly used for evaluating the S/S of heavy metals by

alkali activation are the Toxicity Characteristic Leaching Procedure (TCLP) [72–

75,79,81,84], standard NEN 7341 [85–87] and standard NEN 7345 [55,78,88,89].

The extractant in these tests is water at an acidic pH.

The standard leaching test in Finland is EN 12457-3 [90] which is used as a

quality-control test for fly and bottom ash intended for construction use [91] and

for evaluating its suitability for landfill disposal [92]. This is a two-step extraction

method that uses water as the extraction fluid, but the pH is not controlled.

The sequential leaching test (or sequential extraction) is a non-standard test

originally developed for the fractioning of elements in sediments [93,94]. Here, a

set of chemical reagents are applied to the sample in series and each leaching step

is more drastic than the previous one. There are different versions of the sequential

leaching test, but the one used for fractioning elements in ash [95,96] is a four-step

30

version in which the leachable elements are extracted into four fractions: 1) a water-

soluble fraction, 2) an acid-soluble fraction, 3) an easily reducible fraction and 4)

an oxidizable fraction. In theory it is the water-soluble ions that are extracted in the

first step, metals bonded with weak covalent forces, electrostatically, or to

carbonates in the second step, metals bonded to Fe and Mn oxides in the third step

and metals bonded to organic matter or various sulphides and oxides in the fourth

step. As was stated earlier, the sequential leaching test was originally developed for

the fractioning of sediments, and thus interpretation of the results when it is applied

to FA is not as straightforward, as the bonding of metals in FA differs from that in

sediments. For example, if FA contains a lot of CaO it has a very high pH and the

acetic acid used in the second step of the test may not be strong enough to acidify

the sample. This would prevent the extraction of carbonates even though they might

be present in the FA. Rather than demonstrating the exact bonding mechanisms of

metals in FA, the sequential leaching test should be regarded as a way of generating

useful information about the bio-availability of the metals.

2.3 Granulation

Granulation is a general term for particle size enlargement. A typical raw material

for granulation would be a micron-size powder, which can be formed into granules

a few millimetres in diameter. The pharmaceuticals, food, fertilizer, detergent,

metal ore and plastics industries commonly use granulation technology. There are

a number of reasons why granules may be preferred over a powder [97]: 1)

improved flow properties, 2) prevention of segregation, 3) increased bulk density,

4) improved dusting behaviour, 5) production of mixtures with an uniform

composition, 6) improved product appearance, 7) improved solubility and 8)

improved performance.

There is some inconsistency in the nomenclature in the literature due to the

widespread use of granulation in various industrial sectors [98], but in this thesis

granulation will be taken to imply agglomeration in the presence of a liquid in a high-shear granulator.

2.3.1 Factors affecting granulation

There are several factors which affect the granulation process [99–101]. The initial

particle size distribution is one of the most important properties of the raw material

[102]. If there are particles of several sizes present, granules will be more compact,

31

as the small particles can fill the holes between larger particles. Particle shape

affects how close particles can come to each other, in that it is easier to pack the

particles densely if they are spherical, for instance, and the density and hardness of

the particles will also affect their nucleation, coalescence and breakage.

The impeller speed, granulation time and the amount and viscosity of the

binder liquid have also been found to have a significant effect on the granulation

process [99].

The binder liquid can be added as a spray or in drops, and once it is added its

small drops start to form bridges between the particles (Fig. 5). As more liquid is

added, more particles are bound together. The optimal result is a spherical, surface-

dry granule with no liquid left over inside it.

Fig. 5. Mechanism of high-shear granulation.

In addition to the surface tension of the binder liquid, particles can be bound

together by several other mechanisms: mechanical locking, electrostatic forces, or

the formation of solid bridges between them. Bonds formed by electrostatic forces

are much weaker than those in which a solid bridge has formed [103]. Either strong

or weak granules may be preferred, depending on the application and field of

industry concerned.

All these factors make granulation a sensitive process, and it was for a long

time regarded more as an art than a science [104]. A professional granulation expert

can determine just by looking at and touching the material whether more liquid is

needed, for example, or whether a lower drum speed is required. Experience with

regard to how the material will behave during granulation is the most important

aspect for achieving a successful batch of granules.

32

2.3.2 The high-shear granulator

A schematic diagram of a high-shear granulator similar to the ones used in this

thesis is shown in Fig. 6. The material is granulated in a rotating drum which has a

high-speed impeller inside it which rotates in the opposite direction. The speed of

the drum is typically less than 100 rpm, but the speed of the impeller can be several

thousands of rpm. Thus, the high-shear granulation process is very intensive and

the particles collide with each other at very high speeds.

Fig. 6. Schematic illustration of a high-shear granulator. The impeller inside spins in the

opposite direction to that of the drum. The binder is added by drops from a flask through

a tube on to the powder bed.

33

2.4 Lightweight aggregates

Lightweight aggregates (LWAs) are granular materials which have a loose bulk

density of less than 1.2 cm/g3 or a particle density of less than 2.0 cm/g3 [105].

Other important physical properties are their porosity, water absorption and

strength.

Some natural LWAs such as pumice, scoria and tuff have been used as concrete

aggregates [106], but there is also a need for artificial LWAs due to the increasing

demand for and non-availability of natural ones.

The most common method for producing artificial LWAs is the sintering of

clay at 1200-1400°C in a rotary kiln. This causes the clay to melt and expand and

results in porous granules commonly known as Lightweight Expanded Clay

Aggregate (LECA).

The mining of natural raw materials is expensive and inevitably causes

environmental burdens, so that various alternative raw materials have been

investigated, including FA, slags and other waste materials [107–109]. This

approach nevertheless has one important disadvantage: even if waste materials are

used, high-temperature sintering is itself energy-intensive and thus expensive. Thus,

the search goes on for alternative methods of manufacturing LWAs.

One option is the granulation of waste material together with cement in a

granulator (at room temperature), and promising results have been obtained with a

pelletizing disc [110–114], but even so, the use of cement as a binder is undesirable

due to the high CO2 emissions from its manufacturing process.

One interesting method of bypassing the high-temperature sintering and the

use of cement is to granulate waste material with an alkali activator. So far, there

has been only one study using this approach [115], in which BFS, rice husk ash and

coal FA were together granulated with an alkali activator in a pelletizing disc. In

the present work (Papers I-V) a high-shear granulator was used, which has an

impeller inside, thus employing a different granulation mechanism.

2.5 Lightweight mortar and concrete

The use of LWAs in mortars and concretes is steadily increasing as they have better

properties than normal weight aggregates, including reduced dead weight, higher

insulating coefficients and superior sound-dampening qualities [116].

As LWAs may differ in density and water absorption, the rheology of the

cement mixture, i.e. its workability, may alter. The properties of the LWAs also

34

affect those of the hardened mortars and concretes (such as mechanical strength

and capillarity).

Depending on the binding between the aggregate and the binder, the weakest

aspect of the concrete may be either the aggregate itself or the interfacial transition

zone (ITZ) [117,118]. Soluble components of the aggregates such as silicates,

aluminates and alkalis are especially likely to modify the ITZ and the surrounding

binder gel. The use of geopolymer aggregates in mortars and concretes was studied

in Paper III.

In addition to mortars and concretes, LWAs can be used in earthworks as a road

base material (to prevent frost) and as a flower bed aggregate, as they lower the

density of the soil, thus ensuring better root growth.

35

3 Materials and methods

3.1 Materials

This chapter presents materials and methods used in the thesis work.

3.1.1 FBC FA samples

A total of 8 FBC FA samples were used in the experiments, as presented in Table 1. All

of these were collected from heat and power plants using either a circulating or a

bubbling fluidized bed boiler. The samples were collected from ESPs or from FA silos

into 10-litre containers. FBC boilers may contain several ESPs for flue gases, the first

being labelled here as “ESP-A” and the last as “ESP-C”. The first ESP typically collects

the largest fly ash particles and the last one the smallest. No specific sampling

information is available as the sampling was done by employees of the power plants.

Table 1. FBC fly ash sample numbering, the papers in which the samples were used,

and the fuel mixture for each sample.

FBC FA # #1 #2 #3 #4 #5 #6 #7 #8

Paper I II & IV V V V V III III

Fuel 70% Peat,

30% Wood

50% REF-

50% Biofuel

100%

Peat

50% Forest

residuals,

20% Peat,

20% Bark,

10%

Biosludge

54%

Sludge,

42%

Wood, 4%

Rejects

75% Peat,

25%

Wood

70% Peat,

30%

Wood

70%

Wood,

30% Peat

Sample

collection

ESP-A ESP-all ESP-C ESP-all ESP-all ESP-C ESP-A ESP-A

The fuel mixture used for each FBC boiler is shown on the third row, but the fuel

percentage reported is only indicative and should not be taken as an exact value,

because the volumes in which the fuels are loaded into the boilers are huge and it

is not possible to know the exact fuel ratios at each moment in time.

Recovered fuel (REF) includes packing-material waste such as plastic (not

PVC), cartons, paper and wood collected from industrial and retail suppliers and

was of quality class 1 according to the SFS EN 15359 standard [28]. The biofuel

used with the REF was mainly tree bark.

36

3.1.2 Co-binders

Two metakaolins (Engelhard and Sigma Aldrich) and BFS from two sources

(Finnsementti and ORCEM) were used as co-binders. Coal FA from a PCC plant

was used in Paper I.

Chemical compositions calculated in terms of oxides and particle size

distributions reported as volumetric-based amounts of the co-binders are presented

in Table 2. Both BFS types consisted mainly of CaO and SiO2, but also had ~10

wt% of Al2O3 and MgO present. The combined amounts of SiO2 and Al2O3 in the

metakaolins were over 90 wt%. The coal FA also contained high amounts of SiO2

and Al2O3 and some CaO and Fe2O3.

Metakaolin 2 had by far the smallest particle size, while metakaolin 1, which

was prepared prior to the experiments by the calcination of kaolin, had a slightly

larger median size and a wider particle size distribution. The BFSs had similar

particle size distributions to metakaolin 2. Coal FA had a significantly larger

particle size than any of the other co-binders. CEM II/B-L 32.5 N cement

(CIMPOR InterCement) was used for all the mortar and concrete samples, with

siliceous sand (J.Soares & Filhos, Lda) as an additional aggregate.

Table 2. Chemical compositions and particle sizes of the co-binders.

Co-binder BFS 1 Coal FA Metakaolin 1 BFS 2 Metakaolin 2

Supplier ORCEM Pulverized fuel

power plant

Sigma- Aldrich Finn-

sementti

Engelhard

Paper(s) I I I II & IV II & IV

CaO 39.5 4.9 0.1 38.5 0.2

SiO2 34.5 54.3 59.5 32.3 52.5

Al2O3 9.9 22.9 32.8 9.6 43.5

Fe2O3 0.5 8.0 1.4 1.2 1.1

Na2O 0.4 1.1 0.1 0.5 0.3

K2O 0.3 1.7 0.6 0.5 0.2

MgO 8.1 1.8 0.1 10.2 0.0

P2O5 0.0 0.7 0.0 0.01 0.1

TiO2 1.1 1.2 1.9 2.2 1.8

SO3 3.4 0.8 0.0 4.0 0.0

Cl N/A N/A N/A 0.0 0.0

<10% (µm) 1.0 3.0 1.0 0.9 0.6

<50% (µm) 10.3 30.2 8.0 10.8 2.6

<90% (µm) 31.9 133.7 53.1 51.7 9.1

37

3.1.3 Activators

Potassium silicate (K-Sil), sodium silicate (Na-Sil) and sodium aluminate (Na-Alu)

solutions were used as alkali activators. The K-Sil solution was prepared by mixing

a commercial K-Sil solution (Kasil 2135) from PQ Europe with potassium

hydroxide to obtain SiO2/K2O=1.25 (65 wt% of water), the Na-Sil solution, Zeopol

25, with SiO2/Na2O=2.5 (approximately 66 wt% of water) was obtained from

Huber, and Na-Alu was prepared by mixing a Na-Alu solution from Sigma-Aldrich

with sodium hydroxide (NaOH) pellets to obtain Al2O3/Na2O=0.45 (60 wt% of

water).

3.2 Methods

3.2.1 Characterisation of the materials

The chemical compositions of the FA and co-binders were determined by X-ray

fluorescence (XRF) from a melt-fused tablet produced from 1.5 g of sample melted at

1150 °C with 7.5 g of X-ray Flux Type 66:34 (66% Li2B4O7 and 34% LiBO2).

The trace element concentrations in the FA were determined using an

inductively coupled, plasma-optical emission spectrometer (ICP-OES; Thermo

Electron IRIS Intrepid II XDL Duo, Thermo Scientific). Samples were pre-treated

by microwave-assisted wet digestion using a 3:1 mixture of HNO3 and HCl for 0.5

g of FA.

The selectively soluble contents of SiO2, Al2O3, CaO, and Fe2O3 were

determined as their solubility in a solution consisting of ethylenediaminetetraacetic

acid (EDTA, C10H14N2Na2O8*2H2O) and triethanolamine (TEA, C6H15NO3)

solutions with pH adjusted to 11.6 ± 0.1 by means of additions of 1 M NaOH. The

selectively soluble dissolution method is described in more detail in [119,120]. The

soluble content was determined using the ICP-OES technique.

Free calcium oxide content was determined by dissolving samples in a

specified mixture of butanoic acid, 3-oxo-ethyl ester and butan-2-ol for a 3 h

boiling time. The methodology is described in the EN 451-1 standard [121]. The

reactive CaO content of the FA was determined according to the EN 197-1 standard

[122] and the CaCO3 content by thermo-gravimetric analysis (PrepAsh, Precisa).

The particle size distributions of the FA and co-binders were measured with a

Beckman Coulter LS 13320 device and reported in terms of volumetric-based size

(d10, d50 and d90). The specific surface area was determined with an ASAP 2020

38

Micrometrics device and was based on the physical adsorption of gas molecules on

a solid surface. The results were reported in the form of a Braun-Emmett-Teller

(BET) isotherm. Moisture-% and loss-on-ignition (at 525°C and 950°C) were

determined by thermo-gravimetric analysis (PrepAsh, Precisa).

The crystalline phases of the FBC FA and powdered geopolymer aggregate

samples were identified with a Siemens 5000 X-ray diffractometer (XRD). The step

interval, integration time, and angle interval used were 0.04°, 4 s, and 10°–70° 2 θ,

respectively.

A field emission scanning electron microscope (FESEM, Zeiss Ultra Plus) was

used to analyse the microstructure of the FBC FA samples and the fractured

surfaces of the geopolymer aggregates by placing FA onto carbon tape and coating

it with carbon. The sample distance was 8.5 mm, and the acceleration voltage 15

kV. Cross-sections of the geopolymer aggregates and mortar samples were analysed

by FESEM (Zeiss Sigma and Zeiss Ultra Plus), the granules being mounted in

epoxy resin. After curing for 24 h, 2-mm slices were cut and placed in a 25-mm

(40-mm for mortar samples) diameter plastic mould. The slices were then mounted

in the epoxy resin and left to cure for 24 h. The hardened samples were ground,

polished and coated with carbon to obtain an optimal surface for FESEM analysis.

The sample distance was 5 mm or 8.5 mm and the acceleration voltage was 5 kV

for the Secondary Electron (SE) images and 15 kV for the Back-Scattering Electron

(BSE) images and the Energy Dispersive X-Ray Spectrometer (EDS).

3.2.2 Leaching tests

The water-leachable fractions of hazardous components were determined by the

two-step leaching method specified in the SFS-EN 12457-3 standard [90]. In the

first step the sample is mixed in an end-over-end tumbler for 6 h with a liquid/solid

(L/S) ratio of 2, after which it is mixed for 18 h with L/S = 8. Eluates from both

steps were analysed by the ICP-OES method and a cumulative release of L/S = 10

was calculated. Conductivity and pH of the leachate were measured with the

Denver Instrument model 50. Deviating from the standard, a sample size of 17.5 g

was used instead of 175 g, and the filter-paper retention size was 1 µm instead of

0.45 µm. Granule size fractions between 2 and 4 mm were used for the aggregate

analysis. Duplicate samples of FA and each granule batch were analysed and the

averages were calculated.

The sequential leaching test is a four-step procedure based on the process

outlined in [94] by which leachable elements are divided into four fractions: (1) a

39

water-soluble fraction leachable with H2O, (2) an acid-soluble fraction leachable

with CH3COOH, (3) an easily reduced fraction leachable with HONH3Cl, and (4)

an oxidizable fraction leachable with a mixture of H2O2 and CH3COONH4. An

exception was made here in the first step by adjusting the pH of the water to 4.0

with HNO3 as in [123,124] in order to mimic the leachability effect of acid rain in

the environment.

The reagents, mixing times and temperatures used in the procedure are

summarized in Table 3. The concentration of heavy metals in each leachate was

analysed with ICP-OES. Duplicate samples taken from the FA and each LWA batch

were analysed, and the average was calculated in each case.

Table 3. Sequential leaching test procedure.

Step Fraction Reagents/1 g of sample Mixing time and temperature

F1 Water-soluble 40 ml H2O (pH=4.0) 16 h at 22 °C

F2 Acid-soluble 40 ml 0.1 M CH3COOH 16 h at 22 °C

F3 Easily reduced 40 ml 0.1 M HONH3Cl 16 h at 22 °C

F4 Oxidizable 10 ml 30% H2O2 (evaporation) 1 h at 25 °C

10 ml 30% H2O2 (evaporation) 1h at 85 °C

50 ml 1 M CH3COONH4 16 h at 22 °C

3.2.3 Granulation

Granulation was carried out with Eirich high-shear granulators, since high-shear

granulators can spread viscous fluids (e.g. sodium silicate) and produce more

compact granules than low-shear granulators (e.g. disc pelletizers) [125]. Model

R02 was used in the first experiments (FA #1). The tilt angle of the drum was 33°

and the drum rotating speed was 44 rpm. Preliminary experiments were conducted

to determine a suitable L/S (w/w) window for the raw material mixes, the aim being

to define the optimal parameters for the granulation and alkali activation process

when using peat and wood FA with co-binders. The process was carried out by the

following method:

1. The dry raw materials were weighed, mixed and added to the drum.

2. The drum and the impeller were switched on.

3. Drops of liquid were added to the powder bed rotating inside the drum

4. The process was continued until the target size was achieved and the granules

stopped growing

40

5. Each batch was removed from the drum, sealed in an air-tight plastic bag and

stored at room temperature for later analysis of the granules.

The same procedure was carried out with the other FA samples using a model EL1

apparatus. This has a drum size of 5 litres, and a tilt angle of 10° and drum rotating

speeds of 45 rpm and 170 rpm were used. The impeller was 10 cm in diameter and

was spinning at 900 rpm or 1200 rpm in the opposite direction to the drum.

3.2.4 Physical characterisation of the geopolymer LWAs

The strength of the aggregates was measured by a method used previously for single

granule crushing tests [110,111]. The granule is placed in the steadiest possible position

between steel plates and loaded at a constant deformation rate until it breaks. The

measurements were carried out with Zwick Z100 Roellor Shimadzu AG-IC testing

machines at a compression speed of 0.01 mm/s. The measured granule size varied

between the experiments, as reported in the Results section.

The loose bulk density and size distribution of the aggregate were determined

according to the EN 1097-3 [126] and EN 933-1 standards, respectively [127], and

the particle densities and water absorption were analysed in accordance with the

EN 1097-6 standard [128]. These measures were defined as follows:

1. Loose bulk density, ρL

The density obtained when the mass of dry aggregate filling a specific container

without compaction is divided by the volume of the container.

2. Dry particle density, ρLrd

The ratio obtained by dividing the oven-dried mass of an aggregate sample by the

volume it occupies in water, including the volume of any internal sealed voids and

the volume of any water-accessible voids.

3. Apparent particle density, ρa

The ratio obtained by dividing the oven-dried mass of an aggregate sample by the

volume it occupies in water, including the volume of any internal sealed voids but

excluding the volume of water in any water-accessible voids.

4. Water absorption, W24h

The mass of the absorbed water expressed as a percentage of the oven-dried mass

of the aggregate sample.

41

3.2.5 Preparation of lightweight mortars and concrete

The mortar and concrete samples were prepared with geopolymer LWAs, using

LECA as a reference material. The aggregate diameter was 0-2 mm in for the

mortars and 2-8 mm for the concrete samples. The effect of the aggregate size

distribution was minimized by determining the size distribution of the geopolymer

aggregates and then mixing LECA filler (size 0-3 mm) and LECA 4-12.5 (size 4-

12.5 mm) to obtain a similar size distribution. In the case of the concrete samples

it was necessary to prepare two reference samples (C0a and C0b) due to variation

in the size distribution of the 2-8 mm fractions of the geopolymer aggregates. The

composition of the samples are presented Table 4.

The mortar samples were prepared by the following method:

1. Water, cement and sand were mixed for three minutes.

2. The aggregates and extra water (according to the water absorption and mass of

the aggregates) were added and mixed for three minutes.

3. The mixtures were cast into moulds (4x4x16 cm3 moulds for mortars and

10x10x10 cm3 for the concretes).

4. The mortar samples were hardened in a room at 20±5ºC and 65±5% RH and

the concrete samples were cured at 95±5% RH for 28 days at 20±5ºC.

Suitable mortar and concrete mixtures were first prepared with LECA and then this

was replaced with other aggregates according to the loose bulk volume of the

aggregates instead of the mass, due to the different loose bulk densities of the

aggregates. The aggregates were not pre-wetted prior to addition, as it has been

reported that the water absorption rate of an aggregate decreases markedly after the

first 2–5 min and that the absorption by pre-wetted aggregate after mixing is not

meaningful and only slightly affects the workability of the fresh concrete [129].

42

Table 4. Batch composition of the mortar and concrete samples.

Sample code Aggregate Cement Sand Aggregate W/C

Mortars

(aggregate size 0-2 mm)

wt% wt% Vol% (g/g)

M0 LECA 22 57 21 57

M1 #7 22 57 21 57

M2 #8 22 57 21 57

Concretes

(aggregate size 2-8 mm)

C0a LECA 22 59 19 63

C0b LECA 22 59 19 63

C1 #7 22 59 19 63

C2 #8 22 59 19 63

3.2.6 Mortar and concrete properties

The capillary water absorption of the hardened mortar samples was determined

according to the EN 1015-18 standard [130], and their mortar flexural and

compressive strengths were measured using a Shimadzu AG-IC testing machine

according to the EN 1015-11 standard [131]. The FORM+TEST type Beta 2 3000D

testing machine was used to determine the compressive strength of the concrete

samples according to the EN 12390-3 standard [132]. The elasticity modulus was

measured with a Portable Ultrasonic Nondestructive Digital Indicating Tester

(PUNDIT).

43

4 Results and discussion

4.1 Chemical and microstructural properties of the fly ash samples

All the FA samples had a high CaO content, but that for FA #5 was as high as 49

wt%. A high CaO content is typical of biomass FA as it originates from the organic

matrix of the fuel [133]. The amount of SiO2 varied from 18 wt% (#6) to 45 wt%

(#1), but the Al2O3 content was only around 10 wt% in all the samples. One

additional major component was Fe2O3, which varied between 1 and 20 wt% in the

peat FA. A variable amount of iron is typical of peat and biomass FA [16].

The alkali metals magnesium and phosphorus were determined as minor

components, but a high sulphur content of 20% was detected in FA #6. Some power

plants add lime or limestone to the combustion process, which reduces sulphur

emissions by capturing it in the form of calcium sulphite or calcium sulphate [134].

The FA #6 sample was partly a desulphurization product, which would explain the

high level of SO3.

All the FAs except for #4, which had been wetted prior to storing, were

completely dry and had a low loss-on-ignition. There was a large variation in the

particle size distribution, mainly due to the sampling and the fuel. Samples #3 and

#6 were collected from ESP-C, which gathers the finest fraction of the fly ash, and

thus had a smaller median size, whereas the fly ash particles from waste-based fuels

were larger in size.

More detailed information about the FA samples is presented Table 6. The pH

of the FA was around 11-12, mainly on account of the free CaO, although other

soluble calcium compounds and alkali metal salts can also increase the pH.

The amount of free CaO in the FA varied from 0.2% to 6.3%. FA #2, for

example, contained 4.2% free CaO, which is 15% of the total of 27.7% CaO

analysed in the sample by the XRF. The rest of the calcium may have been present

as CaSO4, CaCl2 or CaCO3, for example, or incorporated in the crystalline and

amorphous silicate phases. This suggests that calcium is a reactive component in

the system and could take part in the hardening reactions.

44

Table 5. The FBC FA samples: the chemical composition (wt%), moisture (wt%), loss-

on-ignition (%wt) and particle sizes.

FBC FA # #1 #2 #3 #4 #5 #6 #7 #8

CaO 11.6 27.7 17.7 22.3 49.3 28 13.8 16.2

SiO2 44.9 32.8 30.7 42.8 27.1 17.9 40.2 42.4

Al2O3 10.7 12.1 8.7 11.4 14.1 5.9 10.1 9.4

Fe2O3 20.1 5.9 23.9 5.6 1.7 7.6 22.3 14.8

Na2O 1.5 2.7 1.1 2.2 0.6 1.5 1.3 1.7

K2O 2.1 1.7 2.8 2.8 0.6 5.4 2.5 3.6

MgO 3 2.6 3.8 4.2 3.1 5.2 2.8 3.7

P2O5 2.6 1.5 4.7 2.5 0.3 4.5 3.3 3.7

TiO2 0.4 1.8 0.2 0.4 0.7 0.3 0.3 0.3

SO3 2 5.5 5.2 3.4 0.8 19.7 2.4 3.2

Cl 0.1 1.2 0.2 0.1 0.5 0.3 0.1 0.2

Moisture [%] 0 0.0 0.2 8.7 0.03 0 0.1 0

LOI 525ºC [%] 0.2 -0.1 0.7 3.1 -0.1 0.1 0.1 0.1

LOI 950ºC [%] 0.5 2.1 1.2 5.6 2 -2.7 1.0 0.5

<10% (µm) 2.4 3.6 1.2 19.3 23.1 1.2 3.4 2.6

<50% (µm) 17.4 29 9.4 61.9 226.4 8.1 20.7 14.7

<90% (µm) 135.6 160.5 29.7 305.1 525 22 56.9 50.7

The results of the selectively soluble leaching test (Table 6) supported the notion of

the reactivity of calcium, in that the selectively soluble CaO content could be as

high as 36% (FA #5), whereas selectively soluble SiO2, Al2O3 and Fe2O3 gave much

lower values, indicating that the main reactive component in the FA was calcium.

45

Table 6. Feed pH, free CaO and selectively soluble content values of the FA samples.

FBC FA # #1 #2 #3 #4 #5 #6 #7 #8

Feed pH

(2.5%)

10.6 12.3 11.0 12.0 12.3 12.0 11.2 11.7

Free CaO

(pH 4) [%]

N/A 4.2 0.2 2.2 6.3 2.0 0.3 0.8

Selectively

soluble CaO

[%]

4.4 19.7 4.7 14.4 36.0 25.2 5.4 6.9

Selectively

soluble SiO2

[%]

6.9 3.3 3.4 7.0 10.1 3.8 3.5 2.4

Selectively

soluble Al2O3

[%]

1.0 2.2 1.0 2.0 5.5 1.9 1.1 1.1

Selectively

soluble Fe2O3

[%]

5.2 0.7 1.3 0.9 0.6 2.0 1.5 1.2

The particle morphologies of FA #3-#6 are shown in Fig. 7. It can be seen that the

particles are very irregular in shape and there are only a few ball-shaped particles

as opposed to coal FA particles (Fig. 3).

46

Fig. 7. 5x magnification of FBC FA #3-#6 (A-D, respectively). The FBC FA particles are

typically very irregular in shape.

4.2 Granulation

The results with regard to the granulation-process are presented below.

4.2.1 Liquid-solid-ratio

The composition of each batch that was granulated, the binder liquid and the

amount of binder liquid (L/S ratio) required to achieve the target aggregate size are

shown in Table 7. K-Sil, Na-Sil and Na-Alu were used as alkali activators. Water

was used as a reference binder.

47

Table 7. Composition of granulation batches, liquid used for granulation and L/S ratio.

FBC FA # Co-binder Liquid L/S ratio (w/w) Target size

(mm)

#1 - H2O 0.43 4-6

#1 - K-Sil 0.41 4-6

#1 - Na-Alu 0.38 4-6

#1 20% BFS 1 K-Sil 0.38 4-6

#1 40% BFS 1 K-Sil 0.39 4-6

#1 20% Coal FA K-Sil 0.39 4-6

#1 40% Coal FA K-Sil 0.34 4-6

#1 20% Metakaolin 1 K-Sil 0.38 4-6

#1 40% Metakaolin 1 K-Sil 0.39 4-6

#2 Na-Sil 0.62 2-4

#2 20% BFS 2 Na-Sil 0.62 2-4

#2 40% BFS 2 Na-Sil 0.57 2-4

#2 20% Metakaolin 2 Na-Sil 0.66 2-4

#2 40% Metakaolin 2 Na-Sil 0.88 2-4

#3 - Na-Sil 1.00 2-4

#4 - Na-Sil 0.60 2-4

#5 - Na-Sil 0.94 2-4

#6 - Na-Sil 0.61 2-4

#7 - Na-Sil 0.71 2-10

#8 - Na-Sil 0.89 2-10

Overall, it was observed that a difference of only a few grams in the amount of

binder resulted in aggregates with totally different forms. If there were few grams

less than the required amount of binder liquid, granulation would not start, but if

there were a few grams more than the optimal amount, all the material would

combine into one huge agglomerate and eventually become a wet slurry. This

phenomenon has also been observed with other raw materials used in granulation

[135,136]. Fig. 8 shows a batch of geopolymer granules prepared from FA #8.

48

Fig. 8. A batch of geopolymer granules made from FA #8.

As binders of different densities were used, the L/S ratio varied to some extent, and

in any case the differences in target size prevent any direct comparison between

L/S ratios and FAs, but some conclusions can still be drawn. The amount of liquid

required (L/S ratio) varied between 0.3 and 1.0, the figure being lower for BFS and

coal FA whereas with metakaolin the L/S ratio increased when the reference FA (#2)

was used. With the peat-wood FA (#1), however, all the co-binders reduced the L/S

ratio, so that it was lower than with brick and block-type samples prepared from

similar FBC FA [60,64]. This was expected, as the material does not have to be a

‘workable’ paste at any point during granulation as is the case with mouldable

geopolymer samples.

4.2.2 Granule growth

With FA #1 the approximate aggregate size was observed every 30 s during

granulation (Fig. 9). No granules formed for a long period of time (up to 10 min),

even though 90% of the total amount of binder liquid had been added. Then,

between 7 and 12 minutes of granulation, a rapid growth of granules was observed,

pointing to an induction-type manner of growth, as noted in [135,136]. This growth

type suggests that granulation will not start until the raw material particles have

been covered with binder liquid, and that the particles are strong and are not

49

significantly deformed so as to coalesce during impact [135]. Rather, growth occurs

when excess binder liquid is squeezed to the surface due to consolidation, enabling

the coalescence of particles under the influence of the surface tension and viscosity

of the binder liquid.

The granules stopped growing after 8-14 minutes of granulation, as all the

binder liquid had been consumed. No more binder liquid was added because the

target size had been achieved, but the granulator was kept on for another 5-10

minutes. The total granulation time was thus 10-20 minutes and the granules were

spherical and surface-dry, i.e. ideal granules were obtained. With all the binder

liquids other than H2O there was hardly any ungranulated material left in the drum

after the granulation process.

Even so, there are some factors that make difficult to set out general guidelines

for alkali activation and granulation processes. As the alkali activation reaction

takes place during granulation, both the L/S ratio and the viscosity of the binder

liquid change in time, and this makes difficult to determine the true end point of

granulation. In our experiments the end of consolidation was chosen as the final

end point, as it was observed that the granules did not grow during the last 5-10

minutes of granulation.

50

Fig. 9. Growth in granule size vs. granulation time. The granules began to increase in

size 6-12 minutes after 90% of the liquid had been added. The growth was rapid,

indicating an induction-type manner of growth. Codes: P100 H2O=#1 + H2O; P100

KSil=#1 + K-Sil; P100 Na-Alu=#1 + Na-Alu; P80 S20= #1 + 20% BFS1 + KSil; P60 S40=#1

+ 40% BFS1 + KSil; P80 C20= #1 + 20% Coal FA + KSil; P60 C40=#1 + 40% Coal FA + KSil;

P80 M20= #1 + 20% Metakaolin 1 + KSil; P60 M40=#1 + 40% Metakaolin 1 + KSil (Paper

I).

Although it has been acknowledged that the particle size distribution affects the

size and strength of the aggregate [100], this is not the only major contributor to

the process. Particle morphology and surface area are two obvious factors that can

enhance or inhibit effective packing of the particles. FBC FA are typically very

irregular in shape (Fig. 7), whereas spherical particles (as are typical of PCC coal

FA) can be densely packed, giving them increased strength. On the other hand,

metakaolin has plate-shaped particles [137], which may inhibit the granulation

process. In addition, the dissolution of reactive material from the surface of the

particles by the alkali activator can modify the granulation process and affect the

packing and coalescence of the particles, thus determining how fast the granules

51

grow. Delayed granule growth was observed with metakaolin as a co-binder,

whereas BFS and coal FA seemed to speed up the start of granule growth.

When different FBC FAs are compared in terms of their L/S ratio no clear

reason can be seen for some having a high ratio and others a low one (Table 7).

Although it seemed that a wider particle size distribution can yield better

granulation results with hazardous FAs (#2-#6), this was not the case in the other

experiments. Neither was there any trend in the consumption of binder liquid or the

surface area of the raw material. These findings highlight the effect of alkali

activation during granulation. The more reactive the material present in the

precursors, the more the granulation process is affected.

4.2.3 Microstructure of the geopolymer aggregates

Cross-sections of the hardened aggregates are shown in Fig. 10. Angular, spherical

and irregular-shaped FBC FA particles can be seen in all the BSE images, but more

round particles originating from coal FA and angular particles from BFS (marked

with arrows) can be seen in the aggregates in which those co-binders were used.

There is a clear difference between the aggregates granulated with water and

those in which an alkali activator was used, in that the particles in the one

granulated with water seem to be bound together mainly by physical interlocking

and are more porous. K-Sil yielded a denser microstructure than Na-Alu. The

aggregates made from waste-based FA were more porous than the peat-wood FA

aggregates, but co-binders reduced the porosity in all cases.

52

Fig. 10. BSE-image of the general appearance of the geopolymer aggregates (FA #1 +

co-binders). Codes: P100 H2O=#1 + H2O; P100 KSil=#1 + K-Sil; P100 Na-Alu=#1 + Na-

Alu; P80 S20= #1 + 20% BFS1 + KSil; P80 C20= #1 + 20% Coal FA + KSil; P80 M20= #1 +

20 % Metakaolin 1 + KSil. (Paper I)

4.3 Geopolymer aggregates

We will now turn our attention to the geopolymerization of FBC fly ash and the

physical properties of the resulting geopolymer aggregates.

53

4.3.1 Geopolymer matrix of the aggregates

XRD patterns for the raw materials and reaction products of FA #1 and co-binders

are presented in Figures 11-14. FBC FA is highly heterogeneous in nature, and some

differences in diffraction patterns are possible between two samples taken from the

same batch. Thus caution should be exercised when comparing the signals found

in X-ray diffraction patterns.

It is evident that peat-wood FA (#1) has more crystalline phases than BFS, coal

FA or metakaolin, and a similar crystalline nature to that of FA sample #1 was also

found here for FA #2-#8.

Fig. 11. XRD patterns for the raw materials and reaction products of FA #1 with different

binders. Codes: Peat-wood ash=FA #1; P100 H2O=#1 + H2O; P100 Na-Alu=#1 + Na-Alu;

P100 KSil=#1 + K-Sil (Paper I).

54

Fig. 12. XRD patterns for the raw materials and reaction products of FA #1 and BFS

mixtures. Codes: Slag= BFS 1; Peat-wood ash=FA #1; P80 S20= #1 + 20% BFS1 + KSil;

P60 S40=#1 + 40% BFS1 + KSil (Paper I).

Fig. 13. XRD patterns for the raw materials and reaction products of FA #1 and coal FA

mixtures. Codes: Peat-wood ash=FA #1; P80 C20= #1 + 20 % Coal FA + KSil; P60 C40=#1

+ 40 % Coal FA + KSi. (Paper I).

55

Fig. 14. XRD patterns for the raw materials and reaction products of FA #1 and

metakaolin mixtures. Codes: Peat-wood ash=FA #1; P80 M20= #1 + 20% Metakaolin 1 +

KSil; P60 M40=#1 + 40% Metakaolin 1 + KSil (Paper I).

Quartz (SiO2), which was the only phase found in all the FAs, contained most of

the silicon present in them (Table 5). This originated from the bed sand of the FBC

boiler, although some might also have been present in the fuels (dirt from the

collection of wood, peat or waste materials). Several calcic phases such as CaSO4,

CaCO3, CaO and Ca silicates were identified, and as calcium was one of the major

components of the FAs it is understandable that it can form different types of

crystalline phases. Aluminium was present in the phases together with silicon, e.g.

as albite (NaAlSi3O8), microcline (KAlSi3O8) and gehlenite (Ca2Al2SiO7), while

iron was mostly present as Fe2O3 (maghemite and hematite).

Amorphous material can be observed as a “hump” between 20° and 40° 2θ

(shown in grey in Figures 11-14). It is unknown what elements the amorphous

phase contains precisely, but silicon, aluminium, calcium and iron are typically the

main components of the amorphous material in FA, metakaolins and BFS

[34,35,138].

The solubility of the components is an essential factor during alkali activation,

since it is only the dissolved components that can react further and form new,

56

strength-increasing structures. Since amorphous phases are generally more soluble

than crystalline phases, due to their less organized crystalline structure and thus

lower lattice energies [139], it is generally the case that the higher the amorphous

material content is, the stronger the geopolymer product will be. This is especially

pronounced with metakaolin and BFS, both of which are typically totally

amorphous, so that geopolymers prepared from them have very high mechanical

strength [9,44].

Geopolymers can have nanocrystalline zeolite-type structure [140] or (Ca)-Al-

Si-hydrate structure depending on the amount of calcium available [141], and the

formation of these structures can be observed in the XRD patterns as a hump around

30° 2θ [138,140,142,143]. It is often difficult to distinguish this new hump, because

amorphous material from the raw material and several crystalline phases can

overlap with it, but the shift in the hump towards 30° 2θ in Figures 12-15 clearly

indicates the success of alkali activation. This shift was not as clearly observable

in the samples containing BFS, since the BFS already had a high incidence of X-

ray-amorphous material around 30° 2θ prior to alkali activation. The new X-ray-

amorphous material is seen in the BSE images as the grey matrix between the FA

particles (Fig. 10).

In addition to the amorphous material, crystalline phases can also dissolve

during alkali activation, the extent of the reactivity being dependent on their

solubility (at a high pH), solubility equilibrium and kinetics. Phases such as CaO

and CaSO4 have high solubility compared to SiO2 which is almost inert. The XRD

results indicate that anhydrite, lime, hydroxylapatite, ettringite, portlandite,

calcium aluminium iron carbonate hydroxide, larnite and mayenite reacted at least

to some extent during alkali activation, and as such are consistent with the free CaO

and selectively soluble content values (Table 6), as all the reactive phases contained

Ca, which would lead to a high selectively soluble CaO content.

No new crystalline phases were identified in any of the samples, which means

that the reactive components mainly formed X-ray amorphous phases. In Ca-rich

systems, a strongly alkaline sodium/potassium silicate solution may initiate a

pozzolanic reaction between CaO and SiO2, which would produce a calcium

silicate hydrate (CSH) phase, typically with broad peaks around 30° and 50° 2θ

[144]. There is some indication of this phase in all of the samples, although the

peaks may have overlapped with CaCO3 signals, for example. Calcium was the

most readily soluble component in the FAs and Si was added in the alkali activator

(Na-Sil), so it is logical that calcium-silicate structures should be formed.

57

The overall granulation-alkali activation process can be summarized as follows

(Fig. 15). During granulation the raw material particles are bound together by the

surface tension of the binder liquid. The alkali activator dissolves reactive material

from the surfaces of these raw material particles and a geopolymer gel starts to

form. Particles with the geopolymer gel on their surface coalesce and bind together

strongly. The process results in spherical, surface-dry granules, and hardening

continues after the granulation process has come to an end.

Fig. 15. Schematic diagram modelling the granulation-alkali activation process. The

model shows (a) the addition of an alkali activator to the precursor particles, (b) wetting

of the particles by the alkali activator, (c) dissolving of the reactive material and (d)

formation of an aluminosilicate gel, which binds the particles together (e). The process

results in spherical granules (f) (Paper I).

4.3.2 Physical properties of geopolymer aggregates

The forces required to break the aggregates after 28 days of hardening are detailed

in Fig. 16. If we first consider the experiments performed with FA #1, it was clearly

the alkali-activated aggregates that were much stronger than those granulated with

water, showing that alkali activation had a major effect and that some strength-

increasing structures had formed. It was also clear that, even though the viscosity

of the binder liquid has an effect on the granulation process, it did not have any

additional effect on the final aggregate strength. This can also be seen in Fig. 10,

where solid bridges formed between the undissolved raw material particles in the

alkali-activated aggregates, whereas the ones granulated with water did not have

58

any solid matrix between the particles. It can also be seen in the XRD results (Fig.

11) that no change in the mineralogy of the aggregates granulated with water

occurred in connection with the granulation, indicating that no reactions occurred.

The use of K-Sil as an alkali activator resulted in higher strength than with Na-

Alu. If it is assumed that additional silicate from the K/Na-Sil is needed in order to

form CSH-type gels, as suggested by the XRD results (Section 4.3.1), this would

mean that an aluminate activator could not produce as much CSH-type gel as a

silicate activator.

Fig. 16. Forces required to break the aggregates after 28 days of hardening (black bars).

The granules were 5-6 mm in diameter in all the aggregates except for #2 (3 mm) and

LECA (7 mm). 5-15 granules were measured in each batch and the average was

59

calculated. The error bars represent confidence intervals for the means at the 95%

confidence level. The black columns represent dry crushing strength and the white

columns wet crushing strength. The strength of the wet #1 aggregates was determined

after 90 days of hardening and those of the #2 aggregates after 28 days of hardening.

All the co-binders increased the strength of the geopolymer aggregates. The co-

binders increase the reactive material content of the precursors and thus enable

more geopolymer gels to form. BFS as a co-binder produced the strongest

aggregates, metakaolin the second strongest and coal FA the third strongest.

A similar strength trend was observed with FA #2. Both co-binders increased

the strength of the aggregates, but BFS did so to a greater extent.

The overall strengths of the hazardous FA types (#2-#6) were lower than those

of the non-hazardous ones (#1, #7, and #8). Heavy metals and anionic salts [56,145]

may inhibit the hardening reactions, which may explain the lower strengths. Also,

the granule size of the FA #2 aggregates (3 mm) may have affected their crushing

strength [110].

One of the most important physical properties of aggregates is their

performance when they get wet. It was observed that the strengths were only

slightly lower after water immersion (Fig. 16) and that the granules did not

disintegrate. In two cases (#1 + 40% Coal FA + K-Sil and #2 + Na-Sil) it was

observed that immersion in water for 7 days slightly increased the strength,

although the differences between the mean strengths in these cases were within the

error margin. Nevertheless, it can be concluded that immersion in water did not

reduce the strength significantly, so that the aggregates would be suitable for use in

civil engineering applications.

The commercial reference material LECA generally had a lower crushing

strength than the geopolymer aggregates. At its best the difference in strength was

almost six-fold in favour of the geopolymer aggregates (83 N (LECA) vs. 490 N

(#1 + 40 % BFS + KSil).

The strength of the geopolymer aggregates can be compared with those of other

cold-bonded aggregates by calculating the compressive strength:

Equation 1 [146] = 2.8 ∗∗

or

Equation 2 [147] = F4 ∗

60

where F is the fracture load (N), and d is the average granule diameter.

Conversion from N/mm2 to megapascals gives compressive strength ranging from

1 MPa (#3 + Na-Sil) to 13 MPa (#1 + 40% BFS 1 + KSil) with Equation 1, while

Equation 2 gives slightly higher compressive strength due to the larger

coefficient in the equation. The compressive strengths varied from 1.4 MPa to 18.5

MPa for #3 + Na-Sil and #1 + 40% BFS 1 + KSil, respectively. The equations

strongly underline the effect of granule size, so that even the #2 aggregates resulted

in values of up to 10 MPa and 15 MPa (#2 + 20% BFS 2 + K-Sil) with equations 1

and 2, respectively.

The compressive strengths determined in this work were similar or even higher

than those reported for other artificial aggregates prepared from waste materials.

González-Corrochano et al. produced aggregates from sludge FA from used motor

oil [148] and a clay-rich sludge sediment [149] by sintering at 1150-1225°C and

calculated their compressive strength using Equation 1, resulting in strengths

between 1.4 MPa and 5.6 MPa in [148] and 0.5-13.3 MPa in [149].

Arslan and Baykal [111] prepared pellets by granulating coal FA with cement

and water in a pelletization disc, but only achieved a compressive strength of 1.4

MPa (calculated with

Equation 2) at best (30 wt% cement + 70 wt% coal FA), while Bui et al. [115]

prepared alkali-activated aggregates with a pelletization disc using class F FA, rice

husk FA and BFS as raw materials and Na-Sil as the binder liquid, obtaining

compressive strengths between 6.0 MPa and 15.7 MPa. Ferone, Colangelo and

Cioffi et al. [112–114,150] used several waste materials as precursors in the

cement-based pelletization process and achieved compressive strengths between 1

MPa and 7 MPa.

All the results discussed above show that geopolymer aggregates exhibit good

physical performance by comparison with artificial aggregates prepared by

sintering or cement-based pelletization. Even the widespread commercial product

LECA gave generally lower strength values.

There was no correlation with the total content ratios (for example SiO2/Al2O3,

or CaO/SiO2 in Table 5) and crushing strength. The selectively soluble SiO2 and

Al2O3 content values correlated with the crushing strength of FAs #3-#6 (Fig. 17),

but this correlation was not found when all the experiments were considered or

when the amounts of reactive components from the co-binders and alkali activators

were included in the calculations. This emphasizes that the strength of the

61

aggregates results from a combination of the packing of particles during

granulation and the amounts of reactive components.

Fig. 17. Crushing strength vs. selectively soluble amounts of SiO2 and Al2O3 in FAs #3-

#6 (Paper V).

Other physical properties of geopolymer aggregates

The loose bulk density, dry particle density, apparent density and water absorption

of the geopolymer aggregates and LECA are presented in Table 8. Standard 13055-

1 [105] states that the loose bulk density must be less than 1.20 g/cm3 or the dry

density less than 2.0 g/cm3 in order to meet the definition of a lightweight aggregate

(LWA). All the geopolymer aggregates comply with both these requirements, so

that it can be stated that all of them were indeed lightweight aggregates. By

comparison, natural aggregates such as sand and rocks have a bulk density of

around 2.5-3.5 g/cm3. Even though the geopolymer aggregates had a low density,

it was still higher than that of the reference material LECA.

The water absorption of the geopolymer LWAs varied between 18% and 36%,

averaging 26%, while the figure for LECA was approximately 30% for granules of

diameter 0-3 mm and 4-12.5 mm. The lower water absorption of FA #2 + 20-40%

BFS 2 + Na-Sil and #3 + Na-Sil indicates that these LWAs had a surface layer that

was less impermeable to water.

62

There was no trend to be found in the density of the aggregates or of the FAs

or co-binders, although the hazardous FA-based aggregates generally had a lower

density than those from peat-wood FA. This may be due to the lower amount of

geopolymer matrix formed between the fly ash particles in these aggregates, thus

leaving more pores inside the aggregates. The specific gravity of the raw material

and the amount of alkali activator can cause differences in density between

geopolymer LWAs, and an additional effect arises from the granulation process, as

the particles can pack together in different ways and can thus affect the aggregate

density.

Table 8. Physical properties of the geopolymer aggregates.

Sample code Loose bulk

density

[g/cm3]

Dry particle

density

(g/cm3)

Apparent

particle

density

(g/cm3)

Water

absorption 24 h

(%)

#1 + H2O 0.93 N/A N/A N/A

#1 + K-Sil 1.14 N/A N/A N/A

#1 + Na-Alu 0.97 N/A N/A N/A

#1 + 20% BFS 1 + K-Sil 1.03 N/A N/A N/A

#1 + 40% BFS 1 + K-Sil 1.08 N/A N/A N/A

#1 + 20% Coal FA + K-Sil 0.9 N/A N/A N/A

#1 + 40% Coal FA + K-Sil 1 N/A N/A N/A

#1 + 20% Metakaolin 1 + K-Sil 0.96 N/A N/A N/A

#1 + 40% Metakaolin 1+ K-Sil 1.04 N/A N/A N/A

#2 + Na-Sil 0.95 1.54 2.75 28.6

#2 + 20% BFS 2 + Na-Sil 1 1.7 2.69 21.4

#2 + 40% BFS 2 + Na-Sil 1.08 1.77 2.63 18.3

#2 + 20% Metakaolin 2 + Na-Sil 0.95 1.47 2.51 28.2

#2 + 40% Metakaolin 2 + Na-Sil 0.93 1.44 2.44 28.5

#3 + Na-Sil 0.97 1.64 2.82 17.8

#4 + Na-Sil 0.68 1.37 2.81 32.1

#5 + Na-Sil 0.88 1.6 2.91 22.5

#6 + Na-Sil 0.61 1.02 2.96 35.6

#7 + Na-Sil 1 1.6 3.2 30.7

#8 + Na-Sil 1 1.9 3.1 21.5

LECA 0-3 mm 0.7 1.1 1.6 29.9

LECA 4-12.5 mm 0.4 0.6 0.8 32.6

63

4.4 Lightweight mortars and concretes

Mortars and concretes were prepared from FA #7 + Na-Sil and #8 + Na-Sil

according to Table 4, using LECA of a similar size distribution as a reference

material. Rheological measurements were performed on the mortar mixtures and

their plastic viscosity and yield stress were determined. Detailed results are

presented in Paper III, and only the conclusions are presented here.

The geopolymer aggregates displayed similar behaviour to that of LECA. This

is a significant observation, since if the rheological behaviour of all mixtures is

similar, the differences in the properties of the hardened samples can only be due

to the components of each mixture.

4.4.1 Physical properties of geopolymer LWA mortars and concretes

Both geopolymer aggregates exhibited a higher compressive strength than LECA

in the mortars and concretes (Fig. 18). Compressive strength of up to 26 MPa was

determined for the FA #8 + Na-Sil mortar, while the flexural strength was

approximately 5 MPa for all samples. The compressive strength for the mortars and

concretes was almost the same, which can be attributed to the fact that LWAs are

typically the weakest component in the system and therefore define its overall

physical performance.

The geopolymer mortars and concretes showed a higher modulus of elasticity

than did the LECA mortars and concretes.

64

Fig. 18. Compressive and flexural strengths of the mortar and concrete samples. The

line and points indicate the modulus of elasticity.

The LECA mortars absorbed more water more quickly than did the geopolymer

aggregate samples, and the FA #8 + Na-Sil mortar had the slowest water absorption,

indicating that water is not able to permeate these aggregates as quickly as it does

others.

Total water absorption was determined after the capillary water absorption test

by immersing the mortar samples under water for 24 h and then surface-drying and

weighing them. The LECA and #7 + Na-Sil samples had a total water absorption

of 10% (Table 9), whereas for #8 + Na-Sil the figure was only 8%. Thus the FA #8

+ Na-Sil mortars not only absorbed water more slowly than the other mortar

samples but their total water absorption was also lower.

The LECA mortars and concretes had lower apparent densities than the

geopolymer aggregate mortars and concretes (Table 9), evidently on account of the

lower density of LECA (Table 8). Lightweight structural concretes must have a

maximum air-dry density of 1920 kg/m3 according to the ASTM C567 standard, so

that geopolymer concretes did not meet this requirement. The density of the

samples was measured immediately after they were taken out of the curing chamber,

however, implying that they still had a high water content, which increased their

weight. One way of reducing the densities of mortar and concrete samples could be

to lower the amount of sand in the mixtures, since this is the main (and heaviest)

component.

65

Table 9. Apparent density and total water absorption of the mortar and concrete

samples.

Sample code Apparent density

(kg/m3)

Total water absorption

[%]

LECA mortar 1731 10.2

M1 1907 10.1

M2 1943 7.9

C0a 1774 N/A

C0b 1763 N/A

C1 2025 N/A

C2 2029 N/A

4.4.2 Microstructure of the geopolymer LWA mortars

FESEM examination of the mortar samples showed the LECA to be much more

porous than the geopolymer aggregates (Fig. 19). This may be attributed to the low

density, high capillarity and low compressive strength of sample M0.

None of the mortars had a clearly visible ITZ. An EDS line analysis (data not

shown) was conducted on multiple aggregate–cement interfaces (6–10

spots/sample). The mortar samples that contained geopolymer aggregates had

higher concentrations of Si and Na in the hydrated cement matrix near the aggregate

surface than did the LECA mortars. These elements originated from the aggregate

binder (Na-Sil), which was the alkali activator used in the granulation process.

Other than that, no differences were found between the samples in the elemental

composition of their cement matrices, although gaps a few tens of microns wide

were observed between the geopolymer aggregate and the cement matrix at some

points, indicating possible shrinkage of the geopolymer aggregates.

The compressive strength of the mortars and concretes (Fig. 18) correlates

quite well with the crushing strength of the aggregates (Fig. 16). This is logical,

since the LWAs are presumably the weakest components and therefore the failure

of a sample will depend on the strength of the aggregate. There might be additional

effects that can increase or decrease from the compressive strength.

Interfacial roughness has been found to have a significant impact on the

mechanical performance of a mortar [118]. In Paper III mine tailing geopolymer

aggregates were also used, and these yielded higher compressive strength in both

66

the mortar and the concrete, even though the crushing strength of the mine tailing

geopolymer aggregates as such was lower than that of the LECA. It was observed

that this material had a less smooth outer layer than the LECA or FAs #7 + Na-Sil

and #8 + Na-Sil, which could explain the unexpectedly high compressive strength.

Also, possible reactions at the aggregate-cement interface could have enhanced the

ITZ [151], thus increasing the compressive strength.

Fig. 19. Secondary electron images of polished cross-sections of the mortar samples

with LECA and FA #7 + Na-Sil geopolymer aggregates. A: aggregate; S: sand particle

(Paper III).

4.5 Stabilization-solidification of heavy metals

FAs #2-#6 were chosen for the S/S experiments on account of their high heavy

metal content (Table 10). In fact all the samples had a generally high heavy metal

content, but in these sample it was particularly obvious with regard to Ba, Cu, Pb

and Zn. FA #2 which was collected from a power plant that uses REF and biofuel,

had substantially higher concentrations of heavy metals than the others.

Table 10. Trace element content (mg/kg) of the hazardous FA samples.

Element #2 #3 #4 #5 #6

As 26 62 10 8.9 160

Ba 2930 1800 1200 800 2480

Cd 31 7.2 3.6 2.5 13

Cr 235 110 72 77 93

67

Element #2 #3 #4 #5 #6

Cu 5020 160 100 380 170

Mo 23 7.8 9.8 1.3 51

Ni 91 87 65 38 71

Pb 1625 170 35 460 130

Sb 945 <3 <3 <3 5.2

Se <3 6.1 3.5 <3 7.7

V 43 110 48 20 150

Zn 4920 950 890 1440 2580

4.5.1 Leachable hazardous components

Barium, chromium, copper, molybdenum, lead, selenium, zinc, chloride, sulphate

and fluoride were leached from the FAs (Table 11), and particularly, high

concentrations were determined for FA #2.

The leachable percentage (leachable/total concentration) remained at or under

1% in most cases, the exceptions being FA #5 12% for Ba, all the FAs 5-44% for

Mo, and FA #3 15%, FA #4 4% and FA #6 12% for Se.

Table 11. Leachable hazardous components (mg/kg) in FAs with a liquid-solid ratio of

10, as determined according to the standard SFS-EN 12457-3 [90].

Element #2 #3 #4 #5 #6

As <0.75 <0.15 <0.15 <0.15 <0.15

Ba 5.09 1.6 2.6 95.7 1.5

Cd <0.1 <0.02 <0.02 <0.02 <0.02

Cr 2.75 1.1 0.4 <0.1 5.7

Cu <0.25 <0.05 <0.05 <0.05 0.06

Mo 4.2 2.7 2.62 0.06 22.54

Ni <0.25 <0.05 <0.05 <0.05 <0.05

Pb 4.09 <0.15 <0.15 <0.15 <0.15

Sb <0.75 <0.15 <0.15 <0.15 <0.15

Se <0.75 0.9 <0.15 <0.15 0.9

V <0.25 <0.05 <0.05 <0.05 <0.05

Zn 17.4 <0.1 <0.1 <0.1 <0.1

Cl N/A 2087 617 951 4822

SO4 28532 25867 471 42 55720

68

Element #2 #3 #4 #5 #6

F N/A <10 <10 17 <10

pH 12.5 9.8 12.3 12.6 12.1

Conductivity [mS/m] N/A 390.0 560.0 940.0 1731

The leachable concentrations in the geopolymer aggregates are presented in Table

12. As FA #2 contained the highest leachable concentrations of heavy metals it was

subjected to more profound analysis. The effects of BFS and metakaolin as co-

binders on the leaching of heavy metals are shown in Table 13.

Table 12. Leachable hazardous components (mg/kg) in geopolymer aggregates with a

liquid-solid ratio of 10, as determined according to the standard SFS-EN 12457-3 [90].

Element #2 + Na-Sil #3 + Na-Sil #4 + Na-Sil #5 + Na-Sil #6 + Na-Sil

As 0.92 2.2 0.8 2.4 1.4

Ba 0.4 0.25 0.54 0.07 0.47

Cd <0.1 0.06 0.03 <0.02 <0.02

Cr 7.04 0.6 1.5 11.3 1.5

Cu 0.59 0.11 0.16 0.16 0.05

Mo 3.66 0.89 2.69 1.08 6.75

Ni <0.25 0.08 <0.05 <0.05 <0.05

Pb <0.75 1.25 0.4 <0.15 <0.15

Sb 9.97 <0.15 <0.15 0.47 <0.15

Se <0.75 1.7 1 <0.15 0.4

V 2.07 2.1 3.13 3.13 4.54

Zn <0.5 <0.1 0.25 0.11 <0.1

Cl N/A 532 256 814 1072

SO4 35480 11931 16984 4933 34886

F N/A <10 <10 <10 10

pH 12.2 11.2 12.1 12.7 11.6

Conductivity [mS/m] N/A 6.4 6.1 10.2 7.5

69

Table 13. Leachable hazardous components (mg/kg) in alkali-activated FA #2 with co-

binders, as determined according to the standard SFS-EN 12457-3 [90].

Element #2 + Na-Sil #2 + 20% BFS 2

+ Na-Sil

#2 + 40% BFS 2

+ Na-Sil

#2 + 20%

Metakaolin 2 +

Na-Sil

#2 + 40%

Metakaolin 2 +

Na-Sil

As 0.92 0.8 <0.75 1.24 1.92

Ba 0.4 <0.25 <0.25 0.7 0.92

Cd <0.1 <0.1 <0.1 <0.1 <0.1

Cr 7.04 <1.0 <1.0 6.42 6.75

Cu 0.59 2.6 1.6 0.91 1.26

Mo 3.66 3.11 2.69 2.64 2.19

Ni <0.25 <0.25 <0.25 <0.25 <0.25

Pb <0.75 <0.75 <0.75 <0.75 <0.75

Sb 9.97 9.47 5.96 9.16 9.89

Se <0.75 <0.75 <0.75 <0.75 <0.75

V 2.07 6.92 9.45 5.36 9.64

Zn <0.5 <0.5 <0.5 <0.5 1.23

Cl N/A N/A N/A N/A N/A

SO4 35480 30370 29232 26818 17209

F N/A N/A N/A N/A N/A

pH 12.2 12.3 12.4 11.9 11.9

In order to determine whether immobilization occurred by physical encapsulation,

FA #2 + Na-Sil aggregates were ground manually to a fine powder in an agate

mortar and a leaching test was conducted. The effect of grinding on leaching is

presented in Fig. 20.

70

Fig. 20. Leaching of metals from FA #2, #2 + Na-Sil and from ground #2 + Na-Sil sample.

‘‘X’’ signifies that the leachable concentration was below the determination threshold.

Modified from Paper IV.

When Na-Sil and co-binders are mixed with a FA, dilution occurs in the

concentrations of FA components. In order to evaluate the true stabilization-

solidification potential, the theoretical leaching of heavy metals was calculated in

relation to the actually determined concentration. The leachable concentrations in

the raw materials were multiplied by the weight percentages of raw material in the

geopolymer aggregates, the impurities in the Na-Sil being determined by ICP-OES.

Figures showing the theoretical and determined leachable concentrations are

presented below.

In addition to the leaching test EN 12457-3, a sequential leaching test was

conducted for FA #2 with and without 40% of co-binders. The concentrations of

each fraction in all the samples are presented in Appendix 1.

The S/S results are presented below for each metal separately.

Arsenic

Increased leaching of arsenic was observed in all the FA geopolymer aggregates.

Its leachability was below the determination limit before alkali activation, but

concentrations between 0.8 mg/kg and 2.4 mg/kg were recorded afterwards (Tables

12 and 13). Geopolymer aggregates with BFS as a co-binder had reduced As

71

leaching, but this was merely because of the dilution effect (Fig. 21). The grinding

of FA #2 + Na-Sil aggregates caused As leaching to increase even more, suggesting

that geopolymer aggregates can partly trap arsenic species by physical

encapsulation (Fig. 21).

Fig. 21. Theoretical leaching vs. determined leaching rate (mg/kg) of arsenic as

assessed using the EN 12457-3 standard. The dashed-line columns (marked with As*)

represent the calculated leaching rate of the metal if its leachability had not been

affected by alkali activation, while the black columns represent the actually determined

leaching rate. ‘‘X’’ signifies that the leachable rate was below the determination limit.

Modified from Paper IV.

The increased leaching of As was probably caused by the formation of an

oxyanionic form which is mobile at a high pH. Oxyanionic species cannot

neutralize the negative charge of silicates and aluminates in the geopolymer matrix,

or precipitate as hydroxides.

Increased leaching of As from geopolymers with coal FA and other precursor

materials has been reported [89,152–154], although quite effective rates of As

immobilization have been also achieved [72,155].

Fernández-Jiménez et al. [73] mentioned that As was associated with iron if

this was present in coal FA, but interestingly, the same effect was not observed if

the iron was added as a Fe2O3 reagent. This suggests that the form of the iron is

important for As sorption. The iron content was more than sufficient in relation to

72

As in all the FAs considered in this thesis, but it was evidently not present in

suitable form for As stabilization. Luukkonen et al. [156] achieved effective

removal of As (III) from spiked mine effluent with metakaolin and BFS

geopolymers, but this case cannot be compared directly with the experiments

reported in this thesis, because the geopolymer in their study had first been

synthesized and was subjected to the sorbent only after proper hardening. In our

case, however, arsenic was competing with all the other components in the system

and thus may not have been adsorbed or absorbed into the geopolymer matrix.

Barium

Excellent stabilization of Ba was found in all the geopolymer aggregates (Table 12),

with the efficiency of Ba immobilization being between 69% and 99%. BFS as a

co-binder had a beneficial effect on immobilization, whereas metakaolin was found

to have a negative effect (Table 13).

Since the leaching rate did not increase when the aggregates were ground (Fig.

20), the immobilization mechanism must have been chemical rather than physical,

although it is possible that Ba could be entrapped inside micron-sized zeolite-type

structures. It is also possible for barium to be precipitated as BaSO4 [157].

Fig. 22. Theoretical leaching vs. determined leaching rate (mg/kg) of barium as

assessed using the EN 12457-3 standard. The dashed-line columns (marked with Ba*)

73

represent the calculated leaching rate of the metal if its leachability had not been

affected by alkali activation, while the black columns represent the actually determined

leaching rate. ‘‘X’’ signifies that the leachable rate was below the determination limit.

Modified from Paper IV.

More insight into the immobilization mechanism was obtained by means of the

sequential leaching test (Fig. 23), in which it was observed that the ‘acid-soluble’

fraction was larger after geopolymerization for Ba, so that it must have at least

partly been stabilized by adsorption forces. Cationic Ba2+ has a positive charge, so

that it could partly neutralize the negative charges of silicates and aluminates in the

geopolymer structures in a manner similar to Ca2+ and adsorb to negatively charged

surfaces. Effective stabilization of Ba has also been reported in other types of

geopolymer materials [72,78,89].

Fig. 23. Sequential leaching fractions of FA #2 with co-binders. Fraction F1: water-

soluble; fraction F2: acid-soluble; fraction F3: easily reduced and fraction F4: oxidizable.

Modified from Paper IV.

Cadmium

The leaching of Cd from FA was below the determination limit (0.02 mg/kg) (Table

11), and even after alkali activation increased Cd leaching was observed only in

FAs #3 and #4, it still remained low (0.06 mg/kg and 0.03 mg/kg, respectively).

74

The leaching rate also remained below the determination limit in the geopolymer

aggregates with co-binders.

Cadmium could be precipitated as a hydroxide after alkali activation, so that it

would have been only slightly soluble at a high pH [158]. This explanation is

supported by Zhang et al. [159], who found that the leaching of Cd from

geopolymers increased if an acidic medium was used in the leaching test.

Chromium

The effect of alkali activation on the leaching of Cr varied between the FA samples.

In FAs #2, #4 and #5 an increase in leaching was observed, whereas in FAs #3 and

#5 the leaching rate decreased by 45% and 74% respectively (Tables 11 and 12).

BFS as a co-binder had a positive effect on Cr immobilization (Fig. 24), but

metakaolin had a negligible effect.

Fig. 24. Theoretical leaching vs. determined leaching rate (mg/kg) of chromium as

assessed using the EN 12457-3 standard. The dashed-line columns (marked with Cr*)

represent the calculated leaching rate of the metal if its leachability had not been

affected by alkali activation, while the black columns represent the actually determined

leaching rate. ‘‘X’’ signifies that the leachable rate was below the determination limit.

Modified from Paper IV.

75

Chromium is one of the more widely studied metals in geopolymer S/S terms, and

both increased [89,153,159–161] and decreased [54,55,76,81,84,152,154,162–164]

chromium leaching has been reported. Likewise, the complexity of Cr

immobilization has been reviewed and discussed in [66] and [71]. In short, the

sulphide present in BFS could partially reduce Cr(VI) to Cr(III), leading to lower

leaching by Cr(OH)3 precipitation. Indeed, a significant decrease in the ‘easily

reduced fraction’ of Cr was observed in the sequential leaching test (Fig. 25),

supporting the “reducible” theory. The sulphur content was high in all the FAs, but

it remained an open question as to whether some FAs lacked the S(-II) species and

thus also lacked a reducing agent, leading to inconsistent results. As seen in Fig.

20, the leaching of Cr increased slightly when ground, indicating some Cr

immobilization by encapsulation.

Fig. 25. Sequential leaching fractions of FA #2 with co-binders. Fraction F1: water-

soluble; fraction F2: acid-soluble; fraction F3: easily reduced and fraction F4: oxidizable.

Modified from Paper IV.

Copper

Alkali activation increased the leaching of copper from all the FAs, although it

remained low and close to the determination limit in every case. The co-binders did

76

not improve its immobilization, but in fact worsened it. Success in Cu

immobilization has been reported elsewhere [54,74,76,80,81,153,154,162,165].

The copper in FAs #2-#6 was water-insoluble (Table 10 and 11) and mainly in

the ‘oxidizable’ fraction (Appendix 1), indicating that it was present mainly as an

oxidizable form (e.g. sulphide). After alkali activation, however, the ‘oxidizable’

fraction decreased significantly and the ‘easily reduced’ fraction increased. This

gives some evidence that oxide species were formed and the rest of the Cu was

incorporated into the geopolymer matrix. Again, the ‘easily-reduced’ fraction

diminished in the presence of BFS, pointing to a possible reduction by sulphide.

Molybdenum

Mixed results were also obtained for Mo. Stabilization was observed in FAs #2, #3

and #6, although in #2 it was due to the dilution effect, while a slight increase in

Mo leaching was detected in FAs #4 and #5. The various co-binders did not have

any effect on the leaching of Mo. Unsuccessful Mo immobilization has also been

reported elsewhere [78,89,154].

Even though the FA had a low total Mo content (1-51 mg/kg), it was largely

water-soluble (5-44%) (Table 11), and the fact that no differences were observed

between the sequential leaching fractions before and after alkali activation suggests

that Mo was insensitive to Eh conditions. A similar finding has been reported by

Ahmed & Buenfeld [166]. Mo can form an oxyanionic species at a high pH after

alkali activation in a similar manner to arsenic, thus showing some increase in

leaching.

Nickel

The leachability of Ni remained below the determination threshold (0.05 mg/kg)

before and after alkali activation (Tables 11 and 12) with the exception of FA #3 +

Na-Sil, for which it was just above the threshold (0.08 mg/kg). No changes in

leaching were detected with any of the co-binders, either.

Nickel was insoluble in water and no significant changes in the sequential

leaching test fractions were detected, showing that Ni was present in the FA in a

form that is insensitive to alkali activation conditions. Nickel is a less common

heavy metal in geopolymer stabilization studies, although Izquierdo et al. [161]

have reported very low Ni mobility regardless of the curing conditions.

77

Lead

The leachable Pb fraction was below the determination limit in all the FA samples

except for FA #2, in which it was 4 mg/kg (Table 11). Excellent immobilization

was achieved in FA #2 after alkali activation, although a slight increase in leaching

was detected in FAs #3 and #4 (1.25 mg/kg and 0.4 mg/kg, respectively (Table 12

and Fig. 26). As the stabilization in FA #2 was very effective even with Na-Sil, no

effect of the co-binders was detected (Table 13), and similarly there was no increase

in leaching upon grinding (Fig. 20). Nonetheless, a decrease in the ‘easily reduced’

fraction was observed (Fig. 27), which indicates a similar reducing effect to that

noted in copper and chromium.

Fig. 26. Theoretical leaching vs. determined leaching rate (mg/kg) of lead as assessed

using the EN 12457-3 standard. The dashed-line columns (marked with Pb*) represent

the calculated leaching rate of the metal if its leachability had not been affected by alkali

activation, while the black columns represent the actually determined leaching rate. ‘‘X’’

signifies that the leachable rate was below the determination limit. Modified from Paper

IV.

Lead is one of the most widely studied heavy metals in geopolymer immobilization

applications, and good results have been reported for coal and waste-based FA and

slags [55,76,78–80,153,154,158–162,165]. There is evidence [159,160,165,167]

78

that Pb can form chemical bonds with the geopolymer matrix or be precipitated as

a hydroxide [157].

Chemical bonding is probably the major immobilization mechanism in this

case, too, since grinding did not increase the leaching of lead and the ‘acid soluble’

fraction was minimal in the sequential leaching test, thus making an adsorption

mechanism unlikely. The increased leaching after alkali activation observed in FAs

#3 and #4 is consistent with the chemical bonding-theory, because Pb being in a

soluble form in FA #2 is actually preferable. If Pb is not readily soluble (as in the

case of FA #3 and #4) it will be liberated only after alkali activator has caused

disintegration of the FA particles. As Pb would be available to be incorporated in

the formation of a geopolymer matrix, this would be “too late” and thus it would

not be able to bind so strongly.

Fig. 27. Sequential leaching fractions of FA #2 with co-binders. Fraction F1: water-

soluble; fraction F2: acid-soluble; fraction F3: easily-reduced and fraction F4:

oxidizable. Modified from Paper IV.

Antimony

The total antimony content was low (<5 mg/kg) in all the FAs except FA #2 (945

mg/kg) (Table 10) and the antimony was water-insoluble in all cases (Table 11).

High leachable amounts (10 mg/kg) were nevertheless determined in FA #2 after

79

alkali activation, whereas the co-binders did not reduce leachability (Table 13).

Antimony was present in the ‘acid-soluble’, ‘easily reduced’ and ‘oxidizable’

fractions of the FAs, but all these fractions decreased drastically after alkali

activation and only the water-soluble fraction increased (Appendix 1). This shows

that the total bioavailable content in the sequential leaching test was much lower

after alkali activation even though the water-soluble content increased.

The increased leachability can be explained by the formation of oxyanionic

species, as in the case of arsenic and molybdenum. Even the reduced sulphur

species from BFS would form soluble compounds with Sb, thus negating the

otherwise beneficial effect of BFS [166].

Selenium

The total Se content was low (<8 mg/kg) in all the FAs, but small amounts (0.9

mg/kg) were leached from FAs #3 and #6 (Table 10). Alkali activation seemed to

increase its leachability, although the amounts were so low that no far-reaching

conclusions can be made from the experiments (Table 11 and 12). When the

geopolymer aggregates were ground, leaching increased (Fig. 20), showing some

degree of physical retention.

Increased leaching of Se was also found by Izquierdo et al. [161]. As with As,

Mo and Sb, selenium can form oxyanionic species, thus revealing unsuccessful

stabilization.

Vanadium

Vanadium was water-insoluble prior to alkali activation (Table 11), but it became

leachable after alkali activation in all cases (Table 12). The various co-binders did

not reduce leaching, but on the contrary, they increased it (Table 13). BFS in

particular worsened the leaching results, and it was also discovered that BFS

increased the vanadium content of all the fractions in the sequential leaching test

as compared with the geopolymer aggregate without BFS (x

Appendix 1). The highest increase was found in the ‘easily reduced’ fraction,

pointing to sensitivity anoxic conditions.

Once again, the formation of oxyanionic species could explain the increased

leaching, an interpretation which is supported by the results of Izquierdo et al. [161].

80

Zinc

Zinc was the most abundant heavy metal present in the FA (Table 10), but it was

leachable only from FA #2 (Table 11). Excellent immobilization was achieved with

alkali activation in FA #2, although minor leachable quantities (<0.25 mg/kg) were

detected in FAs #4 and #5 (Table 12). Unexpectedly, the use of metakaolin as a co-

binder seemed to worsen the immobilization of Zn (Table 13).

Unlike the other heavy metals, Zn was present in relatively high concentrations

in the ‘acid-soluble’ fraction of FA #2 (Fig. 28). This fraction decreased after alkali

activation and decreased even more when BFS was used as a co-binder. This shows

that Zn may have been bound by quite weak interactions in the FA particles and

thus was readily bioavailable. It became much less bioavailable after alkali

activation, however, indicating chemical retention. This was supported by an

experiment in which the leaching did not increase upon grinding (Fig. 20). Alkali

activation has generally been found to be a relatively good method for

immobilizing Zn [54,55,162], although it has been found that Zn leads to reduction

in final compressive strength in brown coal FA geopolymers [162].

Fig. 28. Sequential leaching fractions of FA #2 with co-binders. Fraction F1: water-

soluble; fraction F2: acid-soluble; fraction F3: easily-reduced and fraction F4:

oxidizable. Modified from Paper IV.

81

Sulphates and chlorides

Although the focus in this thesis is on the S/S of heavy metals, anionic salts such

as Cl-, SO42- and F- can also cause environmental hazards. In addition, the

compressive strength of a geopolymer may decrease as anions consume the

available alkali activator moles, thus hindering the geopolymerization reactions

[75,145,168]. As stated earlier, S2- could affect stabilization by reducing some

metals, such as Cr(VI) to Cr(III) [169]. On the other hand, chloride was found to

increase Cr leaching by lowering the stability of chromium hydroxide [76]. The

presence of anionic salts in geopolymers is undesirable, and one option for

removing soluble salts from FA is a preliminary washing treatment, as suggested

by Colangelo et al. [170] and Zheng et al. [29].

Sulphate in particular was present in high quantities in the FA used in this work,

and was highly leachable (Table 11). The presence of simple inorganic, water-

soluble salts such as chlorides and sulphates is common in FBC fly ash [25]. Thus

decreased leaching of chloride was observed in all the FAs after alkali activation,

but mixed results were found for sulphate, the leaching of which decreased in FAs

#3 and #6 but increased in the others. Fluoride leaching was below the

determination limit before and after alkali activation, with the exception of FA #5,

in which it decreased from 17 mg/kg to <10 mg/kg.

4.5.2 Conclusions regarding stabilization-solidification

The stabilization-solidification of hazardous components in FBC FA by alkali

activation varied greatly (Table 14). Barium and chloride were the only components

for which good immobilization was achieved with all the FA samples. The results

for Pb and Zn were also good, although slight increase in leaching was observed in

some cases, while increased leaching of As and V after alkali activation was

detected in all the FAs. Mixed results were obtained for the other elements.

82

Table 14. Conclusions regarding the effects of alkali activation on the leachability of

hazardous components. “+” indicates that lower leachability was observed, “-” that

higher leachability was observed and “<D.L” that the concentration was below the

determination limit before and after alkali activation.

Element #2 + Na-Sil #3 + Na-Sil #4 + Na-Sil #5 + Na-Sil #6 + Na-Sil

As - - - - -

Ba + + + + +

Cd <D.L - - <D.L <D.L

Cr - + - - +

Cu - - - - +

Mo + + - - +

Ni <D.L - <D.L <D.L <D.L

Pb + - - <D.L <D.L

Sb - <D.L <D.L - <D.L

Se <D.L - - <D.L +

V - - - - -

Zn + <D.L - - <D.L

Cl N/A + + + +

SO4 - + - - +

F N/A <D.L <D.L + -

pH + - + - +

Conductivity

[mS/m]

N/A - - - +

The differences in the results are probably caused by FA differences in mineralogy.

The hazardous components may be located on the surface or inside the FA particle

and may be present as several species, so that their availability may vary between

the FAs. An alkali activator can liberate more elements from inside the FA particles

than a leaching solution, and thus the elements that are released later on in the

process may not be as well bound, because there is no place for the ions or no need

for them. These elements will remain loosely bound and therefore leach out easily.

The extent of the release of such components will depend on the FA chemistry and

morphology, and will not be related to the total concentration of the component.

In the cases where effective immobilization was achieved, the stabilization

mechanism is likely to have been chemical rather than physical, as the porosity of

the geopolymer aggregates was high in all cases, so that the water was able to

83

penetrate inside the aggregates. There remains a possibility, however, that some of

the metals were trapped inside micron-scale zeolite-type structures.

Overall, the stabilization results reported in this thesis were quite consistent

with those of studies employing coal or waste FA, slag or metakaolin as the

precursor. In general, cationic species such as Pb and Zn are immobilized better

than anionic species such as As and V. Since these elements would be oxyanionic

at a high pH, they would be unable to neutralize the negative charge of cationic

metals such as Al or Si, or to be precipitated as hydroxides. The use of a lower pH

alkali activator such as Na2CO3 could yield better results in the stabilization of

anionic species.

84

85

5 Summary and concluding remarks

The urgent need for a means of using industrial inorganic waste as a precursor for

new materials is one of the targets of a circular economy. This could be done by

geopolymerization. The ability to simultaneously form an aluminosilicate matrix

and stabilize hazardous components under mild conditions is an ideal property as

far as waste treatment is concerned. So far, coal fly ash (FA) and blast furnace slag

have been under intensive investigation as geopolymer precursors, but the synthesis

of geopolymers from biomass, peat and waste-based FA from fluidized bed

combustion (FBC) boilers has been a virtually unexplored area. The complex

nature of these forms of ash makes their utilization a challenging prospect.

In this study eight FBC FA samples collected from heat and electricity power

plants were used as raw materials for geopolymer aggregates prepared by a

simultaneous alkali activation-granulation process.

The granulation process is fast and results in spherical, surface-dry granules.

The amount of binder liquid consumed in the granulation, the L/S ratio, varies

between the fly ashes. In this thesis, the optimal ratio for the FA samples was

between 0.4 and 1.0 depending on the ash properties.

After granulation, solid geopolymer bridges are formed between the particles

and the granules gain strength. The produced geopolymer aggregates have

satisfactory physical properties and surpass the definition of a lightweight aggregate (LWA) as defined by the EN 13055-1 standard. They also have generally

greater strength and higher density than the commercial reference material, LECA,

but a similar water absorption capacity. It was also shown that mortars and

concretes prepared from geopolymer aggregates have similar workability to those

prepared from LECA, but a higher compressive strength.

When the mineralogy of the geopolymer aggregates was compared with that

of the FA samples, no new crystalline phases were found. There was a change in

the X-ray amorphous material content, however. The new phases formed from Ca,

Si and Al are possibly micron-sized C(A)SH-type gels.

Since the fuels used in generating some of the FA samples were waste-based,

the samples contained heavy metals and their stabilization varied in efficiency. The

best stabilization results were obtained for barium, lead, zinc and chlorine, while

increased leaching after alkali activation was observed especially for arsenic and

vanadium. Varying results were obtained for the other heavy metals.

The efficiency of stabilization evidently depends on the form and location of

the heavy metal. An alkali activator can liberate more elements than water, and

86

some of the elements are not bound very firmly to the geopolymer matrix, so that

they can leach out easily and cannot be stabilized. The extent of leaching depends

on the physical and chemical properties of the FA (its mineralogy and morphology,

for example) and is not related to the total amount of the element.

The results presented here show that geopolymer aggregates with competitive

physical properties can be prepared even from low-reactive FBC FAs containing

heavy metals. The alkali activation-granulation process can increase the utilization

of ash and generate valuable products that could be used as aggregates in mortar

and concrete or in civil engineering. The manufacturing of artificial aggregates

from waste offers an ecological and economical solution to certain waste

management problems and can help to save natural resources. On the other hand,

if high concentrations of heavy metals are present, their bioavailability should be

studied carefully.

87

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Appendix

Appendix 1. Sequential leaching fractions (mg/kg) of #2, #2 + Na-Sil, #2 +40% BFS 2 +

Na-Sil and #2 + 40% metakaolin 2 + Na-Sil. Fraction F1: water-soluble; fraction F2: acid

soluble; fraction F3 easily reduced; fraction F4: oxidizable.

Element #2 F1 #2 F2 #2 F3 #2 F4 #2 + Na-Sil

F1

#2 + Na-Sil

F2

#2 + Na-Sil

F3

#2 +

Na-Sil

F4

As <0.6 <0.6 4.1 2.9 <3 <3 <3 <3.8

Ba 9.6 8.6 64.2 728 1.5 101 85 425.5

Be <0.2 <0.2 <0.2 0.26 <1 <1 <1 <1.25

Cd <0.1 14.2 5.2 1 <0.4 2.3 4.4 0.8

Co <0.1 3.3 3.58 2.7 <0.6 <0.6 3.2 1.4

Cr 1.6 3.1 23.6 49.6 8.2 2.9 3.5 19.8

Cu 0.3 97.6 624 2190 <1 248.6 1060 374.25

Mo 5.2 2.2 0.94 1.42 4 <1 <1 <1.25

Ni <0.2 12.4 11 15.2 <1 4.8 10.6 6.75

Pb 3.9 <1.0 290.4 171 <3 7.4 86.2 90.5

Sb <0.6 142.4 78 143.4 12.6 23.4 16.8 5.0

Se <0.6 <0.66 <0.6 4.4 <3 <3 <3 7

Tn <0.6 <0.6 3.3 10.4 <3 <3 <3 <3.75

Ti <0.6 0.72 30 160.4 <3 <3 4.1 4.5

V <0.2 0.32 16 7.2 2 <1 9.4 2.7

Zn 11.6 530 1286 704 <2 349.8 700 488.25

100

Element #2 + 40%

BFS 2 +

Na-Sil F1

#2 + 40%

BFS 2 +

Na-Sil F2

#2 + 40%

BFS 2 +

Na-Sil F3

#2 + 40%

BFS 2 +

Na-Sil F4

#2 + 40%

Metakaolin

2 + Na-Sil

F1

#2 + 40%

Metakaolin

2 + Na-Sil

F2

#2 + 40%

Metakaolin

2 + Na-Sil

F3

#2 + 40%

Metakaolin

2 + Na-Sil

F4 As <3 <3 <3 <3.75 <3 <3 <3 <3.8

Ba 1.0 72.2 98.8 169.8 1.36 30.6 95.6 273.3

Be <1 <1 <1 <1.3 <1 <1 <1 <1.3

Cd <0.4 <0.4 0.7 2 <0.4 1 2.46 0.7

Co <0.6 <0.6 0.7 1.5 <0.6 <0.6 1.74 1.3

Cr <2 <2 <2 15.5 7.8 2.6 5.6 11.5

Cu <1 3.5 1.9 527.8 2.62 112.2 612 269.8

Mo 2.94 <1 <1 <1.25 2.54 <1 <1 <1.3

Ni <1 1.6 5 4.1 <1 2.7 8.2 6.75

Pb <3 3.78 12 52.3 <3 8.6 67.2 81

Sb 7.6 8 6.2 <3.8 11.8 10 <3 <3.8

Se <3 <3 <3 7.25 <3 <3 <3 6.8

Tn <3 <3 <3 <3.8 <3 <3 <3 <3.8

Ti <3 <3 16.6 <3.8 5.4 <3 <3 <3.8

V 7.6 2.4 36.6 4.2 10.2 <1 6.4 2.6

Zn <2 46.2 191.8 406.3 2.5 178.4 394.8 333.3

101

Original Papers

I Yliniemi J, Nugteren H, Illikainen M, Tiainen M, Weststrate R, Niinimäki J (2016) Lightweight aggregates produced by granulation of peat-wood fly ash with alkali activator. International Journal of Mineral Processing. 149: 42-49

II Yliniemi J, Tiainen M, Illikainen M. (2016) Microstructure and physical properties of lightweight aggregates produced by alkali activation-high shear granulation of FBC recovered fuel-biofuel fly ash. Waste and Biomass Valorization. 7:1235–1244

III Yliniemi J, Paiva H, Ferreira V, Tiainen M, Illikainen M. (2017) Development and incorporation of lightweight waste-based geopolymer aggregates in mortars and concretes. Construction and Building Materials 131: 784-792.

IV Yliniemi J, Pesonen J, Tiainen M, Illikainen M (2015) Alkali activation of recovered fuel–biofuel fly ash from fluidised-bed combustion: Stabilisation/solidification of heavy metals. Waste Management. 43: 273-282.

V Yliniemi J, Pesonen J, Tanskanen P, Peltosaari O, Tiainen M, Nugteren H, Illikainen M (2016) Alkali activation–granulation of hazardous fluidized bed combustion fly ashes. Waste and Biomass Valorization.8: 339-348.

Reprinted with permission from Elsevier (I, III, IV) and Springer (II, V). Original

publications are not included in the electronic version of this thesis.

102

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