Insulating Refractory Materials

INSULATING REFRACTORY MATERIALS FROM INORGANIC WASTE RESOURCES

by

AMANDA JONKER

Submitted in partial fulfillment of the requirements for the DOCTOR TECHNOLOGIAE in the Department Chemistry FACULTY OF SCIENCE TSHWANE UNIVERSITY OF TECHNOLOGY

Supervisor: Dr MJ van der Merwe Co-Supervisor: Prof RI McCrindle

December 2006

I hereby declare that the thesis submitted for the degree D Tech: Ceramics Technology, at the Tshwane University of Technology, is my own original work and has not previously been submitted to any other or quoted are indicated and acknowledged by means of a comprehensive list of references.

A. Jonker

Copyright © Tshwane University of Technology 2006

i

DEDICATED TO MY FAMILY ROELOF, DELMARIE, ZANEL & ROELOF (Jnr.) IZAAK, DELENE, CONNIE & RUNA-MARIE “Sonder julle opoffering sou dit nie vir my moontlik gewees het nie.”

ii

ACKNOWLEDGEMENTS The author would like to express gratitude to: Dr MJ van der Merwe, my supervisor and mentor, for her able supervision, criticism and constant readiness to discuss problems during the course of this work and for proofreading the script and sitting through the night with me. Prof RI McCrindle, my co-supervisor, for his efforts, hard work and dedication in finalising this work. Prof JH Potgieter, for motivating me to further my studies. The Department of Chemistry & Physics as well as the Department of Chemical & Metallurgical Engineering, Tshwane University of Technology, for arranging my duties so that I could pursue my studies. The Ceramics Technology division of the Department of Chemistry & Physics, Tshwane University of Technology, for fulltime use of their laboratories and facilities. The National Research Foundation for the financial support to fulfil my studies. Mr MI Lavere for his assistance and hard work in the laboratory (RIP). Miss W Perrins, Cermalab, for her help during the development stages of the project and assistance in testing. All my B. Tech students, for their assistance during the course of this work. Colleagues, family and friends for their critical opinions, aid and patience.

iii

ABSTRACT The management of inorganic waste produced from diverse forms of industrial activity remains a major problem in many parts of the world. Typical industrial inorganic wastes include coal fly-ash, metallurgical slag, phosphogypsum waste and iron-rich waste. This investigation focused on the use of coal fly-ash, phosphogypsum and ironrich waste as a substitute for natural aluminosilicate raw materials for manufacturing low-cost insulating refractory materials. The physical and chemical properties of inorganic waste materials were characterised during the development of an insulating refractory material. Different mixtures were investigated to find a formulation that had refractory properties. The manufacture of the porous insulating material was studied and adapted so as to achieve a low-cost manufacturing route using a geopolymeric process. The strength of the geopolymeric refractory material developed is double that of the previous materials manufactured, also allowing for the automatisation of the process. The manufacturing process is rapid, with setting times of circa three hours being achieved. In situ, foaming of the geopolymer resulted in high closed porosities, therefore maintaining good thermal conductivities. This in situ forming of the monolithic porous geopolymeric material would further benefit storage handling and mould availability. The aim of developing a porous geopolymeric insulating refractory material using inorganic waste materials as the main ingredient was successfully accomplished.

iv

CONTENT

Page

Declaration

i

Dedication

ii

Acknowledgements

iii

Abstract

iv

List of figures

xi

List of tables

xiii

List of abbreviations

xvi

CHAPTER 1: INTRODUCTION 1.1

Background

1

1.2

Problem statement

6

1.3

Goals of this investigation

7

1.4

Hypothesis

7

1.5

General objectives

7

1.6

Specific objectives

8

1.7

Scope of the thesis

8

CHAPTER 2: LITERATURE SURVEY 2.1

Introduction

9

2.2

Thermal-insulating ceramics

11

2.2.1

General

11

2.2.2

Disadvantages of porous insulating materials

14

2.2.3

Service limiting temperature

15

2.2.4

Thermal conductivity

16

2.2.5

Shrinkage

18

2.2.6

Strength

18

2.3

Firebrick refractories and thermal insulation

20

v

2.4

Conventional silicate-bonded refractories versus Geopolymers

22

2.5

Production of foam geopolymers from waste materials

23

2.5.1.

Geopolymer chemistry

25

2.5.2.

Materials used in geopolymerisation

28

2.6

Inorganic Waste Materials

30

2.6.1

Coal fly-ash

30

2.6.1.1

World production of coal fly-ash

31

2.6.2

Phosphogypsum

32

2.6.3

Iron rich waste

33

2.7

Natural inorganic silicate minerals

34

2.7.1

Phyllosilicates (Greek: phyllon, leaf)

34

2.7.1.1

Kaolin

35

2.7.1.2

Ball clay

37

2.7.1.3

Bentonite

37

2.7.2

Tectosilicates

38

2.7.2.1

Silica

38

2.7.2.2

Feldspar group

39

2.8

Production methods employed in the ceramics industry

40

2.8.1

Burning-additive method

40

2.8.2

‘Gas’ methods

40

2.9

Drying

41

2.10

Firing

42

vi

2.11

Factors affecting the strength of a ceramic body.

42

2.11.1

Chemical or mineralogical composition of the material

42

2.11.2

Porosity

42

2.11.3

The mode of preparation of the material

43

2.11.4

Mode of manufacture of the article

43

2.11.5

Conditions of drying

44

2.11.6

Conditions of firing

44

2.12

Summary

45

CHAPTER 3: METHODS 3.1

Introduction to the evaluation of inorganic materials

47

3.1.1

Determination of the chemical composition of the inorganic

47

materials 3.1.2

Sample preparation of inorganic materials

48

3.1.3

Shrinkage of inorganic materials

48

3.1.4

Strength of inorganic materials

48

3.1.5

Water absorption of inorganic materials

49

3.2

Introduction to developing a ceramic body mixture from waste

49

materials 3.2.1

Sample preparation of ceramic body mixtures

49

3.2.2

Particle size distribution of ceramic body mixtures

50

3.2.3

Flow properties of ceramic body mixtures

50

3.2.4

Extrusion and casting of ceramic body mixtures

51

3.2.5

Firing of ceramic body mixtures

51

3.3

Introduction to the determination of refractory properties of

52

porous ceramic bodies from inorganic waste materials 3.3.1

Procedure

52

vii

3.4

Introduction to the production of insulating geopolymers from

54

waste materials 3.4.1

Procedure

54

3.4.2

Physical properties of insulating geopolymers

55

CHAPTER 4: RESULTS 4.1

Results of the physical test on the inorganic materials

57

4.1.1

Chemical composition of inorganic materials

57

4.1.2

Shrinkages of inorganic materials

58

4.1.3

Strength of inorganic materials

59

4.1.4

Water absorption of inorganic materials

60

4.2

Discussion of the physical properties of the inorganic materials

61

4.3

Conclusion on the inorganic materials

63

4.4

Results of the ceramic body mixtures from waste materials

64

4.4.1

Particle size distribution of ceramic body mixtures

64

4.4.2

Flow properties of ceramic body mixtures

65

4.4.3

Extrusion of the ceramic body mixtures

66

4.4.4

Physical properties of the ceramic body mixtures

67

4.5

Discussion of ceramic body mixtures from waste materials

70

4.6

Conclusion of the ceramic body mixtures from waste materials

72

4.7

Results of the physical and refractory properties of porous

73

ceramic bodies from inorganic waste materials 4.7.1

Chemical and mineralogical composition of porous ceramic

73

bodies 4.7.2

Ash fusion temperatures of porous ceramic bodies

viii

78

4.7.3

Physical properties of porous ceramic bodies

78

4.7.4

Thermal conductivity of porous ceramic bodies

80

4.8

Discussion of physical and refractory properties of porous

81

ceramic bodies from inorganic waste materials 4.9

Conclusion on the physical and refractory properties of porous

82

ceramic bodies from inorganic waste materials 4.10

Physical properties of the prepared geopolymeric insulating

84

materials 4.10.1

Shrinkage of geopolymeric insulating materials

84

4.10.2

Bulk density of geopolymeric insulating materials

85

4.10.3

Strength of geopolymeric insulating materials

85

4.10.4

Porosity of geopolymeric insulating materials

86

4.10.5

Ash fusion temperatures of geopolymeric insulating materials

86

4.10.6

Thermophysical properties of geopolymeric insulating materials

87

4.11

Discussion of the physical properties of the prepared porous

88

geopolymeric insulating materials 4.12

Discussion of the thermophysical properties of the prepared

90

geopolymeric insulating materials 4.13

Conclusion of the physical properties of the prepared

91

geopolymeric insulating materials

CHAPTER 5: CONCLUSIONS AND RECOMMENDATIONS 5.1

Conclusions

92

5.2

Recommendation

94

96

REFERENCES

ix

APPENDIX A: RAW DATA OF CERAMIC PROPERTIES OF

104

INORGANIC MATERIALS

APPENDIX B: RAW DATA OF CERAMIC PROPERTIES OF CERAMIC BODIES

x

116

LIST OF FIGURES Figure 1.1:

Page Al2O3 – SiO2 binary phase diagram as applicable to

3

refractories. 1.2:

Ternary phase diagram of the CaO-Al2O3-SiO2 system.

2.1:

Thermal conductivity of insulating fire brick and insulating

5 16

castables (Carniglia & Barna, 1992). 2.2:

Mechanisms of Geopolymerisation according to Davidtovits,

27

(1991). 2.3:

Typical coal fly-ash (Mantel, 1991).

30

4.1:

Shrinkages of inorganic materials

58

4.2:

Strength of inorganic materials

59

4.3:

Water absorption of inorganic materials

60

4.4:

Particle size analyses of ceramic bodies from waste materials.

64

4.5:

Fluidity of ceramic body mixtures

66

4.6:

Thixothropy of ceramic body mixtures

66

4.7

Firing shrinkage of ceramic bodies

69

4.8:

Strengths of ceramic bodies

69

4.9:

Water absorption of ceramic bodies

70

4.10

Mineralogical analysis of FBI small

74

4.11

Mineralogical analysis of FBI large

75

4.12

Mineralogical analysis of FBO small

76

4.13

Mineralogical analysis of FBO large

77

4.14

Apparent porosity of porous ceramic mixtures

79

4.15

Physical properties of porous ceramic mixtures

80

4.16

Thermal conductivity of porous refractory mixtures

81

4.17

Shrinkage of geopolymeric insulating materials.

84

4.18

Bulk density of geopolymeric insulating materials.

85

xi

4.19

Strength of geopolymeric insulating materials.

85

4.20

Porosity of geopolymeric insulating materials

86

4.21

Ash fusion temperatures of insulating geopolymeric materials

87

4.22

Thermophysical properties of insulating geopolymeric materials

88

5.1

Thermal conductivity of insulating fire brick and insulating

93

castables

xii

LIST OF TABLES Table

Page

2.1

Melting points of refractory oxides (Carniglia & Barna, 1992).

11

2.2

Typical mechanical properties of raw materials used in the formulation of insulating refractories. (data sheet: G&W base and industrial minerals)

36

3.1

List of Inorganic materials tested

47

3.2:

Body mixtures for ceramic bodies

50

3.3:

Body mixtures for porous refractory materials

52

3.4:

Composition of mixtures for sample geopolymer preparation

55

4.1:

Chemical composition of the inorganic materials

57

4.2:

Summary of shrinkages (%) of the inorganic materials

58

4.3:

Summary of strength (MPa) of the inorganic materials

59

4.4:

Summary of water absorption (%) of the inorganic materials

60

4.5:

Flow properties of ceramic body mixtures.

65

4.6

Physical properties of extruded dried samples of ceramic body

67

mixtures 4.7:

Physical properties of the fired FCB ceramic body mixture

67

4.8:

Physical properties of the fired FCI ceramic body mixture

68

4.9:

Physical properties of the fired FPI ceramic body mixture

68

4.10:

Chemical composition of the porous refractory mixtures

73

4.11

78

4.13

Ash fusion temperature test in oxidising atmosphere of porous ceramic refractory Apparent porosity, bulk density and apparent relative density of porous refractory mixtures Cold crushing strength of porous refractory mixtures

79

4.14

Thermal conductivity of porous refractory mixtures

80

4.15:

Ash fusion temperatures for geopolymeric insulating materials

86

4.16:

Thermophysical properties of the geopolymeric insulating material

87

4.12

xiii

79

6.1:

Comparison of physical properties of traditional and geopolymeric

92

porous insulating refractories APPENDIX A: RAW DATA OF CERAMIC PROPERTIES OF INORGANIC MATERIALS A.1:

Physical properties of kaolin and 20 % ball clay fired at 800 °C

104

A.2:

Physical properties of kaolin and 20 % ball clay fired at 850 °C

105

A.3:

Physical properties of kaolin and 20 % ball clay fired at 900 °C

105

A.4:

Physical properties of fly-ash and 20 % ball clay fired at 800 °C

106

A.5:

Physical properties of fly-ash and 20 % ball clay fired at 850 °C

106

A.6:

Physical properties of fly-ash and 20 % ball clay fired at 900 °C

107

A.7:

Physical properties of gypsum and 20 % ball clay air-dried

108

A.8:

Physical properties of gypsum and 20 % ball clay fired at 850 °C

109

A.9:

Physical properties of gypsum and 20 % ball clay fired at 900 °C

110

A.10:

Physical properties of Fe-rich waste and 20 % ball clay fired at 800 °C Physical properties of Fe-rich waste and 20 % ball clay fired at 850 °C Physical properties of Fe-rich waste and 20 % ball clay fired at 900 °C Physical properties of K-feldspar and 20 % ball clay fired at 850 °C Physical properties of K-feldspar and 20 % ball clay fired at 900 °C

111

A.11: A.12: A.13: A.14:

112 113 114 115

APPENDIX B: RAW DATA OF CERAMIC PROPERTIES OF CERAMIC BODIES B.1:

Physical properties of FCB-extruded samples dried at 110 °C

116

B.2:

Physical properties of FCB-extruded samples fired at 900 °C

117

B.3:

Physical properties of FCB-extruded samples fired at 950 °C

118

B.4:

Physical properties of FCB-extruded samples fired at 1000 °C

119

B.5:

Physical properties of FCB-extruded samples fired at 1050 °C

120

B.6:

Physical properties of FCB-cast samples

121

B.7:

Physical properties of FCI-extruded samples dried at 110 °C

122

xiv

B.8:

Physical properties of FCI-extruded samples fired at 900 °C

123

B.9:

Physical properties of FCI-extruded samples fired at 950 °C

124

B.10:

Physical properties of FCI-extruded samples fired at 1000 °C

125

B.11:

Physical properties of FCI-extruded samples fired at 1050 °C

126

B.12:

Physical properties of FCI-cast samples

127

B.13:

Physical properties of FPI-extruded samples dried at 110 °C

127

B.14:

Physical properties of FPI-extruded samples fired at 900 °C

128

B.15:

Physical properties of FPI-extruded samples fired at 950 °C

128

B.16:

Physical properties of FPI-extruded samples fired at 1000 °C

129

B.17:

Physical properties of FPI-extruded samples fired at 1050 °C

130

B.18:

Physical properties of FPI-cast samples

131

xv

LIST OF ABBREVIATIONS %

percentage

AP

apparent porosity

ASG

apparent relative density

av.

average value

b

breadth

BD

bulk density

CCS

cold crushing strength

dia.

diameter

GPIR

geopolymeric porous insulating refractory

h

height

l

length

LOI

loss on ignition

m

mass

MoR

modulus of rupture, (strength)

RSD

percentage relative standard deviation

RT

room temperature

SD

standard deviation

SEM

scanning electron microscope

WA

water absorption

XRD

x-ray diffraction

xvi

CHAPTER 1 INTRODUCTION

1.1

BACKGROUND

Inorganic waste consists of materials such as sand, dust, glass and many synthetically produced aluminosilicates. Large amounts of inorganic waste are available in South Africa in the form of coal fly-ash (Class F), iron-rich waste and phosphogypsum.

Certain applications have been found for some of these

inorganic waste materials but disposal remains a major environmental problem. Applications making use of these inorganic waste materials would be of benefit to the environment. The rapid increase in population in the world and the world economic growth has lead to an increase in demand for energy. Worldwide, coal reserves are the most stable and available fossil energy source. Utilisation of coal as an energy source, however, involves the generation of large amounts of waste material in the form of coal ash (an aluminosilicate material). It is estimated that more than 300 billion tons of coal fly-ash are produced annually (Ciccu et al., 1999). The recycle rate of this ash is low (Kikuchi, 1999). The amount of coal fly-ash produced annually, in the Republic of South Africa, ranks South Africa amongst the highest solid inorganic waste producers, even when compared with countries such as France, Australia, Hungary and Canada. It is estimated that approximately 350 million tons are produced mainly from power/electricity generation (Eskom) and in liquid fuel processing plants (Sasol). It is stored in dumps across South Africa. This amount is growing by 23 million

1

tons annually (Swanepoel & Strydom, 2002).

The use of landfills, the main

option in many countries for coal fly-ash and other inorganic waste disposal, leads to serious pollution (air, water and land pollution), and socio-economic problems. The Lethabo power station, near Vereeniging in the province of Gauteng in South Africa, produces more coal fly-ash than France (4.65 Mega ton per year (Mt.y-1)) or Hungary (4.09 Mt.y-1), and the same as Australia (5.75 Mt.y-1). Matla power station near the town of Kriel in Mpumalanga, South Africa, produces almost as much ash as the whole of Canada (3.15 Mt in 1990). (South African Ceramic Society, 1990). Another solid waste, phosphogypsum, is a by-product of the phosphoric acid process in fertiliser production. Production of one ton of phosphate, results in five ton of phosphogypsum as waste. Phosphogypsum has limited usage due to the presence of undesirable impurities such as phosphor(V) oxide (P2O5), fluoride, organic matter and alkalis contained in it (Lutz, 1995; Kumar, 2003). The main component of the slag emanating from the production of ferrovanadium-alloy is an iron-rich waste containing a substantial amount of iron(III) oxide (Fe2O3).

This iron-rich waste is currently discharged in close

proximity to the point of production. These enormous volumes of unused inorganic industrial waste, because of their nature, create disposal and environmental degradation problems that can potentially result in large-scale air and water pollution. The transport of waste material to dumping sites, inertisation treatments and disposal (Barbieri et al.,1999) results in cost increases of the final manufactured product and has further social and economic implications.

2

It is of economic and social importance to research the use of these solid wastes so as to develop new or alternative applications to exploit them as raw materials for processing useful products rather than dumping them where future inhabitants are at risk of health problems (Ilic et al., 2003).

Figure 1.1: Al2O3 – SiO2 binary phase diagram as applicable to refractories (Eriç & Hejja, 1996)

3

The refractory industry, with a consumption of 1 111 Mt.y-1 of traditional fireclay products, can be a potential major source for the utilisation or application of these aluminosilicate waste materials. The chemical composition of South African coal fly-ash from Lethabo, the main inorganic waste in our investigation, lies in the same Al2O3 – SiO2 binary system as the fireclay currently used in refractories (Hlaváč, 1983), see Figure 1.1. A study of the Al2O3-SiO2 (Figure 1.1) binary phase system is of particular interest as it is an aid in the understanding of mullite formation, which possesses good thermo-mechanical properties. Fireclay refractory materials also belong to the Al2O3-SiO2 binary phase system (Hlaváč, 1983). Mullite, an aluminosilicate mineral, is a technologically attractive material for refractory ceramics, due to its low thermal expansion and conductivity. Properties like chemical inertness and excellent mechanical properties at high temperatures contribute to the attractiveness of mullite and aluminosilicate minerals in applications such as refractory materials (Ildefonse et al., 1999).

If minor oxides in the materials are ignored, the composition of the Lethabo coal fly-ash lies in the centre of the mullite area of the CaO-Al2O3-SiO2 ternary phase diagram (Figure 1.2) Addition of lime will move the composition closely along the lime-anortite tie line in the graph (Hlaváč, 1983). The performance of refractory ceramics depends mainly on the final phase diagram of the combined raw materials and the amount of impurities in the raw materials.

4

Figure 1.2: Ternary phase diagram of the CaO-Al2O3-SiO2 system (Eriç & Hejja, 1996; Mao et al., 2006). The tick curves represent three-phase equilibria with the solid phase. The labeled areas show the liquidus surfaces of various solids. The thin curves represent the isothermal sections. By carefully choosing the proportions of the mix, it should be possible to design porous refractory ceramic materials from inorganic waste that can be used as an ingredient for the manufacturing of refractories (Hlaváč, 1983).

5

More stringent measures for special waste landfills, in combination with the emerging recycling philosophy, have encouraged the policy of the three Rs, which stand for recycle, reuse and reduce. Coal fly-ash due to its mineralogical, physical and chemical composition, and the presence of some elements and compounds, are excellent substitutes for clay in several industries. Using inorganic waste as raw materials has three main advantages. First, the use of a zero to very low cost raw material, secondly, the conservation of natural resources, and finally the elimination of solid waste.

1.2

PROBLEM STATEMENT

Diverse applications in the various possible fields have been identified for the use of coal fly-ash. However, they require development to render the end product more cost-effective and extend applications to reduce the amount of stockpiled waste product. Although there is potential to use coal fly-ash in the refractory industry, conventional fireclay refractories and/or thermal insulation materials, dry pressing or slip casting manufacturing processes are expensive. Despite the fact that several methods have been identified for the production of refractory or/and thermal insulation using coal fly-ash, cost and process variables remain issues that need to be resolved. This study will, therefore, focus on the characterisation and possible utilisation of coal fly-ash and other waste materials as a raw material to develop cost-effective and production-friendly procedures for the manufacture of porous refractory thermal insulating products by casting.

6

1.3

GOALS OF THIS INVESTIGATION

This investigation was launched to determine if the following waste materials could be used in the production of porous thermal insulating refractory materials: •

Coal fly-ash.

•

Phosphogypsum.

•

Iron-rich waste.

The materials used in this investigation, were employed in various compositions to achieve the properties of traditional fireclay products. Fireclay products still hold the largest share in the production of refractory materials.

1.4

HYPOTHESIS

A porous geopolymeric insulating refractory material can be developed using inorganic waste materials as the main ingredient.

1.5

GENERAL OBJECTIVES

In order to be able to manufacture an economic production friendly, porous geopolymeric insulating refractory material, the following should be addressed: •

A porous insulating refractory material needs to be developed that complies with the specifications of a porous insulating refractory material.

•

A process needs to be developed which is easy, economical and

•

Delivers a good product without shifting the waste disposal problem by creating different waste products.

7

1.6

SPECIFIC OBJECTIVES

To achieve the general objectives, the following specific objectives are: •

Characterisation of the physical and chemical properties of the inorganic waste materials.

•

Comparison of different mixtures to achieve a mix formulation with refractory properties.

•

Investigation of a manufacturing process for porous insulating materials in order to achieve an economical manufacturing route.

These goals will be achieved by progressing through the planned procedure of this research.

1.7

SCOPE OF THE THESIS

The thesis covers aspects in the use of coal fly-ash in ceramics, especially in refractory and thermal insulation products. Chapter 1 focuses on the introduction and problem statement of the thesis. The literature survey, Chapter 2, describes possible uses of coal fly-ash in refractory applications and/or thermal insulation products with the emphasis on the usage of geopolymerisation. In, Chapter 3, the methods used for the evaluation of the physical and chemical properties of the different solid inorganic waste materials and refractory mixtures are described. Chapter 4, gives the results and discussions.

The conclusions and the way

forward with recommendations are given in Chapter 5.

8

CHAPTER 2 LITERATURE SURVEY 2.1

INTRODUCTION

Many thousands of years ago, man tamed fire. The use of fire necessitated the use of refractories, materials that will withstand high temperatures without melting. The Egyptians were the first to melt iron in vessels and furnaces, which were simply a hollow of earth filled with iron ore and charcoal.

Radical

advancement in steelmaking technology was brought about by the invention of the Bessemer converter in 1856, a steel vessel lined with refractories. From that time, refractory materials have grown from a craft to an applied science (Hloben, 2000). The term refractory literally means “able to withstand not only heat but in many cases chemical attack, abrasion, thermal shock and rough handling” (Hloben, 2000).

Refractoriness points to the resistance of extreme conditions of heat

(temperature > 1000 °C) and corrosion when hot and molten materials are contained while being transported and/or processed. A high melting point is not the only prerequisite for a refractory material. Energy is an expensive commodity and metallurgical extraction processes tend to be very energy-demanding. An important aspect in refractory material usage is energy conservation. Additionally, high mechanical strength is required to resist load, impact, abrasion and erosion in refractory materials (Hloben, 2000). The melting temperature of refractory compounds is important for several reasons: •

Diffusion of atoms or ions in a solid, generally by complex lattice vacancy migration (Shackelford, 1988), depends exponentially on temperature. 9

•

Vacancy diffusion in oxide compounds becomes significant above 75 % of the absolute melting point (Tammann temperature).

•

Resistance to thermal decomposition correlates to the melting point as well as other properties such as transport properties which include thermal and electrical conductivity (Carniglia & Barna, 1992).

To save energy and obtain acceptable furnace shell temperatures, insulating materials are normally used as a back lining for the vessel. Thermally insulating refractories function by providing stagnant or “dead” gas space, that is, they contain large volume fractions of voids (low bulk density). Since it is impossible to build closed-cell structures into high-void-volume ceramics, these materials are all “open”: - i.e. susceptible to permeation and saturation by hot process liquids and to chemical attack by aggressive gases. It follows that they are not willingly exposed directly to liquids of any kind, nor to condensable vapours, nor gases of more than minor chemical reactivity (Carniglia & Barna, 1992). The prime criterion for insulating material selection are refractoriness and dimensional stability sufficient for the application. The service temperature limit of an insulating refractory material relates to composition, sintering temperature and void volume. Two reasons for interpolating an insulating layer between a hot working lining and the “outside” of the vessel are: •

To cool the back face, e.g. to preserve the mechanical integrity of an enclosing metal shell or for reasons of safety outside a wall or roof; and

•

to reduce the heat flux (thermal conductivity) through the lining and hence improve process fuel economy.

Both motives may apply simultaneously,

though the second usually predominates (Carniglia & Barna, 1992). The melting point of the oxides present in an insulating material is the first of several indicators of how it will behave, thermally, chemically and mechanically at high temperatures (Carniglia & Barna, 1992). Of all the ternary oxide compounds that are possible, only a few have high melting points. A list of oxides that may 10

be considered for industrial refractories, are listed in Table 2.1, which include the melting point of each substance and also gives the approximate Tammann temperature. The melting point will serve as a sufficient basis for considering the thermal stability of refractory mixtures.

Table 2.1: Melting points of refractory oxides (Carniglia & Barna, 1992). Name

Formula

Melting Point

Tammann

(°C)

Temperature (°C)

CaO

2927

2130

2CaO.SiO2

2130

1530

3Al2O3.2SiO2

1920

1380

Forsterite

2MgO.SiO2

1910

1370

Dialuminium silicate

Al2O3.SiO2

1868

1340

Iron chromite

FeO.Cr2O3

~1700

1210

Lime Dicalcium silicate Mullite

The focus of this study will be on coal fly-ash, iron-rich waste and phosphogypsum as inorganic waste materials and kaolin, ball clay and bentonite as natural inorganic materials as the major sources of oxides for the manufacture of insulating materials. First the requirements of a good insulating refractory material will be investigated followed by the investigation of the available waste and natural inorganic material oxides.

2.2

THERMAL-INSULATING CERAMICS

2.2.1 GENERAL In this study emphasis is placed on developing specifically insulating refractory materials, therefore the appropriate literature will be discussed. 11

Insulating bricks are made from a variety of oxides, most commonly fireclay (42 % SiO2 and 53 % Al2O3) or silica. The desirable features of these bricks are their light weight and low thermal conductivity, which usually results from a high degree of porosity. The high porosity of the brick is created during manufacturing by adding a fine organic material to the mix, such as sawdust. During firing, the organic addition burns out, creating internal pores. Another way to accomplish high porosity involves the addition of a foaming agent to the slip. Using this approach, the insulating brick can be cast instead of dry pressed. Additions of lightweight aggregates like diatomite, is another approach. Because of their high porosity, insulating bricks inherently have lower thermal conductivity and lower heat capacity than other refractory materials (Nyikos & King, 1996). Insulating refractories are used as back-up materials, but they can also be used as linings of furnaces where abrasion and wear by aggressive slag and molten metal are not a concern. Where they can be used, insulating materials offer several distinct advantages: •

Decreased heat losses through the furnace lining and less heat loss to the refractory leads to savings in fuel cost

•

The insulating effect causes a more rapid heat-up of the lining and lower heat capacity of the insulating refractory

•

Thinner furnace wall construction to obtain a desired thermal profile

•

Less furnace mass due to the lower mass of the insulating refractory.

A variety of insulating bricks provide a range of thermal efficiencies and strengths.

By composition and property characteristics, lightweight insulating

silica bricks are similar to conventional silica bricks with the exception of density and porosity.

Insulating bricks have a maximum service limit of 1650 °C and are, for example, used in the crowns of glass furnaces and tunnel kilns. Insulating bricks based on fireclay aggregate are also available with a combination of high strength and low 12

thermal conductivity (2.6 – 2.8 W.m-1.K-1) and these bricks offer a maximum service limit in the range of 1150 – 1261 °C. For even higher temperature applications, lightweight, insulating 90 % alumina bricks are used. These bricks possess high strength, good spalling resistance and low permeability (Nyikos & King, 1996). High-temperature processes require a considerable amount of energy. Often the energy consumption for high-temperature processes is used only partially for the actual technical process. An essential part of the energy escapes through the kiln walls into the atmosphere and is consequently lost to the process. In the case of kilns for ceramics, this loss of heat due to its escaping through walls can amount to 15 to 30 % of the total energy consumption required for the sintering process. To keep thermal energy inside the processing room of a thermal plant and prevent its escape into the ambience, special materials for the lining of plants, called high-temperature insulating materials, are necessary.

High-

temperature insulating materials are generally considered to be heterogeneous, multiple-phase, polycrystalline, highly porous refractory ceramics based on inorganic oxide materials, and this type of material often consists of a solid matter skeleton with a continuously dispersed porous phase. Besides energy saving aspects the lightweight construction ensures that the required temperature in high-temperature plants is reached more rapidly, as only a small proportion of the temperature released into the processing vessel/furnace is used for the heating of the walls and can predominantly be used for a balanced heating of the processing vessel/furnace and the loaded material.

Utmost

energetic and economical efficiency for the application of high-temperature insulating material is only reached when insulating material, construction of the kiln and lining technique of the walls, are regarded as an integrated whole. The result generally is a kiln wall consisting of several layers of different insulating materials. 13

2.2.2

DISADVANTAGES OF POROUS INSULATING MATERIALS

Besides considerable advantages of the highly porous insulating material, the following restrictions have to be mentioned as well: •

They show little stability due to their high porosity.

•

Additionally, they show an erosive sensitivity to flowing gases and a low abrasive resistance.

•

The gas permeability of high-temperature insulating material is high.

•

Due to their high, mostly open pores, gases and liquids can penetrate into the materials, thus the corrosion resistance against aggressive gases and melts is low.

•

On account of inferior stability, high temperature gradients and stresses due to low heat conductibility, they show little resistance to thermal shock.

•

They tend to sinter at higher temperatures because of their high porosity, which causes volume stability problems (Schulle & Schlegel, 1991).

Generally, high insulating refractory material is distinguished from lightweight material because of a total porosity of 45 to 75%. Extremely lightweight materials have a porosity of 75 to 85% and ultra-lightweight, high-temperature insulating materials have a total porosity greater than 85%. With respect to application temperature, high-temperature insulating materials can be classified as follows (Schulle & Schlegel, 1991): •

Temperature-resistant heat insulating materials for application temperatures up to 800 °C: these are regarded as thermal insulating materials and not refractory products.

•

Heat resistant insulating materials for application temperatures up to 1100 °C: calcium silicate materials; products from siliceous earth, perlite or vermiculite; silica based microporous heat insulators; alumosilicate fibres.

•

Refractory insulating materials for application temperatures up to 1400 °C: lightweight chamotte and kaolin bricks; lightweight castables; mixed fibres and aluminium oxide fibres. 14

•

High refractory insulating materials for application temperatures up to 1700 °C: lightweight mullite and alumina bricks; lightweight hollow sphere corundum castables and bricks; special high refractory fibres.

•

Ultra-high refractory insulating materials for application temperatures up to 2000 °C:

zirconia lightweight bricks and fibres; non-oxide compounds;

carbon.

International standards classify high-temperature insulating materials according to three criteria. These are (Schulle & Schlegel, 1991): •

The bulk density, and with it the porosity and indirectly the thermal conductivity as well as heat capacity.

•

The temperature (indicated as temperature limit for classification and application) at which the product shows a linear shrinkage of 1 to 7 % and hence volume stability, taking into consideration the maximum application temperature.

•

The main materials components, such as chamotte, silica, basic materials or specials.

Sometimes crushing strength and thermal conductivity are included for classifying high-temperature insulating materials.

2.2.3 SERVICE LIMITING TEMPERATURE The chemical composition, as a basic property of all refractory products, determines the sintering and melting of heat insulators and, the classification temperature.

As most high-temperature insulating materials consist of silica

(SiO2) and alumina (Al2O3) and the liquidus temperature of the SiO2-Al2O3 system (Figure 1.1) increases in the high alumina containing section corresponding to the Al2O3 content, the classification temperature rises with increasing Al2O3content in heat insulating materials. Due to the required volume stability, the 15

increased application temperature asks for a higher bulk density with increased stability and thermal conductivity (Schulle & Schlegel, 1991).

2.2.4 THERMAL CONDUCTIVITY Thermal conductivity, λ, is defined by Carniglia and Barna (1992) as: λ (T) = ρ (T).cp(T).a(T)

(Eq. 2.1)

where ρ is the bulk density, cp the specific heat, a the thermal diffusivity and T the temperature. The unit for thermal conductivity λ is W.m-1.K-1. Unlike the heat capacity, the thermal conductivity of heterogeneous mixtures is intensely sensitive to variations in microstructure. The governing micro structural features being intimately dependant on processing and thus largely uncoupled from composition, there is no reliable “rule of mixtures” for thermal conductivity.

Figure 2.1: Thermal conductivity of insulating fire brick and insulating castables (Carniglia & Barna, 1992). STL indicating the Service Temperature limit (in °F) of the Insulating Fire Brick (IFB) 16

The variation of the thermal conductivity with average body temperature for insulating fire bricks and insulating castables is consolidated in Figure 2.1. Internal heat transportation, and with it heat insulation, in high-temperature insulating materials, are decisively influenced by the structural composition and the temperature.

The effectiveness of the influenced temperature is also

controlled by the structure. Consequently, the structural composition plays a dominating part. As emphasised before, high-temperature insulating materials represent heterogeneous, porous multiple phase bodies.

These materials

facilitate extensive internal heat transportation by means of thermal conduction and heat radiation, which can be summed up as an effective thermal conductivity: •

The porosity, or bulk density, has to be adapted to the temperature of the application, or the temperature gradient, intended to be applied.

The

porosity required for a minimum effective thermal conductivity decreases with increasing temperature of application (Schulle & Schlegel, 1991). •

Porosity exerts the main influence on the effective thermal conductivity.

•

In cases of pure heat conduction the gas-filled pores have a small role to play, the solid matter structures a decisive one.

•

The effective thermal conductivity depends on the thermal conductivity of the pore-free, solid phase.

•

The type of pore gas and the gas pressure influence the thermal conductivity.

•

The pores should be as small as possible and efforts should be made to provide micro-porosity.

•

The microstructure of the solid matter should consist of loosely packed crystal structures and complicated crystal lattices with little symmetry, high defect density, as well as a substantial poly- or micro crystallinity.

•

The microstructure of the solid matter should show little transmission and a high degree of absorption in the infrared wave range.

•

Cracks and coarse pores more than 5 mm have to be avoided. 17

•

The overall structure should not allow gas permeability or at least at only on a small scale.

2.2.5

SHRINKAGE

The shrinkage behaviour of an insulating material is used for evaluating its maximum possible temperature of application. For this reason non-reversible length modification is measured over a long period of time at constant temperatures, the material being heated up on one or all sides in an oxidising atmosphere without corrosive influences. The classification temperature or the limit of application temperature corresponds to the temperature which allows a maximum admissible amount of linear shrinkage.

Most countries have

established different shrinkage standards. For refractory lightweight bricks and concretes there are shrinkages of 1 to 2 % and for refractory fibres 2 to 5 %, sometimes even up to 7 %. The isothermal heating time, required for thermal treatment, also fluctuates between 4 and 24 hours (Schulle & Schlegel. 1991).

A typical refractory is based on a mixture of low shrinkage clays with a small addition of plastic clays, for example ball clay, to ease shaping during manufacture and impart high green strength before firing (Hancock, 1988).

2.2.6 STRENGTH Kruger (1996) reported the development of castable refractories from coal fly-ash and cenospheres which have physical and chemical properties that are inherently beneficial for the manufacture of insulating refractories. Their use imparts excellent flow properties to the product, thus enhancing the placeability of monolithic linings. This phenomenon has been ascribed to the lubricating (ballbearing) effect of the spherical particles. Insulating refractories based on coal flyash exhibit remarkable strength to density ratios, excellent thermal shock resistance and an improved ratio of thermal conductivity to bulk density. Most 18

importantly, they are far more cost-effective than competitive products. In general, the higher the proportion of cenospheres in the product, the better will be the insulation efficiency and the lower the density. Compressive strength is, however, slightly lower at higher cenospheres content. The maximum service temperature of approximately 1250 to 1300 ºC does restrict the use of cenospheres and coal fly-ash to heat insulating or lower-temperature refractories. Careful selection of the particle size distribution of the coal fly-ash or cenospheres ensures optimum particle packing and enables the manufacture of low-shrinkage refractories (Kruger, 1996). The need for energy conservation necessitates insulating refractories with improved performance. The incorporation of cenospheres as part of the formulation has enabled the manufacture of products (Cenref) that have lower thermal conductivity and greater strength, which are lighter than the conventional Moler bricks widely used in industry. A cenosphere refractory can out-perform competitive products. Besides its superior insulation, its low apparent porosity is the most significant advantage. This is ascribed to the fact that the cenosphere refractory consists of isolated spheres lightly fused together; whereas other types of insulating refractories have interconnecting micro channels. Heat diffusion is more efficient along these micro channels than across the isolated air within the spheres. The inability of liquids to penetrate the monolithic cenosphere matrix also gives these refractories superior acid resistant properties. Service temperatures of 1300 ºC have been achieved and formulations have been developed that, at elevated temperatures, provide superior insulation to ceramic fibre. Due to their excellent in-service performance, domestically developed coal fly-ash and cenosphere refractories are gaining popularity (Kruger, 1996).

19

2.3

FIREBRICK REFRACTORIES AND THERMAL INSULATION

The group of aluminium silicate lightweight refractory bricks (fireclay and mullite bricks) is the most important and common group of lightweight refractories. (Hancock, 1988). Raw materials based on Al2O3, SiO2 and sometimes CaO are used to produce these bricks. Raw materials such as clays, kaolin, fireclay, sillimanite, andalusite, kyanite, mullite, alumina, alumina hydrate and corundum are used as a source of alumina (Figure 1.1). In addition to the granulated finegrained raw materials, coarse-grained and porous raw materials are also used. These include lightweight fireclay and hollow spheres (balls) consisting of corundum or mullite. The “burnout” process is applied most often to the production of lightweight refractory bricks. Fine saw dust, petroleum coke, lignite abrasion; fine waste products of cellulose and paperboard (carton) are utilised as organic materials to be burnt out. Burnout materials with low ash content are required in order to prevent negative effects on the hot properties of the refractory materials. The foam process is a further method of production to achieve high porosity refractory materials. Special soaps, saponins and sulfonates are used to make stable foams (Ferguson, 1982). The slurry for the ceramic body is often made separately from the foam emulsion. Foam and slurry are homogenised in an intensive mixer. By the controlled mixing of foam and slurry the required bulk density is adjusted. Lightweight, low density and high strength refractory bricks can be produced by mixing in evaporating substances (naphthalene), which have distinctive differences in their properties when compared with other bricks. Very fine pores guarantee that high dimensional accuracy of lightweight refractory products is achieved by casting, centrifuging or pressing (Hancock, 1988). During casting, the perforated metal moulds (forms) are lined with filter paper before being filled. Sulphite liquor, gypsum or concrete can be added in order to strengthen the 20

mixture and to speed up the setting. The centrifuging process of large blocks is very efficient and ensures excellent dimensional stability. Plastic, semi-dry and dry mixes are shaped by corresponding presses (extrusion, hydraulic or mechanical presses). The bricks, unfinished cylindrical pieces or blanks, are fired in chamber furnaces, bogie hearth furnaces or tunnel kilns. The firing temperature corresponds approximately to the classification temperature indicated by the producers. Due to high drying and firing shrinkage, cutting or grinding is necessary for most brick qualities in order to obtain the standard shapes. Hand forming, vibration or moulding processes produce bricks which are complicated in shape (Hancock, 1988). Otero et al. (2004) reported on the preparation of thermal insulating firebricks from coal fly-ash. Due to its morphological characteristics, physicochemical properties and pozzolanic activity, coal fly-ash has potential for use in the production of refractory insulating bricks in combination with clays, a binder (sodium silicate) and a foaming agent (50 % hydrogen peroxide). The bricks obtained exhibit the appropriate characteristics of mechanical resistance, porosity and thermal conductivity. Vilches et al. (2003) underlined the use of coal fly-ash and titanium waste in thermal insulation and fireproof applications. Plates were prepared from a mixture of coal fly-ash (>50 %) and titanium waste (>35 %).

Exfoliated vermiculite

(800 ºC). Refractories are only the start of yet another field of application for coal fly-ash and its derivatives. Although volumes used are currently modest, these are bound to increase as the refractory, and more especially the user industries, 21

realise the benefits that can be achieved. Development is continuing on these materials and the limits have not yet been reached. More products based on coal fly-ash and cenospheres will soon be seen with even lower thermal conductivities (Kruger, 1996).

Cenospheres are essentially thin-walled glass

spheres with a relative density of less than 1.0. They float on water and are recovered from the surface of ash disposal ponds and are of similar chemical composition to fly ash. Fly ash will be discussed in detail in Section 2.6.

2.4

CONVENTIONAL

SILICATE-BONDED

REFRACTORIES

VERSUS GEOPOLYMERS Previously silicate-bonded materials have been used in refractories. However, recent research projects on inorganic silicate materials have evolved a new product called a geopolymer, which can incorporate large amounts of coal fly-ash in its formulation. A geopolymer is an inorganic aluminosilicate, synthesised from predominantly silicon and aluminium materials of geological origin, or by-products such as coal fly-ash and granulated blast furnace slag (Cheng & Chiu, 2003). Geopolymers are versatile materials which can form composites with almost any material, hence providing the possibility of property amelioration in diverse applications, such as refractory, thermal insulation, fire resistance, etc., by careful addition of selected materials.

Davidovits (1991) pointed out that physical

properties, such as fusion temperature and coefficient of thermal expansion, are a function of the Si:Al ratio. Barbosa and Mackenzie (2003a; 2003b) investigated the thermal behaviour of inorganic geopolymers derived from sodium and potassium polysialate, with different inorganic fillers and found that, in general, properly cured potassium polysialate geopolymer showed little sign of shrinkage and melting up to 1400 ºC. 22

Crystalline

phases,

leucite

(KAlSi2O6)

and

kalsilite

(KAlSiO4),

form

at

approximately 1000 ºC. Silica-rich geopolymers such as potassium polysialatesiloxo materials are friable above 1200 ºC.

Properly cured sodium-based

geopolymers have a melting point around 1300 ºC.

2.5

PRODUCTION OF FOAM GEOPOLYMERS FROM WASTE MATERIALS

Recycling waste materials would aid in the protection of the environment. When the properties of waste products are such that it is possible to use them for high added value applications, these products stand a better change of competing than products made from primary materials. Coal fly-ash, iron-rich wastes and ball clay have chemical and physical properties that, in principle, make them suitable for recycling as geopolymeric materials. The

remarkable

geopolymerisation

achievements include

the

made

production

through of

mineral

geosynthesis polymers

and termed

geopolymers. These inorganic polymeric new materials can polycondense just like organic polymers, at temperatures lower than 100 °C (Hardjito et al., 2004b). Historically (Davidovits, 1991) geopolymerisation involves chemical reactions of aluminosilicate oxides (Al3+ in the fourfold coordination) with alkali polysilicates yielding polymeric Si-O-Al-O- bonds. The amorphous to semi-crystalline three dimensional silico-aluminate structures are of the poly (sialate) type (-Si-O-Al-O-), the poly (sialate-siloxo) type (-Si-O-Al-O-Si-O) and the poly (sialate-disiloxo) type (Si-O-Al-O-Si-O-Si-O-).

Geopolymeric

compounds

involved

in

materials

developed for industrial applications are either crystalline or non-crystalline (amophorous or glassy structures), whereas, several geopolymeric materials of practical interest are non-crystalline. This viewpoint has been debated (Swaddle, 2001; Provis et al., 2005). 23

These new generation of materials, when applied in the pure form, reinforced or with fillers, can be used for storing toxic chemicals or radioactive wastes, manufacturing of special concretes, moulds for moulding thermoplastics and in making tooling in the aluminium alloy foundries and metallurgy. High temperature techniques are no longer necessary to obtain materials that are ceramic-like in their structure and properties. Geopolymers can polycondense just like organic polymers at temperatures lower than 100 °C. As a result, geopolymeric materials are easy to make. Their physical properties make them viable alternatives for many conventional cements and plastics. Their synthesis at low temperatures with no CO2 emissions is energy-efficient and more environmentally friendly than many older materials (Van Jaarsveld, van Deventer & Lukey, 2003). The polycondensation potential of geopolymers is much higher than that of cement-based

materials.

advantageous

properties

Thus, such

as

geopolymer mechanical

materials properties,

possess

many

unique

high-

temperature (1200 °C) properties, long-term durability, easily recycled, an adjustable coefficient of thermal expansion, heavy metal ion-fixation and acid resistance. It is also a “Green Material” because of its low manufacturing energy consumption and low waste gas emission. The chemical bonds of Si-O and Al-O are among the most stable covalent bonds in nature.

Consequently,

geopolymers are considered as one of the candidates to solve the conflict of social development against environmental pollution as they can be utilised in the fields of fire resistance, nuclear wastes solidification, hazardous wastes disposal, binder, fast reparation, decoration, intelligent material and construction (Davidovits, 1991; Van Jaarsveld, van Deventer & Lukey, 2003). Portland cement production is under review due to the high levels of carbon dioxide released to the atmosphere. Geopolymer concrete is a new material that 24

does not need the presence of Portland cement as a binder. Instead, low-cost available materials such as coal fly-ash, that are rich in Si and Al, are used and activated by alkaline liquids to produce the binder.

This also has a positive

effect on the environment (Hardjito et al., 2004a). Since 1972, Davidovits has been developing a kind of mineral polymer material with the structure of a three dimensional (3D) cross-linked polysialate chain (-(Si-O)z-Al-O-) which resulted from the hydrolysation and polycondensation reactions of natural minerals or industrial aluminosilicate wastes such as clays, slag, coal fly-ash and pozzolan with alkaline activators below 150 oC. This “inorganic polymer” material was first named “Polysialate” in 1976 (Zhang, Gong & Lu, 2004). Nine years later, Davidovits coined another term “geopolymer”, in his US Patent, to represent this family of inorganic polymers. The term “geopolymer” has been wildly accepted (Davidovits, Davidovics & Davidovits, 1994; Zhang, Gong & Lu, 2004). A two-step mechanism for the geopolymer reaction was proposed. The first step can be named “activation step” including the dissolution of starting materials and the formation of orthosialate acid in a high pH, basic solution. The second step concerns mainly the further polycondensation between orthosialate acid and surface silanol groups and the formation of the 3D-cross-linked polysialate structure, which can be called the “polycondensation step” (Zhang, Gong & Lu, 2004).

2.5.1. GEOPOLYMER CHEMISTRY Geopolymers are chemically designed as polysialates. Sialate is an abbreviation for silicon-oxo-aluminate. The sialate network consists of SiO4 and AlO4 tetrahedra linked in an alternating sequence by sharing all of the interstitial oxygens. Positive ions (Na+, K+, Li+, Ca2+, Ba2+, NH4+ and H3O+) must be present

25

in the framework cavities to balance the negative charge of Al3+ in four fold coordination. Polysialate has the empirical formula: Mn[(SiO2)z.AlO2]n·wH2O where: M is a cation, usually an alkali, n is a degree of polycondensation, w ≤ 3 and z is 1, 2 or 3 (Comerie & Kriven, 2003). Polysialates are chain and ring polymers with Si4+ and Al3+ in four fold coordination with oxygen, and are amorphous to semi-crystalline. Apart from poly-sialate (-Si-O-Al-O-), poly-sialate siloxo (-Si-O-Al-O-Si-O-) and poly-sialatedisiloxo (-Si-O-Al-O-Si-O-Si-O) chemical groupings are also possible structural units for geopolymers, when the amount of silicate reactant increases in the reaction system (Comerie & Kriven, 2003). Geopolymerisation is exothermic and is given schematically in Figure 2.5. It is assumed that the reactions are carried out through oligomers (dimers or trimers) that provide the actual unit structure of the three dimensional, macromolecular edifices. When geopolymeric polymerisation is carried out at ambient temperature, amorphous or semi-crystalline structures are formed. However, when the geopolymers are synthesised at hydrothermal setting and hardening temperatures, in the 150 oC to 180 oC range, the geopolymeric products are crystalline in structure. The coordination of Si and Al in geopolymers detected by nuclear magnetic resonance (NMR) is four fold and the X-ray diffraction of geopolymeric binder is amorphous with no crystalline peak detectable. The difference between a geopolymeric binder and a geopolymeric product is that the geopolymeric binder is synthesised from a precursor such as 2SiO2.Al2O3 (calcined kaolinite), at ambient temperature. However, geopolymeric products or commercial products are different from the binder, because other materials or metals are involved in the system as an aggregate or reinforcement, such as for example, sand, SiC, and carbon fiber (Comerie & Kriven, 2003).

26

(-)

(Si2O5.Al2O2)n + nH2O KOH.NaOH ———–> n(OH)3–Si–O–Al –(OH)3 *

(-)

*

(-)

n(OH)3–Si–O–Al –(OH)3 KOH.NaOH ———–> (Na.K)(–Si–O–Al –O–)n + 3nH2O * * O O orthosialate (Na.K)–poly(sialate) ———————————————————————— (-)

(Si2O5.Al2O2)n + nSiO2 + nH2O KOH.NaOH ———–> n(OH)3–Si–O–Al –O–Si–(OH)3 * (OH)2 (-)

* KOH.NaOH

*

(-)

*

n(OH)3–Si–O–Al –O–Si–(OH)3 ———–> (Na.K)(–Si–O–Al –O–Si–O–)n + nH2O * * * * (OH)2 O O O ortho(sialate-siloxo) (Na.K)-poly(sialate-siloxo) Figure 2.2: Mechanisms of Geopolymerisation according to Davidtovits, (1991). Pozzolanic materials, high in SiO2 and often also Al2O3 are sufficiently reactive when mixed with water and CaO to produce calcium silicate hydrate (nCaO.mSiO2.wH2O) at ordinary temperatures and thereby act as hydraulic cements. The compound nCaO.mSiO2.wH2O has the properties of a rigid gel. These products can also be obtained from pozzolanic reactions of calcined clays and coal fly-ash. Pozzolanic reactions are accelerated by an increase in temperature and, in particular, the presence of an alkali metal hydroxide. South African coal fly-ash (Class F), low in CaO, is an example of a pozzolanic material (Taylor, 1997). The coal fly-ash can also serve as the reagent for the synthesis of geopolymers, although the reaction path is different from that of pozzolanic reactions. During

27

the synthesis of geopolymers (geopolymerisation) there is a definite interaction between the pozzolanic material with alkaline media and especially aqueous solutions of polysialate (Van Jaarsveld, Van Deventer & Lukey, 2003). The chemistry involved in geopolymerisation is close to that for the synthesis of zeolites, although the resultant products are different in composition and structure.

2.5.2. MATERIALS USED IN GEOPOLYMERISATION Three sources are needed for geopolymer synthesis: raw materials, active filler, and geopolymer liquor (Xu & van Deventer 2002). Raw materials can be industrial wastes, such as coal fly-ash, ball clay, blast furnace slag, red mud, waste glasses, or some natural minerals and rocks. Active filler, mainly supporting Al3+ ions, can be kaolinite or metakaolinite. Geopolymer liquor includes a sodium silicate solution acting as a binder, and alkali hydroxide for the dissolution of raw materials (Cheng & Chiu, 2003). Coal fly-ash is largely composed of glassy, spherical particles. The finest ashes are coarser than typical clays, with the average particle size and clays somewhat above and below two microns, respectively. The coarseness and sphericity of coal fly-ash act to reduce internal surface area when mixed with clays and increase void volume when mixed with aggregate. The introduction of coal fly-ash that possesses no plasticity has a ‘grogging’ effect on the clays. Shrinkage of clay bodies can therefore be lowered by addition of coal fly-ash (Addis, 1994). Ball clays of the best qualities contain 60 % or more of particles less than 0,0005 mm and up to 90% less than 0.001 mm, but many are much coarser. The larger particles in most ball clays are usually quartz, mica and other impurities present in small amounts. The variable and often large proportion of organic

28

matter causing the dark colour of the raw ball clay is mostly present as a film surrounding the clay particles (Cheng & Chiu, 2003). The strength of a geopolymer depends on the nature of the source materials. Geopolymers made from calcined source materials, such as metakaolinite (calcined kaolin), coal fly-ash, slag etc., yield a higher compressive strength when compared to those synthesised from non-calcined materials, such as kaolin clays. The source used for geopolymerisation can be a single material or a combination of several types of materials (Xu & van Deventer, 2002).

A

combination of sodium or potassium silicate and sodium or potassium hydroxide has been widely used as the alkaline activator (Palomo, Grutzeck & Blanco, 1999; van Jaarsveld, van Deventer & Lukey, 2003; Xu & van Deventer 2002; Swanepoel & Strydom 2002), with the activator liquid-to-source material ratio by mass in the range of 0.25-0.35 (Palomo, Grutzeck & Blanco, 1999; Swanepoel & Strydom 2002). Because heat is a reaction accelerator, curing of a fresh geopolymer is carried out mostly at an elevated temperature (Palomo, Grutzeck and Blanco, 1999). When curing at elevated temperatures, care must be taken to minimise the loss of water. Calcined source material of pure geological origin, such as metakaolinite, can be successfully cured at room temperature. (Davidovits, 1994; Barbosa, McKenzie & Thaumaturgy, 2000) Coal fly-ash, as the largest component of the inorganic waste materials, will be investigated with regard to its formation, production and previous applications. The natural inorganic materials will be discussed with regard to their structure and properties.

29

2.6

INORGANIC WASTE MATERIALS

2.6.1 COAL FLY-ASH Coal fly-ash is a solid material extracted by electrostatic and mechanical means from the flue gases of furnaces fired with pulverised bituminous coal (Addis, 1994). Coal ash, a ceramic material, is essentially an aluminosilicate glass with inclusions of mullite, spinel, quartz and lime. The properties of this coal ash are determined mainly by its unique chemical and mineralogical composition. In turn, these are dependent upon the type of coal, as well as the thermodynamic environment prevalent during the combustion processes.

In modern power

stations, the coal is ground to a very fine powder before being injected into the boilers. In the boilers the combustibles burn giving off heat energy to produce steam.

The non-combustibles form the ash.

Due to very high flame

temperatures the ash is in the liquid state in the flame and on cooling solidifies in the form of hollow spheres, as shown in Figure 2.3 (Mantel, 1991).

Figure 2.3: Typical coal fly-ash (Mantel, 1991). 30

While the composition of coal fly-ash produced within any one particular South African power station is remarkably consistent, there are differences between the various power stations. The major source of coal fly-ash in South Africa is the Lethabo power station near Vereeniging. The exact composition of the coal flyash is also dependent on the particular particle size range (Kruger, 1990). The surface area of various ashes varies from 400 – 600 m2.kg-1 (Mantel, 1991). The primary components of power station coal fly-ash are silica (SiO2), alumina (Al2O3) and iron oxide (Fe2O3), with varying amounts of carbon, calcium (as lime or gypsum), magnesium and sulphur (sulphides and sulphates) (Malisch, 1981).

2.6.1.1

World production of coal fly-ash

Worldwide, some authorities forecast coal fly-ash volumes of more than the current world output by as much as 800 x 106 ton by the year 2010, (Swanepoel & Strydom, 2002). In the United Kingdom, approximately 50 % of the coal fly-ash produced is used while in India only 6 % (Satapathy, 2000) despite various efforts in using coal flyash in traditional applications. In India, thermal power plants generate more coal fly-ash than in other countries. It is estimated that currently about 90 megaton of coal fly-ash is generated every year in India alone. Only a small amount of the total coal fly-ash generated is utilised in making bricks or concrete building blocks, or blending with cement (Chandra et al., 2005). According to Ilic et al. (2003), coal-fired power plants in Yugoslavia produce approximately 5 megatons of coal fly-ash per year. Of this only 20 kilotons are currently used in the cement industry for the production of paving slabs, building blocks and ready-mixed concrete. For this reason it is of utmost importance to develop new applications and uses for coal fly-ash. 31

The coal fly-ash used in this study was received from the company Ash Resources.

Coal fly-ash is an inorganic waste material from the coal fired

Lethabo electrical power station, situated near Vereeniging and Sasolburg in the Free State province of South Africa. The oxides present in coal fly-ash make it an ideal raw material. Coal fly-ash will introduce to the mixture the necessary oxides needed to manufacture insulating refractory materials.

2.6.2

PHOSPHOGYPSUM

Phosphoric acid waste gypsum (phosphogypsum) (Smadi, Haddad & Akour, 1999) is a by-product resulting from the phosphoric acid process for manufacturing fertilizers. The phosphogypsum used in this study was obtained from AECI/Kynoch. This material originated from fertilizer production. It consists mainly of CaSO4.2H2O and contains impurities such as P2O5, F- and organic substances. The quantity of phosphogypsum is very large: for each one ton of phosphate (P2O5) produced, there is a co-production of five tons of calcium sulphate (phosphogypsum). The annual world production of this material is 180 million tons. Only 15% of the phosphogypsum is utilised by cement and gypsum industries as a setting moderator for cement and for making gypsum plaster. The remaining 85% of phosphogypsum is not used, causing an environmental problem and creating a need for large areas for disposal. Therefore, attempts were made to use phosphogypsum in applications such as road and rail works fills, stabilisation of base course and building constructions. In addition many other applications of phosphogypsum are sought (Smadi, Haddad & Akour, 1999) as in some jurisdictions, phosphogypsum is considered a radio active waste due to the levels of radon and other radioisotopes present in it, which leads to disposal problems.

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Phosphogypsum (Lutz, 1995) has substantially higher water content than other synthetic gypsums or natural gypsum, often as high as 30%. This is only true immediately after production. This gypsum can also contain varying amounts of residual phosphates, sodium and fluorine compounds, organic products and other impurities depending on how the preceding phosphoric acid process and preparation step are managed. The particle size of this gypsum is usually below 200 micrometer.

Phosphogypsum has low strength and poor adhesive

properties, but is added to bodies to assist in setting of concrete (Mantel, 1991).

2.6.3 IRON RICH WASTE There is little, if any, literature on iron rich waste, as it is a waste product from the vanadium extraction process. Annual world production of vanadium pentoxide averaged 62 200 t between 1980 and 1993.

South Africa’s share of this

production has averaged at 42 %. South Africa’s reserves of vanadium-bearing titaniferous magnesites in the Bushveld complex are vast (Shürmann & Marsh, 1998).

The titaniferous magnitude magnetite of the Bushveld complex is not

amenable to physical beneficiation techniques, it contains sufficient vanadium to permit recovery by the salt-roast and leach process (Shürmann & Marsh, 1998). The smelting stage generates a titanium bearing slag, containing about 15% TiO2 and 75 % Fe2O3. Currently it is stockpiled (Grohmann, 1995). TiO2 and Fe2O3 act as a flux in ceramic materials. Iron-rich waste for this study was obtained as a slag from the vanadium manufacturing company Vametco, situated near Brits in the North-West province of South Africa. All of the above mentioned inorganic waste materials pose a problem to the South African industry with regard to waste disposal. Therefore new applications for these waste materials are continuously sought.

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2. 7 NATURAL INORGANIC SILICATE MINERALS With a few minor exceptions all the igneous rock-forming minerals are silicates, and they constitute well over 90 % of the earth’s crust (Klein & Hurlbut, 1993). According to Klein and Hurlbut (1993) the silicates are formed by the different arrangements of SiO4. When three of the oxygens of a tetrahedron are shared between adjoining tetrahedra, infinitely extending flat sheets are formed with a unit composition of Si2O5. phyllosilicates.

Such sheet silicates are also referred to as

When all four oxygens of a SiO4 tetrahedron are shared by

adjoining tetrahedra, a three-dimensional network with a unit composition of SiO2 is obtained. These framework silicates are also known as tectosilicates. The natural inorganic materials to be discussed in this section belong to phyllosilicates (clay minerals) and the tectosilicates (feldspar).

2.7.1 Phyllosilicates (Greek: phyllon, leaf) As the derivation of the name implies, most of its members have a platy or flaky habit and one prominent cleavage (Klein & Hurlbut, 1993). They are generally soft, of low relative density and may show flexibility or even elasticity of the cleavage lamellae (Buseck, 1983). Most of the members of the phyllosilicates are hydroxyl bearing with the -OH group located in the centre of the 6-fold rings of tetrahedra, at the same height as the unshared apical oxygens in the SiO4 tetrahedra (Klein & Hurlbut, 1993). The phyllosilicates are divided into four major groups: a serpentine group, a clay mineral group, a mica group and a chlorite group (Klein & Hurlbut, 1993). The most important group for this study is the clay mineral group which is further divided into the kaolinite minerals, talc minerals and pyrophyllite minerals.

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Clay is a rock term, and like most rocks it is made up of a number of different minerals in varying proportions (Grim, 1968). Clays also carry implication of very small particle size (