Kirsty_Stevenson_Thesis_The_Retention_of_Caesium_and_Strontium

Kirsty Stevenson
The Retention of Caesium and Strontium in
Cemented Zeolites.
UTS Sydney
BSc(Hons).
1997
i

DECLARATION
I hereby declare that this submission is my own work and that, to the best of my
knowledge and belief, it contains no material previously published or written by another
person (nor material which to a substantial extent has been accepted for the award of
any other degree or diploma of the university or other institute of higher learning),
except where due to acknowledgment is made in the text.
ii

ACKNOWLEDGMENTS
I wish to express my heartfelt gratitude towards Dr. Laurie Aldridge (ANSTO),
Dr. Abhi Ray (UTS), and Dr. Michael Stevens (UTS) for their continual support,
guidance and understanding throughout the course of this thesis.
I wish to thank Mr. Kevan Harder, from the Cement Waste Forms Group at
ANSTO for the time and advice he offered with regard to operations in the Cement
Laboratory.
I also wish to thank Ms Liz Keegan (ANSTO), Mr. Frank Cordaro (ANSTO),
and Mr. Jimmy Keegan (UTS) for the help they have rendered pertaining to the analysis
work.
Last but not least I wish to thank Mrs. Diane Stevenson for her support and for
the use of her computer.
iii

ABSTRACT
The aim of this work was to determine if there were significant changes between
the retention of caesium or strontium in cemented zeolites cured at different
temperatures. The zeolites used were a 65% pure clinoptilolite zeolite, a 90% pure
clinoptilolite zeolite, and synthetic A type zeolite, and two different Portland cements
were used to immobilise these zeolites.
The zeolites were ion-exchanged with caesium and strontium prior to
immobilisation in the hydrated Portland cements. The cemented zeolites were cured for
28 days at room temperature, and for 7 days at 70o
C or 150o
C. The cured cemented
zeolites were then leached in pure water or MCC-1 brine solution at temperatures of
25o
C or 40o
C for 28 days without replacement of the leachant solution. Leachate
aliquots were taken every 3, 7, 14, 21 and 28 days and analysed by Inductively Couple
Plasma – Mass Spectrometry (ICP-MS).
This study shows that (a) zeolites improve the retention of caesium and
strontium in cements, (b) cure temperature can modify the retention of caesium and
strontium in cemented waste forms, and (c) cemented zeolites when leached in the
MCC-1 brine solution exhibit worse caesium and strontium retention than when leached
in pure water.
It was also shown that the retention of caesium and strontium in cemented
zeolites was highly dependant on cement type, zeolite type, and leach temperature.
iv

TABLE OF CONTENTS
Declaration i
Acknowledgments ii
Abstract iii
Table of Contents iv
List of Tables vi
List of Figures viii
Chapter 1
1.0 Introduction 1
Chapter 2
2.0 Literature Survey 3
2.1 The Structure and Properties of Zeolites 3
2.2 Ion Exchange Reactions in Zeolites 5
2.2.1 Ion Exchange of Clinoptilolite 11
2.2.2 Ion Exchange of Synthetic Zeolite A 14
2.2.3 Zeolite Ion Exchange Applications: 16
Treatment of Radioactive Waste Waters
2.3 Cement-Based Immobilisation of Zeolites 18
2.3.1 Composition of Portland Cement 19
2.3.2 Chemistry of Portland Cement Setting 20
2.3.3 Reactions Between Zeolite and Hydrated 23
Portland Cement
2.4 Leaching of Cement-Based Systems 27
2.4.1 Leaching Model 28
2.4.2 Factors Affecting Leachability 30
Chapter 3
3.0 Experimental Procedures 33
3.1 Materials 33
3.1.1 Zeolites 33
3.1.2 Cements 35
3.1.3 Leachant Solutions 36
3.2 Equipment 37
3.3 Procedures 43
3.3.1 Ion Exchange 43
3.3.2 Cementation and Hydration 44
3.3.3 Leaching 47
3.3.4 Analysis 48
v

Chapter 4
4.0 Results and Discussion 49
4.1 No Zeolite 49
4.2 Werris Creek Clinoptilolite 54
4.3 Teague Clinoptilolite 57
4.4 Synthetic Zeolite A 60
Chapter 5
5.0 Summary and Conclusions 64
Chapter 6
6.0 Recommendations for Further Work 65
Chapter 7
7.0 References 66
Chapter 8
8.0 Appendix A 72
vi

LIST OF TABLES
Table 1. Exchange Capacity of Zeolites 8
Table 2. Clinoptilolite: Physical and Chemical Data 12
Table 3. Zeolite A: Physical and Chemical Data 15
Table 4. Chemical Composition of Portland Cement 19
Table 5. Mineral Composition of Portland Cement 19
Table 6. Basic Hydration Reactions of Portland Cement 21
Table 7. Composition of the Aqueous Phase in Cements 22
Table 8. Relative Caesium Retentions of Zeolites After 34
Leaching in Distilled Water for 7Days
Table 9. Composition of Aalborg Lion Brand Portland Cement 35
Table 10. Composition of Berrima Portland Cement 35
Table 11. Mix Compositions 44
Table 12. Retention of Strontium and Caesium in Berrima Cement 50
After 28 Days Leaching
Table 13. Retention of Strontium and Caesium in Aalborg Cement 50
After 28 Days Leaching
Table 14. Retention of Strontium and Caesium in Cement 52
After 28 Days Leaching in Pure Water
Table 15. Retention of Strontium and Caesium in Cemented Zeolites 52
After 28 Days Leaching in Pure Water
Table 16. Retention of Strontium and Caesium in Berrima Cement/ 55
Werris Creek Clinoptilolite After 28 Days leaching
Table 17. Retention of Strontium and Caesium in Aalborg Cement/ 55
Werris Creek Clinoptilolite After 28 Days Leaching
Teague Clinoptilolite After 28 Days Leaching
vii

Table 19. Retention of Strontium and Caesium in Aalborg Cement/ 58
Teague Clinoptilolite After 28 Days Leaching
Wessalith P After 28 Days Leaching
Wessalith 80 After 28 Days Leaching
viii

LIST OF FIGURES
Figure 1. Types of Ion Exchange Isotherms 7
Figure 2. Rate of Uptake of Ammonium Ions 10
Figure 3. Configuration of the T10O20 Units of Tetrahedra 11
in the Framework Structure of Clinoptilolite
Figure 4. (a) The Truncated Cuboctahedron and 14
(b) The Arrangement in the Framework of Zeolite A
Figure 5. The Phases of Portland Cement Setting 22
Figure 6. Ion Exchange Between Clinoptilolite 24
and Ca(OH)2 Solution
Figure 7. Schematic Diagram of Leaching Model 29
Figure 8. Hobart 3-Speed Mixer 38
Figure 9. 4744 General Purpose Acid Digestion Bomb 41
Figure 10. Inductively Couples Plasma – Mass Spectrometer 42
Figure 11. Hydrated Cemented Zeolite Cylinder 46
CHAPTER 1
1.0 INTRODUCTION
ix

Radioactive wastes are generated in a number of different kinds of facilities and arise in a wide
range of concentrations of radioactive materials, and in a variety of physical and chemical forms. There is
also a variety of alternatives for treatment and conditioning of the wastes prior to disposal. Zeolites offer
attractive, cost-effective alternatives for remedial clean up of radioactive wastes. The usefulness of
zeolites for pollution control depends primarily on their ion-exchange capabilities. The ion-exchange
properties directly result from the porous, three-dimensional framework structure of these hydrous
aluminosilicate materials. The framework consists primarily of silica tetrahedra with aluminium
substituting for silicon in the framework to create a net negative charge on the structure. Sodium, calcium,
potassium, and other cations balance the charge and occupy ion-exchange sites in the structure.
Zeolites have been used extensively to remove radioactive caesium and strontium from
contaminated wastewaters. The contaminated zeolites themselves must be conditioned before they can be
disposed of in a near-surface repository. Portland cement can be conveniently used to immobilise zeolites
that contain radioactive ions. The hydration of Portland cement involves a series of exothermic reactions
resulting in the formation of calcium-silicate-hydrate structures.
Since the rate of cement reactions and their phase composition can be altered substantially by the
heat evolution during hydration,i
it is important to establish if cemented zeolites hydrated at different
temperatures do condition zeolites in a similar manner.
The aim of this work is to determine the retention of caesium and strontium in cemented zeolites
cured at room temperature, 70o
C and 150o
C. These conditions resemble those that would be expected by
the hydration of a large mass of cement where the heat of hydration can be in excess of 150o
C.ii
For the
disposal of cemented zeolites in near-surface repositories the ground water temperature is expected to be
in the vicinity of 25o
C. However, some laboratories insist on a leaching at 40o
C to increase the reactivity
of the cemented waste samples.
x

CHAPTER 2
2.0 LITERATURE SURVEY
2.1 The Structures and Properties of Zeolites
Zeolites are crystalline, hydrated aluminosilicates of group I and group II elements, in particular,
sodium, potassium, magnesium, calcium, strontium, and barium. Structurally the zeolites are
“framework” aluminosilicates that are based on an infinitely extending three-dimensional network of
AlO4 and SiO4 tetrahedra linked to each other by sharing all the oxygen atoms.
Zeolites may be represented by the empirical formula:iii
M2/nO•Al2O3•xSiO2•yH2O
In this oxide formula, x is generally equal to or greater than two since AlO4 tetrahedra are joined only to
SiO4 tetrahedra, M is the cation and, n is the cation valence. The framework contains channels and
interconnected voids that are occupied by the cation and water molecules. The cations are quite mobile
and may usually be exchanged, to varying degrees, by other cations. The cation selectivity properties of
zeolites are of great importance in the treatment of radioactive wastes [2.2.3]. Intracrystalline “zeolitic”
water in many zeolites is removed continuously and reversibly. In many other zeolites, mineral and
synthetic, cation-exchange or dehydration may produce structural changes in the framework.
The structural formula of a zeolite is best expressed for the crystallographic unit cell as:
xi

Mx/n[(AlO2)x (SiO2)y]•wH2O
Where M is the cation of valence n, w is the number of water molecules and the ratio y/x usually has
values of 1 – 5 depending upon the structure. The sum (x+y) is the total number of tetrahedra in the unit
cell. The portion within [ ] represents the framework composition.
The characterisation of a previously unknown mineral or synthetic material, such as a zeolite,
requires structural, compositional, and physiochemical information. Zeolites are classified into groups
according to common features of the aluminosilicate framework structures, that is, important properties
have a structural interpretation. The structural-related properties of zeolites include:
1. High degree of hydration and the behaviour of “zeolitic” water
2. Low density and large void volume when dehydrated
3. Stability of the crystal structure of many zeolites when dehydrated.
4. Cation-exchange properties
5. Uniform molecular-sized channels in the dehydrated crystals
6. Various physical properties such as electrical conductivity
7. Adsorption of gases and vapours
8. Catalytic properties
xii

2.2 Ion-Exchange Reactions in Zeolites
The cation-exchange properties of zeolite minerals were first observed more
than a century ago.iv
The exchangeable cations of a zeolite are only loosely bonded to
the tetrahedral framework and can be removed or exchanged easily by washing with a
strong solution of another ion. As such, crystalline zeolites are some of the most
effective known ion-exchangers, with capacities of up to 3 or 4 milliequivalents per
gram.
The ion-exchange capacity is a function of the degree of substitution of
aluminium for silicon in the framework structure; the greater the substitution, the
greater the charge deficiency, and the greater the number of alkali or alkaline earth
cations required for electrical neutrality. In practice, however, the cation-exchange
behaviour is dependant on a number of other factors as well, including;
(1) The nature of the cation species, the cation size, both anhydrous and
hydrated,
and cation charge
(2) The temperature
(3) The concentration of the cation species in solution
(4) The anion species associated with the cation in solution
(5) The solvent
(6) The structural characteristics of the particular zeolite under investigation
xiii

Cation-exchange in zeolites is accompanied by dramatic alteration of stability,
adsorption behaviour and selectivity, catalytic activity and other important physical
properties. The cation-exchange reactionv
can be expressed simply as:
A1
(z) + B2
(s) = A2
(z) + B1
(s)
Where A1
is the cation in the zeolite (z), and B2
is the cation in the solution (s). Cation-exchange
equilibria between a zeolite and a solution are usually depicted by an ion-exchange isotherm, which plots
equivalent molal fraction of the exchanging cation in the zeolite phase (Az) as a function of the equivalent
molal fraction of the exchanging cation in the solution phase (As). The different kinds of selectivities and
isotherm shapesvi
shown in Figure 1 reflect the diversity of zeolite frameworks and the stabilities of
cations in various sites within the structures.
xiv

Figure 1. Types of ion-exchange isotherms for the reaction As + Bz ≡ Az + Bs .6
Five types of isotherms are illustrated: (a) selectivity for the entering
cation over the entire range of zeolite composition; (b) the entering
cation shows a selectivity reversal with increasing equivalent
fraction in the zeolite; (c) selectivity for the leaving cation over the
entire range of zeolite compositions; (d) exchange does not go to
completion although the entering cation is initially preferred. The
degree of exchange,
xmax < 1 where x is the ratio of equivalents of entering cation to the
gram equiv. of Al in the zeolite; (e) hysteresis effects may result from
formation of two zeolite phases.
In certain species, cations may be trapped in structural positions that are relatively inaccessible,
thereby reducing the effective exchange capacity of that species for that ion. Also, cation sieving may
take place if the size of the cation is too large to pass through entry ports into the central cavities of the
structure. Analcime, for example, will exchange almost completely its sodium for rubidium (ionic radius
= 1.49Χ), but not at all for caesium (ionic radius = 1.65Χ).vii
Variations in structure, cation sites, cation
population, and distribution lead to considerable variation in the ion-exchange behaviour. The ultimate
base exchange capacity is observed with zeolites of low Si/Al ratio. Table 1 shows the exchange capacity
of various hydrated zeolites in the powdered form.
Table 1. Exchange Capacity of Various Zeolites7
Zeolite Si/Al Ratio Exchange Capacity
(milliequiv/g)
Chabazite 2.0 3.9
Mordenite 5.0 2.3
Erionite 3.0 3.1
Clinoptilolite 4.5 2.2
xv

Zeolite A 1.0 5.5
Zeolite X 1.25 4.7
Zeolite Y 2.0 3.7
Zeolite T 3.5 2.8
Unlike most non-crystalline ion-exchangers, such as organic resins or inorganic aluminosilicate
gels, the framework of a crystalline zeolite dictates its selectivity towards competing ions, and different
structures offer different sites for the same cation. The hydration spheres of high field-strength ions
prevent their close approach to the seat of charge in the framework; therefore, in many zeolites, ions with
low field strength are more tightly held and selectively taken up from solution than other ions. For
example, in the zeolite clinoptilolite, the small amount of aluminium substituting for silicon in the
framework results in a relatively low ion-exchange capacity (about 2.3 meq/g); however, its cation
selectivity is as follows:viii
Cs > Rb > K > NH4 > Ba > Sr > Na > Ca > Fe > Al > Mg > Li
Thus, clinoptilolite has a decided preference for larger cations.
Synthetic zeolite A, on the other hand, shows a widely different type of cation selectivity, as
evidenced by the following sequences for mono- and divalent cations:ix
Ag > Tl > Na > K > NH4 > Rb > Li > Cs
Zn > Sr > Ba > Ca > Co > Ni > Cd > Hg > Mg
Hence clinoptilolite would be expected to favour caesium removal from solutions and synthetic zeolite A
would be expected to favour strontium removal.
Previous workx
has shown that different zeolites exhibit different rates of uptake of radioactive
ions. The rate of ammonium ion uptake of three zeolites was measured since there was no ion selective
xvi

electrode for caesium, and the selectivity of clinoptilolite zeolite for caesium is similar to ammonium
ions. A clinoptilolite zeolite from America (particle size 35µm) had a similar rate of uptake to that of a
synthetic A type zeolite (particle size 4µm), both zeolite types removed most of the ammonium ions from
solution in a period of 5 minutes. However, a clinoptilolite zeolite from Australia (particle size 73µm) had
a very slow rate of uptake, continuing to remove ammonium ions after 20 minutes [Fig. 2]. This suggests
that the coarser particle size of the clinoptilolite from Australia results in its lower rate of acceptance of
ammonium ions.
Figure 2. Rate of uptake of ammonium ions by zeolites10
2.2.1 Ion-Exchange of Clinoptilolite
Clinoptilolite is classified as a morphologically lamellar zeolite (a group 7 zeolite). The common
structural feature in the framework structures of group 7 zeolites is the special configuration of
tetrahedraxi
shown in Figure 3.9
Each tetrahedron belongs to one of these elements, which contain 4- and
5- rings. These are arranged in sheetlike arrays, which accounts for the cleavage properties of these
zeolites.
xvii

Figure 3. Configuration of the T10O20 units of tetrahedra in the framework structure of
Clinoptilolite9
Clinoptilolite is very stable towards dehydration and readily readsorbs H2O and CO2. Some
varieties adsorb O2 and N2. Table 2 lists the physical and chemical data for clinoptilolite.xii
Table 2. Clinoptilolite: Physical and Chemical Data12
Structure Group:
Reference:
Chemical Composition
Typical Oxide Formula:
Typical Unit Cell Contents:
Variations:
Crystallographic Data
Symmetry:
Space Group:
Unit Cell Constants:
Structural Properties
Framework:
Void volume:
Dehydrated-
Effect of Dehydration:
Largest Molecule Adsorbed:
Kinetic Diameter, Φ, A:
7
pg 139, ref 179-181 Breck
(Na2, K2)O • Al2O3 • 10SiO2 • 8H2O
Na6[(AlO2)6(SiO2)30] • 24H2O
Ca, K, Mg also present; Na, K > Ca
Si/Al, 4.25 to 5.25
Monoclinic Density: 2.16g/cc
I 2/m Unit Cell Volume: 2100A3
a = 7.41
b = 17.89
c = 15.85
∃ = 91°29’
Possibly related to heulandite but not determined
0.34cc/cc Framework Density: 1.71 g/cc
Very stable - in air to 700°C
O2
3.5
xviii

Clinoptilolite displays an ion sieve effect for large organic cations.xiii
It has been shown by Ames
that the degree of exchange decreases with increasing size of the
cation.xiv, xv, xvi
The number of water ions varies linearly with (1) the degree of exchange and (2) the
volume of the organic ion. Clinoptilolite is quite selective for ammonium and caesium ions as compared
to other zeolites. It is the clinoptilolite-type zeolites high affinity for caesium that enables the zeolite to be
used for the removal of caesium from radioactive wasters, even when other ions are present.
2.2.2 Ion-exchange of Synthetic Zeolite A
The aluminosilicate framework of synthetic zeolite A is based upon the double 4-ring structure,
D4R. The aluminosilicate framework of zeolite A is generated by placing the cubic D4R units (Al4Si4O16)
xix

in the centres of the edges of a cube of edge 12.3 A. This arrangement produces truncated octahedral units
centred at the corners of the cube, producing truncated octahedral units centred at the corners of the cube
[Fig. 4].xvii, xviii
The unit cell of a zeolite A contains 24 tetrahedra, 12 Al2O4 and 12 SiO4.
Figure 4. (a) The truncated cuboctahedron and (b) the arrangement in the framework of
zeolite A17,18
Table 3 lists the physical and chemical data for zeolite A.xix
Table 3. Zeolite A: Physical and Chemical Data19
Structure Group:
Reference:
Chemical Composition:
Typical Oxide Formula:
Typical Unit Cell Contents:
Variations:
Crystallographic Data:
Symmetry:
Space Group:
Unit Cell Constants
Structural Properties:
Framework:
3
102,103,105-112,115,172
Na2O • Al2O3 • 2 SiO2 • 4.5 H2O
Na12[(AlO2)12(SiO2)12] • 27 H2O, pseudo cell
and 8X for true cell
Si/Al = ~0.7 to 1.2
Cubic Density: 1.99 g/cc
Pm3m Unit Cell Volume: 1870 A3
a = 12.32 A, pseudo cell
a = 24.64 A for true cell
Cubic array of ∃-cages linked by D4R units
Void Volume: 0.47 cc/cc
xx

Channel System:
Largest Molecule Adsorbed:
Kinetic Diameter, Φ, A:
Framework Density:1.27 g/cc
Three-dimensional
C2H4 at RT, O2 at -183°C
3.9 and 3.6
Zeolite A displays a double ion-sieve action. Only small cations can penetrate the single 6-rings
into the ß-cages. Large organic cations cannot penetrate the 8-rings into the α-cages. Ion-exchange
equilibria in zeolite A have involved mostly univalent and divalent counter ions. Hence, zeolite A has a
strong affinity for strontium, enabling A-type zeolites to be used for radioactive waste treatment.
2.2.3 Zeolite Ion-Exchange Applications: Treatment of Radioactive
Waste Waters
Disposal of radioactive materials from the reprocessing of nuclear fuels is a serious problem.
Earlier workers have found that zeolites may be used to remove long-lived caesium and strontium
isotopes from waste waters.xx
The Three Mile Island nuclear accident illustrates how zeolites were
successfully used to treat radioactive wasters.
At Three Mile Island Nuclear Generating Station an accident resulted in a release of significant
quantities of radioactive fission products (~ 2800 m3
) from the reactor fuel to various parts of the plant.
In particular, large quantities of water containing these radioactive contaminants were produced.xxi
Inorganic zeolites were chosen for the waste water treatment because of their high stability to ionising
radiationxxii
, and their selectivity for radionuclides in solutions containing competing cations such as
sodium [2.2].
The decontamination process involved ion-exchange of the radioactive cations, caesium and
strontium, onto a blend of two zeolites consisting of a naturally occurring zeolite and a synthetic zeolite.
Decontamination of the water in the reactor-building basement removed ≅ 2800,000 Ci of 137
Cs and ≅
12,000 Ci of 90
Sr onto the zeolite media.xxiii
The contaminated zeolite media was then stored and will
eventually be immobilised in a solid form, perhaps by hydrated cement.
xxi

The Three Mile Island accident illustrates the importance of zeolites in the containment of
radioactive wastes through ion-exchange. However, zeolites containing radionuclides 137
Cs and 90
Sr
cannot simply be disposed of because contact with ground waters containing cations could result in
exchange with the zeolite’s radioactive cations. These radionuclides would then enter the ground water.
Hence, it is necessary to immobilise the contaminated zeolites in a solid form both to inhibit water ingress
and to increase the retention of the radionuclides. Cement based materials are the most commonly used
waste forms for solidifying non-high-level radioactive waste.
2.3 Cement-Based Immobilisation of Zeolites
The objectives of immobilisation of radioactive waste are to convert the waste into forms which
are:
1. Leach resistant so that the release of radionuclides will be slow even though flowing
water may contact them.
xxii

2. Mechanically, physically and chemically stable for handling transport and disposal.xxiv,
xxv
The cementing of radioactive waste liquids is known as a suitable and approved process for
conversion into a solid, transportable and final storable form.xxvi
Incorporation of the low and intermediate
level wastes into cement-based systems has been routinely used in the nuclear power industry. Although
cement has several unfavourable characteristics as a solidifying material, such as low volume reduction
and relatively high leachability, it produces many practical advantages: good mechanical characteristics,
low cost, easy operation and radiation and thermal stability. The ability of cement to immobilise zeolites
containing radioactive ions depends on the composition of the cement, the chemistry and mechanisms of
cement setting, and the reactions occurring between the zeolites and the cement.
2.3.1 Composition of Portland Cement
Portland cement is composed chiefly of silica (SiO2), lime (CaO) and alumina (Al2O3), but also
contains small quantities of magnesia (MgO), ferric oxide (Fe2O3), sulfur trioxide (SO3), and other oxides
introduced as impurities in the raw materials used in its manufacture. Typical compositional analyses are
shown in Tables 4 and 5.xxvii
Table 4. Chemical Composition of Portland Cement27
Chemical
Compound
Chemical
Composition (%)
SO3 2.0 – 3.0
SiO2 18 – 24
Al2O3 4 – 8
Fe2O3 1.5 – 4.5
CaO 62 – 67
MgO 0.5 – 4.0
Table 5. Mineral Composition of Portland Cement27
xxiii

Mineral
Compound
Mineral
Composition (%)
C3Sa
45
C2Sb
27
C3Ac
11
C4AFd
8
Free CaO 0.5
CaSO4 3.1
a
3CaO•SiO2 = tricalcium silicate
b
2CaO•SiO2 = dicalcium silicate
c
3CaO•Al2O3 = tricalcium aluminate
d
4CaO•Al2O3•Fe2O3 = tetracalcium aluminoferrite
2.3.2 Chemistry of Portland Cement Setting
A knowledge of Portland cement setting is important to successfully use cement-based
immobilisation for radioactive wastes. The chemistry of cement setting greatly affects the ability of
cement to immobilise zeolites containing radioactive ions. Various theories have been advanced to
explain the setting of cement.xxviii
Two models, the crystallinexxix
model and the osmotic or gelxxx
model,
emphasise different setting mechanisms, and it is important to realise that after 100 years, the setting
mechanism of Portland cement is still not fully understood. In both models, the same basic reactions
occur. In the presence of water, each of the major crystalline compounds hydrates, but the products are
different and their contributions to the final waste form are different. Tricalcium aluminate and sulfates
react almost immediately to form hydrates. If sufficient sulfate is present, the reaction product is hydrated
calcium aluminate sulfate, which coats the surface of the particles, inhibiting further hydration.
The basic hydration reactions are given in Table 6.xxxi
Reaction starts when the cement powder
and water are mixed together. First C3A hydrates, causing the rapid setting that produces a rigid structure.
The ettringite that forms does not contribute to setting, but coats the cement particles and retards setting
reactions. Hydration of C3S and C2S, which account for approximately 75 percent of the cement by
weight, is responsible for strength development after the initial set. The reaction products in both cases
are the same – (1) the C3S2H3 gel, which at temperatures above 150o
C is transformed into crystalline
tobermorite, and (2) crystalline calcium hydroxide.
xxiv

It is important to note that the hydration of cement is an exothermic reaction and is capable of
producing high temperatures, as indicated by the amount of heat evolved on reaction [Table 6].
The four stages in the hydration of Portland cement are illustrated in Figure 4.xxxii
Initially, the
cement grains dispersed in water appear as in Figure 5(a); after two minutes, calcium sulfoaluminate
hydrate begins to form on the surfaces of the grains [Fig. 5(b)]. Two hours later the sulfoaluminate
hydrates, and possibly the other hydrates, begin forming an intermeshing network that causes setting [Fig.
5(c)]. After two days, the network has developed further due to the hydration of calcium silicates, forming
tobermorite and causing hardening [Fig.5(d)]. Portland cements are intensely alkaline, typically having an
internal pH greater than 13. Thus the pore fluid in hydrated cement is not pure water, but is instead an
aqueous solution containing high concentrations of alkaline components. Table 7xxxiii
shows some typical
values.
Table 6. Basic Hydration Reactions of Portland Cement31
Reactants Products Heat Evolved
(cal.g-1
)
C3A + 6H C3AH6 207
C3A + 3CS + 32H C6AS3H32
(ettringite)
347
2C3S + 6H C3S2H3 + 3CH 120
2C2S + 4H C3S2H3 + 3CH
(tobermorite gel)
62
C + H CH 279
xxv

Figure 5. The phases of Portland cement setting32
Table 7. Composition of the aqueous phase in cements33
Element Conc. Range
(g/L)
Ca 0.01 – 0.1
Na 1 – 10
K 2 – 20
Al 0.05 – 0.1
SO4
2-
0.01 – 0.02
2.3.3 Reactions between Zeolite and Hydrated Portland Cement
The reactions occurring between cement and zeolite in the presence of water, resulting in the
formation of low lime C-S-H or other phases are considered to be pozzolanic. Upon suitable activation,
zeolites are the source of pozzolanic activity, and as such, are inherently cementious. A pozzolana may be
simply defined as; xxxiv
“A material which is capable of reacting with calcium hydroxide in the presence of water to
produce cementious compounds.”
Reactive aluminosilicate materials, such as zeolites, form hydrated calcium silicates when
combined with calcium hydroxide in the presence of water.
Ca3SiO5 + yH2O → (CaO)x (SiO2)(H2O)y + zCa(OH)2
Ca2SiO4 + yH2O → (CaO)x (SiO2)(H2O)y + zCa(OH)2
Angus et. al.(1984) suggests that the reaction of calcium hydroxide in cement with zeolites
leads to a series of reactions, which are detrimental to the containment of nuclear wastes in cemented
zeolites.
xxvi

When caesium-exchanged clinoptilolite zeolite is immobilised in Portland cement, a pozzolanic
reaction occurs, leading to the release of caesium. The pozzolanic reaction occurs between the
clinoptilolite and the Ca(OH)2 produced during the cements hydration.
xCa(OH)2 + SiO2 → (CaO)x (SiO2)(H2O)y
In the first reaction, Ca2+
from the pore solution-exchanges in the zeolite. This reaction was
simulated by using saturated Ca(OH)2 solution to exchange with the caesium-loaded zeolite [Fig. 5].xxxv
At caesium loadings where more than 50 percent of the zeolite exchanged sites were filled, the stability of
caesium in the zeolite was diminished due to the rapid exchange of Cs+
for Ca2+
in the pore solution of the
cement. Exchange of Cs+
for the Ca2+
results in leaching of the caesium through the cement-zeolite
system until the attainment of a steady state, by which no further exchange takes place. It was shown that
the zeolite content is greatly diminished, if not destroyed by pozzolanic reactions, resulting in the release
of mobile caesium into the cementious system.
Figure 6. Ion-exchange between clinoptilolite and Ca(OH)2 solution35
To combat this, the use of low caesium loadings (< 50%), low cure temperatures (<100°C), and
the caesium exchanged from pore solutions should improve the caesium retention. The effect of
pozzolanic reactions on the caesium retention can be minimised by low caesium loading and higher
xxvii

zeolite content (so that when zeolites are destroyed there is sufficient zeolite to immobilise the zeolite).
While it was not explicitly stated in the earlier work, the high zeolite loading (1:1) by weight of cement
gave a mix that was very stiff, would not flow, and was very difficult to work. In this work it was decided
to reduce the zeolite loading and use a superplasticizer in order to produce a mix that more useful.
Thus, the principal parameters that must be taken into account are the cure time, cure
temperature, and chemical nature of the cement matrix.
Previous work has shown that different zeolites exhibit different abilities to withstand possible
pozzolanic reactions with hydrated cement.xxxvi
The three zeolites used to determine this were; a 65% pure
clinoptilolite from the Werris Creek deposit in Australia, a 90% pure clinoptilolite from the Teague
deposit in the United States of America, and synthetic zeolite A supplied by Degussa.
After curing at either room temperature or 60o
C for periods of 3, 7, 14, and 28 days (with cement
to zeolite weight ratio of 1:1), X-ray Diffraction analysis of the cemented zeolites showed the existence of
greater amounts of calcium hydroxide in samples cured at lower temperatures.
Thus, higher cure temperatures increases the rate of reaction between zeolite and calcium
hydroxide, resulting in a decrease in the amount of calcium hydroxide in the cement-zeolite matrix. It was
found that the pozzolanic activity of the zeolites with the cement is in the order: Werris Creek
clinoptilolite is less reactive than the Teague clinoptilolite, which is less reactive than the synthetic zeolite
A. Zeolites that are highly pozzolanic exhibit the most reactivity with hydrated cement, leading to the
breakdown of the zeolite and the release of exchanged ions into the environment.
xxviii

2.4 Leaching of Cement-Based Systems
If ground or surface water passes through a material, each constituent dissolves at some finite
rate. Therefore, when cemented zeolites are disposed of under ground they are exposed to ground waters,
and a rate of dissolution can be measured. This process is called leaching, the solution is called the
leachant, and the contaminated solution that has passed through the cemented zeolites is the leachate. The
capacity of the cemented zeolites to leach is called its leachability.
Leaching is a rate phenomenon in which hazardous or other undesirable constituents are
removed from the cemented zeolites and passed into the environment via the leachate. This rate is usually
measured and expressed in terms of concentration of the constituent in the leachate. Interestingly, as the
hazardous constituents leach, the hazard potential gradually diminishes. Thus, if leaching rate remains is
controlled so as not to exceed the allowable environmental standards in the ground or surface water,
leaching will be a beneficial process in the long term. This assumes that the leaching rate remains
constant at an acceptable level, or decreases with time. The factors affecting the leachability of cemented
zeolites include those that originate with the material itself, and those that are a function of the leaching
conditions.
xxix

2.4.1 Leaching Model
Cement is a porous material, containing both open interconnected pores and closed pores. When
a sample is exposed to a leachant solution, liquid-liquid dissolution occurs and the pore solution of the
open pores and the leachate begin to equilibrate.xxxvii
This equilibrium step occurs in a relatively short
period of time, since the diffusion coefficient of liquid-liquid diffusion is on the order of 10-5
cm2
/sec.xxxviii
Once immersed in the leachate, the open pore solution and the leachate begin to lose their
individual character, they are essentially one in the same liquid, and thus may be referred to as the “outer”
pore solution. The solution in the closed pores may be called the “inner” pore solution, because it is
isolated from leachant and is still in equilibrium with the solid. The inner pore solution would remain
unchanged by this initial interaction. Since the outer pore solution has now been diluted, the system now
begins to re-equilibrate and the solid begins to dissolve. Dissolution rates are also expected to be quite
high.
The initial period of rapid exchange is followed by a longer period of slower change in which
competing reactions take place. Continuing dissolution of the surface and diffusion of exchanged ions
through the solid to the sample surface would increase the leachate (outer pore solution) concentration of
these ions. Conversely, adsorption of the exchanged ions onto the various surfaces (possibly negatively
charged due to a decrease in pH), precipitation of a new phase such as carbonates, and a continued
formation of exchange ion-containing phases during hydration would all tend to decrease the
concentration of these ions in the outer pore solution.
Eventually, a steady state or equilibrium between these competing reactions should occur and
the leach and pore solution concentrations should become identical. The inner pore solution remains
relatively unchanged during the process. Figure 8 illustrates the leaching of caesium from a cement-based
waste form in deionised water
xxx

Figure 7. Schematic diagram of leaching model37
2.4.2 Factors Affecting Leachability
As stated previously, the ability of a cement-zeolite matrix to retain exchanged ions depends on
the cure time, cure temperature, and chemical nature of the cement matrix. When exposed to a leachant
solution, the extent to which exchanged ions are retained in the cemented zeolites depends mainly on four
variables; (a) the chemical nature of the cemented zeolites, (b) the temperature of the leachant, and (c) the
chemical nature of the leachant.xxxix
The chemical nature and temperature of the leachant solution greatly affects the leachability of
ions from cement-based systems. Krumhansl et. al. suggests that cement-based systems exposed to a
magnesium-rich brine leachant at different temperatures undergo phase changes.xl
The formation of a new
hydrated gel enriched in magnesium and chloride was the principle reaction observed, resulting in the
replacement of the initial calcium-silicate-hydrate gel (CSH) by a magnesium-silicate-hydrate gel (MSH).
It was found that increased leach temperatures produced a greater amount of alteration in the cement-
based system, causing a decrease in the integrity of the system. Thus, the formation of such phases results
in the degradation of cement which in turn affects the leachability of cement-based systems.
xxxi

Hoyle et. al. suggests that a direct relationship exists between the waste form composition, pore
solution, and the leach behaviour of caesium and strontium-doped, cement-based waste forms.xli
It was
found that caesium retention in cements was improved by increasing the SiO2 and Al2O3 contents, and
decreasing the CaO content of the mixtures. When such a mixture is exposed to a leachant solution, the
caesium concentration in both the pore solution and leachate was found to be minimal after a long period
of time. These results indicate that by increasing the amount of SiO2 and Al2O3 , a greater amount of
caesium is retained in the solid cement matrix.
The interaction between cement-based systems and leachant affects the leachability of the
system. Atkinson et. al. suggests that in cement-based matrices the main reason for different
radionuclides having different leaching characteristics is their different chemical interaction with the solid
phases present in cement.xlii
It was found that the loss of caesium from cement-based waste forms (71% of
the total caesium) was always greater than that of strontium (10% of the total strontium). They also found
that the release of caesium and strontium from cement-zeolite waste forms was very much less than for
ordinary cement waste forms, due to the absorptive properties of the zeolite.
Previous work has shown that the extent to which ions are exchanged with zeolites greatly
effects the retention of the ions in cemented zeolite. Lower exchanges of caesium result in higher caesium
retention in cemented zeolites [Table 8]xliii
. It was also found that, without the zeolites, caesium is easily
leached from hydrated cement.
xxxii

Table 8. Relative caesium retentions of zeolites after leaching in distilled
water for 7 days43
Zeolite
Type
Caesium
Loading
Caesium
Retained
(%)
Werris Creek Low 99.8
Clinoptilolite Medium 99.0
Teague Low 98.0
Clinoptilolite Medium 94.0
Zeolite A Low 84.0
Cement only Low 0.0
No zeolite Medium 12.0
CHAPTER 3
3.0 EXPERIMENTAL PROCEDURE
3.1 Materials
3.1.1 Zeolites
1) Mt Gibbs Zeolite
Type: 65% pure Clinoptilolite
Source: Werris Creek, Australia
Occurrence: Natural
Form: Powder
Range Particle Size: 80% between 6-160µm
Mean Particle Size: 73µm
Unit Cell Contents: (Na2, K2)O.Al2O3.10SiO2.8H2O
Composition: 62% clinoptilolite, 29% quartz, 7% sanidine,
and 2% mordenite
xxxiii

2) Teague Zeolite
Type: 90% pure Clinoptilolite
Source: Teague, United States of America
Occurrence: Natural
Form: Powder
Range Particle Size: 80% between 1-58µm
Mean Particle Size: 35µm
Unit Cell Contents: (Na2, K2)O.Al2O3.10SiO2.8H2O
Composition: 93% Clinoptilolite, and a low % of Quartz and Sanidine
3) Zeolite A
(a) Type: Pure A
Commercial Name: Wessalith P
Source: Degussa Ltd., Germany
Occurrence: Synthetic
Form: Powder
Particle Size: 4µm
Unit Cell Contents: Na12[(AlO2)12(SiO2)12].27H2O
(b) Type: Pure A
Commercial Name: Wessalith 80
Source: Degussa Ltd., Germany
Occurrence: Synthetic
Form: Powder
Particle Size: 8µm
Unit Cell Contents: Na12[(AlO2)12(SiO2)12].27H2OTeague Zeolite
xxxiv

3.1.2 Cements
1) Aalborg Lion Brand Portland Cement
Aalborg Lion Brand cement is a white Portland cement made in Denmark. Table 9 lists the
percent weight composition of the cement.
Table 9. Composition of Aalborg Lion Brand Portland Cement
Chemical % Weight
SO3 1.90
SiO2 24.0
Al2O3 1.90
Fe2O3 0.00
CaO 70.00
MgO 0.00
Na2O 0.00
K2O 0.00
2) Berrima Portland Cement
Berrima Portland cement is a limited shrinkage Portland cement made in Australia by Blue
Circle. Table 10 lists the percent weight composition of the cement.
Table 10. Composition of Berrima Portland Cement
Chemical % Weight
SO3 2.72
SiO2 20.25
TiO2 0.20
Al2O3 4.36
Fe2O3 4.48
CaO 64.14
MgO 0.83
Na2O 0.03
K2O 0.51
3.1.3 Leachant Solutions
1) MCC-1 Brine
Brine leachant (3M salt solution) was used to simulate the effect of ground waters on the
cemented zeolites. Preparation of the brine leachant is as specified by the Materials Characterisation
Centre (MCC).xliv
Dissolve 48.2g KCl, 90.0g NaCl, and 116.0g MgCl2 in about 900ml of water (stirred
xxxv

constantly for 1 hour by magnetic stirrer). The pH of the solution is adjusted to within the range of 6.4 to
6.6 by adding drop wise 0.01M HCl. Pure water is added to make up to 1 litre of solution (using a
volumetric flask). The leachant is stored in polyethylene bottles with tight fitting lids. Before use the
bottles are rinsed with 6M HNO3, pure water, and the freshly prepared leachant.
2) Pure Water
Tap water was purified by a Continental Water Systems Purifier CFL-EQP-0027.
3.2 Equipment
1) Scales
YMC scales were used to measure all weights.
Model Number: JK-200
Serial Number: 91689
Capacity: 200g
2) Oven
A Memmert oven was used to partially dry the exchanged zeolites at 40±1o
C, and cure the
cemented zeolites at 150±1o
C.
Model Number: 400
Serial Number: D06060agnetic Stirrer
3) Magnetic Stirrer
The HI 301N magnetic stirrer was used during solution formulations and the ion
xxxvi

exchange process.
Model Number: HI 301N
Serial Number: 966145
Volts: 220V AC
Frequency: 50/60 Hz
4) Electric Mixer
A Hobart 3-speed mixer was used to mix the cement, zeolite, and pure water
together.
Model Number: N50
Serial Number: PL-19239-1
5) Sample Vials
Polyethylene sample vials (“pop-tops”) were used as moulds and self-sealing containers to store
the cemented zeolites for curing.
Height: 21.5mm
Diameter: 19.0mm
xxxvii

Figure 8. Hobart 3-Speed Mixer
6) Vibrator
An ICAL Syntron vibrator was used in conjunction with a Syntron electrical
controller to remove air bubbles from the cemented zeolites prior to curing.
(a) ICAL Syntron
Type: LP01C
Style: B67613
Serial Number: J70A98084
Volts: 240V
Cycles: 80
Amps: 1-8A
(b) Syntron Electrical Controller
Model Number: SCR-1
Lot Number: VOR
Volts: 240V
Cycles: 50
Amps: 4A
7) Baths
Two Labec water baths were used, one for the curing of the cemented zeolites (set
at 70±o
C), the other for the leaching of the cemented zeolites (set at 40±o
C).
(a) 70o
C Bath
Serial Number: G859
Volts: 240V
Watts: 1000W
xxxviii

(b) 40o
C Bath
Serial Number: H069
Volts: 240V
Watts: 1000W
8) Automatic Pipettes
Finnpipette Digital pipettes were used to extract aliquot’s of leachants and
leachates.
(a) 1-5 ml
Code: 4027040
Lot Number: 10253
(b) 40-200 µl
Code: 4027020
Lot Number: 102050
9) Bombs
Parr stainless steel acid digestion bombs were used to cure cemented zeolite samples in an oven
at 150o
C.
Bomb Number: 4744
Capacity: 45 ml
Maximum Charge: 2.0 g
Maximum Temperature: 250o
C
Maximum Pressure: 1800 psig
Parts List
241AC Spring
264AC2 Hook Spanner
276AC2 Bomb body
277AC Bottom disc
278AC Screw cap
A280AC2 Teflon cup with cover
xxxix

282AC Pressure plate, lower
283AC Pressure plate, upper
A284AC Tumbling ring
A285AC Holding fixture
286AC Corrosion disc
287AC Rupture disc
Figure 9. 4744 General Purpose Acid Digestion Bomb
10) Inductively Coupled Plasma – Mass Spectrometer (ICP-MS)
The Fisons Instruments ICP-MS with VG Plasma Quad was used to determine the
concentration of caesium and strontium in the leachate.
xl

Figure 10. Inductively Coupled Plasma – Mass Spectrometer
3.3 Procedures
3.3.1 Ion-exchange
For ease of experimentalisation radioactive isotopes have not been used in this study. Non-active
caesium and strontium nitrates are used instead. Experiments carried out by Atkinson et. al. used
radioactive traces and there is no evidence that any significant difference would be observed. The
caesium and strontium loadings for this study were calculated based on the International Atomic Energy
Agency’sxlv
classification of intermediate level wastes. The loadings of 1.45g CsNO3 per 1000g cement,
and 1.60g Sr(NO3)2 per 1000g cement, were chosen as an approximate upper limit of intermediate level
wastes. The final mix ratio of zeolite to cement is 1 to 2. The afore mentioned amounts of caesium and
strontium nitrate were dissolved in 1 litre of pure water to which 500g of zeolite was added for ion-
exchange under constant stirring and at room temperature. After a period of 1 hour, the zeolite was
filtered off using a vacuum flask and dried slowly in an oven. The dried exchanged zeolite was stored in
an airtight plastic container until required for cementation.
3.3.2 Cementation and Hydration
Table 11 lists the mix compositions used in this study.
xli

Table 11. Mix Compositions
Code Zeolite Type Zeolite
(±1g)
Cement
Type
Cement
(±1g)
Pure Water
(±1g)
Water:Cement
Ratio
Plasticiser
(±0.1g)
WC/ALB Werris Creek 200 Aalborg 400 168 0.42 4.8
WC/BER Clinoptilolite 200 Berrima 400 168 0.42 4.8
T/ALB Teague 200 Aalborg 400 168 0.42 4.8
T/BER Clinoptilolite 200 Berrima 400 168 0.42 4.8
A1/BER Synthetic A
Wessalith P
200 Berrima 400 168 0.42 4.8
A2/BER Synthetic A
Wessalith 80
200 Berrima 400 168 0.42 4.8
ALB No zeolite 0 Aalborg 400 168 0.42 4.8
BER 0 Berrima 400 168 0.42 4.8
Cement, zeolite, water, and plasticiser (Rheobuild 2000) were mixed in a Hobart 3-speed mixer
based on the ASTMxlvi
specifications. The mixing procedure is as follows:
1) Place the dry paddle and the dry bowl in the mixing position in the mixer. Then
introduce the materials for a batch into the bowl and mix in the following order:
2) Place all the mixing water in the bowl.
3) Add the cement to the water, then start the mixer and mix at low speed (140 ± 5 r/min)
for 30 s.
4) Add the entire quantity of zeolite slowly over a 30 s period, while mixing at low speed.
5) Stop the mixer, add the plasticiser ,and change to medium speed (285 ± 10 r/min), and
mix for 30 s.
6) Stop the mixer and let the mix stand for 30 s. During the first 15 s of this interval,
quickly scrape down into the batch any mix that may have collected on the side of the
xlii

bowl.
7) Finish by mixing for 60 s at medium speed (285 ± 10 r/min).
The cemented zeolites were cast into self-sealing “pop-tops” and vibrated for 1 minute using the
Syntron Vibrator for better compaction. The samples were then allowed to cure under the following
conditions:
1) At room temperature for 28 days, or
2) In a water bath set at 70o
C for 7 days, or
3) Inside a bomb in an oven set at 150o
C for 7 days
It was necessary to cure at this range of temperatures due to the cement heat of hydration, which
can reach up to 100 to 200o
C. Once cured, the cemented zeolites were removed from the “pop-tops” and
were ready for leaching.
xliii

Figure 11. Hydrated cemented zeolite cylinder
3.3.3 Leaching
The leaching test followed the procedure of Zamorani and Serrini.xlvii
The hydrated cemented
zeolite cylinders, of average weight 16g and surface area 24cm2
, were suspended in sealed plastic
containers containing 200ml of either pure water or brine solution under the following conditions:
1) Leaching at room temperature, or
2) Leaching at 40o
C in a water bath.
At 3, 7, 14, 21, and 28 days a 5ml aliquot of leachate was taken, after which 5ml of fresh
leachant was added to the sealed plastic container so as to maintain a constant leachant volume of 200ml.
The 5ml aliquot of leachate was then acidified by 2ml of 17% HNO3 acid solution, and diluted to a total
volume of 10ml by pure water.
3.3.4 Inductively Coupled Plasma – Mass Spectrometer Analysis
The ICP-MS is a sensitive elemental analyser using inductively coupled plasma to produce ions
and a mass spectrometer to separate their quantities. Caesium was measured at a mass of 133 (100%
abundance), and strontium at a mass of 88 (82.56% abundance). Sample preparation for ICP-MS analysis
was as follows; take 0.1ml of the diluted leachate aliquot, add to this 0.1ml of 5ppm In/Th standard, and
xliv

dilute to a total volume of 10ml with 3% HNO3 acid solution. The concentration of caesium and strontium
in the diluted sample was in the range of 1-100ppb and ready for ICP-MS analysis.
CHAPTER 4
4.0 RESULTS AND DISCUSSION
A copy of the ICP-MS data obtained from the analysis of 400 leach samples can be found in
Appendix A. The results show the effects of cement type, zeolite type, cure temperature, leachant
solution, and leach temperature on the retention of caesium and strontium in cemented zeolites.
4.1 No Zeolite
Tables 12 and 13 list the percent retention of caesium and strontium in Berrima
Portland cement and Aalborg Portland cement after 28 days leaching. From these results, the following
trends can be observed;
(a) The caesium retention is less than strontium retention. Atkinson et. al.xlviii
suggests
that strontium behaves like calcium in cementious matrices. Sr2+
is ‘bound’ into the
xlv

matrix much like Ca2+
, whereas Cs1+
is ‘held’ in the matrix. Thus, when exposed to
leachant, the caesium will be more easily lost from the cement matrix.
Table 12. % Retention of strontium and caesium in Berrima cement after 28 days
Leaching
Cure
Temperature
Leachant Leach
Temperature
Sr Retention
(±2%)
Cs Retention
(±2%)
RT Pure Water RT 96 49
40o
C 88 10
Brine RT 94 57
40o
C 90 34
70o
C Pure Water RT 82 4
40o
C 82 0
Brine RT 83 24
40o
C 82 22
150o
Brine RT 83 55
RT = Room Temperature (25o
C)
Table 13. % Retention of strontium and caesium in Aalborg cement after 28 days
Leaching
Cure
Temperature
Leachant Leach
Temperature
Sr Retention
(±2%)
Cs Retention
(±2%)
40o
C 75 19
Brine RT 82 44
40o
C 79 36
70o
40o
C 70 0
Brine RT 79 50
xlvi

40o
C 72 35
150o
Brine RT 73 52
C)
(b) Increasing leach temperature results in a decrease in strontium and caesium retention. Leaching at
40o
C decreases the strontium and caesium retention compared to that at room temperature. This is almost
certainly due to kinetic effects as there was no indication that curing at 70o
C increases the leach rate.
This suggests that both the repository conditions and the curing conditions must be specified
before commencing waste disposal design. It is well known that cements can attain temperatures of 140o
C
during curing and the results shown in Tables 14 and 15 suggest that these relative high temperatures
could have significant effects on the retention of caesium and strontium
(c) Aalborg Portland cement has better retention of caesium at increased leach temperatures than Berrima
Portland cement. However, Berrima Portland cement generally has better caesium and strontium retention
than Aalborg Portland cement. This could be due to the difference between alumina contents of the
cements. As suggested by Hoyle et. al.xlix
, the presence of additional alumina can bind cations into the
cement paste. Berrima Portland cement has twice the amount of alumina content in Aalborg Portland
cement. Thus, it is expected that Berrima Portland cement will have better caesium and strontium
retention than Aalborg Portland cement.
Table 14. % Retention of strontium and caesium in Berrima cement and Aalborg cement
after 28 days leaching in pure water
Cure Temperature Strontium Retention Caesium Retention
(o
C) Berrima Cement Aalborg Cement Berrima Cement Aalborg Cement
RT 96 91 49 50
70 82 74 4 0
150 85 75 54 51
xlvii

Table 15. % Retention of strontium and caesium in cemented zeolites
after 28 days leaching in pure water
Zeolite Type Cure
Temperature
Strontium Retention Caesium Retention
(o
C) Berrima
Cement
Aalborg
Cement
Berrima
Cement
Aalborg
Cement
RT 98 95 99 99
Werris Creek 70 96 94 99 99
150 97 94 99 99
RT 94 97 96 99
Teague 70 98 98 98 98
150 93 72 91 85
Wessalith P Wessalith 80 Wessalith P Wessalith 80
RT 99 99 97 96
Synthetic A 70 99 99 94 93
150 98 96 87 89
(d) Samples cured at different temperatures have different caesium and strontium retentions. The samples
cured at 70o
C have the worst caesium and strontium retention. In particular, no caesium is retained in the
70o
C cured samples. This may be due to the formation of a CSH phase with different structural properties
to the CSH formed at room temperature (ie little ion-exchange sites).
Tobermorite is produced when hydrated cement is cured at temperatures greater than 100o
C, and
CSH gel is produced at temperatures below 100o
C. The 150o
C-cured samples have different caesium and
strontium retentions, indicating that the tobermorite phase has different ion-exchange capacities and thus
different amounts of caesium and strontium.
The CSH phase produced at room temperature can also absorb caesium and strontium, but to a
lesser degree than the tobermorite.
(e) Zeolite additions improve the caesium and strontium retention of cements. The addition of either one
of the three zeolites improves the caesium and strontium retention of both Aalborg and Berrima Portland
xlviii

cement. This is shown in Tables 14 and 15 for the cemented Werris Creek zeolite only, but all of the
zeolites improve the retention of caesium and strontium because of the ion selectivity of the zeolites.
4.2 Werris Creek Clinoptilolite
Tables 16 and 17 list the percent retention of caesium and strontium in Berrima cement/Werris
creek clinoptilolite and Aalborg cement/Werris Creek clinoptilolite after 28 days leaching. From these
results, the following trends can be observed;
(a) When leaching in pure water, the amount of caesium retention is greater than when
leaching in brine. This results due to the theoretical cation selectivity of the
clinoptilolite;
Cs>Rb>K>NH4>Ba>Sr>Na>Ca>Fe>Al>Mg>Li
When leaching in pure water there are no ions for which the clinoptilolite will preferentially exchange for
caesium. However, the brine leachant is a concentrated salt solution rich in magnesium, sodium, and
potassium ions. Clinoptilolite has greater ion selectivity for caesium, however, due to the high
concentration of potassium and sodium ions in the brine leachant, caesium is exchanged for these ions in
order to reach ionic equilibrium between the leachate and cement pore solution.
Na1+
, K1+
⇔ Cs1+
Similarly, the amount of strontium retention decreases (to a lesser extent) when the cemented
clinoptilolite zeolite is leached in brine due to the exchange of strontium ions for magnesium ions.
Mg2+
⇔ Sr2+
xlix

Table 16. % Retention of strontium and caesium in Berrima cement/Werris Creek
clinoptilolite after 28 days leaching
Cure
Temperature
Leachant Leach
Temperature
Sr Retention
(±2%)
Cs Retention
(±2%)
40o
C 96 97
Brine RT 96 84
40o
C 96 75
70o
40o
C 96 99
Brine RT 93 82
40o
C 94 84
150o
Brine RT 96 94
C)
Table 17. % Retention of strontium and caesium in Aalborg cement/Werris Creek after 28
days leaching
Cure
Temperature
Leachant Leach
Temperature
Sr Retention
(±2%)
Cs Retention
(±2%)
40o
C 93 98
Brine RT 92 86
40o
C 79 73
70o
40o
C 92 98
Brine RT 92 88
40o
C 90 84
150o
Brine RT 91 96
C)
(b) The amount of strontium lost when leached in brine is smaller than the amount of caesium lost. This is
due to the reaction between the magnesium ions in the brine and the CSH in the cement. A large
percentage of magnesium ions in the brine react with the CSH to form a new phase, magnesium-silicate-
hydrate (MSH)l
. Thus, the majority of magnesium ions preferentially form MSH in the cement matrix
rather than exchanging with strontium in the zeolite.
l

(c) Table 12 shows that the caesium and strontium retention was greater in the Berrima
cement/Werris Creek clinoptilolite samples than in the Aalborg cement/Werris Creek
clinoptilolite samples. This result may possibly be attributed to the different
compositions of the cements, and the interaction between zeolite as discussed in section
4.1.1(c).
(d) Leaching at 40o
C decreases the amount of strontium and caesium retention compared to that at room
temperature. This is almost certainly due to kinetic effects similar to those discussed in section 4.1.1(b).
(e) Cure temperature did not appreciably increase or decrease the amount of caesium and strontium
retention in the cemented zeolite samples. This suggests that curing of cement at different temperatures
has no effect on the loss of caesium or strontium from the cemented Werris Creek clinoptilolite zeolite.
4.3 Teague Clinoptilolite
Tables 18 and 19 list the percent retention of caesium and strontium in the Berrima
cement/Teague clinoptilolite and the Aalborg cement/Teague clinoptilolite after 28 days
leaching. The following trends can be observed;
(a) Curing at 150o
C decrease the amount of strontium and caesium in the cemented Teague
clinoptilolite zeolites. Interestingly, the Berrima cement/Teague clinoptilolite samples cured at 150o
C
retained more caesium and strontium than the Aalborg cement/Teague clinoptilolite samples. This
indicates that the reduction in caesium and strontium retention is may be due to the interaction between
the zeolites and the hydrated cement.
At temperatures greater than 100o
C, the cement hydration reaction product is tobermorite gel
(calcium-silicate-hydrate + calcium hydroxide). It is possible that this calcium-rich hydration product
reacts with the zeolite, causing the zeolite to breakdown. The more calcium in the cement, the larger the
extent of reaction between the calcium and zeolite. Thus, Aalborg cement/Teague clinoptilolite zeolite
li

samples retain less caesium and strontium than Berrima cement/Teague clinoptilolite zeolite samples
when cured at 150o
C, due to a higher calcium content. Berrima Portland cement has more alumina than
Aalborg Portland cement. This may also account for the reduction in retention in Aalborg Portland
cement.
Table 18. % Retention of strontium and caesium in Berrima cement/Teague clinoptilolite
after 28 days leaching
Cure
Temperature
Leachant Leach
Temperature
Sr Retention
(±2%)
Cs Retention
(±2%)
40o
C 95 93
Brine RT 92 81
40o
C 84 76
70o
40o
C 98 98
Brine RT 95 88
40o
C 87 84
150o
Brine RT 81 70
C)
Table 19. % Retention of strontium and caesium in Aalborg cement/Teague clinoptilolite
after 28 days leaching
Cure
Temperature
Leachant Leach
Temperature
Sr Retention
(±2%)
Cs Retention
(±2%)
40o
C 94 96
Brine RT 82 82
40o
C 87 84
70o
40o
C 98 98
Brine RT 93 90
40o
C 84 86
150o
Brine RT 85 72
C)
lii

(b) Leaching at 40o
C decreases the amount of strontium and caesium retention compared to that at room
temperature.
(c) The retention of caesium and strontium in the cemented zeolite is greater than in brine. This
behaviour is the same as that observed by the Werris Creek clinoptilolite zeolite
(d) The caesium and strontium retention in the cemented Teague clinoptilolite samples was found to be
less than in the cemented Werris Creek clinoptilolite samples. These results are almost certainly due to
the pozzolanic reaction between zeolite and hydrated cement, reducing the ability of the zeolite to retain
caesium or strontium. As was found earlier, the Werris Creek clinoptilolite (73µm) was more resistant to
the pozzolanic reaction than the Teague clinoptilolite (35µm).
4.4 Synthetic Zeolite A
Tables 20 and 21 list the percent retention of caesium and strontium in the Berrima
cement/Wessalith P type A zeolite and the Berrima cement/Wessalith 80 type A zeolite
after 28 days leaching. The following trends can be observed;
liii

(a) The amount of caesium and strontium retained in the cemented zeolite decreases as cure temperature
increases. Samples cured at room temperature were found to retain more caesium and strontium than
those cured at 70o
C and 150o
C.
As the cure temperature increases, so does the level of pozzolanic activity between the zeolite
and the hydrated cement [4.1.2 (a)]. Increased pozzolanic activity results in the degradation of the zeolite,
causing caesium and strontium to be lost into the leachant.
(b) The cemented synthetic A type zeolite has better strontium retention than caesium
retention. This is due to the theoretical ion selectivity of the synthetic A zeolite.
Ag>Tl>Na>K>NH4>Rb>Li>Cs
Zn>Sr>Ba>Ca>Co>Ni.Cd>Hg>Mg
The synthetic zeolite A has very strong ion selectivity for strontium, resulting in better strontium retention
than caesium in the cemented zeolite. Figure 15 illustrates the general effect of cure temperature on the
retention of caesium and strontium in cemented zeolites.
Table 20. % Retention of strontium and caesium in Berrima cement/Wessalith P after 28
days leaching
Cure
Temperature
Leachant Leach
Temperature
Sr Retention
(±2%)
Cs Retention
(±2%)
40o
C 99 96
Brine RT 98 93
40o
C 99 93
70o
40o
C 99 94
Brine RT 99 92
40o
C 98 91
150o
Brine RT 97 83
C)
liv

Table 21. % Retention of strontium and caesium in Berrima cement/Wessalith 80 after 28
days leaching
Cure
Temperature
Leachant Leach
Temperature
Sr Retention
(±2%)
Cs Retention
(±2%)
40o
C 99 94
Brine RT 98 92
40o
C 98 89
70o
40o
C 99 96
Brine RT 98 93
40o
C 98 92
150o
Brine RT 96 85
C)
lv

(c) The cemented Wessalith P synthetic A zeolite has similar strontium and caesium
retentivity to the cemented Wessalith 80 synthetic A zeolite. This may be because the
only difference between the two zeolites is a particle size difference of 4µm. Thus, the
difference in particle size may not be great enough to affect the caesium and strontium
retention ability of the zeolites.
(d) Leaching in brine decreases the caesium retention of the cemented synthetic A zeolite. However, the
brine leaching solution does not significantly effect the strontium in the cemented synthetic A zeolite
The synthetic A zeolite has a higher ion selectivity for strontium ions than for magnesium ions.
Zn>Sr>Ba>Ca>Co>Ni.Cd>Hg>Mg
Thus, the magnesium ions in the brine solution will not preferentially exchange with the
strontium ions in the zeolite. A high percentage of the magnesium ions will in fact react
with the CSH gel in the cement to form MSH gel. Therefore, leaching in brine does not
decrease the strontium retention in the cemented synthetic A zeolite.
(e) The cemented synthetic A zeolite generally has the best strontium retention. These
results can be largely attributed to the different ion selectivities of the zeolites. As
previously stated, the synthetic A zeolite has a higher ion selectivity for strontium than
the two clinoptilolite zeolites. Also, the synthetic zeolite A has a lower Si/Al ratio than
lvi

the clinoptilolite zeolites, resulting in a larger ion-exchange capacity. Thus, cemented
synthetic A zeolite exhibits the best ability for the retention of strontium.
CHAPTER 5
5.0 SUMMARY AND CONCLUSIONS
The retention of caesium and strontium in cemented zeolites was determined.
The results show that cement type, zeolite type, cure temperature, leachant solution, and
leach temperature have measurable effects on the retention of caesium and strontium in
cemented zeolites. A summary of the most significant results follows;
1) Zeolites increase the retention of caesium and strontium in cements. Berrima Portland cement/Werris
Creek clinoptilolite zeolite had maximum caesium retention. Berrima Portland cement/synthetic A
lvii

zeolite had maximum strontium retention. Berrima and Aalborg Portland cement with no zeolite
content had the worst caesium and strontium retention.
2) Cements retain strontium better then caesium. Aalborg and Berrima Portland cement retained greater
amounts of strontium than caesium.
3) The ability of cement-based systems to retain caesium and strontium can be highly
dependant on the cure temperature.
4) The chemical nature of the leachant affects the caesium and strontium retention of the
cement-based system. Cement-based systems leached in pure water have higher retentions of caesium
and strontium than those leached in brine.
CHAPTER 6
6.0 RECOMMENDATIONS FOR FURTHER WORK
1) Curing temperature is directly related to the retention of various cations in hydrated cement. It is
suggested that this is due to the production of non-specific cementious phases, formed at different cure
temperatures. It is vital that these cementious phases be identified so as to fully understand their effect
on cation retention in the cement. Further work may involve curing cements at temperatures between
25o
C - 250o
C, and analysing the hydration products before and after leaching.
2) It is recommended that a more realistic brine-type leachant be used for the production of
in situ leaching results, for example, artificial sea water or a brine solution of lower molarity.
3) Zeolites improve the retention of various cations in hydrated cements. In this study,
zeolite:cement ratios were either 1:2 or 0:1. It is recommended that further work be undertaken to
determine the effect of zeolite:cement ratio on the retention ability of cemented zeolites.
4) Zeolites react with hydrated cements due to their highly pozzolanic nature. This causes the zeolites to
breakdown, resulting in the release of potentially hazardous ions into the environment. Thus,
prevention or retardation of the pozzolanic reactions is necessary.
Future work may include; using slag cement blends, the addition of silica and/or alumina.
lviii

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