Theoretical and experimental study of degradation mechanisms of cement in the repository environment

European Commission Directorate-General For Research And Innovation

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Nuclear energy and safety

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• Publié par: EUROPEAN-COMMISSION-DIRECTORATE-GENERAL-FOR-RESEARCH-AND-INNOVATION
• Nombre de lectures: 12
• Langue: English
• Poids de l'ouvrage: 22 Mo

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ISSN 1018-5593
European Commission
Theoretical and experimental study
of degradation mechanisms of cement
in the repository environment
Report
EUR 17642 EN European Commission
Theoretical and experimental study
of degradation mechanisms of cement
in the repository environment
1 E. Revertegat, F. Adenot, C. Richet, L. Wu
2 F. R Glasser, D. Daminot, S. A. Stronach
CEA-CE Saclay
DCC/DESD/SESD
F-91191 Gif sur Yvette
2 University of Aberdeen
Department of Chemistry
Aberdeen AB24 3UU
United Kingdom
Contract No FI2W-CT90-0035
Final report
Work performed as part of the European Atomic Energy Community
shared cost-programme (1990-94) on 'Management and
storage of radioactive waste'
Task 3: Characterization and qualification of waste forms, packages
and their environment
Directorate-General
Science, Research and Development
1997 EUR 17642 EN LEGAL NOTICE
Neither the European Commission nor any person acting on behalf of the Commission
is responsible for the use which might be made of the following information
A great deal of additional information on the European Union is available on the Internet.
It can be accessed through the Europa server (http://europa.eu.int.)
Cataloguing data can be found at the end of this publication
Luxembourg: Office for Official Publications of the European Communities, 1997
ISBN 92-828-0394-5
© European Communities, 1997
Reproduction is authorized, provided the source is acknowledged
Printed in Luxembourg CONTENTS
GENERAL INTRODUCTION 1
1. KNOWLEDGEOFCEMENTPASTE ATTACK MECHANISMS 4
1.1. Introduction
1.2. Background 4
1.2.1. Compositionof cement
1.2.2. Hydratation of cement 5
1.2.3. Durability of cement 6
1.3. Experimental data acquisition for chlorides, sulfates and carbonates attack 7
1.3.1. Alteration
1.3.2. Identification of zoning and mineralogical characterization 8
1.33. Dimensional and mass variations 40
13.4. Porosity8
1.3.5. Diffusion factor of tritiated water in the degraded zone 62
1.4. Literature assessment 8
1.4.1. Thermodynamic theory
1.4.2.c modelling6
1.4.3. Investigation of solid solutions 99
2. TESTING OF THE LOCAL EQUILIBRIUM HYPOTHESIS 108
2.1. Checking that kinetics is governed by diffusion
2.1.1. Introduction 10
2.1.2. Study of leaching kinetics
2.1.3. Summary 112
2.2. Long term synthetic phases and temperature effects 11
2.2.1. Results3
2.2.2. Discussion
III 3. MODELLING OF THE CEMENT PASTE DEGRADATION 115
3.1. Thermodynamic modelling of cement paste attack
3.1.1. Experimental 11
3.1.2. CaO - Si02 - H2O system at 25 °C 136
3.1.3. Reactions of cement components with saline solutions 170
3.1.4. Concluding discussion and suggestions for future work 194
3.1.5. Calculation of phase diagrams of systems occurring in cement paste 20
3.2. Modelling of cement paste degradation by coupling thermodynamic modelling
of equilibrium and transport by diffusion 22
3.2.1. Modelling6
3.2.2. Numerical solution of model7
3.2.3. Model validation8
3.2.4. Long-term prediction and perspectives 231
REFERENCES 233
ABBREVIATIONS 24
APPENDIX 1 : Compound composition of CLC and chemical analysis of OPC and CLC 244
APPENDIX 2 : The pitzer database5
APPENDIX 3 : The phreeqe algorithm 252
APPENDIX 4 : Source of data used in database
APPENDIX 5 : Instrumentation and techniques used by University of Aberdeen 263
IV GENERAL INTRODUCTION - OBJECTIVES
One of the most important reasons for the current limited use of nuclear power has been
apprehension over the hazards presented by nuclear wastes and their safe disposal. These
wastes can be broadly categorised as high, medium and low level, and different techniques are
required for their disposal. High level wastes arising from reprocessing tend to be aqueous
solutions which can be calcined, and then incorporated into glass and ceramic solids. The
current intention is for these solid blocks to be stored at the waste reprocessing site for at least
50 years prior to disposal in a suitable repository. Medium and low level wastes are of greater
heterogeneity, comprising of solids, liquids, sludges etc. These wastes are frequently wet and
difficult to dewater1. This forces the need for a water-tolerant containment matrix, and cement
is viewed as the material of choice. It is likely to be a major component in the immobilisation
of low and medium level radioactive waste in underground repositories, as both a
solidification matrix and as backfill and construction materials.
Favoured techniques entail the encapsulation of waste by cement in containers such as
stainless steel drums. The exact method used depends upon the nature of the waste but each
proceedure minimises the contamination of external equipment. The use of cement filled
drums enables the safe transportation of the waste to the disposal site. Once at the repository,
the containers are either placed directly into the vaults, or are grouped together and coated in a
cementitious overpack to create large blocks for easier stacking and storage. Finally, once the
repository is full, the cement-lined vaults are backfilled. The backfill may contain Ca(OH)2
and CaC03 as well as cement.
The aim of this multi-barrier approach is two-fold. Firstly, a physical barrier is
provided against waste migration into the biosphere ; the solid cement provides physical
strength to the repository, and inhibits groundwater through flow. The second, and more
important feature is that the cement also provides a chemical barrier.
When cement clinker is hydrated, excess water is used to ensure that the freshly mixed
slurry is plastic and workable. This excess water remains within the cement mass in pores. The
fluid tends to become rich in soluble alkali hydroxides present in the original clinker, and
hydroxide ions, the most readily dissolved of the anions in cement, which combine to form
alkali hydroxides. Thus, the aqueous phase of cements is of high pH, and it is this feature
which makes cement so suitable for waste immobilisation, as many radionuclides have
reduced solubilities at high pH ; also, this high pH environment provides protection from
corrosion for the steel containers. Furthermore, the micorporous, high surface area cement
inhibits the transport of radionuclides out of the repository by adsorption onto surfaces2.
The half lives of many of the radioactive species contained within the cement matrix
are of the order of 103 or 104 years. Therefore, it is of great importance that the high pH
environment provided by the cement is durable, and will outlast its need. It is anticipated that
the pH will decrease over time, both as a result of the leaching of soluble ions by
groundwaters, and through chemical attack by aggressive species contained within the
groundwater and the repository constituents, such as sulfate, chloride and magnesium.
Countering these effects are many cement hydrate phases, which have inherently high solution
pH's. Phases such as Ca(OH)2, C-S-H, C3AH6 and ettringite all supply the aqueous phase with
OH" ions which maintain a high pH long after the original soluble alkalis have been leached
away. It is only after these hydrates, and similar phases have themselves been degraded by
attack and leaching that the pH falls below acceptable levels.
1 The objective of the research programme covered by CEC Contract
No.F12W-CT90.0035 is to evaluate, over a long period, the degradation of the concretes used
in waste disposal conditions, specific to each facility of the countries of the European
Community.
Since these conditions are not yet fully established, this study will attempt to be as
comprehensive as possible.
A model is being developed to quantify the degradation. The input variables, which
will have to be as representative as possible of disposal conditions, must be selected and
quantified according to the specificity of the different facilities.
Three variables have accordingly been selected :
• structural and textural characteristics of the unattacked cement paste,
• ionic concentrations of the aggressive solution liable to be in contact with the concrete,
• temperature of the disposal facility.
Little work has been reported on modelling the time dependence of chemical degradation of
concrete by ground water. On the other hand, the phenomena involved in alteration of rocks by
water and their modelling are well known to geochemists3'7. Thus, it appears that degradation
of a saturated porous medium immersed in water depends on two main consecutive
phenomena with different kinetics (as advective transport does not occur) :
• material transport by diffusion, resulting from concentration gradients between the solid
interstitial solution and the aggressive solution
• dissolution-precipitation chemical reactions, induced by the concentration variations
brought about by diffusion.
The slowest process controls the overall velocity. In many geochemical systems, when the
advective transport is very slow or neglected, the hypothesis of local equilibrium can be
assumed. As hydrated cement is more reactive than natural rocks, it is probable that this
assumption of local equilibrium is valid f