Modelling long-term evolution of cementitious materials used in waste disposal (original) (raw)
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Simulating concrete degradation processes by reactive transport models
Journal de Physique IV (Proceedings), 2006
Cement-based materials are commonly used in the multibarrier systems of radioactive waste repositories. Under the sub-surface environmental conditions they are exposed to during service-life, the chemical composition of the initially highly alkaline cement pore fluid may be altered by the influence of external ions and the leaching of dissolved species present in the cement interstitial solution, both of which processes are mainly ruled by ionic diffusion. Furthermore, the perturbation induced in the local thermodynamic equilibrium of the system yields to a series of dissolution/precipitation reactions which may result in a significant reorganization of the microstructure of concrete, in terms of both the distribution of mineral phases and the physical morphology of the capillary pore network, thus causing the concrete properties to undergo a gradual decline. Therefore, the long-term performance of concrete structures is a relevant issue in relation to the safety assessment of radioactive waste disposals. The analysis of the evolution of concrete degradation is a challenging task. It is also one that stresses the relevance of the development of reliable modeling techniques aimed at the prediction of long-term concrete behavior. The present work deals with the conceptualization of concrete both as a mineral aggregate, thus susceptible to deterioration, and as a porous material, where transport processes are expected to take place. Coupled reactive transport models are required to cope with the highly complex cyclic interactions arising between the chemical reactions which take place in the water-concrete interface and diffusive and advective transport in the aqueous phase. The approach taken herein aims at formulating and testing reactive transport numerical models by reproducing recent experiments reported in the scientific literature. Such procedure is intended to provide insight into the very nature of the phenomena involved, particularly those related to the appropriate methods available to describe ionic diffusion and the accuracy of the constitutive laws (e.g., porosity/permeability, porosity/diffusivity, etc.) developed for cement-based materials.
Modelling chemical degradation of concrete during leaching with rain and soil water types
Cement and Concrete Research, 2010
Percolation of external water through concrete results in the degradation of cement and changes the concrete pore water and solid phase composition. The assessment of long-term degradation of concrete is possible by means of model simulation. This paper describes simulations of chemical degradation of cement for different types of rain and soil water at an ambient earth surface temperature (10°C). Rain and soil water types were derived using generic equations and measurement of atmospheric boundary conditions representative for North-Belgium. An up-to-date and consistent thermodynamic model is used to calculate the geochemical changes during chemical degradation of the concrete. A general pattern of four degradation stages was simulated with the third stage being the geochemically most complex stage involving reactions with calcium-silicate hydrates, AFm and AFt phases. Whereas the sequence of the dissolution reactions was relatively insensitive to the composition of the percolating water, the duration of the different reactions depends strongly on the percolating water composition. Major identified factors influencing the velocity of cement degradation are the effect of dry deposition and biological activity increasing the partial pressure of CO 2(g) in the soil air phase (and thus increasing the inorganic carbon content in the percolating water). Soil weathering processes have only a minor impact, at least for the relatively inert sandy material considered in this study.
2011
Many disposal concepts currently show that concrete is an effective confinement material used in engineered barrier system (EBS) at a number of LLW disposal sites. Cement-based materials have properties for the encapsulation, isolation, or retardation of a variety of hazardous contaminants. However, several questions are raised about the safety of concrete barrier, such as: Could the EBS completely isolate from groundwater? What hydrogeochemical reactions and key aqueous species in ground water affect the degradation of concrete barrier of repository? How the redox processes influence on the formations of degradation materials? To evaluate the geochemical evolution of concrete barrier, reactive chemical transport, and groundwater flow models can be an effective tool. The present study aims to assess hydrogeochemical influences on concrete barrier degradation using reactive chemical transport model with thermodynamic equilibria data in cementitious media. A proposed site for final di...
Coupling Time-Dependent Sorption Values of Degrading Concrete With a Radionuclide Migration Model
ASME 2009 12th International Conference on Environmental Remediation and Radioactive Waste Management, Volume 1, 2009
Safety assessment of radioactive waste disposal facilities is usually carried out by means of simplified models. Abstraction of the numerical model from the real physical environment is done in several steps. One of the most challenging issues in safety assessment concerns the long time scales involved and the evolution of engineered barriers over thousands of years. For some processes occurring in specific engineered barriers the uncertainties related to long time scales are addressed by implementing conservative assumptions in the radionuclide migration models. Other processes such as chemical concrete degradation, however, can be estimated for long time periods by the use of coupled geochemical transport models. For many near-surface disposal facilities, concrete is a very important engineered barrier because it is used in the construction of disposal modules or vaults, in production of high-integrity monoliths and their backfilling and for waste conditioning. Knowledge on the durability of such concrete components and its relation to radionuclide sorption is important for a defensible safety assessment. Chemical degradation typically occurs as the result of decalcification, dissolution and leaching of cement components and carbonation. These reactions induce a gradual change in the solid phase composition and the concrete pore-water composition, from "fresh" concrete porewater with a pH above 13 to a pH lower than 10 for "evolved" porewater associated with fully degraded concrete. In this study the time-dependency of the concrete mineralogy and porewater was coupled with sorption values that are characteristic for the four concrete degradation states: (i) State I with a pH larger than 12.5, controlled by the dissolution of alkali-oxides, (ii) State II with a pH at 12.5 controlled by the dissolution of portlandite, (iii) State III with a pH between 12.5 and 10 when all portlandite has been dissolved and the pore water composition is determined by different cement phases including calcium-silicate hydrates (C-S-H phases), and (iv) State IV with a pH lower than 10 with calcite and aggregate minerals present. Above mentioned pH values are valid for a system with a temperature of 25 o C. Sorption values were obtained from a literature review. The time-dependency of the sorption values R d was implemented in a one-dimensional radionuclide migration model used for release calculations from the planned near-surface disposal facility at Dessel, Belgium. Calculated releases will be discussed for radionuclides typical of low-and intermediate level short-lived (LILW-SL) waste.
Physics and Chemistry of the Earth, Parts A/B/C, 2017
This paper describes a multi-scale approach for the modelling of the degradation of model cement pastes using reactive transport. It specifically aims at incorporating chemistry-transport feedback results from a pore-scale approach into a continuum description. Starting from a numerical representative elementary volume of the model cement paste, which was built according to extensive experimental dedicated chacarterizations, this paper provides three separate descriptions of two different degradations: leaching and carbonation. First, 2D pore-scale simulations are performed and predict degradation depths in very good agreement with experiments. Second, 3D pore scale descriptions of how the microstructre evolves provides accurate description of the evolution of transport properties through degradation. Finally, those latter results are incorporated as a feedback law between porosity and effective diffusion coefficient into a 1D continuum approach of reactive transport. This paper provides pore-scale explanations of why reactive transport modelling has encountered mitigated success when applied to cementitious materials, especially during carbonation or degradations consisting of precipitation reactions. In addition to that, different degradation modellings are in very good agreement with experimental observations.
Modelling Concrete Degradation by Coupled Non-linear Processes
Journal of Advanced Concrete Technology
Concrete in a transuranic (TRU) waste repository is considered a suitable material to ensure safety, provide structural integrity and retard radionuclide migration after the waste containers fail. Modelling of concrete degradation often focuses solely on solid-water chemical reactions and related changes in porosity, however, cracking and damage from steel corrosion will likely exert significant control over the mass-transport properties of concrete. In the current study, coupling between chemical, mass-transport and mechanical, so-called non-linear processes that control concrete degradation and crack development were investigated by coupled numerical models. Application of such coupled numerical models allows identification of the dominant non-linear processes that will control long-term concrete degradation and crack development in a TRU waste repository.
Performance simulation and quantitative analysis of cement-based materials subjected to leaching
Computational Materials Science, 2010
A 3D numerical modelling platform (MuMoCC) developed in a previous work by the authors is applied in this paper to investigate the effect of leaching of some solid phases of cement paste (portlandite and hydrated aluminates or sulfoaluminate phases) on the mechanical and diffusivity performances of cement paste and mortar. The platform is based on a multi-scale approach and uses two numerical tools. First, NIST's CEMHYD3D code is used to simulate 3D Representative Volume Elements of cement paste and mortar. Then mechanical and diffusivity behaviour of the numerical materials are simulated using ABAQUS software. The proposed three-dimensional heterogonous model presents at least two advantages. Firstly, it is able to capture the complexity of the random microstructure of cement-based materials. Secondly, only a few parameters have to be fitted compared to the other existing models, which indicates the relevance of the model. The numerical simulations of leached cement paste and mortar performance highlight and quantify the significant effect of portlandite and hydrated aluminate and sulfoaluminate phases' dissolution on the decrease of elastic modulus and compressive strength and on the increase of ductility and diffusivity. The numerical results show that the leaching of portlandite decreases the compressive strength of a w/c = 0.4 cement paste by a factor of 1.33. The dissolution of portlandite and hydrated aluminates or sulfoaluminate phases involves a decrease by a more important factor (1.86). The leaching of portlandite phase involves an important increase, by a factor of 31, of the effective diffusion coefficient. Using the developed multi-scale modelling and knowing the leaching kinetics values, the mechanical and diffusion performances of cement-based materials can be estimated correctly according to leaching duration.
Applied Geochemistry, 2014
Near-surface cement-based disposal systems for hazardous materials such as radioactive waste will undergo chemical alterations due to interaction with the surrounding environment. One of the most relevant long-term geochemical alteration processes is decalcification or leaching of the cement phases by percolating water. Consequently, the cementitious components of the disposal system will evolve through different chemical degradation states, also altering physical material parameters such as porosity and bulk density and chemical parameter relevant to solute migration such as the solidliquid partition coefficient or distribution ratio. This paper presents a novel approach in which geochemical modeling serves as a fundamental basis for assessing the evolution of geochemical conditions within a cement-based near-surface disposal facility. On one hand, geochemical modelling is used to quantify uncertainties related to the infiltrating water composition and C-S-H degradation model, both of which allow for various conceptualizations of the evolution of retardation factor. On the other hand, the concept of mixed tank reactor is used to represent cement degradation within the entire disposal system. This paves the way to establish a link between the evolution of the geochemical conditions and the evolution of the retardation factor via the knowledge of amount of percolated water through the system. The usefulness of the approach is demonstrated via a number of case studies concerning leaching of radionuclides ( 14 C and 94 Nb) from a cementitious near-surface disposal facility. The studies reveal that there is a large effect of the conceptualization on calculated fluxes from the disposal facility, depending on the type of radionuclide. A crucial factor is the amount of radionuclide mass present in the disposal system when large changes in the retardation factor occur, for instance, when different retardation factors exist in different chemical degradation states.
Cement-based materials have properties for the encapsulation, isolation, or retardation of a variety of hazardous contaminants. Therefore, many disposal concepts currently show that concrete is a confinement material used in engineered barrier system (EBS) at a number of LLW disposal sites. However, several questions are raised about the safety of concrete barrier, such as: Could the EBS be a complete isolation from groundwater? Do hydrogeochemical reactions of ground water affect the degradation of concrete barrier of repository? Whether the redox processes influences on the formations of degradation materials? What are the key aqueous species affecting the concrete degradation processes? Studying the relationship among the geochemical evolution of concrete barrier, reactive transport of solutes, and groundwater flow could gain further insights of these problems. Therefore, the present study aims to compile the thermodynamic equilibria data in cementitious media and incorporate with a reactive chemical transport model for assessing hydrogeochemical influences on concrete barrier degradation. A proposed site for final disposal of LLW located in Daren Township of Taitung County along the southeastern coast has been on the selected list in Taiwan. Concrete is a confinement material in EBS of the proposed site. To investigate the effect of hydrogeochemical environment on concrete degradation in the proposed LLW repository site, a reactive chemical transport model of HYDROGEOCHEM5.0 is applied to simulate the effect of hydrogeochemical evolution on concrete barrier degradation in the proposed site. Simulated results show that the main processes responsible for concrete degradation are species induced from hydrogen ion, sulfate and chloride. The EBS with the side ditch drainage system is an effective design to discharge the infiltrated water and lower the solute concentrations that may induce the concrete degradation. Reduction environment in the EBS can reduce the concentration of Ettringite formation in the concrete degradation processes. Moreover, the chemical conditions in the concrete barriers maintain alkaline condition after 300 years in proposed LLW repository site. This study provides a detailed picture of the long-term evolution of the hydrogeochemical environment in the proposed LLW disposal site in Taiwan.
2011
Cementitious binders are commonly used to solidify/stabilize hazardous wastes prior to disposal in multi-barrier engineered disposal facilities or landfills. Because they are not in equilibrium with other materials in a landfill, cementitious materials usually degrade with time. Reactive transport models may be used to estimate the possible effects of changing geochemical conditions on the transport properties of the cementitious materials, the leaching of chemotoxic elements from the waste form, and alterations of the sorption properties of surrounding materials due to leaching of cement components.