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Theoretical and Experimental Study of Degradation Mechanisms of Cement in the Repository Environment

Objective

The object of the research program is to estimate the long-term corrosion of concrete used for waste disposal and to determine the mechanisms involved. A corrosion model is being developed based on the different mechanisms involved, in which it will be possible to introduce storage conditions specific to each site (type of corrosive solution, cement composition, storage temperature ...).
The objective of the research programme is to determine degradation mechanisms of cement and concrete relevant to the repository environment. It is apparent that throughout the Community, repository environments are not completely quantified. There is, therefore, an urgent need to collect data on the performance of cement which are quantitative but not site-specific; which are applicable to a range of possible sites. The studies will address time dependent changes in the internal constitution of cement based materials and quantify reaction kinetics and mechanisms, based on theoretical and experimental determination of calcium leaching. It will also determine the impact of ground water chemistries and backfills on cement performance. Work is being undertaken on the basic thermodynamics and solution chemistry of cement systems. Working systems which model cement performance have been constructed. These give good agreement between predicted and measured values. The models are, however, chemcially simple and do not enable us to take into account ageing factors, the complexity of ground water, chemistry and interactions between cement and backfill materials.

Globally, the modelling can be divided into 2 parts; the first concentrates on the isothermal thermodynamics of the cement system in order to predict the stable hydrates. The second part deals with kinetics to give the time dependence of the processes described by thermodynamics. Prediction of the alteration of concrete over a 300 year period required the development of a model which integrates the problems of ionic diffusion in the pores of the material as will as kinetics of the dissolution and precipitation processes of the different hydrates. Results obtained for Portland cement clearly show that the cement paste is leached in deionized water according to a diffusion process. The pore solution is in local equilibrium with the solid phases at every point of the paste.
Work programme:

Investigation about degradation mechanisms involved when the cement paste is corroded by a solution including aggressive ions:
- study of phase diagrams to determine which phase may coexist and acquisition of the thermodynamic data necessary to solve the model in the different possible cases (study of equilibria as functions of aggressive ions and temperature)
- study of degradation processes of cement pastes submitted to various aggressive environments (chlorides, sulphates, carbonates)

Testing of the local equilibrium hypothesis (checking that the kinetics is effectively governed by diffusion)

Modelling of cement paste degradation:
- mathematic writing of the model equations built on physico-chemical laws
- numerical resolution of the model by a microcomputer program : in a first step, a simplified algorithm is developed in order to validate the model equations
- model validation
- algorithm extension to resolve any case (modelling of Cl-, sulphate and carbonates attack and taking into account the main reactions of precipitations and dissolutions occurring)

Application to cement clay interaction: resolution of the generalized model and comparison with experimental results

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Commissariat à l'Energie Atomique (CEA)
Address

91191 Gif-sur-yvette
France