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The Performance of Cementitious Barriers in Repositories

Objective

The mechanisms of crack healing within cementitious materials under repository conditions are being investigated. The critical characteristics of cement type, groundwater chemistry, crack geometry and hydraulic head which lead to healing will be identified. The material involved in the closure of cracks will be characterized. An existing model of the crack healing process by carbonate precipitation will be notified. Similar models for crack healing by the precipitation of silicate minerals will be developed. A modified version of a repository source term model which takes account of the development of inhomogeneities will be supplied.
Cementitious materials are likely to be used in waste disposal facilities to help retain radionuclides by acting as chemcial and physical barriers to migration. Current source term calculations are based on the homogeneous repository assumption. This is unlikely to be true in reality since cementitious materials will crack, leading to inhomogeneities in mass transport and chemistry. The impact of these inhomogeneities will depend upon reactions with groundwater that can lead to the healing of cracks, to the benefit of the physical barrier performance, and sealing of concrete surfaces, to the detriment of chemical performance.

An initial set of experiments dealing with the healing of cracks by bicarbonate bearing water have been completed and a second set initiated. The resulting deposits have been investigated using chemical and microscopic means. The previously existing model of the crack healing process is being extended to handle the data from these experiments. Experiments were performed to examine crack closure due to calcium carbonate deposition from simulated groundwater flowing through cracks in hardened cement mortar specimens. The results have shown that the tendency of the cracks to actually close depends mainly on the crack width. The closure of cracks with widths greater than 0.1 mm appears to be uncertain. Some simple experiments on the flow of pure, carbon dioxide free water through high porosity cement mortar have performed. A decrease in the hydraulic conductivity by a factor of about 10 was observed over a period of 60 days. This may possible be due to the migration of the calcium silicate hydrate (CSH) phase within the specimen.

An experimental apparatus for the simulation of long term water flow through materials has been designed and constructed. Relatively high pressure gradients are employed to force significant volumes of water or reactive solutions through small specimens. The interaction of groundwater and cement mineral phases during flow within the pore structure is being modelled together with the interaction between the water flowing in the crack and surrounding backfill.
Work programme:

To meet the objectives the work programme will consist of three main tasks:

Healing of cracks by cementitious materials
- Experiments on crack healing
- Modelling of crack healing

Cement-groundwater interactions
- Diffusive cement-groundwater interactions
- Perfusive cement-groundwater interactions
- Modelling cement-groundwater interactions

Barrier properties of the inhomogeneous repository
- Chemistry varying with position and time
- Effect of fissures on the source term
- Validation of source term model

Coordinator

AEA Technology plc
Address
Harwell Laboratory
OX11 0RA Didcot
United Kingdom

Participants (2)

Bundesanstalt für Materialforschung und Materialprüfung
Germany
Address
Unter Den Eichen 87
12203 Berlin
Risø National Laboratory
Denmark
Address

4000 Roskilde