OR/14/048 Introduction

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Rochelle, C A, Purser, G, and Milodowski, A E. 2014. Results of laboratory carbonation experiments on NRVB cement. British Geological Survey Internal Report, OR/14/048.

Some repository concepts envisage the use of large quantities of cementitious materials — both for repository construction and as a buffer/backfill. Key aspects of these materials are their good mechanical properties, and their ability to buffer pH to alkaline conditions. Such high pH conditions are important as they greatly limit metallic corrosion and radionuclide solubility — and as a consequence, radionuclide migration. Some wastes placed within a subsurface repository will contain a significant amount of organic material (e.g. ion-exchange resins, contaminated clothing etc). Over time, these may degrade to produce carbon dioxide (CO2), which will rapidly react with cement buffer/backfill to produce carbonate minerals such as calcite. The conversion of cement minerals to carbonates will reduce the ability of the buffer/backfill to maintain highly alkaline conditions and as a consequence its ability to limit radionuclide migration. However, the reaction may also alter the physical properties of the buffer/backfill, possibly changing its permeability and strength. Although carbonation reactions might improve some properties, it is currently unclear whether the overall changes due to carbonation will be beneficial to long-term radionuclide immobilisation, or deleterious. The work involved in this study investigates these processes through laboratory experiments. These were conducted at a range of possible future in situ repository conditions, including those expected over glacial timescales, which might influence the form of the CO2 (i.e. dissolved, gas, liquid or supercritical). The results of the experiments will serve as examples with which to test predictive modelling codes that incorporate reaction kinetics, and coupling between geochemical reaction, porosity changes and fluid flow (such as in the BGS-developed PRECIP code).

Two types of experiments were conducted as part of the FORGE work:

- Numerous batch experiments aimed at providing information on changing mineralogy and porosity upon reaction with free phase and
  dissolved gaseous, liquid and supercritical CO2

- A few flow experiments with mainly free phase and dissolved supercritical CO2 aimed at providing information on changing
  permeability upon reaction

This report will only describe results from former of these, with Purser et al. (2013)[1], describing the flow experiments.

References

  1. PURSER, G, MILODOWSKI, A E, HARRINGTON, J F, ROCHELLE, C A BUTCHER, A, and Wagner, D. 2013. Modification to the flow properties of repository cement as a result of carbonation. Procedia Earth and Planetary Science, 7, 701–704.