CO2 migration and reaction in cementitious repositories : a summary of work conducted as part of the FORGE project

Some repository concepts envisage the use of large quantities of cementitious materials – both for repository construction and as a buffer/backfill. However, some wastes placed within a subsurface repository will contain a significant amount of organic material which may degrade to produce carbon dioxide (CO2). This will react with cement buffer/backfill to produce carbonate minerals such as calcite, which will reduce the ability of the buffer/backfill to maintain highly alkaline conditions and as a consequence its ability to limit radionuclide migration. The reaction may also alter the physical properties of the buffer/backfill. The work presented here summarises the findings of a study conducted within the laboratories of the British Geological Survey into the impact of CO2 on a relatively permeable potential repository cement (Nirex reference vault backfill, NRVB). The work investigated reactiontransport processes through elevated pressure laboratory experiments conducted at a range of likely future in-situ repository conditions. These provide information on the reactions that occur, with results serving as examples with which to test predictive modelling codes. Thirty-two static batch experiments were pressurised with either CO2, or with N2 for ‘nonreacting’ comparison tests. Twenty six of these were left to react for durations of between 10-40 days, with six more left to react for a year. The aim of them was to help investigate mineralogical and fluid chemical changes due to the diffusional ingress of CO2 into unconfined NRVB samples measuring 2.5 cm in diameter and 5 cm long. Four flow experiments were also conducted, aimed at quantifying changes in the transport properties of the buffer/backfill cement under likely in-situ conditions as a consequence of carbonation due to the advection of free phase or dissolved CO2.