OR/13/004 Summary

Rochelle, C A, Purser, G, Milodowski, A E, Noy, D J, Wagner, D, Butcher, A, and Harrington, J F. 2013. CO2 migration and reaction in cementitious repositories: A summary of work conducted as part of the FORGE project. British Geological Survey Internal Report, OR/13/004.

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 reaction-transport 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 ‘non- reacting’ 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.

All the cement samples showed rapid reaction with CO₂, manifested by a colour change from grey to light brown. The cement blocks remained intact, even after prolonged exposure to CO₂-rich fluids. Carbonation was associated with an increase in weight by up to 9%, though the samples did not change in overall size. Free phase CO₂ gave slightly more reaction than dissolved CO₂, possibly because of its higher concentration and greater ability to penetrate the samples. In terms of major reactions during carbonation, these were the breakdown of portlandite and calcium silicate hydrate (CSH) phases and the formation of carbonate phases and silica gel. The observed colour change was possibly caused by CO₂-enhanced reaction of small amounts of calcium ferrite minerals in the cement and liberation of the ferric iron to give a ‘rusty’ colour.

Carbonation resulted in a series of reaction fronts that moved through the cement over time. These fronts separated several reaction zones: Zone 1 = minor carbonation with minimal apparent volume change, Zone 2 = partial carbonation and very localised shrinkage, Zone 3 = complete conversion of portlandite and CSH with localised shrinkage associated with the development of calcium carbonate-sealed microfractures, Zone 4 = dissolution of initially-formed carbonate minerals in the outermost parts of the sample by the surrounding, slightly-acidic water. The shrinkage in Zone 2 was expressed as small fractures (typically several mm long), though these do not appear to extend beyond this zone. Zone 3 contained an anastomising ‘3D chickenwire’ meshwork of interconnected, higher-density, carbonate-filled microfractures (typically on a 10s–100s µm scale) that separated silica-rich areas having lower-density and high porosity, and sub-parallel ‘relic’ reaction fronts. The small fractures of Zone 2 appear to have filled with secondary precipitates in Zone 3. Appreciable amounts of a Cl-rich phase were formed at the boundary of Zones 2 and 3, which was aided by the presence of CO₂. The formation of Cl-rich phases within a repository could be beneficial as it might help to immobilise ³⁶Cl leaching from the waste.

Controlled flow-rate carbonation experiments on 5 cm diameter by 5 cm long cores of NRVB cement reveal decreases in overall sample permeability. These reflect porosity reduction due to conversion of portlandite and CSH to secondary carbonate minerals and silica gel. Small discharges of water were also released as a by-product of CSH phase carbonation. Detailed petrographic observations of partly-reacted cement samples show a series of reaction zones as per the static experiments. These observations, coupled with micropermeameter data, show the greatest reductions in porosity and permeability in a very narrow zone at the leading edge of the visible alteration front. Injection of free-phase (gaseous and supercritical) CO₂ resulted in a halving of permeability, whereas use of dissolved CO₂ reduced hydraulic permeability by about 3 orders of magnitude. These reductions could be beneficial within a repository setting, as they reduce the potential for radionuclide migration. Carbonation did not lead to complete blockage of the cement however. CO₂ migration was still possible (though to a reduced extent), so in a repository setting there is still potential for the cement to vent gas with a view to preventing build-up of pressure in and around the waste canisters.

Carbonation features and secondary phases observed in these experiments using a relatively porous/permeable cement, bear many similarities to those found in far lower porosity/permeability borehole cements used in CO₂-storage operations. There are also similarities to samples of naturally-occurring CSH phases which have been naturally-carbonated over prolonged timescales. A number of common carbonation processes may be operating in all these systems.