OR/14/048 Conclusions

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.

Thirty-two static batch experiments were pressurised with either CO₂, or with N₂ for ‘non- reacting’ comparison tests at 20°C or 40°C, and 40 or 80 bar. The aim of them was to help investigate mineralogical and fluid chemical changes due to the diffusional ingress of CO₂ into unconfined samples of Nirex Reference Vault Backfill measuring 2.5 cm in diameter and 5 cm long.

The cement blocks remained intact, even after prolonged exposure to CO₂-rich fluids. Carbonation was associated with an increase in weight of up to 8.5% during CO₂ uptake, 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 (calcite, aragonite and vaterite) and silica gel. All the cement samples showed rapid reaction with CO₂, manifested by a colour change from grey to light brown due to the formation of finely-disseminated free ferric oxide (probably hematite, Fe₂O₃). 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. Petrographic analyses also revealed that some heterogeneities were generated within the cement samples as a result of cement plug casting, leading to heterogeneous reaction and faster carbonation in some parts of the cement samples. Such features may be present within a repository, and should be taken into account when assessing repository performance.

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. It would be useful to understand how these reaction zones evolve over longer timescales, and investigate whether they have the potential to become narrower (or even merge together), with more efficient sealing of porosity that might ‘armour’ the cement from further carbonation.

In terms of radionuclide immobilisation, a mixed picture was found. Cement carbonation led to loss of high pH buffering, and as a consequence might be expected to enhance solubility of metallic radionuclides. Conversely, precipitation of CaCO₃ and densification of the cement would be expected to reduce permeability and lower the potential for radionuclide mobility. In terms of specific radionuclides, carbonation would immobilise ¹⁴CO₂ if that were present. The formation of Cl-rich phases within the cement could be beneficial as it might help to immobilise ³⁶Cl leaching from the waste.

The study highlighted several areas where further investigations could be useful. These include:

• Assessing the likelihood for compositional heterogeneity within the cement as a consequence of settling during and after pouring.
• Quantifying the longevity of the reaction zones identified, and whether they evolve into a single reaction front over long timescales.
• Better defining the likelihood of cement micro-fracturing during carbonation, mechanisms controlling the formation of narrow carbonate precipitation zones, and their impact on permeability.
• Quantifying how efficient secondary phases are at ‘armouring’ cement from further carbonation, and how the permeability of this carbonated zone changes over time.
• Precise identification of the Cl-rich phases forming within the altered cement, and consideration of the impact such phases could have on ³⁶Cl retardation and repository safety functions.

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, and consideration of all these sources of information is needed to help provide an overall picture of cement carbonation over a range of temporal and spatial scales.