OR/13/004 Concluding remarks

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.

The proceeding sections summarise the findings of our experimental study conducted as part of the FORGE project. This work has allowed identification and quantification of processes occurring during cement carbonation, and this will help inform predictive modelling of repository evolution. The buffer/backfill cement appears to have coped with carbonation well, remaining intact, and with some properties apparently improving (such as reductions in permeability). Uptake of Cl by the cement was also identified, and this could also be beneficial in terms of retarding the mobility of ³⁶Cl. The work also confirmed that cement carbonation leads to a loss of pH buffering to alkaline conditions.

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.