Results of laboratory carbonation experiments on Nirex Reference Vault Backfill cement

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 that may degrade to produce carbon dioxide. 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 involved in this study investigates these processes through elevated pressure laboratory experiments conducted at a range of likely future in situ repository conditions. These will provide information on the reactions that occur, with results serving as examples with which to test predictive modelling codes. This report details a series of batch experiments to study carbonation of Nirex Reference Vault Backfil (NRVB) cement. Thirty-two static batch experiments were pressurised with either CO2, or with N2 for ‘nonreacting’ comparison tests at 20°C or 40°C, and 40 or 80 bar. 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. All the cement samples showed rapid reaction with CO2, manifested by a colour change from grey to light brown. Petrographic analysis of the reacted cement revealed that this colour change reflected the breakdown and dissolution of primary calcium ferrite and calcium alumina-ferrite (CAF) cement clinker phases (e.g. brownmillerite, Ca2(Al,Fe)2O5 to form calcium carbonates and finely-disseminated free ferric oxide (probably hematite, Fe2O3), as a result of reaction with CO2 to give a ‘rusty’ colour. It should be noted that his is not an oxidation reaction as the iron is present as Fe3+ in the original cement phases. 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. There is potential therefore, for carbonation to immobilise 14CO2 if that were present. 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, calcium silicate hydrate (CSH) phases, calcium aluminate (or calcium aluminate hydrate) phases, and ettringite-like phases, and the formation of carbonate phases and silica gel. Carbonation also revealed that heterogeneity within the cement samples had a major impact on migration pathways and extent of carbonation. This heterogeneity may have been a result of casting, and was only observed in some of the samples studied. It led to faster carbonation in some areas, and may account for some of the differences observed in the reacted cement samples. Such heterogeneity may be present within a repository, and should be taken into account when assessing repository performance.