Leaching of Nirex Reference Vault Backfill cement by clay, granite and saline groundwaters
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.248
Leaching of Nirex Reference Vault Backfill cement by clay, granite and saline groundwaters Rita G. W. Vasconcelos1, Andres Idiart2, Neil C. Hyatt1, John L. Provis1, and Claire L. Corkhill1* 1 NucleUS Immobilisation Science Laboratory, Department of Materials Science and Engineering, University of Sheffield (UK)
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Amphos 21, Barcelona (ES) * Corresponding author: [email protected]
For the UK geological disposal facility (GDF) concept in a high strength crystalline rock, Nirex Reference Vault Backfill (NRVB) has been considered to use as a cementitious backfill material. The lifetime performance of this high pH material is required to be extremely long, and as a consequence of the interaction with the geological environment (e.g. groundwater), the backfill material will evolve and age with time. In this paper, we present the results of a leaching experiment, where NRVB is placed in contact with three different groundwater compositions representative of granitic, clay and saline groundwater for 35 days. Some differences were observed related with the mineralogy. X-ray Diffraction results showed the formation of more ettringite, mainly in the NRVB samples leached in clay and saline groundwater. An increase of the pH was also observed in the NRVB samples leached with the three different ground waters, being these results in accordance with the reactive transport modelling performed.
Introduction One of the UK conceptual scenarios for the final disposal of intermediate level waste (approximately 450 000 m3) is in a geological disposal facility (GDF) [1]. In this multibarrier concept, the waste would be conditioned (e.g. using cementitious grout), packaged and placed in vaults excavated deep underground, in a hard rock (crystalline geology) [2,3]. Consequently, these vaults would be backfilled with a high pH cementitious material, called the Nirex Reference Vault Backfill (NRVB) [3,4]. NRVB was designed in the early 90s and the composition includes Portland cement, limestone flour and hydrated lime. With this composition, NRVB fulfils some specific characteristics including: maintenance of a high alkaline environment that minimises the solubility and transport of many radionuclides; and provision of high porosity to allow a high sorption capacity and minimise the pressure build-up from the gases generated in the waste forms.
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Cement backfill is expected to maintain a safety function for more than 10,000 years), therefore, it is of crucial importance to understand how interactions with the geological environment, specifically with groundwater, may affect the mineralogical, geochemical and microstructural properties of the NRVB. From predictive modelling, it is hypothesised that when cementit
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