A study of the effect of combined leaching and carbonation processes on physical and transport properties of mortar
In a deep geological repository for the disposal of radioactive waste, gas can be generated by different mechanisms including anaerobic corrosion, radiolysis and microbial degradation. If the gas generation rate is larger than the capacity for the diffusive transport of the dissolved gas, a free gas phase will be formed that enters the Engineered Barrier System (EBS). Within the Belgian concept for geological disposal for intermediate and high level waste, cementitious materials are omnipresent. Hence we need to understand how gas will be transported through these materials. Especially the cementitious backfill may play an important role, as it might be designed for promoting gas storage and facilitating gas release. Based on examples in the UK (Nirex Reference Vault Backfill, NRVB) and Switzerland (M1 and M2 mortars), capillary porosities between 25 to 50 % are targeted. Currently it is considered that gas migration will occur through a classical visco-capillary 2-phase flow mechanism. In order to support argumentation for safe geological disposal of (especially intermediate-level) radioactive waste, a mechanistic understanding of the behaviour of gas in cementitious materials as part of the EBS is needed.
In this context, SCK-CEN has recently completed two relevant PhD projects: (i) an experimental investigation to understand individually the effect of leaching and carbonation behaviour on microstructural properties of hardened cement paste (HCP) and general transport parameters (hydraulic conductivity and diffusivity of dissolved gases) (Phung Q.T, 2015), and (ii) development of a novel microscale simulator based on lattice Boltzmann method/geochemical modelling (LB-PHREEQC) to capture degradation of HCP and its impact on transport (Patel R. et al., 2014). This provides a strong foundation in terms of know-how and experimental and computational infrastructure.
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