Cementitious materials will be used as backfill, fracture grouts, and as matrices for waste encapsulation in many geological disposal facilities for radioactive wastes [1, 2]. The current candidate backfill material for use in a UK Geological Disposal Facility (GDF) in a Higher Strength Rock (HSR) is the Nirex Reference Vault Backfill (NRVB), which was developed in the 1990s [3]. NRVB comprises Ordinary Portland Cement (OPC) with a fine aggregate containing crushed limestone filler (primarily calcium carbonate) and hydrated lime (calcium hydroxide) [3]. The NRVB plays an important role in the multibarrier approach of this disposal concept; one of the main safety functions of the NRVB is to provide a high-pH environment around waste packages, which limits both the corrosion of waste containers and the solubility of many radionuclides. The properties and behaviour of NRVB have been the subject of research over many years and a significant body of understanding has been developed concerning both the chemical and physical properties of freshly cured samples, largely through laboratory experiments [3]. In terms of its performance to fulfil the desired key safety functions of a backfill in a GDF (namely maintaining high pH conditions to minimize waste container corrosion and radionuclide solubilities), the leaching characteristics have been studied [4]. More recent work has been focussed on the impact of ageing and alteration processes on the evolution of cement backfills, such as carbonation [5] and uptake of chloride [6]. However, less attention has been given to modelling NRVB behaviour using reactive-transport approaches. One previous study [7] that was part of a wider programme of work including that presented here, considered NRVB backfill in a ‘vault scale’ simulation. However, the work presented here is different to that study: the focus here is on the interface between backfill and host rock; a wider range of possible cement alteration pathways are considered (including chloride attack, sulphate attack characterised by thaumasite formation and the potential precipitation of secondary clay minerals); model assumptions and uncertainties are considered in a number of ‘variant’ simulations; and one of the groundwater compositions considered has a lower salinity. Reactive transport models are often used to predict behaviour of engineered barrier materials over different spatial scales in radioactive waste disposal systems [7–۱۷] as the timescales of interest for risk assessment purposes, are generally on the order of thousands to hundreds of thousands of years. It is clearly not possible to simulate such extreme time-scales under laboratory conditions, especially for relatively slow processes such as mineral dissolution-precipitations. Although techniques such as using high water to solid ratios, large surface area-to-volume ratios and elevated temperatures can be used to promote reactions, such approaches may introduce conditions that are less representative of the system of interest [18]. However, experimental data are valuable in that even when conditions are imposed to speed-up reactions, they provide an indication of possible cement alteration phenomena, and the potential for reactions observed in experiments to occur under repository conditions can be explored further in modelling studies. Ideally, when developing an understanding of a complex system, experimental and modelling studies should be complementary, with experimental and modelling studies being used to inform one another. Other types of data that can be used to consider longevity of engineered barrier materials include natural and industrial analogues, and several examples have been identified for cementitious materials and cement-rock interaction [19, 20]. The pyro-metamorphic rocks and associated hyperalkaline groundwater plumes of the Maqarin site are of particular interest and have been investigated by various radioactive waste management agencies since 1989 [19, 21, 22].