Modelling reactions between alkaline fluids and fractured rock: The Maqarin natural analogue

Abstract Cement and concrete will be used as fracture grouts, shotcretes, tunnel seals, and as matrices for waste encapsulation in many radioactive waste geological disposal facility concepts. Alteration of the disposal facility host rock and/or swelling clay in waste package buffers and tunnel backfills by alkaline solutions leached from cement/concrete may have implications for system performance. The Maqarin natural analogue has received considerable attention over the last twenty-five years as it represents some of the conditions of interest for a cementitious radioactive waste disposal system; as naturally-occurring high pH waters have been transported through fractures passing through clayey marls, causing mineralogical alteration in the wall-rock and sealing of the fractures as minerals precipitate. In this study, reactive-transport simulations were constructed of the mineral–fluid interactions at the Maqarin site with a particular emphasis on mineral formation in fractures, including armouring of surfaces, and how this can be simulated in geological environments that may host a radioactive waste disposal facility. The evolution of fractures is important as they may act as pathways for water and contaminant transport. In a ‘base case’ reactive-transport model, a pseudo 2-D approach was used to simulate flow along a fracture and diffusion of solutes into the rock matrix. The model predicts that ettringite, thaumasite and C–S–H (jennite and tobermorite), dominate the fracture-filling minerals, whereas alteration products forming at the expense of the primary silicates in the rock matrix include scolecite (Ca–zeolite), ettringite, C–S–H and small amounts of sepiolite, in broad agreement with observations of the rock present at Maqarin. A more realistic conceptual model of mineral precipitation in the fracture has also been developed, whereby minerals precipitated on the fracture walls in an ‘armoured’ layer rather than uniformly throughout the fracture cells, thus impeding diffusion into the rock matrix. In this model only jennite and calcite precipitated in the fracture, eventually filling it. The modelling carried out here confirms that, where alkaline fluids derived from cement degradation come into contact with rocks saturated with neutral pH, bicarbonate-rich groundwaters, sealing of fractures may occur. However, fracture sealing is complex and is dependent upon a range of factors such as: rock composition (mineralogy, grain size, porosity); groundwater composition and flow rates; and mineral reaction kinetics inter alia. Although the precise timing of fracture sealing at Maqarin is unknown, the consideration of a number of uncertainties in reactive-transport modelling conducted here would suggest that sealing is moderately rapid (of the order of hundreds of years). The geochemical modelling approach used here could be applied to potential fractured host rocks for radioactive waste disposal facilities, in order to elucidate the evolution of water flow and contaminant transport pathways.