High-pH plume from low-alkali-cement fracture grouting: Reactive transport modeling and comparison with pH monitoring at ONKALO (Finland)

Grouting of water-conducting fractures with low-alkali cement is foreseen for the potential future repository for spent nuclear fuel in Finland (ONKALO site). A possible consequence is the formation of high-pH solutions which will be able to react with the host rock. Calculations have been performed including the hydration and simultaneous leaching of the grout. The effect of different possible groundwater compositions has been studied. The results show that after grouting, the duration of the initial high-pH peak is short (<0.5 a), which compares well with observations at a test borehole. Magnesium in the groundwater induces the precipitation of brucite at the grout–fracture interface, which consumes OH−. In the longer term, the results show a gradually decaying pH tail (pH < 9) controlled by the precipitation of calcite at the grout–fracture interface. The duration of this tail correlates inversely with the carbonate content of the inflowing groundwater.A major finding of this study is that mineral precipitation controls the formation of a potential high-pH plume by consuming alkalinity and limiting diffusive solute exchange between the grout and the circulating groundwater. As a consequence, assumption of long-term interaction between rocks or engineering-barrier materials with flowing high-pH (>12) solutions may not be very realistic.

► Fracture grouting with a low-pH-cement grout could trigger a high-pH plume.
► The simultaneous hydration and leaching of the grout has been modeled.
► Model results show a very limited formation of a high-pH plume.
► Results are in qualitative agreement with observations (grouting test).
► Assumption of long-lived high-pH plumes moving by advection may be unrealistic.