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Summary:
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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 of the interaction between groundwater and grout is the formation of high-pH solutions which will be able to react with the host rock (gneisses) and alter its mineralogy and porosity.
Following the results of previous work, new calculations simulating the leaching of grout by flowing groundwater in a fracture have been performed. The calculations include the hydration and simultaneous leaching of the grout through diffusive exchange between the porewater in the grout and the flowing water in the fracture. The effect of different potential groundwater compositions has been studied.
The results of the new calculations show that after grouting, the duration of the initial high-pH peak in the fracture is shorter (< 0.5 a) than in previous results (ca. 1.5 a), which compares well with observations at test borehole ONK-KR3 in ONKALO. The inclusion of Mg in the definition of the new fracture groundwater compositions is responsible for the shorter duration of the peak. Mg induces the precipitation of brucite (Mg(OH)2) at the grout-fracture interface, consuming OH- from the solution and reducing its pH.
Maximum calculated pH values of the groundwater after interaction with the grout range between pH 10.3 and 10.9, which are smaller than the maximum measured value (pH 11.5). pH values at later times for all the new cases compare reasonably well with the measurements.
In the longer term, the results show a gradually decaying pH tail (pH < 9), which is controlled by the precipitation of calcite at the grout-fracture interface. The duration of this low-pH tail correlates inversely with the carbonate content of the inflowing groundwater. The longest duration (ca. 800 a) corresponds to the case of a high-salinity brine.
Finally, the results show the formation of a high-porosity narrow band on the grout side of the calcite-sealed grout-fracture interface. The presence of this structure could affect the mechanical stability of the calcite seal. Breaking of the seal could mean a somewhat longer duration of the low-pH tail, although interaction between the groundwater in the fracture and the cement porewater would lead to new calcite precipitation.
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Keywords:
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Low-alkali cement, grout, fracture, flow, diffusion, porosity.
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