Release of Colloids from Injection Grout Silica Sol
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Release of Colloids from Injection Grout Silica Sol Pirkko Hölttä1, Martti Hakanen1, Mari Lahtinen1, Anumaija Leskinen1, Jukka Lehto1 and Piia Juhola2 1 University of Helsinki, Department of Chemistry, P.O. Box 55, 00014 University of Helsinki, Finland. 2 Posiva Oy, Olkiluoto, 27160 Eurajoki, Finland. ABSTRACT Non-cementitious grouts have been tested in Olkiluoto for the sealing of fractures with the small hydraulic apertures. A promising non-cementitious inorganic grout material for sealing the fractures with the apertures less than 0.05 mm is commercial colloidal silica called silica sol. The objective of this work was to determine colloid release from the silica sol gel and stability of silica colloids in different groundwater conditions. To use silica sol as a grout, the injected colloids have to aggregate and form a gel within a predictable time by using a saline solution as an accelerator. Silica sol gel samples were stored in contact with saline and low–salinity groundwater simulates. Release of silica colloids and colloid stability was followed by analyzing the colloid concentration, particle size distribution, concentration of reactive silica, solution pH and zeta potential after one month, half a year and one year. The release and stability of silica colloids were found to be dependent significantly on groundwater salinity. Zeta potential values near zero and an increase in particle size at first and then the disappearance of large particles indicated particle flocculation or coagulation and instable colloidal dispersion in a saline groundwater simulate. In low–salinity groundwater simulate high negative zeta potential, small particle size, and constant size distribution indicate the existence of stable silica colloids. Under prevailing saline groundwater conditions in Olkiluoto no significant release of colloids from silica sol is expected but the possible influence of low–salinity glacial melt waters has to be considered. INTRODUCTION In Finland the final disposal of spent nuclear fuel is investigated by Posiva Oy in Olkiluoto utilizing the underground rock characterization facility ONKALO. The project seeks to obtain information about the bedrock at the site planned for the final disposal repository and assess its safety and test final disposal technology in actual deep underground conditions. Cement is typically used for permeation grouting in hard rock. Because high pH can be harmful for the Engineered Barrier System and cement-based grouts may have limited penetration, noncementitious grouts have been tested for the sealing of fractures with a small hydraulic aperture. A promising non-cementitious inorganic grout material for sealing fractures with apertures of 0.05 mm or less is silica sol, which is commercial colloidal silica manufactured by Eka Chemicals in Bohus, Sweden [1]. The main experiences with colloidal silica come from geotechnical applications like the grouting of soil to increase its liquefaction resistance [2] and to seal narrow fractures in low permeable rock to prevent w
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