A "Colloidal Silica Grout" (CSG) material has been developed as an alternative grouting material for the geological repository of high level radioactive waste. Two main advantages of the CSG are its smaller particle size distribution (< 50 nm) than those of the cementitious materials, and low pH so as to reduce unfavorable effects on the long-term performance assessment of geological disposal system. The CSG is a composed of a mixture of "colloidal silica solution" and KCl as hardening accelerator. The mixture being investigated for grouting materials was focused on SiO₂concentration, grain size and degree of modified surface of silica. Preliminary laboratory experiments were carried out to characterize its fundamental properties from the viewpoint of viscosity, dimensions stability, compressive strength, pressure resistivity and leaching rate of silica. The experiments demonstrate that CSG is expected to become an alternative grouting material for a geological repository.

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