Evaluation of the Changes in Effective Fracture Aperture with Deposition of Calcium Silicate Hydrate on Granite Surface Under Highly Alkaline Saline Conditions Using a Microflow Cell

In the highly alkaline environment caused by cementitious materials around radioactive repositories, C–S–H phases are expected to form on groundwater flow paths and retard radionuclide migration by clogging the flow paths. This study evaluated the effective fracture aperture of a flow path using a microflow cell under highly alkaline saline conditions. Experiments showed a decrease in the effective fracture aperture with time, even under 600 mM NaCl, due to the deposition of C–S–H gels. Furthermore, a mathematical model was proposed to estimate the apparent elution rate of silicic acid and the apparent formation rate of C–S–H gel with consideration given to the deposition and detachment of C–S–H gels in the flow path. These apparent rate constants increased slightly with the NaCl concentration. This may be due to the replacement of H+ on silanol groups with Na+ and the formation of Si–O–Na, which has a large ionic length and angle, and promotes the elution of silicic acid and the deposition of C–S–H gels. These suggest the potential for deposition of C–S–H phases and clogging of groundwater flow paths, even under saline conditions.