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

Abstract

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.