The potential for colloidal silica grout in multi-barrier systems for geological disposal

Over the past decades, researchers and industry have developed multi-barrier approaches to the sealing of deposition holes, tunnels, shafts and boreholes. Research has comprehensively described the hydromechanical behaviour of typical barrier materials, such as cement and compacted bentonite over a wide range of hydromechanical conditions. Where challenges remain for post-closure sealing is in controlling the hydromechanical behaviour at interfaces between materials within the repository. These interfaces include between the host rock and the barrier materials, as well as at the interfaces with materials required for safe construction and operation, such as shotcrete and steel.

 

This study explores the potential for colloidal silica grout as a secondary grouting material for repair of degraded cementitious construction materials and wasteforms and for sealing the interfaces between barriers materials and the host rock. Colloidal silica is an aqueous suspension of silica (SiO2) nanoparticles, with average particle size <100 nm.  The creation of siloxane bonds (Si – O – Si), typically triggered by the addition of an electrolyte accelerator, leads to the formation of a solid-like network of silica nanoparticles in the form of a hydrogel. Previous work at Strathclyde on colloidal silica gel has proved its potential to form low-permeability hydraulic barriers against fluid migration, and to inhibit the diffusion of radionuclides through the gel, making it a promising material for use in retrieval operations.

 

Here we present research to determine the potential for colloidal silica to be used in a range of geological disposal applications. We show that due to its excellent penetrability and controllable gel time, colloidal silica has the potential for repairing fine-aperture cracks within the cementitious materials, at the cement/steel interface, or at the interface between barrier materials and the host rock. We inject colloidal silica injected into fractured cement cores (0.2 and 0.5 mm fracture aperture) and expose them to varying pressure and temperature conditions. Fracture permeability upon water injection is assessed pre- and post-treatment. We find that permeability values after treatment are reduced by three orders of magnitude, thus confirming the potential of colloidal silica for repairing fine-aperture cracks. Further, our experiments show that mechanical strength of the cement is recovered, suggesting that additional C-S-H is produced during grouting. We then discuss the wider potential for colloidal silica as part of a multi-barrier approach to long-term sealing of geological disposal facilities.