Effects of the injection grout Silica sol on bentonite

Silica sol, i.e., colloidal SiO2, may be used as a low-pH injection grout for very fine fractures in the construction of deep geological repositories for radioactive waste in Sweden and in Finland. If the bentonite barrier encounters SiO2-colloid particles under conditions favorable for aggregation, there is concern that it will modify the bentonite barrier at the bentonite/bedrock interface. In this study qualitative experiments were performed with mixed dispersions of SiO2-colloids and bentonite or homo-ionic Na/Ca-montmorillonite. Samples were prepared at different colloid concentrations and treated under various conditions such as low and high ionic strength (0.3 M NaCl), as well as dehydration and redispersing. Free swelling and settling experiments were performed in order to qualitatively compare the conditions in which SiO2colloids affect the bulk/macro properties of bentonite. In order to study specific SiO2-colloid/montmorillonite interactions and preferred type of initial aggregation, dilute dispersions of homo-ionic montmorillonite dispersions mixed with varying concentrations of SiO2-colloids were prepared and selected samples were characterized by PCS, SEM/EDS, AFM and PXRD. The results from this study show that bentonite and montmorillonite particles can be modified by SiO2-colloids when mixed in comparable amounts, due to dehydration or high ionic strength. Some indications for increased colloidal stability for the SiO2colloid modified clay particles were also found. From the AFM investigation it was found that initial attachment of the SiO2-colloids in Na + dominated samples seemed to occur on the edges of the montmorillonite layers. In Ca 2+ dominated samples not subjected to excess NaCl, SiO2colloid sorption onto the faces of the montmorillonite layers was also found. In all, contact between the bentonite barrier and ungelled Silica sol should preferably be avoided.