Dynamic behavior of silicic acid in the co

A main concern about cement materials for the construction of the repository is altering the groundwater to high alkalinity around the repository. Under such chemical condition, the behavior of the silicic acid is one of the key factors to estimate the initial condition of the repository system after the backfill. Since the solubility of silicic acid is extremely large in pH larger than 10, the silicic acid is supersaturated by mixing with the surrounding groundwater (pH 8). Supersaturated silicic acid would produce the polymeric silicic acid even in the presence of a solid phase (i.e., the flow-path surface). Such polymeric silicic acid also deposits gradually onto the flow-path surface while flowing downstream from the repository. However, the precipitation (or deposition) alters the surface to an amorphous one, which in turn strongly affects the sorption behavior of radionuclides. To estimate the spatial spread of the altered surface around the repository, this study examined the dynamic behaviors of supersaturated silicic acid in the co-presence of the solid phase and Ca ions. To determine the concentrations of both soluble silicic acid and its polymeric (colloidal) form in the solution, this study used the silicomolybdenum-yellow method and ICP-AES. As a silicate mineral, Mallinckrodt silicic acid powder (SiO2∙0.23H2O, specific surface area 350 m /g) was used. A Na2SiO3 solution was diluted to a given concentration as a sample solution (pH>10), then the pH value of the solution was adjusted to 8 in the co-presence of the solid phase and Ca ions (1.0 mM). The amount of solid phase was within the range of 0.1 g to 1.0 g. The concentrations of the soluble silicic acid, the polymeric silicic acid and Ca ions in the solution were monitored. This study evaluated the decrease rate, using the first-order reaction equation considering the specific surface area. In the results, the apparent rate constant, k, was 4.9x10 m/s, smaller than that under the Ca ion free condition (k=2.0x10 m/s). This means that Ca ions obstruct the deposition of silicic acid onto the surface. Moreover, the polymeric silicic acid was obviously observed, although the polymeric silicic acid could not be detected under the Ca ion free condition when the initial supersaturated concentration of silicic acid was less than 5 mM. In addition, the concentration of Ca ions in the solution immediately decreased down to 0.4 in WM2010 Conference, March 7-11, 2010, Phoenix, AZ 2 fraction, due to the contact with the solid phase of SiO2. After that, the fraction of Ca ions was kept at 0.4 in the solution. These suggest that Ca ions played two roles in producing the polymeric silicic acid in the presence of the solid phase. One is that Ca ions altered the surface of the SiO2 solid phase by producing CSH gel on the surface of the solid phase. Such alteration affects the chemical property of the surface, also changing the surface area. The other is that the remaining Ca ions in the solution played a role in producing polymeric silicic acid as a pseudo-colloid. (Contrarily, it was confirmed that, when the Ca ion concentration exceeds 10 mM, the supersaturated silicic acid immediately produces the bridged compound and drops on the surface.) To estimate the altered surface due to polymeric silicic acid, we need a more reliable model to estimate these roles of Ca ions at less than 1.0 mM in the presence of silicic acid of a relatively low concentration such as 5 mM.