『Abstract
Cementitious materials used for radioactive waste repository
construction complicate the performance assessment of radioactive
waste systems because the use of cement may greatly alter the
pH (8-13) of groundwater and release constituents such as calcium
ions. Under such conditions, it is important to clarify also the
dynamics behavior of silica (silicic acid), in order to evaluate
the alteration in the chemical and physical properties of the
fractured layer or the host rock surrounding the repository. since
silica undergoes polymerization, precipitation or dissolution
depending on the pH and/or temperature, the behavior of silica
would be greatly complicated in the presence of other ions. This
study is focused on the deposition rates of polysilicic acid and
soluble silicic acid with up to 10-3 M Ca ions. In
the experiment, Na2SiO3
solution (250 mL, pH>10, 298 K) was poured into a polyethylene
vessel containing amorphous silica powder (0.5 g), and a buffer
solution, HNO3, and CaNO3
as Ca ions were sequentially added into the vessel. The pH of
the solution was set to 8. The silica, initially in a soluble
form at pH>10 (1.4×10-2 M), became supersaturated and
either deposited on the solid surface or changed into the polymeric
form. Then the concentrations of both poly- and soluble silicic
acid were monitored over a 40-day period. The decrease of polysilicic
acid became slow with an increase in the concentration of Ca ions
in the range of up to 10-3 M. In general, the addition
of electrolytes to a supersaturated solution accelerates the aggregation
and precipitation of polymeric species. However, the experimental
result showed that polysilicic acid in the presence of Ca ions
is apparently stable in solution, compared with that under a Ca-free
condition. On the other hand, the concentration of soluble silicic
acid in the presence of Ca ions immediately became metastable,
that is, slightly higher than the solubility of soluble silicic
acid. Its dynamic behavior was similar to that in the Ca-free
condition. 』
1. Introduction
2. Experimental
2.1. Samples
2.2. Procedure
2.3. Determination of concentrations of silica and calcium
3. Results
4. Conclusions
References