The migration of cesium-137 through cement formulations applicable to radioactive waste management

A novel method was developed to accurately measure the migration of $\sp{137}$Cs through cementitious materials. Eighteen portland cement formulations containing three levels of cement/fly ash content (100%/0%, 85%/15%, and 60%/40%), two levels of water to cementitious solids ratio (0.33 and 0.43) and three levels of additive (none, silica fume, and latex polymer) were tested. Each formulation was molded into a solid cement specimen containing a centered well space which served as a reservoir for a $\sp{137}$Cs solution. After a predetermined time period, the $\sp{137}$Cs solution was removed and a 2-cm long by 1.27-cm diameter cement core was extracted from the monolith well bottom using a diamond core drill. Each cement core was then cut into successive horizontal slices with a diamond saw blade. The net activity of $\sp{137}$Cs in each slice was determined with a NaI(T1) detector connected to a single-channel analyzer. Analysis of variance (ANOVA) and regression models were used to examine the statistical relationship between different formulations in terms of the $\sp{137}$Cs activity in the cement slices versus the migration distance as indicated by the respective slice depths. A separate cubic regression equation was determined for each formulation at a particular time interval. The generation of regression curves allowed the comparison between different cement formulations and their relative ability to effectively retard the migration of the $\sp{137}$Cs. Results indicated that mixes incorporating 5% silica fume at a low w/s ratio are relatively effective in retarding $\sp{137}$Cs migration when compared to the other formulations tested. These mixes exhibited high $\sp{137}$Cs uptake in the slices in contact with the $\sp{137}$Cs well solution and a dramatic decrease in $\sp{137}$Cs activity in successive slices compared to other mixes. Formulations with a w/s ratio of 0.33 consistently demonstrated better containment of the $\sp{137}$Cs solution compared to those formulations with a w/s ratio of 0.43. However, it was possible to obtain comparable $\sp{137}$Cs retardation at a higher w/s ratio if large amounts of fly ash were added to 5% EMSAC mixes at w/s = 0.43. The addition of 5% latex polymer did not improve the ability of cements to retard $\sp{137}$Cs ion migration under the curing conditions used