Uptake of nickel and strontium by a sulphate-resisting Portland cement

Sr-85 and Ni-63 uptake by a sulfate-resisting Portland cement were investigated under the chemical conditions relevant to a non-evolved cementitious near field. Uptake studies were performed under equilibrium as well as transient chemical conditions in the presence and absence of isosaccharinic acid (ISA). Sr and Ni inventories in cement pore water were found to be controlled by stable isotopes associated with the cement matrix. Under equilibrium conditions, retention at trace levels is due to isotopic exchange between the radioactive and stable isotopes. Under transient conditions, however, uptake processes are superimposed by phase reactions that affect Ni immobilization but not Sr binding. The presence of ISA exerts no measurable effect on radionuclide retention. Sr-85 uptake by cement is attributed to Ca/Sr replacement on calcium silicate hydrates (CSH), whereas the formation of Ni-Al layered double hydroxides may control Ni-63 uptake.