Strontium interaction with unblended and Low-pH cementitious wasteforms: pH-dependent leaching experiments and geochemical modelling

Cementitious materials are widely-applied for the solidification and stabilization of low and intermediate level radioactive waste streams, that often contain strontium-90. Low-pH cement pastes, based on ordinary Portland cement (OPC) supplemented with pozzolanic additives, were found to have better Sr retention capabilities than unblended OPC pastes. However, precise definition of the phases and processes that control Sr retention in unblended and low-pH OPC pastes is still lacking. The present study combines experimental work with thermodynamic modelling to investigate the retention of Sr in unblended pastes compared to low-pH pastes. The unblended and low-pH pastes were prepared each in two formulations: one based on reagent grade alite (tri-calcium silicate) and the other based on OPC. The chemical and mineralogical behavior of the paste were studied by pH-dependent leaching test. The pastes characteristics were compared in order to assess the impact of: (1) the addition of amorphous silica to the pastes and (2) the presence of minor OPC phases (which are absent in the alite-based pastes) on Sr binding. The low-pH systems showed one order of magnitude lower Sr leachability than the unblended systems at the natural pH. Moreover, the leaching curves indicated that the low-pH systems bind Sr effectively at pH > 7 while for the unblended systems binding is effective only at pH > 10. These results are attributed to the difference in Sr binding to low Ca/Si C–S–H, dominant in low-pH systems, versus the high Ca/Si C–S–H, dominant in unblended systems. Additionally, it was shown that Sr binds mainly to the C–S–H phase rather than to minor phases. The leaching results were thermodynamically modelled and correlated successfully with the experimental findings. A novel modelling approach is offered for the consideration of partial pozzolanic reaction and SiO₂-gel formation. The effect of these processes on strontium retention has been successfully demonstrated.