Retention of strontium in high- & low-pH cementitious matrices – OPC vs. model systems

Strontium can be found in low/intermediate level radioactive waste streams, often solidified by Ordinary Portland Cement (OPC) matrices. Amorphous silica is added to improve wasteform performance, leading to low-pH systems. Our aim is to identify the phases controlling strontium immobilization in OPC matrices. The novelty of this work resides in combination between two approaches: 1) Comparing OPC systems to model systems based on alite, yielding matrices containing only major phases, emphasizing the relative role of minor phases; 2) Comparing high- & low-pH pastes. Pastes were characterized using calorimetry, XRD, DTG, SEM/EDS and porosimetry. Leaching experiments were conducted to study Sr retention and diffusivity. Strontium diffusivities in low-pH pastes are lower with respect to high-pH equivalents, despite their higher porosities, due to preferred Sr sorption to low Ca/Si C-S-H. OPC pastes show better Sr retention relative to model systems, suggesting that minor phases (like ettringite and AFm) improve Sr retention. •Sr is better retained in cement-based systems compared with alite-based systems.•Minor cement hydrated phases, AFt and AFm, increase Sr retention.•Sr is better retained in low-pH systems, rich in amorphous silica.•The low-pH effect on leachability is stronger than the minor hydrates effect.•Superposition of these two effects allows better insight into strontium retention.