Cesium immobilization of high pH and low pH belite-rich cement under varying temperature

Abstract

Low-pH cement is being studied in radioactive waste repositories. The belite-rich cement (BRC) recently gained attention due to its higher CO₂ sequestration and low pH attainment under carbonation exposure. Therefore, this study evaluated the effects of pH and temperature on cesium immobilization of BRC. High pH (12.6) and low pH (9.9) BRC were produced via air curing and carbonation treatment, respectively. The high and low pH BRC samples were placed in a leaching environment at 25 °C and 45 °C for 90 days. An inverse correlation between pH and cesium mobilization of BRC was observed. The high pH BRC achieved the lowest effective diffusion coefficient (4.05E-09 cm²/s), whereas the highest value (2.64E-07 cm²/s) was achieved in case of low pH BRC. The physicochemical and morphological properties unveiled the decrease in Si/Ca ratio of gel, precipitation of Ca²⁺ ions in calcite formation, and increment in pore structure connectivity (pore size > 100 nm) in low pH BRC. However, the high pH BRC demonstrated the high Si/Ca ratio in C-S-H gel, hydroxide phases and higher disconnected pores. Thermodynamic modeling revealed the presence of significant carbonated phases beyond 15% CO₂ uptake. The findings contributed to the BRC's feasibility in developing nuclear waste storage facility.

Keywords: Belite rich cement; Cesium; Leaching; PH; Radioactive nuclides.