Uptake and retardation of Cl during cement carbonation

The presence of 36Cl in low- and intermediate-level radioactive waste (L/ILW) is of concern in repository performance assessment. Its mobility and its relatively long half-life (302,000 years) could potentially lead to early release from the waste and its return to the biosphere within the 106 timescale. Experiments have been undertaken to examine the impact of carbonation on the mineralogical and physical properties of NRVB cement in relation to the degradation of organic material in the L/ILW, and with oilwell Type-G cement in relation to borehole sealing for carbon capture and storage. These show that the cements can uptake a significant amount of Cl through the formation of transient secondary calcium chloroaluminate and Cl-rich calcium silicate hydrate phases. The formation of the Cl- rich phases is enhanced by carbonation reactions and also by low temperatures (20 °C). The process may be important in retarding the migration of 36Cl from a repository for L/ILW.