Microscale investigations of Ni uptake by cement using a combination of scanning electron microscopy and synchrotron-based techniques.

Cement is used to condition waste materials and for the construction and backfilling of repositories for low-and intermediate-level radioactive waste. In this study, Ni uptake by hardened cement paste has been investigated with the aim of improving our understanding of the immobilization process of Ni(ll) in cement on the microscale. Information on the cement microstructure, Ni distribution, Ni concentration, and speciation of the Ni phases formed in the cement system and their association with specific cement minerals has been gained by using scanning electron microscopy (SEM) and synchrotron-based micro-X-ray fluorescence (micro-XRF) and micro-X-ray absorption spectroscopy (micro-XAS). The Ni-doped samples were prepared at a water/cement ratio of 0.4 using a sulfate-resisting Portland cement and were hydrated for 30 days. The metal loadings of the system were varied from 50 up to 5000 mg/kg. SEM investigations show that for all metal loadings the Ni phases form rims around inner-calcium silicate hydrates, suggesting a direct association with this cement phase. The micro-XAS measurements further reveal that a mixture of Ni phases form at Ni-enriched regions. Data analysis indicates that Ni(ll) is predominantly immobilized in a layered double hydroxide-type phase (Ni-Al LDH) and only to a minor extent precipitates as Ni-hydroxides (alpha-Ni(OH)2 and beta-Ni(OH)2). At 50 mg/kg Ni loading, however, the p-XAS measurements suggest the presence of an additional Ni species. In the latter system Ni-Al LDH is found in Ni-rich regions, whereas at Ni-poor regions an unknown species is formed.