EXAFS study of Nd(III) uptake by amorphous calcium silicate hydrates (C–S–H)

Abstract

Calcium silicate hydrate (C–S–H) phases control the immobilization of many metal cations in cementitious materials. In this study Nd binding to amorphous C–S–H phases with different Ca/Si (C/S) mol ratios (0.56, 0.87 and 1.54) and Nd loadings (7 and 35 μmol/g), and which had been aged up to 270 days, has been investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. The structural parameters derived from EXAFS were compared with those predicted from bond-valence calculations. The study reveals that Nd may form several species in contact with C–S–H phases. The EXAFS parameters determined in samples after one day of reaction indicate the formation of inner-sphere surface complexes. The Nd–Ca and Nd–Si bond-distances tend to increase with time at both Nd loadings. Changes in the coordination numbers NSi and NCa were found to be dependent on the (C/S) ratio. At the lowest C/S ratio the number of neighboring Si atoms tends to increase with time while the number of neighboring Ca atoms tends to increase with time at highest C/S ratio. No clear trend was observed for the medium C/S ratio. Nd incorporation into the structures of C–S–H phases is assumed to be the dominant immobilization process based on comparison with bond-distances predicted from structural considerations. After prolonged reaction times (⩾45 days) Nd is expected to be predominantly incorporated into the Ca sheets of the C–S–H structure while small portions of Nd might also be taken up by the interlayer. The study suggests that, in the long term, amorphous C–S–H phases are capable of taking up Nd via exchange processes with Ca2+ in the Ca sheets and the interlayer.