Speciation of cadmium in cement Part I.: Cd2+ uptake by C-S-H

Hazardous cadmium can be trapped in C-S-H, the main phase in Portland cement. Two kinds of Cd-containing calcium silicates were synthesized: Cd/Ca silicate hydrates prepared by coprecipitation and Cd-exchanged C S-H. In Cd-exchanged C-S-H, chemical and structural studies (ICP-AES, XRD, extended S-ray absorption fine structure (EXAFS), and nuclear magnetic resonance (NMR)) show that up to 30 wt.% Cd uptake is possible, with little structural change. XRD patterns and Si-29 magic angle spinning (MAS) NMR spectra are similar in Cd-free ee and Cd-containing C-S-H. Cd-EXAFS shows that the majority of Cd2+ atoms are homogeneously distributed within the C-S-H structure, although Cd-113 cross-polarization (CP) MAS NMR cannot discriminate between Cd atoms in main layer location and Cd atoms in interlayer location. In Cd/Ca silicate coprecipitates, the structure is nearly amorphous and the silicate species do not polymerize to a C-S-H-like structure. (C) 2001 Elsevier Science Ltd. All rights reserved.