X-ray absorption spectroscopy study of immobilization processes for heavy metals in calcium silicate hydrates. 2. Zinc

Tricalcium silicate (C3S), the major phase of cement-based materials, conta ins naturally a few hundred parts per million of heavy metals. An X-ray abs orption spectroscopy (XAS) study at the Zn K-edge has been carried out to e lucidate the binding mechanisms of Zn by calcium silicate hydrates, which i s important to predict the long-term behavior of Zn. At low concentration, under the threshold of precipitation of the oxide, oxy hydroxide, and calci um zinc hydroxide, no retardation of the hydration of the C3S has been obse rved for C3S doped with Zn. A previous Si-29 NMR (Moulin, I.; Stone, W. E. E.; Sanz, J.; Bottero, J.-Y.; Mosnier, J-Y.; Haehnel, C. Langmuir 1999, 151 2829-2835) study has demonstrated that the structure of the hydrated CBS ( called C-S-H for calcium silicate hydrates) does not change if Zn is presen t initially or not. XAS experiments have shown that Zn tetrahedra are incor porated within the matrix of C-S-H and linked directly at the end of the si licate chains through Zn-O-Si bonds. The Zn-Si distance is approximate to3. 1 Angstrom. Extended X-ray absorption fine structure results have been conf irmed by ab initio calculations including multiple scattering.