X-ray Absorption Spectroscopy Study of Immobilization Processes for Heavy Metals in Calcium Silicate Hydrates. 2. Zinc

Abstract

Tricalcium silicate (C 3 S), the major phase of cement-based materials, contains naturally a few hundred parts per million of heavy metals. An X-ray absorption spectroscopy (XAS) study at the Zn K-edge has been carried out to elucidate the binding mechanisms of Zn by calcium silicate hydrates, which is important to predict the long-term behavior of Zn. At low concentration, under the threshold of precipitation of the oxide, oxy hydroxide, and calcium zinc hydroxide, no retardation of the hydration of the C 3 S has been observed for C 3 S doped with Zn. A previous 29 Si NMR (Moulin, I.; Stone, W. E. E.; Sanz, J.; Bottero, J.-Y.; Mosnier, J−Y.; Haehnel, C. Langmuir 1999, 15, 2829−2835) study has demonstrated that the structure of the hydrated C 3 S (called C-S-H for calcium silicate hydrates) does not change if Zn is present initially or not. XAS experiments have shown that Zn tetrahedra are incorporated within the matrix of C-S-H and linked directly at the end of the silicate chains through Zn−O−Si bonds. The Zn−Si distance is ≈3.1 Å. Extended X-ray absorption fine structure results have been confirmed by ab initio calculations including multiple scattering.