Tricalcium silicate (C3S), 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 C3S has been observed 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 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 approximate to3.1 Angstrom. Extended X-ray absorption fine structure results have been confirmed by ab initio calculations including multiple scattering.