Lead and zinc retention during hydration of tri-calcium silicate:: A study by sorption isotherms and 29Si nuclear magnetic resonance spectroscopy

In cement-based materials heavy metals contents are rarely larger than a few hundred parts per million. Sorption isotherms of Zn(II) and Pb(II) carried out on nonhydrated (C(3)S) and hydrated (C-S-H) calcium silicate show that lead and zinc have different affinities for calcium silicate in a concentration range lower than the saturation concentration values of PbO and calcium zinc hydroxide. Lead has a much higher affinity than zinc for both nonhydrated and hydrated calcium silicate. Furthermore, the different retention sites of zinc and lead in hydrated calcium silicate have been investigated by (29)Si nuclear magnetic resonance (NMR) spectroscopy. A spectral line analysis clearly shows that structural retention mechanisms are involved for both ions. Indeed, the additional lines at -85.6 or -85.9 ppm that appear in the NMR spectra have been attributed to Q(1Me) sites involving Si-O-Pb and Si-O-Zn bonds, respectively.