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

In cement-based materials heavy metals contents are rarely larger than a fe w hundred parts per million. Sorption isotherms of Zn(II) and Pb(II) carrie d out on nonhydrated (C3S) and hydrated (C-S-H) calcium silicate show that lead and zinc have different affinities for calcium silicate in a concentra tion range lower than the saturation concentration values of PbO and calciu m zinc hydroxide. Lead has a much higher affinity than zinc for both nonhyd rated and hydrated calcium silicate. Furthermore, the different retention s ites of zinc and lead in hydrated calcium silicate have been investigated b y Si-29 nuclear magnetic resonance (NMR) spectroscopy. A spectral line anal ysis clearly shows that structural retention mechanisms are involved for bo th 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.