X-ray absorption spectroscopy study of immobilization processes for heavy metals in calcium silicate hydrates: 1. Case of lead

Trace amounts of heavy metals can be detected in cement. The major source i s due to heavy metals naturally present in some of the raw materials used f or clinkers manufacture. One way to predict the long-term behavior of these metals is to know which types of links occur between heavy metals and majo r elements (Ca, Si, Al, Fe). Since tricalcium silicate (C3S) is the major p hase of cement-base material, a XAS study at the Pb-LIII edge has been carr ied out to elucidate the binding mechanisms of Pb by calcium silicate. At l ow concentration, under the precipitation of the oxide and oxy-hydroxide, a strong retardation of the hydration of the C3S has been observed for C3S d oped with Pb. EXAFS results confirmed by XANES at the Pb-LIII edge indicate that Pb is chemically adsorbed at the surface of the C3S. This Pb layer ac ts as a diffusional barrier to water and leads to the retardation of C3S hy dration. When C3S is previously hydrated without Pb, lead is strongly retai ned by the hydrated C3S (called CSH for calcium silicate hydrate). A previo us Si-29 NMR1 study has demonstrated that Pb is chemically fixed to the CSH structure through a Pb-O-Si bond. XAS experiments have confirmed that Pb i s incorporated within the matrix of CSH and directly linked at the end of t he silicate chains through Pb-O-Si bonds. The Pb-Si distance is approximate to 13.75 Angstrom. All EXAFS results have been confirmed by ab initio calc ulation including multiple scattering.