X-ray Absorption Spectroscopy Study of Immobilization Processes for Heavy Metals in Calcium Silicate Hydrates: 1. Case of Lead

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Trace amounts of heavy metals can be detected in cement. The major source is due to heavy metals naturally present in some of the raw materials used for 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 major elements (Ca, Si, Al, Fe). Since tricalcium silicate (C3S) is the major phase of cement-base material, a XAS study at the Pb−LIII edge has been carried out to elucidate the binding mechanisms of Pb by calcium silicate. At low concentration, under the precipitation of the oxide and oxy-hydroxide, a strong retardation of the hydration of the C3S has been observed for C3S doped 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 acts as a diffusional barrier to water and leads to the retardation of C3S hydration. When C3S is previously hydrated without Pb, lead is strongly retained by the hydrated C3S (called CSH for calcium silicate hydrate). A previous 29Si 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 is incorporated within the matrix of CSH and directly linked at the end of the silicate chains through Pb−O−Si bonds. The Pb−Si distance is ≈3.75 Å. All EXAFS results have been confirmed by ab initio calculation including multiple scattering.