Cement is commonly used in landfills to stabilize waste materials which contain heavy metals and / or radioactive elements, despite that the behavior of the cement matrix during the process of alteration is very little known. The CSH, mainly responsible for the mechanical strength of the hardened cement, is a substantially amorphous or semi-amorphous compound, with a compositional ratio Ca/Si ranging between 0.6 and 2.0. The detailed study of the incorporation of heavy metals in this material and of its thermal behavior is difficult due to the lack of long-range order. On the other hand, in the same range of chemical composition of the CSH, several crystalline phases exist, whose structure has been solved and described in detail in recent years. Such phases occur also in crystalline state in certain types of cement. Among these phases, the members of the tobermorite group are definitely the most representative. The incorporation of various metals (Zn, Co, Sr, Pb, etc.) in the structure of tobermorite has been investigated by several authors, and the products of such incorporation have been so far characterized with spectroscopic methods. Ziegler et al. (2001) hypothesised that Zn-enriched metastable phases formed in treated synthetic tobermorite. They used XAFS (X-ray Absorption Fine Structure) spectroscopy and found that a high concentration of Zn gave rise to a hemimorphite-like phase, whereas low Zn concentration corresponded to intercalation of Zn in the interlayer space of tobermorite. Tommaseo & Kersten (2002) studied the hydration product of tricalcium silicate in presence of zinc oxide. Their EXAFS data suggested that Zn-centred tetrahedra could substitute the end-chain tetrahedra. Our goal is to build a crystal-chemical model for the Ca-Zn cation-exchange reaction in C-S-H through a detailed study of the effects due to the incorporation of Zn within the well-known crystal structure of clinotobermorite (Merlino et al., 2001). Preliminary single crystal X-ray diffraction results indicated that (1) for autoclaved products at high temperature (150°C) the exchange product is essentially hemimorphite; and (2) for mild-treatment, at 70-80°C, a significant amount of Zn partially substitutes the Ca cations in the interlayer space of clinotobermorite (Figure 1). The bad quality of the exchanged single crystal prevented an acceptable structural refinement, but the Fourier synthesis clearly indicated an additional electron maximum at approximately 1 Å from Ca, with reasonable bond-distances with four neighbour oxygen atoms, which could correspond to 0.3 Zn. The final chemical formula for the exchanged phase, obtained through EDS analyses and X-ray diffraction data, is approximately Ca4.7Zn0.3Si6O17·5H2O.

Cation exchange in clinotobermorite: preliminary results

BONACCORSI, ELENA
2014-01-01

Abstract

Cement is commonly used in landfills to stabilize waste materials which contain heavy metals and / or radioactive elements, despite that the behavior of the cement matrix during the process of alteration is very little known. The CSH, mainly responsible for the mechanical strength of the hardened cement, is a substantially amorphous or semi-amorphous compound, with a compositional ratio Ca/Si ranging between 0.6 and 2.0. The detailed study of the incorporation of heavy metals in this material and of its thermal behavior is difficult due to the lack of long-range order. On the other hand, in the same range of chemical composition of the CSH, several crystalline phases exist, whose structure has been solved and described in detail in recent years. Such phases occur also in crystalline state in certain types of cement. Among these phases, the members of the tobermorite group are definitely the most representative. The incorporation of various metals (Zn, Co, Sr, Pb, etc.) in the structure of tobermorite has been investigated by several authors, and the products of such incorporation have been so far characterized with spectroscopic methods. Ziegler et al. (2001) hypothesised that Zn-enriched metastable phases formed in treated synthetic tobermorite. They used XAFS (X-ray Absorption Fine Structure) spectroscopy and found that a high concentration of Zn gave rise to a hemimorphite-like phase, whereas low Zn concentration corresponded to intercalation of Zn in the interlayer space of tobermorite. Tommaseo & Kersten (2002) studied the hydration product of tricalcium silicate in presence of zinc oxide. Their EXAFS data suggested that Zn-centred tetrahedra could substitute the end-chain tetrahedra. Our goal is to build a crystal-chemical model for the Ca-Zn cation-exchange reaction in C-S-H through a detailed study of the effects due to the incorporation of Zn within the well-known crystal structure of clinotobermorite (Merlino et al., 2001). Preliminary single crystal X-ray diffraction results indicated that (1) for autoclaved products at high temperature (150°C) the exchange product is essentially hemimorphite; and (2) for mild-treatment, at 70-80°C, a significant amount of Zn partially substitutes the Ca cations in the interlayer space of clinotobermorite (Figure 1). The bad quality of the exchanged single crystal prevented an acceptable structural refinement, but the Fourier synthesis clearly indicated an additional electron maximum at approximately 1 Å from Ca, with reasonable bond-distances with four neighbour oxygen atoms, which could correspond to 0.3 Zn. The final chemical formula for the exchanged phase, obtained through EDS analyses and X-ray diffraction data, is approximately Ca4.7Zn0.3Si6O17·5H2O.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11568/764044
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