Chemical state analysis of trace heavy metals in concrete and recycling of waste

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It is essential to clarify chemical behavior of harmful heavy metals in cement because of the rise of the tendency that uses waste for raw materials of the cement. The aim of this study is to clarify how a chemical state of a small amount of chromium (58.4ppm) and zinc (205.1ppm) originally contained in the ordinary portland cement changed with hydration.The following findings were obtained by applying XAFS (X-ray absorption fine structure) analysis by the synchrotron radiation to the cement system for the first time.As for the chromium, the reduction reaction from hexavalent to trivalent proceeded with hydration because the peak intensity of hexavalent chromium appearing at the absorption edge of the XANES region was decreased with hydration. This reduction reaction was presumed to be due to the trace divalent iron ion contained in cement. As for zinc, XAFS spectra indicated that in unhydrated cement the intrinsic trace zinc exists at the same local structure as the wurtzite ZnO, in which Zn atoms are surrounded by four O atoms in the first-coordination shell. As the hydration of cement proceeds, only the coordination number of the second shell decreases from 15.4 (unhydrated) to 7.5 (294 day) whereas that of the first shell and the interatomic distances of the both shells remain 4, 0.20nm, and 0.32 nm, respectively. Our results elucidate that zinc exists as a solid solution in unhydrated cement having the same local structure as the wurtzite ZnO, and that this trace zinc hydrolyzes to tetrahydroxozincate (II) ion [Zn(OH)_4]^<2-> with the hydration of cement keeping their fundamental structure of ZnO_4 tetrahedra.