Abstract
In this study, Ni uptake by hardened cement paste has been investigated with the aim of improving our understanding of the immobilization process of Ni(II) in cement and the influence of the hydration time on the Ni speciation on the microscopic scale. Information on the Ni distribution and speciation of the Ni phases formed in the cement system has been gained by employing synchrotron-based μ-X-ray fluorescence (μ-XRF) and μ-X-ray absorption spectroscopy (μ-XAS). The Ni-doped cement samples were prepared at a water/cement ratio of 0.4 and with a metal loading of 5000 mg/kg using a sulphate-resisting Portland cement. The samples were hydrated for six hours and one year to account for the chemical environment in a fresh and aged cement paste, respectively. The μ-XAS measurements reveal that a mixture of Ni phases is formed at single regions of interests, independent of the hydration time. Data analysis further indicates that Ni(II) is predominantly immobilized in a layered double hydroxide (LDH, Ni-Al LDH) and only to a minor extent precipitates as Ni-hydroxides. A comparison of the results from this micro-spectroscopic investigation with those from an earlier macro-spectroscopic study, indicate that the same Ni phases are formed both on the macro-and micro-scale.
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