Cs Speciation in Cements
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Cs Speciation in Cements J. V. Hanna, L. P. Aldridge and E. R. Vance Materials Division, Australian Nuclear Science and Technology Organisation, Menai, NSW 2234, Australia ABSTRACT Solid-state 133Cs magic angle spinning nuclear magnetic resonance (MAS NMR) measurements have been used to investigate Cs speciation in cements designed for immobilizing low-level nuclear wastes. Cs in ordinary Portland cement cured for only 10 days at room temperature appears to only inhabit aqueous solutions, as evidenced by a sharp signal with no sidebands, whether or not substantial quantities of blast furnace slag or silica fume are present. No significant 133Cs spectral differences were observed when slag or silica fume were present for curing periods of 10 days, but differences due to Cs partly inhabiting crystalline sites instead of water solutions were observed after 12 months in the slag-containing cement. Preincorporation of Cs in various zeolites, followed by mixing the zeolites into the cement, leads to enhanced Cs retention in the cement when it is subject to water leaching at 25oC. INTRODUCTION The conditioning of low-level nuclear wastes (LLW) by direct incorporation in cements, as distinct from macroscopic encapsulation in pressed steel containers which are then inserted into cement containers, is being carried out in several countries. The immobilization of radiocesium incorporated in cement alone is difficult, as shown by the Cs leachability in aqueous media being uncomfortably large[1]. Pre-incorporation of Cs in zeolites before incorporation in cement has promise [2-4] but the pozzolanic reaction between the cement and the zeolite can potentially liberate the Cs [5,6]. Other ways of reducing Cs leachability are to admix fly ash, silica fume or blast furnace slag with the cement, possibly also using silica fume and superplasticisers to reduce the porosity [1, 2, 7-11]. It has been well demonstrated that porosity can control the diffusivity of the Cs+ through cement paste (see Atkinson and Nickerson [12] for a demonstration on the increase of diffusivity with the increasing porosity of cement paste via increased water content). However there is considerable evidence that there is binding between Cs ions and some components of the cement paste containing admixtures [11]. In spite of much empirical work in the past, little is known about Cs speciation in these cements. Thus in the present work, we have studied the speciation by solid-state magic-angle spinning 133Cs nuclear magnetic resonance. In particular, we have studied the changes with curing time of Cs speciation in Portland cements containing the types of additives used for LLW immobilization. EXPERIMENTAL Because solid-state NMR was to be a key method in studying Cs speciation, a Portland cement containing few paramagnetic impurities was selected so as not to influence the NMR
spectra. Thus a Danish Super White Portland Cement (Lion Brand) consisting of 82% C3S, 8% C2S, 4% C3A and ~ 1% C4AF was chosen, where these phases have standard cement notation, in which C
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