The use of (micro)-X-ray absorption spectroscopy in cement research A.M. Scheidegger * , M. Vespa, D. Grolimund, E. Wieland, M. Harfouche, I. Bonhoure, R. Da ¨hn Laboratory for Waste Management, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Accepted 31 January 2006 Available online 11 April 2006 Abstract Long-term predictions on the mobility and the fate of radionuclides and contaminants in cementitious waste repositories require a molecular-level understanding of the geochemical immobilization processes involved. In this study, the use of X-ray absorption spectros- copy (XAS) for chemical speciation of trace elements in cementitious materials will be outlined presenting two examples relevant for nuclear waste management. The first example addresses the use of XAS on powdered cementitious materials to determine the local coor- dination environment of Sn(IV) bound to calcium silicate hydrates (C–S–H). Sn K-edge XAS data of Sn(IV) doped C–S–H can be ratio- nalized by corner sharing binding of Sn octahedra to Si tetrahedra of the C–S–H structure. XAS was further applied to determine the binding mechanism of Sn(IV) in the complex cement matrix. The second example illustrates the potential of emerging synchrotron-based X-ray micro-probe techniques for elucidating the spatial distribution and the speciation of contaminants in highly heterogeneous cemen- titious materials at the micro-scale. Micro X-ray fluorescence (XRF) and micro-XAS investigations were carried out on Co(II) doped hardened cement paste. These preliminary investigations reveal a highly heterogeneous spatial Co distribution. The presence of a Co(II)-hydroxide-like phase Co(OH) 2 and/or Co–Al layered double hydroxide (Co–Al LDH) or Co-phyllosilicate was observed. Surpris- ingly, some of the initial Co(II) was partially oxidized and incorporated into a Co(III)O(OH)-like phase or a Co-phyllomanganate. Ó 2006 Elsevier Ltd. All rights reserved. 1. Introduction The long-term immobilization and safe disposal of radio- active and industrial wastes in landfills and deep geological waste repositories is worldwide one of the challenging tasks to endorse the sustainable development of modern civiliza- tion (Spence, 1993; Agency, 1999). Although new technolo- gies focusing on waste minimization and recycling will undoubtedly reduce waste arising in the future, an increase in the amounts of intractable waste will be unavoidable. Thus, for any disposal facility, release of hazardous sub- stances must be limited to the lowest level technically (and economically) achievable. Furthermore, strategies are needed to ensure the safe disposal of these waste forms to minimize their environmental impacts (Levi et al., 1990; Agency, 1999). The present study is focusing on the molecular-level investigation of geochemical processes related to the long-term fate of radioactive and industrial waste in cement-based materials. Mixing ‘fugitive’ hazardous waste products into a cementitious binder system improves the stabilization and the solidification of waste materials (Atkins et al., 1994). Consequently, the migration of radionuclides and other heavy metals from cement-based landfills and waste repositories into the environment can be significantly retarded and possible impacts on the environmental quality can be minimized. For this reason, as well as for engineering purposes, cement-based materi- als play an important role in multi-barrier concepts for the safe disposal of radioactive wastes in underground repositories. For example, approx. 90 wt% of the near-field material of the planned Swiss disposal cavern for intermediate level waste consists of cement and cementitious backfill materials. From a mineralogical standpoint cement consists mainly of calcium (aluminium) silicate hydrates (C(A)SH phases, 50 wt%), portlandite 0956-053X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2006.01.032 * Corresponding author. Tel.: +41 56 310 2184; fax: +41 56 310 4551. E-mail address: Andre.Scheidegger@psi.ch (A.M. Scheidegger). www.elsevier.com/locate/wasman Waste Management 26 (2006) 699–705