Alteration of bentonite by hyperalkaline fluids: A review of the role of secondary minerals David Savage a, * , Colin Walker b , Randy Arthur c , Chris Rochelle d , Chie Oda e , Hiro Takase f a Quintessa Limited, Dalton House, Newtown Road, Henley-on-Thames, RG9 1HG, UK b Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK c Monitor Scientific LLC, 3900 S. Wadsworth Bvd., Denver, CO 80235, USA d British Geological Survey, Keyworth, Nottinghamshire NG12 5GG, UK e JAEA, 4-33 Muramatsu, Tokai-mura, Ibaraki-ken 319-1149, Japan f Quintessa K.K., Queen’s Tower A 7-707, 2-3-1 Minatomirai, Yokohama 220-6007, Japan Received 22 April 2005; received in revised form 11 July 2005; accepted 31 August 2005 Available online 4 October 2006 Abstract Data concerning potential solid products of the interaction of cement pore fluids with bentonite have been reviewed with respect to accurate prediction of bentonite alteration in the long-term. Calcium (aluminium) silicate hydrates (C(A)SH), zeolites, feldspars, hydrox- ides, carbonates, polymorphs of silica, and some sheet silicates (all of varying degrees of crystallinity) are potential products of cement– bentonite interaction. Evidence from natural systems and laboratory studies suggests that most, or all of these phases, may precipitate on timescales of interest to safety assessment of the geological disposal of radioactive wastes. These data indicate that growth kinetics of secondary minerals is equally as important as thermodynamic stability in controlling occurrence. C(A)SH show variable Ca/Si ratio and Al contents. At high pH (>11), the growth of C(A)SH minerals provides a means by which OH  ions from cement pore fluids may be titrated. Although thermodynamic data exist for a number of naturally-occurring crystalline C(A)SH minerals, they are of doubt- ful quality and should be applied with caution in predictive modelling. Zeolites are likely to form at lower pH than for C(A)SH, with the Si/Al ratio of the zeolite decreasing with increasing pH of the fluid. Zeolite stability is also strongly dependent upon silica activity in the fluid phase. Although silica activity in bentonite pore fluids will be spatially (and temporally) variable as hyperalkaline alteration pro- ceeds, it is likely that minerals which could form would be those stable in quartz-saturated or supersaturated fluids. Currently available thermodynamic data for zeolites tend to overestimate their stability, leading to inaccurate predictions of their occurrence. Notwithstand- ing this uncertainty, it is considered that the following secondary minerals are the most likely to form in low temperature cement–ben- tonite systems: calcite, dolomite, chalcedony, C(A)SH of variable Ca/Si ratio, K-feldspar, illite, phillipsite, analcime, clinoptilolite, and heulandite. The relatively more siliceous zeolites (clinoptilolite, phillipsite) are likely to form at lower pH (distal regions of migrating cement pore fluids), whereas C(A)SH, illite, feldspars, and the more aluminous zeolites (analcime, heulandite) are more likely to form at higher pH and hence, the more proximal regions of migrating cement pore fluids. Predominantly Na-, K-bearing solids will be trans- formed to those dominated by Ca as the composition of cement pore fluids evolves with time. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Bentonite; Alteration; Cement; Secondary minerals; Thermodynamic data 1. Introduction Concerns over the chemical compatibility of cement and bentonite in engineered barrier systems for the geological disposal of radioactive wastes necessitate a comprehensive 1474-7065/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pce.2005.08.048 * Corresponding author. Tel.: +44 1491 636246; fax: +44 1491 636247. E-mail address: davidsavage@quintessa.org (D. Savage). www.elsevier.com/locate/pce Physics and Chemistry of the Earth 32 (2007) 287–297