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case, the iron diffusion front in the bentonite can reach a distance of 10 mm from the steel-bentonite
interface, but cronstedtite is limited to precipitate close to this interface and the maximum amount
of cronstedtite predicted in this location is 0.0006 wt %.
Other modelling studies carried out under NF-PRO have emphasised the need to address reaction
kinetics to estimate the long-term degree of bentonite alteration due to interaction with iron [5]. For
example, it is likely that the sequence of alteration of bentonite by Fe-rich fluids will proceed via an
Ostwald step sequence. Although natural systems evidence is not completely analogous to waste
package corrosion scenarios, the low-temperature diagenesis of iron-rich sedimentary rocks shows
that chlorite is the common Fe-silicate in ancient sandstones, but does not occur in recent sediments
(< 1 m.a.), whereas the mixed ferrous-ferric silicates, odinite and cronstedtite occur in recent sediments,
but do not occur in ancient sediments. It may be concluded that although chlorite is the most
likely stable Fe-silicate phase in the iron-bentonite system, its formation is kinetically inhibited, and
occurs through an Ostwald step process via odinite, cronstedtite, and/or berthierine precursors.
These processes of nucleation, growth, precursor cannibalisation, and Ostwald ripening to address
the issues of the slow growth of bentonite alteration products have been incorporated into models of
bentonite alteration under NF-PRO. This, together with incorporation of processes of iron corrosion,
diffusion and sorption of Fe 2+ ions in the iron-bentonite system in the model has enabled the
extrapolation of the results of short-term corrosion experiments to the long-term [5].
Results obtained during NF-PRO have thus challenged the previous wisdom that iron corrosion
would lead to the development of thick corrosion product layers and accumulation of iron in situ. It
is clear from experiments with compacted bentonite conducted under NF-PRO that this is not the
case over experimental timescales, in which only thin corrosion product layers develop, with iron
diffusing and sorbing readily through the bentonite, driven by the concentration gradient between
the internal and external boundary of the bentonite.
4.2 Cement and Concrete
18-month duration batch experiments at 30 and 60 °C showed that the main process observed during
contact of FEBEX bentonite with pH 13.2-13.5 pore fluids was the dissolution of montmorillonite
[6]. Calculated dissolution rates are similar to those measured in other work (10 -12 to 10 -13 mol
m -2 s -1 ). The precipitation of secondary minerals, such as zeolites (K-chabazite and K-phillipsite),
and a mixture of Mg-rich mica (celadonite) and other smectite-type clays was also observed. It has
to be stressed that the main reaction is the formation of K-zeolite and not the conversion of smectite
to mica (illitisation). Batch tests of bentonite interacting with a pH 11 solution showed negligible
reactivity. Calcium silicate hydrate minerals (CSH) were not detected, but equilibrium calculations
based on the speciation of the experimental pore fluids were consistent with the achievement of an
equilibrium state between montmorillonite and a CSH-gel.
Diffusion experiments with cement pore fluids produced a mineralogical alteration of approximately
2.5 mm in the bentonite, but only in those experiments using young cement-derived water
(pH=13.5). This alteration zone was a mixture of poorly ordered Mg-rich minerals (brucite, hydrotalcite
and tri-octahedral Mg-smectite) (Fig. 4). The alteration zone observed in the diffusion experiment
did not evolve with time, mainly due to the reduction of porosity that led to a sharp decrease
in diffusivity. The results of these experiments were successfully modelled with the reaction
transport code, Raiden-3 [7], exhibiting all the main characteristics of the experiment, including
pore blocking, brucite precipitation, minor montmorillonite dissolution, and ion exchange of Mg-
by K-montmorillonite throughout the length of the bentonite.
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