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sure hydration of the bentonite. After 7.6 years, the experiment was dismantled and several parameters
analysed and compared with similar experiments running for 0.5, 1 and 2 years.
Chloride concentration (mmol/100g)
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.5 years
1 year
2 year
7.6 years
0.0
0 10 20 30 40 50 60
Distance from the heater (cm)
Exchangeable cation (meq/100g)
185
50
Na Mg
5.0
Ca K
4.8
45
4.6
40
4.4
4.2
35
4.0
3.8
30
3.6
25
Na 3.4
3.2
20
3.0
0 10 20 30 40 50 60
Distance from the heater (cm)
+
Mg 2+
Ca 2+
K +
Figure 2: a) Chloride concentrations in the bentonite along the experimental columns for different
duration tests obtained by aqueous leaching at 1:4 solid to liquid ratio [2]. The thick black line is
the initial chloride concentration in the pore water; b) Cation exchange population along the experimental
column for the 7.6-years test [2]. Thick straight lines are the reference values of the
cation exchange population in the FEBEX bentonite.
At the end of the experiment, it was discovered that the bentonite was not fully saturated. Full saturation
occurred only in the first 10 cm of the column near the hydration surface; whereas the bottom
end close to the heater was found to be less saturated than at the beginning of the experiment. The
hydration of the upper part of the column caused an increase in bentonite swelling, leading to an
increase of the porosity (interlamellar porosity) associated with the expansion of the interlamellar
space. This process caused a decrease of the bentonite dry density in the upper part of the column
and the opposite effect in the bottom of the column, where a decrease of water content was recorded.
Although no mineralogical alteration was observed after the experiment, different bentonite-water
interaction processes controlling the chemistry of the system were detected. Hydration
caused dilution and advective transport of chlorides (Fig. 2a) and sodium, transport of sulphates
controlled by gypsum solubility and dissolution/precipitation of carbonates. As a consequence, a
modification of the average cation exchange population in smectites was observed, increasing the
MgX2 and KX contents in the bottom of the column (Fig. 2b) at the expense of sodium and calcium.
The migration of chloride through the column can be understood by comparing the results of the
experiments of different duration (Fig. 2a). From this, it can be seen that as hydration proceeds,
fronts of chloride developed (the hydration water is less saline than the initial pore water), [2].
However, an additional effect can be seen for the 7.6 years experiment, where the chloride concentration
of the front increases significantly when it reaches the bentonite near the heater. The reason
for such behaviour is related to: i) there is no possibility for further migration of chloride, so it accumulates
at the bottom of the column due to water movement; and ii) water evaporates due to the
higher temperature in this part of the column and water vapour migrates upwards, thus increasing
the chloride concentration of the remaining pore water.
Assuming that chloride behaves as a conservative element in the system, the geochemical processes
associated with the heating/hydration experiment (sinks and sources for other elements) can be
evaluated by normalising all other chemical components with respect to chloride. The results of
this comparison show that in the upper part of the column carbonate increases, due to the dissolution
of carbonate minerals (calcite). This is consistent with the increase of pH recorded in this sec-
Exchangeable K (meq/100g)