The present investigation
deals with the hydro-mechanical behavior of a mixture composed of
pellets and powder of MX80 bentonite with a proportion of 80/20 in dry
mass. This is one of the studied materials by the French Institute for
Radiation protection and Nuclear Safety (IRSN) within the SEALEX
project, which aims at investigating the long-term performance of
swelling clay-based sealing systems in the context of geological
high-level radioactive waste disposal. This study has been conducted by
following an experimental program covering different scales. Firstly,
the microstructure changes while wetting of a single pellet was
investigated by combining MIP results with μ-CT observations. Results
revealed that swelling of a pellet is due to the development of cracks,
with significant development between 38 and 9 MPa of suction, combined
to swelling of bentonite grains, which is governed by the hydration
mechanisms of smectite at nano-scale. The application of suctions below 9
MPa leads to a significant decrease of the platelet thickness and to an
increase in the disorder of the platelet assembly. Water retention
tests, swelling pressure tests and suction controlled oedometer tests on
the pellet/powder mixture were performed. Similar water retention
properties were observed for the mixture under constant-volume condition
and pellet under free swelling condition under suctions higher than 4
MPa, suggesting that physico-chemical suction prevails on capillary
suction. At lower suctions, constant-volume condition defined a lower
water retention capacity because of the disappearance of macro-pores.
Lower yield stress values than the common pure bentonite mixtures were
found for the pellet/powder mixture for non-zero suctions, showing that
the volume change behavior is governed by the rearrangement and crushing
of pellets, and the loss of the granular structure in the case of zero
suction. Two mock-up tests were performed, aiming at studying two
extreme cases at a global dry density of 1.49 Mg/m3: a homogeneous
pellet/powder mixture fabricated by following a special protocol, and a
strong heterogeneous sample. Results revealed that the radial swelling
pressure depends strongly on the local pellet/powder distribution
combined with the evolution of the hydration front. An anisotropy
swelling was found in both cases, being the axial swelling pressure
lower than the radial one. Moreover, different values of axial pressure
were found between the two tests, even though they have the same global
dry density of samples. In parallel, μ-CT observations were carried out
on the mixture while wetting, revealing a homogeneous sealed sample
after 100 days of hydration. No density gradients were identified at the
investigated resolution (50 μm/voxel) after this long time of
hydration. A new damage model, which takes into account the development
of fissures within a pellet while wetting, was proposed an included to
the well-IVknown double porosity Barcelona Expansive Model (BExM) to
carry out numerical simulations of one mock-up test. The initial
heterogeneous porosity distribution was also considered to reproduce the
anisotropy swelling. The experimental results obtained in this study
will greatly help well understand the response of seals made up of
pellets/powder bentonite mixture in the SEALEX in situ experiment.
Moreover, the constitutive model developed taking into account the
pellet cracking damage and the initial sample heterogeneity allows
significantly improving the prediction of hydomechanical behavior of
seals/plugs made up of this mixture, constituting thus an useful tool
for the safety assessment of the nuclear waste disposal system.