Water-Rock Interaction Mechanisms and Ageing Processes in Chalk - Advances in Data, Methods, Models and Their Applications in Geoscience - page 173

of the pressure in the observation chamber was used to define the level of hygrometry, h r ,
based on the state diagram of water (Fig. 11).
4.2 Microstructural changes under wetting
The changes in microstructure under wetting when passing from h r = 97% (chalk in its
natural state at sampling with w = 20.7%) to h r = 100% are evident when comparing Figs. 12a
and 12b. A reference network has been superposed to the micrograph and the boundary of
one characteristic pore has been plotted. Since conditions in the chamber correspond to h r =
100% ( p = 705 Pa, T = 2°C), hydration takes place as time passes. In Fig. 12b, the same pore is
visualised after the in-situ hydration. As is seen from the two images, hydration produced a
progressive enlargement of the pore boundaries due probably, but not exclusively, to the
loss of capillary bridges between the grains. Progressive saturation of smaller pores is also
observed on the left side of the photo in Fig. 12b. This observation still remains rather
qualitative, though it provides an initial picture of the ongoing phenomena. It should be
emphasised that pore enlargement is certainly amplified by the specific condition reproduced
in the ESEM environment, namely the absence of any external loading and the observation of
the external surface of the sample. It is expected that the extent of this phenomenon could be
less for the inner non-visible pores. It is worth mentioning that the occurrence of pore
enlargement during saturation at zero external applied load is consistent with the swelling
shown in Fig. 10 for test T1 during water injection under low applied vertical stress.
a) b)
Fig. 12. Modifications of the porous network in chalk during wetting: a) initial state, b)
intermediate state before complete saturation
4.3 Saturation/desaturation cycles with ESEM
A series of tests was carried out on samples submitted to saturation/desaturation cycles
following the path indicated in Fig. 11. During these tests a constant temperature condition
was chosen ( T = 2°C). Relative humidity was then modified, changing the level of vacuum
inside the chamber between 705 Pa and 346 Pa, corresponding to an h r varying between
100% and 50% (path A-B-C-D in Fig. 11). Observations were conducted at 1500