Water-Rock Interaction Mechanisms and Ageing Processes in Chalk - Advances in Data, Methods, Models and Their Applications in Geoscience - page 172

4. Microscopic scale: Electron Scanning Environmental Microscope (ESEM)
observations and micro-testing

The electron scanning environmental microscope (ESEM) allows the observation of
microstructural changes of geomaterials in their natural state, under controlled conditions of
temperature and pressure. Unlike the traditional scanning electron microscopy (SEM),
ESEM technology does not require any preliminary treatment of the observed samples (such
as dehydration or conductive coating). This has undeniable advantages in the analysis of the
microstructure of geomaterials. In this study, a FEI Quanta 400® ESEM equipped with a
Deben® microtesting facility has been used as a tool for the microstructural and
micromechanical characterisation of Estreux chalk.
Changes in S rw were reproduced by controlling sample temperature and pressure following
the state diagram of water (Fig. 11), being simultaneously correlated to the corresponding
microstructural evolutions. A further step of the analysis involved the investigation of the
microstructure while the material was subjected to a micromechanical loading, under
constant or variable relative humidity, by means of ESEM micromechanical in situ tests.

h r = 100 %
95 %
85%

1400

LIQUID

1200

1000

60%

800 A  D

50%

600

B

400

C

GAS

200

0

0 2 4 6 8 10 12 14

TEMPERATURE (°C)

Fig. 11. State diagram of water and imposed changes during observation (ii)

Three types of observations were conducted: (i) changes in microstructure under wetting,
(ii) samples submitted to saturation/de-saturation cycles starting from their natural state of
saturation (path Fig. 11), and (iii) samples submitted to unconfined axial compression
microtests under variable states of water saturation (Sorgi & De Gennaro, 2007).

4.1 Sample preparation
Sub-samples were extracted from available blocks of Estreux chalk retrieved from the
underground mine, and these were sealed and stored in a thermo-regulated chamber to
ensure the preservation of in situ conditions in terms of water content.
Observations (i) and (ii) were conducted on sub-samples having a square section (about 10
mm/side) and a thickness varying from 2 mm to 4 mm. These were fixed on the observation
plate inside the ESEM chamber using a carbon conductive glue. Reduced plug thicknesses
ensured a more uniform temperature distribution within the samples, and temperature was
controlled using a thermo-electric cooler (based on Peltier's effect). The corresponding value