Environmental Engineering Reference
In-Depth Information
Figure 10.3
.
Superficial layers of a marble column, SEM observation. Cloister of Saint
Trophim church, Arles, France. Layer enriched in oxalate (weddelite), crystals with a
pyramidal shape, associated with gypsum crystals (lamellar shape) [VER 94b]
It is known (see Chapter 8, section 8.2) that alteration usually leads to a
modification in porosity. By impregnating the sample with a liquid resin that
polymerizes at ambient temperature, and by polishing it, it is possible to visualize
the porous network under backscattered electrons. Using a threshold method, the
image shows just two gray levels: one for the solid matrix and the other for the
porous medium. Image analysis allows us to calculate the porosity. Whereas the
value obtained should be taken with care (see section 10.1.1.1), it is possible to use
cross-sections to observe its evolution of porosity as a function of depth [BEU 93].
SEM images are also able to show the biological origin of some deteriorations,
such as the presence of bacteria close to gypsum underneath the stone surface
(granite [SCH 93], sandstone [KRA 96], microcavities due to fungi on marble
blocks [URZ 93], biological deposits [ELM 93]). On the other hand, the optimal
observation of algae or fungi is only possible after specific preparation (freeze-
drying), with a specific electron microscope that works under partial vacuum, or the
environmental SEM that allows observations under low to very low vacuum.
10.1.1.3
. X microanalysis
This is commonly used with SEMs as it allows us to identify and quantify the
components of the sample being observed. Its principle relies on the detection of
X-rays emitted under the primary electron beam with a microprobe (detector able to
select the X-rays by means of a dispersive wavelength device or dispersive energy
one). The analysis is quick and the volume analyzed may be from tens to around
1 µm
3
, which explains its name.
