After 48 h degradation, some grains present
pseudomorphosis zones revealed by yellowish spots
inside grains in Fig. 22. These spots appear dark in
Fig. 23 under crossed polars, whatever the orientation
of the revolving stage. It is the start of the transformation
from crystalline to amorphous phases.
These attack faces correspond to faces observed
above in optical microscopy in reflected light and
SEM. A global observation of thin sections shows that
the grain outlines stay regular. On the other hand, the
grains have a smaller size than the initial grains. The
different alteration stages of grains reveal the heterogeneous
behaviour of the flint aggregate towards the
alkali–silica reaction.