A very important class of technological porous materials is formed
by building materials. Although these materials are widely used in
practice
they can be seriously damaged by moisture and especially salt. These
enter
by, e.g., driving (contaminated) rain, condensation, run off from roof
and facade, and/or capillary rise of ground water, whereas concrete
structures
are often placed in salt water. Salts do also enter by the use of thaw
salts in deicing of roads. If wet, the material can become
susceptible
to freezing damage. It may also act as a substrate for the growth of
bacteria,
fungi, or algae with possible physical and chemical damages, but also
possible
health risks. Also chloride ions can penetrate, causing corrosion of
steel
reinforcement in concrete structures, which may result in structural
damages.
A large fraction of the absorbed water will leave a material again
because
of drying. While a material is drying salt crystallisation may occur at
the surface, causing defacing (efflorescence), or just under the
surface,
where it may cause structural damages, e.g., delamination, surface
chipping,
or desintegration. Here a few examples are given of damages are given:.
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