Environmental Engineering Reference
In-Depth Information
On a monument, in the presence of a solution containing different cations and
anions, things go in approximately the same way: the less soluble salts always
precipitate soonest and closest to the source, i.e. closer to the ground if the water is
coming from the ground, or closer to a gutter hole if this is the source of water on
the wall. Arnold and Zehnder [ARN 89] have published a clear design featuring the
distribution of salts as a function of height on a rendered wall (see Figure 8.14).
Figure 8.14.
Distribution of salts and degradation occurring close to the ground [ARN 89]
In the same way, when a porous material is moistened with sea water and then
submitted to a slow drying, salts will precipitate at different places: less soluble ones
(e.g. gypsum) will precipitate closer to the surface, and more soluble ones (e.g.
halite) deeper into the material.
8.3.2.2.2. Relative humidity equilibrium (RHE)
At a given temperature, in equilibrium conditions and in a closed environment,
the air above free water can contain water vapor up to a fixed value. This maximum
is called the “water vapor content at saturation” and is expressed in mg/m
3
. Relative
humidity, expressed as a percentage, is defined as 100 times the ratio between the
water vapor content of a certain atmosphere and the water vapor content at
saturation.
If a soluble salt is added to a vessel containing water and placed in a closed
environment, relative humidity will decrease and reach a fixed value, which is
characteristic of the salt. If the salt is added again, the solution will reach saturation.
No more salt will dissolve at this point. As soon as the solution becomes saturated,
the relative humidity stabilizes at a threshold, called the RHE.
RHE is a very important parameter because it accounts for the potential of a
given atmosphere to gain water vapor, or in the contrary, if relative humidity reaches
