Environmental Engineering Reference
In-Depth Information
100%, to let its water vapor condense. Saline solutions trapped in building materials
are very rarely in contact with a closed atmosphere. The atmosphere, for many
reasons, may have a variable relative humidity. If this humidity is higher than the
salt RHE, the salt will be in a dissolved state in the material. If the relative humidity
of the atmosphere is lower than the salt RHE, the salt will crystallize as soon as its
solution reaches saturation. Let us take NaCl as an example: it stays in a solid state
as long as the atmosphere is lower than its RHE, equal to 75%. If relative humidity
exceeds 75%, the salt will trap water from the atmosphere and dissolve. The RHE of
a salt mixture solution is different from that of the individual salts, and cannot easily
be calculated.
In any building, the relative humidity of the air oscillates regularly at a diurnal
and seasonal pace, and also irregularly, following the use of the building. A religious
service or concert induces increases in both temperature and relative humidity
because human respiration involves the exhalation of water.
It is clear that each time the critical RHE threshold (or RHE range in the case of
salt mixtures) is reached, salts present at the surface of the wall will either dissolve
or crystallize. It is generally considered that the more frequent these fluctuations
around RHE, the more salt-linked deterioration will occur. Indeed, salts with an
RHE in the range of diurnal or seasonal oscillations of atmospheric relative humidity
have the potential to cause more aggressive damage than others (see Figure 8.15).
For instance, in October Arnold and Kueng [ARN 85] observed that after cool nights
on a wall exposed to sun, that salts were crystallizing in the morning, and dissolving
in the late afternoon. This situation remains quite rare, because walls have such a
high thermal inertia that it may take days or even weeks for the salts to crystallize.
Figure 8.15.
Sodium sulfate-linked degradation (thenardite and mirabilite) on granite and
kersantite sculptures in Quimper cathedral. These two salts are often considered to be
particularly harmful because their RHE (82% and 94% respectively at 20°C) is located
within the seasonal fluctuation range of relative humidity
