Environmental Engineering Reference
In-Depth Information
cannot enter between clay mineral plates. Hydration implies the compulsory
destruction of the crystalline organization of the non-hydrated phase, followed by
construction of the hydrated form. The harm caused by sodium sulfate is likely to be
due to the higher probability of transitions between the two physical states
(dissolved-crystallized) with respect to other salts in equivalent relative humidity
conditions. Arnold [ARN 76] has, for instance, shown that the transition from one
phase to the other may occur several times per day in a mild climate.
Differential hygric dilatation
Another possible mechanism of damage in the presence of salts is proposed by
Snethlage and Wendler [SNE 97]. These authors report that in the presence of
gypsum, a clay-containing sandstone (Sander, see section 8.2.2.2.1) shows a much
higher hygric dilatation (from 2 mm/m to 7 mm/m). They also show that the Sander
sandstone, if contaminated with sodium chloride and submitted to wetting and
drying cycles between 35% and 90% relative humidity, contracts during wetting and
expands during drying. The unaltered stone behaves in the opposite way; however, it
expands during wetting and contracts during drying. Moreover, the dilatation of the
contaminated stone during drying is irreversible, and increases from one cycle to
another. The same behavior is observed with magnesium sulfate and calcium nitrate.
Barbara Lubelli [LUB 06] obtained very similar results on a lime-cement mortar
contaminated with sodium chloride, consequently showing that the presence of
reactive clay is not necessarily a factor in the dilatation behavior observed. Lubelli
also showed that the behavior of the salt-contaminated specimen is only modified if
the RHE of the salt is crossed (see section 8.3.2.2). She interpreted the blistering
phenomena observed on plasters and renders contaminated with salts, and
reproduced in the lab on her salt-laden mortar specimens, as being due to shear
stresses developing at the interface between zones containing different quantities of
salts.
8.3.2.2.6. Maximum salt loads
In practice, it is often necessary to know the highest level of salt contamination a
stone can bear without any damage. The concept of maximum “bearable” salt load is
quite tricky though, because the behavior of a stone in the presence of salts depends
on its environmental conditions: a salt storage cellar with more than 1% sodium
chloride contamination may be completely stable as long as its atmospheric
environment stays at a relative humidity higher than 75%. The thresholds published
in the literature, that we mention hereafter, can thus only be considered as indicators.
The thresholds indicated by the Mission études et travaux of the French Ministry of
Culture [MIS 03], in France, are 0.1 wt% chlorides, 0.5 wt% nitrates and 0.1 wt%
sulfates in the case of sodium or potassium sulfates, and 5 wt% sulfates in the case
of gypsum. In the German literature, reported by Bourguignon [BOU 09], the
