Environmental Engineering Reference
In-Depth Information
Nitrates and chlorides are in higher quantity in soils that are cultivated or located
close to dwellings in cities and in the countryside. They are often present within the
capillary fringe of walls located close to manure heaps, stables, cemeteries or
ancient toilets. In this last case, they may be found at upper levels, in places where
sewage pipes have broken. The presence of salt is generally easily identified because
salts induce discoloration, such as moist stains and white efflorescence.
Nitrates are formed through the bacterial oxidation of organic nitrogen present in
animal proteins and feces. Sodium chloride or halite is often linked to salt storage in
old buildings [VER 92].
This salt used to be a very important and precious commodity. Halite is also
found in large quantities in buildings close to the sea shore: it penetrates the building
materials via capillary water and sea sprays [CAD 96, CHA 96].
Halite can also be found on materials located close to places where de-icing salts
are used. Antarcticite, another chloride (CaCl 2 .6H 2 O), is also used as a de-icing salt
but generates less damage than halite because its relative humidity equilibrium
(RHE) is very low (30%).
For this reason it is almost never in a crystallized form in temperate countries.
Some very scarce salts can occasionally be found, such as calcium formiate, which
was found be linked to the use of formic acid as a cleaning agent [ZEH 84].
Primary salts form directly; secondary salts are the product of the reaction
between several salts. Arnold and Zehnder [ZEH 89] quote reactions where calcium
and magnesium sulfates, nitrates and chlorides react with sodium and potassium
carbonates.
The products originating from the reaction are sulfates, nitrates, chlorides of
sodium and potassium on one hand, and calcium or magnesium carbonates on the
other. These chemical reactions are very harmful: dangerous salts form in the place
of relatively harmless ones.
The reactions are of the following type:
CaSO 2H O
+
Na CO 10H O
Na SO CaCO
+
+
1 2H O
42
23 2
24
3
2
gypsum +
natron
→ thenardite + calcite + water
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