Environmental Engineering Reference
In-Depth Information
8.3.2.2.3. Hydration
Most salts are anhydrous, which means that their molecule does not contain
water as a constituent (for example NaCl). Hydrated salts contain water. For
instance, calcium sulfate may be crystallized under several forms: gypsum
(CaSO
4
2H
2
O) with two water molecules, bassanite (CaSO
4
½H
2
O) with half a
molecule, and anhydrite (CaSO
4
) without any water. With the same quantity of
cations and anions, hydrated salts take up more room than their less hydrated
counterparts.
Gypsum and bassanite can more or less co-exist because their RHE is the same,
and in addition, is very high. This is not the case for sodium sulfates. Mirabilite, the
decahydrate (Na
2
SO
4
10H
2
O) cannot exist in a crystalline state under relative
humidity of 97% at 15°C or 88% at 30°C. It is replaced by thenardite (Na
2
SO
4
)
under this relative humidity threshold.
8.3.2.2.4. Hygroscopicity
The weight of materials such as pure and non-microporous limestones, remains
practically constant whatever the relative humidity. These materials are not
hygroscopic. Other materials, such as wood show a weight increase as RH increases.
The curve
m
=
f
(RH) shows an inflexion point generally located in the relative
humidity range 30-80%. These materials are hygroscopic [DEL 89a]. Mineral-based
building materials can be hygroscopic if they contain a significant quantity of very
small pores (< 0.1 µm), which trap water as relative humidity increases, and if they
contain clay minerals.
In addition to this, building materials contaminated with salts may become
highly hygroscopic if they contain hygroscopic salts. Halite (NaCl), nitrocalcite
[Ca(NO3)
2
.(H
2
O)
4
], sodium nitrate (NaNO
3
) and bishofite [MgCl
2
(H20)
6
] are all
hygroscopic salts. Migné stone, for instance, if contaminated with 0.5% chlorides, at
a relative humidity of 80% contains seven times more water than if it is free of salts
[SIM 98].
8.3.2.2.5. Crystallization and hydration pressures
The action of salts on porous building materials is still not totally understood.
Until recently, it was considered that there were two main causes of salt decay:
crystallization and hydration pressures.
Crystallization pressure
Correns [COR 49] proposed an equation allowing the calculation of the
crystallization pressure on the basis of simple parameters:
