Environmental Engineering Reference
In-Depth Information
Crystallisation pressure (MPa)
90
halite
80
70
calcite
anhydrite
60
50
niter
thenardite
40
gypsum
30
glauberite
20
Syngenite +
aphtitalite
10
mirabilite
0
1
1,2
1,4
1,6
1,8
2
2,2
2,4
2,6
2,8
3
Supersaturation degree C/Cs
Figure 8.16.
Crystallization pressure of salts as a function of supersaturation
degree of the salt solution at 298.15K (25.15°C) [HAM 93]
Hydration pressure
Some salts can hydrate or dehydrate in response to changes in relative
humidity of the atmosphere. The presence of water in the crystalline network
induces an increase in molar volume. Thenardite/mirabilite (53.3/217.7 cm
3
/mol)
and kieserite/epsomite (56.6/146.7 cm
3
/mol) in particular have this property.
Winkler and Wilhelm [WIN 70] have proposed an equation allowing us to calculate
the pressure generated by this increase in volume:
(
)
PnRT/V Vx . nP/P
=
−
h
a
w
'
In addition to R and T seen in the preceding formula, the equation considers the
number of moles gained during hydration (n), the molar volumes of the two minerals
considered before (V
a
) and after (V
h
) hydration respectively, and the water vapor
pressure in equilibrium with the saturated solutions of both minerals, before (P
w
) and
after (P
w'
) hydration respectively.
The reality of hydration pressure has been questioned by Doehne [DOE 94] on
the basis of the observed behavior of sodium sulfate under environmental scanning
electronic microscope, a device allowing changes in relative humidity and
observation of the sample at microscopic scale at the same time. Doehne has shown
that the transformation of thenardite-mirabilite occurs through the dissolution of
thenardite. There is no increase in volume of an already existing salt. Indeed, the
organization of the atoms in the non-hydrated and hydrated phases are so different
that it is impossible for water to enter the crystalline network, for the same reason it
