Environmental Engineering Reference
In-Depth Information
=
PRT/Vx nC/Cs
[8.2]
0
With P being pressure generated by the crystal growth (MPa), R the ideal gas
constant (8.317 J.K
-1
.mol
-1
), T the temperature (K), V
0
the molar volume of the salt
(cm
3
/mol), C the concentration of the solution during crystallization, and Cs the
concentration of the solution when it reaches saturation.
Using the physical constants of salts [WEA 80], Hammecker [HAM 93]
calculated the crystallization pressure of several salts at 25°C with this formula, as a
function of the supersaturation grade of the solution. For a given temperature and
saturation grade, salts with the smallest molar volumes develop the highest
crystallization pressures (see Table 8.8 and Figure 8.16).
As a consequence, hydrated salts develop lower pressures than their anhydrous
or less hydrated equivalent (see thenardite/mirabilite). Hammecker also observes
that halite is potentially the most harmful salt because its crystallization pressure is
the highest. Finally, he remarks that the crystallization pressure of calcite is very
high, although it is known that this mineral is not the source of any deterioration. He
concludes that factors other than simple crystallization pressure must be taken into
account to explain salt-related degradations.
Molar volume V
0
(cm
3
/mol)
Salt
Anhydrite, CaSO
4
45.5
Aphthitalite, NaK
3
(SO
4
)
2
125.4
Calcite, CaCO
3
36.9
Glauberite, Na
2
SO
4
.CaSO
4
99.3
Gypsum, CaSO
4
.2H
2
O
74.2
Halite, NaCl
27
Mirabilite, Na
2
SO
4
.10H
2
O
217.7
Potassium nitrate, KNO
3
47.8
Syngenite, K
2
Ca
2
(SO
4
)
3
126.3
Thenardite, Na
2
SO
4
53.3
Table 8.8.
Molar volume of salts [WEA 80]
