Effect of Dissolved NaCl on Freezing Curves of
Kaolinite, Montmorillonite, and Sand Pastes
S.A. GRANT, G.E. BOITNOTT, AND A.R. TICE
INTRODUCTION
Chemical thermodynamics has long been used to calculate the heaving pres-
sures and liquid-water contents of frozen ground (Everett 1961, Defay and Prigogine
1966). In most instances, the effects of solutes in the liquid phase have been ne-
glected in the application of chemicalthermodynamic theory to frozen porous
media: the liquid phases are typically assumed to be pure. At equilibrium, ice ac-
cepts few solutes and these at only vanishingly small mole fractions. Most solutes
in frozen ground are in the liquid phase (Hobbs 1974). At temperatures apprecia-
bly below 0C, where most of the water in a waterelectrolyte system is in the ice
phase, liquid solutions tend to be highly concentrated. In these environments, the
effects of solutes on the freezing behavior of ground can be considerable.
In recent years, theoretical and experimental advances have provided the tools
for estimating the thermophysical properties of aqueous electrolyte solutions at
subzero temperatures. The major theoretical advance has been the development
and acceptance of the Pitzer model of electrolyte solutions (Pitzer 1991). The Pitzer
model allows for the calculation of the important physiochemical properties of
simple and complex aqueous electrolyte solutions over a wide range of tempera-
tures, pressures, and compositions. Most importantly for this discussion, the Pitzer
model allows the accurate estimation of electrolyte-solution properties at subzero
temperatures (Spencer et al. 1990, Archer 1992, Marion and Grant 1994). Since many
of the thermophysical properties modelled by the Pitzer model are relative to the
same properties of the solvent, a critical component when modelling the
thermophysical properties of electrolyte solutions is accurate measurements of pure-
solvent properties. Historically, this has presented problems for water at tempera-
tures below its equilibrium freezing point. Novel experimental procedures have
allowed the heat capacity and density of supercooled liquid water to be measured
accurately at temperatures as low as 35C (Angell et al. 1973, Hare and Sorensen
1987). Speedy (1987) has fitted much of the available data on supercooled water to
a family of equations that calculate physicalchemical properties of liquid water
under atmospheric pressure in the temperature range 0 to 45C.
To our knowledge, no one has applied these new capabilities to the long-stand-
ing problem of calculating the effects of solutes on the freezing curves of frozen
ground. The objectives of the research presented here were:
1. To extend the conventional (i.e., pure-phase) chemicalthermodynamic theory
of frozen ground to the liquid phases of aqueous electrolyte solutions
2. To calculate the needed chemicalthermodynamic data from physicalchemi-
cal models of ice, water, and electrolyte solutions
3. To make experimental measurements of liquid-water contents of sand and
clay-mineral pastes with which to test this extension of the model.
In this report, we initially discuss the chemicalthermodynamic methods by
which capillary pressures of either pure or solution liquid phases can be calculated
for frozen porous media. The thermophysical data needed to make these calcula-
tions are presented. We then describe and evaluate the data from a set of experi-
ments we conducted to test this approach to calculating capillary pressures.
Previous Page
