for the review of Gibbs free energies of formation and of
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solubility products of clay minerals). This procedure is justified
if the calculations refer to equilibria in dilute conditions
and in large amounts of water, at the atmospheric pressure
(1 bar); therefore, the activity of water can be assumed to
be equal to one. However, this assumption is not always true,
especially in soils, brines, or capillaries of compacted clay
materials in which water activity can be considered as different
from one. Consequently, water amounts have to be taken
into consideration both in equilibrium reactions, and water
activity has to be considered in thermodynamic calculations