The moisture content of specimens was determined by weighing until to obtain a constant mass so that the equilibrium
state is reached in each test. The constant mass was achieved when the mass variation
between three consecutive weightings, carried out with a time separation of
at least 15 h, becomes less than 0.1% of the total mass of the specimen. The hygroscopic
moisture storage is described using the sorption isotherm curves of the
material, termed hygroscopic curves. These curves establish the relationship
between the moisture content for porous media in volume, V, or per unit mass,
and the relative humidity of air in equilibrium at constant temperature, u, as follows
[20]: