Water resistance refers to the ability of a material to resist water damage. The destruction of material properties by water is reflected in different aspects, the most obvious manifestation is the decrease in mechanical properties of materials. Water resistance is usually expressed by softening coefficient, which is calculated as follows:

In the formula, Kf--the softening coefficient of the material; f1--the compressive strength of the material in the saturated state of water absorption, MPa; f0--the compressive strength of the material in the dry state, MPa.

After the material absorbs water, the bonding force between the internal particles is weakened, resulting in a decrease in the strength of the material to varying degrees. The larger the softening coefficient, the stronger the ability of the material to resist water damage, that is, the stronger the water resistance. The value of the softening factor is between 0 and 1.

The softening factor is an important basis for selecting water-resistant materials. For materials directly used in water or severely dampened, the softening coefficient should not be lower than 0.85; for lightly damp materials, the softening coefficient should not be lower than 0.75. It is generally considered that materials with a softening coefficient greater than 0.85 are water-resistant materials and can be used as waterproof materials.