Ferret's Law

Ferret's Law says that:

S = K (c/[c+e+a])

Where:
S = predicted compressive strength
K = constant
c = absolute volume of cement
e = absolute volume of water
a = absolute volume of air

Air-entrained mortar, just like air-entrained concrete, can benefit from the millions of microscopic bubbles that are produced with an air-entraining admixture. The tiny air bubbles act like miniature ball bearings, making the fresh mixture easier to work. A benefit for hardened mortar or concrete is that the air bubbles give water tiny voids to expand into when it freezes, making the mortar more durable in freeze-thaw conditions.

Too much entrained air can have a negative effect on the compressive strength strength of the mortar. Ferret’s Law could be used to predict the effect that a given ratio of cement to water and air volume will have on the compressive strength. In the Ferret’s Law formula above, the more air volume (or water volume) the lower the compressive number would be relative to a constant based on a real sample.