Other more specialized types of cement, such as CTS' Rapid Set , can be used for additional protection against cracks due to freezing or shrinkage. Fine, light-colored sand. This should be graded to fairly uniform sizes. Its inclusion is especially important since it is the first line of defense against shrinkage. Including some extra fine sand will improve the surface smoothness, but using only such fine sand is likely to cause hairline cracking.
Sand is also significantly less expensive than cement.
Avoid the temptation to use too much however, as doing so will weaken your finished product. Superplasticizers , or high range water reducing admixtures, are highly efficient dispersants or deflocculants which decrease the water requirement in the mortar mix thereby decreasing shrinkage. That is, they help eliminate the main cause of cracked glass mosaic elements. They keep individual cement particles from clumping together which also helps the mortar to flow between mosaic elements. Pigment dispersion is also improved.