Sand is underrated. It provides the structure of plaster and the quality of your sand can make the difference between success and failure. So what makes sand good or bad. In general, good plaster sand should be sharp, with a diversity of particle sizes and clean. As a conservative rule the largest particles in your sand should be no more than half the thickness of your plaster, but preferably would be at least 1 qt. the thickness of your plaster (larger aggregate can provide better structure, resulting in a stronger plaster with less cracking). So if your plaster coat is a half inch, your largest aggregate would ideally be between 1/8 in. and 1/4 in.
Mix with Portland cement and hydrated lime or plastic cement to make base coat stucco
Can be mixed with potting soil or be used for other garden applications
Particle size diversity is important to create good structure and to reduce the amount of binder needed