ASTM C 144 is specified in most architectural specifications for the sand in mortars. This specification allows the use of both manufactured sand and natural sand. The specification states that manufactured sand consists of crushed stone or air-cooled iron blast furnace slag, processed to ensure suitable particle shape as well as gradation. The specification goes on to define the gradation for manufactured sand but not the particle shape. Is particle shape important? If so, what types of particle shapes should be avoided in manufactured sand?
I am not aware of any manufactured sands that have problems related to particle shape. It can theoretically be an issue if the sand consists of many elongated particles rather than spherical ones. Mortars made with sands containing flat or elongated particles can have very different plastic properties than those made with sand containing well-graded spherical particles. I have heard that particle shape, in some cases, may have a greater impact on the properties of plastic or cured mortar than the gradation itself. However, I have not encountered any projects where this was an issue. I do recommend, nevertheless, avoiding any manufactured sands that contain significant amounts of flat or elongated particles.