Regardless of legality, you shouldn't be taking sand as a souvenir. Beaches are a natural resource for both people (recreation) and animals/plants. They provide food, breading grounds, habitats...

Leave no trace.

This is bizarre. Too much for a souvenir. Too little for industrial use. What an odd thing to do.

You can imagine how many vials of Sardinian sand you van sell from 40kg of it