(Often referred to as Poly sand).
Polymeric sand is sand that is used for joints that are large. It contains polymers, which are a type of plastic. This type of sand hardens when wet, while still creating the elasticity of an interlock. Even when it hardens it maintains elasticity. (Caulk has a similar effect.)
If you used regular sand in wide spaced pavers, it’d come out very easily.
This is different than joint sand.
Example:
The crew uses polymeric sand for large joint pavers.
