Why can plastering gypsum use cement mortar instead of traditional plastering?

Mortar consists of cementing material and fine aggregate.

Cement gypsum mortar is a substitute for cement mortar. It uses portland cement as the main cementing material, sand as fine aggregate, and partial gypsum instead of cement to improve the density and workability of mortar.

Plastering gypsum mortar is completely made of gypsum as cementing material, sand as fine aggregate, and modified by adding polymers such as latex powder and cellulose ether, which is mainly used as surface layer in engineering.