gipsGypsum is a mineral that consists largely of the salt calcium sulfate (CaSO4).
Gypsum is soft. With a fingernail one can make a scratch in it.
Natural gypsum is mined e.g. in the basin around Paris.
In construction plaster is used as plaster and in drywall, or as well plaster to cast figures and in medical plaster cast.

Plaster (mortar) is a layer of mortar, a few millimeters to a few centimeters thick, consisting mainly of gypsum, clay or lime, sometimes mixed with sand and other surcharges, mounted on a wall, ceiling or vault.
The purpose of plastering the walls is to flatten or finish (for wall covering, paintwork..).
Other names: lime, whitewash, plaster, stucco.

Tadelakt is a waterproof shine plaster based on natural lime hydrate that around Marrakech (Morocco) is found. He is (sometimes mixed with marble powder) during the drying polished with a (precious gem-) stone and then saturated with olive soap, which made it filled and waterproof. Tadlakt is e.g. used on walls and floors in bathrooms, toilets and kitchens.

Kalei is a thin lime mortar or plaster paint which is applied with a block brush on brick walls. Earlier (by the Romans) based on lime, now also with cement additive and pozzolans (crushed stone or volcanic earthy material). As a filler is used only river sand as it does not contain any salts which may be harmful to the patch. It makes exterior brick walls water resistant, but still breathing and smooth.