Properties of Gypsum Pastes and Mortars

Setting Time of Gypsum Pastes 

The time from first mixing the gypsum and water until the attainment of rigidity is called setting time. Important for shaping the material.

Pure gypsum paste  ~30 min.

Anhydrous gypsum pastes or pastes containing some impurities >30 min.

Plasticity of Gypsum Pastes and Mortars

Plasticity is the ease with which the plaster or mortar can be shaped and spreaded. It should not be too sticky or too stiff.

The plasticity of the gypsum mortar is largely affected by the amount of water in the mix. The plasticity is also affected by the sand content. Inclusion of sand reduces the stickiness of the paste. However, too much sand decreases workability. Approximately 2/3 by weight of the mixture is constituted of sand in gypsum mortars.

Strength of Gypsum Pastes and Mortars

Compressive strength of gypsum plasters and mortars is around 3 or more times greater than their tensile strength.

Plaster of Paris shows typically linear diagram up to fracture

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