The grade of soft-burnt gypsum binders is determined by the compressive strength of sample beams 40x40x160 mm in size, formed from paste with normal density at the age of 2 hours after mixing with water. GOST establishes 12 grades from G-2 to G-25 (the number in the designation corresponds to the compressive strength expressed in MPa). In addition to the compressive strength, the flexural strength, which is significantly lower, is also standardized.
Gypsum strength can be increased by drying. During this process, the dissolved gypsum dihydrate settles in the pores and strengthens the crystalline intergrowth. The strength of gypsum plaster samples dried at temperatures of up to 60°C is 2–2.5 times higher than the strength of wet samples after 2 hours of hardening.
The strength of hard-burnt binders is assessed by special methods. The grade of an anhydrite binder is determined by the compressive strength at the age of 28 days in samples produced from mortars with dry consistency (binder:sand = 1: 3). Four grades of hard-burnt binders are produced — 5, 10, 15, and 20 MPa. The strength of any type of gypsum plaster also depends on the amount of water used for the preparation of the mortar. Therefore, it is possible to increase the strength of the plaster mold by reducing the amount of water in which the gypsum plaster is diluted, but within such limits that the consistency of the diluted gypsum plaster allows the mold to be well-filled. It should be borne in mind that the more complex the mold being made, the more fluid the gypsum plaster must be to ensure proper flow around the model of the mold.