The brand of low-fired gypsum binders is determined by the compressive strength of beam samples 40x40x160 mm in size, formed from a test of normal density at the age of 2 hours after mixing with water. GOST approves 12 grades from G-2 to G-25 (the number in the brand designation corresponds to the compressive strength, expressed in MPa). Along with the compressive strength, bending strength is also normalized, which is significantly lower.
The strength of the gypsum can be increased by drying. In this case, dissolved two-water gypsum settles in the pores and strengthens the crystalline growth. The strength of gypsum samples dried at temperatures up to 60 ° C is 2–2.5 times higher than the strength of wet samples after 2 hours of hardening.
High fired binders have their own strength assessment methods. The brand of anhydrite binder is determined by compressive strength at the age of 28 days in samples from solutions of rigid consistency of 1: 3 composition (binder: sand). Four types of binder are available - 5, 10, 15 and 20 MPa. The strength of any type of gypsum also depends on the amount of water in the preparation of the solution. Therefore, it is possible to obtain an increase in the strength of the gypsum mold also by reducing the amount of water in which the gypsum is bred, but within such a range that the consistency of the diluted gypsum allows the mold to be well filled. It should be borne in mind that the more complex the mold to be manufactured, the more fluid gypsum should be for better flow around the pro-model by which the mold is manufactured.