Gypsum stone is a sulfate rock of sedimentary (chemical) origin, monomineral granular crystalline structure, consisting of the same mineral - two-water calcium sulfate, white or light gray in color, which depends on the accompanying impurities of clay, sand and organic substances.
Gypsum binders obtained from gypsum stone are air binders obtained by heat treatment of raw materials with preliminary or post-calcining to a powder state. Heat treatment can be low temperature (120-180 ° C) and high temperature (600-700 °C).
High-strength gypsum is a material obtained by processing gypsum stone (in the form of crushed stone) at a temperature of 123 ° С with saturated steam or boiling in aqueous solutions of certain salts (e.g., calcium chloride), followed by drying and grinding.
With this treatment, recrystallization of two-water gypsum, composed of small lamellar crystals, into large-crystalline semi-water gypsum of alpha modification, consisting of long needle-shaped or prismatic transparent crystals of hexagonal structure.
High-strength gypsum has high (but lower than conventional gypsum ) water requirements and a fairly high compressive strength after 2 hours after the manufacture of the product. With an increase in the content of hemihydrate in natural raw materials with a corresponding decrease in impurities, the quality of gypsum also increases.
To obtain high-strength gypsum of the first grade, the content of hemihydrate in the raw material is not less than 95%. High-strength gypsum refers to quick-setting (2-20 minutes) and quick-hardening (15-30 minutes).
Heat treatment of gypsum crushed stone for high-strength gypsum binder alpha modification is carried out in vertical autoclaves-steamers with steam supply from the outside or in self-steamers to produce steam due to evaporation of hygroscopic and partly crystallization water. These technological operations were preceded by large and lengthy studies of various specialists.
Moreover, the study of the best techniques and methods for the influence of heat and steam on a raw material product continues with unflagging interest, because this operation is one of the essential factors for improving the quality of finished products as the most important building material.