Gypsum Binding Materials

air-entrained binding materials produced on the basis of semihydrous calcium sulfate or anhydrous calcium sulfate (anhydritic binders).

According to the conditions of their heat treatment and their setting and hardening speeds, gypsum binding materials are divided into two groups: low-fired (quick-setting and quick-hardening), which includes structural and molding gypsum, high-strength gypsum, and cement-plaster-pozzolana binders; and highly fired (slow-setting and slow-hardening), which includes supersulfate cement and highly fired gypsum (anhydrous gypsum plaster).

Structural gypsum is produced by heat treatment of crushed or ground natural gypsum (gypsum rock) in curing pans, rotary kilns, and other technological apparatus at temperatures ranging from 140° to 190° C. Gypsum paste begins to set within 4 to 15 minutes after mixing with water. The compressive strength of structural gypsum may be as high as 10 meganewtons per sq m (MN/m²), or 100 kilograms-force per sq cm (kgf/cm²). Structural gypsum is used to make gypsum products (primarily for interior building components), as well as for plastering and masonry work.

Molding gypsum and high-strength gypsum are produced by basically the same methods used for structural gypsum but from purer raw material; they are characterized by great strength and are used for making various molds and models in the ceramics industry and certain other branches of industry, as well as in the production of finishing materials and architectural components.

Cement-plaster-pozzolana binders, which were proposed by the Soviet scientist A. V. Volzhenskii, are produced by mixing structural gypsum and other types of gypsum binders with portland cement (or portland-pozzolana cement) and an acidic hydraulic additive (tripoli, diatomite, volcanic ash, trass, tuff, and ashes from the roasting of brown coal). These mixed binding materials are distinguished from pure gypsum binding materials by their capacity for hydraulic hardening and by their increased water resistance. Products made from them have significantly less plastic deformation than those made from structural gypsum and other gypsum binders. Cement-plaster-pozzolana binders usually contain 50-75 percent gypsum, 15-25 percent pozzolana additive (with calcium oxide absorption activity of more than 100-250 mg/g). The ratio of portland cement to the pozzolana additive, on which the durability of the products depends, is determined by a special method.

Supersulfate cement is made by firing natural gypsum at a temperature of 600°-700° C and subsequently grinding it together with catalytic additives for hardening (lime, sodium sulfate or bisulfate with iron vitriol or blue vitriol, and so on). It is used for making structural masonry plasters, concretes, scagliola, and decorative products.

Highly fired gypsum (anhydrous gypsum plaster) is produced by firing natural gypsum at a temperature of 800°-1000° C, with subsequent fine grinding; it is utilized for the same purposes as supersulfate cement. In comparison with products made of structural gypsum, articles made of anhydrous gypsum plaster are more resistant to water and tend less toward plastic deformations.