What is the CaSO4 – H2O System?

The CaSO4– H2O system is characterized by five solid phases. Four exist at room temperature: calcium sulfate dihydrate, calcium sulfate hemihydrate, anhydrite III, and anhydrite II. The fifth phase, anhydrite I, only exists above 1180℃, and it has not proved possible to produce a stable form of anhydrite I below that temperature. Table 1 characterizes the phases in the CaSO4– H2O system. The first four phases are of interest to industry.

Physical properties of the CaSO4 – H2O System

Calcium sulfate dihydrate, CaSO4 · 2 H2O, is both the starting material before dehydration and the final product after rehydration. Calcium sulfate hemihydrate, CaSO4 · 1/2 H2O, occurs in two different forms, α and β, representing two limiting states. They differ from each other in their application characteristics, their heats of hydration, and their methods of preparation. The α-hemihydrate consists of compact, well-formed, transparent, large primary particles. The β-hemihydrate forms flaky, rugged secondary particles made up of extremely small crystals. The most important physical properties of the calcium sulfate phases are shown in Table 2.

Laboratory Synthesis

The thermodynamic stability ranges for the calcium sulfate phases are shown in Table 1. Below 40℃, i.e., under normal atmospheric conditions, only calcium sulfate dihydrate is stable. The other phases are obtained at higher temperatures by progressive dehydration of the calcium dihydrate in the following order:

dihydrate→ hemihydrate → anhydrite III → anhydrite II

Under normal atmospheric conditions hemihydrate and anhydrite III are metastable, and below 40℃ in the presence of water or water vapor they undergo conversion to the dihydrate, as anhydrite II does. However, between 40℃ and 1180 ℃ anhydrite II is stable.

To synthesize pure phases in the laboratory, β-hemihydrate is made from the dihydrate by heating at a low water-vapor partial pressure, i.e., in dry air or vacuum, between 45 ?C and 200 ℃. Further careful heating at 50 ℃ in a vacuum or up to ca. 200 ℃ at atmospheric pressure produces β-anhydrite III.

At very low water-vapor partial pressure, if water vapor is released rapidly and particle size is small, β-anhydrite III forms directly from the dihydrate, without formation of an intermediate hemihydrate. The specific surface area of such β-anhydrite III can be up to ten times that of β-anhydrite III.

α-Hemihydrate is obtained from the dihydrate at high water-vapor partial pressure, e.g., above 45 ℃ in acid or salt solutions, or above 97.2 ℃ in water under pressure (e.g., 134℃, 3 bar, 4 h). Further careful release of water at 50 ℃ in a vacuum or at 100 ℃ under atmospheric pressure yields α-anhydrite III.

Anhydrite III is difficult to prepare pure because anhydrite II begins to form above 100 ?C, and anhydrite III reacts readily with water vapor to form hemihydrate.

The β-hemihydrates fromβ-anhydrite III and β-anhydrite III differ in their physical properties. Therefore, hemihydrates from β- anhydrite III should be designated as β-hemihydrate . α-Anhydrite III absorbs water vapor to form α-hemihydrate. Likewise, the hemihydrates, in humid air, reversibly adsorb up to 2 % of their weight in water without converting to dihydrate. This nonstoichiometric water in the hemihydrate can be completely removed by drying at 40 ℃.

Gypsum dehydration kinetics have been investigated by several authors. Neutron and X-ray powder diffraction studies have shown that the dehydration (and hydration) mechanism is strongly topotactic in the temperature range of 20 – 350 ℃. In neutron thermodiffractometry experiments it was found that gypsum decomposes to CaSO4(H2O)0.5, then to anhydrite III, and finally to anhydrite II. With high local steam pressure, a subhydrate with 0.74 H2O was found. According to another paper three phases of the α-hemihydrate type can be prepared as pure samples: CaSO4(H2O)0.6, CaSO4(H2O)0.5, and anhydrite III. The crystal structures of these phases were determined; data for CaSO4(H2O)0.5 and for anhydrite III are listed in Table 2. The structure of the subhydrate CaSO4(H2O)0.6 was found to be monoclinic, space group I121, with a = 1.19845, b = 0.69292, c = 1.27505 nm and β = 90 ?. The density is 2.74 g/cm3. In general the hemihydrate CaSO4(H2O)0.5 is considered to be the kinetically most stable subhydrate. Table 2 also lists the well-established crystallographic data of gypsum and of anhydrite II.

Anhydrite II is formed at temperatures between 200 ℃ and 1180 ℃. Above 1180 ℃, anhydrite I forms; below 1180 ℃ it reverts to anhydrite II.

Another mechanism for conversion of gypsum directly to anhydrite II has been found in the catalytic action of small quantities of sulfuric acid on moist, finely divided gypsum at 100 – 200℃. In this case anhydrite II with orthorhombic crystal structure is produced by neoformation.