Setting Of Gypsum Plaster Research Articles

Bassanite (CaSO 4·0.5H 2O) dissolution and gypsum (CaSO 4·2H 2O) precipitation in the presence of cellulose ethers

The setting of gypsum plaster in the presence of cellulose ether was studied by monitoring the kinetics of bassanite (calcium sulfate–hemihydrate, HH) dissolution and of gypsum (calcium sulfate dihydrate, DH) precipitation. Dissolution rates of various industrial HH vary between 4 and 9×10 −5 mol/m 2 s when normalized to the BET surface area and the content of soluble CaSO 4. An apparent activation energy of about 10–20 kJ/mol indicates a diffusion controlled dissolution mechanism in deionized water as well as in cellulose ether (CE) containing solution. The presence of CE at industrially relevant conditions of 0.25 wt% retards the dissolution rate by 50%. The incubation time for DH nucleation is extended by 100% in presence of 0.25 wt% CE. The subsequent DH growth is significantly affected up to 80% by the presence of CE depending on the degree of supersaturation. Close to equilibrium, a surface reaction controls the growth of DH in pure CaSO 4 solution as well as in the presence of CE. At high degrees of supersaturation, DH growth is controlled by volume diffusion. Since 90% of the HH–DH transformation occurs at high degrees of supersaturation, we conclude that the interaction between CE and HH as well as DH surfaces is weak and the dissolution and precipitation processes are affected primarly due to the diffusion properties of dissolved Ca 2+ and SO 4 2− species in CE solution.

The setting of gypsum plaster

The effect of calcium sulphate presented as soluble anhydrite (γ-CaSO 4 ) on the setting of gypsum plasters was investigated by measurement of the set strength and a microstructural examination of the set plasters. The anhydrite reduced the set strength by disrupting the interlocking crystal matrix of dihydrate needles. Additives, such as the nucleating agent calcium sulphate dihydrate and the accelerator potassium sulphate, increased the rate of the hydration reaction by increasing seeding and the rate of dissolution of calcium sulphate hemihydrate. The additives also alter the crystal habit of the dihydrate needles in the set plaster, resulting in reduced intercrystalline bonding and as a consequence reduced strength (...)

The setting of gypsum plaster

The α- and Β-forms of calcium sulphate hemihydrate, CaSO4·1/2H2O, were characterized using thermal, X-ray diffraction, optical and electron microscopy techniques. The differences between the two forms arose from their mode of production, resulting in different crystal size and habit, and not from different crystal structures. The hydration of CaSO4·1/2H2O to CaSO4·2H2O was investigated using calorimetry, phase analysis and the changes in electrical resistance of the plaster slurries. The reaction occurred in three steps: dissolution, nucleation and precipitation, and finally completion due to depletion of reactants. The differences in reactivity of the α- and Β-forms with water again appeared to be due to their different physical states.

The setting of gypsum plaster

The α- and β-forms of calcium sulphate hemihydrate, CaSO 4 .1/2H 2 O, were characterized using thermal, X-ray diffraction, optical and electron microscopy techniques. The differences between the two forms arose from their mode of production, resulting in different crystal size and habit, and not from different crystal structures. The hydration of CaSO 4 .-1/2H 2 O to CaSO 4 .2H 2 O was investigated using calorimetry, phase analysis and the changes in electrical resistance of the plaster slurries. The reaction occurred in three steps: dissolution, nucleation and precipitation, and finally completion due to depletion of reactants. The differences in reactivity of the α- and β-forms with water again appeared to be due to their different physical states

The effects of some organic acids and salts on the setting of gypsum plaster III. citrates

The effects of some organic acids and salts on the setting of gypsum plaster III. citrates

The effects of some organic acids and salts on the setting of gypsum plaster. II. Tartrates

AbstractIn continuation of previous work on the setting of gypsum plasters,1 study has been made of the effects of tartaric acid and potassium and calcium tartrates on this reaction.Tartaric acid (like acetic acid) has little retarding power, except in the higher concentrations. The marked accelerating effect of the potassium salt is attributed to precipitation of tartrate ions as calcium tartrate, with the formation of potassium sulphate; this is supported by several similarities between the effects of potassium tartrate and those of potassium sulphate, as determined by Smith et al.2 Calcium tartrate has negligible effect on the reaction.

The effects of some organic acids and salts on the setting of gypsum plaster. II. Tartrates

The effects of some organic acids and salts on the setting of gypsum plaster. II. Tartrates

The effects of some organic acids and salts on the setting of gypsum plaster. I. Acetates

AbstractThe properties of one batch of autoclave calcium sulphate hemihydrate have been determined. A study has been made of the effects of various concentrations of acetic acid, and potassium and calcium acetates on (i) the rate of evolution of heat, (ii) the setting expansion, (iii) the setting time, (iv) the compressive strength, (v) pH and (vi) the crystal morphology of this plaster. A theory of Ridge et al. has been tested and is discussed.It has been shown that the three additives have some similarities in their effects on the kinetics of setting and it is concluded that the acetate ion plays a major role in the observed retardation of setting.

Effect of Temperature on the Expansion on Setting of Gypsum Plaster

IT has been shown by Ridge1 that setting-temperature exerts a profound effect on the structure of set gypsum plaster. He postulated that this change in structure would be accompanied by changes in other properties such as the expansion on setting. Experimental evidence showing the expansion of plaster on setting at different temperatures has recently been obtained. The observations were made incidental to the preparation of plaster samples of different pore sizes that were to be used as standard porous bodies.

Setting of Gypsum Plaster

THE initial thickening of slurries of hemi-hydrate type gypsum plasters has been considered by many workers to be due to a colloidal mechanism, whereas the further hardening and final setting have always been attributed to a crystallization process. Recent work supporting the colloidal theory is that of Fischer1, while still later Becherer and Fiedler2 have attributed the initial thickening of plaster slurries to the action of capillary forces causing a packing together of hemi-hydrate particles.