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Structural evolution during the dehydration of gypsum materials - Volume 73 Issue 3 ... Jorgensen, J.D. and Hinks, D.G. (1984) A crystallographic study of the low-temperature dehydration products of gypsum, CaSO 4-2H 2 O; hemihydrate CaSO 4-0.5H 2 O, and γ-CaSO 4. American Mineralogist, ... Thermal dehydration of calcium sulfate dihydrate: ...
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Naturally occurring crystals are calcium sulfate dihydrate, CaSO 4 ·2H 2 O, known as gypsum, and anhydrous calcium sulfate, II-CaSO 4. When gypsum is heated, dehydration occurs to form hemihydrates, CaSO 4 ·1/2H 2 O, followed by anhydrite, III-CaSO 4. The hemihydrates and the anhydrite have two modifications, α and β, respectively.
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Dehydration of gypsum dihydrate to gypsum hemihydrate in liquid phase can be understood in terms of solubility. The cross point of solubility of gypsum dihydrate and hemihydrate are at 97 ± 1 °C in pure water. 9 The hemihydrate is hardly formed, even if the dihydrate is heated in water at 100 °C (Fig. 2).Therefore, it is necessary to use either …
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Gypsum of chemical formula CaSO 4 ·2H 2 O, also called calcium sulfate dihydrate, is a versatile material used in several sectors of activity such as cement industry, plaster manufacturing and agriculture [1, 2].It is also found in the formulation of many useful products like paper, glass and paints [3, 4].Gypsum production is carried out in two different …
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As can be seen in Fig. 11 the dehydration temperature of the hemihydrate increases with sodium content. ... which can be found as calcium sulfate dihydrate or gypsum (CaSO 4 ⋅2H 2 O ...
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Temperature measurements at pre-selected board locations indicated three distinct stages of gypsum dehydration; free moisture evaporation, transformation of calcium sulfate dihydrate to calcium ...
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1. Ultrasonic results indicate gypsum hydration reaction consists of two stages: the dissolution of hemihydrate in the first stage and the nucleation and precipitation of dihydrate in the second stage. 2. Gypsum hydration is strongly influenced by water amount. Spread flow test is suitable to determine the water demand for gypsum hydration.
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THE differential thermograms of calcium sulphate dihydrates (gypsums) show two relatively large endothermic effects in close sequence below 250° C. There is general agreement1,2 that these two effects represent the two-step decomposition of the dihydrate to hemihydrate and to soluble anhydrite. A third small endothermic effect, which is always associated with the second …
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occurrence of dihydrate dehydration at this temperature was detected, evidenced by the disappearance of multiple dih ydrate peaks, especially the peak at 1002 cm 1 .
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Gypsum (calcium sulfate dihydrate, CaSO 4 ·2H 2 O) is the most aqueous phase in the system CaSO 4 –H 2 O and, besides anhydrite II, the stable phase under ambient atmospheric conditions. With increasing temperature gypsum starts to dehydrate and form calcium sulfates with lower H 2 O content. The dehydration process proceeds stepwise and ...
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The dehydration of gypsum CaSO 4.2H 2 O has been studied, at negligible water vapour pressure, by in situ infrared (IR) spectroscopy and by thermogravimetry to determine whether intermediate phases (CaSO 4. n H 2 O) exist, other than the hemihydrate with n =0.5, and also to compare the mechanism of the dehydration process when measured by two …
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The addition of sulfuric acid reduced the dehydration temperature, accelerating the nucleation of anhydrous calcium sulfate and increasing the number of crystal nuclei, resulting in the formation of small anhydrite gypsum crystals. ... In Fig. 10 g, long columnar dihydrate gypsum crystals formed in HPG280–40 % at 7 days, with some ...
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Methods. The dehydration process of FGD gypsum was performed with simultaneous TG/DSC analysis (NETZSCH STA 409 Luxx, Selb/Bavaria, Germany) at N 2 atmosphere with a gas flow of 20 mL min −1.Non-isothermal analysis was carried out at different heating rate (2−20 K min −1).Isothermal analysis was conducted in the temperature range of …
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However only - 35 % of natural gypsum is calcium sulphate dihydrate. Thus on calculating, 35% of the heat of dehydration of the calcium sulphate dihydrate value gives a heat of dehydration of the calcium sulphate dihydrate in natural gypsum of - 140-175 J g- t.
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The plaster of paris is in full Heshui steam or solution, and at certain pressure and temperature, crystal changes, and dihydrate gypsum dehydration generates the α-type semi-hydrated gypsum powder containing l/2 crystal water.At present China produces α high strength semi-hydrated gypsum powder, generally adopts the dewater mode of fine grinding again of dry method …
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the process of dehydration of dihydrate gypsum under hydrothermal conditions. 2 12 223242 5262 2 Θ, [deg] Relative intensity, u.a. DDDD D D D D D D D D D D D D D D D ... Fig. 6, the higher dehydration temperature, the shorter isothermal curing is required for the dihydrate to turn to hemihydrate in full. 5 10 15 20 0 0.5 1 1.5 2 2.5 3 τ,h.
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The dehydration process of gypsum rock was studied under concentrated solar energy by using a Fresnel lens with power density of 260 Wcm 2. Temperatures higher than 700 C were attained for 1 min ...
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A microscope study has been made of the development of crystals of calcium sulphate hemihydrate produced on, or in, sheets of the dihydrate in the form of selenite by calcining under controlled conditions. The crystal forms were shown to be strongly dependent upon both temperature and ambient water vapour pressure. A mechanism for the nucleation and growth …
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Dihydrate gypsum is the dominant phase composition in PG, it is a stable phase at room temperature but can be converted into α-hemihydrate gypsum (α-CaSO 4 ·0.5H 2 O) [16] and β-hemihydrate gypsum (β-CaSO 4 ·0.5H 2 O), also called building gypsum [17] according to different heating methods. α-hemihydrate gypsum is prepared in a ...
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Drying and thermal dehydration of gypsum in the shape of thick circular plate and sphere ware investigated, especially by using a fluidized bed to reduce the film resistance for heat transfer. …
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Mass losses vary between 13.7 and 16.5% and heat of dehydration values between 377 and 420 J g −1 for the dehydration of ground and mixed inhomogeneous gypsum samples. As for the dehydration of CaSO 4 ·2H 2 O, the dehydration of synthetic gypsum proceeds via multi-step reactions. Using a heating rate of 5°C min −1, the very slow …
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During the calcination process of the dihydrate gypsum, there was an occurrence of two dehydration reactions in a temperature range from 120 °C to 360 °C. The first step was the conversion of CaSO 4 ·2H 2 O into CaSO 4 ·1/2H 2 O, followed by dehydration of CaSO 4 ·1/2H 2 O to CaSO 4 [ 39 ].
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During calcination, calcium sulfate subhydrates are formed and, for technical reasons, are mixed with water to form dihydrate again. Therefore, the dehydration process of gypsum and the ...
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In this paper, the effect of two thermal drying regimes of hardened FGD gypsum specimens, with the maximum temperatures of 40 °C and 80 °C and at atmospheric pressure, on the measured …
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The crystal lattice of raw gypsum, i.e., calcium sulfate dihydrate (CaSO 4 ·2H 2 O) contains approximately 21% by weight chemically bound ... This study was the first CFD simulation effort to account for the two main macroscopic effects of gypsum dehydration, namely, the temperature-dependent GP thermophysical properties and the production and ...
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In order to study the physical and mechanical properties of gypsum rock samples in three states (natural, high-temperature dehydration and hydration time), natural gypsum rock was put into high ...
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Herein, an in situ high–temperature synchrotron X–ray diffraction study of gypsum is performed in the temper-ature range of 30 –200 °C to investigate the continuous structural change from …
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Gypsum (calcium sulfate dihydrate) is one of the most used inorganic binding materials in the world. During calcination, calcium sulfate subhydrates are formed and, for technical reasons, are mixed with water to …
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Depending on the mill temperature, the gypsum becomes partially or completely dehydrated to hemihydrate or anhydrite respectively. According to [8] the temperatures at which gypsum dehydration to hemihydrate can take place lie between 95 and 170 °C. The results of [9] indicate that gypsum dehydration can even begin at temperatures below 70 °C.
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Request PDF | Structural evolution of gypsum (CaSO4·2H2O) during thermal dehydration | Herein, an in situ high-temperature synchrotron X-ray diffraction study of gypsum is performed in the ...
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The dehydration of gypsum to hemihydrate has been studied for decades because it is an important model reaction for understanding fluid-triggered earthquakes, and due to the global use of plaster ...
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The dehydration products of gypsum under different temperature and water vapor pressure were investigated by thermodynamic theory. Additionally, the rehydration mechanism …
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temperature dehydrated gypsum rock was treated with different hydration time, and the gypsum rock samples in three states were subjected to ultrasonic test, density test and uniaxial compression test. The results show that the main components of natural gypsum minerals were gypsum dihydrate (71%), dolomite (27%) and potassium chloride (2%). The ...
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Keywords: Gypsum, Hemihydrate, Soluble anhydrite, Dehydration, In situ high–temperature synchrotron X– ray diffraction INTRODUCTION Calcium sulfate dihydrate, gypsum CaSO 4·2H 2O, is the most common sulfate mineral. Therefore, considerable attention has been paid to the dehydration of gypsum. Numerous studies on the mechanisms and products of
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This study explored the production of a medium–low strength α-hemihydrate gypsum (α-HH) from phosphogypsum using an autoclaved method. The influence of slurry concentration, spreading thickness, crystallizing agents, slurry pH, autoclaving temperature, and autoclaving time on the properties of α-HH was investigated. The energy consumption of …
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The dependence of the solubility of gypsum on temperature is nonlinear, reaching a maximum at 43°C. ... the dehydration of gypsum due to the effects. ... dihydrate: Jour. Inorg. Nucl. Chem., v. 33,
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