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No. 1688, Gaoke East Road, Pudong new district, Shanghai, China.
The 'solubility' or solving rate for hemihydrate decreases with higher temperature. Crystals with such behaviour normally have a positive solving energy. Solving hemihydrate is exothermic. For gypsum, a nearly constant solubility can be observed, so the solving energy of gypsum is around zero.
Transforming phosphogypsum (PG) to β-hemihydrate PG with cementitious properties at low calcination temperature is an effective method for large-scale utilization of by …
volume ratio of hemihydrate/free water (0.36 for α- and 0.38 for β-hemihydrate), vn/vw the specific volume ratio of crystal water/free water (0.81 for α- and 0.71 for β-hemihydrate), w the water/hemihydrate ratio, and w0 the water requirement for the hydration reaction (0.186 for both α- and β-hemihydrate).
High-strength gypsum is the recognized designation for α-hemihydrate gypsum, often referred to as α-HH (CaSO 4 ·0.5H 2 O), which is a cementitious material with low water requirement and high strength [8].The strength of α-HH is generally between 25 and 50 N/mm 2, which is three times that of ordinary building plaster, and has superior working performance. α …
With the best calcination temperature of 800 °C, gypsum plaster showed a relatively quick setting and maximum strength with a compact microstructure and low ... converted impurities could hinder the hydration of anhydrite. For preparing β-hemihydrate at low calcination temperatures, Geraldo et al. [28] calcined PG at respective 120 °C, 150 ...
Gypsum and anhydrite are naturally occurring minerals, however the hemihydrate and anhydrite can be obtained by calcination which is a process of chemical alteration by heating the dihydrated gypsum at temperatures of 125 °C-180° C and higher than 300 °C, respectively, as shown in Fig. 11 [30,31,20,32].
gypsum calcination process by building two traditional ovens, assisted by locals masons, and we study how the differences in the ... it has been observed that set α-hemihydrate can obtain a four times higher compression strength than concrete (Arredondo 1961; Ye 2011). ... would be the maximum temperature reached during calcination in ...
β-HH can be obtained by calcination of gypsum (Christensen et al., 2010; Schmidt et al., 2011). 170 Physicochem. Probl. Miner. ... fertilizer industries. Additionally, anhydrite and hemihydrate gypsum are unstable, with the extension of the accumulation time, they slowly hydrate into dihydrate gypsum. ... elevated temperature (above 100 °C ...
process and becomes hemihydrate gypsum (CaSO 4 •½H 2 O) or calcined gypsum; it is also known as beta gypsum (alpha gypsum is made by processing batches of gypsum under elevated pressure). Calcined gypsum or beta gypsum is soft and may easily be ground, forming a powder termed hemihydrate gyp-. Fig. 8.1Big gypsum crystals. (Image courtesy of
The currently commonly used PG pretreatment methods can partially solve the impurity problem, such as using high-temperature calcination to obtain hemihydrate or anhydrous gypsum for use as cement ...
The α-hemihydrate hydration results in a high dihydrate formation; therefore, this type of gypsum is applied in conditions where more significant strengths are necessary (e.g. sculptures and sanitaryware casing) [36], [37]. β-Hemihydrate is obtained through the dry calcination of gypsum in the range of 120 °C–180 °C and it is an easier ...
The preparation of natural and synthetic gypsum prior to calcination, such as drying and grinding. The various calcination methods for β-hemihydrate and α-hemihydrate are reviewed, such as batch kettle, continuous kettle, submerged combustion kettle, …
Method and apparatus for calcining gypsum in a kettle (10) in which the calcined mass is fluidized. Heat for the calcination is supplied by heating rods (46) which are immersed in said bed. Said heating rods (46) supply about 1 British Thermal Unit (Btu) of heat per minute per square inch of surface area of rod. Heat is also supplied to the calcined mass by a hot gas produced by …
The roller mill is a short-time calcining system. Both natural gypsum and mixtures of natural and synthetic raw materials can be ground, dried, calcined and classified in this kind of mill. Bundling several process steps results in highly efficient gypsum processing. Natural gypsum and up to 45% moist FGD gypsum can be fed into a roller mill.
The calcination process of gypsum binder was carried out at four di erent temperatures ranging from 170 to 190 C. The specimens for testing were prepared on the basis of the obtained raw material with a constant water to gypsum ratio of w/g = 0.75. It was noted that the calcination temperature influenced the setting time of the gypsum.
The rate of the gypsum-hemihydrate transformation is dominantly controlled by temperature, T, and the water vapour pressure in the reaction environment, p H2O 16,17,18.
In order to facilitate a better grasp of the calcination process, he explains, I start off by taking raw gypsum and heating it to a temperature of 150°C to 180°C, which produces gypsum in hemihydrate form.
The temperature at which the gypsum binder (calcium sulfate hemihydrate) is industrially produced, between 80 and 180 °C, is often mentioned in the literature …
A process for producing alpha-hemihydrate gypsum from dihydrate gypsum includes feeding a slurry comprising the dihydrate gypsum and water into a heating tube, heating the heating tube at a temperature effective to generate steam and pressure from the water, wherein the steam and pressure are effective to convert the dihydrate gypsum to the alpha-hemihydrate gypsum, and …
When gypsum is heated at about 150–160 °C, it loses approximately three-quarters of its water in a partial dehydration process and becomes hemihydrate gypsum (CaSO 4 •½H 2 O) or calcined gypsum; it is also known as beta gypsum (alpha gypsum is made by processing batches of gypsum under elevated pressure). Calcined gypsum or beta gypsum is soft and …
This suggests that PG undergoes calcination at 400 ℃ to form anhydrite III, which absorbs moisture from the air to become hemihydrate gypsum. Additionally, PG calcined at …
The results show that the obtained hemihydrate gypsum mixing with water can prepare high-performance building gypsum after the FGD gypsum adding 3–5 wt% of CaO is …
Flue-gas desulphurization (FGD) gypsum is a highly prevalent industrial by-product worldwide, which can be an excellent alternative to natural gypsum due to its high content of CaSO4·2H2O. The preparation of α-calcium sulfate hemihydrate is a high-value pathway for the efficient use of FGD gypsum. Here, a dynamic method, or an improved autoclaved process, …
The main objective was to find adequate calcination conditions considering PG transformation into β-hemihydrate gypsum, with minimum energy spent. ... With the best calcination temperature of 800 ...
The results show that the obtained hemihydrate gypsum mixing with water can prepare high-performance building gypsum after the FGD gypsum adding 3–5 wt% of CaO is calcined at 160–220 °C for 1.5–2 h. ... calcination temperature and holding time of FGD gypsum were systematically investigated. ... Here we examined the hydration behavior of ...
Calcium sulphate dihydrate (CaSO4·2H2O or gypsum) is used widely as building material because of its excellent fire resistance, aesthetics, and low price. Hemihydrate occurs in two …
The dehydration products of gypsum under different temperature and water vapor pressure were investigated by thermodynamic theory. Additionally, the rehydration mechanism of soluble anhydrite was also studied by Monte Carlo (MC) simulations. The thermodynamic calculation results reveal that the dehydration mechanism of gypsum significantly depended …
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 form dihydrate again. Therefore, the dehydration process of gypsum and the rehydration of hemihydrate were investigated.
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 ...
At calcination temperatures above 350℃, type III anhydrous gypsum transforms into type II anhydrous gypsum or natural anhydrite characterized by dense and stable crystalline phase …
