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This article addresses the microstructure and mechanical properties of a β-hemihydrate produced gypsum from a combined study of hydration and dehydration. A thin layer of adsorbed water …
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The dissolution of hemihydrate and growth of gypsum is a two-step process: at first, ... and then completely dehydrated to γ-anhydrite at ∼673 K. And furthermore, the gypsum-bassanite and ...
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Plaster, made of calcium sulfate hemihydrate (CaSO 4 •½H 2 O) and/or γ-anhydrite (CaSO 4) is obtained by dehydration of gypsum (CaSO 4 ·2H 2 O) at 100–200 °C. When mixed with water, it dissolves while new gypsum crystals precipitate. Plaster's microstructures affect its reactivity and setting properties, but are poorly understood due to confusion between crystals …
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Gypsum crystal partly dehydrated, cleaved along cleavage plane, with observable dihydrate, α β α-hemihydrate and β-hemihydrate. Yellow circle = still dihydrate, blue circle = alpha hemihydrate, red circle = beta hemihydrate, black arrow = …
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The apparent empirical activation energies (Ea) suggest that the transition from gypsum to hemihydrate is mainly controlled by nucleation and growth mechanism, while the transition from gypsum to γ-anhydrite is mostly followed by phase boundary mechanism. ... and water (3250-3750 cm-1) mol. stretching (v1 and v3) modes, the gypsum dehydrated ...
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The changes of crystal structure of gypsum on heating are evaluated by X-ray diffraction. The XRD data of gypsum, hemihydrate and anhydrite indicate that most of the peaks (2θ versus intensity) are different from each other. There are three major peaks at 11.59°, 20.72° and 29.11° 2θ in gypsum, 14.75°, 29.70° and 31.86° 2θ in hemihydrate and only 25.44° 2θ in …
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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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This paper introduced the characteristics of α high-strength gypsum preparation from FGD gypsum by autoclaved semi-dry process. The compositions of the product in different stages of the reaction and the effects of crystal modifiers on the morphology of the α-HH crystals were examined and compared. The properties of α-HH and the feature of hydration products of α …
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Gypsum is a hydrated calcium sulfate with composition CaSO 4 ·2H 2 O. It commonly occurs as a chalky powder or occasionally as small crystals. ... Dehydrated forms of gypsum range isostructurally from CaSO 4 ·2H 2 O to CaSO 4. The hemihydrate CaSO 4 ·0.5H 2 O is a stable form known commercially as plaster of Paris and in nature as bassanite.
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The application of α-hemihydrate gypsum (α-HH) is limited by several factors, such as a rapid hydration rate, short setting time, poor water resistance, and high cost. Especially because of the ...
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This is because the main composition of PG is dihydrate gypsum (CaSO 4 ⋅2H 2 O), which can be dehydrated to form synthetic hemihydrate gypsum, like natural mineral gypsum. Depending on the temperature and condition of the dehydration process, natural gypsum exists in four different phases, namely hemihydrate gypsum (CaSO 4 ⋅0.5H 2 O), III ...
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The purpose of this search was to understand the hydration of calcium sulfate hemihydrate to gypsum deeply. ESEM was used to observe the dissolution of hemihydrate clearly and continuously. Accordingly, a systematical analysis of the time-dependent phase, ion concentration, particle size distribution and morphology change were conducted during ...
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The preparation of β-hemihydrate gypsum is an important approach for large-scale utilization of phosphogypsum (PG) because of the simple production technology by calcination. ... confirming that only a small amount of PG was dehydrated. The dominant phase is β-hemihydrate gypsum at 60 min, and the weak diffraction peaks of dihydrate gypsum at ...
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Type III anhydrite is also known as dehydrated hemihydrate gypsum. Its crystal phase structure is the same as that of the original hemihydrate gypsum. Type III anhydrite is unstable and will quickly hydrate to hemihydrate gypsum when it comes into contact with moisture in the air. This unstable phase is unavoidable when producing hemihydrate ...
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based on the weight of the calcium sulfate hemihydrate and the calcium sulfate dihydrate. Furthermore, the calcined gypsum may be α-hemihydrate, β-hemihydrate, or a mixture thereof. [0007] In addition to the stucco, the gypsum slurry may also contain other hydraulic materials. These hydraulic materials may include calcium sulfate
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CaSOa and hemihydrate samples used in the 295 K experiments were prepared from synthetic gypsum by dehydration. In the case of hemihydrate, gypsum (CaSO+ ' 2D2O) synthesized …
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dehydration still needs further investigation, especially the behaviour of the β-hemihydrate and gypsum produced from it is still poorly understood. This article focuses on the role of water on the properties of gypsum produced from a β-hemihydrate due to its combined effect on both hydration and dehydration.
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Three main commercial beta calcium sulfate hemihydrate from natural gypsum, phosphogypsum and flue gas desulphurization gypsum, represented as NH, PH, and DH, supplied by Chengdu Hongtai High Strength Gypsum Co. Ltd, Sichuan Longmang Gypsum Industry Development Co. Ltd and Chengdu Landing Environmental Protection Technology …
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The deterioration of buildings and monuments by gypsum is the result of crystallization cycles of this salt. Although gypsum can dehydrate to a hemihydrate, the mineral bassanite, and to an anhydrate, the mineral anhydrite, this reaction occurs in nature on a geological time scale and therefore it is unlikely to occur when gypsum is found on and in …
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However, using gypsum presents challenges due to its moderate water solubility, the formation of swelling clay minerals under certain conditions, and the tendency of dehydrated gypsum to swell upon exposure to water, to name a few.
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KEY WORDS: b-hemihydrate; gypsum board; dehydration; thermal properties; microstructure; mechanical properties List of symbols ... The strength of the dehydrated system was also tested on gypsum prisms according to EN 13279-2 [19] to investigate the effect of the microstructural change. The flexural strength was measured with
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A 'zero-water hemihydrate' is observed before refinement in the higher symmetry γ-anhydrite cell is possible. The waste gypsum materials studied showed significant differences in the temperatures at which key transformation events occurred; these observations raise implications concerning the re-use of by-product gypsum materials.
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The development of a multimillion-dollar gypsum industry was made possible by the following factors: (1) Gypsum is an abundant, naturally pure raw material. (2) It can be dehydrated to the hemihydrate at relatively low temperature using relatively simple equipment. (3) The reaction is virtually 100 percent complete, as is the reverse reaction.
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The dehydration of gypsum can be observed using thermo-gravimetric measurement and differential scanning calorimetry. Depending on the parameters of …
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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 of soluble anhydrite...
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Gypsum-based binders are generally based on the reaction between calcium sulfate hemihydrate (HH) and water to form calcium sulfate dihydrate (DH). Often gypsum can be used as a raw material to obtain hemihydrate. In this study, the obtained alpha hemihydrate consists of large crystals that have more or less lattice defects according to its ...
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Gypsum (calcium sulfate dihydrate, Fluka, 99.9% purity) samples were dehydrated at temperatures from 85 up to 800 °C 25 g of substance was weighed in corundum crucibles of 60 ml size and dehydrated in two different furnace types to cover the complete temperature range.The furnaces were calibrated with thermo elements.
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Request PDF | Discrimination of bassanite and anhydrite III dehydrated from gypsum at different temperatures | Technical OPC normally contains mixed sulphate carriers in varying amounts. Gypsum ...
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Here, we perform fast, time-resolved synchrotron X-ray scattering on natural alabaster samples, finding that a small elastic load accelerates the dehydration reaction …
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Calcium sulphate hemihydrate commonly known as plaster of Paris is extensively used in buildings, ceramics and medical industries. Hydration of hemihydrate is a highly exothermic reaction and occurs as: CaSO 4 · 0.5 H 2 O + 1.5 H 2 O ⇒ CaSO 4 · 2 H 2 O + Q where Q the amount of heat evolved depends on number of factors. During hydration, gypsum …
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