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 …
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A high hemihydrate content results in false setting of the cement, thus a maximum percentage of the hemihydrate is required in gypsum samples [3]. It is shown in the literature that the decomposition of calcium sulphate dihydrate proceeds via two reactions [4, 5] CaS04- 2H20 (s) -. CaS04 -0.5H20 (s) + 1.5Hz0 (g) CaS04 -0.5H20 (s) ~ CaS04 (s ...
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Gypsum_120 120 h 120(±5) °C Gypsum_180 16 h 180(±5) °C Gypsum_250 16 h 250(±5) °C Gypsum_380 16 h 380(±5) °C Gypsum_550 16 h 550(±5) °C For the dehydrated gypsum samples the loss on ignition was determined. To determine the experimental loss on ignition a reference gypsum sample was dehydrated at 800 °C and weighed out.
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Phosphogypsum (PG) is a kind of industrial solid waste with high impurity content and low utilization rate. In this study, α-hemihydrate gypsum (α-HH) was synthesized from PG in H 3 PO 4 –H 2 O solution (wet-process phosphoric acid solution and phosphoric acid solution) under atmospheric pressure. The crystal morphology of α-HH was controlled by adding F and …
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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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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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In this work, the dehydration pathways of gypsum and the rehydration mechanism of soluble anhydrite were mainly investigated by thermodynamic modeling and MC simulations. The …
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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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The obtained dehydrated gypsum contained combined gypsum phases of dihydrate, hemihydrate, and II-anhydrite. Cao et al. [23] also found that the soluble phosphate content decreased after calcination at 500 °C. All the experimental results of previous researches have illustrated that the content of soluble phosphates in the dehydrated PG ...
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Calcium Sulphate Hemihydrate is called Plaster of Paris because it is derived by heating gypsum (calcium sulfate dihydrate) to around 150°C, which removes water and forms the hemihydrate. This process was historically centred in Paris, where large gypsum deposits were found and widely used for making quick-setting materials.
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the time-limiting step in these processes is the step of forming the alpha-hemihydrate gypsum. Even in processes where the dihydrate gypsum slurry is continuously fed into a vessel and alpha-hemihydrate gypsum continuously removed from the vessel, the slurry must remain in the vessel for a residence time on the order of hours, as described above, for the full conversion to alpha …
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It is found that the dehydration of gypsum to anhydrite proceeds via the hemihydrate and γ-anhydrite phases and the interplay and behaviour of these phases has been determined …
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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 was also studied by Monte Carlo (MC) simulations. The thermodynamic calculation results reveal that the dehy …
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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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The gypsum powder was stored for 24 h in a 100 °C oven, where mainly it dehydrated into hemihydrate, next to small amounts of anhydrite. ... The calculated saturation indices from porewater data of both gypsum and hemihydrate are reflecting one another in each sample and follow the sulfur concentrations (Fig. 6). The saturation of hemihydrate ...
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The gypsum is dehydrated tocalcium sulphate hemihydrate (hereafter abbreviated to hemihydrate) before use. The hemihydrate is then eitherrehydrated during the industrial production of plasterboardor left unaltered and sold as plaster in bags.A number of processes have been developed to produce hemihydrateindustrially using a variety of calciner ...
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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 …
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Factors affecting these results include the type of added gypsum (for example when hemihydrate calcium sulfate, i.e. bassanite, is used instead of gypsum, or when solid waste byproducts such as titanium gypsum, phosphorous gypsum, and desulfurization gypsum are used instead of pure gypsum), the type and percentage of fine portion of the soil ...
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This work firstly shows that the observed crystallization inside of gypsum during dehydration is the formation of alpha-hemihydrate. Gypsum (calcium sulfate dihydrate) is one of the most used inorganic binding materials …
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Nesquehonite (NQ) can be activated via thermal treatment under 50–175°C, after which the dehydrated nesquehonite can hydrate to form a plaster-like product, like gypsum-based plaster. Here, we developed different blends of NQ and hemihydrate (dehydrated gypsum) to produce plaster.
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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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Abriel et al. studied the dehydration mechanism of gypsum using neutron and X-ray diffraction (XRD) and proposed a novel gypsum …
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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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The process of converting gypsum into alpha hemihydrate involves heating gypsum at a specific temperature to remove water molecules, resulting in a more concentrated and stable form of calcium sulfate. The term "alpha" in alpha hemihydrate refers to the unique crystal structure formed during this dehydration process. The alpha form exhibits ...
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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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where P H 2 O(gypsum–hemihydrate) is the equilibrium water vapor pressure of the reaction for the transformation of CaSO 4 ·2H 2 O into β-CaSO 4 ·0.5H 2 O, P H 2 O(gypsum–anhydrate) represents the dissociation pressure of β-CaSO 4 ·0.5H 2 O dehydrated to form γ-CaSO 4, and P H 2 O(hemihydrate–anhydrate) naturally denotes the ...
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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 …
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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 …
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Hemihydrate gypsum and the corresponding desulfurized ash from semi-dry flue gas desulfurization were weighed, and the ratio of water/mixture of hemihydrate gypsum and the corresponding desulfurized ash was kept as the constant of 50:1. Water was poured into the mixture and stirred at low speed for 2 min. Finally, 3 to 5 ml clarification ...
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The objective of this study was to assess the feasibility of using the plant-source polymer of the matcha powder as a composite admixture for hemihydrate gypsum. Hemihydrate gypsum was mixed with different contents of matcha powder, and then the water requirement for the normal consistency, setting times, density, strength, hydration and microstructure of the …
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