In the high-temperature and low water vapor pressure region, gypsum dehydrates to form γ-CaSO4 in a single-step process (CaSO4·2H2O γ -CaSO4); with increasing water vapor …
2.2.2 Transition temperature gypsum-anhydrite in water. The purely statistically fitted curves of the data by Krumgalz (Eqs 6, 9) yield a crossing point of the gypsum-anhydrite solubility curve at 45.6°C . This value looks like a compromise between …
The commercial rock gypsum studied generates both hemihydrate and anhydrite at 128°C, and its impurities do not dehydrate until the temperature is increased to 500°C. The dehydration process of ...
The dehydration process of the dihydrate gypsum can be divided into two stages as follows: the transformation of dihydrate gypsum into α-hemihydrate at approximately 130 °C and that of α-hemihydrate into type III anhydrite at approximately 150 °C, as expressed in the following equations:
Gypsum plaster is an inorganic binder that is prepared in a two-step process. The first step is dehydration: heated to 100–200 °C, gypsum (CaSO 4 ·2H 2 O) releases water as vapor [1, 2] and, depending on temperature and water vapor partial pressure (p H2O) [3, 4] forms either hemihydrate (CaSO 4 •½H 2 O) or γ-anhydrite (CaSO 4), the main components of the …
It was also found that gypsum is stable in the low-temperature and high water vapor pressure region and does not dehydrate to form any calcium sulfate hemihydrate. Finally, the rehydration...
With increase in hydration time, the dehydrated gypsum rock sample absorbed water molecules to form dihydrate gypsum, which strengthened the sample, opened after the …
Expected induction times of gypsum and anhydrite precipitating from an evaporating (sea)water pond. The solute concentration of Ca 2 + and SO 4 2 − steadily increases due to evaporation and the solution of a pond will become supersaturated with respect to anhydrite and gypsum (blue and pink curves in the inset, two evaporation rates are ...
relating gypsum and anhydrite zones, a number of differential thermal analyses were made of selected samples. This in-vestigation had negative results, since all the thermograms of gypsum and anhydrite proved to be extremely similar, respectively, indicating constant, or almost constant com-positions of the analyzed materials.
thermochimica acta ELSEVIER Thermochimica Acta 282/283 (1996) 483-492 The thermal dehydration of natural gypsum and pure calcium sulphate dihydrate (gypsum)' D.L. Hudson-Lamb", C.A. Strydom a, J.H. Potgieter b a Department oJ'Chemistry, University of Pretoria, Pretoria, 0002, South Africa b Technical Services, PPC, PO Box 40073, Cleveland …
Gypsum is one of the most widely-used building materials in the world. It is applied in several interior building uses, including in wallboard. Normally, calcium sulphate dihydrate (CaSO 4 •2H 2 O) is used as a raw material and a subhydrate such as hemihydrate (CaSO 4 •0.5H 2 O) or anhydrite (CaSO 4) is produced. The subhydrate is the ...
The results show that H 3 PO 4 and Ca(H 2 PO 4) 2 ·H 2 O could accelerate the anhydrite dissolution and dihydrate gypsum nucleation due to the increased crystal defects and the formation of the nucleation site. The early mechanical strength of anhydrite significantly increased due to its high hydration degree, compact microstructure, and fine ...
gypsum and anhydrite are completely stable in an aqueous solution and therefore, this investigation has experimentally determined the transition line separating the two regions
The hydration rate of SCRM can be efficiently delayed by dihydrate gypsum, although the result is subpar. The material exhibited the lowest fluidity loss in 20 min, the setting time was extended, and the 28-day flexural and compressive strengths were raised by 26.56% and 28.08%, respectively, after adding 8 percent anhydrite.
Below 100 °C, the mechanism consists of a direct dehydration step of gypsum to anhydrite (with no intermediate hemihydrate formation), resulting in the formation of anhydrite …
The water requirement (W R) for normal consistency for anhydrite II at 280°C was 0.68, exhibiting higher flexural and compressive strength compared to calcination at 500°C (W R=0.58), and the hydration rate at 28 days reached 78.5 %. At 280°C, the dihydrate crystals formed by anhydrite II at 28 days exhibited a compact microstructure ...
The thermal dehydration of natural gypsum and pure calcium sulphate dihydrate (gypsum) ... (heating rate 5°C min-'), there is an increase in the hemihydrate and anhydrite content of natural and pure gypsum and possibly CaSO,.0.62 H,O could also be present. ... Calcium sulphate dihydrate electron micrographs show that the decomposition of the ...
The transformation of anhydrite into gypsum proceeds until all the anhydrite present in the rock is converted to gypsum. This process (gypsification of anhydrite) is accompanied by about 61% increase in volume (Butscher et al. 2011, …
The present paper addresses the gypsum hydration study with the theoretical and experimental method. The microstructure of gypsum changes during the hydration reaction. The hydration …
In the low-temperature and high water vapor zone above curve AB in Figure 2, gypsum is thermodynamically stable; in the ABC zone, CaSO 4 ·2H 2 O will partially dehydrate its crystal water to form β-CaSO 4 ·0.5H 2 O, and as the temperature is increased, gypsum …
The effect of K 2 SO 4 activator on the hydration of chemical anhydrite obtained from burned FGD-gypsum has been studied by different experimental techniques. Results obtained show that the degree of hydration increases when the K 2 SO 4 concentrations increase from 0.5 to 3.3 wt%. Their heat evolution rate and maximum value also increase with the …
The activation mechanism of sodium oxalate was analyzed from the standpoint of the crystal growth of gypsum. The addition of Na2C2O4 increased the hydration rate of anhydrite, concentrated the ...
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 …
The results show that the main components of natural gypsum minerals were gypsum dihydrate (71%), dolomite (27%) and potassium chloride (2%). ... and then increased again. The expansion rate and ...
The influence of gypsum is mainly reflected in the last two stages, which are the change in the formation rate of hydration products and the change in morphology. Anhydrite can maintain the hydration acceleration period at 9–12 h, forming needle-shaped ettringite.
The hydration properties of hemihydrate gypsum and anhydrite affect the quality of MPPG-based materials. Anhydrite has a much slower hydration rate and strength development than hemihydrate gypsum, and is commonly blended with cement, strong alkali or alkali sulfate to increase its hydration activity [15], [16], [17]. Many studies have been ...
In Fig. 10 g, long columnar dihydrate gypsum crystals formed in HPG280–40 % at 7 days, with some aggregations observed on the surface and vicinity of the formed dihydrate. These aggregations were likely unhydrated anhydrite II gypsum, as inferred from the previous analysis and the 7-day hydration rate results of HPG280–40 %.
Calcium sulfate can be precipitated as dihydrate (DH) or gypsum (CaSO 4 ·2H 2 O) at low temperatures and as hemihydrate (HH) (CaSO 4 ·0.5H 2 O) or anhydrite (AH) (CaSO 4) at elevated temperatures (Li and Demopoulos, 2005).All calcium sulfate hydrates are relatively insoluble and they are formed wherever calcium and sulfate occur together after the …