Studies in the system calcium sulphate–water. Part I. Kinetics of dehydration of calcium sulphate dihydrate

Ball, Matthew C.; Norwood, L. S.

doi:10.1039/j19690001633

Cited by 51 publications

(29 citation statements)

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“…Since hemihydrate is a metastable phase of the CaSO 4 -H 2 O system under ambient pressure (Yamamoto and Kennedy 1969), it is very likely that it only exists in the heated sample in a very narrow temperature range (less than 10 K). A number of studies on the dehydration of gypsum (Bell and Norwood 1969;Yamamoto and Kennedy 1969;McConnell et al 1987) reported that the dehydration temperature was highly dependent on the heating rate and partial pressure of water vapor. A number of studies on the dehydration of gypsum (Bell and Norwood 1969;Yamamoto and Kennedy 1969;McConnell et al 1987) reported that the dehydration temperature was highly dependent on the heating rate and partial pressure of water vapor.…”

Section: Resultsmentioning

confidence: 99%

Micro-Raman studies of gypsum in the temperature range between 9 K and 373 K

Chio

Sharma

Muenow

2004

American Mineralogist

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Raman spectra were collected for synthetic gypsum (CaSO 4 ⋅2H 2 O) powder between 9 and 373 K under atmospheric pressure with special emphasis on the temperature dependence of the OH-stretching modes. The stretching bands of the water molecules in gypsum were found to shift in opposite directions as a result of the different degree of intermolecular hydrogen-bonding between nonequivalent water H atoms and the O atoms of nearby SO 4 ions. The anharmonic parameters of the OH-stretching modes are calculated using the temperature derivatives measured from the present investigation and existing pressure derivatives. These parameters are -4.7 × 10 -6 K -1 and -0.6 × 10 -6 K -1 for the 3407 and 3494 cm -1 bands, respectively. The dehydration of gypsum into γ-CaSO 4 and the subsequent rehydration of γ-CaSO 4 into hemihydrate are clearly identified in the Raman spectra by the observed variation in Raman shifts of the OH and ν 1 (SO 4 ) bands. The latter increases as the mineral becomes increasingly anhydrous (1007 cm -1 in gypsum; 1014 cm -1 in hemihydrate; 1026 cm -1 in γ-CaSO 4 ), which can be used as a fingerprint for the remote detection of these minerals on planetary surfaces.

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Section: Resultsmentioning

confidence: 99%

Micro-Raman studies of gypsum in the temperature range between 9 K and 373 K

Chio

Sharma

Muenow

2004

American Mineralogist

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“…Typically, the dehydration process is studied by thermogravimetry and the dehydration mechanism is determined from the rate law for water loss (see e.g. Ball and Norwood, 1969;Ball and Urie, 1970). Such methods have their shortcomings, due to problems such as the possible presence of intergranular and surface water, to the empirical nature of the correlation between rate law and reaction mechanism, and also due to the fact that thermogravimetry is a macroscopic technique which cannot determine the mechanism at a local level.…”

Section: Introductionmentioning

confidence: 99%

In situ IR spectroscopic and thermogravimetric study of the dehydration of gypsum

1990

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The dehydration of gypsum CaSO4.2H20 has been studied, at negligible water vapour pressure, by in situ infrared (IR) spectroscopy and by thermogravimetry to determine whether intermediate phases (CaSO4.nH20) exist, other than the hemihydrate with n=0.5, and also to compare the mechanism of the dehydration process when measured by two techniques with very different correlation lengths. Thermogravimetry shows an apparently continuous water loss with an activation energy of 90.3 kJ.mol -~, with no changes in the activation energy as a function of the degree of dehydration. IR spectroscopy on the other hand, clearly shows the existence of three discrete phases, gypsum CaSO4.2H20, hemihydrate CaSO4.0.5H20 and y-CaSO4, with nucleation of each successive phase as dehydration proceeds. There is no evidence to suggest the presence of phases with any intermediate water content.

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“…5). This might be due to a transition from gypsum to soluble anhydrite (CaSO 4 III or γ-CaSO 4 ), occurring under reduced vapor pressure [29][30] even at moderate temperatures [31]. Being strongly hygroscopic, this phase will rapidly rehydrate to β-hemihydrate during sample preparation before the XRD measurement.…”

Section: Temperature (°C)mentioning

confidence: 96%