Universität zu Köln

Ritterbach, Laura (2021). Investigations on dehydration and rehydration processes in the CaSO4 – H2O system at controlled time, temperature and humidity conditions. PhD thesis, Universität zu Köln.

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Abstract

This thesis takes as its primary focus to reinvestigate the dehydration of gypsum (CaSO4 · 2H2O) at moderate to low temperatures with defined H2O partial pressures, and to provide a systematic examination of T− and pH2O-induced rehydration of hemihydrate (mineral bassanite, CaSO4 · 0.5H2O). Therefore, isothermal and isohumid powder XRD and Raman spectroscopic measurements were conducted. The isothermal treatment of gypsum over time ranges up to 60 h at temperatures ≤ 150 ◦C and relative humidities ≤ 5.5% (averagely), yielded the formation of hemihydrate and γ-anhydrite (γ-CaSO4). The process-dependence on particle sizes was evident. β-anhydrite (β-CaSO4, mineral anhydrite) was not found as dehydration product of these measurement series but formed over time ranges up to six months at 60 ◦C/11% RH and 80 ◦C/30% RH. Rehydration of hemihydrate to gypsum was investigated at temperatures ≤ 40 ◦C and relative humidities ≤ 95%. The great influence of applied sample humidities on the hydration rate became apparent. Fraction of conversion vs. time (α − t) plots were derived by quantitative phase analysis by the Rietveld method of several isothermal/ isohumid powder XRD measurements. If suitable, isokinetic and isoconversional kinetic analysis was performed and the applicability discussed. The dehydration process of gypsum to hemihydrate was found to differ with surrounding T and pH2O values. The greater part of the data could be fitted with the nucleation-controlled Johnson-Mehl-Avrami-Erefe’ev-Kolmogorov equation and the autocatalytic Prout-Tompkins model. The hydration process of hemihydrate seems more complex. Exemplary, a time-dependent hydration curve measured at 35 ◦C and 91% relative humidity was fitted with the power law and the 3D diffusion equation according to Ginstling-Brounshtein. Implication of the present findings for de- and rehydration reactions of calcium sulfate phases in the Atacama Desert, Chile, are debated.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Ritterbach, Laura laura.ritterbach@uni-koeln.de UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-536292
Date:	2021
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute of Geology and Mineralog
Subjects:	Earth sciences
Uncontrolled Keywords:	Keywords Language Calcium sulfate phases UNSPECIFIED Dehydration and rehydration processes UNSPECIFIED Solid-state reaction kinetics UNSPECIFIED
Date of oral exam:	11 June 2021
Referee:	Name Academic Title Becker-Bohaty, Petra Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/53629