Contribution
Hydrationswärmen von α-Calciumsulfaten
[Heats of hydration of the α-calcium sulfates]
Kuzel, H.-J.
Neues Jahrbuch für Mineralogie - Abhandlungen Band 156 Heft 2 (1987), p. 155 - 174
22 références bibliographiques
publié: Mar 27, 1987
DOI: 10.1127/njma/156/1987/155
ArtNo. ESP154015602003, Prix: 29.00 €
Kurzfassung
Die Hydratationswärmen der metastabilen α-Calciumsulfate wurden bei 295 K unter isoperibolen Bedingungen mit einem Wärmeleitungskalorimeter gemessen. Für die Reaktionen mit H2O(Π) zu Dihydrat ergaben sich die folgenden ΔH295-Werte: CaSO4 • 0.53H2O - 17709±18 J/Mol, CaSO4 • 0,62H2O - 16981±11 J/Mol und Ca SO4-0,03H2O (AIII) - 28001±33 J/Mol. Der Wärmeumsatz der Hydratation von CaSO4 • 0,03H2O mit H2O(Π) zu Halbhydrat CaSO4 • 0,62H2O wurde bei H2O-Transport über die Gasphase im geschlossenen System zu -11000±27 J/Mol bestimmt. Aus dem in sich geschlossenen Datensatz lassen sich die Enthalpien für weitere Hydratationsreaktionen berechnen. Der H2O-Gehalt von Halbhydrat ist in Abhängigkeit vom Wasserdampfpartialdruck variabel. Im Dampfdruckbereich PH2O/P0 = 0,35 bis 0,92 sind die Einbaugrenzen durch die Zusammensetzungen CaSO4 • 0,53H2O und CaSO4 • 0,62H2O beschrieben. Die Hydratation von AIII zu Halbhydrat erfolgt wahrscheinlich nicht durch kontinuierliche Auffüllung des Kanalwassers in der Kristallstruktur, sondern durch unmittelbare Reaktion, ohne daß Zwischenstufen durchlaufen werden.
Abstract
α-calcium sulfate hemihydrate has been prepared by dehydration of reagent quality CaSO4 • 2H2O in concentrated HNO3 at 36±3°. After rehydration at 35 per cent relative humidity the composition of the phase is CaSO4 • 0.53H2O. The crystals are monoclinic with space group I2. It is shown, that the composition of hemihydrate depends on the water vapour pressure. The amount of structural water in the channels increases from CaSO4 • 0.53H2O at PH2O/P0 = 0.35 to a limiting composition of CaSO4 • 0.62H2O at PH2O/P0 = 0.92. P0 is the water vapour pressure at sample temperature. CaSO4 • 0.62H2O has trigonal symmetry with the space group P31121. Soluble anhydrite (AIII) of composition CaSO4 • 0.03H2O has been prepared by dehydration of hemihydrate for 4 days at 54±4° in a vacuum of 1 • 10-4 bar. Samples of hemihydrate and AIII prepared in this manner always contained less than 0.3 per cent of CaSO4 • 2H2O and less than 0.5 per cent insoluble anhydrite (AII). The heats of hydration of the metastable α-calcium sulfates have been determined in a computer controlled conduction-type isoperibol calorimeter equipped with BiTe solid state elements as thermal fluxmeters at 295 K. The calibration factor of the calorimeter at this temperature was determined to be K(T) = 91.50 ±0.01 /μV/mW. ΔH-determinations with liquid phase hydration of the samples were performed with a sample mass of about 0.6 g and with a water/solid-ratio of 1.5. By direct hydration with H2O(liq) the following heats of reaction have been measured: CaSO4 • 0,53H2O ΔH295 = -17709±18 J/mole CaSO4 • 0,63H2O ΔH295 = -16981±11 J/mole CaSO4 • 0,03H2O (AIII) ΔH295 = -28001±33 J/mole The heat evolved in the reaction of AIII with water vapour to calcium sulfate hemihydrate CaSO4 • 0.62H2O in the closed system is CaSO4 • 0,03H2O (AIII) ΔH295 = -11000±27 J/mole From these values the heats evolved in the transition of monoclinic CaSO4 • 0.53H2O to trigonal CaSO4 • 0.62H2O and in the hydration of soluble anhydrite CaSO4 • 0.O3H2O to hemihydrate CaSO4 • 0.53H2O were calculated to be -728 J/mole and -10272J/mole, respectively. Measured heat flux curves for the hydration reactions are given and the mechanisms which govern the reactions are discussed. Evidence is presented that the hydration of CaSO4 • 0.O3H2O to hemihydrate does not occur by continuous increase of the amount of channel water in the structure but by direct hydration to CaSO4 • 0.53H2O.
Mots-clefs
α-calcium sulfates • heats of hydration • calorimetry