Dehydration gypsum

It is usual, in -hemihydrate preparation by gypsum dehydration, to do a reversion because the fired products may contain some anhydrite 111. Dehydration products after a certain amount of time are placed in a wet atmosphere. Figure 7 shows variations of t an a function of conservation time in a wet atmosphere (81% relative humidity) for a P-hemihydrate, without any previous drying of the products. 

As gypsum dehydration is realized in quite reproducible conditions, the reactor may be used to determine with accuracy, the amount of gypsum contained in a plaster rock or an incompletely hydrated plaster. 

M. Carbone, P. Ballirano, and R. Caminiti, Kinetics of Gypsum Dehydration at Reduced Rressure An Energy Dispersive X-ray Diffraction Study, European Journal of Mineralogy, 20 (2008), 621-627. 

I. Gypsum—162 and 174 °C endotherms, gypsum dehydrates in stages 378 °C exotherm, CaS04 crystallizes and forms anhydrite 1217 °C endotherm, anhydrite undergoes a polymorphic transformation. 

Jowett E.C., Cathles 111 L.M. Davis B.W. 1993 - Predicting depths of gypsum dehydration in evaporitic sedimentary basin. AAPG Bull., 77, 3 402-413... 

The moisture given off by the gypsum dehydration to the atmosphere in the mill, as also the water which may be injected into the mill for cooling its charge during grinding, will react more particularly with the finest particles of the cement formed. As a result, the reactivity of the tricalcium aluminate (C3A) is in part substantially reduced, and if fairly large amounts of moisture are thus released into the mill, the... 

FIGURE 6.6 The differential distribution of the contact strength for the contact between two gypsum dehydrate crystals brought into edge-to-edge contact with a force,/=10 dyn, in solutions of calcium sulfate the various degrees of supersaturation, a, for the contact time t = 100 s (a) the various contact times, t, for supersaturation, a = 1.8 (b). The ordinate depicts the density of the distribution, p. 

Our model implies that the gap has a constant width, h. In order to assess the value of this width rigorously, one would need to analyze the thermodynamic factors, snch as the concentration, and the kinetic factors, such as the rate of diffusion in the gap. One can, however, get an estimate of h on the basis of the available experimental data. For a crude estimation, we can neglect particular values of 02, just assuming that Oj = 03 = <7 O2. From the expressions for b and c, and Vi = 1.2 x 10 cm, we find o 20 erg/cm for gypsum dehydrate at room temperature. Substitution into the equation for b yields h l nm (10 A). This value of h may be viewed as the mean width of the gap between the crystals at the point where the contact bridge is most likely to form. Near the contact zone, the surfaces of the crystals may not necessarily be parallel to each other. The values of o Oi at the nucleus/solution interface are in reasonable agreement with the experimental data reported for the nucleation in solutions, particularly for the nuclei of gypsum [16,54]. 

Similar results were also obtained in other systems, for example, in the case of the adsorption of ethanol on magnesium hydroxide. The adsorption of water is the reason for the reduced strength of all hydrophilic construction materials, such as cements and gypsum dehydrate. 

Isa and Oruno describe a method that enables identification of intermediates (in the gypsum dehydration process) more easily. The method involves the use of simultaneous TG-DTG-DTA under various sealed atmospheres corresponding to three systems—open completely, sealed, and quasi-sealed. Endothermic DTA peaks appear earlier (129 and 133°C) than the point of decreasing TG. This technique, resembling the quasi-isothermal and isobaric thermogravimetry (Q-TG), is superior to the latter in that it needs less of the sample. 

Section 8.0-A Three Step Gypsum Dehydration Process... 

Badens, E., P. Llewellyn, J. M. Fulconis, C. Jourdan, S. Veesler, R. Boistelle and F. Rouquerol (1998). Study of gypsum dehydration by controlled transformation rate thermal analysis (CtCTA) . Journal of Solid State Chemistry 139(1) 37-44. 

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