Geometrical constraints of thermal dehydration of β-calcium sulfate hemihydrate induced by self-generated water vapor

Abstract

The thermal dehydration of calcium sulfate dihydrate exhibits complex reaction behavior, in which the reaction pathway and kinetics vary depending on water vapor pressure (p(H2O)) applied as the atmospheric condition...

An advanced kinetic approach to the multistep thermal dehydration of calcium sulfate dihydrate under different heating and water vapor conditions: kinetic deconvolution and universal isoconversional analyses

“…CS-HH intermediate, the specific shapes of the universal experimental master plots can be explained by the physicogeometrical consecutive process of the SR-PBR, as reported previously. 2,3,38 The magnitude relationship between A and A i values determined by fitting the universal experimental master plots using SB(m, n, p) was A o A 1 o A 2 (Table 2), the same as that observed for the E a and E a,i values indicating the apparent kinetic compensation behavior among the Arrhenius parameters for different thermal dehydration processes. [62][63][64][65][66][67][68][69] Assuming both the rate-limiting step and steady-state approximations in the universal kinetic formalism across different p(H 2 O) values, the slope and intercept of the universal Friedman plot are related to the temperature dependences of the rate constant and equilibrium constant in a complex manner connected with the exponents in the AF.…”

“…8(c)), but comparable with that observed for the thermal dehydration of the CS-HH reagent in a stream of dry N 2 gas and semi-closed system in our previous study. 38 The similar shape of the experimental master plots between the reactions at different p(H 2 O) values implied that the atmospheric and self-generated p(H 2 O) did not influence the geometrical reaction mechanism as expressed by f 2 (a 2 ) in the kinetic equation, characterized by the advancement of the reaction interface from the top surface of the sample bed to the bottom. 2).…”

“…Calcium sulfate dihydrate (CaSO 4 Á2H 2 O, CS-DH, 499.2%, Kanto Chem., Japan) and calcium sulfate hemihydrate (CaSO 4 Á(1/2)H 2 O, CS-HH, 497.8%, Wako Chem., Japan) were of reagent grade and were used in this study without any further purification and sieving. In our previous studies focusing on the effect of self-generated water vapor on the kinetics of thermal dehydration processes, 2,38 these samples were characterized by powder X-ray diffractometry (XRD) and Fourier-transform infrared spectroscopy (FT-IR). The thermal behaviors of these samples were investigated by simultaneous TG-differential thermal analysis (DTA) coupled with mass spectroscopy for the evolved gas and high-temperature XRD techniques.…”

“…39,40 On heating, the thermal dehydration resulting in g-CaSO 4 (hexagonal, S.G. = P6 2 22(180), a = b = 6.9900, c = 6.3400, a = b = 90.000, g = 120.000, ICDD PDF 01-073-1942) [41][42][43] occurred with the relative massloss value, agreeing with the calculated value (20.93%). The CS-HH sample also comprised columnar particles with the dimension comparable with those of the CS-DH sample, 38 but sub-micrometer sized particles were attached on their surfaces. The major component of the CS-HH sample was b-CaSO 4 Á(1/2)H 2 O (hexagonal, S.G. = P3 1 (144), a = b = 6.9314, c = 12.7362, a = b = 90.000, g = 120.000, ICDD PDF 01-076-6905), 44 with some detectable impurities.…”

“…[35][36][37] Preturlan et al 3 reported the universal kinetic description of the thermal dehydration of CS-DH to CS-HH under isothermal conditions at different p(H 2 O) values. Based on numerous kinetic studies on the thermal dehydration of CS-DH and CS-HH, as summarized in recent articles, 2,3,38 the present study aims to find the specific kinetic features of the component thermal dehydration steps of CS-DH to anhydride via the CS-HH intermediate by integrating the cutting-edge kinetic approach with the thermal decomposition of solids, including humiditycontrolled TG, KDA, and universal kinetic description under different heating and p(H 2 O) conditions. By comparing the kinetic results of individual thermal dehydration steps in the universal kinetic description, the specific kinetic feature of each thermal dehydration step was identified, and the kinetic explanation for the variation in the reaction pathway of thermal dehydration of CS-DH with p(H 2 O) was attempted.…”