Solution-Mediated Transformation Kinetics of Calcium Sulfate Dihydrate to α-Calcium Sulfate Hemihydrate in CaCl₂ Solutions at Elevated Temperature

Abstract

α-Calcium sulfate hemihydrate (α-HH), a kind of advanced cementitious material, can be prepared from calcium sulfate dihydrate (DH) in electrolyte solutions. The kinetics of the DH−α-HH transformation in CaCl 2 solutions was investigated to better understand and, hence, control the transformation process. The results showed that the DH−α-HH transformation is a nucleation−growth limited process, following a dispersive kinetic model. The α-HH nucleation and growth were both promoted significantly with the increment of CaCl 2 concentration and temperature. This is due to the enlarged activation entropy change and supersaturation, which were caused by the decreasing water activity and increasing solubility product ratio (K sp,DH /K sp,HH ), respectively. The rate of DH−α-HH transformation in aqueous solutions can be effectively controlled by regulating water activity and temperature. ■ INTRODUCTIONLarge quantities of flue gas desulfurization (FGD) gypsum produced by coal-burning power plants have continuously raised significant environmental concerns. 1,2 The recycling and utilization of these gypsums become rather important for environmental protection and gypsum industry. FGD gypsum is composed mostly by calcium sulfate dihydrate (DH), 3 which can be transformed into α-calcium sulfate hemihydrate (α-HH) in the presence of inorganic or organic additives in aqueous solutions. 3−6 Hence, the preparation of α-HH from FGD gypsum could be a potential alternative for FGD gypsum utilization 3,7 owing to the superior physical property of α-HH. 8 DH dissolution, α-HH nucleation, and growth constitute the solution mediated transformation. 7,9,10 Four principal transformation scenarios are classified on the basis of the solid and liquid component evolution, including dissolution control, 11 growth control, 12 nucleation−dissolution control, 13 and nucleation−growth control. 14 The identification of the limiting step is essential for the control of the DH transformation to α-HH.The DH−α-HH conversion is driven by their solubility difference. According to dissolution equilibrium, solubility product and water activity are two important factors affecting the solubility, which depends heavily on temperature and stock solution. 15−18 For a given temperature, there exists a critical water activity at which both DH and α-HH would reach dissolution equilibrium (i.e., having the same solubility). 19,20 Low water activity makes DH prone to dehydration, attempting to increase availability of the free water molecules. 21 The increment of temperature or acidity (hydrochloric acid concentration) could enlarge the solubility difference between DH and α-HH and, hence, accelerate the conversion. 6,21 However, the underlying relationship between the driving force and operational factors (i.e., electrolyte concentration and temperature) is still not clear.The transformation among calcium sulfate phases versus time generally presents a sigmoidal profile. 8,22,23 Many kinetic models have been proposed to describe the S-shaped transformation curve. ...

Calcium Sulfate Hemihydrate Whiskers Obtained from Flue Gas Desulfurization Gypsum and Used for the Adsorption Removal of Lead

“…This theory supports that supersaturation is the main impetus for α-CSH nucleation and growth. Fu et al revealed that temperature and water activity are important factors affecting the solubility difference [40]. Based on previous research [41], the relationship between solubility and temperature has been verified.…”

Crystallization Kinetics of α-Hemihydrate Gypsum Prepared by Hydrothermal Method in Atmospheric Salt Solution Medium

“…At 98 • C, they were reduced to 1h and 1.5 h, respectively. The reason can be attributed to the increase in the solubility of DH with increasing temperature, resulting in more nucleation centers at higher temperatures [27].…”

“…In the field of crystallization, XRD data is often used to determine the degree of crystallinity. The reaction process can be determined from Raman data [24], thermogravimetric analysis (TGA) [25][26][27], pH value [28], X-ray diffraction (XRD) [29], and atomic absorption spectroscopy (AAS) [28]. Then the empirical data can be used to fit the known dynamic model to understand and predict the appropriate parameters of the control mechanism.…”

Preparation of High Percentage <i>α</i>-Calcium Sulfate Hemihydrate via a Hydrothermal Method

“…α-HH is a metastable phase, which can only maintain its form in a certain period in aqueous solution before further transit to a relatively stable phase. It is well accepted that at a given temperature solid phase that has the lowest solubility in aqueous solution is the stable phase [19] [20] [21]. Daniela et al pointed that AH was the stable phase at temperatures higher than 70˚C in CaSO 4 -based salts because of its low solubility [16].…”

“…In the past decades, numerous investigations have been carried out on the dissolution-recrystallization process of DH and HH [15] [18] [19]. In the hydrothermal method to prepare α-HH, the transformation time and temperature have been considered as very important parameters for purity control and dehydration of α-HH.…”