Metastability Of α-calcium Sulfate Hemihydrate And Crystallization In Alcohols-water Solution

China is abundant in gypsum resource (calcium sulfate dihydrate, CaSO4.2H2O, DH) with a reserve of 57 billion tons, and 90 million tons of them are mined each year. Meantime, quantities of chemical gypsum waste are produced, such as the fuel gas desulfurization (FGD) gypsum (90 wt% DH),70-80 million tons of which were produced every year. Besides DH, gypsum also exists in the phases of hemihydrate (CaSO4.0.5H2O, HH, α or β) and anhydrite (CaSO4, AH). Contrary to the low value of DH and AH, a-HH owns high mechanic strength as a binding material, and superior biocompatibility and biodegradability, and has gained more acceptance in modern building materials, molding and even bone remedy. Accordingly, the preparation of a-HH from DH shows a great economic benefits and market prospect to improve the grade and value of the gypsum resource and promote the high-value-added utilization of chemical gypsum waste. Based on the study on metastability and crystallization theory of a-HH, a novel approach was invented to prepare α-HH from DH in glycerol-water solution. Simultaneously, monodisperse a-HH nanoparticles were synthesized. The achievements shows promising to develop high-quality gypsum products for application in a more widely and advanced field.a-HH is a metastable mineral and can only maintain its phase unchanged in the aqueous solution for a period and then transit into the thermodynamically stable phase of DH or AH. Here we define a new parameter of metastable lifetime (MLT) to characterize the period and describe the metastablity of a-HH in a quantitative way. MLTs in CaCl2 solution were found to decrease with increasing temperature and CaCl2 concentration. The control of the total interfacial Gibbs free energy (△Gs) required for the transition of α-HH to AH is an effective way to tune MLT of a-HH. The results provide key parameters to control the transition of DH to α-HH, and establish new methods to study metastabilty of other materials.In an aqueous solution, water activity is the only determinant of the feasibility of the transition of DH to a-HH, and a lower water activity is thermodynamically required for the transition. The alcohols aqueous solution has a low water activity and the successful transitions in methanol and glycerol aqueous solutions confirm that the alcohols aqueous solution can serve as the reaction medium for a-HH preparation from DH. Thermodynamic preparation window of a-HH in glycerol-water solution was then drawn, which demonstrates the phase-transition rule of DH-a-HH in glycerol-water solution and help the selection of solution condition that can ensure the a-HH to be the target product. This work establishes a novel wet-chemical approach for a-HH synthesis from DH in glycerol-water solution. In this approach, the only use of glycerol and water in the reaction medium benefits a-HH synthesis with superior biocompatibility and purity.In glycerol-water solution, the transition is kinetically unfavorable, which usually takes 510 min or longer. Addition of small amount of non-lattice cations (Mb+:K+, NH4+, Na+, Li+,Mg2+, Zn2+, Cu2+ and Mn2+) can significantly shorten the time to 35min. The solution composition analysis indicates that most of SO42- in solution are paired with Mb+, resulting in an ion pair-directed nucleation of a-HH. By ion pairing, the SO42- concentration in solution is greatly increased, constructing a larger supersaturation and accelerating the transition. However, SO42- should be released from ion pair before participating the a-HH nucleation, the process of which increases the energy barrier for α-HH nucleation and slows down the transition. A strong pairing capability generates a higher supersaturation but increases the difficulty in the release of SO42-. The best accelerator should have the suitable pairing capability, which optimizes its effect on the supersaturation and the release of SO42-. The limiting diffusion coefficient (D0) can serve as an indicator of Mb+ efficiency inregulation of nucleation rate, and the optimized (D0) are 1.69×10-5 cm2 s-1 for M2+ and 3.58×10-5 cm2 s-1 for M+. One with a D0 close to the optimal value can acquire a fast transition.In glycerol-water solution, monodisperse α-HH nanoellipsoids with a length of 600 nm and a width of 300 nm were synthesized through a Na2EDTA-mediated self-assembly of small primary building blocks (α-HH domains:-48 nm). Study on morphology evolution of α-HH reveals that controlled synergy of supersaturation and Na2EDTA is crucial for the development of α-HH into nanoellipsoid. Supersaturation controls the size and number of α-HH domains. A larger supersaturation generates larger number of α-HH domains, which may overload the capability of Na2EDTA to assemble them well, leading to a worse monodispersity. Na2EDTA regulates the supersaturaton through chelating the Ca2+, mediates the self-assembly of a-HH domains and simultaneously directs the orientation of crystal growth. Na2EDTA also serves as the capping agent to stabilize the a-HH nanoellipsoids. Considering the superior biocompatibility and biodegradability of a-HH, these monodisperse a-HH nanoellipsoids shows very promising applications in biomedicine field.