Elsevier

Journal of Crystal Growth

Volume 550, 15 November 2020, 125885
Journal of Crystal Growth

α or β?-hemihydrates transformed from dihydrate calcium sulfate in a salt-mediated glycerol–water solution

https://doi.org/10.1016/j.jcrysgro.2020.125885Get rights and content

Highlights

  • The presence of α-HH and β-HH bulk crystal in a salt-mediated glycerol–water system was confirmed.

  • The difference between α-HH and β-HH was investigated under more comparable conditions.

  • The formation mechanism of the two polymorphs of HH was analyzed.

Abstract

α-and β-hemihydrate calcium sulfate (HH), the most important calcium sulfate phases, are usually achieved by autoclave hydrothermal reaction and calcination in air, respectively. Herein, we report an investigation on preparation of HH from dihydrate calcium sulfate(DH) in a salt-mediated glycerol-water solution. We found that both α-HH and β-HH in forms of apparently single crystals could be obtained at 110 ℃ only by tuning the concentrations of glycerol, Na2EDTA, and NaCl. TG-DSC, XRD, SEM, and Raman spectroscopy were used to identify and characterize the product crystals. The results indicate that the high relative supersaturation of HH could be the key factor determining the generation of β-HH crystal in solution. Glycerol and NaCl generally decrease the water activity, enhance the crystallization kinetics, and therefore allow the presence of a metastable HH phase (especially for compact α-HH bulk crystal). But too high concentration of glycerol (i.e., ≥70%) and NaCl (i.e., ≥0.2 M) would be favorable for β-HH crystallization with obviously defective crystal morphology due to the dramatically increased local relative supersaturation. Na2EDTA could retard the recrystallization but well regulate the morphology of HH bulk crystals.

Introduction

Calcium sulfate is the subject of much research due to its wide scope and large-scale of industrial applications [1], [2], [3]. It occurs in nature mainly in form of three different phases distinguished by the content of crystal water: dihydrate (DH, CaSO4·2H2O), hemihydrate (HH, CaSO4·0.5H2O), and anhydrite (AH, CaSO4). Well control of the phase and crystal morphology is essential for calcium sulfate to be facilely used with desired performance [4], [5]. In particular, hemihydrate, as one of the most extensively produced inorganic cementitious materials, has attracted considerable attention [6], [7]. Among the two polymorphs of calcium sulfate hemihydrate, α-HH that has higher mechanical strength and less water demand during hydration, has drawn much more interest in recent decade than β-HH with reverse properties [8], [9], [10].What determines their distinctive properties has been widely considered to be the differences in their crystal morphology, density, solubility and more specifically crystallite perfection [11], [12]. It is noted that the difference between these two forms of calcium sulfate, to much extent, is attributed to the crystallite perfection—α-HH is composed of well-formed single or bulk calcium sulfate crystals whereas β-HH is composed of fine aggregates. However, a fact that has often been neglected is that most of the comparisons about these differences are almost based on samples from different systems or synthetic methods. Usually, α-HH is prepared by heating DH in saturated steam under elevated temperature and pressure or in aqueous solutions in the presence of certain salts or additives under atmospheric pressure, while β-HH is often prepared by calcining DH at 110–180 °C under atmospheric pressure [13]. One can hardly draw a convincing conclusion when a comparison was based on the analysis of samples from totally different methods or systems. Thus, the crystal structures and transformations of these phases have been studied for decades but remain not fully understood [14], [15]. However, it is evident that there is an industrial demand for the design of efficient protocols for controlling the phase composition, size, and morphology of CaSO4 crystals.

The transformation of DH to α-HH or β-HH is determined by the reaction conditions in a complex manner involving temperature, pressure, and sometimes dissolved electrolytes or organics [16], [17]. Among them, alcohol-water solution method for preparing α-HH from DH has recently attracted more and more attentions. Meanwhile, a trace of organic and inorganic additives can be helpful for gaining product with desirable size and morphology. It has been confirmed that carboxylates like Na2EDTA can change crystal growth habits and regulate crystal morphology by chelating Ca2+ [18], [19]. Also, nonlattice cations (such as monovalent Na+ ions) can shorten the transition time as an accelerator and decrease the phase transfer temperature through tuning the water activity in DH-H2O system [20], [21]. As many noted, alcohol can decrease the water activity to achieve the transition of DH to HH at mild temperature and pressure as well. In particular, glycerol is relatively non-toxic and non-volatile compared with methanol, etc., thus its effect on HH was considered to be stable. Besides, glycerol-water solution has proved to be able to provide a relatively wide scope of metastable region for HH presence in aqueous solution [22]. However, few details about the phase transition and morphology of these two forms of HH in such a region have been discussed.

This paper provides relevant results of research where the transition of both α-HH and β-HH from DH was observed in a glycerol-water solution system and the effects of several factors, such as the concentrations of glycerol, Na2EDTA, and NaCl, on the phase transformation were discussed. The phenomena and discussion may help to enhance our understanding of the phase relationships between the two common polymorphs of calcium sulfate hemihydrate.

Section snippets

Materials

Calcium sulfate dihydrate (99.0 wt%) was purchased from Sinopharm Chemical Reagent Co. Ltd., (Shanghai, China). Ethylenediaminetetraacetic acid disodium salt (Na2EDTA, 99.0 wt%) was purchased from HuiHong Reagents Co., Ltd., (Hunan, China). NaCl (99.5 wt%) was purchased from Sailboat Reagents Co., Ltd, (Tianjin China). The glycerol–water solution was prepared by deionized water and glycerol (99.0 wt%), Tianjin Hengxing Chemical Reagent Co. Ltd., Tianjin, China). All the chemicals used in the

Results and discussion

In order to develop a facile method for α-HH synthesis under mild conditions, several researchers had previously tried to prepare HH from DH using glycerol solution as the phase transfer media in substitution of water [18], [22]. It was revealed that the transition process could involve three stages: dissolution of DH, nucleation, and crystal growth of HH. In pre-experiments, we also used NaCl and Na2EDTA as additives for this DH–glycerol–water system to regulate the phase transfer kinetics and

Conclusions

We have demonstrated the presence of the two types of HH phases with apparently single crystalline structure but different crystalline defects (i.e., compact α-HH and defective β-HH) in a DH–glycerol–water system by simply tuning the concentrations of glycerol, Na2EDTA, and NaCl. The generation of β-HH crystal instead of α-HH crystal could be mainly attributed to the increase of the local relative supersaturation of HH. High concentration of glycerol (i.e., above 70%) would provide a high

CRediT authorship contribution statement

Shishi Yin: Methodology, Software, Data curation, Writing - original draft, Visualization, Investigation, Validation. Liuchun Yang: Conceptualization, Writing - review & editing, Supervision, Resources, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Authors are thankful to the College of Chemistry of Xiangtan University for analytical laboratory instruments used in this study and the repeated guidance from the tutor. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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