Influence of Activity of CaSO 4 ⋅ 2 H 2 O on Hydrothermal Formation of CaSO 4 ⋅ 0 . 5 H 2 O Whiskers

The influence of the activity of calcium sulfate dihydrate (CaSO 4 ⋅2H 2 O) on the hydrothermal formation of CaSO 4 ⋅0.5H 2 Owhiskers was investigated in this paper, using commercial CaSO 4 ⋅2H 2 O as the raw material. The experimental results indicated that the activity of CaSO 4 ⋅2H 2 Owas improved after calcination of the commercial CaSO 4 ⋅2H 2 O at 150C for 6.0 h followed by hydration at room temperature for 1.0 h, corresponding to the decrease of the agglomerated particle sizes from 29.7 μm to 15.1 μm, the increase of the specific surface areas (BET) from 4.75m ⋅g to 19.12m ⋅g and the grain sizes from 95 nm to 40 nm.The active CaSO 4 ⋅2H 2 O produced by the calcination-hydration treatment favored the hydrothermal dissolution of CaSO 4 ⋅2H 2 O, promoting the formation of hemihydrate calcium sulfate (CaSO 4 ⋅0.5H 2 O) whiskers with high aspect ratios.


Introduction
The synthesis of calcium sulfate (CaSO 4 ) whiskers with high aspect ratios and homogeneous morphology has drawn much attention in recent years since they can be used as the reinforcing materials in many fields as plastics, ceramics and paper making, and so forth [1][2][3][4][5][6].
CaSO 4 whiskers were usually prepared by hydrothermal formation of the CaSO 4 ⋅0.5H 2 O whiskers from the CaSO 4 ⋅2H 2 O precursor followed by calcination of the CaSO 4 ⋅0.5H 2 O whiskers at elevated temperatures.Wang et al. prepared CaSO 4 ⋅0.5H 2 O whiskers with an aspect ratio of 5-20 at 115 ∘ C, using the natural gypsum as the reactant [7].Wang et al. found that the use of the superfine CaSO 4 ⋅2H 2 O precursor was essential for the formation of CaSO 4 ⋅0.5H 2 O whiskers with small diameters and prepared CaSO 4 ⋅0.5H 2 O whiskers with a diameter of 0.19 m and an aspect ratio of 98 via hydrothermal conversion of the fine grinded gypsum with an agglomerated size smaller than 18.

Experimental
2.1.Experimental Procedure.Commercial CaSO 4 ⋅2H 2 O with analytical grade was used as the raw material in the experiments.The CaSO 4 ⋅2H 2 O was sintered at 150 ∘ C for 3.0-6.0h, then mixed with deionized water to keep the weight ratio of the solid to water at 1.0-5.0wt%.After being stirred (60 r⋅min −1 ) at room temperature for 1.0 h, the suspension containing 1.0-5.0wt% CaSO 4 ⋅2H 2 O was then treated in an autoclave at 135 ∘ C for 4.0 h.After hydrothermal treatment, the suspension was filtrated and dried at 105 ∘ C for 6.0 h.

Characterization.
The morphology of the samples was detected with the field emission scanning electron microscope (JSM 7401F, JEOL, Japan).The structures of the samples were identified by powder X-ray diffractometer (D8 advanced, Brucker, Germany) using Cu K radiation ( = 1.54178Å).The agglomerated particle sizes of the samples were analyzed with the laser particle analyzer (Micro-plus, Germany).The soluble Ca 2+ and SO 4 2− were analyzed by EDTA titration and barium chromate spectrophotometry (Model 722, Xiaoguang, China), respectively.2 showed that the intensities of the XRD peaks in curve  were weaker than those in curve , revealing that the calcination-hydration treatment promoted the formation of CaSO 4 ⋅2H 2 O with poor crystallinity.The grain sizes of the raw material, the calcination sample, and the hydration sample were estimated as 94.9 nm, 37.5 nm and 39.5 nm, respectively, based on the (020) peaks located at 2 = 11.6 ∘ and the Scherrer equation:  ℎ = / cos , where  ℎ , , , and  represent the grain size, the wavelength of the Cu K (1.54178 Å), the full width at half maximum (FWHM), and the Scherrer constant ( = 0.89), respectively.

Formation of Active CaSO
The BET and the agglomerated particle sizes of the raw material, the calcination sample and the hydration sample, are shown in Figure 3.The BET and the agglomerated particle sizes were 4.75 m 2 ⋅g −1 and 29.7 m for the raw material, 13.37 m 2 ⋅g −1 and 15.5 m for the calcination sample, and 19.12 m 2 ⋅g −1 and 15.1 m for the hydration sample, revealing the increase of BET and the decrease of the agglomerated particle sizes of the samples after calcination and calcinationhydration treatment.The above work showed that the calcination-hydration treatment favored the activation of the CaSO

Conclusion
Active
4 ⋅2H 2 O via Calcination-Hydration Route.The morphology and XRD patterns of the raw material (a), the calcination sample (b), and the hydration sample (c) are shown in Figures 1 and 2, respectively.The CaSO 4 ⋅2H 2 O raw material was composed of the irregular plates (a length of 1.5-20.0m and a width of 3.5-10.0m) and particles (a diameter of 0.5-5.5 m).After the calcination treatment at 150 ∘ C for 6.0 h, the CaSO 4 ⋅2H 2 O raw material was converted to the CaSO 4 ⋅0.5H 2 O irregular rectangle planes with a length of 1.0-10.0m and a width of 0.2-3.0m.The hydration of the CaSO 4 ⋅0.5H 2 O at room temperature led to the formation of CaSO 4 ⋅2H 2 O irregular rectangle planes with a length of 1.0-5.0m and a width of 0.1-2.0m.The data in Figure

Figure 5
shows the variation of the morphology of the samples with hydrothermal reaction time.The commercial CaSO 4 ⋅2H 2 O was converted to CaSO 4 ⋅0.5H 2 O whiskers after 2.0 h of hydrothermal treatment, while the active CaSO 4 ⋅2H 2 O produced by calcination-hydration treatment was changed to CaSO 4 ⋅0.5H 2 O whiskers after 1.0 h of hydrothermal reaction owing to the acceleration of the hydrothermal dissolution-precipitation process.It was also noticed that the diameters of the CaSO 4 ⋅0.5H 2 O whiskers formed from the active CaSO 4 ⋅2H 2 O were much thinner than those from the commercial CaSO 4 ⋅2H 2 O.For example, after 4.0 h of hydrothermal reaction, the CaSO 4 ⋅0.5H 2 O whiskers with a diameter of 1.0-5.0m, a length of 5-100 m, and an aspect ratio of 20-80 were prepared from the commercial CaSO 4 ⋅2H 2 O, while CaSO 4 ⋅0.5H 2 O whiskers with a diameter of 0.1-0.5 m, a length of 30-200 m, and an aspect ratio of 270-400 were produced from the active CaSO 4 ⋅2H 2 O precursor (Figures 5(e) and 5(j)).
1 m at 120 ∘ C [8]. Xu et al. prepared CaSO 4 ⋅0.5H 2 O whiskers with a length of l00-750 m and a diameter of 0.1-3 m at 110-150 ∘ C from the desulfurization gypsum composed mainly of CaSO 4 ⋅2H 2 O (93.45 wt%) and CaCO 3 (1.76 wt%) [9], using H 2 SO 4 to change the CaCO 3 impurity to active CaSO 4 ⋅2H 2 O. Yang et al. prepared calcium sulfate whiskers 50-450 m by hydrothermal treatment of the desulfurization gypsum at 130 ∘ C for 1.0 h in the presence of K 2 SO 4 [10].It was noticed that most of the former work showed that the use of the active CaSO 4 ⋅2H 2 O precursor promoted the hydrothermal formation of CaSO 4 ⋅0.5H 2 O whiskers with high aspect ratios.In this paper a facile calcination-hydration hydrothermal reaction method was developed to synthesize the active CaSO 4 ⋅2H 2 O precursor from the commercial CaSO 4 ⋅2H 2 O and to produce the CaSO 4 ⋅0.5H 2 O whiskers with high aspect ratios at hydrothermal condition.The influences of calcination and hydration on the morphology and structure of CaSO 4 ⋅2H 2 O precursor as well as on the morphology of the CaSO 4 ⋅0.5H 2 O whiskers were studied.
CaSO 4 ⋅2H 2 O precursor improved the morphology of the CaSO 4 ⋅0.5H 2 O whiskers.Active CaSO 4 ⋅2H 2 O was produced by calcination of the commercial CaSO 4 ⋅2H 2 O at 150 ∘ C for 6.0 h followed by hydration at room temperature for 1.0 h.The use of the active CaSO 4 ⋅2H 2 O favored the hydrothermal dissolution of CaSO 4 ⋅2H 2 O and the formation of CaSO 4 ⋅0.5H 2 O whiskers with high aspect ratios, producing CaSO 4 ⋅0.5H 2 O whiskers with a length of 30-200 m, a diameter of 0.1-0.5 m, and an aspect ratio of 270-400.