Influence of Activity of CaSO₄·2H₂O on Hydrothermal Formation of CaSO₄·0.5H₂O Whiskers

Abstract

The influence of the activity of calcium sulfate dihydrate (CaSO₄·2H₂O) on the hydrothermal formation of CaSO₄·0.5H₂O whiskers was investigated in this paper, using commercial CaSO₄·2H₂O as the raw material. The experimental results indicated that the activity of CaSO₄·2H₂O was improved after calcination of the commercial CaSO₄·2H₂O at 150°C 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 4.75 m²⋅g⁻¹ to 19.12 m²⋅g⁻¹ and the grain sizes from 95 nm to 40 nm. The active CaSO₄·2H₂O produced by the calcination-hydration treatment favored the hydrothermal dissolution of CaSO₄·2H₂O, promoting the formation of hemihydrate calcium sulfate (CaSO₄·0.5H₂O) whiskers with high aspect ratios.

1. Introduction

The synthesis of calcium sulfate (CaSO₄) 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 [1–6].

CaSO₄ whiskers were usually prepared by hydrothermal formation of the CaSO₄·0.5H₂O whiskers from the CaSO₄·2H₂O precursor followed by calcination of the CaSO₄·0.5H₂O whiskers at elevated temperatures. Wang et al. prepared CaSO₄·0.5H₂O 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 CaSO₄·2H₂O precursor was essential for the formation of CaSO₄·0.5H₂O whiskers with small diameters and prepared CaSO₄·0.5H₂O whiskers with a diameter of 0.19 μm and an aspect ratio of 98 via hydrothermal conversion of the fine grinded gypsum with an agglomerated size smaller than 18.1 μm at 120°C [8]. Xu et al. prepared CaSO₄·0.5H₂O whiskers with a length of l00–750 μm and a diameter of 0.1–3 μm at 110–150°C from the desulfurization gypsum composed mainly of CaSO₄·2H₂O (93.45 wt%) and CaCO₃ (1.76 wt%) [9], using H₂SO₄ to change the CaCO₃ impurity to active CaSO₄·2H₂O. Yang et al. prepared calcium sulfate whiskers 50–450 μm by hydrothermal treatment of the desulfurization gypsum at 130°C for 1.0 h in the presence of K₂SO₄ [10]. It was noticed that most of the former work showed that the use of the active CaSO₄·2H₂O precursor promoted the hydrothermal formation of CaSO₄·0.5H₂O whiskers with high aspect ratios.

In this paper a facile calcination-hydration hydrothermal reaction method was developed to synthesize the active CaSO₄·2H₂O precursor from the commercial CaSO₄·2H₂O and to produce the CaSO₄·0.5H₂O whiskers with high aspect ratios at hydrothermal condition. The influences of calcination and hydration on the morphology and structure of CaSO₄·2H₂O precursor as well as on the morphology of the CaSO₄·0.5H₂O whiskers were studied.

2. Experimental

2.1. Experimental Procedure

Commercial CaSO₄·2H₂O with analytical grade was used as the raw material in the experiments. The CaSO₄·2H₂O was sintered at 150°C for 3.0–6.0 h, then mixed with deionized water to keep the weight ratio of the solid to water at 1.0–5.0 wt%. After being stirred (60 r·min⁻¹) at room temperature for 1.0 h, the suspension containing 1.0–5.0 wt% CaSO₄·2H₂O 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.

2.2. 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 Kα radiation (𝜆=1.54178Å). The agglomerated particle sizes of the samples were analyzed with the laser particle analyzer (Micro-plus, Germany). The soluble Ca²⁺ and SO₄²⁻ were analyzed by EDTA titration and barium chromate spectrophotometry (Model 722, Xiaoguang, China), respectively.

3. Results and Discussion

3.1. Formation of Active CaSO₄·2H₂O 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 CaSO₄·2H₂O raw material was composed of the irregular plates (a length of 1.5–20.0 μm and a width of 3.5–10.0 μm) and particles (a diameter of 0.5–5.5 μm). After the calcination treatment at 150°C for 6.0 h, the CaSO₄·2H₂O raw material was converted to the CaSO₄·0.5H₂O irregular rectangle planes with a length of 1.0–10.0 μm and a width of 0.2–3.0 μm. The hydration of the CaSO₄·0.5H₂O at room temperature led to the formation of CaSO₄·2H₂O irregular rectangle planes with a length of 1.0–5.0 μm and a width of 0.1–2.0 μm. The data in Figure 2 showed that the intensities of the XRD peaks in curve 𝑐 were weaker than those in curve 𝑎, revealing that the calcination-hydration treatment promoted the formation of CaSO₄·2H₂O 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 2𝜃=11.6° and the Scherrer equation: 𝐷_ℎ𝑘𝑙=𝑘𝜆/𝛽cos𝜃, where 𝐷_ℎ𝑘𝑙, 𝜆, 𝛽, and 𝐾 represent the grain size, the wavelength of the Cu Kα (1.54178 Å), the full width at half maximum (FWHM), and the Scherrer constant (𝐾=0.89), respectively.

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(b)

(c)

Figure 1 

Morphology of raw material (a), calcination (b), and calcination-hydration (c) samples.

Figure 2 

XRD patterns of raw material (1), calcination (2), and calcination-hydration (3) samples. Black square denotes CaSO4·2H2O, black circle denotes CaSO4·0.5H2O.

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 m²·g⁻¹ and 29.7 μm for the raw material, 13.37 m²·g⁻¹ and 15.5 μm for the calcination sample, and 19.12 m²·g⁻¹ and 15.1 μm for the hydration sample, revealing the increase of BET and the decrease of the agglomerated particle sizes of the samples after calcination and calcination-hydration treatment. The above work showed that the calcination-hydration treatment favored the activation of the CaSO₄·2H₂O precursor CaSO₄⋅2H₂O−−−−−−−−−→^150∘C,6.0h_calcinationCaSO₄⋅0.5H₂O_25∘C,1.0h−−−−−−−−→_hydrationCaSO₄⋅2H₂O(active) (1)

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(b)

Figure 3 

BET (a) and the agglomerated particle sizes (b) of the samples 1: commercial CaSO4·2H2O, 2: calcination sample, and 3: calcination-hydration sample.

Hydrothermal formation of CaSO₄·0.5H₂O whiskers from active CaSO₄·2H₂O precursor.

Figure 4 shows the variation of [Ca²⁺] and [SO₄²⁻] with hydrothermal reaction time. Compared with the commercial CaSO₄·2H₂O, the active CaSO₄·2H₂O produced by calcination-hydration treatment was easier to be dissolved at hydrothermal condition, so that [Ca²⁺] and [SO₄²⁻] in the active CaSO₄·2H₂O system were higher than those in the commercial CaSO₄·2H₂O system. The gradual increase of [Ca²⁺] and [SO₄²⁻] within 2.0–3.0 h indicated the faster dissolution of the CaSO₄·2H₂O than the precipitation of CaSO₄·0.5H₂O, while the decrease of [Ca²⁺] and [SO₄²⁻] in the later time revealed the faster precipitation of CaSO₄·0.5H₂O than the dissolution of CaSO₄·2H₂O.

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(b)

Figure 4 

Variation of [Ca2+] (a) and [SO42−] (b) with hydrothermal reaction time. Precursor: 1: commercial CaSO4·2H2O, 2: active CaSO4·2H2O.

Figure 5 shows the variation of the morphology of the samples with hydrothermal reaction time. The commercial CaSO₄·2H₂O was converted to CaSO₄·0.5H₂O whiskers after 2.0 h of hydrothermal treatment, while the active CaSO₄·2H₂O produced by calcination-hydration treatment was changed to CaSO₄·0.5H₂O 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 CaSO₄·0.5H₂O whiskers formed from the active CaSO₄·2H₂O were much thinner than those from the commercial CaSO₄·2H₂O. For example, after 4.0 h of hydrothermal reaction, the CaSO₄·0.5H₂O whiskers with a diameter of 1.0–5.0 μm, a length of 5–100 μm, and an aspect ratio of 20–80 were prepared from the commercial CaSO₄·2H₂O, while CaSO₄·0.5H₂O whiskers with a diameter of 0.1–0.5 μm, a length of 30–200 μm, and an aspect ratio of 270–400 were produced from the active CaSO₄·2H₂O precursor (Figures 5(e) and 5(j)).

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(b)

(c)

(d)

(e)

(f)

(g)

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Figure 5 

Variation of the morphology of the samples with hydrothermal reaction time Precursor: (a)–(e): commercial CaSO4·2H2O, (f)–(j): active CaSO4·2H2O; Time (h): (a), (f) 0.5; (b), (g) 1.0; (c), (h) 2.0; (d), (i) 3.0; (e), (j) 4.0.

Figure 6 shows the schematic drawing for the conversion of the commercial/active CaSO₄·2H₂O to CaSO₄·0.5 H₂O whiskers. The active CaSO₄·2H₂O precursor with small grain size and high BET was formed by calcination-hydration treatment, which accelerated the hydrothermal dissolution of CaSO₄·2H₂O and promoted the formation of CaSO₄·0.5H₂O whiskers with high aspect ratios.

Figure 6 

Schematic drawing for conversion of commercial/active CaSO4·2H2O to CaSO4·0.5H2O whiskers.

4. Conclusion

Active CaSO₄·2H₂O precursor improved the morphology of the CaSO₄·0.5H₂O whiskers. Active CaSO₄·2H₂O was produced by calcination of the commercial CaSO₄·2H₂O at 150°C for 6.0 h followed by hydration at room temperature for 1.0 h. The use of the active CaSO₄·2H₂O favored the hydrothermal dissolution of CaSO₄·2H₂O and the formation of CaSO₄·0.5H₂O whiskers with high aspect ratios, producing CaSO₄·0.5H₂O whiskers with a length of 30–200 μm, a diameter of 0.1–0.5 μm, and an aspect ratio of 270–400.

Acknowledgments

This work was supported by the National Science Foundation of China (no. 51234003 and no. 51174125) and National Hi-Tech Research and Development Program of China (863 Program, 2012AA061602).