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

Authors: S. C. Hou; L. Xiang
Journal: Journal of Nanomaterials
 (2013)

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## 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.


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## Body

## 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.


## 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 Figures1 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: Dhkl=kλ/βcosθ, where Dhkl, 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.Morphology of raw material (a), calcination (b), and calcination-hydration (c) samples.
(a)
(b)
(c)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 Figure3. The BET and the agglomerated particle sizes were 4.75 m2·g−1and 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
(1)CaSO4·2H2O→150°C,6.0hcalcinationCaSO4·0.5H2O→25°C,1.0hhydrationCaSO4·2H2O(active)BET (a) and the agglomerated particle sizes (b) of the samples 1: commercial CaSO4·2H2O, 2: calcination sample, and 3: calcination-hydration sample.
(a)
(b)Hydrothermal formation of CaSO4·0.5H2O whiskers from active CaSO4·2H2O precursor.Figure4 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.Variation of [Ca2+] (a) and [SO42-] (b) with hydrothermal reaction time. Precursor: 1: commercial CaSO4·2H2O, 2: active CaSO4·2H2O.
(a)
(b)Figure5 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)).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.
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(e)
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(j)Figure6 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.


## 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 Figures1 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: Dhkl=kλ/βcosθ, where Dhkl, 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.Morphology of raw material (a), calcination (b), and calcination-hydration (c) samples.
(a)
(b)
(c)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 Figure3. The BET and the agglomerated particle sizes were 4.75 m2·g−1and 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
(1)CaSO4·2H2O→150°C,6.0hcalcinationCaSO4·0.5H2O→25°C,1.0hhydrationCaSO4·2H2O(active)BET (a) and the agglomerated particle sizes (b) of the samples 1: commercial CaSO4·2H2O, 2: calcination sample, and 3: calcination-hydration sample.
(a)
(b)Hydrothermal formation of CaSO4·0.5H2O whiskers from active CaSO4·2H2O precursor.Figure4 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.Variation of [Ca2+] (a) and [SO42-] (b) with hydrothermal reaction time. Precursor: 1: commercial CaSO4·2H2O, 2: active CaSO4·2H2O.
(a)
(b)Figure5 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)).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.
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(j)Figure6 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.



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*Source: 237828-2013-04-09.xml*