Influence of the Process Parameters on the Formation of CaSO4·0.5H2O Whiskers

This paper discussed the influence of the process parameters such as the temperature, the mixing ways, and the molar ratios of the reactants on the morphology of the CaSO 4 ⋅2H 2 O precursors and the CaSO 4 ⋅0.5H 2 O whiskers. The experimental results indicated that CaSO 4 ⋅0.5H 2 O whiskers with a length of 80–310 μm and a width of 0.8–8.0 μm were produced at hydrothermal condition, using CaSO 4 ⋅2H 2 O fine particles as the precursors which were formed by adding Na 2 SO 4 solution into CaCl 2 solution at 25C at the molar ratio of Na 2 SO 4 to CaCl 2 being 0.5 : 1. A lower supersaturation and a higher [Ca2+]/[SO4 2−] molar ratio favored the formation of CaSO 4 ⋅2H 2 O particles with small sizes and the hydrothermal synthesis of CaSO 4 ⋅0.5H 2 O whiskers with high aspect ratios.


Introduction
As a kind of environmental friendly material, the calcium sulfate whisker is widely used as the reinforcing material in many fields such as plastics, ceramics, papers, and cements, owing to its high tensile strength, good electronic isolation, and perfect stability at high temperature or in acidic/alkaline media [1][2][3].Many methods have been developed to synthesize calcium sulfate whiskers, including the hydrothermal route, the normal acidic synthesis route, the microemulsion route, and the microwave route, [4][5][6][7][8][9].The hydrothermal method was getting more and more attention owing to the regular morphology of the product, the moderate condition, and the adjustable process parameters [10,11].
This paper investigated the possibility of the synthesizing of CaSO 4 ⋅0.5H 2 O whiskers via coprecipitation at room temperature followed by hydrothermal treatment, using Na 2 SO 4 and CaCl 2 as the raw materials.The influences of the process parameters for the formation of CaSO 4 ⋅2H 2 O precursors such as the temperature, the molar ratio, and the mixing ways of the raw materials on the morphology of the hydrothermal products were investigated, and the optimized synthesizing condition was suggested.to Na 2 SO 4 was kept as 0.1-10 : 1, and the dripping speed was 3 mL⋅min −1 .The suspension was stirred for 1 h after the mixing of the raw materials, then transferred to a small stainless steel autoclave with an inner volume of 80 mL, heated (3 ∘ C⋅min −1 ) to 120-200 ∘ C, and kept in isothermal condition for 1.0-8.0h.After hydrothermal treatment, the product was cooled to room temperature naturally, filtrated, washed with distilled water and dried at 105 ∘ C for 2 h.

Analysis Method.
The morphology of the products was observed by using the field emission scanning electron microscope (FSEM, JSM 7401F, JEOL, Japan).The composition of the products was characterized by X-ray powder diffractometer (XRD, D/Max2500, Rigaku, Japan), using CuK ( = 1.54148Å).The concentrations of Ca 2+ and SO 4 2− were analyzed by EDTA titration and barium chromate

Results and Discussion
3.1.Influence of Temperature.Figure 1 shows the morphology of the precursors and the hydrothermal products obtained by adding Na 2 SO 4 solution into CaCl 2 solution at different temperatures (25 ∘ C, 60 ∘ C, and 90 ∘ C). Figure 2 shows the XRD patterns of the precursor and the hydrothermal product obtained by mixing the reactants at 25 ∘ C. The data in Figure 1 indicated that the precursors were plate-like particles.The plates formed at 25 ∘ C, 60 ∘ C, and 90 ∘ C were with a length of 3-45 m, 8-80 m, and 12-150 m and a width of 0.8-15 m, 1.5-28 m, and 1.5-45 m, respectively.The increase of temperature led to the increase of the precursor sizes.The data in Figure 1 also showed that the morphology of the hydrothermal products was connected closely with the formation temperature of the precursors.The hydrothermal products were uniform whiskers with lengths of 80-310 m and diameters of 0.8-8 m if the precursor was prepared at 25 ∘ C. Being composed of the mixtures of the whiskers and the rod-like particles, the hydrothermal products with a length of 30-240 m, 30-310 m and a diameter of 1.5-28 m, 4.5-22 m were formed using the precursors formed at 60 ∘ C and 90 ∘ C, respectively.The hydrothermal products became more ununiform and thicker as increases the precursor temperature, from 25 ∘ C to 90 ∘ C.
The data in Figure 2    The change of the morphology of precursor with temperature may be connected with the varying super-saturations at different temperatures.Super-saturation () is defined as follows: where  is the super-saturation, [Ca 2+ ] and [SO 4 2− ] are the practical concentrations of the soluble Ca 2+ and SO 4 2− , respectively, and  sp is the equilibrium constant for the dissolution of CaSO 4 ⋅2H 2 O, which can be calculated out from the HSC software [12].
The influence of temperature on the super-saturation is listed in Table 1 at the mixing time of 1 minute.It was supposed that no CaSO 4 ⋅2H 2 O precipitate was formed within 1 minute of mixing time.
The data in Table 1 showed that the increase of temperature led to the decrease of the  sp of CaSO 4 ⋅2H 2 O. Therefore, the super-saturation at high temperature is bigger than the super-saturation at low temperature.According to the crystallinity theory, the super-saturation is connected closely with the growth rate of the crystals.The bigger the super-saturation, the faster the nucleation rate and the growth rate.It is known from Figure 1 that smaller precursors were formed at lower temperature, indicating that the formation of the CaSO 4 ⋅2H 2 O precursor may be connected mainly with the growth of the crystals: a lower temperature led to a smaller super-saturation and a slower growth rate of crystals, which favored the formation of CaSO 4 ⋅2H 2 O precursors with smaller particles as well as the formation of uniform CaSO 4 ⋅0.5H 2 O whiskers at hydrothermal condition.Therefore, compared with 60 ∘ C and 90 ∘ C, 25 ∘ C was more favorable for the formation of CaSO 4 ⋅2H 2 O precursors with small sizes.

Influence of the Mixing Ways of the Reactants.
The influences of the mixing ways of the reactants at 25 ∘ C on the morphology of the precursors and the hydrothermal products are shown in Figure 3.
The particle sizes of the plate-like precursors connected closely with the mixing ways of the reactants.The lengths of the plates were 15-140 m, 3-45 m, and 10-60 m and the widths of the particles were 1.5-35 m, 0.8-15 m, and 1.5-30 m in the cases of the following mixing ways of the reactants: adding CaCl 2 to Na 2 SO 4 , adding Na 2 SO 4 to CaCl 2 and adding Na 2 SO 4 and CaCl 2 , simultaneously, respectively.The particle sizes of the precursors formed by adding Na 2 SO 4 to CaCl 2 were much smaller than those formed by adding CaCl 2 to Na 2 SO 4 .The aspect ratios of the corresponding hydrothermal products-CaSO 4 ⋅0.5H 2 O whiskers changed with the precursor sizes.The smaller the particle sizes of the precursors, the higher the aspect ratios of the whiskers.CaSO 4 ⋅0.5H 2 O whiskers with a length of 80-310 m and a diameter of 0.8-8.0m were formed at hydrothermal condition using the precursor formed by adding Na 2 SO 4 to CaCl 2 .
The data in Figures 3 and 4(a) indicated that all of the practical super-saturations of the solutions were much bigger than  sp , which favored the quick nucleation and crystal growth.The super-saturations in line a were bigger than those in line b, and the particle sizes of the corresponding precursors in line a were also bigger than those in line b, indicating that a higher super-saturation system favored the formation of bigger precursors.Compared with the case of  2− ] favored the formation of the CaSO 4 ⋅2H 2 O with small particles.The above phenomena indicated that the particle sizes of the precursors were influenced by both the super-saturation and the value of [Ca 2+ ]/[SO 4 2− ].Adding Na 2 SO 4 to CaCl 2 was considered to be a suitable mixing way of reactants for the formation of fine precursor and the whiskers with higher aspect ratios.

Influence of the Molar Ratio of the Reactants.
The influence of the molar ratios of the reactants on the morphology of the precursors and the hydrothermal products is shown in Figure 5.The precursors were prepared by adding Na 2 SO 4 to CaCl 2 at 25 ∘ C.

Conclusion
CaSO 4 ⋅0.5H 2 O whiskers were formed via co-precipitation at room temperature followed by hydrothermal treatment, using CaCl 2 and Na 2 SO 4 as the raw materials.The particle sizes of the CaSO 4 ⋅2H 2 O precursors formed at room temperature connected closely with the process parameters as temperature, the mixing way and the molar ratios of the reactants.The CaSO 4 ⋅2H 2 O precursors with small particle sizes can be formed at the following condition: 25 ∘ C, adding Na 2 SO 4 to CaCl 2 and keeping the molar ratio of Na 2 SO 4 to CaCl 2 as 0.5 : 1 owing to the comparatively low supersaturations and the high values of [Ca 2+ ]/[SO 4 2− ], which favored the hydrothermal formation of the CaSO 4 ⋅0.5H 2 O whiskers with high aspect ratios.
indicated that precursor was composed of CaSO 4 ⋅2H 2 O and the hydrothermal product was composed of CaSO 4 ⋅0.5H 2 O.

Figure 2 :
Figure 2: XRD patterns of the precursor (a) and the hydrothermal product (b).
Figure 4(b) shows the variation of [Ca 2+ ]/[SO 4 2− ] with reaction time.The order of the values of [Ca 2+ ]/[SO 4 2− ] in lines a-c (line b > line c > line a) was in accordance with the order of the particle sizes of the precursors, indicating that a high value of [Ca 2+ ]/[SO 4

Table 1 :
Influence of temperature on the supersaturation of CaSO 4 ⋅2H 2 O.