Synthesis of α-hemihydrate gypsum from cleaner phosphogypsum

Journal of Cleaner Production 195 (2018) 396e405

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Synthesis of a-hemihydrate gypsum from cleaner phosphogypsum Baoguo Ma a, Wenda Lu a, Ying Su a, *, Yubo Li a, Chao Gao a, Xingyang He b a b

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, Hubei, China School of Civil Engineering, Hubei University of Technology, Wuhan 430068, Hubei, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 September 2017 Received in revised form 25 May 2018 Accepted 27 May 2018 Available online 28 May 2018

Phosphogypsum (PG), as a waste by-product with high content and complicated impurities, is difficult to be utilised effectively. Herein, cleaner PG is prepared with “HCleH2SO4 method”, and thus is used to synthesise a-hemihydrate gypsum (a-HH) in CaCl2 solution under atmospheric pressure. Morphology and mechanical strength are controlled by introducing varying dosages of modifier (0e0.06 wt% of the PG). Results show that reaction time extends from 120 min to 390 min and average length/diameter (L/D) ratio of a-HH crystal is decreased from 10.3 to 0.6 with the increase in the modifier dosage from 0 to 0.045 wt%. When the modifier dosage is 0.04 wt%, the converted product shows the best mechanical performance and fine commercial application prospect. The impurity content of a-HH synthesised from cleaner PG significantly decreases to approximately 5% of that from traditional PG. The mechanical strength of a-HH prepared from cleaner PG is better than that from traditional PG. The work presents a new and practical method to utilise PG efficiently. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Waste elimination Cleaner phosphogypsum a-hemihydrate gypsum Morphology control

1. Introduction “Wet method” is an economic process of preparing phosphoric acid and is widely applied in phosphate fertiliser industry. The main chemical reaction of this industrial process can be simplified as follows: Ca5(PO4)3F(s)þ5H2SO4(aq)þ10H2O(l)/3H3PO4(aq)þ 5CaSO4$2H2O(s)þHF(aq)

(1)

Generally, 5 tons of phosphogypsum (PG) are generated for every ton of phosphoric acid production with this traditional method (Jamialahmadi and Müller-Steinhagen, 2000). Most of PG is in form of CaSO4$2H2O with various impurities (such as SiO2, P2O5, F, organic matter, or even heavy metal ions, etc.), which releases its purity and whiteness and makes it hard to be utilised effectively (Garg et al., 2009). The presence of complicated impurities also restricts the application field of PG (Zhang et al., 2016). Although pre-treatment of PG could partly address these issues, it is likely to increase the manufacturing cost and create secondary pollution. With the rapid development of agriculture and fertiliser industry, the worldwide annual emission of PG is up to 280 million tons,

* Corresponding author. E-mail address: [email protected] (Y. Su). https://doi.org/10.1016/j.jclepro.2018.05.228 0959-6526/© 2018 Elsevier Ltd. All rights reserved.

while only 15% of that is utilised and the rest has to be dumped in vast stockpiles (Zhou et al., 2016). Mountains of dumped PG not only occupy considerable land area and require enormous maintenance funds, and it also damage the ecological environment of the surrounding area. The dust causes air pollution and the leach of soluble impurity such as F, PO3 4 or even heavy metal ions, causes land and water pollution. Therefore, producing PG with less impurities is of great environmental importance and urgency. Currently, much work has been done to utilise PG. It has been recycled as building materials (such as gypsum plaster, hardened tile (Zhou et al., 2016), foam concrete (Tian et al., 2016)), lime-fly ash-phosphogypsum binder (Shen et al., 2007), soil stabilization amendments and setting rate controller of Portland cement (Shen et al., 2012). However, the relatively low added value of these products leads to limited transportation radius and application range. Meanwhile, a-calcium sulfate hemihydrate (a-HH) is widely recognised as a high value-added cementitious material for its superior workability, high strength and favourable biocompatibility (Jiang et al., 2016; Singh and Middendorf, 2007). Thus, it has been widely applied in moulding, special binder systems, construction industry (Zhou et al., 2016), orthopaedic and dental fields (Peters et al., 2006), etc. The current commercial processes of a-HH preparation mainly include autoclave method and pressurized aqueous method, each of which is energy intensive due to the elevated temperature and high pressure adopted during manufacture. In

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2. Experimental procedures 2.1. Materials

Fig. 1. Diagrams of (a) traditional “wet method”, (b) “HCleH2SO4 method” and (c) hydrothermal reaction to obtain high-purity and varying performance a-HH.

contrast, autoclave-free method in salt solutions has been increasingly applied because of its mild reaction condition and hence reduced energy consumption (Yang et al., 2012). Herein, cleaner PG instead of traditional PG was prepared from phosphate rocks with “HCleH2SO4 method”. This method and the traditional method were compared as shown in Fig. 1. The main chemical reaction of this process can be simplified as follows:

The main raw material used in this paper is a cleaner PG prepared with “HCleH2SO4 method”. In this process, phosphate rocks (marine sedimentary phosphorite deposit, obtained from Hubei Yihua Fertiliser Co., Ltd, China, and the chemical composition is shown in Table 1) are firstly crushed and grinded. Then the phosphate rock powders are fully acidolysed by slightly excess hydrochloric acid (10 wt%). The slurry is filtered to obtain filtrate and insoluble residues are removed. The insoluble residues contain over 80 wt% SiO2 (the chemical composition is shown in Table 1), which can be used for some other chemical industry applications such as producing silicon fertiliser. Then sulfuric acid (0.5 mol L1) is added into the filtrate within 3 min, standing for 5 h at a stirring rate of 120 rpm, to form dihydrate gypsum precipitate. Finally, the filtrate is used to produce phosphate fertiliser and the precipitate is filtered and washed to gain cleaner PG. All these reactions are performed in the temperature of (25 ± 3)  C. The chemical composition, whiteness, radiological characterisation, XRD patterns and morphology of this as-produced cleaner PG are shown in Table 1-PG4, Table 3-PG4, Figs. 2-a and 3-a. The traditional PG used as control group is obtained directly

Ca5 ðPO4 Þ3 FðsÞ þ 10HClðaqÞ þ 10H2 OðlÞ/3H3 PO4 ðaqÞ þ 5CaCl2 ðaqÞ þ HFðaqÞ 5CaCl2 ðaqÞ þ 5H2 SO4 ðaqÞ þ 10H2 OðlÞ/5CaSO4 $2H2 OðsÞ þ 10HClðaqÞ

The impurity and whiteness of the cleaner PG were proved to be better than that of the traditional PG. The possibility of converting this cleaner PG into a-HH in hydrothermal reaction under atmosphere was demonstrated feasible. The influence of modifier dosage on the morphology of a-HH crystal and the macroperformance of a-HH product was further discussed. The best modifier dosage for superior mechanical performance was found. The differences between a-HH products prepared from cleaner PG and from traditional PG were compared.

(2)

from the industrial process and from Hubei Yihua Fertiliser Co., Ltd, China. The raw materials (phosphate rock) of the traditional PG are the same with the cleaner PG in present work. It is produced as the by-product of “wet method” process of preparing phosphoric acid or phosphate fertiliser. The traditional PG is pretreated by water-washing (which is widely used in industrial production) for ease of utilisation. In the water-washing pretreatment, the traditional PG and water are mixed in a plastic container with the solid/water ratio being 1:2. The slurry is stirred

Table 1 Chemical composition (wt%) and whiteness of raw materials and products.

SO3 CaO SiO2 P2O5 Al2O3 Fe2O3 MgO Na2O K2O SrO TiO2 BaO F Cl Loss whiteness

PG1

PG2

PG3

PG4

Phosphate Rock

Insoluble Residue

Product from PG1

Product from PG4

41.39 ± 0.18 30.32 ± 0.21 6.41 ± 0.13 0.58 ± 0.03 0.473 ± 0.013 0.152 ± 0.008 0.042 ± 0.007 0.068 ± 0.006 0.050 ± 0.003 0.037 ± 0.013 ND ND 0.361 ± 0.018 ND 20.16 ± 0.23 65.6 ± 0.3

39.32 30.45 8.66 0.79 0.49 0.13 0.02 0.03 0.07 NP 0.04 NP NP NP 19.09 NP

36.97 26.76 14.74 4.58 0.68 0.12 0.11 0.43 NP NP NP NP NP NP 18.90 NP

44.68 ± 0.14 33.94 ± 0.18 0.047 ± 0.006 0.103 ± 0.017 0.022 ± 0.003 ND ND ND ND 0.018 ± 0.003 ND ND 0.010 ± 0.001 0.011 ± 0.003 21.18 ± 0.17 92.7 ± 0.2

1.34 ± 0.03 45.04 ± 0.24 10.17 ± 0.14 25.89 ± 0.18 1.93 ± 0.04 0.69 ± 0.03 4.01 ± 0.07 0.36 ± 0.03 0.59 ± 0.03 0.082 ± 0.017 0.147 ± 0.011 0.078 ± 0.008 1.57 ± 0.08 0.037 ± 0.007 7.96 ± 0.13 NT

2.71 ± 0.04 0.638 ± 0.021 81.13 ± 0.36 0.431 ± 0.022 5.86 ± 0.11 1.78 ± 0.03 1.12 ± 0.03 0.383 ± 0.021 1.55 ± 0.03 0.018 ± 0.003 0.903 ± 0.017 0.208 ± 0.012 ND 0.120 ± 0.023 3.13 ± 0.11 NT

51.10 ± 0.27 39.06 ± 0.19 2.36 ± 0.08 0.82 ± 0.04 0.081 ± 0.019 0.076 ± 0.012 ND ND ND 0.052 ± 0.007 ND ND 0.082 ± 0.013 0.072 ± 0.006 6.31 ± 0.17 78.3 ± 0.2

53.35 ± 0.16 40.18 ± 0.14 0.053 ± 0.008 0.066 ± 0.013 ND ND ND ND ND 0.017 ± 0.002 ND ND ND 0.042 ± 0.008 6.27 ± 0.11 93.5 ± 0.4

PG1: traditional PG obtained from Hubei Yihua Fertiliser Co., Ltd, China. PG2: the value taken from Zhou et al. (2016). PG3: the value taken from Yang et al. (2016). PG4: cleaner PG. These chemical composition and whiteness data are shown as “mean value ± standard deviation of the mean”. “ND” denotes not detected, “NP” denotes not provided in the cited paper, “NT” denotes not tested.

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Table 2 W/H ratio and mechanical strength of a-HH prepared and other researchers’ result.

Modifier dosage/% Average L/D ratio Reaction time/min Crystal water content/% W/H ratio 2 h bending strength (MPa) 3d bending strength (MPa) 3d compressive strength (MPa)

A

B

C

D

E

F

G

H

a-HH-1

a-HH-2

0 10.3 120 6.4 0.53 5.2 ± 0.1 5.8 ± 0.1 16.9 ± 0.4

0.01 6.4 150 6.2 0.48 4.2 ± 0.1 4.9 ± 0.1 18.0 ± 0.2

0.02 4.7 190 6.4 0.37 6.9 ± 0.1 7.6 ± 0.1 26.2 ± 0.5

0.03 2.9 210 6.3 0.33 6.9 ± 0.2 8.1 ± 0.1 35.8 ± 0.3

0.04 1.2 310 6.3 0.33 7.1 ± 0.1 8.2 ± 0.1 38.3 ± 0.4

0.045 0.6 390 6.7 0.35 6.7 ± 0.2 7.2 ± 0.1 27.4 ± 0.6

0.05 NT NT 18.7 NT NT NT NT

0.06 NT NT 19.2 NT NT NT NT

1.4 1.5 180 6.3 0.35 6.0 ± 0.2 6.8 ± 0.1 28.7 ± 0.5

NP NP NP NP 0.3e0.41 5.6e6.3 5.7e8.8 15.1e25.3

a-HH-1: the a-HH synthesised from PG1. a-HH-2: the value taken from Guan et al. (2011). The mechanical strength data are present as “mean value ± standard deviation of the mean”. “NP” denotes not provided in the cited paper, “NT” denotes not tested.

Table 3 Radioactivity of different materials.

CRa (Bq$kg1) CTh (Bq$kg1) CK (Bq$kg1) IRa Ig

PG1

PG4

a-HH from PG1

a-HH from PG4

Limit of radionuclides from GB6566-2010

21.2 28.4 267.4 0.11 0.23

17.7 25.1 189.2 0.09 0.19

18.6 22.7 286.7 0.09 0.21

15.2 26.2 174.3 0.08 0.18

e e e 1.0 1.0

“d” means the value is not required in the GB6566-2010.

Fig. 2. XRD patterns for PG1 and PG4. “D” marks denote the locations of the DH peaks, “S” marks denote the locations of the a-SiO2 peaks.

at 60 rpm for 20min, and then stands for another 10min. Then the supernatant water is discharged. The same process was repeated for 3 times. Finally, the washed PG is dried for further use and characterisation. Obviously, the discharged waste water in the water-washing pre-treatment process, which is difficult to dispose, can cause environmental problems. The chemical composition, whiteness, radiological characterisation, XRD patterns and morphology of this traditional PG are shown in Table 1PG1, Table 3-PG1, Figs. 2-b and 3-b. Compared with the traditional “wet method”, the “HCleH2SO4 method” adds a step of acidolysis with hydrochloric acid before the reaction of phosphate rock and sulfuric acid, thus the ultimate byproduct PG contains less impurities (Table 1) and is more likely to be used with higher value and less pollution. To simplify the study, the “impurity content” is calculated by adding all the tested impurities of the chemical composition. Precisely, the impurity contents of PG1, PG2 and PG3 (different kinds of traditional PG) are (8.14 ± 0.23) wt%, 10.19 wt% and 20.66 wt%, respectively. In contrast, the PG4 (the cleaner PG) contains merely (0.21 ± 0.03) wt% impurities. Meanwhile, the whiteness of the PG4 (92.7 ± 0.2) is much higher than that of the PG1 (65.6 ± 0.3). The XRD pattern of the cleaner PG (Fig. 2-a) shows all peaks are indexed corresponding to CaSO4$2H2O phase (JCPDS card 33e0311) and no impurity phase is examined, the 2theta value of main peaks are 11.726 , 20.810 , 23.474 , 29.184 , corresponding to lattice plane of (020), (021), (040), (041), indicating the main phase of dihydrate gypsum. In comparison, the XRD pattern of the traditional PG (Fig. 2-b) shows the main phase is CaSO4$2H2O. Meanwhile, characteristic peaks of a-SiO2 (2theta value is 26.686 ) is detected demonstrating the existence of a-SiO2 phase. SEM image (Fig. 3-a) displays morphology of cleaner PG is rhomboid, no obvious defects are found on the crystal facets. Some layered structure can be found, which can be explained as the water molecules alternating with calcium sulfate layers forming the layered structure thus making it easy to cleave (Ballirano and Melis, 2009). Radioactivity results (Table 3) show that internal exposure index (IRa ¼ 0.09) and external exposure index (Ig ¼ 0.19) of the cleaner PG fully meet the requirement of Chinese standard GB6566-2010 (IRa1.0, Ig1.0).

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Fig. 5. Crystal water content of samples obtained during different time intervals (the same samples of Fig. 4) and calculated conversion ratio of the reaction.

2.2. Synthesis of a-HH

Fig. 3. SEM images of PG1 and PG4.

The modifier used in the experiment is sodium citrate. It is a kind of polycarboxylic salt whose FTIR spectra is shown in Fig. 11-c. All of the other chemicals used in this study are of analytically pure grade which are purchased from Sinopharm Chemical Reagent Co., Ltd. Distilled water was used throughout the experiments.

Fig. 4. XRD patterns of samples obtained during the conversion process for the reaction time of 5min, 30min, 60min, 90min, 120min and 150min. “D” marks denote the locations of the DH peaks, and “H” marks denote the locations of the HH peaks.

Non-autoclave hydrothermal reaction was adopted to synthesise a-HH. The main steps include hydrothermal reaction, filtration, washing, drying and characterisation. The specific procedure is as follows: The reaction was performed in a 1000 mL 3-neck flask at a stirring rate of 60 rpm and in a temperature of 97  C, which was separately monitored by electric mixer and heating equipment with temperature control system. The 200 g raw material was weighed and dispersed in 400 g CaCl2 solution at a concentration of 26 wt%. Samples were withdrawn at different time intervals and observed under a digital microscope (KH-7700; Hirox Co. Ltd., Tokyo, Japan) at 700 times magnification to investigate whether the

Fig. 6. SEM images of samples obtained during the conversion process of different time intervals (the same samples of Fig. 4). (a):5min, (b):30min, (c):60min, (d):90min, (e):120min, (f):150min.

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Fig. 9. SEM images of the converted crystals with different dosage of modifier. The dosage: (a) ¼ 0, (b) ¼ 0.01%, (c) ¼ 0.02%, (d) ¼ 0.03%, (e) ¼ 0.04%, (f) ¼ 0.045%, (g) ¼ 0.05%, (h) ¼ 0.06%. Fig. 7. DSC/TG curves for (a) the cleaner PG and (b) the converted product.

raw material had any change of morphology. The reaction was considered as finished when the picture shown by digital microscope contained 95% or more hexagonal prism shaped crystals. During the hydrothermal reaction procedure, 20 ml samples were withdrawn at different time intervals and immediately filtrated and washed with hot water of 90  C using a syringe filter with 0.45 mm

cellulose membrane to acquire clean solid samples and avoid the probable hydration. Subsequently, the separated solid samples at different time intervals were rinsed and dispersed with ethyl alcohol and dried at 80  C for 5 h. Ultimately, the dried samples were kept in a desiccator for further characterisation. To modify the crystal morphology and improve the macroproperties of the as-prepared product, different dosages of modifiers were added in the reaction system with other conditions remaining consistent. The same method was used to determine the reaction time (reaction time was limited to 600min considering the efficiency of actual industrial utilisation). The solid samples are filtrated, washed, dried and kept in the same way for further characterisation. To further understand the advantages and disadvantages of using cleaner PG in hydrothermal reaction, traditional PG (PG1) was chosen as raw material to synthesise a-HH. The same method was adopted. The characters of the obtained a-HH samples from PG1 were compared with that from the cleaner PG.

2.3. Characterisation

Fig. 8. (a) (b) (c) SEM image of the converted crystals (the same sample in Fig. 7-b), together with (d) schematic representation of the crystal.

The chemical composition was determined using a wavelength dispersive X-ray fluorescence spectrometer (XRF, Axios advanced, PANalytical B.V, Netherlands). In each of the chemical composition test, six samples were tested and the ultimate data were shown as “mean value ± standard deviation of the mean”. Standard deviation

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Water-hemihydrate weight ratio (W/H ratio) of standard consistency of the prepared samples was tested according to Gypsum plasters-Determination of physical properties of pure paste (GB/T 17669.4e1999). Specimens used to determine bending and compressive strength of different ages (2 h, 3d) were prepared, maintained and tested according to Gypsum plasters-Determination of mechanical properties (GB/T 17669.3e1999, eqv. ISO 3051:1974). The mechanical strength was tested by an automated breaking and compression resistance tester (WYA-300, Xiyi Building Material Instrument Factory, China). In the mechanical strength test, the paste was prepared at the tested W/H ratio and was cast into six 40 mm  40 mm  160 mm iron moulds and compacted by jolting. After setting for 30 min, the prisms were demoulded and immediately stored in an environmental chamber at 20 ± 2  C and 90 ± 5%RH. At curing ages of 2 h (2 h), three of the prisms were tested and the mean value of the three figure was recognised as the ultimate 2 h bending strength. At curing ages of 3 days (3d), the left three prisms were tested for 3d bending strength and the six broken half prisms were tested for 3d compressive strength with loading area of 0.0016 m2. Standard deviation of the mean was used to describe the standard uncertainty of these mechanical strength data. The radiological characterisation of the prepared samples was tested according to Limits of radionuclides in building material (GB6566-2010). Samples were tested by a low background multichannel gamma ray spectrometer (GEM40P4-76, ADVANCED EASUREMENT TECHNOLOGY inc., America). The activity concentration of 226Ra, 232Th and 40K was tested. IRa and Ig were used to determine whether the products can be applied in building industry.

Fig. 10. Effect of modifier dosage on (a) length, diameter and (b) average L/D ratio of the converted crystals.

0

1 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rP N B C ðx xÞ2 C B i¼1 i of the mean BSx=rootðNÞ ¼ was used to describe the NðN1Þ C @ A standard uncertainty of these data. The whiteness was tested by a Whiteness Meter (PNe48B, Pnshar Co. Ltd, China) which is according to Method for measurement of whiteness of building materials and non-metal mineral products (GB5950-2008). In each of the whiteness test, six samples were tested and the ultimate data were shown as “mean value ± standard deviation of the mean”. X-ray diffraction analysis (XRD; D8 Advance, Bruker Inc., Germany) was performed with CuKa radiation at a scanning rate of 10 /min in the 2theta range from 5 to 70 . The morphology was examined using a field emission scanning electron microscopy (FE-SEM; QUANTA FEG 450, FEI Ltd., America). The information of crystal size and length/diameter (L/D) ratio of prepared crystals was obtained on the basis of measuring 200 crystals through digital microscope (KH-7700; Hirox Co. Ltd., Japan). Thermal analysis was performed on a simultaneous thermogravimetry and differential scanning calorimeter (TG-DSC; STA449F3, NETZSCH Group, Germany) for further identifying phase and determining the crystal water content. Chemical groups were characterized using a Fourier transform infrared spectrophotometer (FTIR; NEXUS; Thermo Nicolet Corporation, America) with scanning range from 500 to 4000 cm1.

Fig. 11. FTIR spectra of (a) the a-HH samples prepared in the presence of different dosage of modifiers, (b) enlarged Fig. from the wavenumbers of 2600 cm1 to 4000 cm1 and (c) the modifier.

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IRa ¼

Ig ¼

CRa 200

CRa C C þ Th þ K 370 260 4200

(3)

(4)

Where IRa and Ig denotes internal exposure index and external exposure index. CRa, CTh and CK denotes the activity concentration of 226 Ra, 232Th and 40K (Bq$kg1). 200 denotes the limit of activity concentration of 226Ra only in the condition of internal exposure (Bq$kg1). 370, 260 and 4200 denotes the limit of activity concentration of 226Ra, 232Th and 40K only in the condition of external exposure (Bq$kg1).

3. Results and discussions 3.1. Analysis of crystal transformation in hydrothermal reaction XRD, crystal water content and SEM (Figs. 4e6) of samples with different time intervals were tested to determine the crystal transformation in hydrothermal reaction. As shown in Fig. 4, only characteristics of calcium sulfate dihydrate (DH) and/or calcium sulfate hemihydrate (HH) was tested, shrink of DH peaks and emerge of HH peaks could be observed throughout the reaction, indicating the hydrothermal reaction is a progress of DH to HH without other phase involvement. Given that the molecular weight of DH and HH is approximately 172.18 amu and 145.15 amu, respectively, conversion ratio (the content of HH in the solid samples) of the reaction can be defined and calculated by:

R¼

nðHHÞ 36:03  172:18C ¼ nðHHÞ þ nðDHÞ 27:02ð1  CÞ

(5)

Where R, n(HH), n(DH), and C denotes conversion ratio, numbers of HH particles, numbers of DH particles, and crystal water content. The calculated results of different time intervals are shown in Fig. 5. As shown in Figs. 4e6, the 10 min sample’s XRD pattern indicates characteristics of DH, its crystal water content is 21.0 wt% (corresponding to calculated conversion ratio of approximately 0), and it has a rhomboid morphology (similar to that of the raw material). These results reveal there is no reaction in the first 10min. After 30 min sample, its XRD pattern shows that peaks of HH occur, and its crystal water content is 19.6 wt% (corresponding to calculated conversion ratio of approximately 10.5%). The SEM image of the 30 min sample shows circular edges apparent in the surface of some of the DH crystals compared to the former straight edge, and a small amount of hexagonal prism crystals (a characteristic morphology of calcium sulfate hemihydrate crystal (Jiang et al., 2016)) appear. As for the 60 min and 90 min samples, relative intensities of DH peaks weaken and those of HH become obvious. The crystal water content of the sample declines further. HH crystal size enlarges distinctly compared to the 30 min sample. The SEM image of 90 min sample shows DH crystals disappear and HH crystals grow larger with many fracture surfaces in the polar areas. As for 120 min and 150 min samples, the XRD patterns, crystal water content information and SEM images are basically similar. XRD patterns indicate all the diffraction peaks of DH disappear and the remaining peaks are all corresponding to HH. The crystal water content is 6.2 wt% and 6.3 wt%, respectively (close to the theoretical value of HH: 6.21 wt%). The HH crystal’s morphology is perfect hexagonal prism. This indicates that the 120 min and 150 min samples have been transformed into HH before 120 min.

These results imply that the hydrothermal reaction can be explained as dissolution-crystallization process which is also confirmed by other literature (Feldmann and Demopoulos, 2012; Pan et al., 2013; Mao et al., 2014; Zhao et al., 2016; Li and Demopoulos, 2006). Specifically, DH crystal dissolves gradually in the reaction, which can be inferred from the phenomenon of shrink of DH diffraction peaks, release of crystal water content and circular edge appearing in the DH crystal. HH crystal crystalizes in certain period of the reaction, which can be inferred from the phenomenon of emergence of DH diffraction peaks, release of crystal water content and appearance of hexagonal prism crystals. Meantime, the growth of the HH crystal is conspicuous, and the growth rate in length direction (c axis direction as shown in Fig. 8-d) is faster than that in diameter direction (the direction perpendicular to c axis in Fig. 8-d). In length direction it can be seen the 30 min crystal of 4e23 mm has grown to 115e273 mm and in diameter the 2e8 mm crystal has grown to 5e24 mm (Fig. 6), which results in the average L/D ratio of the ultimate HH crystal displaying 10.3. And this can be explained that the preferential growth direction of HH is along the c-axis (Luo et al., 2010). 3.2. Analysis of converted crystal XRD analysis can differentiate whether the composition phase is DH or HH. However, it cannot differentiate explicitly whether the HH is a type or b type. Therefore, thermal analysis and SEM analysis were performed to further analyse the converted HH crystal. Fig. 7-a shows the DSC and TG curves of the cleaner PG. The TG mass change is 21.03%. The DSC curve displays endothermic peak at 157.0  C and 163.1  C, denoting a two-step release of firstly 1.5 and secondly 0.5 water molecule in the DH crystal. Less apparently, an exothermic peak at 371.5  C can be examined reflecting the crystal form transition of anhydrite. These results show the main phase of cleaner PG is DH, which corresponds with the former analyses. The DSC curve of the 120 min sample (Fig. 7-b) displays the characteristic peaks of a-HH (Pan et al., 2013; Kubota and Mullin, 1995; Hou et al., 2014), with a distinctive exothermic peak at around 175.6  C immediately following the endothermic peak at around 156.1  C. Meanwhile, there is no characteristic peak of DH phase. Both results confirm that DH crystals have been completely transformed to a-HH crystal phase. SEM images of different magnification (Fig. 8-a, b, c) were taken to expose more details of the converted crystals. As shown, the aHH crystals prepared through hydrothermal reaction display a length of 106e286 mm (whose average value is 217 mm) and a diameter of 6e27 mm (whose average value is 21 mm). The morphology of the converted crystal is hexagonal prism with two poles (both of which are composed of three pentagonal crystal planes) and with six elongated pentagonal crystal planes as prism planes. For ease of viewing, schematic representation of a-HH crystal is illustrated in Fig. 8-d. As the schematic represents, the top planes belong to {1101} family of planes, the prism planes belong to {1100} family of planes. The crystal lattice of a-HH consists of repeating, ionically bonded Ca and SO4 atoms in “CaeSO4eCaeSO4” chains, which are hexagonally arranged and form a framework parallel to the c axis in continuous channels with a diameter of about 4.5 Å, where one water molecule is attached to every two calcium sulfate molecules (Dantas et al., 2007; Pan et al., 2013). Therefore, the spontaneously grown a-HH crystal without any modifier displays morphology of long hexagonal prism. The W/H ratio, bending and compressive strength of the a-HH as prepared were tested (Table 2, group A) and the result was not favourable. It has been demonstrated that there are various crystal

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forms of a-HH such as whisker, column, rod and plate (Kong et al., 2012). And different morphology of a-HH crystal leads to different W/H ratio and mechanical strength (Guan et al., 2011). Therefore, studies should be done to change the morphology of a-HH and enhance its mechanical performance. 3.3. Modification of a-HH crystal’s morphology Different dosage of modifier (0.01%, 0.02%, 0.03%, 0.04%, 0.045%, 0.05%, 0.06%, 0.08% mass percent of the raw material) was added with other conditions consistent to adjust the morphology and size of a-HH crystals, and the mechanism of the modifier on growth of a-HH crystal was investigated. Reaction time and crystal water content were tested (Table 2) to determine the rate of hydrothermal reaction and whether the reaction was finished. As shown, when the dosage is from 0 to 0.045% (group A to F) crystal water content of the converted product varies from 6.2% to 6.7%, close to the theoretical value of HH 6.21%, indicating the reaction from DH to HH is basically finished. Meanwhile, the reaction time extends as the dosage increases. When the dosage is more than 0.045% (group G and H) crystal water content of the converted crystals in 600 min is 18.7% and 19.2% respectively, indicating these two reactions are too slow to finish within 600 min and reaction of group H is even slower than that of group G. It can be inferred that the modifier has an inhibiting effect in the reaction, and the more modifier introduced the stronger the inhibiting effect for the conversion from DH to a-HH. In order to analyse the effect of modifier dosage on crystal morphology, SEM images of converted crystal with different modifier dosage were taken (Fig. 9) and the crystal size information based on digital microscope (KH-7700; Hirox Co. Ltd., Tokyo, Japan) was illustrated in Fig. 10. As shown, when modifier dosage increases from 0 to 0.04% (Fig. 9, group a-e), the a-HH crystals get shorter, its diameter enlarges and the average L/D ratio lessens. When the modifier dosage is more than 0.04% (Fig. 9, group f-h), the converted a-HH crystal length, diameter and average L/D ratio reduce with increasing dosage. The ultimate external shape of crystal depends on the relative growth rate of different crystal planes or crystal directions, which could be altered by adding modifiers to the reaction system (Pan et al., 2013; Kong et al., 2012). The as-obtained a-HH crystals with different morphology and size have further confirmed the theory. The variation in a-HH crystal shape can be explained as a result of selective adsorption of modifier on specific crystal planes to influence the relative growth rate of certain crystal orientation. It has been demonstrated that the Ca2þ and SO2 4 contents vary in different crystal planes (the content of Ca2þ is higher in the top plane than that of the prism plane), resulting in the top plane of aHH crystal being slightly positively charged (Zhao et al., 2016; Hou et al., 2014; Kong et al., 2012). Therefore, hydroxyl groups of modifier molecules tend to adsorb more onto the top planes with Ca2þ to form complexes. The growth of the top plane along c-axis is then inhibited and the relative growth rates of different crystal orientations change. Consequently, a-HH crystal with smaller L/D ratio is obtained. As the modifier dosage increases, the morphology of a-HH crystal changes from long hexagonal prism (Fig. 9, group a) to short hexagonal prism (Fig. 9, group b-e) and even to hexagonal platelet (Fig. 9, group f-h). Interestingly, when the dosage ranges from 0.045 wt% to 0.06 wt %, the top planes are basically perfect and the top planes of these aHH crystals approximately belong to {0001} instead of {1101}. Defects are found on some of the a-HH crystal’s top planes with the modifier dosage ranging from 0.01 wt% to 0.04 wt%. The prism planes are basically perfect. Such facts further demonstrate that the effect of the modifier on the a-HH crystal’s top planes is greater

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than that on the prism planes. Further scrutinizing the top plane areas, we find that it is the central zone rather than the rounding zone that contains these defects. As traditional crystal growth theory indicates, the growth of a-HH crystal is a result of the Ca2þ and SO2 4 in the solution diffusing and overlying on the planes of crystal. It is reasonable to assume that when modifier concentration is relatively low (0.01e0.04 wt% in this research system), in the initial stage of crystal growth, the modifier can adsorb onto the top planes, inhibiting crystal length direction growth and giving rise to acceleration of the relative growth rate of diameter direction and the area of top planes expand radially. Hence, more modifier is needed to adsorb onto the emergent areas of the top planes. As the modifier runs out the emergent areas of the top planes (rounding area) are less likely to be adsorbed by modifier, and the growth rate of these areas tends to be faster than that of the central area. Eventually, defect areas could be found in some of the crystal’s top planes. But, when the concentration of modifier is relatively high (0.045e0.06 wt% in this research system), the modifier can adsorb onto the top planes of the crystals throughout the crystal growth progress, resulting in few defects on examination. As the quantity of modifier adsorbed on the central area is more than that of the rounding area, the ultimate top planes belong to {0001} family of planes instead of {1101} family of planes. The samples obtained with different modifier dosage (0, 0.02 wt %, 0.04 wt%, 0.06 wt%, the same sample of group a, c, e, g of Fig. 9) were further characterized by FTIR analysis (Fig. 11). The modifier is a polycarboxylic salt whose FTIR spectra is shown in Fig. 11-c. Two obvious characteristic peaks at 2962 cm1 and 2921 cm1 are examined, indicating the modifier contains eCH2e groups. It can be inferred that, if the modifier adsorbed onto the top planes, carboxylic anion would form complex with Ca2þ and eCH2e groups would remain invariant. So eCH2e groups can be the characteristic of the a-HH crystal adsorbed by modifier. To get more detailed information of the probable adsorption of modifier on the planes of a-HH crystal, FTIR spectra is magnified from 2600 cm1 to 3000 cm1 (Fig. 11-b). There are two evident peaks at (2920e2921) cm1 and (2851e2853) cm1 in all the samples except the blank group a, which indicates characteristic peaks of symmetric stretch and antisymmetric stretch of eCH2e groups. And the mutual reaction of modifier and a-HH crystal is further verified. 3.4. Relationship of morphology and mechanical strength For all the cementitious materials, a-HH included, mechanical strength is a crucial index which influence their application prospect. So, mechanical strength with different curing time of the above a-HH samples was tested to study the relationship of crystal morphology and mechanical strength. These results (Table 2) together with the morphology information (Figs. 9 and 10) show that as modifier dosage increases from 0 to 0.04 wt% (group A, B, C, D, E), the W/H ratio displays a decreasing trend and the bending and compressive strength of different curing time displays an increasing trend. However, the 0.045 wt% modifier dosage product (group F) shows an increase of W/H value and decrease of mechanical strength compared to its former group (group E). When the dosage is higher than 0.45 wt% (group G, H), since the conversion cannot be finished within the given time (600 min), these products contain a mass of dihydrate phases which have no cementitious behaviour. Therefore, the W/H ratio and mechanical strength of these products are meaningless and not tested and compared. In summary, when the modifier dosage is 0.04 wt%, the converted product shows the highest mechanical strength. This result can be explained as the a-HH crystal morphology is controlled by introducing different dosage of modifier, hence the W/H ratio of a-HH is changed, which further changes the porosity

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created by the evaporation of excess water in hardened materials, resulting in changes to mechanical strength. It can be preliminarily concluded from the given information that when the average L/D ratio of a-HH crystals range from 1 to 3 (group D, E), the W/H value is lower (0.32) and mechanical strength is higher than under other conditions, probably because this kind of a-HH has a relatively low specific surface area. These conclusions are in good agreement with the conclusions of other researchers (Jiang et al., 2016; Peters et al., 2006; Tang et al., 2010). Further comparing group D and E, we find when the modifier dosage is 0.04 wt% of the raw material, the converted product shows an average L/D ratio of approximately 1 and the mechanical strength is the highest (namely, its 2 h bending strength is (7.1 ± 0.1) MPa, 3d bending strength (8.2 ± 0.1) MPa, 3d compressive strength (38.3 ± 0.4) MPa). In addition, it can be inferred that the effect on the hardened product behaviour by crystal defects in the obtained product tend to be weaker than that of the average L/D ratio, considering that the strength values of group D and E (whose crystals contain a mass of defects) are larger than that of group F (whose crystals contain few defects). For ease of investigation only L/D ratio is taken into account in this study. However, the crystal morphology is a complicated index including L/D ratio, distribution of particle size and defects in the crystal, each of which has a potential to influence the mechanical strength of a-HH’s hardened specimen. Therefore, more work should be done to fully identify this relationship. 3.5. Comparison between a-HH synthesised from cleaner PG and from traditional PG A traditional PG which had been pre-treated by water-washing (PG1) was chosen as the raw material of a control group to further study the advantages or disadvantages of using cleaner PG as raw material in hydrothermal reaction. As proved above, a-HH with average L/D ratio of 1e3 prepared from the cleaner PG displays superior performance (Fig. 9-e and Table 2eE). To simplify the comparison, a-HH with average L/D ratio of 1e3 was synthesised from PG1 and was compared with the a-HH of group E. As shown, the purity and whiteness of these two a-HH product differs radically (Table 1). The impurity content of a-HH synthesised from PG1 (3.54 ± 0.18 wt%) is 18 times higher than that from the cleaner PG (0.18 ± 0.03 wt%) and the whiteness of a-HH from PG1 is lower (78.3 ± 0.2 vs. 93.5 ± 0.4). It can be explained that in the dissolution-crystallization process of hydrothermal reaction, the soluble impurities of PG can dissolve and partly recrystallize along with a-HH crystals. So, the impurity content of a-HH product is closely related to its raw material, the difference between PG raw materials leads to the difference between a-HH products. In addition, as the radioactivity results (Table 3) show, IRa and Ig of a-HH products from PG1 and the cleaner PG all fall far below the limit of radionuclides from GB6566-2010, which confirms these a-HH products can be applied in the building industry in China. Modifier dosage of PG1 to a-HH (1.4 wt%) is more than that of the group E (0.45 wt%). It can be inferred that in the same reaction condition, the cation content in PG1 system is higher. Some cation impurities can react with the carboxylic anion of modifier. Therefore, more modifiers are needed to obtain a-HH with similar L/D ratio. Reaction time of PG1 to a-HH (180 min) is shorter than that of the group E (310 min). The XRD pattern of converted products from PG1 is shown in Fig. 12, which indicates that the main phase of the converted products is CaSO4$0.5H2O and a-SiO2 can be detected. The crystal morphology manifests that the crystal size of a-HH from PG1 is smaller and agglomeration exists extensively, compared to Fig. 9-e. It can be inferred that some of the impurities in PG1 can act as crystal nucleus thus accelerate the crystal transformation and

Fig. 12. XRD pattern of converted products from PG1. “H” marks denote the locations of the HH peaks, “S” marks denote the locations of the a-SiO2 peaks.

limit the crystal size. The relatively quick-grown and small-sized aHH crystals are more likely to agglomerate with each other. Furthermore, the adsorption of soluble phosphate impurity on the crystal surface (most likely calcium dihydrogen phosphate complex) can also be held responsible for the crystal morphology deterioration (Feldmann and Demopoulos, 2013) (see Fig. 13). As to the macro-performance (Table 2), the W/H ratio is a little higher (0.35 vs. 0.33). The 2 h bending strength, 3d bending strength and 3d compressive strength are about 15%, 17% and 25% lower than that of the counterpart, respectively. The mechanical strength is likely to be influenced by the morphology and impurity content of the a-HH product. Though the average L/D ratio of a-HH from PG1 resembles that from the cleaner PG, its crystal size is smaller. The smaller crystal size results in higher specific surface area and could lead to higher W/H ratio and lower mechanical strength. Furthermore, higher impurity content of a-HH from PG1 probably influences the hydration and hardening process of a-HH product and leads to inferior mechanical performance. Considering the practical industrial application, shorter reaction time is favourable. However, the higher modifier dosage can increase production cost. The lower whiteness and higher impurity

Fig. 13. SEM images of converted crystals from PG1.

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content would restrict the application area of the product. The inferior mechanical strength is not favourable. In addition, traditional PG contains considerable amount of SiO2 (Table 1 and Fig. 2), which can lead to abrasion to the equipment. 4. Conclusions Herein, a-HH with various morphology and performance was prepared by hydrothermal reaction using cleaner PG prepared with “HCleH2SO4 method”. Differences in a-HH between cleaner PG and traditional PG were discussed. The conclusions are: (1) The reaction time extends from 120 min to 390 min and the average L/D ratio of converted crystal displays a diminution from 10.3 to 0.6 as the modifier dosage increases from 0 to 0.045 wt%. When the modifier dosage is higher (0.05 wt% or 0.06 wt%), the reaction cannot be finished within the given time (600 min). In addition, defects are found in top planes but not in side planes of the a-HH crystal with 0.01e0.04 wt% modifier introduced. (2) When the modifier dosage is 0.04 wt%, the converted product shows the highest mechanical strength (i.e., 2 h bending strength equals (7.1 ± 0.1) MPa, 1d bending strength equals (8.2 ± 0.1) MPa, 3d compressive strength equals (38.3 ± 0.4) MPa). (3) Compared to the traditional PG, converting cleaner PG to aHH needs more time. However, in terms of the a-HH product from cleaner PG, the impurity content significantly decreases to only about 5% of that from traditional PG. The whiteness increases from (78.3 ± 0.2) to (93.5 ± 0.4). And the 2 h bending strength, 3d bending strength and 3d compressive strength are approximately 18%, 20% and 33% higher than that from traditional PG, respectively. Acknowledgment This work was financially supported by the Major Scientific and Technological Innovation Project of Hubei Province (project No. 2015ACA060). References Ballirano, P., Melis, E., 2009. Thermal behaviour and kinetics of dehydration of gypsum in air from in situ real-time laboratory parallel-beam X-ray powder diffraction. Phys. Chem. Miner. 36 (7), 391e402. https://doi.org/10.1007/ s00269-008-0285-8. Dantas, H.F., Mendes, R.A.S., Pinho, R.D., Soledade, L.E.B., Paskocimas, C.A., Lira, B.B., 2007. Characterization of gypsum using TMDSC. J. Therm. Anal. Calorim. 87 (3), 691e695. https://doi.org/10.1007/s10973-006-7733-9. Feldmann, T., Demopoulos, G.P., 2012. Phase transformation kinetics of calcium sulfate phases in strong CaCl2, HCl solutions. Hydrometallurgy 129e130, 126e134. https://doi.org/10.1016/j.hydromet.2012.08.015. Feldmann, T., Demopoulos, G.P., 2013. Influence of impurities on crystallization kinetics of calcium sulfate dihydrate and hemihydrate in strong HCl-CaCl2 solutions. Ind. Eng. Chem. Res. 52 (19), 6540e6549. https://doi.org/10.1021/ ie302933v. Garg, M., Jain, N., Singh, M., 2009. Development of alpha plaster from phosphogypsum for cementitious binders. Construct. Build. Mater. 23 (10), 3138e3143.

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