H. TAN et al.: EFFECTS OF IMPURITIES FROM PHOSPHOGYPSUM ON THE CRYSTAL GROWTH ...
633–637
EFFECTS OF IMPURITIES FROM PHOSPHOGYPSUM ON THE
CRYSTAL GROWTH OF CALCIUM SULFATE HEMIHYDRATES
VPLIV NE^ISTO^ KALCIJEVEGA FOSFATA (SADRE) NA RAST
KRISTALOV KALCIJEVEGA SULFAT-POLHIDRATA
Hongbin Tan
1,2,3*
, Faqin Dong
3**
, Jinfeng Liu
1
, Jin Wang
1
, Xiaochun He
1
1
Southwest University of Science and Technology, School of Materials Science and Engineering, State Key Laboratory of
Environment-friendly Energy Materials, Qinglong Road 59, Mianyang 621010, China
2
Shaanxi University of Technology, Shaanxi Engineering Center of Metallurgical Sediment Resource, Hanzhong Shaanxi 723000, China
3
Southwest University of Science and Technology, Ministry of Education, Key Laboratory of Solid Waste Treatment and Resource Recycle,
Qinglong Road 59, Mianyang Sichuan 621010, China
*hb-t@163.com (H. Tan), **fqdong@swust.edu.cn (F. Dong)
Prejem rokopisa – received: 2018-04-24; sprejem za objavo – accepted for publication: 2018-05-10
doi:10.17222/mit.2018.082
The influences of organic compounds, supernatant, calcium phosphate and calcium fluoride on the morphology of calcium
sulfate hemihydrate were investigated in hydrothermal conditions by using calcium sulphate dihydrate of analytical grade as the
raw material, while the organic compounds and supernatant were obtained from phosphogypsum (PG). The influence of the
supernatant on the crystal growth was greater than the organic compounds. The crystal diameter increased and the aspect ratio
decreased, while the added amounts of calcium fluoride and calcium phosphate increased. The influence of calcium phosphate
on the crystal growth was more obvious than with calcium fluoride. Extremely non-uniform whiskers were observed when the
amount of calcium fluoride and calcium phosphate were5%and3%( w/%), respectively.
Key words: phosphogypsum; calcium sulfate hemihydrate; crystal morphology; impurities
Avtorji so v pri~ujo~em prispevku raziskovali vplive organskih spojin, plavajo~ih ostankov, kalcijevega sulfata in kalcijevega
fluorida na morfologijo kalcijevega sulfat polhidrida v hidrotermalnih pogojih. Kot surovino so uporabili sulfat dihidrat anali-
ti~ne ~istosti. S to sintezo so dobili organske spojine in plavajo~e ostanke iz fosfatnega gipsa (PG). Vpliv plavajo~ih ostankov na
rast kristalov je bil ve~ji kot vpliv organskih spojin. Premer kristalov je nara{~al, medtem ko je razmerje med dol`ino in
premerom kristalov padalo z nara{~anjem dodatka kalcijevega fluorida oz. kalcijevega fosfata. Vpliv kalcijevega fosfata na rast
kristalov je bil o~itnej{i kot vpliv jedavca (kalcijevega fluorida). Vlakna skrajno nepravilnih oblik so nastajala, ko je bila
vsebnost kalcijevega fluorida in kalcijevega fosfata5%oz.3%.
Klju~ne besede: fosfatni gips (sadra), kalcijev sulfat polhidrat, kristalna morfologija, ne~isto~e
1 INTRODUCTION
Phosphogypsum (PG) is a major solid waste when
phosphoric acid (H
3
PO
4
) is manufactured using a wet
acid process, of which gypsum (CaSO
4
·2H
2
O) is the
major component.
1
About 4.5–5 kg of PG is generated
for every kg of P
2
O
5
produced. Almost 55 million tons of
PG are generated annually in China and its output is
estimated to be around 280 million tons worldwide per
year.
2,3
Currently, PG is applied in fields, as soil-stabili-
zation amendments, agricultural fertilizers, cement retar-
ders, building bricks/blocks and cementitious binders.
3,4
However, the reuse proportion of PG is lower than 10 %,
while the vast majority of PG is dumped in large
stockpiles, which are exposed to weathering processes
without any treatment.
3
PG contains metals, organic
substances and other potentially toxic elements, which
have potential environmental impacts.
5
Therefore, the
effective utilization of PG cannot only save the natural
gypsum, but protect the environment.
6
It will be of
interest to see the transformations of PG to obtain
high-value-added gypsum products.
7
Although six different gypsum phases are known,
only anhydrous calcium sulfate (AH, CaSO
4
), calcium
sulfate hemihydrate (HH, CaSO
4
·0.5H
2
O), and calcium
sulphate dihydrate (DH, CaSO
4
·2H
2
O) are the usually
encountered three phases.
6
Among gypsum products, HH
crystals have high economic value and have been widely
utilized.
Recently, much more attention has been paid to the
morphological control of HH crystals, since many
applications are dependent on the crystal morphology.
8
For example, acicular HH crystals (whiskers) are widely
used as the reinforcing agent in many fields, such as in
rubbers, plastics, adhesives, friction materials, paper-
making and environmental protection.
9–11
Short columnar
crystals are used as a cementitious material for its supe-
rior workability and high strength, such as in ceramics,
molding, special binder systems, industrial arts and
architecture, as well as in the construction industry.
7
The morphology of HH is usually influenced by pro-
cess parameters and organic/ inorganic additives.
12–14
Han et al. investigated the influence of
Na
2
HPO
4
·12H
2
O on the hydrothermal formation of HH
Materiali in tehnologije / Materials and technology 52 (2018) 5, 633–637 633
UDK 620.1:661.842:543.38 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(5)633(2018)

whiskers from calcium sulfate dihydrate (CaSO
4
·2H
2
O)
at 135 °C.
15
They observed that HH whiskers were pre-
pared at 135 °C for 2 h by adding 8.72 × 10
–3
mol/L
Na
2
HPO
4
·12H
2
O. When the P
2
O
5
content was more than
0.6 %, it was difficult to obtain thin and high-aspect-ratio
whiskers.
16
He et al. prepared HH whiskers in glycerol
solution, where the glycerol plays an important role in
promoting the dissolution of CaSO
4
·2H
2
O and guiding
the growth of CaSO
4
·0.5H
2
O whiskers along the c-axis.
6
The PG usually contains P, F, and organic com-
pounds, which will affect the morphology of HH. But the
details need to determined. In this work, the influences
of organic compounds and the supernatant of PG,
calcium phosphate and calcium fluoride on the morphol-
ogy of HH were investigated.
2 EXPERIMENTAL PART
Commercial calcium sulphate dihydrate, calcium
phosphate, calcium fluoride and sulphuric acid of analy-
tical grade were provided by Chengdu Kelong Chemical
Reagent Co. Ltd., China. PG was provided by Guangxi
Luzhai Chemical fertilizer Co. Ltd., China.
The X-ray diffraction (XRD) pattern and chemical
composition of the as-received PG are shown in Figure 1
and Table 1, respectively. The PG contained 50.37 % of
mass fractions of SO
3
, 39.60 % CaO, 7.99 % SiO
2
,
1.07 % P
2
O
5
, etc. The PG phases were mainly
CaSO
4
·2H
2
O (85.68 %), a minor amount of poorly
crystalline CaSO
4
·0.5H
2
O, and crystalline SiO
2
.
A 100 g of PG was added to 2000 mL of water and
stirred to obtain an organic compounds foam. The orga-
nics mainly came from the floatation reagent of phos-
phorite, where the floatation reagent is a surface-active
agent. The supernatants were obtained from leaching PG
for (2, 4 and 6) h. The chemical compositions of the
supernatants are shown in Table 2, where the super-
natants contained some impurities (e.g., P, F, Na, etc).
The iron content was almost unchanged, while the
leaching time increased.
Table 1 Chemical composition of PG (w/%)
CaO SO
3
SiO
2
P
2
O
5
Al
2
O
3
Fe
2
O
3
K
2
O SrO MgO
39.60 50.37 7.99 1.07 0.51 0.21 0.13 0.08 0.04
The FT-IR spectra of the organics foam and the
supernatants are shown in Figure 2. It is clear that the
bands at 3270 cm
–1
and 1639 cm
–1
are assigned to the
OH of the water stretching modes and bending modes,
respectively. The band at 1120 cm
–1
can be assigned to
the organics stretching or bending modes. The band at
400 cm
–1
can be assigned to the hydrated ion stretching
or bending modes.
2.0 g DH and 40 mL water/supernatant were added
into a 50-mL Teflon stainless-steel autoclave. To some of
solution, calcium phosphate, calcium fluoride or the
organics were added. The reaction systems were stirred
with a glass rod, and then aged at 140 °C for 2 h. Finally,
the autoclaves were cooled down to room temperature
naturally, and the samples were filtered and then washed
with hot distilled water and dried at 100 °C for 5 h.
The morphologies of samples were characterized by
scanning electron microscopy (SEM, TM-1000, Hitachi,
Japan). The phases of the PG and the samples were
identified by X-ray powder diffraction (XRD) (X’pert
PRO, PANalytical, Netherlands) using Cu-K
 radiation
H. TAN et al.: EFFECTS OF IMPURITIES FROM PHOSPHOGYPSUM ON THE CRYSTAL GROWTH ...
634 Materiali in tehnologije / Materials and technology 52 (2018) 5, 633–637
Figure 2: FT-IR spectra of the supernatant leached for: a) 2 h, b) 6 h
and c) the organics foam of PG Figure 1: XRD pattern of received PG
Table 2 Chemical compositions of the supernatants (mg/L)
Leaching time (h) As Ca K Mg Mn Na P Si Sr F Cl
2 0.16 371.28 8.53 7.39 0.641 17.79 2.14 1.49 2.54 0.26 10.27
4 0.16 322.70 8.50 7.25 0.59 16.74 2.04 1.25 2.25 0.27 10.29
6 0.18 311.10 8.27 10.08 0.63 16.90 2.46 1.30 2.40 0.21 10.29

(k = 0.154178 nm). The chemical compositions of the
supernatants were tested with an ion chromatograph
(Compact IC pro 881, Metrohm, Switzerland) and induc-
tively coupled plasma atomic emission spectroscopy
(ICP) (ICAP 6500, ThermofiShher scientific, UK).
3 RESULTS AND DISCUSSION
(1) Effects of PG impurities on the crystal growth
The SEM micrographs of the samples prepared
with/without PG impurities are shown in Figure 3. The
HH whiskers were obtained without the addition of PG
impurities (Figure 3a), with a 0.5–1.5 μm diameter and a
30–40 aspect ratio.
The formation mechanism of calcium sulfate
hemihydrates whiskers was a dissolution-recrystalli-
zation. The formation of the whiskers underwent a
three-stage process. Firstly, the homogeneous nucleation
of calcium sulfate hemihydrate took place; secondly,
self-assembly of calcium sulfate hemihydrate aggregated
co-oriented along their c axes; finally, the crystalline
grain grown occurred and the whiskers formed.
17
Cal-
cium sulfate hemihydrate normally crystallizes in the
whiskers (1D shape) because the crystal lattice of cal-
cium sulfate hemihydrate consists of –SO
4
–Ca–SO
4
–Ca–
chains, with each S atom and four O atoms forming a
tetrahedron. These chains are hexagonally arranged and
form a framework parallel to the c axis, where one water
molecule is attached to every two calcium sulfate
molecules.
18
The HH whiskers were obtained by adding organics,
where the organics were obtained by adding PG to water
and stirring to produce the organics foam. The HH
crystals were obtained with about 0.2–4 μm diameter and
an about 10–20 aspect ratio (Figure 3b).
The HH whiskers were obtained by adding super-
natant, where the supernatant was obtained by leaching
PG. The HH crystals were obtained with 0.2–4 μm in
diameter and an about 1–6 aspect ratio (Figure 3c).
The mechanism of impurities on the crystal morphol-
ogy control as through selective adsorption on specific
crystal planes to alter their surface energy. During the
hydrothermal process, the impurity molecules tend to
absorb on the polar crystal faces rather than on the
prismatic faces, which would inhibit the crystal growth
along the plane.
15–18
According to the above results, the influence of the
supernatant on the HH crystal growth was greater than
the organics. In these impurities, PO
4
3–
and F
–
can react
with Ca
2+
to form difficult-to-dissolve compounds
(Ca
3
(PO
4
)
2
, CaF
2
), which can firmly adsorb on the HH
crystal surfaces and hinder the crystal growth. On the
other hand, the impurity compounds were also possibly
the crystal nucleus for the HH and influenced the HH
crystal growth.
(2) Effects of calcium fluoride/calcium phosphate on
the crystal growth
The SEM micrographs of the samples prepared by
adding calcium fluoride are shown in Figure 4.T h e
H. TAN et al.: EFFECTS OF IMPURITIES FROM PHOSPHOGYPSUM ON THE CRYSTAL GROWTH ...
Materiali in tehnologije / Materials and technology 52 (2018) 5, 633–637 635
Figure 4: SEM micrographs of the samples prepared by adding cal-
cium fluoride: a) 0.5 %, b) 1.0 %, c) 3.0 %, d) 5.0 %, e) 10.0 %,
f) 15.0 %
Figure 3: SEM micrographs of the samples prepared with/without PG impurities: a) without the impurities, b) with the organics foam, c) with the
supernatant

influence on the crystal growth was little by adding a
0.5 % calcium fluoride. The diameter increased and the
aspect ratio decreased, while the addition amount of
calcium fluoride increased. The crystals were obtained
by adding 15 % of mass fractions of calcium fluoride,
with 0.5–3.5 μm diameter and a 6–30 aspect ratio.
The SEM micrographs of the samples prepared by
adding calcium phosphate are shown in Figure 5. The
crystals, with a 15–30 aspect ratio and a 0.6–2.0 μm
diameter were obtained by adding 0.5 % calcium phos-
phate. And some short crystals were also observed.
Some particles of calcium sulfate were observed by
adding 1.0 % calcium phosphate. The crystals were
obtained by adding 15 % calcium phosphate, with
1.0–3.5 μm diameter and a 4–10 aspect ratio.
The pK
sp
of calcium sulphate dihydrate, calcium
fluoride and calcium phosphate are 6.29, 9.79 and 28.70,
respectively. The dissolved PO
4
3–
to from the calcium
phosphate can absorb on the crystal surface and hinder
the crystal growth. The dissolved F
–
to from the calcium
fluoride can also absorb on the crystal surface and hinder
the crystal growth. The influence of calcium fluoride on
the crystal growth was not more obvious than calcium
phosphate, because the solubility of the calcium fluoride
is higher than the calcium phosphate.
On the other hand, extremely un-uniform whiskers
can be observed when the amount of calcium fluoride
was 5 % and the amount of calcium phosphate was 3 %.
The XRD patterns of the samples prepared without,
with the organics, the supernatant, calcium fluoride and
calcium phosphate are shown in Figure 6. The peaks of
the samples presented analogous diffraction angles,
indicating that the phases of all the samples were same
calcium sulfate hemihydrate (CaSO
4
·0.5H
2
O). The peaks
of calcium phosphate and calcium fluoride were not
obvious, because the peaks were much weaker than the
HH peaks.
According to the above result, the HH crystal growth
was most affected by the soluble phosphorus of the
supernatant. The soluble phosphorus can be removed by
using water to wash the PG.
6
The effect of soluble
phosphorus on the crystal growth can also be decreased
by adding CaO in the PG.
16
4 CONCLUSIONS
The HH crystals were obtained by adding organics
with an about 0.2–4 μm diameter and an about 10–20
aspect ratio. The HH crystals were obtained by adding
the supernatant, with a 0.2–4 μm diameter and an about
1–6 aspect ratio. The influence of the supernatant on the
HH crystals’ growth was greater than the organics.
The crystal diameter increased and aspect ratio
decreased when the added amount of calcium fluoride
and calcium phosphate increased, respectively. The
influence of calcium phosphate on the crystal growth
was more obvious than the calcium fluoride. Extremely
non-uniform whiskers can be observed when the
amounts of calcium fluoride and calcium phosphate were
5and3%( w/%), respectively.
Acknowledgements
This work was supported by the Sichuan Science and
Technology Development Program of China
H. TAN et al.: EFFECTS OF IMPURITIES FROM PHOSPHOGYPSUM ON THE CRYSTAL GROWTH ...
636 Materiali in tehnologije / Materials and technology 52 (2018) 5, 633–637
Figure 6. XRD patterns of the samples prepared: a) without the
impurities, with b) the organics form, c) the supernatant d) calcium
fluoride and e) calcium phosphate
Figure 5. SEM micrographs of the samples prepared by adding
calcium phosphate: a) 0.5 %, b) 1.0 %, c) 3.0 %, d) 5.0 %, e) 10.0 %,
f) 15.0 %

(2017GZ0380) and Natural Science Foundation of
Southwest University of Science and Technology
(18zx7101).
5 REFERENCES
1
O. Hentati, N. Abrantesd, A. L. Caetano, S. Bouguerra, F. Gonçalves,
J. Römbkeg, R. Pereira, Phosphogypsum as a soil fertilizer:
Ecotoxicity of amended soil and elutriates to bacteria, invertebrates,
algae and plants, J. Hazar. Mater., 294 (2015), 80–89,
doi:10.1016/j.jhazmat.2015.03.034
2
M. Contreras, R. Pérez-López, M. J. Gázquez, V. Morales-Flórez, A.
Santos, L. Esquivias, J. P. Bolívar, Fractionation and fluxes of metals
and radionuclides during the recycling process of phosphogypsum
wastes applied to mineral CO2 sequestration, Waste Manage., 45
(2015), 412–419, doi:10.1016/j.wasman.2015.06.046
3
Y. Shen, J. Qian, J. Chai, Y. Fan, Calcium sulphoaluminate cements
made with phosphogypsum: Production issues and material proper-
ties, Cem. Concr. Compos., 48 (2014), 67–74, doi:10.1016/
j.cemconcomp.2014.01.009
4
M. Zielinski, Influence of constant magnetic field on the properties
of waste phosphogypsum and fly ash composites, Constr. Build.
Mater., 89 (2015), 13–24, doi:10.1016/j.conbuildmat.2015.04.029
5
A. A. Cuadri, F. J. Navarro, M. García-Morales, J.P. Bolívar, Valori-
zation of phosphogypsum waste as asphaltic bitumen modifier, J.
Hazar. Mate., 279 (2014), 11–16, doi:10.1016/j.jhazmat.2014.06.058
6
H. He, F. Dong, P. He, L. Xu, Effect of glycerol on the preparation of
phosphogypsum-based CaSO4·0.5H2O whiskers, J. Mater. Sci., 49
(2014), 1957–1963, doi.10.1007/s10853-013-7825-4
7
B. Guan, L. Yang, H. Fu, B. Kong, T. Li, L.Yang,  -calcium sulfate
hemihydrate preparation from FGD gypsum in recycling mixed salt
solutions, Chem. Eng. J., 174 (2011), 296–303, doi:10.1016/
j.cej.2011.09.033
8
X. Mao, X. Song, G. Lu, Y. Xu, Y. Sun, J. Yu, Effect of additives on
the morphology of calcium sulfate hemihydrate: Experimental and
molecular dynamics simulation studies, Chem. Eng. J., 278 (2015),
320–327, doi:10.1016/j.cej.2014.10.006
9
X. Feng, Y. Zhang, G. Wang, M. Miao, L. Shi, Dual-surface modifi-
cation of calcium sulfate whisker with sodium hexametaphos-
phate/silica and use as new water-resistant reinforcing fillers in
papermaking, Powder Tech., 271 (2015), 1–6, doi:10.1016/j.powtec.
2014.11.015
10
Z. Zhu, L. Xu, G. Chen, Effect of different whiskers on the physical
and tribological properties of non-metallic friction materials, Mater.
Des., 32 (2011), 54–61, doi:10.1016/j.matdes.2010.06.037
11
X. Chen, L. Yang, J. Zhang, Y. Huang, Exploration of As(III)/As(V)
uptake from aqueous solution by synthesized calcium sulfate
whisker, Chin. J. Chem. Eng., 22 (2014), 1340-1346, doi:10.1016/
j.cjche.2014.09.018
12
C. Hazra, S. Bari, D. Kundu, A. Chaudhari, S. Mishra, A. Chatterjee,
Ultrasound-assisted/biosurfactant-templated size-tunable synthesis of
nano-calcium sulfate with controllable crystal morphology, Ultrason.
Sonochem., 21 (2014), 1117–1131, doi:10.1016/j.ultsonch.2013.
12.020
13
J. Mao, G. Jiang, Q. Chen, B. Guan, Influences of citric acid on the
metastability of  -calcium sulfatehemihydrate in CaCl2 solution,
Coll. Surf. A: Phys. Eng. Aspects, 443 (2014), 265–271,
doi:10.1016/j.colsurfa.2013.11.023
14
B. Kong, J. Yu, K. Savino, Y. Zhu, B. Guan, Synthesis of  -calcium
sulfate hemihydrate submicron-rods in water/n-hexanol/CTAB
reverse microemulsion, Coll. Surf. A: Phys. Eng. Aspects, 409
(2012), 88–93, doi:10.1016/j.colsurfa.2012.05.041
15
Q. Han, K. Luo, H. Li, L. Xiang, Influence of disodium hydrogen
phosphate dodecahydrate on hydrothermal formation of hemihydrate
calcium sulfate whiskers, Particuology, 17 (2014), 131–135,
doi:10.1016/j.partic.2013.10.002
16
F. Dong, Z. Huang, H. Tan, C. Wu, P. He, Effect of additives on
calcium sulfate hemihydrate whiskers morphology from calcium
sulfate dehydrate and phosphogypsum, Mater. Manuf. Process, 31
(2016), 2037–2043, doi:10.1080/10426914.2016.1176184
17
F. Li, J. Liu, G. Yang, Z. Pan, X. Ni, H. Xu, Q. Huang, Effect of pH
and succinic acid on the morphology of  -calcium sulfate Hemi-
hydrate synthesized by a salt solution method, J. Crys. Grow., 374
(2013), 31–36, doi:10.1016/j.jcrysgro.2013.03.042
18
Y.-W. Wang, C. Meldrum, Additives stabilize calcium sulfate hemi-
hydrate (bassanite) in solution, J. Mater. Chem., 22 (2012),
22055–22062, doi:10.1039/C2JM34087A
H. TAN et al.: EFFECTS OF IMPURITIES FROM PHOSPHOGYPSUM ON THE CRYSTAL GROWTH ...
Materiali in tehnologije / Materials and technology 52 (2018) 5, 633–637 637