Effect of Molecular Structure of Organic Acids on the Crystal Habit Of

crystals

Article Eﬀect of Molecular Structure of Organic Acids on the Crystal Habit of α-CaSO4·0.5H2O from Phosphogypsum

Xianbo Li 1,2,3 and Qin Zhang 1,2,3,* 1 Mining College, Guizhou University, Guiyang 550025, China; [email protected] 2 National & Local Joint Laboratory of Engineering for Eﬀective Utilization of Regional Mineral Resources from Karst Areas, Guiyang 550025, China 3 Guizhou Key Lab of Comprehensive Utilization of Nonmetallic Mineral Resources, Guiyang 550025, China * Correspondence: [email protected]

Received: 22 November 2019; Accepted: 4 January 2020; Published: 6 January 2020

Abstract: The organic acid crystal modifiers play an important role in the control of the crystal habit of α-hemihydrate gypsum (α-CaSO 0.5H O) from phosphogypsum, but the molecular structure 4· 2 characteristics of crystal modifiers have not been clarified, which makes it difficult to judge whether an organic acid has the ability to regulate the crystal habit of α-CaSO 0.5H O directly. In this work, 4· 2 the effect of organic acids with different molecular structures on the crystal habit of α-CaSO 0.5H O 4· 2 and its adsorption differences onto the α-CaSO 0.5H O surface were explored. The results show 4· 2 that the molecular structure characteristics of crystal modifiers contain two or more carboxylic groups (COOH) that are separated by two methylene or methine groups. Furthermore, organic acids with the regulation ability can adsorb on the surface of α-CaSO 0.5H O and change its growth 4· 2 habit. With the increase in the crystal modifier concentration, the α-CaSO 0.5H O crystals are 4· 2 shortened in length and enlarged in width, resulting in the decrease in aspect ratio and the increase in compressive strength. Conversely, when the adsorption of ineffective organic acids on the surface of α-CaSO 0.5H O was not detected, the α-CaSO 0.5H O crystals remained long hexagonal prisms. 4· 2 4· 2 These results have guiding significance for the screening of novel organic acid crystal modifiers.

Keywords: α-hemihydrate gypsum; crystal habit; organic acid; crystal modiﬁer; molecular structure; phosphogypsum

1. Introduction Phosphogypsum (PG) is an industrial solid waste produced during the production of phosphoric acid from phosphate ore [1,2], which is an important renewable gypsum resource. The main constituent of PG is calcium sulfate dihydrate (CaSO 2H O), but it also contains impurities such as soluble 4· 2 phosphate, co-crystallized phosphorus, soluble fluoride, and organic matters as well as heavy metals and radionuclides [3–6]. The utilization of PG is severely restricted by these harmful impurities, so the comprehensive utilization rate is only 39.7% [7]. Currently, the preparation of α-hemihydrate gypsum (α-CaSO 0.5H O) using PG as the raw material is an important direction for its high value-added 4· 2 utilization. α-CaSO 0.5H O has excellent mechanical strength, work performance, and environmental 4· 2 performance, which has been widely used in precision casting, high-end ceramics, functional filler, decorative materials, and so on [8,9]. However, the impurities have a detrimental effect on the preparation of α-CaSO 0.5H O; the soluble phosphate and co-crystallized phosphorus can decrease 4· 2 the slurry pH and retard the hydration of α-CaSO 0.5H O, and then reduce the strength of hardened 4· 2 gypsum. Similarly, the soluble fluoride can decrease the gypsum strength [10]. Therefore, PG should be pretreated to remove the soluble impurities. In addition, the radionuclides of PG are derived from

Crystals 2020, 10, 24; doi:10.3390/cryst10010024 www.mdpi.com/journal/crystals Crystals 2020, 10, 24 2 of 15 phosphate ores, mainly Ra226, Th232, and K40, and the radioactivity of PG must meet the standard of GB 6566-2010 (limits of radionuclides in building materials) in order for it to be used as a building material. Notably, the crystal habit is one of the most important factors for affecting the mechanical strength of α-CaSO 0.5H O[11–14]. Compared with the needle-shaped α-CaSO 0.5H O, the short columnar 4· 2 4· 2 α-CaSO 0.5H O with a low aspect ratio and a large width shows better mechanical strength [15]. 4· 2 Therefore, the key to the preparation of α-CaSO 0.5H O is to select a suitable crystal modifier to 4· 2 control the crystal habit of α-CaSO 0.5H O. 4· 2 Many studies have proven that some organic acids can be used as a crystal modifier to regulate the crystal shape of α-CaSO 0.5H O particles. For instance, the short hexagonal prism 4· 2 α-CaSO 0.5H O, with an aspect ratio of 1.0–1.4, could be obtained by adding succinic acid 4· 2 (C H O )[15–18] and sodium succinate (C H O Na 6H O) [19]. The pilot scale tests further 4 6 4 4 4 4 2· 2 demonstrated that α-CaSO 0.5H O with an aspect ratio of 2.8–3.2 can be synthesized by adding 4· 2 a small amount of C H O and sodium borate (Na B O 5H O) [20]. In the presence of maleic acid 4 6 4 2 4 7· 2 (C H O ), the crystal habit of α-CaSO 0.5H O evolved from a needle-like whisker to a short equiaxial 4 4 4 4· 2 crystal [21]. The crystal habit of α-CaSO 0.5H O crystals can be influenced by adding citric acid 4· 2 (C H O H O) [22] and sodium citrate (Na C H O 2H O) [12,13]; the aspect ratio of α-CaSO 0.5H O 6 8 7· 2 3 6 5 7· 2 4· 2 crystal is decreased from 10.3 to 0.6 with the increase in the sodium citrate dosage from 0 to 0.045 wt% [23]. Recently, Li et al. [24] found that L-aspartic acid (C4H7NO4) is an effective crystal modifier for regulating the crystal shape of α-CaSO 0.5H O. Additionally, Wang et al. [25] demonstrated that 4· 2 disodium ethylenediamine tetraacetate (C10H14N2Na2O8) influences the crystal habit and nucleation of α-CaSO 0.5H O. In particular, Badens et al. [26] investigated the influence of different organic additives 4· 2 on the morphology of gypsum crystals where the results showed that there is an obvious relationship between the rate retarding effect and structural matching between the crystal faces involved and the conformation of the molecule of the additive. The acicular gypsum crystals in pure solution transformed into flat crystals with much more twinned crystals in the presence of malic acid at 1000 ppm. Nevertheless, not all organic acids possess the ability to control the crystal shape of α- CaSO 0.5H O, and only a few reports have been published regarding the molecular structure 4· 2 characteristics of organic acids as crystal modifiers. This makes it difficult to determine, without testing, whether an organic acid has the ability to regulate the crystal habit of α-CaSO 0.5H O, 4· 2 and a considerable amount of experimental work is needed to confirm the effectiveness of organic acids. Therefore, the effects of organic acids with different molecular structures on the crystal habit and mechanical properties of α-CaSO 0.5H O were systematically investigated. The organic 4· 2 acids used in this paper can be divided into three families: benzenedicarboxylic acids, amino acids, and straight-chain dicarboxylic acids. According to the comparative research of benzenedicarboxylic acids and straight-chain dicarboxylic acids, the effect of different carbon chain lengths between two carboxylic groups (COOH) on the crystal habit of α-CaSO 0.5H O can be clarified. Based on the 4· 2 amino acids, the effect of the number of COOH and the feasibility of the amino group (NH2) instead of a hydroxyl group (OH) linked to the carbonyl group (C=O) on the crystal morphology can be clearly defined. Furthermore, the adsorption differences of organic acids on the surface of α-CaSO 0.5H O 4· 2 particles were also discussed. The findings have a certain guiding significance for the screening or design and synthesis of novel organic acid crystal modifiers purposefully.

2. Materials and Methods

2.1. Materials

Analytical grade calcium chloride (CaCl2) and organic acids were obtained from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. PG was collected from the Wengfu (Group) Co. Ltd., Guizhou Province, China. The x-ray diffraction analysis indicated that the main phase of PG is CaSO 2H O with a little quartz (SiO ) and co-crystallized phosphorus (CaHPO 2H O). The contents 4· 2 2 4· 2 of the CaSO 2H O, soluble phosphate and soluble fluorine were 91.32 wt%, 0.18 wt%, and 0.01 wt%, 4· 2 Crystals 2020, 10, 24 3 of 15 respectively [21]. In order to reduce the influence of soluble impurities on the preparation process and mechanical strength of α-CaSO 0.5H O, the soluble phosphate and soluble fluoride were solidified 4· 2 by adding 0.6 wt% quicklime (mass ratio of lime to PG), and the removal rates were 90.45% and 36.36%, respectively. Radionuclides exceeding the standard of GB 6566-2010 are one of the important factors restricting the utilization of solid waste resources. PG generally contains a small amount of radionuclides, and the radioactivity level of PG varies from region to region. Therefore, the specific activity of PG should be tested before it is used. The results are shown in Table1 where it can be seen that the radioactivity of PG meets the requirements of the main materials for building and class A decorative materials (GB 6566-2010). Therefore, PG can be used to prepare α-CaSO 0.5H O cementitious materials. 4· 2 Table 1. Radioactivity of phosphogypsum (PG).

Radioactivity Nuclide Speciﬁc Activity Internal External Items Exposure Index Exposure Index 226Ra 232Th 40K PG 61.0 2.3 3.3 0.31 0.17 Main materials for building 1.0 1.0 ≤ ≤ Class A decorative 1.0 1.3 materials ≤ ≤

2.2. Preparation of α-CaSO 0.5H O Crystals 4· 2 The atmospheric salt solution method was adopted to prepare α-CaSO 0.5H O powders. 4· 2 The hydrothermal reaction was carried out in the CaCl2 solution, and the original solution pH was 8.6. First, 1.5 L of 2.97 M CaCl2 and diﬀerent concentrations of organic acid mixed solutions were preheated by oil bath in a 2.0 L three-mouth ﬂask. The solution temperature was monitored by a thermometer inserted into the reactor and maintained with a deviation of 0.5 C. Then, 500 g of ± ◦ pretreatment PG was added into the mixed solutions when the temperature reached 95 ◦C, and the slurry in the reactor was stirred at a rate of 200 r/min. During the experiments, 10 mL of the suspension was sampled at 30 min intervals to observe the crystal habit of the reaction product.

2.3. Characterization Methods The crystal habit of the α-CaSO 0.5H O particles was observed by a scanning electronic microscope 4· 2 (SEM, SU8010, Hitachi, Japan). The length and width of the α-CaSO 0.5H O particles were measured 4· 2 by the image analysis software Image-Pro Plus, and at least 100 particles were counted to calculate the average aspect ratio of α-CaSO 0.5H O particles. The length, width, and aspect ratio data are shown 4· 2 as the “mean value standard deviation of the mean”. ± Fourier transform infrared spectroscopy (FTIR, VERTEX 70, Bruker, Germany) was adopted to clarify the adsorption of organic acids on the surface of α-CaSO 0.5H O particles. The α-CaSO 0.5H O 4· 2 4· 2 powders prepared with diﬀerent organic acids were further ground to ~5 µm and then mixed with 1 KBr. The spectra were recorded with a scanning range from 400 to 4000 cm− at room temperature. The adsorption of the organic acids on the surface of the α-CaSO 0.5H O particles was analyzed by 4· 2 the change in the characteristic adsorption peaks in the spectra. The as-prepared α-CaSO 0.5H O powders were mixed with deionized water to form a slurry. 4· 2 Then, the slurry was poured into iron molds, which were vibrated ten times to remove bubbles in the slurry. After being maintained for 2 h, the iron molds were removed, and the plasters were cured for 24 h at 20 2 C and 65 5% RH. Finally, the plasters were dried to a constant weight in an ± ◦ ± oven with the temperature set to 40 1 C, and the compressive strength of the hardened pastes ± ◦ were determined by a microcomputer-controlled compressive testing machine (YAW-300B, Zhejiang Yingsong Instrument and Equipment Manufacturing Co. Ltd., China). Crystals 2020, 10, 24 4 of 15

Crystals3. Results 2020, 1 and0, x FOR Discussion PEER REVIEW 4 of 15

3.1.3.1. Effect Effect of of Benzenedicarboxylic Benzenedicarboxylic Acid Acid on on the the Crystal Crystal Habit Habit of of αα--CaSOCaSO4·0.5H0.5H2OO 4· 2 The benzenedicarboxylic acid (BA, HOOCC6H4COOH) has three kinds of isomers: phthalic acid The benzenedicarboxylic acid (BA, HOOCC6H4COOH) has three kinds of isomers: phthalic (PA),acid (PA),isophthalic isophthalic acid (IPA) acid, (IPA),and terephthalic and terephthalic acid (TA), acid the (TA), molecular the molecular structural structural formulas formulas of which of arewhich shown are shown in Figure in Figure 1. The1. PA, The PA,IPA IPA,, and and TA TA molecules molecules contain contain two two COOH, COOH, but but the the separation betweenbetween the the two two COOH COOH are are different different.. At At this this time time,, using using BA BA as as a a crystal crystal modifier modifier for for the the preparation preparation ofof αα--CaSOCaSO4·0.5H0.5H2OO has has not not been been reported reported yet, yet, based based on on the the available available literature. literature. 4· 2

FigureFigure 1. 1. MoleculaMolecularr structural structural formula formula of of (a (a) )PA, PA, (b (b) )IPA IPA,, and and ( (cc)) TA TA..

The effects of BA on the crystal habit and size of α-CaSO 0.5H O crystals are presented in Figure2 The effects of BA on the crystal habit and size of α-CaSO44··0.5H22O crystals are presented in Figure and Table2, respectively. The crystal habit of α-CaSO 0.5H O crystals were significantly affected by 2 and Table 2, respectively. The crystal habit of α-CaSO4· 4·0.5H2 2O crystals were significantly affected PA concentration. In the absence of PA, α-CaSO 0.5H O exists as a long columnar crystal, the average by PA concentration. In the absence of PA, α-CaSO4· 42·0.5H2O exists as a long columnar crystal, the length and width of which were 59.04 14.08 µm and 11.04 3.64 µm, respectively, while the aspect average length and width of which were± 59.04 ± 14.08 μm and± 11.04 ± 3.64 μm, respectively, while ratio reached 5.74 1.84. In addition, the XRD pattern of the reaction product matched well with the the aspect ratio reache± d 5.74 ± 1.84. In addition, the XRD pattern of the reaction product matched well standard pattern of α-CaSO 0.5H O (Joint Committee on Powder Diffraction Standards (JCPDS) card with the standard pattern of4· α-CaSO2 4·0.5H2O (Joint Committee on Powder Diffraction Standards 41-0224), and no characteristic peaks of CaSO 2H O (JCPDS card 33-0311) were present (Figure3). (JCPDS) card 41-0224), and no characteristic peaks4· of2 CaSO4·2H2O (JCPDS card 33-0311) were present The results indicate that the PG was completely transformed into α-CaSO 0.5H O. (Figure 3). The results indicate that the PG was completely transformed into4· α-CaSO2 4·0.5H2O. The α-CaSO 0.5H O crystals were shortened in length and enlarged in width with an increase in The α-CaSO44·0.5H· 22O crystals were shortened in length and enlarged in width with an increase PA concentration. In the presence of 0.07 mM PA, the average length of the α-CaSO 0.5H O particles in PA concentration. In the presence of 0.07 mM PA, the average length of the4 α· -CaSO2 4·0.5H2O shortened to 42.56 10.02 µm, the width increased to 16.09 3.55 µm, and the aspect ratio was particles shortened to± 42.56 ± 10.02 μm, the width increased to ±16.09 ± 3.55 μm, and the aspect ratio decreased to 2.72 0.70. With a further increase in PA concentration to 0.20 mM, the average length was decreased to 2.72± ± 0.70. With a further increase in PA concentration to 0.20 mM, the average and width of the α-CaSO 0.5H O particles were 32.19 6.87 µm and 20.73 5.26 µm, respectively, length and width of the 4α· -CaSO2 4·0.5H2O particles were± 32.19 ± 6.87 μm ± and 20.73 ± 5.26 μm, and the corresponding aspect ratio decreased to 1.61 0.39. Accordingly, it can be concluded that respectively, and the corresponding aspect ratio decrease± d to 1.61 ± 0.39. Accordingly, it can be PA is an effective crystal modifier to control the crystal habit of α-CaSO 0.5H O. However, it is concluded that PA is an effective crystal modifier to control the crystal habit4· of2 α-CaSO4·0.5H2O. interesting to note that the growth habits of α-CaSO 0.5H O were nearly unchanged at 0.20 mM IPA However, it is interesting to note that the growth habit4· s of2 α-CaSO4·0.5H2O were nearly unchanged and TA; α-CaSO 0.5H O remained a long columnar crystal, the average width was close to 10 µm, at 0.20 mM IPA and4· TA2; α-CaSO4·0.5H2O remained a long columnar crystal, the average width was and the aspect ratios were 4.24 1.43 and 4.54 1.60, respectively. Consequently, the crystal shape of close to 10 μm, and the aspect ratios± were 4.24 ± 1.43 and 4.54 ± 1.60, respectively. Consequently, the α-CaSO 0.5H O particles cannot be regulated by IPA and TA. crystal shape4· of2 α-CaSO4·0.5H2O particles cannot be regulated by IPA and TA.

Crystals 2020, 10, x FOR PEER REVIEW 5 of 15 Crystals 2020, 10, 24 5 of 15 Crystals 2020, 10, x FOR PEER REVIEW 5 of 15

Figure 2. Effect of BA on the crystal habit of CaSO4·0.5H2O. (a) 0 mM; (b) PA: 0.07 mM; (c) PA: 0.13 FiguremM; (d 2.) PA:Eﬀ ect0.20 of mM; BA on(e) theIPA: crystal 0.20 mM; habit (f of) TA: CaSO 0.20 0.5HmM. O. (a) 0 mM; (b) PA: 0.07 mM; (c) PA: 0.13 Figure 2. Effect of BA on the crystal habit of CaSO44·0.5H· 2O2 . (a) 0 mM; (b) PA: 0.07 mM; (c) PA: 0.13 mM;mM; ( (dd)) PA: PA: 0.20 0.20 mM; mM; ( (ee)) IPA: IPA: 0.20 0.20 mM; ( f) TA: 0.20 mM.mM. Table 2. Effect of benzenedicarboxylic acid (BA) on the length, width, and aspect ratio of α- Table 2. Eﬀect of benzenedicarboxylic acid (BA) on the length, width, and aspect ratio of Table 2. Effect of benzenedicarboxylicCaSO acid4·0.5H (BA)2O on particles the length,. width, and aspect ratio of α- α-CaSO4 0.5H2O particles. · CaSO4·0.5H2O particles. BA Concentration/(mM) Average Length/μm Average Width/μm Aspect Ratio BA Concentration/(mM) Average Length/µm Average Width/µm Aspect Ratio BA Concentration/(mM)0 Average59.04 Length ± 14.08/μm Average11.04 Width ± 3.64/μm Aspect5.74 ± Ratio 1.84 0 59.04 14.08 11.04 3.64 5.74 1.84 0.070 59.0442.56 ± ± 14.08 10.02 11.0416.09± ± ± 3.64 3.55 ±5.742.72 ± ± 1.84 0.70 PA 0.07 42.56 10.02 16.09 3.55 2.72 0.70 PA ± ± ± 0.070.130.13 42.56 39.3939.39 ± ±10.029.63 9.63 18.0216.0918.024.91 ± ± 3.55 4.91 2.36 2.722.360.91 ± ± 0.70 0.91 PA ± ± ± 0.130.200.20 39.39 32.1932.19 ± ± 9.636.87 6.87 20.7318.0220.73±5.26 ± 4.915.26 1.61 2.361.610.39 ± ± 0.91 0.39 ± ± ± IPAIPA 0.200.20 0.20 41.8332.1941.83 ± 12.72 6.8712.72 10.6020.73±10.603.81 ± 5.26 3.81 4.24 1.614.241.43 ± ± 0.39 1.43 ± ± ± IPATA TA0.200.20 0.20 41.83 40.3040.30 ± ± 12.7211.51 11.51 9.4610.609.462.96 ± 3.812.96 4.54 4.244.541.60 ± ± 1.43 1.60 ± ± ± TA 0.20 40.30 ± 11.51 9.46 ± 2.96 4.54 ± 1.60

▼ ▼ (b) ▼-CaSO ·0.5H O ▼ ▼ 4 2 (b) ■ -SiO ▼-CaSO2 ·0.5H O ▼ 4 2 ■ -SiO2 ▼ The reaction product The reaction product ▼ ▼ ■ ▼ ■ ▼ ▼ ▼ ■ ▼ ■ ▼ (a)

Intensity/Counts (a)

Intensity/Counts

JCPDS card of α-CaSO4·0.5H2O

0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 250 60 70 80 90 2 Figure 3. XRD patterns of (a) the JCPDS card of α-CaSO4 0.5H2O and (b) the reaction product prepared Figure 3. XRD patterns of (a) the JCPDS card of α-CaSO4·0.5H· 2O and (b) the reaction product prepared without organic acid. Figurewithout 3. XRDorganic pattern acid.s of (a) the JCPDS card of α-CaSO4·0.5H2O and (b) the reaction product prepared without organic acid.

CrystalsCrystals 20202020,, 1100,, x 24 FOR PEER REVIEW 66 of of 15 15 Crystals 2020, 10, x FOR PEER REVIEW 6 of 15 The effect of the BA concentration on the compressive strength of hardened gypsum The effect of the BA concentration on the compressive strength of hardened gypsum (CaSO4 2H2O) (CaSOThe4·2H effect2O) hydrated of the fromBA concentrationα-CaSO4·0.5H 2 onO is the shown compressive in Figure strength 4. As shown of hardened in Figure gypsum 4,· it is hydrated from α-CaSO4 0.5H2O is shown in Figure4. As shown in Figure4, it is beneficial to improve beneficial(CaSO4·2H to2O) improve hydrated the· from compressive α-CaSO4 ·0.5H strength2O is of shown α-CaSO in4 ·0.5H Figure2O 4. prepared As shown with in PA. Figure Compared 4, it is the compressive strength of α-CaSO 0.5H O prepared with PA. Compared with the blank experiment, withbeneficial the blank to improve experiment, the compressivethe compressive4· strength2 strength of α increase-CaSO4·0.5Hd from2O 13.1 prepared MPa to with 22.5 PA.MPa Comparedwith a PA the compressive strength increased from 13.1 MPa to 22.5 MPa with a PA concentration of 0.20 mM. concentrationwith the blank ofexperiment, 0.20 mM. the However, compressive when strength the concentration increased from of 13.1 IPA MPa or TA to 22.5was MPa0.20 with mM, a thePA However, when the concentration of IPA or TA was 0.20 mM, the compressive strength was only 10 MPa. compressiveconcentration strength of 0.20 was mM. only However, 10 MPa. when Therefore, the concentration the addition ofof IPA or or TA TA cannotwas 0.20 improve mM, the Therefore, the addition of IPA or TA cannot improve the mechanical strength of α-CaSO4 0.5H2O, mechanicalcompressive strength strength of was α-CaSO only 410·0.5H MPa.2O, Therefore,and they are the not addition appropriate of IPA as or crystal TA cannot modifier improves· for the and they are not appropriate as crystal modifiers for the preparation of α-CaSO4 0.5H2O by atmospheric preparationmechanical strengthof α-CaSO of4 ·0.5Hα-CaSO2O by4·0.5H atmospheric2O, and they salt aresolution. not appropriate as crystal· modifiers for the preparationsalt solution. of α-CaSO4·0.5H2O by atmospheric salt solution. 25 25

20 PA 20 PA

15 15

10 10

Compressive strength/MPa 5

0 0 0 0.07 0.13 0.20 0.20 0.20 0 0.07BA concentration/mM0.13 0.20 0.20 0.20 BA concentration/mM

Figure 4. EffectEffect of BA concentration on the compressive strength of CaSO 4·0.5H0.5H22OO.. Figure 4. Effect of BA concentration on the compressive strength of CaSO4·0.5H· 2O. 3.2. Effect of Amino Acid on the Crystal Habit of α-CaSO 0.5H O 3.2. Effect of Amino Acid on the Crystal Habit of α-CaSO44·0.5H· 22O 3.2. Effect of Amino Acid on the Crystal Habit of α-CaSO4·0.5H2O InIn order order to to investigate investigate the the feasibility feasibility of amino of amino acids acidsas crystal as crystal modifier modifiers,s, L-aspartic L-aspartic acid (L-Asp, acid

HOOC(L-Asp,In orderCH(NH HOOC–CH(NH to investigate2)CH22COOH),)–CH the2 feasibility–COOH), L-glutamic L-glutamicof amino acid (L acid-Glu, acids as (L-Glu,HOOC crystal HOOC–CH(NH CH(NHmodifier2)s, (CHL-aspartic22))–(CH2COOH) acid2)2–COOH),, ( andL-Asp, L- aHOOCandsparagine L-asparagineCH(NH (L-Asn,2) (L-Asn,CH O=C2 COOH),(NH O=C2)(NH CH L-2g )CH(NHlutamic CH2 CH(NH 2)acid–COOH) 2(L)–COOH)-Glu, were HOOC wereapplied appliedCH(NH to regulate to2) regulate(CH the2)2 COOH) thecrystal crystal ,habit and habit Lof- aofsparagineα-CaSO (L0.5H-Asn,O. O=C The molecular(NH2) CH structural2 CH(NH2 formulas)–COOH) are were shown applied in Figure to regulate5. The L-Aspthe crystal contains habit one of α-CaSO4·0.5H4· 2O.2 The molecular structural formulas are shown in Figure 5. The L-Asp contains one NHαNH-CaSO22 andand4·0.5H two two COOH,2 COOH,O. The and andmolecular the the two two structural COOH COOH are are formulas separated separated are by shown a – –CHCH in2––CH(NH CH(NHFigure 5.22))– –The group group. L-Asp. Compared Compared contains with withone theNHthe L2 L-Asp, and-Asp, two the the COOH, only only difference di andfference the two is is that thatCOOH the the two are two COOHseparated COOH are are by separated separateda –CH2– CH(NHby by a – a(CH –(CH2)–2 )group2-2CH(NH)2-CH(NH. Compared2)– group2)– group with for thefor theL--Asp,Glu L-Glu molecule.the molecule. only difference The The L-Asn L-Asn is that contain contains the stwo a COOH aCOOH COOH andare and separated a aC=O, C=O, and andby thea the– (CH two two2 ) functional2- functionalCH(NH2 )– groups groups group are arefor separatedtheseparated L-Glu by molecule. aa –CH–CH22–CH(NH – TheCH(NH L-Asn22)–)– group. containgroup. In sIn a fact, fact, COOH the the only andonly di a ff difference C=O,erence and between between the two L-Asn L functional-Asn and and L-Asp groupsL-Asp is a NH isare a2 NHseparatedinstead2 instead of anby of OHa –anCH linked OH2–CH(NH linked to the 2to C)–= thegroupO function. C=O. In function fact, Although the. Althoughonly we difference reported we reported earlierbetween thatearlier L- L-AspAsn that and is L an-LAsp-Asp eff ectiveis is an a effectiveNHcrystal2 instead modifier crystal of modifieran [24 OH], L-Glu linked [24] and, Lto- L-AsnGlu the and C=O with L -functionAsn similar with. structures Althoughsimilar structures have we reported not ha beenve earliernot investigated. been that investigated. L-Asp is an effective crystal modifier [24], L-Glu and L-Asn with similar structures have not been investigated.

Figure 5. Molecular structural formulas of (a) L-Asp, (b) L-Glu, and (c) L-Asn. Figure 5. Molecular structural formulas of (a) L-Asp, (b) L-Glu, and (c) L-Asn. Figure 5. Molecular structural formulas of (a) L-Asp, (b) L-Glu, and (c) L-Asn. The crystal habit and size variations of α-CaSO4 0.5H2O at different amino acid concentrations are The crystal habit and size variations of α-CaSO· 4·0.5H2O at different amino acid concentrations presented in Figures6 and7, respectively. Clearly, the crystal habit of α-CaSO4 0.5H2O is significantly are presentedThe crystal in habit Figure ands 6size and variations 7, respectively. of α-CaSO Clearly,4·0.5H 2O the at crystaldifferent habit amino ·of acid α-CaSO concentrations4·0.5H2O is affected by L-Asp. In the presence of 2.50 mM L-Asp, the average width of α-CaSO4 0.5H2O particles are presented in Figures 6 and 7, respectively. Clearly, the crystal habit of α-CaSO· 4·0.5H2O is significantlyincreased to 27.63 affected7.52 by µ Lm,-Asp. and In the the aspect presence ratio decreased of 2.50 mM to 1.21 L-Asp,0.30. the Therefore, average width it is inferred of α- CaSOsignificantly4·0.5H2O affected particles± by increase L-Asp.d to In 27.63 the presence± 7.52 μm, of and 2.50 the mM aspect L-Asp,± ratio the decrease averaged to width 1.21 ± of0.30. α- that L-Asp has the ability to control the crystal habit of α-CaSO4 0.5H2O. Interestingly, although the CaSO4·0.5H2O particles increased to 27.63 ± 7.52 μm, and the aspect· ratio decreased to 1.21 ± 0.30. Therefore,concentrations it is ofinferred L-Glu and that L-Asn L-Asp were has theobviously ability greater to control than the that crystal of L-Asp, habit they of αcould-CaSO not4·0.5H change2O. Interestingly,Therefore, it isalthough inferred the that concentrations L-Asp has the of L ability-Glu and to controlL-Asn were the crystal obviously habit greater of α- CaSOthan that4·0.5H of2 O.L- the crystal habit of α-CaSO4 0.5H2O. The average width was about 11 µm, and the range of aspect Asp,Interestingly, they could although not change the concentrationsthe· crystal habit of of L α-Glu-CaSO and4·0.5H L-Asn2O. were The averageobviously width greater was than about that 11 of μm, L- andAsp, the they range could of not aspect change ratio the was crystal 5–6. habit Like of IPA, α-CaSO even4·0.5H if the2O. L -TheGlu average contains width two COOH,was about it can11 μm,not and the range of aspect ratio was 5–6. Like IPA, even if the L-Glu contains two COOH, it cannot

Crystals 2020, 10, 24 7 of 15 ratio was 5–6. Like IPA, even if the L-Glu contains two COOH, it cannot regulate the crystal habit of α-CaSOCrystals0.5H 2020, O10, fromx FOR PEER acicular REVIEW crystal to short columnar crystal when the two COOH are separated7 of 15 by Crystals4· 2020,2 10, x FOR PEER REVIEW 7 of 15 more than two methylene/methine groups. Inversely, Teng et al. [27] synthesized α-CaSO4 0.5H2O regulate the crystal habit of α-CaSO4·0.5H2O from acicular crystal to short columnar crystal when ·the whiskersregulate with the high crystal aspect habit ratios of α-CaSO of 2004·0.5H at atmospheric2O from acicular pressure crystal with to short the columnar assistance crystal of L-Glu. when Therefore, the two COOH are separated by more than two methylene/methine groups. Inversely, Teng et al. [27] the L-Glutwo COOH is unable are separated to convert by the more morphology than two methylene of α-CaSO/methine4 0.5H groups2O from. Inversely, needle-like Teng shape et al. to[27] a short synthesized α-CaSO4·0.5H2O whiskers with high aspect ratios of 200 at atmospheric pressure with synthesized α-CaSO4·0.5H2O whiskers with high aspect ratio· s of 200 at atmospheric pressure with columnarthe assistance shape. of Furthermore, L-Glu. Therefore, although the L-Glu the is COOH unable andto convert C=O the are morphology separated byof α a-CaSO –CH24·0.5H–CH(NH2O 2)– the assistance of L-Glu. Therefore, the L-Glu is unable to convert the morphology of α-CaSO4·0.5H2O groupfrom like needle the L-Asp,-like shape L-Asn to a also short cannot columnar regulate shape.the Furthermore, crystal habit although of α-CaSO the COOH4 0.5H and2O C=O because are the from needle-like shape to a short columnar shape. Furthermore, although the COOH· and C=O are NH2 separatedgroup does by a not –CH have2–CH(NH the same2)– group role like as the L OH-Asp group., L-Asn Similarly,also cannot Tanregulate et al. the [22 crystal] found habit that of long separated by a –CH2–CH(NH2)– group like the L-Asp, L-Asn also cannot regulate the crystal habit of columnarα-CaSO crystals4·0.5H2 withO because uniform the NH diameters2 group does could not be have obtained the same by addingrole as the acrylic OH group acid (CH. Similarly,2=CH–COOH), Tan α-CaSO4·0.5H2O because the NH2 group does not have the same role as the OH group. Similarly, Tan et al. [22] found that long columnar crystals with uniform diameters could be obtained by adding whichet al. contained [22] found only that one long COOH columnar and crystals could with not convertuniform thediameterα-CaSOs could4 0.5H be 2obtainedO from by long adding columnar acrylic acid (CH2=CH–COOH), which contained only one COOH and · could not convert the α- whiskersacrylic to acid short (CH columnar2=CH–COOH), crystals. which However, contained the onlycrystal one habit COOH of α and-CaSO could4 0.5H not2 O convert can be the controlled α- CaSO4·0.5H2O from long columnar whiskers to short columnar crystals. However,· the crystal habit by citricCaSO acid4·0.5H and2O ethylenefrom long diaminecolumnar tetraacetic whiskers to acid short (EDTA) columnar [28 ],crystals. which However, contain three the cr andystal four habit COOH, of α-CaSO4·0.5H2O can be controlled by citric acid and ethylene diamine tetraacetic acid (EDTA) [28], respectively.of α-CaSO Thus,4·0.5H2O it can be be controlled concluded by citric that theacid crystaland ethylene modiﬁer diamine must tetraacetic contain acid two (EDTA) or more [28] COOH,, which contain three and four COOH, respectively. Thus, it can be concluded that the crystal modifier which contain three and four COOH, respectively. Thus, it can be concluded that the crystal modifier and themust separation contain two between or more twoCOOH, COOH and inthe the separation crystal between modiﬁer two molecule COOH cannotin the crystal be more modifier than two must contain two or more COOH, and the separation between two COOH in the crystal modifier methylenemolecule/methine cannot groups.be more than two methylene/methine groups. molecule cannot be more than two methylene/methine groups.

Figure 6. Effect of amino acid concentration on the crystal habit of CaSO4·0.5H2O. (a) L-Asp: 1.25 mM; FigureFigure 6. E 6.ﬀ ectEffect of aminoof amino acid acid concentration concentration on the the crystal crystal habit habit of ofCaSO CaSO4·0.5H4 0.5H2O. (2aO.) L (-aAsp:) L-Asp: 1.25 mM; 1.25 mM; (b) L-Asp: 2.50 mM; (c) L-Glu: 4.44 mM; (d) L-Glu: 8.88 mM; (e) L-Asn: 2.52 mM;· (f): L-Asn: 12.60 mM. (b) L-Asp:(b) L-Asp: 2.50 2.50 mM; mM; (c) ( L-Glu:c) L-Glu: 4.44 4.44 mM; mM; ((dd)) L-Glu:L-Glu: 8.88 8.88 mM; mM; (e ()e L)- L-Asn:Asn: 2.52 2.52 mM; mM; (f): L (f-Asn:): L-Asn: 12.60 12.60mM. mM. 80 9 80 9 8 70 Length 8 70 Length Width 7 60 Width 7 60 Aspect ratio Aspect ratio 6 50 6 50 5 40 5 40 4 4

30 Aspect ratio 30 3 Aspect ratio 3 Length and width/μm 20 Length and width/μm 2 20 2 10 1 10 1 0 0 0 1.25 2.50 4.44 8.88 2.52 12.60 0 1.25 2.50 4.44 8.88 2.52 L-Asp L-Glu L-Asn12.60 L-Asp L-Glu L-Asn Amino acid concentration/mM Amino acid concentration/mM Figure 7. Eﬀect of amino acids on the length, diameter, and aspect ratio of CaSO4 0.5H2O. · Crystals 2020, 10, x FOR PEER REVIEW 8 of 15

Figure 7. Effect of amino acids on the length, diameter, and aspect ratio of CaSO4·0.5H2O. Crystals 2020, 10, x FOR PEER REVIEW 8 of 15 CrystalsThe2020 effect, 10, 24 of L-Asp, L-Glu, and L-Asn on the compressive strength of the hardened pastes8 of 15 Figure 7. Effect of amino acids on the length, diameter, and aspect ratio of CaSO4·0.5H2O. hydrated from α-CaSO4·0.5H2O is shown in Figure 8. The results show that the compressive strength increases with an increase in the L-Asp concentration. In the presence of 2.50 mM L-Asp, the The e effectﬀect of of L-Asp, L-Asp, L-Glu, L-Glu and, and L-Asn L-Asn on the on compressive the compressive strength strength of the hardened of the hardened pastes hydrated pastes compressive strength of α-CaSO4·0.5H2O reached 30.2 MPa. On the contrary, the compressive hydratedfrom α-CaSO from 0.5Hα-CaSOO is4·0.5H shown2O is in shown Figure 8in. TheFigure results 8. The show results that show the compressive that the compressive strength increases strength strength decrease4· d along2 with the increase in L-Glu or L-Asn concentrations, with the value being increaseswith an increase with an in increase the L-Asp in concentration.the L-Asp concentration. In the presence In the of presence 2.50 mM of L-Asp, 2.50 themM compressive L-Asp, the lower than 10 MPa. Therefore, the addition of L-Glu or L-Asn does not improve the mechanical compressivestrength of α -CaSO strength0.5H of αO-CaSO reached4·0.5H 30.22O MPa. reache Ond the30.2 contrary, MPa. Onthe compressive the contrary, strength the compressive decreased strength. 4· 2 strengthalong with decrease the increased along in35 with L-Glu the or increase L-Asn concentrations,in L-Glu or L-Asn with concentrations, the value being with lower the than value 10 being MPa. lowerTherefore, than the 10 addition MPa. Therefore, of L-Glu theor L-Asn addition does of not L-Glu improve or L the-Asn mechanical does not improvestrength. the mechanical strength. 30 35 L-Asp 25 30 20 L-Asp 25 15 20 10 15

Compressive strength/MPa 5 10

Compressive strength/MPa 0 5 1.25 2.50 4.44 8.88 2.52 12.60 Amino acid concentration/mM 0 Figure 8. Effect of amino1.25 acid concentration2.50 4.44 on the compressive8.88 2.52 strength12.60 of CaSO4·0.5H2O. Amino acid concentration/mM 3.3. Effect of Straight-Chain Dicarboxylic Acid on the Crystal Habit of α-CaSO4·0.5H2O Figure 8. Effect of amino acid concentration on the compressive strength of CaSO 0.5H O. Figure 8. Effect of amino acid concentration on the compressive strength of CaSO44··0.5H2O. Dicarboxylic acids with different lengths of carbon chain such as oxalic acid (OA), propanedioic 3.3. Effect of Straight-Chain Dicarboxylic Acid on the Crystal Habit of α-CaSO4 0.5H2O acid3.3. Effect(PD), ofsuccinic Straight acid-Chain (SA) Dicarboxylic, and maleic Acid acid on (MA)the Crystal are shown Habit ofin α Figure-CaSO 49·0.5H·. What2O these dicarboxylic acids Dicarboxylicall have in common acids with is that diff theyerent contain lengths two of carbonCOOH, chain but the such separation as oxalic between acid (OA), the propanedioic two COOH Dicarboxylic acids with different lengths of carbon chain such as oxalic acid (OA), propanedioic areacid different, (PD), succinic being separated acid (SA), by and zero, maleic one acidmethylene (MA) aregroup shown, and in two Figure methylene9. What groups these dicarboxylic for OA, PD, acid (PD), succinic acid (SA), and maleic acid (MA) are shown in Figure 9. What these dicarboxylic andacids SA, all respectively. have in common Compared is that theywith containSA, the twoonly COOH, difference but thein the separation MA is that between it contains the two a double COOH acids all have in common is that they contain two COOH, but the separation between the two COOH bond.are di fferent, being separated by zero, one methylene group, and two methylene groups for OA, PD, are different, being separated by zero, one methylene group, and two methylene groups for OA, PD, and SA, respectively. Compared with SA, the only difference in the MA is that it contains a double bond. and SA, respectively. Compared with SA, the only difference in the MA is that it contains a double bond.

Figure 9. Molecular structural formula of (a) OA, (b) PD, (c) SA and (d) MA. Figure 9. Molecular structural formula of (a) OA, (b) PD, (c) SA and (d) MA. The effect of dicarboxylic acids on the crystal habit of α-CaSO 0.5H O is presented in Figure 10, 4· 2 and the corresponding length, width, and aspect ratio are shown in Table3. In the presence of The effect of dicarboxylic acids on the crystal habit of α-CaSO4·0.5H2O is presented in Figure 10, 0.11 mM OA andFigure PD, 9. the Molecular corresponding structural aspect formula ratios of (a of) OA,α-CaSO (b) PD,0.5H (c) SAO and particles (d) MA. were 5.58 1.85 and the corresponding length, width, and aspect ratio are shown 4in· Table2 3. In the presence of± 0.11 and 5.19 1.45, respectively, with the average width of 10 µm. Therefore, the OA and PD cannot mM OA and± PD, the corresponding aspect ratios of α-CaSO4·0.5H2O particles were 5.58 ± 1.85 and regulateThe theeffect crystal of dicarboxylic habit of α-CaSO acids on0.5H the crystalO, and habit we can of α infer-CaSO that4·0.5H the2 separationO is presented between in Figure the two 10, 5.19 ± 1.45, respectively, with the average4· width2 of 10 μm. Therefore, the OA and PD cannot regulate andCOOH the incorresponding the crystal modifier length, width molecule, and cannot aspect beratio lower are thanshown two in methyleneTable 3. In groups.the presence The of results 0.11 the crystal habit of α-CaSO4·0.5H2O, and we can infer that the separation between the two COOH in mM OA and PD, the corresponding aspect ratios of α-CaSO4·0.5H2O particles were 5.58 ± 1.85 and theagree crystal with modifier previously molecule studies, can forno example,t be lower Liu than et al.two [29 methylene] also found groups that. OAThe didresults not agree modify with the 5.19crystal ± 1.45 morphology, respectively, of α with-CaSO the average0.5H O, width which of was 10 μm. still Therefore, acerose or the linear OA clavate,and PD can andnot the regul crystalate previously studies, for example,4· Liu 2 et al. [29] also found that OA did not modify the crystal themorphology crystal habit of α of-CaSO α-CaSO4 0.5H4·0.5H2O had2O, and no significantwe can infer changes that the with separation the addition between of PD. the However, two COOH when in morphology of α-CaSO4·0.5H· 2O, which was still acerose or linear clavate, and the crystal morphology the concentrationcrystal modifier of SAmolecule was 0.11 can mM,not be the lower average than width two enlargedmethylene to groups 17.67 . 6.71The µresultsm, and agree the aspect with of α-CaSO4·0.5H2O had no significant changes with the addition of PD.± However, when the previouslyratio decreases studies, to 3.12 for example,1.10. With Liu a further et al. [29] increase also in found SA concentration that OA did tonot 0.22 modify mM, the the particle crystal ± morphologysize of α-CaSO of α-0.5HCaSOO4·0.5H decreased2O, which rapidly, was still the acerose average or width linear reducedclavate, and to 1.47 the crystal0.54 morphologyµm, and the 4· 2 ± ofaspect α-CaSO ratio4·0.5H reached2O uphad to 9.40no significant3.10. When changes the concentration with the addi of MAtion was of 0.17PD. mM, However, the average when width the ±

Crystals 2020, 10, x FOR PEER REVIEW 9 of 15 concentration of SA was 0.11 mM, the average width enlarged to 17.67 ± 6.71 μm, and the aspect ratio decreasesCrystals 2020 to, 10 3.12, 24 ± 1.10. With a further increase in SA concentration to 0.22 mM, the particle size9 of of 15 α-CaSO4·0.5H2O decreased rapidly, the average width reduced to 1.47 ± 0.54 μm, and the aspect ratio reached up to 9.40 ± 3.10. When the concentration of MA was 0.17 mM, the average width enlarged enlarged to 18.15 7.24 µm, with the lower aspect ratio of 2.42 1.09. Hence, SA and MA can be used to 18.15 ± 7.24 μm,± with the lower aspect ratio of 2.42 ± 1.09. Hence,± SA and MA can be used as crystal modifiers,as crystal modiﬁers,and the cis and-conformation the cis-conformation MA shows MA a better shows control a better effect. control The eﬀ reasonect. The is reason that the is double that the double bond helps to improve the complexing electron coordination ability of COOH and Ca on the bond helps to improve the complexing electron coordination ability of COOH and Ca on the crystal crystal surface of α-CaSO4 0.5H2O[30,31]. Furthermore, the concentration of SA should be controlled surface of α-CaSO4·0.5H2O ·[30,31]. Furthermore, the concentration of SA should be controlled within within a certain range. a certain range.

FigureFigure 10. 10. EffectEﬀect of of straight straight-chain-chain dicarboxylic dicarboxylic acid acid on on the the crystal crystal habit habit of of CaSO CaSO4·0.5H0.5H2OO.. (a (a) )OA: OA: 0.11 0.11 4· 2 mM;mM; ( (bb)) PD: PD: 0.11 0.11 mM; mM; ( (cc)) SA: SA: 0.11 0.11 mM; mM; ( (dd)) SA: SA: 0.22 0.22 mM; mM; ( (ee)) MA: MA: 0.06 0.06 mM; mM; ( (ff):): MA: MA: 0.17 0.17 mM mM..

TableTable 3. 3. EffectEﬀect of of straight straight-chain-chain dicarboxylic dicarboxylic acid acid on on the the length, length, diameter diameter,, and and aspect aspect ratio ratio of α of- α-CaSO4 0.5H2O particles. CaSO4·0.5H· 2O particles. Average Average Width DicarboxylicDicarboxylic Acids Concentration Acids /mM Average Average Width AspectAspect Ratio Length/µm /µm Concentration/mM Length/μm /μm Ratio OA 0.11 51.31 12.17 9.86 2.74 5.58 1.85 OA 0.11 51.31 ±± 12.17 9.86± ± 2.74 5.58± ± 1.85 PD 0.11 51.87 10.17 10.53 2.75 5.19 1.45 PD 0.11 51.87 ±± 10.17 10.53± ± 2.75 5.19± ± 1.45 0.11 50.14 10.87 17.67 6.71 3.12 1.10 SA 0.11 50.14 ±± 10.87 17.67± ± 6.71 3.12± ± 1.10 SA 0.22 13.89 7.33 1.47 0.54 9.40 3.10 0.22 13.89 ±± 7.33 1.47± ± 0.54 9.40± ± 3.10 0.06 59.19 11.11 13.68 4.10 4.68 1.52 MA 0.06 59.19 ±± 11.11 13.68± ± 4.10 4.68± ± 1.52 MA 0.17 39.35 14.69 18.15 7.24 2.42 1.09 0.17 39.35 ±± 14.69 18.15± ± 7.24 2.42± ± 1.09

TheThe effect eﬀect of of OA, OA, PD, PD, SA SA,, and and MA MA on on the the compressive compressive strength strength of of hardened hardened pastes pastes is is shown shown in in FigureFigure 1111.. The The results results show show that that the the compressive compressive strength strength of of αα--CaSOCaSO4·0.5H0.5H2OO prepared prepared with with OA OA and and 4· 2 PDPD was was less less than than 10 10 MPa. MPa. Hence, Hence, the the addition addition of of OA OA or or PD PD do doeses not not improve improve the the mechanical mechanical strength. strength. ItIt is is worth worth noting noting that that the the compressive compressive strength strength increase increasedd to to 25.3 25.3 MPa MPa in in the the presence presence of of 0.11 0.11 mM mM SA, SA, butbut with with a a further further increase increase in in S SAA concentration concentration to to 0.22 0.22 mM, mM, the the particle particle size size and and compressive compressive strength strength ofof αα--CaSOCaSO4·0.5H0.5H2OO sharply sharply decrease decreased.d. In In addition addition,, the the compressive compressive strength strength of αα-CaSO-CaSO4·0.5H0.5H2OO 4· 2 4· 2 preparedprepared with with 0.17 0.17 mM mM MA MA reache reachedd 30.4 30.4 MPa.

Crystals 2020, 10, 24 10 of 15 Crystals 2020, 10, x FOR PEER REVIEW 10 of 15

30 OA 25

Compressive strength/MPa 5

0 0.11 0.11 0.11 0.22 0.06 0.17 Straight-chain dicarboxylic acid concentration/mM Figure 11. Eﬀect of straight-chain dicarboxylic acid concentration on the compressive strength of Figure 11. Effect of straight-chain dicarboxylic acid concentration on the compressive strength of α- α-CaSO4 0.5H2O. CaSO4·0.5H· 2O. In conclusion, when the separation between the two COOH in the molecular structure of organic In conclusion, when the separation between the two COOH in the molecular structure of organic acids is lower or higher than two methylene or methine groups and the number of COOH is less than acids is lower or higher than two methylene or methine groups and the number of COOH is less than two, the organic acids do not have the ability to regulate the crystal habit of α-CaSO4 0.5H2O. That is two, the organic acids do not have the ability to regulate the crystal habit of α-CaSO4·0.5H· 2O. That is to say, the molecular structure characteristics of organic acid crystal modiﬁers are that they contain to say, the molecular structure characteristics of organic acid crystal modifiers are that they contain two or more COOH and the two COOH are separated by two methylene or methine groups. two or more COOH and the two COOH are separated by two methylene or methine groups.

3.4. Control Mechanism of Organic Acids for the Crystal Habit of α-CaSO4 0.5H2O 3.4. Control Mechanism of Organic Acids for the Crystal Habit of α-CaSO4·0.5H· 2O The FTIR spectra of α-CaSO4 0.5H2O particles prepared with different crystal modifiers are The FTIR spectra of α-CaSO4·0.5H· 2O particles prepared with1 different crystal modifiers are presented in Figure 12. As shown in Figure 12a, the peak at 1622 cm− belongs to the bending vibrations presented in Figure 12. As shown in Figure 12a, the peak at 1622 cm−1 belongs 1 to the bending1 of water molecules in α-CaSO4 0.5H2O, and the two peaks positioned at 3557 cm− and 3610 cm− vibrations of water molecules in ·α-CaSO4·0.5H2O, and the two peaks positioned1 at 3557 cm−1 and 3610 are attributed to the O–H stretching vibration [32]. The peak at 466 cm− is the symmetrical bending cm−1 are attributed2 to the O–H stretching vibration1 [32]. The peak1 at 466 cm−1 is the symmetrical vibration of υ2 SO4 −. The peaks located at 1152 cm− and 1093 cm− are ascribed to the asymmetrical bending vibration of υ2 SO42−2. The peaks located at 11521 cm−1 and 1 1093 cm−1 are ascribed to the stretching vibration of υ3 SO4 −. The peaks at 660 cm− and 599 cm− are the asymmetrical bending asymmetrical stretching2 vibration of υ3 SO42−. The peaks at 6601 cm−1 and 599 cm−1 are the asymmetrical vibrations of υ4 SO4 − [33]. In addition, the peak at 799 cm− is the stretching vibration peak of P–O [22]. bending vibrations of υ4 SO42− [33]. In addition, the peak at 799 cm−1 is the stretching vibration peak The spectra of α-CaSO4 0.5H2O prepared with L-Asp are presented in Figure 12b. The peak of P–O [22]. 1 · appearing at 1443 cm− can be assigned to the symmetric stretching vibration of COO− in amino acid. The spectra of α-CaSO4·0.5H1 2O prepared with L-Asp are presented in Figure 12b. The peak The peak located at 779 cm− is due to the bending vibration of COO−, which is considered to be appearthe characteristicing at 1443 absorptioncm−1 can be peakassigned of short-chain to the symmetric fatty acids stretching [24]. As vibration can be seenof COO from− in Figure amino 12 acid.c,d, The peak located at 7791 cm−1 is due to the bending vibration of COO−, which is considered to be the the peak at 1421 cm− is the symmetric stretching vibration peak of COO−, indicating that the crystal characteristic absorption peak of short-chain fatty acids [24]. As can be seen from Figures 12c,d, the modifiers have been adsorbed on the surface of the α-CaSO4 0.5H2O particles. peak at 1421 cm−1 is the symmetric stretching vibration peak· of COO−, indicating that the crystal modifiers have been adsorbed on the surface of the α-CaSO4·0.5H2O particles.

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3608 (c) 1421

3555

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1625

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660

599

3610

599

1004

1093

1152

1093 4000 3500 3000 2500 2000 1500 1152 1000 500 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber/cm-1 Wavenumber/cm

Figure 12. Fourier transform infrared spectroscopy (FTIR) spectra of CaSO4 0.5H2O particles prepared Figure 12. Fourier transform infrared spectroscopy (FTIR) spectra of CaSO4··0.5H2O particles prepared withFigure di ff12.erent Fourier crystal transform modifiers. infrared (a) Blank spectroscopy test; (b) L-Asp; (FTIR) ( spectrac) PA; and of CaSO (d) MA.4·0.5H2O particles prepared with different crystal modifiers. (a) Blank test; (b) L-Asp; (c) PA; and (d) MA. As PD, IPA, L-Glu, and L-Asn do not have the ability to control the crystal shape of α-CaSO4 0.5H2O As PD, IPA, L-Glu, and L-Asn do not have the ability to control the crystal shape· of α- particles,As PD, FTIR IPA, was L-Glu also, usedand L to-Asn analyze do not the have adsorption the ability of these to control organic the acids crystal on shape the surface of α- CaSO4·0.5H2O particles, FTIR was also used to analyze the adsorption of these organic acids on the ofCaSOα-CaSO4·0.5H42O0.5H particles2O. The, FTIR results was arealso shownused to in analyze Figure the 13 .adsorption Compared of withthese theorganic FTIR acids spectra on the of surface of α-·CaSO4·0.5H2O. The results are shown in Figure 13. Compared with the FTIR spectra of αsurface-CaSO of4 0.5H α-CaSO2O prepared4·0.5H2O. without The results a crystal are shown modifier, in Figure the positions 13. Compared and numbers with ofthe the FTIR characteristic spectra of α-CaSO4·0.5H· 2O prepared without a crystal modifier, the positions and numbers of the characteristic absorptionα-CaSO4·0.5H peaks2O prepared of α-CaSO without4 0.5H a2 crystalO prepared modifier, with the PD, positions IPA, L-Glu, and num andbers L-Asn of the did characteristic not change, absorption peaks of α-CaSO4·0.5H· 2O prepared with PD, IPA, L-Glu, and L-Asn did not change, indicatingabsorption that peaks they of could α-CaSO not4·0.5H be adsorbed2O prepared onto the with surface PD, IPA, of α-CaSO L-Glu4, 0.5Hand 2 LO-Asn particles did ornot adsorbed change, indicating that they could not be adsorbed onto the surface of α-CaSO4··0.5H2O particles or adsorbed byindicating physical that absorption. they could not be adsorbed onto the surface of α-CaSO4·0.5H2O particles or adsorbed by physical absorption.

(e)

(d)

(c)

(b) (a)

Transmitance/% (a)

Transmitance/%

466

3557

1621

3557

660

1621

3610

660

3610

599

1004

599

1004

1093

1152

1093 4000 3500 3000 2500 2000 1500 1152 1000 500 4000 3500 3000 2500 2000 1500 1000 500 -1 Wavenumber/cm-1 Wavenumber/cm Figure 13. FTIR spectra of CaSO4 0.5H2O particles prepared with different organic acids. (a) Blank test; Figure 13. FTIR spectra of CaSO·4·0.5H2O particles prepared with different organic acids. (a) Blank (Figureb) PD; 13. (c) IPA;FTIR ( dspectra) L-Glu; of and CaSO (e)4 L-Asn.·0.5H2O particles prepared with different organic acids. (a) Blank test; (b) PD; (c) IPA; (d) L-Glu; and (e) L-Asn. To further reveal the microcosmic differences between organic acids with and without the ability to regulateTo further the crystalreveal the habit microcosmic of α-CaSO differences0.5H O, the between separation organic between acids thewith COOH and without (d the ability) in 4· 2 COOH–COOH to regulate the crystal habit of α-CaSO4·0.5H2O, the separation between the COOH (dCOOH–COOH) in dito ffregulateerent organic the crystal acids habit was measuredof α-CaSO using4·0.5H the2O, Cambridgethe separation Serial between Total Energythe COOH Package (dCOOH (CASTEP)–COOH) in ofdifferent Materials organic Studio acids 8.0 was software measured based using on the the density Cambridge functional Serial Total theory Energy (DFT), Package the results (CASTEP) of which of Materials Studio 8.0 software based on the density functional theory (DFT), the results of which are areMaterials shown Studio in Table 8.04 .software As can based be seen on from the density Table4, functional the d COOH-COOH theory (DFT)of crystal, the results modifiers of which with theare shown in Table 4. As can be seen from Table 4, the dCOOH-COOH of crystal modifiers with the ability to abilityshown toin regulateTable 4. theAs shapecan be of seenα-CaSO from Table0.5H O4, suchthe d asCOOH SA,-COOH MA, of PA,crystal and modifier L-Asp weres with 4.327 the Å, abi 3.785lity Å,to 4· 2 regulate the shape of α-CaSO4·0.5H2O such as SA, MA, PA, and L-Asp were 4.327 Å , 3.785 Å , 3.946 regulate the shape of α-CaSO4·0.5H2O such as SA, MA, PA, and L-Asp were 4.327 Å , 3.785 Å , 3.946

Crystals 2020, 10, x FOR PEER REVIEW 12 of 15

CrystalsÅ , and2020 4.154, 10, 24 Å , respectively, which were close to the separation between Ca atoms (dCa-Ca = 124.187 of 15 Å ) on the end surface of α-CaSO4·0.5H2O (1 1 1), and the difference value (∆d = dCa-Ca − dCOOH-COOH) was not more than 0.5 Å . However, the dCOOH-COOH of OA and PD were obviously less than 4.187 Å , and 3.946 Å, and 4.154 Å, respectively, which were close to the separation between Ca atoms (dCa-Ca the ∆d values reached 1.517 Å and 1.322 Å , respectively. Conversely, the dCOOH-COOH of IPA, TA, and = 4.187 Å) on the end surface of α-CaSO4 0.5H2O (1 1 1), and the difference value (∆d = dCa-Ca L-Glu were significantly greater than dCa·-Ca, and the ∆d values reached 0.933 Å , 2.831 Å , and 1.746− Å , dCOOH-COOH) was not more than 0.5 Å. However, the dCOOH-COOH of OA and PD were obviously less respectively. Therefore, when the dCOOH-COOH of organic acids is distinctly lower (Figure 14a) or higher than 4.187 Å, and the ∆d values reached 1.517 Å and 1.322 Å, respectively. Conversely, the dCOOH-COOH than (Figure 14b) dCa-Ca, the organic acids cannot interact with Ca atoms on the (111) face of α- of IPA, TA, and L-Glu were significantly greater than dCa-Ca, and the ∆d values reached 0.933 Å, CaSO4·0.5H2O, and only when the dCOOH-COOH matches the dCa-Ca do the organic acids possess the 2.831 Å, and 1.746 Å, respectively. Therefore, when the dCOOH-COOH of organic acids is distinctly lower ability to regulate the crystal habit of α-CaSO4·0.5H2O. Similarly, Badens et al. [26] found that the (Figure 14a) or higher than (Figure 14b) d , the organic acids cannot interact with Ca atoms on the separation between calcium ions of theCa-Ca faces (120) and (111) of crystal gypsum (4.0 Å ) matched the (111) face of α-CaSO 0.5H O, and only when the d matches the d do the organic acids distance between 4two· oxygen2 ions of the two carboxylicCOOH-COOH groups of the moleculeCa-Ca of citric acid and MA, possess the ability to regulate the crystal habit of α-CaSO 0.5H O. Similarly, Badens et al. [26] found and the distance between the two oxygen ions of the active4· sites2 was much more important than the that the separation between calcium ions of the faces (120) and (111) of crystal gypsum (4.0 Å) matched distance of two first neighbors of calcium ions on the gypsum crystal. Furthermore, although the the distance between two oxygen ions of the two carboxylic groups of the molecule of citric acid and dCOOH-C=O of L-Asn (4.073 Å ) was close to dCa-Ca, the L-Asn contained only one COOH, so it cannot MA, and the distance between the two oxygen ions of the active sites was much more important than stably adsorb on the (111) face by two COOH. These analysis results are in agreement with the SEM the distance of two first neighbors of calcium ions on the gypsum crystal. Furthermore, although the observation and FTIR tests. dCOOH-C=O of L-Asn (4.073 Å) was close to dCa-Ca, the L-Asn contained only one COOH, so it cannot stably adsorb on the (111) face by two COOH. These analysis results are in agreement with the SEM Table 4. COOH separation of different organic acids. observation and FTIR tests. L- L- L- Organic Acid OA PD SA MA PA IPA TA Table 4. COOH separation of different organic acids. Asp Asn Glu COOH separation Organic Acid OA2.670 PD2.865 SA4.327 MA3.785 PA3.946IPA 5.120TA 7.018 L-Asp 4.154 L-Asn 4.073L-Glu 5.933 /Å COOH separation /Å 2.670 2.865 4.327 3.785 3.946 5.120 7.018 4.154 4.073 5.933

Figure 14. Schematic diagram of OA (a) and L-Glu (b) on the (111) surface of CaSO4 0.5H2O. Figure 14. Schematic diagram of OA (a) and L-Glu (b) on the (111) surface of CaSO· 4·0.5H2O. The model of α-CaSO 0.5H O crystal habit regulated by the crystal modifier is shown in Figure 15. 4· 2 In our previousThe model study, of α the-CaSO adsorption4·0.5H2O ofcrystal the MA habit molecule regulated on theby the (111), crystal (110), modifier and (010) is crystalshown faces in Figure of 15. In our previous study, the adsorption of the MA molecule on the (111), (110), and (010) crystal α-CaSO4 0.5H2O were studied using DFT calculations, and the results indicated that the (111) face was faces · 4 2 the most of stable α-CaSO chemical·0.5H adsorptionO were studied surface using and DFT presented calculations, the strongest and the bonding results indicated interaction that with the MA, (111) andface the was adsorption the most of stable MA onchemical the (110) adsorption and (010) surface faces was and relatively presented weak the strongest [21]. Therefore, bonding the interaction crystal modifierwith MA, can and preferentially the adsorption adsorb of MA on on the the (111) (110) face and and (01 retard0) faces its was growth relatively in the weak length [21] direction.. Therefore, Moreover,the crystal with modifier an increase can preferentially in the crystal adsorb modifier on concentration,the (111) face and the αretard-CaSO its0.5H growthOcrystals in the length are 4· 2 shorteneddirection. in Moreover,length and withenlarged an increase in width, in resulting the crystal in the modifier decrease concentration, in aspect ratio. the α-CaSO4·0.5H2O crystals are shortened in length and enlarged in width, resulting in the decrease in aspect ratio.

Crystals 2020, 10, 24 13 of 15 Crystals 2020, 10, x FOR PEER REVIEW 13 of 15

Figure 15. Model of CaSO4 0.5H2O crystal habit regulated by the crystal modifier. Figure 15. Model of CaSO4·0.5H· 2O crystal habit regulated by the crystal modifier. 4. Conclusions 4. Conclusions Organic acids with different molecular structures were adopted to regulate the crystal habit of α-CaSOOrganic0.5H acidsO,and with the different adsorption molecular differences structure of organics were acidsadopted on theto regulate surface oftheα -CaSOcrystal habit0.5H ofO 4· 2 4· 2 αparticles-CaSO4·0.5H were2O, clarified. and the The adsorption conclusions differences are as follows. of organic acids on the surface of α-CaSO4·0.5H2O particlesThe were molecular clarified. structure The conclusions characteristics are ofas organicfollows. acid crystal modifiers are that they contain two or moreThe COOH molecular and thatstructure the two characteristics COOH are separated of organic by twoacid methylene crystal modifier or methines are groups. that they The contain crystal twomodifiers or more can COOH adsorb and on that the surfacethe two ofCOOHα-CaSO are separated0.5H O particles by two methylene and change or its methine growth groups habit.. With The 4· 2 crystalthe increase modifiers in the can crystal adsorb modifier on the concentration surface of α-CaSO such as4·0.5H PA,2 L-Asp,O particles SA, andand MA, change the itsα-CaSO growth0.5H habit.O 4· 2 Withcrystals the are increase shortened in the in length crystal and modifier enlarged concentratio in width, resultingn such inas the PA, decrease L-Asp, in SA aspect, and ratio MA, and the theα- CaSOincrease4·0.5H in2 compressiveO crystals are strength. shortened Organic in length acids and without enlarged the controlin width, ability resulting such in as the PD, decrease IPA, L-Glu, in aspectand L-Asn ratio neither and the adsorb increase on in the compressive surface of α -CaSOstrength.0.5H OrganicO particles, acids without nor change the control its crystal ability habit such and 4· 2 asimprove PD, IPA, the L compression-Glu, and L-Asn strength. neither adsorb on the surface of α-CaSO4·0.5H2O particles, nor change its crystal habit and improve the compression strength. Author Contributions: X.L. performed the experiments and wrote the manuscript. Q.Z. conceived and designed Authorthe experiments. Contributions: All authors X.L. performed have read the and experiments agreed to the and published wrote the version manuscript of the. Q.Z. manuscript. conceived and designed the experiments. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant number U1812402. Funding:Conflicts ofThis Interest: researchThe was authors funded declare by nothe conflicts National of Natural interest. Science Foundation of China, grant number U1812402.

ReferencesConflicts of Interest: The authors declare no conflicts of interest.

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