Synthesis, Characterization and Formation Mechanism ... - Springer Link

Vol.30 No.1 MA Jiayu et al: Synthesis, Characterization and Formation Mechanism of ...

76 DOI 10.1007/s11595-015-1104-y

Synthesis, Characterization and Formation Mechanism of Friedel’s Salt (FS: 3CaO·A12O3·CaCl2·10H2O) by the Reaction of Calcium Chloride with Sodium Aluminate MA Jiayu1, 2, LI Zhibao3, JIANG Yuehua4, YANG Xiaoping1 (1. National Engineering and Technology Research Centre for Aluminium & Magnesium Electrolysis Facilities, Guiyang Aluminium Magnesium Design & Research Institute Co., Ltd., Guiyang 550081, China; 2. Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430073, China; 3. Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China; 4. Shenyang Aluminium & Magnesium Engineering & Research Institute Co., Ltd., Shenyang 110001, China) Abstract: The synthesis of Friedel’s salt (FS: 3CaO・A12O3・CaCl2・10H2O) by the reaction of calcium chloride with sodium aluminate was investigated. Factors affecting the preparation of Friedel’s salt, such as reaction temperature, initial concentration, titration speed, aging time and molar Ca/Al ratio were studied in detail. XRD, SEM images and particle size distribution show that the reaction temperature, aging time and molar Ca/Al ratio have significant effect on the composition, crystal morphology, and average particle size of the obtained samples. In addition, the initial CaCl2 concentration and NaAlO2 titration speed do not significantly influence the morphology and particle size distribution of Friedel’s salt. With the optimization of the operating conditions, the crystals can grow up to a average size of about 28 m, showing flat hexagonal (or pseudohexagonal) crystal morphology. Moreover, two potential mechanisms of Friedel’s salt formation including adsorption mechanism and anion-exchange mechanism were discussed. In the adsorption mechanism, Friedel’s salt forms due to the adsorption of the bulk Cl- ions present in the solution into the interlayers of the principal layers, [Ca2Al(OH-)6·2H2O]+, in order to balance the charge. In the anion-exchange mechanism, the free-chloride ions bind with the AFm (a family of hydrated compounds found in cement) hydrates to form Friedel’s salt by anion-exchange with the ions present in the interlayers of the principal layer, [Ca2Al(OH-)6· 2H2O]+-OH-. Key words: synthesis; characterization; formation mechanism; Friedel’s salt

1 Introduction Layered double hydroxides (LDH) are an important class of materials currently receiving considerable attention for a wide variety of applications in catalysis and environmental remediation [1-5] . In particular, Friedel’s salt [Ca 2 Al(OH) 6 ]Cl・2H 2 O, also called hydrocalumite, and abbreviated hereafter as Ca2AlCl is structurally one of the best understood layered double hydroxides and can serve as a model for other less ordered LDH [6,7]. Friedel’s salt was for the first time mentioned by Friedel in 1897[8], who ©Wuhan University of Technology and SpringerVerlag Berlin Heidelberg 2015 (Received: Oct. 19, 2013; Accepted: Nov. 16, 2014) MA Jiayu(马家玉): Ph D; E-mail: [email protected] Funded by International Science & Technology Cooperation Program of China (No.2013DFB70220), the National Natural Science Foundation of China (No.21076212) and the Natural Science Foundation of Guizhou Province of China (No. [2014]2003)

studied the reactivity of lime with aluminum chloride. The structure of Friedel’s salt has been reported by Filipaksis et al[9] and more recently by Suryavanshi[10]. The formation of Friedel’s salt can be imagined by an ordered replacement of one Ca 2+ ion out of the three in a layer of Ca(OH)2 by an A13+ ion. The reason for the replacement is not clear. Maybe the Al atoms are smaller in size compared to Ca atoms and hence stabilize the crystal structure. And the above replacement with an ionic charge imbalance is caused in each “principal layer,” [Ca 2Al(OH -) 6·2H 2O] +. To balance the unit positive charge a Cl- ion occupies the site above A13+ in the gap between the OH- ions. Thus, the composition of the principal layer becomes [Ca2Al(OH-)6·2H2O]Cl. The unit cell of Friedel’s salt consists of two such principal layers of the above composition, each with a Cl- ion in the interlayer to balance the charge. The application of Friedel’s salt has been

Journal of Wuhan University of Technology-Mater. Sci. Ed. www.jwutms.net Feb.2015

reported in the open literature. Yauri and Gougar[11,12] reported that Friedel’s salt is formed in cements rich in tricalcium aluminate and is known as a phase used world-wide for the disposal of radioactive, hazardous and mixed wastes. Miyata [13] found that Friedel’s salt would be capable of accumulating anions by ion exchange. Walcarius et al[14] reported that Friedel’s salt is used as a novel electrode modifier for the accumulation of iodide species. Recent years, Friedel’s salt attracted widely attention in dealing with heavy metals from wastewater because of its higher adsorption efficiency and lower production cost compared with other adsorbents[15-17]. For example, Dai et al[18] examined the adsorption behaviors of Friedel’s salt for Cr (VI) from aqueous solution at different concentrations and various initial pHs. Zhang et al[19] found Friedel’s salt synthesized at low temperature was effective for the removal of arsenic from water with relatively fast kinetics. Recently, Liu et al[15] reported that Friedel’s salt could effectively adsorb zinc from wastewater. Most recently, Zhang et al[16] found that the removal rate of Cd2+ from wastewater with initial concentration of 10 mg/L was up to 94.34% with the adsorption capacity of 301.9 mg/g when Friedel’s salt dosage was 0.03 g/L. Besides, Friedel’s salt was proved by the authors[17] to have higher desilication capacity than conventional desilication agents. Previous reports indicate that the morphology and particle size of Friedel’s salt are of particular interest for optimization of ion exchange properties[13-19]. However, until this study was carried out, the preparation, morphology, and particle size of Friedel’s salt had not yet been studied systematically. In this study, the synthesis of Friedel’s salt by the reaction of calcium chloride (a waste by-product in the alkali industry) with sodium aluminate was investigated. The experimental parameters affecting the preparation of Friedel’s salt, such as reaction temperature, initial concentration, titration speed, aging time and molar Ca/Al ratio were studied in detail. The reaction involved was as follows: (1)

2 Experimental 2.1 Materials All chemical reagents CaCl2, NaOH and A1(OH)3

77

used in the experiments were of analytical grade from Chemical Co. of Beijing and were used without further purification. The water used in all experimental work for solution preparation, dilution, crystal washing etc, was double distilled water (conductivity < 0.1 μS・cm-1) unless otherwise specified. 2.2 Preparation of Friedel’s salt The experimental apparatus used in these experiments was a mixed tank reactor. The reactor was made from glass and had a volume of about 1 liter. It was baffled and equipped with a two-flat-blade turbine mixer. The mixer was connected to a directcurrent motor that allowed the speed of the mixer to be varied between 0 and 500 rpm. The temperature of the solution was controlled during the experiments with a band heater on the outside of the reactor. Preparation experiments were conducted in the mixed tank reactor. A standard volume (300 mL) of CaCl2 solution in the reactor was heated to a certain temperature with the aid of the circulation of hot oil. Solution temperature was monitored with a thermometer. Upon attainment of the desired temperature, addition of 300 mL NaAlO 2 solution (αk=3.2) was started simultaneously with the initiation of stirring at 300 rpm. Stirring was provided by a motor drive and a 2-blade radial impeller was used. The location of the impeller and 4 baffles attached to the lid provided uniform mixing. During the CaCl2-NaAlO2 reaction, NaAlO2 addition by titration was done using a peristaltic pump. When the addition procedure was completed, stirring continued for a certain time. Then the white precipitate was collected, filtered and washed with distilled water three times (to remove any possible ionic remnants), and finally dried in an oven at 323 K for 10 h. 2.3 Characterization The structure and morphology of the synthesized samples were examined by using X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). X-ray powder diffraction (XRD, X’Pert PRO MPD, PANalytical, Netherlands) patterns were recorded on a diffractometer (using Cu Kα radiation) operating at 40 kV/30 mA. A scanning rate of 0.02o/ s was applied to record the patterns in the 2θ angle range from 10o to 90o. The morphology and particle size of the as-synthesized samples were examined by a scanning electron microscope (SEM, JEOL-JSM6700F). Size distributions of particles were examined by a laser diffraction particle size analyzer (LS-13-320).

78


3 Results and discussion

3.1.2 Effect of reaction temperature on morphology

3.1 Effect of the reaction temperature Temperature can strongly affect the solution precipitation process and the composition of the obtained products. In the Kuzel study [20], he found both a monoclinic α-3CaO・Al2O3・CaCl2・10H2O and a rhombohedral β-3CaO・Al2O3・CaCl2・10H2O. The α-form changes to the β-form reversibility at 301 K. The β-form of Friedel’s salt persists up to 473 K at which four water molecules lost. Similarly, reaction temperature should play a very important role in this precipitation. 3.1.1 XRD observation

Fig.1 displays the XRD patterns of the precipitates obtained in the range of 303-353 K. All peaks in the XRD patterns of the obtained samples are in good agreement with the Friedel’s salt reference data (JCPDS-78-2051)[13]. The sharp diffraction peaks imply a good crystallinity of our target compound, Friedel’s salt.

Fig.2 displays the XRD patterns of the precipitates obtained in the range of 373-473 K. Ca8Al4(OH)24(CO3) Cl2(H2O)9.6 appears at 373 K at which carbonation of Friedel’s salt occurred[21]. When the reaction temperature is in the range of 373-473 K, the precipitates are the mixtures of 3CaO・A1 2 O 3 ・CaCl 2 ・10H 2 O and Ca8Al4(OH)24(CO3)Cl2(H2O)9.6.

Fig.3 Influence of different reaction temperatures on SEM morphologies of precipitates: (a) and (b) 303 K; (c) and (d) 353 K; (e) and (f) 373 K; (g) and (h) 423 K; (i) and (j) 453 K; (k) and (l) 473 K


Fig.3 provides a set of typical SEM images corresponding to Friedel’s salt obtained from different reaction temperatures. As can be observed, these morphologies drastically change with the variation of the reaction temperatures. In the range of 303-353 K, flat hexagonal (or pseudohexagonal) crystals, about 15 mm maximum dimension, are observed by SEM photomicrographs. Besides, serious aggregation and small non-uniform growths are seen in Figs.3(a)-(f). When the reaction temperature increases from 423 to 453 K, as shown in Figs.3(g)-(j), the maximum dimension gradually increases to 6 8 mm and the particles exhibit smooth surfaces. However, with further increasing the temperature up to 473 K, the surface of particles is covered by some small particles and serious aggregation is seen in Figs.3(k) and(l). 3.1.3 Effect of reaction temperature on particle size distribution

79

Crystallinity, crystal size and crystal morphology are the major indicators of crystal quality. Besides particle morphology, a great attention is paid to monitoring the particle size distribution of the obtained precipitates[22]. Fig.4 displays the particle size distribution of the obtained Friedel’s salt particles in the range of 303-473 K. At low reaction temperature (303 K), the curve is very irregular and has four peaks, a large one and three very slight ones, respectively, and the average particle size (D0.5) of Friedel’s salt particles is ~ 9 m. When the reaction temperature is up to 353 K, the curve is still irregular, and the average particle size decreases to 6.32 m. Moreover, further increasing the temperature up to 373 K results in a slight decrease of the size, and it becomes 5.85 m. When the reaction temperature is in the range of 423-453 K, the curve becomes smooth. It is shown that the average particle size increases drastically to 19.566, 18.745  m,

80


respectively and the span ((D0.9-D0.1)/D0.5) decreases to 1.639, 1.763, respectively. So the size distributions are sharpened. The particle size distribution of the obtained Friedel’s salt particles is uniform at 423-453 K. The fact that Friedel’s salt crystals obtained in the range of 423-453 K has high quality which is coincident with the SEM results (Figs.3(g)-(j)). However, with the increase of the reaction temperature up to 473 K, the curve becomes irregular and the average size of the particles decreases to 16.832 m with a increased span 2.304, as can be seen in Fig.4(f). 3.2 Effect of the initial CaCl2 concentration

Fig.6 Influence of different initial CaCl2 concentrations on SEM morphologies of precipitates: (a) 0.25 M; and (b) 1.5 M

For an industrial process, it is obvious that the reagent should be used as concentrated as possible to get larger yield of product[23]. To investigate the effect of the initial CaCl2 concentration, two concentrations of 0.25 M and 1.5 M CaCl2 were investigated at 353 K. In all cases, the XRD analysis of the obtained crystals confirmed the presence of only Friedel’s salt (Fig.5). SEM image analysis was performed to check the crystal morphology. The results are shown in Fig.6. As can be seen from the SEM images, the samples from different initial CaCl2 concentrations are flat hexagonal (or pseudo-hexagonal) with some serious aggregation and small non-uniform growths. The crystals obtained at low initial concentration of CaCl2 (0.25 M) consist of large crystals with D 0.5 7.697  m, as shown in Fig.7(a).The average particle size D0.5 of Friedel’s salt

particles decreases to 5.024 m with increasing initial CaCl2 concentration up to 1.5 M. This behavior may be attributed to the fact that with higher concentration of CaCl2, higher supersaturation develops, which in turn gives rise to fast prime nucleation and subsequently formation of small crystals[24].

3.3 Effect of NaAlO2 titration speed The state of supersaturation is an essential feature of all crystallization operations. When the precipitation is in the unstable zone, the spontaneous crystallization will occur and a large number of crystal nuclei are formed [25]. In order to keep the crystallization occurred in the metastable zone, two rates of NaAlO2 addition, i e,1.25 mL/min, 10 mL/ min, were investigated under otherwise identical conditions. The obtained precipitates were examined by XRD (Fig.8) and found to contain only Friedel’s salt. The typical images and particle size distribution of the particles synthesized at different NaAlO2 titration speeds are shown in Figs.9 and 10. According to the images, the titration speed has insignificant influence on the supersaturation environment and the quality of crystals. Apparently the faster the titration speed (10 mL/min), the higher the supersaturation, hence the more extensive homogeneous nucleation is vis-à-vis growth[26], while the crystals with small size with D0.5 5.133 m and span 3.716, as shown in Fig.10(a) are


81

produced. On the other hand, when the reaction occurs at lower supersaturation by titration at a rate of 1.25 mL/min, the growth rate of the nuclei is higher than the nucleation rate, while the crystals grow into lager one with D0.5 6.986 m and span 1.783 (Fig.10(b)).

3.4 Effect of the aging time According to the “Ostwald ripening” rule, many small crystals form in a system initially but slowly disappear except for a few that grow larger, at the expense of the small crystals. The smaller crystals act as

Fig.9 Influence of different NaAlO2 titration speeds on SEM morphologies of precipitates: (a) 10 mL/min; and (b) 1.25 mL/min

Fig.12 Influence of different aging time on SEM morphologies of precipitates: (a) 0 h; and (b) 10 h

82


“nutrients” for the bigger crystals[27]. To study the crystal growth process, a series of aging time experiments were carried out. The precipitates collected at various aging time were examined by XRD (Fig.11) and found to contain only Friedel’s salt. Figs.12 and 13 provide the typical SEM images and particle size distribution corresponding to the samples from different aging time. It was found that the aging time has significant influence on the quality of crystals. When the aging time increases from 0 to 10 h, as shown in Fig.12, flat hexagonal (or pseudohexagonal) particles are produced, whilst the obtained samples show much larger particles with increasing aging time, as shown in Fig.13. 3.5 Effect of molar Ca/Al ratio

salt by the reaction of calcium chloride with sodium aluminate should comply with the two mechanisms. 3.6.1 Anion -exchange mechanism The first step is the formation of the interlayers of AFm hydrates which may be expressed by the following equation: (2) The OH- ions in the interlayers are particularly prone to ion exchange[30,31]. Thus, the anion-exchange reaction occurring between the OH- in the interlayers of AFm hydrates and the free-chloride ions in the solution to form Friedel’s salt is expressed as follows: (3) The amount of OH- in moles occurring during chloride binding is equal to the amount of chloride ions bound in moles. Hence, the problem of ionic charge imbalance in the solution does not arise. 3.6.2 Adsorption mechanism First, the formation of the interlayers of the principal layer, [Ca 2Al(OH -)6·2H 2O] +, of the AFm structure is expressed as follows: (4)

Molar Ca/Al ratio can strongly affect the solution precipitation process and the composition of the obtained products. Fig.14 displays the XRD patterns of the samples obtained from different molar Ca/Al ratios. For the synthesized sample with Ca/Al ratio of 1, the XRD pattern indicates that it is a mixture of Friedel’s salt and Al(OH)3. All peaks in the XRD patterns of the obtained samples with Ca/Al ratios of 2 and 3 are in good agreement with the Friedel’s salt reference data (JCPDS-78-2051) [13]. However, with further increasing the molar Ca/Al ratio up to 4, the obtained sample is a mixture of Friedel’s salt and 3CaO・Al2O3・0.16Ca( OH)2・0.83CaCl2, identified by XRD. The results are consistent with the prior study[28]. 3.6 The mechanism of Friedel’s salt formation The mechanism of LDH formation in cement pastes has not been unambiguously identified. Essentially, two mechanisms have been proposed: an adsorption mechanism and an anion-exchange mechanism[29]. Likewise, the formation of Friedel’s

Then, the bulk of the free-chloride ions from the CaCl2 solution are directly adsorbed in the interlayers of [Ca 2 Al(OH - )6·2H 2 O] + in order to balance the charge arising due to the replacement of a Ca2+ ion by an Al3+ ion[31]. The adsorption of Cl- ions leads to the formation of Friedel’s salt, which may be expressed by the following equation: (5)

4 Conclusions Friedel’s salt was prepared under different synthesis conditions. It has been shown that reaction temperature is of the prime importance besides the molar Ca/Al ratio: at lower temperatures (303-353 K), Friedel’s salt with hexagonal morphology was obtained and the average size D 0.5 of Friedel’s salt particles was about 9  m. With the increase of the reaction temperature from 373 to 453 K, a mixture of 3CaO・A1 2O 3・CaCl 2・10H 2O and Ca 8Al4(OH) 24(CO 3)


Cl 2 (H 2 O) 9.6 was obtained, and the average size increased drastically to about 19 m. However, when the temperature reached 473 K, the obtained sample exhibited serious aggregation and the average size of the particles decreased to 16.832  m. When the molar Ca/Al ratio was 1, the obtained sample was a mixture of Friedel’s salt and Al(OH)3. The obtained samples with molar Ca/Al ratios of 2 and 3 were only Friedel’s salt. However, with further increasing the molar Ca/Al ratio up to 4, 3CaO・Al2O3・0.16Ca(OH)2・ 0.83CaCl2 diffraction peaks were appeared in sample. It was also found that the aging time has significant influence on the quality of crystals. With the increase of the aging time from 0 to 10 h, flat hexagonal (or pseudohexagonal) particles with lager size were produced. Besides, the initial CaCl2 concentration and NaAlO2 titration speed do not significantly influence the morphology and particle size distribution of Friedel’s salt. Compared with Friedel’s salt formed in concrete under the condition of erosion, Friedel’s salt synthesized in this paper at low temperature has higher purity, lower degree of crystallinity and bigger specific surface area, indicating better adsorption capacity [16-18, 28]. Tw o p o t e n t i a l m e c h a n i s m s o f F r i e d e l ’s salt formation were discussed. In the adsorption mechanism, the bulk of the free-chloride ions from the CaCl2 solution were directly adsorbed in the interlayers of [Ca 2 Al(OH  )] 6 ·2H 2 O] + in order to balance the charge arising due to the replacement of a Ca2+ ion by an Al3+ ion, leading to the formation of Friedel’s salt. In the anion-exchange mechanism, Friedel’s salt was formed by an anion-exchange between the OH in the interlayers of AFm hydrates and the free-chloride ions in the solution. References [1] [2] [3]

[4]

[5]

[6]

[7]

Roy A de. Lamellar Double Hydroxides[J]. Mol. Cryst. Liq. Cryst., 1998, 311(1): 173-193 Vaccari A. Clays and Catalysis: a Promising Future [J]. Appl. Clay Sci., 1999, 14(4): 161-198 Rives V, Ulibarri M A. Layered Double Hydroxides (LDH) Intercalated with Metal Coordination Compounds and Oxometalates [J]. Coord. Chem. Rev., 1999, 181(1): 61-120 Inacio J, Forano C, Besse J P. Adsorption of MCPA Pesticide by MgAl Layered Double Hydroxides [J]. Appl. Clay Sci., 2001, 18(5-6): 255-264 Rousselot I, Palvadeau P, Besse J P. Insights on the Structural Chemistry of Hydrocalumite and Hydrotalcite-Like Materials: Investigation of the Series Ca2M3+(OH)6Cl・2H2O (M3+: Al3+, Ga3+, Fe3+, and Sc3+) by X-ray Powder Diffraction[J]. J. Solid State Chem., 2002, 167(1): 137-144 Roussel H, Briois V, Elkaim E, et al. Cationic Order and Structure of [Zn−Cr−Cl] and [Cu−Cr−Cl] Layered Double Hydroxides: a XRD and EXAFS Study [J]. J. Phys. Chem. B, 2000, 104(25): 5 915-5 923 Roussel H, Briois V, Elkaim E, et al. Study of the Formation of the Layered Double Hydroxide [Zn−Cr−Cl] [J]. Chem. Mater., 2001, 13(2): 329-337

[8] [9] [10]

[11]

[12]

[13] [14]

[15] [16]

[17]

[18]

[19]

[20]

[21]

[22] [23] [24] [25]

[26] [27]

[28]

[29]

[30]

[31]

83

Friedel P M. Surunchloro-Aluminate de Calcium Hydrate Semaclantpar Compression [J]. Bull. Soc. Fr. Mineral, 1897, 19(20): 122-136 Filippakis S, Terzis A, Burzlaff H. The Crystal Structure of Ca2Al(OH)6Cl・2H2O[J]. Zeit. Kris., 1987, 181(1-4): 29-34 Suryavanshi A K, Scantlebury J D, Lyon S B. Mechanism of Friedel’s Salt Formation in Cements Rich in Tricalcium Aluminate [J]. Cem. Concr. Res., 1996, 26(5): 717-727 Birnin-Yauri U A, Glasser F P. Friedel’s Salt, Ca2Al(OH)6(Cl, OH): Its Solid Solutions and Their Role in Chloride Binding[J]. Cem. Concr. Res., 1998, 28(2): 1 713-1 723 Gougar M L D, Scheetz B E, Roy D M. Ettringite and C-S-H Portland Cement for Waste Ion Immobilization: a Review[J]. Waste Manage., 1996, 16(4): 295-303 Miyata S. Anion-Exchange Properties of Hydrotalcite-Like Compounds [J]. Clays Clay Miner., 1983, 31(4): 305-311 Walcarius A, Lefevre G, Rapin J P. Voltammetric Detection of Iodide after Accumulation by Friedel’s Salt [J]. Electroanalysis, 2001, 13(4): 313-320 Liu Q, Li Y, Zhang J, et al. Effective Removal of Zinc from Aqueous Solution by Hydrocalumite [J]. Chem. Eng. J., 2011, 175: 33-38 Zhang J J, Zhao H, Cao H B, et al. Adsorption Kinetics of Friedel's Salt for Removal of Cd2+from Low Concentration Wastewater [J]. The Chinese Journal of Process Engineering, 2012, 12(4): 590-595 Ma J Y, Li Z B, Zhang Y, et al. Desilication of Sodium Aluminate Solution by Friedel’s Salt (FS: 3CaO・Al 2O 3・CaCl 2 ・10H 2 O) [J]. Hydrometallurgy, 2009, 99(3-4): 225-230 Dai Y C, Qian G R, Cao Y L, et al. Effective Removal and Fixation of Cr(VI) from Aqueous Solution with Friedel’s Salt [J]. J. Hazard. Mater., 2009, 170(2-3): 1 086-1 092 Zhang D N, Jia Y F, Ma J Y, et al. Removal of Arsenic from Water by Friedel’s Salt (FS: 3CaO・Al2O3・CaCl2・10H2O) [J]. J. Hazard. Mater., 2011, 195(15): 398-404 Kuzel H J. X-ray Investigations of Some Complex Calcium Aluminum Hydrates and Related Compounds[C]. Suryavanshi AK, Hager WH. Proceedings of the Fifth International Congress on Chemistry of Cements, Tokyo: Cement Association of Japan, 1969 Goni S, Guerrero A. Accelerated Carbonation of Friedel's Salt in Calcium Aluminate Cement Paste [J]. Cem. Concr. Res., 2003, 33(1): 21-26 Mersmann A. Crystallization Technology Handbook [M]. New York: Marcel Dekker Inc., 2001: 342-351 Cheng W T, Li Z B. Precipitation of Nesquehonite from Homogeneous Supersaturated Solutions[J]. Cryst. Res. Technol., 2009, 44(9): 937-947 Mersmann A. Supersaturation and Nucleation [J]. Chem. Eng. Res. Des., 1996, 74(7): 812-820 Wang Y, Li Z B, Demopoulos G P. Controlled Precipitation of Nesquehonite (MgCO 3 ・3H 2 O) by the Reaction of MgCl 2 with (NH4)2CO3[J]. J. Cryst. Growth., 2007, 310(5): 1 220-1 227 Boistelle R, Astier J P. Crystallization Mechanisms in Solution [J]. J. Cryst. Growth, 1988, 90(1-3): 14-30 S Girgin. Crystallization of Alpha-Calcium Sulphate Hemihydrate by Aqueous Reaction of Calcium Chloride with Sulphuric Acid [D]. Montreal: McGill University, 2006 Brown P, Bothe J. The System CaO-Al2O3-CaCl2-H2O at 23±2 ℃ and the Mechanisms of Chloride Binding in Concrete [J]. Cem. Concr. Res., 2004, 34(9): 1 549-1 553 Jones M R, Macphee D E, Chudek J A. Studies Using 27Al MAS NMR of AFm and AFt Phases and the Formation of Friedel's Salt [J]. Cem. Concr. Res., 2003, 33(2): 177-182 Dosch W, Keller H. Phase Equilibria and Cement Hydration[C]. Idom GM, Fordos Z. Proceedings of the Sixth International Congress on Chemistry of Cements. Moscow: Cement Association of Russia, 1976 Daimdot D, Glasser F P. Thermodynamic Investigation of the CaOAl2O3-CaSO4-H2O System at 25 ℃[J]. Cem. Concr. Res., 1993, 23(1): 221-238