1. Introduction

Calcium carbonate (CaCO₃) is one of the world’s most abundant materials. It has several different crystalline forms calcite, aragonite and vaterite are anhydrous crystalline polymorphs; hexahydrate and monohydrate are hydrates form. In addition, an amorphous hydrated calcium carbonate ^[1]. Practical, applications of the PCC are determined by its properties such as specific surface area, pore sizes, particle size distribution, polymorphic phase, purity. Calcite has crystallographic rhombohedra structure and is the most thermodynamically stable polymorph in ambient conditions. Aragonite (orthorhombic structure) forms at high temperatures, while vaterite (hexagonal structure) is the most unstable polymorph and can accompany calcite if the precipitation occurs at low temperature. The control of the precipitation process, aiming to obtain a specific morphology of the PCC is extensively studied. The operating conditions such as temperature, pH and supersaturating degree can favor the nucleation and growth of a given polymorph ^[2].

Calcium carbonate has great interest due to its wide industrial applications, including paper, paints, inks, plastics, adhesives, rubbers, pharmaceuticals, dental care products, cleaning agents, food stuff ^{[3, 4]}.

In relation to the preparation of nanoparticles, the most important aspect is the control of the polymorphism, particle size and morphology of the material. Nanoparticles with new properties such as mechanical, optical, biochemical, and catalytic have attracted many of research interest in the past years. This is mainly due to the strong functional dependence of its properties on the shapes of materials ^[5]. Calcium carbonate nanoparticles have numerous applications as biomaterials with different morphological structures. It is used in medicine, pharmaceutical industries, and drug delivery systems ^[6].

Many of works have studied the effect of organic and inorganic compounds on process of crystallization and modification of CaCO₃ such as, surfactant ^[7], glycine ^[8], polyols(ethylene glycol, glycerol, and erythritol) ^[9], cationic biosurfactants ^[10], cetyltrimethylammonium bromide (CTAB) ^[11], polyacrylic acids ^{[12, 13]}, Polyacrylamide ^[14], fatty acids and its salts ^{[15, 16, 17]}, methyl methacrylate ^[18], polyethylene oxide ^{[19, 20]}, polymaleic acid ^{[21, 22]}, poly (styrene sulfonic acid) sodium salt ^[23], heavy alkylbenzene sulfonate ^[24], Polypeptide and polyaspartic acid ^[25], alginate hydrogels ^{[26, 27, 28]}, Cellulose derivatives ^[29], inorganic additives ^[30], polysilicic acid ^[31] have been used. Also, Calcium carbonate was used for preparation of nanocomposites ^[32].

There are different techniques used for the preparation of calcium carbonate nanoparticles such as the precipitation of homogeneous solutions ^[33], water in- oil-in-water emulsions ^[34], mechanochemical and sonochemical syntheses ^{[35, 36]}, continuous gas-liquid membrane ^[37], double injection ^[38], precipitation ^[39].

Polycarboxylate water-reducer is known as polycarboxylate superplasticizer. It is a kind of copolymer which molecular structures graft carboxylic acid. Its branched structure is considered as “comb” or “graft” which is formed by polyethylene oxide, and contained other functional groups as carboxylic and sulfonic groups ^[40]. In the present paper, we are aimed at synthesis spherical vaterite, Where Polycarboxylate-type superplasticizer was used as stabilizer and capping agent.

2. Experimental

All chemicals are commercially available and analytical grade used without further purification. The raw materials used for synthesis were 95% CaCl₂ and 99% Na₂CO₃, were obtained from Al-Nasser Company for Chemicals Co. (Egypt) and polycarboxylate-type superplasticizer was obtained from the Egyptian British company for special chemicals, 6^th October City, Egypt. Appearance, Light brown viscous liquid; Density, 1.18 ± 0.02 g/cm³; pH =6~8; Solid content, 40±1.5. Double distilled water was used in all experiments. The Figure 1 is shown the chemical structure of polycarboxylate-type superplasticizer.

Infrared spectra of polycarboxylate are depicted in Figure 2. The analysis result of spectra is as follows: an O-H of alcohol stretching, vibration bond at 3700–3300 cm⁻¹; The signals at 2960 cm^-1 are attributed to -CH₃- and -CH₂-^[41], pack of 1640 cm⁻¹ prove the existence of ester bond; The peaks at around 1460 cm^-1 stands for -CH₂- ^[41], the signal at 1100 cm^{-1 [42]}, which proves that it contained the side chain of polyethylene glycol methyl ether in the molecular structure. Also, the signals at 1460 and 1354 cm^-1 are the characteristic absorptions of PEO group ^[43], The peak at 946 cm⁻¹ could respectively be assigned to the this configuration and the trans-configuration of RCH=CHR′ group ^[44], the bond 620 cm⁻¹ is C-S stretching vibration ^[45].

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Figure 1. Chemical structure of Polycarboxylate-type superplasticizer

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Figure 2. FT-IR spectrum of Polycarboxylate-type superplasticizer

Calcium carbonate precipitation experiments were carried out at 25°C, rapidly mixing, under constant and vigorous stirring (3000 RPM), equal volumes of 1.0 M Na₂CO₃ (pH =11.2) reacts with 0.5M CaCl₂ (pH = 6.7) solutions in the presence of 2% polycarboxylate-type superplasticizer. The precipitated colloidal phase was filtered immediately and washed with acetone. Then the solid was dispersed in deionized water and filtered, this process repeated three times in order to rinse the particles. After the last time particles were dried in the microwave oven. The products were slightly ground for analysis ^[46].

The phases of calcium carbonate products were analyzed from the Fourier Transform Infrared (FT-IR) spectroscopy. Fourier transform Infrared (FT-IR) spectra were measured using a Perkin Elmer 880 FT IR spectrometer with the KBr pellet method. The sample was good ground and thoroughly mixed to homogenize them. The homogenized sample was mixed with dry KBr in 1:100 mass ratios and pellet were pressed at 5 Tons. The spectra of the sample pellets were recorded by using pure KBr pellet as the blank. The X-ray diffraction (XRD) studies were performed to study the crystalline phases of synthesized calcium carbonate products. The XRD spectra were recorded on (M/S. Shimadzu Instruments, Japan) diffractometer XRD 7000 with Ni filtered Cu Kα as a radiation source at 2θ scan speed of 4◦ min⁻¹, k = 0.1540562 nm. The samples were carefully ground to a fine powder and thoroughly mixed to homogenize them and their powder XRD spectra were recorded. The resulted XRD patterns were analyzed using X Powder 12 software with the aid of ICDD PDF 2 database. Morphologies of the products were observed with the help of JEOL JSM 6360 DLA, Japan, Scanning Electron Microscope (SEM) and Transmission Electron Microscope ((TEM; Hitachi, H-800).

The molar content (%) of CaCO₃ polymorphs can be calculated according to the intensity of the (221) plane of aragonite, the (104) plane of calcite and the (110) plane of vaterite ^[47]:

For mixture composed of calcite, aragonite and vaterite:

(1)

(2)

(3)

Where X_A, X_V and X_C are the molar content (%) of aragonite, vaterite and calcite, respectively. I_A ²²¹, I_C ¹⁰⁴ and I_V¹¹⁰ are the XRD intensity of the (221), (104) and (110) plane of aragonite, calcite and vaterite, respectively.

3. Results

IR spectroscopy was convenient and useful to distinguish the different crystal phase of calcium carbonates, it was used to monitor effect of polycarboxylate - type superplasticizer on the crystallization of calcium carbonate. The molecular structure of calcium carbonates, containing two ions of calcium and carbonate, were simple and the vibration of carbonate ion in different calcium carbonate structures could be discriminated by IR spectra. The absorption bands of carbonate are divided into four areas: ν₁ (symmetric stretching) at 1080 cm^-1; ν₂ at (out of- plane bending) 870 cm^-1; at ν₃ (doubly degenerates planar asymmetric stretching) 1400 cm^-1 and ν₄ (doubly degenerate planar bending) at 700 cm^{-1 [30, 39]}. The FTIR spectra of PCC products in the absence (a) and in the presence (b) of polycarboxylate are illustrated in Figure 3. For the sample without polycarboxylate (a) has bands centered at 712, 873 and 2522 cm^-1 reveal the presence of calcite. While in the present of polycarboxylate (b) appears new bands centered at 1085 cm^-1, 878 cm^-1, and 745 cm^-1 and a split peak at1440 and 1490 cm^-1 in vaterite ^[48]. Also, IR spectrum of sample (b) showed strong peaks at 3396, 2956 and 2916 cm⁻¹ corresponding to OH, C–H and C–C vibrations respectively as indicated the presence of organic material ^[17]. These values compare with published IR data, produced by FT-IR of KBr pellets of 1420 cm^-1 (ν3), 876 cm^-1 (ν₂), and 714 cm^-1 (ν₄) for calcite and 1090 cm^-1 (ν₁), 878/850 cm^-1 (ν₂), and 747/741 cm^-1 (ν₄) for vaterite ^[49]. Diagnostic peaks were therefore 712 cm^-1 for calcite, and 747 and 1085 cm^-1 for vaterite. Therefore, according to these results, it can be concluded that the dominant phase of all the PCC products in the presence of polycarboxylate superplasticizer is vaterite.

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Figure 3. FTIR spectrum of CaCO₃ in absence of polycarboxylate (a) and in presence of polycarboxylate(b)

X- ray diffraction was used to identify the crystalline phase of the prepared samples and the results are presented in Figure 4. XRD spectra of all samples of crystals obtained in the absence and presence of polycarboxylate - type superplasticizer are shown peaks characteristic of CaCO₃. The XRD peaks located at 2θ values of 29.4, 36.0, 39.4, 43.1, 47.4 and 48.5, which can be assigned to be due to those of calcite, were obtained in the absence of polycarboxylate - type superplasticizer (JCPDS 01-072-1937). In the presence of polycarboxylate - type superplasticizer the existence of calcite and vaterite (JCPDS 01-072-1937 and 00-033-0268) was demonstrated by XRD (Figure 4). The main characteristic peaks of vaterite at 2θ of 24.92°, 26.99° and 32.78° correspond to the (110), (112) and (114) crystallographic planes, respectively ^[50], with no aragonite is detected. The relative contents of a different crystalline form in calcium carbonate were measured by XRD, were 91.9% calcite and 8.04% vaterite for sample prepared without PSS, white for sample prepared with PSS 10, 6% calcite and 89.4 % vaterite. It can be seen that, in the presence of polycarboxylate - type superplasticizer, vaterite and calcite polymorphs occur, while in the absence of polycarboxylate - type superplasticizer, calcite is almost the only crystal form. This is mainly due to the polymer implying the inhibition of crystallization and a reduction of crystallinity. Thus, PSS in the concentration up to 2 % in this system leads to the vaterite structure which remains stable for prolonged time ^[51]. The above results indicate that PSS favors the formation of vaterite, which agrees with the results obtained in the fast-reaction crystallization experiment carried out at ambient temperature ^{[52, 53]}.

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Figure 4. XRD patterns of calcium carbonate samples prepared without (a) and with (b) PSS., □ calcite, ● vaterite

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Figure 5. SEM images of CaCO3 samples (a, b) calcite particles prepared without PSS and (c, d) vaterite particles prepared with PSS

Figure 5 shows that CaCO₃ were synthesized in the presence and absence of PSS. Figure 5 (a, b) indicates the morphology of CaCO₃ crystal prepared by adding PSS. The results exhibit that in the absence of the PSS system, CaCO₃ only formed common very crystalline, cubic crystal calcite crystal structure. On the other hand The PSS as modifier has an effect on crystal growth as shown in Figure 5 (c, d), the spherical morphology of final vaterite calcium carbonate are formed. These particles are in nanoscale with the average particle size ranging from 15 to 26 nm as estimated using TEM. The vaterite particles have been aggregated during preparation.

TEM was also used to study the calcium carbonate crystal particles. Figure 6 (a) and Figure 6 (b) shows the TEM image of prepared calcium carbonate in the presence of PSS that reveal a spherical shape of calcium carbonate vaterite polymorphs. The nano crystal particles synthesized in the presence of PSS with individual particles measuring 15–26 nm, in addition, the shape of the particles is clear, and the difference in the particle size. PSS has been reported to strongly effect on the size and morphology of calcium carbonate nanoparticles during synthesis.

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Figure 6. TEM Micrographs of CaCO3 formed in the presence of PSS

4. Discussion

The calcite, vaterite, and aragonite are different in their crystallographic characteristics. But they composed of calcium ions and carbonate ions. Their difference lies in the ions location in the crystals, which is formed during the crystallization. In a normal condition three phases to form, three types of crystals grow simultaneously. The component of products is determined by their activity. With change of external condition, the balance of competition would change, resulting in an increase of one phase and a decrease of another phase. The growth of calcite is largely dependent on the strength of a solution, while it has less dependence on the solution velocity and ionic strength ^[54]. The vaterite grows as aggregated particles of small vaterite particles. The aggregation is mainly dependent on the density of vaterite nuclei, which is determined by the supersaturation as well as the transformation of vaterite to calcite and aragonite. The aragonite grows through an oriented attachment of small aragonite particles ^[55]. Vaterite has a different crystalline shape such as hexagonal and mono-crystalline plates, although it is rarely obtained in solution. Also, Vaterite can precipitate from solution as spherical particles spontaneously. However the mechanism of spherical particle growth is not fully recognized and is even quite often returned to be the result of nano-aggregation of precursor crystals.

Since vaterite is lowest dense phase and its nucleation rate is faster than that of calcite. But the vaterite nuclei are not thermodynamically stable that dissolving in the solution to precipitate as calcite. However, at high supersaturation, the dissolution of vaterite decreases and leads to the formation of metastable spherical particles of aggregates vaterite ^[56]. So, the direct mixing experiment with high supersaturation leads to massive formation of spherical vaterite. The massive formation of vaterite is also related to the short time of reaction because aging helps the transformation of vaterite to calcite.

Based on the Ostwald’s phase rule, the least stable ACC is nucleated first and then it transforms to the most stable calcite through intermediate vaterite during the synthesis of PCC ^[2]. The transformation of ACC to vaterite takes place by rapid dehydration of ACC to reorganize the poorly ordered structure of ACC ^[57]. Kinetic studies of PCC formation given in the literature reveal that the transformation of vaterite to calcite occurs through dissolution of vaterite followed by recrystallization to achieve calcite structure ^{[57, 58, 59]}. The dissolution of vaterite is mainly controlled by the diffusion rate of Ca²⁺and CO₃^-2 ions in the solution ^[58]. The formation of calcite from vaterite is favored by slow diffusion rates and low concentrations of Ca²⁺ and CO₃^-2 ions ^[58]. Also, the reaction temperature affects the formation of vaterite. The best temperature to stabilize vaterite is 40 ^○C and the amount of vaterite has been decreased with increasing temperature. This may be due to the increase of dissolution of vaterite with the increasing temperature.

The polycarboxylate - type superplasticizer is an amphiphilic copolymer that can form micelles in aqueous solution, this micelle is dynamically stable in aqueous solution. The micelles concentrate with PSS hydrophobic cores and hydrophilic side chain shells in solutions. It is speculated that block copolymer based micelles work as “pseudonuclei” for the formation of calcium carbonate nano-crystals nuclei.

The influence of organic additives on the nucleation and crystal growth rates and its polymorphic forms, morphology, and particle size was thought to be caused by a decrease in the Ca²⁺ ion concentrations because of their reaction with carboxyl containing groups. Also, to be the adsorption of polymer on the different faces of CaCO₃ crystals, and the differences in the strength of this adsorption because of the composition of polymers i.e. The number and nature of functional groups, including polar ones, and the molecular weight, determine the final polymorphic form of the calcium carbonate precipitate ^{[60, 61]}.

In general, there are existing a thermodynamic/kinetic balance between calcite and metastable vaterite that can controlled by organic additives. It is well known that metastable vaterite could transform readily and irreversibly into stable calcite form through a solvent-mediated process ^[62]. The stable vaterite particles can be produced in the presence of – COO −- functionalized which can stabilize the vaterite surfaces. The surface density of –COO − functional group of the organic additive is important to control CaCO₃ morphologies and polymorphs. The formation of vaterite crystal becomes favorable to increase the density of -COO- groups, due to increasing the electrostatic interaction between the carboxylic group and the Ca²⁺. Also, the position of the carboxylate group in the polymers plays an important role in their ability to bind CaCO₃ clusters. The parallel-oriented carboxylate groups are important for the binding to CaCO₃ clusters ^{[63, 64]}.

According to Colfen’s results ^[65], the side chains of polymer are not expected to exhibit any affinity for CaCO₃ and have the potential to control the degree of surface activity. The side chains may disperse and stabilize the inorganic particles. The carboxylate groups of PSS polymer can be expected to interact with crystallized CaCO₃ through electrostatic interaction. The (0 0 1), (1 0 1), and (1 1 0) planes of vaterite are positively charged and contain exposed Ca²⁺; thus, the carboxylate groups of PSS polymer can absorb on these planes ^[66]. The vaterite surface can be stabilized by the carboxylate groups of polymers that block the transformation of vaterite to calcite that might account for the kinetic promotion of the metastable polymorph owing to the interaction between Ca²⁺ and the carboxylate groups.

There are many studies have shown that the effect of organic additive on the stabilization of the vaterite polymorph. Jada et al. have suggested that the amount of each crystalline species, vaterite or calcite, is a function of the concentration and molecular weight of polyacrylate when CaCO₃ precipitation occurs ^[67]. Ouhenia et al., also reported that the crystallization of vaterite is favored in the presence of polyacrylic acid up to 50 ◦C ^[68]. Song has been shown that, the CaCO₃ films obtained on the surface of chitosan films, mainy consisted of vaterite, in the presence of high concentration of polyacrylic acid, which suggests the presence of PAA plays an overwhelming part in stabilizing the vaterite ^[69].

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Figure 7. Shown the Schematic illustration of phase transformation process of the vaterite nanoparticles with PSS

The – COO − functionalized additive in our case PSS was effectively adsorbed on the surface of the vaterite particles, which causes the stabilization of vaterite and the crystalline dissolution and recrystallization could have been inhibited with PSS segment. The carbonyl groups could interact with Ca²⁺ ions through coordination effect and form pss/Ca²⁺ chelate. While the CO₃²⁻ ions encounter with the PSS/Ca²⁺ chelate, the Ca²⁺ ions, on the one hand, act as the nucleating sites, on the other hand, take part in the nucleation of ACC. In the presence of pss the Ca²⁺ ions is chelated by PSS and form the PSS/Ca²⁺ chelate. The CO₃²⁻ ions capture the Ca²⁺ ions and induce the formation of ACC particles, as soon as they meet with the PSS/Ca²⁺ chelate. The ACC particles transform into metastable vaterite instantly, which is negatively charged on (001) planes. With the aging time prolongs, the PSS molecules rearrange on the surfaces of vaterite particles and modulate the formation of lenticular aggregates through hydrogen bonding effect, as shown in Figure 7.

5. Conclusion

Vaterite microspheres are synthesized by a precipitation route assisted by PSS. The vaterite crystal phase of calcium carbonate is stabilized by PSS. The crystal growth process of vaterite microspheres is supposed that the nanoparticles formed and aggregated to microsphere assisted by PSS. As a result of adsorption of PSS on the surface of the vaterite particles, which causes the stabilization of vaterite and the crystalline dissolution and recrystallization could have been inhibited with PSS segment.

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