Sustainable Production and Physicochemical Characteristics of Calcium Sulfate Dihydrate Prepared from Waste Eggshells

Abstract

Gypsum products (calcium sulfate dihydrate, CaSO₄·2H₂O) were synthesized through an eco-friendly and low-cost process by two different renewable calcium carbonate sources (CaCO₃), hen and duck eggshell wastes, with product yields obtained of 84.73 and 87.74%, respectively. The X-ray fluorescence results indicated that calcium oxide (CaO) and sulfur trioxide (SO₃) are the major elemental components of CaSO₄·2H₂O prepared from both calcium sources. The Fourier transform infrared results confirmed the vibrational characteristics of SO₄²⁻ and H₂O functional groups in the chemical structure of the prepared samples. The X-ray diffraction patterns of CaSO₄·2H₂O prepared from both calcium sources confirmed the sample’s crystal structure as well as the chemical formula, after comparing them to the standard powder diffraction file. The crystallite sizes of CaSO₄·2H₂O products were calculated from the experimental diffraction peak through the Scherrer equation and found to be 19–20 nm. The positive preferential growth (Pg) value highlighted the excellent stability of the synthesized CaSO₄·2H₂O. The scanning electron microscopic results showed the agglomeration particles of hen- and duck-CaCO₃ raw agents, whereas plate-like particles were observed for hen- and duck-CaSO₄·2H₂O products but the particle sizes were different.

1. Introduction

Calcium sulfate, a ceramic material, is widely used for various applications. It has been used as a binder material for building construction [1], bone graft material [2], periodontal disease treatment [3], endodontic lesions [4], alveolar bone loss [5], maxillary sinus augmentation [6], filler for plastic [7] and rubber [8], coating [9], desiccant [10], coagulant [11], catalyst [12], and bleaching agent [13]. Four calcium sulfate forms have been observed and previously reported: calcium sulfate dihydrate (CaSO₄·2H₂O, gypsum) [13], α- and β-calcium sulfate hemihydrate (CaSO₄·0.5H₂O, bassanite) [14], and calcium sulfate anhydrous (CaSO₄, anhydrite) [15]. Moreover, two different anhydrous polymorphs, β-CaSO₄ (insoluble anhydrite, natural anhydrite form) and γ-CaSO₄ (soluble anhydrite), were also reported [16]. The phase transformation, crystal structure, morphology, and growth rate of calcium sulfate depend on different physicochemical conditions, i.e., temperature, pressure, solutions, and other ionic minerals [17,18]. Calcium sulfates can be obtained from different preparation processes, i.e., formed from an aqueous sulfuric acid [19], flue gas desulfurization [20], reverse microemulsion [21], and chemical precipitation [22]. In turn, their characteristics (color, hardness, and density), crystal structure, solubility, optical property (translucence and refractive index), and applications depend on the preparation conditions [18].

Eco-friendly production and consumption for sustainable development are considered important topics. Furthermore, the depletion of the environment is intensifying the need to reduce pollution. In that context, “sustainability” is related to the reliability of resources and ecology, which connects to society, economies, and environments. At present, one-third of the total food we produce is cast off as waste, and this waste affects the environment [23]. Accordingly, the utilization of waste food may form part of the approach to ensuring a sustainable environment. Waste eggshells are one of the most discarded foods, generated from consumption as well as from various industries, i.e., drugs, cosmetics, etc. They offer an important calcium source since the main component of eggshells is calcium carbonate (CaCO₃), which varies from 95 to 97% [24], mainly in the form of calcite polymorph > 94 wt%, which is considered a rich natural calcium source [25].

Eggshells have been applied in various fields of materials science, mostly with industrial and biomedical applications [26]. Meng and Deng [27] demonstrated a novel application of waste eggshells as a multifunctional reaction system to regulate the reactants and solution pH. They proposed the idea of transferring waste eggshells into a unique reaction system for the synthesis of Co₉S₈ nanorod arrays. Furthermore, due to its pore structure, eggshell is an excellent secondary raw material for the preparation of value-added products with a fine powder, such as porous solid catalysts [26]. Since the main chemical component of eggshells is CaCO₃, which can then be calcined, CaO may be obtained, one of the most promising metal oxides with high basicity. This basicity property highlights that eggshell-derived CaO can be used as a heterogeneous catalyst in biodiesel production [28]. Previously, eggshell was applied as an adsorbent for the removal of hydrogen sulfide (H₂S) from wastewater [29]. Additionally, eggshells are very useful green and economic adsorbents due to their availability and absence of toxic elements. Another novel application of eggshell waste is as a calcium (Ca²⁺) source to prepare hydroxyapatite (a calcium phosphate ceramic material with the chemical formula Ca₁₀(PO₄)₆(OH)₂) [30]. This calcium phosphate form, hydroxyapatite, has been applied as a biomaterial for bone and dental replacement because of its natural characteristics and its incorporation into bone tissues [31]. Moreover, beyond those already reported, the use of food waste for the production of other compounds will also help reduce the pollution effect of waste on the environment, with the subsequent conversion of the waste into a highly valuable product having the added benefit of reducing the production cost of that product.

In addition to the preparation of hydroxyapatite, the use of eggshells has also been reported as a secondary material for the preparation of other important agricultural- and chemical-based compounds, i.e., monocalcium phosphate (Ca(H₂PO₄)₂·H₂O), dicalcium phosphate (CaHPO₄⋅2H₂O), and tricalcium phosphate (Ca₃(PO₄)₂) [32]. Eggshell was also used as a Ca²⁺ source in the preparation of calcium sulfate compounds, e.g., CaSO₄·2H₂O (gypsum) [23], which is an important fertilizer that provides Ca²⁺ and SO₄²⁻ nutrients for plants. Gypsum is obtained from two sources: natural and synthetic gypsums. Synthetic gypsum is produced as a waste product or by-product in a range of industrial processes. Recently, a study reported the synthesis of gypsum from a reaction between bassanite, eggshells, and Pila globosa shells with sulfuric acid, which was carried out with different process conditions, acid concentrations, and time lengths [23,24,26,27,28,29].

Egg production in Thailand displays significant growth. According to the statistical data reported by the Information and Communication Technology Center (Department of Livestock Development, Ministry of Agriculture and Cooperatives, Thailand), in 2022, Thailand had more than 66 and 17 million hens and ducks, which produced more than 51 and 12 million hen and duck eggs per day, respectively. These quantities create lots of hen and duck eggshell waste. Against that background, the synthesis of value-added compounds from renewable natural resources is the driving force for environmental sustainability. Bringing the two together, this work aimed to use bio-waste eggshells as raw materials (a Ca²⁺ source) for the preparation of valuable products (CaSO₄·2H₂O) for environmental sustainability, specifically products to be used as soil treatments and fertilizers. The demand for fertilizer is greatly influenced by the demand for food, which is ever-growing due to the significantly increasing global population. CaSO₄·2H₂O is suitable to use as a fertilizer component as it is 150 times more soluble in water than CaCO₃ and is used to minimize crusting, lessen erosion, and reduce phytotoxicity, as well as to treat sodium-enriched soil [23]. In this study, the physicochemical characteristics of CaSO₄·2H₂O samples were investigated by using X-ray fluorescence (XRF, an analytical technique used to determine the elemental composition of the samples), X-ray diffraction (XRD, a technique used for phase identification of the samples), Fourier-transform infrared (FTIR) spectroscopy (a technique used to record an infrared spectrum of absorption or transmission of the samples and confirm the presence of the functional groups), and scanning electron microscopy (SEM, a type of electron microscope that photographs the morphology of a sample by scanning the surface with a focused beam of electrons). This article reports an easy and low-cost process of using two different renewable calcium carbonate sources (CaCO₃)—hen and duck eggshell wastes—to produce cheap gypsum products to be used in the agroindustry.

2. Experiments

2.1. Materials and their Synthesis

Distilled water prepared by the Department of Chemistry, School of Science, King Mongkut’s Institute of Technology Ladkrabang, Thailand was used throughout this study. Sulfuric acid (H₂SO₄, 98% w/w, 18 mol/L, AR grade) was purchased from Sigma-Aldrich. Hen and duck eggshells were collected from the bakery shops and canteens at the School of Science. Each eggshell type (duck or hen) was random, soaked, and cleaned with tap and distilled water several times, exposed to sunlight under ambient air for 2 days (drying), and ground with a 450 rpm ball milled process (MTI Corporation, Richmond, CA, USA, TMAX-XQM planetary ball milling) for 120 min using binary mixtures of two zirconium oxide (ZrO₂) ball sizes (2 and 10 mm) under dry conditions with an eggshell:ball weight ratio of 1:10, creating hen- or duck-eggshell CaCO₃ powders.

For the preparation of calcium sulfate, concentrated H₂SO₄ (18 mol/L, 98% w/w) was first diluted with distilled water to prepare 10 mol/L (~54% w/w), and diluted H₂SO₄ was then used as the SO₄²⁻ source, whereas the eggshell powder was used as the Ca²⁺ source (CaCO₃). We gradually added 20 mL of diluted H₂SO₄ solution (~0.2 mole) to 20 g of each CaCO₃ (~0.2 mole) powdered from duck or hen eggshell. This created a molar ratio between the eggshell powder and H₂SO₄ of 1:1. The preparation reaction was conducted under ambient conditions without temperature control. The mixed suspension was stirred (C-MAG HS 7, IKA magnetic stirrer, Staufen, Germany) at 100 rpm until no more carbon dioxide (CO₂) evolved. Equation (1) demonstrates the chemical reaction between H₂SO₄ solution and eggshell CaCO₃ powder, which is the same reaction as in previous research [23,24,26,27,28,29] but the process conditions, acid concentration, and time consumption are different.

H₂SO₄(aq) + CaCO₃(s) → Ca²⁺(aq) + SO₄²⁻(aq) + nH₂O(l) + CO₂(g)↑

(1)

The calcium sulfate product (calcium sulfate hydrate, CaSO₄·nH₂O) was precipitated in a powder form, as demonstrated in Equation (2), when the solvent (H₂O) was evaporated in an oven (Binder FD 115) at 60 °C for 48 h.

Ca²⁺(aq) + SO₄²⁻(aq) → CaSO₄·nH₂O(s) + mH₂O(g)↑

(2)

No additional chemicals were used in the calcium sulfate formation, ensuring a sustainable development by avoiding unnecessary precipitating reagents. The usage of waste eggshells as a raw reacting material also supports the sustainable production of CaSO₄·nH₂O products. Three repetitions were performed, and an average value was reported. After evaporation and formation had occurred completely, the product (CaSO₄·nH₂O) was kept in a desiccator for the sample characterization process.

2.2. Characterization

An XRF spectrometer (Bruker, Karlsruhe, Germany, SRS 3400) was employed to investigate the elemental compositions of the raw materials (hen and duck eggshell powders, CaCO₃) as well as the synthesized calcium sulfate samples (CaSO₄·nH₂O) using operation conditions of 100 mA current and 50 kV voltage. An XRD (Bruker, D8 Advance) diffractometer was employed to characterize the crystallographic characteristics of the sample (conditions: 40 mA and 40 kV). The diffraction database (powder diffraction file, PDF) generated by the International Centre for Diffraction Data (ICDD) [33] was used to confirm the experimental diffraction patterns of the sample. The diffraction was recorded from 5 to 60° (2θ angles) with a 0.01° increment and 1 s/step as the scan speed. An FTIR spectrometer (PerkinElmer, Waltham, MA, USA, Spectrum GX) was employed to record the infrared absorption spectra of the sample, and the recorded spectra were used for the identification of the functional groups (i.e., CO₃²⁻, SO₄²⁻, and H₂O) that existed in the sample structure. Each sample was first mixed with spectroscopic-grade potassium bromide powder and then pressed to form a small sample pellet for infrared measurement [34]. Infrared spectra of the samples were recorded at 4000–400 cm⁻¹ (mid-infrared, MIR, region) using a resolution of 2 cm⁻¹ and 64 scans. An SEM (Zeiss, Oberkochen, Germany, LEO VP1450) was used to observe the sample morphology after the gold coating technique.

3. Results and Discussion

3.1. Elemental Composition

The elemental compositions of the raw CaCO₃ materials (hen and duck eggshell powders) and their reacted products (CaSO₄·nH₂O) were measured by the XRF technique, and the results are listed in

Table 1. Elemental composition of raw CaCO₃ materials (hen eggshell: HES; duck eggshell: DES) and CaSO₄·nH₂O products derived from HES and DES (three repetitions were conducted, and average values are reported).

Elemental Composition		Elemental Contents/wt%
		HES	DES	HES-CaSO4·nH2O	DES-CaSO4·nH2O
Calcium oxide	CaO	97.00	97.40	39.70	39.70
Sulfur trioxide	SO3	0.64	1.08	58.80	59.40
Sodium oxide	Na2O	0.21	0.17	0.10	0.07
Magnesium oxide	MgO	0.93	0.37	0.76	0.29
Aluminum oxide	Al2O3	0.13	0.04	0.06	0.02
Silicon dioxide	SiO2	0.36	0.13	0.17	0.05
Phosphorus pentoxide	P2O5	0.51	0.62	0.34	0.40
Potassium oxide	K2O	0.10	0.05	0.05	0.03
Ferric oxide	Fe2O3	0.07	0.034	0.03	0.03
Strontium oxide	SrO	0.06	0.01	0.02	0.03
Chlorine	Cl	0.02	0.02	−	−
Cupric oxide	CuO	−	0.01	−	−

. The major elemental component of both hen and duck eggshell powders is CaO, and we recorded contents of 97.0 and 97.4 wt%, respectively. These confirmed the chemical presentation of CaCO₃ [23]. However, other compounds (mainly in oxide forms, as shown in Table 1) were also observed with total contents of 3.01 and 2.53 wt% for the hen and duck CaCO₃ samples, respectively. These experimental values are in agreement with the results reported in the literature. Hossain et al. [23] used the wavelength-dispersive XRF technique to evaluate the elemental composition of eggshells, and the results showed that CaO was the major component (97.0%). They also observed MgO (~1.08%), P₂O₅ (~0.457%), and Na₂O (~0.17%) as minor compounds. CuO is not present in hen eggshells but is present in duck eggshells, which may be due to hens’ and ducks’ different diets (with different mineral contents) in animal husbandry.

The CaSO₄·nH₂O products synthesized from hen and duck eggshell powders were composed of two major elemental components: CaO (39.7 wt%) and sulfur trioxide (SO₃, 58.8 and 59.40 wt%). Other minor compounds in oxide forms were also observed, with total contents of 1.54 and 0.928 wt% for HES-CaSO₄·nH₂O and DES-CaSO₄·nH₂O, respectively. As listed in Table 1, the raw CaCO₃ materials (HES and DES) powdered from hen and duck eggshells were found to have an insignificant difference in their CaO (97.00 and 97.40 wt%) contents. These raw CaCO₃ materials with the same content of CaO (39.70 wt%), after being reacted with the SO₄²⁻ source (H₂SO₄), were observed in both the HES-CaSO₄·nH₂O and DES-CaSO₄·nH₂O products. Although both products showed the same content of CaO, a slight difference in the SO₃ content (0.6 wt%) was observed, which might have arisen from Ca²⁺ sources that initially contained different SO₃ contents (0.644 and 1.08 wt% for hen and duck eggshells, respectively). The lowest content of minor components (0.928 wt%) indicated the highest purity of the DES-CaSO₄·nH₂O product. Hence, it could be inferred that the preparation of CaSO₄·nH₂O by using duck eggshell waste as a renewable source provided the highest purity of the product. In addition, toxic metals, i.e., lead, arsenic, cadmium, and chromium, were not found in the hen and duck eggshell wastes (HES and DES), highlighting their green characteristics for use as eco-friendly and low-cost renewable sources for the production of value-added products like hydrated calcium sulfate (CaSO₄·nH₂O).

3.2. Reaction Times and Product Yields

The “reaction time” was considered the time taken to dissolve eggshell CaCO₃ powders in H₂SO₄ solution completely, then generate a calcium sulfate (Ca²⁺-SO₄²⁻) salt solution, as demonstrated in Equation (1).

Table 2. Reaction times and product yields of CaSO₄·nH₂O derived from hen or duck eggshell powders and 50% w/w H₂SO₄ (three repetitions were conducted, and average values are reported).

Samples	Reaction Time/s	Product Yields/%
Hen-eggshell-derived CaSO4·nH2O	62	84.73
Duck-eggshell-derived CaSO4·nH2O	55	87.74

illustrates the reaction time recorded during the generation of salt solution.

As described in Section 2.1, to achieve the complete chemical reaction, the molar ratio required between eggshell CaCO₃ powder and H₂SO₄ solution was 1:1, forming Ca²⁺-SO₄²⁻-H₂O solution. We observed that the temperature of the reaction increased, demonstrating that the dissolution process is an exothermic reaction. As can be seen in Table 2, there was an insignificant difference between the reaction times observed for the hen and duck eggshell systems; however, the results indicated that duck eggshell waste should be selected as the Ca²⁺ source used to rapidly prepare Ca²⁺-SO₄²⁻ solution in a large-scale system.

As shown in Equations (1) and (2), the yields (Y_p/%) of CaSO₄·nH₂O products prepared from eggshell CaCO₃ raw materials were computed by using Equation (3) [29]:

$$Y_P=\frac{w{t}_{ocs}}{w{t}_{tcs} }\times 100\%$$

(3)

where wt_ocs and wt_tcs (g) are the weights of CaSO₄·nH₂O observed and in theory, respectively.

Table 2 lists the yields of CaSO₄·nH₂O products synthesized from the two different eggshell wastes and 50% w/w H₂SO₄. The yields of the prepared CaSO₄·nH₂O products derived from hen and duck eggshells are 84.73 and 87.74%, respectively. The product yields reported in this work are higher than that reported in a previous work (<80%) [29]. These results highlight that to achieve the highest CaSO₄·nH₂O yield on a plant scale with a short reaction time, duck eggshell waste should be selected as the starting material for the CaSO₄·nH₂O preparation. The reaction times of the gypsum products synthesized from the two kinds of eggshell wastes in this work are shorter than those reported in previous research (24 h) [23,24].

3.3. Vibrational Spectroscopy

The vibrational characteristics of the functional groups of the samples were observed and identified using an FTIR spectrometer, and the spectra of CaCO₃ and CaSO₄·nH₂O in the MIR range (4000−400 cm⁻¹) are demonstrated in Figure 1 and Figure 2, respectively. The functional group associated with the eggshell CaCO₃ samples is the carbonate (CO₃²⁻) anion, whereas sulfate (SO₄²⁻) and water (H₂O) are the functional groups of the CaSO₄·nH₂O products. Therefore, the vibrational characteristics of the samples were characterized and mainly assigned based on the asymmetric stretching, symmetric stretching, and bending vibrational modes of CO₃²⁻, SO₄²⁻, and H₂O.

CaCO₃ can crystallize in three anhydrous polymorphs, e.g., calcite, aragonite, and vaterite, [35]; however, calcite is the most stable form under ambient conditions (room temperature and 1 atm) [36], while aragonite is slightly observed [37] and vaterite is formed under high specific conditions [38]. The FTIR spectroscopic technique was previously applied in the literature to quantify the vibrational modes of calcite and aragonite CaCO₃ polymorphs [37]. Therefore, the spectroscopic technique was selected in this work to identify the crystal structure (polymorph) of CaCO₃ milled from hen and duck eggshells, and the infrared spectra of CaCO₃ samples are demonstrated in Figure 1. The vibrational bands were considered based on the vibrational positions of trigonal planar CO₃²⁻, which consists of four absorption modes: asymmetric (υ₃) and symmetric (υ₁) stretching modes between C and O atoms, and out-of-plane (υ₂) and in-plane (υ₄) bending modes between C and two O atoms. Another two vibrational characteristic combinations—between υ₁ and υ₃, and between υ₁ and υ₄—were also previously reported [37].

Calcite and aragonite CaCO₃ polymorphs show slightly different absorption positions overall, but various absorption positions between calcite and aragonite overlap [36,37]. Previously, three absorption positions of 670, 1021, and 1094 cm⁻¹ of hen eggshell CaCO₃ powder or 668, 1019, and 1093 cm⁻¹ of duck eggshell CaCO₃ powder were assigned as the vibrational characteristics of an aragonite CaCO₃ polymorph [36,37], whereas, in this study, other absorption positions were assigned as the vibrational characteristics of calcite CaCO₃. This revealed that both eggshell waste powders mainly contained the calcite CaCO₃ polymorph, with a very small aragonite CaCO₃ amount. The vibrational characteristics of the milled CaCO₃ samples are listed in

Table 3. Vibrational characteristics and their absorption positions of the hen eggshell CaCO₃ (hen-CaCO₃) and duck eggshell CaCO₃ (duck-CaCO₃) powders.

Vibrational Characteristics	Positions/cm−1
	Hen-CaCO3	Duck-CaCO3
CO32−
Asymmetric C−O stretching (υ3)	1421, 1262	1420, 1259
Symmetric C−O stretching (υ1)	1094, 1021	1093, 1019
O−C−O out-of-plane bending (υ2)	876, 803	876, 800
O−C−O in-plane bending (υ4)	713, 670	710, 668
υ2 + υ3 + υ4 combination	2964, 2876	2962, 2873
υ1 + υ3 combination	2516	2513
υ1 + υ4 combination	1797	1796
H2O
Asymmetric (υ3) and symmetric (υ1) O−H stretching	3427	3291
H−O−H bending (υ2)	1654	1656
CO2
Asymmetric C-O stretching	2367, 2343	2361, 2342

.

Figure 2 shows the infrared spectra of the CaSO₄·nH₂O products derived from hen and duck eggshell CaCO₃ raw materials. The vibrational modes (asymmetric stretching, symmetric stretching, and bending) of SO₄²⁻ and H₂O were mainly considered and assigned.

Takahashi et al. [39] carried out factor group analysis of gypsum (CaSO₄·2H₂O) and reported that the symmetric stretching vibration (υ₁ (A₁, single mode, one component)) of the SO₄²⁻ ions belongs to the A_u symmetric species. The bending vibration (υ₂ (E, doubly degenerate mode, two components)) also belongs to A_u symmetry. Both the asymmetric stretching (υ₃ (F₂, triply degenerate mode, three components)) and bending (υ₄ (F2, triply degenerate mode, three components)) vibrations belong to two symmetric species: A_u (one component) and B_u (two components). This previously obtained information was used for the assignment of the vibrational characteristics of the synthesized CaSO₄·2H₂O. All observed vibrations of CaSO₄·nH₂O products derived from hen and duck eggshell CaCO₃ powders, as listed in

Table 4. Vibrational characteristics and their absorption positions for CaSO₄·nH₂O products derived from hen and duck eggshell CaCO₃ raw materials: hen-CaSO₄·nH₂O and duck-CaSO₄·nH₂O.

Vibrational Characteristics	Positions/cm−1
	Hen-CaSO4·nH2O	Duck-CaSO4·nH2O
SO42−
Asymmetric S−O stretching (υ3)	1153, 1131	1156, 1135
Symmetric S−O stretching (υ1)	1014	1013
υ1 + υ3 combination	2137	2139
O−S−O bending (υ4)	674, 612, 594	675, 610, 596
O−S−O bending (υ2)	510, 456	509, 451
H2O
Asymmetric (υ3) and symmetric (υ1) O−H stretching	3412	3407
H−O−H bending (υ2)	1642	1640
Vibrations (rocking, wagging, twisting)	885, 850, 510, 456	886, 851, 509, 451
CO2
Asymmetric C–O stretching	2360	2361, 2342

, were interpreted. Two vibrational positions at 510 and 456 cm⁻¹ were assigned as the υ₂ mode of SO₄²⁻ [39]. In addition, these two bands were also assigned as the vibrational modes (rocking, wagging, and twisting) of H₂O [40]. Three vibrational positions at 674, 612, and 594 cm⁻¹ were assigned as the υ₄ mode of SO₄²⁻. This observation is in agreement with the vibrational characteristics of CaSO₄·2H₂O reported by Kayabaş and Yildirim [41]. The peak at 674 cm⁻¹ is prominent, whereas the two peaks at 612 and 594 cm⁻¹ are weaker. This phenomenon is in agreement with the results reported by Takahashi et al. [39]. The intensities of these three peaks exhibit remarkable polarization dependence, indicating that the direction of the transition moment of the 674 cm⁻¹ peak is parallel, while the transition moments of the 612 and 594 cm⁻¹ peaks are perpendicular to the c-axis. Three modes—υ₁ (symmetric O−H stretching), υ₂ (H−O−H bending), and υ₃ (asymmetric O−H stretching)—of H₂O in CaSO₄·2H₂O crystals are in the A_u and B_u symmetry species. A peak at 3412 cm⁻¹ is assigned as both the asymmetric (υ₃) and symmetric (υ₁) O−H stretching modes (belonging to B_u symmetry) of H₂O, whereas a peak at 1642 cm⁻¹ is assigned as the H−O−H bending (υ₂) mode of H₂O. All these observed vibrational bands are in agreement with results reported by Salvadori et al. [42] and Hossain et al. [23], which confirm the presence of SO₄²⁻ and H₂O. Other vibrational modes and their positions of SO₄²⁻ and H₂O, as well as CO₂ (adsorbed gas on the surface of the sample), are listed in Table 4.

3.4. Crystallographic Characteristics

As described in the vibrational spectroscopic results, calcite and aragonite polymorphic crystals of CaCO₃ were observed. Therefore, another powerful technique, XRD, was used for the characterization of different CaCO₃ polymorphs [36,37]. The diffraction patterns (relationships between the XRD intensity and 2θ angle) of hen and duck eggshell waste powders are demonstrated in Figure 3a.

Similar diffraction patterns between hen and duck eggshell waste powders were observed (Figure 3a), indicating that both materials (hen and duck eggshell powders) had the same crystallographic characteristics. All diffraction peaks were consistent with the standard diffraction data, PDF-ICDD #47-1743 [43], revealing that only the calcite CaCO₃ polymorph was observed, which is the natural polymorph of CaCO₃ present in the eggshell [44]. No diffraction peak of the aragonite CaCO₃ polymorph was observed. This result might have been since there was a very small amount of aragonite CaCO₃, which was not observed in the XRD technique [44]. Smooth diffraction baselines with high diffraction intensity peaks, as exhibited in Figure 3a, indicated the high crystallinity of CaCO₃ powders obtained from both hen and duck eggshells. Figure 3b shows the crystal structure of CaCO₃ in the calcite polymorph [15]. The lattice parameters a and c of calcite CaCO₃ are 4.9896 and 17.0610 Å, with a cell volume of 367.8537 ų [37]. The crystallite sizes (S_c) of the samples were calculated by using the main diffraction (hkl lattice plane: 104; 2θ angles: 29.40° and 29.41° for hen and duck eggshell CaCO₃ samples) through the Scherrer equation (Equation (4)) [45].

$$S_c=\frac{0.89\lambda }{\beta cos\theta }$$

(4)

where λ is the X-ray wavelength (1.5406 Å), β is the full width at half-maximum (FWHM) value of the selected XRD diffraction peak, and θ is the angle of the selected diffraction peak.

The calculated crystallite sizes for the (104) calcite-lattice plane are listed in

Table 5. Crystallite sizes (S_c) of the prepared samples calculated from lattice planes (hkl), full width at half-maximum (FWHM, β), and diffraction peaks (2θ, θ).

Samples	hkl Planes	β/°	2θ/°	θ/°	Sc/nm
Hen-CaCO3	(104)	0.13763	29.40	14.700	18.67
Duck-CaCO3		0.12451	29.41	14.705	20.48
Hen-CaO	(220)	0.18188	37.66	18.830	7.54
Duck-CaO		0.17062	37.62	18.810	8.04
Hen-CaSO4·2H2O	(020)	0.07774	11.67	5.835	19.57
Duck-CaSO4·2H2O		0.07863	11.68	5.840	19.30
Hen-CaSO4	(200)	0.11701	25.42	12.710	11.84
Duck-CaSO4		0.12425	25.47	12.735	11.19

for hen and duck eggshell waste CaCO₃ powders, respectively.

Other crystallographic data (observed 2θ positions, diffraction intensities, the standard d-spacing values, and hkl lattice planes [46]) of hen and duck eggshell CaCO₃ powders are listed in

Table 6. Crystallographic data of hen and duck eggshell CaCO₃ powders.

2θ/°		Diffraction Intensity/a.u.		d-Spacing/Å	hkl Planes/-
Hen-CaCO3	Duck-CaCO3	Hen-CaCO3	Duck-CaCO3
23.40	23.08	8587	9654	3.8556	(012)
29.40	29.41	143,135	147,710	3.0364	(104)
31.49	31.43	2717	3090	2.8448	(006)
35.97	35.99	10,136	14,082	2.4951	(110)
39.42	39.42	16,789	19,140	2.2851	(113)
43.17	43.17	13,453	20,007	2.0948	(202)
47.13	47.12	3795	5420	1.9278	(024)
47.52	47.49	12,098	16,631	1.9131	(018)
48.53	48.50	16,047	15,125	1.8758	(116)
56.58	56.59	1801	4286	1.6260	(211)
57.44	57.40	5223	6935	1.6043	(122)

. These obtained data—specifically the higher intensities of all diffraction peaks, except for the (116) lattice plane—indicated that good crystallinity was obtained for the duck eggshell CaCO₃ sample.

The diffraction patterns of the products obtained from the calcination process of raw hen and duck eggshell waste powders, as demonstrated in Figure 4a, showed three crystalline phases, consisting of CaO (lime), calcium hydroxide (Ca(OH)₂, portlandite), and calcite CaCO₃. Diffraction peaks at 32.47°, 37.62°, and 54.10° (Figure 4a) were assigned to the (111), (200), and (220) lattice planes of CaO based on PDF-ICDD #74-1226 [47], and Figure 4b shows the crystal structure of CaO [48]. Conventionally, a small fraction of CaO material can adsorb atmospheric H₂O, resulting in the formation of Ca(OH)₂ crystals due to the exposure of CaO to the air. The presence of Ca(OH)₂ was confirmed by PDF-ICDD #01-1079 with 2θ results of 18.26, 34.23, and 47.37° for the (001), (101), and (102) lattice planes of Ca(OH)₂ [49]. A diffraction peak at 29.66° (for both hen-and duck CaCO₃) was observed, which is the highest diffraction intensity of calcite CaCO₃, corresponding to the (104) lattice plane [37]. The crystallite sizes of CaO samples calculated from the Scherrer equation are listed in Table 5.

The experimental diffraction pattern and the crystal structure [50] of the synthesized CaSO₄·nH₂O product are demonstrated in Figure 5a,b, respectively. All diffraction peaks (Figure 5a) observed for the synthesized products were consistent with the standard diffraction of PDF-ICDD #33-0311, demonstrating the diffraction characteristics of gypsum (CaSO₄·2H₂O) [23,51].

Given the similarity of the diffraction patterns of the hen and duck CaSO₄·2H₂O products, we surmised that both prepared products had the same structural and chemical characteristics, which were those of gypsum (CaSO₄·2H₂O). The characteristic peaks of other phases were not observed in the diffraction patterns, indicating that CaSO₄·2H₂O might be formed as a single phase or that other phases with small amounts were not observed by the XRD technique. The observed 2θ positions, diffraction intensities, standard d-spacing values, and hkl lattice planes of hen- and duck-eggshell-derived CaSO₄·2H₂O samples are listed in

Table 7. Crystallographic data of CaSO₄·2H₂O samples derived from hen and eggshell CaCO₃ raw materials.

2θ/°		Diffraction Intensity/a.u.		d-Spacing/Å	hkl Planes/-
Hen-CaSO4·2H2O	Duck-CaSO4·2H2O	Hen-CaSO4·2H2O	Duck-CaSO4·2H2O
11.67	11.68	2699	1051	7.6000	(020)
20.79	20.78	1030	394	4.2811	(021)
23.41	23.43	360	129	3.8000	(040)
29.14	29.17	804	316	3.0642	(041)
31.15	31.17	390	139	2.8730	(−221)
32.13	32.19	84	32	2.7875	(−112)
33.41	33.45	279	114	2.6828	(220)
34.57	34.64	66	30	2.5964	(−151)
36.02	36.11	97	33	2.4957	(−202)
36.67	36.60	55	34	2.4521	(022)
40.69	40.69	121	59	2.2174	(151)
43.39	43.64	136	62	2.0860	(−152)
45.54	45.56	51	29	1.9912	(−171)
47.94	47.96	132	53	1.8985	(080)
50.40	50.35	94	47	1.8113	(062)
55.24	55.20	47	24	1.6643	(−262)
56.80	56.80	85	48	1.6207	(190)

.

The crystallite sizes calculated by using Scherrer’s equation for the hen and duck CaSO₄·2H₂O samples are listed in Table 5. Pinto et al. [52] synthesized pure CaSO₄·2H₂O by using CaCl₂ and Na₂SO₄ as the Ca²⁺ and SO₄²⁻ sources, respectively. The lattice parameters of CaSO₄·2H₂O were then calculated and found to comprise a, b, and c values of 5.675, 15.214, and 6.284 Å, respectively. CaSO₄·2H₂O crystallizes in a monoclinic crystal system with the space group C2/c (#15) [23]. The relative intensity (Ri), an important parameter, was also calculated in this work from the experimental XRD data. The Ri for the highest diffraction intensity ((020) plane) was calculated from the other three strong lattice planes—(021), (040), and (041)—using Equation (5) [23]:

$$R_i=\frac{In{t}_{020}}{In{t}_{021}+In{t}_{040}+In{t}_{041}}$$

(5)

where Int₍₀₂₀₎, Int₍₀₂₁₎, Int₍₀₄₀₎, and Int₍₀₄₁₎ are the intensities of the diffraction peaks of the (020), (021), (040), and (041) lattice planes, respectively.

The Ri values of the hen and duck CaSO₄·2H₂O samples were 1.23 and 1.25, respectively. In order to compare the Ri values, the Ri of the diffraction database (PDF-ICDD #33-0311) was also calculated for the same lattice planes and found to be 0.52. The preferential growth (Pg) parameter of the (020) plane was then calculated using Equation (6) [23]:

$$P_g=\frac{{Ri}_{ss}-{Ri}_{sd}}{{Ri}_{sd}}$$

(6)

where Ri_ss and Ri_sd are the relative intensities of the synthesized sample (CaSO₄·2H₂O) and the standard data (PDF-ICDD #33-0311, Ri = 0.52), respectively.

The Pg values for the synthesized hen and duck CaSO₄·2H₂O samples were calculated and found to be 4.54 and 4.62, respectively. The positive values we obtained suggested that the growth along the (020) lattice plane was stable and favorable, indicating good crystal stability [23].

Figure 6a demonstrates the diffraction patterns of products obtained from the calcination step (900 °C, 2 h) of CaSO₄·2H₂O. All diffraction peaks show the structural characteristics of the anhydrite (CaSO₄) phase. As described in the Section 1, two polymorphs of CaSO₄ were reported, β-CaSO₄ and γ-CaSO₄, which are the stable and metastable forms, respectively [16]. The hydration rate of the γ polymorph (CaSO₄(s) + nH₂O(g) → CaSO₄·nH₂O(s)) is greater than that of the β polymorph; consequently, if the γ form comes in contact with atmospheric water, its rehydration process will occur, preventing the formation of the γ form [53].

All diffraction peaks of both CaSO₄ samples, obtained from hen- and duck-eggshell-derived CaSO₄·2H₂O, were consistent with PDF-ICDD #72-0916. This standard diffraction confirmed that the product of the calcination process of CaSO₄·2H₂O is the orthorhombic CaSO₄ phase with β polymorphic crystals. The crystal structure of β-CaSO₄ [54] is shown in Figure 6b. The crystallite sizes of β-CaSO₄ samples calculated from the Scherrer equation are listed in Table 5.

3.5. Morphological Characteristics

The morphological characteristics of hen and duck eggshell CaCO₃ raw materials and their reaction products (CaSO₄·2H₂O) were imaged with a magnification of 15 kx, and the resulting micrographs of the samples are presented in Figure 7. The micrographs of the CaCO₃ particles obtained from the milling process of raw hen and duck eggshell powders present an agglomeration of CaCO₃ particles, causing different particle sizes. These different particle sizes were obtained since milling is a mechanical process, commonly employed for material size reduction through centrifugal forces [55]. During the milling process, some fractions of eggshells were milled completely, resulting in small powdered particles, whereas other fractions were not milled completely, resulting in large powdered particles. In addition, during the rotation process, small particles were randomly moved and located on the surface of large particles.

The micrographs of the CaSO₄·2H₂O samples synthesized from the reaction between hen or duck eggshell CaCO₃ raw material and H₂SO₄, as demonstrated in Figure 7, show plate-like particles. This finding is not consistent with the results reported in the literature. Mbogoro et al. [56] used commercial CaCl₂·2H₂O and Na₂SO₄ as Ca²⁺ and SO₄²⁻ sources, respectively, to form CaSO₄·2H₂O crystals. The same concentration of Ca²⁺ and SO₄²⁻ salts of 0.56 mol/L was prepared as stock solutions and then those were employed to prepare growth solutions with five different Ca²⁺:SO₄²⁻ free-ion ratios of 2.5:18.8, 3.4:13.7, 6.9:6.8, 13.6:3.4, and 17.8:2.5, corresponding to the ionic ratios (r) of 0.13, 0.25, 1.01, 4.00, and 7.12, respectively. At low ionic ratios (r < 1, SO₄²⁻-rich solutions), plate-like CaSO₄·2H₂O crystals were observed, whereas needle-like crystals were observed at an equal stoichiometry (r = 1) and at high ionic ratios (r < 1, Ca²⁺-rich solutions) [56]. Moreover, Madeja et al. [57] also used commercial CaCl₂·2H₂O and Na₂SO₄ solutions to prepare CaSO₄·2H₂O using titration precipitation and then investigated the influences of various additives (small molecules, homo- and co-polymers) on the morphological characteristics of CaSO₄·2H₂O. Among the additives, a co-polymer of vinylpyrrolidone (PVP) and acrylic acid (PAA) acted as an important modifier for gypsum development. CaSO₄·2H₂O particles with tilted stacking edges and pseudo-hexagonal plates were obtained [57]. In this work, irregular plate-like CaSO₄·2H₂O particles were observed by using an ionic ratio between Ca²⁺ (hen or duck eggshell CaCO₃) and SO₄²⁻ (H₂SO₄) of 1 (as described in “Materials and their Synthesis”, Equations (1) and (2)) without an additive addition step. These results demonstrate that different formation conditions cause different morphological characteristics [56,57].

4. Conclusions

Gypsum (CaSO₄·2H₂O) products were synthesized successfully using two kinds of eggshells (hen and duck) with a short time period and easy process, generating low-cost products. The experimental and standard diffraction patterns confirmed that the value-added product from the eggshell waste powders is CaSO₄·2H₂O (gypsum). When CaSO₄·2H₂O is thermally treated, it can completely dehydrate to form CaSO₄ (anhydrite, β polymorph). The diffraction patterns also indicated that the sample phase will change from a monoclinic (CaSO₄·2H₂O) to an orthorhombic (CaSO₄) crystal structure. The morphological characteristics of CaCO₃ show an irregular shape with different agglomeration particle sizes, whereas plate-like particles were observed for CaSO₄·2H₂O. The crystallite sizes calculated from the diffraction peak with the highest intensity for CaCO₃, CaO, CaSO₄·2H₂O, and CaSO₄ were 18−21, 7−8, 19−20, and 11−12 nm, respectively.

This article has reported an easy and low-cost method with no environmental effect, which may be applied to produce cheap CaSO₄·2H₂O products to be used in the agroindustry, forming part of zero-waste sustainability efforts.