The Effect of the Addition of Eggshell Residues in Mass Formulation for Ceramic Coating

: In this study, we developed formulations of a ceramic coating from clay, kaolin, quartz, talc and feldspar as a standard formulation with the addition of eggshell residue to improve the mechanical characteristics of the product. The addition of eggshell residue is justiﬁed as it will contribute to ﬁlling the formulation’s interstices. It would also help decrease the sintering temperature due to the high presence of calcium oxide in its composition. Samples with the ceramic coating (45% by weight of feldspar; 30% by weight of clay; 15% by weight of kaolin; 7% by weight of quartz; 3% by weight of talc; and additions of 5%, 10% and 20% by weight of eggshell residue) were pressed uniaxially at 70 MPa for 30 s; dried at 110 ◦ C for 24 h; and sintered at 1000 ◦ C, 1100 ◦ C and 1200 ◦ C. The main mineralogical phases (microcline, mullite, quartz and anorthite) of the sintered samples were identiﬁed by X-ray diffraction (XRD). After evaluating the physical-mechanical properties (water absorption, linear shrinkage, apparent porosity and resistance to ﬂexion), it was observed that the incorporation of eggshell residue (5%, 10% and 20%) resulted in a signiﬁcant loss of the desired physical and mechanical properties. A loss of over 50% of mechanical strength was obtained.


Introduction
The search for alternative natural resources for the civil construction sector is a growing challenge with the increasing demand for construction materials [1,2]. A sustainable approach is needed to face the challenge of the scarcity of these materials in the coming years, which will be aggravated by the excessive use of non-renewable resources [3][4][5]. Some alternatives to exploiting non-renewable natural resources include reusing building materials, replacing building materials with low environmental impact materials, and adopting eco-friendly materials such as recycled materials or waste [5][6][7]. In addition, adopting more efficient construction techniques, such as modular construction, can also benefit the sector as it can significantly reduce the waste of building materials [7,8]. Therefore, a holistic approach is needed to ensure that the construction sector becomes more sustainable and efficient.
A recent study published in 2022 entitled "Development of eco-ceramic wall tiles with bio-CaCO 3 from eggshells waste" investigated the feasibility of using eggshell waste as a source of calcium for the preparation of ceramic glaze [8]. The study found that eggshell waste could be used as an alternative source of calcium, which could pave the way for further investigation into using eggshells in ceramic tiles.
The ceramics industry is looking for alternative materials to produce ceramic tiles, and adding eggshell waste can be a potential solution for sustainability, cost reduction and waste reduction [9][10][11]. In addition, the high concentration of calcium may contribute to the reduction in the sintering temperature of the ceramic coating, thus making the process less costly for the industry [10][11][12].
There are studies on using eggshell waste in other construction materials, such as concrete and mortar [11][12][13]. However, further exploration of the potential for eggshell residue in ceramic tiles may help to understand its viability and impact on ceramic tile mass properties, which may contribute to increasing the sustainability efforts of the ceramics industry and reducing waste [12,13].
The search by research centres for reducing waste and increasing the sustainability of the ceramic industry indicated that the eggshell residue contains calcium carbonate, which can potentially improve the durability and hardness of the ceramic coating mass, leading to reduced water absorption and greater mechanical strength [14][15][16]. More research may be needed to fully understand the physical implications of adding eggshell waste to the ceramic tile mass.
One of the viable alternatives for mass formulations for ceramic coating is the incorporation of eggshell residue because it provides a possible source of raw materials at a low cost and with low environmental impact [15,16]. The eggshell residue has an enormous concentration of calcium carbonate, which is an essential component in the manufacture of ceramics; however, the excessive use of calcium compounds in a ceramic tile putty can increase the alkalinity of the mix, which can negatively affect the physical and chemical characteristics of the putty, leading to problems such as cracking or damage to the total surface [16,17]. Furthermore, eggshell waste is biodegradable, which means it poses no risk to the environment [18]. The residue can also be used as a binding agent in the manufacture of ceramics, helping to improve mechanical strength and durability [19].
The objective of using eggshell residue in a ceramic coating mass would be to evaluate the possibility and effectiveness of using this material as an environmental treatment agent and as a source to improve the physical properties. The proposed study would determine the resulting material's physical, chemical and mechanical properties and compare the results with other materials used for the same purpose. It is expected that the results obtained can contribute to the development of new, more sustainable and effective methods of environmental treatment, as well as to the improvement of materials for ceramic coating.

Raw Material
The clay, kaolin, feldspar, and quartz were purchased from Risi Ceramics Products in Cunha, State of São Paulo, Brazil. Talc was collected from mineral deposits in Dirceu Arcoverde in Piauí, Brazil. The eggshell residue was collected in bakeries in Teresina in Piauí.
For the experimental elaboration of this work, the raw materials presented in Table 1 were used.
All materials were dried to hygroscopic humidity, ground in a ball mill for 48 h and passed through a No. 44 µm sieve. The raw materials were characterised in crystalline phases by X-ray diffraction (XRD) via particle size distribution and chemical analysis using the X-ray fluorescence (XRF) technique.
The chemical composition of the raw materials was measured using X-ray fluorescence (Shimadzu, EDX 720, Kyoto, Japan). The mineralogical phases of the raw materials and sintered samples were identified by X-ray diffraction (Shimadzu XRD 6000, Kyoto, Japan), with X-ray wavelength (Cu = 40 Kv/30 mA) in the range of 2θ of 10 • to 70 • and with the JCPDS database. Particle size distribution was determined by laser diffraction (Cilas, 1064LD, Orleans, France). The thermal behaviours were evaluated by thermogravimetry (TG)/derived thermogravimetry (DTG) (DTG Shimadzu, TA 60H, Kyoto, Japan), with a heating rate of 5 • C/min, under a nitrogen gas atmosphere as standard.

Sample Preparation
The compositions were formulated based on a standard composition (45 wt % feldspar, 30 wt % clay, 15 wt % kaolin, 7 wt % quartz and 3 wt % talc), in which essentially 5 wt %, 10 wt % or 20 wt % of eggshell waste was added to improve the technical characteristics of the final product. The nominal compositions of the ceramic formulations (wt %) and their respective nomenclatures are summarised in Tables 2 and 3. The default composition was based on the literature [19][20][21]. Table 2. Nomenclature and nominal composition (% by weight) of the porcelain mass with the addition of eggshell waste studied in this research.

Raw Material Composition (wt %)
Feldspar 45  Clay  30  Kaolin  15  Quartz  7  Talc  3   Table 3. Nomenclature and nominal composition (wt %) of the standard formulation with the addition of eggshell waste studied in this research.

Raw Material Composition (wt %)
Standard Formulation 95 90 80 Eggshell waste 5 10 20 The selected raw materials were calculated to provide 5.0 kg of standard composition material. The raw materials were homogenised wet in a ball mill (SOLAB ball mill) in a 1:1 ratio (one portion of water to one portion of composition) for 24 h. The samples were formed in a metallic prismatic matrix (10 mm × 10 mm × 80 mm). The samples were obtained from 18 to 20 g of powder in a hydraulic press at a pressure of 750 kg/cm 2 , with a force of around 6.0 tons applied for 30 s. Ten samples of each formulation were produced for each temperature. Quality control was performed by selecting the six best samples, considering the absence of small cracks, size and conformation quality. After pressing and drying, the green densities for samples of all compositions ranged from 1.80 to 2.00 g/cm 3 . Notably, these values are below those usually used for forming porcelain tile compositions (around 2.0 to 2.1 g/cm 3 ) [19]. Finally, the pressed samples were dried at 110 • C for 24 h and sintered in a conventional electric furnace.

Characterisation of Samples after Sintering Treatment
The sintering protocol consisted of a heating rate equal to 5 • C/min; 30 min at the final temperatures of 1000 • C, 1100 • C and 1200 • C; and natural cooling to room temperature.
The powders of the burnt compositions were characterised regarding the crystalline phases by X-ray diffraction (DRX); this method determines the crystalline structure of the sintered compositions. This analysis is used to determine the formation of crystalline phases and provide information about the crystalline structure of the samples. It is a fast and easy-to-use method that provides detailed information about the crystalline structure of sintered materials. Scanning electron microscopy was also used to verify the distribution of totally open and closed porosity, the normal and fracture surface, and the material's morphology. Scanning electron microscopy is then used to detect and map the porosity distribution in the material. The obtained data makes it possible to determine the total porosity, pore dimensions and other related properties. This technique can also be used to measure material permeability.
For the execution of this experimental method, a small portion of the test specimen was used for each of the following compositions: standard; standard with the addition of 5%, 10% and 20% burnt eggshell residue at 1200 • C. Subsequently, each part was subjected to fracture to allow for the analysis of the internal microstructure using the Shimadzu SSX-550 scanning electron microscope, without any prior grinding or polishing treatment. This experimental method was conducted with the aim of elucidating the microstructural characteristics associated with the incorporation of eggshell residue in ceramic test specimens.
The fired samples were characterised using the Archimedes method to measure water absorption and apparent porosity. Flexural strength measurement was performed using the three-point flexion test. The procedures for carrying out these tests are all supported by standards [22][23][24][25][26][27][28].

Linear Retraction (RL)
Linear shrinkage considers the variation in the linear dimension of the ceramic body, in percentage, after the sintering step. A variation with a positive value characterises a retraction of the dimension initially considered. Otherwise, it is considered that the ceramic body underwent expansion. The procedure adopted was the measurement with a calliper of the lengths of the specimens before and after sintering. The equation below was used to calculate the linear shrinkage values after firing: RL is the linear shrinkage value, in percentage, of the specimen after sintering; L0 is the length value of the sample before sintering; and Lf is the specimen's value after the sintering process.

Water Absorption (AA)
Water absorption is the percentage value of the mass of water absorbed by the body after sintering. The water absorption test was carried out as follows: the specimens were weighed immediately after leaving the oven on an analytical balance. Consecutively, they were submerged in distilled water for 24 h in a glass container. After this time, they were removed from the container, and excess surface water was removed with a damp cloth and immediately weighed to verify the variation of their new masses. The water absorption value, in mass percentage, was obtained using the equation below.
where AA is the water absorption in percent; Mu is the mass of the specimen saturated in water; Mq is the mass of the dry sample. After calculating the water absorption of each sample, the arithmetic mean of the values obtained for each group was calculated.

Apparent Porosity (AP)
The calculation of the apparent porosity provides the probable percentage of the volume of open pores, after sintering, of the specimens concerning their total volume. Obtaining this value was calculated as follows: after weighing the specimens to calculate the water absorption, the mass of the immersed samples was also measured using the hydrostatic balance method. With the three values, Mu, Mq and immersed body mass Mi, the equation below was used to obtain the percentage value of the apparent porosity: PA is the calculated value of the apparent porosity, and Mi is the mass of the specimen immersed in water.

Three-Point Bending Failure Stress (TRF)
Flexural rupture stress refers to the material's resistance to simple flexion by the threepoint method, according to the method proposed by VICAT. To measure this property, a universal testing machine, model AG-I 250 KN, from Shimadzu, was used, operating at a speed of 0.5 mm/min, according to the method proposed by the standard (American Society for Testing and Materials) (ASTM). Tests were performed on six specimens for each >formulation to obtain the results, and the final value was given by the arithmetic mean of these values. The calculations were performed automatically by the software Trapezium 1.14 from Shimadzu. The equation below provides the results: where TRF is the breakdown tensile (kgf/cm 2 ); P is the load reached at the moment of failure (kgf); L is the distance between the supports of the specimen; b is the width of the sample; and h is the height of the sample. Figure 1a shows that the clay used contained phases of quartz (SiO 2 ) (JCPDS: 46-1045) and kaolinite Al 2 (Si 2 O 5 )(OH) 4 (JCPDS: 78-2110), which provided malleability to the mass and mechanical strength of the final product [29][30][31].

Characterisation of Raw Materials
Image 1b shows kaolin, which resulted in sintered parts with a lighter shade. Additionally, kaolinite is a fundamental carrier of aluminium oxide (Al 2 O 3 ) (JCPDS: 10-0173) [29]. In the vitrification phase, the ceramic mass regulates the balance of reactions, culminating in the formation of mullite, which acts as a "bone structure", increasing resistance [30]. The kaolin used as raw material in the sintered samples consisted of the kaolinitic phase Al 2 (Si 2 O 5 )(OH) 4 (JCPDS: 78-2110).
Image 1c shows the mineral feldspar, which can play a crucial role in the composition of porcelain tile ceramic masses. This mineral provides ceramics with high gresification, high mechanical strength and low porosity after firing [31,32]. The composition of the feldspar used in the samples consisted of the following phases: potassium feldspar-(K, Na)AlSi 3 O 8 (JCPDS 10-0357) and sodium feldspar-Na(AlSi 3 O 8 ) (JCPDS 10-0357). Figure 1a shows that the clay used contained phases of quartz (SiO2) (JCPDS: 46-1 and kaolinite Al2(Si2O5)(OH)4 (JCPDS: 78-2110), which provided malleability to the and mechanical strength of the final product [29][30][31]. Image 1b shows kaolin, which resulted in sintered parts with a lighter shade. A tionally, kaolinite is a fundamental carrier of aluminium oxide (Al2O3) (JCPDS: 10-0 [29]. In the vitrification phase, the ceramic mass regulates the balance of reactions, cu nating in the formation of mullite, which acts as a "bone structure", increasing resist  Figure 1d shows quartz that can maintain a siliceous "skeleton" in the mass when the other components soften due to increased temperature [30]. Furthermore, quartz, an essential regulator between silica (SiO 2 ) (JCPDS: 46-1045) and alumina (Al 2 O 3 ) (JCPDS: 10-0173), contributes to the formation of mullite (3Al 2 O 3 .2SiO 2 ) (JCPDS: 79-1276), a phase that increases the mechanical resistance of the product [32,33]. The quartz used is a (SiO 2 ) phase (JCPDS: 46-1045), with no evidence of impurities and/or clay minerals within the detection limits of X-ray analysis.

Characterisation of Raw Materials
The talc in Figure 1e is considered flow energy, and its presence increases the fusibility forming a eutectic with the feldspar, which produces a large amount of the liquid phase with low viscosity, helping in the densification and the reduction in the porosity [21]. The talc used was constituted by the following phases: dolomite-CaMg(CO 3 ) 2 (JCPDS 05-0622) and talc-(Mg 3 Si 4 O 10 (OH) 2 (JCPDS 29-1493). Figure 1f shows the eggshell residue used as an alternative additive in the composition of porcelain tiles. The XRF technique showed that the raw materials have a high degree of purity, as shown in Table 4. A possible benefit of adding eggshell residue to a ceramic coating mass is its potential contribution to increasing mechanical resistance and the final product's durability and thermal insulation capacity. In addition, the eggshell residue is an abundant source of calcium and other elements, which can reduce the production costs of ceramic materials. However, it is essential to remember that incorporating the residue in the dough can change the physical and chemical properties of the product.  Table 4 summarizes the chemical compositions of the raw materials (clay, kaolin, quartz, talc, feldspar and eggshell residue). The main oxides identified in clay and kaolin were SiO 2 (49.50% and 50.44%) and Al 2 O 3 (46.44% and 46.15%), respectively. These oxides usually originate from the structure of clay minerals and free silica [33]. Furthermore, a high K 2 O content (3.03%) was detected in kaolin. The presence of K 2 O is significant because it is a known flux and decreases the sintering temperature, providing economic benefits for the industry [33,34]. Quartz contained SiO 2 (99.88%) as the main constituent, thus presenting excellent purity, which indicates its potential as a fluidity agent and its role in the formation of "bones" in the finished ceramic coating [32,33]. In addition, quartz helps reduce the ceramic mass maturation temperature, reducing energy consumption [33]. The high contents of CaO (22.06%) and MgO (37.13%) identified in talc are associated with the presence of dolomite; both CaO and MgO are essential to reduce refractoriness [34]. However, a high fire loss can be attributed to the thermal decomposition of calcium carbonate and the release of gases [34]. The more significant amount of K 2 O (8.41%) and Na 2 O (5.80%) present in the feldspar contributes to the formation of the glassy phase during sintering, increasing the densification and ceramic resistance of the ceramic body and decreasing the porosity after sintering. Adding eggshell residue can reduce the refractoriness of the ceramic piece due to the high concentration of CaO (98.36%).
Figure 2a-f show the granulometry of each raw material used in the experiment. Its average values, D 50 , are around 10 to 20 µm. The exception is talc particles with a D 50 of approximately 40 µm, which is above the recommended value for producing porcelain tiles-around 20 µm [21]. However, the amount of talc in the dough is only 3% by weight, so talc should not significantly change the final composition after homogenization. Figure 2a-f show, respectively, that the clay particles had an average size of 11.36 (µm); the kaolin particles were 21.28 (µm), the feldspar particles were 11.52 (µm), the quartz particles were 13.89 (µm), the talcum particles were 39.66 (µm); and the eggshell waste particles were 14.20 (µm). Coarser particles can interfere with the kinetics of sintering reactions, drastically influencing the sintering step of the product [35,36]. Smaller particle sizes provide more significant surface areas and reactivity between particles, favouring kinetic reactions and the diffusion process during phase transformations [37].
2a-f show, respectively, that the clay particles had an average size of 11.36 (µm); the kaolin particles were 21.28 (µm), the feldspar particles were 11.52 (µm), the quartz particles were 13.89 (µm), the talcum particles were 39.66 (µm); and the eggshell waste particles were 14.20 (µm). Coarser particles can interfere with the kinetics of sintering reactions, drastically influencing the sintering step of the product [35,36]. Smaller particle sizes provide more significant surface areas and reactivity between particles, favouring kinetic reactions and the diffusion process during phase transformations [37]. Figure 3a-f provides TG/DTG data for clay, kaolin, feldspar, quartz, talc and eggshell residue. The TG/DTG curve of Figure 3a obtained from the clay illustrates that the mass loss occurred in two stages. In the first stage, the mass loss was equal to 1.54%, which was observed in the temperature range from 22 to 200 °C. The origin of the first thermal event was associated with the loss of free and adsorbed water. A mass loss of 12.87% was observed during the second step (300-800 °C). This second phase can be attributed to losing clay minerals' organic matter and hydroxyl groups. The small peaks observed around 800 to 1200 °C were due to metakaolinite transformation into a spinel-like structure [37][38][39]. In Figure 3b, kaolin showed a significant mass loss (15.01%) between 400 and 800 °C, which was probably associated with the dehydroxylation of kaolinite and mica, transforming it into metakaolinite. The total mass losses for clay and kaolin were 15.85% and 16.20%, respectively. Above 900 °C, mullite nucleation occurs with the release of β -quartz from the previously created amorphous structure [37]. In Figure 3c, feldspar showed a minimum mass loss of 2.24%. Figure 3a-f provides TG/DTG data for clay, kaolin, feldspar, quartz, talc and eggshell residue. The TG/DTG curve of Figure 3a obtained from the clay illustrates that the mass loss occurred in two stages. In the first stage, the mass loss was equal to 1.54%, which was observed in the temperature range from 22 to 200 • C. The origin of the first thermal event was associated with the loss of free and adsorbed water. A mass loss of 12.87% was observed during the second step (300-800 • C). This second phase can be attributed to losing clay minerals' organic matter and hydroxyl groups. The small peaks observed around 800 to 1200 • C were due to metakaolinite transformation into a spinel-like structure [37][38][39]. In Figure 3b, kaolin showed a significant mass loss (15.01%) between 400 and 800 • C, which was probably associated with the dehydroxylation of kaolinite and mica, transforming it into metakaolinite. The total mass losses for clay and kaolin were 15.85% and 16.20%, respectively. Above 900 • C, mullite nucleation occurs with the release of β -quartz from the previously created amorphous structure [37]. In Figure 3c, feldspar showed a minimum mass loss of 2.24%.
Furthermore, carbonates, sulfates and organic matter were not found. The critical event between 1160 and 1200 • C was probably due to the emergence of the liquid phase of potassium feldspar occurring due to the phase change from microcline to leucite [38][39][40]. Figure 3d confirms the presence of some characteristic quartz peaks. The first event characterises the release of free water between 30 and 100 • C with a mass loss of 0.49%. In the range between 400 and 600 • C, there was a mass loss of 0.92%, related to the allotropic transformation of α-quartz to β-quartz. Between 1000 and 1200 • C, a mass loss of 0.14% was associated with transforming kaolinite into metakaolinite, thus crystallising quartz [38,39]. The total mass loss shown in the test was approximately 2.23%. The talc TG/DTG curve in Figure 3e had a total mass loss of 34.37%. That was mainly related to eliminating CO 2 from decarbonising dolomite, which is present in large amounts in the mineral. The mass loss of 27.55% between 600 and 900 • C refers to the release of CO 2 from dolomite and the decarbonation of talc as dolomite mainly comprises calcium and magnesium carbonates. Figure 3f observes that the eggshell residue had an important total mass loss of 49.25%. From 900 • C, there was stabilization in mass loss, and between 200 and 400 • C, a mass loss of 4.58% is observed. Said mass loss is attributed to the thermal decomposition of the organic matter present in the sample, which, in turn, results in the release of gases, such as carbon dioxide (CO 2 ) and water vapor (H 2 O). Minerals 2023, 13, x FOR PEER REVIEW 9 of 15 Furthermore, carbonates, sulfates and organic matter were not found. The critical event between 1160 and 1200 °C was probably due to the emergence of the liquid phase of potassium feldspar occurring due to the phase change from microcline to leucite [38][39][40]. Figure 3d confirms the presence of some characteristic quartz peaks. The first event characterises the release of free water between 30 and 100 °C with a mass loss of 0.49%. In the range between 400 and 600 °C, there was a mass loss of 0.92%, related to the allotropic transformation of α-quartz to β-quartz. Between 1000 and 1200 °C, a mass loss of 0.14% was associated with transforming kaolinite into metakaolinite, thus crystallising quartz [38,39]. The total mass loss shown in the test was approximately 2.23%. The talc TG/DTG curve in Figure 3e had a total mass loss of 34.37%. That was mainly related to eliminating CO2 from decarbonising dolomite, which is present in large amounts in the mineral. The mass loss of 27.55% between 600 and 900 °C refers to the release of CO2 from dolomite and the decarbonation of talc as dolomite mainly comprises calcium and magnesium carbonates. Figure 3f observes that the eggshell residue had an important total mass loss of 49.25%. From 900 °C, there was stabilization in mass loss, and between 200 and 400 °C, a mass loss of 4.58% is observed. Said mass loss is attributed to the thermal decomposition of the organic matter present in the sample, which, in turn, results in the release of gases, such as carbon dioxide (CO2) and water vapor (H2O).   Anorthite and mullite are desirable in ceramic materials due to their excellent mechanical properties.

Mineralogical Phases and Physical-Mechanical Properties of Sintered Samples
As the industry uses production process temperatures between 1190 • C and 1220 • C as a limit for producing ceramic tiles, we decided to analyse the 1200 • C process. Therefore, the phase transformations that occurred during the firing cycles were observed. Figure 4a presents the results obtained in the mineralogical analysis by the X-ray diffraction of the specimens synthesised from the standard formulation, where feldspar occurs as a fusion agent. The phases identified by X-ray diffraction were quartz (SiO 2 ) (JCPDS: 46-1045) and mullite (3Al 2 O 3 .2SiO 2 ) (JCPDS: 79-1276). In the XRF of the raw materials that constitute the formed formulation, this comes from the kaolin in the formulation, thus contributing to a more significant formation of the glassy phase and consequently increasing the mechanical resistance of the product. Mullite is formed from the epithelium and amorphous aluminium silicates obtained at 985 • C from clays such as kaolinite [39][40][41]. The formation of mullite contributes significantly to the mechanical strength of the part [42][43][44]. Figure 4b,c show the results obtained in the mineralogical analysis by the X-ray diffraction of the specimens synthesised from the formulations by adding 5% and 10% of eggshell residue.
The compositions with the addition of 5% (Figure 4b) and 10% (Figure 4c  As the industry uses production process temperatures between 1190 °C and 1220 °C as a limit for producing ceramic tiles, we decided to analyse the 1200 °C process. Therefore, the phase transformations that occurred during the firing cycles were observed. Figure 4a presents the results obtained in the mineralogical analysis by the X-ray diffraction of the specimens synthesised from the standard formulation, where feldspar occurs as a fusion agent. The phases identified by X-ray diffraction were quartz (SiO2) (JCPDS: 46-1045) and mullite (3Al2O3.2SiO2) (JCPDS: 79-1276). In the XRF of the raw materials that constitute the formed formulation, this comes from the kaolin in the formulation, thus contributing to a more significant formation of the glassy phase and consequently increasing the mechanical resistance of the product. Mullite is formed from the epithelium and amorphous aluminium silicates obtained at 985 °C from clays such as kaolinite [39][40][41]. The formation of mullite contributes significantly to the mechanical strength of the part [42][43][44]. Figure 4b,c show the results obtained in the mineralogical analysis by the X-ray diffraction of the specimens synthesised from the formulations by adding 5% and 10% of eggshell residue.
A new phase (microcline) appeared in the 5% and 10% formulations in the composition with the addition of 20% eggshell residue. That can be explained due to CaO, sodium oxide (Na2O) and potassium oxide together with silica and alumina of the amorphous A new phase (microcline) appeared in the 5% and 10% formulations in the composition with the addition of 20% eggshell residue. That can be explained due to CaO, sodium oxide (Na 2 O) and potassium oxide together with silica and alumina of the amorphous phase formed from the decomposition of clay minerals, forming these silicates and aluminosilicates of calcium, sodium and potassium. Figure 5 shows the impact of sintering temperature (1000 • C, 1100 • C and 1200 • C) on the physical-mechanical properties (linear shrinkage, apparent porosity, water absorption and flexural strength) of the standard formulation and the formulations with the addition of eggshell residue (5%, 10% and 20%). Figure 5a shows that the linear shrinkage of all formulations progressively increased when the sintering temperature was raised from 1000 • C to 1200 • C.
In Figure 5b, the apparent porosity decreased regressively with the increase in temperature. We can see that the standard formulation acquired an apparent porosity much lower than that of the formulations with eggshell residue. With the addition of calcium carbonate (CaCO 3 ) in the standard composition, there was an increase in the porosity of the material since CaCO 3 decomposes during burning, producing carbon dioxide (CO 2 ) and leaving voids, causing the formation of pores. This burning process in the presence of CaCO 3 leads to an increase in the formation of pores in different sizes and shapes. Furthermore, the amount and distribution of CaCO 3 within the ceramic structure can significantly influence the mechanical and physical properties of the material [45][46][47]. In Figure 5c, water absorption decreases regressively with increasing temperature. We can see that the standard formulation acquired much lower water absorption than the formulations with eggshell residue. Figure 5d indicates that the mechanical strength of the standard formulation progressively increased with temperature. However, the same phenomenon does not occur for formulations with the presence of eggshell residue, where the effect is exactly the opposite: a decrease in the resistance of the part. This can be explained by the decomposition of calcium carbonate into calcium oxide (CaO) and carbon dioxide (CO 2 ) with increasing temperature during ceramic firing (CaCO 3 CaO + CO 2 ). This reaction forms pores in the ceramic, increasing its apparent porosity and water absorption.
phase formed from the decomposition of clay minerals, forming these silicates and aluminosilicates of calcium, sodium and potassium. Figure 5 shows the impact of sintering temperature (1000 °C, 1100 °C and 1200 °C) on the physical-mechanical properties (linear shrinkage, apparent porosity, water absorption and flexural strength) of the standard formulation and the formulations with the addition of eggshell residue (5%, 10% and 20%). Figure 5a shows that the linear shrinkage of all formulations progressively increased when the sintering temperature was raised from 1000 °C to 1200 °C. In Figure 5b, the apparent porosity decreased regressively with the increase in temperature. We can see that the standard formulation acquired an apparent porosity much lower than that of the formulations with eggshell residue. With the addition of calcium carbonate (CaCO3) in the standard composition, there was an increase in the porosity of the material since CaCO3 decomposes during burning, producing carbon dioxide (CO2) and leaving voids, causing the formation of pores. This burning process in the presence of CaCO3 leads to an increase in the formation of pores in different sizes and shapes. Furthermore, the amount and distribution of CaCO3 within the ceramic structure can significantly influence the mechanical and physical properties of the material [45][46][47]. In Figure  5c, water absorption decreases regressively with increasing temperature. We can see that The addition of eggshell residue significantly contributed to the increase in water absorption and the decrease in the mechanical strength of the piece. However, there was no significant difference in the physical-mechanical properties when the concentration of eggshell residue was 5%, 10% and 20%. In general, it is observed that the addition of eggshell residue promoted an increase in the water absorption and apparent porosity and a significant reduction in the linear shrinkage and mechanical resistance of the incoming ceramic piece.
When comparing the results of the mechanical resistance of the ceramic coating with the addition of eggshell residue, it was possible to verify that the formulation with the addition of 5% of the residue obtained a resistance that was 27.56% lower on average than the standard formulation. When adding 10% of eggshell residue, the reduction was 30.22% lower, and when adding 20% of eggshell residue, it was possible to obtain 36.80% less than the standard formulation. Thus, it is concluded that the addition of eggshell residue in the standard formulation would impair the mechanical strength. Figure 6a-d show the SEM images acquired from the compositions after sintering at 1200 • C. The occurrence of a glassy phase with small pores can be seen in Figure 6a. According to the images presented, it can be concluded that the microcline phase is stable and improves the mechanical properties of ceramics, reducing failures and cracks in the ceramics. Pores appear in Figure 6b-d, and there is an intensification as the amount of eggshell residue increases.
When comparing the results of the mechanical resistance of the ceramic coating with the addition of eggshell residue, it was possible to verify that the formulation with the addition of 5% of the residue obtained a resistance that was 27.56% lower on average than the standard formulation. When adding 10% of eggshell residue, the reduction was 30.22% lower, and when adding 20% of eggshell residue, it was possible to obtain 36.80% less than the standard formulation. Thus, it is concluded that the addition of eggshell residue in the standard formulation would impair the mechanical strength. Figure 6a-d show the SEM images acquired from the compositions after sintering at 1200 °C. The occurrence of a glassy phase with small pores can be seen in Figure 6a. According to the images presented, it can be concluded that the microcline phase is stable and improves the mechanical properties of ceramics, reducing failures and cracks in the ceramics. Pores appear in Figure 6b-d, and there is an intensification as the amount of eggshell residue increases.

Conclusions
The incorporation of eggshell residue in ceramic masses is considered a sustainable alternative as it can help to reduce the environmental impact of ceramic production and the amount of waste that needs to be discarded. However, when it comes to porcelain tiles, the addition of eggshell residue has not proved to be a viable solution. This is because the incorporation of eggshell residue significantly altered the physical-mechanical properties of the sintered samples. Regarding the physical indices observed in the ceramic piece, it was noticed that the incorporation of eggshell residues resulted in alterations in its physical properties. Specifically, linear burn shrinkage was virtually unchanged, but there was a 51.58% increase in apparent porosity and an approximately 51.37% increase in water absorption. In addition, there was an average decrease of 43.82% in mechanical strength. These results suggest that the incorporation of eggshell residue negatively influences the formulations under analysis.
It is concluded that the incorporation of eggshell residue in ceramic masses can be a sustainable alternative to reduce waste, but it did not prove to be viable for porcelain tiles. The physical-mechanical properties of the sintered samples deteriorated significantly, with a significant loss of mechanical strength. In addition, the incorporation of eggshell residue also caused negative impacts on the physical properties of the ceramic piece, such as increased porosity and water absorption and decreased mechanical strength.

Conclusions
The incorporation of eggshell residue in ceramic masses is considered a sustainable alternative as it can help to reduce the environmental impact of ceramic production and the amount of waste that needs to be discarded. However, when it comes to porcelain tiles, the addition of eggshell residue has not proved to be a viable solution. This is because the incorporation of eggshell residue significantly altered the physical-mechanical properties of the sintered samples. Regarding the physical indices observed in the ceramic piece, it was noticed that the incorporation of eggshell residues resulted in alterations in its physical properties. Specifically, linear burn shrinkage was virtually unchanged, but there was a 51.58% increase in apparent porosity and an approximately 51.37% increase in water absorption. In addition, there was an average decrease of 43.82% in mechanical strength. These results suggest that the incorporation of eggshell residue negatively influences the formulations under analysis.
It is concluded that the incorporation of eggshell residue in ceramic masses can be a sustainable alternative to reduce waste, but it did not prove to be viable for porcelain tiles. The physical-mechanical properties of the sintered samples deteriorated significantly, with a significant loss of mechanical strength. In addition, the incorporation of eggshell residue also caused negative impacts on the physical properties of the ceramic piece, such as increased porosity and water absorption and decreased mechanical strength.

Data Availability Statement:
The data presented in this study are available only by the requesting them from the corresponding author due to the further work in progress.

Conflicts of Interest:
The authors declare no conflict of interest.