*3.1. Characterization of Raw Materials*

Table 2 presents the XRF results of the raw materials. It is observed that the chemical composition of clays is predominantly based on SiO<sup>2</sup> and Al2O3, which is typical of clay minerals. The presence of Fe2O<sup>3</sup> is important, as it offers the reddish after-firing color typical of ceramic artifacts [27]. The presence of TiO<sup>2</sup> helps in the surface hardness of the material, which can improve its mechanical properties, but it makes the material more fragile [28,29]. It can also act as a coloring oxide, clearing the ceramic piece after firing. The presence of K2O, MgO, Na2O and CaO is important because these alkaline and earthy alkaline elements act in the formation of the liquid phase, which is the main sintering mechanism of ceramic materials [6,30]. The loss of ignition, above 10%, is problematic because it is an indication of the presence of organic matter that can increase the material's shrinkage and porosity, hence the importance of using corrective materials such as sand and glass waste. The use of two types of clay is justified to minimize dependence on a single deposit and to make the material less heterogeneous.

The chemical composition of natural sand is predominantly based on SiO2, which is compatible with its mineralogical composition based on quartz. The glass waste, however, has predominantly SiO<sup>2</sup> in its composition, which is typical of glass. This material is probably in an amorphous form, contributing to the reduction of the material's porosity. There is also a high content of Na2O (6.86%) and CaO (15.54%). These components aid in the formation of a liquid phase and improve strength, as they reduce the porosity of the material. The high content of CaO can cause bleaching in ceramic pieces, producing other static patterns for the pieces [31,32].

Figure 1 presents the results of the granulometry of the raw materials, while Figure 2 presents the SEM of the glass waste. It is easy to observe that the particle size of glass waste is close to the particle size of natural sand, which is another result that indicates the feasibility of using glass waste as a substitute for sand in ceramic materials. Regarding the morphology of the waste, an irregular pattern can be seen in Figure 2, due to the amorphism of the material, and particles with a high specific surface area, which is important to increase the resistance of the ceramic materials after firing. TiO2 1.29 1.21 0.47 0.01 K2O 1.01 0.99 1.50 0.36 MgO 0.55 0.61 0.62 2.53 Na2O 0.34 0.24 0.84 6.86

is also a high content of Na2O (6.86%) and CaO (15.54%). These components aid in the formation of a liquid phase and improve strength, as they reduce the porosity of the material. The high content of CaO can cause bleaching in ceramic pieces, producing other

**Composition Clay 1 (wt%) Clay 2 (wt%) Natural Sand (wt%) Glass Waste (wt%)**  SiO2 47.04 49.34 81.10 72.58 Al2O3 32.56 30.71 11.90 1.82 Fe2O3 3.48 3.66 1.20 0.55


**Table 2.** Chemical composition by XRF of raw materials.

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static patterns for the pieces [31,32].


L.O.I., loss of ignition.

**Figure 1.** Granulometry of raw materials. **Figure 1.** Granulometry of raw materials.

**Figure 2.** SEM of the glass waste. **Figure 2.** SEM of the glass waste.

fulfill the same roles as natural sand is an advantage in this regard.

too excessive. This will be discussed further later in this text.

**0 200 400 600 800 1000**

Temperature (°C)


dl/Lo (%)

31ºC

 M0 M5 M10 M20

**Figure 3.** Dilatometry results.

the use of glass waste promotes the formation of larger amounts of liquid phase than natural sand, due to the presence of amorphous silica. Around 600 °C, the first relevant event occurs, which is when the glass enters to a softening point, showing a high reduction in its viscosity [33,34]. This is also evident by the optical dilatometry shown in Figure 3. The ceramic mass with 0% glass waste, and which has only natural sand, suffers an adverse event at this same temperature due to the allotropic transformation of the quartz, which can cause defects in ceramic materials [35]. The use of an amorphous material that can

Another important event takes place between 800 and 1000 °C, where the glass fusion occurs. These temperatures promote greater formation of the liquid phase, which can help the ceramic properties due to the reduction of porosity [33]. The excess of this phase can be harmful, due to the high shrinkage that can cause excessive defects in the manufacturing stage [36]. Thus, it is necessary to evaluate the results obtained after firing, especially for the mass containing 20% of glass waste, to verify that the retraction obtained was not

889°C

1124ºC

1028ºC

844ºC

828ºC

*3.2. Characterization of Ceramic Masses before Firing* 

#### *3.2. Characterization of Ceramic Masses before Firing* event at this same temperature due to the allotropic transformation of the quartz, which

*3.2. Characterization of Ceramic Masses before Firing* 

**Figure 2.** SEM of the glass waste.

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Figure 3 presents the results of the dilatometry of the ceramic masses, revealing that the use of glass waste promotes the formation of larger amounts of liquid phase than natural sand, due to the presence of amorphous silica. Around 600 ◦C, the first relevant event occurs, which is when the glass enters to a softening point, showing a high reduction in its viscosity [33,34]. This is also evident by the optical dilatometry shown in Figure 3. The ceramic mass with 0% glass waste, and which has only natural sand, suffers an adverse event at this same temperature due to the allotropic transformation of the quartz, which can cause defects in ceramic materials [35]. The use of an amorphous material that can fulfill the same roles as natural sand is an advantage in this regard. can cause defects in ceramic materials [35]. The use of an amorphous material that can fulfill the same roles as natural sand is an advantage in this regard. Another important event takes place between 800 and 1000 °C, where the glass fusion occurs. These temperatures promote greater formation of the liquid phase, which can help the ceramic properties due to the reduction of porosity [33]. The excess of this phase can be harmful, due to the high shrinkage that can cause excessive defects in the manufacturing stage [36]. Thus, it is necessary to evaluate the results obtained after firing, especially for the mass containing 20% of glass waste, to verify that the retraction obtained was not too excessive. This will be discussed further later in this text.

Figure 3 presents the results of the dilatometry of the ceramic masses, revealing that the use of glass waste promotes the formation of larger amounts of liquid phase than natural sand, due to the presence of amorphous silica. Around 600 °C, the first relevant event occurs, which is when the glass enters to a softening point, showing a high reduction in its viscosity [33,34]. This is also evident by the optical dilatometry shown in Figure 3. The ceramic mass with 0% glass waste, and which has only natural sand, suffers an adverse

**Figure 3. Figure 3.**  Dilatometry results. Dilatometry results.

Another important event takes place between 800 and 1000 ◦C, where the glass fusion occurs. These temperatures promote greater formation of the liquid phase, which can help the ceramic properties due to the reduction of porosity [33]. The excess of this phase can be harmful, due to the high shrinkage that can cause excessive defects in the manufacturing stage [36]. Thus, it is necessary to evaluate the results obtained after firing, especially for the mass containing 20% of glass waste, to verify that the retraction obtained was not too excessive. This will be discussed further later in this text.

Figure 4 presents the extrusion prognostic results. It is observed that all the studied ceramic masses are within the acceptable extrusion region. This point is important for using the glass waste in place of the natural sand because problems during the extrusion step will impact the properties of the ceramic material before and after firing [8]. This indicates that the behavior of the waste is non-plastic, compatible with natural sand, which also behaves this way.

Figure 5 shows the results of apparent density during the drying and linear drying. Note that the apparent density of the specimens increased with the use of the glass protector. This characteristic is because the greater the density, the greater the mass of the material occupying the same volume. This information can help to reduce the porosity of ceramic pieces after firing [37]. Regarding linear drying, note that glass reduces shrinkage. This is also positive due to the formation of particles, which will probably increase after the retraction of ceramic pieces and the firing due to the liquid phase. The reduction in drying shrinkage can compensate the high shrinkage after firing [38].

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Figure 4 presents the extrusion prognostic results. It is observed that all the studied ceramic masses are within the acceptable extrusion region. This point is important for using the glass waste in place of the natural sand because problems during the extrusion step will impact the properties of the ceramic material before and after firing [8]. This indicates that the behavior of the waste is non-plastic, compatible with natural sand,

**Acceptable Extrusion**

Figure 4 presents the extrusion prognostic results. It is observed that all the studied ceramic masses are within the acceptable extrusion region. This point is important for using the glass waste in place of the natural sand because problems during the extrusion step will impact the properties of the ceramic material before and after firing [8]. This indicates that the behavior of the waste is non-plastic, compatible with natural sand,

Figure 5 shows the results of apparent density during the drying and linear drying.

**Figure 4.** Extrusion prognosis. **Figure 4.** Extrusion prognosis. the retraction of ceramic pieces and the firing due to the liquid phase. The reduction in drying shrinkage can compensate the high shrinkage after firing [38].

which also behaves this way.

**30**

**35**

**40**

which also behaves this way.

**Figure 5.** Results of (**a**) apparent drying density; (**b**) linear drying. **Figure 5.** Results of (**a**) apparent drying density; (**b**) linear drying.

*3.3. Characterization of Ceramic Masses after Firing* 

**800 900 1000**

Temperature (ºC)

(**a**)

**1.5**

**1.6**

**1.7**

**1.8**

Apparent Burning Density (g/cm3

)

**1.9**

**2.0**

**2.1**

 M0 M5 M10 M20

Figure 6 shows the post-fire density and post-fire linear shrinkage results. It is ob-

material increases, as does the linear shrinkage. This behavior is the pattern of this type of material and has already been reported by other authors [7,39]. However, the glass waste promoted an increase in the densification of the ceramic material, which is directly related to the formation of a liquid phase and greater sintering of resistant phases in the material. A negative characteristic is that the use of the waste promoted an increase in the firing shrinkage, which was already expected based on the other results. Compositions 10% and 20% show shrinkage above 4% at a firing temperature of 1000 °C. This can be a problem during the manufacture of ceramic parts due to defects in the developed products. The other retraction values, however, are within the standards established by other authors, such as Girondi et al. (2020) [9], Delaqua et al. (2020b) [10], and Delaqua, et al. (2022) [22]. **3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0**

**Linear Drying (%)**

#### *3.3. Characterization of Ceramic Masses after Firing 3.3. Characterization of Ceramic Masses after Firing*

**M0 M5 M10 M20**

**Formulations (%)** (**b**) **Figure 5.** Results of (**a**) apparent drying density; (**b**) linear drying.

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Figure 6 shows the post-fire density and post-fire linear shrinkage results. It is observed that as the calcination temperature increases, the apparent density of the ceramic material increases, as does the linear shrinkage. This behavior is the pattern of this type of material and has already been reported by other authors [7,39]. However, the glass waste promoted an increase in the densification of the ceramic material, which is directly related to the formation of a liquid phase and greater sintering of resistant phases in the material. A negative characteristic is that the use of the waste promoted an increase in the firing shrinkage, which was already expected based on the other results. Compositions 10% and 20% show shrinkage above 4% at a firing temperature of 1000 ◦C. This can be a problem during the manufacture of ceramic parts due to defects in the developed products. The other retraction values, however, are within the standards established by other authors, such as Girondi et al. (2020) [9], Delaqua et al. (2020b) [10], and Delaqua, et al. (2022) [22]. Figure 6 shows the post-fire density and post-fire linear shrinkage results. It is observed that as the calcination temperature increases, the apparent density of the ceramic material increases, as does the linear shrinkage. This behavior is the pattern of this type of material and has already been reported by other authors [7,39]. However, the glass waste promoted an increase in the densification of the ceramic material, which is directly related to the formation of a liquid phase and greater sintering of resistant phases in the material. A negative characteristic is that the use of the waste promoted an increase in the firing shrinkage, which was already expected based on the other results. Compositions 10% and 20% show shrinkage above 4% at a firing temperature of 1000 °C. This can be a problem during the manufacture of ceramic parts due to defects in the developed products. The other retraction values, however, are within the standards established by other authors, such as Girondi et al. (2020) [9], Delaqua et al. (2020b) [10], and Delaqua, et al. (2022) [22].

**Figure 6.** Results of (**a**) apparent burning density; (**b**) linear shrinkage. **Figure 6.** Results of (**a**) apparent burning density; (**b**) linear shrinkage.

Figure 7 presents the water absorption results, in which the mass containing only natural sand (0%) does not meet the limits for tiles, since at all temperatures, it presents water absorption greater than 20%. The masses containing glass waste, especially the one with 20% of the material, meet the water absorption requirements for both tiles and blocks,

ings in the manufacture of ceramics. The reduction of water absorption is related to the reduction of porosity and to the formation of liquid phase. As observed in Figure 3, from 600 °C onward, the glass already reaches its softening point, and therefore, it already has a functional liquid phase mechanism. At 1100 °C, glass still meets its operating point, updated, even more with the interconnection mechanisms of the materials [13,33]. These results do not prove the feasibility of using the glass waste, as they are used to use this

Tiles 20% (NBR ABNT 15.310/09)

**800 900 1000**

Temperature (ºC)

Bricks 8 - 25% (NBR ABNT 15.270/17)

material.

Water Absorption - WA (%)

 M0 M5 M10 M20

**Figure 7.** Water absorption results.

**0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5**

**Linear Shrinkage - LS (%)**

(2-4%) High shrinkage

 M0 M5 M10 M20

Figure 7 presents the water absorption results, in which the mass containing only natural sand (0%) does not meet the limits for tiles, since at all temperatures, it presents water absorption greater than 20%. The masses containing glass waste, especially the one with 20% of the material, meet the water absorption requirements for both tiles and blocks, already in the calcination at 800 ◦C [7]. This is a great advantage because it provides savings in the manufacture of ceramics. The reduction of water absorption is related to the reduction of porosity and to the formation of liquid phase. As observed in Figure 3, from 600 ◦C onward, the glass already reaches its softening point, and therefore, it already has a functional liquid phase mechanism. At 1100 ◦C, glass still meets its operating point, updated, even more with the interconnection mechanisms of the materials [13,33]. These results do not prove the feasibility of using the glass waste, as they are used to use this material. water absorption greater than 20%. The masses containing glass waste, especially the one with 20% of the material, meet the water absorption requirements for both tiles and blocks, already in the calcination at 800 °C [7]. This is a great advantage because it provides savings in the manufacture of ceramics. The reduction of water absorption is related to the reduction of porosity and to the formation of liquid phase. As observed in Figure 3, from 600 °C onward, the glass already reaches its softening point, and therefore, it already has a functional liquid phase mechanism. At 1100 °C, glass still meets its operating point, updated, even more with the interconnection mechanisms of the materials [13,33]. These results do not prove the feasibility of using the glass waste, as they are used to use this material.

**800 900 1000**

**Temperature (ºC)**

(**b**)

**Figure 6.** Results of (**a**) apparent burning density; (**b**) linear shrinkage.

(1-2%) Moderate shrinkage

Figure 7 presents the water absorption results, in which the mass containing only natural sand (0%) does not meet the limits for tiles, since at all temperatures, it presents

(0-1%) Low shrinkage

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**Figure 7. Figure 7.** Water absorption results. Water absorption results.

Figure 8 presents the flexural strength results. It is observed that the 20% composition meets the requirements of blocks and tiles already at a temperature of 800 ◦C, proving the savings highlighted in Figure 7. That is, it does not make much sense to propose calcination at higher temperatures if in this calcination range the composition of 20% already meets the requirements for water absorption and flexural strength, promoting savings in the firing stage. The same results are not possible without the use of glass waste, since the 0% composition does not meet the 6.5 MPa limit for tiles nor at the highest calcination temperature [8,10]. The results obtained are consistent with those highlighted by other authors and can be attributed to the formation of a liquid phase, as previously discussed [32].

Based on the discussion of the results, the effect of glass waste on ceramic materials is clear. It is observed that the role of natural sand is to act as a source of quartz for ceramic materials. In ceramic materials, quartz has the function of controlling shrinkage and reducing defects in the material. Quartz replacement, therefore, should be limited so as not to exacerbate these problems. However, the use of glass waste contributes feldspar to the ceramic material. This component, as highlighted in the introduction, contributes to the typical ternary composition of ceramic materials, containing clay, feldspar, and quartz [15,16]. The presence of glass waste contributes to a decrease in the firing temperature and an increase in the apparent density, resulting in a reduction in porosity

and an increase in flexural rupture strength. Conversely, glass waste causes defects in ceramic materials, which is why its content should be limited to 20%, as proven in this research. At 1000 ◦C, the introduction of 20% of the glass waste caused an increase in flexural rupture strength from approximately 5 to almost 20 MPa. Conversely, it caused an increase in linear shrinkage from 2.5% to approximately 7.0%, which leads to an increase in material defects, as will be discussed below. Figure 9 shows a picture of the 20% specimens calcined at 1000 ◦C, where the formation of defects in the material, which, although they did not affect the strength gain, make the material of low aesthetic value. Therefore, the use of lower temperatures is more advantageous, since at 800 ◦C, it is possible to obtain values compatible with the main applications of ceramic materials. meets the requirements of blocks and tiles already at a temperature of 800 °C, proving the savings highlighted in Figure 7. That is, it does not make much sense to propose calcination at higher temperatures if in this calcination range the composition of 20% already meets the requirements for water absorption and flexural strength, promoting savings in the firing stage. The same results are not possible without the use of glass waste, since the 0% composition does not meet the 6.5 MPa limit for tiles nor at the highest calcination temperature [8,10]. The results obtained are consistent with those highlighted by other authors and can be attributed to the formation of a liquid phase, as previously discussed [32].

Figure 8 presents the flexural strength results. It is observed that the 20% composition

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**Figure 8.** Flexural rupture strength results. **Figure 8.** Flexural rupture strength results.

Based on the discussion of the results, the effect of glass waste on ceramic materials is clear. It is observed that the role of natural sand is to act as a source of quartz for ceramic materials. In ceramic materials, quartz has the function of controlling shrinkage and reducing defects in the material. Quartz replacement, therefore, should be limited so as not to exacerbate these problems. However, the use of glass waste contributes feldspar to the ceramic material. This component, as highlighted in the introduction, contributes to the typical ternary composition of ceramic materials, containing clay, feldspar, and quartz Figure 9a shows a typical defect of ceramics produced with clays containing high levels of Fe2O<sup>3</sup> called black heart. This defect is essentially due to the high iron content present in the clay, which occurred in all compositions calcined at 1000 ◦C in this research. It is observed that the thermal effect is a catalyst for the black heart. Thus, an essential action to reduce these defects is to use lower calcination temperatures [40]. It can be seen from the results presented above that the use of glass waste makes it possible to reduce the calcination temperature without affecting the properties evaluated. Thus, an efficient action to reduce these defects is the use of glass waste to improve sintering in ceramic materials.

[15,16]. The presence of glass waste contributes to a decrease in the firing temperature and an increase in the apparent density, resulting in a reduction in porosity and an increase in flexural rupture strength. Conversely, glass waste causes defects in ceramic materials, which is why its content should be limited to 20%, as proven in this research. At 1000 °C, the introduction of 20% of the glass waste caused an increase in flexural rupture strength The formation of surface defects, highlighted in Figure 9b, is related to the high linear shrinkage values of this composition, shown in Figure 6b, and to the sintering mechanism promoted by glass waste. This sintering mechanism is potentiated at a temperature of 1000 ◦C, because as illustrated in Figure 3, at this temperature range, the glass waste melts, causing the surface defects observed in the material [4].

from approximately 5 to almost 20 MPa. Conversely, it caused an increase in linear shrinkage from 2.5% to approximately 7.0%, which leads to an increase in material defects, as will be discussed below. Figure 9 shows a picture of the 20% specimens calcined at 1000 °C, where the formation of defects in the material, which, although they did not affect the strength gain, make the material of low aesthetic value. Therefore, the use of lower temperatures is more advantageous, since at 800 °C, it is possible to obtain values compatible

Figure 9a shows a typical defect of ceramics produced with clays containing high

present in the clay, which occurred in all compositions calcined at 1000 °C in this research.

with the main applications of ceramic materials.

causing the surface defects observed in the material [4].

materials.

(**a**)

(**b**)

**Figure 9.** Macroscopic analysis of the 20% composition: (**a**) inner surface; (**b**) external surface. **Figure 9.** Macroscopic analysis of the 20% composition: (**a**) inner surface; (**b**) external surface.

It is observed that the thermal effect is a catalyst for the black heart. Thus, an essential action to reduce these defects is to use lower calcination temperatures [40]. It can be seen from the results presented above that the use of glass waste makes it possible to reduce the calcination temperature without affecting the properties evaluated. Thus, an efficient action to reduce these defects is the use of glass waste to improve sintering in ceramic

The formation of surface defects, highlighted in Figure 9b, is related to the high linear

shrinkage values of this composition, shown in Figure 6b, and to the sintering mechanism promoted by glass waste. This sintering mechanism is potentiated at a temperature of 1000 °C, because as illustrated in Figure 3, at this temperature range, the glass waste melts,

Figure 10 presents the SEM results of the ceramic masses. Figure 10a,b shows the difference between the 0% and 20% compositions calcined at 800 °C, where in Figure 10b, the presence of a more compact and dense structure is visible. This can be attributed to the sintering promoted by the glass waste. Figure 10c,d shows the effects of calcination at 1000 °C in the 0% and 20% formulations, where the formation of a glassy phase in the 20% composition is clear. This is directly related to the use of glass, which promotes the formation of a liquid phase and reduces the porosity of the material. This happens due to the melting process of the glass waste, observed in Figure 3. In this temperature range, the glass becomes liquid, densifying the matrix and filling the pores not accessible by the sand that the glass waste replaces. Thus, the mechanism involved is liquid phase diffusion, Figure 10 presents the SEM results of the ceramic masses. Figure 10a,b shows the difference between the 0% and 20% compositions calcined at 800 ◦C, where in Figure 10b, the presence of a more compact and dense structure is visible. This can be attributed to the sintering promoted by the glass waste. Figure 10c,d shows the effects of calcination at 1000 ◦C in the 0% and 20% formulations, where the formation of a glassy phase in the 20% composition is clear. This is directly related to the use of glass, which promotes the formation of a liquid phase and reduces the porosity of the material. This happens due to the melting process of the glass waste, observed in Figure 3. In this temperature range, the glass becomes liquid, densifying the matrix and filling the pores not accessible by the sand that the glass waste replaces. Thus, the mechanism involved is liquid phase diffusion, which fills the accessible pores of the ceramic material and contributes to the flexural rupture strength [33]. The images are compatible with the results obtained by Figures 7 and 8.

which fills the accessible pores of the ceramic material and contributes to the flexural rupture strength [33]. The images are compatible with the results obtained by Figures 7 and

8.

(**b**)

**Figure 10.** *Cont.*

(**c**)

(**d**)

#### **4. Conclusions 4. Conclusions**

Based on the results obtained, the following conclusions were summarized: Based on the results obtained, the following conclusions were summarized:


As perspectives for future work, the following stand out: (i) evaluate the influence of the granulometry of the glass waste on the properties of red ceramics; (ii) evaluate the effect of using 15% and 25% of glass waste as a substitute for sand; (iii) carry out durability and degradation tests on the red ceramic containing glass waste.

**Author Contributions:** Methodology, C.V., G.D. and J.M.; formal analysis G.D. and J.M.; resources S.M. and C.V.; formal analysis, H.C. and F.V.J.; writing—original draft preparation, G.D., J.M. and M.M.; writing—review and editing, C.V., G.D., M.M. and H.C.; supervision, C.V.; project administration, C.V.; funding acquisition, C.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro: E-26/200.847/2021 and Conselho Nacional de Desenvolvimento Científico e Tecnológico: 301634/2018.1.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank FAPERJ and CNPQ as well as NewTemper for the waste and the Laboratory of Advanced Materials—LAMAV/UENF for the support.

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