*3.4. Abrasive Wear*

In terms of abrasive wear, for the application of ornamental stones on floor coverings, Chiodi Filho and Rodrigues [27] classify the material quality as: high traffic floor (<1.5 mm), medium traffic floor (<3 mm), and low traffic floor (<6 mm). Following technological parameters described above, AS-EP and AS-PU low wear values shown in Table 6 classify them for use in high traffic floors because of an efficient rearrangement in the minerals structure and texture during their agglutination by resins. A determining factor for this excellent result, which influenced the characteristics mentioned above, was the use of the highest close-package mixture to prepare the plates.

**Table 6.** Amsler wear associated with the thickness reduction of the artificial stones developed (AS-EP and AS-PU), CS, and NS.


However, the natural quartzite stone value of 0.35 mm wear was the smallest found on the same route, possibly due to the presence of large mineral quartz crystals in the stone. Among the ornamental stones, quartz is one of the hardest minerals found (Mohs = 7) [38].

We compared the results of this work with some other works that cited that they developed artificial stones using the same method of vacuum, compression, and vibration. Silva et al. [10] produced an artificial stone based on calcite marble waste and 20% w epoxy resin. The artificial stone showed less than 1.5 mm wear on the 1000 m runway, suitable for high traffic floors. Carvalho et al. [29], when evaluating an artificial stone based on 20% w epoxy resin and 80% steel residue from the electrostatic precipitator from the sintering step, found 1.04 and 2.16 mm thickness reduction and 2.16 mm for 500 and 1000 m. The artificial stone was considered suitable for medium traffic floors.

Ribeiro et al. [40] developed an artificial stone using polyester resin and dolomitic marble waste. The developed artificial stone underwent a 5.23 and 8.83 mm thickness reduction for 500 and 1000 m. The wear presented by this material was related to its great porosity because the interfacial adhesion between the particles and the matrix were proven not to be good. It is observed that wear results differ according to types of wastes and resins as well as variables of the manufacturing method.

#### *3.5. Hard Body Impact Resistance 3.5. Hard Body Impact Resistance*

Hard body impact resistance was performed in order to assess the AS-EP and AS-PU level of cohesion and toughness as well as how much energy is needed to dissipate until the material breaks, depending on the maximum drop height of objects supported in its surface. With this result, it is possible to scale the plates in the appropriate size to the usage requirements. Hard body impact resistance was performed in order to assess the AS-EP and AS-PU level of cohesion and toughness as well as how much energy is needed to dissipate until the material breaks, depending on the maximum drop height of objects supported in its surface. With this result, it is possible to scale the plates in the appropriate size to the usage requirements.

The average rupture height and dissipated energy of the artificial stone AS-PU is 4.4 times greater than that presented by AS-PE, with a 0.39 m height and 3.86 J energy (Table 7). Compared to the epoxy resin, the polyurethane resin has a less rigid behavior that can affect some properties such as flexural strength. On the other hand, it increases AS-PU toughness, precisely because it dissipates more energy. The porosity of AS-EP may also facilitate crack formation, since the pores are naturally stress concentrators [41]. The average rupture height and dissipated energy of the artificial stone AS-PU is 4.4 times greater than that presented by AS-PE, with a 0.39 m height and 3.86 J energy (Table 7). Compared to the epoxy resin, the polyurethane resin has a less rigid behavior that can affect some properties such as flexural strength. On the other hand, it increases AS-PU toughness, precisely because it dissipates more energy. The porosity of AS-EP may also facilitate crack formation, since the pores are naturally stress concentrators [41].

**Table 7.** Resistance to hard body impact values. **Table 7.** Resistance to hard body impact values.


Chiodi Filho and Rodrigues [27] proposed a stones classification according to their impact resistance in rupture height, ranging as: very low (<0.30 m), low (0.30–0.50 m), medium (0.50–0.70 m), high (0.70–0.95 m), and very high (>0.95 m) rupture height, and ABNT NBR 15844 [28] establishes that the stones must support a minimum height of 0.3 m. The AS-PU supported height greater than that specified by the standard and by Costa et al. [41] (0.95 m), proving its excellent quality and its feasibility to be applied in areas where there is a higher falling load incidence, such as supermarkets, airports, and industries in general. impact resistance in rupture height, ranging as: very low (<0.30 m), low (0.30–0.50 m), medium (0.50–0.70 m), high (0.70–0.95 m), and very high (>0.95 m) rupture height, and ABNT NBR 15844 [28] establishes that the stones must support a minimum height of 0.3 m. The AS-PU supported height greater than that specified by the standard and by Costa et al. [41] (0.95 m), proving its excellent quality and its feasibility to be applied in areas where there is a higher falling load incidence, such as supermarkets, airports, and industries in general.

The natural quartzite resisted a rupture height of 0.20 m, resulting in 1.96 J of energy, so the artificial stone impact resistance was higher than that of the natural stone. Silva et al. [10] and Gomes et al. [15] developed artificial marble and granite with epoxy resin, which presented cracks at heights of 0.43 m and 0.4 m with 4 J of activation rupture energy, similar to that of AS-EP. The natural quartzite resisted a rupture height of 0.20 m, resulting in 1.96 J of energy, so the artificial stone impact resistance was higher than that of the natural stone. Silva et al. [10] and Gomes et al. [15] developed artificial marble and granite with epoxy resin, which presented cracks at heights of 0.43 m and 0.4 m with 4 J of activation rupture energy, similar to that of AS-EP.

#### *3.6. Microstructure 3.6. Microstructure*

Figures 6 and 7 present SEM micrographs of the fracture surface sections of AS-EP and AS-PU, respectively, after the three-point flexural test. Figures 6 and 7 present SEM micrographs of the fracture surface sections of AS-EP and AS-PU, respectively, after the three-point flexural test.

**Figure 6. Figure 6.** SEM micrographs of the fracture surface sections of AS-EP ( SEM micrographs of the fracture surface sections of AS-EP (**aa**) 50×; (**b**)100×. ) 50×; (**b**)100×.

*Sustainability* **2022**, *14*, x FOR PEER REVIEW 14 of 18

**Figure 7.** SEM micrographs of the fracture surface sections of AS-PU (**a**) 50×; (**b**)100×. **Figure 7.** SEM micrographs of the fracture surface sections of AS-PU (**a**) 50×; (**b**)100×.

Figure 6a,b show AS-EP's smoother surface, evidencing a more defined fracture plane that is characteristic of a highly cohesive material. The arrow indicates few evident pores, proving the low water absorption both for the artificial stone with polyurethane and epoxy. According to Debnath et al. [38], the maximum performance of the system occurs through an optimal mineral load wetting by epoxy resin. Therefore, the greater the interface between the matrix and the load, the better the material mechanical properties, this interaction being directly linked to the adhesive bond strength provided by effective wetting in the interfacial regions. Figure 6a,b show AS-EP's smoother surface, evidencing a more defined fracture plane that is characteristic of a highly cohesive material. The arrow indicates few evident pores, proving the low water absorption both for the artificial stone with polyurethane and epoxy. According to Debnath et al. [38], the maximum performance of the system occurs through an optimal mineral load wetting by epoxy resin. Therefore, the greater the interface between the matrix and the load, the better the material mechanical properties, this interaction being directly linked to the adhesive bond strength provided by effective wetting in the interfacial regions. Figure 6a,b show AS-EP's smoother surface, evidencing a more defined fracture plane that is characteristic of a highly cohesive material. The arrow indicates few evident pores, proving the low water absorption both for the artificial stone with polyurethane and epoxy. According to Debnath et al. [38], the maximum performance of the system occurs through an optimal mineral load wetting by epoxy resin. Therefore, the greater the interface between the matrix and the load, the better the material mechanical properties, this interaction being directly linked to the adhesive bond strength provided by effective wetting in the interfacial regions. Figure 7a,b show AS-PU's fracture surface SEM micrograph, illustrating the material

Figure 7a,b show AS-PU's fracture surface SEM micrograph, illustrating the material filling all interstices between the larger grains. Bubbles not eliminated by the vacuum process appear within the matrix, which were already confirmed by the porosity test. It is possible that the bubbles were also generated during the polymerization process between the isocyanate (NCO) and hydroxyl (OH) groups, in which occur parallel reactions mainly involving NCO + H2O forming high molecular weight compounds such as poly (urethanes/ureas) with excellent mechanical properties. The NCO + H2O expansion reaction results in urea formation releasing CO2 and forming bubbles. The release of curing process CO2, added to the water vapor release at the time of pressing, are evidence of defects formation [42]. Figure 6b points out a quartz grain that fractured after the artificial stone was produced. Figure 7a,b show AS-PU's fracture surface SEM micrograph, illustrating the material filling all interstices between the larger grains. Bubbles not eliminated by the vacuum process appear within the matrix, which were already confirmed by the porosity test. It is possible that the bubbles were also generated during the polymerization process between the isocyanate (NCO) and hydroxyl (OH) groups, in which occur parallel reactions mainly involving NCO + H2O forming high molecular weight compounds such as poly (urethanes/ureas) with excellent mechanical properties. The NCO + H2O expansion reaction results in urea formation releasing CO<sup>2</sup> and forming bubbles. The release of curing process CO2, added to the water vapor release at the time of pressing, are evidence of defects formation [42]. Figure 6b points out a quartz grain that fractured after the artificial stone was produced. filling all interstices between the larger grains. Bubbles not eliminated by the vacuum process appear within the matrix, which were already confirmed by the porosity test. It is possible that the bubbles were also generated during the polymerization process between the isocyanate (NCO) and hydroxyl (OH) groups, in which occur parallel reactions mainly involving NCO + H2O forming high molecular weight compounds such as poly (urethanes/ureas) with excellent mechanical properties. The NCO + H2O expansion reaction results in urea formation releasing CO2 and forming bubbles. The release of curing process CO2, added to the water vapor release at the time of pressing, are evidence of defects formation [42]. Figure 6b points out a quartz grain that fractured after the artificial stone was produced.

#### *3.7. Petrography 3.7. Petrography*

*3.7. Petrography*  Petrographic slides were analyzed for possible physical features that could decrease the mechanical properties of the material, such as microcracks and pores. Quartz crystals, when observed in natural light (polarized plane), are low relief, colorless, and have no pleochroism or cleavage. In polarized light, quartz grains are anhydrous with a prismatic habit, exhibiting wavy extinction and low interference colors. Therefore, as recognizable Petrographic slides were analyzed for possible physical features that could decrease the mechanical properties of the material, such as microcracks and pores. Quartz crystals, when observed in natural light (polarized plane), are low relief, colorless, and have no pleochroism or cleavage. In polarized light, quartz grains are anhydrous with a prismatic habit, exhibiting wavy extinction and low interference colors. Therefore, as recognizable in Figures 8 and 9, all minerals present in the artificial stone of quartzite are quartz. Petrographic slides were analyzed for possible physical features that could decrease the mechanical properties of the material, such as microcracks and pores. Quartz crystals, when observed in natural light (polarized plane), are low relief, colorless, and have no pleochroism or cleavage. In polarized light, quartz grains are anhydrous with a prismatic habit, exhibiting wavy extinction and low interference colors. Therefore, as recognizable in Figures 8 and 9, all minerals present in the artificial stone of quartzite are quartz.

**Figure 8.** Petrographic slides of artificial stone with quartzite with epoxy AS-EP (**a**) and quartzite with polyurethane AS-PU (**b**).

with polyurethane AS-PU (**b**).

**Figure 9.** Petrographic slide showing failure in load allocation failure of artificial stone with quartz and polyurethane (AS-PU). **Figure 9.** Petrographic slide showing failure in load allocation failure of artificial stone with quartz and polyurethane (AS-PU).

**Figure 8.** Petrographic slides of artificial stone with quartzite with epoxy AS-EP (**a**) and quartzite

Figure 8a shows the AS-EP slide image, evidencing the optimal load distribution in the matrix, with coarse grains forming a network filled by the matrix and the medium grains and mixed with fine grains. Moreover, in the matrix dispersed region, the presence of small voids (pores) is noticeable. Figure 8b shows the AS-PU slide image, with arrows indicating the presence of small voids. The horizontal arrow indicates a space between the grains, pointing out that the load distribution could have been more efficient. Figure 8a shows the AS-EP slide image, evidencing the optimal load distribution in the matrix, with coarse grains forming a network filled by the matrix and the medium grains and mixed with fine grains. Moreover, in the matrix dispersed region, the presence of small voids (pores) is noticeable. Figure 8b shows the AS-PU slide image, with arrows indicating the presence of small voids. The horizontal arrow indicates a space between the grains, pointing out that the load distribution could have been more efficient.

Figure 9 shows the image of the AS-PU petrographic slide, with the occurrence of microcracks. The microcracks can be attributed to a high compression pressure that, instead of packing the material, as it should, compressed it until it broke [4]. Figure 9 shows the image of the AS-PU petrographic slide, with the occurrence of microcracks. The microcracks can be attributed to a high compression pressure that, instead of packing the material, as it should, compressed it until it broke [4].

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

The artificial stone is made up of 85% w of quartzite, epoxy resin (AS-EP), and polyurethane resin (AS-PE), using the "vacuum, vibration and compression" method. From the experimental results, the following conclusions can be drawn: The artificial stone is made up of 85% w of quartzite, epoxy resin (AS-EP), and polyurethane resin (AS-PE), using the "vacuum, vibration and compression" method. From the experimental results, the following conclusions can be drawn:


The method adopted for the production of artificial rock demonstrates that it has the potential to be manufactured, which could reduce the negative environmental impacts

caused by the natural rock industry. This method is currently the most adopted by researchers and the industry, and has shown better results, using vacuum, compression, and vibration (CVC). Higher mechanical properties and a more cohesive microstructure were exhibited by materials produced by VCV, with residues of marble, crushed stone, quartz, and glass.

The definition of the mixture with the highest packing factor, based on the variation of the particle size percentages, using the Simplex-lattice design (SLD) mathematical model, provided greater cohesion between the particles in studies aimed at artificial rocks. They calculated the minimum resin content, TMR (%), which allowed to reduce the resin content used, and they realized through their results that compacting the particles prevents the formation of pores in the samples, thus reducing the absorption of water and reinforcing the structure, compressed.

The development of artificial rock using materials from renewable sources has already been studied, obtaining satisfactory properties with the use of waste from the ornamental rock industry, with zero toxicity and without releasing gases during its production process—that is, a product totally ecological; it has been made possible through the substitution of resins derived from petroleum for resins of natural source.

**Author Contributions:** Methodology, C.P.A. and E.A.S.C.; writing—original draft preparation, E.A.S.C. and G.N.S.B.; writing—review and editing, E.A.S.C. and A.R.G.d.A.; resources, C.M.F.V., S.N.M. and M.C.B.G.; supervision, C.M.F.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the State University of the Northern Fluminense (UENF), partially financed by CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior-Brazil), and provided additional financial by CNPq (Coordenação Nacional de Pesquisa). The participation of A.R.G.A. was sponsored by FAPERJ through the research fellowships proc. no: E-26/210.150/2019, E-26/211.194/2021, E-26/211.293/2021, and E-26/201.310/2021 and by CNPq through the research fellowship PQ2 307592/2021-9. The participation of M.C.B.G. was sponsored by FAPES through the research fellowships proc. n<sup>o</sup> : 80857019 and 84376732.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This work was made possible with the assistance of the Foundation of Support for Research in the State of Rio de Janeiro (E-26/200.847/2021) and (E-26/202.387/2021). They also thank UENF, FAPERJ, CNPq, CAPES, FAPES, CETEM AND IFES for the space and analyses carried out.

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