Comparison between Synthetic and Biodegradable Polymer Matrices on the Development of Quartzite Waste-Based Artificial Stone

Abstract

The development of artificial stone from the agglutination of polymeric resin using industrial wastes can be a viable alternative from a technical, economic, and sustainable point of view. The main objective of the present work was to evaluate the physical, mechanical, and structural properties of artificial stones based on quartzite waste added into a synthetic, epoxy, or biodegradable polyurethane polymer matrix. Artificial stone plates were produced through the vacuum vibration and compression method, using 85 wt% of quartzite waste. The material was manufactured under the following conditions: 3 MPa compaction pressure and 90 and 80 °C curing temperature. The samples were characterized to evaluate physical and mechanical parameters and microstructure properties. As a result, the artificial stone plates developed obtained ≤0.16% water absorption, ≤0.38% porosity, and 26.96 and 10.7 MPa flexural strength (epoxy and polyurethane resin, respectively). A wear test established both artificial quartzite stone with epoxy resin (AS-EP) and vegetable polyurethane resin (AS-PU) high traffic materials. Hard body impact resistance classified AS-EP as a low height material and AS-PU as a very high height material. The petrographic slides analysis revealed that AS-EP has the best load distribution. We concluded the feasibility of manufacturing artificial stone, which would minimize the environmental impacts that would be caused by this waste disposal. We concluded that the production of artificial rock shows the potential and that it also helps to reduce environmental impacts.

1. Introduction

Developing countries that have accelerated industrialization and urbanization processes are producing billions of tons of waste per year. As a major global ornamental stone producer, Brazil is also a major waste generator, due to the daily extraction of thousands of tons of blocks in quarries. However, much of this type of waste is discarded in an uncontrolled manner, causing environmental problems [1,2].

The waste generated from the extraction and processing of ornamental stones represents around 83% of raw material losses. To produce 330 m² of slabs (average from a 10 m³ block), approximately 30 m³ of stone is extracted from the massif, of which 20 m³ remains in the quarry as waste. Most of them are thick blocks, normally non-standard, irregular, and faulty. In addition, stone chips and hulls from the remaining stocks block rigging, as they can have direct use in the smaller plates, less value tile production, or other structural or decorative pieces [3].

Some of these losses are inevitable, as they are linked to the natural stone quality, the deposit characteristics, or the cutting process. However, they can be reduced by improving deposit knowledge, by mine planning, and, mainly, by transforming the waste generated into by-products [3]. Even though the artificial stone’s unit price (m²) and the volume used are relatively high compared to the natural materials, this waste destination, as a by-product, is still the application with the best cost benefit [4].

Artificial stones have aroused the interest of consumers due to certain advantages compared to natural stones. According to data from the Brazilian Association of Dimension Stones Industry—ABIROCHAS, in 2019, Brazilian exports of artificial stones totaled 12.75 thousand tons. Regarding imports, 70,000 tons present average prices of US$ 530/t and US$ 1470/t, respectively, while natural stones presented an average price of US$ 523/t in the same year. Even with a slightly higher price than natural stones, the artificial stone import is much greater than the export [5]. Amongst the advantages of this material are the mechanical and aesthetic properties as well as the use of wastes that reduces irregular disposal, valorizing a traditionally unwanted product, creating jobs, and contributing to economic development [6].

1.1. Artificial Stone

Artificial stones are produced by a high percentage of particles agglomerated by a small percentage of polymeric material. Marble, granite, glass crystals, and quartz sand are examples of stones that, when particulate, are considered natural aggregates. Artificial stone’s mechanical properties are considered to be higher than those of natural stone because of the artificial stone’s lower water absorption and porosity. As such, artificial stone is a more suitable option for demanding work conditions, such as floors and walls [7,8,9,10].

These materials are molded by an aggregate mixture, compacted and agglomerated, which is placed into a mold through mechanical pressure and/or vibration, using vacuum-assisted or open molding methods. The process currently used by industries for the compact artificial stone production is called “vacuum vibro-compression”. In this process, the mineral fillers are mixed with a polymeric resin and, afterwards, the mass is compacted under vibration and vacuum. The materials are generally subjected to a heat treatment at temperatures in the range of 70 to 110 °C, in order to promote the final curing of the resin, that is, the interlocking between the polymeric chains [11,12].

Suta et al. [13] used the method of vibration and compaction in a vacuum environment to produce a compact projected stone slab whose main solid components were glass waste and fine granite aggregate. The final product showed <0.02% water absorption and 51 MPa flexural strength, which were higher than those of natural construction slabs. The research results revealed that vibration was the most important processing step to adjust the aggregate orientation to become more compacted.

Lee and Shin [14] reported fiberglass/PET composites produced under the influence of vacuum, mold temperature, and cooling rate. The authors observed that specimens produced without vacuum during the preheating stage registered values of approximately 1.9% porosity, whereas specimens produced with the aid of vacuum recorded much lower values of 0.3–0.4% porosity.

The use of vacuum in manufacturing artificial stone plates is extremely important, since it facilitates the removal of air that, in the process of molding the composite, could be attached to the load and the mass, lowering the plate porosity degree. Once porosity harms the performance of artificial materials, the use of vacuum helps to produce better quality artificial materials [15].

1.2. Development of New Materials Based on Artificial Stones

The creation of novel materials using waste from the ornamental stone industry is economically interesting, in addition to meeting a sustainable ideology in the industrial environment. Today, many researchers use petroleum-based resins, wastes from natural stones cutting process, fine gravel particles, steel, rubber, PET, and iron ore mixed up with epoxy or polyester resin in artificial stone development, obtaining stones with superior properties when compared to natural stones [4,8,9,10,16].

Erfan [17] developed artificial stone with the incorporation of calcite marble residue and epoxy resin. The results showed 0.01% water absorption and a good filler/resin adhesion.

The use of a polymer matrix from a renewable source, which does not emanate toxic substances, is also another attractive alternative in view of the environmental impact attenuation. Gomes et al. [18] developed artificial stone with waste from natural granite cutting and polyurethane resin, and obtained an artificial stone with properties superior to natural granite.

1.3. Objective and Originality of the Research

The ornamental stone sector, which through its production and extraction generates about 40 to 60% of waste, was investigated [19]. It is estimated that there is a loss of 84% from the beginning to the final production of commercial dimension stone slabs. As Brazil is one of the largest ornamental stones producers, there is still a lack of concern to avoid waste and a neglect with the waste destination, even though companies in the sector are responsible for developing projects for their recovery and reuse [15].

Quartzite waste was obtained as raw stone, with a massive structure and coarse grain, usually cracked and milk white in color. The material is petrographically called “Quartzolite or quartzites” due to the presence of more than 90% silicon, and they are classified as “Class II B—Inert” wastes.

Vegetable oil-derived polyurethanes have been extensively studied in recent years, as a non-polluting alternative to replace petroleum-derived polyurethanes [20]. The development of new polyester polyols and prepolymers derived from fatty acids cause this polyurethanes class to present themselves with different chemical, physical, and mechanical properties, in addition to being from a renewable source [20]. It is important for the development of a new material such as artificial stone to use biodegradable natural resin and quartzite residue.

Epoxy resins are thermo-rigid materials widely used in adhesives, matrices for composites, electrical materials, and materials for coatings, among other applications, as well as in the production of artificial stones, because of their excellent adhesive, mechanical, thermal, and electrical properties [21]. For these reasons, it is important to investigate the influence of both types of synthetic and biodegradable resins on the development of artificial stone.

This new alternative, in addition to possibly reducing raw materials costs since the waste is costless (excluding its transportation costs), would also transform the waste back into raw material. The polyurethane resin used as a binder originates from castor oil, a renewable vegetable source, and together with the inert waste of natural quartzite stone, it allows the manufacture of a totally ecological and sustainable artificial stone [20].

Epoxy resin is an excellent thermoset polymer to be used as a binder in artificial stone development. The mechanical properties can be accentuated, providing an optimal adhesion to the quartz waste particles. As the epoxy resin is characterized by good chemical resistance, it will generate an environmentally harmless artificial stone with adding value to wastes that would be discarded while reducing the production costs [9].

This work’s main goal is to compare the development of artificial stones from quartzite waste, using epoxy and plant polyurethane resins as binders, with good physical and mechanical properties. This waste is also compared with the performance of our artificial stone and with other artificial stones in order to confirm the technical feasibility of producing these novel sustainable materials with economic potential.

2. Materials and Methods

2.1. Materials

The “quartzite” natural stone (NS) waste, commercially called “Cristallo”, was supplied by Pettrys company, located in Cachoeiro de Itapemirim, ES, Brazil. The raw stone material was collected directly from the company.

As a comparison material, a commercial stone (CS) called “Branco Aldan” was used. It was supplied by Empresa Guidoni, located in São Domingos do Norte, ES, Brazil. The commercial artificial stone is composed of quartz powder, additives, and polyester resin, information provided by the company.

The quartzite waste was classified by the sieves 8 to 200 mesh and divided into three different particle sizes. The larger particles (coarse) were classified in the range of 2.38 to 2.00 mm, the medium particles were between 2.00 and 0.630 mm, and fine particles were less than 0.630 mm [7,8,9,21].

The waste particles were agglutinated by two types of resins. The natural resin, vegetable polyurethane derived from castor oil, is bicomponent, that is, it results from the mixture between a component A, prepolymer (methylene diphenyl diisocyanate), with 1.22 g/cm³ relative density, and a component B, a polyol (castor oil-base) with a 0.96 g/cm³ typical density. The resin in 1:1.2 ratio of component B was supplied by the company IMPERVEG (Aguaí, Brazil). The other resin, epoxy, was a bisphenol A diglycidyl ether-MC130 and the hardener, FD129, triethylenetetramine (TETA), used in proportion of 13 wt% of resin, were both supplied by the company EPOXYFIBER (Rio de Janeiro, Brazi).

2.2. Determination of the Highest Packaging Granulometric Composition

Based on three ranges of grains obtained, 10 different mixtures with different percentages of rough, medium, and fine particles were proposed (

Table 1. Experiment matrix to determine apparent dry best-packed.

Composition	Large (Coarse)	Medium	Fine
1	0	0	1
2	0	1	0
3	1	0	0
4	0	1/2	1/2
5	1/2	1/2	0
6	1/2	0	1/2
7	1/3	1/3	1/3
8	2/3	1/6	1/6
9	1/6	1/6	2/3
10	1/6	2/3	1/6

).

Figure 1 shows a complete ternary diagram developed in the experimental numeric-modeling grid Simplex (Simplex-Lattice Design) [22] to obtain greater packaging in ternary mixtures. To determine the proportion of the greatest packaging of the 10 mixtures, all were tested based on the standard ABNT/MB-3388 Brazilian standard (1991)—Determination of the minimum index void ratio of non-cohesive [23].

The mixtures were placed in a device with a container of 1013.24 cm³ and a 10 kg piston was used on the material, which was subjected to vibration through a vibrating table. This procedure was repeated three times for each of the 10 compositions. As it can be seen in Table 1, mixture 8 (4/6 rough, 1/6 medium, and 1/6 fine particles), was the one with the greatest apparent dry density representing the most close-packed one. Consequently, it was chosen for the production of artificial stone plates.

As described by Ribeiro [11], it was necessary to calculate the minimum amount of resin (MAR) necessary for the artificial stone production, using Equations (1) and (2):

$$VV\%=\left(1-\frac{{\rho }_{PA}}{{\rho }_Q}\right)\ast 100$$

(1)

where:

VV% = Void volume present in the mixture of particles;

${\rho }_{PA}$= Apparent density of particles, calculated by the packaging method;

ρQ = Quartzite density, calculated by pycnometry.

From obtaining the void volume (VV%) value, it was possible to calculate the minimum amount of resin (MAR), through Equation (2) below:

$$MAR\%=\frac{VV\%\ast {\rho }_{resin}}{VV\%\ast {\rho }_{resin}+\left(100-VV\%\right)\ast \rho Q_{}}$$

(2)

where:

MAR% = Minimum amount of resin to fill the void volume;

$VV\%$ = Void volume present in the mixture of particles;

${\rho }_{resin}$ = Epoxy resin and polyurethane resin density;

ρQ = Quartzite density, calculated by pycnometry.

2.3. Production of Artificial Stone Plates

Artificial stone plates with a dimension of 100 × 100 × 10 mm were developed, with epoxy and natural vegetable polyurethane polymer resins, using the vacuum, vibration, and compression method. Initially, the quartzite particles were dried in an oven for 24 h at 100 °C to release moisture, then weighed and mixed with the resin in a vibration system under vacuum. Two plates were made of each resin in the dimensions 200 × 200 × 10 mm, for the hard body impact test required by the standard.

The mixture was taken to a Marcone MA 098-A hydraulic press for the plates production, with 3 MPa compression pressure and temperature of 90 °C for plates produced with epoxy resin and 80 °C for plates produced with polyurethane resin [7,24]. After pressing, the mold was disconnected from the vacuum system and cooled to room temperature to remove the plate. The plates with 85 wt% of artificial quartzite stone were developed with epoxy resin (AS-EP) and vegetable polyurethane resin (AS-PU), were subjected to a finishing step by polishing with sandpaper, and then cut with diamond disc to prepare samples for the tests according to the standards.

2.4. Characterization of Artificial Stone Plates

The apparent density and porosity as well as the water absorption were evaluated based on Annex B of the ABNT/NBR 15845 Brazilian standard, which establishes the applications of stone materials for the coating of building constructions [25].

The three-point flexural strength test was performed on our INSTRON universal testing machine, model 5582, based on F Annex of ABNT/NBR 15845 Brazilian standard [25]. The specimen’s dimensions were 100 × 25 × 10 mm and the test was performed under 0.25 mm/min loading rate, 100 kN load cell, and 80 mm distance between the two points (Figure 2).

A stone bend strength is directly related to the stone’s material porosity, structure, and texture. The test was carried out in dry conditions. The first stage consists of putting the specimens in a ventilated oven for 48 h at 70 °C and waiting 1 h for them to cool down to room temperature. Figure 3 shows the specimen before and after the bend test.

The second stage is performed by applying a slow and steady load with increment speed defined by the standards. The bend strength is calculated by Equation (3), and then the arithmetic average is calculated.

$$R=\frac{3PL}{2b{d}^2}$$

(3)

where:

R = bend rupture stress (MPa);

P = rupture load (N);

L = distance between the action cleavers (mm);

b = width of the specimen after test (mm);

d = minimum thickness of the specimen (mm).

Abrasive wear tests were performed using a MAQTEST Amsler equipment on three samples with 70 × 70 × 40 mm, according to ABNT/NBR 12042 Brazilian standard [26]. The samples’ thicknesses were measured before the wear test and then measured again after abrasive wearing suffered in 500 and 1000 m track.

The hard body impact test was performed using three 10 × 200 × 200 mm samples, according to Annex H of ABNT/NBR 15845 Brazilian standard [25]. It consists of releasing a 1 kg steel ball, at increasing heights, from 20 cm on, over the sample. The stone plate, seated on a sand mattress, receives the impacts until it cracks and rupture occurs. The height at which the material breaks occur is used to calculate the breaking energy using Equation (4).

$$W=m\ast g\ast h$$

(4)

W = breaking energy (J);

m = steel ball weight (Kg);

g = gravity acceleration (9.806 m/s²);

h = breaking height (m).

The microstructure of the fracture surface of bend-ruptured specimens was analyzed by a scanning electron microscope (SEM) in a model Super Scan SSX-550 from Shimadzu. The samples were prepared using an adhesive carbon tape enveloped by a gold surface.

Petrography, the main diagnostic technique for pathologies of stone materials, is the analysis of thin sections of stone by transmitted light microscopy, in order to identify, among other things, changes of minerals and microcracks. Using Buehler Petrothin Section System sectioning equipment, the petrographic slides of AS-EP and AS-PU were prepared. The analysis was performed by a geologist in a ZEISS polarized light petrographic microscope.

3. Results

3.1. Determination of the Highest Packaging Mixture and the Minimum Amounf of Resin

To determine the materials’ vibration density, 10 different mixtures from the three granulometric ranges were proposed, as shown in

Table 2. Results of vibration density.

Mixture	Large (Coarse)	Medium	Fine	Vibration Density (g/cm3)
1	0	0	1	1.37 ± 0.01
2	0	1	0	1.53 ± 0.01
3	1	0	0	1.13 ± 0.01
4	0	1/2	1/2	1.82 ± 0.02
5	1/2	1/2	0	1.57 ± 0.04
6	1/2	0	1/2	1.64 ± 0.06
7	1/3	1/3	1/3	1.68 ± 0.01
8	2/3	1/6	1/6	1.93 ± 0.01
9	1/6	1/6	2/3	1.87 ± 0.01
10	1/6	2/3	1/6	1.41 ± 0.01

.

The mixture with the highest vibration density determined the proportion of the waste’s granulometric ranges used to manufacture the artificial stone plates. Higher vibration density means better packing and, therefore, fewer voids, which contributes to improving the mechanical properties of the final product. Therefore, mixture number 8, with 1.93 g/cm³, which, according to the mathematical model presented in Figure 1, corresponds to 67% coarse, 17% medium, and 16% of fine particles, was chosen to manufacture the AS plates. With the optimized apparent dry density (composition number 8), the volume of voids for the load studied was calculated, and with this data, the minimum resin content for the resins was used, according to

Table 3. Resin contents of prepared stones.

Artificial Quartzite Stone	Quartzite Charge
volume of voids (%)	25.95%
Density (g/cm3)	2.609
	Epoxy Resin	Vegetable Polyurethane Resin
Density (g/cm3)	1.134	1.078
Minimum amount of resin to fill the void volume (%)	13.22	12.65
Resin content used (safety) (%)	15	15

.

3.2. Water Absorption, Density, and Apparent Porosity

Table 4. Physical properties of apparent density, water absorption, and apparent porosity of the artificial stones developed (AS-EP and AS-PU), the commercial stone and the natural stone.

Physical Properties
	AS-EP	AS-PU	“Branco Aldan” Commercial Stone (CS)	Cristallo “Natural Stone” (NS)
Water absorption (%)	0.16 ± 0.06	0.14 ± 0.06	0.05 ± 0.01	0.26 ± 0.53
Apparent porosity (%)	0.38 ± 0.15	0.31 ± 0.13	0.13 ± 0.03	0.68 ± 1.38
Density (g/cm3)	2.35 ± 0.03	2.22 ± 0.04	2.41 ± 0.01	2.63 ± 0.01

shows the apparent density, water absorption, and apparent porosity values of the artificial stones developed with 85 wt% of quartzite waste and both epoxy resin (EP), 85 wt% of quartzite waste and polyurethane resin (AS-PU), natural stone “Cristallo” (NS), and the commercial stone “Branco Aldan” (CS).

According to the results in Table 4, it can be observed that, as expected, both the artificial stones and the commercial stone had lower density than the natural stone. This is attributed to the artificial stone’s composition, consisting of low-density polymers, producing a lighter material and consequently reducing logistical costs [21]. Manufacturers of artificial stones report density varying in the range of 2.4 to 2.5 g/cm³ [9]. AS-EP and AS-PU densities were below this range probably because of the low densities of the polymer resins, epoxy 1.20 g/cm³ and polyurethane 1.08 g/cm³.

The commercial stone “Branco Alda” has 2.41 g/cm³ density that is within the range of the artificial stone’s manufacturers and superior to that of developed stones. The commercial stone uses a polyester resin with 1.18 g/cm³ density as a binder, which can be explained by changes in the values in the production process variables [21]. It is worth mentioning that these results were within the range found by Lee et al. [4], in which by varying compression pressure, vacuum, and vibration frequency, obtained an artificial stone with density values ranging from 2.03 to 2.45 g/cm³.

The porosity values of the natural stone were above the Chiodi Filho and Rodriguez [27] recommendation, which can be explained by the natural occurrence of flaws and cracks in the material. Consequently, these defects generate a mechanical strength decrease. The commercial stone obtained better values, which is directly related not only to the waste particle variables in the size distribution process but also to the manufacturing processing and to the additive added to the mixture, which promoted better adhesion and modified the resin properties [28,29].

Gomes et al. [20] developed an artificial stone with granite waste and polyurethane resin, using the same technique as the present work, and produced a stone with 0.42% apparent porosity and 0.19% water absorption, relatively higher than those of AS-PU and AS-EP. The artificial stones’ low porosity content may have contributed to an excellent adhesion of the granite particles/polymer matrix, with the voids being filled by the resin, forming a material as homogeneous as possible.

Water absorption includes liquids percolation through these voids. Therefore, once the pores are not 100% interconnected through the cracks, water absorption values will always be lower than the apparent porosity ones [30]. For the artificial stones developed, the water absorption values of 0.16% in AS-EP and 0.14% in AS-PU indicate superior properties when compared to natural stone, which justifies the use of the proposed method.

Chiodi Filho and Rodrigues [27] include the water absorption index as one of the three most important technological parameters (flexion, wear, and absorption). All stones obtained water absorption ≤0.4%, a requirement suggested for stone selection, which are considered as stone class A1, for indoor and outdoor environments, with frequent wetting and low to high pedestrian traffic for floors [31]. For applications as coating, the ASTM C615 [32] standard indicates that, for granites and marble, the value must be ≤0.4% while the NBR 15845 [25] standard indicates it must be ≤0.2%.

Borsellino et al. [31], also used epoxy resin in their research and obtained 0.16% water absorption values. Among some works reported, water absorption between 0.04–0.67% is considered an ideal range for building materials for wall and floor coverings [4,10,16,29,33,34,35].

3.3. Three-Point Flexural Strength

Table 5. Results of three-point flexural strength of the artificial stones developed (AS-EP and AS-PU), the commercial stone, the natural stone, and the pure resins.

	Three-Point Flexural Strength (MPa)
AS-EP	27.96 ± 1.86
AS-PU	10.77 ± 0.64
“Branco Aldan” Commercial Stone (CS)	56.25 ± 2.62
Epoxy Resin	93.6 ± 4.7
Vegetable Polyurethane Resin	23 ± 5.3
“Cristallo” Natural Stone (NS)	14.32 ± 1.79

presents the three-point flexural strength values of the artificial stones (AS-EP and AS-PU), commercial artificial stone, and the pure resins, epoxy, and natural vegetable polyurethane resin.

The natural PU resin has lower flexural strength (23 MPa) compared to epoxy resin (93 MPa), which may be related to the polyurethane resin degree of polymerization. However, the study of the stoichiometric quantity between the NCO/OH ratio, responsible for the degree of polymerization, which involves the main reaction sites present in the prepolymer and polyol, was not carried out. Reactions with an excess isocyanate may occur along with parallel reactions that harden the material. On the other hand, reactions with an excess of hydroxyls cause the polymer to soften through the reduction in intercrossed bonds [36], which explains why AS-PU obtained a low flexural strength compared to the other tested stones.

In their study, Mileo et al. [37] classified polyurethane as a ductile material on account of the low flexural strength and large deformation without breakage at the maximum applied load. It may presuppose an inadequate cure or the bubbles excess caused by both the presence of moisture and a plasticizing effect. This validates the natural polyurethane non-rupture, the AS-PU at the maximum applied load, and the appearance of bubbles.

Figure 4 shows typical flexural stress x strain curves, obtained from three-point flexural tests. Comparing the behavior of the two polymeric resins and of the developed artificial stones, it is possible to observe that the addition of load contributed to the material hardening. This is an expected behavior, considering that the incorporation of rigid particles in a polymeric matrix generally increases the material’s flexural modulus [4]. Maximum performance can be achieved if the polymer adhesion to the reinforcement is perfect. The stronger the matrix/particle interface, the better the mechanical properties of the developed artificial stone [38].

Figure 5 shows the confidence interval (average ± standard error) for the artificial stone flexural strength test of artificial, natural, and commercial stones.

The ABNT/NBR-15844 Brazilian standard [28] stipulates that granite stones for coating applications must have at least 10 MPa from the three-point flexural strength test. For coating applications, the ASTM C880 [39] standard indicates that, for granites and marble, the flexural strength must be ≥8.7 MPa, and the NBR 15845 standard [25] demands ≥10 MPa. Chiodi and Rodriguez [27] reported that ornamental stones with flexural strength above 20 MPa are classified as high quality for coating applications.

The AS-PU, despite the 10.7 MPa flexural strength, according to the referred standards and its specifications, can be used for civil construction. The “Branco Aldan” artificial commercial stone obtained better flexural strength, which can be attributed to the industrial and continuous manufacturing process as well as the type of additive, the granulometry, and the material used as aggregate, quartz powder, which is one of the hardest minerals (Mohs = 7).

Carvalho et al. [8] and Gomes et al. [9] developed artificial stone based in (quarry and red ceramic) wastes and epoxy resin with flexural strength values of 30 and 32 MPa, very close to those obtained in the AS-EP, of the same matrix, evidencing the influence of the polymeric binder in the final composite strength.

Gomes et al. [20] manufactured artificial stones using 85% granite particles and 15% polyurethane resin derived from castor oil, using the same methodology. The result was an 18 MPa maximum flexural strength, which was higher than AS-PU, possibly because this artificial stone may not have achieved a satisfactory cure.

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

Material	Wear Thickness Reduction (mm)
	500 m	1000 m
AS-EP	0.58	1.20
AS-PU	0.60	1.21
“Branco Aldan” Commercial Stone (CS)	0.50	1.13
“Cristallo” Natural Stone (NS)	0.15	0.35

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.

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

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. Resistance to hard body impact values.

Materials	Rupture Height (m)	Rupture Energy (J)
AS-EP	0.39	3.86
AS-PU	1.72	16.95
“Branco Aldan” Commercial Stone (CS)	>2.0	>19.62
“Cristallo” Natural Stone (NS)	0.2	1.96

). 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].

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.

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

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

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 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

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 Figure 8 and Figure 9, all minerals present in the artificial stone of quartzite are quartz.

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].

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:

• AS-EP and AS-PU developed with 85 wt% of quartzite waste are a lightweight coating with 2.35 and 2.22 g/cm³ density, respectively;
• The porosity was lower than 0.5%, classifying both AS-EP and AS-PU as high-quality lining materials for civil construction. The 0.16% and 0.14% water absorption of AS-EP and AS-PU were also lower than ≤0.4%, a suggested requirement for indoor and outdoor environments, with frequent wetting and low to high traffic on floors;
• AS-EP artificial stone achieved a bend strength of 27.96 ± 1.86 MPa, classifying AS-EP as a high-quality material, due to its bend strength being higher than >20 MPa; compared to the other studied stones, AS-EP had a bend strength twice higher than NS and 50% lower than CS. AS-PU obtained a low bend strength, evidencing a probable inadequate cure or the excess of bubbles, as shown by the SEM; through the abrasive wear test, the low thickness (<1.5 mm) reductions observed in the artificial stone (1.20 mm for AS-EP and 1.21 mm for AS-PU) indicate that both stones can be used for intense traffic floor areas. The NS thickness reduction of 0.35 mm was the lowest, possibly owing to the presence of large mineral quartz crystals in the NS. The hard body impact resistance classified AS-EP as a material for low heights and AS-PU as a material for very high heights enabling its use in areas with a high risk of falling loads, such as supermarkets, airports, and industries in general;
• The petrographic slides analysis revealed that AS-EP has the best load distribution, ensuring greater compaction and presence of microcracked grains during the process of vibration and compression. As demonstrated, the development of artificial stones from wastes of ornamental stones productive chain associated with mining and processing, in terms of its technical feasibility, is proven.

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.