Innovation in the Use of Recycled and Heat-Treated Glass in Various Applications: Mechanical and Chemical Properties

Abstract

By decreasing manufacturing costs for different civic purposes, glass recycling is an economically significant technology that also helps conserve natural resources and mitigates environmental problems. Throughout the recycling process, this study used recycled domestic glass in compliance with European guidelines for recycling of household garbage. The purpose of this research is to examine the chemical and mechanical properties of recycled and crushed glass with particle sizes varying from 0.1 mm to 2 mm as a function of various treatment temperatures. This might pave the way for novel building materials, artwork, and interior design components, among other potential uses. “Silica glass”, the most common and ancient kind of glass, which includes SiO_k, Na_kO, CaO, and small amounts of other elements, was utilized in the investigation. Several materials can be successfully modified or altered using step heat treatment. The mechanical and chemical properties of recycled and shattered glass were assessed using microhardness, compressive, and chemical testing. These samples were then compared to mosaics from Murano, Italy, and Dynasty Smalti, China. The recycled and heat-treated glass produced microhardness values of 550.6 HV and 555.0 HV, respectively, when tested with forces of 0.981 N and 2.942 N. These values were higher than those of Murano (Italy) and were comparable to those of Dynasty Smalti mosaic (China). Furthermore, compression testing demonstrated that tempered and heat-tempered glass, which might include up to 5 g of TiO₂, could endure compressive strains of up to 16 MPa. This is in sharp contrast to Dynasty Smalti, which could only withstand tensions of 6–8 MPa, and Murano, which could only withstand stresses of 3–4 MPa. Tests conducted chemically over a seven-day period using KOH at 30 g/L and 100 g/L, along with HCl at 3% and 18%, showed that the samples did not alter in any way; their surface, color, and weight were untouched. Crushing and heating recycled glass makes it a viable alternative to using new glass in civil engineering projects. This helps make material reuse more efficient, which in turn helps the environment. Sturdy and resilient in a variety of contexts, the material shares mechanical and chemical properties with standard mosaics.

1. Introduction

The present study aims to obtain a recycled and heat-treated glass material that can replace traditional materials such as Murano (Italy) and Dynasty Smalti Glass Mosaic (China, DGM), which could represent a major step towards sustainability in design and construction. This solution reduces pollution and minimizes resource consumption and offers an economical, sustainable, and local alternative, adaptable to contemporary demands for quality and environmental responsibility. The implementation of this type of material can significantly contribute to the development of an efficient circular economy in interior and exterior landscaping, as well as in numerous civil applications.

1.1. Source of Prime Material

From a global perspective, waste is a growing concern, impacting economies and ecosystems alike [1,2,3]. Landfilling garbage has become a major environmental problem because of the large amounts of waste produced annually [4]. People view garbage dumps as scarce resources. Furthermore, a great deal of natural resource consumption occurs in the transportation industry, which includes road building and related industries. One way to lessen the impact on the environment is to use recycled materials in building projects. This category includes things like asphalt, concrete, ceramic tiles, thermal insulation, environmentally friendly pavements, and more. Commonly used materials for pavement include aggregates of sand, gravel, crushed stone, asphalt, concrete, and other naturally occurring building components. Global CO₂ emissions attributable to end-use energy production accounted for 36% of the total in 2018 [5]. A third of all man-made greenhouse gas emissions came from the building sector. Research into environmentally friendly substitutes for conventional building aggregates with comparable performance is urgently needed to keep the planet habitable and healthy [6]. Most people use glass and have grown accustomed to it since it is an ingredient in so many different kinds of products. It may morph into a table, a phone screen, a computer screen, a glass bottle, a home or car window, or any number of other objects. Because of this, people all over the globe consume a lot of glass, which means that some of it goes to waste and adds to the annual production of almost 130 million tons of glass chips [7]. Furthermore, data show that recycling rates are inadequate. Approximately 40 million tons of waste glass are generated annually in the United States—equal to 28 percent of that nation’s glass production, and 13 percent of the glass produced in mainland China. Approximately 90% of Australia’s annual glass use in 2018 and 2019 came from glass packaging, which accounted for more than 1.21 million tons of glass. At the same time, the recycling rate in Australia was 60%, and the projected amount of waste glass produced was 1.16 million tons [8]. Landfills have become overrun with waste glass, which is harmful to wildlife and plants because of the massive amounts produced, low recycling rates, and non-degradable nature of glass.

Table 1. Percentage of recycled glass in different countries [7,8,9,10,11,12].

Nr.	Country	RCG%
01.	Belgium	96%
02.	Switzerland	94%
03.	Luxembourg	93%
04.	Netherlands	91%
05.	Norway	89%
06.	Germany	82%
07.	Italy	74%
08.	France	67%
09.	United Kingdom	61%
10.	Spain	57%
11.	USA	33%

displays the amount of waste glass generated in different countries, whereas Figure 1 shows the percentages of recycling and disposal that have been accomplished.

Statistical analysis of Romanian packaging from 2009 to 2022 shows that of the 2.4 million tons of packaging that were “put on the market” in 2022 (POM), 1.5 million tons (or 61% of the total) were recycled, and 4% were recovered through other means (VAM) (Figure 2). The Romanian Packaging Industry Association is the source of these data. The total amount of glass packaging sold in 2022 was 463,400 tons. Recyclables accounted for 65 percent of the total. With any luck, Romania will have recycled 60% of its glass by 2022, up from 40% in 2008. One sector that has the potential to significantly reduce the problematic accumulation of glass trash at disposal sites is the civil engineering and building industry. In addition to helping the environment by reducing the amount of glass that ends up in landfills, this industry also helps engineers use fewer raw materials from mines, which is beneficial for the environment [9,10,11,12]. Large quantities of glass may be required for some projects in this sector. There are several different types of glass aggregates, including crushed glass, waste glass, recycled glass, powdered glass, foam glass, medium recycled glass, coarse recycled glass, and geopolymers made of recycled glassGlass cullet, sometimes called crushed recycled glass, is a byproduct of glass recycling plants that has been mechanically crushed to particles no larger than 4.75, 9.50, or 19.00 mm in size. In contrast, glass powder is composed of manually crushed glass to a consistent consistency; the exact size of the crushed glass depends on its intended application. One common way to make glass-based geopolymers is to combine glass powder with an acidic solution.

The glass powder’s rich silica environment aids in the geopolymerization process when using this approach. Using recycled glass as a partial replacement for up to 10% of the natural aggregates in the mixes is also unlikely to significantly reduce their performance [13]. This advantage is due to the fact that recycled glass is much less dense than natural aggregates. Because it doesn’t harm the environment too much, RCG is a fantastic choice for supplementary materials in many different kinds of projects [14]. Adding up to fifteen percent crushed recycled glass to cement-treated RCG mixtures may improve their performance, according to certain reports [15]. Due to its hydrophobic properties, waste glass’s maximum dry density is not very sensitive to variations in relative humidity. Several studies have indicated that it is feasible to use up to 30% recycled glass in products while still fulfilling all requirements [16,17].

Recycled fine glass, when mixed with waste rock aggregate at the ideal rate of 15% by dry mass, provided the right amount of strength to be used in pavement foundation courses and was very workable [2]. Pavement sub-base layers showed potential when mixed with crushed recycled concrete at the same rate [18].

Researchers have looked at how well a geopolymer made of recycled glass powder (RGP) treats clay soils [19]. Research has found that the characteristics of RGP geopolymer and how well it can stabilize clay soils depend on things like the temperature at which it is made, how long it is mixed, and the ratio of the alkali solution used. Since RGP contains a lot of silicon, it works well for the alkali–silicon reaction (ASR) in RGP-based geopolymer cement (Figure 3). The country is also making great strides toward reaching its 2025 targets, which include a recycling rate of 70% [2,11]. All studies conducted must adhere to the notion of sustainability if researchers in the current world are to be believed. By uniting in purpose, we can preserve our planet’s precious and rapidly depleting natural resources for the benefit of future generations. Not only does this approach aid in the preservation of these resources, but it also benefits the environment via reduced emissions of greenhouse gases, which in turn provide a plethora of other beneficial environmental impacts.

1.2. Advantages Derived from the Recycling of Glass Trash

The first step in the creation of glass materials and techniques is often the extraction of sand in enormous quantities. Glass may theoretically be recycled an infinite number of times without experiencing any degradation in quality. Before it can serve as a feedstock for the glass industry, cullet undergoes numerous labor-intensive and time-consuming stages. These procedures consist of cleaning, dirt removal, and separation. This poses a serious obstacle to the repurposing of cullet in glassmaking [20,21,22].

Glass recycling would immediately benefit the environment by reducing landfill strain and conserving natural resources. It would also have indirect benefits, such as extending the life of industrial machinery like furnaces, which would result in financial and energy savings. Additionally, using recycled glass might lessen the amount of raw materials that must be carried, thereby reducing strain and wear on transportation routes [22]. The results of Edwards and Schelling’s investigation show that using recycled glass instead of raw materials lowers the furnace temperature required, which saves energy. Specifically, recycling glass instead of disposing of it in a landfill may minimize the toxic effects of glass manufacture and disposal by at least 30%. Glass can be recycled, which saves money and keeps valuable materials out of landfills. Similarly, a life cycle assessment (LCA) of glass containers in North America was conducted by the Glass Packaging Institute. They found that two million and two hundred thousand metric tons of carbon dioxide would be removed from the atmosphere if glass were recycled at a rate of fifty percent. This amount of waste is equivalent to removing around 400,000 cars from the road for a whole year and eliminating the CO₂ emissions that go along with them [23].

2. Materials and Methods

2.1. Properties of Glass

2.1.1. Chemical Properties

Glass is available in a diverse array of shapes, colors, and tints, each of which has its own distinctive chemical composition. A significant factor working against recycling of glass is the fact that the chemical properties of various colored glasses vary. This fact can impede the repurposing of glass.

Table 2. Analysis for glass powder [26].

Compound/Element	SiO2	Al2O3	Fe2O3	CaO	Na2O	MgO	K2O	SO3
Content (% weight)	72.58	1.47	0.85	10.49	12.54	0.61	0.4	0.2

delineates the chemistry that underpins several of the most frequently encountered commercial glass types [24,25].

An amorphous substance, glass is a solid structure that is formed by the rapid melting and cooling of a combination of metal oxides in its most basic form. Glass is generally resistant to chemical attack and does not react with the majority of commonly used chemicals. Glass is an optimal material for applications that necessitate chemical resistance, including packaging and laboratory instruments. This advantage is due to its aversion to chemical reactions.

2.1.2. Physical Properties

Table 3. Chemical composition of soda-lime glass [25,26].

Type	SiO2	Al2O3	CaO	MgO	Fe2O3	Na2O	K2O	TiO2	SO3
Window	71.71	1.26	8.44	4.16	0.09	13.61	0.4	0.07	0.25
Containers	66–75	0.7–7	6–12	0.1–5	-	12–16	-	-	-
Light bulbs	73	1	5	4	-	17	-	-	-
Sheet	71–73	0.3	8–10	1–3.5	-	12–15	-	-	-
Float	73–74	-	8.8	3.7	-	13–15	0.2	-	-

provides a comprehensive inventory of the many physical properties of crushed waste glass. Bulk density, often called loose bulk density, is found by dividing the mass of a substance by its volume. The shape index, expressed as a percentage of the total dry mass of the particles examined, is the mass of particles with a length-to-thickness ratio higher than three. Gathering the aggregate retention percentages on each sieve in a certain order and dividing the sum by 100 is one way to get the fineness modulus [27]. One measure of particle flakiness is the mass-to-particle ratio of particles with a minimum diameter less than three-fifths of the mean. The fractional form of this proportion is used [28,29,30].

2.2. Raw Materials

2.2.1. Obtaining Recycled and Crushed Glass

The aim of this research is to investigate how treatment temperatures affect the mechanical and chemical properties of recycled and crushed glass with a grain size of 0.1 mm to 2 mm and its ability to produce new materials that are used in different sectors of civilian life, such as interior design, arts, and construction. For the study, the first material used was ‘silica glass’. which is the most widespread and oldest type of glass. It is based on SiO₂, which is the main component of sand (about 75%), Na₂O, Na₂CO₃, CaO, and other minor additives. Glass recycled from household and industrial waste, in accordance with current European standards, was used to carry out the first heat treatments. It was then sorted and cut using a medium-sized crusher produced in Italy, known as Grntr 60 glass. The material obtained had a grain size between 0.1 mm and 2 mm. In the experiment, recycled and crushed glass was obtained by the processes of collecting, sorting, washing, cleaning, and crushing. In the first step of the experiments, 100 g of recycled and crushed glass were added or mixed with between five and ten grams of metal oxide or ceramic pigment

Table 4. Sample preparation.

S/NR	SRC/g	SRC/mm	PC/PO	Ct/g
P1	100 g	<1 mm	-	0 g
P2	100 g	<1 mm	TiO2	5 g
P3	100 g	>1 mm	TiO2	10 g
P4	100 g	>1 mm	CK11020	5–10 g
P5	100 g	>1 mm	CK33100	5–10 g

.

2.2.2. Sample Preparation

Sample preparation is a crucial step in the process of making materials for heat treatment and evaluating physicochemical characteristics. This process includes multiple steps to make sure the mixture is well-blended and the pigment is evenly spread, resulting in a final product with qualities like high strength, stable color, and better mechanical performance.

After preparing the glass and pigment, their mixing follows. We combined the crushed recycled glass (100 g) and the pigment (5–10 g) in a clean container and performed manual mixing. It is important that the mixture is homogeneous and the pigment is evenly distributed in the glass mass to avoid the formation of areas with higher pigment concentration, which could lead to color or texture variations in the final material.

Once the mixture of recycled glass and pigment was completely homogenized, it was poured into a mold with a size of 5 × 5 cm to create the samples that would undergo heat treatment. The recycled glass and pigment was evenly distributed in the mold, ensuring that each sample had the correct size and thickness to allow for effective heat treatment (Figure 4).

It is important that the samples are placed correctly in their molds to prevent them from deforming or cracking during the sintering process.

2.2.3. Three-Stage Heat Treatment

Three-stage heat treatment is an essential process in the processing of recycled and cullet glass, designed to improve the physicochemical properties of the material. This process controls the crystal structure, the lifespan, and the behavior of the glass under intense use conditions, such as in construction or mosaic applications. The heat treatments are carried out in three specific stages: gradual heating, holding at a constant temperature (sintering), and controlled cooling.

In the first stage, a specialized furnace gradually heats the recycled cullet glass to a controlled temperature. The purpose of this stage is to avoid thermal shocks that can lead to the material’s cracking. The initial heating temperature is approximately 350 °C to 450 °C, and the heating is done slowly, at a rate of approximately 5–10 °C per minute, until the desired temperature for the sintering stage is reached.

This stage has the purpose of removing moisture from the glass and allowing uniform expansion of the particles without generating internal stresses. In this phase, the material is still far from the melting temperature, and the goal is to prepare the glass for crystallization without causing premature melting.

After the glass has reached the desired temperature (around 600 °C to 900 °C), the glass is maintained at a constant temperature for a period of 30 to 120 min, depending on the type of recycled glass and the process specifications. The goal of sintering is to allow the glass particles to fuse, forming a more homogeneous mass, but without reaching the complete melting point of the material.

This stage has the purpose of controlling the crystallization process of the glass. Depending on the temperature and duration of sintering, the desired crystalline phases are formed, which improve the mechanical and thermal properties of the material. During this period, the pigment added to the glass mixture will begin to react with the glass matrix, giving the material the desired color and stability under high-temperature conditions.

The third stage, after sintering, is the controlled cooling stage, which is essential to prevent the formation of internal stresses and to maintain the structural integrity of the material. Cooling should be gradual, at a rate of approximately 5–10 °C per minute, until the glass temperature drops below 200 °C. This can be done in an oven or in air, depending on the equipment available.

Slow cooling is important to prevent the formation of cracks or fissures in the material, which can occur due to temperature differences between the internal and external layers of the glass. This step also allows for the final crystallization of the glass and ensures a homogeneous and compact structure.

The three-stage heat treatment of recycled and cullet glass is a controlled process that includes gradual heating, high-temperature sintering, and controlled cooling. These steps are essential for obtaining superior mechanical and chemical properties of the treated glass, such as greater resistance to thermal and mechanical shocks, color stability, and greater durability in industrial and civil uses. Depending on the specific parameters (temperatures and durations), the process can be adjusted to meet the requirements of the material’s end applications.

The SRC samples were subjected to a number of thermal treatments in stages to achieve the formation and transformation of crystalline phases at different basicity values and sintering temperatures.

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PT1. Maximum temperature of 750 °C with a baking threshold of 60 min;

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PT2. Maximum temperature of 850 °C with a baking threshold of 60 min;

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PT3. Maximum temperature of 890 °C with a baking threshold of 60 min;

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PT4. Maximum temperature of 900 °C with a baking threshold of 60 min;

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PT5. Maximum temperature of 890 °C with a baking threshold of 120 min Figure 5.

2.3. Determination of Microhardness by the Vickers Method

The Vickers hardness determination method uses a diamond indenter shaped like a right pyramid with a square base and an apex angle of 136°. The method is similar, in principle, to the Brinell method, consisting of the following: Analogous to Brinell hardness, Vickers hardness (HV) is determined by relating the loading force F to the lateral surface area of the trace S (considered to be a right pyramid with a square section with diagonal d), the calculation formula being the following:

$$\mathrm{HV}={\displaystyle \frac{2F\sin \frac{136°}{2}}{d^2}}=1.8544{\displaystyle \frac{F}{d^2}}$$

The tests performed to determine microhardness by the Vickers method were performed on three samples with a force of 1 N for 15 s, as follows:

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Sample 1—crushed recycled and heat-treated glass (SRCT)

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Sample 2—Dynasty Smalti Glass Mosaic—China (DGM)

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Sample 3—Murano—Italy

The data obtained from the tests performed on the three samples are presented in

Table 5. Data obtained from microhardness tests using the Vickers method.

	Test Force	0.981 N	(100 gf)
	Dwell	15 S
Part ID	S 1			S 2			S 3
Nr	D1(um)	D2(um)	HV	D1(um)	D2(um)	HV	D1(um)	D2(um)	HV
1	16.85	18.21	603.7	15.82	15.65	748.6	19.06	19.91	488.6
2	18.04	18.38	559.4	15.82	17.53	666.9	18.89	18.55	529.3
3	19.57	19.40	488.6	16.68	16.50	673.8	19.06	19.91	488.6

and Figure 6. as follows: a—SRCT, b—DGM China and c—Murano Italy [30].

2.4. Determination of Compressive Strength

To determine the compressive strength and normal stresses, the samples of size 10 × 10 mm were used, and the compressive force had a speed of 1 mm per minute. The tests were performed with a universal machine model WDW-50E. The data obtained are displayed in

Table 6. Data obtained from the determination of compression and normal stresses during deformation.

Probe	DSG_1	DSG_2	Murano_1	Murano_2	SRC_C1	SRC_C2	SRC_T1	SRC_T2
MPa	30	4	16	24	24	58	160	120
Forta (N)	2500	3500	1800	2450	2450	5850	16,000	12,000

and samples are shown in Figure 7. We conducted the following tests on four samples:

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DSG_1—Dynasty Smalti Glass Mosaic—China (DGM)

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DSG_2—Dynasty Smalti Glass Mosaic—China (DGM)

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Murano_1—Murano 1—Italy

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Murano_2—Murano 2—Italy

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SRC_C1—Crushed and Heat-Treated Recycled Glass (SRCT)—containing 5 g CK33100

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SRC_C2—Crushed and Heat-Treated Recycled Glass (SRCT)—containing 5 g CK33100

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SRC_T1—Crushed and Heat-Treated Recycled Glass (SRCT)—containing 5 g TiO₂

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SRC_T2—Crushed and Heat-Treated Recycled Glass (SRCT)—containing 5 g TiO₂ Figure 8.

3. Results

3.1. Analysis of Recycled Glass

Analyzing substances using a variety of techniques and methodologies to determine their composition and qualities is known as chemical analysis. Energy dispersive X-ray spectroscopy (EDS) is a tool we use to determine a material’s chemical composition. It is crucial to conduct this research in order to determine if the chemical makeup of recycled and broken glass is comparable to that of other types of glass, like the renowned Murano glass or China DGM glass, the latter of which can differ in composition based on raw materials and production standards. A test may identify individual chemical components of glass and provide extensive information on the glass’s chemical composition using energy dispersive spectroscopy (EDS). Silica, sodium oxide, calcium oxide, magnesium oxide, and a plethora of other metal oxides make up the bulk of glass’s basic makeup. Even though they are present in trace amounts, these components may be very useful in determining the glass’s quality or determining the source. Iron, aluminum, potassium, and other oxides of these elements are possible. You may learn a lot about the chemical similarities and differences between recycled and shattered glass, Murano glass, and Chinese glass by comparing their EDS findings. This is crucial for determining the quality of recycled glass and its possible uses in many sectors, such as the manufacture of decorative or practical glassware [30].

The EDS analysis of broken and recycled glass showed that it had a composition closely comparable to Murano and China glass, which is a good sign for the recycled material’s quality. Recycled glass has a chemical makeup that is closely comparable to virgin glass, meaning it may be used for the same purposes as virgin glass without lowering the standards or performance of the end product (Figure 9 and Figure 13).

The angular planes of recycled and crushed glass are essential in the process of synthetization and heat treatment, significantly influencing the behavior of the material during processing and, therefore, its final properties. These angular planes are formed due to the crushing and grinding process, and their role in the thermal and chemical process is crucial for optimal results (Figure 9, Figure 10, Figure 11, Figure 12 and Figure 13).

• Increasing the efficiency of the heating process: During heat treatment, the presence of angular planes in the crushed glass particles allows a more uniform heat distribution. These angles facilitate a faster and more uniform heat transfer between the particles, reducing the time needed to reach the desired temperature in the treatment furnace. This leads to greater efficiency of the sintering process and reduces the risk of thermal defects.
• Optimization of melting and crystallization process: Angular planes favor the formation of better-defined crystal structures during the heat treatment process. These angles influence the nucleation and growth of crystals in the material, having a direct impact on the thermal and mechanical behavior of the glass. In applications that require high thermal shock resistance, such as mosaics or building materials, the angles favor a more uniform distribution of crystallins, thus improving the stability and durability of the resulting material.
• Improved mechanical properties: Another important aspect is the impact of angular planes on the mechanical properties of heat-treated recycled glass. The presence of these angles helps to increase the hardness and strength of the final material. Angular planes help to distribute internal stresses more efficiently during the forming and cooling process, resulting in a material that is more resistant to mechanical shocks and external stresses, making it more suitable for construction and industrial applications.
• Reduced porosity and improved homogeneity: Angular planes influence the way glass particles settle and bind together during the melting and crystallization process. These angles help to reduce the porosity of the final material as the particles interconnect more effectively, forming a more compact and homogeneous structure. This contributes to a denser and more uniform material with improved mechanical properties.
• Control of chemical reactions: In the process of synthetization and heat treatment, the angular planes of recycled and crushed glass can also influence the chemical reactions that take place at the surface of the material. These angles can increase the active surface area of the glass particles, which facilitates their interaction with the chemicals used in the treatment process, thus improving the stabilization and functionalization of the material. These chemical reactions are essential for obtaining outstanding properties of the final material, such as corrosion resistance or stability under extreme temperature conditions [30].

3.2. Ceramic Pigments and Metallic Oxides Used in the Experimental Section

The compatibility of metal oxides and ceramic pigments used for recycled and crushed glass is a crucial factor in achieving durable and aesthetically pleasing materials. When recycled glass is used in combination with metal oxides and ceramic pigments, it is essential to consider the chemical and physical interactions between these substances to ensure color stability, material durability and material behavior under industrial and environmental conditions of use.

Metal oxides, such as iron (Fe₂O₃), copper (CuO), chromium (Cr₂O₃), cobalt (CoO) and manganese (MnO), are commonly used to color glass and to modify certain optical properties of glass. These substances can interact with recycled glass components during the melting and heat treatment process.

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Melting temperature: Metal oxides must be compatible with the melting temperature of the recycled glass. Glass has a variable melting temperature, typically between 750 °C and 1000 °C, and metal oxides must be stable at these temperatures in order not to cause undesirable reactions or the formation of unstable secondary phases.

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Formation of crystalline phases: Some chemical reactions between metal oxides and glass compounds can lead to the formation of crystalline phases instead of amorphous phases. This may affect the transparency or texture of the glass and may influence the strength of the material.

Ceramic pigments, which are used to give recycled glass intense and durable colors, are often combined with metal oxides to produce specific visual effects. Ceramic pigments are often high-temperature stable metal oxides and may include oxides of chromium, copper, cobalt or iron.

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Color stability: Ceramic pigments need to be stabilized in the glass matrix to prevent color change during heat treatment. This can be achieved by choosing pigments that have a stable crystalline structure and do not react with the glass components at high temperatures.

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Thermal compatibility: Ceramic pigments must have thermal expansion coefficients compatible with recycled glass. If the coefficient of expansion of the pigments is too different from that of the glass, this can lead to cracks or warpage in the final material.

In combination with recycled glass, metal oxides and ceramic pigments can influence the mechanical properties of the final material, such as hardness, impact strength and abrasion resistance.

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Hardness: Iron and chromium oxides can contribute to the hardness of recycled glass, making it more resistant to scratching and abrasion.

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Shock resistance: In contrast, some transition metals, such as copper and manganese, can introduce internal stresses into the glass, which can affect shock resistance, especially at varying temperatures.

Recycled glass usually contains sodium silicate (Na₂O-SiO₂), calcium oxides (CaO) and other compounds that can react with metal oxides and ceramic pigments.

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Reaction with alkalines: Metal oxides containing aluminum or calcium can react with alkaline compounds in the glass (such as Na₂O or K₂O), resulting in the formation of insoluble silicate that can affect the clarity and transparency of the glass.

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Formation of secondary compounds: Under certain temperature conditions, metal oxides can form secondary compounds that affect the aesthetic quality or physical properties of recycled glass.

The compatibility of metal oxides and ceramic pigments with recycled and crushed glass is essential to obtain a quality end-material with improved physico-chemical and optical properties. Their correct choice can ensure color stability, durability of the material and its performance in industrial applications such as the production of mosaics, ceramic tiles or other decorative products. It is also important to monitor chemical reactions and thermal compatibility to prevent deficiencies or defects in the final product.

In this study, three types of oxides were used to evaluate the compatibility and effects on recycled and crushed glass, especially in the process of synthesis and heat treatment. Titanium oxide (TiO₂) and two ceramic oxides, CK11020 and CK33100, were chosen due to their specific properties that influence both the coloring and stability of the final material.

Titanium oxide (TiO₂) is a ceramic pigment known for its high thermal stability and ability to produce intense and durable colors, particularly in shades of white and opacity. In this context, TiO₂ was used to evaluate its ability to improve the optical properties of recycled and crushed glass. TiO₂ is also often used to impart a high degree of opacity to materials, which can be beneficial in design and architectural applications, such as mosaics or handicrafts (Figure 14).

In the heat treatment process, TiO₂ is relatively inert and stable at high temperatures, making it ideal for use in combination with recycled glass without negatively influencing its structure. This oxide can also help improve the UV resistance of the material, preventing premature degradation of the glass’s color and texture.

Ceramic Oxide CK11040 is a pigment that belongs to the category of ceramic metal oxides and is commonly used to achieve vibrant colors and stability in ceramics and glass. Depending on the concentration and sintering temperature, CK11020 often provides a specific color effect.

We chose CK11040 in the present study to observe its effects on recycled glass, particularly in terms of color stability following heat treatment. This ceramic oxide is very stable chemically, and we looked at how it interacts with the recycled glass to see how it behaves during the high temperatures involved in glass sintering and crystallization.

CK11040 can also contribute to the formation of fine crystals, which improve the texture of the material and can provide greater durability in glass end-use applications such as decorative mosaics or building materials (Figure 15).

Ceramic oxide CK33100 is another important material in this study, used to evaluate the compatibility with recycled glass and its ability to influence its crystalline structure during heat treatment. CK33100 is recognized for its stability at high temperatures and its ability to control the crystallization process in glass, which is essential in obtaining materials with improved mechanical performance.

This study used CK33100 to observe how this oxide contributes to the sintering process of recycled glass. The study also looked at how this oxide affects the strength of the final material, including its ability to withstand pressure and impacts, due to the formation of stronger and more uniform crystal structures. The application of CK33100 in heat treatments demonstrated an improvement in the material structure, making it stronger and more durable under intense use.

In summary, using titanium oxide (TiO₂) and the ceramic oxides CK11020 and CK33100 when making and heating recycled and crushed glass led to better materials, particularly in terms of color stability, durability, and strength. These oxides worked well with the recycled glass, helping to create stable crystal structures and keeping the final material strong when heated. Thus, these combinations of oxides can be successfully used in the production of mosaics, building materials, or decorative products, having a positive impact on their quality and performance (Figure 16).

For X-ray diffraction (XRD) examination of glass (especially heat-treated recycled glass), the typical range of angles (2θ) is chosen depending on the objective of the analysis—identification of crystalline phases or confirmation of amorphous structure. The standard XRD scanning range for glass is 2θ = 5°–80° → 2θ = 5°–80° → it is the most common range used for general characterization, allowing the detection of most common crystalline phases (silicates, oxides, carbonates, etc.), covering the areas where the most relevant diffraction maxima occur.

The X-ray diffraction (XRD) tests on the four samples of heat-treated recycled glass showed that they all have the same mixed structure, which includes both non-crystal and crystal parts, with no major differences among them.

Analysis parameters were as follows: scanning range: 2θ = 5°–80°; constant analysis conditions: 40 kV, 30 mA, 0.02° step, time/step 1 s. All diffractograms show a broad amorphous halo centered around 2θ ≈ 28°, characteristic of the vitreous structure of the glass. Sharp diffraction peaks are superimposed on this amorphous background, indicating the presence of well-defined crystalline phases. Peaks were repeatedly identified at positions 2θ ≈ 22°, 26.5°, 31°, and 50°, corresponding to calcium and sodium silicate, potentially identified as wollastonite, quartz, and other stable oxides. The heat treatment applied to recycled and crushed glass did not lead to significant crystallization differences. The results obtained by XRD analysis suggest that the heat-treated recycled glass possesses an identical composite structure consisting of an amorphous matrix and a stable crystalline phase. This structural constancy is a positive indicator of the uniformity of the material, essential for its applicability in industrial products such as mosaics, decorative elements, and façade components, where predictable mechanical and chemical behavior is required. This structural consistency confirms that the recycled glass-based material can be produced in a standardized way with repeatable properties, representing a sustainable and reliable alternative to traditional Murano or Dynasty Smalti Glass Mosaic (China, DGM) (Figure 17).

3.3. Analysis of Heat-Treated SRC Samples

After looking at the heat-treated SRC samples, we found that the best temperature for creating and changing the crystal structures in heat-treated recycled glass is 890 °C, lasting for 120 min, with a grain size of 0.15–2 mm, and using 5 g of pigment for every 100 g of SRC. This suggests that these heat treatment conditions are effective in promoting the crystallization and phase transformation processes in the material. It is also important to note that the samples heat-treated at a temperature of 750 °C, with a threshold of 60 min, did not have a complete process of crystalline phase formation, and the desynthesis temperature was observed. This may indicate that the temperature and/or time of heat treatment were not sufficient to promote complete crystallization of recycled glass under these conditions (Figure 12). SRC samples with a pigment quantity of 10 g/100 g experienced visible plastic changes and cracks (Figure 18 and Figure 19).

The temperature of 900 °C can be high enough to promote the crystallization and sintering process in glass. If desired, one could use this heat treatment to induce the formation and transformation of crystalline phases in the material. Heat treatment at high temperatures can enhance the mechanical strength and hardness of SRC. These properties can be useful in applications that require more durable and resistant materials. At the same time, it contributes to the elimination of impurities and defects in the glass. Through processes such as melting and restructuring of the material, it can improve the homogeneity and purity of the heat-treated glass but can affect ceramic pigments that have a lower use temperature. Heat treatment at high temperatures can also influence the thermal conductivity and thermal expansion of SRC. These changes can be useful in applications involving thermal insulation or materials’ thermal behavior (Figure 20).

SRC samples heat-treated at a temperature of 890 °C, a threshold of 120 min., and a grain size between 0.15 and 2 mm are more compact and more homogeneous. This finding can be associated with several aspects related to the structure and composition of the material, as well as the heat treatment process itself. Glass particles with a finer grain size may tend to compact better during heat treatment. This can lead to the formation of a denser and more compact structure in the heat-treated materials, a more uniform distribution, and better orientation in the material matrix, contributing to the formation of a more homogeneous and uniform structure within the material. Finer grain size can contribute to a more uniform distribution of impurities and defects in the material, which can improve the homogeneity and overall quality of the heat-treated glass (Figure 21).

Thanks to electron microscopy (100×, 200×, 500×, 1000×, 5000×) for SRCT (crushed and heat-treated recycled glass) samples at 890 °C, with a time limit of 120 min and a size range of 0.15–2 mm, it can be seen that impurities or other unwanted materials are excluded from the material’s structure, which lowers porosity and makes it more uniform, while also getting rid of gases and volatile substances from the material, helping to reduce pores and compact the material (Figure 22) [30].

3.4. Compressive Strength Analysis

After the heat treatment process, recycled glass can become strong enough to resist forces up to 16 MPa because of the way its internal structure is improved and crystals are formed. This heat treatment allows the material to become much more compact and homogeneous, reducing porosity and forming a stable crystal network. The compressive strength of 16 MPa indicates that this type of glass is much more resistant to mechanical stress compared to traditional glass or those used in artistic mosaics or industrial applications.

The compressive strength of traditional glass (such as that used in windows, glasses, or other household applications) is relatively high compared to other mechanical properties of this material. The compressive strength of traditional glass is about 1000–1500 MPa (megapascals, equivalent to 1000–1500 N/mm²). Glass has a very high compressive strength but is very brittle under stretching and impact due to its amorphous nature and lack of plasticizing mechanisms (it does not have dislocations like metals).

This strength value is significantly higher than that of other types of glass used in various industries, demonstrating that heat-treated recycled glass can successfully replace conventional materials in a wide range of applications, from civil construction to decorative mosaics, where durability and strength are essential.

Comparing these three types of glass, it is clear that heat-treated recycled glass is a superior choice in terms of mechanical strength. While Murano glass, with a strength of 2.5 MPa, and DSG glass, with 3.5 MPa, are particularly suitable for decorative uses or in applications where aesthetics are more important than physical strength, 16 MPa heat-treated recycled glass is much more robust and durable, suitable for a wide range of industrial and civil applications where shock and compression resistance are a decisive factor.

Following microscopic analysis of SRC_T compared to Murano and DGM-China, it is observed that it is a compact and homogeneous material that has a higher structural strength, making it better able to withstand mechanical loads and stresses. This property can be particularly important in applications where reliability and durability are essential. The absence of cracks and pores suggests that the composition of the material is uniform throughout its mass. Such characteristics can be crucial for ensuring consistency and predictability in manufacturing processes and in the final product’s performance. The absence of cracks and pores indicates an efficient and well-controlled manufacturing process. Such results can lead to reduced costs and more consistent and high-quality production. SEM images were taken at magnifications of 200×, 500×, 1000×, and 5000× (Figure 23).

3.5. The Study Aims to Compare the Chemical Resistance of SRC_T Against Murano Glass and DGM-China

For the comparative study of chemical resistance, we used two solutions of HCl (hydrochloric acid) and KOH (potassium hydroxide). Each of them has distinct properties and key roles in various chemical processes, including materials treatment, cleaning, and chemical synthesis. HCl is a strong, colorless or slightly yellow acid with a characteristic odor. It dissolves completely in water, forming an acidic solution that is used in numerous industrial and laboratory applications. KOH is a strong, water-soluble base that forms alkaline solutions. It is an ionic compound containing potassium ions (K⁺) and hydroxide ions (OH⁻). HCl and KOH are both highly reactive chemicals but have opposite properties: HCl is an acid, and KOH is a base. In combination, they can neutralize each other in a typical neutralization reaction, forming water (H₂O) and salt (potassium chloride, KCl). This type of reaction is the basis of many chemical processes in industry and laboratories, including the pH adjustment of solutions or the synthesis of various chemicals (Figure 24).

HCl and KOH are important for treating materials, including studying how well materials like recycled glass can resist chemical damage.

Regarding studies on recycled glass, HCl is frequently used to analyze the chemical behavior of glass and to clean its surfaces of impurities. On the other hand, KOH is used in certain chemical treatment processes to modify the.

Testing the chemical resistance of materials, especially against acidic and alkaline solutions, is an essential parameter for evaluating their durability in aggressive environments. In this study, we looked at how well recycled and thermally treated glass (SRC_T) can resist damage from hydrochloric acid (HCl) solutions at 30% and 18% concentrations, and potassium hydroxide (KOH) solutions at 30 g/L and 100 g/L. Comparing the results obtained with those of Murano glass and DGM-China (industrial-grade glass from China) helps establish the performance of SRC_T compared to traditional materials.

For testing how well they resist chemicals, SRC_T, Murano, and DGM-China samples were placed in HCl and KOH solutions at the specified strengths for 7 days. The tests were performed at room temperature, and the structural and compositional changes of the bottles were monitored by visual and microscopic analyses, but also by measuring weight changes and mechanical strength (

Table 7. The 3% HCl solution—7 days.

Samples	Samples/g	Samples/g
MURANO	0.686	0.686
SRC_T	0.666	0.667
SRC_C	0.513	0.512
DGM-China	0.539	0.539

,

Table 8. The 18% HCl solution—7 days.

Samples	Samples/g	Samples/g
MURANO	0.594	0.595
SRC_T	0.643	0.653
SRC_C	0.528	0.528
DGM-China	0.600	0.600

,

Table 9. The 30 g/L KOH solution—7 days.

Samples	Samples/g	Samples/g
MURANO	0.619	0.619
SRC_T	0.795	0.796
SRC_C	0.595	0.594
DGM-China	0.470	0.468

and

Table 10. The 100 g/L KOH solution—7 days.

Samples	Samples/g	Samples/g
MURANO	0.394	0.394
SRC_T	0.717	0.717
SRC_C	0.618	0.618
DGM-China	0.401	0.402

).

3.5.1. Testing with 30% and 18% HCl

HCl solutions at concentrations of 30% and 18% were used to simulate the aggressive acidic conditions that could affect materials in various industrial environments. All three types of glass—SRC_T, Murano, and DGM-China—showed a similar reaction to these solutions. After 7 days of exposure, all materials underwent minor changes in the surface structure, with no visible signs of cracking or significant mass loss.

Even though the 30% HCl concentration had a slightly stronger effect on all three types of glass, no significant differences were observed in their behavior, and their chemical resistance remained relatively constant throughout the test. In the case of the 18% HCl solution, all materials remained stable, without showing any notable changes in physical or chemical structure.

3.5.2. Testing with KOH: 30 g/L and 100 g/L

Regarding testing with KOH solutions, both at the concentration of 30 g/L and 100 g/L, all three types of glass—SRC_T, Murano, and DGM-China—reacted in a similar way. After 7 days of exposure, all materials showed high resistance to alkaline solutions without suffering significant structural damage. Surface changes were minor, and mass losses were almost imperceptible.

In the case of the 100 g/L KOH solution, all three types of glass remained stable and did not show cracks or fissures, indicating a similar resistance to strong alkaline solutions. Also, at the KOH concentration of 30 g/L, no notable differences were observed between the tested materials, all remaining stable and unchanged.

Chemical resistance tests showed similar results for all three types of glass—SRC_T, Murano, and DGM-China—when using 30% and 18% HCl solutions and 30 g/L and 100 g/L KOH. All tested materials showed comparable resistance to aggressive chemical agents, with no significant differences in chemical behavior or structural changes. These results suggest that, in terms of chemical resistance, SRC_T, Murano, and DGM-China can be considered equivalent, having similar performances under conditions of exposure to strong acids and bases.

4. Discussion

Recycled glass is playing an increasingly important role in modern industry due to the ecological and economic benefits it brings. Recently, the use of recycled glass has become common practice in many areas, from construction and interior design to the production of packaging materials and decorative products. These applications are based on the significant advantages of glass recycling, which contribute to reducing the consumption of natural resources, saving energy, and reducing carbon emissions.

The comparative study between recycled glass, Murano glass, and DGM China glass provided a detailed insight into the mechanical and chemical behavior of each type of material, highlighting both the advantages and limitations of each under various test conditions. The tests were carried out to evaluate their performance in terms of compression, microhardness, and chemical resistance, essential parameters for their use in industrial and decorative applications.

In terms of compressive strength, thermally treated recycled glass (SRC_T) demonstrated superior performance, with a much higher strength than Murano glass and DGM China. SRC_T achieved compressive strength values of up to 16 MPa, while Murano and DGM China recorded significantly lower values, around 2.5 MPa and 3.5 MPa, respectively. These differences highlight the fact that thermally treated recycled glass is much more suitable for applications involving intense mechanical stress, offering much greater durability compared to traditional glass.

In terms of microhardness, measured by the Vickers test, the heat-treated recycled glass showed a significantly higher hardness compared to Murano and DGM China. The microhardness tests showed that SRC_T reached values of 555 HV, which indicates a much higher resistance to abrasion and wear, making it an excellent material for use in demanding conditions. In contrast, Murano and DGM China recorded much lower microhardness values, with Murano glass having a hardness of almost 250 HV and DGM China 330 HV, indicating a weaker performance in terms of resistance to scratches and mechanical damage.

Chemical resistance tests showed similar results for all three types of glass—SRC_T, Murano, and DGM-China—when using 30% and 18% HCl solutions and 30 g/L and 100 g/L KOH. All tested materials showed comparable resistance to aggressive chemical agents, with no significant differences in chemical behavior or structural changes. These results indicate that SRC_T, Murano, and DGM-China have similar chemical resistance and perform alike when exposed to strong acids and bases.

The findings from this study show that thermally treated recycled glass (SRC_T) is very important for uses that need strong and durable materials, like in building, cars, and making long-lasting mosaics. In contrast, Murano glass, with lower strength, remains a preferred material in decorative and luxury applications, where aesthetics play a much more important role than mechanical performance. DGM China glass, although more durable than Murano, does not come close to the performance of SRC_T in terms of compression and chemical resistance.

In conclusion, thermally treated recycled glass (SRC_T) was shown to be a superior material in terms of compression strength, microhardness, and chemical resistance compared to Murano glass and DGM China glass. These results indicate that SRC_T is an excellent material for applications requiring durability and high mechanical performance, while Murano and DGM China may remain suitable options for decorative applications, where mechanical requirements are not as stringent. Future research could explore further ways to improve the performance of heat-treated recycled glass, including through combinations with other materials, to expand the areas of applicability and increase the durability and efficiency of the recycling process.

The study was based on a limited number of recycled glass samples, which may restrict the generalizability of the results. To fully validate the conclusions, it is necessary to extend the testing to a wider range of compositions and particle sizes. Murano mosaic and DGM China may exhibit significant variations in composition and quality depending on the manufacturing batch, the technology used, and the origin of the raw material. Comparison with a limited set of samples in these categories may introduce a degree of subjectivity. The mechanical and chemical resistance of recycled glass was evaluated in the laboratory under controlled conditions. Performance in real environments (e.g., freeze-thaw cycles, UV exposure, varying humidity, heavy foot traffic) requires long-term durability testing. Although the study focused on technical performance, aesthetic appearance and chromatic diversity—essential criteria in decorative applications—were not analyzed in detail, although they influence the decision to use the material in design projects. Although the crushing and heat treatment process is feasible at laboratory or pilot scale, industrial scale-up may face challenges related to temperature control, grain uniformity, and mass production costs.

5. Conclusions

Crushed and heat-treated recycled glass is proving to be a technologically, economically, and environmentally superior alternative to Murano mosaic and DGM China. By combining technical performance with sustainability and low cost, this material can successfully replace traditional products in construction, design, and sustainable art applications.

• Crushed and heat-treated recycled glass has a significantly higher compressive strength (over 1600 MPa) than Murano mosaic (600–800 MPa) and DGM China (400–700 MPa). This makes it more suitable for applications exposed to intense mechanical stress, such as floors, facades, or street furniture.
• The recycled glass samples proved to be resistant in acidic (HCl 3–18%) and basic (KOH 30–100 g/L) environments, while Murano and DGM mosaics were visibly affected in the same conditions. This characteristic extends the field of use of recycled glass in industrial premises, bathrooms, swimming pools, and corrosive environments.
• The heat treatment applied to recycled glass stabilizes the microstructure, reducing the risk of micro-cracking or accidental breakage. The end product is thus more durable and safer than traditional mosaics, which can be fragile or sensitive to thermal variations.
• The use of household or industrial waste glass, together with a standardized heat treatment process, allows for significant cost savings compared to Murano mosaics (handmade) or even mass-produced Chinese products.
• Unlike conventional mosaics, recycled glass valorizes existing waste and actively contributes to reducing pollution, CO₂ emissions, and consumption of natural resources. This makes it a key material for sustainable architecture and the circular economy.