Recycling technologies and applications of borosilicate glasses

Structural materials and reinforcements

Some studies focused on using BSG as part of composite materials for enhancing mechanical performance. Trusty and Boccaccini showed that stainless steel fiber reinforced glass matrix composites are feasible with recycled BSG obtained from fluorescent tubing. ⁴⁰ Ponsot et al. produced layered GC tiles from PGW and iron-rich slags by one-step firing at 900 °C. ⁴¹ The resultant materials included a porous core and dense glaze, showing improved mechanical strength (∼40 GPa Young's modulus, ∼30 MPa bending strength), near-zero water absorption, and cytocompatibility. Ventilated facades and construction applications were indicated as potential applications of these GC composites. Heriyanto et al. prepared polymeric glass composite panels, adding BSG powders below 108 µm and without remelting. ³⁸ The addition of BSG resulted in interesting mechanical properties, such as flexural strength up to 48 MPa and compressive strength around 101 MPa, comparable to natural and engineered stone products used in kitchen benchtops and floor tiles. Moreover, the addition of BSG enhanced the panel's durability and aesthetics. Oraimi et al. studied aluminum matrix composites reinforced with BSG labware. ⁴² The tensile strength increased from 70.3 MPa (for the as-cast material) to 117.3 MPa in the metal-BSG composite, while compressive strength increased from 513.7 MPa to 700.67 MPa. Similar studies were performed by Dey et al., who investigated Al2024 alloy composites reinforced with BSG particles (38–52 μm) from laboratory scraps to enhance mechanical and tribological properties. ⁴³ Using stir casting, composites with up to 9 wt% BSG showed over 30% improvement in hardness, increased tensile strength, and better wear resistance. The fracture mode shifted from ductile to mixed ductile-brittle due to the hard glass particles. These studies demonstrated an increase in the mechanical properties of the composite materials, indicating that BSGW could be used as a potential sustainable reinforcement for structural materials.

Finally, we mention the work of Pezzato et al. on the corrosion and wear resistance of 6061 Al alloy with the addition of GW, including ABSG from LCD screens, SLG, and BSG. ⁴⁴ This Al alloy is widely utilized across various fields such as aerospace and automotive industries, military and defense, construction and architecture, etc. However, in humid or maritime climates, the natural oxide layer, which confers good corrosion resistance, is destroyed, and a more resistant protection layer needs to be applied. GW was indicated as a promising and inexpensive resource, and glass particles were added to the alkaline silicate-based electrolyte used for treating the Al alloy by plasma electrolytic oxidation. LCD glass and BSG particles showed better incorporation and sealing activity than SLG; however, the LCD glass was the best material in terms of corrosion and wear resistance, as well as for microhardness.

Cement, concrete, and asphalt systems

The addition of GW to cement, concrete, and asphalts is generally considered a down-cycling process when no improved functionalities or relevant role is played by the GW. However, many studies report on the BSGW up-cycling of these materials.

For instance, Powezcka et al. ⁴⁵ and Kotsai et al. ⁴⁶ investigated the influence of BSG cullet and powder (Termisil^® and Pyrex^®) in cement composites. It has been shown that at low levels of addition (∼2%), the thermal stability and compressive strength were improved, with a maximum of 48.6 MPa after 28 days. However, a decrease in strength is observed at higher glass contents. Meanwhile, Kotsai reported three times higher pozzolanic activity for BSG than for SLG, which was explained by the higher boron and lower alkali content. Moreover, the compressive strength increased by ∼10% for low addition (2–5%) of BSG with 0.5% of synthetic silica. Ahmad et al. studied BSG as a partial replacement of aggregate in asphalt mixes and reported that 10% BSG enhances rutting and skid resistance but causes moisture sensitivity at high percentages. Figure 7 presents these results, highlighting the changes in rut depth and skid resistance associated with increasing BSG content. ⁴⁷

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Figure 7.

(top) Maximum rut depth and (bottom) skid resistance before and after polishing for asphalts containing different amounts of BSG. Reproduced with permission from. ⁴⁷

Concrete-like GCs developed using silicon carbide powder (10–40%) and BSG (60–90%) were reported by Elesho et al. ⁴⁸ The composites made of SiC (40%) and BSG (60%) sintered at 900 °C for 3 h had the highest compressive strength (34 MPa) and lowest water absorption (3.74%), meeting the standards for pavements and wall cladding requirements. The improved mechanical properties were associated with the presence of mullite crystals as the major phase. The authors also concluded that these GC composites present better properties than materials like traditional concrete used for pavements. A more general overview of GW in concrete is given by Guo et al., who emphasized how a consensus on the effects of GW in concrete has not been achieved yet, and, while some researchers reported improved properties of concretes after GW addition, others claimed the opposite. ⁴⁹ Glass particle size and percentage were indicated as the main parameters determining the effect of GW on concretes. In particular, a smaller glass particle size is beneficial because it suppresses the alkali-silica reaction expansion, thus avoiding cracking and long-term damage.

Alkali-activated and geopolymer materials

A different approach to BSGW recycling is the alkali activation, which consists of the partial dissolution of glass fine powders in a mildly basic solution of NaOH or KOH. This alkaline environment attacks the glass surface, causing it to hydrate and form a gel network at relatively low temperatures around 40°C, a process referred to as cold consolidation. ⁵⁰ This alkali-activated glass acts as a binder, enabling the incorporation of industrial residues such as quartz sand, Plasmastone, and ceramic mud. Depending on the formulation, the resulting materials range from dense bricks to lightweight foams, suitable for thermal and acoustic insulation. Figure 8 shows several examples of these materials, showing unfired products, low-temperature-fired samples, and foam structures. This processing route avoids high-temperature firing and demonstrates strong mechanical performance, offering a circular and energy-efficient alternative for GW valorization. A similar processing method was employed by Cammelli et al. ⁵¹ to prepare building materials from alkali-activated PGW and to produce conglomerates by combining it with foundry sands. The alkali-activated PGW exhibited compressive strengths above 40 MPa for pure glass binders and around 10 MPa when foundry sands were included. The strength-to-density ratio was reported to be comparable to commercial insulating lightweight concretes. Moreover, promising results were obtained regarding the stabilization of inorganic toxic pollutants, making these conglomerates a suitable alternative for building and insulating applications. Taveri et al. obtained glass-based geopolymers from fly ash and end-of-life PGW under highly alkaline conditions. ⁵² Different compositions were obtained by adjusting the ratio of fly ash to PGW glass from 70–30 to 30–70 wt%. The 70–30 material exhibited flexural strength of up to 13 MPa and has been toughened by enhancing it with cellulose fibers. To conclude this section, we mention the interesting review paper by Zafar et al. on the use of alkali-activation of seven glass wastes, including BSG, and demonstrating how this technique is a versatile method to produce porous materials and glass foams at relatively low temperatures. ⁵³

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Figure 8.

Examples of construction materials from BSG/mud mixtures: a-c) unfired product (b, c: after boiling test); d) low temperature fired materials; e) foam. Reproduced with permission from. ⁵⁰

Architectural and decorative applications

To conclude this section, we report on BSG, which has been successfully applied in architectural applications or as decorative glass. Pavlushkina et al. integrated copper and iron-rich waste products from electrochemical industries into low-melting-temperature sodium-BSG to decorate concretes. ⁵⁴ Their surfaces exhibited a smooth finish featuring strong adhesion to concrete, and bright colors even in the absence of traditional reducing agents. Although this work does not mention the use of recycled BSGW directly, similar procedures could be achieved using this GW. For example, cullets from PGW and copper-rich sludge were mixed to obtain light-absorbing-colored glasses possessing maximum transmittance in the range 460–510 nm and a strong absorption band in the range 760–780 nm, thus conferring a blue tone. ⁵⁵ These colored glasses also showed physicochemical stability, with properties similar to the parent glasses. López et al. ⁵⁶ reported on a method to recycle fiberglass made of E-glass, a type of ABSG with good insulating properties, into glass-ceramic materials suitable for architectural applications. The first step consists of thermolysis at 550°C to eliminate organics from the original fiberglass-polyester composite, and approximately 99% of the final product consists of glass. These glass fibers are then mixed with 5 wt% of Na₂O and melted at 1450 °C to form a glass; finally, GCs are obtained by heat treatment around 1013°C. Although the mechanical strength of the fibers decreases by around 80%, this method offers an alternative for creating new products such as tiles for wall cladding and flooring. On the other hand, Eliza Scholten, in her Master's Thesis, proposed utilizing high-grade BSGW for dry-interlocking cast glass components in architectural applications implemented in Casa da Música (Portugal). ⁵⁷ On a similar topic, Bristogianni et al. proposed the kiln-casting technique, ⁵⁸ a low-energy ceramic manufacturing process based on unsorted or dirty glass powder sintered between 750–1200 °C, reducing CO₂ emissions drastically and yielding decorative materials and interlocking components, see Figure 9.

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Figure 9.

(Top) Wide range of kiln-cast glasses and (bottom) interlocking systems from different GW streams. Reproduced with permission from. ⁵⁸

Thermal and sound insulation

Several studies were conducted on the conversion of BSGW to foamed glass or GCs for thermal insulation. For example, Lee showed that waste LCD glass (an ABSG) could be converted into foamed glass with outstanding thermal insulation properties characterized by a low thermal conductivity of <0.054 W/m·K and relatively high strength. ⁶⁰ Taurino et al. recycled BSG of washing machine glasses into GC foams exhibiting thermal insulating properties with very high porosity (∼78–79%) and low density, ⁶¹ while Chinnam et al. reported on porous GCs made from plasma vitrified wastes and PGW (10 wt%). ⁶² By mixing and sintering the material with distinct particle sizes at different temperatures, the authors obtained GCs with 65% porosity and ∼20 µm pore size for sintering at 950°C, suitable for thermal sound insulation applications. Cetin et al. proposed a low-cost method for producing a lightweight and thermally insulating façade from mining waste and both SLG and BSG. ⁶³ A highly porous ceramic body was created by directly sintering mixtures of mining tailings and SLG, which showed good mechanical strength due to uniform porosity and new crystal formations at 1000°C. However, water absorption remained very high, around 42%. To address this, a dense surface glaze was applied using BGS powders, and properties like color and shrinkage were improved by blending these with zircon mineral powder and vitrified waste mixtures. D’Amore et al. presented a novel method for producing insulating materials from recycled BSGW (5–20% w/v) and alginate, a natural polymer derived from seaweed, using water as solvent. ⁶⁴ First, a glass-alginate hydrogel is prepared, and then a freeze-drying process at −80°C allows a porous foam structure to be formed, see Figure 10. Unlike traditional glass foam production, this technique operates at low temperatures, making it environmentally friendly and energy-efficient. The best insulation for not-sintered glass foams showed higher sound absorption than rock wool in the medium to high frequency range (1250–3150 Hz).

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Figure 10.

(Left) not-sintered glass foam from laboratory glassware using a freeze-drying process and (right) macro structure detail. Reproduced with permission from. ⁶⁴

Assefi et al. ⁶⁵ and Zhang et al. ⁶⁶ focused on generating foamed glass from e-waste and SLG mixed with BSG. Their results determined that a blend of optimal foaming agents and sintering conditions was able to generate materials with high porosity and low thermal conductivity, appropriate for lightweight insulation applications. Figure 11 presents an example of these foamed panels before and after the foaming process, illustrating the structural changes achieved through their optimized procedures. Suzuki et al. used hydrothermal treatment for the synthesis of hydrate-based porous ceramics from blast furnace slag and BSG. ⁶⁷ Instead, Hmood et al. used the replica technique to create GC sponges from Pyrex^® waste with a maximum porosity of 65% and emphasized the promise of BSG to prepare porous structural components, even though with low mechanical strength (0.145 MPa). ⁶⁸ Ceramic foams prepared by mixing BSGW and wolframite tailings as raw materials with SiC as a foaming agent were also reported. ⁶⁹ The optimum composition produced foams with high compressive strength (3.75 MPa), minimum density, and well-regulated porosity.

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Figure 11.

As-prepared panel from LCD waste used for mechanical properties characterization before foaming (a) and after foaming (b), (c). Reproduced with permission from. ⁶⁵

Filters and membranes

The high chemical resistance and stability of BSG make it a favorable option for filtration and membrane applications. BSG fiber membranes have already been used in electrochemical cells for the separation of gases at 300°C, ⁷⁰ with evidence of long-term stability in corrosive media. Recent research focused on the use of BSGW to prepare membranes and filters, for example, Mehta et al. ⁷¹ and Elsayed et al. ⁷² explored alkali activation and additive manufacturing to produce porous foams and membranes from PGW. These were shown to have application in dye adsorption and environmental remediation, with Mehta even incorporating TiO₂ nanoparticles for photocatalysis. Ghoranneviss et al. created porous glass filters from BSG by partial sintering, producing pore diameters and porosity that are tunable for their use in filtration. ⁷³ The sample with a particle size of 40–63 µm exhibited 30% porosity and an average pore size of 5.4 µm, making it suitable for filtration applications. On the other side, Hujova et al. combined vitrified bottom ash with BSG to create chemically stable GC foams to immobilize Cr³⁺, and with potential for heavy metals immobilization. ⁷⁴ An upcycling method to produce glass microspheres from BSG medical vials using flame synthesis was reported by Larionau et al. ⁷⁵ The glass microspheres showed excellent hydrolytic stability, ideal for water filtration and as a polymer filler.

Magnetic and electromagnetic functional materials

BSG has been used extensively to create magnetic GCs and materials with engineered electromagnetic characteristics. For example, Ponsot et al. mixed PGW with metallurgical slags (made of Fe, Si, and Ca) to produce ferrimagnetic GC tiles at low temperatures (∼900–1000 °C), due to the crystallization of magnetite crystals.. ⁷⁶ The materials also possessed good porosity, water resistance, chemical durability, and the ability to withstand rapid induction heating, suggesting their use in building tiles but also as induction heating elements. Similarly, Gonçalves Avancini et al. prepared magnetite-rich nanocrystalline GCs from BSG laboratory waste and iron-rich metallurgical byproducts. ⁷⁷ Different BSG (80–55 wt%) and metallurgical (20–45 wt%) ratios were melted at 1500°C, and the GCs obtained after crystallization at 700–900°C. Their optimized sample, containing 45% metallurgical waste and heat-treated at 800 °C, exhibited high magnetic saturation (42 emu/g) with low coercivity, making them useful for treatment of hyperthermia, microwave absorption, and magnetic resonance imaging contrast agents rather than permanent magnets. Monich et al. demonstrated that electrical conductivity and electromagnetic shielding (∼8–10 dB at high frequency) were enhanced by firing PGW with municipal waste residue (Plasmastone, rich in SiO₂, Al₂O₃, CaO, and Fe₂O₃) in a nitrogen atmosphere at 800–1000°C. ⁷⁸ These Fe₃O₄-containing GCs possess high dielectric permittivity (up to 1500 at 100 Hz), therefore, they have potential applications for electronics and electromagnetic interference shielding.

The growing electronics and 5G communications demand for highly efficient microwave PCB substrates able to ensure fast and secure signal transmission. BSGW was also used to prepare low-dielectric-loss and low-cost BSG-reinforced polytetrafluoroethylene (PFTE) composites for electronic substrates.^79,80 The BSGW was milled to a particle size in the range 25–106 µm, and then added to PTFE through a dry powder processing technique. The best performance was obtained using a particle size of the BSGW of 63 µm, showing a CTE of 60 × 10⁻⁶°C⁻¹, low moisture absorption (0.02%), a tensile strength of 12.92 MPa, and a dielectric constant ε’= 2.11 and a dielectric loss tanδ = 0.0011 at 10 GHz, appointed as suitable for microwave substrate applications. Liu et al. developed cost-effective and sustainable ultralow-temperature co-fired GCs from BSGW (Pyrex^®-type) with K₂O and BaO additives, for electronic packaging applications. ⁸¹ The starting powders were synthesized through a ball-milling process and then pressed into cylindrical and rectangular shapes, and finally sintered at 650°C to obtain the GCs, with mullite and cristobalite as the major crystalline phases. The addition of K₂O provoked the crystallization of quartz with subsequent transformation into cristobalite, while BaO limited small-radius ions migration and improved the dielectric properties. The resulting GCs exhibited dielectric losses from 3.90 to 7.60 × 10⁻³, depending on the modifier addition, values much lower than that of the glass (12 × 10⁻³). Low dielectric losses are indeed important to minimize heating and signal degradation. Moreover, the CTE was adjusted to fall within the 6–9 × 10⁻⁶ °C⁻¹ range, similar to alumina, a common ceramic material used in electronic packaging. Figure 12 summarizes these CTE values, highlighting the compatibility of the recycled-glass-derived materials with standard packaging requirements.

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Figure 12.

Thermal expansion coefficients of different glasses synthesized from recycled BSG. Reproduced with permission from. ⁸¹

Luminescent and optoelectronic materials

Recycled BSG, SLG, or silica gel waste has also been used in the creation of photonic glass. Cheong et al. prepared Eu³⁺-doped Zn-BSG glasses by mixing SLG waste with ZnO and B₂O₃. ⁸² Doping with Eu³⁺ produced a high red-orange emission (612 nm) with high color purity (99.9%), suitable properties for applications in warm light LED. In a different study, the authors mixed Na₂O (20 mol%) and Bi₂O₃ (0–5 mol%) with BSGW from high-pressure sodium lamps (75–80 mol%), ⁸³ which is a kind of Pyrex^® glass. The addition of Bi₂O₃ to BSG glass disrupts the glass network and promotes the formation of NBOs.^84,85 As a result, the material's polarizability and refractive index also increase. Moreover, the optical band gap decreases while direct electron transitions are enhanced, making these glasses suitable for photonic applications such as infrared transmission, nonlinear optics, and ultrafast optical switching. Buasri et al. used recycled BSGW from labware to prepare luminescent materials doped with Dy³⁺, Sm³⁺, and Dy³⁺/Sm³⁺. ⁸⁶ The authors sintered pellets prepared by mixing the powdered materials using a microwave-assisted process at 800 W for 15 min, offering a rapid and energy-efficient fabrication method. Sm³⁺-doped samples exhibited typical reddish-orange emission with a peak at 601 nm when excited at 401 nm, while Dy³-doped glasses showed blue (483 nm) and yellow (568 nm) emissions under 388 nm excitation. Sm³⁺/Dy³⁺ co-doped glasses emitted in the white region under 388 nm excitation, demonstrating that the emission color could be tuned by adjusting excitation wavelength and dopant ratios.

The excellent chemical stability, low CTE, and high resistance to thermal shock of BSG are also relevant parameters for high-temperature sintering processes. In this respect, recycled BSG labware was used to prepare phosphor-in-glass (PiG) materials, ⁸⁷ an increasingly important type of material to develop efficient luminescent devices.^88,89 SrA_l2O₄:Eu²⁺, Dy³⁺ phosphor, one of the most studied and efficient long-lasting materials, ⁹⁰ was added to BSGW at a weight ratio of 10 wt%. These PiG materials were pressed into pellets and sintered in a conventional way in an electric furnace (700–1000 °C for 1 h) and in a microwave oven (800 W, 6–14 min). Figure 13 illustrates the preparation workflow and the resulting samples obtained by the two sintering routes, highlighting the differences between conventional and microwave processing. Only the furnace-treated samples provoked the crystallization of the cristobalite phase, while the microwave sintering inhibited it. The best performance was obtained for sintering at 800°C, while degradation of the luminescence occurred for sintering at 1000°C due to partial conversion of Eu²⁺ to Eu³⁺. Both conventional and microwave sintering produced practically the same results, the latter being a much faster and energy-efficient method. A similar study on the use of GW to prepare PiG materials for automotive lighting applications was reported by Choi et al.; however, in that case, the authors used recycled SLG instead of BSG. ⁹¹

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Figure 13.

Schematics of (a) the preparation of GW fine powder and (b) the preparation of luminescent samples by conventional heating and microwave heating. Reproduced with permission from. ⁸⁶

To conclude this section on luminescent materials from recycled GW, we mention some studies where BSGs were prepared from silica gel waste, a valuable SiO₂ source. For example, Manyum et al.^92,93 and Rittisut et al. ⁹⁴ added various RE dopants (Eu³⁺, Sm³⁺, Dy³⁺ and Tb³⁺) to borosilicate matrices, achieving tunable luminescence (orange, white, green) under UV and X-ray excitation.

Catalysis

BSG is also a valuable material for catalytic applications, both as a support for the real catalysts or as a catalyst itself. The chemical stability and high temperature resistance of BSG make it a suitable material in this field, replacing the more expensive silica glass. Early studies from Takahashi et al. explored the use of porous glass prepared from BSG as a support for nickel catalyst in the hydrogenation of olefins (1-hexenes and 1-octenes). ⁹⁵ By varying the Al₂O₃ content in the starting glass, the pore size varied in the range of 0.8 to 1.9 nm. The authors observed that porous BSG can be tailored to create catalyst supports with unique molecular shape selectivity and is therefore a valuable candidate for heterogeneous catalysis. In recent years, several studies have focused on BSGW as a robust and efficient support for metal or metal oxide catalysts. Other studies demonstrated that sodium BSG provides high surface area, porosity, and chemical stability, making it ideal for dispersing and stabilizing Cu and Ag nanoparticles.^96,97 These composites were effective in reducing nitroarenes and dyes, and in synthesizing tetrazoles, with excellent recyclability and green synthesis using plant extracts. Mehrjouei et al. used BSG as a support for immobilized TiO₂ in a falling film reactor for photocatalytic water treatment. ⁹⁸ The system showed high degradation efficiency, long-term stability, and superior performance compared to polymethyl methacrylate (PMMA) supports. Similarly, PGW was upcycled into 3D-printed gyroid scaffolds coated with TiO₂, see Figure 14, achieving complete dye degradation and maintaining high activity over multiple cycles. ⁹⁹ Javed et al. coated BSGW with iron and cobalt oxides for catalytic ozonation of textile wastewater. ¹⁰⁰ This Co-doped BSG showed the highest removal efficiency and stability, confirming the effectiveness of BSG as a support in advanced oxidation processes. Instead, Jacob et al. used porous glass as a support for Cu catalysts in microwave-assisted azide–alkyne cycloaddition. ¹⁰¹ The system achieved high selectivity and reusability, offering a green alternative to traditional supports like alumina or silica. Other authors developed a copper nanoparticle-doped SLG tablet catalyst for nitrophenol reduction. ¹⁰² The tablets, made from SLG waste, were easy to handle, highly efficient, and reusable, demonstrating the potential of GW as a direct catalytic material without the need for additional supports.

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Figure 14.

(a) Stereomicroscope images of the BSG gyroid scaffolds prepared from slurries with different solid loadings and sintered at various temperatures. (b) BSG gyroid scaffolds with a higher porosity after sintering at different temperatures. (c) Reference three-dimensional gyroid models adopted for masked stereolithography scaffolds (d) Comparison of translucency of two samples under LED light. Reproduced with permission from. ⁹⁹

Following a different approach, some studies focused on converting GW into active catalytic materials instead of using it as support for the real catalysts. Vadery et al. ¹⁰³ and Foroutan et al. ¹⁰⁴ synthesized solid base catalysts from BSG and SLG waste, respectively, for biodiesel production. These catalysts, mainly composed of sodium silicate Na₂SiO₃, achieved high yields (>99%) under mild conditions and remained effective over several reuse cycles. The processes were scalable and environmentally friendly, using waste cooking oil or chicken fat as feedstocks.

Application of BSG in biomedicine

BSG beads have been used extensively as chemically inert, uniform collectors in column studies of bacterial adhesion and transport. Studies of the ‘90 s reported on the use of 40 μm BSG beads to enable good quantification of bacterial collision efficiency, illustrating intra-population heterogeneity and ionic strength influences on adhesion.^105,106 These studies pointed out the utility of BSG for simulating porous media in modeling microbial migration, with implications for groundwater safety and bioremediation. Vera Barrios et al. synthesized a BSG compound with bacteriostatic effect against E. coli, with potential application in antimicrobial coatings. ¹⁰⁷ On the other side, Jain et al. explored Cu-doped BSG synthesized using SiO₂ (20 mol%) and CaO (24.5 mol%) extracted from rice husk and egg-shells, respectively, and adding B₂O₃ (31 mol%), Na₂O (19.5–24.5 mol%), and CuO (0–5 mol%) before melting the batch at 1200°C for 30 min. ¹⁰⁸ Hydroxyapatite (HAp) formation and biocompatibility were achieved through in vitro biomineralization when Cu-doped BSG specimens were immersed in simulated body fluid at 37 °C for up to 21 days. These materials confirmed the potential of Cu-doped BSG for bioactive and antibacterial properties; however, a precise control of the composition is necessary, because higher CuO contents compromised the mechanical performance and induced cytotoxicity.

BSG from labware and SLG from microscope slides were explored as a binder for dental casting investments, ¹⁰⁹ where the reuse of the mold was possible. Although BSG-based molds had lower expansion and compressive strength (< 2 MPa) compared to SLG (∼ 9 MPa), they were found to be adequate for use at high temperatures. On the other side, SLG resulted in better casting accuracy, transition temperature, and improved fit. BSG was also indicated as a potential reinforcing agent in bioceramic composites for bone implants. Catalgol et al. prepared BSG particles with a size less than 100 μm by milling laboratory glassware and then blended from 5 to 10 wt% of these BSG particles with bovine HAp. ¹¹⁰ The mixed powders were then sintered at 1000–1300 °C, achieving Vickers hardness (233 HV) and compressive strength (117 MPa) comparable with cortical bone, the best results being obtained for the addition of 5 wt% of BSG particles and for sintering at 1200°C. The results highlight the double advantage of BSG in mechanical reinforcement and eco-friendly recycling of materials.

Despite the interesting applications of BSG in biomedicine, the use of recycled glass faces significant technical and regulatory challenges, especially for PGW. The unique chemical composition of BSG complicates separation and remelting, and PGW often carries drug residues, coatings, labels, and adhesives that require intensive cleaning before their use, especially in biomedicine. Indeed, regulatory compliance requires PGW to meet stringent standards to ensure patient safety, including the complete removal of toxic substances, full traceability, and validated processes. There are guidelines and regulations, such as the USP <660 > on the hydrolytic resistance of glasses, as well as EMA and FDA regulations that generally restrict the use of recycled glass in direct drug contact unless safety is unequivocally demonstrated. ¹¹¹ This makes reuse for primary pharmaceutical packaging extremely difficult. To this goal, several sterilization methods are available and can be separated into chemical and physical processes, as shown in Figure 15.

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Figure 15.

Types of sterilization in the pharmaceutical industry for ready-to-use packaging. ¹¹²

Among the physical methods, dry heat is quite common, and it involves exposing glass containers to temperatures above 160°C to eliminate pyrogens and microbial contaminants. Some glass companies apply this technique to Type I borosilicate vials, ensuring compatibility with heat-sensitive biologics and vaccines while maintaining structural integrity. ¹¹³ Another method is the steam sterilization (autoclaving), which combines high-pressure steam and heat to achieve rapid and reliable sterilization. It is a fast, safe, and cost-effective solution for large-scale sterilization of PG bottles and vials. Gamma irradiation is another interesting sterilization process that applies to drugs that cannot be removed with heat, thus offering a non-thermal alternative. This process sterilizes glass containers without exposing them to high temperatures, making it suitable for pre-filled syringes and ampoules containing sensitive biologics. Moreover, this method applied to BSG is advantageous because glasses retain strength and chemical inertness during irradiation, ensuring sterility without compromising the container or glass performance. Other technologies include UV irradiation, effective for surface sterilization, which has limited penetration in the interior of the glass. Regarding the chemical methods, the use of ethylene oxide, formaldehyde, or H₂O₂ is common. Hybrid systems combining plasma and UV are also under investigation. In any case, the sterilization method must demonstrate microbiological effectiveness, compatibility with glass properties, and absence of harmful residues. Another aspect worth mentioning is related to BSG durability. Indeed, BSG can degrade over repeated cycles, especially using autoclave sterilization, due to surface scratches, which reduce tensile strength. Therefore, PGW can be sterilized, but their integrity is also an important aspect to consider, especially when vials or bottles are desired for reusing. However, for open-loop recycling, which is the most promising alternative for BSGW, the GW is usually in powder form, so fractures and mechanical defects in vials and bottles are less critical.

Composition–shielding correlations of borosilicate glasses

BSG is a reference host matrix for high-level nuclear waste in several national programs because it combines good chemical durability, thermal stability, and the ability to accommodate a wide range of radionuclides and fission products. ¹¹⁴ Its amorphous structure allows the incorporation of diverse elements, including fission products, which ceramics cannot accommodate easily due to their rigid crystal lattices and strict stoichiometric requirements. In vitrification, waste oxides are incorporated into the borosilicate network, producing a homogeneous solid that immobilizes radionuclides over geological time scales and reduces the risk of leaching under repository conditions. Moreover, the low CTE of BSG enhances resistance to thermal shock, making it a safe host for nuclear waste immobilization.

Beyond waste immobilization, BSG is also a key material for radiation shielding. Shielding performance strongly correlates with glass composition, particularly density and content of high-Z oxides. Commercial BSG products, which contain mainly low-Z elements (B, Si, Na), are insufficient for gamma-ray shielding. Therefore, increasing the fraction of heavy metal oxides, such as Bi₂O₃, BaO, PbO, or rare-earth oxides, raises mass density and the mass attenuation coefficient, reduces the half-value layer (HVL) — that is, the thickness of a material required to reduce the intensity of an X-ray or gamma-ray beam to 50% of its original value — and improves gamma attenuation.^115,116 From a structural point of view, these modifiers affect the silicate and borate network, for example, by creating NBOs and promoting the transformation of BO₃ to BO₄ units. These modifications increase the packing density, and the higher effective atomic number resulting from heavy-metal oxides explains the improved shielding.

Neutron attenuation, in contrast, depends strongly on elements with high neutron capture or scattering cross sections, such as ¹⁰B, ¹⁵⁷Gd, and certain rare-earth ions like ¹⁴⁹Sm. Therefore, boron content and specific neutron-absorbing dopants dominate the neutron shielding performance. Practical glass formulations need to satisfy both requirements, maintaining sufficient B₂O₃ (and in some cases Gd₂O₃ or similar oxides) for neutron protection while incorporating enough heavy oxides to ensure adequate gamma shielding.

Traditionally, lead-containing glasses have been used for gamma shielding due to their high attenuation, but they pose significant environmental and durability concerns. ¹¹⁷ Instead, BSG offers superior chemical durability, environmental safety, and compatibility with complex nuclear waste compositions. ¹¹⁸ While lead-based and other heavy-metal oxide glasses can achieve higher mass attenuation coefficients and thinner shields, their lower chemical stability and handling risks limit long-term applicability. Some studies show that both lead-based and lead-free heavy-metal glasses provide effective shielding across a wide photon energy range (1 keV–100 GeV), with lead-free compositions sometimes outperforming lead glasses at medium and high energies. ¹¹⁹ Optimized BSG provides a compromise: its shielding efficiency is slightly lower than that of the densest heavy-metal systems but remains sufficient for repository and interim storage conditions, while offering the corrosion resistance and compositional flexibility necessary to safely immobilize diverse high-level waste streams.

Radiation shielding applications

Several studies have examined how BSG made from GW responds structurally and mechanically to radiation exposure. As an example, ultrasonic and FTIR techniques showed that gamma irradiation induces depolymerization in barium-bismuth BSG, reducing rigidity due to increased NBOs. ¹²⁰ Instead, ElBatal et al. confirmed that slag-derived BSG and GCs exhibit high chemical durability and radiation resistance, particularly when crystalline phases like wollastonite and diopside are formed. ¹²¹ More recently, Chen et al. ¹²² demonstrated that sodium BGS undergoes systematic mechanical and structural modifications under heavy-ion irradiation, such as reductions in hardness and modulus that saturate at high doses, swelling of up to ∼4% depending on composition, and a marked increase in crack resistance. These effects were linked to a transformation of BO₄ to BO₃ units revealed by Raman and infrared spectroscopy, highlighting the key role of boron units in controlling radiation-induced changes in BSGs.

A significant body of work has focused on enhancing the radiation shielding capabilities of BSG through compositional modifications and recycling strategies. Al-Buriahi et al. demonstrated that adding Y₂O₃ (up to 20 wt%) ¹²³ or B₂O₃ (up to 60 wt%) ¹²⁴ to BSGW improves gamma and neutron shielding, density, and hardness, making these inexpensive materials suitable for nuclear safety applications, see Figure 16. Similarly, BSG glass was incorporated into metakaolin-based geopolymers, enhancing their shielding performance against various radiation sources and demonstrating that eco-friendly materials for radiation-shielding concrete in nuclear facilities are possible by recycling BSG. ¹²⁵ Quite remarkably, the addition of 30 wt% of BSG offered a better neutron shielding than some existing materials. On the other side, Kurtulus et al. ¹²⁶ and Kurudirek et al. ¹²⁷ also demonstrated that Bi₂O₃-doped PGW or BSG from high-pressure sodium lamp glasses significantly improve shielding while maintaining acceptable mechanical properties. Other authors prepared glasses mixing PGW (70 mol%) with Na₂O (20 mol%), ZnO (10 mol%), and BaO (0–25 mol%) and melting the batch at 1100°C. ¹²⁸ These high-performance shielding materials showed that BaO doping enhances X- and gamma-rays attenuation properties, the best results being obtained for 25 mol% of BaO. On the other hand, Kim et al. showed the benefits of LCD GW in enhancing thermal neutron shielding of cements. ¹²⁹ From 10 to 20 wt% of GW was added to the powder mixture, replacing ordinary Portland cement. While no relevant improvements on fast neutron shielding were observed, higher contents of the LCD glass resulted in lower thermal neutral counts, which means an effective shielding against low-energy neutrons. Lee et al. ¹³⁰ reported on the use of Duran^® BSGW as a fine aggregate. Powders with an average size of approximately 13 μm were incorporated into cement mortar mixtures, demonstrating enhanced thermal neutron shielding and improved compressive strength through the pozzolanic reactions of the finely ground glass powder.

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Figure 16.

(top) Different glass samples for radiation shielding applications made from recycled BSG and with different proportions of boron oxide (wt%) (a) 0% (BB), (b) 40% (BB-40), (c) 50% (BB-50), and (d) 60% (BB-60). (bottom) Comparison of fast neutron removal cross-section of the BB glasses with other neutron shielding materials. Reproduced with permission from. ¹²⁴

The reuse of industrial and municipal waste in BSG production for nuclear applications is another relevant strategy to address waste revalorization. In some cases, BSGs were prepared by a combination of recycled materials, such as SLG or municipal solid waste (MSW), with the addition of specific compounds such as B₂O₃ or heavy metals. For instance, Humaid et al. developed lead-BSG glass, adding B₂O₃ to SLG obtained from vehicle and PbO obtained from battery waste, showing that increasing PbO content significantly boosts shielding efficiency, outperforming commercial materials and reaching a maximum fast neutron removal cross section of 0.124 cm⁻¹ for a PbO content of 40 mol%. ¹³¹ Hussein (2024) et al. prepared BSG by the addition of B₂O₃ (20 wt%), SrO (20 wt%), and Na₂O (10 wt%) with CeO₂ and Gd₂O₃ dopants to MSW, obtaining higher mass attenuation coefficients and better structural stability under gamma irradiation for the Ce-Gd system. ¹³² The starting MSW was first burned at 750°C for 3 h to remove organic compounds, and then the batch was melted at 1450°C. The good shielding properties of these glasses against gamma irradiation were attributed to the highly compacted and rigid network of these glasses. Additional studies confirmed the viability of using MSW ash and BSG powder in shielding applications.^133,134

Cesium immobilization

Cesium and actinides immobilization in BSG matrices is a key concern in nuclear waste management. Hamodi et al. compared ABSG and graphite-glass composites, finding that ABSG offers superior Cs⁺ retention and corrosion resistance under thermal leaching. ¹³⁵ Even though the ABSG reported in this study was synthesized in a laboratory, its composition is quite close to some commercial PGW, making this residue a valuable option for nuclear materials, despite some slight compositional modifications are necessary. In a different study, Lago et al. introduced a low-temperature method for Cs⁺ immobilization (up to 20 wt%) using CsOH alkali activation of PGW with a solid loading of 60 wt%, resulting in the formation of the boro-pollucite phase (CsBSi₂O₆), a very stable phase for Cs⁺ immobilization. ¹³⁶ Additional sintering at 700°C for 1 h further enhanced the stabilization of Cs⁺ due to its immobilization in the glass phase and resulting in a leaching below 0.482 ppb.

Recovery of critical elements and raw materials

Critical Elements (CE) are substances that are essential for modern technologies and economic functions, but also face a risk of supply disruption, and their recovery is becoming an important field of research. ¹³⁷ Among CE, rare-earth elements (REE) are extremely important because they are among the most technologically relevant elements for modern applications, and for several of them, the supply risk is high, at least in the EU.^138,139

Jiang et al. developed a two-step hydrometallurgical process to recover La, Y, and Gd from spent BSG optical glass ¹⁴⁰ provided by Hiraki Glass (Japan). The method uses NaOH and HCl treatments to convert REE BSG into soluble RE chlorides, achieving nearly 100% recovery. The authors state that this method is simpler and more convenient than mineral-based extraction and applicable to ophthalmic glass and optical fibers. In a different study, a phase separation technique by remelting colored SLG with B₂O₃ to produce BSG was used to recover metals from GW. ¹⁴¹ Heavy metals like Co and Cr concentrate in the borate phase, which is leached with nitric acid, leaving behind high-purity porous silica glass and allowing metals to be recovered. A similar procedure was applied by Xing et al. to cathode ray tube glass, using B₂O₃-induced phase separation to recover Ba and Sr with over 98% efficiency. ¹⁴² The process also produces high-silica porous glass and enables partial recovery of B₂O₃ from the leachate (Figure 17). Although BSG was not directly used, this study highlights the importance of these glass compositions in promoting phase separation and facilitating the recovery of critical elements.

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Figure 17.

Photographs of the phase separation products and corresponding acid leaching residues obtained with different additional amounts of B₂O₃ using a temperature of 1100°C and a holding time of 30 min. (a) 10%; (b) 20%; (c) 30%; (d) 40%. (e)–(h) are the corresponding photographs of (a)–(d) after leaching of the BSG. Reproduced with permission from. ¹⁴²

Other authors recovered indium, an important CE, from LCD panels by leaching GW with oxalic acid and thermally converting the extract into InBO₃ nanoparticles. ¹⁴³ In a deep analysis on critical raw materials from the ceramics industry, García-Ten et al. indicated the importance of BSG as a source of boron, thus making BSG a valuable waste not only for improved applications but also as a source from which CE can be recovered. ¹⁴⁴ Indeed, boron compounds are difficult to replace with other fluxing raw materials due to toxicity issues (e.g., lead) or because alternatives like bismuth and strontium are listed as CE by the EU. Therefore, the authors indicate that the best candidates for boron substitution are BSGW. A study by Sullivan Porras et al. reported on the preparation of hydrophobic sand from BSGW and palm oil sludge fatty acids. ¹⁴⁵ The sand reduced water infiltration and hydraulic conductivity, making it suitable for landfill covers and water harvesting. Finally, Lee and Park reported a procedure to extract elemental silicon from crushed BSG via magnesium reduction and then mixed with carbon black. ¹⁴⁶ The as-obtained Si/MgO/C composite demonstrated superior reversible capacity (∼487 mAh/g) and cycle stability, indicating its great potential as an environmentally friendly anode material for lithium-ion batteries. While the main goal was to extract elemental silicon from BSG glasses, other components of the glass, such as B₂O₃ and Al₂O₃, are not removed during processing and remain in the final composite as lithium-inactive oxides. These oxides play a beneficial role by acting as structural buffers that mitigate volume expansion during cycling, thereby enhancing the mechanical stability and long-term performance of the electrode.

Borosilicate glass waste as raw materials for the ceramics and glass industry

A different approach, involving the use of BSG or ABSG wastes as fluxing agents or structural components in the ceramics and glass industries, has been explored by other authors. For example, Koca et al. replaced potassium feldspar with BSGW in porcelain, reducing sintering activation energy from 166 to 43 kJ/mol. ¹⁴⁷ The fluxing oxides (B₂O₃ and Na₂O) promote vitrification and lower energy consumption. Instead, Kim demonstrated that up to 11% LCD cullets can be added to flint and low-iron flat glass without significantly affecting viscosity, liquidus temperature, or optical properties. ¹⁴⁸ The cullet improves water resistance and light transmission. Still on the same residues, LCD glasses were used to replace feldspar in sanitaryware ceramics. ¹⁴⁹ The sintered bodies showed dense microstructures, low water absorption, and stable thermal expansion, thus indicating these ABSG as useful resources. The application of LCD glasses in glass wool production was also studied, and up to 20% replacement was feasible, with no harmful leaching and 6% energy savings. ¹⁵⁰ Lassinanti Gualtieri et al. incorporated 41 wt% GW, made of 34% BSG and 7% SLG, into porcelain stoneware tiles, lowering firing temperature by 140 °C and reducing energy consumption. ¹⁵¹ The tiles met industrial standards and showed reduced residual stress. Instead, Rozenstrauha et al. combined sewage sludge and ABSG with illitic clay to produce porous and dense ceramics, and up to 25% sludge addition improved porosity, water absorption, and bulk density. ¹⁵² In a separate study, tube light and BSGW were added to opaque glazes, enhancing opacity, whiteness, and chemical resistance, with higher values obtained at 15 wt% glass addition. ¹⁵³ Zircon crystals were the major phase, along with minor contents of wollastonite and diopside. The crystallite size decreased with glass addition, and surface crystallization dominated. The authors also concluded that the addition of GW can play a relevant role in enhancing the glaze's properties at low cost.