Porous Glass for Thermal Insulation in Buildings with a Focus on Sustainable Materials and Technologies: Overview and Challenges

Abstract

In response to environmental challenges and primary resource scarcity, sustainable approaches that rely on recycling and reusing waste materials are becoming valuable and highly appealing options in modern society. This paper deals with the usage of porous glass and glass-ceramic products derived from waste in the field of thermal insulation in buildings. After providing an overview of the current state of the art with a focus on existing commercial products and related manufacturing methods (foaming strategies), this review discusses the emerging trends toward greener approaches, including the use of by-products or waste substances as foaming agents (e.g., eggshells or mining residues), the use of vitrified bottom or fly ashes from municipal solid waste incinerators as starting materials, the application of surface treatment to reduce post-processing temperatures, and the promise of additive manufacturing technologies in this field. The increased use and spread of sustainable practices are expected to significantly contribute to glass recycling, to minimize landfilling, and to generally reduce energy consumption as well as greenhouse emissions.

1. Introduction

The use of properly designed thermal insulating materials in residential and commercial buildings not only contributes to create a more comfortable living and working environment, but it has also been widely recognized as a conceptually easy strategy to reduce energy losses. This material-based, “inherent” (or “passive”) type of thermal insulation is useful for potentially reducing energy losses over the whole year while maximizing the efficiency of cooling and heating “active” systems (plants) [1]. Furthermore, the use of proper thermal insulating materials in construction carries two additional advantages, i.e., (i) the decrease in running cost of electricity consumed by cooling and heating systems, and (ii) the reduction of the fixed costs related to the initial installation of cooling and heating plants [2]. It is worth underling that, besides in carbon emissions, with a positive impact on the environment [3]. In this regard, electricity generation stands as the top contributor to global CO₂ emissions. According to the 2023 report published by the Energy Information Administration [4], electricity generation accounts for about one-third of global carbon emissions, and residential consumption of purchased electricity is forecast to increase between about 14 and 22% from 2022 to 2050 across all cases—including the heating and cooling domestic systems reaching between 5.9 and 6.3 quadrillion British thermal units (quads).

Therefore, the development of thermal insulating materials able to tackle the above-mentioned challenges will be of utmost importance over the next few decades.

Many types of thermal insulating materials for buildings are available on the market and/or have been experimentally proposed; according to their nature, they are typically classified in six major groups (Figure 1) [5]:

(1)

Organic materials;

(2)

Inorganic materials;

(3)

Metallic or metallized reflective membranes;

(4)

Aerogels;

(5)

Thermal insulators from waste;

(6)

Composite materials.

It should be underlined that such a classification, although being commonly accepted, suffers from some overlaps among the groups. For example, just to mention a few, materials in class (4) formally belong to class (2) (because aerogels are actually silicate materials); class (5) materials can encompass materials from classes (1) (e.g., natural waste polymers, like vegetal fibers from agri-food industry) and (2) (e.g., waste glass or vitrified bottom/fly ashes from municipal solid waste incinerators (MSWIs)).

If we want to adopt a simplified categorization, most of the available thermal insulating materials for building can substantially fall under the following basic types: inorganic materials (e.g., glass/rock wool, ceramic and glass products), organic materials (natural polymers such as cellulose, cotton, wood, cane, etc., and synthetic polymers such as polystyrene, polyethylene, polyurethane, etc.), and composites. Polymers are generally known to be good thermal insulating materials due to their relatively stable physical and chemical properties as well as versatility of processing and shaping. However, mechanical properties are low and make these materials unsuitable for concurrent load-bearing applications, which require improvement by the addition of inorganic fillers to enhance strength or the design of composite products [6].

There are also some advanced thermal insulating materials/systems, including the vacuum insulation panels (VIPs), aerogels, and nanomaterials like nanocellulose, which show great promise for use in building applications. These materials, however, still need to be optimized as they suffer from various drawbacks such as low mechanical properties, high cost, and potential toxicity. Major limitations of all VIPs include high cost, lack of flexibility, and possible changes in thermal conductivity under severe boundary conditions. In fact, air and moisture diffusion through the barrier envelope and into the core of the panel can lead to an irreversible increase in thermal conductivity over time, which is otherwise very low (0.004 W/(m·K)) [7]. VIPs have a long service life in perfect conditions (>50 years) but also exhibit a fragile nature and require protection against perforation of the foil, which carries additional costs.

Aerogels are produced using the sol–gel method and exhibit a unique microstructure of nano-sized particles with mesopores having diameters ranging from 2 to 50 nm and ultra-high porosity (>99 vol.%). Aerogels can also be used as a core for VIPs [8]. The thermal conductivity of aerogels was found to be quite low, ranging between 0.017 and 0.04 W/(m·K) [9]. Despite these attractive properties, the widespread commercial use of aerogels is limited due to the high production cost. The expensiveness is mainly due to the cost of raw materials (high-purity chemicals) and supercritical drying processes, the latter also carries a healthy risk for operators. On the other hand, advancements in aerogel production technologies as well as the push toward mass production suggest that aerogel thermal insulating products will be fully competitive with conventional thermal insulating materials in the coming years.

Some studies have also been performed to design thermal insulating materials starting from agri-food and industrial waste. This approach has some important added value. First, cost reductions of the thermal insulating materials can be inherently achieved by using natural materials and/or wastes as a part of the main matrix. Second, this also implies an obvious reduction of CO₂ emission. Finally, a possible solution to the waste management problem is found by reusing the waste materials as an alternative to combustion or landfilling [10]. Different wastes like agri-food residues, textile waste, scrap rubber, and waste paper can be blended together and structured into cheap products for thermal and even acoustic insulation, with performances comparable to those of commercial options. Specifically, the use of nanomaterials, like nanocelluloses prepared from wood biomass, is another increasing trend in the design of thermal insulating materials [11]. A key challenge to tackle in the case of biopolymer-based thermal insulating materials is their poor resistance to fire and their moisture sensitivity. Furthermore, mechanical properties are often unsatisfactory if these materials are used alone.

Although the above-mentioned advanced solutions for thermal insulation in buildings are indeed fascinating and promising, there is still room for improvement in the field of more conventional materials, too. Within the class of inorganic materials for thermal insulation, glass and glass-ceramic foams provide a unique combination of attractive properties, including low density and low thermal conductivity that are comparable to those of insulating polymers, accompanied by higher durability and mechanical properties (especially strength) [12]. Using recycled or, more generally, glass derived from waste is highly appealing in the circular economy approach. It helps to reduce the consumption of energy and resources as well as the greenhouse emissions associated with the production of “primary” glass by melting. In fact, it was estimated that 16.9 MJ of waste heat and 0.57 kg of CO₂ are produced for the processing of 1 kg of clear glass [13].

This review article aims at providing a comprehensive picture of the state of the art and the challenges ahead related to “sustainable” porous glass products for thermal insulation in construction. Porous glass-ceramics obtained, for example, through sinter-crystallization of the waste-derived parent material are also considered. A comparison with the properties of ceramic materials used for the same purpose will also be reported for the sake of completeness.

2. Factors Affecting the Functional Properties

2.1. Porosity and Porous Microstructure

The relationship between the pore structures and the key properties of porous glass is a complex interplay that significantly influences thermal conductivity, mechanical strength, and fire resistance. Porous glass, characterized by its own pore size, porosity, pore shape, and distribution, exhibits varied properties based on these structural parameters. Understanding these relationships is crucial for optimizing porous glass for specific applications.

With regard to thermal properties, higher porosity generally leads to lower thermal conductivity due to increased air content, which acts as an insulator. However, the size of the pores also plays a critical role. Smaller pores can reduce thermal conductivity more effectively than larger ones, as they disrupt heat flow more efficiently [14,15].

The shape and distribution of pores affect the path of heat flow. Spherical and evenly distributed pores tend to lower thermal conductivity more than irregularly shaped or unevenly distributed pores [16,17].

In general, porous ceramics may exhibit a wide range of thermal conductivity values, influenced by factors such as porosity, pore size, and material composition (see also Section 2.2). Some quantifications are presented in Table 1. Porous silicon carbide (SiC) ceramics, for instance, have thermal conductivities ranging from 37.9 to 5.8 W/(m·K)) as porosity increases from 30% to 63% [18]. It should be noted that SiC has a significantly higher conductivity compared to the majority of ceramics, which makes it an exception in this class of materials. On the contrary, the thermal conductivity of porous anorthite ceramics is notably low, between 0.018 and 0.13 W/(m·K), with porosities ranging from 69% to 91% [19]. Similarly, porous mullite ceramics demonstrate thermal conductivities as low as 0.09 W/(m·K) at high porosities of 73 to 86 vol.% [20]. Porous alumina ceramics, prepared using starch as a pore-forming agent, show a thermal conductivity that decreases with increasing porosity, fitting a modified exponential relation [21]. Porous yttria-stabilized zirconia (YSZ) ceramics achieve ultra-low thermal conductivities as low as 0.06 W/(m·K), with porosities between 52 and 76 vol.% [22]. The thermal conductivity of fibrous ceramics decreases from 0.18 to 0.06 W/(m·K) as porosity increases from 73 to 90 vol.% [23]. These variations highlight the critical role of porosity and microstructural characteristics in determining the thermal properties of porous ceramics, with higher porosity generally leading to lower thermal conductivity across different ceramic types [24,25,26].

Focusing on the applications in construction, foamed glasses and cellular ceramics are widely used not only for thermal but also for acoustic insulation in buildings due to their high porosity, which significantly reduces thermal conductivity and enhances sound absorption. The porosity of these ceramic- or glass-based construction materials typically ranges from about 60 to over 96 vol.%, depending on the specific type and preparation method. For instance, high closed porosity foamed ceramics prepared from coal gangue waste exhibit a porosity of 83.95 vol.%, which is beneficial for reducing thermal conductivity to 0.11 W/(m·K) [27]. Similarly, Al₂O₃ hollow glass sphere foam ceramics achieve a porosity above 94%, resulting in a thermal conductivity as low as 0.0244 W/(m·K), classifying them as superinsulating materials [28]. Glass-ceramic foams produced from zeolite-poor rock and eggshells show a density range from 0.54 to 1 g/cm³, with thermal conductivities between 0.07 and 0.4 W/(m·K) [29]. High-entropy ceramic foams exhibit porosities between 90.13 and 96.13 vol.%, with ultralow thermal conductivities ranging from 0.0343 to 0.0592 W/(m·K) [30]. For sound absorption, cellular ceramic foams with porosities of about 60–75 vol.% are optimized for frequencies between 200 and 4000 Hz, demonstrating that higher porosity generally improves sound absorption performance [31]. Lightweight ceramic foams with porosities from 74.1 to 83.7 vol.% also show enhanced sound absorption, with larger pores providing superior performance [32]. These examples illustrate the versatility and effectiveness of foam glasses and ceramic foams in building insulation applications, with porosity being a critical factor in their thermal and acoustic properties.

Furthermore, it should be pointed out that the fire resistance of porous glass is closely linked to its thermal insulation properties. Materials with lower thermal conductivity due to high porosity and small pore sizes can better resist heat penetration, enhancing fire resistance [14,33].

With regard to mechanical properties, increased porosity typically results in decreased mechanical strength, as the solid matrix supporting the structure is reduced. However, the presence of smaller, well-distributed pores can mitigate this effect by providing a more uniform stress distribution [34,35]. Smaller pores can enhance mechanical strength by preventing the coalescence of cracks, while larger pores may act as stress concentrators, leading to failure under load [33,34]. Highly interconnected pore networks can compromise mechanical integrity, whereas isolated pores may help maintain strength [36,37].

Table 1. Thermal conductivity and porosity values of selected porous glass-based and ceramic materials.

Material	Thermal Conductivity (W/(m·K))	Porosity (vol.%)	Reference
Porous silicon carbide (SiC) ceramics	37.9–5.8	30–63	[18]
Porous anorthite ceramics	0.018–0.13	69–91	[19]
Porous mullite ceramics	0.09	73–86	[20]
Porous yttria-stabilized zirconia (YSZ) ceramics	0.06	52–76	[22]
Highly porous fibrous ceramics	0.18–0.06	73–90	[23]
Glass wool	0.03	-	[38]
Vitrified bottom ash-based porous granules	0.13	-	[39]
40 wt.% coal fly ash and 60 wt.% waste glass with 30 wt.% borax and 0.5 wt.% calcium carbonate	0.36	-	[40]
Aerogel	0.017–0.04	-	[9]
Al2O3 hollow glass sphere foam	0.0244	94	[28]
Glass-ceramic foams produced from zeolite-poor rock and eggshells	0.07–0.4	Only the density is reported: 0.54–1 g/cm3	[29]
High-entropy ceramic foams	0.0343–0.0592	90.13–96.13	[30]

Table 1. Thermal conductivity and porosity values of selected porous glass-based and ceramic materials.

Material	Thermal Conductivity (W/(m·K))	Porosity (vol.%)	Reference
Porous silicon carbide (SiC) ceramics	37.9–5.8	30–63	[18]
Porous anorthite ceramics	0.018–0.13	69–91	[19]
Porous mullite ceramics	0.09	73–86	[20]
Porous yttria-stabilized zirconia (YSZ) ceramics	0.06	52–76	[22]
Highly porous fibrous ceramics	0.18–0.06	73–90	[23]
Glass wool	0.03	-	[38]
Vitrified bottom ash-based porous granules	0.13	-	[39]
40 wt.% coal fly ash and 60 wt.% waste glass with 30 wt.% borax and 0.5 wt.% calcium carbonate	0.36	-	[40]
Aerogel	0.017–0.04	-	[9]
Al2O3 hollow glass sphere foam	0.0244	94	[28]
Glass-ceramic foams produced from zeolite-poor rock and eggshells	0.07–0.4	Only the density is reported: 0.54–1 g/cm3	[29]
High-entropy ceramic foams	0.0343–0.0592	90.13–96.13	[30]

2.2. Material Composition

The presence of certain impurities or additives can influence the fire resistance by altering the thermal stability of the glass matrix [14]. Various studies have demonstrated that the incorporation of specific additives can enhance thermal performance by modifying the microstructure and pore characteristics of the materials.

The addition of nano-sized TiO₂ powders in silica insulation media resulted in a 46% reduction in thermal conductivity at high temperatures due to improved radiation scattering efficiency [41]. This effect deserves to be mentioned for the sake of completeness, although it is appreciable at a significantly higher thermal range (around 800 °C) than the common temperatures reached in buildings. Therefore, it is of interest only in very special cases, such as fire propagation.

Incorporating chlorides into the charge mixture of porous glass ceramics led to a finely porous structure, achieving a minimum thermal conductivity of 0.065 W/(m·K) [42].

The incorporation or inherent presence of Fe₂O₃ in CaO-MgO-Al₂O₃-SiO₂ porous glass-ceramics facilitated the formation of a closed pore structure, which significantly reduced thermal conductivity and enhanced mechanical strength [43]. This effect will be discussed in more detail in the Section 3 of the present review.

The introduction of organic additives in granular glass-ceramic materials improved the insulation properties by optimizing the pore formation process during high-temperature firing [44]. The generation of pores through the burn-off of sacrificial organic phases will be discussed in more detail in Section 3 and Section 4.2 of the present review.

While these findings highlight the benefits of specific additives in enhancing thermal insulation, it is essential to consider that excessive impurities may adversely affect the structural integrity and thermal performance of the materials (in other words, higher porosity yields a better thermal insulating effect and worse mechanical properties). Therefore, balancing composition is crucial for optimizing the properties of porous glass structures.

While the above points highlight the direct relationships between pore structures and material properties, it is important to consider the trade-offs involved. For instance, enhancing mechanical strength by reducing porosity often leads to increased thermal conductivity, which may not be desirable for thermal insulation applications. Conversely, optimizing for low thermal conductivity might compromise mechanical strength. Therefore, the design of porous glass must balance these competing requirements to achieve the desired performance for specific applications.

3. Porous Glasses for Thermal Insulation in Buildings: Overview and Market Survey

Porous glasses and glass-ceramics can be used in a plethora of high-tech applications such as filtering and adsorption in agriculture and industry, catalysis, and construction [45]. For use as thermal insulators, porous ceramics and glasses are required to exhibit a thermal conductivity of less than 0.2 W/(m·K) [20,40,46]. Multiple studies indicate that less than 0.2 W/(m·K) is a common requirement for porous ceramics used in thermal insulation. For example, the allowable thermal conductivity range for porous ceramics like Y₂SiO₅ and Al₂O₃ has been reported as 0.081–0.283 W/(m·K) [46], and a conductivity below 0.2 W/(m·K) is typically necessary for insulation applications [20]. In order to obtain such low thermal conductivity, it is essential for the porous ceramic or glass material to contain a large volume of void space (i.e., high porosity).

Perhaps the most commonly used glass-based thermal insulating product is glass wool, in which the porosity is attributed to the void spaces among the fibers. Glass wool is a sustainable material for construction because it can be produced with up to 80% of recycled glass. Glass wool is typically produced from borosilicate glass at a temperature around 1400 °C, where the heated mass is pulled through rotating nozzles, thus creating fibers [47]. According to current European regulations [48], glass wool must be biosoluble in order to avoid the risk of toxicity to humans upon inhalation (carcinogenicity). While glass wool products are relatively well-established and available on the market for decades owing to bio-safety (e.g., [49]) and significantly low thermal conductivity (around 0.03 W/(m·K) [38]), other types of porous glasses are still under investigation and development. In this regard, cellular glass, also known as foamed glass, represents a wide class of products characterized by low density, dictated by their porous or even highly porous structure (60–85 vol.%), which is particularly suitable for thermal and acoustic insulation in buildings. Load-bearing capabilities can be allowed or not depending on the material and pore characteristics. Other remarkable properties include excellent resistance to fire; thermal, chemical, and dimensional stability; and resistance to water and moisture. The higher production cost of foamed glass compared to some polymeric options still represents a limiting factor for the widespread diffusion of these materials, which can however be mitigated by the use of waste materials (e.g., recycled glass).

The alveolar (also called “cellular”) structure of foamed glass consists of a dispersion of macropores with dimensions typically varying between 10 µm and 5 mm, divided by interpore glass walls. The type and morphology of the pores, combined with the microstructural characteristics that can be obtained in these materials, are key to determine the potential applications of the cellular glass. Specifically, the pores (or cells) can be either completely isolated from each other, yielding a product with a totally closed porosity, or interconnected, leading to a predominantly open porosity.

Two main processes are typically used to fabricate glass foams: the first relies on the direct introduction of gas into the molten glass, while the second involves the sintering of fine glass powders using viscous flow accompanied by the foaming of the glass mass due to the decomposition or oxidation of specific additives acting as “foaming agents” (e.g., silicon carbide, polymethylmethacrylate, calcium carbonate, etc.). In general, carbonates and sulfates give rise to decomposition reactions, while oxidation takes place between carbon-containing substances (e.g., C, SiC) and oxygen in the atmosphere of the oven. Both reactions cause the emission of gas that forms bubbles inside the viscous mass, thus allowing the foaming process to occur. Concurrently with or after the foaming process, thermal consolidation takes place via sintering. Especially in an industrial scenario, large amounts of starting glasses from various compositions/sources (including waste) are processed. The foaming of glass from waste can be complex due to the tendency of these materials to devitrify during heating (sinter-crystallization). The presence of a porous structure corresponds to a high specific surface area, which promotes surface nucleation of crystal nuclei. As a result, crystallization and sintering/foaming processes may take place simultaneously. In this regard, a comprehensive study was reported by Tulyaganov et al. [50] who produced highly porous (>87 vol.%) glass-ceramic foams using cullet from sheet glass (i.e., broken pieces of clear glass) as starting material and 0.25–1 wt.% SiC as a pore-forming agent (Figure 2). Moderate amounts of alkali earth aluminosilicate glass powders (1–10 wt.%) were also added as they tended to crystallize into diopside and anorthite, thus increasing the compressive strength of the foams up to 2.6 MPa.

Another study on the use of SiC as a pore-forming agent was reported by Bernardo et al. [51] who produced glass-ceramic foams from waste glass fibers. MnO₂ particles were added to the mixture to promote the oxidation of SiC and, thus, the development of gaseous products at temperatures below 1000 °C.

Problems of microstructural inhomogeneity in the foam can be overcome by adding soda–lime glass to the batch. These additions, which may come from clear recycled glass, allow decreasing viscosity of the melt upon heating and the foaming to occur without being hindered by crystallization.

Cellular glass can also be obtained by foaming processes mediated by the presence of appropriate oxides, with lower costs of production. This is the typical case of glass derived from waste containing a high amount of Fe₂O₃, in which iron has a valence state +3. Ferric oxide plays an important role in foaming processes: in fact, upon being reduced to ferrous oxide (FeO) and passing to a valence state of +2, it releases oxygen that generates large pores in the fired body, thus yielding a density decrease [52]. Iron oxide then acts as an oxidation promoter for carbon-based foaming agents and, depending on its amount in the mixture, can also influence the magnetic and electrical properties of the final product. In this regard, a well-known example is represented by the production process of the “Foamglas^®” cellular glass developed by the Pittsburgh Corning Company (now Owens Corning, Toledo, OH, USA), in which a mixture consisting of recycled glass and carbon black as a foaming agent is sintered; an iron-rich glass fraction is also added to promote carbon oxidation [52].

Porosity in cellular glass can be open and interconnected or closed with isolated pores; in the former case, glass products are typically permeated by fluids (liquid or gas) and mostly used for filtration, separation processes, etc. On the other hand, glass foams characterized by a predominantly closed porosity are more commonly used for thermal and acoustic insulation. They resist compressive loads and do not allow water or other liquids to penetrate inside, making them also useful against the proliferation of fungi, mold, and other micro-organisms. Foamglas^® belongs to the latter category and can be processed to make sheets, panels, and other special shapes to perform particular functions. It has to be noted, however, that the effective thermal conductivity of porous structures with open pores is smaller than that of porous structure with closed pores due to the larger thermal resistance of the solid matrix when the porosity and the thermal conductivity of the solid matrix are the same [53]. In general, glass foams are incombustible, which is an advantage over polymeric insulators.

In the Foamglas^® production process, the mixture made up of recycled glass, and “new” glass is pulverized in a ball mill together with the foaming agent (carbon black). Then, the mix is cast into molds to fill them partially and, subsequently, thermally treated inside an oven at a temperature of 850 °C to allow foaming and sintering processes to occur. Upon completion of this operation, the glass foam blocks are passed through a cooling oven where they undergo a long annealing treatment in order to eliminate internal thermal stress and stabilize the product; the blocks are then cut to obtain the final shape/size and eventually packed and stored for selling and commercialization.

In summary, the main features of commercial Foamglas^®—which should also be ideally exhibited by any other thermal insulating porous glass-derived product—include the following:

• Impermeability to water (it does not absorb humidity and does not swell, remaining dimensionally/geometrically stable over time);
• Incombustibility (in case of fire, it does not spread flames, burn-off, or develop harmful gases);
• High thermal insulation (or low thermal conductivity);
• Resistance to parasites, rodents, microorganisms, and bacteria (this is due to its inorganic nature and can be an advantage over polymers);
• Constant compressive strength, even over a long period, and possible use also in load-bearing conditions (structural applications);
• Dimensional stability to variations in temperature (low coefficient of thermal expansion) and humidity, so that no cracks or shrinkage are generated;
• Resistance to acids and organic solvents;
• Ease of processing, cutting, and shaping;
• Recyclability (glass foams keep their properties unaltered for long periods of time, so they can be reused even after disposal as a filler or insulating granulate);
• Very low density (between 100 and 170 kg/m³).

In addition to the Foamglas^® products described above, there are other materials under commercial development belonging to the porous glass family and obtained from waste, and the fields of application for these materials are of considerable interest for the building and industrial sectors.

Specifically, Misapor^® products (Zizers, Switzerland) are not manufactured in panels but in the form of cellular glass gravel, with piece sizes ranging from 5–10 mm to 50–75 mm. The foamed glass gravel is obtained through a continuous process in which the starting recycled glass is crushed to reach particle dimensions between 75 and 150 μm that are subsequently mixed with the foaming agent. The prepared mixture is then passed through an oven at a temperature of about 950 °C on a conveyor belt to be finally crushed in the form of porous gravel. An interesting field of application of Misapor^® products is the introduction of foamed gravel in concrete mixes to obtain lightweight building materials with both structural and thermal insulating properties.

The company Liaver (Ilmenau, Germany) also developed the Reapor^® product, which is a panel obtained by sintering glass foam granules (or pellets). It is produced by inserting the starting mixture into a pelletizer that feeds a rotary kiln where the foaming of the granules takes place at temperatures between 750 and 900 °C; the obtained glass foamed granules can be used in the production of insulating panels by sintering the glass foamed granules together. Due to its acoustic insulation properties, Reapor^® also finds various applications in the field of industrial plants and machines as well as a motorway barrier.

The major fields of application of commercial porous materials deriving from recycled glass are summarized in

Table 2. Cellular glass products and applications.

Product	Applications
Foamglas® (without mechanical loads)	- Facades - Internal insulation
Foamglas® (with mechanical loads)	- Flat and sloping roofs - Facades - Insulation of floors and perimeters - Metal roofs and special roofs - Internal insulation for walls and ceilings
Misapor®	- Coupling with concrete - Perimeter insulation and foundation slabs - Roof insulation - Vertical insulation of walls in contact with the ground
Reapor®	- Fire insulation - Insulation of railway tunnels and ventilation shafts - Insulation of industrial plants and machines (thermal and acoustic) - Indoor insulation (thermal and acoustic) - Highway barriers (thermal and acoustic)

. An overview of these commercial products is also displayed in Figure 3.

The major advantages of cellular glasses with closed pores emerge when compared to traditional insulation materials, such as polymeric foams (polystyrene and polyurethanes), which are flammable and exhibit poor structural strength. Furthermore, cellular glass allows minimizing some criticalities, such as the formation of thermal bridges in the insulation of facades or foundations. In addition, being impermeable and having high resistance to water vapor diffusion allows insulating internal environments without the risk of condensation and formation of mold, thereby avoiding damage from humidity and infiltrations. Finally, the low density of cellular glass also carries additional advantages such as design flexibility, lower construction times and costs, reduced manual processing, and lower transportation costs as compared to other massive building materials like stones or concrete.

4. Toward the Future: New Sources of Waste Materials, Technologies, and Approaches

4.1. The Potential of Vitrified Ashes from MSWIs

It cannot be ignored that, at present, the environmental impact associated with the fabrication of “conventional” ceramic or glass insulating foams may still be quite high due to the overall energy consumption and the inherent cost of raw materials to be used, thus leaving room for further improvement [54]. In the particular case of Foamglas^®, for example, the advantage of reusing recycled glass is counterbalanced by (i) the addition of a secondary glass fraction derived from an expensive melting process in order to incorporate ferric oxide acting as an oxidizer and (ii) the need for a thermal treatment of foaming and sintering. The additional component (i) is useful in providing oxygen by being converted into ferrous oxide, in turn reacting with carbon black that is used as a foaming agent. Oxidizing compounds may be used as additive in mixtures of glass and foaming agent, but they still imply additional costs and a strict control of the process [55].

In order to maximize the environmental benefits related to using glass-based foams for thermal insulation, focus can be directed to the starting material to be foamed. Raw materials can encompass recycled glass from different sources or even vitrified bottom and/or fly ashes. The composition of the ashes—and, hence, of the vitrified product—may vary within a large range depending on the source of waste that is incinerated; there is also a significant variability depending on the geographic location (i.e., from country to country). Significant amounts of bottom ashes are continuously produced by MSWIs and then often confined in controlled landfills [12]. An interesting alternative to landfilling involves bottom ash vitrification, which allows reducing the volume of these residues significantly (up to 80%), thus extending the life of landfills, and makes the melted glass slag chemically stable and non-toxic [56]. Bottom ashes are essentially formed by metal oxides and, therefore, can be considered as suitable raw materials for the preparation of glass. In the field of construction materials, vitrified bottom ashes have been reused to produce glass-ceramic tiles for decorative applications [57], glass fibers [58], and low-cost granules for lightweight concrete [59], as well as to partially replace ordinary Portland cement and/or sand in mortars [60]. The last strategy is noteworthy as the production of ordinary Portland cement, which is the most commonly used binder worldwide in concrete and mortar design, accounts for 5 to 7% of global CO₂ emissions. Therefore, policies contributing to the reduction or replacement of cement consumption would have a significant impact on reducing the greenhouse gases [61]; comprehensive overviews of this topic were recently reported in [62,63]. Vitrified bottom ash-based porous granules with mechanical strength higher than that of Foamglas^® (2 vs. 1 MPa) and thermal conductivity around 0.13 W/(m·K) have been recently produced using a foaming process, in which vitrified Cinderlite^® particles (Iris Ambiente Srl, Conselve, Italy) were mixed with borax and calcium carbonate acting as fluxing and foaming agent, respectively [39]. Uniaxially pressed cylindrical compacts of powders (diameter 10 mm) were thermally treated at 820 °C to remove the pore-forming agents and sinter-crystallize the glass particles, thus obtaining porous glass-ceramics. Bloating of iron-containing vitrified bottom ashes from MSWIs was also reported, using an approach similar to that adopted for producing commercial Foamglas^® [64].

The use of fly ashes, although being more problematic compared to bottom ashes due to the higher content of leachable heavy metals (e.g., Zn, Pb, Cd, Cr, etc.), has also been considered as an additional fraction in the manufacturing of ceramic or glass-ceramic foams mainly consisting of recycled glass; a good overview of this topic was recently provided in [65]. Fernandes et al. [66] fabricated highly porous (82–84 vol.%) and relatively strong (compressive strength 2.40–2.80 MPa) glass-ceramic foams using a mixture containing around 80 wt.% of sheet glass and 20 wt.% of fly ash with low added amounts of carbonates (1–2 wt.%) as foaming agents. The sintering temperature was set in the range of 750 to 950 °C, which was above the softening point of glass, thereby ensuring the necessary low viscosity for effective foaming, and was also consistent with the decomposition temperatures of carbonates. However, thermal functional properties were not assessed in that work.

In another study, Zhu et al. [67] explored the simultaneous use of borax and calcium carbonate as fluxing and foaming agents, respectively, in a mixture of coal fly ash and waste glass. In an effort to study the influence of processing parameters on the properties of the final porous product, it was observed that the bulk density and compressive strength of porous ceramics increased with the increment of sintering temperature, while the apparent porosity decreased, as expected (Figure 4). The best results, in term of mechanical properties (compressive strength > 5 MPa with bulk density of 0.46 g/cm³) and low thermal conductivity (0.36 W/(m·K)) of the glass-ceramic foams, were achieved using 40 wt.% coal fly ash and 60 wt.% waste glass in the mixture, along with 30 wt.% borax and 0.5 wt.% calcium carbonate, with sintering at 800 °C for 45 min. However, thermal conductivity was above the threshold (0.2 W/(m·K) [40]) recommended for good thermal insulating materials.

Another key factor to be considered is the particle size of ashes, which can significantly affect the sinterability and final properties of sintered glass-ceramics. It was shown that the sintering of smaller fly ash particles was easier compared to that for larger ones as the activation energy of the former was lower [68]. However, regardless of particle size, the flexural strength increased while the porosity and water absorption of sintered bodies decreased with increasing sintering temperature. It was also reported that when the fly ash size decreased, the linear shrinkage, bulk density, compressive strength, acid resistance, and thermal conductivity of lightweight insulation materials increased, but the apparent porosity decreased [69].

Therefore, using vitrified waste materials can be of interest for the production of thermal insulating glass or glass-ceramic components for construction applications. In order to skip the vitrification process, the use of ashes as-such was also proposed. For example, Li et al. [70] produced porous insulating ceramics by gel-cast foaming using fly ash as the raw material followed by thermal treatment at 1300 °C for 2 h. Compressive strength and thermal conductivity were 3.27 MPa and 0.13 W/(m·K), respectively, with the latter being acceptable for thermal insulating applications. However, a high-temperature treatment was still necessary—with relevant energy consumption. In addition, the risk of heavy metal release from the non-vitrified ashes could be a concern. Optimizing the vitrification process by mixing the fly ashes with cullet as a source of silica in an attempt to decrease the treatment temperature is an alternative approach that also allows for obtaining safer products from a toxicological viewpoint [71].

The main features of waste sources and vitrified products from the studies discussed above are provided in

Table 3. Selection of studies about MSWI ash vitrification: main features of wastes and vitrified products.

Type of Waste	Composition of Waste	Resulting Vitrified Product	Composition of the Vitrified Product and Crystalline Phases (If Glass-Ceramic)	Reference
Glass powder from obsolete cathode ray tubes		Glass	-	[55]
Different dry bottom ashes (BA) from MSWIs and wastes from an aluminum alloy industry (AW)		Glass, glass-matrix composites		[57]
Bottom ash (BA) from MSWIs, sludge, cullet glass		Glass, glass fibers	-	[58]
Waste glass (clear and colored)		Glass, also combined with mortar	-	[59]
Bottom ashes from MSWIs W1: vitrified bottom ashes collected from a Japanese MSWI equipped with a direct melting system (DMS) W2: bottom ashes collected from Lomello MSWI (Alessandria, Italy) and placed outdoors for 3 months under ambient humidity and CO2 (this step (weathering) is recommended by a large body of research on the stabilization of heavy metals in bottom ashes)	-	Glass, glass-ceramics	W1 contains gehlenite W2 contains quartz, gehlenite, anorthite, hematite, calcite	[60]
Dry bottom ash (BA) from MSWIs of Bergamo and Vercelli (Italy) were vitrified		Glass-ceramics		[64]
Fly ash (FA) from thermal power plant + cullet of commercially produced sodium-calcium-silicate sheet glass Commercial dolomite and sludge from a marble cutting- polishing plant consisting of calcite were used as foaming gents		Glass-ceramic	Crystalline phases: quartz, tridymite, pargasite, and augite	[66]
Coal fly ash (FA) from a thermal power plant + waste glass Borax added as fluxing agent, calcium carbonate chosen as foaming agent		Glass-ceramics	Crystalline phases: diopside, quartz	[67]
Coal fly ash (FA) from a thermal power plant in three size fractions: CFA = 12.6 μm 20CFA = 7.5 μm 40CFA = 4.9 μm		Glass-ceramics	Crystalline phases: quartz, mullite and hematite, with amorphous metastable glassy phase forms	[68]
Fly ash, flint clay, kyanite, clay, saw dust + organic binder polyvinyl alcohol (2 wt.% solution)		Glass-ceramics	Crystalline phase: mullite	[69]
Fly ash floating beads with polycarboxylate (dispersant), dextrin (binder), sodium carboxymethyl cellulose (thickener), and sodium alkyl sulfate (foaming agent)		Glass-ceramics	Amount of crystalline phases of porous glass-ceramics prepared at different temperatures	[70]
Fly ashes from a waste-to-energy plant from two different sources (FA1, FA2) mixed in 50 wt.% combination with glass cullet and silica sand		Glass-ceramics	Crystalline phases: wollastonite (traces)	[71]

.

4.2. The Search for “Sustainable” Pore-Forming Agents

The “sustainability” of porous foams from waste glass can be further increased by paying special attention when selecting the foaming additives. At present, one of the most-commonly used is SiC, which, however, may be quite expensive and have a negative impact on the overall cost/benefit balance. On the other hand, SiC could be sourced from the waste originated by the polishing procedures (e.g., waste grit paper) of many other “hard” materials, such as traditional ceramics, glass, and stones [66]. By-products of common industrial processes, such as carbonates and carbon-containing waste, can also be used as foaming agents to be added in glass-based mixes [72,73]. Other inorganic “waste foaming agents” include dolomite (by-product of mining activities) [66] and eggshells [74], which are entirely constituted by CaCO₃. In all these cases, foaming should take place in a pyroplastic mass, the viscosity of which is dictated by the softening of glass powders [75] (Figure 5). Therefore, the optimal thermal treatment conditions indeed vary depending on the glass used, and the preferable viscosity range for foaming remains an arguable issue. In fact, different authors described different foaming “windows” depending on the specific material investigated. For example, Fernandes et al. [74] studied how to develop glass foams using pulverized cathode ray tube waste glass and eggshells in the thermal range of 600 to 800 °C and reported that the best condition was when foaming occurred while the melt viscosity was within 10⁷–10⁸ Pa⋅s (Figure 6).

Using industrial waste, such as ash and slag from power stations, to produce porous glass is recognized a highly interesting approach. The process involves preparing a batch mixture with broken glass, ash, slag, and additional crystallization initiators like chromium oxide and zirconium dioxide. This method results in foam glass with a uniform porous structure and varying crystalline phases, contributing to its thermal insulation properties [76].

Foamed glass with thermal and acoustic insulation properties can be produced from a blend of 97% glass waste and 3% expanding agents, such as carbon and manganese oxides from spent alkaline batteries. This process involves heating the mixture in a ventilated furnace, resulting in glass foam with moderate thermal and acoustic insulation properties (Figure 7) [77].

The use of agro-industrial residues, such as rice husk ash, in combination with waste glass powder, has been shown to produce closed-pore glass foams with excellent thermal insulation properties. The addition of rice husk ash reduces the viscosity of the glass, facilitating pore formation and enhancing the insulating capabilities of the material (Figure 8) [78].

Porous glass-ceramics can be produced from granite dust and maize cob using one-step sintering technology. This method results in materials with high porosity and low thermal conductivity, making them viable for thermal insulation of residential buildings [79].

In addition to inorganic agents, organic substances are also used in foaming procedures. The most effective organic foaming agents used in the production of porous glass for thermal insulation applications include glycerol, eggshell, coffee ground waste, and yerba mate, among others. Glycerol is frequently utilized due to its ability to form a carbon phase during thermal decomposition, which aids in the foaming process by reacting with sulfate ions in the glass to release gases, thus enhancing porosity [80,81]. Additionally, glycerol derived from biodiesel production, which contains impurities such as methanol and fatty acids, has been shown to produce foam glass with favorable thermal properties when combined with sodium carbonate [82]. Natural waste materials like eggshells and coffee grounds also serve as effective foaming agents. Eggshells, when used with soda–lime–silica glass, result in excellent thermal insulation properties, while coffee grounds, although less effective, still contribute to the foaming process [83]. Yerba mate waste, when combined with pulverized glass from green bottles, has been shown to produce glass foams with high porosity and low thermal conductivity, making it a viable option for thermal insulation [84]. Other organic compounds such as acetic acid, gelatin, starch, saccharose, and paraffin have also been explored for their potential as pore-forming agents, with varying degrees of success depending on the specific composition and processing conditions [85].

Pine scales have also been used as a pore-forming agent in the production of glass foams from waste glass bottles. This method results in foams with high porosity and low thermal conductivity, suitable for thermal insulation applications [86] (Figure 9).

The use of these organic foaming agents not only enhances the thermal insulation properties of foam glass but also supports sustainable practices by utilizing waste materials, thereby contributing to a circular economy [78]. Overall, the choice of foaming agent significantly influences the thermal, mechanical, and structural properties of the resulting foam glass, making it crucial to select the appropriate agent based on the desired application and performance criteria.

Looking at the fate of the foaming agents during and after the thermal treatment, it cannot be ignored that all the substances mentioned above will produce CO and CO₂ gases, thus yielding an increased greenhouse effect. In order to avoid this drawback, the use of NaOH as a pore-forming agent in sintered waste glass or glass-ceramic products was proposed as a possible alternative [87]; however, the overall cost behind the production of NaOH should be also considered to definitely assess whether this approach is less impactful on the environment. Mixing secondary aluminum ash with a mixture of bottom/fly ashes and pickling sludge was also found beneficial to gas generation. Bloating was attributed to the oxidation of AlN in the aluminum ash, yielding the development of N₂, NO, and NH₃ when the temperature reached 700 °C, thereby allowing the production of porous glass-ceramics [88].

4.3. The Potential of Alkali-Activation Treatment

The fact that the foaming and consolidation (sintering) processes are concurrent phenomena may give rise to some criticalities, such as the difficult control of the final pore shape/size; hence, the functional properties (e.g., mechanical behavior, permeability, thermal conductivity) are dictated by the pore characteristics. Furthermore, if vitrified ashes are used—for the production of which a high-energy vitrification process was previously applied—the method of transformation into porous glass or glass-ceramic products should be particularly simple and affordable to comply with the circular economy paradigm.

In this regard, a new foaming approach was proposed relying on the alkali activation of glass or glass-containing mixtures. For example, porous glass microspheres can be fabricated from alkali-activated fiber glass waste. The activation process involves using highly concentrated alkali solutions, which promote pore formation during heat treatment, resulting in microspheres with potential applications in thermal insulation [89]. The alkali activation of glass powder aqueous suspensions [90,91] or glass/slag mixtures [92,93] was also reported. This method comprised two phases: (i) first, a controlled alkaline attack (e.g., using KOH) was applied to the glass particles, and, thus, the suspensions underwent progressive hardening through the partial dissolution of glass and the formation of calcium-silicate-hydrated (C-S-H) compounds as the binding phase; and (ii) then, before reaching complete setting, the mixtures were foamed by vigorous mechanical stirring with the help of a surfactant. These “green” foams eventually underwent sintering or sinter-crystallization (if the glass was prone to surface nucleation). Shrinkage, geometrical alterations, and pore reshaping may take place during sintering. However, if crystallization is important and concurrent to densification, the increase in the viscosity of the softened glass caused by crystal inclusions can “freeze” the microstructural evolution; thus, the open-cell 3D architecture of the porous greens is preserved in the final sintered samples. The pore size and shape can be properly designed by varying the solid/liquid ratio in the suspension, by setting the parameters of mechanical stirring at low temperature (foaming process), and/or by applying a “pre-curing” treatment.

In summary, sinter-crystallization is decoupled from the foaming process and can occur at relatively moderate temperatures (800–900 °C). This strategy was successfully applied to vitrified bottom ashes from MSWIs (Figure 10) [94], blends of waste soda–lime glass, porcelain stoneware residue, plasma-processed asbestos-containing waste [95], and vitrified mixtures of fly ashes/bottom ash slag [96]. In all these cases, the compressive strength (2–6 MPa) was suitable for applications in construction, and the leaching tests revealed a very low release of potentially toxic heavy metals, thus fulfilling the international recommendations for the safe use of these materials [97]. However, thermal properties were not reported and should be investigated in the future.

4.4. The Potential of Additive Manufacturing

As previously underlined, the total porosity, pore size distribution, and pore interconnectivity play a key role in dictating the final properties of porous ceramics and glasses. Thus, careful design and control of all these aspects should be performed. Direct foaming allows tailoring, to some extent, the porous structure (e.g., pore size and distribution), but this process cannot produce porous glasses or glass-ceramics with complex and well-defined shapes. Furthermore, geometrical variations (volumetric expansion) due to foaming may also be not fully predictable and controllable, unless a closed mold is used (like in the production of Foamglas^®). The foams produced typically require additional post-sintering steps (e.g., cutting, polishing) to fit the desired geometry [98]. In this regard, additive manufacturing has emerged over the last years as a valuable group of technologies for producing three-dimensional (3D) components with complex shapes, customizable microstructures, and precise control of their internal architecture as well as overall shape and size. The final geometry of 3D-printed products is regular, eliminating the need for rectification/cutting and ultimately reducing the volume of waste generated. In a typical approach, the component is first designed in CAD (computer-aided design) and then given as an input virtual model to the printing system (CAM—computer-aided manufacturing). A wide variety of 3D printing techniques are available to ceramics/glass manufacturers, including selective laser sintering, robocasting, and vat photopolymerization [99]. Indeed, additive manufacturing can enter the circular economy approach through the use of recycled materials, including ceramics, glasses, and composites thereof, as well as sustainable approaches. A comprehensive overview of this specific topic was recently provided by Villa et al. [100].

While some special, high-added-value glasses (e.g., bioactive glasses [101]) have been processed using additive manufacturing and investigated in both the literature and industry, attempts to print recycled or, in general, waste glass are relatively scarce. Most of relevant studies use waste glass as a replacement for natural aggregates [102] or cement paste [103] in 3D-printed concrete. Both approaches are valuable as the former allows saving natural resources (sand, crushed stones) and the latter is useful to reduce the energy consumption and CO₂ emissions associated with cement production. Pulverized cullet was also used as a precursor material in the 3D printing of geopolymer concrete based on fly ashes and slag because of its beneficial influence on the geo-polymerization rate and extrudability [104]. Other more sophisticated examples include the extrusion of suspensions of ground waste glass, gelatin, and agar [105] as well as borosilicate glass and Pluronic F127 [106] to fabricate porous glass panels and rings, respectively, using robocasting.

With regard to porous glass, Guzi de Moraes et al. [107] reported the production of foam-like glass structures with robocasting, using bentonite (6.5–9.5 wt.%) to overcome the problem of low plasticity of recycled glass powders (median particle size <4 μm). After being fired at 700 °C for 1 h, the materials exhibited total porosity of up to 53 vol.%, pore sizes in the range of 1090 to 1870 μm, and a compressive strength of 18 MPa (Figure 11); however, thermal conductivity was not assessed. Therefore, additive manufacturing deserves to be further investigated in the field of thermal insulating materials for building as a valuable strategy to produce ceramic, glass, glass-ceramic products, or even waste glass-containing (polymer-matrix or polymer-bonded) composites.

While these innovative fabrication techniques offer promising solutions for producing thermal insulation materials from waste, challenges remain in optimizing the mechanical properties and scalability of these porous materials for widespread use. Additionally, the environmental impact of the production processes and the long-term performance of the materials in real-world applications require further investigation. Nonetheless, the use of waste materials in manufacturing porous glass for thermal insulation aligns with sustainable development goals and offers a pathway toward more environmentally friendly construction practices.

5. Conclusions and Challenges

Glass is a highly versatile material used in our lives, the applications of which range from daily to high-tech applications. Glass is fully recyclable as it can be reprocessed indefinitely without undergoing any significant loss of performance. The transformation of cullet or even vitrified bottom/fly ashes from MSWIs into porous products for thermal insulation is convenient provided that the additional processing is relatively simple and cheap. Furthermore, if we look at the circular economy approach, some specific considerations are necessary.

Innovative techniques for manufacturing porous glass from waste materials for thermal insulation in buildings have been explored extensively in recent research. These techniques focus on utilizing various waste materials, such as industrial by-products, glass waste, and agro-industrial residues, to produce foam glass and other porous materials with desirable thermal insulation properties. The use of waste materials not only reduces production costs but also addresses environmental concerns by promoting recycling and reducing landfill waste.

The common methods used to produce porous glass or glass-ceramics for thermal insulation—even at an industrial level—belong to the family of foaming strategies, which always require a high-temperature consolidation step (sintering of glass particles). This additional energy consumption can be justified by the high added value of the final product; moreover, the temperatures used for sintering are typically lower than those required for direct re-melting of recycled glass. Recent studies have proposed the use of vitrified bottom or fly ashes as starting materials for the foaming process; in this case, the large energy consumption associated with the vitrification process is somehow balanced by the advantages of reducing landfilling and obtaining a toxicologically safe final product, from which the release of heavy metals is minimized. In general, the use of bottom or fly ashes to produce foamed glass/glass-ceramics not only provides a valuable path to recycle industrial waste but also offers economic and environmental benefits. This approach reduces the need for natural raw materials that are commonly used in primary glass production (e.g., quartz or silica sand, limestone, and other carbonates), thereby preserving natural resources while mitigating the environmental impact associated with hazardous ash disposal, such as landfilling in dedicated sites preceded by special treatment to make these materials as chemically inert as possible.

Before being used in foaming procedures, recycled glass or vitrified material should be carefully screened from a compositional viewpoint for wise selection; furthermore, the particle size as well as the type and amount of pore-forming agent(s) have to be taken into account to obtain an optimal result. In fact, the characteristics of glass or glass-ceramic foams depend on a delicate balance between viscous flow sintering and gas evolution that is dictated by some factors like the composition and particle size of raw materials, the heat-treatment temperature, the heating rate, and the gas atmosphere in the furnace. Crystallization may occur concurrently to sintering, thus affecting the final physical, mechanical, and thermal properties of the foam (e.g., density, compressive strength, and thermal conductivity). In order to avoid excessive crystallization while maintaining an acceptable control on both foaming and crystallization, waste glasses with a limited tendency toward crystallization should be selected as starting materials, or mixtures combining waste glass from different sources (e.g., vitrified bottom/fly ashes and cullet) can be prepared. Using waste foaming agents, e.g., by-products from mining industry (dolomite, limestone) or cooking (eggshells), can further enhance the “green” fingerprint of the process.

Low-temperature strategies to promote foaming, such as alkali activation followed by mechanical stirring, are potentially applicable to any kind of waste glass and contribute to reducing the energy consumption of the additional pore-forming process.

Big hopes are recently rising from the application of additive manufacturing technologies in the circular economy approach. At present, however, there is a paucity of reports specifically focusing on 3D-printed waste glass. In principle, additive manufacturing allows saving raw materials as well as achieving a better control on pore shape and pore size distribution. These methods are also scalable, allowing mass production with high reproducibility of the goods. On the other hand, if obtaining cellular glass for thermal insulation is the specific goal, it is likely that foaming approaches are still more affordable than 3D printing methods, which may require—especially when implemented on large scales—high investment costs in terms of equipment. Furthermore, there are some important sustainability-related issues that need to be fully considered, such as the formulation of the printable ink. In fact, the use of polymer-based slurries or inks, for example in robocasting and vat photopolymerization, can lead to additional CO₂ emissions generated during post-printing treatments, like high-temperature debinding that aims at removing all the organics. Other aspects deserving consideration within the framework of the circular economy philosophy include the appropriate selection of polymers for designing the ink—bio-based formulations could be an option—and the minimization of waste solvents, for example by using aqueous slurries.

However, these open issues reveal a great opportunity for future research and development toward greener strategies for the fabrication of porous glasses and glass-ceramics for a more sustainable world. Indeed, there are specific challenges in the manufacturing of porous glass that must be addressed. The first challenge is related to material consistency and quality. The variability in waste material composition can lead to inconsistencies in the final product properties. For example, the use of different waste sources like granite dust and maize cob can result in varying porosity and mechanical strength [79]. The second challenges concern energy consumption. The high temperatures required for sintering and foaming processes can lead to significant energy consumption, which may offset some of the environmental benefits of using waste materials [108]. The third challenge is about the mechanical properties. In fact, while high porosity is desirable for insulation, it often results in reduced compressive strength, as seen in materials with compressive strengths ranging from 0.7 to 9.7 MPa [79].

To overcome these challenges, several solutions are being explored. Optimizing the process parameters by adjusting temperature, time, and materials ratios can help optimize the balance between porosity and strength. For instance, varying the processing temperature and the amount of pore-forming agents can control the properties of anorthite ceramics [109]. Furthermore, innovative techniques like colloidal suspension foaming allows for precise control over pore size and structure, enhancing the mechanical properties of the final product [110]. Lastly, incorporating special additives can improve the mechanical properties and reduce the energy required for processing [111].

Therefore, while the use of waste materials for producing porous glass insulation presents several challenges, it also offers significant environmental benefits. The development of innovative manufacturing techniques and optimization of existing processes can help overcome these challenges, making porous glass a viable and sustainable option for building insulation. The integration of waste materials not only reduces the environmental impact but also provides a cost-effective solution for thermal insulation needs.