FOAM GLASS SYNTHESIZED EXCLUSIVELY FROM WASTES

Purpose: The objective of this research was to synthesize and characterize foams glass using glass waste as a glass matrix and marble waste as a foaming agent. Theoretical framework: Foams glass are examples of very porous materials, the result of the insertion of a gaseous phase into a solid phase and are mostly used as thermal and acoustic insulators. Foam glass are obtained by adding a foaming agent to finely ground glass and heating it to temperatures above its softening point. Ornamental stones are basically subdivided into granites and marbles. Carbonate stone are generally included as marbles. The processing of these stones generates large amounts of fine waste. Brazil generates around 2.5 million tons of stone waste per year. Marble waste has great potential to be used as a foaming agent in the synthesis of foams glass. Method/design/approach: The glass and marble wastes were characterized by chemical, mineralogical, granulometric, thermal and morphological analysis. Foams glass were synthesized with 3, 6, 9 and 12% by mass of marble waste in mass of glass waste, fired at temperatures of 750, 800, 850 and 900°C. The characterization of the foam glass was carried out through analysis of volumetric expansion, density, porosity, and the microstructure evaluated by microscopy. Results and conclusion: The waste characterization results proved that they are capable of synthesizing foams glass for the desired purposes, glass waste as a glass matrix and marble waste as a foaming agent. The synthesized foam glass showed excellent volumetric expansion with density and porosity values compatible with those of commercial foam glass. Research implications: The use of wastes as raw material in the production of foams glass, thus aiming to reduce the consumption of natural resources, as well as reducing the environmental impact caused by wastes. Originality/value: Synthesize foams glass exclusively with wastes. It is also one of the waste of ornamental stone, generated in the order of millions of tons.


INTRODUCTION
Cellular solids are very porous materials, resulting from the insertion of a gaseous phase into a solid phase, which can be ceramic, metallic, polymeric or composite, with materialdependent characteristics of this solid, as well as morphological parameters resulting from the processing.Foam glass are examples of these glass materials and are mainly used as thermal and acoustic insulators due to their characteristics of low density, low thermal conductivity, freezing resistance, non-toxicity, non-flammability and chemical inertia (Colombo, 2005;Scheffler and Colombo, 2005).
According to Pittsburgh Corning Foamglas Insulation (2023) foam glass is a rigid but lightweight insulation material composed of millions of completely sealed glass cells, each of which forms an insulating space.This all-glass enclosed structure provides an unparalleled combination of physical properties that are ideal for pipes and equipment installed on the surface or underground, in open air or enclosed spaces, with temperatures ranging from -268 °C to +482 °C: • Water resistant in liquid or vapor form • Non-corrosive • Non-combustible and non-absorbent combustible liquids • Resistant to most industrial reagents • Its dimensions are stable under a variety of temperature and humidity conditions • Superior compressive strength • Resistant to bio-organisms, microbes and mold • Does not contain fiber, CFC or HCFC Foam glass are obtained by adding a foaming agent to the finely ground glass and heated at temperatures above its softening point.This temperature is maintained until the gas released by the foaming agent is captured in the glass structure, forming a large amount of pores (Pokorny et al., 2012).
Knowledge of the chemical composition of the glass and its behavior is necessary and fundamental for the planning of the vitrifiable mixture.The formation of the foams glass, on the other hand, depends on the liberation of the gases during firing, and the properties depend on its structural characteristics, which, in turn, are conditioned to the chemical composition of the foaming agent and to the thermal processing to which they were submitted.
Several research studies have investigated the use of different wastes as raw material for foam glass, both for use as a glass matrix and for foaming agent (Yin et al., 2016;Zilli et al., 2015;Souza et al., 2017;Silva/et al., 2016).
Ornamental and cladding stones are basically subdivided into granites and marbles.Like granites, they fit, in general, into silicatic stone, while the marbles encompass the carbonaceous stone.The main carbonaceous stone include limestones and dolomites, which are sedimentary stone composed mainly of calcite (CaCO3) and dolomite (MgCa(CO3)2) (Sampaio e Almeida, 2008).
The production process of ornamental stone involves complexity from the exploration of the deposits, through processing (sawing and polishing) to storage and transport.In all subsystems there are always causes and impacts on the environment, water, air and soil (Calmon, 2007).
In the processing of the blocks in the sawmills, the waste generated is 40% of the volume of the block processed, 26% of it being very fine waste mixed with the inputs from the sawdust and 14% of it being coarse waste, in the form of a shell.It is estimated, then, that in the processing are generated around 1.5 Mt of fine waste (stone dust) and almost 1 Mt of coarse waste, shell mills and shavings, annually in Brazil (Campos et al., 2013).
Investing in sustainability contributes to the growth of industries and assists in longterm improvements due to growing global interest in the environmental issue, particularly with regard to the responsible use of natural resources, as well as in the proper management of industrial waste (Martínez, 2022).
Various research have studied the feasibility of producing eco-efficient materials for use in civil construction, incorporating wasyes of ornamental stones as a raw material, instead of natural resources, but no research has been found in the literature on the development of foam glass with these wastes.Almeida et al. (2023), Barreto et al. (2023), andBabisk et al. (2013) studied the use of silicate ornamental stone wastes in the production of red ceramics.Gomes et al. (2020) and Babisk et al. (2019) developed glass using silicate and carbonate stone waste.Gadioli et al. (2023) and Agrizzi et al. (2022) produced artificial stone using waste from silicate stone.
The objetive of this research was to characterize solid wastes, synthesize and characterize foams glass obtained using waste sodo-lime glass as a glass matrix and marble waste as a foaming agent, aiming to reduce the consumption of natural resources, as well as reducing the environmental impact caused by wastes.

METHODOLOGY
The raw materials used in this research were: glass waste from beverage packaging as a glass matrix and marble waste as a foaming agent.

Preparation and Characterization of Raw Materials
A glass of beverage packaging was collected, washed and dried in a laboratory oven, later fragmented, crushed, ground and sieved.The marble processing waste was collected in a sawmill, dried in an oven and disassembled.
For synthesis of foams glass, the waste of the glass of the beverage packaging was ground in a ball mill for about two hours and then sieved at 200 mesh.
The characterization of the wastes was carried out by means of scanning electron microscopy (SEM), particle size and size distribution particle size analysis, real density, mineralogical and crystalline phase analysis by X-ray diffraction (DRX), chemical analysis by X-ray fluorescence (FRX) and thermogravimetric (TG)/differential exploratory calorimetry (DSC).
The real density of the glass waste was determined by pycnometrics according to NBR 6508 (ABNT, 1984).The morphology of the marble waste was analyzed by MEV under a TM3030Plus HITACHI microscope and the particle size analysis was determined using a MASTERSIZER 2000 equipment from Malvern Instruments.
DRX analyzes of the wastes were performed by the powder method, in a Bruker-AXS D8 Advance Eco equipment, Cu Κα radiation (40kV/25mA), 2θ ranging from 5 to 80° step and time of 1s per step.The identification of the chemical components carried out by FRX in an Xray fluorescence spectrometer of Philips model PW 2400.
The thermal analysis of the marble waste was performed on a simultaneous TG/DSC equipment of the METTLER TOLEDO model TG/DSC 1 STAR and System and the glass waste in a thermogravimetric analyzer with DSC model NETZSCH STA 409 PC/PG.

Preparation of Foam Glass
Compounds containing 3, 6, 9 and 12% by mass of marble waste were prepared and homogenized for each mass of glass waste, each containing a total of 100 g, both sieved at 200 mesh.
The compositions were moistened with water, using 5% polyvinyl alcohol (PVA) as a binder and passed in 20 mesh sieve, later conformed in cylindrical steel matrix (20 mm diameter) with 30 MPa using the hydraulic press RIBEIRO -P30T.
Sixteen test pieces were prepared for each composition and subsequently dried in two stages, the first in the air for 24 hours, and the second in an oven at 100°C for 24 hours.Next, four test bodies of each composition were fired at temperatures of 750, 800, 850 and 900°C.
The firing was done in electric furnace FL-1300 -INTI FURNACES, with a heating rate of 10°C per minute until reaching the desired temperature, 30 min of plateau time and natural cooling by the inertia of the furnace.

Characterization of Foam Glass
The measurements were carried out with the aid of a digital caliper marked MITUTOYO (0.01mm resolution) and the masses were determined by means of a digital scale Shimadzu model S3000 (0.01g resolution).
The density of the foams glass was obtained the ceramic bodies after firing.The calculations were made on the basis of Equation 1, Where   is the density of the foam glass, m the mass (g) and V the volume (cm 3 ) of the test body.
The percentage of volumetric expansion was performed by calculating the volume of the test bodies, before and after firing, and using Equation 2,   the volume of the ceramic body after firing (cm 3 ) and   the volume of the ceramic body to green (cm 3 ), after drying steps.
The percentage of porosity was determined using the value of the apparent density and the value of the real density of the glass waste used to form them, according to Equation 3.Where   is being the density of foam glass and   being the real density of the glass.
The morphological analysis of the synthesized foams glass was carried out by means of a laser scanning confocal microscope (CLSM), OLYMPUS brand, model LEXT OLS4000 3D and a stereoscopic magnifying glass.

Wastes Characterization
The morphology of the marble waste is shown in Figure 1.It is observed that the particles of this waste have edges of angular shapes, this morphology assists the fusion process, and is characteristic of the wastes that come from the process of processing ornamental stones.Figure 2 shows the granulometry of the marble waste, where the size and the distribution of the particle sizes were analyzed.The particles showed a distribution with values between d10 = 1.97 µm (that is, 10% of the particles are less than 1.97 µm), d50 = 18.35 µm and d90 = 77.62 µm.
This particle size favors its use, since its application would be direct, without the need to undergo any process of particle reduction, because according to Scheffer and Colombo (2005), in order to better promote the process of the formation of the foam glass, the glass powders and the foaming agent must present particles smaller than 4 mm.The real density of the glass waste was 2.51 g/cm 3 and is close to that of the literature for soda-lime glass (Akerman, 2014).
The X-ray diffractograms of the wastes are shown below in Figures 3 and 4. It is observed in the diffracatrogram of the glass waste (Figure 3) that there is no presence of crystalline peaks, the spectrum shows a typical amorphous band around 27°, where the peak of greatest intensity of quartz is located, and therefore comes from the presence of silica present in this type of glass.This result characterizes the glass as amorphous, it does not present longrange order in its atomic arrangement.It can be observed in Figure 4 that the marble waste is composed of the minerals dolomite (ICDD-PDF: 00-036-0426) and magnesium calcite (ICDD-PDF: 01-089-1305), (MgCa(CO3)2) and ((Mg0.06Ca0.94)(CO3)),respectively.They are two carbonates, which when heated result in thermal decomposition, which is a chemical reaction where one chemical substance decomposes into at least two other substances.In this case, from the carbonates, CO2 is released, the retention of this gas in the glass structure that will lead to the expansion of the foam glass and consequent generation of the desired porosity.Table 1 shows the identification of the chemical components of the glass waste and the marble waste.The chemical composition presented by the glass studied is typical of soda-lime glasses and is in accordance with the literature.According to Akerman (2014), soda-lime glasses are generally composed of 69-72% SiO2, 13-16.5% Na2O, 9.5-16.5% CaO and 0.9-2.3%Al2O3, the amounts of oxides vary depending on the desired application.Some authors report 8 the presence of MgO oxides in up to 4% and K2O in up to 1%, in flat glass, lamps and packaging.The presence of Fe2O3, which is a colorant oxide (green), is also mentioned in up to 0.1%.The presence of 0.2% SO3 is probably due to fuel contamination (Shelby, 2005).
The marble waste consists mainly of the oxides of calcium (35.2%) and magnesium (18.3%), and according to Rêgo (2007), according to its composition, can be classified as being dolomitic calcitic, depending on the contents of these oxides, as well as the ratio between them.
The waste presented a high loss to fire (41.6%) common to carbonaceous stones, which is due to the decarbonation of magnesian calcite and dolomite, minerals identified previously in the DRX analysis.This decomposition of the carbonates qualifies this waste to act as a foaming agent in the synthesizing of foams glass.The curves of the thermal analyzes are shown below in Figures 5 and 6 for the glass waste and marble waste, respectively.For the glass waste, it can be observed in the TG curve that there is no loss of mass and in the DSC curve the start and softening of the glass at the temperature of 690°C and the increase of viscosity with the start of the melting process from 800°C.
It is observed for the marble waste, that the loss of mass was around 40%, this result corroborates the loss to fire presented previously in the analysis by FRX.This significant loss in mass is mainly related to the CO2 output from the carbonate decomposition during firing between 700 and 850°C, where there is a steep slope of the TG curve and also the endothermic peak of the DSC curve is observed.
In order for the foam glass process to be successful, it is essential that the gases, formed by decomposition of the foaming agent, are released into the glass matrix between the sintering and softening temperatures of the glass, ensuring that the gases are not lost by the open porosity of the ceramic body (foam glass) and also that the viscosity of the glass phase is sufficient to trap the bubbles formed by decomposition of the foaming agent (Mugoni et al., 2015;König et al., 2016).

Characterization of Foams Glass
Figure 7 shows the glass and porous appearance of the foam glass synthesized with glass and marble wastes.The graph in Figure 8 shows the average values and standard deviations of the volumetric expansion for the synthesized foams glass, depending on the content of incorporated marble waste and the firing temperature.
The marble waste, with the introduction of dolomite and magnesium calcite, two carbonates, promoted the release of CO2 due to decarbonation, and this gas was retained in the glass matrix, resulting in the expansion of the foam glass in all the formulations and temperatures studied.
Volumetric expansions of more than 792, 674, 582, and 388% were observed for incorporations of 3, 6, 9, and 12% marble waste, respectively, at 850°C.The largest expansions obtained at temperatures between 800 and 900°C are justified due to the low viscosity of the glass at these temperatures, as well as the carbonate decomposition range, as previously presented in the thermal analyzes of the wastes (Figures 5 and 6).Considering mean values, it can also be observed that the volumetric expansion tends to decrease with the increase of incorporation of marble waste.Research reports that excessive amounts of foaming agent tend to limit the capacity of foaming in the glass matrix, which validates the lower expansion of the samples produced with higher% in weight of marble waste (Rangel et al., 2021;Teixeira, 2016).For for larger quantities, the expansion of foams decreases as a probable result of the excessive increase of generated gases, in this case the release of CO2, increasing the internal pressure and consequently breaking the walls of the porous structure.Losing the gaseous phase, the foam glass loses volume.
This rupture allows the exhaust of the generated gases, with consequent densification of the samples, as can be seen below in Figure 9 which presents the mean values and standard deviations of density for the synthesized foams glass, depending on the content of incorporated marble waste and the firing temperature.Density values evolve with increasing incorporation of marble waste.
According to Scheffer and Colombo (2005) comercial foams glass have densities between 0.1 and 0.3 g/cm 3 .It can be observed that the foams glass synthesized with the waste of marble reached the densities of the commercial foams glass, with the exception of the composition with 9% of the waste of marble fired at 750°C and the foams with incorporations of 12% of the waste of marble, proving that this quantity of foaming agent is excessive.As the density increases, the porosity decreases, as can be seen in Figure 10.Among the main properties of commercial foams glass is porosity, between 85 and 95% (Scheffer and Colombo, 2005).All the densities of the foams glass synthesized were present in this range, taking into account commercial values, except for those with 9 and 12% of the incorporation of marble waste fired at 750°C.
The decomposition of the carbonates led to the generation of gases and the elimination of CO2, which remained retained in the glass matrix generating porosity.After the bodies begin to soften under expansion pressure occurs the foaming process that generates large amounts of pores in the ceramic bodies.Figure 11 presents the microstructure of foams glass synthesized with wastes.The particles are linked together leading to the formation of cell walls and the small pores, noticeable at the lowest temperatures, disappear or are transformed into large pores gradually with sintering support and an increase in temperature, evidencing the coalescence of the pores and the increase in volumetric expansion, seen previously.This is probably due to the fact that the glass phase, being more viscous at lower temperatures, presents greater resistance to rupture of the pore wall that is expanding by the evolution of the decarbonation and release of CO2 in the glass matrix.Thus, the pores expand more, without losing this gaseous phase through the rupture of the pore wall.
With the increase in temperature, the lower viscosity of the glass phase and the high foaming due to the gaseous phase produced by the marble waste in the ceramic body, it is contributed to the decrease in volume expansion after reaching a maximum, mainly as regards the amount of incorporation of the foaming agent (marble waste), leading to the formation of holes in the walls of the pores through which the gaseous phase evolves out of the ceramic body.Losing the gaseous phase, the foam glass loses volume, porosity and increases its density, as discussed previously.

CONCLUSIONS
Based on the results of the wastes characterization analyzes, it can be concluded that the container glass waste is a fully amorphous sodo-lime type glass with suitable thermal behavior to be used as a glass matrix.The marble waste is characterized as calcitic dolomitic, composed of carbonates, with granulometry, morphology and thermal behavior suitable for use as a foaming agent.
The synthesized foams glass showed excellent volumetric expansion with density and porosity values compatible with those of commercial foams glass, with the exception of the foams produced with the incorporation of 12% of the marble waste.
This research confirms the potential of these wastes as raw material for making foams glass.It is important to emphasize that the use of waste as a raw material in the production of eco-efficient materials reduces the consumption of natural resources, as well as enabling an environmentally correct disposal of waste, such as ornamental stones, generated in the order of millions of tons.

Figure 1 .
Figure 1.Micrographs by MEV of the marble waste.Source: Authors (2023) Figure2shows the granulometry of the marble waste, where the size and the distribution of the particle sizes were analyzed.The particles showed a distribution with values between d10 = 1.97 µm (that is, 10% of the particles are less than 1.97 µm), d50 = 18.35 µm and d90 = 77.62 µm.This particle size favors its use, since its application would be direct, without the need to undergo any process of particle reduction, because according to Scheffer andColombo (2005), in order to better promote the process of the formation of the foam glass, the glass powders and the foaming agent must present particles smaller than 4 mm.Souza et al. (2017),Teixeira et al. (2017) andMarangoni et al. (2014) studied other types of waste, with the purpose of use as a foaming agent in the production of foams glass, the waste studied needed to undergo grinding or grinding, which brings cost to the process for its use.

Table 1 .
Chemical composition of the wastes (% by mass).