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Vermiculite materials are widely used as insulation material in the industry, fire protection systems, and fire stoves. The high dimensional stability of vermiculite boards ensures that thermal runways in the insulation lining are avoided. Vermiculite boards for high-temperature applications are produced from thermally expanded vermiculite and alkali silicate binder. Here, we show the impact of potassium silicate binder and vermiculite source on the dimensional stability of vermiculite boards. The vermiculite boards expand at 500–830 °C and shrink stepwise at 1000–1250 °C and 1400 °C, respectively, when heated at 10 °C min−1 in ambient air. The expansion process is discussed in relation to the boards containing non-expanded particles and metal carbonates. The evolved gas analysis indicates that the CO 2 release from metal carbonates is the potential cause of the expansion of some vermiculite types. The shrinkage is initiated by the dehydroxylation of the clay structure. The binder promotes crystallisation during the shrinkage, ceasing the shrinkage process at higher temperatures. The expansion and shrinkage of vermiculite boards are anisotropic and depend on the board density and vermiculite origin. These findings provide new insights into the expansion and shrinkage behaviour of vermiculite boards. [ABSTRACT FROM AUTHOR]

Cathode ray tube (CRT) panel glass is processed into glass foams by using carbon and Mn 3 O 4 as foaming agents. We investigate the foaming ability and foaming mechanism of the mixture by varying the composition and the temperature. Carbon load of 5 wt% inhibits the sintering of the glass powder, while 2 wt% carbon promotes the coalescence of pores during foaming. Carbon load of ≤0.4 wt% is suitable for achieving low density foams (0.15–0.23 g cm-3) with closed pores (75–100%). Foaming is initiated by the reaction between carbon and Mn 3 O 4 and this is the main source of melt expansion. Reduction of cations in the glass melt, including Mn3+ dissolved from Mn 3 O 4 , makes an important contribution to the melt foaming at >800 °C. The main gaseous product is CO 2 (67–95 vol%). CO appears above 800 °C and the concentration increases with temperature. [ABSTRACT FROM AUTHOR]

Abstract: Foam glasses are usually prepared through a chemical approach, that is, by mixing glass powder with foaming agents, and heating the mixture to a temperature above the softening point (106.6 Pa s) of the glass. The foaming agents release gas, enabling expansion of the sintered glass. Here, we use a physical foaming approach to prepare foam glass. First, closed pores filled with inert gases (He, Ar, or N2) are physically introduced into a glass body by sintering cathode ray tube (CRT) panel glass powder at high gas pressure (5‐25 MPa) at 640°C and, then cooled to room temperature. The sintered bodies are subjected to a second heat treatment above the glass transition temperature at atmospheric pressure. This heat treatment causes expansion of the pores due to high internal gas pressure. We found that the foaming ability strongly depends on the gas pressure applied during sintering, and on the kinetic diameters of the gases. The pressure for attaining maximum expansion, that is, lowest density and highest porosity, is found to be around 20 MPa. [ABSTRACT FROM AUTHOR]

The process of foaming glass is very dependent on the chemical composition of the glass. In this study we used a foaming-agent/oxidizing-agent couple and a crystallization inhibitor to foam cullets of flat, container and CRT-panel glass. Foamed glass with a density of 110–120 kg m –3 , a thermal conductivity of 50–52 mW m –1 K –1 and a homogeneous pore structure was obtained from a mixture of panel glass, 0.33 wt% carbon and 4.45 wt% Fe 2 O 3 . We also showed that it is possible to fabricate foamed glass with the same density or pore structure as mentioned above by adding up to 50 wt% container cullet or 70 wt% flat glass to the mixture. In the foamed samples with a low content of panel glass, crystals form, resulting in an increased open porosity, density and inhomogeneous pore structure. The crystallization can, however, be inhibited by adding calcium phosphate, so enabling the preparation of high-quality foamed glass from flat glass or flat/container-glass mixture. The pore gas is predominantly CO 2 and the pressure inside the pores is 0.36–0.47 bar. The reduced effect of the composition on the foaming process suggests that there is a great potential for stabilizing the production of foamed glass and ensuring the product's quality. [ABSTRACT FROM AUTHOR]

Phosphates play a role in different foaming processes. In slag foaming, the phosphate can lower the surface tension and increase the foam life time. In the sintering–foaming process, sodium phosphates are added as “foam stabilizers”. Though, phosphates show documented effect in slag foaming, the effect of phosphate in the sintering–foaming process still needs to be clarified. In this work, we investigate the influence of alkali phosphates (Li 3 PO 4 , Na 3 PO 4 , and K 3 PO 4 ) on the glass foaming process, foam density, and glass transition temperature ( T g ) of waste silicate glasses. The results show that the T g of foam glasses decreases with increasing the alkali phosphate content, and with decreasing the alkali ion radius. The results, therefore, indicate that alkali phosphates can be used to decrease the heat-treatment temperature, however, only K 3 PO 4 and Na 3 PO 4 prove to be promising candidates to obtain low density foam glasses. [ABSTRACT FROM AUTHOR]

The understanding of the thermal transport mechanism of foam glass is still lacking. The contribution of solid- and gas conduction to the total thermal conductivity remains to be determined. In many foam glasses, the solid phase consist of a mix of an amorphous and a crystalline part where foaming agents can be partially dissolved into the glass structure. We investigate the influence of incorporation of residues from foaming agents (MnO 2 and Fe 2 O 3 ) on the solid conductivity of cathode ray-tube (CRT) panel glass. We have prepared samples by sintering and melt-quenching technique to obtain samples containing glass and crystalline foaming agents and amorphous samples where the foaming agents are completely dissolved in the glass structure, respectively. Results show that the samples prepared by sintering have a higher thermal conductivity than the samples prepared by melt-quenching. The thermal conductivities of the sintered and the melt-quenched samples represent an upper and lower limit of the solid phase thermal conductivity of foam glasses prepared with these foaming agents. The content of foaming agents dissolved in the glass structure has a major impact on the solid thermal conductivity of foam glass. Hence, the solid thermal conductivity of foam glass can be optimized by altering the foaming agent and its content. [ABSTRACT FROM AUTHOR]

The gas-releasing reaction is the most important process in the preparation of foam glass. In this paper we investigated the gas-releasing reactions by means of thermogravimetry coupled with mass spectrometry. We used carbon (activated charcoal and carbon black) and/or manganese oxides (MnO 2 , Mn 2 O 3 , and Mn 3 O 4 ) as the foaming additives. We show that manganese oxides have different functions in the foaming process. The thermal decomposition of MnO 2 below the sintering temperature has a negative impact on the foaming process as it shifts the foaming to higher temperatures, increases the mass-loss rate, leading to open pores, and burns out the carbon. When foaming in an oxidizing atmosphere, the carbon is burnt out by the oxygen from the atmosphere. Instead, Mn 2 O 3 can be used as the foaming agent in an oxidizing atmosphere. In the oxygen-free atmosphere, Mn 3 O 4 can be used as the oxidizing agent , supporting the oxidation of carbon and the foaming process. The redox equilibrium of manganese (Mn 2+ /Mn 3+ ), influenced by the oxygen partial pressure in the pores and physically dissolved oxygen in the glass, shows the strongest influence on the foaming process. The CO/CO 2 ratio in the evolved gases depends on the carbon source and the temperature. [ABSTRACT FROM AUTHOR]

The production of silicate glass foam allows diverse resources and waste materials to be used in the production. Testing of such large palette of materials complicates and prolongs the optimisation process. Therefore, it is crucial to find a universal criterion for foaming silicate glass melts which can offer a practical starting point for the optimisation procedure. The melt viscosity might be the most important parameter for controlling the foaming process and the glass foam density. In this work, we attempt to define a viscosity range in which foaming of different glasses results in a maximum of foam expansion. The expansion maximum is obtained for different glasses (labware, E-glass, CRT panel, soda-lime-silica) by foaming with CaCO 3 at isokom temperature and from literature data. In general, the viscosity window was found to be within 10 4 –10 6 Pa s when foaming with MnO 2 or metal carbonates (CaCO 3 , Na 2 CO 3 , MgCO 3 , SrCO 3 , dolomite) whereas SiC requires higher temperatures and correspondingly lower viscosities (10 3.3 –10 4.0 Pa s). These findings can help assessing the implementation of new resources in the glass foam production. [ABSTRACT FROM AUTHOR]

Optical monitoring techniques can record in situ the size of glass samples during a dynamic heating process. This allowed us to study the sintering and expansion rate of panel glass from cathode ray tube using MnO2 as foaming agent. We show the maximum expansion rate of glass melt foaming ( in situ value) correlates to 50% closed porosity, and we define a universal temperature window for foaming glass melts based on the sintering-expansion curves obtained with a heating microscope. The sample size obtained at the maximum expansion rate can be used to quickly evaluate various foaming parameters such as type and concentration of foaming agent, glass composition, and particle size to obtain foam glass with high porosity and closed pores. Using this approach, we show that the foaming of bottle glass is preferentially conducted at a SiC concentration of 1-4 wt%. [ABSTRACT FROM AUTHOR]

We have prepared low-density foam glasses from cathode-ray-tube panel glass using carbon and MnO 2 as the foaming agents. The effect of the glass particle size on the foaming process, the apparent density and the pore morphology is revealed. The results show that the foaming is mainly caused by the reduction of manganese. Foam glasses with a density of < 150 kg m − 3 are obtained when the particle size is ≤ 33 μm (D50). The foams have a homogeneous pore distribution and a major fraction of the pores are smaller than 0.5 mm. Only when using the smallest particles (13 μm) does the pore size increase to 1–3 mm due to a faster coalescence process. However, by quenching the sample from the foaming to the annealing temperature the pore size is reduced by a factor of 5–10. The foams with an apparent density of < 200 kg m − 3 are predominantly open-porous. The foams exhibit a thermal conductivity as low as 38.1 mW m − 1 K − 1 at a density of 116 kg m − 3 . For the investigated foam glasses, there exists a great potential to further decrease their thermal conductivity by increasing the closed porosity and by changing the trapped gas to CO 2 . [ABSTRACT FROM AUTHOR]

We prepare foam glass from cathode ray tube (CRT) panels using MnO 2 as foaming agent at different temperatures for various durations. The reduction of MnO 2 to Mn 2 O 3 leads to formation of O 2 gas, and hence, causes initial foaming. The Mn 2 O 3 particles dissolve into the glass melt and subsequently reduce, causing further formation of O 2 gas and foaming of the glass melt. Increasing the treatment temperature and time enhances foam expansion, Mn 2 O 3 dissolution, and lowers the closed porosity. Once the foam reaches a percolated stage, the foam continues to grow. This is caused by nucleation of new bubbles and subsequent growth. We discuss evolution of pore morphology in terms of pore number density, pore size and closed porosity. The thermal conductivity of the foam glasses is linearly dependent on density. The heat transfer mechanism is revealed by comparing the experimental data with structural data and analytical models. We show that the effect of pore size, presence of crystal inclusions and degree of closed porosity do not affect the overall thermal conductivity. [ABSTRACT FROM AUTHOR]

We prepared low-density foam glasses from cathode-ray-tube panel glass using carbon and MnO 2 as the foaming agents. We investigated the influence of the carbon and MnO 2 concentrations, the glass-powder preparation and the foaming conditions on the density and homogeneity of the pore structure and the dependence of the thermal conductivity on the foam density. The results show that the moderate foaming effect of the carbon is greatly improved by the addition of MnO 2 . A density as low as 131 kg m −3 can be achieved with fine glass powder. The foam density has a slight dependence on the carbon and MnO 2 concentrations, but it is mainly affected by the foaming temperature and the time. The thermal conductivity of the foam-glass samples is lower than that of commercial foam glasses with the same density. The lowest value was determined to be 42 mW m −1 K −1 for a foam glass with a density of 131 kg m −3 . A further improvement in the closed porosity could potentially decrease the thermal conductivity even further, and thus our approach has great potential in terms of a thermal insulation material. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the kinetics and dynamics of the reaction between Na2CO3 and the cathode ray tube panel glass powder at 923–1173K. The reaction causes foaming of the glass melt. After the reaction, the T g decreases with increasing Na2CO3 content and reaches a minimum value of T g. However, this T g value is even lower than that of the homogeneous bulk glass with the same chemical composition. The lower T g of the foam glass could be attributed to inhomogeneous incorporation of Na in the glass, leading to Na-rich domains that cause an overall decrease of T g. Remarkably, after 5min treatment at 1073K, the T g drops by 120K, indicating that the reaction between Na2CO3 and glass is very fast. Increasing treatment duration causes a slight increase of T g likely due to both a more homogeneous Na distribution and the compositional change of the glass as a result of Na2SrSi2O6 crystal formation. [Copyright &y& Elsevier]

Abstract: We prepared foam glasses from cathode-ray-tube panel glass and CaCO3 as a foaming agent. We investigated the influences of powder preparation, CaCO3 concentration and foaming temperature and time on the density, porosity and homogeneity of the foam glasses. The results show that the decomposition kinetics of CaCO3 has a strong influence on the foaming process. The decomposition temperature can be modified by varying the milling time of the glass–CaCO3 mixture and thus for a specific CaCO3 concentration an optimum milling time exists, at which a minimum in density and a homogeneous closed porosity are obtained. Under the optimum preparation conditions the samples exhibit a density of 260kg/m3. The thermal conductivity of the foam glass was measured to be 50–53mW/(mK). The observed dependence of the foaming process on the decomposition kinetics of the foaming agent can be applied as a universal rule for foaming processes based on thermal decomposition. [Copyright &y& Elsevier]

• Commercial expanded vermiculite bonded with potassium silicate. • Expansion process and vermiculite origin affects hydration ability. • Forsterite and leucite form at high temperatures. • Thermal stability of boards depend on the vermiculite origin. Expanded vermiculite bonded with potassium silicate finds use as high-temperature insulation boards. The thermal properties of the vermiculite boards are critical for their applications and expected lifetime. This work reports on the processing and thermal properties of vermiculite boards (400–600 kg m−3) prepared from three different commercial vermiculite sources. Commercial vermiculite is first expanded through flash-heating, then mixed with potassium silicate and then finally pressed to boards. The untreated- and expanded vermiculite and the boards are studied with X-ray diffraction, differential scanning calorimetry, thermogravimetry, heating microscope and electron microscope. The expansion process affects the ability of the expanded vermiculite to reabsorb moisture and the ability is dependent on vermiculite source. The vermiculite boards undergoes several physical-chemical transitions at elevated temperatures. Water from both clay minerals and binder is lost at <250 °C. At 800–1200 °C, the dehydroxylation of clay minerals first occurs, then the boards shrink, and forsterite (Mg 2 SiO 4) and leucite (KAlSi 2 O 6) crystallise subsequently. At 1200–1420 °C, the leucite melts and the boards shrink again. The crystallisation- and melting temperatures depend on the vermiculite source. These findings provide practical guidelines for processing vermiculite into lightweight boards. [ABSTRACT FROM AUTHOR]

• Foaming couples for foaming CRT panel glass in air and argon atmosphere. • Best foaming effect in air achieved with MnO 2 , open-porous foam. • Beneficiary role of oxidizing agent on foaming in oxygen-free atmosphere revealed. • Best foaming effect in argon with carbon black and Mn 3 O 4 , closed-porous foam. • Pore-gas analyzed as in major part CO 2. We report on the use of foaming agent couples to foam cathode-ray-tube panel glass and uncover the influence of the carbon type, Mn x O y and furnace atmosphere on the foaming effect and physical properties of the foams. In oxidizing atmosphere, carbon burns out before the foaming starts and the best foaming effect is achieved with MnO 2. The samples are open porous and thermal conductivity is 47 mW m–1 K–1 at 161 kg m–3. In oxygen-free atmosphere, oxidation of carbon is facilitated by Mn 3 O 4 as an oxidizing agent. Activated carbon with a high reactivity leads to the formation of CO and open porosity. Thermal conductivity of the closed-porous sample with CO 2 in the pores, produced with carbon black and Mn 3 O 4 , reaches 50 mW m–1 K–1 at 204 kg m–3 and exhibits the highest potential for improving the thermal insulation further by decreasing the density. [ABSTRACT FROM AUTHOR]

• Glass foams are made from CRT panel cullet, Mn 3 O 4 , carbon and alkali phosphates. • Pore structures are analyzed by X-ray tomography. • High porosity glass foams are obtained with varying average pore size. • The impact of pore size on the thermal conductivity is discovered. The thermal conductivity (λ) of glass foams is thought to depend on pore size. We report on the impact of pore size, determined using X-ray microtomography, and percentage porosity on the λ of glass foams. Glass foams were prepared by heating powder mixtures of obsolete cathode ray tube (CRT) panel glass, Mn 3 O 4 and carbon as foaming agents, and K 3 PO 4 as additive, to a suitable temperature above T g , and subsequent cooling. Here, we report for the first time a correlation between λ and pore size in the range 0.10–0.16 mm showing a decrease from 57 to 49 mW m−1 K−1 with increasing the pore size for glass foams with porosities of 87–90%. This indicates that the pore structure should be optimized in order to improve the insulating performance of glass foams. [ABSTRACT FROM AUTHOR]

Glass foams are attractive thermal insulation materials, thus, the thermal conductivity (λ) is crucial for their insulating performance. Understanding the foaming process is critical for process optimization. Here, we applied high-speed synchrotron X-ray tomography to investigate the change in pore structure during the foaming process, quantifying the foam structures and porosity dynamically. The results can provide guidance for the manufacturing of glass foams. The 3D pore structures were also used to computationally determine λ of glass foams using image-based modelling. We then used the simulated λ to develop a new analytical model to predict the porosity dependence of λ. The λ values of the glass foams when the porosity is within 40% to 95% predicted by the new model are in excellent agreement with the experimental data collected from the literature, with an average error of only 0.7%, which performs better than previously proposed models. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Low density open-porous-type foamed glass was prepared. • Nucleation agent TiO 2 secured low content of closed pores. • Contributions to the effective thermal conductivity were evaluated. • Solid thermal conductivity contributes 50% to the effective thermal conductivity. Sustainable development focused on energy and material savings is driving attempts to decrease the thermal conductivity of heat insulation materials. Foamed glass is one of the most sustainable thermal insulation materials and has unique, load-bearing properties. In this study, contributions to the effective thermal conductivity were quantified for an open-porous-type foamed glass. Foamed samples were prepared by heating a mixture of waste flat glass, the foaming additives carbon and Mn 3 O 4 , and the nucleation agent TiO 2. The density of the foamed glass was 117 kg m−3 (total porosity 95.8 vol%) with a closed porosity of only 5.8 vol%. The latter was minimized by promoting partial crystallization. The thermal conductivity was 54.3 mW m−1 K−1 at atmospheric pressure. Fitting the pressure dependence of the thermal conductivity gave a base thermal conductivity of 26.7 ± 0.4 mW m−1 K−1, and a gaseous contribution of 27.7 ± 0.5 mW m−1 K−1 with a pore size of 0.77 ± 0.07 mm. Thus, the base thermal conductivity, which is attributed mainly to the thermal conduction in the solid phase, is much higher than in the case of fibrous glass insulation products. [ABSTRACT FROM AUTHOR]

• Development of low-density open- and closed-porous foamed glasses. • Homogeneous pore structure with >90% of open or closed pores. • Open porosity achieved by partial crystallization of the glass. • Low thermal conductivity (37.5–65.6 mW m−1 K−1) measured using heat-flow meter. • Analysis of the difference in the thermal conductivity. Future developments in foamed glass should focus on improving the thermal insulation properties, controlling the foamed structure and the type of porosity. In this study, we report on the synthesis and properties of foamed glasses with open-porous and closed-porous structures prepared from waste cathode-ray-tube (CRT) panel and soda-lime-silica glasses with the addition of carbon black, and manganese or iron oxides. The samples exhibit a homogeneous structure with an average pore size in the range 0.5–1.35 mm and open porosity above 91% or closed porosity above 92%. The open porosity was obtained by partial crystallization of the glass during the foaming process. The thermal conductivity, measured using a heat flow meter at 10 °C, is 57.2–65.6 mW m−1 K−1 at 116–143 kg m−3 for the open-porous samples and 37.5–56.2 mW m−1 K−1 at 107–245 kg m−3 for the closed-porous samples. The conduction in the solid phase contributes the major part to the effective thermal conductivity (52–72%). The conduction in the gaseous and solid phases, respectively, is higher in the open-porous foamed glass due to the different gas composition (air) and the different solid composition (glass composition and crystalline content). The developed foamed glasses also have high mechanical strength, making them suitable for load-bearing applications. [ABSTRACT FROM AUTHOR]