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• Mixture of glass powder and water glass is atmosphere sensitive. • Reaction with CO 2 results in the formation of carbonates. • Newly formed carbonates are the main source of expansion gas (CO 2). • New mechanism of foaming with water glass proposed. Water glass is a common additive in the process of direct foaming of glass, however, the currently accepted foaming mechanism lacks experimental validation. A foaming mixture of waste cathode ray tube panel glass and water glass was investigated for its expansion and gas evolution behavior via hot stage microscopy and thermogravimetry coupled with mass spectroscopy, respectively. The evolution of a significant amount of CO 2 during the heating of the foaming mixture was detected and an overwhelming presence of CO 2 within the pores of the foamed glass was confirmed with gas chromatography. With this investigation, we reveal an underlying foaming mechanism, where a carbonate phase forms after mixing water glass with the glass powder and later acts as the foaming agent. The unveiled mechanism could help further research in the field of foamed glass, with the potential to produce more sustainable materials with better performance via less energy-demanding processes. [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]