Gas-releasing reactions in foam-glass formation using carbon and MnxOy as the foaming agents

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 (MnO2, Mn2O3, and Mn3O4) as the foaming additives. We show that manganese oxides have different functions in the foaming process. The thermal decomposition of MnO2 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, Mn2O3 can be used as the foaming agent in an oxidizing atmosphere. In the oxygen-free atmosphere, Mn3O4 can be used as the oxidizing agent, supporting the oxidation of carbon and the foaming process. The redox equilibrium of manganese (Mn2+/Mn3+), 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/CO2 ratio in the evolved gases depends on the carbon source and the temperature.