Insights into Reaction Mechanism of Na2CO3 in Foaming Process of Cullet Powder: - Awarded as the Best Poster (1st Prize)

Soda (Na2CO3) is one of the major raw materials for producing most of the daily used glasses. It often acts as effective fluxing agent in glass melting and foaming agent for producing lightweight glass materials due to its release of CO2 at elevated temperatures. When Na2CO3 is added to a batch prior to glass melting, the resulting decomposition product Na2O has strong impact on the melt viscosity, phase separation and crystallisation behaviour, and also mechanical properties of the final glass. This is relatively well understood in literature. In contrast, when Na2CO3 is added as a foaming agent together with glass cullet powder, the impact of the resulting Na2O on the glass structure at the foaming temperatures has not been well understood. Here, we show that Na2O can be readily incorporated into the network structure of the glass phase at temperatures much lower than normal batch melting temperatures. This is verified by a sharp decrease of the glass transition temperature (Tg) of the resulting foam glass when the foaming temperature is increased from 650 to 800 °C for various contents of Na2CO3 (up to 22 wt%). Upon further raising the foaming temperature, Tg reaches a plateau for lower Na2CO3 content, whereas it slightly increases again for the higher Na2CO3 due to possible crystallization. The sharp decrease in Tg implies that Na2CO3 reacts readily with the cullet powder at rather low temperatures. The released CO2 at an optimum temperature causes ideal foaming effect in the glass, making the studied cullet powder potentially suitable for producing insulation materials. Finally, we discuss these results based on supplementary x-ray diffraction and scanning electron microscopy analyses.