Foam glass using sodium hydroxide as foaming agent: Study on the reaction mechanism in soda-lime glass matrix

Abstract Foam glasses have potential of using in several fields due to their high porosity. Usually, the foam glasses are prepared though chemical approach where the powdered glass is mixed with foaming agents at high temperatures. However, in such approach, the emission of gaseous pollutants (e.g. CO 2 ) from the most used foaming agents can be harmful to the environment. Thus, understanding the mechanisms of foam formation is quite important to eliminate the drawbacks afore mentioned, as well as, give us more information on the processing steps. Herein, we propose a new chemical route to obtain foam glasses from soda-lime glass waste using sodium hydroxide and borax as foaming and flow agents, respectively. The reaction mechanism was investigated by thermogravimetric and differential thermal analysis, mass spectroscopy, mercury porosimetry, optical dilatometry, X-ray diffraction, and infrared spectroscopy. At the glass transition temperature (587 o C), the crystallization of a new phase of hydrated sodium and calcium silicate (Na 2 CaSi 2 O 6 .2H 2 O) was observed. As demonstrated by mass spectrometry, when the mixture was heated to a temperature above the softening point, the foaming occurred only due to steam released from the hydrated sodium and calcium silicate. This foaming process provided a foam glass with 86% of porosity, low permeability and bulk density of 0.3 g/cm 3 . The results indicate the proposed approach is environmentally safe, besides being relatively cheap and easy to prepare.