Innovative use of thermally treated cement-asbestos in the production of foaming materials: Effect of composition, foaming agent, temperature and reaction time

The management of asbestos-containing wastes in European Union (EU) is still based on landfilling, which, in the long term, may pose a problem of dispersion of their carcinogenic fibres into the air and water. Therefore, solutions based on inertization of asbestos fibres and subsequent recycling are preferred. In this study, following the principles of circular economy, thermally treated cement-asbestos (KRY⋅AS) and glass wastes (RG) were used as main raw materials to manufacture foam glass, given its potential to incorporate waste materials. However, the low carbon compound content of both raw materials and the high amount of thermostable crystalline phases contained in KRY-AS made the introduction of foaming agents and powerful fluxing agents necessary. Contrary to what was expected, the addition of Na2CO3 as fluxing agent was observed to be ineffective in the tested compositions, because of the devitrification of crystalline phases, namely åkermanite (Ca2MgSi2O7) and combeite (Na2Ca2Si3O9), which reduced the formation of vitreous phase, limiting the foaming process. Li2CO3 as alternative fluxing agent was chosen, despite being aware of its current higher cost. The methodology developed has allowed to investigate the influence of the RG:KRY⋅AS ratio, the type and content of the fluxing agents and the content and size of the foaming agent (SiC), to establish the optimum firing conditions (maximum temperature and soaking time), as well as to unveil the main foaming mechanisms involved. Finally, specimens with good properties were obtained with a composition with high waste material content, namely (values in weight) 55% RG, 40% KRY-AS, 5% Li2CO3 as fluxing agent and 2.5% SiC-1500 as foaming agent, when fired at temperatures and soaking times of 1100 ◦C and 1 min respectively. The obtained product presented values of apparent density (0.55–0.68 g/cm3) and compressive strength (0.56 MPa) comparable with those reported in the literature for foam glasses obtained from wastes and close to commercial foam glasses. Despite the technical and economic limitations of the process, this work opens up new possibilities in a circular economy context for waste materials which encountered serious problem to be recycled and mostly ended up in landfills