# Glass-Ceramic Foams from Alkali-Activated Vitrified Bottom Ash and Waste Glasses
## 1 Introduction  
The impacts of human activities and the consequential waste production have become one of the most discussed topics in the 21 st century. Currently, waste management aims to tackle landfilling through recycling and incineration. However, both of these options to some degree yield byproducts that are still destined for landfill. The reuse of cullet by remelting into new glass products is an excellent example of imperfect recycling. Within the EU alone, in 2014, the production of glass waste was close to 18.5 million tonnes, but it is estimated that \(\sim\)25% of this waste remained unrecovered [1]. Even the adoption of the most sophisticated recycling systems does not imply the full recovery of soda-lim glass (SLG), especially concerning finely powdered fractions, enriched in contaminants, which are landfilled [2], along with specialty glasses, like opal [3], sheet [4], LCD [5], or pharmaceutical [6] glasses, or glass fibers [3]. Mixed municipal waste is, by definition, unrecyclable. It is the most abundant waste category due to ever-growing consumption, and in this case, incineration is single-handedly favored [7]. A fundamental byproduct of incineration consists of bottom ash, which is 20-25% of the waste input volume. Such ash, 90% of which is composed of minerals and glass [8; 9], is in turn subjected to secondary thermal treatments, mainly aimed at metal recovery, with formation of metal-free vitrified bottom ash (VBA) [10]. It is worth mentioning that in Europe, 10 countries are directly landfilling VBA, and only 5 countries utilize it for more than 80% of the produced amount [11]. If utilized, VBA is used for civil engineering projects (i.e., construction of highways) [11], with only limited vaporization. However, in recent years, new business has emerged in the post-treatment technologies of bottom ashes and in the production of new materials from the perspective of \"end-of-waste\" philosophy [12]. In the past, VBA was investigated in products such as tiles and bricks [1]. More recent studies have been concerned with reuse of cellular glass-based materials, which are particularly attractive from an environmental perspective. Energy and emissions savings in reusing waste-derived glasses (instead of glasses from conventional raw materials) are accompanied by savings in their long service life. Glass-based foams are thermal and acoustic insulators with much greater durability than polymer foams [13]. [PERSON] et al. [13] successfully prepared ceramic foams by pressing and sintering VBA mixed with foaming agents, such as borax and calcite. [PERSON] et al. [14], on the contrary, could develop foams from VBA without foaming additives. Foams were prepared by the intensive mechanical stirring of glass suspensions in weakly alkaline solutions, followed by curing at near room temperatures, and sintering with concurrent crystallization. The alkaline activation was not intended to determine an extensive dissolution (as occurring in inorganic polymers, developed by means of solutions with higher alkalinity and including synthetic components such as alkali silicates [15]). It was specifically aimed at the formation of gels at the surface of glass powders, in turn enhancing the viscosity of suspensions and stabilizing the cellular structures developed upon stirring [16]. Due to its origin, VBA might contain hazardous metals (Cr, Ni, Pb, Hg, etc.) and corrosive pollutants (Cl\({}^{-}\), F\({}^{-}\), SO\({}_{3}\)\({}^{2-}\)), so any upcycling product must also be studied from the aspect of chemical stability [17; 18]. Studies on VBA-derived glass-ceramic foams have yielded generally safe materials, that is, with very low leaching values [10; 13; 14; 19]. In any case, any processes should be verified according to the variability of chemical compositions. In this study, we focused on the production of glass-ceramic foams from relatively silica-poor VBA, exploiting the above-mentioned process of weak alkali activation and mechanical foaming, before viscous flow sintering.