# Glass-Ceramic Foams from Alkali-Activated Vitrified Bottom Ash and Waste Glasses
## 3 Results and Discussion  
The VBA demonstrated different behavior from results obtained in previous studies [10; 14]. In fact, \"green\" foams from alkali activation and mechanical foaming, after drying, were extremely brittle. This could be interpreted as the consequence of limited gel formation at the surface of VBA particles, upon attack in weakly alkaline solution. No transformation could be inferred from FTIR spectroscopy (not reported, for the sake of brevity) or diffraction analysis, as shown in Figure 1a. VBA appears to be fully amorphous, with a distinct halo centered at 20-30\({}^{\circ}\), except for traces attributed to magnesium silicate (olivine, Mg\({}_{2}\)SiO\({}_{4}\), PDF#87-2039) and magnesium aluminate (spinel, (Mg\({}_{0.75}\)Al\({}_{0.25}\))(Al\({}_{0.875}\)Mg\({}_{0.125}\))\({}_{2}\)O\({}_{4}\), PDF#86-0088). \begin{table}
\begin{tabular}{c c c c} \hline  & **Vitrified Bottom Ash** & **Soda-Line Glass** & **Borosilicate Glass** \\  & **(VBA)** & **(SLG)** & **(BSG)** \\ \hline SiO\({}_{2}\) & 32.1 & 71.9 & 72 \\ B\({}_{2}\)O\({}_{3}\) & - & - & 12 \\ CaO & 33.5 & 7.5 & 1 \\ Na\({}_{2}\)O & - & 14.3 & 6 \\ MgO & 4.3 & 4.0 & - \\ Al\({}_{2}\)O\({}_{3}\) & 27.5 & 1.2 & 7 \\ Fe\({}_{2}\)O\({}_{3}\) & 0.3 & - & - \\ K\({}_{2}\)O & 0.2 & - & 2 \\ TiO\({}_{2}\) & 0.6 & - & - \\ Residues & 1.6 & 1.1 & - \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition (in wt %) of waste streams. Stronger foams could be obtained, already in the \"green\" state, with SLG and BSG addition. As shown in Figure 1a, SLG actually contained some mineral impurities, such as quartz (PDF#79-1906), sodium, and potassium aluminosilicates (albite, NaAlSi\({}_{3}\)O\({}_{8}\), PDF#09-0466; microcline, KAlSi\({}_{3}\)O\({}_{8}\), PDF#19-0932) and iron silicate (hedenbergetic, CaFeSi\({}_{2}\)O\({}_{6}\), PDF#71-1501). Such impurities disappeared after alkaline attack; the VBA-SLG mixture underwent hardening according to the formation of a semicrystalline gel, featuring a hydrated sodium carbonate phase (thermonatrite, Na\({}_{2}\)CO\({}_{3}\):H\({}_{2}\)O, PDF#76-0910), as already found in previous experiments with VBA of different chemical composition [14]. The matching of most peaks in the pattern of activated VBA-SLG mixture with those of thermonatrite is further demonstrated in Figure 1b. BSG glass was completely amorphous. Interestingly, owing to the different chemical formulation, the amorphous halo was placed at a much lower 20 than in the case of VBA. Probably due to limited dissolution upon the alkaline attack, the signal persisted in the pattern of activated VBA-BSG mixture (see the shoulder at 20-22\({}^{\circ}\) in the top pattern in Figure 1a). BSG addition also led to the formation of a semicrystalline gel, with the presence of thermonatrite (Figure 1b). The broad peak at 20-30\({}^{\circ}\) suggests the formation of an additional phase that is compatible with the formation of hydrated calcium silicate (tobermorite, Ca\({}_{5}\)(Si\({}_{6}\)O\({}_{16}\))(OH)\({}_{2}\), PDF#89-6458). The glass addition also had a fundamental impact on the viscous flow sintering. The DSC plot shown in Figure 2 suggests that VBA presents a narrow sintering window, with a first crystallization peak at T\({}_{\text{cryst,1}}\)\(\sim\) 940 \({}^{\circ}\)C, only 100 \({}^{\circ}\)C above the glass transition temperature (T\({}_{\text{g}}\)\(\sim\) 840 \({}^{\circ}\)C). Therefore, the viscous flow of softened VBA, upon heating above the T\({}_{\text{g}}\), could be compromised by the strong viscosity increase associated with crystal precipitation [23]. This explains why the foams from pure VBA did not undergo a significant consolidation upon firing at 950 \({}^{\circ}\)C. The foams, which were already extremely brittle in the \"green\" state, could be crushed even by gentle hand pressure, after firing.