# Glass-Ceramic Foams from Alkali-Activated Vitrified Bottom Ash and Waste Glasses
## 3 Results and Discussion
At 950 \({}^{\circ}\)C, as shown in Figure 3, VBA predominantly crystallized into gehlenite (Ca\({}_{2}\)Al\({}_{2}\)SiO\({}_{7}\), PDF#79-2421), a relatively silica-poor calcium aluminosilicate, consistent with the low silica content of the starting VBA. For both types of glass addition, the intensity of distinctive gehlenite diffraction lines decreased substantially by more than 30% (amount of glass added). Such a decrease would be expected in the hypothesis of glasses simply providing an inert liquid phase, acting as a glue for VBA particles. SLG and BSG undergo softening well below the T\({}_{\text{g}}\) of VBA (560 \({}^{\circ}\)C for SLG and 720 \({}^{\circ}\)C for BSG [24]), and they modified the crystallization sequence substantially. Figure 1: XRD analysis of starting materials and foams formed after alkali activation: (**a**) overall comparison; (**b**) detailed view of the patterns of the products of activation of VBA mixed with SLG and BSG glasses. The addition of SLG led to consolidation at temperatures of 900 \({}^{\circ}\)C and higher. However, as shown in Figure 4a, gehlenite was already formed at 850 \({}^{\circ}\)C, along with wollastonite (CaSiO\({}_{3}\), PDF#89-6458). The wollastonite peaks increased with increasing firing temperature. Moreover, above 900 \({}^{\circ}\)C, the formation of nepheline (NaAlSiO\({}_{4}\), PDF#76-1864) and some modification in the structure of gehlenite could not be excluded. Gehlenite belongs to melilite phases, which are known to form a wide range of solid solutions. The peaks at 850 \({}^{\circ}\)C were compatible with gehlenite, whereas the pattern at 850 \({}^{\circ}\)C was compatible with gehlenite. Figure 3: Decrease of gehlenite diffraction peaks, at 950 \({}^{\circ}\)C, caused by the addition of SLG and BSG glass. Figure 2: DSC plot for VBA powder, showing glass transition temperature (T\({}_{\rm g}\)) and crystallization temperature (T\({}_{\rm cryst}\)). 950 \({}^{\circ}\)C had a better match with Cr-modified gehlenite (Ca\({}_{2}\)Al(Al(Si\({}_{0.974}\)Cr\({}_{0.026}\))O\({}_{7}\), PDF#77-1147). Cr\({}^{3+}\) inclusions in the melilite crystals are common, and were previously investigated due to the possible enhancement of acoustic and nonlinear characteristics of this crystal group [25]. The formation of solid solutions was also detected in the case of wollastonite, since the peaks matched better with Fe-containing variant (Ca\({}_{2.87}\)Fe\({}_{0.13}\)(SiO\({}_{3}\))\({}_{3}\), PDF#83-2198), from 950 \({}^{\circ}\)C.  
The formation of wollastonite, as a transformation of gehlenite, is not surprising. [PERSON] and [PERSON] [26], for example, produced wollastonite-based glass ceramics by mixing SLG with a gehlenite-based crystallization promoter (deriving from the firing of kaolin clay, CaCO\({}_{3}\), BaCO\({}_{3}\), and ZnO). VBA mixed with SLG likely acted similarly to the crystallization promoter, where at the same time the diffusion of aluminum ions in the glass may justify the formation of nepheline. The curved background of the diffraction patterns of samples from VBA-BSG, shown in Figure 4b, suggests the presence of a residual amorphous phase. The VBA-BSG interaction was much more substantial than that of VBA-SLG, which was already expressed by the more intense wollastonite crystallization at 850 \({}^{\circ}\)C. More importantly, above 900 \({}^{\circ}\)C, wollastonite solid solution became the main phase. Interestingly, nepheline crystallized at 850 \({}^{\circ}\)C, but with increasing temperature it was replaced by labradorite (Ca-Na feldspar, Ca\({}_{0.64}\)Na\({}_{0.35}\)(Al\({}_{1.63}\)Si\({}_{2.37}\)O\({}_{8}\)), PDF#83-1371). The more intensive interaction between VBA and BSG compared to that between VBA and SLG could be attributed to the different chemical formulation. BSG contains a higher content of Al\({}_{2}\)O\({}_{3}\), a lower amount of Na\({}_{2}\)O, and negligible CaO contribution.