# Production of Porous Ceramic Materials from Spent Fluorescent Lamps
## 3 Results and Discussion
### Activation of Coarse Powders (SFL > 20 \(\mu\)m)  
The first series of experiments concerned the activation of an aqueous suspension of coarse SFL powders with NaOH, used in the same molarity (2.5M) adopted with the first experiments on 'inorganic gel casting' of soda-line glass [2]. This approach had been successfully extended to a number of waste-derived glasses for manufacturing highly porous foams already at room temperature by frothing [17; 18]. Figure 2a illustrates the typical structure obtained from 'inorganic gel casting' applied to a waste-derived glass (glass from the plasma processing of municipal solid waste) [27]: a system of interconnected pores is determined after the consolidation of glass particles around gas bubbles. Figure 2b clearly testifies that when operating with coarse SFL, on the contrary, a similar foamed structure could not be developed. The poor control of the microstructure in the green state could be ascribed to the particular morphology of SFL powders, consisting of edgy  
Figure 1: Mineralogical analysis of SFL fractions. platelets, which probably complicated the mutual sliding. Anyway, the gelation, with simultaneous freezing of interstitial porosity, is evident from Figure 2c, showing particles 'glued' by a thin surface layer. The reduction of NaOH molarity (1 M), again, did not determine any foaming; as shown by Figure 2d, however, the binding of adjacent particles was confirmed. The setting of suspensions was studied by means of infrared spectroscopy (in absorbance mode) and mineralogical analysis, as illustrated by Figure 3a,b. From FTIR analysis (Figure 3a), it can be clearly noted that the material before activation and after firing at 700 \({}^{\circ}\)C had nearly identical spectra, whereas the activated state was distinguishable for a much larger band in the 3000-3500 cm\({}^{-1}\) and for a quite neat absorption peak centred at approximately 1450 cm\({}^{-1}\). Such signals are consistent, according to the identification of bands performed by [PERSON] et al. [28], with the formation of a hydrated calcium silicate (C-S-H) gel and carbonate compounds, respectively. Figure 2: (**a**) An example of ‘green’ foam from frothing of an alkali-activated suspension of waste-derived glass; (**b**) a porous structure from frothing the suspension of coarse SFL powders in 2.5 M NaOH aqueous solution; (**c**) SFL powders bound by gel from activation with 2.5 M NaOH aqueous solution; (**d**) SFL powders bound by gel from activation with 1 M NaOH aqueous solution. Calcium carbonate (calcite) was already present in the as-received state (see Figure 1 and bottom pattern of Figure 3b). This specific phase was the only to disappear, undergoing decomposition, upon firing at 700 \({}^{\circ}\)C (see the top pattern in Figure 3b); the rest of the crystal phases (Y\({}_{2}\)O\({}_{3}\), fluorapatite, sandornite) remained unchanged. The activation did not lead to any transformation of calcite, considering the unaltered height of the relative diffraction maxima. The sharp peak at 1450 cm\({}^{-1}\), in the FTIR spectrum, was thus attributed to the contribution of an extra carbonated phase. A possible explanation could be represented by the formation of amorphous calcium carbonate mixed with amorphous C-S-H gel or amorphous C-S-H gel with some structural modifications [29]. The bands, centred at 2900 cm\({}^{-1}\) and visible only in the hardened glass suspension, were attributed to C-H vibrations of the organic surfactant [30]. The change in the activating solution (from 2.5 M to 1 M NaOH) had some consequences on the nature of the compounds responsible for the same setting, as illustrated by Figure 3c,d. In the FTIR pattern of hardened glass suspension (Figure 3c), the large band in the 3000-3500 cm\({}^{-1}\) was visible again, while the absorption peak at approximately 1450 cm\({}^{-1}\) was much reduced.