# Production of Porous Ceramic Materials from Spent Fluorescent Lamps
## 3 Results and Discussion
### Activation of Fine Powders (SFL < 20 \(\mu\)m)  
The application of the second processing conditions (1 M NaOH) to fine powders, instead of coarse powders, did not allow any setting. The low quantity of glass (inferred from Figure 1) evidently did not suffice for the gelation of suspensions. In an attempt to promote the gelation, the fine fraction of phosphors was added with waste glass deriving from the purification of soda- lime container glass (SLG) cullet. The SFL/SLG weight proportion was kept at 9:1 so that the glass amount remained quite limited. Anyway, the glass addition sufficed in promoting the gelation, in turn, exploited for the development of 'green' foams, consisting of a multitude of adjacent pores (each corresponding to the consolidation of the starting suspension around air bubbles, incorporated by intensive mechanical stirring), as illustrated by Figure 5a,b. The 'gluing' of particles is evident, in particular, from Figure 5c,d.  
As shown by Figure 6a, the FTIR spectrum for the activated mixture differed from starting material and fired material only for the band attributed to C-S-H compounds (3000-3500 cm\({}^{-1}\)), to the surfactant (2900 cm\({}^{-1}\)) and to carbonate compounds (1450 cm\({}^{-1}\)). The signals were quite weak, consistently with the limited amount of SLG; such constraint is the reason for no appreciable changes in the diff  
Figure 5: Microstructural details of ‘green’ foams from fien SFL powder/SLG mixture after activation at low temperature: (**a,b**) overall cellular structure; (**c,d**) binding of adjacent particles.  
A reduced solid loading probably promoted the solution/solid interaction. The same bands attributed to C-S-H, surfactant, and carbonates were slightly more intense (Figure 6c), but again, no newly formed compound could be detected from mineralogical analysis (Figure 6d). A possible explanation could be represented by the formation of amorphous calcium carbonate mixed with amorphous C-S-H gel.  
The softening of the SLG additive enabled the consolidation of foams upon firing at 700 \({}^{\circ}\)C; according to the limited glass content, however, there was no collapse of the cellular structure determined already in the green state, as shown by Figure 7a,b. The effective flow of SLG is evident from Figure 7c,d, showing the inclusion of fine particles in a glass mass. Figure 7d, in particular, is interesting for the light-coloured spots associated (in backscattered electron images) with heavy elements, such rare earth elements: phosphor particles were not simply embedded in the glass but, according to their shaded contours, underwent some interdiffusion with the matrix.  
Figure 6: The evolution of samples from fine SFL powder/SLG mixture after activation, low-temperature foaming and drying, prepared according to different solid content: (**a,b**) 68%; (**c,d**) 66%.  
Unlike those from coarse powders, foams from a fine fraction of SFL did not exceed any leaching limit for heavy metal elements (see Table 3). The limited densification of struts (Figure 5c,d) evidently limited the compressive strength of the new foams (not exceeding 2.2 MPa, and corresponding to a strength-to-density ratio not exceeding 2.6 MPa\(\cdot\)cm\({}^{3}\)/g), as reported by Table 2. However, the obtained cellular materials, for the abundant and tortuous porosity (well above 70 vol%), may be interesting when applied as not-load-bearing thermal and acoustic insulators. Both density and compressive strength of foams from fine fraction of SFL actually fall in the typical range of commercial insulators, such as aerated concrete (density from 0.4 to 0.9 g/cm\({}^{3}\), compressive strength from 1.2 to 1.9 MPa) [33].