The optimal foaming temperature of soda-lime glass waste with SiC/AlN foaming agent was found experimentally between 850 and 950 ℃. In this temperature range, SiC and AlN oxidize according to Eqs. (1, 2) [20,21,22] releasing gas bubbles through the low-viscosity glass medium. These gases are entrapped through the viscous glass medium leading to expanding the glass matrix giving rise to high-quality cellular materials. The physical, mechanical, thermal, and microscopic properties of the resulting foams have been precisely studied, and the results are presented in subsequent sections.
Figures 2, 3, and 4 display the FESEM photomicrographs of the resulting “GF2.5–7.5” glass foams sintered at 850, 900, and 950 ℃, respectively. The obtained foams were crack-free and they presented predominantly closed porosity with different geometries that varied between hexagonal-, elliptical-, and pentagonal-shaped structures. FESEM photomicrographs were analyzed using ImageJ software and the variations of mean pore diameter and exact cellularity with the sintering temperature and foaming agent content are depicted in Figs. 5 and 6, respectively. The obtained results showed a good compatibility and a reciprocal relationship between the mean pore diameter and cellularity, where the former increased when the latter decreased and vice versa. This inverse relationship between average pore diameter and microcellularity is a logic, where the number of pores per inch (exact cellularity) diminishes as the pores grow larger. As shown in Fig. 6, the average cellularity of the three foams “GF2.5–7.5” at 850 ℃ was approximately 43 ± 20 PPI, a value which declined to 21 ± 10 and 14 ± 6 PPI at 900 and 950 ℃, respectively. In contrast, the average pore diameter of the foams varied from 0.46 to 1.2 mm at 850 ℃; it went up to 1.355 ± 0.545 and 1.925 ± 0.625 mm at 900 and 950 ℃, respectively. Such increase in the pore diameter with the sintering temperature could be explained in terms of a decrease in the glass viscosity at a higher sintering temperature and the increase in gas pressure in the pores which, in turn, force the pores to develop and the entire glass to expand further. Regardless of the sintering temperature, the mean pore diameter of the produced foams increased with the increase in foaming agent content from 2.5 to 5.0 wt.%, and subsequently decreased after increasing the SiC/AlN content above 5 wt.%. This can be explained as follows: (1) when the foaming agent content is low (2.5 wt.%), a small number of gases is evolved leading to small pore diameter and low porosity (2) The average aperture size increased when the foaming agent content increased to 5 wt.% due to the release of more gases, and coalescence between adjacent pores, a phenomenon that forced the pores to grow larger [7, 23,24,25] (3) Beyond 5 wt.% foaming agent, The glass viscosity enhancement occurred by incorporation of the oxidation products of the foaming agent (Al2O3 and SiO2) seems to be more responsible for the reduced bubble size. A similar trend for the variation of the mean pore diameter with the foaming agent content was observed in several previous works. In light of these findings, the sintering temperature and the amount of pore-forming agents are the primary factors controlling the porosity variation in the final cellular foam products.
Phase composition evolution
Powder XRD patterns of glass foam samples “GF2.5–7.5” fired at 850, 900, and 950 ℃ were displayed in Fig. 7a, b, c, respectively. The resulting foams had an amorphous nature with a wide halo in the 2θ range from 15 to 35° which is characteristic for the amorphous silica with silanol group (Si–OH) [26, 27]. Nonetheless, partially crystalline phases with very low-intensity diffraction peaks were observed in these glass foams, such as: (1) cristobalite phase (SiO2, at 2θ = 21.72°, #JCPDS#01-082-0512) that was mainly detected in “GF2.5” specimen (2) diopside phase (CaMg(SiO3)2, at 2θ = 29.92 and 35.58°, #JCPDS#00–017-0318) which was dectected in “GF5.0–7.5” specimens. Diopside and cristobalite crystallization was reported in a number of glass foams produced from glass cullet and various foaming agents [7, 12, 28,29,30,31].
The densification parameters reflect what happened for the microstructure of the produced foams during sintering. Figures 8 and 9, respectively, show the dependence of bulk density and total porosity of the fabricated foams on the sintering temperature and foaming agent content. Apparently, the bulk density decreased throughout the studied temperature range, and contrarily the total porosity increased with increasing SiC/AlN content from 2.5 to 5 wt.%. This can be easily explained in terms of the formation of larger quantities of gas bubbles as the amount of foaming agent increases . With the SiC/AlN content increase beyond 5 wt.%, a reverse trend occurred, where the density increased and the porosity decreased, featuring V and Ʌ-shaped trends, respectively. The decrease in porosity at the high SiC/AlN content (< 5 wt.%( was attributed to the generation of excessive gas bubbles, breakdown of the struts, and collapse of the structure. At 850 ℃, foamed samples expanded slightly to 3.5 times compared to the green body, giving rise to foam with slightly high density (0.35 ± 0.5 g/cm3) and low porosity (89 ± 2 vol.%), suggesting that the viscosity at this foaming temperature was still relatively high. When the foaming temperature increased to 900–950 ℃, foamed samples were expanded clearly to 5–5.5 times compared to the green body, resulting in foam with quite low density (0.25 ± 0.01 g/cm3 at 900 ℃ & 0.19 ± 0.01 g/cm3 at 950 ℃) and high porosity (92 ± 0.5 vol.% at 900 ℃ & 93.5 ± 0.5 vol.% at 950 ℃). The most efficient foaming effect was registered for samples containing 5 wt.% SiC/AlN, where GF5.0–900 has expanded to around 5 times compared to the green body, registering 92 vol.% porosity, 0.24 g/cm3 bulk density, 12 PPI cellularity, and 1.91 mm pore diameter, whereas GF5.0–950 specimen has expanded to about 5.5 times, recording 94 vol.% porosity, 0.18 g/cm3 bulk density, 10 PPI cellularity, and 2.54 mm pore diameter. Based on the densification results, the best sintering temperature was 900 to 950 ℃, and the optimal foaming content was ranged from 2.5 to 5.0 wt.%.
Cold crushing strength (CCS)
The variation in the CCS of the resulting foams as a function of sintering temperature and SiC/AlN content is illustrated in Fig. 10. The CCS values of the resulting foamed specimens correlated well with their bulk density values. The foamed specimens at 850 ℃ demonstrated the highest CCS values at the expense of their elevated densities. The glass foams sintered at 850 ℃ had compressive strength values between 1.5 and 4.5 MPa, whereas the CCS values decreased to 0.7–1.7 MPa and 0.3–0.8 MPa at 900 and 950 ℃, respectively. Such decrease in the CCS values corresponds to the porosity enhancement and density decline at higher sintering temperatures, since the compressive strength of porous ceramics is basically dependent on the ceramic particles around pores (effective load-bearing struts) which decrease with the increase in porosity at higher sintering temperatures [32, 33]. These findings are in agreement with the reported data in references [13, 34]. The obtained properties for the waste-derived foams synthesized in this work compare well with those displayed by commercially available glass foams in terms of bulk density (0.18–0.4 g/cm3), total porosity (89–94 vol.%), and compressive strength (0.3–4.5 MPa) .
Figure 11 shows the variation of the thermal conductivity of the produced foams with the sintering temperature and SiC/AlN content. The thermal conductivity of glass foams had an inverse proportion with porosity as well as aperture size and homogeneity . This means that the thermal insulation of the glass foams is improved with the increase in porosity and aperture size provided that the pores are uniformly distributed and orderly arranged since the less evenly distributed pores in the glass foam, the smaller the thermal resistance and, consequently, the worse the thermal insulation. Depending on foaming agent content as well as foaming temperature, thermal conductivity of the resulting glass foams ranged between 0.09 and 0.16 W/m K. These values are comparable to those obtained for the foamed materials prepared from the soda-lime glass waste and AlN foaming agent .