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Influence of the glass–calcium carbonate mixture's characteristics on the foaming process and the properties of the foam glass
Available online at www.sciencedirect.com
ScienceDirect Journal of the European Ceramic Society 34 (2014) 1591–1598
Influence of the glass–calcium carbonate mixture’s characteristics on the foaming process and the properties of the foam glass Jakob König a,b,1 , Rasmus R. Petersen a , Yuanzheng Yue a,∗ a b
Section of Chemistry, Aalborg University, Sohngårdsholmsvej 57, DK-9000 Aalborg, Denmark Advanced Materials Department, Joˇzef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
Received 19 August 2013; received in revised form 9 December 2013; accepted 10 December 2013 Available online 3 January 2014
Abstract We prepared foam glasses from cathode-ray-tube panel glass and CaCO3 as a foaming agent. We investigated the influences of powder preparation, CaCO3 concentration and foaming temperature and time on the density, porosity and homogeneity of the foam glasses. The results show that the decomposition kinetics of CaCO3 has a strong influence on the foaming process. The decomposition temperature can be modified by varying the milling time of the glass–CaCO3 mixture and thus for a specific CaCO3 concentration an optimum milling time exists, at which a minimum in density and a homogeneous closed porosity are obtained. Under the optimum preparation conditions the samples exhibit a density of 260 kg/m3 . The thermal conductivity of the foam glass was measured to be 50–53 mW/(m K). The observed dependence of the foaming process on the decomposition kinetics of the foaming agent can be applied as a universal rule for foaming processes based on thermal decomposition. © 2013 Elsevier Ltd. All rights reserved. Keywords: Foam glass; Cathode ray tube; Thermal decomposition; Thermal conductivity
1. Introduction The disposal of obsolete cathode ray tubes (CRTs) has become a global environmental problem.1 The amount of CRT waste increases worldwide, but no proper recycling scheme for CRT glass has been widely applied. CRT glass represents up to two-thirds of a TV’s weight and in the UK alone more than 100,000 tons of CRT glass have been disposed of annually since 2003.2,3 The possibilities for recycling CRT glass are limited, due to high chemical quality requirements of glass production and the declining production of new CRTs. Moreover, the recycling possibilities are hindered by the presence of lead in part of the CRT. A color CRT consists, in general, of two types of glass: barium/strontium-containing glass (the panel glass) and lead-containing glass (the funnel and neck glass). In the past decade new separation methods have been developed with the aim being to increase the amount of recycled CRT glass.4,5
∗
Corresponding author. Tel.: +45 9940 8522. E-mail addresses: [email protected] (J. König), [email protected] (Y. Yue). 1 Permanent address: Advanced Materials Department, Joˇ zef Stefan Institute, SI-1000 Ljubljana, Slovenia. 0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.12.020
The strategy is first to separate the lead-containing glass from the lead-free panel glass. The latter can be recycled as nonhazardous glass in foam glass,6 ceramic tiles and bricks,2,7 while the former can be used as a flux in smelting furnaces or cleaned of lead by a vitrification process.2,5 However, the recycling of CRT glass remains limited and a large proportion of CRT glass is still being landfilled.2,7,8 Hence, existing recycling methods have to be improved or a new method has to be developed. Here, we will focus on foam-glass production as it is one of the most promising recycling technologies for CRT glass. Foam glass is used as a thermal and acoustic insulating material in buildings, road construction and industry.9 Foam glass is produced by mixing glass powder with a foaming agent and subsequently heating the mixture above the glass-softening point. During the heat treatment the foaming agent releases gases that form dispersed bubbles in the softened glass. The expanding gas bubbles increase the volume of the sample, thus forming a typical porous lightweight product. After the heat treatment the glass is cooled again in order to freeze-in the bubbles in the glass melt. Gaseous products are generated by thermal decomposition, e.g., carbonates and sulfates, or through the oxidation of carbonbased agents, i.e., pure carbon (coal, graphite, carbon black,
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etc.), carbohydrates and SiC. The composition of the released gases depends on the foaming agent used and on the composition of the glass. The gases evolved as a result of the decomposition of carbonates or sulfates consist mainly of CO2 and SO2 , respectively. These low-heat-conducting gases contribute positively to the overall heat-insulating properties of the foam glass. Carbon-based foaming agents react with the glass to form a CO/CO2 mixture, but the formation of other gas species is possible, especially when using SiC as a foaming agent.10 The CO/CO2 volume ratio depends on the temperature, but also on the amount of oxygen available in the melt. The residuals of foaming agents or their reaction with the glass have an impact on the foam’s morphology and the glass network. Metal oxides from decomposed carbonates and sulfates can work as a nucleation agent for crystallization during the foaming. Similarly, the formation of SiO2 from SiC will change the viscosity of the glass. Such occurrences can have a negative impact on the melt’s expansion and the foam’s homogeneity. The use of carbon as a foaming agent can lead to reduction of the glass melt, while carbon residuals will not incorporate into the glass network.11 However, due to a change in the valence state of multivalent ions, some changes in the glass network will occur. Foam glass is mainly produced from different types of glass cullet, i.e., container glass,12 flat glass13 and lamp glass,14 but it can also be produced from other glassy materials, like glass from fly ashes15 and amorphous volcanic rocks.16 In order to obtain foams with a low density, glass powders have to be milled to fine powders.9,17 Moreover, the foaming agent’s particle size and concentration need to be optimized to produce homogeneous foams with a low density. In general, the amount of foaming agent added is much higher than theoretically required.6,9,18 Only a few studies have reported on the preparation of foam glass from CRT glass cullet19,20 or from part of it.6,21 In these studies, different foaming agents were used. Applying reducing foaming agents (SiC and TiN) in combination with a lead-containing cullet resulted in a reduction of the cationic lead to metallic lead,8,22,23 which increases its leachability and thus reduces the applicability of the products.19,24 On the other hand, if CaCO3 is used, the lead oxide remains incorporated in the glass network.16,18,25 The amount of added CaCO3 was between 1 and 7 wt%; however, there does not seem to be any trend indicating the optimum CaCO3 concentration. The holding time at elevated temperature should be short in order to prevent the growth and coalescence of the pores, and thus faster heating and cooling is favorable. Another parameter that has a great impact on the foam density and homogeneity of the pore structure is the particle size of the glass powder.9 It was shown that when foaming with carbon the density of the foam glass decreases rapidly with a glass particle size below 100 m.17 On the other hand, an optimum in the glass particle size at 80 m was observed when dolomite was used as the foaming agent.26 In the literature it is common to report the initial particle size of the glass and the foaming agent used, but the glass and the foaming agent are frequently homogenized by milling them together, thereby obscuring the real particle size being employed.
Table 1 Chemical composition of the CRT panel glass. Oxide
wt%
SiO2 Na2 O K2 O BaO SrO ZrO2 Al2 O3 PbO Others
60.1 7.9 7.1 9.5 8.2 1.9 2.5 0.05 2.8
The aim of this work is to highlight the importance of the particle size of the glass and especially the homogenization step on the foaming process and the resulting foam-glass properties. We explore the dependence of the density, porosity and homogeneity of the foamed glass prepared from waste CRT panel glass and CaCO3 on the particle size, milling time, foaming temperature and time. By changing the milling time of the powder mixture, the kinetics of the CaCO3 decomposition were modified. Based on the results, the optimum foaming-agent concentration is discussed. In addition, we measured the thermal conductivity of the prepared foams. 2. Experimental Lead-free CRT panel glass from obsolete color televisions was supplied by a local electronic-waste-management company (Averhoff A/S, Aarhus, Denmark). The composition of the glass was measured using an X-ray fluorescence spectrometer (PW2400, PANalytical, The Netherlands) with SuperQ software. Oxide standards (WROXI, PANalytical) were used for the quantification. The chemical composition of the panel glass shown in Table 1 is in accordance with the reported panelglass compositions.8 Pieces of the panel glass were cleaned with demineralized water in order to remove the coatings on the inner side of the glass. The glass was then crushed in a jaw crusher (BB51, Retsch, Germany), and subsequently dry milled using a planetary ball mill (PM 100, Retsch, Germany). An agate ball mill with 20-mm balls was used. The glass powder was sieved below 63 m. Powder mixtures were prepared by adding 1–10 wt% of CaCO3 (D90 = 8 m; 99.9%, AppliChem, Germany) to the sieved glass powder (<63 m). The powder mixtures were prepared either by nondestructive mixing for 12 h in a vertically spinning carousel (referred to as 0 min milling time) or by milling in the ball mill. The milling conditions used were always the same (the amount of powder, the number of balls, the rotation speed). Then the powder mixtures were uniaxially pressed into disk-shaped pellets. A pressure of 40 MPa was applied for the small samples (1 g, 13 mm diameter) and 30 MPa was used for the large samples (15–35 g, 35 mm diameter). A minimum of three samples were made for each of the preparation and foaming conditions. The pellets were placed in an electrical laboratory-chamber furnace and heated at 10 ◦ C/min to various temperatures in the range 755 to 815 ◦ C. The holding time varied from 5 to 30 min. Afterwards, the samples were cooled at
J. König et al. / Journal of the European Ceramic Society 34 (2014) 1591–1598 Table 2 Particle size (D50 and D90) of the panel glass–CaCO3 mixtures milled for different times. Milling time (min)
D50 (m)
D90 (m)
0 20 45 55
22 6.3 5 4.6
50 18.5 15 14
6 ◦ C/min from the maximum temperature to the glass-transition temperature (Tg ) of 530 ◦ C, and subsequently cooled at 1 ◦ C/min to room temperature. The temperature in the foaming zone was calibrated using an external thermocouple. The particle size was measured with a laser granulometer (Microtrack II, Leeds & Northrup, UK) and the D50 and D90 values for different milling times are given in Table 2. The apparent (ρapp ) and pycnometer (ρpyc ) densities were measured by using Archimedes’ principle in water and a helium pycnometer (ULTRAPYC 1200e, Quantachrome Instruments, USA), respectively. Apparent densities were not affected by the intake of water due to open porosity; mass gain was lower than 0.004 g, which corresponds to an experimental error of 0.1%. The glass powder density (ρpowder ) of the initial glass and the foamed glass samples with different CaCO3 contents was measured with the pycnometer. The addition of CaCO3 only slightly changed the density and all the samples showed a density in the range 2.74 ± 0.02 kg/m3 . Thus the average powder density of 2.74 kg/m3 was used in the calculations. The percentage of open pores (OP) and the percentage of closed pores (CP) in the foam volume (Vfoam = VCP + VOP + Vglass ) was calculated from the apparent, pycnometer and powder densities according to: ρapp × 100 TP [%] = 1 − ρpowder CP [%] =
ρapp ρapp − ρpyc ρpowder
× 100
OP [%] = TP − CP where TP is the percentage of the total porosity in the foam sample. The sintering behavior was investigated using a heating microscope (Hesse Instruments, Germany). The temperature at which the structure of the sample closes as a result of the viscous flow sintering of the glass particles was approximated from the sintering curves during dynamic heating. A thermogravimetric analysis (TGA) was performed on small compacts using a Jupiter 449 C, Netsch, Germany. Due to foaming in the crucible only small amount of mixtures could be tested (∼15 mg). This proved not to be satisfactory to overwhelm small fluctuations in the baseline. Therefore, the TGA results are not interpreted quantitatively. Water content in the milled powders due to adsorption from atmosphere was low, below 0.3%. During the TGA and the heating microscopy the samples were fired in an air atmosphere and the heating rate was 10 ◦ C/min. X-ray powder diffraction
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(XRD; Empyrean, PANalytical, The Netherlands) was used to identify possible crystalline phases. The foamed samples were cut, photographed and the homogeneity of the pore structure was visually evaluated. The thermal conductivity was measured with a surface probe (25 × 25 mm2 ) at 10 ◦ C using a heat-transfer analyzer (ISOMET Model 2104 Heat Transfer Analyzer, Applied Precision, Slovakia). The accuracy of the equipment in the low lambda range was tested by measuring commercial expanded polystyrene and foam glass with known values. The obtained results were in perfect agreement with the declared values. 3. Results and discussion For a successful foaming process it is essential that the gases are released in the sintered body when the porous structure is already closed. Only then can the gases be entrapped in the glass matrix. The decomposition temperature of pure CaCO3 in an air atmosphere is reported to be around 700 ◦ C, which is well above the softening point of the CRT panel glass (∼590 ◦ C).6 However, the thermal stability of the CaCO3 decreases when mixed with glass. To get a deeper insight into the sintering and decomposition kinetics of the glass and the glass–CaCO3 mixtures, heating microscopy and thermogravimetric analyses were applied. The results are shown in Fig. 1. The sieved CRT panel-glass powder densifies at around 690 ◦ C, while with additional milling the sintering temperature decreases by 25 ◦ C (Fig. 1a). The difference in the sintering temperature of the powders milled for 20–55 min is small, since the configuration of the ball mill does not allow a further significant reduction in the particle size. The TGA data on the decomposition of CaCO3 in air show that the first weight percent of the mass loss occurs at 670 ◦ C (Fig. 1b). This indicates that part of the CaCO3 decomposes before the glass particles sinter and close the porous structure. Moreover, with prolonged milling of the glass–CaCO3 mixture the decomposition of the carbonate is shifted to even lower temperatures. Thus in the sample milled for 55 min only a fraction of the carbonate remains in the mixture at the point where the porous structure closes. Furthermore, for the sample milled for 20 min the decomposition of CaCO3 is very rapid above 650 ◦ C. By milling the mixture, the contact area between the glass and the CaCO3 particles increases, because the particle size decreases. Thus the increased contact area and the decreased thermal stability of the CaCO3 in contact with the glass decrease the decomposition temperature of the CaCO3 . The heating-microscope data suggest that the sintering behavior related to the particle size of the glass powders should have a minor effect on the foaming process. However, in contrast, the CaCO3 decomposition kinetics that is affected by the mixture milling is expected to strongly influence the foaming process. The influence of the CaCO3 concentration on the apparent and pycnometer densities of the foams from powders homogenized by mixing in a carousel is shown in Fig. 2. The samples were foamed at 815 ◦ C for 5 min. For the apparent density a minimum at 4 wt% CaCO3 is seen, while for the pycnometer density there is a broad minimum for concentrations between 1.5 and 4 wt%. There is a larger difference between the apparent and
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Fig. 1. (a) Sintering curves for the panel-glass powders with different particle sizes (D90 values); (b) thermogravimetric curves of CaCO3 and of powder mixtures with 4 wt% of CaCO3 milled for different time periods. In (b) the vertical line marks, for the milled samples, the approximate temperature at which the porous structure closes.
Fig. 2. Influence of CaCO3 concentration on the foam densities for the samples homogenized by mixing in a carousel and foamed at 815 ◦ C for 5 min.
the pycnometer density for samples with 2 and 4 wt% of CaCO3 compared to the rest of the samples. However, the addition of 10 wt% of CaCO3 halted the foaming process almost completely as the density of the samples was above 1600 kg/m3 . The morphology of the foams changes with the CaCO3 concentration (Fig. 3a–c). Samples with 1–2 wt% of CaCO3 (Fig. 3a and b) exhibit a dense sintered glass shell around a foamed core. The sample with 1 wt% CaCO3 developed a thicker shell, while the foamed core consists of fine homogeneous pores (Fig. 3a). In the sample with 4 wt% CaCO3 (Fig. 3c) the glass shell is no longer visible. The homogeneity of the pore structure decreases with the increasing addition of CaCO3 . The theoretical concentration of CaCO3 needed to obtain a foam with 90% porosity (i.e., a density of approximately 270 kg/m3 ) was calculated to be 0.4 wt%, assuming a pressure of 1 atm in the pores at the
Fig. 3. Characteristic appearance of the foam samples. (a–c): Samples prepared by mixing, with 1, 2 and 4 wt% of CaCO3 , respectively (815 ◦ C, 5 min); (d) sample prepared by mixing of fine glass powders (D90 = 15 m) with 4 wt% of CaCO3 , (785 ◦ C, 5 min); (e and f) samples prepared by milling, with 4 wt% of CaCO3 , 45 and 55 min milling time, respectively (785 ◦ C, 5 min); (g and h) – samples prepared by milling, with 4 wt% of CaCO3 , 45 min milling, foamed at 785 ◦ C for 15 and 30 min, respectively; (i) 30 g sample with 4 wt% of CaCO3 , 45 min milling (795 ◦ C, 5 min). The ruler for all foam images is shown in right bottom corner.
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foaming temperature (815 ◦ C).9 However, despite much larger additions of CaCO3 than theoretically needed, i.e., 2.5–25 times, the lowest apparent density obtained was 350 kg/m3 . The sintered glass shell of the samples with a low CaCO3 content, i.e., 1–2 wt% CaCO3 , indicates that on the surface of the sample CaCO3 was decomposed before the glass particles sintered and closed the porous structure. In contrast to the surface, part of the CaCO3 in the interior was enclosed by the softened glass and foamed the sample core. The TGA and heating-microscope results (Fig. 1) revealed that CaCO3 starts to decompose before the glass particles sinter. Thus the air atmosphere in the pores of the compressed sample is replaced by the evolved CO2 gas. The partial pressure of the CO2 in the interior of the sample is higher compared to the surface of the sample, which is in contact with the air atmosphere, and influences the decomposition equilibrium of the CaCO3 . Thereafter, the decomposition of the CaCO3 is faster on the surface of the sample. All the carbonate on the surface is decomposed before the porous structure closes and a sintered glass shell is formed by the viscous flow sintering of CaO-enriched glass. In contrast to the surface, part of the CaCO3 in the interior is enclosed by the glass melt and foams the sample core. With an increasing CaCO3 concentration the shell becomes thinner as the surface layer with the completely decomposed carbonate becomes thinner, and is no more visible for the sample with 4 wt% of CaCO3 (Fig. 3c). In this sample, part of the CaCO3 also remains available for foaming on the surface. However, the interior becomes inhomogeneous due to a large amount of formed gas from the carbonate decomposition.27 Consequently, the pore walls start to break and part of the pores becomes open, which is seen from the difference in the apparent and pycnometer density. In the sample with 10 wt% of CaCO3 the porous structure does not close completely and the released gases are able to escape to the atmosphere. Most probably the sintering of the sample with 10 wt% of CaCO3 is hindered by the large content of CaCO3 . However, no crystalline phases were found within the detection limit of the XRD analysis. This indicates that all the CaO was dissolved in the glass matrix during the heat treatment. The optimum CaCO3 concentration in the samples prepared by mixing was found to be 4 wt% and for this concentration the influence of the glass particle size on the foaming process was investigated. For this purpose, glass powders were additionally milled for different periods of time and the particle size of the so-obtained powders was measured. Again, the powder mixtures were homogenized by mixing in a carousel. From Fig. 4 it can be seen that the glass particle size does not have a distinct influence on the density. The change in the apparent density is small, while the pycnometer density of the samples prepared from finer powders increases to above 740 kg/m3 . Moreover, the samples’ homogeneity deteriorates and large voids appear in all of the samples (typical appearance is shown in Fig. 3d). Smaller particles sinter at a lower temperature (Fig. 1a); therefore, more CaCO3 remains in the sintered sample and the released gases break some of the pore walls,27 thus increasing the pore connectivity and therewith the pycnometer density. The glass particle size is an important parameter in the preparation of foamed glass
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Fig. 4. Influence of the panel-glass particle size on the apparent and pycnometer densities of foam-glass samples with 4 wt% of CaCO3 , foamed at 785 ◦ C for 5 min. The powder mixtures were homogenized by mixing in a carousel.
and has a strong influence on the density of the foam.9,17 It was shown that the foam density monotonously decreases with a decreasing particle size. However, these results were obtained when foaming with carbon-based compounds. On the other hand, an optimum in the intermediate particle size of the glass powders was observed when foaming with a thermal decomposition process using dolomite as the foaming agent.26 The results obtained in the present study also suggest that the particle size of the glass powder is not the controlling parameter when CaCO3 is used as the foaming agent. To investigate the impact of the homogenization performed by milling, the sample mixtures were milled for different periods of time. The influence of the milling time on the apparent and pycnometer density of the foams for the samples with 4 wt% of CaCO3 is shown in Fig. 5. A milling time between 0 and 30 min does not influence the apparent density significantly, while a gradual decrease in the pycnometer density is observed. A further increase in the milling time induces two anomalies in the trend. At 37 min of milling, both densities increase, but a further extension of the milling time to 45 min causes a minimum in the densities. At the minimum the apparent and pycnometer
Fig. 5. Influence of the milling time on the apparent and pycnometer densities of the foam glass samples with 4 wt% of CaCO3 foamed at 785 ◦ C for 5 min.
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J. König et al. / Journal of the European Ceramic Society 34 (2014) 1591–1598 Table 3 Apparent and pycnometer densities of the foam-glass samples with 4 wt% of CaCO3 milled for 45 min and foamed at different temperatures for different holding times.
Fig. 6. Influence of the milling time on the apparent and pycnometer densities of the foam-glass samples with 2 and 10 wt% of CaCO3 foamed at 785 ◦ C for 5 min.
densities are almost the same, which indicates the closed-pore structure of the sample. When milling for a time longer than 45 min both densities increase, while the difference remains negligible. The lowest density values obtained were around 260 kg/m3 . Such behavior could be associated with the decrease of the decomposition temperature of the CaCO3 with the milling time. Up to the local maximum at 37 min, the effective amount of CaCO3 that remains available for foaming is high. Moreover, since the decomposition rate of the CaCO3 is high, part of the pore walls break and hence, the pycnometer density is much higher than the apparent density. At the local maximum the samples typically exhibit large void(s) in the interior (similar to that shown in Fig. 3d), and consequently, the consistency of both densities of these samples is lower. On the other hand, the consistency of the samples milled for 45 min was very good. Moreover, the pore structure of these samples was found to be homogeneous (Fig. 3e). The effective amount of CaCO3 available for foaming in the samples milled for 45 min was optimal for the characteristics of the powder mixture, i.e., for the particle size of the glass and CaCO3 powders. When milling for a time longer than 45 min, the effective amount of CaCO3 available for foaming decreases and the density increases, but the homogeneity of the pore structure improves further (Fig. 3f). If the above explanation of the relation between the milling time and the effective amount of CaCO3 available for foaming holds, a similar dependence of both densities on the milling time should also be observed for the samples with a different CaCO3 concentration. Fig. 6 shows such a dependence for samples with 2 and 10 wt% of CaCO3 . Almost identical behavior was observed as in sample with 4 wt% of CaCO3 , where the optimum milling time is shorter and longer, respectively, for the samples with the initially lower and higher CaCO3 concentrations. Moreover, the trend of the homogeneity of the foams was similar, exhibiting a homogeneous porosity for the minimum density. Furthermore, the values of the density in the minima for samples with different CaCO3 contents are comparable, suggesting that there is no optimum CaCO3 concentration, but an optimum milling time that provides a suitably effective amount of CaCO3 enclosed in the sintered glass matrix. Such a preparation results
Temperature (◦ C)
Time (min)
Apparent density (kg/m3 )
Pycnometer density (kg/m3 )
755 785 785 785 815
5 5 15 30 5
312 259 259 391 301
312 261 288 435 311
in low-density foams with a homogeneous, closed-pore structure. In a mixture with a smaller amount of CaCO3 the optimum effective amount of CaCO3 is achieved after a shorter milling time, compared to a mixture with a larger amount of CaCO3 , since the amount of excess CaCO3 that needs to decompose before the point where the porous structure closes is smaller. The results for the influence of the milling time on CaCO3 decomposition (Fig. 1b) and the foaming process (Figs. 4 and 5) show that the decomposition kinetics of the foaming agent governs the foaming process. The observed dependence of the foaming process on the decomposition kinetics of the foaming agent could be applied as a universal rule for foaming processes based on thermal decomposition. The foaming temperature and time affect the foaming process through the viscosity of the glass, the pressure inside the pores and the decomposition kinetics of the foaming agent as well as through secondary effects like the growth and coalescence of pores. The influence of the foaming temperature and time on the foam’s properties was investigated for a mixture with 4 wt% of CaCO3 milled for the optimum time, i.e., 45 min (Table 3). When the foaming temperature is increased or decreased by 30 ◦ C from 785 ◦ C for the 5-min-foamed samples, the apparent density increases by 15–20%. For the decreased temperature both densities are the same, indicating closed porosity, while for the increased temperature the pycnometer density increases in comparison with the apparent density as the pores become more open. The increase in the densities when increasing the foaming temperature above the optimal temperature is related to collapse of foam due to drop in the viscosity and secondary effects, i.e. pore coalescence and pore opening.6,9 When prolonging the foaming time at 785 ◦ C from 5 to 15 min the apparent density stays at the same level, while the pycnometer density increases. Increasing the foaming time to 30 min sees a strong increase in both densities. With an increase in the foaming temperature or time the pore structure coarsens and becomes less homogeneous, accompanied by an increase in the wall thickness of the pores (Fig. 3g and h). These data show that the powder mixture is sensitive to small variations in the foaming conditions. The TGA curves show that the gas-evolving reactions in samples milled for 45 min finish below 700 ◦ C (Fig. 1b). It follows that the pores formed, which are initially small, grow with increasing temperature due to an increase of the gas pressure and a decrease in the viscosity of the glass with temperature. A secondary effect, which accelerates with temperature and time, is the coalescence of the pores, favored by a decrease in the
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Table 4 Total, open and closed porosities of the foam-glass samples with different initial CaCO3 concentration and process parameters. CaCO3 (wt%)
Milling (min)
Temperature (◦ C)
Time (min)
Total porosity (%)
Closed porosity (%)
Open porosity (%)
1 1.5 2 4 10 4 4 4 4 4 4 4 4 4 4 4 2 10
0 0 0 0 0 20 30 37.5 42.5 45 47.5 55 45 45 45 45 22.5 60
815 815 815 815 815 785 785 785 785 785 785 785 755 785 785 815 785 785
5 5 5 5 5 5 5 5 5 5 5 5 5 15 30 5 5 5
74.8 83.7 84.8 86.6 41.4 87.0 86.9 85.4 89.3 90.5 88.2 81.3 88.6 90.5 85.7 89.0 89.8 90.1
71.2 79.8 71.3 64.7 41.0 69.4 76.4 65.7 74.2 89.8 85.5 78.8 88.6 80.5 75.6 85.8 85.3 88.7
3.6 3.9 13.5 21.9 0.4 17.6 10.5 19.7 15.1 0.7 2.7 2.5 0.0 10.0 10.1 3.2 4.5 1.4
Table 5 Thermal conductivity, densities and porosities of large foam samples with 4 wt% CaCO3 , milled for 45 min and foamed at 795 ◦ C for 5 min. Thermal conductivity (mW/(m K))
Apparent density (kg/m3 )
Pycnometer density (kg/m3 )
Total porosity (%)
Closed porosity (%)
50.2 51.3 53.1
240 267 242
319 313 362
91.2 90.2 91.2
66.4 75.5 58
surface area of the system. Thereafter, the holding time in the high-temperature range, where the viscosity of the glass is low, should be short, i.e., a short foaming time and higher heating and cooling rates. A shorter holding time at elevated temperatures is also favorable from the point of view of energy consumption. The open- and closed-porosity fractions of the investigated samples are given in Table 4. The open porosity reflects the difference between the apparent and the pycnometer density; the densities are the same in the case of a completely closed porosity. The highest fraction of closed porosity in combination with a high total porosity is found for the samples milled for the optimum time (45 min) and foamed at a moderate temperature (785 ◦ C) for a short time (5 min). The samples foamed at a lower temperature or milled for a longer time also exhibit largely closed porosity; however, the density of these samples is higher. For use in insulating applications, the pores should be closed, which has a beneficial impact on both the thermal and mechanical properties of the foams.9 Considering the thermal properties, the difference in the thermal conductivity between CO2 (closed pores) and air (open pores) in the room-temperature range is approximately 9 mW/(m K). Since foam glass consists of more than 90% of gaseous phase, the difference in the contribution of the conduction of the gaseous phase to the overall thermal conductivity in a completely open- or closed-pore sample is almost the same as the above-mentioned value.28 Therefore, a closed porosity filled with CO2 is advantageous. When using CaCO3 as a foaming agent, CO2 is the only possible evolved gas, which needs to be entrapped in the pores. To evaluate the thermal conductivity,
larger samples with 60 mm diameter and 30–40 mm thickness were prepared. The pore structure in the interior of such a sample is shown in Fig. 3i. However, some larger and open pores were present in the samples, which is reflected in the significant fraction of open porosity, i.e., 58–75% (Table 5). Due to sensitivity of the foaming process, foaming conditions for larger samples should be further adapted to improve closed porosity. Nevertheless, the thermal conductivity measurements showed values of 50–53 mW/(m K) for the apparent density of 240–270 kg/m3 . Since part of the pores in the prepared samples are open and thus contain air, the thermal conductivity of a sample with completely closed porosity would be even lower. For the best heat-insulating performance the thermal conductivity values should be further reduced to below 40 mW/(m K). However, compared to the literature17 and to commercial products,29 the thermal conductivity of the foams prepared in this study is considerably lower for the same density; or the same thermal insulation is achieved at a higher density and hence results in better mechanical properties. This indicates the high potential of the investigated system for heat-insulating applications. To decrease the foam density and thus improve the thermal insulation properties, the kinetics of CaCO3 decomposition has to be adapted. This can be achieved by using coarser CaCO3 powders to decrease the decomposition rate. 4. Conclusions The results showed that the decomposition kinetics of CaCO3 as the foaming agent governs the foaming process. If the
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decomposition mainly occurs above the temperature at which the glass particles sinter to form a closed structure, more CaCO3 remains available for foaming in the sintered body. The large amount of gases released breaks part of the pore walls, resulting in an inhomogeneous, partially open pore structure. If the decomposition mainly occurs below the sintering temperature, the majority of the CaCO3 decomposes before the structure is closed and the released gases escape into the surrounding atmosphere. Only the remaining CaCO3 foams the softened glass. In this case the pore structure is closed, homogeneous and fine. The CaCO3 decomposition temperature can be modified by varying the milling time of the glass–CaCO3 mixture. With an increase in the milling time the decomposition temperature decreases, thus decreasing the effective amount of CaCO3 available for foaming. For a specific CaCO3 concentration an optimum milling time for the mixture exists, at which the minimum in the density is obtained accompanied by a homogeneous closed porosity. In contrast, the particle size of the glass was found to have only a marginal influence on foaming. Optimized samples with a homogeneous closed porosity exhibit a density of approximately 260 kg/m3 . A thermal conductivity of 50–53 mW/(m K) was measured for samples with an apparent density of 240–270 kg/m3 , which is considerably lower than the values reported for foams with a similar density. The observed dependence of the foaming process on the decomposition kinetics of the foaming agent could be applied as a universal rule for foaming processes based on thermal decomposition. Acknowledgement The authors would like to thank Mr. Christian Prinds (Danish Technological Institute, Aarhus) for the thermal conductivity ˇ measurements and Ms. Andreja Sestan (Joˇzef Stefan Institute) for the heating-microscope measurements. Financial support from the Danish National Advanced Technology Foundation (Højteknologifonden) under Grant number 012-2011-3 is gratefully acknowledged. References 1. Poon CS. Management of CRT glass from discarded computer monitors and TV sets. Waste Manage 2008;28:1499. 2. Industry Council for Electronic Equipment Recycling. Materials recovery from waste cathode ray tubes (CRTs). In: Project code: GLA15-006. The Waste & Resource Action Programme. 2004. 3. Smith AS. Recycled CRT panel glass as an energy reducing fluxing body additive in heavy clay construction products. In: Project code: GLA35-005. The Waste & Resource Action Programme. 2006. 4. www.mogensen.de/en/roentgensortierung.htm [accessed 30.07.13]. 5. www.nulifeglass.com/index.htm [accessed 30.07.13]. 6. Bernardo E, Albertini F. Glass foams from dismantled cathode ray tubes. Ceram Int 2006;32:603–8.
7. Andreola F, Barbieri L, Corradi A, Lancellotti I, Falcone R, Hreglich S. Glass-ceramics obtained by the recycling of end of life cathode ray tubes glasses. Waste Manage 2005;25:183–9. 8. Méar F, Yot P, Cambon M, Ribes M. The characterization of waste cathoderay tube glass. Waste Manage 2006;26:1468–76. 9. Scarinci G, Brusatin G, Bernardo E. Glass foams. In: Scheffler M, Colombo P, editors. Cellular ceramics: structure, manufacturing, properties and applications. Weinheim: WILEY-VCH Verlag; 2005. p. 158–76. 10. Köse S [PhD Thesis] Untersuchungen zur Blähdynamik des Schaumglases. Zürich, Switzerland: Eidgenössische Technische Hochschule (ETH); 1981 [in German]. 11. Pantano CG, Singh AK, Zhang H. Silicon oxycarbide glasses. J Sol–Gel Sci Technol 1999;14:7–25. 12. Ducman V, Kovaˇcevi´c M. The foaming of waste glass. Key Eng Mater 1997;132–136:2264–7. 13. Guo HW, Wang XF, Gong YX, Liu XN, Gao DN. Improved mechanical property of foam glass composites toughened by glass fiber. Mater Lett 2010;64:2725–7. 14. Lotov VA. Making foam glass based on natural and technogenic aluminosilicates. Glass Ceram 2012;68:302–5 [translated from Steklo i Keramika 2011:68:34–7]. 15. Steiner AC [PhD Thesis] Foam glass production from vitrifies municipal waste fly ashes. Eindhoven, The Netherlands: Eindhoven University of Technology; 2006. 16. Saakyan ER. Multifunctional foam glasses from volcanic glassy rocks. Glass Ceram 1991;48:3–5 [translated from Steklo i Keramika 1991;48:5–6]. 17. Ketov AA. An experience of reuse of a glass cullet for production of foam structure material. In: Proceedings of the International Symposium on Recycling and Reuse of Glass Cullet. 2001. p. 1–7. 18. Chen MJ, Zhang FS, Zhu JX. A novel process for foam glass preparation from waste CRT panel glass. In: Proceedings of the Recycling of Electronic Waste II. 2011. p. 97–102. 19. Guo HW, Gong YX, Gao SY. Preparation of high strength foam glass–ceramics from waste cathode ray tube. Mater Lett 2010;64:997–9. 20. Bernardo E, Scarcini G, Hreglich S. Foam glass as a way of recycling glasses from cathode ray tubes. Glass Sci Technol 2005;78:7–11. 21. Chen MJ, Zhang FS, Zhu JX. Lead recovery and the feasibility of foam glass production from funnel glass of dismantled cathode ray tube through pyrovacuum process. J Hazard Mater 2009;161:1109–13. 22. Méar F, Yot P, Viennois R, Ribes M. Mechanical behaviour and thermal and electrical properties of foam glass. Ceram Int 2007;33:543–50. 23. Méar F, Yot P, Cambon M, Ribes M. The changes in lead silicate glasses induced by the addition of a reducing agent (TiN or SiC). J Non-Cryst Solids 2005;351:3314–9. 24. Yot PG, Méar FO. Characterization of lead, barium and strontium leachability from foam glasses elaborated using waste cathode ray-tube glasses. J Hazard Mater 2011;185:236–41. 25. Méar F, Yot P, Cambon M, Caplain R, Ribes M. Characterisation of porous glasses prepared from cathode ray tube (CRT). Powder Technol 2006;162:59–63. 26. Pokorny A, Vicenzi J, Bergmann CP. Influence of particle size of raw materials in the microstructure and properties of vitreous foam. In: Proc: Congress SAM/CONAMET. 2007. p. 1027–32 [in Portuguese]. 27. Spiridonov YA, Orlova LA. Problems of foam glass production. Glass Ceram 2003;60:313–4 [translated from Steklo i Keramika 2003:60:10–1]. 28. Placido E, Arduini-Schuster MC, Kuhn J. Thermal properties predictive model for insulating foams. Infrared Phys Technol 2005;46:219–31. 29. www.foamglas.com/ [accessed 30.07.13].
ScienceDirect Journal of the European Ceramic Society 34 (2014) 1591–1598
Influence of the glass–calcium carbonate mixture’s characteristics on the foaming process and the properties of the foam glass Jakob König a,b,1 , Rasmus R. Petersen a , Yuanzheng Yue a,∗ a b
Section of Chemistry, Aalborg University, Sohngårdsholmsvej 57, DK-9000 Aalborg, Denmark Advanced Materials Department, Joˇzef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
Received 19 August 2013; received in revised form 9 December 2013; accepted 10 December 2013 Available online 3 January 2014
Abstract We prepared foam glasses from cathode-ray-tube panel glass and CaCO3 as a foaming agent. We investigated the influences of powder preparation, CaCO3 concentration and foaming temperature and time on the density, porosity and homogeneity of the foam glasses. The results show that the decomposition kinetics of CaCO3 has a strong influence on the foaming process. The decomposition temperature can be modified by varying the milling time of the glass–CaCO3 mixture and thus for a specific CaCO3 concentration an optimum milling time exists, at which a minimum in density and a homogeneous closed porosity are obtained. Under the optimum preparation conditions the samples exhibit a density of 260 kg/m3 . The thermal conductivity of the foam glass was measured to be 50–53 mW/(m K). The observed dependence of the foaming process on the decomposition kinetics of the foaming agent can be applied as a universal rule for foaming processes based on thermal decomposition. © 2013 Elsevier Ltd. All rights reserved. Keywords: Foam glass; Cathode ray tube; Thermal decomposition; Thermal conductivity
1. Introduction The disposal of obsolete cathode ray tubes (CRTs) has become a global environmental problem.1 The amount of CRT waste increases worldwide, but no proper recycling scheme for CRT glass has been widely applied. CRT glass represents up to two-thirds of a TV’s weight and in the UK alone more than 100,000 tons of CRT glass have been disposed of annually since 2003.2,3 The possibilities for recycling CRT glass are limited, due to high chemical quality requirements of glass production and the declining production of new CRTs. Moreover, the recycling possibilities are hindered by the presence of lead in part of the CRT. A color CRT consists, in general, of two types of glass: barium/strontium-containing glass (the panel glass) and lead-containing glass (the funnel and neck glass). In the past decade new separation methods have been developed with the aim being to increase the amount of recycled CRT glass.4,5
∗
Corresponding author. Tel.: +45 9940 8522. E-mail addresses: [email protected] (J. König), [email protected] (Y. Yue). 1 Permanent address: Advanced Materials Department, Joˇ zef Stefan Institute, SI-1000 Ljubljana, Slovenia. 0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.12.020
The strategy is first to separate the lead-containing glass from the lead-free panel glass. The latter can be recycled as nonhazardous glass in foam glass,6 ceramic tiles and bricks,2,7 while the former can be used as a flux in smelting furnaces or cleaned of lead by a vitrification process.2,5 However, the recycling of CRT glass remains limited and a large proportion of CRT glass is still being landfilled.2,7,8 Hence, existing recycling methods have to be improved or a new method has to be developed. Here, we will focus on foam-glass production as it is one of the most promising recycling technologies for CRT glass. Foam glass is used as a thermal and acoustic insulating material in buildings, road construction and industry.9 Foam glass is produced by mixing glass powder with a foaming agent and subsequently heating the mixture above the glass-softening point. During the heat treatment the foaming agent releases gases that form dispersed bubbles in the softened glass. The expanding gas bubbles increase the volume of the sample, thus forming a typical porous lightweight product. After the heat treatment the glass is cooled again in order to freeze-in the bubbles in the glass melt. Gaseous products are generated by thermal decomposition, e.g., carbonates and sulfates, or through the oxidation of carbonbased agents, i.e., pure carbon (coal, graphite, carbon black,
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etc.), carbohydrates and SiC. The composition of the released gases depends on the foaming agent used and on the composition of the glass. The gases evolved as a result of the decomposition of carbonates or sulfates consist mainly of CO2 and SO2 , respectively. These low-heat-conducting gases contribute positively to the overall heat-insulating properties of the foam glass. Carbon-based foaming agents react with the glass to form a CO/CO2 mixture, but the formation of other gas species is possible, especially when using SiC as a foaming agent.10 The CO/CO2 volume ratio depends on the temperature, but also on the amount of oxygen available in the melt. The residuals of foaming agents or their reaction with the glass have an impact on the foam’s morphology and the glass network. Metal oxides from decomposed carbonates and sulfates can work as a nucleation agent for crystallization during the foaming. Similarly, the formation of SiO2 from SiC will change the viscosity of the glass. Such occurrences can have a negative impact on the melt’s expansion and the foam’s homogeneity. The use of carbon as a foaming agent can lead to reduction of the glass melt, while carbon residuals will not incorporate into the glass network.11 However, due to a change in the valence state of multivalent ions, some changes in the glass network will occur. Foam glass is mainly produced from different types of glass cullet, i.e., container glass,12 flat glass13 and lamp glass,14 but it can also be produced from other glassy materials, like glass from fly ashes15 and amorphous volcanic rocks.16 In order to obtain foams with a low density, glass powders have to be milled to fine powders.9,17 Moreover, the foaming agent’s particle size and concentration need to be optimized to produce homogeneous foams with a low density. In general, the amount of foaming agent added is much higher than theoretically required.6,9,18 Only a few studies have reported on the preparation of foam glass from CRT glass cullet19,20 or from part of it.6,21 In these studies, different foaming agents were used. Applying reducing foaming agents (SiC and TiN) in combination with a lead-containing cullet resulted in a reduction of the cationic lead to metallic lead,8,22,23 which increases its leachability and thus reduces the applicability of the products.19,24 On the other hand, if CaCO3 is used, the lead oxide remains incorporated in the glass network.16,18,25 The amount of added CaCO3 was between 1 and 7 wt%; however, there does not seem to be any trend indicating the optimum CaCO3 concentration. The holding time at elevated temperature should be short in order to prevent the growth and coalescence of the pores, and thus faster heating and cooling is favorable. Another parameter that has a great impact on the foam density and homogeneity of the pore structure is the particle size of the glass powder.9 It was shown that when foaming with carbon the density of the foam glass decreases rapidly with a glass particle size below 100 m.17 On the other hand, an optimum in the glass particle size at 80 m was observed when dolomite was used as the foaming agent.26 In the literature it is common to report the initial particle size of the glass and the foaming agent used, but the glass and the foaming agent are frequently homogenized by milling them together, thereby obscuring the real particle size being employed.
Table 1 Chemical composition of the CRT panel glass. Oxide
wt%
SiO2 Na2 O K2 O BaO SrO ZrO2 Al2 O3 PbO Others
60.1 7.9 7.1 9.5 8.2 1.9 2.5 0.05 2.8
The aim of this work is to highlight the importance of the particle size of the glass and especially the homogenization step on the foaming process and the resulting foam-glass properties. We explore the dependence of the density, porosity and homogeneity of the foamed glass prepared from waste CRT panel glass and CaCO3 on the particle size, milling time, foaming temperature and time. By changing the milling time of the powder mixture, the kinetics of the CaCO3 decomposition were modified. Based on the results, the optimum foaming-agent concentration is discussed. In addition, we measured the thermal conductivity of the prepared foams. 2. Experimental Lead-free CRT panel glass from obsolete color televisions was supplied by a local electronic-waste-management company (Averhoff A/S, Aarhus, Denmark). The composition of the glass was measured using an X-ray fluorescence spectrometer (PW2400, PANalytical, The Netherlands) with SuperQ software. Oxide standards (WROXI, PANalytical) were used for the quantification. The chemical composition of the panel glass shown in Table 1 is in accordance with the reported panelglass compositions.8 Pieces of the panel glass were cleaned with demineralized water in order to remove the coatings on the inner side of the glass. The glass was then crushed in a jaw crusher (BB51, Retsch, Germany), and subsequently dry milled using a planetary ball mill (PM 100, Retsch, Germany). An agate ball mill with 20-mm balls was used. The glass powder was sieved below 63 m. Powder mixtures were prepared by adding 1–10 wt% of CaCO3 (D90 = 8 m; 99.9%, AppliChem, Germany) to the sieved glass powder (<63 m). The powder mixtures were prepared either by nondestructive mixing for 12 h in a vertically spinning carousel (referred to as 0 min milling time) or by milling in the ball mill. The milling conditions used were always the same (the amount of powder, the number of balls, the rotation speed). Then the powder mixtures were uniaxially pressed into disk-shaped pellets. A pressure of 40 MPa was applied for the small samples (1 g, 13 mm diameter) and 30 MPa was used for the large samples (15–35 g, 35 mm diameter). A minimum of three samples were made for each of the preparation and foaming conditions. The pellets were placed in an electrical laboratory-chamber furnace and heated at 10 ◦ C/min to various temperatures in the range 755 to 815 ◦ C. The holding time varied from 5 to 30 min. Afterwards, the samples were cooled at
J. König et al. / Journal of the European Ceramic Society 34 (2014) 1591–1598 Table 2 Particle size (D50 and D90) of the panel glass–CaCO3 mixtures milled for different times. Milling time (min)
D50 (m)
D90 (m)
0 20 45 55
22 6.3 5 4.6
50 18.5 15 14
6 ◦ C/min from the maximum temperature to the glass-transition temperature (Tg ) of 530 ◦ C, and subsequently cooled at 1 ◦ C/min to room temperature. The temperature in the foaming zone was calibrated using an external thermocouple. The particle size was measured with a laser granulometer (Microtrack II, Leeds & Northrup, UK) and the D50 and D90 values for different milling times are given in Table 2. The apparent (ρapp ) and pycnometer (ρpyc ) densities were measured by using Archimedes’ principle in water and a helium pycnometer (ULTRAPYC 1200e, Quantachrome Instruments, USA), respectively. Apparent densities were not affected by the intake of water due to open porosity; mass gain was lower than 0.004 g, which corresponds to an experimental error of 0.1%. The glass powder density (ρpowder ) of the initial glass and the foamed glass samples with different CaCO3 contents was measured with the pycnometer. The addition of CaCO3 only slightly changed the density and all the samples showed a density in the range 2.74 ± 0.02 kg/m3 . Thus the average powder density of 2.74 kg/m3 was used in the calculations. The percentage of open pores (OP) and the percentage of closed pores (CP) in the foam volume (Vfoam = VCP + VOP + Vglass ) was calculated from the apparent, pycnometer and powder densities according to: ρapp × 100 TP [%] = 1 − ρpowder CP [%] =
ρapp ρapp − ρpyc ρpowder
× 100
OP [%] = TP − CP where TP is the percentage of the total porosity in the foam sample. The sintering behavior was investigated using a heating microscope (Hesse Instruments, Germany). The temperature at which the structure of the sample closes as a result of the viscous flow sintering of the glass particles was approximated from the sintering curves during dynamic heating. A thermogravimetric analysis (TGA) was performed on small compacts using a Jupiter 449 C, Netsch, Germany. Due to foaming in the crucible only small amount of mixtures could be tested (∼15 mg). This proved not to be satisfactory to overwhelm small fluctuations in the baseline. Therefore, the TGA results are not interpreted quantitatively. Water content in the milled powders due to adsorption from atmosphere was low, below 0.3%. During the TGA and the heating microscopy the samples were fired in an air atmosphere and the heating rate was 10 ◦ C/min. X-ray powder diffraction
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(XRD; Empyrean, PANalytical, The Netherlands) was used to identify possible crystalline phases. The foamed samples were cut, photographed and the homogeneity of the pore structure was visually evaluated. The thermal conductivity was measured with a surface probe (25 × 25 mm2 ) at 10 ◦ C using a heat-transfer analyzer (ISOMET Model 2104 Heat Transfer Analyzer, Applied Precision, Slovakia). The accuracy of the equipment in the low lambda range was tested by measuring commercial expanded polystyrene and foam glass with known values. The obtained results were in perfect agreement with the declared values. 3. Results and discussion For a successful foaming process it is essential that the gases are released in the sintered body when the porous structure is already closed. Only then can the gases be entrapped in the glass matrix. The decomposition temperature of pure CaCO3 in an air atmosphere is reported to be around 700 ◦ C, which is well above the softening point of the CRT panel glass (∼590 ◦ C).6 However, the thermal stability of the CaCO3 decreases when mixed with glass. To get a deeper insight into the sintering and decomposition kinetics of the glass and the glass–CaCO3 mixtures, heating microscopy and thermogravimetric analyses were applied. The results are shown in Fig. 1. The sieved CRT panel-glass powder densifies at around 690 ◦ C, while with additional milling the sintering temperature decreases by 25 ◦ C (Fig. 1a). The difference in the sintering temperature of the powders milled for 20–55 min is small, since the configuration of the ball mill does not allow a further significant reduction in the particle size. The TGA data on the decomposition of CaCO3 in air show that the first weight percent of the mass loss occurs at 670 ◦ C (Fig. 1b). This indicates that part of the CaCO3 decomposes before the glass particles sinter and close the porous structure. Moreover, with prolonged milling of the glass–CaCO3 mixture the decomposition of the carbonate is shifted to even lower temperatures. Thus in the sample milled for 55 min only a fraction of the carbonate remains in the mixture at the point where the porous structure closes. Furthermore, for the sample milled for 20 min the decomposition of CaCO3 is very rapid above 650 ◦ C. By milling the mixture, the contact area between the glass and the CaCO3 particles increases, because the particle size decreases. Thus the increased contact area and the decreased thermal stability of the CaCO3 in contact with the glass decrease the decomposition temperature of the CaCO3 . The heating-microscope data suggest that the sintering behavior related to the particle size of the glass powders should have a minor effect on the foaming process. However, in contrast, the CaCO3 decomposition kinetics that is affected by the mixture milling is expected to strongly influence the foaming process. The influence of the CaCO3 concentration on the apparent and pycnometer densities of the foams from powders homogenized by mixing in a carousel is shown in Fig. 2. The samples were foamed at 815 ◦ C for 5 min. For the apparent density a minimum at 4 wt% CaCO3 is seen, while for the pycnometer density there is a broad minimum for concentrations between 1.5 and 4 wt%. There is a larger difference between the apparent and
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Fig. 1. (a) Sintering curves for the panel-glass powders with different particle sizes (D90 values); (b) thermogravimetric curves of CaCO3 and of powder mixtures with 4 wt% of CaCO3 milled for different time periods. In (b) the vertical line marks, for the milled samples, the approximate temperature at which the porous structure closes.
Fig. 2. Influence of CaCO3 concentration on the foam densities for the samples homogenized by mixing in a carousel and foamed at 815 ◦ C for 5 min.
the pycnometer density for samples with 2 and 4 wt% of CaCO3 compared to the rest of the samples. However, the addition of 10 wt% of CaCO3 halted the foaming process almost completely as the density of the samples was above 1600 kg/m3 . The morphology of the foams changes with the CaCO3 concentration (Fig. 3a–c). Samples with 1–2 wt% of CaCO3 (Fig. 3a and b) exhibit a dense sintered glass shell around a foamed core. The sample with 1 wt% CaCO3 developed a thicker shell, while the foamed core consists of fine homogeneous pores (Fig. 3a). In the sample with 4 wt% CaCO3 (Fig. 3c) the glass shell is no longer visible. The homogeneity of the pore structure decreases with the increasing addition of CaCO3 . The theoretical concentration of CaCO3 needed to obtain a foam with 90% porosity (i.e., a density of approximately 270 kg/m3 ) was calculated to be 0.4 wt%, assuming a pressure of 1 atm in the pores at the
Fig. 3. Characteristic appearance of the foam samples. (a–c): Samples prepared by mixing, with 1, 2 and 4 wt% of CaCO3 , respectively (815 ◦ C, 5 min); (d) sample prepared by mixing of fine glass powders (D90 = 15 m) with 4 wt% of CaCO3 , (785 ◦ C, 5 min); (e and f) samples prepared by milling, with 4 wt% of CaCO3 , 45 and 55 min milling time, respectively (785 ◦ C, 5 min); (g and h) – samples prepared by milling, with 4 wt% of CaCO3 , 45 min milling, foamed at 785 ◦ C for 15 and 30 min, respectively; (i) 30 g sample with 4 wt% of CaCO3 , 45 min milling (795 ◦ C, 5 min). The ruler for all foam images is shown in right bottom corner.
J. König et al. / Journal of the European Ceramic Society 34 (2014) 1591–1598
foaming temperature (815 ◦ C).9 However, despite much larger additions of CaCO3 than theoretically needed, i.e., 2.5–25 times, the lowest apparent density obtained was 350 kg/m3 . The sintered glass shell of the samples with a low CaCO3 content, i.e., 1–2 wt% CaCO3 , indicates that on the surface of the sample CaCO3 was decomposed before the glass particles sintered and closed the porous structure. In contrast to the surface, part of the CaCO3 in the interior was enclosed by the softened glass and foamed the sample core. The TGA and heating-microscope results (Fig. 1) revealed that CaCO3 starts to decompose before the glass particles sinter. Thus the air atmosphere in the pores of the compressed sample is replaced by the evolved CO2 gas. The partial pressure of the CO2 in the interior of the sample is higher compared to the surface of the sample, which is in contact with the air atmosphere, and influences the decomposition equilibrium of the CaCO3 . Thereafter, the decomposition of the CaCO3 is faster on the surface of the sample. All the carbonate on the surface is decomposed before the porous structure closes and a sintered glass shell is formed by the viscous flow sintering of CaO-enriched glass. In contrast to the surface, part of the CaCO3 in the interior is enclosed by the glass melt and foams the sample core. With an increasing CaCO3 concentration the shell becomes thinner as the surface layer with the completely decomposed carbonate becomes thinner, and is no more visible for the sample with 4 wt% of CaCO3 (Fig. 3c). In this sample, part of the CaCO3 also remains available for foaming on the surface. However, the interior becomes inhomogeneous due to a large amount of formed gas from the carbonate decomposition.27 Consequently, the pore walls start to break and part of the pores becomes open, which is seen from the difference in the apparent and pycnometer density. In the sample with 10 wt% of CaCO3 the porous structure does not close completely and the released gases are able to escape to the atmosphere. Most probably the sintering of the sample with 10 wt% of CaCO3 is hindered by the large content of CaCO3 . However, no crystalline phases were found within the detection limit of the XRD analysis. This indicates that all the CaO was dissolved in the glass matrix during the heat treatment. The optimum CaCO3 concentration in the samples prepared by mixing was found to be 4 wt% and for this concentration the influence of the glass particle size on the foaming process was investigated. For this purpose, glass powders were additionally milled for different periods of time and the particle size of the so-obtained powders was measured. Again, the powder mixtures were homogenized by mixing in a carousel. From Fig. 4 it can be seen that the glass particle size does not have a distinct influence on the density. The change in the apparent density is small, while the pycnometer density of the samples prepared from finer powders increases to above 740 kg/m3 . Moreover, the samples’ homogeneity deteriorates and large voids appear in all of the samples (typical appearance is shown in Fig. 3d). Smaller particles sinter at a lower temperature (Fig. 1a); therefore, more CaCO3 remains in the sintered sample and the released gases break some of the pore walls,27 thus increasing the pore connectivity and therewith the pycnometer density. The glass particle size is an important parameter in the preparation of foamed glass
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Fig. 4. Influence of the panel-glass particle size on the apparent and pycnometer densities of foam-glass samples with 4 wt% of CaCO3 , foamed at 785 ◦ C for 5 min. The powder mixtures were homogenized by mixing in a carousel.
and has a strong influence on the density of the foam.9,17 It was shown that the foam density monotonously decreases with a decreasing particle size. However, these results were obtained when foaming with carbon-based compounds. On the other hand, an optimum in the intermediate particle size of the glass powders was observed when foaming with a thermal decomposition process using dolomite as the foaming agent.26 The results obtained in the present study also suggest that the particle size of the glass powder is not the controlling parameter when CaCO3 is used as the foaming agent. To investigate the impact of the homogenization performed by milling, the sample mixtures were milled for different periods of time. The influence of the milling time on the apparent and pycnometer density of the foams for the samples with 4 wt% of CaCO3 is shown in Fig. 5. A milling time between 0 and 30 min does not influence the apparent density significantly, while a gradual decrease in the pycnometer density is observed. A further increase in the milling time induces two anomalies in the trend. At 37 min of milling, both densities increase, but a further extension of the milling time to 45 min causes a minimum in the densities. At the minimum the apparent and pycnometer
Fig. 5. Influence of the milling time on the apparent and pycnometer densities of the foam glass samples with 4 wt% of CaCO3 foamed at 785 ◦ C for 5 min.
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J. König et al. / Journal of the European Ceramic Society 34 (2014) 1591–1598 Table 3 Apparent and pycnometer densities of the foam-glass samples with 4 wt% of CaCO3 milled for 45 min and foamed at different temperatures for different holding times.
Fig. 6. Influence of the milling time on the apparent and pycnometer densities of the foam-glass samples with 2 and 10 wt% of CaCO3 foamed at 785 ◦ C for 5 min.
densities are almost the same, which indicates the closed-pore structure of the sample. When milling for a time longer than 45 min both densities increase, while the difference remains negligible. The lowest density values obtained were around 260 kg/m3 . Such behavior could be associated with the decrease of the decomposition temperature of the CaCO3 with the milling time. Up to the local maximum at 37 min, the effective amount of CaCO3 that remains available for foaming is high. Moreover, since the decomposition rate of the CaCO3 is high, part of the pore walls break and hence, the pycnometer density is much higher than the apparent density. At the local maximum the samples typically exhibit large void(s) in the interior (similar to that shown in Fig. 3d), and consequently, the consistency of both densities of these samples is lower. On the other hand, the consistency of the samples milled for 45 min was very good. Moreover, the pore structure of these samples was found to be homogeneous (Fig. 3e). The effective amount of CaCO3 available for foaming in the samples milled for 45 min was optimal for the characteristics of the powder mixture, i.e., for the particle size of the glass and CaCO3 powders. When milling for a time longer than 45 min, the effective amount of CaCO3 available for foaming decreases and the density increases, but the homogeneity of the pore structure improves further (Fig. 3f). If the above explanation of the relation between the milling time and the effective amount of CaCO3 available for foaming holds, a similar dependence of both densities on the milling time should also be observed for the samples with a different CaCO3 concentration. Fig. 6 shows such a dependence for samples with 2 and 10 wt% of CaCO3 . Almost identical behavior was observed as in sample with 4 wt% of CaCO3 , where the optimum milling time is shorter and longer, respectively, for the samples with the initially lower and higher CaCO3 concentrations. Moreover, the trend of the homogeneity of the foams was similar, exhibiting a homogeneous porosity for the minimum density. Furthermore, the values of the density in the minima for samples with different CaCO3 contents are comparable, suggesting that there is no optimum CaCO3 concentration, but an optimum milling time that provides a suitably effective amount of CaCO3 enclosed in the sintered glass matrix. Such a preparation results
Temperature (◦ C)
Time (min)
Apparent density (kg/m3 )
Pycnometer density (kg/m3 )
755 785 785 785 815
5 5 15 30 5
312 259 259 391 301
312 261 288 435 311
in low-density foams with a homogeneous, closed-pore structure. In a mixture with a smaller amount of CaCO3 the optimum effective amount of CaCO3 is achieved after a shorter milling time, compared to a mixture with a larger amount of CaCO3 , since the amount of excess CaCO3 that needs to decompose before the point where the porous structure closes is smaller. The results for the influence of the milling time on CaCO3 decomposition (Fig. 1b) and the foaming process (Figs. 4 and 5) show that the decomposition kinetics of the foaming agent governs the foaming process. The observed dependence of the foaming process on the decomposition kinetics of the foaming agent could be applied as a universal rule for foaming processes based on thermal decomposition. The foaming temperature and time affect the foaming process through the viscosity of the glass, the pressure inside the pores and the decomposition kinetics of the foaming agent as well as through secondary effects like the growth and coalescence of pores. The influence of the foaming temperature and time on the foam’s properties was investigated for a mixture with 4 wt% of CaCO3 milled for the optimum time, i.e., 45 min (Table 3). When the foaming temperature is increased or decreased by 30 ◦ C from 785 ◦ C for the 5-min-foamed samples, the apparent density increases by 15–20%. For the decreased temperature both densities are the same, indicating closed porosity, while for the increased temperature the pycnometer density increases in comparison with the apparent density as the pores become more open. The increase in the densities when increasing the foaming temperature above the optimal temperature is related to collapse of foam due to drop in the viscosity and secondary effects, i.e. pore coalescence and pore opening.6,9 When prolonging the foaming time at 785 ◦ C from 5 to 15 min the apparent density stays at the same level, while the pycnometer density increases. Increasing the foaming time to 30 min sees a strong increase in both densities. With an increase in the foaming temperature or time the pore structure coarsens and becomes less homogeneous, accompanied by an increase in the wall thickness of the pores (Fig. 3g and h). These data show that the powder mixture is sensitive to small variations in the foaming conditions. The TGA curves show that the gas-evolving reactions in samples milled for 45 min finish below 700 ◦ C (Fig. 1b). It follows that the pores formed, which are initially small, grow with increasing temperature due to an increase of the gas pressure and a decrease in the viscosity of the glass with temperature. A secondary effect, which accelerates with temperature and time, is the coalescence of the pores, favored by a decrease in the
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Table 4 Total, open and closed porosities of the foam-glass samples with different initial CaCO3 concentration and process parameters. CaCO3 (wt%)
Milling (min)
Temperature (◦ C)
Time (min)
Total porosity (%)
Closed porosity (%)
Open porosity (%)
1 1.5 2 4 10 4 4 4 4 4 4 4 4 4 4 4 2 10
0 0 0 0 0 20 30 37.5 42.5 45 47.5 55 45 45 45 45 22.5 60
815 815 815 815 815 785 785 785 785 785 785 785 755 785 785 815 785 785
5 5 5 5 5 5 5 5 5 5 5 5 5 15 30 5 5 5
74.8 83.7 84.8 86.6 41.4 87.0 86.9 85.4 89.3 90.5 88.2 81.3 88.6 90.5 85.7 89.0 89.8 90.1
71.2 79.8 71.3 64.7 41.0 69.4 76.4 65.7 74.2 89.8 85.5 78.8 88.6 80.5 75.6 85.8 85.3 88.7
3.6 3.9 13.5 21.9 0.4 17.6 10.5 19.7 15.1 0.7 2.7 2.5 0.0 10.0 10.1 3.2 4.5 1.4
Table 5 Thermal conductivity, densities and porosities of large foam samples with 4 wt% CaCO3 , milled for 45 min and foamed at 795 ◦ C for 5 min. Thermal conductivity (mW/(m K))
Apparent density (kg/m3 )
Pycnometer density (kg/m3 )
Total porosity (%)
Closed porosity (%)
50.2 51.3 53.1
240 267 242
319 313 362
91.2 90.2 91.2
66.4 75.5 58
surface area of the system. Thereafter, the holding time in the high-temperature range, where the viscosity of the glass is low, should be short, i.e., a short foaming time and higher heating and cooling rates. A shorter holding time at elevated temperatures is also favorable from the point of view of energy consumption. The open- and closed-porosity fractions of the investigated samples are given in Table 4. The open porosity reflects the difference between the apparent and the pycnometer density; the densities are the same in the case of a completely closed porosity. The highest fraction of closed porosity in combination with a high total porosity is found for the samples milled for the optimum time (45 min) and foamed at a moderate temperature (785 ◦ C) for a short time (5 min). The samples foamed at a lower temperature or milled for a longer time also exhibit largely closed porosity; however, the density of these samples is higher. For use in insulating applications, the pores should be closed, which has a beneficial impact on both the thermal and mechanical properties of the foams.9 Considering the thermal properties, the difference in the thermal conductivity between CO2 (closed pores) and air (open pores) in the room-temperature range is approximately 9 mW/(m K). Since foam glass consists of more than 90% of gaseous phase, the difference in the contribution of the conduction of the gaseous phase to the overall thermal conductivity in a completely open- or closed-pore sample is almost the same as the above-mentioned value.28 Therefore, a closed porosity filled with CO2 is advantageous. When using CaCO3 as a foaming agent, CO2 is the only possible evolved gas, which needs to be entrapped in the pores. To evaluate the thermal conductivity,
larger samples with 60 mm diameter and 30–40 mm thickness were prepared. The pore structure in the interior of such a sample is shown in Fig. 3i. However, some larger and open pores were present in the samples, which is reflected in the significant fraction of open porosity, i.e., 58–75% (Table 5). Due to sensitivity of the foaming process, foaming conditions for larger samples should be further adapted to improve closed porosity. Nevertheless, the thermal conductivity measurements showed values of 50–53 mW/(m K) for the apparent density of 240–270 kg/m3 . Since part of the pores in the prepared samples are open and thus contain air, the thermal conductivity of a sample with completely closed porosity would be even lower. For the best heat-insulating performance the thermal conductivity values should be further reduced to below 40 mW/(m K). However, compared to the literature17 and to commercial products,29 the thermal conductivity of the foams prepared in this study is considerably lower for the same density; or the same thermal insulation is achieved at a higher density and hence results in better mechanical properties. This indicates the high potential of the investigated system for heat-insulating applications. To decrease the foam density and thus improve the thermal insulation properties, the kinetics of CaCO3 decomposition has to be adapted. This can be achieved by using coarser CaCO3 powders to decrease the decomposition rate. 4. Conclusions The results showed that the decomposition kinetics of CaCO3 as the foaming agent governs the foaming process. If the
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J. König et al. / Journal of the European Ceramic Society 34 (2014) 1591–1598
decomposition mainly occurs above the temperature at which the glass particles sinter to form a closed structure, more CaCO3 remains available for foaming in the sintered body. The large amount of gases released breaks part of the pore walls, resulting in an inhomogeneous, partially open pore structure. If the decomposition mainly occurs below the sintering temperature, the majority of the CaCO3 decomposes before the structure is closed and the released gases escape into the surrounding atmosphere. Only the remaining CaCO3 foams the softened glass. In this case the pore structure is closed, homogeneous and fine. The CaCO3 decomposition temperature can be modified by varying the milling time of the glass–CaCO3 mixture. With an increase in the milling time the decomposition temperature decreases, thus decreasing the effective amount of CaCO3 available for foaming. For a specific CaCO3 concentration an optimum milling time for the mixture exists, at which the minimum in the density is obtained accompanied by a homogeneous closed porosity. In contrast, the particle size of the glass was found to have only a marginal influence on foaming. Optimized samples with a homogeneous closed porosity exhibit a density of approximately 260 kg/m3 . A thermal conductivity of 50–53 mW/(m K) was measured for samples with an apparent density of 240–270 kg/m3 , which is considerably lower than the values reported for foams with a similar density. The observed dependence of the foaming process on the decomposition kinetics of the foaming agent could be applied as a universal rule for foaming processes based on thermal decomposition. Acknowledgement The authors would like to thank Mr. Christian Prinds (Danish Technological Institute, Aarhus) for the thermal conductivity ˇ measurements and Ms. Andreja Sestan (Joˇzef Stefan Institute) for the heating-microscope measurements. Financial support from the Danish National Advanced Technology Foundation (Højteknologifonden) under Grant number 012-2011-3 is gratefully acknowledged. References 1. Poon CS. Management of CRT glass from discarded computer monitors and TV sets. Waste Manage 2008;28:1499. 2. Industry Council for Electronic Equipment Recycling. Materials recovery from waste cathode ray tubes (CRTs). In: Project code: GLA15-006. The Waste & Resource Action Programme. 2004. 3. Smith AS. Recycled CRT panel glass as an energy reducing fluxing body additive in heavy clay construction products. In: Project code: GLA35-005. The Waste & Resource Action Programme. 2006. 4. www.mogensen.de/en/roentgensortierung.htm [accessed 30.07.13]. 5. www.nulifeglass.com/index.htm [accessed 30.07.13]. 6. Bernardo E, Albertini F. Glass foams from dismantled cathode ray tubes. Ceram Int 2006;32:603–8.
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