Preparation and characterization of high compressive strength foams ...

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Sep 26, 2005 - tion of anorthite (CaAl2Si2O8) and diopside (CaMgSi2O6). The glass-ceramics produced after heat treatment of AD- glass powder compacts at ...
J Porous Mater (2006) 13: 133–139 DOI 10.1007/s10934-006-7014-9

Preparation and characterization of high compressive strength foams from sheet glass D.U. Tulyaganov · H.R. Fernandes · S. Agathopoulos · J.M.F. Ferreira

Received: April 18, 2005 / Revised: September 26, 2005 C Springer Science + Business Media, LLC 2006 

Abstract High compressive strength glass foams were produced using sheet glass cullet with the aid of 1 wt.% SiC powder, as gassing agent, and the incorporation of small amounts of an alkali earth aluminosilicate glass powder (AD), which is intrinsically prone to be crystallised to anorthite and diopside. The amount of SiC used as well as the mean particle sizes of the powders of both glasses and SiC were lower than those used in earlier studies. The experimental results showed that homogenous microstructures of large pores could be obtained by adding 1 wt.% SiC. The compressive strength of the glass foams was considerably increased when the incorporated AD-glass was higher than 1 wt.%. It is concluded that the presence of the AD glass is beneficial for the produced glass foams because of the formation of a well packed honeycomb structure which features an optimal distribution of pentagonal- and hexagonal-like shaped pores surrounded by dense struts. The crystallization of wollastonite and diopside inside the struts should also have a positive impact on the mechanical behaviour of the produced porous glass foams.

Keywords Glass waste . Foam . Compressive strength

1. Introduction A great amount of glass wastes derives from sheet glasses used for windows (cullet). Only in the UK, cullet is estimated to reach about 0.5 million tones per year [1]. The utilization of cullet for producing glass foams, whose industrial production features high processing cost, is an interesting direction of glass technology. The principal feature of glass foams is their closed pore structure, characterized by the pore size distribution and the shape of the pores [2]. Glass foams are light weight and non toxic, they feature high thermal and sound insulation properties, and exhibit good machining ability and high water resistance. Glass foams can be largely applied as building materials, whereby huge amounts of waste glasses can be consumed. There are quite few studies on the production of foams from recycling glass [3, 4], but new approaches have been recently proposed [5–7]. Glass foams are commonly produced via sintering of powdered glass at high temperatures with the aid of incorporated gassing agents. There are glass foaming technologies based on reduction-oxidation reactions aided by carbon where powdered sulphates act as gassing agents, according to the flowing generic chemical equation [5, 8]: 2− glass-SO2− 4 + 2C → glass-S2 + CO + CO2

D.U. Tulyaganov · H.R. Fernandes · S. Agathopoulos · J.M.F. Ferreira () Department of Ceramics and Glass Engineering, University of Aveiro, CICECO, 3810-193 Aveiro, Portugal e-mail: [email protected] D.U. Tulyaganov Scientific Research Institute of Space Engineering, 700128, Tashkent, Uzbekistan


This technology possesses serious environmental concerns due to the significant amount of H2 S containing in the porous cells, while the materials produced exhibit poor mechanical strength. To avoid sulphur, Ketov [5] has alternatively proposed the use of oxidizing agents, which can produce steam at the stage of the preparation of the raw materials. The formation of polysilicic acids favours the evolution of steam at Springer


temperatures above 500–600◦ C, close to the pyroplastic state of glass. Powder of SiC (1–5 wt.% with respect to the total mass; grain size 40 μm) and fly ashes from fuel oil (1–3%) have been proposed by Brusatin et al. [7] as alternative gassing agents for producing glass foams from cullet. The processing parameters affect the cellular structure and the properties of the resultant materials. In the case of the SiC-glass foams, the heat treatment schedule comprised heating up to 900◦ C, holding for 30 minutes at this temperature, followed by heating up to 950◦ C, and holding at this temperature for 30 minutes. In the case of the fly ash-glass foams, the heat treatment comprised heating to 800◦ C–900◦ C, holding for 20 minutes and annealing at 600◦ C. The SiC-glass foams exhibited higher degree of microstructural homogeneity than the fly ash-glass foams. The microstructural inhomogeneity of the latter case was attributed to the significant difference between the particle size of the powders of the fly ash (∼10– 20 μm) and the glass (75–150 μm). The SiC-glass foams had significantly higher apparent density (0.50–0.65 g/cm3 ) than the typical glass foams used in insulating applications (0.16–0.20 g/cm3 ). That study [7] has provided, however, little information about the role of SiC in the formation of glass foams as well as the control of the properties of the resultant foam materials. Moreover, the heating rates used were not reported. In the present work, glass foams were produced at laboratory scale using glass powder from sheet glass cullet. SiC powder aimed as gassing agent. In comparison with earlier studies [7], the mean particle sizes of the powders of the glass and SiC were significantly smaller and the added amount of SiC was lower. To enhance mechanical properties of glass foams, a powdered alkali-earth aluminosilicate glass, named as AD, was also incorporated in appropriate quantities. The composition of this glass was 47.04% SiO2 , 15.49% Al2 O3 , 0.29% Fe2 O3, 24.32% CaO, 9.70% MgO, 0.04% Na2 O, 0.69% K2 O, 0.01% TiO2 , 2.02% P2 O5 , and 0.37% CaF2 (in this paper, all the compositions refer to wt.%). An earlier study [9] on sintering behaviour and crystallization of this AD glass has shown that heat treatment at 850–920◦ C results in crystallization of anorthite (CaAl2 Si2 O8 ) and diopside (CaMgSi2 O6 ). The glass-ceramics produced after heat treatment of ADglass powder compacts at 850–920◦ C had a highly homogeneous microstructure and high mechanical strength. The good properties of these glass-ceramics were attributed to the uniform precipitation of fine crystals of anorthite and diopside in the matrix of the AD parent glass. To evaluate the potential of this technology with raw materials used in ceramic and glass industry for further industrial exploitation, seven different compositions were tested. The effect of composition on the properties of the produced glass foams, such as compressive strength, apparent density, Springer

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porosity, coefficient of thermal expansion (CTE), along with the microstructural characteristics and the phases formed is presented and discussed.

2. Materials and experimental procedure Cullet of commercially produced sodium-calcium-silicate sheet glass had a chemical composition of 70.64% SiO2 , 0.68% Al2 O3 , 0.18% Fe2 O3 , 9.93% CaO, 3.55% MgO, 13.66% Na2 O, 0.29% K2 O, and 0.21% SO3 (the data were obtained form the manufacturer). After removing impurities, cullet was crushed, using a crushing machine, and dry milled, using a porcelain ball mill (weight ratio of glass/porcelainballs = 1/2). The resulting powder, named as C, had a mean particle size of about 9 μm (Fig. 1). To produce the AD glass, natural raw materials used in ceramic and glass industries, such as quartz, sand, calcite, magnesite, and kaolin, were used. Powders of reactive grade NH4 H2 PO4 and CaF2 were also used as sources of P2 O5 and fluorine, respectively. After mixing of the batch in a ball mill, the homogenous mixture was melted in corundum crucibles in an electric furnace at 1400–1420◦ C for 1 hour. Glass frit was obtained by quenching of glass melt in cold water. The frit was dried and then dry-milled in a high-speed porcelain mill until a mean particle-size of about 13 μm was achieved (Fig. 1). Table 1 presents some properties of the cullet glass (C) and the AD glass. Note that the reported properties of the Table 1 Some properties of the cullet (C) and the AD-glass Property

Cullet (C)

AD-glass [9]

Density (g/cm3 )



CTE (×10−7 K−1 ) 100–400◦ C



Transformation temperature (◦ C)



Dilatometric softening point (◦ C)



This value has been determined between 20◦ C and 600◦ C [9]

Fig. 1 Particle size distributions of the milled powders of cullet (C), AD-glass and SiC

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Table 2 Batch compositions Composition





























AD glass were measured in an earlier study [9] using bulk samples obtained by casting of molten glass in stainless steel moulds at room temperature with no annealing. Commercially available SiC powder, F-500 (Elektroschmelzwerk, Kempten, GmbH, Germany) with a mean particle size of about 17 μm (Fig. 1) was used as gassing agent. Table 2 shows the batch compositions of the seven investigated compositions. The mixtures of the powders of C, AD and SiC were dry mixed in a cylindrical rotary mixer for 30 min. The powders were put in stainless steel moulds and heat treated in air according to the following schedule: Heating (5 K/min) up to 900◦ C, holding for 30 minutes, further heating (5 K/min) up to 950◦ C and holding for 30 minutes. The produced glass foams were easily removed from the moulds and easily machined with a saw and grinding sandpapers. The sintering behaviour of the C and AD glass powders were separately studied using pellets of ∼10 mm in diameter and ∼10 mm in height prepared by dry pressing (80 MPa). The same schedule of heat treatment was followed. The following characterization techniques were employed. The particle size distribution was measured by light scattering equipment (Coulter LS 230, UK, Fraunhofer optical model). The apparent density of materials was determined either by Archimedes method (immersion in ethylenoglycol) or by measuring the weight and the dimensions of the produced materials. The compression strength of cubic samples of ∼30 mm edges, placed between parallel plates of stainless steel, was measured in a Shimadzu machine (Trapezium 2, Japan, displacement 0.5 mm/min). Five different samples from each composition were tested. The maximum load of the first plateau of the stress-strain plots divided by the cross sectional area was considered as the crushing strength [10, 11]. The use of intermediate rubber layers placed between the porous samples and the metallic plates of the apparatus did not perceptibly alter the presenting results. Dilatometry measurements (Bahr Thermo Analyse, DIL 801 L, Germany) were carried out up to 800◦ C (heating rate 5 K/min). X-ray diffraction analyses (XRD, Rigaku Geigerflex D/Mac,

C Series, Cu Ka radiation, Japan) and microstructure observations (SEM, Hitachi S-4100, Japan) were also carried out.

3. Results and discussion Sintered glass powder (C) compacts had a vitreous aspect with a hemi-spherically shaped smooth surface. Their apparent density was 1.32 g/cm3 and the porosity was calculated as ∼ 48% (using the density value of Table 1). Well distributed big pores of 0.5–0.7 mm in diameter were observed at the fracture surfaces. On the other hand, AD-glass powder compacts were transformed into dense (∼5% porosity) glassceramics with an apparent density of 2.79 g/cm3 , which is higher than the density of the parent glass (Table 1). Owing to the fact that glass cullet (C) mainly comprises Na2 O, CaO and SiO2 , the diagrams of Fig. 2 [12] approximately

Fig. 2 Iso-viscosity curves (in poise) for Na2 O-CaO-SiO2 glasses at 900◦ C and 1000◦ C, obtained from [12]



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Fig. 3 General aspect of the produced glass foams (see Table 2)

estimate the viscosity of C-glass as 103.5 –103 Pa·s between 900◦ C and 1000◦ C. At the same temperature range, the viscosity of the AD glass has been experimentally determined in an earlier study as 107 –105.5 Pa·s [13]. In the light of these results, it must be expected that the additives of AD glass in the C glass should reduce the expansion of the generated gas bubbles at 900–950◦ C, resulting in increasing densification of the whole system at temperatures above the Tg (720◦ C, Table 1). Figure 3 shows the typical aspect of the glass foams of all the investigated compositions after rectifying. Evidently, the amounts of SiC and AD-glass in the batch considerably influenced the size, the shape and the distribution of the pores. The glass foams 1 and 2 (i.e. 0.25% and 0.5% SiC, respectively) had rather inhomogeneous microstructures with irregular pores with a high scattering in their size (2.5–4.0 mm). The increase of SiC to 1% in foam 3 resulted in slightly smaller and more uniformly sized pores, improving the homogeneity of porous structure (i.e. narrower pore size distribution). Since 1% SiC efficiently resulted in highly porous and homogenous glass foams, this amount of SiC (1%) was selected for incorporation in the other four investigated compositions, which only feature increasing amount of AD glass (Table 2). The general aspect of the glass foam 4 resembles the foam 3 (the diameter of the round pores is slightly smaller), indicatSpringer

ing that the small amount of 1% AD had a negligible effect on the microstructure of the investigated glass foams. Increasing amount of AD to 3% (foam 5) resulted in glass foams with narrower scattering of pore sizes. Well packed honeycomb structures of pentagonal- and hexagonal-like shaped pores, surrounded by well sintered struts, were observed in the glass foams 6 and 7 (5% and 10% AD-glass, respectively). The resulting pore sizes were smaller than in the previous foams, specifically ∼2.0–2.5 mm and ∼1.0–1.5 mm for the foams 6 and 7, respectively. Consequently, glass powder of sheet glass cullet seems to be very good for foaming technology while powder of AD glass can benefit sinterability between Tg and crystallisation temperature (Tc ). Figure 4 shows typical plots of compressive strength vs strain for the glass-foams with 1% SiC. In all cases, there is an almost linear regime in the beginning (up to 0.5–1.0 mm displacement), which might be assigned to elastic deformation, followed by a plateau, which should be attributed to the occurrence of cracking events. Beyond a certain threshold of displacement (about 4 mm), the plateau of compressive strength abruptly descends to a lower level. Obviously, this steep decrease of compressive stress after the higher plateau is due to extensive collapse of numerous struts in the porous structure. In the case of the richer AD foams, the threshold between the upper and the lower plateau is generally shifted to longer displacements.

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Fig. 5 Dilatometry curves of glass foams of the compositions 3, 5, 6 and 7

Fig. 4 Plots of compressive strength vs strain for the glass foams 3, 4, 5, 6 and 7

Comparison of the curves of Fig. 4 suggests that high amount of AD-glass in the batch considerably endures the resultant glass foam. This conclusion is verified by the maximum values of compressive strength, the average stress of the first (upper) plateau and the enhanced capability for tolerating longer displacements before the total collapse of the structure. These results agree fairly well with the general aspect of the foams (Fig. 3), where the incorporation of AD-glass in high amounts (foams 5–7) significantly decreased the pores’ size and improved the homogeneity of microstructure. Table 3 presents the highest values of compressive strength, the porosity and the apparent density of the foams. Evaluation of these results in the light of Fig. 3 reveals that higher apparent density values correspond to smaller cells, while higher porosity values seemingly reflect larger size of pore cells. The values of CTE and the softening temperatures of the investigated materials presented in Table 3 were calculated from the dilatometry curves shown in Fig. 5. In general, the foams with higher amounts of AD-glass have higher CTE values than the foam of composition 3, which contains no AD.

The foam of composition 6 (5% AD) exhibited the highest thermal resistance with a softening point at 571◦ C. XRD analysis (Fig. 6) showed that quartz and wollastonite were the only crystalline phases formed in the glass foams 1 to 3, containing 0.25%, 0.50% and 1% of SiC. Increasing SiC content caused an increase of the intensity of the peaks of quartz and a decrease of the intensity of wollastonite peaks. The incorporation of AD-glass apparently enhanced the formation of wollastonite, caused the crystallization of a new pyroxene phase (diopside), and considerably weakened the intensity of quartz peaks. Although anorthite and diopside are intrinsically crystallized from the AD glass between 850◦ C and 920◦ C [9], in the present study anorthite was not detected in the XRD spectra of the compositions which incorporated AD-glass. The presence of SiC, which exhibits the most intensive diffraction peak at 35.73◦ (ICDD card 29-1131), can not be doubtlessly confirmed in the XRD spectra because the X-ray spectrum of wollastonite exhibits two strong diffraction lines at 35.57◦ and 35.74◦ (ICDD card 29-0372), and diopside has a strong peak at 35.71◦ (ICDD card 41-1370). Moreover, SiC was incorporated in a very low concentration (1%) in the whole batch and is expected to be oxidized during the heat treatment releasing carbon dioxide (that causes the foaming effect) and forming SiO2 . Note that the experimental results showed that even very small added amounts of SiC have considerably influenced the foaming process.

Table 3 Properties of sintered glass foams

Composition Property






Bulk density (g/cm3 )






Apparent density (g/cm3 )






Porosity (%)


Compressive strength (MPa) ◦


CTE 100–500 C (10


K )

Dilatometric softening point (◦ C)

0.790 6.83 556

92.35 0.532

90.55 2.066 8.21 559

90.05 2.601 8.03 571

87.90 2.132 7.92 551



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Fig. 6 XRD spectra of sintered glass foams [ICDD cards: Quartz “q” (SiO2 ) 82-1556; wollastonite “w” (CaSiO3 ): 29-0372 for the compositions 1, 2 and 3 and 27-0088 for the compositions 6 and 7; diopside “d” (Ca(Mg,Al)(Si,Al)2 O6 ): 41-1370; The full scale of the intensity axis is 5000 cps; the intensities of the spectra have not been normalized]

It is generally accepted that viscous sintering is the dominant sintering mechanism of soda-lime silica glasses in the range of 600◦ C–800◦ C. Accordingly, densification starts as soon as the glass reaches the softening temperature at which the atoms have enough mobility to diffuse towards any vacancy concentration gradient [14, 15]. In the case of AD glass, densification satisfactorily occurred within the range of 750–800◦ C, while crystallization should take place at temperatures above 800◦ C [9, 14]. Microstructure observations (Fig. 7) revealed the presence of tiny pores in the struts. Extensive observation of the investigated samples showed that the amount of these pores seemingly decreases with increasing the amount of AD glass (the struts of the foams 6 and 7 contained fewer pores). Therefore, beyond the total amount of large pores (observed in Fig. 3), the microstructure of struts should also play an important role in the overall mechanical properties of the produced glass foams (Fig. 4, Table 3). These observations agree fairly well with the sintering experiments made in the absence of SiC with pure C and AD-glass powder compacts (reported in the beginning of Section 3), whereby AD-glass results in denser materials than in the case of C glass powder compacts. The aforementioned physical, thermal and mechanical properties of the investigated samples indicate the alkaliearth aluminosilicate glass powder as a suitable material for reinforcing foams made of sheet-glass cullet aiming at structural and insulating applications. The compressive strength of the foams containing 3% and 5% AD glass powder was Springer

Fig. 7 Microstructures of struts between pores of the compositions (a) 3, (b) 6, and (c) 7

2.5 to 3 times higher than those of the AD-free one (foam 3) with the same added amount of SiC. Although the improved compressive strength is consistent with the slight increase of the apparent density (0.202 g/cm3 , 0.239 g/cm3 and 0.252 g/cm3 , for the compositions 3, 6 and 7, respectively), such an improvement would not be attributed only to density variation. In fact, compressive strength values of the same order of magnitude have been reported for glass foams made from cullet but with a significantly higher apparent density of

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0.50–0.65 g/cm3 [7]. Consequently, the most probable cause of the improved compressive strength of the foams containing >1% AD-glass is the well packed honeycomb structure with an optimal distribution of pentagonal- and hexagonallike shaped pores surrounded by denser struts. The crystallization of wollastonite and diopside inside the struts should also have a positive impact on the mechanical behaviour of the produced porous glass foams. 4. Conclusions The possibility of producing glass foams, suitable for structural and insulating applications in building constructions, by using recycling sheet glass cullet along with SiC powder as gassing agent has been presented. The incorporation of the alkali earth aluminosilicate glass powder AD in an amount more than 1% improved sinterability and increased the compressive strength of glass foams (0.239– 0.309 MPa), most probably due to the formation of a well packed honeycomb structure with an optimal distribution of pentagonal- and hexagonal-like shaped pores surrounded by dense struts. The crystallization of wollastonite and diopside in the glass matrix of the struts should also have a positive impact on the mechanical behaviour of the produced glass foams. Acknowledgments This work was financially supported by CICECO and the Portuguese Foundation of Science and Technology (FCT).


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