Corresponding author, e -mail: lucianpaunescu16gmail.com [603000]

*Corresponding author, e -mail: [anonimizat]
COMPARATIVE ANALYSIS OF THE OWN EXPERIMENTAL
TECHNIQUES OF PRODUCING THE FOAMED GLASS -CERAMIC

Abstract: The paper presents experimental results obtained by a team of researchers from
the company Daily Sourcing & Research SRL Bucharest in the field of producing the foamed
glass -ceramic from waste bottle glass, coal ash and silicon carbide as foaming agent. The
originality of the experiments consists in the use of electricity or microwave energy, unlike
all techniques known worldwide consumers of fossil fuel. The product, obtained with low
energy consumptions and very low pollutants emissions, has physical and mechanical
characteristics of an insulating material, i. e. high porosity, low thermal conductivity and an
adequate compressive strength.

Keywor ds: glass -ceramic, waste bottle glass, sintering, foaming agent, microwave, electric
resistance oven .

1. INTRODUCTION

The foamed glass -ceramic is a polycrystalline material with high porosity, low thermal conductivity and adequate
mechanical strength, constituting an innovative technique of silicate wastes revaluation by their sintering at high
temperatures in presence of an additive with role of foaming agent [1]. Worldwide, the technology is not developed
on large scale, but several concerns of experi mental manufacturing this product type are known, using as replacer
of insulating materials in construction.

A glass -ceramic with high porosity of 85 vol.% is obtained from cathode ray tube glass and oil shale ash, as silicate
wastes, with addition of 5 w t.% calcium carbonate and 5 wt.% limestone, as foaming agents [1, 2, 3].

An advanced technology of Pittsburgh Corning Company [4, 5], industrial applied in Tessenderlo Plant (Belgium),
uses recycled cullet glass as raw material and black carbon as foaming agent to obtain a glass -ceramic named
“Foamglas” used as sealants, coatings, jacketing, adhesives etc. The sintering temperature is 1050 – 1100 șC.

Another method to o btain foamed glass -ceramic [1, 6 ] is based on the sintering process at around 1000 șC o f a
finely ground mixture of waste bottle glass (80 wt.%) and coal ash (20 wt.%) as raw material and silicon carbide
(between 2 – 5 wt.%) as foaming agent. The porosity of material is between 70 – 90 vol.% and the compressive
strength is 1.2 – 1.7 MPa.

A manufacturing method of foamed glass -ceramic was tested in Brazil [7 ] using waste bottle glass (50 – 70 wt.%)
and banana leaves (30 – 50 wt.%) as foaming agent. The sintering temperature varies between 700 – 850 șC,
soaked for 30 minutes. The product has p orosity in the range 58.5 – 87.5 vol.% and compressive strength between
1.17 – 3.50 MPa.

A technol ogy tested in China [8, 9 ] uses waste quartz sand, coal gangue and sintering additives to obtain glass –
ceramic foam. The sintering temperature is 1140 șC, soaked for 1 hour. The characteristics of the foamed material
are: apparent density – 0.39 g/ cm3, porosity – 87.5 vol.%, thermal conductivity – 0.085 W/ m·K and compressive
strength – 2.4 MPa.

Waste soda lime glass (50 – 65 wt.%), iron -rich copper slag ( 32 – 47 wt.%) and silicon carbide (3 wt.%) as foaming
agent are used to manufacture glass -ceramic fo am in another method [10 ]. The sintering temperature is between
800 – 1000 șC. The glass -ceramic has porosity between 65 – 70 vol.% and the compressive str ength reaches 9
MPa.

A method which uses a very high weight proportion of waste glass (99 wt.%) and 1 wt.% calcium carbonate as
foaming agent is presented in [11 ]. The sintering temperature is 850 șC. The foamed glass -ceramic has a high
porosity of 85.1 v ol.%, very low thermal conductivity of 0.031 W/ m·K and its compressive strength is between
0.7 – 1.6 MPa.

The techniques of manufacturing foamed glass -ceramic described above use as energy source the conventional
heating system with fossil fuel. Information on energy consumptions of these processes is not offered by literature.
Theoretically, considering the required energy for the sintering process and its thermal efficiency (0.35 – 0.50), in
conditions of a continuous operation industrial plant, the specific energy consumption can be estimated in the
range 2.03 – 2.90 GJ/ t or 564 – 806 kWh/ t. Also, the pollutants and greenhouse gas emissions, characteristic of
the fossil fuel combustion, must be taken into account.

Lately, the microwave heatin g, known as a fast, clean and economic process, began to be industrially applied on
small scale plants in some processing techniques of glass, organics, ceramics, polymers, metals and composites.
The microwaves are applicable when at least one of the raw m aterial mixture components is a microwave
susceptor. Comparing to the well known domestic microwave ovens, with the installed power of 0.7 – 1.3 kW, the
industrial ov ens have magnetrons, whose total power is in the range 3 – 6 kW , but existing the possibil ity to design
ovens with much higher powers [12]. The li terature refers to several microwave techniques of processing different
glass -ceramic types in some systems ( Li-Al-Si, SiO -Fe2O3-B2O3, CaO -ZrO 2-SiO 2, R2O-Al2O3-B2O3-SiO 2, Ca-Mg-
aluminosilicate with SiC), but these have no connection with the paper topic.

2. EXPERIMENTAL WORK

2.1. Methodology of experimentation

Certain temperature range favors the oxidation or decomposition of the foaming agent (depending on its nature).
During these chemical reactions, a gaseous compound (most often, carbon dioxide) is released inside the sintered
material, producing closed and/ or open bubbles. In this way, t he porosity of material is generated.

The foaming agent adopted in our experiments was silicon carbide. In the te mperature range 950 – 1150 șC [6 ],
the silicon carbide is oxidized as the following equations:

SiC + 3/2 O 2 = SiO 2 + CO (1)

SiC + 2 O 2 = SiO 2 + CO 2 (2)

The technological equipments chronologically used in experiments were: a 4 kW electri c resistance heating oven
(Fig. 1a), a 5 kW microwave reactor (Fig.1b ) and a 0.8 kW microwave oven (Fi g.1c), own conception designed or
adapted, in the case of the last.

a
b

c

Fig. 1 The heating equipment used to produce the foamed glass -ceramic
a – 4 kW electric resistance heating oven; b – 5 kW microwave reactor; c – 0.8 kW microwave oven.

The electric resistance heating oven is powered with 4 kW electric resistan ce embedded into a helical channel
placed in a ceramic refractory support around the cylindrical crucible. Inside the crucible is introduced the mould
with the powered mixture, as pellets obtained by pressing at 100 MPa. The crucible and the mould with its cover
are made by refractory steel. The process temperature is measured with a Cromel -Alumel thermocouple introduced
axially inside the mould through the central orifice of the cover. The adopted sintering temperature is soaked with
a temperature regulator.

The microwave reactor is powered with five magnetrons (two placed at the bottom of the reactor cavity and three
placed in the side wall of the cavity. A silicon carbide crucible with the thickness wall of 20 mm and the height of
100 mm is mounted into the reactor cavity. Inside the crucible it is introduced the mould containing the powered
material pressed at maximum 20 MPa. The upper area of the reactor is thermally protected with ceramic fiber. The
measurement of the sintering temperature is performed in the same way than at the electric resistance oven.

The microwave oven is an adapted domestic oven to operate at high temperature of around 1000 șC. The powered
mixture is loaded into a silicon carbide mould, provi ded with a cover made from the same material. The mould is
placed on a support, which is rotated around its axis during the operation of the magnetron. To avoid the thermal
losses, the mould is protected with ceramic fiber. The same way to measure and to l ead the thermal regime in the
oven is applied.

The methodology of experimentation is the following:

The powered mixture is heated up to the adopted sintering temperature programmed on the regulator with the
speed allowed by the used heating equipment. Th us, the heating speed in the electric resistance oven is in the range
8.9 – 12.1 șC/ min., in the microwave reactor is between 8.6 – 13.1 șC/ min and in the microwave oven is between
24.4 – 26.5 șC/ min. In the case of the microwave reactor, the heating sp eed had the largest values range, due to
the using of the all five magnetrons up to about 900 șC, followed by the use of only the three magnetrons placed
in the side wall of the reactor cavity up to the sintering temperature. The soaked time at the sinteri ng temperature
was experimentally varied in very large limits, from 5 min. up to 101 min., following an optimal homogenization
of temperature in the entire volume of material. The adopted cooling speed was 5.0 – 5.5 șC/ min., but the specific
conditions le d to modify this range. In the case of the electric resistance oven, it was necessary to forced the coolin g
due to the high thermal inertia of the ceramic refractory support. In the case of the microwave oven, the cooling
speed was increased to 10.9 – 15.5 șC/ min. from the technological reasons.

2.2. Raw materials

The experiments were based on waste bottle colorless glass and coal ash, in different weight proportions, as raw
materials. The chemical composition of the two wastes is shown in Table 1.

Table 1. Chemical composition of raw materials
Raw
materials Chemical composition, wt.%
SiO 2 Al2O3 Fe2O3 CaO MgO Na2O K2O
Coal ash 46.5 23.7 8.6 7.9 3.2 6.0 4.1
Waste
bottle
glass
71.8
1.9
–
12.0

1.0
13.3
–

The waste bottle colorless glass w as selected, cleaned, broken, finely ground in a ball mill and sieved in several
grain size fractions: below 63 μ m, below 130 μm and below 300 μm , tested separately. The coal ash , brought from
Paroseni thermal power station at grain size between 6 3 – 80 μm, was ground in the ball mil and sieved below 63
μm, being used separately in experiments in the both fractions.

The silicon carbide, used as foaming agent, purchased with the grain size between 63 – 80 μm , was ground in the
ball mill and sieved below 32 μm, being used separately in experiments in the both grain size fractions .

2.3. Characterization of the foamed glass -ceramic samples

The foamed glass -ceramic samples, resulted after the sintering process described above, were tested in laboratory
to determine the physical, mechanical and structural characteristics. The characterizations were performed in
Daily Sourcing & Research SRL Bucharest, Faculty of Applied Chemistry and Materials Science of Univer sity
“Politehnica” of Bucharest and Metallurgical Research Institute, aiming apparent density, porosity, volumetric

proportion of closed and open pores, compressive strength, thermal conductivity, hydrolytic stability and
crystallographic structure of the glass -ceramic samples.

The apparent density was determined by the gravimetri c method with the picnometer [13 ]. The porosity was
calculated by the comparison method of true and apparent densities of the mater ial, experimentally measured [14 ].
The volumetri c proportion of the open and closed pores from the sample structure was determined by the m ethod
of its water immersion [15 ]. Determining the thermal conductivity was performed by the guarded -comparative –
longitudinal heat flow technique, according to ASTM E 1225 – 04. The compressive strength was measured with
an uniaxial hydraulic press. The hydrolytic stability of the samples was measured by the standard procedure ISO
719:198 5 with a 0.01M HCl solution [1 6, 17 ]. The crystallographic structure of the glass -ceramic samples was
investigated with the X-ray diffraction method (XRD) , according to the standard EN 13925 – 2: 2003.

3. EXPERIMENTAL RESULTS AND DISCUSSION

The experimentation of producing the foamed glass -ceramic carried on in the company Daily Sourcing & Research
SRL Bucharest on the three heating equipment types, presented above.

The functional parameters of the sintering and foaming process of the powder raw material mixtures, in different
variants of weight composition, are shown in Table 2.

Table 2. Parameters of the sintering process
Variant Mixture composition, wt%/
Grain size, μm Powered
mixture
quantity/
Sintered
material
quantity
g Sintering
temperature
șC Soaking
time
min. Heating/
Cooling
speed
șC/ min. Specific
consumption
of electricity
kWh/ kg Waste
bottle
glass Coal ash Silicon
carbide
A. Electric resistance heating oven
A1 75.3/
< 63 22.5/
< 63 2.2/
< 32 25.0/ 24.0 970 17 10.2/ 3.0 166.7
A2 78.5/
< 63 19.2/
< 63 2.3/
< 32 37.0/ 35.0 970 25 10.2/ 2.8 120.3
B. Microwave reactor
B1 77.0/
< 63 18.5/
< 63 4.5/
< 32 67.2/ 63.8 980 73 11.8/ 3.9 163.0
B2 77.0/
< 63 18.5/
< 63 4.5/
< 32 59.7/ 56.7 980 73 12.2/ 2.8 180.4
B3 85.0/
< 300 10.0/
< 63 5.0/
63 – 80 60.0/ 57.0 987 96 16.7/ 4.7 150.9
B4 90.0/
< 300 5.5/
< 63 4.5/
63 – 80 60.0/ 57.0 987 100 9.9/ 4.8 145.6
B5 91.0/
< 300 4.5/
< 63 4.5/
63 – 80 158.0/
150.0 995 94 13.4/ 5.0 52.2
B6 95.5/
< 63 – 4.5/
< 32 62.2/ 59.1 970 96 15.0/ 5.7 137.7
B7 76.0/
< 130 19.0/
63 – 80 5.0/
63 – 80 160.0/
152.0 990 64 13.0/ 5.3 51.0
B8 80.0/
< 130 15.0/
63 – 80 5.0/
63 – 80 150/
143 985 54 13.1/ 4.9 45.2
B9 95.0/
< 130 – 5.0/
63 – 80 148/
141 957 48 9.1/ 4.9 46.8
C. Microwave oven
C1 76.0/
< 130 19.0/
63 – 80 5.0/
63 -80
119.5/
118.9 990 34 26.5/
15.5 6.6
C2 80.0/
< 130 15.0/
63 – 80 5.0/
63 – 80 125.0/
111.6 980 6 24.4/
11.8 5.0

C3 84.0/
< 130 11.0/
63 – 80 5.0/
63 – 80 120.0/
119.2 968 10 25.8/
10.9 5.3
C4 89.0/
< 130 6.0/
63 – 80 5.0/
63 – 80 120.0/
118.9 965 12 25.3/
12.0 5.3
C5 95.0/
< 130 – 5.0/
63 – 80 120.0/
119.0 963 10 25.0/
10.9 5.2

The experiments, indifferently the used heating equipment, yielded from a weight ratio between the coal ash and
waste glass of about ¼, being tested variants with increasingly lower ratios, up to zero. The foaming agent (silicon
carbide) was used in low weight proportions, between 2 – 5%, according to the optimal values recommended in
literature [6]. The first experiments carried on in the electric resistance oven used silicon carbide proportions at
the lowe r limit of the above range, i. e. 2.0 – 2.3 wt.%. It was experimentally observed that this limit is not adequate
for the foaming process in the conditions offered by the electric resistance oven, all the more so the experiment
with 2.0 wt.% silicon carbide did not produce the foaming of the material, this being unloaded from the oven only
sintered. On the other hand, the experiments performed in the electric resistance oven highlighted that the higher
temperatures than 1020 șC are not indicated, the best re sults being obtained at 970 șC. Also, the soaking time at
the sintering temperature, initially tested at 5 min., had to be increased to about 30 min., to obtain a good
homogenization of the temperature in the entire volume of the material.

The need of pre ssing the powder raw material mixture to about 100 MPa, recommended in [6], created
technological problems, leading the loading in the mould of very low quantities of pellets and excessively high
specific consumptions of electricity.

The experiments perfo rmed in the electric resistance oven allow to identify, in a first stage, the optimal parameters
of the manufacturing process of the foamed glass -ceramic, which were taken into account in the folloving
experiments.

The most different tests were conducted in the microwave reactor, aiming priority to ensure the temperature
homogenization into the material mass by stopping the magnetrons placed at the bottom of reactor at 900 șC and
operating up to 970 – 995 șC only with the magnetrons placed in the side wall . The pressing technique at 100 MPa
of the powder material as cylindrical pellets was complet ely eliminated and replaced with a slight ly pressing at
maximum 20 MPa of the entire material quantity, directly into the mould. In this way, the used raw material
quantities increased significantly, reaching maximum 158 g.

The magnetrons operation failed to ensure the structural homogeneity of t he obtained material samples, though
the soaking time at the sintering temperature was increased up to over 90 min. Moreo ver, by reducing the
processing degree of the raw material, especially, the waste glass (diminishing the grain size from < 63 μm to <
300 μm), the upper area of samples remained completely unfoamed and the layer thickness reached oven 15 mm .
By increasing the powder material quantity in the mould at 158 g (variant B5), the upper area was foamed as the
rest sample volume, due to the direct contact of the foamed material with the hot surface of the mould cover.

Other variants were tested in the microwave reactor in conditions of loading in the mould of quantities of powder
mixture of 148 – 160 g, which by foaming at the adopted sintering temperature (between 957 – 990 șC) touch the
hot inner surface of the cover. This experiments set was performed with was te bottle glass, in proportions of 76 –
95 wt.% , processed at a grain size below 130 μm. The coal ash, in proportions between 0 – 19 wt.% and the silicon
carbide (5 wt.%) had identical grain size fractions, of 63 – 80 μm.

The use of the domestic microwave oven, adapted to operate at high temperatures, ensured the required structural
homogene ity of the sintered material. T hough the installed power of the oven is much lower than in the previous
cases, the method of loading the powder material in silicon carb ide moulds was energy effective. Because the
silicon carbide is a microwave susceptor material, the heating of the mixture was made directly, without other heat
losses. Moreover, the rotating around the own axis of the mould allows its homogeneous heating. The heating
speed increased significantly. The soaking time at the sintering temperature was reduced at about 10 min. Also,
the cooling speed was increased.

Between the samples of foamed glass -ceramic, experimentally obtained by heating in the electric o ven, the
microwave reactor and the microwave oven, 16 samples were selected to be characterized by the methods
described in Chapter 2.3. Generally, the selection criterions of these samples were the pores distribution
homogeneity, low apparent density and high porosity.

According to the images from Fig. 2 – 17, all samples have the pores structure homogeneously distributed,
indifferently the adopted heating technique, the weight proportion and the processing degree of raw materials.

Fig. 2. Variant A1

Fig. 3 . Variant A2

Fig. 4 . Variant B1

Fig. 5 . Variant B2

Fig. 6 . Variant B3

Fig. 7 . Variant B4

Fig. 8 . Variant B5

Fig. 9. Variant B6

Fig. 10. Variant B7

Fig. 11. Variant B8

Fig. 12. Variant B9

Fig. 13. Variant C1

Fig. 14. Variant C2

Fig. 15. Variant C3

Fig. 16. Variant C4

Fig. 17. Variant C5

In Table 3 the physical and mechanical characteristics of samples obtained in the three used heating equipments
are shown.

Table 3. Physical and mechanical characteristics of the samples
Variant Apparent
density
g/ cm3 Porosity
vol.% Open/ Closed
pores ratio Compressive
strength
MPa Thermal
conductivity
W/ m·K
A. Electric resistance heating oven
A1 0.34 82.1 19.70/ 80.30 3.2 0.062
A2 0.33 82.6 19.50/ 80.5 0 3.5 0.060
B. Microwave reactor
B1 0.55 71.1 16.67/ 83.33 6.6 0.085
B2 0.47 75.3 10.05/ 89.95 7.0 0.073
B3 0.35 81.4 15.05/ 84.95 4.0 0.059
B4 0.40 78.9 16.03/ 83.97 3.8 0.061
B5 0.73 61.6 16.55/ 83.45 6.9 0.089
B6 0.35 81.6 10.99/ 89.01 3.9 0.057
B7 0.46 75.8 15.33/ 84.67 6.2 0.054
B8 0.48 74.7 15.89/ 84.11 6.4 0.058
B9 0.45 76.3 14.90/ 85.10 4.3 0.050
C. Microwave oven
C1 0.34 82.1 15.92/ 84.08 3.8 0.039

C2 0.32 83.2 15.66/ 84.34 3.9 0.038
C3 0.34 82.0 16.01/ 83.99 3.5 0.044
C4 0.34 82.0 14.73/ 85.27 3.2 0.043
C5 0.33 82.5 15.38/ 84.62 2.8 0.040

The physical characteristics (apparent density and porosity) are slight ly different depending on the above
criterions, but their values are framed in limited ranges (0.32 – 0.55 g/ cm3 – apparent density and 71.1 – 83.2
vol.% – porosity), corresponding to the requirements imposed for the insulating materials in construction. Be yond
these limits is placed the sample B5, with 91.0 wt.% waste glass (< 300 μm), 4.5 wt.% coal ash (< 63 μm) and 4.5
wt.% silicon carbide (grain size between 63 – 80 μm), which has a fine and homogeneous grain size, but an
apparent density of 0.73 g/ cm3 and, implicitly, low porosity of 61.6 vol.%.

The high compressive strength of this sample (6.9 MPa) is distinct comparing to all samples with similar weight
composition of raw material, indifferently the oven in which were processed. Generally, the relat ive high
mechanical strength (over 6 MPa) for the foamed glass -ceramic characterizes the samples obtained with
proportions of waste glass of 76 – 80 wt.% and coal ash over 15 wt.% (the samples B1, B2, B7, B8). The samples
obtained in the microwave oven, wh ich have very high porosities (82.0 – 83.2 vol.%), indifferently the raw material
composition, are characterized by compressive strengths with much lower values, corresponding to the raw
material proportions mentioned above (3.8 – 3.9 MPa), though these ar e adequate for the industrial application s
field. The samples proceeding from raw material with very high proportions of waste glass (95 wt.%) and very
low (or without) proportions of coal ash have very small values of the compressive strength (B5, B9, C5) .

The lowest values of the thermal conductivity belong to the samples obtained after the sintering process in the 8
kW microwave oven (between 0.038 – 0.044 W/ m·K), in conditions in which the processing degree of raw
materials and foaming agent was not t he best. The highest values of thermal conductivity resulted in the case of
the samples B5, B1, A2 and A1 , the last two being the single samples with adequate characteristics produced in
the electric resistance oven.

The XRD analysis performed on the samp les with 2.2 – 2.3 and, respectively, 4.5 – 5.0 wt.% silicon carbide
indicated wollastonite -2M (CaSiO 3) as main crystalline phase after the sintering process. Cristobalite was not
detected in the foamed material, though would have been to exist, having int o accou nt the high proportion of SiO 2
in the raw material mixture (over 60 wt.%).

The t ests for determining the hydrolytic stability of samples, using 0.15 ml of 0.01M HCl solution to neutralize
the extracted Na 2O, showed that the stability joins in the hydrolytic class 2, the extracted Na 2O equivalent being
in the range 34 – 57 μg.

4. ECONOMIC EFFECT AND THE IMPACT ON ENVIRONMENT

As previously noted in Chapter 1, the energy source used worldwide to produce foamed glass -ceramic is the fossil
fuel and the specific energy consumptions are not shown in literature. Theoretically, taking into account the energy
requirement of the process and a thermal efficiency corresponding to the continuous industrial process in the range
0.35 – 0.50, the specific consum ption was estimated at 2.03 – 2.90 GJ/ t or 564 – 806 kWh/ t. Analyzing the
electricity consumptions of the experimental processes described in the paper, it results that the sintering process
in the 0.8 kW microwave oven is obviously the most economical p rocess. The specific energy consumptions are
in the range 5.0 – 6.6 kWh/ kg (see Table 2), much lower than the consumptions achieved in the microwave
reactor (45.2 – 180.4 kWh/ kg) and the electric resistance oven (120.3 – 166.7 kWh/ kg), due to, primarily , the
use of silicon carbide moulds which is a microwave susceptor material, allowing the directly heating of material.
However, the thermal efficiency of the processes carried out in the microwave oven, being a discontinuous
experimental process, has very low values in the range 0.042 – 0.056.

Equating the conditions of carr ying out the discontinuous sintering process from the microwave oven at the level
of a typical industrial continuous process, with the thermal efficiencies noted above, it results calc ulated energy
consumptions in the range 560 – 739 kWh/ t , lower comparing to the consumptions estimated for the industrial
processes consumers of fossil fuel with 0.7 – 8.3%, the upper limit of the range corresponding to the thermal
efficiency of 0.50, whi ch characterizes an industrial process with low heat loss into the environment and an
advanced technological recovery of the secondary energy resources.

In point of the environmental protection, the replacement of burning the fossil fuels by the use of mi crowave
energy, in order to achieve technological heating, is favorable . The greenhouse gas and other pollutants as nitrogen
oxides or carbon monoxide, emitted by the combustion of fossil fuels are eliminated, taking into consideration
only the indirect emissions of greenhouse gas resulted in the primary process of producing electricity in thermal
power stations.

In the other hand, the manufacturing technique of glass ceramic, used as replacer of different materials in
construction, from the waste bottle glass, constitutes a viable revaluation solution of this waste existing on large
scale.

5. CONCLUSIONS

The foamed glass -ceramic constitutes a polycrystalline material obtained by the thermal treatment at high
temperature of different types of industri al silicate waste.

In experiments, waste bottle glass (in high proportions of over 76 wt.%) and coal ash (0 – 19 wt.%), as raw
materials, as well as silicon carbide (in low proportions of 2 – 5 wt.%), as foaming agent, were used.

The heating equipments e xperimentally used were a 4 kW electric resistance oven , a 5 kW microwave reactor and
a 0.8 kW microwave oven of the company Daily Sourcing & Research SRL Bucharest.

The best experimental results (physical, mechanical and structural characteristics and specific energy
consumption) were obtained in the microwave oven, using a silicon carbide mould. The powder raw material is
directly heated due to the silicon carbide is a microwave susceptor material.

The foamed glass -ceramic experimentally obtained in the microwave oven has a high porosity of over 82 vol.%,
low thermal conductivity between 0.038 – 0.044 W/ m·K and adequate compressive strength of 2.8 – 3.9 MPa,
corresp onding to the requirements of insulating materials used in construction. The specific energy consumption
is in the range 5.0 – 6.6 kWh/ kg, obtaining in conditions of a discontinuous process.

The main crystalline phase after sintering was identified by XR D analysis as wollastonite -2M (CaSiO 3).

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