Preparation of glass ceramic foams for thermal insulation applications from coal fly ash and waste glass

Construction and Building Materials 112 (2016) 398–405

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Preparation of glass ceramic foams for thermal insulation applications from coal fly ash and waste glass Mengguang Zhu a, Ru Ji b,⇑, Zhongmin Li a, Hao Wang a, LiLi Liu a, Zuotai Zhang a,c,⇑ a

Department of Energy and Resources Engineering, College of Engineering, Peking University, 100871 Beijing, PR China School of Mechanical Engineering, University of Science and Technology Beijing, 100083 Beijing, PR China c School of Environmental Science and Engineering, South University of Science and Technology of China, Shenzhen 518055, PR China b

h i g h l i g h t s
Glass ceramic foams were prepared by using coal fly ash and waste glass.
Effects of material ratio and sintering temperature were systematically investigated.
Energy conservation evaluation was simulated with the software of EnergyPlus.

a r t i c l e

i n f o

Article history: Received 24 December 2015 Received in revised form 6 February 2016 Accepted 25 February 2016

Keywords: Coal fly ash Glass ceramic foams Thermal insulation material

a b s t r a c t Glass ceramic foams were prepared by direct foaming method, using coal fly ash and waste glass as the main materials, borax and calcium carbonate as fluxing agent and foaming agent, respectively. The effects of coal fly ash additions, foaming time, heating rate and sintering temperature on the bulk density, porosity, mechanical properties and thermal conductivity were systematically investigated. The optimum parameters to prepare the glass ceramic foams were obtained at 800 °C for 45 min with 40 wt.% coal fly ash, 60 wt.% waste glass, 30 wt.% borax and 0.5 wt.% calcium carbonate. The specimens prepared this way have a low bulk density (as low as 0.46 g/cm3), exhibiting considerable compressive strength (exceeding 5 MPa) and low thermal conductivity (about 0.36 (W/m K)). The energy saving effect using glass ceramic foams was evaluated by EnergyPlus software, indicating that the prepared glass ceramic foams show good energy conservation effect for building thermal insulation materials. The preparation of glass ceramic foams using solid wastes may provide a promising way to prepare thermal insulation material, considering the advantages in both economic and environmental aspects. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Glass ceramic foams, which are porous heat-insulating and soundproof materials, have attracted great interest and have been applied in many areas such as building, chemistry and defense fields [1]. Glass ceramic foams have excellent properties such as low density, low thermal conductivity, incombustibility, etc. Furthermore, they show better thermal insulation and weatherability properties compared to organic thermal insulation materials, especially polymeric foams which may cause serious problems related to the fire hazard, short life, environmental toxicity and adhesive incompatibility [2]. Currently, numerous technologies have been ⇑ Corresponding authors at: School of Mechanical Engineering, University of Science and Technology Beijing, 100083 Beijing, PR China (R. Ji), Department of Energy and Resources Engineering, College of Engineering, Peking University, 100871 Beijing, PR China (Z. Zhang). E-mail addresses: [email protected] (R. Ji), [email protected] (Z. Zhang). http://dx.doi.org/10.1016/j.conbuildmat.2016.02.183 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.

developed to produce glass ceramic foams, such as replica, sacrificial template and direct foaming methods [3], etc, with different kinds of solid waste used as the raw materials, like waste glass and fly ash [4–7], metallurgical slag [8–10], municipal solid waste [11], polishing porcelain stoneware tile residue [12], etc. Among them, the most straightforward and widely used way is direct foaming, namely, sintering a mixture of raw materials and a small amount of additives, which is also the method employed in the present investigation. These additives commonly referred to as pore-foaming agents, which generate porosity by emitting gas frothing the glass ceramic melt during reaction at elevated temperatures [7]. Meanwhile, the pore size and structure of glass ceramic foams can be significantly influenced by the concentration and the type of the pore-foaming agent [13]. Coal fly ash is the by-product of coal combustion produced in power stations [14]. As is known to all, China’s energy structure is dominated by coal. With the rapid economic development, more

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than one billion tonnes of coal are burned annually to meet the increasing electricity demand. Consequently, the annual coal fly ash generation continues growing and is anticipated to reach 580 million tonnes by 2015 [15]. However, only parts of the enormous waste residue are utilized, primarily in cement industry and building materials field. The residual coal fly ash is generally disposed in ponds or landfill without any treatment, which not only occupies vast land but also results in serious environmental pollution. For example, potentially toxic substances in coal fly ash could leach into soils and groundwater and accumulate in the food chain [16]. Waste glass is another enormous solid waste derived from glass manufacture process and municipal solid waste. To be specific, waste glass accounts for 15–30% of glass production and for 4% of municipal solid waste [17], with millions tonnes of waste glass generated in China each year. Although it has been used in the manufacturing of the original glassware, the proportion of waste glass added in the process is limited by risks of contamination and degradation of quality. Further, the disposal of waste glass is therefore an urgent issue and the production of glass ceramic foams from waste glass has been proposed [18,19]. Considering both fly ash and waste glass contain large amount of SiO2, Al2O3 and CaO [20], which is similar to the components of glass ceramic foams, our research was therefore motivated, with coal fly ash and waste glass as the raw materials. Meanwhile, borax and calcium carbonate were used as fluxing agent and foaming agent, respectively. Macroscopic properties, such as the bulk density, porosity, compressive strength and thermal conductivity of the foams are tested in detail. At last, a building energy simulation software (EnergyPlus) was used to evaluate the energy saving effect of the glass ceramic foams and the possibility of using the material as a thermal insulation material for buildings.

Fig. 1. XRD patterns of the coal fly ash and waste glass.

Table 2 Raw materials compositions (wt.%) of samples. Sample

Coal fly ash

Waste glass

Borax

Calcium carbonate

B35 B40 B45 B50 B55

35 40 45 50 55

65 60 55 50 45

30 30 30 30 30

0.5 0.5 0.5 0.5 0.5

The obtained green samples were heated to 450 °C for 30 min in a muffle furnace with air atmosphere to remove residue water and prevent the samples from rupturing caused by the uneven thermal distribution [10]. Subsequently, the samples were heated to 600–900 °C for 45 min with a heating rate of 20 °C/min, then cooled down to room temperature with a natural cooling rate. 2.3. Characterization techniques

2. Experimental procedure

The total porosity was calculated from the following equation:

2.1. Raw materials

%Porosity ¼ ð1  bulk density=powder densityÞ  100 Coal fly ash (below 200 meshes) was obtained from a thermal power plant at Shuozhou city, Shanxi Province, China and waste glass (200 meshes) was gathered from waste sheet glass. Waste glass can provide sufficient amorphous phase, which is necessary for good heating insulation performance [21]. Chemical compositions (wt.%) of these two raw materials were analyzed by X-ray fluorescence (XRF) and shown in Table 1. Borax (Na2B4O710H2O) was added as fluxing agent to lower the softening temperature of mixture [22]. Calcium carbonate was chosen as foaming agent, which can decompose at around 800 °C and the CO2 gas generated by calcium carbonate will be besieged by the softened glass phase, resulting in a porous structure in the samples. The crystalline phases were identified using X-ray diffraction (XRD), and the XRD patterns of coal fly ash and waste glass are shown in Fig. 1. The major crystalline phases in coal fly ash are anhydrite, quartz and mullite. The existence of anhydrite is owing to the in-furnace desulphurization process.

ð1Þ

In Eq. (1), the bulk density and powder density of materials were measured by Archimedes method and pycnometer method (50 ml capacity), respectively. The specimens for compressive strength test were cut using a hacksaw and refined with SiC abrasive paper to form disks of U20 mm  7 mm. The compressive strength of the foams was measured using a universal testing machine (Suns Shenzhen, China) with a crosshead speed of 2 mm/min. Scanning electron microscope (SEM, S-4800, Hitachi) was used to analyze the size and morphology of pores. The crystalline phases in the raw materials and prepared foams were investigated by X-ray diffraction using Rigaku D/max 2550PC X-ray (CuKa, scanning rate: 8°/ min, scanning range: 10–80°). Finally, thermal analysis was conducted with a thermal analyzer (DRL-III, Instrument and meter Co., Xiangtan, China), adopting the heat flux technique.

3. Results and discussion 2.2. Sample preparation

3.1. Effect of coal fly ash content

In all samples, 30 wt.% of borax and 0.5 wt.% of calcium carbonate were added in the raw materials respectively. Coal fly ash and waste glass were mixed in different proportions, and the samples were named as B35, B40, B45, B50 and B55, respectively. Raw materials compositions and chemical compositions of samples were shown in Tables 2 and 3, respectively. The raw material mixtures were homogenized in porcelain jars with a planetary ball mill for 10 h. Finally, the batches were prepared by uniaxial dry-pressing into disks with a diameter of 22 mm, thickness of 10 mm, using a pressure of 10 MPa.

Fig. 2 shows the evolution of compressive strength, bulk density and porosity with the different coal fly ash content for samples sintered at 800 °C. In addition, the typical surface appearance of glass ceramic foams can be seen from the inserted images in Fig. 2(a). Apart from sample B35, it can be seen that both the density and compressive strength increase while the porosity decreases with

Table 1 The chemical compositions (wt.%) of coal fly ash and waste glass. Raw materials

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

TiO2

SO3

P2O5

LOI

Coal fly ash Waste glass

35.42 66.55

39.40 1.55

2.63 0.26

10.04 8.71

1.85 4.55

0.14 17.26

0.40 0.55

1.19 0.027

4.62 0.281

0.217 0.022

4.09 0.24

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Table 3 Chemical compositions (wt.%) of samples. Sample

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

TiO2

SO3

P2O5

B2O3

LOI

B35 B40 B45 B50 B55

42.8 41.6 40.4 39.2 38.0

11.4 12.8 14.3 15.8 17.2

0.8 0.9 1.0 1.1 1.2

7.1 7.1 7.2 7.2 7.3

2.8 2.7 2.6 2.5 2.4

8.7 8.1 7.4 6.8 6.1

0.4 0.4 0.4 0.4 0.4

0.3 0.4 0.4 0.5 0.5

1.4 1.6 1.7 1.9 2.1

0.1 0.1 0.1 0.1 0.1

0.2 0.2 0.2 0.2 0.2

1.2 1.4 1.5 1.7 1.8

Fig. 2. Effects of coal fly ash the content (Sample number: B35, B40, B45, B50, B55) on the (a) compressive strength, (b) bulk density and porosity of samples sintered at 800 °C.

increasing coal fly ash content. It is interesting to note that sample B40 has the lowest density and highest porosity. This can be explained as follow. For sample B35, which made of 65% waste glass and 35% fly ash, the ratio between CaO, Al2O3 and SiO2 gets more closely to the composition of eutectic point in the ternary diagram of CaO, Al2O3 and SiO2 system [11]. Thus the softening temperature of B35 is the lowest among these samples. The low softening temperature leads to a low viscosity of sample B35 which hinder the development of pores at 800 °C. Small pores disappeared or developed to large pores, and the product gas by decomposition of CaCO3 discharges outward (Fig. 2(a)). As a result, sample B35 has a low porosity. By contrast, sample B40 contain more fly ash (40%) than B35, the viscosity is increased appropriately at 800 °C. The increased viscosity makes more gas to remain in the sample and form more closed pores. Therefore, the porosity increased in sample B40. However, by adding more fly ash in B45–B55, the higher viscosities and less liquid phase not only impede outflow of generated gas but also impede the growth of pores. As consequence, these samples have lower porosity than B40. In addition, a good positive correlation between compressive strength and bulk density can be observed obviously. However, there is an anomaly in the composition containing 35 wt.% of fly ash, i.e. sample B35. As aforementioned, sample B35 with lower coal fly ash content has a lower softening temperature and the viscosity of the molten glass would also decrease. The pores were easy to merge and form into larger pores, or finally escaped, leaving pocking marks (Fig. 2(a)) on the surface. The irregular sizes of pores inside and flaws on the surface formed in this process have an adverse effect on compressive strength. This could explain the abnormal performance of sample B35 with high density but low compressive strength. In order to study the morphology variation with coal fly ash content, SEM analysis was conducted, as shown in Fig. 3. It can be seen that the pore size decreases with increasing content of coal fly ash. Samples with low coal fly ash content (B35, B40) illustrate

inhomogeneous microstructures and irregular sized pores (0.2– 1.5 mm) with relatively bigger pore size and thicker cell walls (struts). By contrast, samples with comparatively high coal fly ash content (B50, B55) demonstrate smaller and more uniformly sized pores with improved homogeneity of porous structure. The type of porous structure and cell walls play an important role in the resulting mechanical strength [23]. Microstructure observations revealed the presence of tiny pores in the struts, which limited the strength of glass ceramic foams [24]. Further observation of the obtained samples showed that the amount of these tiny pores seemingly decreases with increasing amount of coal fly ash, which is beneficial for the mechanical strength due to the reduced probability of existing critical flaws. This is part of the reason why the compressive strength increases with increasing amount of coal fly ash (Fig. 3). From above discussions (Figs. 2 and 3), high density, low porosity, more homogeneous pore size and few flaws in cell walls result in high compressive strength. Since sample B40 has the lowest density, highest porosity and relatively high compressive strength (Fig. 2), this composition was selected for further research. 3.2. Effect of holding time and heating rate The effects of holding time on the compressive strength, bulk density and porosity of sample B40 are shown in Fig. 4. It can be seen that the bulk density decreases slightly while the porosity increases a bit with the extension of holding time at 800 °C. At the same time, compressive strength rises first followed by a decrease with the increase of holding time. As shown in Fig. 4(a), pore size of sample which was held for 45 min is more homogeneous than that for 10 min, which can be used to explain the increase of compressive strength. With prolonging holding time to 90 min, coalescence phenomenon occurred, i.e., smaller pores likely intended to dissolve in larger pores, favored by the decrease of the surface energy of the system [25]. The coalescence of pores resulted in less uniform pore size and more flaws, which were

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Fig. 3. The SEM images of samples with different coal fly ash content: (a) B35, (b) B40, (c) B45, (d) B50 and (e) B55) sintered at 800 °C.

Fig. 4. Effects of the heating rate on (a) compressive strength, (b) bulk density and porosity of samples containing 40 wt.% coal fly ash (B40) sintered at 800 °C.

contributed to the remarkable decrease in compressive strength of the samples with prolonging holding time to 90 min. Fig. 5 illustrates that the bulk density decreases and the porosity increases, while the compressive strength rises first and falls

later with the extension of heating rate at 800 °C. It is observed that, the change rule is similar as the effect of holding time. The sample B40 sintered at 800 °C with a heating rate of 20 °C/min has the highest compressive strength, favorable porosity and bulk

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Fig. 5. Effects of the heating rate on (a) compressive strength, (b) bulk density and porosity of samples containing 40 wt.% coal fly ash (B40) sintered at 800 °C.

density attributed to more uniformly pores size and less coalescence than others.

3.3. Effect of foaming agent content In Fig. 6, we present the evolution of the bulk density, compressive strength and thermal conductivity of sample B40 sintered at 800 °C with increasing foaming agent content. As shown in Fig. 6, the compressive strength decreases dramatically from 25 to 11 MPa even if the incorporation of foaming agent is 0.1 wt.% compared to those without foaming agent. The bulk density of the sample without foaming agent reaches a maximum value of 2.42 g/cm3 which is equivalent to the general density of densified porcelain stoneware tile. As a consequence of more CO2 gas emission brought about by increasing content of foaming agent during the sintering process, the strength of the pore walls was not enough to counterbalance the gas pressure [26]. Therefore, anomalous pore morphology, cracks, and even the collapsed ceramic body will markedly weaken the compressive strength. Meanwhile, more gas lead to lower density and thermal conductivity. It is also worth pointing out that the density and thermal conductivity appear to be stationary with the foaming agent content increased from 0.5 to 1 wt.%, however, the compressive strength decreases sharply from 5.2 to 0.5 MPa. On the basis of this fact, the appropriate content of foaming agent is 0.5 wt.%.

Fig. 6. The bulk density, compressive strength and thermal conductivity of sample B40 sintered at 800 °C as a function of the content of foaming agent.

3.4. Effect of sintering temperature The samples B35–B55 were sintered at different temperatures from 600 to 900 °C, and the bulk density of the obtained foams are shown in Fig. 7. Generally, the bulk density initially increased with increasing temperature, reaching a maximum value of 1.9– 2.2 g/cm3 at 650 °C. It then abruptly decreased to 0.6–1.1 g/cm3 at 700 °C and finally decreased slightly with further enhancement of the sintering temperature. It can therefore be concluded that, among samples B35–B55, sample B40 has the lowest density about 0.45 g/cm3 at elevated temperature. The samples are not well-sintered at 600 °C (below the softening temperature), which are in a loose state due to the lack of liquid phase formation during sintering process. However, the samples show the obvious matrix densification and volume shrinkage at 650 °C as a result of liquid phase generation and foaming gas insufficient. However, it should be noted that, as sample B35 has a lower softening point owing to lower coal fly ash content, the matrix densification stage can occur at lower temperature. The effects of sintering temperature on the compressive strength and bulk density of samples containing 40 wt.% coal fly ash (B40) was investigated, and the results are shown in Fig. 8. The compressive strength and bulk density show similar tendency with increasing temperature, more specifically, they all increase rapidly to a maximum value from 600 to 650 °C and decrease with any further increase in the sintering temperature. The samples sintered at 600 °C are in a loose state with a poor compressive strength. With the increasing of sintering temperature to 650 °C,

Fig. 7. The bulk density of foams as a function of the sintering temperature.

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Fig. 8. Effects of the sintering temperature on the compressive strength and bulk density of samples containing 40 wt.% coal fly ash (B40).

the growing liquid phase contributed to the densification and shrinkage of samples, resulting in the highest compressive strength and bulk density about 13 MPa and 2.1 g/cm3, respectively. When the sintering temperature is over 700 °C, the calcium carbonate decomposition reaction intensifies, the liquid phase generation and the densification correspondingly accelerate. Hence, the generated CO2 gas becomes more difficult to escape out, causing the generation of more and larger closed pores in the bulk samples. Consequently, at 700 °C, the compressive strength declines and bulk density drops dramatically to 11 MPa and 0.7 g/cm3, respectively. Moreover, the sample sintered at 800 °C possesses basically the lowest bulk density of 0.46 g/cm3 and a relative high compressive strength about 5.2 MPa. The porosity and thermal conductivity of sample B40 as a function of the sintering temperature are demonstrated in Fig. 9. According to Fig. 8, samples sintered at 600 °C are in a loose state, so they have a relative high porosity about 30 vol.% (Fig. 9). When the sintering temperature increases to 650 °C, the porosity decreases sharply to 4 vol.%. When the sintering temperature is over 650 °C, the porosity increases dramatically to 62 vol.% and stabilizes at about 80 vol.% due to the formation of extensive pores. In the meantime, the thermal conductivity shows an adverse tendency because of the negative correlation between porosity and thermal conductivity. The thermal conductivity of well foamed sample at 800 °C is around 0.36 W/(m K).

Fig. 9. Porosity and thermal conductivity of glass ceramic foams containing 40 wt.% coal fly ash (B40) as a function of the sintering temperature.

403

The XRD patterns of sample B40 sintered at various temperatures are illustrated in Fig. 10. Weak peaks corresponding to quartz could be observed for glass ceramic foams sintered at low temperature (700–750 °C). With increasing temperature up to 800 °C, the prominent new phase, diopside (CaMgSi2O6) was observed, agreeing well with previous researches [5,27,28]. When the sintering temperature reached 850 °C, the intensity of the peaks corresponding to diopside phase is increased in contrast to the decreasing intensity for quartz, indicating the formation of diopside at the expense of quartz phase. The crystallization of diopside inside the struts have a positive effect on the mechanical behavior of the glass ceramic foams [29]. What’s more, the disappearance of quartz is also beneficial for better mechanical properties of glass ceramic foams. It is reasoned that the quartz phase transition was accompanied by volume change during cooling process, thus resulting in worse mechanical properties [30]. It should be noted that the existence of small amount of unburned coal in fly ash may result in the decomposition of anhydrite at relatively low temperature [31]. The disappearance of mullite phase from XRD results may be due to two reasons. First, the relative content of fly ash decreases with the addition of waste glass, and second, the small amount of mullite may be fused into silicate glass by solid-liquid state reaction from the thermodynamical viewpoint. Fig. 11 shows the ternary diagram of SiO2-RO-R2O system and the compositions of the sample B40 and samples of other researchers (reference numbers and their typical sintering temperatures are labeled on the diagram). Compared to samples of other researchers [4,6,28,32,33], the contents of Na2O (alkali oxides) in our newly proposed glass ceramic foam system are much higher. As seen from Fig. 11, at the temperature of 800 °C, the liquid phase forms in sample B40 located area d, while area b and c are still solid phases. Therefore, the sintering temperature in our research is lower than that of other researches marked in the Factsage phase diagram. From the economic considerations, the lower sintering temperature is an advantage due to the reduction of energy consumption.

3.5. Energy conservation evaluation The energy saving effect of the glass ceramic foams was evaluated by EnergyPlus [34,35], which is a well-recognized and accepted open-source simulation program used for building energy analysis and thermal load simulation. EnergyPlus calculates thermal loads of buildings by the heat balance method, which takes

Fig. 10. XRD patterns of sample B40 sintered at various temperatures (700 °C, 750 °C, 800 °C, 850 °C).

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Fig. 11. Ternary diagram of SiO2-RO-R2O system with the sample B40 and samples of other researchers, where RO = CaO and MgO (alkaline earth oxides), R2O = Na2O and K2O (alkali oxides).

Fig. 12. The building model for energy simulation.

into account all balances on indoor and outdoor surfaces and transient heat conduction through building construction [36]. As shown in Fig. 12, a 9-storey building with a total building area of 7383.9 square meters is used as the simulation model for

energy calculation in the present research. The traditional external walls are composed of three layers: a 20 mm cement mortar, a 200 mm reinforced concrete and a 20 mm composite mortar. Thermal conductivity of each layer is 0.97 W/(m K), 1.95 W/(m K) and 0.65 W/(m K), respectively. The glass ceramic foam external walls are also composed of three layers, the middle layer is replaced with glass ceramic foams and the other two layers are the same as traditional external walls. According to China building thermal design partition, we choose three representative cites in different climatic regions: Haerbin (frozen zone), Beijing (cold zone) and Guangzhou (hot summer and warm winter zone) to evaluate the energy saving effect of the glass ceramic foams. Fig. 13(a) illustrates the simulation results of monthly heating and cooling loads in different climatic regions for different external walls. It can be seen that glass ceramic foam walls reduce the heating and cooling loads significantly in Haerbin in winter due to the huge temperature difference indoor and outdoor. As for Guangzhou, which is located at a low latitude, the energy conservation is especially pronounced in summer for insulating the hot air. As is shown in Fig. 13(b), the application of glass ceramic foam wall

Fig. 13. Heating and cooling loads in different climatic regions for different external walls: (a) monthly loads and (b) annual loads.

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is expected to reduce the heating and cooling loads by 18.5%, 10.5% and 3.4%, respectively. In China, the annual floor spaces of newly built residential buildings are approximately 2 billion square meters. Only if the glass ceramic foam walls can be applied to one-tenth of these buildings, there will be remarkable annual energy conservation of 9.3  104 MJ, which is the equivalent of 8  105 tonnes of standard coal. Since the glass ceramic foams we prepared in the laboratory have not actual applied in the construction, the above evaluation and estimate are just theoretical calculation without the actual experimental data. 4. Conclusions In present work, glass ceramic foams with high mechanical strength and low thermal conductivity were fabricated by direct foaming method. Fly ash content, sintering temperature and foaming agent content have significant effects on the bulk density, porosity, compressive strength and thermal conductivity. The samples sintered at 800 °C with 40 wt.% coal fly ash and 0.5 wt.% foaming agent show the best complex properties. Analysis of macroscopic properties and microstructure shows that lower coal fly ash content or higher temperature resulted in more liquid phase, which are beneficial for the foaming of specimen, the decrease of bulk density, compressive strength and thermal conductivity. When the sintering temperature and coal fly ash were fixed, the bulk density, compressive strength and thermal conductivity all decrease with the increasing foaming agent content. Additionally, the bulk density and porosity are not sensitive to holding time or heating rate, while compressive strength reached its maximum at 45 min and 20 °C/min, respectively. The prominent new phase, diopside shows up at the expense of quartz phase at elevated temperature, which is beneficial for better mechanical properties of glass ceramic foams. According to the simulation calculation results of EnergyPlus, the glass ceramic foams obtained in this work show good energy conservation effect for construction insulation materials. It should be noted that since the glass ceramic foams we prepared in the laboratory have not applied in the actual construction, the evaluation was just theoretical calculation. Whether from the perspective of economic or environmental aspects, the production of glass ceramic foams for building thermal insulation materials can be a promising way to reuse coal fly ash and waste glass. Acknowledgements Supports by the Key Projects in the National Science & Technology Pillar Program (2013BAC14B07) and the National Natural Science Foundation of China (51522401, 51472007 and 51272005) are acknowledged. The paper was also supported by the Fundamental Research Funds for the Central Universities (FRF-TP-15-085A1). References [1] E. Bernardo, R. Castellan, S. Hreglichb, I. Lancellottic, Sintered sanidine glassceramics from industrial wastes, J. Eur. Ceram. Soc. 26 (15) (2006) 3335–3341. [2] R. Ji, Z. Zhang, Y. He, L. Liu, X. Wang, Synthesis, characterization and modeling of new building insulation material using ceramic polishing waste residue, Constr. Build. Mater. 85 (2015) 119–126. [3] E.C. Hammel, L.R. Ighodaro, O.I. Okoli, Processing and properties of advanced porous ceramics: an application based review, Ceram. Int. 40 (10) (2014) 15351–15370. [4] J. Bai, X. Yang, S. Xu, W. Jing, J. Yang, Preparation of foam glass from waste glass and fly ash, Mater. Lett. 136 (2014) 52–54. [5] H.R. Fernandes, D.U. Tulyaganov, J.M.F. Ferreira, Preparation and characterization of foams from sheet glass and fly ash using carbonates as foaming agents, Ceram. Int. 35 (1) (2009) 229–235.

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