Thermal Insulation Monolith of Aluminum Tobermorite Nanosheets

Sep 23, 2015 - Abstract Image. A thermal insulation monolith of aluminum tobermorite nanosheets was prepared by a facile method of one-step hydrotherm...
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Research Article pubs.acs.org/journal/ascecg

Thermal Insulation Monolith of Aluminum Tobermorite Nanosheets Prepared from Fly Ash Jilin Bai, Yuanzhi Li,* Lu Ren, Mingyang Mao, Min Zeng, and Xiujian Zhao State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, P. R. China S Supporting Information *

ABSTRACT: A thermal insulation monolith of aluminum tobermorite nanosheets was prepared by a facile method of one-step hydrothermal reaction and molding of a high energy ball milled slurry of fly ash, sodium bentonite, calcium hydroxide, and sodium water glass, followed by drying at ambient pressure. The monolith was characterized by XRD, FTIR, TG-DSC, SEM, TEM, BET, mercury porosimeter, and AFM. The addition of both sodium bentonite and sodium water glass plays a crucial role in preventing shrinkage and improving the porosity of the monolith. The monolith has the microstructure of randomly oriented and multually interwoven aluminum tobermorite nanosheets among which there are numerous macropores. Interestingly, the aluminum tobermorite nanosheets can be exfoliated by ultrasonication treatment to provide single-layer ultrathin nanosheets of aluminum tobermorite with a thickness of 1.18 nm and an aspect ratio of ∼1000. The monolith of aluminum tobermorite nanosheets has low apparent density (0.077 g cm−3) and very low thermal conductivity (0.03793 W m−1 K−1). The low thermal conductivity of the monolith is attributed to its high porosity or pore volume due to the presence of numerous macropores among aluminum tobermorite nanosheets. KEYWORDS: Fly ash, Thermal insulation, Monolith, Nanosheet, Aluminum tobermorite

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cost. For this purpose, the prerequiste is to find inexpensive and abundant raw materials to prepare thermal insulation materials. Fly ash is solid waste generated in coal-fired power stations. Hundreds of million of tons of fly ash are produced in the world each year.10 Currently, fly ash is mainly disposed in landfills, which brings serious environmental problems such as air particulate pollution and groundwater pollution by soluble poisonous components in fly ash.10 On the other hand, fly ash contains many useful elements such as silicon, aluminum, iron, calcium, and other metal elements. It is very important to realize the resource utilization of fly ash and solve the fly ashinduced environmental pollution problem. A number of approaches have been developed to utilize fly ash, which include utilization in cement, concrete, foamed concrete blocks,11−13 etc. Recently, there have been reports about the utilization of fly ash to prepare functional materials with higher added value such as sorbents of CO2 and heavy metal ions,14−17 zeolite by hydrothermal treatment of fly ash with NaOH solution,18−21 ceramics,22 polymer composites,23 and thermal insulation material.24−27 For example, Liu et al. utilize attapulgite, fly ash, and poly(acrylic acid) to prepare nanocomposite hydrogels with selective adsorbents for Pb2+ ion.17 Ray et al. utilized recycled polypropylene and fly ash to prepare polymer composites with enhanced impact strength.23 Feng et

INTRODUCTION

Energy and environmental problems are two major challenges for a sustainable society. Building energy consumption accounts for more than 10% of the total energy consumption in the world.1 In China, it accounts for ∼25% of the national total energy consumption. The utilization of high performance thermal insulation materials provides an efficient approach for substantially reducing the building energy consumption, thus considerably decreasing the environmental burden generated by high energy consumption. Currently, organic polymer foams such as polystyrene foam and polyurethane foam are widely used as building thermal insulation materials. However, organic polymer foams with low utilization temperature are inflammable, easily leading to building fire. Inorganic thermal insulation materials, such as foamed concrete, porous calcium silicate, mineral wool, expanded perlite, etc.,2 which are incombustible and have high utilization temperature, are alternatives to the polymer foams. However, they usually have higher thermal conductivity and larger apparent density. By designing nanoporous inorganic thermal insulation materials, their thermal conductivity could be considerably reduced.3−9 A typical example is silica aerogel with a thermal conductivity of 0.017−0.041 W K−1 m−1, which has been regarded as the best insulation material.6−9 But silica aerogel is brittle and very expensive, so it cannot be used in large scale building insulation. For building insulation, the challenge is to develop high perfomance inorganic thermal insulation materials with low © XXXX American Chemical Society

Received: August 3, 2015 Revised: September 16, 2015

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DOI: 10.1021/acssuschemeng.5b00808 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 1. Composition of Fly Ash and Sodium Bentonite composition (wt %)

a

sample

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

SO3

P2O5

calcination weight loss

fly ash sodium bentonite

49.47 53.73

34.15 18.19

4.82 6.64

3.44 3.53

0.91 3.01

0.81 1.74

0.70 2.19

1.23

0.57 0.10

1.93a 9.80

Loss on ignition (LOI). were obtained on an infrared spectrometer (Nicolet 6700). The thermogravimetric analysis and differential scanning calorimetry (TGDSC) were conducted on a thermal analyzer (STA449c/3/G). SEM images were obtained on a scanning electron microscope (ULTRA PLUS-43-13). TEM images were obtained on an electron microscope (JEM-2100F). AFM was performed on an atomic force microscope (Veeco Multimode VIII). BET surface area and meso/micropore volume of the samples were measured on ASAP2020 using N2 adsorption at −196 °C. The macropore volume of the samples was measured on an Autopore IV 9500 mercury porosimeter. The thermal conductivity of the monoliths was measured on a thermal constant measuring apparatus (TPS2500S) by the hot disk method at ambient temperature. The compression strength of the monolith was measured on a MTS ceramic test system (Model 810).

al. prepared thermal insulation material with thermal insulation conductivity of 0.0495 W m−1 K−1 from fly ash by a foaming and slip casting process followed by calcination at high temperature (1000 °C).24,26 Ul Haq et al. prepared thermal insulation foam with thermal insulation conductivity of 0.075 W m−1 K−1 from fly ash by a microwave foaming method.25 The thermal conductivity of the thermal insulation materials prepared from fly ash reported is relatively higher (0.0495− 0.256 W m−1 K−1). It is scientifically and technologically significant to develop a method of substantially reducing the thermal conductivity of the fly ash-based thermal insulation materials. Herein, we report a novel and facile approach of hydrothermal reaction and molding in one step for preparing a thermal insulation monolith using fly ash as the major raw material, followed by drying at ambient pressure. By this method, we successfully prevent the shrinkage of the monolith during the stages of both hydrothermal reaction and drying, which is commonly observed phenomena for drying porous materials at ambient pressure. The obtained monolith, composed of randomly oriented aluminum tobermorite nanosheets, has very low thermal conductivity (0.03793 W m−1 K−1). To our best knowledge, this has been the lowest thermal conductivity for fly ash-based thermal insulation materials reported up to now. Interestingly, the aluminum tobermorite nanosheets can be exfoliated by ultrasonication treatment to give single-layer ultrathin nanosheets of aluminum tobermorite with 1.18 nm thickness.

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RESULTS AND DISCUSSION Preparation and Characterization. The fly ash used was analyzed by using an X-ray fluorescence spectrometer. As shown in Table 1, the composition of the fly ash is very complicated. The major components of the fly ash are SiO2 (49.5 wt %) and Al2O3 (34.1 wt %). According to the ASTM C618 standard (United States standard for coal fly ash use in concrete), the used fly ash belongs to Class F. In the present work, we use the fly ash as a SiO2 source to prepare thermal insulation materials. The thermal insulation monolith (denoted as TIM-A) was prepared by a facile one-step hydrothermal reaction and molding of a high energy ball milled slurry of fly ash, sodium bentonite, calcium hydroxide, and sodium water glass, followed by drying at ambient pressure (see Experimental Section). In the preparation, we added a known amount of inexpensive and abundant sodium bentonite, of which composition is shown in Table 1. Figure 1A gives the XRD pattern of TIM-A. As shown in Figure 1A, TIM-A is mainly composed of aluminum tobermorite (Ca5Si5Al(OH)O16·5H2O, PDF 19-0052) in addition to a small amount of calcium carbonate (PDF 05-586). Tobermorite has a layered structure consisting of calcium layer flanked on both sides by linear silicate dreierketten chains, which are formed by alternating bridging and pairing SiO4 tetrahedra. H2O molecules are present between these layers with interlayer spacing of 1.1 nm.28,29 Al is believed to incorporate tobermorite by substituting Si in the bridging tetrahedral.30,31 The formation of calcium carbonate is attributed to the reaction between Ca(OH)2 and CO2 in air in the process of high energy ball milling and the hydrothermal reaction. The other components existing in the raw materials are not detected by XRD due to their low contents. Figure 1B shows the FTIR spectra of TIM-A. The sharp band at 954 cm−1 and three shoulder bands at 1170, ∼1040, and 914 cm−1 are attributed to the asymmetric and symmetric Si−O stretching vibrations. The band around 1642 cm−1 is assigned to H−O−H bending vibration of adsorbed H2O and H2O in the interlayer of tobermorite. The broad band around ∼3400 cm−1 is attributed to O−H stretching vibration.32,33 The band at 1420 cm−1 is due to CO32− vibration,34 indicating the

EXPERIMENTAL SECTION

Preparation. A total of 0.025 g of polyoxyethylene octylphenyl ether (OP-10), 0.5 g of sodium bentonite, and 1.75 g of fly ash were added into 75 mL of distilled water in an agate pot. The agate pot was placed on a planetary ball mill. After 2.5 h of high energy ball milling, 1.375 g of calcium hydroxide was added into the mixture, and continuously milled for 1.5 h. Then, 1.5 g of 35.4 wt % water glass (Na2O·3.4SiO2) was added into the mixture, and well stirred. The slurry was transferred to a cuboid mold consisting of stainless steel slices wrapped by Teflon film. The mold was put in a stainless steel autoclave with Teflon lining. The autoclave was heated to 190 °C and kept at 190 °C for 12 h in an electric oven. After being cooled to room temperature, the obtained monolith was taken out from the mold and dried at 190 °C for 6 h in the electric oven. The obtained thermal insulation monolith is denoted as TIM-A. The thermal insulation monolith of TIM-B and TIM-C was prepared by the same procedure as that of TIM-A except for no addition of sodium bentonite (TIM-B) and increasing the amount of sodium bentonite from 0.5 to 1.0 g (TIM-C). The thermal insulation monolith of TIM-D was prepared by the same procedure as that of TIM-A except for no addition of OP-10. The thermal insulation monolith of TIM-E and TIM-F was prepared by the same procedure as that of TIM-A except for no addition of sodium water glass (TIM-E) and increasing the amount of sodium water glass from 1.5 to 2.0 g (TIM-F). Characterization. The element composition of the samples was determined on a X-ray fluorescence spectrometer (Axios Advanced). XRD patterns of the samples were obtained on a Rigaku Dmax X-ray diffractometer using Cu Kα radiation. FTIR spectra of the samples B

DOI: 10.1021/acssuschemeng.5b00808 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 2. Camera picture of TIM-A before (A) and after the drying (B) and SEM(C, D) of the TIM-A sample.

observation suggests that the pores in the wet monolith are mechanically strong enough to resist the capillary stress. In order to reveal why the pores in the wet monolith can resist the capillary stress at the drying stage, the morphology of TIM-A was characterized by SEM and TEM. As shown from the low magnification SEM image (Figure 2C) and higher magnification SEM image (Figure 2D), the sample of TIM-A is made of randomly oriented nanosheets with thicknesses of ∼21 nm and sizes of several micrometers, among which there are numerous macropores with different sizes. The nanosheets are mutually supported and interwoven, resulting in the mechanically stable macropore structure of the monolith. In addition, compared to micropores and mesopores, the larger sizes of the macropores in the monolith of TIM-A result in lower capillary stress according to eq 1. The mechanically stable macropore structure and lower capillary stress accounts for the no shrinkage of the monolith during the drying stage. The pore structure of TIM-A is quantitatively characterized by N2 adsorption/desorption at −196 °C and a mercury porosimeter. Figure 3 shows the N2 adsorption−desorption isotherm of TIM-A. There is a desorption hysteresis from P/P0 = 0.7 to 1.0 (Figure 3A) due to the capillary condensation in the micropores and mesopores of TIM-A. Figure 3B presents the pore size distribution of TIM-A. The pore size of TIM-A ranges from 1.7 to 216 nm. The average pore size and total volume of TIM-A are 8.3 nm and 0.44 cm3 g−1, respectively (Table 2). The BET surface area of TIM-A is 91.6 m2 g−1. The macropore size distribution of TIM-A measured by mercury porosimeter is shown in Figure 3C. The sizes of the macropores in TIM-A, formed by randomly oriented nanosheets, range from 339 nm to 5.96 μm. The median pore size and intrusion volume of TIM-A are 8.5 μm and 5.99 cm3 g−1, respectively. The monolith of TIM-A has very low apparent density (0.0767 g cm−3). Its porosity is as high as 93.3% (Table 2). The morphology of TIM-A was further characterized by TEM and AFM. For the TEM and AFM observations, the monolith was crushed, and the resultant powder was added to alcohol, ultrasonicated, and dispersed on a TEM grid for TEM observation or on a mica slice for AFM observation. As shown in Figure 4A, the nanosheets with thickness of ∼21 nm observed by SEM are a close stack of thinner nanosheets. HRTEM reveals that the thinner nanosheet has lattice spacing

Figure 1. XRD pattern (A), FTIR spectra (B), and TG-DSC curve (C) of TIM-A.

presence of carbonate. This observation is in agreement with the presence of CaCO3 in TIM-A concluded by XRD. TIM-A is characterized by TG-DSC (Figure 1C). There is an endothermic peak at 65.6 °C accompanied by a 3.26% weight loss due to the desorption of H2O physically adsorbed on TIMA. There is a broad endothermic peak from 100 to 280 °C accompanied by a 2.67% weight loss, which is attributed to the desorption of H2O in the interlayer of tobermorite. An exothermic peak around 653 °C is observed owing to the decomposition of CaCO3. Its corresponding weight loss is 6.35%. According to the weight loss, the content of CaCO3 in TIM-A is calculated to be 14.4%. Figure 2 shows the camera picture of TIM-A before and after drying. The wet monolith obtained by the hydrothermal reaction, as shown in Figure 2A, has the same size as the slurry of the raw materials in the mold without shrinkage at the hydrothermal reaction stage. Usually, drying wet porous materials at ambient pressure results in considerable shrinkage due to the collapse of pores induced by capillary stress, which depends on the surface tension (γ) and the pore radius (r).33,35 Δp =

−2γ cos θ r

(1)

where θ is the contact angle. The shrinkage leads to a significant increase in thermal conductivity for porous materials. Therefore, it is challenging to develop an ambient pressure drying strategy of preventing the shrinkage at the drying stage for synthesizing thermal insulation materials with high porosity. Interestingly, compared to the wet monolith, no detectable shrinkage of the monolith was observed during the stage of drying at 190 °C and ambient pressure (Figure 2B). This C

DOI: 10.1021/acssuschemeng.5b00808 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 4. TEM (A), HRTEM (B), AFM (C), and height-length contour (D) across the dashed white line of panel C of the TIM-A sample.

XRD and HRTEM reveal that the nanosheets in TIM-A is composed of aluminum tobermorite (Ca5Si5Al(OH)O16· 5H2O). XRD, TG-DSC, and FTIR verify the presence of calcium carbonate. However, as discussed above, the composition of the raw materials such as fly ash, sodium bentonite, calcium hydroxide, and sodium water glass, including Al, Ca, Si, O, Na, K, Mg, Fe, etc., is complicated. The produced TIM-A should include Na, K, Mg, and Fe except for Al, Ca, Si, O, and C in aluminum tobermorite and CaCO3. To answer this question and provide direct evidence of the distribution of the elements in TIM-A, the sample of TIM-A is analyzed by EDX mapping. Figure 5 gives SEM image with EDX element mapping of TIM-A. As shown from Figure 5, Ca, Al, Si, and O are well distributed on the nanosheets. The uniform distribution of C indicates that CaCO3 is well dispersed in TIM-A. These observations are in accordance with the result of XRD, HRTEM, TG-DSC, and FTIR. EDX element mapping of Na, K, Mg, and Fe for TIM-A is shown in Figure 6. Na and K are well distributed while Mg and Fe are segregated. This result suggests that Si and Al sources in the raw materials react with Ca(OH)2 to form aluminum tobermorite nanosheets, while Mg and Fe sources in fly ash and sodium bentonite do not react with Ca(OH)2 during the hydrothermal reaction, thus resulting in their segregation in the form of MgO and Fe2O3. The reason is as follows:

Figure 3. Adsorption−desorption isotherm (A) and BJH desorption pore size distribution (B) by N2 adsorption−desorption and intrusion volume by mercury porosimeter (C) for TIM-A.

of 0.365 nm corresponding to the {020} facet of aluminum tobermorite (Figure 4B). The TEM observation is confirmed by AFM. Nanosheets observed in SEM can be exfoliated by ultrasonication treatment in alcohol to be ultrathin nanosheets (Figure 4C). From the height−length contour across the dashed white line on the two ultrathin nanosheets (Figure 4D), the average thickness of the ultrathin nanosheets is estimated to be 1.18 nm, which is consistent with the interlayer spacing corresponding to the {002} facet of aluminum tobermorite. This interesting result indicates that a single {002} facet layer of aluminum tobermorite can be obtained by exfoliating an aluminum tobermorite nanosheet with ultrasonication treatment. The aspect ratio of the ultrathin aluminum tobermorite nanosheet is approximately 1000.

Ca(OH)2 = Ca 2 + + 2OH−

(2)

SiO2 + 2OH− = SiO32 − + H 2O

(3)

Table 2. Apparent Density, Porosity, BET Surface Area, Average Pore Size, and Pore Volume of Monolith Samples Ni adsorption/desorption

mercury porosimeter

sample

apparent density (g cm−3)

porosity (%)

surface area (m2 g−1)

total pore volume (cm3 g−1)

average pore size (run)

macropore volume (cm3 g−1)

median pore size (jun)

TIM-A TIM-B TIM-E

0.077 0.089 0.315

93 3 86.1 77.7

91.6 50.7 40.7

0.45 0.21 0.19

8.2 11.0 s.o

5.99 2.57 1.20

8.5 26.7 4.2

D

DOI: 10.1021/acssuschemeng.5b00808 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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sodium bentonite cannot react with OH−, thus resulting in their segregation in form of Fe2O3 and MgO. Role of Sodium Bentonite and OP-10. For the preparation of TIM-A by the hydrothermal reaction between fly ash and Ca(OH)2, we added sodium bentonite and sodium water glass. To reveal the role of sodium bentonite, the thermal insulation monolith of TIM-B was prepared by the same procedure as TIM-A except no addition of sodium bentonite. In this case, there is a shrinkage of 12.2 vol % compared to TIM-A in the stage of the hydrothermal reaction for the slurry of fly ash, calcium hydroxide, and sodium water glass (Figure 7A). The shrinkage arises from the precipitation of the solid

Figure 5. SEM image with EDX element mapping of Al (yellow), Ca (cyan), Si (green), O (red), and C (olive) for TIM-A.

Figure 7. Camera picture and SEM image of TIM-B (A, B, C) and TIM-E (D, E, F).

materials in the slurry accompanying with water bleeding in the process of the hydrothermal reaction. However, drying the wet monolith of TIM-B at 190 °C and ambient temperature did not result in a further shrinkage (Figure 7B). The SEM image shows that TIM-B also has a morphology of randomly oriented nanosheets among which there are many macropores (Figure 7C). The presence of many macropores in TIM-B accounts for no shrinkage during the drying stage due to the lower capillary stress. TIM-B has a higher apparent density (0.089 g cm−3) than TIM-A (0.077 g cm−3) as shown in Table 2. N2 adsorption−desorption shows that the BET surface area and the micropore/mesopore volume of TIM-B decrease to 50.7 m2 g−1 and 0.21 cm3 g−1, respectively, as compared to those of TIM-A (90.6 m2 g−1, 0.45 cm3 g−1). The measurement of the mercury porosimeter shows that the macropore volume of TIM-B considerably decreases from 5.99 (TIM-A) to 2.57 cm3 g−1. The considerable reduction of pore volume results in a decrease in the porosity from 93.3% to 86.1% (Table 2). These results reveal that the addition of sodium bentonite plays an important role in preventing shrinkage and improving the porosity of the monolith. The reason why the addition of

Figure 6. SEM image with EDX element mapping of Na, K, Mg, and Fe for TIM-A.

Al 2O3 + 4OH− = 2AlO2− + 2H 2O

(4)

5Ca 2 + + 5SiO32 − + AlO2− + 6H 2O = Ca5Si5Al(OH)O16 ·5H 2O + OH−

(5)

The ionization of Ca(OH)2 in the slurry of fly ash, sodium bentonite, calcium hydroxide, and sodium water glass generates strong basicity (OH−) (2). The Si source (e.g., acidic oxide SiO2) and Al source (e.g., amphoteric oxide Al2O3) in fly ash and sodium bentonite react with OH− to produce SiO32− and AlO2− (3, 4), respectively. Ca2+ reacts with SiO32− and AlO2− to produce aluminum tobermorite under the hydrothermal condition (5). However, the Fe source (e.g., basic oxide Fe2O3) and Mg source (e.g., basic oxide MgO) in fly ash and E

DOI: 10.1021/acssuschemeng.5b00808 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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hydroxide, and sodium bentonite. The shrinkage is also due to the precipitation of the solid materials in the slurry accompanying with water bleeding in the process of the hydrothermal reaction. However, no obvious shrinkage is observed during the drying stage for TIM-E (Figure 7E). SEM image shows that TIM-E also has morphology of randomly oriented nanosheets among which there are many macropores (Figure 7F). The presence of many macropores in TIM-E accounts for the no shrinkage during the drying stage. TIM-E has much higher apparent density (0.31 g cm−3) than TIM-A and TIM-B owing to the shrinkage at the stage of the hydrothermal reaction (Table 2). N2 adsorption−desorption shows that the BET surface area and the micropore/mesopore volume of TIM-E decrease to 40.7 m2 g−1 and 0.19 cm3 g−1, respectively. The measurement of mercury porosimeter indicates that the shrinkage during the hydrothermal reaction stage results in a significant decrease in the macropore volume of TIM-E from 5.99 (TIM-A) to 1.20 cm3 g−1. The significant reduction of pore volume results in a decrease in the porosity of TIM-E to 77.7%. These results reveal that the addition of water glass plays a crucial role in avoiding shrinkage and improving the porosity of the prepared monolith, which are very important for improving the thermal insulation property. The reason why the addition of water glass bentonite can avoid shrinkage during the hydrothermal reaction stage is attributed to its reaction with Ca(OH)2 to produce CaSiO3 (6). This reaction considerably increases the slurry consistency, preventing the precipitation of the solid materials in the slurry. Under the stage of hydrothermal reaction, the ionization of CaSiO3 (Ksp = 1.14 × 10−8) generates Ca2+ and SiO32− (7), which participate in reaction 5 to produce aluminum tobermorite.

sodium bentonite can prevent shrinkage during the hydrothermal reaction stage is attributed to its strong hygroscopicity and expansibility due to its layered structure, 36 thus considerably increasing the slurry consistency and preventing the precipitation of the solid materials in the slurry. To determine the optimum addition amount of sodium bentonite, the thermal insulation monolith of TIM-C was prepared by the same procedure as that of TIM-A except for increasing the amount of sodium bentonite from 0.5 to 1.0 g. In this case, no shrinkage during the stage of both the hydrothermal reaction and drying is observed (Figure S1A, B). But the porosity of TIM-C decreases from 93.3% to 84.8%, which results in the increase in thermal conductivity (Figure 8).

Na 2O·nSiO2 + nCa(OH)2 = nCaSiO3 + 2Na + + 2OH− + (n − 1)H 2O CaSiO3 = Ca 2 + + SiO32 −

(6) (7)

To determine the optimum addition amount of sodium water glass, the thermal insulation monolith of TIM-F was prepared by the same procedure as that of TIM-A except for increasing the amount of sodium water glass from 1.5 to 2.0 g. In this case, a slight shrinkage during the stage of both the hydrothermal reaction and drying is observed, and some cracks at the edge of the monolith are observed (Figure S1E, F). The porosity of TIM-F decreases from 93.3% to 84.8%, which results in the increase of thermal conductivity (Figure 8). In addition, the hydrothermal reaction temperature also considerably affects the property of the monolith. The thermal insulation monolith of TIM-G was prepared by the same procedure as that of TIM-A except for the hydrothermal reaction at the lower temperature (160 °C). The porosity of TIM-G decreases from 93.3% to 80.9%, which results in the increase in thermal conductivity (Figure 8). It should be noted that the composition of fly ash varies significantly from one source to another. If we want to apply the present approach to prepare the thermal insulation monolith using other fly ash instead of the present fly ash, we must first analyze the composition and then adjust the mass ratio of the raw materials according to the composition of the obtained aluminum tobermorite monolith (Ca5Si5Al(OH)O16· 5H2O) as discussed above. Thermal Conductivity. The thermal conductivity of the assynthesized monoliths was measured and compared with

Figure 8. Thermal conductivity of the samples (A); standard deviations of TIM-A, TIM-B, TIM-C, TIM-D, TIM-E, TIM-F, TIMG, polystyrene foam board, and fly ash-based foamed concrete block are 0.0008, 0.0022, 0.0011, 0.0005, 0.0067, 0.0012, 0.0011, 0.0010, 0.0176, respectively. The temperature evolution of the back side of the samples under the illumination of the Xe lamp (B); inset is the illustration setup for measuring the temperature evolution.

The thermal insulation monolith of TIM-D was prepared by the same procedure as that of TIM-A except for no addition of OP-10. In this case, no shrinkage during the stage of both the hydrothermal reaction and drying is observed (Figure S1C, D). But the porosity of TIM-D decreases from 93.3% to 85.1%, which results in the increase in thermal conductivity (Figure 8). This result indicates that the addition of a small amount of OP10 also plays an important role in improving the porosity of the monolith. Role of Sodium Water Glass. To reveal the role of sodium water glass, the thermal insulation monolith of TIM-E was prepared by the same procedure as TIM-A except for no addition of sodium water glass. As shown in Figure 7D, no addition of sodium water glass leads to a significant shrinkage of 73.5 vol % compared to TIM-A in the stage of the hydrothermal reaction for the slurry of fly ash, calcium F

DOI: 10.1021/acssuschemeng.5b00808 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

ambient temperature (27 °C) with increasing the illumination time. After 45 min illumination, it reaches to an equilibrium temperature (46 °C) as shown in Figure 8B. In striking contrast, polystyrene foam board and the present thermal insulation monolith produced from fly ash demonstrate much better thermal insulation performance than foamed concrete foam block. For the polystyrene foam board, its back side temperature reaches to a lower equilibrium temperature (36.5 °C) only after 20 min illumination. The back side temperature of TIM-A reaches to an equilibrium temperature (39 °C) after 25 min illumination. This observation indicates that TIM-A has good thermal insulation performance close to the polystyrene foam board. We measured the compression strength of the monolith of TIM-A. Its compression strength is 0.12 MPa, which is the almost same as that of polystyrene foam panel.

several typical building thermal insulation materials documented. As shown in Figure 8, among the prepared monoliths, the thermal conductivity of TIM-A has the lowest thermal conductivity (0.03793 W m−1 K−1). TIM-B prepared by the same procedure as TIM-A except for no addition of sodium bentonite has higher thermal conductivity (0.04141 W m−1 K−1). TIM-E prepared by the same procedure as TIM-A except for no addition of water glass has the highest thermal conductivity (0.07342 W m−1 K−1). The lowest thermal conductivity of TIM-A is ascribed to its highest porosity (93.3%, Table 1) as compared to that of TIM-B (86.1%) and TIM-E (77.7%) because the diffusion of air confined in pores is significantly retarded, considerably reducing the convective heat transfer of air.1,3,24 The highest porosity of TIM-A is due to the presence of numerous macropores among the randomly oriented and interwoven nanosheets, resulting in its highest macropore volume (5.99 cm3 g−1, Table1). The mesopore/ micropore volume of TIM-A (0.45 cm3 g−1) higher than TIM-B (0.21 cm3 g−1) and TIM-E (0.19 cm3 g−1) also pays contribution to its lowest thermal conductivity. But its contribution is relatively low as the mesopore/micropore volume is much lower than the macropore volume of TIM-A (Table 2). The much lower thermal conductivity of TIM-B than TIM-E is mainly attributed to its higher macropore volume (2.57 cm3 g−1) than that of TIM-E (1.20 cm3 g−1) as TIM-B has mesopore/micropore volume similar to TIM-E. The very low thermal conductivity of TIM-A is close to that of organic polymer foams such as polystyrene foam (0.03−0.04 W m−1 K−1) and polyurethane foam (0.020−0.03 W m−1 K−1), which are widely used as building thermal insulation materials, industry equipment, electrical equipment (e.g., diverse domestic appliance), etc. But the main disadvantages of organic polymer foams are their low utilization temperature and inflammability. Incombustible silica aerogel, which has been believed as the best inorganic thermal insulating materials,6−9 has very low thermal conductivity (e.g., silica aerogel: 0.017− 0.041 W m−1 K−1). However, silica aerogel is very expensive and difficult to be prepared in large scale as risky super drying is required for their preparation. Thus, it is only used in special cases. Recently, we reported a method of bubble-controlled and interfacial hydrolysis of SiF4 for the large scale production of silica hollow sphere foam that has very low thermal conductivity (0.0325 W m−1 K−1).5 However, forming a monolith from the silica hollow sphere foam as power by adding binder leads to a considerable increase in thermal conductivity. The very low thermal conductivity of TIM-A (0.03793 W m−1 K−1) is comparable to that of the reported silica aerogel and hollow sphere silica foam. It is much less than the foamed concrete blocks (0.10−0.30 W m−1 K−1) and the silicate thermal insulation materials (0.0495−0.256 W m−1 K−1) prepared by using fly ash as raw material reported by several research groups.24−27 We measured the temperature evolution of the back side of TIM-A, the fly ash-based foamed concrete block, and polystyrene foam panel with the same size of a thickness of 2.5 cm and a length of 5 cm under the illumination of a Xe lamp as simulation solar light, which has similar spectra to solar light.37 The measurement setup is illustrated in the inset of Figure 8B. The light intensity of the Xe lamp on the surface of the thermal insulation sample is 100 mW cm−2, equivalent to the intensity of mid-day sunlight. For the fly ash-based foamed concrete block, which is major building wall material in China, the temperature of its back side gradually increases from

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CONCLUSION In summary, the thermal insulation monolith of aluminum tobermorite nanosheets is prepared by a facile method of onestep hydrothermal reaction and molding using fly ash as the major raw material, followed by drying at ambient pressure. The monolith has the microstructure of randomly oriented and multually interwoven aluminum tobermorite nanosheets among which there are numerous macropores. The aluminum tobermorite nanosheets can be exfoliated by ultrasonication treatment to provide single-layer ultrathin nanosheets of aluminum tobermorite with a thickness of 1.18 nm and apsect ratio of ∼1000. The aluminum tobermorite has low apparent density (0.077 g cm−3) and very low thermal conductivity (0.03793 W m−1 K−1). The low thermal conductivity of the monolith is attributed to its high porosity or pore volume due to the presence of numerous macropores among aluminum tobermorite nanosheets. The aluminum tobermorite monoith has promising application as a building insulation material alternative to the currently used inflammable polymer foams due to its low thermal conductivity, facile production, and low cost. The present method provides an important and promising approach of realizing the resource utilization of fly ash as large volumes of industrial solid waste for building energy savings and solving the fly ash-induced environmental problem.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00808. Camera picture of the monoliths (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Research and Development Project of Hubei Province (2013BAA045) and National Natural Science Foundation of China (21473127, 21273169).

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REFERENCES

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DOI: 10.1021/acssuschemeng.5b00808 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.5b00808 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX