Silica based aerogel synthesis from fly ash and bottom ash: The effect of synthesis parameters on the structure

Abstract

This research aims to demonstrate the synthesis of silica based hydrophobic aerogel from fly ash, bottom ash and the pure silica source, tetraethyl orthosilicate (TEOS). The silica solutions were obtained from the fly ash and bottom ash by alkali fusion reaction, and the synthesis of materials was based on the base-catalyzed sol-gel process. The unreacted materials were removed by washing with hexane and the drying step was carried out with the ambient pressure drying method, which is safer, cheaper and more environmentally friendly. The modification process for the hydrophobic aerogel was completed during the synthesis step, with no further hydrophobization being performed. Characterization of the synthesis samples was achieved by using Fourier transform infrared spectroscopy (FT-IR), contact angle and density measurements. X-ray diffractometer (XRD), differential thermal analysis and thermogravimetry (DTA-TG) and N₂ adsorption/desorption analyses were also performed for optimum samples.

Keywords

Aerogel fly ash bottom ash alkali fusion hydrophobicity

1 Introduction

Aerogels are a nano-porous form of silica and is essentially solid fume with very high relative pore volume. The first step in the production of aerogel usually involves carrying out the sol-gel process at low temperatures. With this process, the material exhibits different properties according to the type of drying and synthesis conditions [1, 2]. Aerogel was first introduced in 1932 by Steven S. Kistler, a former chemistry professor at Pacific College in California, after he had replaced the solvent in the structure by air without altering the structure of the gel [3]. Different methods are used for the classification of aerogels. In terms of appearance, aerogels can be classified as monolith, dust or film, while in terms of preparation method they can be classified as aerogel, xerogel, cryogel, or aerogel related materials [4, 5]. The most appropriate approach, however, in classifying aerogels is to classify the material according to its composition. Aerogels are divided into single component aerogels and aerogel composites. Single component aerogels are mostly produced from inorganic materials, such as silicon oxide, aluminum oxide, zirconium oxide, titanium oxide and tungsten oxide, but there are also organic aerogels produced from organic materials [6 –8]. Aerogel composites are aerogels that have an interpenetrating network of different chemical components. The preparation of aerogel composites involves the optimal synthesis of parameters informing the single component aerogels, such as, pH, solvents, temperatures, and pressures [4].

Silica aerogels have received significant attention by researchers due to their superior properties, such as having a specific surface area of between 600–1000 m²/g, a bulk density of between 80–240 g/dm³, heat conductivity below 0.02 W/mK, a low dielectric constant (<2.0), high porosity (95%), hydrophobicity, and strong optical performance [9 –11].

Since their discovery, aerogels have been classified under many different application areas, including electronics, pharmaceutics, filler, insulation, and chemistry [12 –20]. These and other potential application areas that have been described over the years have led to the recent huge increase in aerogel synthesis. However, because of the high costs involved in the customary supercritical production process and the preferred use of expensive raw materials production on an industrial scale has been difficult. To offset these cost-related problems, it is important that a more environmentally friendly and inexpensive process be applied for the industrial production of aerogels and that, cheaper and easily available raw materials be used [19].

In recent years, the synthesis of silica based hydrophobic aerogels by ambient pressure drying and the use of industrial wastes as raw material have been attracting wide attention. In a relatively new study by Bhagat et al. monolithic silica aerogels were synthesized, via ambient pressure drying, by the acid–base sol–gel polymerization of methyltrimethoxysilane, which has very low density and high specific surface area [21]. In 1995, Prakash et al. synthesized aerogel films from tetraethoxysilane (TEOS) by using a two-step acid/base catalyzed procedure at ambient pressure, and the synthesized aerogels were treated by applying a low temperature dip-coating process [22]. In addition to the studies carried out via ambient pressure using a pure silicon source, other have been performed using industrial wastes in a supercritical environment. Tang et al. in their study were able to obtain sodium silicate from rice hull ash. Since then, this type of sodium silicate has been used as the silica source for silica aerogel production under supercritical conditions [23]. Industrial fly ash, water glass, bio-based starch, and kaolin have also been alternatively used as silica sources. In order to reduce fabrication costs and thereby be able to carry out commercial production of silica based aerogels, it is important that cheap silica sources are used, along with an ambient condition drying process [24 –27].

As known, fly ash and bottom ash are largely derived from the millions of tons of waste produced by coal-burning thermal power plants [28]. These waste ashes mostly consist of SiO₂, Al₂O₃, Fe₂O₃, and MgO, and smaller amounts of CaO, SO₃, K₂O, Na₂O and P₂O₅.

In this study, silica based aerogel samples were synthesized under moderate environment conditions from the fly ash and bottom ash of a coal burning thermal power plant. The drying process was accomplished at ambient pressure instead of under supercritical conditions; this process time was shortened by combining the ultrasonic method and the one step sol-gel method.

2 Materials and methods

Regarding the chemicals used in the study, tetraethyl orthosilicate (C₈H₂O₄Si, 99.9%), hydrochloric acid (HCl, 37%), sulphuric acid (H₂SO₄, 95–97%), ammonia (NH₃, 25%), and hexane (C₆H₁₄, ≥99%) were procured from Merck, while ethanol (C₂H₆O, 25%) and trimethylchlorosilane (C₃H₉ClSi, ≥99%) were supplied from Sigma Aldrich. Sodium hydroxide for the alkali fusion reaction was obtained from Labor Technic. The fly ash and bottom ash samples provided by Seyitomer Thermal Power Plant (Celikler Holding, Kutahya, Turkey) were used as alternative silicon sources. The fly ash samples were grey in color, and the bottom ash samples had a darker hue of grey due to the presence of unburned carbon. Both the fly ash and the bottom ash were dried at 105°C for 12 hours, then ground by using a Retsch RM 100 agate mortar (Retsch GmbH & Co KG, Germany) and sieved with a calibrated sieve, according to ASTM standards (Fritsch Analysette 3 Spartan Pulverisette 0 Vibratory Siev), to be below 90μm. The fly ash and bottom ash were identified by XRD (PANalytical Xpert Pro XRD), which uses a Cu-Kα tube (λ= 0.153 nm). The parameters used for the XRD analysis were 45 kV, 40 mA, 0.02° step size, 0.6 s scan step time, and 2θ scan range of 3–80°. Inorganic Crystal Structure Database (ICSD) was used in order to identify the raw materials. Running three repetitions, the samples were analyzed in the range of Na-U elements using X-ray fluorescence (XRF) (Rigaku, NEX CG, Canada) to determine the contents of materials. Chemical analyses and determination of the concentrations of the solutions obtained from the fly and bottom ashes as a result of the alkali fusion reaction were performed using a Perkin Elmer Optima 2100 DV model inductively coupled plasma optical emission spectrometry (ICP/OES) spectrometer (PerkinElmer Inc., MA, USA). The determination conditions were set to a power of 1.45 kW and a plasma flow of 15.0 L/ min. Si, Al and Na concentrations were measured with ICP-OES in obtained silica solutions from both the fly ash and the bottom ash.

Functional bond properties of all synthesized aerogel samples were studied by conducting four repetitions in a Perkin Elmer Spectrum One FTIR spectrometer. Applying the FT-IR technique, the measurement range was selected as 4000-450 cm^–1. To prepare the samples for analysis, the pellet method was applied. For the pelletization, the sample and the KBr were milled until they became homogeneous at a weight ratio of 1/100, and then they were pressed under 10 tons of pressure for 5 minutes using a hydraulic press.

The properties of the synthesized silica based aerogel samples were subjected to thermal analysis for determination of thermal stability in a Perkin Elmer Pyris Diamond DTA-TG thermogravimetry instrument. The thermogravimetric analyses of the samples were carried out at an oxygen flow rate of 200 ml/min by using a platinum crucible, which was heated from 35°C to 800°C at a rate of 10°C/min.

Porosity of the samples was determined on a Micromeritics ASAP 2020. The samples were degassed at 120°C for six hours. Brunauer-Emmett-Teller (BET) surface areas were calculated using adsorption data. The desorption data were used to determine the size distribution according to the Barrett-Joyner-Halenda (BJH) method.

Contact angle measurements of all synthesized aerogel samples were performed using a RoHs optical microscope and PicPick software. The densities of the obtained silica aerogels were calculated based on their mass to volume ratio [29].

2.1 Preparation of silica solution from fly ash and bottom ash

Alkali fusion reaction was performed using NaOH for the alkali medium for the purpose of obtaining silicon solution. The reaction conditions were adapted from the results of the research conducted by Chandrasekar et al. [30]. Fly ash and bottom ash were mixed with NaOH separately at NaOH/ash weight ratios of 0.8, 1, 1.2, 1.4, 1.6, 1.8 and 2 in a 550°C oven for 1 hour. Acquired solids were then mixed with a given amount of distilled water and agitated at 25°C and 150 rpm for 24 hours in an incubator shaker. The final mixture was separated from the mother liquor by using blue band filter paper under vacuum. ICP-OES analyses were performed to determine the concentrations of the Si, Al, and Na elements of the filtrated liquid. The most efficient ratios were determined according to the silicon concentrations.

2.2 Preparation of silica based hydrophobic aerogels

Silica based hydrophobic aerogels were synthesized using the solutions obtained from both fly ash and bottom ash via the alkali fusion method. First, the pH of the silicon solution was adjusted to the desired value by using sulphuric acid in a polypropylene flask under magnetic stirring. Different mass ratios of silicon solution/total material were determined. With the addition of an ultrasonic homogenization step, the general synthesis process was performed using the procedure shown by Li et al. in their study [31]. TMCS: ethanol: H₂O: HCl were added to the mixture in molar ratios of 0.36:3.1:1.2:7×10^–4. An ultrasonic homogenizer was used for 15 minutes with 1-1 pulsation for homogenous mixing of all reactants before reflux. Mixture was then refluxed in the back-cooled reflux column for a designated temperature and time to facilitate the reaction by increasing the surface area. The solution was mixed with ethanol, water and ammonia at a prepared molar ratio of 1:7: 5×10^–4, respectively. The acquired mixture aged at the designated temperature and time. Wet gels underwent a hexane wash and were filtered to remove unreacted materials. Lastly, the solution was dried according to the constant or gradual temperature drying program.

In addition, for the synthesis of aerogel from pure silica source TEOS, pH of 9, a silicon solution/total mass ratio of 0.25, a reflux temperature of 80°C, a reflux time of 1 hour, aging at 25°C for 24 hours and constant drying conditions were used.

2.3 Effect of reaction parameters on the aerogel synthesis

The pH, silicon solution/total material mass ratio, reflux time, reflux temperature, aging time, aging temperature and drying method were all investigated to examine their effects on the synthesis and the properties of the synthesized samples.

2.3.1 Effect of pH

In the synthesis of the samples, all other parameters were fixed for the investigation of pH effect. The synthesized samples are designated as pH-FA-X and pH-BA-X in the Synthesis and Result sections (X: pH value (3, 5, 7, 9, and 11)). FA symbolizes the fly ash and BA symbolizes the bottom ash. In the steps of the synthesis that followed, the most efficient pH value, as determined according to the results of the pH investigation, was used. Figure 1 depicts the graphs of the density and contact angle values of FA and BA for different pH values.

Fig.1

The effect of pH in a) density, b) contact angle.

As can be seen from the graph results, the lowest density was found in pH 9 for both FA and BA. The density of aerogel synthesized from FA was found to be 0.285 g/cm³, while that of aerogel synthesized from BA was found to be 0.276 g/cm³. Considering that lower density means higher porosity, pH 9 was used in the subsequent steps. According to contact angle measurements, hydrophobic aerogels were successfully synthesized for all the pH values of FA and BA.

2.3.2 Effect of silicon solution/total material mass ratio

The effect of the silicon solution/total material mass ratio to the density of the material was investigated, and the contact angles of the synthesized aerogels were measured. The synthesized samples are designated as ST-FA-X and ST-BA-X in the Synthesis and Result sections, with X taking the values of 0.25, 0.5 and 0.75. Figure 2 presents the graphs of FA and BA for different silicon/total materials mass ratio. In the synthesis, all other parameters were fixed for the investigation of the effect of the silicon solution/total material mass ratio.

Fig.2

The effect of silicon/total materials mass ratio in a) density, b) contact angle.

As can be seen from the graphs, the lowest density was found when X = 0.25 for both FA and BA. Furthermore, it was determined that the density of ST-FA-0.25 was 0.097 g/cm³ and that of ST-BA-0.25 was 0.120 g/cm³, both of which are lower than the densities of pH-FA-9 and pH-BA-9. Although hydrophobic aerogels were synthesized at all ratios, the silicon/total materials mass ratio was determined to be 0.25 for both raw materials. Final results showed that the value of density increased with the increasing of the silicon solution/total material mass ratio.

2.3.3 Effect of reflux temperature

In the investigation of reflux temperature, pH 9 and a silicon/total materials mass ratio of 0.25 were used and all other parameters were fixed. The synthesized samples are designated as RT-FA-X and RT-BA-X in the Synthesis and Result sections (X can be 80, 100 and 120°C). Density and contact angle values for different reflux temperatures are given in Fig. 3.

Fig.3

The effect of reflux temperature in a) density, b) contact angle.

The graphs show that hydrophobic aerogels were produced at all reflux temperatures, with the best results being obtained, in terms of density, at 80°C. It was also found that the density of RT-FA-80 was 0.097 g/cm³ and that of RT-BA-80 was 0.120 g/cm³. The reflux temperature was therefore taken as 80°C in the subsequent steps.

2.3.4 Effect of reflux time

To observe the effect of reflux time on the synthesis, the optimum values found in the previous steps were used and all other parameters were fixed. The synthesized samples are designated as RM-FA-X and RM-BA-X in the Synthesis and Result sections (X can be 30, 60 and 90 min). Density and contact angle values for different reflux times are given in Fig. 4.

Fig.4

The effect of reflux time in a) density, b) contact angle.

As can be seen from the graph results, the lowest density was found in the 60 minute reflux time for both FA and BA.

2.3.5 Effect of aging temperature

Aging is another parameter that has an effect on aerogel synthesis. Aging temperature must be below the degradation temperatures of reactants. The synthesized samples are designated as AT-FA-X and AT-BA-X in the Synthesis and Result sections, with X taking values of 25, 50 and 70°C. Figure 5 shows the graphs of FA and BA for different aging temperatures.

Fig.5

The effect of aging temperature in a) density, b) contact angle.

The graphs show that hydrophobic aerogels were produced at all aging temperatures, with only a few degrees difference. Furthermore, there was a slight change in the density depending on the aging temperature. The best results were obtained at 25°C, in terms of density, for both FA and BA. While the aging temperature for the BA sample was not very effective, for FA, the density increased as the aging temperature increased.

2.3.6 Effect of aging time

In studies on aging time, it is imperative that the aging temperature be kept constant. The synthesized samples are designated as AM-FA-X and AM-BA-X in the Synthesis and Result sections (X can be 0, 6, 12, 24, and 48 hours). Figure 6 shows the graphs of FA and BA for different aging times.

Fig.6

The effect of aging time in a) density, b) contact angle.

The graphs show that hydrophobic aerogel samples were synthesized from FA and BA at all aging times, except at the 0 h for BA (AM-BA-0). The best result for BA was found at the 24 h and for FA at the 0 h aging time. Taking these results into consideration, it was observed that all of the samples which successfully synthesized were hydrophobic.

2.3.7 Effect of drying program

To determine the drying effect of the drying type, constant drying was carried out at 50 ° C for 12 h, while gradual drying was carried out at 60, 80, 120 and 180°C for 2 h at each temperature. Results from this investigation found that the type of drying did not have any significant effect on the density and contact angle of the material.

3 Results and discussion

3.1 Characterization of FA and BA

The XRD patterns of the FA and BA are shown in Fig. 7. The crystalline phases of BA mainly consist of quartz (SiO₂, PDF no: 01-089-8938) and W∖PIstite (Fe_0.922O, PDF no: 01-079-2176). The XRD analysis of FA showed that the crystalline phases were primarily quartz (PDF no: 03-065-0466), maghemite (Fe₂O₃, PDF no: 00-039-1346), and mullite (Al_5.65Si_0.35O_9.175, PDF no: 01-082-1237). These results were also verified by the XRF analysis, as shown in Table 1.

Fig.7

XRD patterns of BA and FA.

Table 1

Chemical composition of ashes

Compound	Content in FA (wt%)	Content in BA (wt%)
SiO₂	47.50	58.30
Al₂O₃	15.40	13
Fe₂O₃	11.60	10.20
MgO	8.08	9.10
CaO	8.33	4.82
SO₃	5.10	2.30
K₂O	2.05	1.15
Na₂O	1.79	1.04
P₂O₅	0.22	0.16

Regarding the XRF analysis results, the chemical composition of the ashes is shown in Table 1, where it can be seen that the ashes were largely comprised of SiO₂, Al₂O₃, Fe₂O₃, MgO, and CaO and small amounts of SO₃, K₂O, Na₂O, and P₂O₅.

3.2 Compositions of the alkali extracted solutions

Table 2 lists the measured Si, Al, and Na ion concentrations in the extracted solutions prepared from different weight ratios of NaOH/ash fusion products. For the sake of ensuring the accuracy of the study, two parallel samples were prepared and analyzed by using of ICP-OES, the averages of which are given in Table 2.

Table 2
Compositions of the alkali extracted solutions

FLY ASH BOTTOM ASH

NaOH/Ash (g/g) Si (ppm) Al (ppm) Na (ppm) Si (ppm) Al(ppm) Na (ppm)

0.8 6213±301.22 121.95±8.41 101485±8223.65 1754±77.78 57335±5.43 145000±10465.2

1 10246.5±839.81 207.6±6.20 110050±8980 4761±239 85.06±4.81 146150±70.71

1.2 14580±876.80 282.05±11.95 152050±4171.90 5718±568.5 94.52±7.61 143000±14283.56

1.4 17260±353.50 269.9±24.32 172200±1697 8481±719.83 155.47±12.63 225900±22627.4

1.6 21910±1216.20 1118.5±43.13 245700±15273.50 10211.5±888.83 162.9±14.57 167000±18243

1.8 20780±989.95 1423.5±95.45 231250±23122.40 2653.5±154.85 244.95±12.94 135100±9333.8

2 20690±961.66 2295.5±51.62 308700±29698.48 2658±155.56 137.8±12.02 93925±5706.35

FLY ASH	BOTTOM ASH
0.8	6213±301.22	121.95±8.41	101485±8223.65	1754±77.78	57335±5.43	145000±10465.2
1	10246.5±839.81	207.6±6.20	110050±8980	4761±239	85.06±4.81	146150±70.71
1.2	14580±876.80	282.05±11.95	152050±4171.90	5718±568.5	94.52±7.61	143000±14283.56
1.4	17260±353.50	269.9±24.32	172200±1697	8481±719.83	155.47±12.63	225900±22627.4
1.6	21910±1216.20	1118.5±43.13	245700±15273.50	10211.5±888.83	162.9±14.57	167000±18243
1.8	20780±989.95	1423.5±95.45	231250±23122.40	2653.5±154.85	244.95±12.94	135100±9333.8
2	20690±961.66	2295.5±51.62	308700±29698.48	2658±155.56	137.8±12.02	93925±5706.35

As can be seen from the obtained results, the highest Si content was found in the solution extracted from the NaOH/ash ratio of 1.6 for FA and BA. Furthermore, the Si content in the solutions increased by increasing the concentration of NaOH. In contrast, the lowest Si contents were found in the solution extracted from the NaOH/ash ratio of 0.8.

3.3 Characterization of aerogel samples synthesized at optimum conditions

To determine contact angles, which are given in Fig. 8, a droplet of water was placed on the smooth sample surface. The contact angle measurement showed that the water droplet maintained an almost spherical shape on the aerogel powder surface. The contact angles of the samples were as follows: TEOS based aerogel was 126.22°, AM-FA-0 was 103.78°, and AT-BA-25 was 99.9°. The photographs clearly show that the aerogel samples have a hydrophobic character. It is well known that the hydrophobicity of silica aerogels affects the properties of the material, such as porous structure and thermal conductivity, in such a way as to improve its utility [28].

Fig.8

Contact angles of aerogels a) TEOS aerogel, b) AT-BA-25, c) AM-FA-0^′?

Both hydrophobic and non-hydrophobic silica aerogel have an amorphous structure, which can be seen from the XRD pattern of the synthesized samples in Fig. 9 [32]. The XRD diffractogram of the synthesized aerogel samples shows a broad peak located at approximately 2θ= 24°, which suggests an amorphous characteristic of SiO₂. The XRD pattern does not feature any peaks that could be defined as crystal for three of the synthesized aerogel samples.

Fig.9

XRD pattern of the synthesized samples.

In Fig. 10, the FTIR spectra of the synthesized silica aerogel samples are given. The peaks at 3436–3468 cm^–1 and at 1633-1634 cm^–1 can be attributed to physically absorbed OH groups during the pelletization of the samples in preparing for FTIR [30]. The absorption peaks near 1082–1084 cm^–1 and 757–793 cm^–1 are due to the asymmetric-symmetric stretching vibrations of Si –O– Si bending, which can appear in any silica samples. The absorption peaks observed at around 2965 cm^–1 are the result of a terminal –CH₃ group, and the bands around 1256 cm^–1 and 845–867 cm^–1 correspond to the Si–C bonds, which are the effect of surface modification by TMCS. The absorption band of the Si–C bond and the Si–CH₃ bond, which shows the successful surface modification of hydrophobic samples, is the main source of the hydrophobic characteristics of the samples [33 –35].

Fig.10

FTIR spectra of the synthesized samples.

The textural properties of the synthesized optimum samples were investigated through N₂ adsorption–desorption analysis. The N₂ adsorption–desorption isotherms obtained at 77 K are depicted in Fig. 11. These isotherms present the adsorbed gas amount as a function of the partial pressure (P/P₀). The maximum amount of N₂ was adsorbed by the TEOS-based aerogel sample. The isotherms reveal that all the samples were of type IV, which is characteristic of mesoporous materials [36]. The desorption cycle of AT-BA-25 and AT-FA-0 showed a hysteresis loop for all the synthesized samples, which can generally be explained by the capillary condensation occurring in the pores at a pressure below the saturation pressure gas [37]. Desorption cycle of TEOS based aerogel also exhibited a type IV isotherm and H1 super position hysteresis loop [38]. In terms of the optimum samples, the surface areas were found to be 943.834 m²/g for TEOS aerogel, 338.296 m²/g for AT-BA-25, and 365.437 m²/g for AM-FA-0.

Fig.11

N₂ adsorption/desorption isotherms of a) AM-FA-0, b) AT-BA-25, c) TEOS aerogel.

Figure 12 shows the pore size distributions of the aerogel powders prepared with different silicon sources. All the samples showed a pronounced peak in the mesoporous region, where the pores are between the size of 2–50 nm and the mesopores are maintained in the structure of the synthesized samples [2].

Fig.12

Pore size distribution profiles of the aerogel samples a) AM-FA-0, b) AT-BA-25, c) TEOS aerogel.

As can be seen in the pore size distribution profiles of samples, in the TEOS based aerogel sample, two distinctive peaks were observed, with the sharp peak attributed to the existence of small mesopores and the broad peak attributed to the existence of large mesopores [37].

The thermal stability of the hydrophobic aerogels was tested at temperatures from 35 to 800 °C in a DTA-TG analyzer, the results of which are given in Fig. 13. It was found that the synthesized aerogel samples retained their hydrophobicity up to a temperature of 250°C, due to the oxidation, precipitated by the reagent TMCS, of the methyl groups responsible for the aerogel hydrophobicity. The FA sample was observed to have continued its mass loss up to 750°C. The weight loss observed up to 200 °C resulted from the removal of the residual solvent and water molecules [21].

Fig.13

TG-DTG of aerogel samples a)TEOS based aerogel b)BA and FA aerogels.

4 Conclusions

In this study, hydrophobic silica based aerogels were synthesized by using a combination of the ultrasonic method and the one step sol-gel method, with the fly ash and bottom ash of a coal burning thermal power plant. To investigate the potential use of fly ash and bottom ash in aerogel synthesis, the performance of the samples produced from the waste was compared with the performance of the samples produced from pure silica. Parameters that have an effect on the synthesis of hydrophobic silica aerogel as well as on the properties of synthesized samples from fly ash and bottom as, such as pH, silicon/total material mass ratio, reflux time, reflux temperature, aging time, aging temperature and drying method, were investigated. The results showed that the best parameters for both silicon sources (i.e. FA and BA) were a pH of 9, a silicon solution/total mass ratio of 0.25, a reflux temperature of 80°C and a reflux time of 1 hour. Aging at 25°C gave the best result for both BA and FA at 24 and 0 hours, respectively. The type of drying was found to not have any significant effect on the properties of synthesized samples. In terms of the optimum samples, the surface areas were found to be 943.834 m²/g for TEOS aerogel, 338.296 m²/g for AT-BA-25, and 365.437 m²/g for AM-FA-0, while the density of aerogels were found to be 0.07 g/cm³ for TEOS aerogel, 0.12 g/cm³ for AT-BA-25, and 0.09 g/cm³ for AM-FA-0.

The elemental contents of fly ash and bottom ash are similar, but they differ in terms of chemical composition and amount of constituents. These differences cause a variety of physical properties, such as density and contact angle of the material.

The results show that fly ash and bottom ash are suitable source materials for the synthesis of aerogels, and thereby, are recommended, as an economically practical solution to the otherwise expensive raw materials typically used, for large scale production of silica based aerogels.

References

Pierre

A.C.

and Pajonk

G.M.

, Chemistry of aerogels and their applications, Chem Rev 102 (2002), 4243–4265.

Sarawade

P.B.

, Kim

J.K.

, Kim

K.H.

and Kim

H.T.

, High specific surface area TEOS-based aerogels with large pore volume prepared at an ambient pressure, Appl Surf Sci m254 (2007), 574–579.

Aegerter

M.A.

, Leventis

and Koebel

M.M.

, Aerogels Handbook, Springer New York Dordrecht Heidelberg London; 2011.

, Zhou

, Zhang

and Shen

, A special material or a new state of matter: A review and reconsideration of the aerogel, Materials 6 (2013), 941–968.

Hrubesh

L.W.

, Aerogels: The world’s lightest solids, Chem Ind 17 (1990), 824.

Zhichao

, Zhicheng

and Manwei

, Synthesis of a new organic aerogel, Chinese Journal of Polymer Science 14 (1996), 127–133.

Peikolainen

A.L.

, Caballero

F.P.

and Koel

, Low-Density Organic Aerogels from Oil Shale by-Product 5-Methylresorcinol, Oil Shale 25 (2008), 348–358.

Tan

, Fung

B.M.

, Newman

J.K.

and Vu

, Organic aerogels with very high impact strength, Adv Mater 13 (2001), 9.

Dorcheh

A.S.

and Abbasi

M.H.

, Silica aerogel; synthesis, properties and characterization, Journal of Materials Processing Technology 199 (2008), 10–26.

10.

Wagh

P.B.

and Ingale

S.V.

, Comparison of some physico-chemical properties of hydrophilic and hydrophobic silica aerogels, Ceramics International 28 (2002), 43–50.

11.

Pajonk

G.M.

, Elaloui

, Chevalier

and Begag

, Optical-transmission properties of silica aerogels prepared from polyethoxidisiloxanes, Journal of Non-Crystalline Solids 210 (1997), 224–231.

12.

Hrubesh

L.W.

and Poco

J.F.

, Thin aerogel films for optical, thermal, acoustic and electronic applications, Journal of Non-Crystalline Solids 188 (1995), 46–53.

13.

Meena

A.K.

, Mishra

G.K.

, Rai

P.K.

, Rajagopal

and Nagar

P.N.

, Removal of heavy metal ions from aqueous solutions using carbon aerogel as an adsorbent, Journal of Hazardous Materials B122 (2005), 161–170.

14.

Smirnova

, Suttiruengwong

and Arlt

, Improvement of the solubility of poor soluble drugs by adsorption on silica aerogels, J Non-Cryst Solids 350 (2004), 54–60.

15.

, Zheng

, Wu

, Liang

, Li

and Fu

, Improving electrochemical performance of polyaniline by introducing carbon aerogel as filler, Physical Chemistry Chemical Physics 12 (2010), 3270–3275.

16.

Beatens

, Jelle

B.P.

and Gustavsen

, aerogel insulation for building applications: A state-of-the-art review, Energy and Buildings 43 (2011), 761–769.

17.

Jensen

K.I.

, Schultz

J. M.

, Kristiansen

F. H.

. Development of windows based on highly insulating aerogel glazings, Journal of Non-Crystalline Solids 350 (2004), 351–357.

18.

Schultz

J.M.

, Jensen

K.I.

and Kristiansen

F.H.

, Super insulating aerogel glazing, Solar Energy Materials and Solar Cells 89 (2005), 275–285.

19.

Schmidt

and Schwertfeger

, Applications for silica aerogel products, Journal of Non-Crystalline Solids 225 (1998), 364–368.

20.

Hrubesh

L.W.

, Aerogel applications, Journal of Non-Crystalline Solids 225 (1998), 335–342.

21.

Bhagat

S.D.

, Oh

C.S.

, Kim

Y.H.

, Ahn

Y.S.

and Yeo

J.-G.

, Methyltrimethoxysilane based monolithic silica aerogels via ambient pressure drying, Microporous Mesoporous Mater 100 (2007), 350–355.

22.

Prakash

S.S.

, Brinker

C.J.

and Hurd

A.J.

, Silica aerogel films at ambient pressure, J Non-Cryst Solids 190 (1995), 264–275.

23.

Tang

and Wang

, Preparation of silica aerogel from rice hull ash by supercritical carbon dioxide drying, J Supercrit Fluids 35 (2005), 91–94.

24.

Cheng

, Xia

, Luo

, Li

, Guo

and Wei

, Effect of surface modification on physical properties of silica aerogels derived from fly ash acid sludge, Colloids and Surfaces A: Physicochem Eng Aspects 490 (2016), 200–206.

25.

Firoozmandan

, Moghaddas

and Yasrebi

, Performance of water glass-based silica aerogel for adsorption of phenol from aqueous solution, J Sol-Gel Sci Technol 78 (2016), 67–75.

26.

Wang

, Sánchez-Soto

, Abt

, Maspoch

M.L.

, Santana

O.O.

. Microwave-crosslinked bio-based starch/clay aerogels, Polym Int 65 (2016), 899–904.

27.

Mengmeng

W.H.

, Chena

L.W.

, Zhanga

, Li

, Wang

, Zhao

. Preparation of hydrophobic silica aerogel with kaolin dried at ambient pressure, Colloids Surf A 501 (2016), 83–91.

28.

Shi

, Liu

J.X.

, Song

and Wang

Z.Y.

, Cost-effective synthesis of silica aerogels from fly ash via ambient pressure drying, J Non-Cryst Solids 356 (2010), 2241–2246.

29.

Sarawade

P.B.

, Kim

J.K.

, Hilonga

and Kim

H.T.

, Production of low density sodium silicate-based hydrophobic silica aerogel beads by a novel fast gelation process and ambient pressure drying process, Solid State Sciences 12 (2010), 911–918.

30.

Chandrasekar

, You

K.S.

, Ahn

J.W.

and Ahn

W.S.

, M Synthesis of hexagonal and cubic mesoporous silica using power plant bottom ash, Microporous Mesoporous Mater 111 (2008), 455–462.

31.

, Cao

, Huo

and He

, One-step synthesis of hydrophobic silica aerogel via in situ surface modification, Mater Lett 87 (2012), 146–149.

32.

Fricke

, Aerogels; Ed., Springer, Berlin; 1986.

33.

Wang

, Ma

, Song

and Preparation

Russ.

, and characterization of silica aerogels from diatomite via ambient pressure drying, J Phy Chem A 88 (2014), 1196–1201.

34.

W.C.

, Lu

A.H.

and Guo

S.C.

, Control of mesoporous structure of aerogels derived from cresol-formaldehyde, J Colloid Interf Sci 254 (2002), 153.

35.

Bhagat

S.D.

, Kim

Y.H.

, Suh

K.H.

, Ahn

Y.S.

, Yeo

J.G.

and Han

J.H.

, Superhydrophobic silica aerogel powders with simultaneous surface modification, solvent exchange and sodium ion removal from hydrogels, Microporous Mesoporous Mater 112 (2008), 1504–509.

36.

Amirkhani

, Moghaddas

and Malmir

H.J.

, Candida rugosa lipase immobilization on magnetic silica aerogel nanodispersion, RSC Journals 6 (2016), 12676–12687.

37.

Sarawade

P.B.

, Shao

G.N.

, Quang

D.V.

, Kim

H.T.

. Effect of various structure directing agents on the physicochemical properties of the silica aerogels prepared at an ambient pressure, Appl Surf Sci 287 (2013), 84–90.

38.

Rao

A.P.

, Rao

A.V.

and Pajonk

G.M.

, Synthesis and characterization of superhydrophobic silica and silica/titania aerogels by sol–gel method at ambient pressure, Appl Surf Sci 253 (2007), 6032–6040.

FLY ASH			BOTTOM ASH
NaOH/Ash (g/g)	Si (ppm)	Al (ppm)	Na (ppm)	Si (ppm)	Al(ppm)	Na (ppm)
0.8	6213±301.22	121.95±8.41	101485±8223.65	1754±77.78	57335±5.43	145000±10465.2
1	10246.5±839.81	207.6±6.20	110050±8980	4761±239	85.06±4.81	146150±70.71
1.2	14580±876.80	282.05±11.95	152050±4171.90	5718±568.5	94.52±7.61	143000±14283.56
1.4	17260±353.50	269.9±24.32	172200±1697	8481±719.83	155.47±12.63	225900±22627.4
1.6	21910±1216.20	1118.5±43.13	245700±15273.50	10211.5±888.83	162.9±14.57	167000±18243
1.8	20780±989.95	1423.5±95.45	231250±23122.40	2653.5±154.85	244.95±12.94	135100±9333.8
2	20690±961.66	2295.5±51.62	308700±29698.48	2658±155.56	137.8±12.02	93925±5706.35