Advanced fabrication of lightweight aerogels from fly ash for thermal insulation

r (DW).
Fabrication of FA aerogels
Initially, the mixture of PVA and CMC solution with
a solute mass ratio of 2:0.5 is prepared in DW which
is heated to 80 C. Then, FA with determined weight
is dispersed into the PVA/CMC solution by agitating
for 3 hours at 80 C to form a homogenous mixture.
After that, the mixture is agitated for another 30 min-
utes at room temperature before being frozen at -50
C within 4 hours for gelation. Finally, the sample
is freeze-dried under vacuum condition for 48 hours
to obtain lightweight FA aerogel. The FA aerogels are
fabricated with different FA contents of 3.0, 4.0, and
5.0 wt.%.
Characterization
The density of FA aerogels is determined by measur-
ing their weight and volume via diameter and thick-
ness. The porosity (j) of the aerogels is calculated by
(Eq. 1) based on their density (ra) and average den-
sities of components (rb).
j =

1 ra
rb

100 (1)
rb =
CFA+CCMC+CPVA
CFA
rFA
+
CCMC
rCMC
+
CPVA
rPVA
(2)
where CFA, CPVA, and CPVA are the content of FA,
PVA, and CMC, respectively. rFA that is the density
of FA powder after separation is about 2.26 g/cm3.
Morphology of FA aerogels is characterized by field
emission-scanning electron microscopy (FE-SEM)
Hitachi S4800. The specimens are coated with a thin
layer of Pt to enhance the resolution of the images cap-
tured.
The thermal conductivity of the prepared aerogels
is measured by TCi C-Therm Thermal Conductivity
Analyzer (Fredericton, NB, Canada) with the modi-
fied transient plane sourcemethod. The average value
is obtained after three measurements for each sample
at ambient temperature.
The thermal stability of FA aerogels is investigated by
thermogravimetric analysis by LabSys Evo TG/DSC
1600 Setaram in air. The samples are heated from
room temperature to 800 C with the heating rate of
10 C/min. The change in weight of specimens fol-
lowing the temperature is recorded.
The compressive strength of the fabricated aerogels is
determined by Instron 5500 (Norwood, USA). Dur-
ing the test, the specimens are under a loading rate of
1.0 mm/min.
638
Science & Technology Development Journal – Engineering and Technology, 4(1):637-644
Figure 1: Particle distribution andmorphology of FA before (a) and after (b) separation by cyclone system.
The SEM images are captured at the same magnification of 1000x.
Table 1: Chemical composition of FA before and after separation
Sample Content (wt.%) LOI* (%)
SiO2 Al2O3 Fe2O3 Others
RFA 55.07 19.10 7.25 18.58 13.0
SFA 59.58 27.31 5.93 7.18 2.0
*LOI: Loss of ignition
RESULTS ANDDISCUSSION
Morphology of FA aerogels
A combination of FA particles which are separated by
cyclone and green binders including PVA and CMC
successfully generates FA aerogels (Figure 2a) by eco-
friendly and cost-effective freeze-dryingmethod. The
sublimation of ice, which is formed at the gelation
step, leaves small holes inside the aerogels as can be
seen from Figure 2b-d. These holes are intercon-
nected and irregular because the freezing process oc-
curs naturally leading to diversity in ice crystal sizes.
However, as the network of pores is created, the FA
aerogels contain a lot of air making them lightweight
with an extremely low density of 0.072 – 0.093 g/cm3
as shown in Table 2. Moreover, with increasing FA
content, the aerogels become denser because of fewer
air pockets and a decrease in pore size. It is predicted
that the porosity of FA aerogels also decreases.
Because of the porous network inside, the fabricated
aerogels have a high porosity of 94.94 – 95.78%. As
higher FA is used, a decrease in porosity of the aero-
gel is witnessed because of an increase in the num-
ber of particles taking up space inside the aerogel. In
contrast, the density of the FA aerogel increases since
more andmore FA particles are presented in the same
unit volume of material. Compared to previous silica
aerogels from FA via sol-gel route and ambient dry-
ing by Qin et al. (0.0915 – 0.2379 g/cm3) and Wu et
al. (0.099 g/cm3), the FA aerogels in this work exhibit
much lower density 1,8. Furthermore, the procedure
of synthesizing our FA aerogels uses no alkaline sol-
vents for pretreatment of FA to obtain pure silicate
and expensive organic chemicals for surface modifi-
cation of silica gel before ambient drying as published
in previous studies. Hence, the fabrication of FA aero-
gels saves rawmaterials cost, is more environmentally
friendly and the total synthesis time is reduced 20%
compared to the previous works1,8.
Thermal insulation of FA aerogels
The potential heat insulation of FA aerogels is evalu-
ated by their thermal conductivity which is measured
at ambient temperature (25 C). Overall, the FA aero-
gels show low thermal conductivity of 0.040 – 0.047
W/m.K at 24.1 C, indicating their excellent heat in-
sulation (Table 3). The heat transfer through aerogel
is contributed by the conduction of the solid-state net-
work, conduction of gas phase, and radiation within
pores9. Air is one of the excellent thermal insulators
due to its extremely low thermal conductivity of 0.026
W/m.K10. Combining the outstanding thermal insu-
lation properties of air and the high porosity of aero-
gels above 94%, the air captured inside is themain rea-
son why the aerogels have such low thermal conduc-
tivity below 0.05 W/m.K.
With increasing FA content from 3.0 to 5.0 wt.%, the
porosity of FA aerogels decreases causing the effi-
ciency of gas-phase conduction to decrease, and thus,
their thermal conductivity increases from 0.040 to
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Science & Technology Development Journal – Engineering and Technology, 4(1):637-644
Figure 2: Aphotograph of ultra-lightweight FA aerogel (a) and SEM images of FA aerogels with different FA
contents: (b) 3.0 wt.%, (c) 4.0 wt.%, and (d) 5.0 wt.%.
Table 2: Summary of density and porosity of fabricated FA aerogels
Sample FA content (wt.%) Density (g/cm3) Porosity (%)
FAA1 3.0 0.072 0.003 95.78 0.15
FAA2 4.0 0.083 0.002 95.35 0.12
FAA3 5.0 0.093 0.004 94.94 0.24
Table 3: Thermal conductivity and compressivemodulus of FA aerogels
Sample Thermal conductivity (W/m.K) Young’s modulus (kPa)
FAA1 0.040 0.001 67.73 0.14
FAA2 0.044 0.001 157.63 0.08
FAA3 0.047 0.002 254.75 1.00
0.047 W/m.K. Moreover, the thermal conductivity of
our FA aerogels is comparable to some commercial in-
sulationmaterials such asmineral wool (0.030 – 0.040
W/m.K), fiberglass (0.033 – 0.044W/m.K), expanded
polystyrene (0.030 – 0.040 W/m.K) and other silica
aerogels from gold mine (above 0.2 W/m.K), coal
gangue (0.026 W/m.K), and dislodged sludge (0.030
– 0.087W/m.K) 11–14. It can be claimed that the aero-
gels from FA are a promising candidate for practical
heat insulation applications.
Thermal stability of FA aerogels
Regarding the TGA results presented in Figure 3a, a
gradual increase in FA concentration plays an impact
on the thermal stability of FA aerogels. In the atmo-
spheric condition containing air, the FA aerogels ex-
hibit two main phases of mass change by the temper-
ature as follows: (i) 80 – 100 C and (ii) 250 – 475
C.The weight loss of 3 – 5% in all samples at the first
stage is due to the evaporation of water adsorbed into
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Science & Technology Development Journal – Engineering and Technology, 4(1):637-644
the aerogels because of hydrophilic hydroxyl groups
on PVA and CMC matrixes. From 100 to below 250
C, the weight of FA aerogels remains unchanged in-
dicating that the materials can withstand the temper-
ature of up to 250 C.
However, a significant weight loss of about 40% is wit-
nessed in the next period as the temperature grad-
ually increases to 475 C. This thermal decomposi-
tion of FA aerogels is because of the degradation of
binders including PVA and CMC with the decompo-
sition temperature of 250 and 350 C, respectively,
and the decomposition of CaCO3 and residual coal
of original coal in FA15. According to the DTA graph
in Figure 3b, the thermal decomposition of FA aero-
gels is an exothermic process because the DTA curves
have upward peaks, in which the FAA1 gives the high-
est heat released of nearly 40 mV at 430 C.
At higher temperatures from500 to 800 C, theweight
of all samples is kept constant since the specimens
then consist of only FA and ash resulting from the
previous oxidative degradation of PVA and CMC. Al-
though all samples have the same initial decomposi-
tion temperature, FAA3 exhibits the highest remain-
ing weight percentage of about 62% at 500 C because
its original FA content is the highest among the tested
concentrations.
Figure 3: TGA (a) andDTA (b) patterns of FA aero-
gels at various FA concentrations of 3.0, 4.0, and
5.0 wt.%.
Mechanical strength of FA aerogels
The results of the compressive strength of FA aerogels
are summarized in Table 2 and Figure 4. As increas-
ing FA content from 3.0 to 5.0 wt.%, Young’s modulus
of aerogels increases from 67.73 to 254.75 kPa, indi-
cating their better durability under loading of 1.000
N than previous aerogels from wastes such as sugar-
cane bagasse (88 kPa), pineapple leaf fibers (1.64 –
5.34 kPa), recycled polyethylene terephthalate (1.16 –
2.87 kPa), spent coffee grounds (5.41 – 15.62 kPa), sil-
ica – cellulose (86 – 169 kPa)16–19. At the first 10%
of strain known as linear elastic region, the stress in-
creases with increasing FA content, in which that for
aerogel composed of 5.0 wt.% is highest20. However,
the sample containing 4 wt.% FA has the same stress
at 10% strain as the one having 5 wt.% FA. This may
be because the porous structure of FAA2 collapses
then, causing a considerable increase in the compres-
sive stress.
CONCLUSION
For the first time, FA has been successfully converted
into durable and thermal insulation aerogels by us-
ing eco-friendly binders such as polyvinyl alcohol and
carboxymethyl cellulose and freeze-drying technique.
The fabricated FA aerogels are lightweight with low
density and high porosity since the porous structure
is formed after sublimation of distilled water without
any damage. A special feature of FA aerogels in this
work is their outstanding compressibility over pre-
vious aerogels which have been synthesized before.
Therefore, FA aerogel is demonstrated to be a promis-
ing candidate for heat insulation in practice.
ACKNOWLEDGEMENT
The research is funded by Ho Chi Minh City Founda-
tion for Science and Technology Development, under
grant number 120/2019/HĐ-QPTKHCN.We also ac-
knowledge the support of time and facilities from Ho
Chi Minh City University of Technology (HCMUT),
VNU-HCM, and Institute for Tropicalization and En-
vironment for this study.
ABBREVIATION
BET: Brunauer–Emmett–Teller
CMC: Carboxymethyl cellulose
DTA: Differential thermal analysis
DW: Distilled water
FA: FA
LOI: Loss of ignition
PVA: Polyvinyl alcohol
RFA: Raw FA
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Science & Technology Development Journal – Engineering and Technology, 4(1):637-644
Figure 4: Stress and strain curves of FA aerogels with increasing FA concentration under the loading of 1
kN.
SEM: Scanning electron microscopy
SFA: Separated FA
TGA:Thermogravimetric analysis
TMCS: Trimethylchlorosilane
COMPETING INTERESTS
The authors declare that they have no competing in-
terests.
AUTHORS’ CONTRIBUTIONS
Nga Hoang Nguyen Do: Conceptualization, Visu-
alization, Writing – original draft. Huy Gia Tran:
Methodology, Investigation, Formal analysis. Huong
Ly Xuan Doan: Characterization, Data analysis.
Nghiep Quoc Pham: Data curation, Resources. Kien
Anh Le: Validation, Writing – review & editing.
Phung Kim Le: Writing – review & editing, Funding
acquisition, Project administration.
REFERENCES
1. Wu X, Fan M, Mclaughlin JF, Shen X, Tan G. A novel low-cost
method of silica aerogel fabrication using fly ash and trona
orewith ambient pressure drying technique. Powder Technol.
2018;323:310–322. Available from: https://doi.org/10.1016/j.
powtec.2017.10.022.
2. Yao Z, Ji XS, Sarker PK, Tang JH, Ge LQ, Xia MS, Xi YQ. A com-
prehensive review on the applications of coal fly ash. Earth-
Sci Rev. 2015;141:105–121. Available from: https://doi.org/10.
1016/j.earscirev.2014.11.016.
3. Tiwari M, Bajpai S, Dewangan UK, Tamrakar RK. Suitability of
leaching test methods for fly ash and slag: A review. J Radiat
Res Appl Sci. 2015;8:523–537. Available from: https://doi.org/
10.1016/j.jrras.2015.06.003.
4. Zaman A, Huang F, JiangM,WeiW, Zhou Z. Preparation, prop-
erties, and applications of natural cellulosic aerogels: A re-
view. Energ Built Environ. 2020;1:60–76. Available from: https:
//doi.org/10.1016/j.enbenv.2019.09.002.
5. Gurav J, Jung IK, Park HH, Kang ES, Nadargi DY. Silica aerogel:
Synthesis and applications. J Nanomater. 2010;2010:409310.
Available from: https://doi.org/10.1155/2010/409310.
6. Nguyen CV, Lambert P, Tran QH. Effect of vietnamese fly ash
on selected physical properties, durability and probability
of corrosion of steel in concrete. Materials. 2019;12(4):593.
PMID: 30781510. Available from: https://doi.org/10.3390/
ma12040593.
7. Shi F, Liu JX, Song K,Wang ZY. Cost-effective synthesis of silica
aerogels fromfly ash via ambient pressure drying. J Non-Cryst
Solid. 2010;365:2241–2246. Available from: https://doi.org/10.
1016/j.jnoncrysol.2010.08.005.
8. Qin X, Xu J, Feng Y, Tahmasebi A, Yu J. An experimental
study on production of silica aero-gel using fly ash from coal-
fired power plants. Adv Mat Res. 2014;p. 1010–1012. Avail-
able from: https://doi.org/10.4028/www.scientific.net/AMR.
1010-1012.943.
9. Long LY,Weng YX,Wang YZ. Cellulose aerogels: Synthesis, ap-
plications, and prospects. Polymers. 2018;10:623. Available
from: https://doi.org/10.3390/polym10060623.
10. Do NHN, Luu TP, Thai QB, Le DK, Chau NDQ, Nguyen ST, Le
PK, Phan-Thien N, Duong HM. Heat and sound insulation ap-
plications of pineapple aerogels from pineapple waste. Mater
Chem Phys. 2020;242:122267. Available from: https://doi.org/
10.1016/j.matchemphys.2019.122267.
11. Illera D, Mesa J, Gomez H, Maury H. Cellulose aerogels for
thermal insulation in buildings: Trends and challenges. Coat-
ings. 2018;8:345. Available from: https://doi.org/10.3390/
coatings8100345.
642
Science & Technology Development Journal – Engineering and Technology, 4(1):637-644
12. Mermer KN, Yilmaz MS, Ozdemir OD, Piskin MB. The synthesis
of silica-based aerogel from gold mine waste for thermal in-
sulation. J ThermAnal Calorim. 2017;129(3):1807–1812. Avail-
able from: https://doi.org/10.1007/s10973-017-6371-8.
13. Zhu P, Zheng M, Zhao S, Wu J, Xu H. A novel environmen-
tal route to ambient pressure dried thermal insulating sil-
ica aerogel via recycled coal gangue. Adv Mater Sci Eng.
2016;2016:9831515. Available from: https://doi.org/10.1155/
2016/9831515.
14. Zhu L, Wang Y, Cui S, Yang F, Nie Z, Li Q, Wei Q. Prepara-
tion of silica aerogels by ambient pressure drying without
causing equipment corrosion. Molecules. 2018;23(8):1935.
PMID: 30072663. Available from: https://doi.org/10.3390/
molecules23081935.
15. Santos RP, Martins J, Gadelha C, Cavada B, Albertini AV, Ar-
ruda F et al. Coal fly ash ceramics: Preparation, character-
ization, and use in the hydrolysis of sucrose. Sci World J.
2014;2014:154651. Available from: https://doi.org/10.1155/
2014/154651.
16. Thai QB, Nguyen ST, Ho DK, Tran TD, Huynh DM, Do NHN, Luu
TP, Le PK, Le DK, Phan-Thien N et al. Cellulose-based aerogels
from sugarcane bagasse for oil spill-cleaning and heat insula-
tion applications. Carbohydr Polym. 2020;228:115365. PMID:
31635729. Available from: https://doi.org/10.1016/j.carbpol.
2019.115365.
17. Koh HW, Le DK, Ng GN, Zhang X, Phan-Thien N, Kureemun
U, DuongHM. Advanced recycled polyethylene terephthalate
aerogels from plastic waste for acoustic and thermal insula-
tion applications. Gels. 2018;4:43. PMID: 30674819. Available
from: https://doi.org/10.3390/gels4020043.
18. Zhang X, Kwek LP, Le DK, Tan MS, Duong HM. Fabrication and
properties of hybrid coffee-cellulose aerogels from spent cof-
fee grounds. Polymers. 2019;11(12):1942. PMID: 31779069.
Available from: https://doi.org/10.3390/polym11121942.
19. Feng J, Le D, Nguyen ST, Nien VTC, Jewell D, Duong HM.
Silica-cellulose hybrid aerogels for thermal and acoustic in-
sulation applications. Colloids Surf A Physicochem Eng Asp.
2016;506:298–305. Available from: https://doi.org/10.1016/j.
colsurfa.2016.06.052.
20. Ma H, Zheng X, Luo X, Yi Y, Yang F. Simulation and analysis of
mechanical properties of silica aerogels: From rationalization
to prediction. Materials. 2018;11:214. PMID: 29385745. Avail-
able from: https://doi.org/10.3390/ma11020214.
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Tạp chí Phát triển Khoa học và Công nghệ – Kĩ thuật và Công nghệ, 4(1):637-644
Open Access Full Text Article Bài nghiên cứu
1Khoa Kỹ thuật Hóa học, Trường Đại
học Bách khoa TP.HCM, 268 Lý Thường
Kiệt, Quận 10, Thành phố Hồ Chí Minh,
Việt Nam
2Đại học Quốc gia Thành phố Hồ Chí
Minh, Phường Linh Trung, QuậnThủ
Đức, Thành phố Hồ Chí Minh, Việt Nam
3Viện Nhiệt đới Môi trường, 57A Trương
Quốc Dung, Quận Phú Nhuận, Thành
phố Hồ Chí Minh, Việt Nam
Liên hệ
Lê Thị Kim Phụng, Khoa Kỹ thuật Hóa học,
Trường Đại học Bách khoa TP.HCM, 268 Lý
Thường Kiệt, Quận 10, Thành phố Hồ Chí
Minh, Việt Nam
Đại học Quốc gia Thành phố Hồ Chí Minh,
Phường Linh Trung, Quận Thủ Đức, Thành
phố Hồ Chí Minh, Việt Nam
Email: phungle@hcmut.edu.vn
Lịch sử
 Ngày nhận: 29-10-2020
 Ngày chấp nhận: 28-12-2020 
 Ngày đăng: 13-02-2021
DOI : 10.32508/stdjet.v3i4.786 
Tổng hợp aerogel siêu nhẹ từ tro bay định hướng cách nhiệt
Đỗ Nguyễn Hoàng Nga1,2, Trần Gia Huy1,2, Đoàn Lý Xuân Hương1,2, PhạmQuốc Nghiệp3, Lê Anh Kiên3,
Lê Thị Kim Phụng1,2,*
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TÓM TẮT
Lần đầu tiên một quy trình thân thiện môi trường và hiệu quả để tổng hợp aerogel có giá trị kỹ
thuật cao từ tro bay (FA) được xây dựng hoàn chỉnh bằng cách phân tán các hạt FA vào hỗn hợp
chất kết dính polyvinyl alcohol (PVA) và carboxymethyl cellulose (CMC) có khả năng phân hủy sinh
học và phương pháp sấy thăng hoa. Ảnh hưởng của hàm lượng FA đến các tính chất vật lý, hình
thái học, độ bền cơ học và độ dẫn nhiệt của FA aerogel cũng được nghiên cứu. Vật liệu FA aerogel
siêu nhẹ được tổng hợp có khối lượng riêng thấp (0,072 – 0,093 g/cm3) với độ xốp lớn (94,94 –
95,78%). Hình thái học của FA aerogel thể hiện sự phân bố đồng đều của các hạt tro bay trong
mạng lưới liên kết giữa PVA và CMC hình thành nên cấu trúc rỗng xốp với kích thước lỗ rỗng là 2-5
mm. Vì vậy, FA aerogel có tính chất cách nhiệt tốt với độ dẫn nhiệt rất thấp (0,040 – 0,047 W/m.K)
ở nhiệt độ phòng và áp suất thường. Bên cạnh đó, độ dẫn nhiệt của tro bay aerogel tương đương
với các vật liệu cách nhiệt thương mại như bông khoáng, bông thủy tinh, polystyrene và các loại
silica aerogel khác từ chất thải. Thêm vào đó, mô-đun nén của tro bay aerogel từ 67,73 – 254,75
kPa, chứng tỏ đặc tính cơ học nổi trội của vật liệu dưới lực nén 1 kN theo phương thẳng đứng. Kết
quả thí nghiệm cho thấy độ bền cơ học của tro bay aerogel cao hơn so với các aerogel từ chất thải
khác như bã mĩa (88 kPa), sợi lá dứa (1,64 – 5,34 kPa), sợi polyethylene terephthalate tái chế (1,16
– 2,87 kPa), bã cà phê (5,41 – 15,62 kPa), silica – cellulose (86 – 169 kPa). Như vậy, FA aerogel là vật
liệu cách nhiệt siêu nhẹ đầy tiềm năng.
Từ khoá: Tro bay, Aerogel, Cách nhiệt, Hình thái học, Độ bền cơ học
Trích dẫn bài báo này: Nga D N H, Huy T G, Hương D L X, Nghiệp P Q, Kiên L A, Phụng L T K. Tổng hợp 
aerogel siêu nhẹ từ tro bay định hướng cách nhiệt. Sci. Tech. Dev. J. - Eng. Tech.; 4(1):637-644.
644
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