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Sodium silicate insulating foam reinforced with acid-treated fly ash

Article  in  Materials Letters · February 2018

DOI: 10.1016/j.matlet.2018.01.150

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 Materials Letters 218 (2018) 56–59

Contents lists available at ScienceDirect

Materials Letters
journal homepage: www.elsevier.com/locate/mlblue

Sodium silicate insulating foam reinforced with acid-treated fly ash

Siqian Zhang a, Yu-Ri Lee a, Ji-Whan Ahn b, Wha-Seung Ahn a,⇑
a
Department of Chemistry and Chemical Engineering, Inha University, Incheon 402-751, Republic of Korea
b
Korea Institute of Geoscience and Mineral Resource (KIGAM), 124 Gwahang-no, Daejeon 305-350, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history: A set of insulating foams based on sodium silicate were prepared using acid-treated fly ash as an additive.
Received 15 December 2017 It was found that the HCl-treated fly ash was able to remove the undesirable transition/alkali metal impu-
Received in revised form 25 January 2018 rities up to 74%. Furthermore, the effect of the fly ash amount and modulus (SiO2/Na2O mole ratio) on the
Accepted 27 January 2018
thermal conductivity, density, and compressive strength of the product was evaluated. The insulating
Available online 1 February 2018
foam prepared using 12.5 wt% acid-treated fly ash and modulus of 1.8 exhibited a low thermal conduc-
tivity of 0.0428 W/m
K, density of 156.3 kg/m3, and high compressive strength of 1.12 MPa.
Keywords:
Ó 2018 Elsevier B.V. All rights reserved.
Insulation foam
Sodium silicate
Acid-treated fly ash
Thermal conductivity
Compressive strength

1. Introduction et al. [10], most of the impurities in fly ash can be removed by acid
treatment, and HCl is more effective than either H2SO4 or HNO3. In
Fly ash is primarily produced by burning coal in power stations, addition, 3 M HCl was reported to be sufficient for the removal of
and its improper disposal is a serious burden on the environment fly ash impurities [11].
[1]. Apart from the safe disposal of fly ash, recycling it as an indus- In this work, samples of power plant fly ash were acid-treated
trially useful product is an attractive option. Fly ash can be used as and used for the reinforcement of a sodium silicate-based insulat-
a low-cost adsorbent for the removal of pollutants such as NOx [2], ing foam. Physical properties of the product were examined to
SOx [3], and mercury [4]. Additionally, fly ash can also be a poten- evaluate the effect of acid treatment and the modulus (SiO2/Na2O
tial additive for the synthesis of insulating foams [5]. mole ratio).
Currently, the demand for inorganic thermal insulation materi-
als is increasing, and those prepared using cheap sodium silicate as
a source have attracted significant attention because silicate foams 2. Experimental
can be prepared via a simple sintering method with low energy
requirement [6]. Compared to other insulating materials such as 2.1. Chemicals
ceramic [7] and glass foams [8], the insulating foam synthesized
using sodium silicate has a lower compressive strength. Therefore, Sodium silicate (Na2O 10.6 wt%, SiO2 26.5 wt%) was pur-
Li et al. [5] used raw fly ash as an additive in the synthesis of chased from Sigma Aldrich. HCl solution (36 wt%) and sodium
sodium silicate-based insulating foams, and obtained the desired hydroxide were purchased from Duksan in Korea. Fly ash was
product with significantly improved compressive strength. obtained from the Samcheonpo power plant (pulverized coal
Typically, iron and calcium oxides, and potentially hazardous firing-type boiler) in Korea.
elements such as S, Mn, and Pb are present as impurities in fly
ash. As these impurities can adversely affect the health and envi- 2.2. Preparation of insulating foam
ronment, it is important to remove impurities from fly ash for
downstream applications [9]. Since acid treatment can remove First, a mixture of fly ash and HCl solution (3 M) in a 1:2 wt ratio
impurities in fly ash relatively easily, it is worthwhile to re- was stirred at 60 °C for 3 h, followed by filtration and drying at 100
examine the sintering process by using acid-treated fly ash to °C for 12 h. Subsequently, 50 g of sodium silicate was weighed in a
improve the quality of insulating foam. According to Panitchakarn beaker with different amounts of sodium hydroxide and acid-
treated fly ash. Here, sodium hydroxide was used to adjust the
⇑ Corresponding author. modulus (SiO2/Na2O molar ratio) of the sodium silicate. The mod-
E-mail address: whasahn@inha.ac.kr (W.-S. Ahn). ulus of sodium silicate and the amount of acid-treated fly ash were

https://doi.org/10.1016/j.matlet.2018.01.150
0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
 S. Zhang et al. / Materials Letters 218 (2018) 56–59 57

varied from 1.6 to 2.2 and 5 to 15 wt%, respectively. The mixture And the average particle size of the ash was ca. 100 lm. After acid
was stirred for 6 h, and poured into a mold and sintered at 450 treatment, the impurity levels of Fe2O3, CaO, and other oxides
°C for 0.5 h in a muffle furnace. For comparison, the insulating decreased significantly from 15.5 to 4.1 wt%, implying that the
foams without fly ash and with 12.5 wt% of raw fly ash without amount of impurity in the fly ash was removed up to 74%. In addi-
acid treatment were prepared at the same modulus of 1.8. tion, the amount of Al2O3 was also reduced from 18.4 to 12.6 wt%.
The foaming process can be affected by the sintering tempera-
ture and the heating rates. This process has been previously opti-
2.3. Characterization
mized by Li et al. [5]; the ideal sintering temperature and
heating rate were determined to be 450 °C and 5 °C/min, respec-
The chemical compositions of the raw and acid-treated fly ash
tively. Therefore, we adopted the same experimental conditions
were analyzed by X-ray fluorescence (XRF; Phillips, Axios, Japan)
in this work, and took into account the effect of two other critical
spectrometry. The particle size was measured by a Zeta Potential
variables, namely, the amount of acid-treated fly ash and the mod-
Analyzer (ELS-Z, Japan). The density of the prepared foams (10
ulus, on the thermal conductivity, density, and compressive
specimens) was determined by quality divided the corresponding
strength of the prepared foam.
volume (q = m/v). The compressive strength of the prepared foams
Fig. 1 shows the physical properties of the three different foam
was measured with a mechanical stress test machine (DUT-2TC,
samples: without fly ash (sample 1), with raw fly ash (sample 2),
Daekyung engineering, Korea) at a cross-head speed of 1 mm/
and with acid-treated fly ash (sample 3). At the same sintering
min. The dimensions of the specimen were 1  1  1 cm3. Thermal
temperature and heating rate, addition of raw fly ash led to an
conductivity of the prepared foams was measured using the TCi
increase in the density, compressive strength, thermal conductiv-
thermal conductivity tester (C-Therm Technologies, Canada). The
ity, and viscosity of the foam (Table 1). This was evident in the
viscosity of the mixtures in three samples before calcination was
higher density of sample 2 (120.74 kg/cm3) as compared to that
estimated with a viscometer (Cap 2000+, Blookfield, USA). Scan-
of sample 1 (70.37 kg/cm3), and the corresponding compressive
ning electron microscopy (SEM) was performed for morphological
strength also increased from 0.43 MPa (sample 1) to 0.69 MPa
analysis using a Hitachi S-4300 model (Japan).
(sample 2). These changes were attributed to the fact that the addi-
tion of fly ash increased the viscosity of the sodium silicate solu-
3. Results and discussion tion, which reduced the foaming capacity and led to a lower
porosity [12]. A lower porosity of the foam implies fewer voids,
Table S1 summarizes the compositions in oxide form of fly ash which leads to a comparatively higher thermal conductivity of
as analyzed by XRF spectrometry, which revealed that Si and Al the whole material because the thermal conductivity of a solid is
were the primary components of fly ash with diverse impurities. significantly higher than that of air within the voids [13].

Fig. 1. Density, compressive strength, thermal conductivity, and viscosity of the samples of 1, 2, and 3 at modulus = 1.8 and 12.5 wt% fly ash loading (I is the standard
deviation in the measurements).
58 S. Zhang et al. / Materials Letters 218 (2018) 56–59

Table 1
Physical properties of the insulating foams.

Samples Experimental conditions Density (kg/cm3) Compressive strength (MPa) Thermal conductivity (W/m
K) Viscosity (Pa.s)
a
Fly ash Fly ash loading (wt%) Modulus
1 – – 1.8 70.37 0.43 0.0320 1.84
2 Raw 12.5 1.8 120.74 0.69 0.0405 2.57
3 Acid-treated 12.5 1.8 156.33 1.12 0.0428 3.67
4 Acid-treated 5.0 1.8 84.82 0.51 0.0370 –
5 Acid-treated 7.5 1.8 100.75 0.62 0.0400 –
6 Acid-treated 10.0 1.8 131.82 0.89 0.0419 –
7 Acid-treated 15.0 1.8 175.09 1.46 0.0750 –
8 Acid-treated 12.5 1.6 123.54 0.70 0.0410 –
9 Acid-treated 12.5 2.0 166.46 1.24 0.0490 –
10 Acid-treated 12.5 2.2 176.34 1.39 0.0592 –
a
Modulus refers to the SiO2 /Na2O mole ratio.

Consequently, the thermal conductivity of sample 2 was higher water led to abundant volume expansion. The decreasing pore
(0.0405 W/m
K) than that of sample 1 (0.0320 W/m
K). When sizes of samples 1, 2, and 3 were in agreement with the trend of
acid-treated fly ash was added, the density, thermal conductivity, increasing viscosity. For sample 1, largest pores were formed
and compressive strength of sample 3 increased further to because of the high foaming capacity and slow solidification pro-
156.33 kg/cm3, 0.0428 W/m
K, and 1.12 MPa, respectively. This cess, which provided sufficient time and space for the pores to
was explained by the partial removal of Al after the acid treatment, grow. When raw fly ash was used in sample 2, its viscosity
which increased the SiO2/Al2O3 ratio in the precursor sodium sili- increased, which impeded the pore expansion process. Lastly, the
cate solution. The higher SiO2/Al2O3 ratio resulted in a higher pop- addition of acid-treated fly ash led to a further decrease in the
ulation of Si–O–Si bonds, which are stronger than the Al–O–Si foaming capacity of sample 3 and consequently, it had the smallest
bonds, and the viscosity of the sodium silicate solution also pore size among the three samples. Fig. 2(d) shows the photograph
increased [14–16]. Sample 3 achieved a tangible increase in its of sample 3.
compressive strength at the cost of moderate increase in density Table 1 lists the properties of the insulating foams produced
and thermal conductivity; compared to sample 2, the density and using acid-treated fly ash under different experimental conditions.
thermal conductivity of sample 3 had increased by 29% and 6%, The thermal conductivity, density, and compressive strength all
respectively, while its compressive strength was increased by 62%. increased with the increasing amount of acid-treated fly ash (5–
Fig. 2 shows the SEM images of the foam samples. Well- 15 wt%) and modulus (1.6–2.2) of sodium silicate, in agreement
developed pores of different sizes were observed in all three sam- with the previous report [5]. Finally, it was established that the
ples, since condensation of sodium silicate and the evaporation of addition of 12.5 wt% of acid-treated fly ash at the modulus of 1.8

Fig. 2. SEM images of the samples of (a) 1, (b) 2, and (c) 3, and (d) photograph of sample 3.
 S. Zhang et al. / Materials Letters 218 (2018) 56–59 59

was the optimum condition for preparing an insulating foam with Appendix A. Supplementary data
sufficiently low thermal conductivity and high compressive
strength. Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.matlet.2018.01.150.

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