Fly ash based lightweight wall materials incorporating ... · Fly ash based lightweight wall...

Construction and Building Materials 249 (2020) 118728

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Construction and Building Materials

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Fly ash based lightweight wall materials incorporating expanded perlite/SiO2 aerogel composite: Towards low thermal conductivity

https://doi.org/10.1016/j.conbuildmat.2020.1187280950-0618/� 2020 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors.E-mail addresses: [email protected] (Y. Zhang), [email protected] (N.

Zhang).

Feixu Chen a, Yihe Zhang a,⇑, Jingang Liu a, Xinke Wang a, Paul K. Chu b, Bohua Chu a, Na Zhang a,⇑aBeijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology,China University of Geosciences, Beijing 100083, ChinabDepartment of Physics and Department of Materials Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong, China

h i g h l i g h t s

� Fly ash based lightweight wall materials (LWM) were prepared towards low thermal conductivity of 0.050 W/(m�K).� Expanded perlite (EP)/SiO2 aerogel composites were synthesized via impregnation method.� EP/SiO2 aerogel composite can significantly decrease the thermal conductivity of LWM.� Fly ash based LWM composed of EP/SiO2 aerogels can be utilized as thermal insulating materials in building for energy saving.

a r t i c l e i n f o

Article history:Received 1 November 2019Received in revised form 3 March 2020Accepted 11 March 2020

Keywords:Fly ashSiO2 aerogelExpanded perliteLightweight wall materialsLow thermal conductivity

a b s t r a c t

Fly ash based lightweight wall materials (LWM) incorporating expanded perlite (EP)/SiO2 aerogel com-posite were successfully prepared towards low thermal conductivity. As a functional padding, silica aero-gel with density of 119 kg/m3 and thermal conductivity of 0.0277W/(m�K) was synthesized by a two-stepcatalytic method and drying procedures under mild conditions. EP/SiO2 aerogel composite was furthersynthesized via impregnation method to load the SiO2 aerogel on the surface of EP. The bulk densityand compressive strength of the prepared fly ash based LWM incorporating the optimized 20% EP/SiO2aerogel composite reached 335 kg/m3 and 0.88 MPa, respectively, and the thermal conductivity was sig-nificantly reduced to 0.050 W/(m�K). Hence, the fly ash based LWM consisting of EP/SiO2 aerogel compos-ite could be efficiently useful as building insulation thermal materials.

� 2020 Elsevier Ltd. All rights reserved.

1. Introduction

Energy consumption has been a major problem in the currentsociety of human development. As we all know, the constructionindustry consumes a lot of energy and emits a lot of carbon diox-ide. Reducing building energy consumption is a key componentof achieving carbon emission reduction commitments. Energy-saving contribution envelope can reach 40–65% in building energyconservation [1]. Generally, wall insulation materials are thoughtto be important factors influencing the building energy conserva-tion. Selecting efficient energy-saving wall insulation materials isan important way to realize the building energy-saving insulation[2–6].

In recent years, people have made considerable efforts on theresearch of new energy-saving materials. Lightweight wall mate-

rial (LWM) not only possesses considerable strength but also helpsto reduce consumption of raw material and self-weight. In general,the raw materials of lightweight concrete include charcoal, blastfurnace slag powder, coal gangue and fly ash. The lightweight con-crete has been widely studied owing to many outstanding proper-ties such as light weight, heat insulation, low cost and low power[7]. In cementitious composites, lightweight aggregate (LWA) areusually all kinds of fillers, which can decrease bulk density andimprove mechanical and functional properties. The lightweightaggregate includes natural materials and processed or syntheticmaterials. Natural materials mainly include palm oil shells andpumice. Processed or synthetic materials mainly include expandedpolystyrene beads (EPS), expanded shale, expanded perlite (EP),glass microspheres, volcanic ash and waste glass [8,9]. Amongthese materials, expanded perlite (EP) stands out for its uniqueproperties such as good chemical stability, small bulk densityand small thermal conductivity, so it could be widely employedin developing lightweight concrete [10–12].

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2 F. Chen et al. / Construction and Building Materials 249 (2020) 118728

Silica aerogel with interconnected nano-particulate buildingblocks is the most common aerogel [13]. It has many excellentphysical properties including high specific surface area, high poros-ity, small thermal conductivity, small density, low dielectric con-stant and large acoustic impendence [14,15]. These propertiesmake silica aerogels possess many application values such as effi-cient super thermal insulating systems [16,17], drug delivery [18],acoustic insulation, oil spill cleanup [19], catalyst supports [20]and other high-technology fields [21–24].

Building materials with SiO2 aerogel or expanded perlite (EP)have been studied for their energy-saving properties. Su et al.[25] investigated expanded perlite/cement composite by isother-mal calorimetry and thermogravimetric analysis, and they drewthe conclusion that EP delayed the hydration rate of lightweightconcrete. Ramakrishnan et al. [26] studied the thermal propertiesof concrete mixed with paraffin/expanded perlite (EP) composite.They found that paraffin/expanded perlite (EP) composite couldremarkably increase the thermal energy saving and thermal inertiaof concrete. Julio et al. [27] studied the lightweight concrete basedthermal rendering materials mixed with subcritical drying silicaaerogels. The density and thermal conductivity reached 410 kg/m3 and 0.085 W/(m�K) by completely using the designed aerogelinstead of silica sand. Ng et al. [28] investigated aerogel mortar(AIM) containing up to 80 vol% aerogels using the reduced ultra-high performance concrete (UHPC) formulation. They found thatwhen 50 vol% of the aerogel was added into the AIM sample, itsthermal conductivity reduced to 0.55 W/(m�K) with the compres-sive strength of 20 MPa. Hanif et al. [29] made use of fly ash ceno-spheres and aerogels to prepare a nanostructured highly porousmaterial to be used as lightweight aggregate. According to thedosage of fly ash and aerogel, the thermal conductivity of light-weight concrete was as low as 0.3197 W/(m�K) with the flexuralstrength of 4.94–3.66 MPa and compressive strength of 23.54–18.63 MPa.

In order to solve the aerogel’s problem of fragility and EP’s prob-lem of absorbing water easily, aerogel/expanded perlite thermalinsulation composite has been proposed and studied in recentyears. Jia et al. [30] fabricated a thermal insulation composite ofaerogel/expanded perlite (AEP) through filling the aerogel into por-ous structure of expanded perlite (EP), resulting in the thermalconductivity of AEP decreased by 14.7–31.8% compared to that ofEP. Wang et al. [31] developed a recipe to synthesize aerogel/EPinsulation material (EPA) with thermal conductivity of 0.034 W/(m�K) under ambient pressure drying, and it was found that thetime and reagents dosage for synthesizing EPA were less than thatfor aerogel. They further used the EPA as aggregate in concrete, andfound that both the strength properties and thermal conductivityof concrete composites decreased with the increasing additioncontent of EPA; the concrete incorporating 100 vol% graded EPAhad a compressive strength of 3.71 MPa and thermal conductivityof 0.098 W/(m�K) [32]. Peng and Yang [33] also synthesizedaerogel-modified expanded perlite by filling the aerogel into thepores of expanded perlite, and nanoporous structure with higherspecific surface area was found in the resultant aerogel-modifiedexpanded perlite. A hybrid use of aerogels and inexpensive fillerssuch as expanded perlite, pumice, and glass bead is thought tobe a promising way to decrease the thermal conductivity but retainthe strength of lightweight concrete. Zeng et al. [34] prepared ther-mally insulating cement-based composites by using glass bead andnano-silica aerogel as coarse and fine fillers, respectively, in whicha pre-wetting step was conducted on the nano-silica aerogels toattain the aimed fluidity of fresh cement-based materials. Theyfound that the incorporation of nano-silica aerogel promoted thecement hydration and even accelerated the carbonation of hydra-tion products of cement, because the highly porous structure ofnano-silica aerogel made the cement-based composite matrix

more loosely compacted. Considering the high water absorptionbehavior of lightweight aggregate resulted from its porous struc-ture, pre-wetting the lightweight aggregate before concrete mixingis generally adopted to improve the workability of fresh concreteand compressive strength of lightweight concrete. Kabay andAköz [35] investigated effects of pre-wetting methods (pre-soaking, water-soaking and vacuum-soaking) on the properties offresh and hardened pumice lightweight concrete. Their resultsindicated that the lightweight concretes with water-soakedpumice and vacuum-soaked pumice had better workability, highercompressive strength and lower drying shrinkage than that withpre-soaked pumice. Top et al. [36] used pre-wetted expanded per-lite to produce fly ash-based lightweight geopolymer concrete, andthey found that pre-wetting of expanded perlite aggregates helpedto reduce the chemical usage by 32.5%. Moreover, it was reportedby Shen et al. [37] that internal curing with the pre-wetted light-weight aggregate significantly reduced the cracking potential oflightweight concrete at early age. When the lightweight aggregatesare used for producing lightweight structural concrete, an anti-earthquake effect has been noticed due to reduction of weightand vertical forces resulting in a decrease in inertia affecting theconcrete structure, thus it is beneficial to use lightweight concretein earthquake resistant buildings [38].

It is known that fly ash has become a main raw material for theproduction of lightweight concrete, the thermal conductivity ofwhich is generally difficult to achieve 0.050 W/(m�K). In this work,a novel fly ash based lightweight wall material (LWM) incorporat-ing expanded perlite (EP)/SiO2 aerogel composite was preparedtowards low thermal conductivity of 0.050 W/(m�K). In order tosynthesis a lightweight wall material with low thermal conductiv-ity, an orthogonal experiment was conducted to acquire high-performance SiO2 aerogels as functional padding firstly. In theorthogonal experiment, several aspects including the density, ther-mal conductivity, hydrophobicity, pore structure and thermal sta-bility were studied, and the optimal conditions for producing SiO2aerogels with low density and low thermal conductivity wereexplored. The SiO2 aerogel was subsequently used as an excellentinsulation material loaded onto the EP to obtain a low thermal con-ductivity composite under normal temperature and pressure. Fur-thermore, using fly ash as the main raw material, thermalinsulating materials towards low density and low thermal conduc-tivity were prepared through adding different contents of EP/SiO2aerogel composites. The properties of fly ash based lightweightwall materials incorporating EP/SiO2 aerogel composites wereinvestigated systematically. This work is hopefully to provide anovel method for producing lightweight wall materials towardslow thermal conductivity of 0.050 W/(m�K).

2. Experimental details

2.1. Materials

The raw materials for making lightweight wall materialsinclude silica aerogel, expanded perlite, fly ash, foaming agentand alkali activator. The fly ash used in this work was Class F flyash according to ASTM C618-17a [39], which was taken from Zibothermal power plant located in Shandong Province of China. SiO2,Al2O3, CaO and Fe2O3 contents in the fly ash were 52.5, 25.3, 5.2and 6.4 wt%, respectively. The main mineralogical phases of thefly ash were quartz, plagioclase, microcline and hematite with per-centages of 59, 19, 14 and 8 wt%, respectively. Expanded perlite(porous structure, 40 mesh) obtained from Yushun InsulationMaterial Factory located in Shandong Province of China, was uti-lized as a supporting material. The main chemical components ofEP were SiO2 (73 wt%) and Al2O3 (16 wt%) with an amorphous

F. Chen et al. / Construction and Building Materials 249 (2020) 118728 3

phase. The foaming agent used was hydrogen peroxide (H2O2, con-centration of 30%), which was purchased from Xilong ScientificCompany Ltd., China. Water glass with modulus (MS) of 1.6 wasprepared by mixing sodium silicate (MS = 2.4) and sodium hydrox-ide (1 mol/L) in a certain proportion. The silica aerogel materialwas prepared by using the following materials: Tetraethoxysilane(TEOS, 28%) and ammonium hydroxide (NH4OH, 25%) was pur-chased from Xilong Scientific Company Ltd., China. Sulfuric acid(H2SO4, 98%), n-hexane and ethanol (EtOH, 99.8%) were purchasedfrom Beijing Chemical Reagent Factory Ltd., China.Trimethylchlorosilane (TMCS, 98%) was purchased from Sino-pharm Chemical Reagent Company Ltd., China. Distilled water (re-sistivity: 18 MX) was produced by automatic dual water distiller(Smart-Q, HITECH Company Ltd., Shanghai, China).

2.2. Sample preparation

2.2.1. Preparation of silica aerogelThe procedures of preparing silica aerogel are illustrated in

Fig. 1. The precursor was TEOS, and the solvent was anhydrousethanol (EtOH). The TEOS and EtOH were mixed under stirring,and the molar ratio of TEOS:EtOH:H2O is 1:5:5. Sulfuric acid(H2SO4) and ammonia (NH3�H2O) were used as the catalysts inthe two-step catalytic reaction to synthesize the silica sol. H2SO4and distilled water were added and stirred for several hours at acertain temperature. Then ammonia and distilled water wereadded at 25 �C. After being aged in ethanol and distilled waterfor 24 h at room temperature, the alcogel was rinsed with ethanolfor 24 h two times. After that, the alcogel was surface-modifiedwith a mixture of n-hexane and TMCS (TMCS concentration:10 wt%) for 24 h two times, and it was subsequently solventexchanged with n-hexane for 24 h two times. Finally, the alcogelwas treated at a high temperature up to 160 �C to produce theSiO2 aerogel under an ambient pressure.

Fig. 1. Preparation of sil

Table 1 shows the four-factor (H2SO4/TEOS molar ratio, hydrol-ysis time, hydrolysis temperature and NH4OH/TEOS molar ratio)and three-level orthogonal experiments for determining the influ-ences of synthesis parameters on the properties of SiO2 aerogels.

2.2.2. Preparation of EP/SiO2 aerogel compositesBased on the optimal conditions of preparing SiO2 aerogels

(H2SO4/TEOS molar ratio of 0.8 � 10�3:1, hydrolysis time of 24 h,hydrolysis temperature of 25 �C, NH4OH/TEOS molar ratio of6.0 � 10�3:1, please see Section 3.1.4), EP/SiO2 aerogel compositeswere further prepared at EP/SiO2 aerogel volume ratio of 1:1, theprocess of which is presented in Fig. 2. The procedure of preparingEP/SiO2 aerogel composites was similar to silica aerogel, which isdescribed as follows: 1) TEOS and EtOH were mixed by stirringat TEOS:EtOH:H2O molar ratio of 1:5:5. 2) H2SO4 and distilledwater were added and stirred for 24 h at a certain temperatureof 25 �C. 3) Ammonia and distilled water were added at 25 �C, thenthe expanded perlite was added into the liquid sol and stirred untilthe gel became alcogel. 4) The alcogel was aged in ethanol and dis-tilled water for 24 h at room temperature, subsequently it wasrinsed with ethanol once every 24 h for two times. 5) The alcogelwas surface-modified with a mixture of n-hexane and TMCS (TMCSconcentration of 10 wt%) once every 24 h for two times, and it wassubsequently solvent exchanged with n-hexane once every 24 h fortwo times. 6) After a series of solvent exchanging and surface mod-ifying, the rinsed alcogel was dried by cascade raising temperatureat an ambient pressure to finally obtain the EP/SiO2 aerogelcomposites.

2.2.3. Preparation of fly ash based lightweight wall materialscomposed of EP/SiO2 aerogel

The procedure of preparing the fly ash based lightweight wallmaterials (LWM) composed of EP/SiO2 aerogel is schematically dis-played in Fig. 3. There were five groups named L1 to L5, and each

ica aerogel samples.

Fig. 2. Preparation process of EP/SiO2 aerogel composites.

Table 1Orthogonal experiment design for the synthesis of silica aerogels (L9 (34)).

Factors Level 1 Level 2 Level 3

A/H2SO4/TEOS molar ratio 0.4 � 10�3: 1 0.8 � 10�3: 1 1.2 � 10�3: 1B/Hydrolysis time (hours) 4 12 24C/Hydrolysis temperature (�C) 25 45 65D/NH4OH/TEOS molar ratio 4.0 � 10�3: 1 6.0 � 10�3: 1 8.0 � 10�3: 1


group had 120 g fly ash. Appropriate amount of water glass(MS = 1.6) and water were firstly added into the fly ash to prepareslurry at a water/solid ratio of 0.46. Secondly, the EP/SiO2 aerogelcomposites with mass ratios of 0%, 5%, 10%, 20% and 30% wereblended with the fly ash slurry in each group, respectively. Thirdly,through adding the H2O2 foaming agent (5 wt% dosage) with addi-tion of 0.4 wt% foam stabilizer into the slurry and stirring thor-oughly, the obtained uniform slurry was poured into a triplemold with dimensions of 40 mm � 40 mm � 40 mm. The sampleswere demoulded after 24 h. Finally, the samples were cured undera constant temperature of 25 �C and relative humidity of 95% for along period of 28 days.

2.3. Sample characterization

The compressive strength of lightweight wall materials (LMW)samples was measured using a CMT4304 universal tester. The bulkdensity of the materials was directly calculated from the mass tovolume ratio, in which the mass was measured on a microbalance(Mettler Toledo ME104E) with precision of 10�5 g. Three samplesin each group were prepared for the tests of compressive strength,bulk density and thermal conductivity, and they were dried in anoven at 100 �C to constant weight before bulk density and thermalconductivity test. The thermal conductivity was measured accord-ing to ASTM C1113/C1113M-09 [40] by using transient hot wire

method at 25 �C (TC3100, XIATECH, China). Transient hot wiremethod has been widely used to measure the thermal conductivityof materials due to its characteristics of simple, fast and accurate. Itrequires a smaller contact area and less time than the steady-statemethod. The SiO2 aerogel powder was filled in a measuring vesselwith size of 20 mm � 30 mm � 60 mm before testing the thermalconductivity, while the sample size of fly ash based LMW subjectedto thermal conductivity test was 40 mm � 40 mm � 40 mm. Thedried fly ash based LMW sample was cut in half in the middle,and the hot wire harness was immediately pressed between them.Pressure should be applied during testing to keep the tested sam-ples in very close contact.

The microstructure of the aerogels was observed by field-emission scanning electron microscopy (SEM, Hitachi SU8020,Japan). The pore size distribution and specific surface area of aero-gels were determined by the Barrett–Joyner–Halenda (BJH) andBrunaure–Emmitt–Teller (BET) techniques (QuantachromeASIQM0002-4, USA). The chemical structure was performed withFourier transformation infrared spectroscope (FTIR, PerkinElmerSpectrum 100). The hydrophobicity of aerogels was assessed bythe static contact angle, which was measured by the water dropleton the aerogel surface (POWEREACH JC2000D, China). The thermalstability was assessed by thermogravimetric analysis at a heatingrate of 10 �C/min from room temperature to 800 �C in air (TG, Net-zsch TG-209C, Germany).

3. Results and discussion

3.1. Properties and microstructure of the silica aerogels

3.1.1. Density and thermal conductivity of the SiO2 aerogelsThe density results of silica aerogels by the orthogonal experi-

ments are shown in Table 2. It shows that the NH4OH/TEOS molarratio (factor D) is an important factor affecting the density of SiO2aerogels, and the H2SO4/TEOS molar ratio (factor A) and hydrolysistime (factor B) have obvious effects as well. The hydrolysis temper-ature (factor C) shows the slightest influence. The SiO2 aerogel(A2B2C3D1) prepared with H2SO4/TEOS molar ratio of0.8 � 10�3:1, hydrolysis time of 12 h, hydrolysis temperature of65 �C, and NH4OH/TEOS molar ratio of 4.0 � 10�3:1 has the small-est density of 0.1084 g/cm3.

The thermal conductivity results of SiO2 aerogels by the orthog-onal experiments are presented in Table 3. It shows that the H2SO4/TEOS molar ratio (factor A) is a significant factor influencing thethermal conductivity of SiO2 aerogels. In addition, the NH4OH/TEOS molar ratio (factor D) also has obvious effect on the thermalconductivity. However, the hydrolysis time (factor B) and hydroly-sis temperature (factor C) show slight influences on the thermalconductivity of SiO2 aerogels. The SiO2 aerogel (A2B3C1D2) pre-pared with H2SO4/TEOS molar ratio of 0.8 � 10�3:1, hydrolysistime of 24 h, hydrolysis temperature of 25 �C, and NH4OH/TEOSmolar ratio of 6.0 � 10�3:1 has the lowest thermal conductivityof 0.0277 W/(m�K).

In order to explore the comprehensive influences of hydrolysison the properties of SiO2 aerogels, sample 1 (A1B1C1D1) and sam-ple 3 (A1B3C3D3) with prominently different process conditions

Fig. 3. Preparation process of the fly ash based lightweight wall material composed of EP/SiO2 aerogel.

Table 2Density results of the silica aerogels by the orthogonal experiments.

No. A B C D Density (g/cm3)

#1 1 1 1 1 0.1260#2 1 2 2 2 0.1269#3 1 3 3 3 0.2211#4 2 1 2 3 0.2042#5 2 2 3 1 0.1084#6 2 3 1 2 0.1190#7 3 1 3 2 0.1772#8 3 2 1 3 0.1924#9 3 3 2 1 0.2142Average k1 0.158 0.169 0.146 0.150Average k2 0.144 0.143 0.182 0.141Average k3 0.195 0.185 0.169 0.206Range, R 0.051 0.042 0.036 0.065

Table 3Thermal conductivity results of the silica aerogels by the orthogonal experiments.

No. A B C D Thermal conductivity � 10�3 (W�m�1�K�1)#1 1 1 1 1 30.6#2 1 2 2 2 28.0#3 1 3 3 3 29.1#4 2 1 2 3 28.3#5 2 2 3 1 28.4#6 2 3 1 2 27.7#7 3 1 3 2 30.5#8 3 2 1 3 31.3#9 3 3 2 1 33.6Average k1 29.23 29.80 29.87 30.87Average k2 28.13 29.23 29.97 28.73Average k3 31.80 30.13 29.33 29.57Range, R 3.67 0.90 0.64 2.14


are selected for more measurement in the following sections. Inaddition, sample 5 (A2B2C3D1) and sample 6 (A2B3C1D2) withlower density and thermal conductivity are used for comparisonand identification of the optimal hydrolysis conditions.

3.1.2. Hydrophobicity of the SiO2 aerogelsThe hydrophobicity of SiO2 aerogel samples was assessed after

ambient pressure drying, and the water contact angles on the sam-ple surface are shown in Fig. 4.

Fig. 4. Water contact angles on the silica aerogels: (a) Sample 1, (b) Sample 3, (c)Sample 5, and (d) Sample 6.


As seen from Fig. 4, sample 1 shows a contact angle of 117.7�and that on sample 6 is 121.7� disclosing hydrophobic characteris-tics. Sample 3 shows a contact angle of 110.8� and that on sample 5is 112.0�. The contact angles on the samples 1 and 6 are larger thanthose on samples 3 and 5. A bigger contact angle suggests betterhydrophobicity which depends on the surface morphology andmethyl groups on the aerogel surface.

Considering that TMCS could react with Si-OH groups on the sil-ica hydrogel to form hydrophobic Si-O-Si(CH3)3 terminal groups onsurface of the SiO2 aerogels during surface modification, FTIR wasperformed between 4500 cm�1 and 400 cm�1 to identify the func-tional groups and surface chemical change after the orthogonalexperiments. Fig. 5 displays the FTIR spectra of silica aerogel sam-ples of 1, 3, 5 and 6. As shown in Fig. 5, the bands at 1091 cm�1 and463 cm�1 correspond to symmetrical and asymmetrical stretchingvibrations of Si–O–Si [41]. The bands around 2971 cm�1 and1397 cm�1 related to C–H from Si-CH3 confirm silylation [42,43].The absorption peaks observed at 854 cm�1 and 616 cm�1 are asso-ciated with Si–C from Si–(CH3)3 modified by TMCS containing

Fig. 5. FTIR spectra of the SiO2 aerogels.

three non-hydrolysable –CH3 groups [44–46]. The special Si–Cbonds can be identified from the FTIR spectra. The intense Si–Cand C–H absorption peaks indicate that the gel skeleton has suffi-cient non-hydrolysable organic substitution groups after surfacemodification, yielding the hydrophobic characteristics.

It should be noticed that the bands observed at 1620 cm�1 and3412 cm�1 from samples 3 and 5 are due to bending of H–OH andasymmetrical stretching of –OH, respectively, which suggests thata small amount of surface –OH groups are left and the hydrophobictreatment for the aerogels is not sufficient [47], whereas the –OHbands observed from samples 1 and 6 are not obvious. It provesthat the unreacted –OH groups of samples 3 and 5 are more thanthose of the samples 1 and 6. This agrees well with the result ofwater contact angles on the silica aerogels, supporting that the sur-face chemical groups play a leading role for the hydrophobicity ofsilica aerogels. With more Si–(CH3)3 groups replacing –OH groups,the remaining „Si–OH groups lead to the shift of water contactangle to larger values.

3.1.3. Pore structure of the SiO2 aerogelsFig. 6 depicts SEM micrographs of the samples 1, 3, 5 and 6. The

SiO2 aerogels have a 3D nanoporous structure with a uniform sizedistribution. Fig. 6 (a) and (b) show that the aerogel is dense andless porous. The aerogel particles cluster together, suggesting sub-stantial shrinking during drying and loss of micropores. The SEMmicrographs of samples 1 and 3 reveal small pieces and powdersas a result of volume shrinkage during ambient pressure drying.Besides, it can be observed that the size of primary particles andclusters increases from sample 6 to sample 5 thus producing differ-ent pore size. Fig. 6 (c) and (d) show that sample 6 has more micro-pores than sample 5. It can be related to the larger dosage ofammonium hydroxide leading to faster gelation of the sol. Becauseof the larger condensation rate of the primary particles, the subse-quent number of largest clusters is reduced resulting in small par-ticle size and small pore size.

Furthermore, the N2 adsorption and desorption isotherms andpore size distribution of the SiO2 aerogels are determined to ascer-tain the effects of various factors. Table 4 presents the results ofpore volume, most probable pore diameter and specific surfacearea of the SiO2 aerogels. The pore volume of sample 1 is4.733 cm3/g, and the most probable pore size and specific surfacearea are 24.7 nm and 809.7 m2/g, respectively, as shown in Table 4.Compared with the samples 3, 5 and 6, the degree of hydrolysisand condensation is quite low for the sample 1 owing to the smal-ler molar ratios of H2SO4/TEOS and NH4OH/TEOS. Moreover,hydrolysis is incomplete for the sample 1 because of the short timeand low temperature, resulting in longer chains composed of smal-ler silica particles, which produces a weaker network with largepore volume and relatively large pore size.

It can be seen from Tables 2 and 4 that sample 3 has the largestdensity of 0.2211 g/cm3 and the lowest specific surface area of715.3 m2/g. Because the molar ratio of NH4OH/TEOS, hydrolysistime and hydrolysis temperature of sample 3 are all high in thedesigned experiment, the hydrolysis and condensation reactionsare thought to be relatively violent. Therefore, the internal struc-ture of obtained silica sol is not uniform with big primary particlesand large silica skeleton. Furthermore, a lot of –OH is unreactedthereby generating more volume shrinkage and inner structurecollapse during drying due to capillary stress. Hence, the obtainedsilica aerogel of sample 3 is less porous, and its pore volume andmost probable pore size are 4.225 cm3/g and 34.2 nm, respectively.

Fig. 7 depicts the nitrogen adsorption–desorption isotherms ofsample 5 and sample 6. The physisorption isotherms of differentsamples exhibit similar hysteresis loops indicating the porous sil-ica network. The adsorption–desorption isotherms of type IV indi-

Fig. 6. SEM micrographs of the SiO2 aerogels: (a) Sample 1, (b) Sample 3, (c) Sample 5, and (d) Sample 6.

Table 4Pore volume, most probable pore diameter and surface area of the SiO2 aerogels.

Pore volume(cm3/g)

Most probable porediameter (nm)

Surfacearea (m2/g)

Sample 1 4.733 24.7 809.7Sample 3 4.225 34.2 715.3Sample 5 3.759 24.8 836.2Sample 6 3.593 3.4 920.3


cates the presence of mesoporous. The hysteresis loops are of typeH3 for slit-like interparticle pores [48–52].

Fig. 8 presents the pore size distributions of sample 5 and sam-ple 6 calculated by the BJH desorption isotherm. It can be seen that

Fig. 7. Nitrogen adsorption/desorption isotherms of the SiO2 aerogels: (a) Sample 5, amethod.)

the pore size distribution of the SiO2 aerogels is in the mesoporousrange (pore size of 8.6–50 nm). Fig. 8 (a) shows that the sample 5has a peak at 24.8 nm with narrow pore size distribution. Thespecific surface area and pore volume of sample 5 are 836.2 m2/gand 3.759 cm3/g respectively as shown in Table 4. The molar ratioof H2SO4/TEOS and hydrolysis time of sample 5 are moderateresulting in modest particle size to build a strong network. Thehigh hydrolysis temperature accelerates hydrolysis and createsfavorable conditions for subsequent condensation. Besides, thesmaller molar ratio of NH4OH/TEOS retards condensation, andthe aerogel particles are less clustered forming loose porous struc-ture with a relatively uniform pore size. It minimizes shrinkageduring drying and so the porous structure can be retained.

nd (b) Sample 6. (The N2 adsorption/desorption results are calculated by the BJH

Fig. 8. Pore size distributions of the SiO2 aerogels calculated by the BJH desorption isotherm: (a) Sample 5 and (b) Sample 6.


On the other hand, Fig. 8 (b) shows that the most probable porediameter of sample 6 is 3.4 nm with a narrow pore size distribu-tion. The specific surface area of sample 6 is 920.3 m2/g as shownin Table 4. The molar ratios of H2SO4/TEOS and NH4OH/TEOS ofsample 6 are moderate, and the degree of hydrolysis and conden-sation is controlled. The long hydrolysis time allows sufficienthydrolysis of TEOS at 25 �C and the reaction rate is also moderate.As a result, the Si-OH of silica sol could react thoroughly, and thegel has a modest particle size and strong network. Because thereis few –OH in the gel, the mitigated shrinkage results in the nano-porous structure with uniform pore size distribution, large specificsurface area and small pore size of SiO2 aerogels.

3.1.4. Thermal stability of the SiO2 aerogelsFig. 9 presents the TGA curves of the silica aerogel samples of 1,

3, 5 and 6. As shown in Fig. 9, the mass of the silica aerogels decli-nes and the total weight loss is as high as 19% as the temperatureraised from 30 �C to 800 �C, but there is no obvious differenceamong these samples. Evaporation of the organic solvent andwater leads to the small weight loss up to 300 �C. The massdegrades abruptly around 300 �C. Between 300 �C and 700 �C,the main weight loss occurs, which can be related to oxidation of–CH3 groups and decomposition of chlorine. When the tempera-ture is sufficient, the �Si–CH3 groups are oxidized to give off heat[53,54]. About 1% weight loss is observed from 700 �C to 800 �C.

As seen from Fig. 9, few weight loss can be observed up to300 �C from the TGA curves of samples 1 and 3 because thesetwo samples have bigger pores. The pores constitute the mainchannels for the discharged liquid, and the pore size distributionis more beneficial to draining of the pore liquid during drying.Therefore, samples 1 and 3 exhibit favorable thermal stabilitybelow 300 �C. The TGA curve shows 13% weight loss ending upat 800 �C for sample 6, which has the least mass loss among thesesilica aerogel samples. Combining the pore structure analysis insection 3.1.3, it is known that the space in the porous structureof sample 6 is so small that organic matters in the aerogels are pro-tected by the SiO2 skeleton thus making oxidation difficult at a lowtemperature.

Based on a series of discussions above, it can be found that it iseasy to obtain a mesoporous structure with weaker network com-posed of smaller silica particles and large pore size by mild hydrol-ysis and condensation reactions (such as sample 1). When thehydrolysis and condensation reactions are active, the acquired sil-ica aerogels are more inclined to be dense, less porous and not uni-form with big primary particles (such as sample 3). Moreover,

combination of fierce hydrolysis reaction and mild condensationreaction can promote gaining small-density aerogels with modestparticle size and a strong network (such as sample 5). Most impor-tantly, the silica aerogels with small density, low thermal conduc-tivity, favorable hydrophobicity and thermal stability can beprepared successfully through moderate hydrolysis and condensa-tion reactions (such as sample 6). In addition, a higher hydrolysistemperature of 65 �C reduces the hydrolysis time of the SiO2 aero-gels. From viewpoint of energy consumption, the favorable hydrol-ysis temperature is 25 �C. Accordingly, the optimal conditions toproduce low-density and low-thermal-conductivity SiO2 aerogelsare: H2SO4/TEOS molar ratio of 0.8 � 10�3:1, hydrolysis time of24 h, hydrolysis temperature of 25 �C, and NH4OH/TEOS molarratio of 6.0 � 10�3:1.

3.2. Properties of fly ash based lightweight wall materials (LWM)composed of EP/SiO2 aerogel

3.2.1. Compressive strength of the fly ash based LWM composed of EP/SiO2 aerogel

Fig. 10 shows the compressive strength results of groups L1–L5cured for 7 days and 28 days, which illustrates the influence ofadded amounts of EP/SiO2 aerogel composites on the compressivestrength of lightweight wall materials. The 28-days compressivestrength from L1 to L5 is 1.16, 1.09, 0.93, 0.88, 0.76 MPa, respec-tively. The results show that the compressive strength of the light-weight wall materials curing for 7 days are lower than those curingfor 28 days. When the curing time increases, the chemical reactionbetween alkali activator and fly ash becomemore sufficient, result-ing in a more stable structure (-O-Si-O-Si-O-Al-O-) in the light-weight wall materials. With increasing addition of EP/SiO2aerogel composites, the compressive strength of the samples curedfor 28 days trends to reduce gradually. Owing to the presence of EPstrengthening the wall and large content of EP/SiO2 aerogel com-posites as aggregates, the compressive strength decrement withthe increasing content of EP/SiO2 aerogel composites is small. Withthe increasing content of insulation aggregate of EP/SiO2 aerogelcomposites, the amount of gelling material used to fill and bondis reduced, and the compressive strength of the wall material isweakened as well. If the content of aggregate is continued toincrease, a ‘‘loose” and ‘‘collapse” phenomenon would appear, ulti-mately leading to loss of strength [55,56]. As a result, 5%-20% con-tents of EP/SiO2 aerogel composites show ideal compressivestrength at curing time of 28 days.

L1 L2 L3 L4 L50

200

400

Bul

k D

ensi

ty(k

g/m

3 )

Sample

curing time(7d)curing time(28d)

Fig. 11. Bulk density results of the LWM samples from L1 to L5.

Fig. 9. TGA curves of the silica aerogels: (a) Sample 1, (b) Sample 3, (c) Sample 5, and (d) Sample 6.

L1 L2 L3 L4 L50.0

0.2

0.4

0.6

0.8

1.0

1.2

Com

pres

sive

Str

engt

h(M

Pa)

Sample

curing time(7d)curing time(28d)

Fig. 10. Compressive strength results of the samples from L1 to L5.


3.2.2. Bulk density of the fly ash based LWM composed of EP/SiO2aerogel

The bulk density results of groups L1–L5 are displayed inFig. 11. It shows that with the increasing content of EP/SiO2 aerogel

composites, the bulk densities for both 7 and 28 days reduce uni-formly with small decrements. Because the EP/SiO2 aerogel com-posites are obviously lighter than fly ash, and they also do notreact with the alkali activator, when more and more fly ash is


replaced by the EP/SiO2 aerogel composites, the extra lightweightwall materials with lower bulk density can be obtained. The bulkdensity results at curing time of 28 days look slightly lower thanthose at curing time of 7 days. It is thought that the chemical reac-tions did not complete and the water inside was not fully evapo-rated within the curing period of 7 days. The reduced bulkdensities at 28 days could be due to the water loss inside the flyash based LWM during cuing from 7 to 28 days. Accordingly, con-sidering both of the lightweight and mechanical properties, whenthe content of EP/SiO2 aerogel composites reaches 20%, the bulkdensity and compressive strength become balanced relatively.

3.2.3. Thermal conductivity of the fly ash based LWM composed of EP/SiO2 aerogel

The most important property of insulation materials is thermalconductivity. Fig. 12 presents thermal conductivities of the fly ashbased lightweight wall materials of groups L1–L5 cured for 28 days.As shown in Fig. 12, the thermal conductivities of the LWM sam-ples gradually decrease when the content of EP/SiO2 aerogel com-posite increases. At this stage, the content of cementing material issufficient to be able to package and fill between the insulationaggregates. It is precisely because the confined tiny pores of insu-lation aggregate effectively adsorb the gas molecules inside thematerial and reduce their convective action. Moreover, the effectof radiation heat transfer can be suppressed as the pores of wallmaterials increase. Therefore, when the content of the insulationaggregate of EP/SiO2 aerogel composite increases, the thermal con-ductivity decreases correspondingly. Furthermore, as previouslymentioned, EP is viewed originally as a good insulation material,the thermal conductivity of which is around 0.048 W/(m�K). Afterit was coupled with the SiO2 aerogel, the ultra-low thermal con-ductivity of SiO2 aerogels was re-obtained with an amplificationeffect, and thus the thermal conductivity of EP/SiO2 aerogel com-posite can be lower than EP by adding SiO2 aerogel. When theEP/SiO2 aerogel composites were doped into the fly ash as a light-weight concrete aggregate, the insulation effect of the fly ash basedlightweight wall material was further enhanced. The thermal con-ductivity of the group L1, L2, L3, L4 and L5 is 0.064, 0.058, 0.055,0.050 and 0.046 W/(m�K) respectively as shown in Fig. 12. Thethermal conductivity of the fly ash based lightweight wall materialcontaining 30% EP/SiO2 aerogel composite is reduced 28% com-pared with that of the blank sample. These results indicate that

Fig. 12. Thermal conductivity results of the LWM samples (L1–L5) cured for28 days.

the EP/SiO2 aerogel composites yield lower thermal conductivity,and hence they can be used as a preferred insulation aggregatein the lightweight wall material.

In order to investigate whether chemical reaction occurs in theEP/SiO2 aerogel composite, FTIR analysis was conducted on the EP,SiO2 aerogel and EP/SiO2 aerogel composite, and the correspondingFTIR spectra are presented in Fig. 13. As shown in Fig. 13, asym-metric and symmetric Si–O–Si vibrations are observed around1070 cm�1 and 475 cm�1 in the spectrum of SiO2 aerogel. Thestretching and deformation modes of Si–C bond can be observedat 840 cm�1. Two strong characteristic peaks of C–H bond at1401 cm�1 and 2960 cm�1 are also observed. The absorption bandsof C–H bond and Si–C bond indicate the surface modification ofSiO2 aerogels. The main characteristic peaks of EP are observedaround 475 cm�1 and 1078 cm�1 in the spectrum of EP. It can beseen that almost all absorption peaks of EP and SiO2 aerogel appearin the spectrum of EP/SiO2 aerogel composite without any newabsorption peaks, indicative of no chemical reaction between EPand SiO2 aerogel. The behavior of individual chemical stability forEP and SiO2 aerogel has also been noticed in the other research[30]. Fig. 13 also presents the SEM image of EP/SiO2 aerogel com-posite. It can be observed that the micro-scale open pores on thesurface of EP was covered irregularly by the SiO2 aerogels. Becausethe capillary formed in the micro-scale open pores of EP couldabsorb the SiO2 hydrosol during the preparation of EP/SiO2 aerogelcomposite, the SiO2 aerogels were successfully synthesized in theopen pores of EP. It is thought that EP can provide a safe skeletonfor the SiO2 aerogels, and simply retain SiO2 aerogels in poresthrough capillary and surface tension forces to prevent SiO2 aero-gel leaking from the composite materials [31,57].

3.2.4. Thermal stability of the fly ash based LWM composed of EP/SiO2aerogel

As the thermal conductivity of fly ash based lightweight wallmaterials containing 10%, 20% and 30% EP/SiO2 aerogel compositeswas around 0.050 W/(m�K), the thermal stability of samples L3, L4and L5 was subsequently tested. Fig. 14 presents the TGA curves ofthe samples L3, L4 and L5. As can be seen from Fig. 14, these threesamples containing different content of EP/SiO2 aerogel compositesonly change roughly with the mass loss of 15%. However, it can beseen that different content of EP/SiO2 aerogel composites incorpo-ration in the fly ash based LMW reflects different thermal stabilityaround 200 �C. Because the surface of EP/SiO2 aerogel contains

Fig. 13. FTIR spectra of EP, SiO2 aerogel, EP/SiO2 aerogel composite, and SEM imageof the EP/SiO2 aerogel composite.

200 400 600 80084

88

92

96

100

30%20%

Wei

ght(%

)

Temperature(°C)

10% 20% 30%

10%

Fig. 14. TGA curves of LWM samples L3, L4 and L5 (containing 10%, 20% and 30% EP/SiO2 aerogel composites, respectively).


organic hydrophobic factors which is inflammable, the thermalstability of samples L3, L4 and L5 is negatively correlated withthe added amount of EP/SiO2 aerogel composite.

3.2.5. Hydrophobicity of the fly ash based LWM composed of EP/SiO2aerogel

Fig. 15 presents water contact angles on the sample surface ofEP, EP/SiO2 aerogel composite, the fly ash based LWM samples L1and L4. As shown in Fig. 15 (b), the contact angle of EP/SiO2 aerogelcomposite was 85.15�, which was significantly improved com-pared with that of EP. The SiO2 aerogel has extraordinary

Fig. 15. Water contact angles on the samples: (a) EP, (b) EP/SiO2 aerogel composite, (c(containing 20% EP/SiO2 aerogel composites).

hydrophobicity which can be found in Fig. 4. It can largely improvethe hydrophobicity of EP through filling the SiO2 aerogels in theopen pores of EP, and thus the application of EP/SiO2 aerogel com-posite could be expanded due to its hydrophobic character. Fig. 15(c) shows that the hydrophobicity of fly ash based LWM withoutEP/SiO2 aerogel composite was very poor with a water contactangle of 28.9�. However, when 20% EP/SiO2 aerogel compositeswere added to the fly ash based LWM, the hydrophobicity ofLWM increased significantly, as shown in Fig. 15 (d), with the con-tact angle rising from 28.9� to 58.14�.

4. Conclusions

Fly ash based lightweight wall materials towards low thermalconductivity of 0.050 W/(m�K) have been prepared and character-ized in this work. In order to achieve the excellent performance,SiO2 aerogels were considered as padding, and orthogonal experi-ments were conducted to synthetize SiO2 aerogels. The effects ofhydrolysis conditions of SiO2 aerogels have been studied systemat-ically. The results illustrate that hydrolysis parameters such as theH2SO4/TEOS molar ratio, hydrolysis time, hydrolysis temperature,and NH4OH/TEOS molar ratio have great influences on densityand thermal conductivity of SiO2 aerogels. The prior four-factorand three-level orthogonal experiments indicate the followingoptimal conditions to prepare SiO2 aerogels with small density of0.119 g/cm3 and low thermal conductivity of 0.0277 W/(m�K):H2SO4/TEOS molar ratio of 0.8 � 10�3:1, hydrolysis time of 24 h,hydrolysis temperature of 25 �C, and NH4OH/TEOS molar ratio of6.0 � 10�3:1.

Foaming agent was used to obtain porous structure of fly ashbased lightweight wall materials. For the insulating lightweightaggregate, EP was utilized as a supporting material and SiO2 aero-gel was used as a hydrophobic insulating material to prepare EP/SiO2 aerogel composites. By gelling on the EP surface without

) LWM Sample L1 (without EP/SiO2 aerogel composites), and (d) LWM Sample L4


chemical reaction, SiO2 aerogel could be filled in the pores of EP.FTIR analysis results confirmed that the structure of SiO2 aerogelsand EP remained unchanged in the composite material. The flyash based lightweight wall materials incorporating 20% EP/SiO2aerogel composite possessed proper compressive strength of0.88 MPa and bulk density of 335 kg/m3. Meanwhile, its thermalconductivity is around 0.050 W/(m�K). It is thought that the pre-pared fly ash based lightweight wall material incorporating EP/SiO2 aerogel composite can be utilized as a thermal insulatingmaterial in building for energy saving.

CRediT authorship contribution statement

Feixu Chen: Investigation, Data curation, Writing - originaldraft. Yihe Zhang: Conceptualization, Supervision. Jingang Liu:Investigation, Resources. XinkeWang: Investigation, Visualization.Paul K. Chu: Writing - original draft. Bohua Chu: Investigation,Data curation. Na Zhang: Validation, Writing - review & editing,Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing finan-cial interests or personal relationships that could have appearedto influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the financial support fromthe National Key Research and Development Program of China [No.2017YFC0703100], the National Natural Science Foundation ofChina [No. 51974283 and 51604026] as well as FundamentalResearch Funds for the Central Universities [No. 2652017339].

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Fly ash based lightweight wall materials incorporating expanded perlite/SiO2 aerogel composite: Towards low thermal conductivity1 Introduction2 Experimental details2.1 Materials2.2 Sample preparation2.2.1 Preparation of silica aerogel2.2.2 Preparation of EP/SiO2 aerogel composites2.2.3 Preparation of fly ash based lightweight wall materials composed of EP/SiO2 aerogel

2.3 Sample characterization

3 Results and discussion3.1 Properties and microstructure of the silica aerogels3.1.1 Density and thermal conductivity of the SiO2 aerogels3.1.2 Hydrophobicity of the SiO2 aerogels3.1.3 Pore structure of the SiO2 aerogels3.1.4 Thermal stability of the SiO2 aerogels

3.2 Properties of fly ash based lightweight wall materials (LWM) composed of EP/SiO2 aerogel3.2.1 Compressive strength of the fly ash based LWM composed of EP/SiO2 aerogel3.2.2 Bulk density of the fly ash based LWM composed of EP/SiO2 aerogel3.2.3 Thermal conductivity of the fly ash based LWM composed of EP/SiO2 aerogel3.2.4 Thermal stability of the fly ash based LWM composed of EP/SiO2 aerogel3.2.5 Hydrophobicity of the fly ash based LWM composed of EP/SiO2 aerogel

4 ConclusionsCRediT authorship contribution statementDeclaration of Competing InterestAcknowledgementsReferences

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