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Accepted Manuscript
A novel building material with low thermal conductivity: Rapid
synthesis of foam concrete reinforced silica aerogel and energy
performance simulation
Sijia Liu , Kunmeng Zhu , Sheng Cui , Xiaodong Shen , Gang Tan
PII:
DOI:
Reference:
S0378-7788(18)31342-2
https://doi.org/10.1016/j.enbuild.2018.08.014
ENB 8750
To appear in:
Energy & Buildings
Received date:
Revised date:
Accepted date:
30 April 2018
13 July 2018
7 August 2018
Please cite this article as: Sijia Liu , Kunmeng Zhu , Sheng Cui , Xiaodong Shen , Gang Tan ,
A novel building material with low thermal conductivity: Rapid synthesis of foam concrete reinforced silica aerogel and energy performance simulation, Energy & Buildings (2018), doi:
https://doi.org/10.1016/j.enbuild.2018.08.014
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ACCEPTED MANUSCRIPT
Highlights
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Foam concrete reinforced silica aerogel (FC-SA) has been prepared using a combined
sol-gel route, vacuum impregnation method and fast ethanol supercritical drying
technique.
The foam concrete reinforced silica aerogel has a surface area of 405.3 m2/g.
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The thermal conductivities of FC-SA is as low as 0.049 W穖-1稫-1 (30 癈).
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In cold and hot areas, the use of FC-SA to replace traditional concrete materials can
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greatly reduce both of space heating/cooling energy consumption and cooling water
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consumption.
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A novel building material with low thermal conductivity: Rapid
synthesis of foam concrete reinforced silica aerogel and energy
performance simulation
Sijia Liua,b,c, Kunmeng Zhu a,b, Sheng Cui a,b, Xiaodong Shen a,b*, Gang Tan c*
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials
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a
Science and Engineering, Nanjing Tech University, Nanjing 210009, China
b
Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites,
c
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Nanjing Tech University, Nanjing 210009,China
Department of Civil and Architectural Engineering, University of Wyoming, Laramie, WY
82071, USA
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*Corresponding Authors
Tel.: +86 25 83587234; fax: +86 25 83221690
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E-mail address: xdshen@njtech.edu.cn (X. Shen)
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Tel.: +01 3077662017; fax: +01 3077662221
E?mail address: gtan@uwyo.edu (G. Tan)
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Abstract
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With the increasing number of commercial buildings in the United States, the energy
consumption for space air conditioning is continuously rising. Considering the external
building envelope an important role to generate cooling and heating loads, we developed a
new high performance building material for building envelope application. Hence, a novel
foam concrete reinforced SiO2 aerogel (FC-SA) material was synthesized via sol-gel
technique?vacuum impregnation method and rapid supercritical drying process. The samples
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were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM),
thermogravimetric / differential scanning calorimetry (TG-DSC), N2 adsorption-desorption
test (BET) and transient plane source method (TPS). The prepared composite had a high
degree of aerogel filling (74% volume of matrix) and the aerogel component still maintained a
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porous nanostructure. Besides, FC-SA showed a large specific surface area of 405.3 m2/g and
high pore volume of 1.28cm3/g. Meanwhile, the introduction of aerogel has little effect on the
mechanical strength of the matrix material. According to the test results, the thermal
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insulation performance of foam concrete has been found greatly improved with silica aerogel
composition. The thermal conductivity of FC-SA composite was measured as low as 0.049
W穖-1稫-1 at room temperature (30 癈), which was a 48.4% decrease from foam concrete.
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Finally, the energy saving simulation results showed that in cold and hot areas, the use of
FC-SA to replace traditional concrete materials can greatly reduce both of energy
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consumption and cooling water consumption.
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Keywords
Super insulation material; Porous material; Silica aerogel; Foam concrete; Building insulation;
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Energy saving
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1. Introduction
In the United States, more than 19% of national primary energy has been consumed by
commercial buildings [1]. In particular, space cooling and heating accounts for about 29% of
the energy use in these buildings[2], with additional energy consumption from distribution
devices such as pumps for chilled water. According to Hans?s study, the commercial buildings
were projected to increase by 25% until 2020 and by 44% until 2030 [3]. How to cut down
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the increase of energy consumption and greenhouse gas emission caused by the growing
commercial buildings is a problem worthy of studying. The building external envelope is a
structure that isolates the indoor environment from the changing outdoor environment and
thus it plays an important role in affecting cooling and heating loads [4-6]. Many researchers
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around the world have carried out studies on improvements in the building envelope and
reduction of associated building energy consumption. Zhang et al. [7] coated high-reflectivity
materials onto the building external walls, and the illustrative example in Chengdu showed it
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can save 15.2% electricity consumption in a whole summer. Chan et al. [8] took Hong Kong
as an example to study the impact of commercial building envelopes on cooling energy
consumptions in the sub-tropical climate, the energy effective building envelope they
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designed can save as much as 35% and 47% of total and peak cooling demands. Lei et al. [6]
investigated the energy performance of building envelopes integrating phase change materials
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(PCMs) with a phase change temperature of 28 癈 for cooling load reduction in Singapore,
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and results showed PCMs can reduce energy usage in a range of 21-32% through the whole
year. Therefore, isolating indoor environment from outdoor environment change using higher
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insulation or other material to building envelope is an effective and common way to reduce
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the building energy consumption.
The common construction of commercial buildings have steel or concrete frames plus a
non load-bearing external wall system[8], and the typical external wall system contains a
concrete layer and an insulation layer. Building insulation materials mainly have teo
categories: organic and inorganic materials [9]. The most commonly used organic materials
including polyisocyanurate (PIR) [10], polystyrene (PS) board [11-13], polyurethane [14, 15],
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extruded polystyrene (XPS) [13, 16], etc. They are manufactured by extrusion, foaming,
expanding and other process. However, due to the poor flame resistance and prone to fire, the
use of organic materials can bring potential safety risks. There has been many cases of fire
disasters in buildings, such as the fire in a London apartment and the Beijing CCTV building,
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which were caused by the burning of external insulation materials. Inorganic insulation
materials mainly include asbestos fiber mat [17], glass fiber mat [18], rock wool [19], calcium
silicate board [20], etc. However, these materials may absorb moisture so that their thermal
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conductivity increases while the insulation performance decreases. Furthermore, the structure
of concrete plus insulation generally causes construction time and cost consuming. [21]. Foam
concrete can be prepared by chemical foaming, molding and machining, which has the
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characteristics of low bulk density (<600 Kg/m3), low thermal conductivity (0.1-0.2W穖-1稫-1)
and long service life [22-24]. The specific advantage of foam concrete is its lightweight,
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which benefits supporting structures including the foundation and walls of lower floors [25].
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Due to the porous structure, foam concrete provides a high degree of thermal insulation and
considerable material savings compared to normal concrete. Foam concrete is expected to be
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a new green building material for construction, but its thermal resistance is not so good to
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compete with traditional insulation material [26], which, however, can be further improved.
Aerogel is a well-known three-dimensional nanoporous material [27-30]. It has been
extensively used for thermal insulation [31-33], catalysis [34-36], biomedical [37, 38] and
adsorption [39-41] owing to its high surface area, high porosity and low thermal conductivity.
As one of the most frequently used aerogels, silica aerogel has extremely high specific area
(up to 1000m2/g) and porosity (98%-99%), and it is known as the world?s lowest density solid
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material [42-44]. Because the solid and gas heat conduction in silica aerogel is largely
restrained thanks to its nanoporous network structure, silica aerogel can serve as a super
thermal insulation material due to its very low thermal conductivity (as low as
0.012W穖-1稫-1)[32, 45]. In the last few years, silica aerogel has been developed into a
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variety of products for the building material market and has been extensively promoted [46,
47]. However, because of brittle property, most silica aerogels haven?t been widely directly
used. The fiber mat reinforced silica aerogel is a kind of market available products but they
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still cannot withstand compressive pressure or shear stress [48, 49]. Some researchers doped
aerogel particles to cement to improve the thermal insulation property of the concrete
structure (see Table 1). Fickler et al. [50] developed aerogel-concrete material by embedding
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silica aerogel granules in a high strength cement matrix, which achieved high compressive
strength between 3.0 MPa and 23.6 MPa and thermal conductivities between 0.16 W/m稫 and
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0.37 W/m稫. Gao et al. [51] prepared aerogel-incorporated concrete (AIC) with a thermal
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conductivity of ?0.26 W/m稫 and a compressive strength of ?8.3 MPa at an aerogel content
of 60 vol%. Although doping aerogel into concrete is a straightforward and simple method,
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the thermal conductivity of aerogel doped concrete is still too high to meet the requirement of
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working as insulation materials. Therefore, there has been lacking of a good solution to use
silica aerogel for high performance insulation material in building envelope.
Table 1 Properties of samples in reference work
Properties
Fickler [50]
Gao [51]
Serina [52]
Compressive strength (MPa)
3.0-23.6
?8.3
20.0
Thermal conductivity (W/m稫)
0.16-0.37
?0.26
?0.55
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In this paper, a new material of foam concrete and silica aerogel (FC-SA) for building
application has been developed using a combined sol-gel method, vacuum impregnation
process and fast ethanol supercritical drying technique. Incorporating silica aerogel into foam
concrete not only further improves the thermal property of the foam concrete, but also
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provides a load-bearing structure to support aerogel material. This work has studied the
thermal insulation performance of the lab-developed FC-SA material and simulated the
building energy saving from application of FC-SA to building envelope for large commercial
2.
Fabrication of FC-SA material
2.1. Synthesis
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buildings.
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Foam concrete reinforced silica aerogel (FC-SA) was synthesized by the sol-gel technique,
vacuum impregnation method and rapid supercritical extraction (RSCE) method based on our
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previous research [53]. Fig. 1 illustrates the schematic overview of synthesizing process.
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Tetraethylorthosilicate (TEOS) was used as silicon source, nitric acid (HNO3) and ammonia
solution (NH3稨2O) were used as catalysts, absolute ethyl ethanol (EtOH) was used as solvent
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and deionized water (H2O) as hydrolysis agent. All of the reagent and solvents were analytical
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grade and used as received without further purification. Foam concrete (FC) were used as
hard matrix and gotten from Langfang Tuo Xin thermal insulation material Co., Ltd.
Fig. 1 Schematic illustration of the synthesizing process
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Firstly, TEOS, EtOH, H2O and HNO3 were mixed with a molar ratio of 1: 16: 6.5: 0.001,
after an hour of stirring at 50 癈, and adjusted pH of solution to 6-7 by using ammonia to
form a silica sol. Then the foam concrete was placed in a vacuum reactor; silica sol was
sucked into the vessel after vacuuming; and this dipping process was repeated three times to
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ensure that the sol and the matrix were fully compounded. According to our previous method
[53], the sol saturated composite can be directly supercritically dried by ethanol without the
need for gelling and aging. The foam concrete reinforced silica aerogel composite was finally
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obtained after the reaction vessel was cooled to room temperature. Compared with the
traditional aerogel preparation process, the fabrication period of our rapid method was
shortened to 5-10 hours, which not only greatly shortened the preparation time but also
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avoided the waste of ethanol in the aging process.
2.2. Characterization and test method
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X-ray diffraction (XRD) patterns were obtained using CuK?1 radiation (?=0.15406nm) with
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a Rihaku Miniflex-600. Scanning electron microscopy (SEM) was carried out on a
LEO-1530VP field emission scanning electron microscope. The Brunauer-Emmett-Teller
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(BET) analysis was measured by nitrogen adsorption/desorption technique, using a
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Micrometrics ASAP2020 surface area and pore distribution analyzer at liquid nitrogen
temperature. The flexural strength and compressive strength were measured for samples with
a normal size of 50�� mm3 at a cross-head of 1mm/min, using an INSTRON 3382
testing machine. The thermal conductivities of FC and FC-SA samples were tested using a
Hot Disk-2500 thermal constant analyzer and the thermal insulation performance was tested
by an in-house setup device. In the thermal insulation performance test, the hot panel was
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heated to a preset temperature, and then samples were pressed against the hot panel and the
change of cold surface temperature was recorded by thermocouples, for which the entire
testing process lasted 24h. The energy saving and cooling tower water saving was evaluated
by EnergyPlus simulation [54] .
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3. Results and discussion
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3.1 Structural evolution of FC-SA
Fig. 2 Schematic diagram of structural evolution of FC-SA
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Fig. 2 shows the schematic diagram of structural evolution of foam concrete and single cell.
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Since the foam concrete has many closed pores, it is difficult to introduce SiO2 into the matrix
directly, so vacuum impregnation process was used to open these closed cells. In addition, the
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sol has not been gelled during the pressurization phase of RSCE process, and the remaining
closed cells can be further opened by the pressure difference. Prior to compositing with the
aerogel, there are visible millimeter-sized holes in the interior of the foamed concrete in
which the gas molecules undergo thermal motion, and the heat is transferred by the collision
of the gas molecules with each other and the collision with the cell walls. After sol-gel and
drying process, the internal spaces of the single foam cell are separated by the nano-porous
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structure of the silica aerogel, which achieves the replacement of the visible holes by the
nano-pores. According to Jennings's [55] study, the mean free path of air at 25 癈 is 66.35 nm,
which is similar to the average pore size of aerogel. Therefore, the gas molecules are confined
to the network of aerogel, greatly attenuating the effects of heat conduction caused by gas
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thermal movement, so as to achieve the improvement of heat insulation performance.
Fig. 3 XRD patterns of foam concrete and FC-SA
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3.2 X-ray diffraction
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The XRD patterns obtained for foam concrete and FC-SA are depicted in Fig. 3. Foam
concrete was produced by using cement, blast-furnace slag, quicklime, sulphoaluminate
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cement, water and foaming agent. So the peaks appearing in foam concrete can be indexed to
limestone and dehydrate gypsum, as the hydration reaction product of gypsum, ettringite was
also observed in the spectrum. Silica aerogel is well known to be amorphous [56], hence no
other diffraction peaks were detected after foam concrete compounded with it.
3.3 Morphology and microstructure
Fig. 4 provides the optical photograph and SEM images of foam concrete and FC-SA,
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Fig.4(f) are the detail view of silica aerogel component in FC-SA. From Fig. 4(a) and (b), the
appearance color of foam concrete is milky white, which is result from the high calcium
carbonate content of matrix, FC has a hive-like structure composed of different size pores,
which including open and closed holes. As can be seen from Fig. 4(c) and (d), the pores of
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foam concrete have been completely filled and covered by pale blue silica aerogel, which
indicated that the silica sol can be sufficiently absorbed into the matrix by vacuum
impregnation process. According to Fig. 4(e) and (f), silica aerogel firmly attaches to the
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foam concrete and it still retains the nano-porous network therein, giving an important
structural feature to achieve improved thermal insulation property. A summary of the textural
properties of RSCE, FC and FC-SA is represented in Table. 2, RSCE-A is the pure SiO2
aerogel prepared by rapid supercritical extraction in our previous work [53], and it is used for
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comparison. The density of foam concrete changed from 0.325g/cm3 to 0.392g/cm3 after
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composited with aerogel, while that of pure silica aerogel was 0.09 g/cm3, so that the aerogel
from SEM.
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filled 74% of the volume of the entire material. This result is consistent with the observation
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Table 2 The properties of RSCE-A, FC and FC-SA
Density
(g/cm3)
BJH surface
area (m2/g)
Average pore
diameter (nm)
Pore volume
(cm3/g)
Thermal conductivitya
(W穖-1稫-1)
RSCE-A
0.090
961.9
13.8
3.53
0.027b
FC
0.325
-
-
-
0.095
FC-SA
0.392
405.3
16.7
1.28
0.049
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Sample
a: at 30 癈
b: data of RSCE-A/glass fiber
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Fig. 4 Optical photographs and SEM micrographs of foam concrete (a, b) and
FC-SA (c, d, e, f)
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3.4 Thermal stability analysis
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Thermal gravimetric analysis (TGA) of FC-SA was measured under air atmosphere from
25 癈 to 1000 癈 and the TG-DSC curve was showed in Fig. 5. First, it can be seen that an
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apparent weight loss occurred below 100癈 owing to the expulsion of water and ethanol from
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the porous structure. Then, the constant weight loss at 100 - 600 癈 results from the
dehydration of dehydrate gypsum and the decomposition of ettringite. In addition, we can
observe a small decrease in weight and a sharp exothermic peak in DSC at 265.2 癈, which is
the characteristic of typical silica aerogels. Ethanol supercritical drying gives aerogel
hydrophobicity properties, while -CH3 group on the aerogel surface can be oxidized to ?OH at
this temperature, and the composite is also changed from hydrophobicity to hydrophilic
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gradually. Another exothermic peak and a mass weight loss of 10.85% were observed at 630 720 癈 corresponding to decomposition of calcium carbonate, indicating that the matrix
skeleton was damaged, where silica aerogel shrank in volume and lost its porous properties
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[57]. In general speaking, the FC-SA can be applied to the circumstances as high as 600 癈.
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Fig. 5 TG-DSC curve under air of FC-SA
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3.5 Porous structure and mechanical strength analysis
Brunauer-Emmett-Teller (BET) gas sorptometry measurements were conducted to examine
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the porous nature of the FC-SA sample and Fig. 6 (a) exhibits the nitrogen
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adsorption/desorption isotherms and pore-size distribution. The isotherms is identified as
type-IV, which is typical characteristic of mesoporous materials [58]. Hysteresis loops with
shapes that are intermediate between typical H-1 and H2-type isotherms are observed, which
is believed to be related to the capillary condensation associated mesoporous, consistent with
the observation results from the SEM image. Compared to pure silica aerogel, FC-SA has a
wider distribution of pore size, which is due to the non-uniform supercritical extraction result
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from the restriction of foam concrete. As shown in Table 2, the surface area, average pore size
and pore volume of FC-SA is 405.3 m2/g, 16.7nm and 1.28cm3/g, respectively. As we all
know that the macropores can?t be effectively detected via nitrogen adsorption method, yet
some macropores do actually exist in the composite sample, the actual pore volume was
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underestimated accordingly.
As can be seen from Fig. 6(b), the flexural strength and compression strength of FC-SA
were 0.62 and 1.12 MPa, respectively. The introduction of aerogel actually affected the
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mechanical strengths of foam concrete. The flexural strength of sample decreased after
compounding, which was due to the partial destruction of matrix by the vacuum impregnation
process. However, the increase in compressive strength can be attributed to the supporting of
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aerogel-skeleton.
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Fig. 6 (a) Nitrogen adsorption/desorption isotherm (pore-size distribution curve inset) and (b)
mechanical strength of FC-SA
3.6 Characteristics of thermal insulation performance
Fig. 7 (a) shows the total thermal conductivities (?t) of the foam concrete and FC-SA
composite measured at different temperatures up to 200 癈 under air atmosphere using a Hot
Disk-2500 thermal constant analyzer. All the ?t of samples increases gradually with the rise of
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temperature, but FC-SA sample shows significantly lower thermal conductivity at all
temperatures, indicating that aerogel plays an important role in the composite. It is worth
mentioning that the ?t of FC-SA composite is as low as 0.049 W穖-1稫-1 at room temperature
(30 癈), which is a 48.4% decrease compared to the foam concrete.
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Table 3 shows the compressive strength and thermal conductivity property comparison
between this work and the common building envelope insulation materials. As it can be seen,
FC-SA is superior to porous geopolymer in both thermal and mechanical performance.
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Compared to foam ceramic, FC-SA has lower compressive strength but much better thermal
resistance. As for rock wool and polyethylene, their thermal insulation property is slight better
than that of FC-SA but without weight bearing capability.
Table 3 Properties of samples in this work and reference work
Compressive strength Thermal conductivity
(MPa)
(W/m稫)
This work
1.12
0.049
Porous geopolymer [59] 0.82
0.074
Foam ceramic [60]
5.0
0.36
Rock wool [61]
-
0.037
Polyethylene [61]
-
0.041
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Fig. 7 Thermal conductivity test results (a) and thermal insulation performance test results (b.
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200癈; c. 300癈; d. 400癈) of foam concrete and FC-SA
The thermal insulation performance of the FC and FC-SA samples was tested by using an
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in-house built thermal insulation test setup as shown in Fig. 8 with a molybdenum silicide bar
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array as heater. The test involved temperature measurements to both surfaces of the sample,
each of which contains three measuring points. The surface on the heater side was called ?hot
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surface? while the one in contact with the cover plate was called ?cold surface?. During the
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test, the temperature of the hot surface was maintained at constant (200癈, 300癈 and 400癈),
and the temperature variation of the cold surface was recorded with time. As illustrated in Fig.
7 (c)-(d), the cold surface temperatures of FC-SA were always lower than that of the foam
concrete. Besides, we can clearly see that FC-SA has a lower heating rate from the local
enlarged drawing, implying that silica aerogel component plays a favorable shielding effect
on the gaseous phase heat conduction. After 24 hours, the heat transfer tended to be stable and
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temperature difference between two samples in the three tests was 4.4 癈, 11.8 癈 and
16.8 癈, respectively. In high-temperature thermal industry, even 1癈 temperature difference
could bring about a great change of energy consumption [62]. Therefore, the improvement of
the heat insulation performance of FC-SA is of great significance for energy saving.
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Furthermore, if infrared opacifier is incorporated into aerogel component, the thermal
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conductivity of composite sample at high temperature may have a further reduction.
Fig. 8 Schematic of thermal insulation performance test setup
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3.7 Energy saving simulation
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The energy saving and water saving of the FC-SA application to large commercial
buildings were evaluated by EnergyPlus, which is a whole building energy simulation
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program that used to model energy consumption ? for heating, cooling, ventilation, lighting
and plug / process loads - and water use in buildings [60]. EnergyPlus calculates heat
consumption of buildings by the heat balance method, which takes into account all balances
on outdoor and indoor surfaces and transient heat conduction through building construction. A
typical large office building model originated from the US DOE (Department of Energy)
building codes program [63] was used. The building has a total floor area of 498,588 ft2.
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Three simulation cases were modeled for (1) the traditional external walls (baseline)
composed of two layers: an 8-inch normal concrete with a thermal conductivity of 2.31
W穖-1稫-1 and an insulation layer meeting ASHRAE Standard 90.1-2013 [64] requirement, (2)
baseline wall (i.e., 8-inch normal concrete) without insulation layer (named with ?B w/o
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Insul?), and (3) 8-inch exterior FC-SA wall only, respectively. Six locations in the United
States were simulated representing different climatic conditions: Albuquerque, AZ (dry and
mild), Burlington, VT (warm summer and long cold winter), Chicago, IL (temperate
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continental climate), Miami, FL (tropical maritime climate), Phoenix, NM (hot summer and
warm winter) and San Francisco, CA (mediterranean climate) to evaluate the energy saving
potential of the FC-SA. The climate zones and insulation R-values for each location are listed
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in Table. 4.
Table 4 Climate zones and insulation requirements for baseline case of the six locations [64]
Climate zone
Albuquerque
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City
Insulation R-value
5B
R-11.4
6A
R-13.3
Chicago
5A
R-11.4
Miami
1A
N/R
Phoenix
3B
R-7.6
San Francisco
3C
R-7.6
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Burlington
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Fig. 9 Space heating/cooling annual savings and saving ratios in different locations (a. FC-SA
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to Baseline; b. FC-SA to B w/o Insul)
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Fig. 10 Water savings and saving ratios from cooling tower in different locations (a. FC-SA to
Baseline; b. FC-SA to B w/o Insul)
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Fig. 9 illustrates the simulation results of annual energy savings in different locations for
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FC-SA application to a typical large office building. Fig. 9 (a) is the energy saving results of
the FC-SA case to the baseline case and Fig. 9 (b) is the FC-SA case to the B w/o Insul case.
The annual energy savings consist of savings from space cooling and space heating. It can be
seen that in Burlington and Chicago as both has a cold winter, FC-SA saves a lot of space
heating energy, leading to 5.09% and 6.64% energy saving in a whole winter season,
respectively. Moreover, FC-SA can also help reduce cooling energy consumption in the hot
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summer of Miami and Phoenix, for which the whole energy saving rate in summer is 4.02%
and 1.8%, respectively. The anomaly is that the space heating saving rate in Miami is as high
as 79.97%; but due to the low total energy consumption in winter, it doesn?t have the
significance of discussion. As for Albuquerque and Phoenix, the energy saving magnitude is
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much lower. However, the results for San Francisco show that the building?s energy
consumption of space cooling is increased. Given that this location is the Mediterranean
climate [65], and the outdoor temperature is relatively mild in summer and winter, the better
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thermal insulation effect can hold back the heat transfer through the exterior wall in nighttime.
Compared to the B w/o Insul case, which has a bare 8-inch normal concrete wall, as seen in
Fig. 9 (b), the insulation effect of FC-SA play a great role in building HVAC energy
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consumption. In Albuquerque, Burlington and Chicago, the energy savings of the FC-SA wall
against bare normal concrete wall increased by 8.8 times, 9.9 times and 8.1 times respectively.
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Taking Burlington as an example, the building can save energy consumption up to
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891.1MW穐 a year, which is the equivalent of the annual electricity consumption of 90
families in the US [66]. Besides energy consumption from space heating and cooling, the
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cooling system in the modeled large office building consumes water to dissipate waste heat
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through cooling towers. Therefore, saving cooling energy will result in cooling tower water
saving too. Fig. 10 shows the simulation results of annual water savings. Fig. 10 (a) is for the
results of the FC-SA case to the baseline case and Fig. 10 (b) is for the FC-SA case to the B
w/o Insul case. As we can see, water saving has almost the same trend with space cooling
saving. With FC-SA wall, the cooling water consumption can be reduced by 6.07% and 2.14%
in Miami and Phoenix compared to the baseline case. In contrast, compared to the B w/o Insul
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case, the water saving of the FC-SA wall against bare normal concrete wall increased by 4
times in Phoenix, and the annual cooling water saving at Phoenix is up about 2,536 m3, which
is equivalent to the annual water consumption of 7 families in the US. In all, if FC-SA can be
applied in areas with long and cold winter or hot summer, it can reduce the building
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air-conditioning energy consumption and cooling water usage greatly.
4. Conclusions
A novel foam concrete reinforced silica aerogel material that has the combined advantages
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of high porous aerogel and load-bearing foam concrete was successfully synthesized by the
sol-gel method, vacuum impregnation process and rapid ethanol supercritical drying
technique. The microstructure, high temperature stability, mechanical properties and thermal
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insulation performance of samples have been characterized and/or tested. For building
applications, the foam concrete reinforced aerogel material plays two roles: (1) working as
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exterior walls because of its good compressive strength, and (2) super insulation to replace
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traditional insulation layer. The conclusions from this work were summarized as follows:
(1) Silica aerogel can be evenly distributed in the porous structure of FC, and the volume
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filling rate of aerogel to FC matrix is up to 74%.
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(2) The surface area of FC-SA is as high as 405.3 m2/g and it also has high pore volume of
1.28cm3/g, which provides a structural basis for the improvement of thermal insulation
property.
(3) The flexural strength and compression strength of FC-SA are 0.62 MPa and 1.12 MPa,
respectively; this gives aerogel the ability to withstand pressure and shear stress.
(4) The thermal conductivity of FC-SA at 30癈 is as low as 0.049 W穖-1稫-1, which is 48.4%
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lower than that of FC due to the changed porous structures and heat transfer mechanisms of
FC caused by silica aerogel.
(5) According to the simulation results of EnergyPlus, the FC-SA obtained in this work
shows good energy conservation effect for building envelope application. In Burlington and
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Chicago with a cold winter, FC-SA saves a lot of space heating energy, leading to 90.5 MW穐
(5.09%) and 98.3 MW穐 (6.64%) energy saving in a whole winter, respectively.
(6) In hot areas like Miami and phoenix, the use of FC-SA to replace traditional concrete
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materials can not only reduce space cooling energy consumption of 80.7 MW穐 (6.07%) and
27.2 MW穐 (2.14%) but also save cooling water usage amount of 1,122.4 m3 (6.62%) and
634.1 m2 (8.05%), respectively.
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Because of the unique combined advantage of mechanical and thermal properties of FC-SA
that outperform other materials in previous work, the newly developed FC-SA material has
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demonstrated a promising future of building exterior wall applications. On the other hand, this
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study has further expanded the product categories of aerogel insulation materials and provided
a technically feasible aerogel application in buildings for thermal insulation. In the next
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research, the low-cost preparation of FC-SA could be a focus. For instance, low/no-cost fly
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ash can be used as a raw material for both aerogel and foam matrix with simplified
preparation method.
Acknowledgements
This work was financially supported by the Industry Program of Science and Technology
Support Project of Jiangsu Province (BE2016171), the Program for Changjiang Scholars and
Innovative Research Team in University (No.IRT_15R35), the Major Program of Natural
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Science Fund in Colleges and Universities of Jiangsu Province (15KJA430005), National
Natural Science Foundation of China (51702156), the Natural Science Foundation of Jiangsu
Province-China (BK20161002). This project is also supported by the funding from the
Wyoming State Legislator through the School of Energy Resources, University of Wyoming,
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USA.
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