Experimental investigation on thermal conductivity of aerogel-incorporated concrete under various hygrothermal environment

Energy 188 (2019) 115999

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Experimental investigation on thermal conductivity of aerogelincorporated concrete under various hygrothermal environment Yingying Wang a, b, *, Jinjin Huang b, Dengjia Wang a, b, Yanfeng Liu a, b, Zejiao Zhao b, Jiaping Liu a a b

State Key Laboratory of Green Building in Western China, Xi'an University of Architecture and Technology, No.13 Yanta Road, Xi'an, 710055, PR China School of Building Services Science and Engineering, Xi'an University of Architecture and Technology, No.13 Yanta Road, Xi'an, 710055, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 April 2019 Received in revised form 25 July 2019 Accepted 22 August 2019 Available online 4 September 2019

Since aerogels have low density and thermal conductivity, mixing aerogels with concrete can effectively improve the thermal insulation of concrete materials. In this study, aerogel-incorporated concrete (AIC) with different aerogel volume admixtures were prepared. The porosity and pore distribution characteristics of AICs were obtained using scanning electron microscope and microparticle mercury porosimeter. The variation of thermal conductivity with temperature (20e90
C) under dry state and the effect of humidity (35
C, 0e100% relative humidity) on the thermal conductivity of AICs were studied experimentally. Meanwhile, the water absorption coefficient and sorption isotherms were obtained. The results show that the thermal conductivity of AIC changes with the content of aerogel following a quadratic function. The highest reduction in thermal conductivity was 79.3% with the increase in the content of aerogel. Within the temperature range of 20e90
C, the thermal conductivity of AIC increases with the increase in temperature. The highest increase in thermal conductivity was 15.5%. The change in thermal conductivity with humidity can be fitted to a cubic polynomial function. When the relative humidity changes from 0% to 100%, the variation in thermal conductivity of AIC0 is the lowest (19.51%). Additionally, the corresponding highest variation in the thermal conductivity was observed for AIC60 (76.33%). © 2019 Elsevier Ltd. All rights reserved.

Keywords: Aerogel-incorporated concrete Thermal conductivity Experimental investigation Various hygrothermal environment

1. Introduction 1.1. Aerogel-incorporated concrete (AIC) In the early 1930s, Kistler [1] first synthesized aerogels. Aerogels can be prepared using several base materials such as alumina, chromium, tin oxide, carbon and silica. Silica aerogel is a nanoporous material, which is made of silica [2]. Silica aerogel's 94e99% volume is occupied by air, has low density, low thermal conductivity and high permeability [3]. Due to their easy preparation and reliability, silica aerogels are the most common materials available on the market. Aerogel-enhanced materials are novel materials, which are prepared by adding a certain amount of aerogel to another material to improve their performance during the

* Corresponding author. State Key Laboratory of Green Building in Western China, Xi'an University of Architecture and Technology, No.13 Yanta road, Xi'an, 710055, PR China. E-mail address: [email protected] (Y. Wang). https://doi.org/10.1016/j.energy.2019.115999 0360-5442/© 2019 Elsevier Ltd. All rights reserved.

manufacturing process. Aerogel-enhanced materials with a thermal conductivity of less than 0.02 W/m$K are generally considered super insulating materials [4]. Due to high thermal insulation, aerogel-enhanced materials have been widely researched in recent years. The aerogel-incorporated concrete (AIC) prepared by incorporating aerogel particles in cement has a lower thermal conductivity. However, the hydrophobicity and ultra-low density of aerogel particles make it easy to float during the concrete-making process. Due to this reason, adding aerogel to concrete remains challenging. The idea of embedding silica aerogel particles in a cement matrix was first proposed by Ratke [5]. Dai et al. studied the effect of doping additives on the thermal insulation properties of aerogel composites [6]. Kim's research shows that methanol can help silica aerogel particles to fully mix with cement and coexist in the cement matrix [7], however, without any pretreatment of silica aerogel particles, Gao [8] adds the aerogel to the cement mortar to replace the amount of sand by volume ratio, and added a super-plasticizer to precast concrete to obtain a well-mixed paste. The results

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showed that, when the aerogel content was 60%, the concrete density was 1.0 g/cm3, whereas its thermal conductivity was 0.26 W/m$K. Meanwhile, the compressive strength of the concrete was 8.3 MPa. After Gao et al., aerogel enhancement technology was further developed. Welsch et al. studied the mathematical models which were used to describe the relations between the structure of aerogel concrete and their properties. The results show that High Performance Aerogel Concrete (HPAC) shows an improved correlation between dry bulk density and compression strength as well as compression strength and thermal conductivity [9], which shows that the aerogel concrete has reliable development potential. Therefore, in order to accurately simulate building energy consumption and calculate coupled heat and moisture transfer process, the hygrothermal property parameters of aerogel concrete need to be obtained. In this paper, the relationship between thermal conductivity of aerogel concrete and temperature and humidity is further studied, and their wet property parameters are also tested to analyze the influence of moisture absorption characteristics on thermal conductivity. 1.2. Thermal conductivity and hygrothermal characteristics For the building envelopes exposed to different temperatures and relative humidity, the mechanical properties and thermal insulation of concrete materials are of particular significance [10]. Therefore, many scholars have studied the compressive strength and thermal conductivity of AIC [8,11,12]. Liu et al. studied the relationship among density, compressive strength, and thermal conductivity of AIC when the silica aerogel particles were added in the proportion of 10e60% [11]. In some previous works [8], when the volume fraction of aerogel was varied between the range of 0e60%, the density of AIC was found to lie within the range of 1000e2300 kg/m3. In a previous work [7], silica aerogel powder was added to cement slurry, and the results showed that, compared to the aerogel-free concrete, the thermal conductivity of the aerogel-enhanced cement reduced by 75% when the aerogel's mass content was 2.0% (wt.%). However, studies on the hygrothermal characteristics of AIC at different temperatures and humidities are scarce in literature. Berardi et al. conducted a large number of studies on aerogel materials through experiments. They made aerogel plasters [13] and studied the long-term thermal conductivity of aerogelenhanced insulating materials under different laboratory aging conditions [14]. Moreover, a thin aerogel-enhanced product was installed in the opaque and transparent envelope to simulate changes in building energy consumption [15]. The research results show that high relative humidity has great impact on the thermal properties of aerogel reinforcements, and the application of aerogel products can save up to 34% in overall building energy. This means that the aerogel-incorporated materials have a good development prospect. However, in actual use, the building material is generally in an environment with a certain temperature and humidity, and the hygrothermal environment has a significant influence on the most important thermal properties (thermal conductivity) of materials. Therefore, in this paper, the thermal conductivity under different temperatures and humidity, water absorption coefficient and sorption isotherms of aerogel concrete are further studied. It is hoped to contribute to the prediction of building energy consumption, the calculation of coupled heat and moisture transfer processes and the development of aerogel-incorporated concrete. Under different temperature and humidity conditions, the storage, evaporation and migration of wet components in the internal pores of building materials change the heat transfer process of the envelope structure into a three-phase (solid, liquid and gas)

heat conduction process, which leads to non-negligible changes in the heat transfer process. Therefore, if the effect of moisture transmission is neglected, the use of various parameters of concrete's dry state for energy consumption calculations will inevitably lead to inaccuracies in the calculated results. This not only causes waste of energy, but also leads to a reduction in building comfort. The determination of the hygrothermal properties of building materials is the premise of the calculation of coupled heat and moisture transfer processes. However, the hygrothermal properties of AIC are reported scarcely in literature. Therefore, the experimental study on the thermal conductivity, water absorption coefficient and sorption isotherms of AIC with different volumetric proportions of aerogels was carried out in this work. Thermal conductivity refers to the heat flux of the material in per square meter of surface area and per unit thickness of 1 m under the condition of temperature gradient of 1 K, which represents the heat transfer capacity of material [16]. The thermal conductivity of concrete has been widely studied [7]. However, the influence of temperature and humidity on the thermal conductivity of concrete matrix is necessary for accurate heat transfer calculations for the building. Moreover, the porosity and pore inhomogeneity of concrete can result in a decrease in its own thermal conductivity [17e19] and a change in hygroscopicity, which result in fluctuations of thermal conductivity at different temperatures and humidity. Nguyen and Beaucour et al. studied the effects of moisture content and temperature on the thermal conductivity of light-weight aggregate concrete. Their works showed that the thermal conductivity and the specific heat of light-weight aggregate concrete exhibit a significant dependence on moisture content [20]. Some previous studies [21e24] reported the thermal conductivity of light-weight concrete at different temperatures and moisture contents. Hanif et al. studied the mechanical and thermal insulation properties of green light-weight cement composites containing aerogel and fly ash hollow microspheres [25]. Some scholars have also studied the effect of high temperature conditions on the thermal conductivity of concrete [26]. A previous work [20] has shown that the thermal conductivity of concrete increases with the increase in temperature, however the thermal conductivity begins to decrease after the temperature exceeds 50e60
C, which is due to the change in specific heat with temperature. Nosrati et al. reported the hygrothermal characteristics of aerogel-reinforced insulation under different humidity and temperature conditions. The results showed that the thermal conductivity of the material under high temperature and high humidity conditions is 100% higher than that of the dry state [13]. Therefore, ignoring the influence of temperature and humidity on thermal conductivity will lead to problems, such as inaccurate calculations of a building's energy consumption and increased condensation risk [27]. The water absorption coefficient is the mass of water absorbed by the material per face area and per square root of time through capillary action [28]. The water absorption coefficient is similar to the capillary transmission coefficient [29], which is an important parameter for quantifying the weight fraction of water in concrete holes and simulating the heat and water transfer in materials [30]. An accurate understanding of the water absorption coefficient of concrete can help predict the growth of mold in building envelope [31,32] or improve the indoor air quality [33]. In addition, the sorption isotherms and the water absorption coefficient of the material can help obtain the hygrothermal behavior of the material under equilibrium and dynamic conditions [34]. Sorption isotherm refers to the relation curve of moisture content and relative humidity of the environment when the material and the environment reach the wet equilibrium at the specified temperature [35]. The sorption isotherms of concrete materials play an important role in evaluating the durability of building

Y. Wang et al. / Energy 188 (2019) 115999

envelopes [36], understanding the porous nanostructures of materials [37], and simulating the indoor air temperature and humidity fluctuations [38e41]. In order to study the effect of water content on heat transfer processes in concrete and water storage performance, Jerman and Cerny [42] determined the porosity, thermal conductivity and sorption isotherms of aerated concrete. Promis [43] established a model based on the relationship between the adsorption and desorption isotherms. Lakatos et al. analyzed the influence of moisture content on the thermal properties of aerogel blankets and tested the moisture absorption characteristics of several common materials [44,45]. The thermal conductivities of several aerogel materials were tested by the hot plate method and the heat flow meter method. The water absorption coefficients and isothermal adsorption curves of solid brick, lightweight and other materials at different temperatures were tested by the climate chamber method, and the influence of water absorption on the thermal properties of the materials was also studied. The results show that the amount of water absorbed by the aerogel material is independent of temperature after 24 h of wetting. The building's energy demand increases significantly with increasing water content, this is mainly because the moisture content has a significant impact on the thermal conductivity of building materials. Therefore, in this paper, the thermal conductivity under different temperatures and relative humidity was tested, the water absorption coefficient and sorption isotherms of the materials were also tested at constant temperature. 1.3. Scope of this research In engineering, building envelope is usually exposed to the environment with different temperatures and relative humidities, due to which, the thermal conductivity of materials will inevitably change, resulting in a deviation in building's energy consumption from the pre-designed estimated value. In addition, the hygrothermal property parameters of materials are the key to perform calculations regarding heat and moisture coupled transfer processes of building envelope. Therefore, in this paper, on the basis of previous works [8], an AIC was successfully fabricated by replacing sand in concrete with silica aerogel particles in different volumetric ratios. The porosity and pore distribution of synthesized AIC were obtained using scanning electron microscope and microparticle mercury porosimeter. The variation in thermal conductivity with temperature under dry state and the effect of humidity on the thermal conductivity of the material were studied experimentally. At the same time, the water absorption coefficient and the sorption isotherms were obtained. The works can help explore the theoretical basis about the hygrothermal properties of AIC. 2. Experimental measurement In this work, AIC was prepared by replacing the volume of sand in concrete with 0e60% (vol%) of silica aerogel particles. In order to test its hygrothermal properties, samples of different sizes were prepared according to the requirements of the test device. The pore size distribution and porosity were determined to analyze the differences in microstructures. Thermal conductivity at different temperatures and humidity was determined, and used to study the effect of temperature and humidity on the thermal conductivity of AIC. The water absorption coefficient was determined in the water absorption experiments, which was used to reflect the effect of capillary action on the water absorption rate. Meanwhile, the sorption isotherms were obtained experimentally, and used to analyze the dependence of amount of adsorbed water on temperature and humidity.

3

2.1. Preparation of test samples Gao et al. used aerogel to replace sand in concrete with 60% volume to prepare aerogel concrete with compressive strength of 8.3 MPa and thermal conductivity of 0.26 W/m･K. In order to make the aerogel concrete has a reliable compressive strength, maintain the uniform mixing of the aerogel particles in the cement matrix without affecting its low thermal conductivity, in this paper, based on the research of Gao, successfully produced an AIC by replacing the volume of sand in concrete with various volumes of silica aerogel particles. Moreover, an appropriate amount of acrylamide was added and low alkalinity anti-sulfate cement was used. This contributes to the complete and uniform distribution of aerogel particles in the cement matrix without affecting the strength of aerogel concrete. The AIC samples were prepared by mixing sand, cement, silica aerogel particles, acrylamide, superplasticizer and water. Based upon the testing instrument's requirements, the samples had two sizes of 300 mm  300 mm and 100 mm  100 mm. The details about the sample preparation are reported in Table 1. In the nomenclature of samples (AIC0~AIC60), the numbers represented the corresponding amount of silica aerogel particles (0e60%). The cement was low alkalinity anti-sulfate cement (L$SAC42.5). The sand was a medium sand with the particle size of 0.35e0.5 mm and density of 2600 kg/m3. The silica aerogel particles were the product of the Cabot Co., Ltd., USA. (called P100), and had the particle size of 0.4e1.2 mm and the stacking density of 60e150 kg/m2. The preparation processes of the samples was shown in Fig. 1. First, the cement and sand were evenly mixed in the mixer. Then, the aerogel was added for the dry mix, which is conducive to maintaining the integrity of aerogel particles in the cement base [7,8]. During the stirring process, water and super-plasticizer were slowly added. Then, acrylamide was mixed into the water to evenly disperse the silica aerogel particles [46]. Finally, the uniformly mixed slurry was poured into the mold, placed in a room at the temperature of 20e25
C, and cured for 28 days. 2.2. Microstructure Based on uniform porous material system, heat transfer studies have demonstrated that density or porosity plays a key role in determining the apparent thermal conductivity [47]. For materials with larger porosity, the effect of moisture content on thermal conductivity is more obvious [19]. In order to analyze the influence of microstructure of the material on the hygrothermal properties, the porosity and the microstructure of AIC were obtained experimentally. 2.2.1. Scanning electron microscope As shown in Fig. 2, The area marked as “A” in Fig. 2 represents the aerogel particles. The natural section of the AIC was observed when the magnification was 100  , 1000 and 5000 using a scanned electron microscope (Quanta 200, with a maximum resolution of 3.5 nm and an effective magnification of 100000  ). The internal structure of the concrete with aerogel content at 60% and without aerogel were compared in Fig. 2(a). We can see that the hydrated product changes due to the addition of aerogel, which changes the internal structure of the concrete. Additionally, the aerogel can be seen evenly distributed in the cement base. The internal structure of concrete with an aerogel content of 10%e50% were compared in Fig. 2(b). It can be observed at 100 magnification that the pore size of the material becomes smaller with the increase in the content of aerogel, which may be due to the reason that some of the aerogel particles are crushed into powder during the solidification process and fill the larger pores. At 1000

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Table 1 Mixing ratio of AIC with different silica aerogel volumes (300 mm  300 mm  30 mm  1, and 100 mm  100 mm  30 mm  2). Sample

AIC0 AIC10 AIC20 AIC30 AIC40 AIC50 AIC60

Cement

Sand

Water

DK-R3

Acrylamide

Aerogel

Aerogel fraction

Sand

Density

Porosity

kg

kg

kg

kg

kg

kg

vol%

wt.%

vol%

kg/m3

%

1 1 1 1 1 1 1

7 6.3 5.6 4.9 4.2 3.5 2.8

0.7 0.7 0.7 0.7 0.7 0.7 0.7

0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.015 0.015 0.015 0.015 0.015 0.015 0.015

0 0.02827 0.05654 0.08481 0.11308 0.14135 0.16962

0 10% 20% 30% 40% 50% 60%

0.00% 0.35% 0.77% 1.26% 1.87% 2.63% 3.61%

100% 90% 80% 70% 60% 50% 40%

1825.739 1689.981 1766.636 1678.872 1537.956 1352.101 1183.211

24.6616 29.3431 30.6183 32.1705 38.3047 43.3756 40.9234

Note: DK-R3 represents the model of the super-plasticizer.

Fig. 1. Preparation process of the sample.

magnification, we can see that most of the aerogel particles are still completely and evenly distributed in the cement matrix, which shows that our improvement has achieved good results. At 5000 magnification, it can be observed that the hydration products and internal structure of the concrete are similar, and the concrete becomes loose after the addition of aerogels, due to which, the density and thermal conductivity of AIC are reduced. Furthermore, the water absorption coefficient may be reduced due to the hydrophobicity of the aerogel.

2.2.2. Mercury porosimeter The total porosity of AIC0~AIC60, measured in the experiments, is presented in Table 1. The results showed that, with the increase in the content of aerogel, the porosity of AIC gradually increased. Compared with 0% and 50% aerogel contents, the porosity of AIC50 increased by 75.89%. However, when the aerogel content was 60%, the porosity of AIC60 was 5.65% lower than that of AIC50. This may be due to the fact that the aerogel content of AIC60 is too high, which resulted in the filling of some of the larger pores in the cement matrix with aerogel aggregates, and thus, the porosity was reduced. Fig. 3(a) shows the specific division of the concrete aperture, which represents the pore volume of a certain aperture and its percentage. Due to the limitation of instruments, when the pore size is greater than 6  105 nm, further subdivision is impossible. The results showed that the pore size distribution of ACI0~ACI60 exhibit similar trends, and the peak number of pores appears at 600e1000 nm. In addition, with the increase in the content of aerogel, the proportion of pore size in 200e1000 nm gradually increased, while the proportion of other pore diameter ranges gradually decreased. This means that the addition of aerogel changes the pore size. Furthermore, small and large pores gradually move towards the middle pores. The pore structure of the concrete could be divided into harmless pores (<20 nm), less harmful pores (20e200 nm) and harmful pores (>200 nm), which represented impermeability, less

permeability and high permeability, respectively [48]. Fig. 3(b) shows the division of the concrete aperture range, which represents the total volume of the holes and their percentages within a certain range. When the content of aerogel is 0e40%, the number of AIC pore size at 0e1000 nm gradually increases. However, when the content of aerogel is 40e60%, the number of AIC pore size at 20e200 nm gradually increases. Additionally, the number of AIC pore size at 200e1000 nm gradually decreases. This means that, as the aerogel content increases, the permeability of the AIC first increases and then, decreases.

2.3. Thermal conductivity test under different temperature conditions Temperature is one of the most important factors affecting the thermal conductivity. Particularly, for concrete made of siliceous aggregates, its thermal conductivity increases with the increase in ambient temperature, peaks at about 50e60
C, and then, decreases with further increase in temperature [20]. This is due to the reason that the specific heat of light-weight concrete changes with temperature in dry state, and thus, affects the thermal conductivity of the material [20]. In order to study the effect of temperature on the thermal conductivity of concrete with different aerogel contents, the thermal conductivity of AIC with the temperature range of 20e90
C was studied experimentally. As shown in Fig. 4(a), testing the thermal conductivity of materials at different temperatures requires constructing an external environment with a constant temperature. Therefore, the steadystate test is selected. The test device is a steady-state thermal conductivity tester GHP456 produced by NETZSCH Co., Ltd., Germany. The measurement range of the device was 0.01e2 W/m$K, while the measurement accuracy was ±0.1%. The size of the sample to be tested was 300 mm  300 mm  30 mm. Fig. 4(b) shows the schematic of the device. Two dry test pieces were placed on the hot plate and the lower cold plate. The instrument was closed to sandwich the test piece between the cold and

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5

Fig. 2. Scanning electron micrograph at 100  , 1000 and 5000 magnifications. (a) AIC0 and AIC60; (b) AIC10 to AIC50.

hot plates. After the desired temperature was set, the instrument adjusted the temperature of cold and hot plates using a water bath. The heat was transferred from hot plate to upper and lower cold plates. The thermal conductivity of the material was calculated using the “Fourier heat conduction law”, which is represented by Equation (1).

lðlt Þ ¼

f$d AðT1  T2 Þ

(1)

where l is the thermal conductivity or apparent thermal conductivity ðlt Þ of the material (in W/m$K), f is the average heating power of the metering part of the heating unit (in W), d is the average thickness of the test piece (in m), A is metering area (in m2), and T1 and T2 are the average temperatures of the hot and cold sides of the test piece, respectively (in K). 2.4. Thermal conductivity test under different humidity conditions The thermal conductivity of porous materials is obtained by converting all types of heat transfer (including heat transfer in solid, liquid and gas) into heat conduction of the materials [49e51]. Therefore, the thermal conductivity of porous media is closely

related to its internal moisture content. In order to study the thermal conductivity of AIC with the change in humidity and moisture conditions, the thermal conductivities of AIC at 35
C with the relative humidity of 0%, 30%, 50%, 70%, 85% and 100% were determined. In order to reflect the changes in hygroscopic properties of AIC with the content of aerogel, the sorption isotherms of AIC at 35
C were obtained. 2.4.1. Thermal conductivity test under different humidity conditions When measuring the thermal conductivity under different humidity conditions, the material was placed in a constant temperature and humidity chamber. This was done to achieve a heatmoisture balance in an environment having stable relative humidity. The instrument with the model number of KMF115 was produced by Binder Co., Ltd., Germany. The temperature was controlled within the range of 10e100
C, while the humidity could be adjusted within the range of 0e100%. The accuracy was ±2% for temperature and ±3% for relative humidity (RH). As shown in Fig. 5, using the transient thermal conductivity tester (TC3000) produced by Xi'an Xiaxi Co., Ltd., China, the sensor was inserted in a constant temperature and humidity chamber to measure the thermal conductivity of the material. The instrument can quickly test the thermal conductivity of materials [52,53] based

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Incremental Intrusion [mL/g]

0.024

AIC 0 AIC 10 AIC 20 AIC 30 AIC 40 AIC 50 AIC 60

0.020 0.016 0.012 0.008 0.004 0.000 8

Percentage increment [%]

7

Percentage VD/VP [%]

(a)

AIC 0 AIC 10 AIC 20 AIC 30 AIC 40 AIC 50 AIC 60

6 5 4 3 2 1 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

logD[nm] 65 60 55 50 45 40 35 30 25 20 15 10 5 0 0.00

AIC 0 AIC 10 AIC 20 AIC 30 AIC 40 AIC 50 AIC 60

D[nm]

0~20

20~50

50~100

100~200

200~1000

1000~60000

Specific volume[mL/g]

0.02 0.04 0.06 0.08 0.10 0.12

(b) 0.14

AIC 0 AIC 10 AIC 20 AIC 30 AIC 40 AIC 50 AIC 60

Fig. 3. (a) Relative increments in the internal pore volumes of AIC0~AIC60 and the percentage of total pores based upon these increments; (b) Relative volume and volume percentage of total pores in different pore sizes.

upon the transient plane source principle. The test range was 0.005e10 W/m$K with the accuracy of ±3%. The size of the specimen was 100 mm  100 mm  30 mm. The main process of the experiment is as follows. (1) Two samples of AIC were placed in a blast drying oven, and dried at 105
C to constant weight. The process was stopped until the mass change measured by three consecutive intervals of 12 h did not exceed 1%. Finally, the dried sample was covered using a plastic wrap and cooled to room temperature. At this point, the initial quality was recorded. (2) The material was placed in a constant temperature and humidity chamber and the corresponding temperature and

humidity were set. The change in the quality of the material was monitored after 7 days under each working condition. The observations were made every 24 h until the change in mass for three consecutive observations was less than 0.1%, which was considered as the equilibrium point. (3) The thermal conductivity of the material in equilibrium state was measured using the transient thermal conductivity tester. Ten sets of each material were measured, and the average value was taken as the thermal conductivity of the material under the set working condition.

Y. Wang et al. / Energy 188 (2019) 115999

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Fig. 4. Test device for determining the thermal conductivity at different temperatures (GHP456 of NETZSCH). (a) Test device diagram; (b) Schematic of the test device.

Fig. 5. Test device for determining the thermal conductivity under different humidity conditions.

2.4.2. Isothermal hygroscopic tests The isothermal hygroscopic test is used to study the change in the moisture content of the material with relative humidity at a certain temperature. The change in moisture content of the material during the equilibration process was obtained according to the mass change of the sample recorded in experiments described in Section 2.4.1. The quality of the sample was recorded, and the moisture content of the specimen under the working condition was calculated according to Equation (2).

M¼

m  m0 m0

2.5. Water absorption experiments The water absorption coefficient of the material can be used to evaluate the effect of capillarity action on the rate of water absorption of the building material. As shown in Fig. 6, this study analyzed the water absorption coefficient of AIC with different aerogel contents using a self-built experimental device. According to the ASTM-C1794-2015 standard [28], the calculation of water absorption coefficient is divided into parts (see Equations (3) and (4)). If liquid water appears on the surface of the sample within 24 h, it is calculated using Equation (3). If liquid water does not appear on the surface of the sample within 24 h, it is calculated using Equation (4).

(2)

where M represents the moisture content (in kg/kg), m represents the mass of the sample at equilibrium (in kg) and m0 represents the initial mass of the sample (in kg).

Dm;tf  Dm0 0

Aw ¼

pffiffiffiffi tf

(3)

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Y. Wang et al. / Energy 188 (2019) 115999

Fig. 6. Test device for water absorption coefficient of the material.

Dm;tf Aw;24 ¼ pffiffiffiffiffiffi 24

(4)

where, Aw and Aw;24 represent the hourly water absorption coefficients calculated using the two methods, respectively pffiffiffi (in kg =ðm2 $ hÞ), tf is the duration of the water absorption test (in h), and Dm;tf represents the difference in the quality between the 2 unit area specimen at time tf and the starting time (inp kg/m ). The ffiffi experiment results in a plot of Dmt as a function of t , whereas 0 Dm0 is the Y-intercept of the plot. According to the ASTM-C1794-2015 standard [28], the water absorption area of the sample is not less than 100 cm2. In this experiment, two samples with the sizes of 100 mm  100 mm  30 mm were taken. The samples were dried, wrapped in plastic wrap and placed in a room for natural cooling. After the samples were cooled to room temperature, the plastic wrap was removed and the side of the samples was sealed with melted paraffin. The sample was placed on the iron frame, and deionized water was injected into the container up till the mark of the sealing tape. This way, it was ensured that the water surface did not exceed 5 ± 2 mm of the bottom surface of the sample. During the experiments, the relative humidity of the indoor environment was maintained at 50 ± 5% RH, whereas the quality of the material was determined periodically. The observations were made at 1 h, 2 h, and 4 h, which were the necessary measurement points until liquid water appeared on the surface of the sample. Alternatively, it was observed that the mass increase of the sample after 8 h interval did not exceed 1 g/m2 and the experiment was considered to have completed.

3. Results and discussion 3.1. Effect of aerogel content on thermal conductivity Since aerogels have low density and low thermal conductivity [12], the most intuitive performance after blending them with concrete is the change in concrete's density. In order to characterize

the effect of the content of aerogel on the density of concrete, the density of AIC after reaching the heat-moisture equilibrium at different humidity levels was determined. The corresponding results are shown in Fig. 7(a). Two samples were tested for each concrete, and the measured standard deviation of density was found to be lying within the range of 0.38e1.1%, which proved the reliability of the results. As shown in Fig. 7(b), with the increase in the content of aerogel, the density of concrete decreased, which is similar to the results reported in some previous works [8, 9, 28]. The reason why the density at the aerogel content of 20% is higher than 10% is due to the volume deviation of the mold for producing the AIC10 test piece. During the manufacturing process, due to the requirements of the thermal conductivity test, we focused on ensuring that the bottom surface of the specimen remained flat. As can be seen from Fig. 8, such an abnormality was not found in the change of thermal conductivity, which means that the volume measurement of the AIC10 sample is larger than the actual volume, resulting in a smaller calculation value of the density here. However, its hygrothermal parameters are not affected, so we neglect this reduction point in the study of density. The difference in concrete's density caused by the content of aerogel decreases with the increase in relative humidity. When RH ¼ 100%, the density difference is the smallest (31.2%), while when RH ¼ 0%, the density difference is the largest (35.19%). Conversely speaking, the difference of concrete density caused by different humidity levels increases with the increase in the content of aerogel. The difference in density is minimal (3.01%) in the absence of aerogel, whereas the difference in density is the greatest (9.36%) when the aerogel content is 60%. Compared with Nosrati and Berardi's research [13,15], the aerogel was directly added to the plaster and the volume of the aerogel was up to 90%, which may reduced the strength of the plaster. In the study of Gao [8], the 60% volume of sand was replaced by aerogel in the concrete, which gave sufficient strength (8.3 MPa) with a thermal conductivity of 0.26 W/m$K. Therefore, in this paper, AIC was made on the basis of Gao. In order to improve the uniformity of aerogel particles and cement mixing, an appropriate amount of acrylamide was added and low alkalinity anti-sulfate cement was used. When the humidity was 0%, the thermal conductivity of AIC60 was lowest at 0.18 W/m$K and was 79.3% lower than that without aerogel (See Fig. 12), which was better than the 75% reduction in Kim's [7] study. In addition, it can be observed from Fig. 2 that aerogel particles were evenly distributed in cement base regardless of the content of aerogel. Fig. 8 shows the concrete's thermal coefficient as a function of aerogel content at different humidities. It can be seen that, no matter what the humidity is, the thermal conductivity decreases significantly with the increase in the content of aerogel and presents a relationship to which a quadratic function could be fitted. A previous study [8] has reported similar results under dry state. However, due to the lack of data, the fitting of the results was not carried out. However, with the increase in humidity, the rate of change of AIC's thermal conductivity, caused by different contents of aerogel, showed a decreasing trend. The thermal conductivity decreases by 79.3% when RH ¼ 0%, whereas the thermal conductivity decreases by 65.25% when RH ¼ 85%. Similarly, as shown in the table of Fig. 8 with the increase in humidity, the ratio of the thermal conductivity in the humid state to that in dry state (relative thermal conductivity: lRH =ld ) increased. The peak of the change in relative thermal conductivity occurred at RH ¼ 85%. However, when the humidity was increased to 100%, the change in thermal conductivity was unusual. As the aerogel content increases, the rate of change of AIC's thermal conductivity increases to 69.5%, whereas that of AIC's relative thermal conductivity

Y. Wang et al. / Energy 188 (2019) 115999

9

2400 RH=0% RH=30% RH=50% RH=70% RH=85% RH=100%

2200 Error rate

2000 0.49% ~ 1.03% 1800

Error rate Error rate 0.53% ~ 1.03% Error rate 0.49% ~ 1.06% 0.50% ~ 1.03% Error rate

0.49% ~ 1.03% Error rate 0.48% ~ 1.03% Error rate

3

Density [kg/m ]

1600 1400

0.38% ~ 1.07%

1200 1000 800 600 400 200 0

0% (a)

40% 20% 50% 30% 10% Volume fraction of aerogel incorporated

60%

1900

RH=0% RH=30% RH=50% RH=70% RH=85% RH=100%

1800

1600 1500

AIC60 Highest change:9.36%

AIC 0 Lowest change:3.01%

3

Density [kg/m ]

1700

1400

RH=100%, Lowest rate of change:31.2% 1300

RH=0%, Highest rate of change:35.19%

1200 0%

10%

(b)

20%

30%

40%

50%

60%

Aerogel volume percentage

Fig. 7. (a) Density of concrete with different aerogel volumes; (b) Variation in the density of AIC with humidity.

decreases to 47.54%. This may be because the liquid water content increases at RH ¼ 100% and the influence of humidity on the thermal conductivity is higher at low aerogel content due to the hydrophobicity of the aerogel [12].

3.2. Thermal conductivity as a function of temperature The table in Fig. 9 shows the thermal conductivity of AIC at different temperatures. It can be seen that the thermal conductivity of AIC increases with the increase in temperature. Among all the samples, the change rate of AIC10 was lowest (9.41%), whereas that of AIC30 was the highest (15.5%). The difference in concrete's

thermal conductivity caused by the content of aerogel first decreases, and then, increases with the increase in ambient temperature. Among them, the different of AIC's thermal conductivity is the highest at 20
C (72.61%), whereas that is the lowest at 60
C (72.24%). However, the variation does not follow any particular mathematical function. In the study of Nguyen et al. [20], the thermal conductivity of lightweight concrete under the condition of 5e50
C was tested. The results show that the thermal conductivity increases nonlinearly with temperature due to changes in specific heat with temperature, and previous studies have shown that the thermal conductivity decreases at 50e60
C. Moreover, Berardi and Nosrati

10

Y. Wang et al. / Energy 188 (2019) 115999

Thermal conductivity [W/m K]

1.1

Relative thermal conductivity AIC20 AIC30 AIC40

Relative humidity

AIC0

AIC10

AIC50

AIC60

growth rate

30%

1.00533

1.02213

1.03841

0.96264

1.10287

0.98984

1.32267

31.57%

50%

0.99262

1.06193

1.00827

0.97556

1.22496

1.12491

1.48945

50.05%

70%

1.00007

1.0419

0.98622

0.97804

1.17112

1.15779

1.59658

59.65%

85%

1.00797

1.10601

1.06883

0.9834

1.32176

1.22128

1.69177

67.84%

100%

1.19514

1.23458

1.19534

1.17506

1.45071

1.4266

1.76326

47.54%

1.1

1.0

1.0

0.9

0.9

0.8

0.8

0.7 0.6 0.5 0.4 0.3 0.2 0.1

0.7

RH = 0% RH = 30% RH = 50% RH = 70% RH = 85% RH = 100%

-69.5% -79.3%

2

2

y=-0.95x -0.58x+0.866.r =0.95 2

0.5 0.4

2

y=-0.7x -0.68x+0.881.r =0.97 2 2 y=-0.73x -0.59x+0.874.r =0.99 2

2

0.3

2

2

0.2

y=-0.45x -0.72x+0.877.r =0.98 2 2 y=-0.75x -0.55x+0.895.r =0.99 y=-0.83x -0.70x+1.038.r =0.99

0%

0.6

10%

20%

30%

40%

50%

60%

0.1 70%

Volume fraction of aerogel incorporated Fig. 8. Thermal conductivity of concrete at different humidities as a function of the volume of aerogel. (The growth rate in the table represents the increase in relative thermal conductivity of AIC60 relative to AIC0.)

et al. studied the thermal conductivities of aerogel composites [13,14], and linearly fitted the changes of thermal conductivities of the materials under the condition of 10e50
C, and the results were relatively reliable. However, the thermal conductivity of aerogel concrete at higher temperature needs to be further researched. Although the incorporation of a large amount of aerogel will reduce the density of the concrete, the volume of large pores may be reduced. Therefore, as the temperature increases, whether its thermal conductivity will decrease like porous materials is the focus of this paper. Fig. 9 shows the results for the ratio of thermal conductivity at different temperatures to 20
C (relative thermal conductivitylT =l20 ). It can be seen that the thermal conductivity of AIC increases with the increase in temperature. On the one hand, this is due to the change in material's specific heat. Previous studies [20] have shown that, when the temperature increases to 50
C, the specific heat of light-weight concrete increases. On the other hand, the factor that may affect the increase in thermal conductivity is the presence of trapped water in the pores [20]. As the temperature increases, the water vapors inside the material will migrate, which also leads to an increase in the thermal conductivity of the material.

However, this effect will be greatly reduced for the dry material. However, further comparison shows that the thermal conductivity increased irregularly with temperature for different contents of aerogel. With the increase in temperature within the range of 20e30
C, the thermal conductivity of AIC60 showed the highest growth rate, whereas AIC10 was the slowest within the temperature range of 30e50
C. The fastest rate of increase was for AIC30, whereas AIC0 showed the slowest rate. This may be due to the difference in the distribution of aerogel in the cement matrix, because different processes and aerogel particle sizes will lead to changes in the uniformity of aerogel distribution. Therefore, the rate of change of concrete's thermal conductivity with temperature based upon different contents of aerogel needs to be further studied. 3.3. Effect of moisture content on thermal conductivity 3.3.1. Moisture balance process of materials under different relative humidity At 35
C, the change in moisture content of AIC with time was determined experimentally. Under each working condition, the

Y. Wang et al. / Energy 188 (2019) 115999

1.15

Temperature [ ]

AIC0

Thermal conductivity at different temperatures [W/mK] AIC10 AIC20 AIC30 AIC40

AIC50

AIC60

growth rate

20

0.8551

0.74039

0.70541

0.6576

0.39759

0.32867

0.23421

72.61%

30

0.86716

0.74919

0.71619

0.67048

0.40646

0.33478

0.23992

72.33%

40

0.87518

0.75801

0.72709

0.6828

0.41065

0.33992

0.24246

72.30%

50

0.88369

0.76768

0.73842

0.69347

0.41518

0.34406

0.24529

72.24%

60

0.9027

0.78012

0.76059

0.71423

0.42352

0.35203

0.25059

72.24%

70

0.9176

0.79114

0.77488

0.72901

0.42466

0.35595

0.25416

72.30%

80

0.92899

0.80176

0.79243

0.74155

0.43536

0.35893

0.25673

72.37%

90

0.93899

0.81005

0.80178

0.75952

0.43841

0.36579

0.25981

72.33%

AIC 0 AIC 10 AIC 20 AIC 30 AIC 40 AIC 50 AIC 60

Maximum difference rate:5.27%

The fastest change:AIC 30

Maximum difference rate:3.97%

Maximum difference rate:3.65% Maximum difference rate:2.99%

20

1.10

11

T

/

Maximum difference rate:2%

1.05

Maximum difference rate:1.43% Maximum difference rate:1.22%

1.00 20

30

The slowest change:AIC 10

40

50

60

70

80

90

T [°C] Fig. 9. The thermal conductivities of concrete and relative changes at different temperatures. (The growth rate in the table represents the increase in thermal conductivity of AIC0 relative to AIC60.)

sample gradually reached equilibrium after 7 days. Therefore, the quality of the sample was monitored starting from the seventh day, and the equilibrium can be reached in 10e12 days under low relative humidity conditions. However, under high relative humidity conditions, the equilibrium was reached in about 18e20 days. The variation in moisture content with time is shown in Fig. 10. The results show that, at low relative humidity, the mass of AIC changes by around 0.77% after 7 days, whereas it changes by around 0.01% after 12 days. At high relative humidity, the rate of change in mass after 18 days is about 0.03%. Comparing the results presented in Fig. 10 (a) and (b), it can be seen that, as the aerogel content increases, the equilibrium time increases slightly at high relative humidity, which may be due to the difference in their porous structure, which resulted in different resistances for water in the process of infiltration [56,57]. Therefore, the equilibrium time for AIC60 is longer than that for AIC0 at high relative humidity. The equilibrium time for AIC0 is 18 days, whereas the final equilibrium time for AIC60 is 21 days. 3.3.2. Thermal conductivity under different moisture contents Due to the storage, evaporation and migration of liquid water in the pores of concrete, the heat transfer process will become threephase (solid, liquid, and gas) heat conduction. Previous studies have shown that the thermal conductivity of concrete increases with the increase in water content [21e24], especially for the light-weight concrete. The thermal conductivity of the material increases linearly with the increase in the content of moisture [54,55]. In order to study the variation of thermal conductivity of AIC with moisture

content, the thermal conductivity of AIC under different moisture contents was obtained experimentally. As shown in Fig. 11, although the thermal conductivity of the AIC is positively correlated with the moisture content, the change rate of the thermal conductivity is much smaller with the increases of aerogel content. This result is different from that observed by Berardi et al. [13], where the thermal conductivity of aerogelenhanced plasters presents a primary function with moisture content, or that studied by Liu et al. [19], where the thermal conductivity of lightweight concrete presents a power function with moisture content. However, the rate of increase in thermal conductivity drops down with the increase in the content of aerogel. Moreover, the rate of decrease in thermal conductivity increases with the increase in the content of aerogel, which indicates that different of thermal conductivity caused by moisture content decrease with the increase in the content of aerogel. Fig. 12 shows the variation in moisture content and thermal conductivity of concrete with different aerogel volumes at various humidity levels. The results show that the moisture content has exhibits three distinct stages during its growth. These three stages are the medium-speed growth, slow-speed growth and high-speed growth, which is similar to the variation in water absorption of the material. However, the difference in thermal conductivity of concrete for different aerogel contents gradually reduces (except for 100% RH), and this different is increased at RH ¼ 100%. Because the moist air is easily converted into liquid water at 100% relative humidity, higher porosity and water absorption rapidly increase the moisture content of the material, which partially offsets the action

12

Y. Wang et al. / Energy 188 (2019) 115999

0.050 0.045

AIC 0 at 30% RH AIC 0 at 50% RH AIC 0 at 70% RH AIC 0 at 85% RH AIC 0 at 100% RH AIC10 at 30% RH AIC10 at 50% RH AIC10 at 70% RH AIC10 at 85% RH AIC10 at 100% RH AIC20 at 30% RH AIC20 at 50% RH AIC20 at 70% RH AIC20 at 85% RH AIC20 at 100% RH AIC30 at 30% RH AIC30 at 50% RH AIC30 at 70% RH AIC30 at 85% RH AIC30 at 100% RH

Balance point: day 20

Mass moisture content[kg/kg]

0.040 0.035

Balance point: day 18

0.030 0.025

Balance point: day 12

0.020 0.015 0.010 Balance point: day 10

0.005 0.000

0

2

4

6

8

(a)

10

12

14

16

18

20

22

Time[d]

0.08 0.07

Mass moisture content[kg/kg]

AIC40 at 30% RH AIC40 at 50% RH AIC40 at 70% RH AIC40 at 85% RH AIC40 at 100% RH AIC50 at 30% RH AIC50 at 50% RH AIC50 at 70% RH AIC50 at 85% RH AIC50 at 100% RH AIC60 at 30% RH AIC60 at 50% RH AIC60 at 70% RH AIC60 at 85% RH AIC60 at 100% RH

Balance point: day 21

0.06

Balance point: day 18

0.05

Balance point: day 21

0.04 Balance point: day 12

0.03 0.02 0.01 0.00

Balance point: day 10

0

(b)

2

4

6

8

10

12

14

16

18

20

22

24

26

Time[d]

Fig. 10. Equilibrium process of concrete with different volumes of aerogel at different humidity levels. (a) AIC0, AIC10, AIC20 and AIC30; (b) AIC40, AIC50 and AIC60.

of the hydrophobicity of aerogel particles, due to which, the rate of change in thermal conductivity increases.

3.3.3. Thermal conductivity as a function of humidity Fig. 13(a) shows the thermal conductivity of concrete with different volumes of aerogel as a function of relative humidity. The relative humidity affects the thermal conductivity by affecting the moisture content inside the material [19,20,58], and the thermal conductivity is a function of density and ambient relative humidity [59]. Liu and Ma [19] studied the relationship between moisture content and thermal conductivity. The paper indicates that the thermal conductivity changes linearly or with a power function with moisture content. Nguyen et al. [20] studied the thermal conductivity of several lightweight concretes as a function of

linearity with moisture content. Berardi et al. [13] reported a linear function of the thermal conductivity and moisture content of aerogel-enhanced plasters. In these studies, the wet state of the sample was mostly obtained by immersion or placed in a selfcontained constant humidity device. Therefore, the moisture content of the sample was mostly in the state of liquid water or gasliquid coexistence, and the precision was low. However, in this paper, different aerogel content concretes were placed in a precision instrument with constant temperature and humidity to achieve a moisture balance state, and the instrument's accuracy for temperature and humidity controlled was within 3%. Furthermore, the change of thermal conductivity with relative humidity was obtained, and the results can be fitted as cubic polynomial function.”

Y. Wang et al. / Energy 188 (2019) 115999 1.2 1.1

Thermal conductivity [W/m K]

1.0 0.9 0.8 0.7 0.6

1.2

AIC 0 AIC 10 AIC 20 AIC 30 AIC 40 AIC 50 AIC 60

1.1 -11.88%

1.0

-8.08%

0% aerogel

0.9

-9.52%

0.8

10% aerogel

-33.68%

20% aerogel

0.6

-23.68%

0.5 0.4 0.3 0.2

0.7

30% aerogel

0.5

-49.52%

40% aerogel

0.4

50% aerogel

0.3 0.2

60% aerogel

0.1 -0.02 -0.01

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.1 0.09

0.08

Mass moisture content [kg/kg] Fig. 11. Variation in thermal conductivity of AIC as a function of moisture content at 35
C and at 0%, 30%, 50%, 70%, 85%, and 100% RH.

13

humidity is increased to 100%, the content of liquid water inside the material is greatly increased, and the thermal conductivity increases rapidly. Since the water vapor adsorbed by the material are more easily condensed into liquid water at high relative humidity, the hygroscopic state can be reflected by the adsorption curve [13,60]. Fig. 13(b) shows the relative thermal conductivity (lRH =ld ) of AIC with the change in relative humidity. The relative thermal conductivity indicates the ratio of the thermal conductivity of the concrete at a certain humidity to the dry state. It can be seen from Fig. 13(b) that the relative thermal conductivity of AIC increases with the increase in relative humidity, the change in relative thermal conductivity was the highest (76.33%) when the content of aerogel was 60%. When the content of aerogel was 30%, the change in relative thermal conductivity was the lowest (17.51%). In the dry state, the growth rate of thermal conductivity was the highest (383.01%) with the decrease of aerogel content, whereas that was the smallest (187.78%) under RH ¼ 85%.

3.4. Water absorption coefficient

0.04 0.03

% : 15 ge an ch ge

0.05

era

0.06

7.2

AIC 0 AIC 10 AIC 20 AIC 30 AIC 40 AIC 50 AIC 60

0.07

Av

Mass moisture content [kg/kg]

0.08

0.02 0.01

Averag

e chan

% ge:92.5

e ch Averag

8% ange:7

0.00

RH

Thermal conductivity [W/m K]

0.0

0%

30%

50%

70%

85%

-11.8% -79.3%

-72.8%

-68.9% -11.8%

-66.9% -11.8%

-65.3%

100%

0.2 0.4 0.6 0.8 1.0 1.2

Fig. 14 shows the changes in the moisture absorption (DMt ) per unit surface area of AIC over time. According to the ASTM-C17942015 standard [28], AIC0 and AIC10 reached osmotic equilibrium within 4 h and liquid water appeared on the surface of the sample, which indicates that the material's permeability was too high to have capillary absorption. Under these conditions, the water absorption coefficient could not be obtained. AIC20 and AIC50 reached equilibrium at 4 h. AIC30, AIC40 and AIC60 reached equilibrium at 8 h. The change rate of moisture absorption (DMt ) of the samples slowly decrease with the change in time, and liquid water has transferred to the upper surface of the samples within 24 h. Under these conditions, the water absorption coefficients of AIC20, AIC50, AIC30, AIC40 and AIC60 could be calculated using Equation (3). The water absorption coefficient of AIC20 and AIC50 were calculated:

-69.5%

Fig. 12. Variation in moisture content and thermal conductivity of concrete with different aerogel volumes at various humidity levels (RH: Relative Humidity).

AwðAIC20Þ ¼ 2:637 kg

. pffiffi . pffiffi m2 s ; AwðAIC50Þ ¼ 2:923 kg m2 s

The water absorption coefficient of AIC30, AIC40 and AIC60 were calculated: Looking at Fig. 13(a), it can be observed that the thermal conductivity increases with the increase in relative humidity. The process can be divided into three stages. In the first stage, the thermal conductivity increases steadily, then the growth rate becomes gentle, while in the final stage, the thermal conductivity increases rapidly. Moreover, a polynomial function is fitted to the results. There are two reasons for this change. On the one hand, it can be seen from the moisture absorption results (Fig. 19) that, as the relative humidity increases, the moisture content changes slowly, whereas the thermal conductivity increases slowly. On the other hand, the trapped water in concrete may exist in a vaporliquid equilibrium [20]. In the first stage, part of the humid air enters the inside of the hole, resulting in an increase in the thermal conductivity. In the second stage, the moist air begins to fill the closed pores and creates a pressure between the pore walls and the moist air. The air layer between the moist air and the pore walls is equivalent to a layer of thermal resistance, which reduces the heat transfer efficiency and decreases the rate of increase of thermal conductivity. In the third stage, the pores gradually begin to coexist as vapor-liquid, which reduces the heat transfer resistance, and the thermal conductivity increases again. Especially when the relative

AwðAIC30Þ ¼ 1:79 kg

. pffiffi . pffiffi m2 s ; AwðAIC40Þ ¼ 2:194 kg m2 s ;

AwðAIC60Þ ¼ 1:852 kg

. pffiffi m2 s

According to the calculated results, when the aerogel content was 30e50%, the water absorption coefficient of AIC increased, and the growth rate was 63.8%. When the aerogel content was 60%, the water absorption coefficient of AIC decreased by 36.53%. This indicates that, as the aerogel content increases, the permeability of AIC first decreases, and then, increases. When the aerogel content was 0e10%, the water absorption coefficient could not be obtained due to excessive permeability. When the aerogel content was 60%, the porosity of AIC decreased and the hydrophobicity increased, resulting in a decrease in permeability again. This was due to the very high content of aerogel in the AIC. Further comparison with Figs. 14 and 13 shows an interesting phenomenon. The change in water absorption characteristics of aerogel concrete has a certain relationship with the thermal conductivity. Lakatos's research presented that both the heat flux (increasing) and the thermal resistance (decreasing) are changing

14

Y. Wang et al. / Energy 188 (2019) 115999

1.3

AIC 0 AIC 10 AIC 20 AIC 30 AIC 40 AIC 50 AIC 60

1.2

Thermal conductivity [W/m K]

1.1 1.0

3

2

y=1.187x -1.4x +0.377x+0.874. 3 2 y=0.721x -0.84x +0.296x+0.756. 3 2 y=1.026x -01.26x -0.0378x+0.725. 3 2 y=0.736x -0.77x +0.2141x+0.67. 3 2 y=0.462x -0.60x +0.326x+0.406. 3 2 y=0.15x -0.00x +0.04x+0.33. 3 2 y=0.05x -0.1x +0.23x+0.18.

2

r =0.85 2 r =0.91 2 r =0.97 2 r =0.85 2 r =0.86 2 r =0.86 2 r =0.99

0% aerogel 10% aerogel

0.9

20% aerogel

0.8

30% aerogel

0.7 0.6

40% aerogel

Smooth phase

0.5

50% aerogel

0.4 60% aerogel

0.3 0.2 0.1

(a)

0%

20%

40%

60%

80%

100%

Relative humidity 1.8

1.8 1.7 1.6 1.5

AIC 0 AIC 10 AIC 20 AIC 30 AIC 40 AIC 50 AIC 60

1.7 1.6 1.5

1.4

1.4

The fastest growth rate:76.33%

d

/ RH

120%

1.3

1.3

1.2

1.2

1.1

1.1

1.0 0.9 0%

(b)

The slowest growth rate:17.51% 20%

40%

60%

80%

100%

1.0 0.9 120%

Relative humidity

Fig. 13. (a) Thermal conductivity of concrete with different volumes of aerogel as a function of relative humidity; (b) Relative thermal conductivity of concrete with different volumes of aerogel changes with relative humidity.

with at least 20% if they contain at least 20% moisture [44]. However, an increase in aerogel will reduce the thermal conductivity of the material, but an increase in the water absorption coefficient caused by the aerogel will increase the thermal conductivity of the material in the wet state. In the study of this paper, the water absorption characteristics make the thermal conductivity increase lower than the aerogel's thermal conductivity reduction, because no matter what humidity, we have not observed thermal conductivity of concrete with high aerogel content was higher than that of concrete with low aerogel content.

3.5. Sorption isotherms Previous studies have shown that the permeability of concrete is determined by the porosity and connectivity of the internal pores, which are related to the passage of various transmission media [61]. Generally, higher water absorption reflects that the concrete has higher porosity and permeability [62]. The sorption isotherm is a key characteristic reflecting the water absorption performance of the material, and different sorption isotherms can be obtained at different temperatures. However, in the study of Lakatos [44], the sorption isotherms of materials at three different temperatures were tested. The results show that there is no strong correlation

Y. Wang et al. / Energy 188 (2019) 115999 6

5

Penetration time: 8 hours

ACM 0 ACM 10 ACM 20 ACM 30 ACM 40 ACM 50 ACM 60

Penetration time: 4 hours

2

Mt[kg/m ]

4

3

Penetration time: 2 hours 2

2

y=2.654x-0.784.r =0.99 2

y=2.20x-0.8. r =0.97 2 y=2.29x-1.2. r =0.98 2

y=2.958x-0.716.r =0.99 2 y=1.96x+0.01.r =0.97

1

0

0

2

4

6

8

10

12

14

T[ h ] Fig. 14. Water absorption process of AIC.

between water absorption and temperature with the increase of time. In the study of Berardi and Nosrati [13,14], the moisture content of aerogel composites was also tested under different humidity conditions, and similar fitting results were obtained for different materials. However, the sorption isotherm is the key parameter in the calculation of coupled heat and moisture transfer process of buildings, and different moisture content has obvious influence on the thermal conductivity. Therefore, in order to obtain the influence of different moisture content on the thermal conductivity of concrete with different aerogel content, the moisture content of AIC under different relative humidity at 35
C was tested, and the process of sorption isotherm was analyzed. As shown in Fig. 15, the sorption isotherms of concrete with different aerogel contents were obtained experimentally. The results show that the moisture content of AIC0 to AIC60 shows similar variation trend with humidity. The process can be divided into three steps. With the increase in relative humidity, the moisture content first increases at a slow pace, then it increases at a medium pace, and finally, the rate of increase is high. This is because as the water absorption increases, liquid water is gradually formed in the internal pores of the material [63], due to which, a condensationevaporation dynamic equilibrium process [64,65] occurs inside the material. In the first step, the relative humidity of the environment is low. As the wet air penetrates, the internal face of the

Mass moisture content[kg/kg]

0.07 0.06 0.05

ACM ACM10 ACM20 ACM30 ACM40 ACM50 ACM60

Growth rate:143.2%

Stage 3 1 Stage

0.04 0.03 0.02

Aerogel volume content: 0% to 60%

0.08

Stage Stage 1 2 Stage Stage 1 1

0.01 0.00 0%

20%

40%

60%

80%

Relative humidity Fig. 15. Isothermal adsorption curve.

100%

120%

15

pores in the material gradually forms a water molecule adsorption layer, resulting in an increase in moisture content; in the second step, as the relative humidity of the environment increases, the partial pressure of water vapor is increased, the pores of the material are saturated with the humid air, resulting in a slightly slow in the growth of the moisture content. In the third stage, when the humidity gradually reaches 100%, the humid air in the pores of the material is easily converted into liquid water, and the adsorption layer of water molecules on the internal face of the pores begins to condense, resulting in a rapid increase in moisture content. When the relative humidity changes from 0% to 100%, the rate of change in moisture content of AIC0 was the smallest (90.7%), while that of AIC60 was the largest (93.4%). The results show that the influence of aerogel content on the material becomes more and more obvious with the increase in humidity. At 50% RH, the moisture content of AIC increased by 98.82% with the increase in the content of aerogel. At 70% RH, the increase in the moisture content of AIC was around 105.24%, while the corresponding value was 157.2% for 100% RH. In the first step, the moisture content of AIC varies from 0 to 0.006 kg/kg, while in the second and third steps, the variation is from 0.005 to 0.014 kg/ kg and from 0.012 to 0.077 kg/kg, respectively. 4. Conclusions In this study, by replacing the volume of sand in concrete with 10%, 20%, 30%, 40%, 50% and 60% volume proportions of silica aerogel particles, aerogel-incorporated concrete (AIC) with different contents of aerogel was successfully prepared and its hygrothermal characteristics were studied. The work included the study of pore distribution characteristics of AIC, changes in thermal conductivity of AIC with temperature (within the range of 20e90
C) in dry state, influence of humidity on the thermal conductivity of AIC (at 35
C, and within the relative humidity range of 0e100%), water absorption coefficient and the sorption isotherms. Based upon the results, following conclusions are drawn. C With the increase in aerogel's content, the thermal conductivity of AIC decreased significantly, and the thermal conductivity changed with the content of aerogel following a quadratic function. In the dry state at 35
C, the thermal conductivity increased by 79.3% when the aerogel content was increased from 0% to 60%. C With the increase in relative humidity, the difference in concrete's density caused by the change in the content of aerogel decreased. When RH ¼ 0%, the density difference was the largest at 35.19%. When RH ¼ 100%, the density difference was the smallest and had the value of 31.2%. Conversely speaking, as the aerogel content increased, the difference in concrete's density caused by the change in humidity increased. When the aerogel content was 0%, the density difference was the smallest at 3.01%. When the aerogel content was 60%, the difference in density was the largest and had the value of 9.36%. C Within the temperature range of 20e90
C, the thermal conductivity of AIC increases with the increase in temperature. With the increase in temperature, the thermal conductivity of AIC30 showed the highest increase of 15.5%. However, the difference in thermal conductivity caused by the change in aerogel's content decreased with the increase in temperature. When the aerogel content changes from 0% to 60%, the rate of drop of thermal conductivity is the highest at 20
C, and has the value of 72.61%. C The thermal conductivity of AIC is a cubic polynomial function of relative humidity, which is similar to the sorption

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Y. Wang et al. / Energy 188 (2019) 115999

isotherms of the material. The process can be divided into three steps. When the aerogel content changes from 0% to 60%, the ratio of decrease of thermal conductivity is the largest at 79.3% in dry state, whereas this value is the smallest at 65.25% for RH ¼ 85%. When the relative humidity changes from 0% to 100%, the ratio of change of thermal conductivity of AIC0 is the lowest at 19.51%, whereas that of AIC60 is the highest at 76.33%. C With the increase in aerogel content, the variation in AIC water absorption coefficient is positively correlated with the porosity of the material. When the aerogel content is 30e50%, the water absorption coefficient of AIC increases and the growth rate is 63.8%. When the aerogel content is 60%, the water absorption coefficient of AIC decreases by 36.53%. Comparing the aerogel content of 30% and 50%, the porosity of AIC increased by 34.83%. However, when the aerogel content is 60%, the porosity of AIC decreases by 5.65%. Acknowledgements This work was supported by the National Key R&D Program of China (Grant number 2016YFC0700400), the National Natural Science Foundation of China (Grant numbers 51878534, 51590911, and 51678468) and Shaanxi Provincial Association for Science and Technology Young Talents Promotion Program (20180414). References [1] Riffat SB, Qiu G. A review of state-of-the-art aerogel applications in buildings. Int J Low-Carbon Technol 2012;8:1e6. [2] Liang LH, Li BW. Size-dependent thermal conductivity of nanoscale semiconducting systems. Phys. Rev. B 2006;73:1e4. [3] Hrubesh LW. Aerogels: the world's lightest solids. Chem. Ind. 1990;24:824e7. [4] Cuce E, Cuce PM, Wood CJ, Riffat SB. Toward aerogel based thermal superinsulation in buildings: a comprehensive review. Renew Sustain Energy Rev 2014;34:273e99. [5] Ratke L. Herstellung und Eigenschaften eines neuen Leichtbetons: Aerogelbeton. Beton-und Stahlbetonbau. 2008;4:236e43. [6] Dai Yan-Jun, Tang Yu-Qing, Fang Wen-Zhen, Zhang Hu, Tao Wen-Quan. A theoretical model for the effective thermal conductivity of silica aerogel composites. Appl Therm Eng 2018;128:1634e45. [7] Kim Sughwan, Seo J, Cha J, Kim Sumin. Chemical retreating for gel-typed aerogel and insulation performance of cement containing aerogel. Constr Build Mater 2013;40:501e5. [8] Gao T, Jelle BP, Gustavsen A, Jacobsen S. Aerogel-incorporated concrete: an experimental study. Constr Build Mater 2014;52(2):130e6. [9] Welsch T, Held MS, Milow B. High performance aerogel concrete. Proc. 12th Conference on Advanced Building Skins, Bern 2017:591e9.  [10] Ng S, Jelle BP, Ingunn L, Sandberg C, Gao T, Wallevik OH. Experimental investigations of aerogel-incorporated ultra-high performance concrete. Constr Build Mater 2015;77:307e16. [11] Zhao HL, Ding YD, Wang F, Deng ZP. Thermal insulation material based on SiO2 aerogel. Constr Build Mater 2016;122:548e55. [12] Fickler S, Milow B, Ratke L, Schnellenbach-Held M, Welsch T. Development of high performance aerogel concrete. Energy Procedia 2015;78:406e11. [13] Nosrati RH, Berardi U. Hygrothermal characteristics of aerogel-enhanced insulating materials under different humidity and temperature conditions. Energy Build 2018;158:698e711. [14] Berardi U, Nosrati RH. Long-term thermal conductivity of aerogel-enhanced insulating materials under different laboratory aging conditions. Energy 2018;147:1188e202. [15] Berardi U. Aerogel-enhanced systems for building energy retrofits: insights from a case study. Energy Build 2018;159:370e81. [16] GB 50176-2016. Housing and urban-rural development of the People's Republic of China, Code for thermal design of civil building. Beijing: China Architecture & Building Press; 2016 [in Chinese]. [17] Dell'Isola M, d'Ambrosio FR, Alfano, Giovinco G, Ianniello E. Experimental analysis of thermal conductivity for building materials depending on moisture, content. Int J Thermophys 2012;33(8e9):1674e85. [18] Qiu L, Zou HY, Tang DW, Wen DS, Feng YH, Zhang XX. Inhomogeneity in pore size appreciably lowering thermal conductivity for porous thermal insulators. Appl Therm Eng 2018;130:1004e11. [19] Liu YF, Ma C, Wang DJ, Wang YY, Liu JP. Nonlinear effect of moisture content on effective thermal conductivity of building materials with different pore size distributions. Int J Thermophys 2016;37(6):56.

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