Development of a novel sulphoalumitate cement-based composite combing fine steel fibers and phase change materials for thermal energy storage

Accepted Manuscript

Development of a novel sulphoalumitate cement-based composite combing fine steel fibers and phase change materials for thermal energy storage Guochen Sang , Yanzhou Cao , Min Fan , Geyang Lu , Yiyun Zhu , Qin Zhao , Xiaoling Cui PII: DOI: Reference:

S0378-7788(18)31400-2 https://doi.org/10.1016/j.enbuild.2018.10.039 ENB 8869

To appear in:

Energy & Buildings

Received date: Revised date: Accepted date:

7 May 2018 16 September 2018 18 October 2018

Please cite this article as: Guochen Sang , Yanzhou Cao , Min Fan , Geyang Lu , Yiyun Zhu , Qin Zhao , Xiaoling Cui , Development of a novel sulphoalumitate cement-based composite combing fine steel fibers and phase change materials for thermal energy storage, Energy & Buildings (2018), doi: https://doi.org/10.1016/j.enbuild.2018.10.039

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HIGHLIGHTS A novel fine steel fiber reinforced cement-based thermal energy storage composite was developed.

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GM-SSPCM was fabricated by heating mixing of paraffin, low density polyethylene and flake graphite.

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Mechanical strength of STESC is improved with the reinforcement of steel fibers.

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The thermal conductivity of SF-STESC at different temperatures was measured and analyzed.

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Heat storage/release performances of STESCs are enhanced with fine steel fibers.

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Development of a novel sulphoalumitate cement-based composite combing fine steel fibers and phase change materials for thermal energy storage Guochen Sang *, Yanzhou Cao, Min Fan, Geyang Lu, Yiyun Zhu, Qin Zhao, Xiaoling Cui School of Civil Engineering and Architecture，Xi’an University of Technology, Xi’an 710048,China

ABSTRACT

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To increase the mechanical strength and thermal energy storage/release efficiency, fine steel fibers and graphite-modified shape stabilized phase change materials (GM-SSPCM) were added into sulphoaluminate cement mortar. Paraffin, low density polyethylene and flake graphite were heating mixed to produce GM-SSPCM. Fine steel fibers were used to reinforce sulphoaluminate cement-based thermal energy storage composite (STESC) for improving mechanical strength and thermal conductivity. The thermophysical and microstructure of GM-SSPCM, and the thermal and mechanical properties of steel fiber reinforced sulphoaluminate cement-based thermal energy storage composite (SF-STESC) were investigated. The results indicated that about 50% paraffin could be effectively encapsulated in GM-SSPCM with multi-level space network structure. And, the steel fiber can increase the mechanical and thermal properties of SF-STESC. When a 3.5 vol% steel fiber was added, the 28-day compressive strength and flexural strength of the SF-STESC were increased by 7.3% and 40.6%, also the compressive/flexural strength ratio was decreased by 21.6%. The three-dimensional reinforcement of steel fibers reduced the volume shrinkage of the composites. In addition, the thermal conductivity of SF-STESC increases with the increase in volume fraction of the steel fibers. When the steel fiber volume fraction increases from 0 to 3.5%, the thermal conductivity of SF-STESC is increased by 51.3% while the inner paraffin is in solid state and 84.5% while the inner paraffin is in liquid state. The results of thermal energy storage/release performance tested using a self-designed setup showed that the steel fiber reinforced STESC leads to a high thermal energy storage/release rate. Keywords: Thermal energy storage; Sulphoaluminate cement-based composite; Fine steel fiber; Mechanical and

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thermal properties; Thermal energy storage/release performance

Abbreviation:

Graphite-modified shape stabilized phase change material

STESC

Sulphoaluminate cement-based thermal energy storage composite

SF-STESC

Steel fiber reinforced sulphoaluminate cement-based thermal energy storage composite

PCM

Phase change material

SF

Steel fiber

SAC

Sulphoaluminate cement

SP

Polycarboxylate superplasticizer

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GM-SSPCM

1. Introduction

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Latent heat storage systems have the characteristics of high heat storage density and storage/release thermal energy at a nearly constant temperature [1, 2]. When the systems are applied in buildings, not only energy utilization efficiency can be improved, but also the mismatch between thermal energy supply and consumption can be decreased [3]. Cement-based composite incorporated PCMs is one of typical latent heat storage composites which is composed of PCM, supporting matrix and other additives [4]. Paraffin has been considered as a promising PCM to be used in latent heat storage systems for its unique characteristics, such as high latent heat, chemical inertness, no segregation, non-toxic, commercial availability and low cost [5, 6]. Similar to other solid-liquid PCM, paraffin exhibits the issues of liquid leakage during phase change. In recent years, the encapsulation of shape stabilized paraffin has been studied. Cheng et al. [7] and Liu et al. [8] showed that liquid paraffin leakage problem can be effectively solved by mixing melted paraffin with melted polyethylene to prepare shape * Corresponding author. Tel.: +86-29-8231 2519; fax: +86-29-8231 2519. E-mail address: [email protected] (G.C. Sang)

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stabilized phase change materials. However, when PCMs are incorporated into cement-based matrix, both the mechanical strength and thermal conductivity of composite were reduced significantly [9]. Zhang et al. [10] conducted an experimental study about the influence of PCM dosage on the mechanical properties of cement-based composite. It was found that the compressive strength was reduced by 58.4% after 30 wt% of PCM was incorporated. A negative effect of PCM on the compressive strength and thermal conductivity were also observed by Cui et al. [11]. They reported that 20 wt% PCM additive decreased the compressive strength and flexural strength of cement mortar by 49.6% and 33.3%, meanwhile, the thermal conductivity coefficient of PCM cement mortar was reduced by 39.9% and 40.1% respectively, when the inner PCM in solid and liquid state. Lecompte et al. [12] investigated the mechanism that paraffin impacting on the mechanical and thermal properties of cement-based composites through experimental and theoretical analysis. Their results confirmed that low thermal conductivity of paraffin between 0.1-0.2 Wm-1K-1 led to low thermal conductivity of composite, and the paraffin have no contribution to the mechanical strength, which can be considered as voids in cement-based matrix. Low mechanical strength will limit the structural application of thermal energy storage cement-based PCM composites, while low thermal conductivity will lead to a low heat storage/release rate. Improvement of charging/discharging behaviour is highly required for most latent heat thermal energy applications. Several studies have focused on enhancing thermal conductivities of cement-based PCM composites by mixing high thermal conductivity fillers in PCM, such as carbon materials [13, 14] and metal materials [15]. Nevertheless, increasing the thermal conductivity of PCMs cannot significantly improve the thermal energy charging/discharging rate of the PCM composites due to the low thermal conductivity of cement mortar of 0.6-0.8 Wm-1K-1 [16]. Because steel fiber possesses high tensile strength and thermal conductivity, in recent years, it has been suggested using steel fibers to enhance the mechanical and thermal properties of cement-based composites such as cement mortar and concrete [17, 18]. Sevil et al. [19] carried out a study on steel fiber reinforced mortar and showed that the addition of a low dosage of steel fiber (2.0% by volume ) could evidently increase the mechanical properties of cement pastes. The influence of steel fiber on the thermal conductivities of steel fiber reinforced concrete were investigated by Liu et al. [20] and the results showed that the thermal conductivity of steel fiber reinforced concrete evidently increases as steel fiber volume fraction increases in the range of 0.5%-1.5%. However, to the best of authors' knowledge, there is few researches on the use of steel fibers to enhance thermal property as well as the mechanical strength of cement-based PCM composites. With regard to mechanical properties, another route on improving the mechanical properties of cement-based PCM composites is employing high mechanical strength materials to serve as matrix. To date, most studies on cement-based thermal energy storage composites were focused on using Ordinary Portland Cement (OPC) as matrix material. However, compared with OPC, sulphoaluminate cement (SAC) has the characteristics of high strength[21], high volume stability [22] and low carbon dioxide emission [23], etc. Therefore, in the past decades, the SAC has been received increasing attention because of its excellent mechanical properties [24]. However, to the best of our knowledge, few researches employing SAC as matrix material to prepare thermal energy storage composites were reported. Up to date, for cement-based thermal energy storage composite, the improving of mechanical properties and heat storage/release rate are highly required and challenging for latent heat thermal energy applications [25]. In this research, a novel fine steel fiber reinforced SAC-based thermal

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energy storage composite (SF-STESC) was proposed. To create SF-STESC, SAC mortar was used as matrix material and graphite-modified shape stabilized phase change material (GM-SSPCM) was incorporated into SAC mortar, and fine steel fibers were introduced to increase mechanical strength and heat transfer efficiency. Fig. 1 shows the innovation of this research by a schematic drawing of fine steel fibers in mortar matrix incorporated with GM-SSPCM. It is worth pointing out that adding fine steel fibers into SAC-based PCM composite is an efficient way to develop high performance and low-CO2 emission composite for enlarge scope of structures application and efficiency of thermal energy storage in buildings.

Fig. 1. Schematic drawing of the performance enhancement of SF-STESC by fine steel fiber and the application of SF-STESC in construction. ① The mechanical strength of matrix is improved by steel fiber reinforcement; ② Thermal conductivity of composite is enhanced by fine steel fibers; ③ Cracks inside the composite are inhibited

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by fiber bridging.

2. Experimental

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2.1 Preparation of GM-SSPCM

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In this study, the following materials were used for the preparation of GM-SSPCM: paraffin (Nanyang Hannuowei Petrochemical Co., Ltd), low density polyethylene (LDPE, Sinopec Beijing Yanshan Petrochemical Co., Ltd.), flake graphite (Tianjin Hengxing Chemical Reagent Manufacturing Co., Ltd.). The thermo-physical properties of paraffin are shown in Table 1. LDPE with a melting point of 150 °C was used as a supporting material and flake graphite composed of 10-50μm scales were used as an additive to enhance thermal conductivity of SSPCM. The morphology of flake graphite is shown in Fig. 2.

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Table 1

Thermo-physical properties of the paraffin Thermo-physical properties

Parameters

Melting temperature range (℃)

18.6-24.1

Solidifying temperature range (℃)

19.9-15.6

Enthalpy (J/g)

173.0 (fusion) / 178.4(crystallization) 3

Density (kg/m ) [Note] l stands for liquid, s stands for solid.

740 (l) / 825 (s)

Fig. 2. The images of graphite

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The mass ratio of paraffin, LDPE and graphite was kept at 6:4:0.4. The mixture was heated and mechanical stirred at 150℃ for more than 30 minutes to ensure uniform mixing, the mixed mixture was then allowed to cool and solidify at room temperature. Specimens were divided into two types of different sizes. Small particle specimens with diameter less than 2.5 mm were used for the preparation of STESC, these particles were achieved by crushing the solidified mixture with a crusher. Cylindrical specimens with diameters of 30 ± 2 mm and thicknesses of 15 ± 1 mm were used to test thermal conductivity, which were achieved by casting using specific test molds. The preparation process of GM-SSPCM is illustrated in Fig. 3.

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Fig. 3. Schematic drawing of GM-SSPCM preparation process

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2.2 Preparation of SF-STESC 2.2.1 Materials

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Grade 42.5 SAC was used as a binder, which comply with GB20472-2006 (China national standard: Sulphoaluminate cement), the density of SAC is 2.84 g/cm3. The chemical compositions and performance of SAC are listed in Tables 2 and 3, respectively. Copper-plated fine steel fibers (SF) with an average diameter of 0.19mm and a length of 6.0mm supplied by Anshan Changhong Technology Development Co., Ltd. The properties of SF are given in Table 4 and the morphology is shown in Fig. 4. A high-range water reducing agent, polycarboxylate superplasticizer (SP) with a density of 1.06 g/cm3, was adopted to adjust cement slurries with similar workability. The China ISO standard sand, with a density of 2.60 g/cm3, was employed as fine aggregate in the composites. Table 2 Chemical composition of the SAC

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Components

CaO

SiO2

Al2O3

Fe2O3

MgO

SO3

Na2O

K2O

Loss

Contents (wt%)

42.35

8.46

33.38

3.12

2.29

9.53

0.21

0.05

0.61

Table 3 Properties of the SCA Setting Time（minute）

Compressive Strength（MPa）

Flexural strength（MPa）

Initial Setting

Final Setting

1d

3d

28d

1d

3d

28d

21

195

32.5

43.2

45.9

6.1

6.7

7.2

Properties of the SF Density 3

Length

Aspect ratio

0. 20 mm

6.0 mm

30.0

Thermal conductivty coefficient 40.0 Wm-1K-1

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7.85 g/cm

Diameter

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Table 4

Fig. 4. The images of SF

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2.2.2 Mix proportion of SF-STESC

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The mixture proportions of SF-STESC composites are listed in Table 5. For all specimens, the water/cement ratio and cement/sand ratio were fixed at 0.45 and 1.0, respectively. The mass percentages of GM-SSPCM were kept at 50wt% of cement. Fine steel fibers were added for enhancing mechanical strength and thermal conductivity of composites and four different volume fractions at 0%, 1.5%, 2.5% and 3.5% were adopted. For instance, the SF-STESC-1.5 mixture specimen contains fine steel fiber of 1.5% by volume. Table 5

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Mixture proportions Specimens

SAC (g)

Sand (g)

SF/ (vol/%)

GM-SSPCM (g)

SP (g)

W/C

SF-STESC-0

500

500

0

250

4

0.45

SF-STESC-1.5

500

500

1.5

250

4

0.45

SF-STESC-2.5

500

500

2.5

250

4

0.45

SF-STESC-3.5

500

500

3.5

250

4

0.45

[Note] "W/C" represents the mass ratio of water to SAC. “vol/%” represents the volume percentage of SF in the composite.

2.2.3 Preparation method of hardened SF-STESC specimens Since the SF-STESC has different components compared with ordinary cement mortar, a special mixing process was utilized. Firstly, cement, sand, fine steel fibers, SP and the GM-SSPCM were weighted and dry mixed for 1 min in a mixer. Secondly, water premixed with

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SP was poured into the mixer and they were mixed for an additional 3 min. Thirdly, the fresh slurry with the proper fluidity and dispersed steel fibers was cast in moulds and compacted using a vibration table. Finally, the cast STESC specimens were cured at standard condition with temperature at 20±1℃ and relative humidity at 99% until they were tested at specified ages. Specimens were divided into three types of different sizes and the photographs of specimens are shown in Fig. 5. Among them, the specimens with 40mm×40mm×160mm (Fig. 5 (a)) were used to test compressive strength, flexural strength and volume stability. Cylindrical specimens (Fig. 5 (b)) with diameters of 30 ± 2 mm and thicknesses of 15±1 mm were tested to obtain thermal conductivity. Another type of specimens with 300mm (L) × 300mm (W) ×30mm (T) (Fig. 5 (c)) were used for determination of thermal energy storage/release performance.

Fig. 5. Photographs of specimens with different sizes

2.3 Test methods

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2.3.1 Morphology and microstructure of graphite, SSPCM and SF-STESC

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Scanning electron microscope (SEM, Type: JSM-6700F) was utilized to observe the morphology of graphite and SF-STESC under low vacuum condition with an accelerating voltage of 20 kV. SEM specimens were vacuum dried at 50℃ for 24 hours and treated with the surface gold spray prior to observation. In addition, laser confocal scanning microscope (LCSM, Type: LEXT OLS4000) was also used to reveal the structural characterization of GM-SSPCM instead of SEM. Due to the phase change temperature of paraffin is only about 20℃, the SSPCM structure may be damaged to some extent by the melting and evaporation of paraffin under vacuum drying conditions. Therefore, using SEM to analyze the structural features of GM-SSPCM has some limitations. Unlike using SEM, the surface of LCSM specimens were not treated with other good conductive materials, such as gold and platinum, etc., so the obtained images can more clearly show the original characteristics. In this study, both SEM and LCSM were utilized. The purpose is to get the best results by complementing the two analytical techniques. 2.3.2 Thermal capacity of GM-SSPCM The phase change temperature and latent heat of GM-SSPCM were determined using differential scanning calorimetry (DSC, Type: NETZSCH 200F3). The tests were performed under a nitrogen atmosphere and specimens were tested in a temperature range of -30℃to 50℃ at a heating and cooling rate of 5℃/min. 2.3.3 Mechanical strength of SF-STESC According to the China national standard of GB/T 17671-1999, the compressive strength and flexural strength of SF-STESC were tested at the age of 1 day, 3 days and 28 days, respectively. In

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this study, the mechanical properties of SF-STESC were evaluated by the compressive strength, flexural strength at different ages, and the ratio of compressive strength to flexural strength at the age of 28 days.

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2.3.4 Volume stability of SF-STESC A vibrating wire strain gauge (Type: xhx-21) was employed to conduct the volume stability determination of SF-STESC. When the specimens were cured at standard condition for 12 hours, they were removed from the moulds and initial lengths were recorded. After that, the specimens were cured in a room with relativity humidity at 50 ± 5% and temperature at 24 ± 1℃. In this process, the changes of length of the specimens were recorded continually for 40 days.

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2.3.5 Thermal conductivity of SSPCM and SF-STESC To test the thermal conductivity, a thermal conductivity meter (Type: DZDRS) was employed with the transient plane source method. The measuring range of the thermal conductivity meter is from 0.005-300Wm-1K-1. Cylindrical specimens with diameters of 30 ± 2 mm and thicknesses of 15 ± 1 mm were used. Each set of test specimens has two identical ones, and the top and bottom of each specimen was smoothed. Before performing of the thermal conductivity test, the specimens were dried at 50℃ for 24 hours to eliminate the influence of moisture on the test results. Then, thermal conductive silicone was applied between the contact surfaces of the upper and lower specimens to ensure heat transfer sufficiently between the test probe and test specimens. In order to determine the thermal conductivity of specimens at two different states, i.e. the incorporated paraffin is at solid or liquid states, the test was performed at temperatures of 30℃ and 10℃ respectively. Each specimen was tested three times and the average value was calculated.

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2.3.6 Thermal energy storage/release performance of SF-STESC Thermal energy storage/release performance of latent heat storage composite is one of the most important thermal properties. However, there is no standardized method can be carried out to test the thermal energy storage/release performance. Some researchers often use a self-designed cube box as an experimental setup to evaluate the thermal energy storage/release performance of the tested specimen by comparatively analyzing the characteristics of the "indoor" air temperature fluctuation in the experimental setup with different specimens inside. The test process includes a heating process and a cooling process. Referring to the testing methods reported in the literatures [11, 13, 26], we used a self-designed setup to evaluate the thermal energy storage/release performance of SF-STESC by analyzing the characteristics of "indoor" air temperature variation with time. Although this experimental setup is not similar to the actual building, the purpose that we designed this setup to display the thermal energy storage/release performance of SF-STESCs. The specimens of SF-STESC with dimensions of 300 mm (L) × 300 mm (W) × 30 mm (T) were employed. The experimental setup is sketched in Fig. 6.

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(a) Sectional view

(b) Component diagram

diagram.

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Fig. 6. Schematic of thermal energy storage/release performance test setup: (a) Sectional view. (b) Component

The experimental setup consisted of a test room in a cold environment at a constant

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temperature of -15C. The test room was a cube with dimensions of 300×300×300 mm3, which was constructed using six 30mm thick expanded polystyrene (EPS) boards. The connection between EPS boards was sealed with polyurethane foam adhesive. The EPS is a low thermal conductivity material and often used in energy-saving buildings as insulating material. In this experiment, the EPS boards in the test room refer to the energy-saving envelope of the actual building and the cold room represents the cold winter outdoor environment. In the test room, a 25W iodine tungsten filament lamp (used as a heat source) was located at the center of the top wall and thermocouples (LR5021, HIOKI) were arranged at the center of the test room and the inner surface of the tested panel. Besides, foil paper was used to cover thermocouples to avoid direct radiation. The thermal energy storage/release performance of SF-STESC was evaluated by testing the rate and amplitude of indoor air temperature changes in test room with the test specimens. The testing process consists of a cooling and a heating process, in which the indoor air temperature of the test room was monitored and recorded. The tests were conducted by the following steps. (1) The test room installed test specimen panel was kept at -15℃ in the cold room for 12 hours for the allowance of sufficient time to reach steady-state conditions. (2) Heat source (lamp) was turned on to preheat the test room for 35 min. (3) Then, the heat source was turned off to begin the first cooling stage, which lasted for 30 min. Cooling was achieved by the cold environment outside the test room. (4) After 30 min of cooling, the lamp was turned on to heat the test room and the heating time was also 30 min. In this test, the cooling and heating cycle was carried out without interruption. At the same time, the indoor air temperature in the test room were continuously recorded at 1 min intervals. Based on the collected temperature data, the thermal energy storage/release performance of SF-STESC panels would be analyzed.

3. Results and discussion 3.1. Morphology and microstructure of GM-SSPCM In this study, LDPE was innovatively used as a supporting material in the preparation of GM-SSPCM. The morphology and microstructure of SSPCM observed by LSCM were shown in Fig. 7(a). It can be seen that, after the completion of heating, mixing, solidifying and other preparation steps, a LDPE multi-level space network structure had been developed with paraffin

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(b) Particles

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(a) LDPE network structure

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being contained inside. Then, after crushing, the GM-SSPCM appeared as particles with a size of less than 2.5 mm as shown in Fig. 7(b). In this study, flake graphite was used as modifier to enhance the thermal conductivity of SSPCM. Flake graphite was distributed evenly in SSPCM composite, as shown in Fig. 7(c).

(c) Graphite distribution in GM-SSPCM

Fig.7. LSCM images of GM-SSPCM: (a) LDPE network structure, (b) Particles, (c) Graphite distribution

3.2. Thermal capacity of GM-SSPCM and SF-STESC SF-STESC is composed of GM-SSPCM, matrix material and fine steel fibers. Among them, GM-SSPCM is the provider of latent heat storage capacity of SF-STESC. In this study, the thermal properties such as phase change temperature and enthalpy of paraffin, GM-SSPCM and SF-STESC-3.5 were tested by DSC. The DSC curves of Paraffin, GM-SSPCM and SF-STESC-3.5

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are presented in Fig. 8. Paraffin GM-SSPCM SF-STESC-3.5

1.0

Tpeak=24.1ºC

Tpeak=23.7ºC

H=173.0J/g

H=85.19J/g

Endo

Tpeak=18.1ºC H=13.22J/g

0.0 Tpeak=15.1ºC

-1.0

H=10.42J/g

Tpeak=16.5ºC

Exo

Tpeak=15.6ºC

H=90.42J/g

H=178.4J/g

-2.0 -40

-20

0 20 Temperature / ºC

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2.0

40

60

Fig. 8. DSC curves of Paraffin, GM-SSPCM and SF-STESC-3.5

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As shown in Fig. 8 that there are two peaks in each of the fusion and crystallization processes in the DSC curves of paraffin and GM-SSPCM. The left peak means the solid-solid phase change and the right peak means solid-liquid phase change. Therefore, the latent heat is the sum of the solid-solid and solid-liquid phase change latent heat. It can be seen from Fig. 8m that the enthalpy and peak phase change temperature of paraffin are determined to be 173.0 J/g and 24.1℃for the fusion process and 178.4 J/g and 15.6℃ for the crystallization process. In addition, it can be seen from the DSC curve of GM-SSPCM that the enthalpy and peak phase change temperature for fusion process and crystallization process are 85.19 J/g, 23.7℃ and 90.42 J/g, 16.5℃, respectively. Cui et al. [11] and Zhang et al. [27] used the encapsulation ratio (Eq. (1)) to evaluate effective encapsulation ratio of paraffin within the microcapsules, which can also indicate the loading content of paraffin in microcapsules. According to the parameters listed in Table 1, the fusion enthalpy of paraffin is 173 J/g before mixing with flake graphite and being encapsulated by LDPE shell to prepare GM-SSPCM. In this study, the encapsulation ratio of GM-SSPCM can be calculated by following equation base on the enthalpies of paraffin and GM-SSPCM. R=

∆𝐻𝑚,𝐺𝑀−𝑆𝑆𝑃𝐶𝑀 ∆𝐻𝑚,𝑝𝑎𝑟𝑎𝑓𝑓𝑖𝑛

× 100%

(1)

where R is the encapsulation ratio, ∆𝐻𝑚,𝐺𝑀−𝑆𝑆𝑃𝐶𝑀 is the measured fusion enthalpy of the GM-SSPCM, and ∆𝐻𝑚,𝑝𝑎𝑟𝑎𝑓𝑓𝑖𝑛 is the fusion enthalpy of the paraffin.

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Using Eq.(1), the encapsulation ratio of GM-SSPCM is calculated to be 49.2%. According to the determined enthalpy of paraffin and the mass ratio of paraffin, LDPE and graphite to fabricate GM-SSPCM, the enthalpy of GM-SSPCM can be calculated to be 57.7%. The measured value of GM-SSPCM enthalpy is lower than the theoretical value, probably because the paraffin used in this study has a low melting temperature (about 20℃), so there is a small amount of paraffin lost during GM-SSPCM preparation, sampling and testing. However, the encapsulation ratio of GM-SSPCM in this study is very close to the 52.2% encapsulation ratio of the graphite-modified microencapsulated paraffin prepared by Cui et al. using interfacial polymerization method [11]. The DSC curve of SF-STESC-3.5 is showed in Fig. 8. Since the paraffin dosage in SF-STESC-3.5 is less than that in GM-SSPCM, the solid-solid phase change peak is not obvious. The enthalpy and peak melting temperature of SF-STESC-3.5 are 13.22 J/g and 18.1℃for the

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fusion process and 10.42 J/g and 15.1℃ for the crystallization process, respectively. Furthermore, the peak melting temperature of SF-STESC-3.5 is lower slightly than that of GM-SSPCM, but it still suitable to for use in heating buildings. The enthalpy of SF-STESC-3.5 is close to the 11.4 J/g thermal energy storage capacity of heat storage cement mortar prepared by Li et al [13].

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3.2 Mechanical strength of SF-STESC Steel fibers are usually used as reinforcement to enhance the mechanical properties of cement-based materials [19, 28]. In this study, fine steel fibers were dispersed in STESC, with the aim of improving the mechanical strength and thermal conductivity. Fig. 9 shows the flexural and compressive strengths of SF-STESC with different volume fractions (0 vol/%, 1.5 vol/%, 2.5 vol/% and 3.5 vol/%), at 1 day, 3 days and 28 days. It can be seen in Fig. 9(a) that the flexural strengths at 1 day, 3 days and 28 days of SF-STESC-0 without SF were 2.02 MPa, 2.18 MPa and 2.54 MPa respectively. However, the flexural strength of SF-STESC increased significantly at all ages, when SFs were added. When SF was added about 3.5 vol/%, the flexural strengths at 1 day, 3 days and 28 days were 2.93 MPa, 3.17 MPa and 3.57 MPa respectively. Compared to the SF-STESC without SF, the flexural strengths increased by 45.0%, 45.4% and 40.6% at 1day, 3 days and 28 days, respectively. The experimental data demonstrate that the mechanical strength of SF-STESCs has an increasing trends with an increase of the volume fraction of SF and SF-STESCs has promising mechanical properties. Compared with the results of the 28-day compressive strength 5.1 MPa and flexural strength 2.01 MPa of heat storage OPC mortar (with polypropylene fiber reinforcement) in the literature [13], it can be found that although PCM dosage in SF-STESC-3.5 is more higher than the former, the compressive strength and flexural strength are 73.5% and 77.6 % higher than that, respectively. The incorporation of SFs contributed to the flexural strength enhancement of the test specimens due to the bonding interaction between steel fibers and the mortar paste. However, the effect of SF volume fraction on compressive strength of SF-STESC seemed to be insignificant. It can be seen in Fig. 8(b) that the compressive strengths of SF-STESC-0 (0 vol/%) at 1 day, 3 days and 28 days were 5.58 MPa, 6.51Mpa and 8.25MPa respectively. When the SF volume fractions increased from 0 to 1.5%, 2.5% and 3.5%, the compressive strength of SF-STESC at the same ages increased slightly. As shown in Fig. 9(b) that the compressive strengths of SF-STESC-3.5 specimens at 1d, 3d and 28d were 6.04 MPa, 6.89 MPa and 8.85MPa, which were 8.3%, 5.8% and 7.3% higher than SF-STESC-0 specimens at the same ages. It can be found that, by comparing the mechanical properties changes of SF-STESCs with SF volume fraction increasing, the flexural strength increasing rate was higher than that of compressive strength. Furthermore, the ratio of compressive to flexural strength was not constant, but demonstrated a regular change pattern with the increase of the SF volume fraction as shown in Fig. 10. It can be observed that, when the SF volume fraction increased from 0 to 3.5%, the compressive/flexural strength ratio at 28-day curing ages decreased by 21.9% from 3.2 to 2.5. The results in Fig. 10 revealed that the flexural strength is more sensitive to SF dosage than the compressive strength. The reasons for the decrease in the compressive/flexural strength ratio of SCTESM might be explained by the following: (a) The compressive strength depends on the physical and mechanical properties of the components [29]. According to the research proposed by Wu et al. [30], the adhesion of SFs to the cement hydration product interface resulted in the improvement of mechanical interlocking between cement hydration products. Meanwhile, however, with the

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increase in the SF volume fraction, the porosity and the quantities of interfacial transition zones (ITZs) between SFs and matrix increased, as shown in Fig. 11, which resulted in the adverse effects on the compressive strength of SF-STESC. Combined with the above factors, the mechanism of the slight improvement of the steel fiber to the compressive strength can be clarified. (b) The bonding area between SF and matrix increased with the increasing of SF volume fraction, which leads to increasing of SF bridging strength. The report by Yoo et al. [18] supports the above argument, according to Yoo et al. [18] , in order to pull out the fibers with a higher bonding area, a higher force is required, so a higher fiber bridging strength is activated. 4.0

Flexural strength / MPa

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1d 3d 28d

3.5 3.0 2.5 2.0

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0.0

1.5

2.5

3.5

Volume fraction of SF / %

(a) Flexural strength 10.0

1d 3d 28d

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(b) Compressive strength

3.5 Ratio of compressive to flexural strength

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Fig. 9. Mechanical properties of SF-STESC: (a) flexural strength; (b) compressive strength

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3.0 2.8 2.7 2.5

2.5

2.0

1.5 0

2.5 1.5 Volume fraction of SF / %

3.5

Fig. 10. Influence of SF volume fraction on the ration of compressive to flexural strength

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Fig. 11. SEM image of ITZs and pores of SF-STESC

3.3 Volume stability

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The influence of SFs on the volume stability of SF-STESC was examined by comparing the specimen SF-STESC-0 without SF and the specimen SF-STESC-3.5 which has the highest SF content. The volume stability of the specimens of SF-STESC-0 and SF-STESC-3.5 were measured for 40 days. Comparison of volume changes of two specimens within 40 days is shown in Fig. 12. 0.02

SF-STESC-0 SF-STESC-3.5

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20 Curing ages / d

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Fig. 12. Comparison of volume stability of SF-STESC-0 and SF-STESC-3.5

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The length changes of SF-STESC-0 is shown in Fig. 12. The curve of length change demonstrates that the volume shrinkage of SF-STESC-0 specimen mainly occurred in the first 3 days, and then developed into a volume stable state and the volume shrinkage was steady at 0.02% relative to the initial state. The curve of length changes of SF-STESC-3.5 in Fig. 12 demonstrate that the length change of SF-STESC-3.5 was here for the first 14 days and then began to stabilize with a volume shrinkage percentage of 0.016%. In comparison with SF-STESC-0, the maximum volume shrinkage of SF-STESC-3.5 decreased by 20.0%. The length changes of the two specimens in Fig. 12 indicate that the SFs incorporated into cement mortar matrix can reduce the degree of volume shrinkage during the hardening process of SF-STESC. The volume shrinkage of cement-based composite is mainly caused by plastic shrinkage cracks [31]. The SFs added in STESC specimens can inhibit the cracks development in the hardening process of SCA matrix, including reducing the width and amount of cracks. Similar results were reported by Brandt [32] that the addition of randomly distributed fibers in the mixture can provide three-dimensional reinforcement to control cracks propagation. As a result, the

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volume shrinkage percentage of SF-STESC-3.5 is less than that of SF-STESC-0 which does not contain SFs. The SFs distributed in the matrix can delay the first crack appearance and slow the development of cracks. Therefore, specimens reinforced with SFs showed a decrease in volumetric shrinkage and a delay in volume stability. This attributed to the adhesion between the SFs and surrounding matrix, as demonstrated in previous studies [31, 33]. In addition, it is noteworthy that in addition to the contribution of SF to the volume stability of STESC, the lower volume shrinkage of SAC also plays an important role. The research findings reported in the published literature [26] indicate that the volume shrinkage of the OPC based heat storage material with the same dosage of the PCM in the present study is about 0.05% at 40 days curing ages. By comparison, although the volume shrinkage of SF-STESC-0 is larger than that of SF-STESC-3.5 modified by SF, it is significantly less than that of OPC based heat storage materials with a PCM dosage of approximately equivalent. 3.4 Thermal conductivity of SF-STESC 3.4.1 Thermal conductivity measurement

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The thermal conductivity of SF-STESC is an important influencing factor in heat transfer, which is also an important factor influencing the thermal energy storage/release performance. The SF-STESC is a composite including pure mortar (matrix), GM-SSPCMs, and SFs. The thermal conductivities of mortar, GM-SSPCM and SF-STESCs were measured at different temperatures. As shown in Table 1 that the melting temperature range of paraffin is 18.6-24.1℃ and solidifying temperature range is 19.9-15.6℃，so the paraffin is considered in a solid state at 10℃and a liquid state at 30℃. Based on calorimetric analysis, thermal conductivities of the SF-STESC were determined at 10℃ and 30℃，which were considered as being representative of thermal conductivities with paraffin in the solid and liquid state respectively. The average values from three different tests are listed in Table 6. It can be seen from Table 6 that the thermal conductivities of specimens increase with the increasing of SF volume fraction. When the volume fraction of SFs increases from 0 to 3.5%, the thermal conductivities of the SF-STESC increase by 51.3% and 84.5% at 10℃ and 30℃, respectively, they reaches 0.794 W/m·K and 1.062 W/m·K. The test results showed that the thermal conductivity of the STESC can be improved by the addition of SFs. Meanwhile, the data in Table 6 shows that the thermal conductivity of the specimen at 30℃ is higher than that of 10℃. Compared with the data at 10℃, the thermal conductivities of specimen SF-STESC-0, SF-STESC-1.5, SF-STESC-2.5 and SF-STESC-3.5 in the 30℃ increased by 9.7%, 12.3%, 21.3% and 33.8%, respectively. Table 6.

Thermal conductivities of matrix, GM-SSPCM and SF-STESC at 10C and 30℃ Specimens

Thermal conductivity / Wm-1K-1 10℃

30℃

SF-STESC-0

0.525

0.576

SF-STESC-1.5

0.693

0.779

SF-STESC-2.5

0.734

0.890

SF-STESC-3.5

0.794

1.062

Pure mortar

0.925

0.932

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GM-SSPCM

0.368

0.405

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The effect of temperature on thermal conductivity of SF-STESC is mainly due to the difference of thermo-physical property of each component at different temperatures. The SF-STESC is composed of pure mortar (no GM-SSPCM and SFs added), GM-SSPCMs, and SFs. It can be seen from Table 6 that temperature has little effect on the thermal conductivity of pure mortar, but has an significant effect on that of GM-SSPCM. When the temperature was increased from 10℃ to 30℃,the thermal conductivity of GM-SSPCM was increased by 10.1% from 0.368 W/m·K to 0.405. It can be seen in literature [12] that the thermal conductivity of paraffin is about 0.21 W/m·K in solid state and 0.15 W/m·K in liquid state. Liquid paraffin has lower thermal conductivity and density than that of solid paraffin (as shown in Table 4). Therefore, it can be deduced that, there are no gaps between the paraffin and its supporting material when the paraffin is in a liquid state. However, when atmospheric temperature is lower than the solidifying point of paraffin, the solidifying of paraffin will lead to its volume shrinkage which results in the emergence of voids gaps between the paraffin and LDPE supporting material. This phenomenon is verified in available literature [34] that there is no contact thermal resistance between the liquid paraffin and its encapsulation material, but there is contact thermal resistance after paraffin solidification. The conclusions in the literature [34] are consistent with the experimental results in this study and can reveal the mechanism that the thermal conductivities of the SF-STESCs at 30℃ are evidently higher than that of 10℃ in this study.

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3.4.2 Thermal conductivity prediction Establishing a suitable thermal conductivity theoretical model is necessary for the calculation and prediction of the thermal conductivity of composite materials, which has attracted the attention of many researchers. Meshgin et al. [35] proposed a multi-phase and multi-scale model to analyze the thermal conductivity of concrete incorporated phase change material. Bederina et al. [36] established a model for estimating the thermal conductivity of heterogeneous materials on the basis of knowing the conductivity of each component and its concentration. Shi et al. [34] developed a numerical model to predict the thermal conductivity of form-stable fiber composite concrete containing dispersed phase change materials, and validated the numerical model with experimental data. Based on previous studies, Liu et al. [20] proposed a multi-phase predication model to calculate the thermal conductivity of fiber reinforced cement-based composites. However, few researchers reported works on prediction of the influence of steel fiber volume fraction on the thermal conductivity of composite mortar with PCMs in solid and liquid states. To predict the thermal conductivity, the SF-STESC can be assumed as a two-phase composite material, including the cement composite incorporated GM-SSPCM particles (CCG) and the dispersed phase of SFs. Therefore, a thermal conductivity predication model was proposed in the paper [37] that investigated the thermal conductivity of phase change material containing carbon fibers. The thermal conductivity prediction model is shown in Eq. (2). −1/3

1/3

1/3

𝐾𝑆𝑇𝐸𝑆𝐶 = *(𝑉𝐶𝐶𝐺 − 𝑉𝐶𝐶𝐺 )𝜇𝐾𝑆𝐹 + 𝐾𝐶𝐶𝐺 + 𝑉𝐶𝐶𝐺

(2)

Where 𝐾𝑆𝑇𝐸𝑆𝐶 is the thermal conductivity of SF-STESC specimens, 𝑉𝐶𝐶𝐺 refers to the volume fraction of the CCG, 𝐾𝑆𝐹 is the thermal conductivity of SF, 𝐾𝐶𝐶𝐺 represents the thermal conductivity of CCG, and 𝜇 is an empirical parameter. The volume fraction and thermal conductivity of CCG and SF are listed in Table 7.

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Table 7 The volume fraction and thermal conductivity of CCG and SF Components

Volume fraction / %

Thermal conductivity W/mK

SF-STESC-0

SF-STESC-1.5

SF-STESC-2.5

SF-STESC-3.5

10℃

30℃

CCG

100

98.5

97.5

96.5

0.525

0.576

SF

0

1.5

2.5

3.5

40.000

40.000

Note: The thermal conductivity of CCG was derived from the test data of SF-STESC-0 specimens in Table 5. Thermal conductivities of SF were referenced in the literatures [20, 38], and ignored the influence of 20 ℃ change on the thermal conductivity.

1.00 0.90 0.80 0.70 0.60

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Experimental value ( at 10 oC) Calculate vaule (at 10 oC) Calculate vaule (at 30 oC) Experimental value ( at 30 oC)

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Thermal conductivity / Wm-1K-1

1.10

0.50 0.0

1.5 2.5 Volume fraction of SFs / %

3.5

Fig. 13. Comparison of the calculated values and experimental values

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The predicted values are compared with the experimental results, as shown in Fig. 13. It is noted from Fig. 13 that the experimental data compared reasonably well with the model calculation data. The calculated results from the existing model are closer to the experimental results. Therefore, the thermal conductivity of SF-STESC can be accurately predicted using the existing model, which can simplify the analysis of the thermal conductivity of SF-STESC with inner paraffin in both solid and liquid states. 3.5 The thermal energy storage/release performance of SF-STESC

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In the published literatures [11, 13, 26], the researchers used similar test methods to test the thermal properties of the phase change heat storage cement mortars and characterize the adjustment abilities of the tested mortars to the "indoor" air temperature. Their research mainly focused on the influence of the increase of PCM dosage on indoor air temperature fluctuation. However, the effect of the increase of the thermal conductivity on the thermal energy storage/release performance were not studied. Theoretically, the higher the thermal conductivity of the thermal energy storage mortar, the higher the rate of heat storage/release. Nevertheless, for the best of our knowledge, there are no reports on the experimental results of the difference in indoor air temperature regulation of thermal energy storage cement mortar panel with different thermal conductivity. In this study, the thermal energy storage/release performance of SF-STESC was evaluated by analyzing the characterizes of indoor air temperature fluctuations in the test room model with a SF-STESC panels located inside (as shown in Fig. 6). The data in Table 6 shows that the volume fractions of the SF are linear with the thermal conductivities of the specimens, and the thermal conductivity of SF-STESC-3.5 is the highest whereas that of SF-STESC-0 is the lowest. Therefore, SF-STESC-3.5 and SF-STESC-0 were used for comparative experiments to investigate the influence of SF dosage on the thermal energy

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storage/release rate of the SF-STESC. The experimental results are presented in Fig. 14. Preheating

40

Cooling

Heating 3.1ºC

SF-STESC-0 SF-STESC-3.5

0 -10

Temperature / ºC

10

40

20

B

1.5ºC

24 22 20 18 16 14

-20 0

Paraffin freezing temperature range

42 44 46 Time / min

40

60 Time / min

48

Paraffin melting temperature range

28 26 24 22 20 18 16 67

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20

Temperature / ºC

Temperature / ºC

30

68 69 70 Time / min

80

71

100

Fig. 14. The indoor temperature curves of test room with SF-STESC-0 panel or SF-STESC-3.5 panel

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According to the specified test procedure, when the test room installed test specimen reach steady-state conditions at -15℃, the heat source was turned on to preheat the test room. The preheating process lasted about 35 minutes and then the heat source was turned off to begin the first cooling stage, which lasted for 30 min. During the preheating stage, the indoor air temperature of SF-STESC-3.5 is gradually higher than that of SF-STESC-0, and the difference between the two indoor air temperatures starts at about 20 °C. In the cooling process, as can be seen in Fig. 14, the indoor air temperature of SF-STESC-3.5 panel and SF-STESC-0 decreased rapidly in the initial stage, but both of them slowed down significantly when the indoor air temperature dropped to about 20℃ (as shown in the A region). At this point, an interesting phenomenon emerged that the decrease rate of the indoor air temperature of SF-STESC-3.5 panel was significantly slower than that of SF-STESC-0. After 30 minutes of cooling, the indoor air temperature of SF-STESC-3.5 panel was 1.5 ℃ higher than that of SF-STESC-0. According to the thermo-physical properties of paraffin (Table 1), it is known that the initial solidifying temperature of paraffin is 19.9℃. Therefore, when the ambient temperature drops to 20℃, it can be deduced the paraffin incorporated in the panels begins to crystallize and release latent heat, thereby inhibiting the decrease of indoor air temperature. However, the low thermal conductivity can reduce the energy utilization efficiency during heat charging and discharging processes [12, 39, 40]. Therefore, the latent heat discharge rate of SF-STESC-0 panel is less than that of SF-STESC-3.5. This characteristic was verified in this experiment that the inhibitory ability of SF-STESC-3.5 was higher than that of SF-STESC-0 after the ambient temperature was less than 20℃. In the heating process, as shown in Fig. 14, the indoor air temperature of both specimen panels increased rapidly in the initial stage, but the rate of indoor air temperature rise slow down obviously after the air temperature reached about 20℃ (as shown in the B region). The thermo-physical parameters of the paraffin in Table 1 indicate that the paraffin has a initial melting temperature of 18.6 ℃ . When the ambient temperature is higher than the initial melting temperature, the paraffin incorporated into the STESC panel will begin to melt and absorb the thermal energy in the air, resulting in a decrease in the rise rate of indoor air temperature. Due to the difference in endothermic efficiency, the indoor temperature of the SF-STESC-3.5 panel was 3.1℃ lower than that of SF-STESC-0 when the heating process lasted 30 minutes. The results of

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the comparison analysis indicated that the thermal conductivity of the STESC panel has evidently effected on thermal energy storage/release performance. The SF-STESC-3.5 panel has a higher thermal conductivity, it shows a better performance in inhibiting indoor air fluctuations. The report by Koizumi [41] supports this phenomenon that employing high conductivity materials in the mixes can reduce melting time and consequently improve thermal energy storage/release performance. According to the test results of thermal and mechanical properties of SF-STESCM-3.5, it can be inferred that the composite can be used as a wall and a floor in a building thermal energy storage system. For buildings containing such energy storage composites, during the process of outdoor heat transfer to indoor, the PCM melts and absorbs heat, slowing down the indoor air temperature and storing energy. When the indoor air temperature drops, the molten PCM solidifies, and at the same time releases heat to the room to relieve the indoor air temperature from drastically changing, so that the indoor air temperature is maintained at around 20 °C. Furthermore, the effect of this system is more remarkable under the condition of short-term temperature mutation and day-night temperature change.

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4. Conclusions

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In this paper, a novel SF-STESC was prepared with SAC, GM-SSPCM and fine steel fibers. The mechanical properties, volume stability, thermal conductivity and thermal performance of the SF-STESC were experimentally tested and measured. LSCM images of the GM-SSPCM revealed that paraffin can be encapsulated in the LDPE multi-level space network matrix, and the flake graphite are homogeneously dispersed among them as well. The enthalpy and peak phase change temperature of GM-SSPCM are determined to be 85.19 J/g and 23.7℃for the fusion process and 90.42 J/g and 16.5℃ for the crystallization process, respectively. Moreover, the encapsulation ratio of the GM-SSPCM is 49.2%. Both the compressive strength and flexural strength of the SF-STESC showed an increasing trends with an increase of the volume fraction of SF, but the compressive strength increased slightly while the flexural strength increased significantly. Compared with the specimen without SF, the 28-days compressive strength and flexural strength of SF-STESC containing 3.5% (volume fraction) SF increased by 7.3% and 40.6%, respectively. In addition, the SFs can also enhance the volume stability of STESC by providing three-dimensional reinforcement to control cracks propagation. The addition of SFs contributes to the increase of thermal conductivity. When volume fraction of SF is up to 3.5%, thermal conductivity of the SF-STESC was increased by 51.3% and 84.5% with the inner paraffin in solid and liquid states, respectively. Moreover, for a constant SF dosage, the thermal conductivity of a SF-STESC whose inner PCM is in the liquid state is higher than that in solid state. The thermal conductivities of STESCs can be predicted by the mathematical model. The effect of the addition of SFs on the improvement of thermal energy storage/release performance of STESC was verified through experimental tests. SF-STESC-3.5 has a higher thermal conductivity, it shows a better performance in inhibiting indoor air fluctuations. Comparing to the STESC (without SF), the maximum indoor air temperatures of SF-STESC-3.5 was reduced by 3.1℃ and minimum indoor air temperatures was increased by 1.5℃. Acknowledgements

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This work was financially supported by the National Nature Science Foundation of China (under the grant No. 51678482). References [1] E. Rodriguez-Ubinas, L. Ruiz-Valero, S. Vega, J. Neila, Applications of Phase Change Material in highly energy-efficient houses, Energy and Buildings, 50

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