Cement & Concrete Composites 55 (2015) 139–144

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Cement & Concrete Composites

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Enhanced performance of high temperature aluminate cementitiousmaterials incorporated with Cu powders for thermal energy storage

http://dx.doi.org/10.1016/j.cemconcomp.2014.08.0060958-9465/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors. Tel./fax: +86 25 83587252 (C. Lu). Tel./fax: +86 2583172128 (Z. Xu).

E-mail addresses: chhlu@njtech.edu.cn (C. Lu), zzxu@njtech.edu.cn (Z. Xu).

Huiwen Yuan, Yu Shi, Chunhua Lu ⇑, Zhongzi Xu ⇑, Yaru Ni, Xianghui LanState Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China

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

Article history:Received 4 November 2013Received in revised form 17 May 2014Accepted 7 August 2014Available online 28 September 2014

Keywords:Cementitious materialThermal energy storageCu powdersProperties

Cementitious materials have been extensively developed in thermal energy storage system of solar ther-mal power. This paper deals with the volume heat capacity, thermal conductivity, thermal expansioncoefficient, and compressive strength of aluminate cementitious thermal energy storage materials withthe addition of metal Cu powders. The specimens were subjected to heat-treatment at 105, 350, and900 �C, respectively. In the heating process, Cu powders gradually oxidized to Cu2O and CuO, providinga so-called mass compensation mechanism for the composite paste. Meanwhile, it indicates that volumeheat capacity and thermal conductivity both increase with increasing Cu powders content and decreasewith the rising temperature. The optimum thermal properties were obtained at 15 wt% Cu powders load-ing. In addition, Calorimetric Test, XRD, TG–DSC, and MIP are performed for characterizing the hydrationrates, the phases, the mass/heat evolution, and the pore distribution, respectively.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Thermal energy storage systems (TES) of solar thermal powerplants can avoid a series of instability problems and increase elec-tricity production [1–4]. There are several kinds of thermal energystorage materials which have been considered for storage systems.Liquid materials such as synthetic oil are unreasonable to be usedas a large volume storage material due to high vapor pressure athigh temperature, which are usually applied as heat transfer med-ium [5,6]. Nitrate molten salt is the most common materialemployed in the TES systems, nonetheless, the corrosive andexpensive investment problems still remain [7–10]. Solid sensibleheat storage materials store energy in a material without phasechange in the temperature range of the storage process, whichare considered to be simple designs, stable conditions and easeof control [11,12].

Traditional concrete materials play a significant role in the con-struction industry. Moreover, high temperature concrete com-posed of high alumina materials was identified as havingpotential for sensible heat storage media as they are low cost com-posite materials for solar thermal storage systems [13–15]. Thereare some advantages of aluminate cement including low pH-value

during hydration, high early strength development and high chem-ical resistance [16–18]. An undesirable property of cement paste isits relatively low thermal conductivity and volume heat capacitythat strongly suppress the energy charging/discharging rates andenergy storage ability. Much more attention has been paid to solarenergy storage materials to attempt to improve thermal andmechanical performance [19,20]. Previous research has been car-ried out on thermal properties improvement, such as the additionof graphite powders [21]. Graphite contributes to the increasedthermal conductivity and volume heat capacity mainly at 350 �C.When the temperature increases to 900 �C, the graphite powderswould oxidize to carbon dioxide and gradually overflow, which isnot good for the improvement of thermal properties.

This paper outlines the influence of the incorporation of differ-ent contents of metal Cu powders on the thermal and mechanicalproperties of the composite pastes. The desirable properties of thecomposite pastes after heat-treatment at 105, 350, and 900 �C arediscussed, respectively. In addition, Calorimetric Test, X-ray Pow-der Diffraction (XRD), thermogravimetry–differential scanning cal-orimetry (TG–DSC), and Mercury Intrusion Porosimetry (MIP) wereobtained to characterize the hydration rates/heat, the phases, themass/heat evolution, and the pore distribution respectively. Theoptimum properties of the composite pastes incorporating metalCu powders will express better performance for further prepara-tion of high temperature concrete thermal storage materials andlay theory foundation for actual project of thermal energy storagein solar thermal power plants.

140 H. Yuan et al. / Cement & Concrete Composites 55 (2015) 139–144

2. Experimental

2.1. Materials and sample preparation

Our composite pastes were fabricated using aluminate cementas matrix material which was partly substituted by Cu powders.Cu powders with high thermal conductivity of 390 W m�1 K�1

and volume heat capacity of 3.395 MJ m�3 K�1 were used toimprove thermal properties of the composite materials [22]. TheSEM image and the XRD pattern of Cu powders were shown inFig. 1. The chemical composition of aluminate cement is presentedin Table 1. It is obvious that this cement has high Al2O3 amount inthe blend. The main mineral phase of the cement is CaO�Al2O3(CA),then the minor phase is CaO�2Al2O3(CA2), 2CaO�Al2O3�SiO2(C2AS)and 12CaO�7Al2O3(C12A7) 1 wt% of high-performance polycarbox-ylate typed PCA-II (China, Jiangsu Sobute New Materials LimitedCompany) was used for achieving water reducing and uniformdispersion.

The five mix designs are designated as 0Cu, 1Cu, 5Cu, 10Cu, and15Cu which represent cement pastes incorporating 0 wt%, 1 wt%,5 wt%, 10 wt%, and 15 wt% of Cu powders, respectively. 0Cu wasconsidered as a reference. The water to binder ratio was uniformlyset at 0.22. Then the composite pastes were cast for volume heatcapacity (thermal conductivity), thermal expansion coefficient,and compressive strength, performing with moulds of 48 mm �20 mm � 80 mm, 5 mm � 5 mm � 50 mm, and 20 mm � 20 mm �20 mm, respectively. The moulds were covered with plastic wrapin order to limit the water evaporation and were moistened for24 h. The specimens were demoulded and cured in water at thetemperature of 25 �C for 7 days. Then the specimens were heatedat 105, 350, and 900 �C for 6 h, respectively. Both thermal/mechan-ical properties and the characterizations of the composite pastesafter heating at these three temperatures were listed in the presentpaper, which might be the vital evaluation index for thermalenergy storage materials.

2.2. Test methods

Volume heat capacity and thermal conductivity were measuredby thermal conductivity constant tester (TPS2500, Hot Disk Ltd.,Sweden) with Probe 5465 at 25 �C, and thermal expansion coeffi-cient was measured by thermal expansion coefficient apparatus(RPZ-03P, Institute of refractories Luoyang, China) at a heating rateof 5 �C min�1. Compressive strength was measured using auto-matic pressure test machine (HualongWHY-200, Hualong Ltd.,China) at a rate of 500 N s�1. The hydration rates/heat were deter-mined by isothermal conduction calorimetry (TAM Air, Thermo-metric, Sweden). In this method, 4 g cement powder or mixturepowder (cement and Cu powder) was placed in the calorimetriccell, and 2 g of deionized water was poured into a solution cell.XRD (Rigaku D/Max-2500, Rigaku Ltd., Japan) was performed inthe angular range 2h from 15� to 60�, with a step size of 0.02�and scan speed of 10�/min. The TG–DSC tests (STA449C, Netzsch,Germany) were carried out in N2 atmosphere at a heating rate of20 �C min�1 ranging from room temperature to 900 �C. FESEMimages were obtained on a ‘Hitachi S-4800’ instrument using atest voltage of 15 kV on dried samples coated with gold. MIP

Table 1Chemical compositions (wt%) of aluminate cement.

Materials CaO SiO2 Al2O3 Fe2O3 R2O LOI

Aluminate cement 38.79 7.17 51.68 2.07 0.29 0.30

LOI: loss on ignition.

(PM-60-GT, Quantachrome Ltd., America) was used to obtain thepore distribution of the composite pastes.

3. Results

3.1. Thermal properties

3.1.1. Volume heat capacityVolume heat capacity describes the ability of a given volume of

a substance to store internal energy while undergoing a given tem-perature, but without a phase transition. Volume heat capacities ofthe composite pastes incorporating Cu powders after heat-treat-ment at 105, 350, and 900 �C are shown in Table 2. It indicated thatthe volume heat capacity increased with the addition of Cupowders contents and decreased with the rise of the temperature.After heating at 105 �C, the volume heat capacity value of thepastes incorporating with 15 wt% Cu powders increased to2.536 MJ m�3 K�1, which was 7% higher than that of pure paste.Then further increasing heat-treatment temperature to 350 �C, thevolume heat capacity of the pastes decreased compared with thepastes heated at 105 �C. Even so, the highest volume heat capacityof the pastes composite 15 wt% Cu powders at 350 �C still exhibited17% higher than that of pure paste. Afterwards, the volume heatcapacity of the pastes composite 15 wt% Cu powders after heat-treatment at 900 �C was 31% higher than that of pure paste.

3.2. Thermal conductivity

Thermal conductivities of the composite pastes incorporatingCu powders after heat-treatment at 105, 350, and 900 �C are shownin Table 3. It can be noticed that the thermal conductivityincreased with the addition of Cu powders contents and decreasedwith the rise of the temperature as well as volume heat capacity.Among the content of Cu powder chosen in this study, the optimalthermal conductivity values after heat-treatment were obtained atCu powders content of 15 wt%, which showed 24%, 50%, and 51%higher than that of pure paste at 105, 350, 900 �C, respectively.

3.2.1. Thermal expansion coefficientThe thermal expansion coefficient curves of the composite

pastes incorporating different contents of Cu powders at elevatedtemperature are shown in Fig. 2. The measurement temperatureranges from 100 to 900 �C. The results were obtained after preheat-ing at 900 �C for 6 h. It can be noted that when the temperaturewas around 400 �C thermal expansion coefficient of sample 15Cushowed an obvious fluctuation due to the oxidation reaction ofCu powders gradually intensified along with the increase of thetemperature. The specimens could keep stable ranging from 500to 900 �C. The average value of thermal expansion coefficient of0Cu, 1Cu, 5Cu, 10Cu, and 15Cu was about 8.5 � 10�6 �C�1, 8.7 �10�6 �C�1, 8.0 � 10�6 �C�1, 7.7 � 10�6 �C�1, and 7.8 � 10�6 �C�1,respectively.

3.3. Compressive strength

Compressive strength curves of the composite pastes incorpo-rating different contents of Cu powders after heat-treatment at dif-ferent temperatures are shown in Fig. 3. It gave an overall view ofthe compressive strength change. It can be observed that when theCu powders content was 1 wt%, the compressive strengthincreased apparently the most at all heat-treatment temperatures,which was 52%, 53%, and 41% higher than that of pure paste,respectively. Then despite the fact that further increase of Cu pow-ders decreased the compressive strength gradually, the pastesincorporating Cu powders still presented higher compressive

Fig. 1. The SEM image (a) and the XRD pattern (b) of Cu powders.

Table 2Volume heat capacity of the composite pastes incorporating different contents of Cupowders after heat-treatment at different temperatures.

Sign 105 �C (MJ m�3 K�1) 350 �C (MJ m�3 K�1) 900 �C (MJ m�3 K�1)

0Cu 2.365 1.615 1.1531Cu 2.376 1.637 1.1745Cu 2.424 1.708 1.27210Cu 2.475 1.811 1.38815Cu 2.536 1.898 1.506

Table 3Thermal conductivity of the composite pastes incorporating different contents of Cupowders after heat-treatment at different temperatures.

Sign 105 �C (W m�1 K�1) 350 �C (W m�1 K�1) 900 �C (W m�1 K�1)

0Cu 1.187 0.621 0.5421Cu 1.190 0.691 0.5735Cu 1.220 0.724 0.69210Cu 1.340 0.867 0.71115Cu 1.474 0.932 0.819

Fig. 2. Thermal expansion coefficient curves of the composite pastes incorporatingdifferent contents of Cu powders at elevated temperature.

Fig. 3. Compressive strength of the composite pastes incorporating differentcontents of Cu powders after heat-treatment at different temperatures.

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strength than 0Cu. As a matter of comparison, the gap in compres-sive strength between the pastes heated at 350 �C and the pastesheated at 900 �C gradually becomes smaller.

3.4. Characterization

3.4.1. Calorimetric curvesIn light of the high thermal properties of the composite pastes

incorporating 15 wt% Cu powders, the calorimetric curves of sam-ple 0Cu and 15Cu specimens are shown in Fig. 4. Soon after mixedwith water, an initial rapid exothermic reaction appears due to asurface hydrolysis and the release of ion into solution (Stage I). Itindicated that the addition of Cu powders decreased the initialreaction rate. This is followed by the deceleration of heat evolution(Stage II). Then the acceleration period (Stage III) occurred afterabout 15 h, and it indicated the maximum heat liberation ratesof sample 0Cu and 15Cu arrived at 4.13 and 3.05 m W g�1 whenthe reaction time reached 23.0 and 26.3 h, respectively. Subse-quently, the rate of heat decreased slowly (Stage IV), and reachedequilibrium (Stage V). The total heat of 0Cu and 15Cu after hydra-tion for 48 h were 244.15 and 208.72 J g�1, respectively.

3.4.2. XRD patternsFig. 5 shows the XRD patterns of pure Cu powders and the com-

posite pastes incorporating different contents of Cu powders afterheat-treatment at 105, 350, and 900 �C. The main diffraction peaksof Cu2O (JCPDS No. 78-2076) and part diffraction peaks of CuO(JCPDS No. 80-1268) were observed after heating at 350 �C(Fig. 5(a)). When the heat-treatment temperature further

Fig. 4. The calorimetric curves of pure paste and the composite pastes incorporating 15 wt% Cu powders: (a) rate of heat, and (b) total heat.

Fig. 5. XRD patterns of pure Cu powders after heat-treatment (a) and the composite pastes incorporating different contents of Cu powders after heat-treatment at 105 �C (b),350 �C (c), and 900 �C (d).

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increased to 900 �C, Cu powders (JCPDS No. 89-2838) finally trans-formed to CuO completely. However, the transformation of Cupowders in the solid pastes appeared to suffer the restriction fromthe hydration products due to the isolation from the air. Thehydration products of CA in pure paste and the pastes with Cupowders mainly were 3CaO�Al2O3�6H2O (C3AH6, JCPDS No. 77-240) and Al2O3�3H2O (AH3, JCPDS No. 29-41), which were shownin Fig. 5(b). The diffraction peaks intensity of Cu increased appar-ently with the increased incorporation of Cu. After heating at

350 �C for 6 h, the Cu2O diffraction peaks appeared gradually, andthe residual diffraction peaks of Cu still existed (Fig. 5(c)). Besides,a new dehydrated product 3CaO�Al2O3�SiO2�4H2O (C3ASH4, JCPDSNo. 32-151) was formed as well. The diffraction peaks at 29.4�and 33.3� were associated with 3CaO�Al2O3 (C3A) produced fromthe dehydration of the C3AH6 phase. Then observing Fig. 5(d), inthe pastes only the characteristic peaks of CuO were detected,which indicated that the Cu powders underwent an oxidation reac-tion completely. At the same time, some new mineral phases began

Fig. 6. The TG/DSC curves of sample 0Cu (a) and sample 15Cu (b).

Fig. 7. The pore distribution curves of sample 0Cu and sample 15Cu after heating at 105 �C (a), 350 �C (b), and 900 �C (c).

H. Yuan et al. / Cement & Concrete Composites 55 (2015) 139–144 143

to appear, such as CaO�Al2O3(CA), CaO�2Al2O3(CA2), and 12CaO�7Al2O3(C12A7).

3.4.3. TG/DSC curvesFig. 6 shows the TG–DSC curves of sample 0Cu and sample

15Cu. The measurement temperature ranged from room tempera-ture to 900 �C. As information given from Fig. 6(a) that there weretwo main endothermic peaks at 282.2 and 696.8 �C, respectively.The hydration products in aluminate cementitious system werequite complex. As a consequence, the endothermic peaks observedwere the results of the overlapped peaks. For example, rangingfrom 200 �C to 400 �C the dehydration of AH3 and C3AH6 are bothinvolved in the paste, pointing out the endothermic peak atapproximately 282.2 and 300.0 �C, respectively. And a series ofthe phases such as CA, CA2, and C12A7 were obtained after thedecomposition reaction ranging from 600 to 800 �C. It indicatedthat the dehydration of the products led to 13.7% mass loss in all.Similarly, the endothermic peaks associated with the dehydrationof AH3 and C3AH6 in the composite paste with Cu powders alsoappeared around 280 �C (Fig. 6(b)). The overall mass change beforeabout 500 �C can reach 10.0%. According to the XRD patterns inFig. 5, Cu in the solid pastes oxidized to Cu2O after heating at350 �C and then Cu2O oxidized to CuO after heating at 900 �C.The continuous oxidation process can lead to the weight increase.The TG curve of the composite paste demonstrated that the totalmass increase was 1.5%.

4. Discussion

The thermal and mechanical properties of the composite pastesare greatly affected by incorporating Cu powders, which is

elaborated as follows. First, the improved volume heat capacityof the composite pastes compared with sample 0Cu (pure paste)can be easily understood by the addition of Cu powders withhigher volume heat capacity mentioned above (Table 2). Afterheating at 105 �C, the Cu phase existed stably in the pastes(Fig. 5(b)), thereby giving rise to an improved volume heat capacityalong with the increased Cu powders. It is well known that the the-oretical volume heat capacity of water is 4.3 MJ m�3 K�1. Hence,the removal of a large amount of chemical-bonding water fromthe paste would also decrease the thermal and mechanical proper-ties after heating at 350 �C (Fig. 5(c)). In addition, in the heat-treatment process at 350 �C part of Cu powders oxidized to Cu2O.The theoretical volume heat capacity of Cu2O is 2.7 MJ m�3 K�1

[22], which is lower than that of Cu. Even so, Cu2O provided a masscompensation mechanism for the heated pastes by obtaining theoxygen, which might improve the pastes density in some degree(Fig. 6). When the heat-treatment temperature further increasedto 900 �C, Cu totally transformed into CuO (Fig. 5(d)). Normally,the increased heat-treatment temperature enlarged the pore distri-bution of sample 0Cu especially at 900 �C, as is shown in Fig. 7c.While the pore distribution of sample 15Cu obviously decreasedafter heating at 900 �C (Fig. 7(c)). Second, despite the Cu powdersadmixture have excellent thermal conductivity, the thermal con-ductivity optimization can hardly follow a linear trend like volumeheat capacity. Herein, the structure of the composite pastes incor-porating Cu powders might contribute more to the thermal con-ductivity evolution. Third, the addition of Cu powders increasedthe compressive strength especially at the Cu powders content of1 wt%. The reason can be the following: the stiffness of Cu powdersis higher than that of the pastes and the mixture of Cu powderswith the pastes is favorable to the compressive strength in theory.However, the addition of Cu powders suppressed the combination

144 H. Yuan et al. / Cement & Concrete Composites 55 (2015) 139–144

situation of water, which was one of most important factors deter-mining the hardened pastes structure. Besides, the incorporation ofCu powders also slowed down the release of ion into solution(Fig. 4a, Stage I). When the content of Cu powders reached a certainvalue, the increased Cu powders decreased the compressivestrength gradually.

5. Conclusions

In summary, the thermal/mechanical properties and some char-acterization of the aluminate cementitious composite materialincorporating metal Cu powders for thermal energy storage werereported. It is desirable that with the increase of Cu powders con-tent the thermal properties of the composite pastes improve.When the content of Cu powders was 15%, volume heat capacityand thermal conductivity of the composite paste reached1.506 MJ m�3 K�1 and 0.819 W m�1 K�1 after heating at 900 �C.The pastes incorporating Cu powders presented higher compres-sive strength than sample 0Cu. On the other hand, with theincrease of heat-treatment temperature Cu powders gradually oxi-dized to Cu2O and CuO, indicating a mass compensation mecha-nism. On the whole, the addition of Cu powders was beneficial toaluminate cementitious composite material especially at high tem-perature, which is available for the further development of thermalstorage materials for solar power plants.

Acknowledgements

The authors would like to express sincere thanks to PriorityAcademic Program Development of Jiangsu Higher EducationInstitutions (PAPD) and the Independent Research Topics of StateKey Laboratory of Materials-Oriented Chemical Engineering(ZK201211) for Financial Support.

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