Effect of Nanosilica on Mechanical and Thermal Properties of Cement Composites for Thermal Energy Storage Materials

Effect of Nanosilica on Mechanical and Thermal Properties of Cement Composites for Thermal Energy Storage Materials

Available online at www.sciencedirect.com ScienceDirect Energy Procedia 79 (2015) 10 – 17 2015 International Conference on Alternative Energy in Dev...

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Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 79 (2015) 10 – 17

2015 International Conference on Alternative Energy in Developing Countries and Emerging Economies

Effect of Nanosilica on Mechanical and Thermal Properties of Cement Composites for Thermal Energy Storage Materials Pongsak Jittabuta* a

Physics and General Science Program, Faculty of Science and Technology, Nakhon Ratchasima Rajabhat University, Nakhon Ratchasima, 30000, Thailand

Abstract This research was presented the mechanical and thermal properties of cement-based composite for thermal energy storage materials. The effects of nanosilica particle size and concentration determined by mixing three nanosilica particle sizes of 12, 50 and 150 nm, using nanosilica were of 1-5 wt%. Thermal properties coefficients were tested using a direct measuring instrument with surface probe (ISOMET2114). The influence of nanosilica on the performance, such as compressive strength, bulk density, thermal conductivity, volume heat capacity and thermal diffusivity of hardened composite cement pastes were studied for future solar thermal energy materials with better performance. According to the development of thermal storage materials and their application environment requirement in solar thermal power, the specimens were subjected to heat at 350oC and 900oC. There were observed that, before heating, the compressive strength is optimized at nanosilica amount of 4wt% with nanosilica particle size of 50 nm at the age of 28 days. Moreover, after heating at 350oC and 900oC, the thermal conductivity and volume heat capacity of the cement paste enriched with nanosilica were significantly lesser than that of the before heating one. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility the Organizing Committee of 2015 AEDCEE. Peer-review under responsibility of theofOrganizing Committee of 2015 AEDCEE

Keywords: Thermal properties, mechanical properties, cement composites, thermal energy storage, nanosilica

1. Introduction

Solar thermal power is being attractive to many researchers around the world because of its clean energy. Four main sections are required in the solar thermal power plant: concentrator, receiver, transportstorage media system and power conversion device. There is no sunshine at night and limitation of solar

* Corresponding author. Tel.: +66 44009009-1303. E-mail address: [email protected].

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Organizing Committee of 2015 AEDCEE doi:10.1016/j.egypro.2015.11.454

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energy available on cloudy days. So thermal energy storage (TES) systems balancing energy supply are designed to collect more solar energy during a sunny day, which is necessary component of four sections. Typical storage media for sensible heat consists of molten nitrate salt, rocks and pebbles or concrete [1, 2]. Thermal energy storage systems of four main sections can facilitate the integration of solar thermal power plants into the electrical grids by smoothing out fluctuations, thus avoiding instability problems and increasing electricity production [3]. Solid sensible materials store thermal energy through inherent characteristics with the change of temperature, and exhibit non-toxicity and non-corrosiveness. Cementitious materials have been extensively studied as a promising type of sensible energy storage materials due to abundant source, good thermal stability, and good thermal properties. Some efforts are made to further optimize cementitious thermal storage materials and improve the storage efficient [4-6]. Studies on nanoparticles in cementitious-concrete materials have started since the early 2000s [5, 7]. Larger surface area due to theirs nanoparticles size can maximize the surface activity. The use of nanoparticles in cementitious-concretes materials significantly modifies their behavior not only in the fresh but also in the hardened conditions as well as the physical, mechanical and microstructure development. So, we introduced nanosilica into cement composites materials and prepared solar thermal storage composite materials. The mixtures of nanosilica were prepared with the cement replacement of 15wt%. by mixing three nanosilica particle with average particle sizes of 12, 50 and 150 nm, respectively. As well, they were heated at 350oC and 900oC respectively. Todate, few studies have been reported on thermal and mechanical properties in assessment of cement composites for thermal energy materials. 2. Experimental details and testing analysis

Ordinary Portland Cement (OPC) which obtained from SCG Experience Company Limited of Thailand conforming to ASTM C150 standard was used. The mixtures of nanosilica were prepared with the cement replacement of 1%, 2%, 3%, 4% and 5% by weight. The water-cementitious ratio (W/C) was 0.5 and three contents of nanosilica particle with average particle sizes of 12, 50 and 150 nm, respectively. The paste samples of 10cm×10cm×10cm were prepared, cured and their thermal property measured at the curing ages of 28 days. In addition, three specimens were tested for bulk density in accordance with the standard of ASTM B962-08 [8] and thermal property coefficients were tested using a direct measuring instrument with surface probe (ISOMET2114, Applied Precision Ltd.), see Fig. 1. The compressive strength of three specimens were tested in accordance with the standard of ASTM C109 [9]. All paste samples of 10cm×10cm×10cm were prepared, cured and their compressive strength measured at the curing ages of 28 days. Furthermore, the selection of heat treatment temperature depends strongly on the actual operation conditions in solar energy application. In this paper, we mainly chose two heat treatment temperatures of 350oC and 900oC respectively. The heat treatment time is 6 hours [6].

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Fig. 1. ISOMET apparatus and its surface lead. 3. Results and discussions 3.1. Bulk Density of The Pastes The variation of the paste density when adding nanosilica is shown in Fig. 2. It can be clearly seen that for all nanosilica sizes, the addition of nanosilica reduced the sample density in comparison to the control cement paste. For the cement pastes with 12 nm nanosilica (Fig. 2a), the density monotonously decreases with increasing amount of nanosilica. On the other hand, the cement pastes with 50 nm and 150 nm nanosilica (Fig. 2b and 2c) showed a drop in density for the nanosilica addition of 1-2% and a slightly increase for 3-4% and a drop again at 5%. Overall, it can be concluded that the density has a tendency to decrease with an addition of nanosilica. This observation is different from the previous works [10, 11] where the increase in density was observed when nanosilica was added which was due to the filler effect of nanosilica. However, the earlier studies exploited the addition of superplasticizer in the mixture to control the flow rate. The superplasticizer can reduce the amount of water required in mixing the cement pastes but the amount of superplasticizer in each mixture is different. In the present study, we intend to investigate the effect of nanosilica addition on the mechanical and thermal properties without superplasticizer to control all other parameters under the same condition. As a result, with the same W/B ratios, the cement pastes become thicker for higher nanosilica concentration.

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Fig. 2. Bulk density of 7 and 28 days cement paste with addition of nanosilica (a) 12 nm, (b) 50 nm, (c) 150 nm. 3.2. Compressive Strength of Pastes The results of compressive strengths of nanosilica added cement pastes are shown in Fig. 3. In order to compare the effect of nanosilica sizes, we plot the strength of cement paste with 12, 50 and 150 nm nanosilica in the same plot and separate the measurement at the age of 7 days and 28 days in Fig. 3a and 3b, respectively. There are many interesting points regarding this plot. Firstly, the compressive strength increases to the maximum point with the addition of certain amount of nanosilica and then drops when excessive nanosilica is introduced. For 12 nm nanosilica, the pastes show the maximum strength at 1% addition while for 50 and 150 nm sizes, the strength is maximized at 4% nanosilica. Secondly, the size of nanosilica has a significant effect on the strength. The cement paste with 50 nm nanosilica shows the higher strength than the others, with the maximum strength of >50% higher than the control cement paste. The possible reasons for this experimental result are discussed as follows. When nanosilica was added, it stimulated the hydration reaction of the cement particles and form C-S-H gel resulting in the higher compressive strength. Moreover, nanosilica has pozzolanic reaction with the Ca(OH) 2 crystals to form additional C-S-H phase. This prevents the Ca(OH)2 crystals from growing and strengthens the pastes. However, the excessive nanosilica absorbed water required for the cement hydration process, reducing the final C-S-H product. The nanosilica particle may also surround cement particles making them inaccessible to water. Also, when the nanosilica is not well dispersed, as in the case for excessive

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nanosilica content, the aggregated nanosilica can create weak zone in form of voids and lowered the compressive strength. Since 12 nm nanosilica is the most reactive, it is required only 2% to obtain the maximum strength and more than that amount leads to the adverse effect. On the other hand, for less reactive nanosilica with the particles size of 50 and 150 nm, it requires about 4% for the maximum strength. Furthermore, the strength of cement paste is related to the density. Since the density of pastes with 12 nm nanosilica is lower than those with 50 and 150 nm for high nanosilica%, the strength of paste with 12 nm nanosilica also drops. In addition, for all samples regardless the particle size and concentration, the compressive strength is higher for the samples cured at 28 days due to the advance in hydration. 60 (a)

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Compressive Strength (MPa)

Compressive Strength (MPa)

45 35 30 25

20 Silica 150 nm Silica 50 nm Silica 12 nm

15

10 5 0

0

1

2 3 NanoSilica (%)

4

5

(b)

50 40 30 Silica 150 nm Silica 50 nm Silica 12 nm

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10 0 0

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2 3 Nanosilica (%)

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Fig.3. Compressive strength cement paste added nanosilica with different particle size, measured at (a) 7 days and (b) 28 days. 3.3. Thermal Conductivity of Pastes The thermal conductivity of pastes with different quantity of nanosilica before and after heatings were shown in Fig. 4. The result was shown that the thermal conductivity of cement pastes slightly decrease with the increment of quantity. However, after heating both at 350 and 900 oC for 6 hours the thermal conductivities reduce approximately 38%. Accordingly, the heat-treatment indeed affects the thermal conductivity of pastes. Conspicuously, the thermal conductivity of composite pastes could keep a higher value than that of pure paste that without nanosilica. Especially at the nanosilica quantity of 4 wt%, the thermal conductivity of pastes heated at 900oC was 0.647 W/mK, which is close to the thermal conductivity value of pastes at 350oC. Although the overall thermal conductivity has not been improved greatly, the comparatively better value of composite pastes with nanosilica were shown a basic candidate. Consequently, application of nanosilica cement paste in building constructions may be an interesting solution in order to improve sustainability and building energy efficiency.

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Thermal Conductivity (W/mK)

1.20

Before Heating 350 900

1.00 0.80

0.60 0.40

0.20 0.00

0

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2 3 4 Ratio of nanosilica praticles (%)

5

Fig.4. Thermal conductivity of pastes with different quantity of nanosilica before and after heating. 3.4. Volume Heat Capacity of Pastes After heating volume heat capacity of all pastes display of inorgainc solid materials is related to inherent materials compositions and material porosity. Decrease of volume heat capacity after heating my be due to inherent materials compositions. However, volume heat capacity still has a bit increases after adding nanosilica to the pastes, which is avilable for the optimization of the performance. Fig. 5 has shown that, before heating the cement paste, the volume heat capacity of every ratio of nanosilica particle admixed into cement paste remained constant at 1.7 MJ/m3K. Whereas, after heating them at 350oC and 900oC for 6 hours, the volume heat capacity of them reached to the maximum volume at 1.34 and 1.028 MJ/m3K (at the ratios of nanosilica particle between 3-4 wt%.) respectively.

Volume Heat Capacity MJ/m 3K)

2.00

1.80 1.60 1.40 1.20 1.00 0.80

Before Heating 350 900

0.60 0.40 0.20 0.00

0

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2 3 4 Ratio of nanosilica praticles (%)

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Fig.5. Volume heat capacity of pastes with different quantity of nanosilica before and after heating.

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3.5. Thermal Diffusivity of Pastes The difference in thermal diffusivity values of composite pastes enriched with nanosilica is likely to be caused by the dehydration of the hydration products. Fig. 6 has shown that, before heating the cement paste, the thermal diffusivity of every ratio of nanosilica particle admixed into cement paste, the thermal diffusivity of every ratio of nanosilica particle admixed into cement paste remained constant around 0.9-1 μm2/s. While, after heating them at 350oC and 900oC for 6 hours, the thermal diffusivity remained constant around 0.5-0.6 μm2/s. The addition of nanosilica was beneficial to the composite pastes.

Thermal diffusivity (μm 2/s)

1.20 1.00 0.80 0.60 0.40

Before Heating 350 900

0.20 0.00 0

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2 3 4 Ratio of nanosilica praticles (%)

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Fig.6. Thermal diffusivity of pastes with different quantity of nanosilica before and after heating.

4. Conclusions 1. The compressive strength increases to the maximum values with the addition of certain amount of nanosilica due to the highly reactive nature of nanosilica which stimulates the hydration reaction. 2. The 12 nm nanosilica particle with the largest surface area is the most reactive and can enhance the strength even with only 1 % by weight substitution. Higher content of nanosilica decreases the strength due to the lack of water for hydration reaction, the creation of weak zone from nonuniform distribution and the reduction of density. 3. For the use of nanosilica cement composites for thermal insulation, it requires both strength and low thermal conductivity. Hence, the cement paste with 50 nm nanosilica of 4-5% is the most suitable composition which provides high compressive strength and relatively low thermal conductivity. 4. The best of thermal properties, the thermal conductivity in the rage of 0.42-0.57W/mK (after heating at 900oC), the volume heat capacity in the rage of 1.63-1.73 MJ/m3K (non-heating) and the thermal diffusivity in the rage of 0.84-0.94μm2/s (after heating at 900oC) respectively. These properties of cement composite from my research were illustrated that available and beneficial for future composite thermal storages.

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Acknowledgements This study is financially supported by the Sustainable Infrastructure Research and Development Center (SIRDC), a research unit in Khon Kaen University and Nakhon Ratchasima Rajabhat University. References [1] G. Antoni, et al., State of the art on high temperature thermal energy storage for power generation. Part 1-Concepts, materials and modellization, Sustainable Energy Rev 2010;14:31-55. [2] D. Laing, et al., Economic Analysis and Life Cycle Assessment of Concrete Thermal Energy Storage for Parabolic Trough Power Plants, J. Sol. Energy Eng 2010;132(4):041013. [3] D. Laing, et al., Solid Media Thermal Storage Development and Analysis of Modular Storage Operation Concepts for Parabolic Trough Power Plants, J. Sol. Energy Eng 2007;130(1):011006. [4] H. Li, et al., Economic assessment of the mobilized thermal energy storage (M-TES) system for distributed heat supply, Appl. Energy 2013;104:178-186. [5] H. Yuan, et al., Mechanical and thermal properties of cement composite graphite for solar thermal storage materials, Solar Energy 2012;86(11):3227-3233. [6] H. Yuan et al., Influence of nano-ZrO2 on the mechanical and thermal properties of high temperature cementitio, Construction and Building Materials 2013; 48:6-10. [7] J.M. Fernández, et al., Influence of nanosilica and a polycarboxylate ether superplasticizer on the performance of lime mortars, Cem. Concr. Res. 2013;43:12-24. [8] ASTM B962-08, Standard Test Methods for Density of Compacted or Sintered Powder Metallurgy (PM) Products Using Archimedes’ Principle, ASTM International, West Conshohocken, PA, 2008, www.astm.org [9] ASTM C109 / C109M-13, Standard Test Method for Compressive Strength of Hydraulic Cement Mortars (Using 2-in. or [50mm] Cube Specimens), ASTM International, West Conshohocken, PA, 2013, www.astm.org [10] Li H., Xiao H.G., Yuan J., Ou J.P., Microstructure of cement mortar with nano-particles, Compos Part B-Eng 2004;35:185189. [11] Berra M., Carassiti F., Mangialardi T., Paolini A.E., Sebastiani M., Effects of nanosilica addition on workability and compressive strength of Portland cement pastes, Constr Build Mater 2012;35:666-675.

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