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Effect of foaming gas and cement type on the thermal conductivity of foamed concrete
Construction and Building Materials 231 (2020) 117197
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of foaming gas and cement type on the thermal conductivity of foamed concrete Tian Li a, Fangmei Huang a, Jiang Zhu a, Jinhui Tang b,c, Jiaping Liu a,b,c,⇑ a
College of Materials Science and Engineering, Chongqing University, Chongqing 400045, PR China Jiangsu Sobute New Materials Co., Ltd., Nanjing 211103, China c State Key Laboratory of High Performance Civil Engineering Materials, Nanjing 211103, China b
a r t i c l e
i n f o
Article history: Received 17 April 2019 Received in revised form 11 September 2019 Accepted 6 October 2019
Keywords: Foamed concrete Foaming gas Cement type Thermal conductivity
a b s t r a c t This study has been carried out to investigate the effect of the thermal conductivity of foaming gas and cement type on that of foamed concrete. The results show that the foamed concrete employed a gas having lower thermal conductivity as foaming gas or prepared from a type of cement with lower thermal conductivity takes on lower thermal conductivity. And the thermal conductivity of foamed concrete is directly proportional to the thermal conductivity of foaming gas. In addition, with the length of age, the thermal conductivity of the foamed concrete whose foaming gas is not air gradually approaches to those using air as foaming gas at the initial stage. Using the foaming gas or cement with lower thermal conductivity to prepare foamed concrete may be a feasible as well as effective way to improve the thermal insulation of foamed concrete. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction In some countries, building energy consumption has exceeded 35% of total energy consumption [1,2]. Targeting to reduce the building energy consumption, many countries have introduced relevant policies, including the use of insulation materials [3–5]. Insulation materials are a porous lightweight material and can be categorized into two categories [6,7]. One is organic insulation materials and the other is inorganic insulation materials. In the past few decades, organic insulation materials had been widely used because of its advantages, such as recyclable and save resource [8]. However, some fire disasters have made people aware of organic insulation materials exist fire safety hazard [9,10]. Inorganic insulation materials, therefore, have gradually replaced organic insulation materials been used in building exterior insulation systems recently due to its good fire-resistance performance. One of the commonly used inorganic insulation materials is foamed concrete. Foamed concrete is a cement-based material prepared by physical foaming or chemical foaming [11,12]. However, compared to organic insulation material, foamed concrete shows higher thermal conductivity, which restricts its application at some
⇑ Corresponding author at: College of Materials Science and Engineering, Chongqing University, Chongqing 400045, PR China. E-mail address: [email protected] (J. Liu). https://doi.org/10.1016/j.conbuildmat.2019.117197 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
extent [13]. Therefore, decreasing the thermal conductivity of foamed concrete has practical significance. Foamed concrete can be roughly regarded as a combination of hardened cement paste and pores, wherein the formation of pores is caused by foaming agent [14]. Therefore, the properties of foaming agent and cement may have a certain effect on the thermal conductivity of foamed concrete. Li et al. [15], Ma et al. [16], Fu et al. [17] and Chen et al. [18] studied the foamed concrete from magnesia phosphate cement, respectively. The corresponding foaming agents are hydrogen peroxide, sodium carbonate, zinc powders and protein. Their results show that the thermal conductivity of foamed concrete prepared from magnesia phosphate cement presents a significantly difference due to the type of foaming agent and the foamed concrete whose foaming agent is sodium bicarbonate displays the lowest thermal conductivity. Huang et al. [19] undertook a study of Portland cement-based foamed concrete employing hydrogen peroxide as foaming agent. Jiang et al. [20] investigated the properties of foamed concrete based on ordinary Portland cement using animal protein as foaming agent. From Huang et al. and Jiang et al. results, it can be found that, at the same density, the thermal conductivity of the two Portland cement-based foamed concretes are very close. It is interesting to note that the gas introduced by the above mentioned foaming agents are different. The gas introduced by protein is air, the gas generated by metal powers is hydrogen, the gas generated by hydrogen peroxide is oxygen, and the gas produced by
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T. Li et al. / Construction and Building Materials 231 (2020) 117197
sodium bicarbonate is carbon dioxide. Here, we refer to this kind of gas introduced by foaming agent as foaming gas. The foaming gas would cause a porous structure in foamed concrete and fill in the pores of foamed concrete. The thermal conductivity of four foaming gases, at 20 °C, is shown in Table 1[21,22]. Based on the above reported results and the thermal conductivity of four gases, it is reasonable to believe that foaming gas may be a critical factor affecting the thermal conductivity of foamed concrete. Zhang et al. [23] and Pan et al. [24] respectively prepared geopolymer foamed concrete and Portland cement foamed concrete using the same foaming agent. However, their results show that the thermal conductivity of foamed concrete prepared from different types of cement takes on a considerable difference. From the results of Chen et al. [18] and Jiang et al. [20], it is also found that the thermal conductivity of foamed concrete from magnesia phosphate cement is higher than Portland cement-based foamed concrete. In the above comparison cases, one of the main variables is the type of cement. Therefore, the type of cement may be another key factor affecting the thermal conductivity of foamed concrete. In this paper, the effect of foaming gas and cement type on the thermal conductivity of foamed concrete was investigated. We carried out this study by combining theoretical analysis with experimental verification, and four foaming gases and three types of cement were employed. Hope that the results of this study can provide a theoretical foundation for improving the thermal insulation of foamed concrete.
Table 2 Chemical composition of the OPC, SAC and MgO (% by mass). Chemical composition
OPC
SAC
MgO
SiO2 CaO Fe2O3 Al2O3 P2O5 TiO2 MgO Na2O K2O SO3 LOI
21.82 64.93 4.53 4.36 – – 1.06 0.15 0.39 1.89 0.87
15.55 47.14 1.45 21.47 – 0.91 2.36 0.20 0.37 10.53 0.12
6.93 3.35 1.42 0.57 0.24 87.21 – – – 0.28
2. Experimental 2.1. Materials The cementitious materials employed for prepare foamed concrete are magnesium phosphate cement (MPC), ordinary Portland cement (OPC, 42.5R) and sulfoaluminate cement (SAC). The raw materials of MPC are dead-burned magnesia (MgO), potassium dihydrogen phosphate (KDP) and borax (B). The mass ratio of KDP to MgO is 0.25, in the meantime the mass ratio of B to MgO is 0.12. The specific surfaces of the OPC, SAC and MgO are 353, 432 and 230 m2/kg, respectively. The chemical compositions of them are listed in the Table 2. The purity of the KDP and B is more than 95% and the X-ray diffractions (XRD) of them are shown in Fig. 1. The four foaming gases employed are air, hydrogen, oxygen and carbon dioxide. The corresponding foaming agents employed are protein (P), zinc powders (Zn), hydrogen peroxide (H2O2) and sodium bicarbonate (NAC), respectively. The protein-based foaming agent is foamed to a density of 40 kg/m3 for use. The NAC used is a chemically pure reagent. The content of H2O2 employed is 30%. In addition, calcium stearate (CaS) is employed as foam stabilizer. 2.2. Mix proportions The mix proportions for investigate the effect of foaming gas and cement type on the thermal conductivity of foamed concrete are presented in Table 3 and Table 4, respectively. 2.3. Specimen preparation The specimens were prepared following these steps according to the mix compositions (Table 3, Table 4). Firstly, cement, CaS,
Fig. 1. The XRD patterns of the KDP and B.
and MnO2 (if necessary) were casted into a vertical-axis planetary mixer and mixed at low speed. After mixing for 1 min, water was added to the planetary mixer followed by another 2.5 min mixing. Then, foaming agent or the prepared foam was introduced and mixed at high speed for 30 s. Finally, the foamed concrete paste was placed into the slab molds (300 mm 300 mm 30 mm), and then the specimens were transferred into a curing room after demolded. 2.4. Measurement methods 2.4.1. Thermal conductivity The thermal conductivity of foam concrete was measured according to ASTM C 518 [25]. The measured specimens were the foamed concrete slab with size 300 mm 300 mm 30 mm. Before the thermal conductivity was measured, the cured specimens were dried in a 45 vacuum oven until the mass remained unchanged. The thermal conductivity presented in this paper is the average of three specimens and the accuracy is 0.001 W/(mK). 2.4.2. Pore size distribution The investigation of pore size distribution was performed according to the following steps. Firstly, capturing the crosssectional picture of sample and transforming it into binary form.
Table 1 Thermal conductivity of four gases at 20 °C (W/(mK)). Gas
air
hydrogen
oxygen
carbon dioxide
Thermal conductivity
0.0242
0.1760
0.0240
0.0143
3
T. Li et al. / Construction and Building Materials 231 (2020) 117197 Table 3 Mix compositions for investigate the effect of foaming gas. No.
Composition of mixture per m3
Dry density (kg/m3)
MPC (kg)
H2O (kg)
P (kg)
Zn (kg)
NAC (kg)
H2O2 (kg)
MnO2 (kg)
CaS (kg)
I-400 I 500 I 600 I 700 I 800 I 900 II-400 II 500 II 600 II 700 II 800 II 900 III-400 III 500 III 600 III 700 III 800 III 900 IV-400 IV 500 IV 600 IV 700 IV 800 IV 900
337 421 506 590 674 759 337 421 506 590 674 759 337 421 506 590 674 759 337 421 506 590 674 759
40.4 50.5 60.7 70.8 80.9 91.1 40.4 50.5 60.7 70.8 80.9 91.1 40.4 50.5 60.7 70.8 80.9 91.1 40.4 50.5 60.7 70.8 80.9 91.1
1.80 1.72 1.65 1.58 1.50 1.43 – – – – –
– – – – –
– – – – –
– – – – –
– – – – –
– – – – –
– – – – –
– – – – –
– – – – –
4.2 4.0 3.9 3.8 3.6 3.4 – – – – –
– – – – –
– – – – –
– – – – –
– – – – –
5.5 5.3 5.1 4.8 4.6 4.4 – – – – –
11.3 10.9 10.5 10 9.6 9.2
6.1 7.7 9.2 10.8 12.8 13.8
0.62 0.77 0.92 1.08 1.23 1.38 0.62 0.77 0.92 1.08 1.23 1.38 0.62 0.77 0.92 1.08 1.23 1.38 0.62 0.77 0.92 1.08 1.23 1.38
405 503 607 706 809 902 403 506 602 711 804 908 409 507 606 703 811 906 403 506 607 710 812 918
Table 4 Mix compositions for investigate the effect of cement type. No.
Composition of mixture per m3
Dry density (kg/m3)
MPC (kg)
OPC (kg)
SAC (kg)
H2O (kg)
P (kg)
CaS (kg)
I-400 I-500 I-600 I-700 I-800 I-900 II-400 II-500 II-600 II-700 II-800 II-900 III-400 III-500 III-600 III-700 III-800 III-900
337 421 506 590 674 759 – – – – – – – – – – – –
– – – – – – 333 417 500 583 667 750 – – – – – –
– – – – – – – – – – – – 286 357 429 500 571 643
40.4 50.5 60.7 70.8 80.9 91.1 173.2 216.8 260.0 303.2 346.8 390.0 157.3 196.4 236.0 275.0 314.0 353.6
1.80 1.72 1.65 1.58 1.50 1.43 1.64 1.48 1.32 1.16 1.00 0.84 1.70 1.55 1.40 1.26 1.11 0.96
0.62 0.77 0.92 1.08 1.23 1.38 0.61 0.77 0.92 1.07 1.23 1.38 0.53 0.66 0.79 0.92 1.05 1.18
Secondly, identifying and computing the average diameter of the white area (i.e. pore size) adopting Image-pro plus 6.0 software. Finally, the pore sizes were analyzed statistically. The typical cross-sectional picture and its binary form are presented in Fig. 2. 3. Results and discussion 3.1. Foaming gas 3.1.1. Theoretical analyses Some scholars analyzed the thermal conductivity of foamed concrete and summarized an empirical equation (Eq. (1)) [26,27].
k¼
ðk2 k1 Þq1 þ k1 ð5q2 4q1 Þz
ð1Þ
where: k is the thermal conductivity of foamed concrete; k1 and k2 are the thermal conductivity of foaming gas and cement paste,
405 503 607 706 809 902 404 502 604 709 803 906 402 508 598 702 807 903
respectively; q1 and q2 are the dry density of foamed concrete and cement paste, respectively; z is the ratio of the closed pores to all the pores of foamed concrete, and the value of z is equal to 1 when all the pores are closed. In this section, we theoretically calculated the thermal conductivity of foamed concrete employing Eq. (1) and the results are shown in Fig. 3. The cement used is MPC and the thermal conductivity of MPC paste at dry density of 2100 kg/m3 is 1.236 W/(mK). The foaming gases employed are air, oxygen, hydrogen and carbon dioxide. In the calculation process, it is assumed that all the pores are closed and the pore structures of all prepared foamed concrete are the same. From Fig. 3, it can be seen that, for a given foaming gas, the thermal conductivity of foamed concrete increases with dry density. In addition, this figure also shows that, in terms of foamed concrete with a given density, the thermal conductivity of whose employed hydrogen as foaming gas is the highest and the thermal conductivity of whose using carbon dioxide as foaming gas is the lowest.
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T. Li et al. / Construction and Building Materials 231 (2020) 117197
Fig. 2. The typical cross-sectional picture and its binary form.
air hydrogen
Thermal conductivity (W/(m·k))
0.30
oxygen carbon dioxide
0.25 0.20 0.15 0.10 0.05
400
500
600
700
800
900
3
Dry density (kg/m ) Fig. 3. The calculated thermal conductivity of foamed concrete with different foaming gas.
Therefore, it can be concluded that, for a given dry density, the foamed concrete employed a gas having lower thermal conductivity as foaming gas takes on lower thermal conductivity.
3.1.2. Experimental analyses In order to verify whether the conclusion of the theoretical analyses is correct, the related experiment was carried out. The foamed concrete from MPC was prepared according to the mix proportions (Table 3). The corresponding foaming gases are air, oxygen, hydrogen and carbon dioxide, respectively. The thermal conductivity was measured on the 7th day of curing since the hydration of MPC is basically completed on the 7th day [28–30]. In this work, we measured the thermal conductivity at 7 d, 9 d, 11 d, 13 d, and 15 d and the results are presented in Fig. 4. From Fig. 4, it can be seen that, for a given foaming gas and given age, the thermal conductivity of foamed concrete increases with dry density. This figure also indicates that, for a given density, the fluctuation of thermal conductivity of the foamed concrete employed air or oxygen as foaming gas with age is very small, which can be ignored. However, the thermal conductivity of the foamed concrete employed hydrogen as foaming gas decreases first with age and then remains unchanged, and the thermal conductivity of the foamed concrete employed carbon dioxide as foaming gas increases first with age and then would not change any more. The reason for this phenomenon might be that foamed concrete contains unclosed pores and closed pores and the difference in thermal conductivity between foaming gases. The foaming gas filled in the unclosed pores can exchange with the air in the
atmosphere and is replaced gradually by air with the increase of age. The foaming gas contained in the closed pores, however, remains unchanged with age. Since the value of thermal conductivity of oxygen is approximately equal to that of air, the thermal conductivity of the foamed concrete whose foaming gas is oxygen does not change with age. On the other hand, as a result of the thermal conductivity between hydrogen (or carbon dioxide) and air has a large difference, therefore the thermal conductivity of foamed concrete employed hydrogen or carbon dioxide as foaming gas exhibits the above phenomenon with age. In addition, by comparing the results shown in Fig. 4 with the calculation results shown in Fig. 3, it can be found that, for a given foaming gas, the measured value of thermal conductivity of the foamed concrete is not equal to the theoretical calculated value. The reason for this might be that the pores of foamed concrete are not all closed pores. For observe the influence of thermal conductivity of foaming gas on the thermal conductivity of foamed concrete more clear and intuitive, the measured results of thermal conductivity of the foamed concrete, at 15 d, with different type of foaming gas were ploted in figure (Fig. 5). From Fig. 5, it can be seen that the thermal conductivity of the foamed concrete employed hydrogen as foaming gas is the highest and the thermal conductivity of foamed concrete employed carbon dioxide as foaming gas is the lowest. This result concurs with the result of Section 3.1.1. According to the reported literatures [31–33], the pore size distribution can affect the properties of foamed concrete. To this end, the pore size distribution of the foamed concrete at a dry density of 600 kg/m3 was surveyed and the results are presented in Fig. 6. From Fig. 6, it can be seen that the pore size distributions and average pore size of the foamed concrete with different type of foaming gas are very close. Therefore, here, it can be roughly believed that the factor affecting the thermal conductivity of foamed concrete is only the thermal conductivity of foaming gas. From the above analysis, it can be roughly understood that, for a given dry density, the smaller the thermal conductivity of the foaming gas is, the smaller the thermal conductivity of the prepared foamed concrete is. For clarify the relation between the thermal conductivity of foaming gas and the thermal conductivity of foamed concrete, the thermal conductivity of the foamed concrete is plotted as a function of the thermal conductivity of the foaming gas in Fig. 7. It can be seen from Fig. 7 that, for a given dry density, the thermal conductivity of the foamed concrete is directly proportional to the thermal conductivity of foaming gas. The relationship of them can be expressed using the following equation:
y ¼ a þ b x; R2 ¼ 0:9233 0:9994
ð2Þ
where: y = the thermal conductivity of foamed concrete; x = the thermal conductivity of foaming gas; a and b are constants. According to the value of dry density, the value of R2 varies from 0.9233 to 0.9994, indicating a strong relationship between the thermal con-
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T. Li et al. / Construction and Building Materials 231 (2020) 117197
0.36
0.36
7d 13 d
0.30
9d 15 d
0.33
11 d Thermal conductivity(W/(m⋅k))
Thermal conductivity(W/(m⋅k))
0.33
0.27 0.24 0.21 0.18 0.15 0.12
500
600
700
800
9d 15 d
11 d
0.27 0.24 0.21 0.18 0.15 0.12
400
7d 13 d
0.30
900
400
500
600
700
(a)
(b) 0.35
0.50 7d 13 d
0.40
9d 15 d
0.30
11 d
Thermal conductivity(W/(m⋅k))
0.45
Thermal conductivity(W/(m⋅k))
900
Dry density(kg/m )
Dry density(kg/m )
0.35 0.30 0.25 0.20 0.15
800 3
3
400
500
600
700
800
900
3
7d 13 d
9d 15 d
11 d
0.25 0.20 0.15 0.10 0.05
400
500
600
700
800
900
3
Dry density(kg/m )
Dry density(kg/m )
(c)
(d)
Fig. 4. The thermal conductivity of the foamed concrete from MPC and the foaming gases are air (a), oxygen (b), hydrogen (c) and carbon dioxide (d), respectively.
Thermal conductivity(W/(m⋅k))
0.40
air oxygen hydrogen carbon dioxide
0.35 0.30 0.25 0.20 0.15 400
500
600
700
800
900
3
Dry density(kg/m ) Fig. 5. The thermal conductivity of the foamed concrete, at 15 d, with different type of foaming gas.
ductivity of foamed concrete and the thermal conductivity of foaming gas.
3.2. Cement type 3.2.1. Theoretical analyses In this part, we still adopted Eq. (1) to theoretically analyze the effect of cement type on the thermal conductivity of foamed con-
crete. The assumptions in this part are the same as that of Section 3.1.1, i.e. all the pores are closed and the pore structures of all prepared foamed concrete are the same. In addition, the thermal conductivity of OPC paste and SAC paste, at a dry density of 2100 kg/m3, were also measured. The measured results were 0.4544 W/(mK) and 0.4307 W/(mK), respectively. The thermal conductivity calculated of foamed concrete from different type cement is shown in Fig. 8. From Fig. 8, it can be seen that, for a given foaming gas, the thermal conductivity of SAC-based foamed concrete is the lowest and the thermal conductivity of MPC-based foamed concrete is the highest. This result concurs with the measured result of the thermal conductivity of cement pastes. It also can be seen from this figure that the thermal conductivity of the foamed concrete whose the cement is OPC is closed to those the cement is SAC. In addition, this figure also shows that, for a given cement type, the thermal conductivity of the foamed concrete employed air as foaming gas is lower than the thermal conductivity of the foamed concrete employed hydrogen as foaming gas. This result does accord with the conclusions in section 3.1. From this section, it can be concluded that the foamed concrete prepared from the cement having a lower thermal conductivity shows lower thermal conductivity.
3.2.2. Experimental analyses The authors conducted the related experiment for verify the conclusion from Section 3.2.1. The foamed concrete from different type cement was prepared according to Table 4 and the thermal conductivity of specimens was measured after cured 28 d. The
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T. Li et al. / Construction and Building Materials 231 (2020) 117197
0.300 Air Oxygen Hydrogen Carbon dioxide
Percent (%)
40
30
20
10
0.285 Average pore size (mm)
50
0.270 0.255 0.240 0.225 0.210 0.195
0
0.180 0.15-0.20
0.20-0.25
0.25-0.30
0.30-0.35
0.35-0.40
Air
Oxygen
Hydrogen
Carbon dioxide
Foaming gas
Pore size (mm)
0.40
400kg/m3 700kg/m3
500kg/m3 800kg/m3
600kg/m3 900kg/m3
0.35 MPC
Thermal conductivity (W/(m⋅k))
Thermal conductivity of foamed concrete (W/(m·k))
Fig. 6. The pore size distribution and average pore size of the foamed concrete with a dry density of 600 kg/m3.
0.35 0.30 0.25 0.20 0.15 0.00
0.03
0.06
0.09
0.12
0.15
0.18
Thermal conductivity of foaming gas (W/(m·k))
OPC
SAC
0.30 0.25 0.20 0.15 0.10 0.05
400
500
600
700
800
900
3
Dry density (kg/m ) Fig. 7. The relationship between the thermal conductivity of foaming gas and the thermal conductivity of the prepared foamed concrete.
0.20
The pore size distributions of foamed concrete from different type cement were also surveyed, and the foamed concrete with a dry density of 600 kg/m3 was taken as an example for illustration. The results are shown in Fig. 10. From Fig. 10, it can be seen that the pore size distributions and average pore size of foamed concrete from different type cement are also very close. This indicates that the effect of the pore structure on the thermal conductivity of foamed concrete can be ignored.
Thermal conductivity (W/(m⋅k))
0.18 MPC-Air SAC-Air OPC-Air
0.16
Fig. 9. The measured thermal conductivity of foamed concrete from different type cement.
0.14 0.12 0.10 0.08
4. Conclusion
0.06
This paper studied the effect of the thermal conductivity of foaming gas and cement type on that of foamed concrete. From the above results, we draw the following conclusions:
0.04 400
500
600
700
800
900
3
Dry density (kg/m ) Fig. 8. The thermal conductivity calculated of foamed concrete from different type cement.
results are shown in Fig. 9. From this graph, it is clear that the thermal conductivity of MPC-based foamed concrete is the highest and the thermal conductivity of SAC-based foamed concrete is the lowest. In addition, the thermal conductivity of OPC-based foamed concrete is close to the thermal conductivity of SAC-based foamed concrete. This result concurs with the conclusion in Section 3.2.1.
The thermal conductivity of foaming gas and cement type can affect the thermal conductivity of foamed concrete. The thermal conductivity of foamed concrete is directly proportional to the thermal conductivity of foaming gas and the relationship of them shows a high correlation coefficient. Due to the presence of unclosed pores, the thermal conductivity of the foamed concrete whose foaming gas is hydrogen or carbon dioxide gradually approaches to those using air as the foaming gas with the increase of age at the initial stage. Foamed concrete prepared from a type of cement with lower thermal conductivity takes on lower thermal conductivity.
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T. Li et al. / Construction and Building Materials 231 (2020) 117197
0.300
50 MPC OPC SAC
0.270 Average pore size (mm)
Percent (%)
40
0.285
30
20
10
0.255 0.240 0.225 0.210 0.195
0
0.180
0.15-0.20
0.20-0.25
0.25-0.30
0.30-0.35
0.35-0.40
Pore size (mm)
MPC
OPC
SAC
Cement type
Fig. 10. The pore size distribution and average pore size of the foamed concrete, at a dry density of 600 kg/m3, from different type cement.
Conflict of interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service or company that could be construed as influencing the position presented in or the review of the manuscript entitled. Acknowledgement Authors appreciate the research support from the National Natural Science Foundation of China (Grant No.: 51738004). References [1] X.P. Zhang, X.M. Cheng, Energy consumption, carbon emissions, and economic growth in China, Ecol. Econ. 68 (10) (2009) 2706–2712. [2] K.H. Yang, K.H. Lee, Tests on high-performance aerated concrete with a lower density, Constr. Build. Mater. 74 (2015) 109–117. [3] M. Lu, J.H.K. Lai, Building energy: a review on consumptions, policies, rating schemes and standards, Energ. Proce. 158 (2019) 3633–3638. [4] E. Aydin, D. Brounen, The impact of policy on residential energy consumption, Energy 169 (2019) 115–129. [5] U. Berardi, A cross-country comparison of the building energy consumptions and their trends, Resour. Conserv. Recy. 123 (2017) 230–241. [6] L. Aditya, T.M.I. Mahlia, B. Rismanchic, A review on insulation materials for energy conservation in buildings, Renew. Sust. Energ. Rev. 73 (2017) 1352– 1365. [7] A.M. Papadopoulos, State of the art in thermal insulation materials and aims for future developments, Energy Build. 37 (2005) 77–86. [8] H.-R. Kymäläinen, A.-M. Sjöberg, Flax and hemp fibres as raw materials for thermal insulations, Build Environ. 43 (2008) 1261–1269. [9] M. Yu, G. Zhu, Q. Meng, Experimental study and analysis of XPS vertical countercurrent fire spread, Proce. Eng. 211 (2018) 945–953. [10] H. Sun, Y. Pan, J. Wang, et al., Flame spread characteristics of inclined extruded polystyrene thermal insulation material, Proce. Eng. 211 (2018) 651–658. [11] K. Ramamurthy, E.K.K. Nambiar, G.I.S. Ranjani, A classification of studies on properties of foam concrete, Cem. Concr. Compos. 31 (6) (2009) 388–396. [12] Y.H.M. Amran, N. Farzadnia, A.A.A. Ali, Properties and applications of foamed concrete; a review, Constr. Build. Mater. 101 (2015) 990–1005. [13] H. Yang, Y. Jiang, H. Liu, D. Xie, C. Wan, H. Pan, S. Jiang, Mechanical, thermal and fire performance of an inorganic-organic insulation material composed of hollow glass microspheres and phenolic resin, J. Colloid. Interf. Sci. 530 (2018) 163–170.
[14] E.P. Kearsley, P.J. Wainwright, The effect of high fly ash content on the compressive strength of foamed concrete, Cem. Concr. Res. 31 (2001) 105–112. [15] T. Li, Z. Wang, T. Zhou, Y. He, F. Huang, Preparation and properties of magnesium phosphate cement foam concrete with H2O2 as foaming agent, Constr. Build. Mater. 205 (2019) 566–573. [16] C. Ma, B. Chen, Experimental study on the preparation and properties of a novel foamed concrete based on magnesium phosphate cement, Constr. Build. Mater. 137 (2017) 160–168. [17] X. Fu, Z. Lai, X. Lai, Z. Lu, S. Lv, Preparation and characteristics of magnesium phosphate cement based porous materials, Constr. Build. Mater. 127 (2016) 712–723. [18] Y. Li, B. Chen, New type of super-lightweight magnesium phosphate cement foamed concrete, J. Mater. Civ. Eng. 27 (1) (2014) 04014112. [19] Z. Huang, T. Zhang, Z. Wen, Proportioning and characterization of Portland cement-based ultra-lightweight foam concretes, Constr. Build. Mater. 79 (2015) 390–396. [20] J. Jiang, Z. Lu, Y. Niu, J. Li, Y. Zhang, Study on the preparation and properties of high-porosity foamed concretes based on ordinary Portland cement, Mater. Des. 92 (2016) 949–959. [21] S. Bretsznajder, Chapter 5-thermal conductivity of gases, prediction of transport and other physical properties of fluids, 1971, 240–288. [22] C.L. Yaws, Chapter 6-Thermal Conductivity of Gas-Inorganic Compounds, Transport Properties of Chemicals and Hydrocarbons, 2009, 292–298. [23] Z. Zhang, J.L. Provis, A. Reid, H. Wang, Mechanical, thermal insulation, thermal resistance and acoustic absorption properties of geopolymer foam concrete, Cem. Concr. Compos. 62 (15) (2009) 97–105. [24] Z. Pan, H. Li, W. Liu, Preparation and characterization of super low density foamed concrete from Portland cement and admixtures, Constr. Build. Mater. 72 (2014) 256–261. [25] ASTM C518, Standard test method for steady-state thermal transmission properties by means of the heat flow meter apparatus, 2004. [26] S. Wang, Y. Chen, Y. Yang, et al., Summary of the calculation formula for the performance of foamed cement, New. Build. Mater. 7 (2017). [27] T. Li, Research on Key Influence Factors and Preparation of High Performance Foam Concrete, Chongqing University, 2018. [28] M.A. Haque, B. Chen, Research progresses on magnesium phosphate cement: A review, Constr. Build. Mater. 211 (2019) 885–898. [29] D.V. Ribeiro, G.R. Paula, M.R. Morelli, Use of microwave oven in the calcination of MgO and effect on the properties of magnesium phosphate cement, Constr. Build. Mater. 198 (2019) 619–628. [30] J. Li, Y. Ji, L. Zhang, B. Liu, Resistance to sulfate attack of magnesium phosphate cement-coated concrete, Constr. Build. Mater. 195 (2019) 156–164. [31] T.T. Nguyen, H.H. Bui, T.D. Ngo, G.D. Nguyen, M.U. Kreher, F. Darve, A micromechanical investigation for the effects of pore size and its distribution on geopolymer foam concrete under uniaxial compression, Eng. Fract. Mech. 209 (2019) 228–244. [32] A.A. Hilal, N.H. Thom, A.R. Dawson, On entrained pore size distribution of foamed concrete, Constr. Build. Mater. 75 (2015) 227–233. [33] F. Xua, G. Gu, W. Zhang, H. Wang, X. Huang, J. Zhu, Pore structure analysis and properties evaluations of fly ash-based geopolymer foams by chemical foaming method, Ceram. Int. 44 (2018) 19989–19997.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effect of foaming gas and cement type on the thermal conductivity of foamed concrete Tian Li a, Fangmei Huang a, Jiang Zhu a, Jinhui Tang b,c, Jiaping Liu a,b,c,⇑ a
College of Materials Science and Engineering, Chongqing University, Chongqing 400045, PR China Jiangsu Sobute New Materials Co., Ltd., Nanjing 211103, China c State Key Laboratory of High Performance Civil Engineering Materials, Nanjing 211103, China b
a r t i c l e
i n f o
Article history: Received 17 April 2019 Received in revised form 11 September 2019 Accepted 6 October 2019
Keywords: Foamed concrete Foaming gas Cement type Thermal conductivity
a b s t r a c t This study has been carried out to investigate the effect of the thermal conductivity of foaming gas and cement type on that of foamed concrete. The results show that the foamed concrete employed a gas having lower thermal conductivity as foaming gas or prepared from a type of cement with lower thermal conductivity takes on lower thermal conductivity. And the thermal conductivity of foamed concrete is directly proportional to the thermal conductivity of foaming gas. In addition, with the length of age, the thermal conductivity of the foamed concrete whose foaming gas is not air gradually approaches to those using air as foaming gas at the initial stage. Using the foaming gas or cement with lower thermal conductivity to prepare foamed concrete may be a feasible as well as effective way to improve the thermal insulation of foamed concrete. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction In some countries, building energy consumption has exceeded 35% of total energy consumption [1,2]. Targeting to reduce the building energy consumption, many countries have introduced relevant policies, including the use of insulation materials [3–5]. Insulation materials are a porous lightweight material and can be categorized into two categories [6,7]. One is organic insulation materials and the other is inorganic insulation materials. In the past few decades, organic insulation materials had been widely used because of its advantages, such as recyclable and save resource [8]. However, some fire disasters have made people aware of organic insulation materials exist fire safety hazard [9,10]. Inorganic insulation materials, therefore, have gradually replaced organic insulation materials been used in building exterior insulation systems recently due to its good fire-resistance performance. One of the commonly used inorganic insulation materials is foamed concrete. Foamed concrete is a cement-based material prepared by physical foaming or chemical foaming [11,12]. However, compared to organic insulation material, foamed concrete shows higher thermal conductivity, which restricts its application at some
⇑ Corresponding author at: College of Materials Science and Engineering, Chongqing University, Chongqing 400045, PR China. E-mail address: [email protected] (J. Liu). https://doi.org/10.1016/j.conbuildmat.2019.117197 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
extent [13]. Therefore, decreasing the thermal conductivity of foamed concrete has practical significance. Foamed concrete can be roughly regarded as a combination of hardened cement paste and pores, wherein the formation of pores is caused by foaming agent [14]. Therefore, the properties of foaming agent and cement may have a certain effect on the thermal conductivity of foamed concrete. Li et al. [15], Ma et al. [16], Fu et al. [17] and Chen et al. [18] studied the foamed concrete from magnesia phosphate cement, respectively. The corresponding foaming agents are hydrogen peroxide, sodium carbonate, zinc powders and protein. Their results show that the thermal conductivity of foamed concrete prepared from magnesia phosphate cement presents a significantly difference due to the type of foaming agent and the foamed concrete whose foaming agent is sodium bicarbonate displays the lowest thermal conductivity. Huang et al. [19] undertook a study of Portland cement-based foamed concrete employing hydrogen peroxide as foaming agent. Jiang et al. [20] investigated the properties of foamed concrete based on ordinary Portland cement using animal protein as foaming agent. From Huang et al. and Jiang et al. results, it can be found that, at the same density, the thermal conductivity of the two Portland cement-based foamed concretes are very close. It is interesting to note that the gas introduced by the above mentioned foaming agents are different. The gas introduced by protein is air, the gas generated by metal powers is hydrogen, the gas generated by hydrogen peroxide is oxygen, and the gas produced by
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T. Li et al. / Construction and Building Materials 231 (2020) 117197
sodium bicarbonate is carbon dioxide. Here, we refer to this kind of gas introduced by foaming agent as foaming gas. The foaming gas would cause a porous structure in foamed concrete and fill in the pores of foamed concrete. The thermal conductivity of four foaming gases, at 20 °C, is shown in Table 1[21,22]. Based on the above reported results and the thermal conductivity of four gases, it is reasonable to believe that foaming gas may be a critical factor affecting the thermal conductivity of foamed concrete. Zhang et al. [23] and Pan et al. [24] respectively prepared geopolymer foamed concrete and Portland cement foamed concrete using the same foaming agent. However, their results show that the thermal conductivity of foamed concrete prepared from different types of cement takes on a considerable difference. From the results of Chen et al. [18] and Jiang et al. [20], it is also found that the thermal conductivity of foamed concrete from magnesia phosphate cement is higher than Portland cement-based foamed concrete. In the above comparison cases, one of the main variables is the type of cement. Therefore, the type of cement may be another key factor affecting the thermal conductivity of foamed concrete. In this paper, the effect of foaming gas and cement type on the thermal conductivity of foamed concrete was investigated. We carried out this study by combining theoretical analysis with experimental verification, and four foaming gases and three types of cement were employed. Hope that the results of this study can provide a theoretical foundation for improving the thermal insulation of foamed concrete.
Table 2 Chemical composition of the OPC, SAC and MgO (% by mass). Chemical composition
OPC
SAC
MgO
SiO2 CaO Fe2O3 Al2O3 P2O5 TiO2 MgO Na2O K2O SO3 LOI
21.82 64.93 4.53 4.36 – – 1.06 0.15 0.39 1.89 0.87
15.55 47.14 1.45 21.47 – 0.91 2.36 0.20 0.37 10.53 0.12
6.93 3.35 1.42 0.57 0.24 87.21 – – – 0.28
2. Experimental 2.1. Materials The cementitious materials employed for prepare foamed concrete are magnesium phosphate cement (MPC), ordinary Portland cement (OPC, 42.5R) and sulfoaluminate cement (SAC). The raw materials of MPC are dead-burned magnesia (MgO), potassium dihydrogen phosphate (KDP) and borax (B). The mass ratio of KDP to MgO is 0.25, in the meantime the mass ratio of B to MgO is 0.12. The specific surfaces of the OPC, SAC and MgO are 353, 432 and 230 m2/kg, respectively. The chemical compositions of them are listed in the Table 2. The purity of the KDP and B is more than 95% and the X-ray diffractions (XRD) of them are shown in Fig. 1. The four foaming gases employed are air, hydrogen, oxygen and carbon dioxide. The corresponding foaming agents employed are protein (P), zinc powders (Zn), hydrogen peroxide (H2O2) and sodium bicarbonate (NAC), respectively. The protein-based foaming agent is foamed to a density of 40 kg/m3 for use. The NAC used is a chemically pure reagent. The content of H2O2 employed is 30%. In addition, calcium stearate (CaS) is employed as foam stabilizer. 2.2. Mix proportions The mix proportions for investigate the effect of foaming gas and cement type on the thermal conductivity of foamed concrete are presented in Table 3 and Table 4, respectively. 2.3. Specimen preparation The specimens were prepared following these steps according to the mix compositions (Table 3, Table 4). Firstly, cement, CaS,
Fig. 1. The XRD patterns of the KDP and B.
and MnO2 (if necessary) were casted into a vertical-axis planetary mixer and mixed at low speed. After mixing for 1 min, water was added to the planetary mixer followed by another 2.5 min mixing. Then, foaming agent or the prepared foam was introduced and mixed at high speed for 30 s. Finally, the foamed concrete paste was placed into the slab molds (300 mm 300 mm 30 mm), and then the specimens were transferred into a curing room after demolded. 2.4. Measurement methods 2.4.1. Thermal conductivity The thermal conductivity of foam concrete was measured according to ASTM C 518 [25]. The measured specimens were the foamed concrete slab with size 300 mm 300 mm 30 mm. Before the thermal conductivity was measured, the cured specimens were dried in a 45 vacuum oven until the mass remained unchanged. The thermal conductivity presented in this paper is the average of three specimens and the accuracy is 0.001 W/(mK). 2.4.2. Pore size distribution The investigation of pore size distribution was performed according to the following steps. Firstly, capturing the crosssectional picture of sample and transforming it into binary form.
Table 1 Thermal conductivity of four gases at 20 °C (W/(mK)). Gas
air
hydrogen
oxygen
carbon dioxide
Thermal conductivity
0.0242
0.1760
0.0240
0.0143
3
T. Li et al. / Construction and Building Materials 231 (2020) 117197 Table 3 Mix compositions for investigate the effect of foaming gas. No.
Composition of mixture per m3
Dry density (kg/m3)
MPC (kg)
H2O (kg)
P (kg)
Zn (kg)
NAC (kg)
H2O2 (kg)
MnO2 (kg)
CaS (kg)
I-400 I 500 I 600 I 700 I 800 I 900 II-400 II 500 II 600 II 700 II 800 II 900 III-400 III 500 III 600 III 700 III 800 III 900 IV-400 IV 500 IV 600 IV 700 IV 800 IV 900
337 421 506 590 674 759 337 421 506 590 674 759 337 421 506 590 674 759 337 421 506 590 674 759
40.4 50.5 60.7 70.8 80.9 91.1 40.4 50.5 60.7 70.8 80.9 91.1 40.4 50.5 60.7 70.8 80.9 91.1 40.4 50.5 60.7 70.8 80.9 91.1
1.80 1.72 1.65 1.58 1.50 1.43 – – – – –
– – – – –
– – – – –
– – – – –
– – – – –
– – – – –
– – – – –
– – – – –
– – – – –
4.2 4.0 3.9 3.8 3.6 3.4 – – – – –
– – – – –
– – – – –
– – – – –
– – – – –
5.5 5.3 5.1 4.8 4.6 4.4 – – – – –
11.3 10.9 10.5 10 9.6 9.2
6.1 7.7 9.2 10.8 12.8 13.8
0.62 0.77 0.92 1.08 1.23 1.38 0.62 0.77 0.92 1.08 1.23 1.38 0.62 0.77 0.92 1.08 1.23 1.38 0.62 0.77 0.92 1.08 1.23 1.38
405 503 607 706 809 902 403 506 602 711 804 908 409 507 606 703 811 906 403 506 607 710 812 918
Table 4 Mix compositions for investigate the effect of cement type. No.
Composition of mixture per m3
Dry density (kg/m3)
MPC (kg)
OPC (kg)
SAC (kg)
H2O (kg)
P (kg)
CaS (kg)
I-400 I-500 I-600 I-700 I-800 I-900 II-400 II-500 II-600 II-700 II-800 II-900 III-400 III-500 III-600 III-700 III-800 III-900
337 421 506 590 674 759 – – – – – – – – – – – –
– – – – – – 333 417 500 583 667 750 – – – – – –
– – – – – – – – – – – – 286 357 429 500 571 643
40.4 50.5 60.7 70.8 80.9 91.1 173.2 216.8 260.0 303.2 346.8 390.0 157.3 196.4 236.0 275.0 314.0 353.6
1.80 1.72 1.65 1.58 1.50 1.43 1.64 1.48 1.32 1.16 1.00 0.84 1.70 1.55 1.40 1.26 1.11 0.96
0.62 0.77 0.92 1.08 1.23 1.38 0.61 0.77 0.92 1.07 1.23 1.38 0.53 0.66 0.79 0.92 1.05 1.18
Secondly, identifying and computing the average diameter of the white area (i.e. pore size) adopting Image-pro plus 6.0 software. Finally, the pore sizes were analyzed statistically. The typical cross-sectional picture and its binary form are presented in Fig. 2. 3. Results and discussion 3.1. Foaming gas 3.1.1. Theoretical analyses Some scholars analyzed the thermal conductivity of foamed concrete and summarized an empirical equation (Eq. (1)) [26,27].
k¼
ðk2 k1 Þq1 þ k1 ð5q2 4q1 Þz
ð1Þ
where: k is the thermal conductivity of foamed concrete; k1 and k2 are the thermal conductivity of foaming gas and cement paste,
405 503 607 706 809 902 404 502 604 709 803 906 402 508 598 702 807 903
respectively; q1 and q2 are the dry density of foamed concrete and cement paste, respectively; z is the ratio of the closed pores to all the pores of foamed concrete, and the value of z is equal to 1 when all the pores are closed. In this section, we theoretically calculated the thermal conductivity of foamed concrete employing Eq. (1) and the results are shown in Fig. 3. The cement used is MPC and the thermal conductivity of MPC paste at dry density of 2100 kg/m3 is 1.236 W/(mK). The foaming gases employed are air, oxygen, hydrogen and carbon dioxide. In the calculation process, it is assumed that all the pores are closed and the pore structures of all prepared foamed concrete are the same. From Fig. 3, it can be seen that, for a given foaming gas, the thermal conductivity of foamed concrete increases with dry density. In addition, this figure also shows that, in terms of foamed concrete with a given density, the thermal conductivity of whose employed hydrogen as foaming gas is the highest and the thermal conductivity of whose using carbon dioxide as foaming gas is the lowest.
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T. Li et al. / Construction and Building Materials 231 (2020) 117197
Fig. 2. The typical cross-sectional picture and its binary form.
air hydrogen
Thermal conductivity (W/(m·k))
0.30
oxygen carbon dioxide
0.25 0.20 0.15 0.10 0.05
400
500
600
700
800
900
3
Dry density (kg/m ) Fig. 3. The calculated thermal conductivity of foamed concrete with different foaming gas.
Therefore, it can be concluded that, for a given dry density, the foamed concrete employed a gas having lower thermal conductivity as foaming gas takes on lower thermal conductivity.
3.1.2. Experimental analyses In order to verify whether the conclusion of the theoretical analyses is correct, the related experiment was carried out. The foamed concrete from MPC was prepared according to the mix proportions (Table 3). The corresponding foaming gases are air, oxygen, hydrogen and carbon dioxide, respectively. The thermal conductivity was measured on the 7th day of curing since the hydration of MPC is basically completed on the 7th day [28–30]. In this work, we measured the thermal conductivity at 7 d, 9 d, 11 d, 13 d, and 15 d and the results are presented in Fig. 4. From Fig. 4, it can be seen that, for a given foaming gas and given age, the thermal conductivity of foamed concrete increases with dry density. This figure also indicates that, for a given density, the fluctuation of thermal conductivity of the foamed concrete employed air or oxygen as foaming gas with age is very small, which can be ignored. However, the thermal conductivity of the foamed concrete employed hydrogen as foaming gas decreases first with age and then remains unchanged, and the thermal conductivity of the foamed concrete employed carbon dioxide as foaming gas increases first with age and then would not change any more. The reason for this phenomenon might be that foamed concrete contains unclosed pores and closed pores and the difference in thermal conductivity between foaming gases. The foaming gas filled in the unclosed pores can exchange with the air in the
atmosphere and is replaced gradually by air with the increase of age. The foaming gas contained in the closed pores, however, remains unchanged with age. Since the value of thermal conductivity of oxygen is approximately equal to that of air, the thermal conductivity of the foamed concrete whose foaming gas is oxygen does not change with age. On the other hand, as a result of the thermal conductivity between hydrogen (or carbon dioxide) and air has a large difference, therefore the thermal conductivity of foamed concrete employed hydrogen or carbon dioxide as foaming gas exhibits the above phenomenon with age. In addition, by comparing the results shown in Fig. 4 with the calculation results shown in Fig. 3, it can be found that, for a given foaming gas, the measured value of thermal conductivity of the foamed concrete is not equal to the theoretical calculated value. The reason for this might be that the pores of foamed concrete are not all closed pores. For observe the influence of thermal conductivity of foaming gas on the thermal conductivity of foamed concrete more clear and intuitive, the measured results of thermal conductivity of the foamed concrete, at 15 d, with different type of foaming gas were ploted in figure (Fig. 5). From Fig. 5, it can be seen that the thermal conductivity of the foamed concrete employed hydrogen as foaming gas is the highest and the thermal conductivity of foamed concrete employed carbon dioxide as foaming gas is the lowest. This result concurs with the result of Section 3.1.1. According to the reported literatures [31–33], the pore size distribution can affect the properties of foamed concrete. To this end, the pore size distribution of the foamed concrete at a dry density of 600 kg/m3 was surveyed and the results are presented in Fig. 6. From Fig. 6, it can be seen that the pore size distributions and average pore size of the foamed concrete with different type of foaming gas are very close. Therefore, here, it can be roughly believed that the factor affecting the thermal conductivity of foamed concrete is only the thermal conductivity of foaming gas. From the above analysis, it can be roughly understood that, for a given dry density, the smaller the thermal conductivity of the foaming gas is, the smaller the thermal conductivity of the prepared foamed concrete is. For clarify the relation between the thermal conductivity of foaming gas and the thermal conductivity of foamed concrete, the thermal conductivity of the foamed concrete is plotted as a function of the thermal conductivity of the foaming gas in Fig. 7. It can be seen from Fig. 7 that, for a given dry density, the thermal conductivity of the foamed concrete is directly proportional to the thermal conductivity of foaming gas. The relationship of them can be expressed using the following equation:
y ¼ a þ b x; R2 ¼ 0:9233 0:9994
ð2Þ
where: y = the thermal conductivity of foamed concrete; x = the thermal conductivity of foaming gas; a and b are constants. According to the value of dry density, the value of R2 varies from 0.9233 to 0.9994, indicating a strong relationship between the thermal con-
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T. Li et al. / Construction and Building Materials 231 (2020) 117197
0.36
0.36
7d 13 d
0.30
9d 15 d
0.33
11 d Thermal conductivity(W/(m⋅k))
Thermal conductivity(W/(m⋅k))
0.33
0.27 0.24 0.21 0.18 0.15 0.12
500
600
700
800
9d 15 d
11 d
0.27 0.24 0.21 0.18 0.15 0.12
400
7d 13 d
0.30
900
400
500
600
700
(a)
(b) 0.35
0.50 7d 13 d
0.40
9d 15 d
0.30
11 d
Thermal conductivity(W/(m⋅k))
0.45
Thermal conductivity(W/(m⋅k))
900
Dry density(kg/m )
Dry density(kg/m )
0.35 0.30 0.25 0.20 0.15
800 3
3
400
500
600
700
800
900
3
7d 13 d
9d 15 d
11 d
0.25 0.20 0.15 0.10 0.05
400
500
600
700
800
900
3
Dry density(kg/m )
Dry density(kg/m )
(c)
(d)
Fig. 4. The thermal conductivity of the foamed concrete from MPC and the foaming gases are air (a), oxygen (b), hydrogen (c) and carbon dioxide (d), respectively.
Thermal conductivity(W/(m⋅k))
0.40
air oxygen hydrogen carbon dioxide
0.35 0.30 0.25 0.20 0.15 400
500
600
700
800
900
3
Dry density(kg/m ) Fig. 5. The thermal conductivity of the foamed concrete, at 15 d, with different type of foaming gas.
ductivity of foamed concrete and the thermal conductivity of foaming gas.
3.2. Cement type 3.2.1. Theoretical analyses In this part, we still adopted Eq. (1) to theoretically analyze the effect of cement type on the thermal conductivity of foamed con-
crete. The assumptions in this part are the same as that of Section 3.1.1, i.e. all the pores are closed and the pore structures of all prepared foamed concrete are the same. In addition, the thermal conductivity of OPC paste and SAC paste, at a dry density of 2100 kg/m3, were also measured. The measured results were 0.4544 W/(mK) and 0.4307 W/(mK), respectively. The thermal conductivity calculated of foamed concrete from different type cement is shown in Fig. 8. From Fig. 8, it can be seen that, for a given foaming gas, the thermal conductivity of SAC-based foamed concrete is the lowest and the thermal conductivity of MPC-based foamed concrete is the highest. This result concurs with the measured result of the thermal conductivity of cement pastes. It also can be seen from this figure that the thermal conductivity of the foamed concrete whose the cement is OPC is closed to those the cement is SAC. In addition, this figure also shows that, for a given cement type, the thermal conductivity of the foamed concrete employed air as foaming gas is lower than the thermal conductivity of the foamed concrete employed hydrogen as foaming gas. This result does accord with the conclusions in section 3.1. From this section, it can be concluded that the foamed concrete prepared from the cement having a lower thermal conductivity shows lower thermal conductivity.
3.2.2. Experimental analyses The authors conducted the related experiment for verify the conclusion from Section 3.2.1. The foamed concrete from different type cement was prepared according to Table 4 and the thermal conductivity of specimens was measured after cured 28 d. The
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T. Li et al. / Construction and Building Materials 231 (2020) 117197
0.300 Air Oxygen Hydrogen Carbon dioxide
Percent (%)
40
30
20
10
0.285 Average pore size (mm)
50
0.270 0.255 0.240 0.225 0.210 0.195
0
0.180 0.15-0.20
0.20-0.25
0.25-0.30
0.30-0.35
0.35-0.40
Air
Oxygen
Hydrogen
Carbon dioxide
Foaming gas
Pore size (mm)
0.40
400kg/m3 700kg/m3
500kg/m3 800kg/m3
600kg/m3 900kg/m3
0.35 MPC
Thermal conductivity (W/(m⋅k))
Thermal conductivity of foamed concrete (W/(m·k))
Fig. 6. The pore size distribution and average pore size of the foamed concrete with a dry density of 600 kg/m3.
0.35 0.30 0.25 0.20 0.15 0.00
0.03
0.06
0.09
0.12
0.15
0.18
Thermal conductivity of foaming gas (W/(m·k))
OPC
SAC
0.30 0.25 0.20 0.15 0.10 0.05
400
500
600
700
800
900
3
Dry density (kg/m ) Fig. 7. The relationship between the thermal conductivity of foaming gas and the thermal conductivity of the prepared foamed concrete.
0.20
The pore size distributions of foamed concrete from different type cement were also surveyed, and the foamed concrete with a dry density of 600 kg/m3 was taken as an example for illustration. The results are shown in Fig. 10. From Fig. 10, it can be seen that the pore size distributions and average pore size of foamed concrete from different type cement are also very close. This indicates that the effect of the pore structure on the thermal conductivity of foamed concrete can be ignored.
Thermal conductivity (W/(m⋅k))
0.18 MPC-Air SAC-Air OPC-Air
0.16
Fig. 9. The measured thermal conductivity of foamed concrete from different type cement.
0.14 0.12 0.10 0.08
4. Conclusion
0.06
This paper studied the effect of the thermal conductivity of foaming gas and cement type on that of foamed concrete. From the above results, we draw the following conclusions:
0.04 400
500
600
700
800
900
3
Dry density (kg/m ) Fig. 8. The thermal conductivity calculated of foamed concrete from different type cement.
results are shown in Fig. 9. From this graph, it is clear that the thermal conductivity of MPC-based foamed concrete is the highest and the thermal conductivity of SAC-based foamed concrete is the lowest. In addition, the thermal conductivity of OPC-based foamed concrete is close to the thermal conductivity of SAC-based foamed concrete. This result concurs with the conclusion in Section 3.2.1.
The thermal conductivity of foaming gas and cement type can affect the thermal conductivity of foamed concrete. The thermal conductivity of foamed concrete is directly proportional to the thermal conductivity of foaming gas and the relationship of them shows a high correlation coefficient. Due to the presence of unclosed pores, the thermal conductivity of the foamed concrete whose foaming gas is hydrogen or carbon dioxide gradually approaches to those using air as the foaming gas with the increase of age at the initial stage. Foamed concrete prepared from a type of cement with lower thermal conductivity takes on lower thermal conductivity.
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T. Li et al. / Construction and Building Materials 231 (2020) 117197
0.300
50 MPC OPC SAC
0.270 Average pore size (mm)
Percent (%)
40
0.285
30
20
10
0.255 0.240 0.225 0.210 0.195
0
0.180
0.15-0.20
0.20-0.25
0.25-0.30
0.30-0.35
0.35-0.40
Pore size (mm)
MPC
OPC
SAC
Cement type
Fig. 10. The pore size distribution and average pore size of the foamed concrete, at a dry density of 600 kg/m3, from different type cement.
Conflict of interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service or company that could be construed as influencing the position presented in or the review of the manuscript entitled. Acknowledgement Authors appreciate the research support from the National Natural Science Foundation of China (Grant No.: 51738004). References [1] X.P. Zhang, X.M. Cheng, Energy consumption, carbon emissions, and economic growth in China, Ecol. Econ. 68 (10) (2009) 2706–2712. [2] K.H. Yang, K.H. Lee, Tests on high-performance aerated concrete with a lower density, Constr. Build. Mater. 74 (2015) 109–117. [3] M. Lu, J.H.K. Lai, Building energy: a review on consumptions, policies, rating schemes and standards, Energ. Proce. 158 (2019) 3633–3638. [4] E. Aydin, D. Brounen, The impact of policy on residential energy consumption, Energy 169 (2019) 115–129. [5] U. Berardi, A cross-country comparison of the building energy consumptions and their trends, Resour. Conserv. Recy. 123 (2017) 230–241. [6] L. Aditya, T.M.I. Mahlia, B. Rismanchic, A review on insulation materials for energy conservation in buildings, Renew. Sust. Energ. Rev. 73 (2017) 1352– 1365. [7] A.M. Papadopoulos, State of the art in thermal insulation materials and aims for future developments, Energy Build. 37 (2005) 77–86. [8] H.-R. Kymäläinen, A.-M. Sjöberg, Flax and hemp fibres as raw materials for thermal insulations, Build Environ. 43 (2008) 1261–1269. [9] M. Yu, G. Zhu, Q. Meng, Experimental study and analysis of XPS vertical countercurrent fire spread, Proce. Eng. 211 (2018) 945–953. [10] H. Sun, Y. Pan, J. Wang, et al., Flame spread characteristics of inclined extruded polystyrene thermal insulation material, Proce. Eng. 211 (2018) 651–658. [11] K. Ramamurthy, E.K.K. Nambiar, G.I.S. Ranjani, A classification of studies on properties of foam concrete, Cem. Concr. Compos. 31 (6) (2009) 388–396. [12] Y.H.M. Amran, N. Farzadnia, A.A.A. Ali, Properties and applications of foamed concrete; a review, Constr. Build. Mater. 101 (2015) 990–1005. [13] H. Yang, Y. Jiang, H. Liu, D. Xie, C. Wan, H. Pan, S. Jiang, Mechanical, thermal and fire performance of an inorganic-organic insulation material composed of hollow glass microspheres and phenolic resin, J. Colloid. Interf. Sci. 530 (2018) 163–170.
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