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Properties of foamed concrete containing water repellents
Construction and Building Materials 123 (2016) 106–114
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Properties of foamed concrete containing water repellents Cong Ma, Bing Chen ⇑ Department of Civil Engineering, Shanghai Jiaotong University, Shanghai 200240, PR China
h i g h l i g h t s A foamed concrete modified by water repellent with low water absorption is prepared. The effects of water repellent on the physical and mechanical properties are investigated. The hygroscopic fitted curves from KUM and CUB models are established.
a r t i c l e
i n f o
Article history: Received 26 May 2016 Received in revised form 27 June 2016 Accepted 29 June 2016
Keywords: Foamed concrete Water repellent Compressive strength Sorptivity Hygroscopicity
a b s t r a c t It is generally known that the physical and mechanical properties would be greatly degraded after water or moisture transferring into the foamed concrete. In this study, a foamed concrete with a low density of about 550 kg/m3 is prepared using ordinary Portland cement, and three types of water repellents including potassium trimethylsilanolate (PT), calcium stearate (CS) and siloxane-based polymer (SP) are employed to decrease the water absorption of the foamed concrete. The effects of the water repellent on the mechanical and physical properties of the foamed concrete, such as 7-day and 28-day compressive strength, thermal conductivity, sorptivity and hygroscopicity, are studied. The laboratory results indicate that the water repellents improve the compressive strength to some extent without sacrificing the thermal insulation property of the foamed concrete. The sorptivity evaluated by 48-h water absorption and strength retention coefficient (RS) is significantly improved as increasing the content of water repellent. When 1.0% SP is used, the water absorption and RS value of the foamed concrete with 28-day strength of 1.77 MPa and thermal conductivity of 0.150 W/m K are 2.5% (by volume) and 0.989, respectively. In addition, the contents of hygroscopic moisture [W(u)] also decrease with the increasing content of water repellent. The hygroscopic fitted curves with high coefficients of determination obtained the KUM and CUB models have been proved applicable in exploring the relationship of the W(u) to the relative humidity. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction In China, the energy consumption of construction industry has roughly 35 percent of total energy consumption in society [1]. In the past several years, lightweight concrete, aerated concrete and foamed concrete with excellent thermal insulating properties have been used as exterior wall materials and studied by many researchers [2–7]. In addition, the foamed concrete with high fluidity and low cement and aggregate usage is also applied to sandwich structures, earth-remaining walls and running tracks or playgrounds [8,9]. The common used foamed concrete is defined as pre-foamed foam concrete by adding the projected amount of foam into cement slurry.
⇑ Corresponding author. E-mail address: [email protected] (B. Chen). http://dx.doi.org/10.1016/j.conbuildmat.2016.06.148 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
The first modern foamed concrete prepared by Portland cement was reported in the twenty-twenties [10]. During the past decades, the improvement of foaming technology and the application of high-efficiency superplasticizers, foam stabilizers and early strength agents have substantially improve the engineering properties of the foamed concrete. And many researchers have studied the preparation methods, fresh properties, physical properties and mechanical properties in detail [4,6,11–15]. It is notable that the transfer of moisture or water into the foamed concrete has drawn increasing attention due to the significant impact of the climate change [13,16]. The permeability of the foamed concrete which depends upon its porosity and pore structure (such as pore diameter and continuity) is the main factor in determining the transport of moisture or water [17]. To understand the mechanism of water permeation in foamed concrete, sorptivity and hygroscopicity characterized by water and moisture (water vapour) absorption are the applicable
C. Ma, B. Chen / Construction and Building Materials 123 (2016) 106–114
parameters [18]. The properties of the foamed concrete after water immersion and hygroscopicity tests have been studied [19–22]. It can be concluded that the water or moisture absorption has significantly negative influence on the physical and mechanical properties of the foamed concrete, which may result in energy waste and failure cracks. As reported earlier, types of foam agents, densities and mineral supplementary materials for the foamed concrete are also the factors influencing the sorptivity and hygroscopicity. Several researches have studied the moisture dynamics of foamed concrete and established some phenomenological models to guide the design of the foamed concrete [17,19,21]. Even so, the water absorption of foamed concrete prepared by cement, sand and fly ash with the dry density of about 900 kg/m3 is about 30% (by volume of the concrete) and the moisture absorption of lightweight concrete prepared by cement and lightweight sand with the dry density of about 950 kg/m3 is approximately 30 kg/m3 under a relative humidity of 95% [13,18]. This implies that the foamed/lightweight concrete prepared at the optimal design also has a high water or moisture absorption. However, there are few studies reported on the improvement methodology of water absorption of foamed concrete. In this study, ordinary Portland cement and silica fume are employed as cementitious materials to prepare foamed concrete with low dry density of about 550 kg/m3. To decrease the water absorption, three different water repellents have been introduced into foamed concrete. The physical and mechanical properties have been tested and analyzed in laboratory. And the effects of water repellent on compressive strength and thermal conductivity are investigated. Based on the results of water immersion and hygroscopic tests, some numerical models are established and a foamed concrete with low water absorption is obtained.
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of water repellent on the properties of foamed concrete. Three types of water repellents are selected and they are potassium trimethylsilanolate (PT), calcium stearate (CS) and a type of siloxane-based polymer (SP), respectively. SP powder are prepared in the laboratory and CS is chemically pure. The detailed compositions of each mixture for contrast specimen and three series of specimens are presented in Table 2. 2.2. Specimen preparation A paddle mixer is used to produce foamed concrete mixture by adding the projected volume of foam. The different series of specimens are prepared by adding different types of water repellents as shown in Table 2. The preparation of the mixture includes three steps. Firstly, the predetermined amount of water was mixed with cementitious materials and admixtures and the stirring process lasts about 3 min in order to obtain a homogeneous mixture. And then, PP fibers are introduced into the mixture and mixed for about 5 min. Finally, the projected volume of foam is added immediately into the fluid mixture and the mixture is continued to mix in high speed until the foam is uniformly distributed in the mixture. Then, the fluid foamed concrete is poured into the cube PVC moulds (150 mm 150 mm 150 mm) and the surface of each specimen is smoothed by hand only. Subsequently, the moulds are transferred to standard curing room with a temperature of 22 ± 2 °C and a relative humidity of 97 ± 2%. Within the initial 12 h of curing, the moulds are covered with wet linen in order to keep the volume stability of the foamed concrete. After 3 days of curing, the specimens are demoulded and stored in the curing room until testing age. 2.3. Property measurement
2. Experimental procedure 2.1. Materials Ordinary Portland cement (OPC) with a 28-strength of 56.5 MPa and silica fume (SF) achieved from Elken Materials are the selected cementitious materials to prepare low-density foamed concrete. Table 1 lists the chemical compositions of OPC and silica fume. The polypropylene (PP) fiber with a tensile strength of 800.0 MPa and modulus of 8.0 GPa is employed to improve the toughness of foamed concrete, and its length and diameter are 15.0 mm and 100.0 lm, respectively. In order to obtain an acceptable workability of the mixture, a type of naphthalene-based superplasticizer (SL) is introduced into each specimen. In addition, the composites of early strength and foam stabilizer agents (NAF) prepared in the laboratory are used. A protein-based foaming agent which can provide a more stable bubble network is used, and the density of the foam is about 70 kg/m3. The purpose of this study is to prepare a foamed concrete with excellent sorptivity and hygroscopicity and investigate the effects
Table 1 Chemical composition of binders (% by weight). Oxide
OPC
SF
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 LOI (Loss on ignition)
21.6 4.13 4.57 64.44 1.06 0.11 0.56 1.74 0.76
92.40 0.80 0.50 0.91 0.27 – – – 2.0
2.3.1. Compressive strength, thermal conductivity, Before thermal conductivity tests, the specimens after 28 days of standard curing are oven-dried in a vacuum oven at 45 ± 2 °C. After 48 h of oven drying, the specimen mass is weighed out every 12 h until the mass variation among three consecutive testing results not more than 0.1% of the total mass of specimens. The finally constant mass is used to calculate the dry density of the foamed concrete specimens and the calculated values are presented in Table 2. The thermal conductivity of each specimen is measured by using a KD2 Pro thermal properties analyzer (Decagon Devices, Inc., USA). This equipment has an electronically handheld controller and sensors of different sizes which should be inserted into the foamed concrete specimens. Each needle-like (single-needle or double-needle) sensor consists of a thermistor and a heating element. In this study, a single-needle sensor with a diameter of 2.4 mm and a length of 100 mm is used and it can measure the thermal conductivity ranging from 0.02 W/m K to 2.0 W/m K. In order to insert the sensor into the oven-dried specimens, a hole of the same size with the sensor should be drilled. Each measurement process continues at least 5 min and at least four measurements are taken for each foamed concrete specimen to ensure the accuracy of ±5%. At 7 and 28 days of curing, the specimens are took out from the curing room. An MTS servo hydraulic testing machine with a capacity of 100 kN is employed to measure the compressive strength of foamed concrete. The loading rate is fixed at 0.5 mm/min and the strength is the mean value of five measurements to confirm the reproducible of experimental results. In addition, the strength values which are beyond ±15% of the mean strength value should not be used and the compressive strength presented in this study is the average of at least three testing results.
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Table 2 Mixture compositions. Series
Mix No.
Target density (kg/m3)
Composition of mixture (per m3) Binder (kg) OPC
SF
3
Water (kg)
PP fiber (kg)
Foam (m )
SL (kg)
NAF (kg)
PT (kg)
CS (kg)
SP (kg)
Dry density (kg/m3)
CON
1
550
368
32
140
0.4
0.8
1.6
3.6
0
0
0
546
Series I
2 3 4 5 6
550 550 550 550 550
368 368 368 368 368
32 32 32 32 32
140 140 140 140 140
0.4 0.4 0.4 0.4 0.4
0.8 0.8 0.8 0.8 0.8
1.6 1.6 1.6 1.6 1.6
3.6 3.6 3.6 3.6 3.6
0.8 1.6 3.2 4.0 4.8
0 0 0 0 0
0 0 0 0 0
552 561 547 548 559
Series II
7 8 9 10 11
550 550 550 550 550
368 368 368 368 368
32 32 32 32 32
140 140 140 140 140
0.4 0.4 0.4 0.4 0.4
0.8 0.8 0.8 0.8 0.8
1.6 1.6 1.6 1.6 1.6
3.6 3.6 3.6 3.6 3.6
0 0 0 0 0
0.8 1.6 3.2 4.0 4.8
0 0 0 0 0
548 553 551 555 562
Series III
12 13 14 15 16
550 550 550 550 550
368 368 368 368 368
32 32 32 32 32
140 140 140 140 140
0.4 0.4 0.4 0.4 0.4
0.8 0.8 0.8 0.8 0.8
1.6 1.6 1.6 1.6 1.6
3.6 3.6 3.6 3.6 3.6
0 0 0 0 0
0 0 0 0 0
0.8 1.6 3.2 4.0 4.8
557 547 553 549 542
2.3.2. Sorptivity The sorptivity of the foamed concrete can be evaluated by two parameters: water absorption and strength retention coefficient. Water absorption can be obtained by water immersion tests and the purpose of the soaked condition is to simulate the negative impact of rainy weather on the foamed concrete. After 28 days of curing, the specimens should be oven-dried to constant mass at the same condition as mentioned before, and then immersed in deionized water in the standard curing room. After 0.25, 0.5, 1, 2, 3, 6, 12, 24 and 48 h of soaking, the values of water absorption by volume can be calculated by the following equation [13,24].
wT ¼
mT m0 V qw
ð1Þ
where wT is the water absorption by volume at a desired soaking time, mT is the weight of specimen after immersion, m0 is the weight of the oven-dried specimen, V is the volume of the specimen, and qw is the density of water at standard condition. After 48 h of soaking, the surface of the soaked specimens is wiped with a dry cloth; subsequently, compressive strength tests are performed on these soaked specimens. The strength retention coefficient can be expressed as the following expression.
RS ¼
fs f 28
ð2Þ
where RS is the strength retention coefficient of the foamed concrete, f28 is the 28-day compressive strength of specimens before water immersion, and fS is the compressive strength of specimens after 48 h of soaking. 2.3.3. Hygroscopicity Similar with the sorptivity tests, all the specimens for hygroscopicity tests should be oven-dried. Hygroscopicity tests in this study follow the Standard ISO 12571:2000 [23]. A climatic testing chamber illustrated in the standard is prepared in the laboratory to investigate the hygroscopic sorption of the foamed concrete. This simple instrument can make the experimental condition with a stable temperature (22 ± 0.5 °C) and a variation range of relative humidity from 9.2% to 100%. The final moisture absorption of the foamed concrete specimens is obtained at an approximate equilibrium state. The characteristic of the equilibrium state is that the mass variation among three consecutive weightings which are measured every 12 h is not more than 0.1% of the total mass of
the oven-dried specimens. The moisture absorption reflects the amount of water vapour in the air of the climatic testing chamber permeating into the foamed concrete specimens and it can be calculated as the following equation [13,24].
WðuÞ ¼
mðuÞ m0 V
ð3Þ
where W(u) is the moisture absorption by weight, m(u) is the weight of specimen after hygroscopicity tests, m0 is the weight of the oven-dried specimen, and V is the volume of the specimen. 3. Results and discussion 3.1. Compressive strength and thermal conductivity The effects of different types of water repellents on dry density of the foamed concrete are illustrated in Table 2. As shown in Table 2, the dry density of contrast specimen is 546 kg/m3 and the dry density of different series of specimens with water repellent varies from 542 kg/m3 to 562 kg/m3. That is to say, the maximal value is only 20 kg/m3 higher than the minimal value. This implies that the addition of water repellent in the foamed concrete has little influence on the foam stability, hence the dry density. In addition, it can be observed that there is no predictable pattern to explain the relationship of dry density with the content of water repellent. Fig. 1 shows the effects of PT content on the compressive strength at 7 days and 28 days of curing and thermal conductivity of the foamed concrete. As shown in Fig. 1, the 7-day and 28-day strength increase almost linearly as the PT content increasing from 0 to 1.0%. The 7-day and 28-day strengths of the foamed concrete specimens with 1.0% PT are 1.12 MPa and 1.33 MPa, which are 38.3% and 23.1% higher than those of contrast specimens. However, the compressive strength decreases slightly when the PT content is greater than 1.0%. The possible reason may be that PT can participate in the cement hydration reaction and play a role of surfactant which can increase the amounts of hydration products and improve the structure compactness. When the PT content is excess, its hydrophobic effect is more significant which may lead to the reduction of water participating in hydration reaction. The thermal conductivity generally increases with the increase of PT content from 0 to 1.0% initial, and then decreases slightly as the PT content going beyond 1.0%, which is approximately consistent with the
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0.160
7-day strength 28-day strength Thermal conductivity
0.155
1.2
0.150 1.0
0.145
Thermal conducticity (W/mK)
Compressive strength (MPa)
1.4
0.8
0.140 0.0
0.2
0.4
0.6
0.8
1.0
1.2
in Fig. 3. As seen in Fig. 3, the compressive strength increases as increasing the SP content from 0 to 1.2% in general, which is different from the strength development observed in Figs. 1and 2. The 7-day and 28-day strength of specimens with 1.0% SP are 1.53 MPa and 1.77 MPa, which are 88.9% and 63.9% higher than those of contrast specimens. It also can be found that the specimens with 1.0% or 1.2% SP have the highest compressive strength among the three different series of specimens. The influence of SP content on the thermal conductivity is irregular and it is difficult to seek the relationship between thermal conductivity and compressive strength. The maximal thermal conductivity of specimens with 0.2% SP is 0.153 W/m K which is about 2.7% higher than that of contrast specimens. And the thermal conductivity of specimens with 1.0% SP is 0.150 W/m K. This implies that the addition of SP can significantly improve the compressive strength without sacrificing the thermal insulation property of the foamed concrete.
PT content (%)
tendency presented in compressive strength. However, the thermal conductivity of the foamed concrete specimens with 1.0% PT is 0.154 W/m K, which is only 3.4% higher than that of contrast specimen. This implies that the application of PT in the foamed concrete has little impact on its thermal insulation property. Fig. 2 shows the effect of CS content on the compressive strength at 7 days and 28 days of curing and thermal conductivity of the foamed concrete. As shown in Fig. 2, the strength development of the foamed concrete has a similar tendency with that presented in Fig. 1. The strength increases as increasing the CS content initial, and then decrease by a slight when the CS content is greater than 1.0%. The 7-day and 28-day strengths of specimens with 1.0% CS are 1.24 MPa and 1.68 MPa, which are 53.1% and 55.6% higher than those of contrast specimens. It should be noted that the 28-strength of specimens with 1.0% CS is much higher than that of specimens with 1.0% PT. The thermal conductivity also generally increases with the increase of the CS content due to the improvement of compressive strength, which has been concluded in many researches [4–6,14,15]. Nevertheless, the range of thermal conductivity is very small, and the maximum value at the CS content of 1.0% is 0.159 W/m K which is only 6.7% higher than the minimal value. The 7-day and 28-day strength and thermal conductivity of the foamed concrete with different contents of SP are presented
The sorptivity of the foamed concrete can be evaluated by the amount of water absorption and the decrease in compressive strength after a predetermined soaking time [16]. The effect of PT content on the water absorption of the foamed concrete are presented in Fig. 4. As shown in Fig. 4, the water absorption of most of the foamed concrete specimens dramatically increases within the initial 6 h of soaking time, and then the water absorption keeps nearly constant. The 6-h and 48-h water absorption of contrast specimen are 56.5% and 68.5%, respectively. This indicates that the permeation of water into the foamed concrete occurs immediately after water immersion of the specimens. The possible reason may be that there are many inter-connected pores in the lightweight foamed concrete. This observation is consistent with the conclusions reported in the previous researches [24,25]. It can be observed that the water absorption decreases gradually as increasing the PT content from 0.2% to 1.2%. The water absorption of the specimen with only 0.4% PT at 6 h and 48 h of soaking are 29.1% and 32.8%, which are only 51.5% and 47.9% of the water absorption of contrast specimen at the same soaking time. Accordingly, the strength retention coefficient (RS) of specimens with 0.4% PT is 0.754, which is 18.7% higher than that of contrast specimens (Fig. 5). It can be also observed from Fig. 5 that the RS value increases significantly with the decrease in water absorption. As the PT content increasing from 0.2% to 1.2%, the water absorption decreases from 54.3% to 15.6% and the RS value increases from
0.155 1.4 0.150 1.2
1.0
0.145
0.8
Compressive strength (MPa)
1.8
1.6
0.160
7-day strength 28-day strength Thermal conductivity
0.160
7-day strength 28-day strength Thermal conductivity
Thermal conducticity (W/mK)
Compressive strength (MPa)
1.8
3.2. Sorptivity
0.155
1.6
1.4 0.150 1.2
0.145
1.0
Thermal conducticity (W/mK)
Fig. 1. The effect of PT content on 7-day and 28-day compressive strengths and thermal conductivity for Series I.
0.8 0.140
0.140 0.0
0.2
0.4
0.6
0.8
1.0
1.2
CS content (%) Fig. 2. The effect of CS content on 7-day and 28-day compressive strengths and thermal conductivity for Series II.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
SP content (%) Fig. 3. The effect of SP content on 7-day and 28-day compressive strengths and thermal conductivity for Series III.
C. Ma, B. Chen / Construction and Building Materials 123 (2016) 106–114
CON 0.8% PT
0.2% PT 1.0% PT
80
0.4% PT 1.2% PT
CON 0.8% CS
60
Water absorption,wT (%)
Water absorption,wT (%)
80
40
20
0
0
10
20
30
40
40
20
0
10
20
Soaking time (hours)
RS
0.8 40 0.7
20 0.6
0.9
1.2
0.5
PT content (%) Fig. 5. The relationship of water absorption and strength retention coefficient to PT content.
0.695 to 0.893. This implies that the addition of PT can improve the after-soaking compressive strength and the water resistance of the foamed concrete. As analyzed before, PT has little impact on the dry density and thermal insulation of the foamed concrete, that is, the foam quantities keep constant. Hence, the possible mechanisms for the improvement of water resistance by adding a water repellent into the foamed concrete are the increase of the disconnected pores and the change of the hydrophilic surfaces of some hydration products to hydrophobic surfaces. The influence of CS content on the water absorption of the foamed concrete is shown in Fig. 6. The addition of CS has more significant effect on the water absorption in comparison to PT, and the 48-h water absorption of specimen with 0.2% CS is 37.4%, which is almost half of that of contrast specimen. It is notable that the 48-h water absorption of specimens with 1.0% and 1.2% CS are both less than 10%, which indicates that the sorptivity of the foamed concrete can be markedly improved. The 48-h water absorption of specimen with 1.0% CS is 9.4%, which is 47.2% of that of specimen with 1.0% PT. Likewise, the RS value increases gradually from 0.716 to 0.933 as decreasing the water absorption from 37.4% to 8.9% (Fig. 7). It can be concluded that CS can also improve the water resistance of the foamed concrete, which is consistent with the previous research [26].
Water absorption, wT (%)
0.9
60
RS values
Water absorption, wT (%)
1.0
wT
RS
0.6
50
80
wT
0.3
40
Fig. 6. The water absorption of specimens CON and Series II with different contents of CS.
1.0
80
0.0
30
Soaking time (hours)
Fig. 4. The water absorption of specimens CON and Series I with different contents of PT.
0
0.4% CS 1.2% CS
60
0
50
0.2% CS 1.0% CS
0.9
60
0.8 40 0.7
RS values
110
20 0.6
0.5
0 0.0
0.3
0.6
0.9
1.2
CS content (%) Fig. 7. The relationship of water absorption and strength retention coefficient to CS content.
Fig. 8 shows the effect of SP content on water absorption of the foamed concrete. It can be seen that a small addition of SP can tremendously decrease the water absorption, and the 48-h water absorption of specimens with 0.2% SP is 13.9% which is only 20.3% of that of contrast specimens. Analogously, the water absorption decreases with the increase of SP content, and the RS value also increases significantly with the decrease of water absorption (Fig. 9). When the SP content is 1.0%, the 48-h water absorption and RS value of the foamed concrete are 2.5% and 0.989, respectively. This illustrates that the foamed concrete with a certain content of SP has extremely low water absorption. On the basis of the above analysis, it can be imagined that there might be a mathematical relationship between the RS value with the water absorption. Fig. 10 shows the relationship of the RS value to the water absorption of the foamed concrete modified by adding water repellent. As presented in Fig. 10, there is an approximately linear decrease in the RS value with the increasing water absorption. The linearly fitted curve and formula with high coefficient of determination (R2 = 0.930) are also shown in Fig. 10. That is to say, the RS value can be estimated by the 48-h water absorption on the premise of the little difference in dry density of the foamed concrete.
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80
CON 0.8% SP
0.2% SP 1.0% SP
50
0.4% SP 1.2% SP
CON 0.8% PT
0.2% PT 1.0% PT
0.4% PT 1.2% PT
W(ϕ) (kg/m3)
Water absorption, wT (%)
40 60
40
30
20
20
10
0
0 0
10
20
30
40
0
50
20
40
60
80
100
Relative humidity, ϕ (%)
Soaking time (hours) Fig. 8. The water absorption of specimens CON and Series III with different contents of SP.
Fig. 11. The hygroscopic moisture contents of specimens Series I at different relative humidities.
50 1.0
80
CON 0.8% CS
0.2% CS 1.0% CS
0.4% CS 1.2% CS
40
RS
0.8
40 0.7
W(ϕ ) (kg/m3)
wT RS values
Water absorption, wT (%)
0.9 60
20
0
0.0
0.3
0.6
0.9
1.2
30
20
0.6
10
0.5
0 0
20
Fig. 9. The relationship of water absorption and strength retention coefficient to SP content.
60
80
100
Fig. 12. The hygroscopic moisture contents of specimens Series II at different relative humidities.
50
1.0
CON 0.8% SP
40
RS =1-0.0059⋅wT , R 2 =0.930
W(ϕ) (kg/m3)
0.9
RS values
40
Relative humidity, ϕ (%)
SP content (%)
0.8
0.2% SP 1.0% SP
0.4% SP 1.2% SP
30
20
0.7
10
0.6 0
20
40
60
80
Water absorption, wT (%) Fig. 10. The relationship and fitted curve of the strength retention coefficient to the water absorption of the foamed concrete.
0 0
20
40
60
80
100
Relative humidity, ϕ (%) Fig. 13. The hygroscopic moisture contents of specimens Series III at different relative humidities.
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3.3. Hygroscopicity The contents of hygroscopic moisture [W(u)] of the foamed concrete specimens Series I with PT at a predetermined relative humidity are shown in Fig. 11. It can be observed from Fig. 11 that the W(u) increases markedly with the increasing relative humidity and decreases gradually as increasing the PT content. At the relative humidities of 18.7%, 57.9% and 100%, the W(u) values of the contrast specimens are 16.5 kg/m3, 28.6 kg/m3 and 46.4 kg/m3, respectively. The W(u) values of specimens with 0.2% and 1.0% PT at the relative humidity of 100% are 39.7 kg/m3 and 26.5 kg/m3, which are 14.4% and 42.9% lower than that of the
50
CON Average for Series I Average for Series II Average for Series III
W(ϕ) (kg/m3)
40
30
20
10
0 0
20
40
60
80
100
Relative humidity,ϕ (%) Fig. 14. Hygroscopic fitted curves for the average values by the KUM model.
50
CON Average for Series I Average for Series II Average for Series III
W(ϕ) (kg/m3)
40
contrast specimens at the same relative humidity. Similarly, the W(u) decreases significantly with the increase of the CS content initial, and then the W(u) decreases by a slight as the CS content going beyond 0.8% (Fig. 12). The W(u) values of the foamed concrete specimens with 0.2% CS under relative humidities of 18.7%, 57.9% and 100% are 11.2 kg/m3, 21.3 kg/m3 and 35.1 kg/m3, which are 67.9%, 74.5% and 75.6% of the hygroscopic moisture of contrast specimens under the corresponding relative humidity. Moreover, the W(u) values of specimens Series II with 0.8%, 1.0% and 1.2% CS are 23.5 kg/m3, 21.6 kg/m3 and 20.8 kg/m3, respectively. Fig. 13 shows the effect of SP contents on the hygroscopicity of the foamed concrete and the W(u) at the relative humidity of 100% decreases dramatically from 46.4 kg/m3 to 24.3 kg/m3 as the SP content increasing from 0 to 0.2%. And the W(u) values of specimens 0.4% and 1.2% SP at the relative humidity of 100% are 20.5 kg/m3 and 17.6 kg/m3, which indicates that there is only a small decrease in the W(u) values when the SP content is greater than 0.2%. This observation is different from the water absorption of specimens Series III with high content of SP obtained in Fig. 8. The W(u) reflects the amount of moisture (water vapour) absorption on the inner surface of pores in the foamed concrete and the absorption mainly depends upon the magnitude of adhesive force between pore surface and water vapour. When the hydrophilic surfaces of pores are translated to hydrophobic ones by adding water repellent, the adhesive force decreases, resulting in the decrease in the moisture absorption. When the addition of water repellent is greater, the adhesive force between the hydrophobic pore surface and water vapour may even be stronger than the molecular attraction force of the water vapour. Therefore, the W(u) decreases slightly as the content of water repellent going beyond a certain value. Nevertheless, the molecular attraction force of liquid water is much higher than the adhesive force between the hydrophobic pore surface and liquid water. This is the possible reason for the difference between the effects of water repellent on the water absorption and moisture absorption of the foamed concrete. As reported in previous researches [27–29], three theoretical models studied on sorption isotherms curves can be used to obtain hygroscopic fitted curves. Among them, the KUM and CUB models are best fit to analyze the relationship of the moisture absorption of the foamed concrete to the relative humidity [24]. The KUM model established by Kumaran is expressed as follows.
u
30
WðuÞ ¼
20
where A, B and C are the constants of the KUM model. The CUB model is a cubic equation and its expression is proposed as the following function.
10
WðuÞ ¼ a u3 þ b u2 þ c u
0 0
20
40
60
80
100
Relative humidity, ϕ (%) Fig. 15. Hygroscopic fitted curves for the average values by the CUB model.
ð4Þ
Au2 þ Bu þ C
ð5Þ
where a, b and c are the constants of the CUB model. To observe the accuracy of the KUM and CUB models in fitting hygroscopic curves, the average values of moisture absorption of different series of specimens are first analyzed. The hygroscopic fitted curves for the average values by the KUM and CUB models are shown in Figs. 14 and 15, respectively. It can be seen that the data
Table 3 The constants and R-square of the fitted curves for the average values of each Series. Average for Seires
KUM model A (10
CON I II III
2.23 2.64 4.49 5.78
4
)
CUB model 2
B
C
R
0.039 0.050 0.074 0.096
0.535 0.999 1.044 1.320
0.989 0.988 0.995 0.994
a (104)
b
c
R2
1.027 0.588 0.513 0.411
0.0168 0.0099 0.0083 0.0066
1.118 0.711 0.569 0.449
0.998 0.998 0.998 0.998
113
C. Ma, B. Chen / Construction and Building Materials 123 (2016) 106–114 Table 4 The constants and R-square of the fitted curve for each specimen. Series
Specimen
KUM model A (10
4
)
CUB model B
C
R
a (104)
b
c
R2
2
CON
–
2.23
0.039
0.535
0.989
1.027
0.0168
1.118
0.998
Series I
0.2% 0.4% 0.8% 1.0% 1.2%
PT PT PT PT PT
2.08 2.51 3.26 3.24 1.94
0.041 0.047 0.058 0.058 0.043
0.648 0.824 1.029 1.286 1.735
0.971 0.989 0.992 0.990 0.985
0.939 0.671 0.551 0.460 0.320
0.0157 0.0114 0.0091 0.0076 0.0058
1.025 0.802 0.649 0.571 0.506
0.995 0.998 0.999 0.996 0.991
Series II
0.2% 0.4% 0.8% 1.0% 1.2%
CS CS CS CS CS
2.99 3.68 4.63 6.43 6.64
0.051 0.064 0.080 0.100 0.100
0.829 0.945 0.996 1.097 1.564
0.995 0.997 0.991 0.993 0.993
0.658 0.552 0.515 0.450 0.391
0.0108 0.0091 0.0084 0.0071 0.0060
0.767 0.638 0.562 0.469 0.409
0.998 0.998 0.997 0.991 0.997
Series III
0.2% 0.4% 0.8% 1.0% 1.2%
SP SP SP SP SP
5.28 5.28 5.77 6.11 6.52
0.085 0.091 0.098 0.101 0.105
0.959 1.201 1.291 1.608 1.820
0.993 0.992 0.991 0.995 0.994
0.528 0.437 0.411 0.347 0.330
0.0084 0.0072 0.0067 0.0056 0.0052
0.553 0.482 0.449 0.393 0.367
0.995 0.997 0.996 0.999 0.999
points are mainly on the fitted curves and the constants in Eqs. (4) and (5) obtained from the fitted curves are listed in Table 3. The high coefficients of determination imply that the KUM and CUB models are appropriate to describe the hygroscopic behavior of the foamed concrete modified by water repellent. In addition, the fitted curves from CUB model are a bit more precise than those from KUM model. The constants in KUM and CUB models of the fitted curves for each specimen with water repellent are presented in Table 4. Each fitted curve has high coefficient of determination (>0.9), and the relationship of the constants to the content of water repellent can also be obtained. For example, the A values for Series II decrease approximately linearly with the increasing CS content and the B and C values generally increase with CS content. 4. Conclusions The mechanical and physical properties of the foamed concrete with potassium trimethylsilanolate (PT), calcium stearate (CS) and siloxane-based polymer (SP) including 7-day and 28-day compressive strength, thermal conductivity, sorptivity and hygroscopicity are systematically studied. Based on the analysis on the laboratory results, the following conclusions can be drawn: 1. The water repellent has little impact on the foam stability and dry density of the low-density foamed concrete. The compressive strength can also be improved by adding water repellent, and the optimal improvement often occurs at the content of water repellent of 1.0%. The 28-day strength of the foamed concrete with a certain content of SP is higher than that of specimens with the same content of TP and CS. Moreover, the thermal conductivities of the foamed concrete with different contents of water repellents vary from 0.149 W/m K to 0.159 W/m K. The 28-day strength and thermal conductivity of the foamed concrete with 1.0% SP are 1.77 MPa and 0.150 W/m K, respectively. 2. The effect of water repellent on the sorptivity of the foamed concrete is evaluated by water absorption and strength retention coefficient (RS) after 48 h of soaking. The water absorption decreases significantly as the content of water repellent increasing from 0.2% to 1.2%. When the same content of water repellent is employed, the 48-h water absorption of the foamed concrete modified by SP is the lowest. And the 48-h water absorption of the foamed concrete with 1.0% SP is 2.5% (by volume), which is about only 3.6% of that of the foamed concrete without water repellent.
3. The RS increases markedly with the increasing content of water repellent. Likewise, the addition of SP has the most significant effect on the RS. The RS value of the foamed concrete with 1.0% SP is 0.989, which is 55.7% higher than that of the foamed concrete without water repellent. The relationship of the RS to the water absorption indicates that the RS value approximately linearly with the increase of the water absorption. And the fitted curve and function with a coefficient of determination of 0.930 are achieved. 4. The application of water repellent can reduce the contents of hygroscopic moisture [W(u)] of the foamed concrete. The W (u) decreases gradually as the PT and CS contents increasing from 0.2% to 1.2% and decreases slightly with the increasing SP content from 0.2% to 1.2%. The difference in the effects of water repellent on water and moisture absorption may be explained by the values of the adhesive force between the pore surface and liquid water or water vapour. The fitted curves with high coefficients of determination have been proposed to describe the relationship of the W(u) to the relative humidity. Acknowledgments This research work was financially supported by the National Natural Science Foundation of China, Grant No. 51378309. 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] T.Y. Lo, W.C. Tang, H.Z. Cui, The effects of aggregate properties on lightweight concrete, Build. Environ. 42 (8) (2007) 3025–3029. [3] Z. Wu, Y. Zhang, J. Zheng, Y. Ding, An experimental study on the workability of self-compacting lightweight concrete, Constr. Build. Mater. 23 (5) (2009) 2087–2092. [4] B. Chen, J. Liu, Experimental application of mineral admixtures in lightweight concrete with high strength and workability, Constr. Build. Mater. 22 (6) (2008) 1108–1113. [5] O. Kayali, B. Zhu, Chloride induced reinforcement corrosion in lightweight aggregate high-strength fly ash concrete, Constr. Build. Mater. 19 (4) (2005) 327–336. [6] B. Chen, Z. Wu, N. Liu, Experimental research on properties of high-strength foamed concrete, J. Mater. Civ. Eng. 24 (1) (2011) 113–118. [7] 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. [8] 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. [9] P.J. Tikalsky, J. Pospisil, W. MacDonald, A method for assessment of the freezethaw resistance of preformed foam cellular concrete, Cem. Concr. Res. 34 (5) (2004) 889–893.
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[10] N. Narayanan, K. Ramamurthy, Structure and properties of aerated concrete: a review, Cem. Concr. Compos. 22 (5) (2000) 321–329. [11] K.H. Yang, K.H. Lee, J.K. Song, M.H. Gong, Properties and sustainability of alkaliactivated slag foamed concrete, J. Clean. Prod. 68 (2014) 226–233. [12] S.K. Lim, C.S. Tan, O.Y. Lim, Y.L. Lee, Fresh and hardened properties of lightweight foamed concrete with palm oil fuel ash as filler, Constr. Build. Mater. 46 (2013) 39–47. [13] J.J. del Coz Díaz, F.P. Álvarez Rabanal, P.J. García Nieto, J. Domínguez Hernández, B. Rodríguez Soria, J.M. Pérez-Bella, Hygrothermal properties of lightweight concrete: experiments and numerical fitting study, Constr. Build. Mater. 40 (2013) 543–555. [14] 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. [15] J. Jiang, Z. Lu, Y. Niu, Y. Zhang, Study on the preparation and properties of highporosity foamed concretes based on ordinary Portland cement, Mater. Des. 92 (2016) 949–959. [16] E.P. Kearsley, P.J. Wainwright, Porosity and permeability of foamed concrete, Cem. Concr. Res. 31 (5) (2001) 805–812. [17] A. Benazzouk, O. Douzane, M. Quéneudec, Transport of fluids in cement-rubber composites, Cem. Concr. Compos. 26 (1) (2004) 21–29. [18] E.K.K. Nambiar, K. Ramamurthy, Sorption characteristics of foam concrete, Cem. Concr. Res. 37 (9) (2007) 1341–1347. [19] M.J. Shannag, Characteristics of lightweight concrete containing mineral admixtures, Constr. Build. Mater. 25 (2) (2011) 658–662.
[20] E. Yasßar, Y. Erdog˘an, Strength and thermal conductivity in lightweight building materials, Bull. Eng. Geol. Environ. 67 (4) (2008) 513–519. [21] X. Lü, Modelling of heat and moisture transfer in buildings: II. Applications to indoor thermal and moisture control, Energy Build. 34 (10) (2002) 1045–1054. [22] J.J. del Coz Díaz, P.J.G. Nieto, J.D. Hernández, A.S. Sánchez, Thermal design optimization of lightweight concrete blocks for internal one-way spanning slabs floors by FEM, Energy Build. 41 (12) (2009) 1276–1287. [23] ISO 12571:2000 Standard, Hygrothermal performance of building materials and products, Determination of hygroscopic sorption properties. [24] C. Ma, B. Chen, Properties of a foamed concrete with soil as filler, Constr. Build. Mater. 76 (2015) 61–69. [25] A. Kampala, S. Horpibulsuk, Engineering properties of silty clay stabilized with calcium carbide residue, J. Mater. Civ. Eng. 25 (5) (2012) 632–644. [26] A.K. Suryavanshi, R.N. Swamy, Development of lightweight mixes using ceramic microspheres as fillers, Cem. Concr. Res. 32 (11) (2002) 1783–1789. [27] H.M. Künzel, Simultaneous heat and moisture transport in building components, one- and two-dimensional calculation using simple parameters PhD dissertation, IRB-Verlag Fraunhofer-Institut für, Bauphysik, Stuttgart (Germany), 1995. [28] K.M. Kumaran, Heat, Air and Moisture Transport in Envelope Parts Final report, Task 3 material properties, Leuven, Internal report, IEA, ECBCS, 1996. [29] UNE-EN ISO 10456:1999, Building materials and products. Procedures for determining declared and design thermal values (ISO 10456:1999).
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Properties of foamed concrete containing water repellents Cong Ma, Bing Chen ⇑ Department of Civil Engineering, Shanghai Jiaotong University, Shanghai 200240, PR China
h i g h l i g h t s A foamed concrete modified by water repellent with low water absorption is prepared. The effects of water repellent on the physical and mechanical properties are investigated. The hygroscopic fitted curves from KUM and CUB models are established.
a r t i c l e
i n f o
Article history: Received 26 May 2016 Received in revised form 27 June 2016 Accepted 29 June 2016
Keywords: Foamed concrete Water repellent Compressive strength Sorptivity Hygroscopicity
a b s t r a c t It is generally known that the physical and mechanical properties would be greatly degraded after water or moisture transferring into the foamed concrete. In this study, a foamed concrete with a low density of about 550 kg/m3 is prepared using ordinary Portland cement, and three types of water repellents including potassium trimethylsilanolate (PT), calcium stearate (CS) and siloxane-based polymer (SP) are employed to decrease the water absorption of the foamed concrete. The effects of the water repellent on the mechanical and physical properties of the foamed concrete, such as 7-day and 28-day compressive strength, thermal conductivity, sorptivity and hygroscopicity, are studied. The laboratory results indicate that the water repellents improve the compressive strength to some extent without sacrificing the thermal insulation property of the foamed concrete. The sorptivity evaluated by 48-h water absorption and strength retention coefficient (RS) is significantly improved as increasing the content of water repellent. When 1.0% SP is used, the water absorption and RS value of the foamed concrete with 28-day strength of 1.77 MPa and thermal conductivity of 0.150 W/m K are 2.5% (by volume) and 0.989, respectively. In addition, the contents of hygroscopic moisture [W(u)] also decrease with the increasing content of water repellent. The hygroscopic fitted curves with high coefficients of determination obtained the KUM and CUB models have been proved applicable in exploring the relationship of the W(u) to the relative humidity. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction In China, the energy consumption of construction industry has roughly 35 percent of total energy consumption in society [1]. In the past several years, lightweight concrete, aerated concrete and foamed concrete with excellent thermal insulating properties have been used as exterior wall materials and studied by many researchers [2–7]. In addition, the foamed concrete with high fluidity and low cement and aggregate usage is also applied to sandwich structures, earth-remaining walls and running tracks or playgrounds [8,9]. The common used foamed concrete is defined as pre-foamed foam concrete by adding the projected amount of foam into cement slurry.
⇑ Corresponding author. E-mail address: [email protected] (B. Chen). http://dx.doi.org/10.1016/j.conbuildmat.2016.06.148 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
The first modern foamed concrete prepared by Portland cement was reported in the twenty-twenties [10]. During the past decades, the improvement of foaming technology and the application of high-efficiency superplasticizers, foam stabilizers and early strength agents have substantially improve the engineering properties of the foamed concrete. And many researchers have studied the preparation methods, fresh properties, physical properties and mechanical properties in detail [4,6,11–15]. It is notable that the transfer of moisture or water into the foamed concrete has drawn increasing attention due to the significant impact of the climate change [13,16]. The permeability of the foamed concrete which depends upon its porosity and pore structure (such as pore diameter and continuity) is the main factor in determining the transport of moisture or water [17]. To understand the mechanism of water permeation in foamed concrete, sorptivity and hygroscopicity characterized by water and moisture (water vapour) absorption are the applicable
C. Ma, B. Chen / Construction and Building Materials 123 (2016) 106–114
parameters [18]. The properties of the foamed concrete after water immersion and hygroscopicity tests have been studied [19–22]. It can be concluded that the water or moisture absorption has significantly negative influence on the physical and mechanical properties of the foamed concrete, which may result in energy waste and failure cracks. As reported earlier, types of foam agents, densities and mineral supplementary materials for the foamed concrete are also the factors influencing the sorptivity and hygroscopicity. Several researches have studied the moisture dynamics of foamed concrete and established some phenomenological models to guide the design of the foamed concrete [17,19,21]. Even so, the water absorption of foamed concrete prepared by cement, sand and fly ash with the dry density of about 900 kg/m3 is about 30% (by volume of the concrete) and the moisture absorption of lightweight concrete prepared by cement and lightweight sand with the dry density of about 950 kg/m3 is approximately 30 kg/m3 under a relative humidity of 95% [13,18]. This implies that the foamed/lightweight concrete prepared at the optimal design also has a high water or moisture absorption. However, there are few studies reported on the improvement methodology of water absorption of foamed concrete. In this study, ordinary Portland cement and silica fume are employed as cementitious materials to prepare foamed concrete with low dry density of about 550 kg/m3. To decrease the water absorption, three different water repellents have been introduced into foamed concrete. The physical and mechanical properties have been tested and analyzed in laboratory. And the effects of water repellent on compressive strength and thermal conductivity are investigated. Based on the results of water immersion and hygroscopic tests, some numerical models are established and a foamed concrete with low water absorption is obtained.
107
of water repellent on the properties of foamed concrete. Three types of water repellents are selected and they are potassium trimethylsilanolate (PT), calcium stearate (CS) and a type of siloxane-based polymer (SP), respectively. SP powder are prepared in the laboratory and CS is chemically pure. The detailed compositions of each mixture for contrast specimen and three series of specimens are presented in Table 2. 2.2. Specimen preparation A paddle mixer is used to produce foamed concrete mixture by adding the projected volume of foam. The different series of specimens are prepared by adding different types of water repellents as shown in Table 2. The preparation of the mixture includes three steps. Firstly, the predetermined amount of water was mixed with cementitious materials and admixtures and the stirring process lasts about 3 min in order to obtain a homogeneous mixture. And then, PP fibers are introduced into the mixture and mixed for about 5 min. Finally, the projected volume of foam is added immediately into the fluid mixture and the mixture is continued to mix in high speed until the foam is uniformly distributed in the mixture. Then, the fluid foamed concrete is poured into the cube PVC moulds (150 mm 150 mm 150 mm) and the surface of each specimen is smoothed by hand only. Subsequently, the moulds are transferred to standard curing room with a temperature of 22 ± 2 °C and a relative humidity of 97 ± 2%. Within the initial 12 h of curing, the moulds are covered with wet linen in order to keep the volume stability of the foamed concrete. After 3 days of curing, the specimens are demoulded and stored in the curing room until testing age. 2.3. Property measurement
2. Experimental procedure 2.1. Materials Ordinary Portland cement (OPC) with a 28-strength of 56.5 MPa and silica fume (SF) achieved from Elken Materials are the selected cementitious materials to prepare low-density foamed concrete. Table 1 lists the chemical compositions of OPC and silica fume. The polypropylene (PP) fiber with a tensile strength of 800.0 MPa and modulus of 8.0 GPa is employed to improve the toughness of foamed concrete, and its length and diameter are 15.0 mm and 100.0 lm, respectively. In order to obtain an acceptable workability of the mixture, a type of naphthalene-based superplasticizer (SL) is introduced into each specimen. In addition, the composites of early strength and foam stabilizer agents (NAF) prepared in the laboratory are used. A protein-based foaming agent which can provide a more stable bubble network is used, and the density of the foam is about 70 kg/m3. The purpose of this study is to prepare a foamed concrete with excellent sorptivity and hygroscopicity and investigate the effects
Table 1 Chemical composition of binders (% by weight). Oxide
OPC
SF
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 LOI (Loss on ignition)
21.6 4.13 4.57 64.44 1.06 0.11 0.56 1.74 0.76
92.40 0.80 0.50 0.91 0.27 – – – 2.0
2.3.1. Compressive strength, thermal conductivity, Before thermal conductivity tests, the specimens after 28 days of standard curing are oven-dried in a vacuum oven at 45 ± 2 °C. After 48 h of oven drying, the specimen mass is weighed out every 12 h until the mass variation among three consecutive testing results not more than 0.1% of the total mass of specimens. The finally constant mass is used to calculate the dry density of the foamed concrete specimens and the calculated values are presented in Table 2. The thermal conductivity of each specimen is measured by using a KD2 Pro thermal properties analyzer (Decagon Devices, Inc., USA). This equipment has an electronically handheld controller and sensors of different sizes which should be inserted into the foamed concrete specimens. Each needle-like (single-needle or double-needle) sensor consists of a thermistor and a heating element. In this study, a single-needle sensor with a diameter of 2.4 mm and a length of 100 mm is used and it can measure the thermal conductivity ranging from 0.02 W/m K to 2.0 W/m K. In order to insert the sensor into the oven-dried specimens, a hole of the same size with the sensor should be drilled. Each measurement process continues at least 5 min and at least four measurements are taken for each foamed concrete specimen to ensure the accuracy of ±5%. At 7 and 28 days of curing, the specimens are took out from the curing room. An MTS servo hydraulic testing machine with a capacity of 100 kN is employed to measure the compressive strength of foamed concrete. The loading rate is fixed at 0.5 mm/min and the strength is the mean value of five measurements to confirm the reproducible of experimental results. In addition, the strength values which are beyond ±15% of the mean strength value should not be used and the compressive strength presented in this study is the average of at least three testing results.
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C. Ma, B. Chen / Construction and Building Materials 123 (2016) 106–114
Table 2 Mixture compositions. Series
Mix No.
Target density (kg/m3)
Composition of mixture (per m3) Binder (kg) OPC
SF
3
Water (kg)
PP fiber (kg)
Foam (m )
SL (kg)
NAF (kg)
PT (kg)
CS (kg)
SP (kg)
Dry density (kg/m3)
CON
1
550
368
32
140
0.4
0.8
1.6
3.6
0
0
0
546
Series I
2 3 4 5 6
550 550 550 550 550
368 368 368 368 368
32 32 32 32 32
140 140 140 140 140
0.4 0.4 0.4 0.4 0.4
0.8 0.8 0.8 0.8 0.8
1.6 1.6 1.6 1.6 1.6
3.6 3.6 3.6 3.6 3.6
0.8 1.6 3.2 4.0 4.8
0 0 0 0 0
0 0 0 0 0
552 561 547 548 559
Series II
7 8 9 10 11
550 550 550 550 550
368 368 368 368 368
32 32 32 32 32
140 140 140 140 140
0.4 0.4 0.4 0.4 0.4
0.8 0.8 0.8 0.8 0.8
1.6 1.6 1.6 1.6 1.6
3.6 3.6 3.6 3.6 3.6
0 0 0 0 0
0.8 1.6 3.2 4.0 4.8
0 0 0 0 0
548 553 551 555 562
Series III
12 13 14 15 16
550 550 550 550 550
368 368 368 368 368
32 32 32 32 32
140 140 140 140 140
0.4 0.4 0.4 0.4 0.4
0.8 0.8 0.8 0.8 0.8
1.6 1.6 1.6 1.6 1.6
3.6 3.6 3.6 3.6 3.6
0 0 0 0 0
0 0 0 0 0
0.8 1.6 3.2 4.0 4.8
557 547 553 549 542
2.3.2. Sorptivity The sorptivity of the foamed concrete can be evaluated by two parameters: water absorption and strength retention coefficient. Water absorption can be obtained by water immersion tests and the purpose of the soaked condition is to simulate the negative impact of rainy weather on the foamed concrete. After 28 days of curing, the specimens should be oven-dried to constant mass at the same condition as mentioned before, and then immersed in deionized water in the standard curing room. After 0.25, 0.5, 1, 2, 3, 6, 12, 24 and 48 h of soaking, the values of water absorption by volume can be calculated by the following equation [13,24].
wT ¼
mT m0 V qw
ð1Þ
where wT is the water absorption by volume at a desired soaking time, mT is the weight of specimen after immersion, m0 is the weight of the oven-dried specimen, V is the volume of the specimen, and qw is the density of water at standard condition. After 48 h of soaking, the surface of the soaked specimens is wiped with a dry cloth; subsequently, compressive strength tests are performed on these soaked specimens. The strength retention coefficient can be expressed as the following expression.
RS ¼
fs f 28
ð2Þ
where RS is the strength retention coefficient of the foamed concrete, f28 is the 28-day compressive strength of specimens before water immersion, and fS is the compressive strength of specimens after 48 h of soaking. 2.3.3. Hygroscopicity Similar with the sorptivity tests, all the specimens for hygroscopicity tests should be oven-dried. Hygroscopicity tests in this study follow the Standard ISO 12571:2000 [23]. A climatic testing chamber illustrated in the standard is prepared in the laboratory to investigate the hygroscopic sorption of the foamed concrete. This simple instrument can make the experimental condition with a stable temperature (22 ± 0.5 °C) and a variation range of relative humidity from 9.2% to 100%. The final moisture absorption of the foamed concrete specimens is obtained at an approximate equilibrium state. The characteristic of the equilibrium state is that the mass variation among three consecutive weightings which are measured every 12 h is not more than 0.1% of the total mass of
the oven-dried specimens. The moisture absorption reflects the amount of water vapour in the air of the climatic testing chamber permeating into the foamed concrete specimens and it can be calculated as the following equation [13,24].
WðuÞ ¼
mðuÞ m0 V
ð3Þ
where W(u) is the moisture absorption by weight, m(u) is the weight of specimen after hygroscopicity tests, m0 is the weight of the oven-dried specimen, and V is the volume of the specimen. 3. Results and discussion 3.1. Compressive strength and thermal conductivity The effects of different types of water repellents on dry density of the foamed concrete are illustrated in Table 2. As shown in Table 2, the dry density of contrast specimen is 546 kg/m3 and the dry density of different series of specimens with water repellent varies from 542 kg/m3 to 562 kg/m3. That is to say, the maximal value is only 20 kg/m3 higher than the minimal value. This implies that the addition of water repellent in the foamed concrete has little influence on the foam stability, hence the dry density. In addition, it can be observed that there is no predictable pattern to explain the relationship of dry density with the content of water repellent. Fig. 1 shows the effects of PT content on the compressive strength at 7 days and 28 days of curing and thermal conductivity of the foamed concrete. As shown in Fig. 1, the 7-day and 28-day strength increase almost linearly as the PT content increasing from 0 to 1.0%. The 7-day and 28-day strengths of the foamed concrete specimens with 1.0% PT are 1.12 MPa and 1.33 MPa, which are 38.3% and 23.1% higher than those of contrast specimens. However, the compressive strength decreases slightly when the PT content is greater than 1.0%. The possible reason may be that PT can participate in the cement hydration reaction and play a role of surfactant which can increase the amounts of hydration products and improve the structure compactness. When the PT content is excess, its hydrophobic effect is more significant which may lead to the reduction of water participating in hydration reaction. The thermal conductivity generally increases with the increase of PT content from 0 to 1.0% initial, and then decreases slightly as the PT content going beyond 1.0%, which is approximately consistent with the
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0.160
7-day strength 28-day strength Thermal conductivity
0.155
1.2
0.150 1.0
0.145
Thermal conducticity (W/mK)
Compressive strength (MPa)
1.4
0.8
0.140 0.0
0.2
0.4
0.6
0.8
1.0
1.2
in Fig. 3. As seen in Fig. 3, the compressive strength increases as increasing the SP content from 0 to 1.2% in general, which is different from the strength development observed in Figs. 1and 2. The 7-day and 28-day strength of specimens with 1.0% SP are 1.53 MPa and 1.77 MPa, which are 88.9% and 63.9% higher than those of contrast specimens. It also can be found that the specimens with 1.0% or 1.2% SP have the highest compressive strength among the three different series of specimens. The influence of SP content on the thermal conductivity is irregular and it is difficult to seek the relationship between thermal conductivity and compressive strength. The maximal thermal conductivity of specimens with 0.2% SP is 0.153 W/m K which is about 2.7% higher than that of contrast specimens. And the thermal conductivity of specimens with 1.0% SP is 0.150 W/m K. This implies that the addition of SP can significantly improve the compressive strength without sacrificing the thermal insulation property of the foamed concrete.
PT content (%)
tendency presented in compressive strength. However, the thermal conductivity of the foamed concrete specimens with 1.0% PT is 0.154 W/m K, which is only 3.4% higher than that of contrast specimen. This implies that the application of PT in the foamed concrete has little impact on its thermal insulation property. Fig. 2 shows the effect of CS content on the compressive strength at 7 days and 28 days of curing and thermal conductivity of the foamed concrete. As shown in Fig. 2, the strength development of the foamed concrete has a similar tendency with that presented in Fig. 1. The strength increases as increasing the CS content initial, and then decrease by a slight when the CS content is greater than 1.0%. The 7-day and 28-day strengths of specimens with 1.0% CS are 1.24 MPa and 1.68 MPa, which are 53.1% and 55.6% higher than those of contrast specimens. It should be noted that the 28-strength of specimens with 1.0% CS is much higher than that of specimens with 1.0% PT. The thermal conductivity also generally increases with the increase of the CS content due to the improvement of compressive strength, which has been concluded in many researches [4–6,14,15]. Nevertheless, the range of thermal conductivity is very small, and the maximum value at the CS content of 1.0% is 0.159 W/m K which is only 6.7% higher than the minimal value. The 7-day and 28-day strength and thermal conductivity of the foamed concrete with different contents of SP are presented
The sorptivity of the foamed concrete can be evaluated by the amount of water absorption and the decrease in compressive strength after a predetermined soaking time [16]. The effect of PT content on the water absorption of the foamed concrete are presented in Fig. 4. As shown in Fig. 4, the water absorption of most of the foamed concrete specimens dramatically increases within the initial 6 h of soaking time, and then the water absorption keeps nearly constant. The 6-h and 48-h water absorption of contrast specimen are 56.5% and 68.5%, respectively. This indicates that the permeation of water into the foamed concrete occurs immediately after water immersion of the specimens. The possible reason may be that there are many inter-connected pores in the lightweight foamed concrete. This observation is consistent with the conclusions reported in the previous researches [24,25]. It can be observed that the water absorption decreases gradually as increasing the PT content from 0.2% to 1.2%. The water absorption of the specimen with only 0.4% PT at 6 h and 48 h of soaking are 29.1% and 32.8%, which are only 51.5% and 47.9% of the water absorption of contrast specimen at the same soaking time. Accordingly, the strength retention coefficient (RS) of specimens with 0.4% PT is 0.754, which is 18.7% higher than that of contrast specimens (Fig. 5). It can be also observed from Fig. 5 that the RS value increases significantly with the decrease in water absorption. As the PT content increasing from 0.2% to 1.2%, the water absorption decreases from 54.3% to 15.6% and the RS value increases from
0.155 1.4 0.150 1.2
1.0
0.145
0.8
Compressive strength (MPa)
1.8
1.6
0.160
7-day strength 28-day strength Thermal conductivity
0.160
7-day strength 28-day strength Thermal conductivity
Thermal conducticity (W/mK)
Compressive strength (MPa)
1.8
3.2. Sorptivity
0.155
1.6
1.4 0.150 1.2
0.145
1.0
Thermal conducticity (W/mK)
Fig. 1. The effect of PT content on 7-day and 28-day compressive strengths and thermal conductivity for Series I.
0.8 0.140
0.140 0.0
0.2
0.4
0.6
0.8
1.0
1.2
CS content (%) Fig. 2. The effect of CS content on 7-day and 28-day compressive strengths and thermal conductivity for Series II.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
SP content (%) Fig. 3. The effect of SP content on 7-day and 28-day compressive strengths and thermal conductivity for Series III.
C. Ma, B. Chen / Construction and Building Materials 123 (2016) 106–114
CON 0.8% PT
0.2% PT 1.0% PT
80
0.4% PT 1.2% PT
CON 0.8% CS
60
Water absorption,wT (%)
Water absorption,wT (%)
80
40
20
0
0
10
20
30
40
40
20
0
10
20
Soaking time (hours)
RS
0.8 40 0.7
20 0.6
0.9
1.2
0.5
PT content (%) Fig. 5. The relationship of water absorption and strength retention coefficient to PT content.
0.695 to 0.893. This implies that the addition of PT can improve the after-soaking compressive strength and the water resistance of the foamed concrete. As analyzed before, PT has little impact on the dry density and thermal insulation of the foamed concrete, that is, the foam quantities keep constant. Hence, the possible mechanisms for the improvement of water resistance by adding a water repellent into the foamed concrete are the increase of the disconnected pores and the change of the hydrophilic surfaces of some hydration products to hydrophobic surfaces. The influence of CS content on the water absorption of the foamed concrete is shown in Fig. 6. The addition of CS has more significant effect on the water absorption in comparison to PT, and the 48-h water absorption of specimen with 0.2% CS is 37.4%, which is almost half of that of contrast specimen. It is notable that the 48-h water absorption of specimens with 1.0% and 1.2% CS are both less than 10%, which indicates that the sorptivity of the foamed concrete can be markedly improved. The 48-h water absorption of specimen with 1.0% CS is 9.4%, which is 47.2% of that of specimen with 1.0% PT. Likewise, the RS value increases gradually from 0.716 to 0.933 as decreasing the water absorption from 37.4% to 8.9% (Fig. 7). It can be concluded that CS can also improve the water resistance of the foamed concrete, which is consistent with the previous research [26].
Water absorption, wT (%)
0.9
60
RS values
Water absorption, wT (%)
1.0
wT
RS
0.6
50
80
wT
0.3
40
Fig. 6. The water absorption of specimens CON and Series II with different contents of CS.
1.0
80
0.0
30
Soaking time (hours)
Fig. 4. The water absorption of specimens CON and Series I with different contents of PT.
0
0.4% CS 1.2% CS
60
0
50
0.2% CS 1.0% CS
0.9
60
0.8 40 0.7
RS values
110
20 0.6
0.5
0 0.0
0.3
0.6
0.9
1.2
CS content (%) Fig. 7. The relationship of water absorption and strength retention coefficient to CS content.
Fig. 8 shows the effect of SP content on water absorption of the foamed concrete. It can be seen that a small addition of SP can tremendously decrease the water absorption, and the 48-h water absorption of specimens with 0.2% SP is 13.9% which is only 20.3% of that of contrast specimens. Analogously, the water absorption decreases with the increase of SP content, and the RS value also increases significantly with the decrease of water absorption (Fig. 9). When the SP content is 1.0%, the 48-h water absorption and RS value of the foamed concrete are 2.5% and 0.989, respectively. This illustrates that the foamed concrete with a certain content of SP has extremely low water absorption. On the basis of the above analysis, it can be imagined that there might be a mathematical relationship between the RS value with the water absorption. Fig. 10 shows the relationship of the RS value to the water absorption of the foamed concrete modified by adding water repellent. As presented in Fig. 10, there is an approximately linear decrease in the RS value with the increasing water absorption. The linearly fitted curve and formula with high coefficient of determination (R2 = 0.930) are also shown in Fig. 10. That is to say, the RS value can be estimated by the 48-h water absorption on the premise of the little difference in dry density of the foamed concrete.
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C. Ma, B. Chen / Construction and Building Materials 123 (2016) 106–114
80
CON 0.8% SP
0.2% SP 1.0% SP
50
0.4% SP 1.2% SP
CON 0.8% PT
0.2% PT 1.0% PT
0.4% PT 1.2% PT
W(ϕ) (kg/m3)
Water absorption, wT (%)
40 60
40
30
20
20
10
0
0 0
10
20
30
40
0
50
20
40
60
80
100
Relative humidity, ϕ (%)
Soaking time (hours) Fig. 8. The water absorption of specimens CON and Series III with different contents of SP.
Fig. 11. The hygroscopic moisture contents of specimens Series I at different relative humidities.
50 1.0
80
CON 0.8% CS
0.2% CS 1.0% CS
0.4% CS 1.2% CS
40
RS
0.8
40 0.7
W(ϕ ) (kg/m3)
wT RS values
Water absorption, wT (%)
0.9 60
20
0
0.0
0.3
0.6
0.9
1.2
30
20
0.6
10
0.5
0 0
20
Fig. 9. The relationship of water absorption and strength retention coefficient to SP content.
60
80
100
Fig. 12. The hygroscopic moisture contents of specimens Series II at different relative humidities.
50
1.0
CON 0.8% SP
40
RS =1-0.0059⋅wT , R 2 =0.930
W(ϕ) (kg/m3)
0.9
RS values
40
Relative humidity, ϕ (%)
SP content (%)
0.8
0.2% SP 1.0% SP
0.4% SP 1.2% SP
30
20
0.7
10
0.6 0
20
40
60
80
Water absorption, wT (%) Fig. 10. The relationship and fitted curve of the strength retention coefficient to the water absorption of the foamed concrete.
0 0
20
40
60
80
100
Relative humidity, ϕ (%) Fig. 13. The hygroscopic moisture contents of specimens Series III at different relative humidities.
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C. Ma, B. Chen / Construction and Building Materials 123 (2016) 106–114
3.3. Hygroscopicity The contents of hygroscopic moisture [W(u)] of the foamed concrete specimens Series I with PT at a predetermined relative humidity are shown in Fig. 11. It can be observed from Fig. 11 that the W(u) increases markedly with the increasing relative humidity and decreases gradually as increasing the PT content. At the relative humidities of 18.7%, 57.9% and 100%, the W(u) values of the contrast specimens are 16.5 kg/m3, 28.6 kg/m3 and 46.4 kg/m3, respectively. The W(u) values of specimens with 0.2% and 1.0% PT at the relative humidity of 100% are 39.7 kg/m3 and 26.5 kg/m3, which are 14.4% and 42.9% lower than that of the
50
CON Average for Series I Average for Series II Average for Series III
W(ϕ) (kg/m3)
40
30
20
10
0 0
20
40
60
80
100
Relative humidity,ϕ (%) Fig. 14. Hygroscopic fitted curves for the average values by the KUM model.
50
CON Average for Series I Average for Series II Average for Series III
W(ϕ) (kg/m3)
40
contrast specimens at the same relative humidity. Similarly, the W(u) decreases significantly with the increase of the CS content initial, and then the W(u) decreases by a slight as the CS content going beyond 0.8% (Fig. 12). The W(u) values of the foamed concrete specimens with 0.2% CS under relative humidities of 18.7%, 57.9% and 100% are 11.2 kg/m3, 21.3 kg/m3 and 35.1 kg/m3, which are 67.9%, 74.5% and 75.6% of the hygroscopic moisture of contrast specimens under the corresponding relative humidity. Moreover, the W(u) values of specimens Series II with 0.8%, 1.0% and 1.2% CS are 23.5 kg/m3, 21.6 kg/m3 and 20.8 kg/m3, respectively. Fig. 13 shows the effect of SP contents on the hygroscopicity of the foamed concrete and the W(u) at the relative humidity of 100% decreases dramatically from 46.4 kg/m3 to 24.3 kg/m3 as the SP content increasing from 0 to 0.2%. And the W(u) values of specimens 0.4% and 1.2% SP at the relative humidity of 100% are 20.5 kg/m3 and 17.6 kg/m3, which indicates that there is only a small decrease in the W(u) values when the SP content is greater than 0.2%. This observation is different from the water absorption of specimens Series III with high content of SP obtained in Fig. 8. The W(u) reflects the amount of moisture (water vapour) absorption on the inner surface of pores in the foamed concrete and the absorption mainly depends upon the magnitude of adhesive force between pore surface and water vapour. When the hydrophilic surfaces of pores are translated to hydrophobic ones by adding water repellent, the adhesive force decreases, resulting in the decrease in the moisture absorption. When the addition of water repellent is greater, the adhesive force between the hydrophobic pore surface and water vapour may even be stronger than the molecular attraction force of the water vapour. Therefore, the W(u) decreases slightly as the content of water repellent going beyond a certain value. Nevertheless, the molecular attraction force of liquid water is much higher than the adhesive force between the hydrophobic pore surface and liquid water. This is the possible reason for the difference between the effects of water repellent on the water absorption and moisture absorption of the foamed concrete. As reported in previous researches [27–29], three theoretical models studied on sorption isotherms curves can be used to obtain hygroscopic fitted curves. Among them, the KUM and CUB models are best fit to analyze the relationship of the moisture absorption of the foamed concrete to the relative humidity [24]. The KUM model established by Kumaran is expressed as follows.
u
30
WðuÞ ¼
20
where A, B and C are the constants of the KUM model. The CUB model is a cubic equation and its expression is proposed as the following function.
10
WðuÞ ¼ a u3 þ b u2 þ c u
0 0
20
40
60
80
100
Relative humidity, ϕ (%) Fig. 15. Hygroscopic fitted curves for the average values by the CUB model.
ð4Þ
Au2 þ Bu þ C
ð5Þ
where a, b and c are the constants of the CUB model. To observe the accuracy of the KUM and CUB models in fitting hygroscopic curves, the average values of moisture absorption of different series of specimens are first analyzed. The hygroscopic fitted curves for the average values by the KUM and CUB models are shown in Figs. 14 and 15, respectively. It can be seen that the data
Table 3 The constants and R-square of the fitted curves for the average values of each Series. Average for Seires
KUM model A (10
CON I II III
2.23 2.64 4.49 5.78
4
)
CUB model 2
B
C
R
0.039 0.050 0.074 0.096
0.535 0.999 1.044 1.320
0.989 0.988 0.995 0.994
a (104)
b
c
R2
1.027 0.588 0.513 0.411
0.0168 0.0099 0.0083 0.0066
1.118 0.711 0.569 0.449
0.998 0.998 0.998 0.998
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C. Ma, B. Chen / Construction and Building Materials 123 (2016) 106–114 Table 4 The constants and R-square of the fitted curve for each specimen. Series
Specimen
KUM model A (10
4
)
CUB model B
C
R
a (104)
b
c
R2
2
CON
–
2.23
0.039
0.535
0.989
1.027
0.0168
1.118
0.998
Series I
0.2% 0.4% 0.8% 1.0% 1.2%
PT PT PT PT PT
2.08 2.51 3.26 3.24 1.94
0.041 0.047 0.058 0.058 0.043
0.648 0.824 1.029 1.286 1.735
0.971 0.989 0.992 0.990 0.985
0.939 0.671 0.551 0.460 0.320
0.0157 0.0114 0.0091 0.0076 0.0058
1.025 0.802 0.649 0.571 0.506
0.995 0.998 0.999 0.996 0.991
Series II
0.2% 0.4% 0.8% 1.0% 1.2%
CS CS CS CS CS
2.99 3.68 4.63 6.43 6.64
0.051 0.064 0.080 0.100 0.100
0.829 0.945 0.996 1.097 1.564
0.995 0.997 0.991 0.993 0.993
0.658 0.552 0.515 0.450 0.391
0.0108 0.0091 0.0084 0.0071 0.0060
0.767 0.638 0.562 0.469 0.409
0.998 0.998 0.997 0.991 0.997
Series III
0.2% 0.4% 0.8% 1.0% 1.2%
SP SP SP SP SP
5.28 5.28 5.77 6.11 6.52
0.085 0.091 0.098 0.101 0.105
0.959 1.201 1.291 1.608 1.820
0.993 0.992 0.991 0.995 0.994
0.528 0.437 0.411 0.347 0.330
0.0084 0.0072 0.0067 0.0056 0.0052
0.553 0.482 0.449 0.393 0.367
0.995 0.997 0.996 0.999 0.999
points are mainly on the fitted curves and the constants in Eqs. (4) and (5) obtained from the fitted curves are listed in Table 3. The high coefficients of determination imply that the KUM and CUB models are appropriate to describe the hygroscopic behavior of the foamed concrete modified by water repellent. In addition, the fitted curves from CUB model are a bit more precise than those from KUM model. The constants in KUM and CUB models of the fitted curves for each specimen with water repellent are presented in Table 4. Each fitted curve has high coefficient of determination (>0.9), and the relationship of the constants to the content of water repellent can also be obtained. For example, the A values for Series II decrease approximately linearly with the increasing CS content and the B and C values generally increase with CS content. 4. Conclusions The mechanical and physical properties of the foamed concrete with potassium trimethylsilanolate (PT), calcium stearate (CS) and siloxane-based polymer (SP) including 7-day and 28-day compressive strength, thermal conductivity, sorptivity and hygroscopicity are systematically studied. Based on the analysis on the laboratory results, the following conclusions can be drawn: 1. The water repellent has little impact on the foam stability and dry density of the low-density foamed concrete. The compressive strength can also be improved by adding water repellent, and the optimal improvement often occurs at the content of water repellent of 1.0%. The 28-day strength of the foamed concrete with a certain content of SP is higher than that of specimens with the same content of TP and CS. Moreover, the thermal conductivities of the foamed concrete with different contents of water repellents vary from 0.149 W/m K to 0.159 W/m K. The 28-day strength and thermal conductivity of the foamed concrete with 1.0% SP are 1.77 MPa and 0.150 W/m K, respectively. 2. The effect of water repellent on the sorptivity of the foamed concrete is evaluated by water absorption and strength retention coefficient (RS) after 48 h of soaking. The water absorption decreases significantly as the content of water repellent increasing from 0.2% to 1.2%. When the same content of water repellent is employed, the 48-h water absorption of the foamed concrete modified by SP is the lowest. And the 48-h water absorption of the foamed concrete with 1.0% SP is 2.5% (by volume), which is about only 3.6% of that of the foamed concrete without water repellent.
3. The RS increases markedly with the increasing content of water repellent. Likewise, the addition of SP has the most significant effect on the RS. The RS value of the foamed concrete with 1.0% SP is 0.989, which is 55.7% higher than that of the foamed concrete without water repellent. The relationship of the RS to the water absorption indicates that the RS value approximately linearly with the increase of the water absorption. And the fitted curve and function with a coefficient of determination of 0.930 are achieved. 4. The application of water repellent can reduce the contents of hygroscopic moisture [W(u)] of the foamed concrete. The W (u) decreases gradually as the PT and CS contents increasing from 0.2% to 1.2% and decreases slightly with the increasing SP content from 0.2% to 1.2%. The difference in the effects of water repellent on water and moisture absorption may be explained by the values of the adhesive force between the pore surface and liquid water or water vapour. The fitted curves with high coefficients of determination have been proposed to describe the relationship of the W(u) to the relative humidity. Acknowledgments This research work was financially supported by the National Natural Science Foundation of China, Grant No. 51378309. 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] T.Y. Lo, W.C. Tang, H.Z. Cui, The effects of aggregate properties on lightweight concrete, Build. Environ. 42 (8) (2007) 3025–3029. [3] Z. Wu, Y. Zhang, J. Zheng, Y. Ding, An experimental study on the workability of self-compacting lightweight concrete, Constr. Build. Mater. 23 (5) (2009) 2087–2092. [4] B. Chen, J. Liu, Experimental application of mineral admixtures in lightweight concrete with high strength and workability, Constr. Build. Mater. 22 (6) (2008) 1108–1113. [5] O. Kayali, B. Zhu, Chloride induced reinforcement corrosion in lightweight aggregate high-strength fly ash concrete, Constr. Build. Mater. 19 (4) (2005) 327–336. [6] B. Chen, Z. Wu, N. Liu, Experimental research on properties of high-strength foamed concrete, J. Mater. Civ. Eng. 24 (1) (2011) 113–118. [7] 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. [8] 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. [9] P.J. Tikalsky, J. Pospisil, W. MacDonald, A method for assessment of the freezethaw resistance of preformed foam cellular concrete, Cem. Concr. Res. 34 (5) (2004) 889–893.
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