Synergism effects of coconut diethanol amide and anionic surfactants for entraining stable air bubbles into concrete

Construction and Building Materials 237 (2020) 117625

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Synergism effects of coconut diethanol amide and anionic surfactants for entraining stable air bubbles into concrete Guangcheng Shan a,b, Shuang Zhao a,b, Min Qiao a,b,⇑, Nanxiao Gao a,b, Jian Chen a,b, Qianping Ran a,c,⇑ a

State Key Laboratory of High Performance Civil Engineering Materials, Jiangsu Sobute New Materials Co. Ltd., Nanjing 211103, PR China Bote New Materials Taizhou Co., Ltd., Taizhou 225474, PR China c School of Material Science and Engineering, Southeast University, Nanjing 211189, Jiangsu, PR China b

h i g h l i g h t s
The interactions between the anionic and nonionic surfactants were studied.
There are strong interactions on the interfaces of the mixed surfactants.
The mixed surfactants have higher foaming stability in aqueous solutions.
They also have higher air content stability in both cement mortars and concretes.
The study shows great benefit to understanding the influence of mixed surfactants on the interfaces and stability of air bubbles in concrete.

a r t i c l e

i n f o

Article history: Received 23 July 2019 Received in revised form 5 November 2019 Accepted 14 November 2019

Keywords: Surfactants Coconut diethanol amide Concrete Bubbles Air voids

a b s t r a c t Entraining tiny and stable bubbles into cement and concrete is becoming more and more important with the complex composition of cement and concrete. In this work, multi-component surfactants are compounded to improve the air-void stability in fresh concrete. The surface tensions, foam properties of their solutions, and the air contents and air-void parameters of the fresh and hardened cement mortars were tested. The results show that there are strong interactions on the interface of multi-components by calculating the important parameters of surface activity. When coconut diethanol amide molecules are introduced into the interface for co-assembly, the electrostatic repulsion between the anionic surfactant molecules is effectively diminished, thereby decreasing the interfacial free energy and making the interface more stable. The multi-component surfactants induced smaller bubbles with larger amounts in aqueous solutions, which also have higher stability of air bubbles in both cement mortars and concretes. So it is of great practical significance to compound nonionic and anionic surfactants to improve the air-void stability in concrete. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Concrete is known as a porous structural material, the pore structure of concrete effectively affects the freeze-thaw resistance, workability and structural stability of concrete [1,2]. The most widely adopted approach to control the pore structure of concrete is to introduce surfactants as air entraining agents [3–6]. Surfactants molecules containing both hydrophilic and hydrophobic groups can adsorb and self-assemble at the gas-liquid interface and have a tendency to reduce the surface tension of distilled

⇑ Corresponding authors at: State Key Laboratory of High Performance Civil Engineering Materials, Jiangsu Sobute New Materials Co. Ltd., Nanjing 211103, PR China. E-mail addresses: [email protected] (M. Qiao), [email protected] (Q. Ran). https://doi.org/10.1016/j.conbuildmat.2019.117625 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

water [7–9]. They can entrain a mass of air bubbles in fresh concrete [10,11], however, the bubbles are inherently unstable. The air/water interfaces of the air bubbles present high surface energy and the air bubbles tend to aggregate together and then broke up [1,12], resulting in poor bubble stability in fresh concrete. In recent years, because of the increasing complexity of the concrete compositions and the influence of salts and adsorption, the bubbles in fresh concrete introduced by air entraining agents is getting more and more unstable. Therefore, it is of great practical significance to compound multi-component surfactants to improve the air-void stability in fresh concrete. As a multi-component air controlled concrete admixture, there is bound to take place a complex interaction among various surfactants. Although the interaction has been widely recognized in the field of surfactants and biological medicine [13–15], it has rarely

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G. Shan et al. / Construction and Building Materials 237 (2020) 117625

been reported in the field of concrete. Naqvi et al. [16] reported the interfacial co-assembly interactions of the cationic geminizwitterionic surfactants, which indicated that the co-assembly arrangement of the interface between the two surfactant molecules significantly reduced the interfacial free energy and improved the stabilization of the interface. Golemanov et al. [17] demonstrated that the stability of the interface and the viscoelasticity of foam films were improved by the ternary interfacial coassembly interactions among the anionic, zwitterionic and nonionic surfactant, observably enhanced the thickness of the foams and showed higher foaming stability. These investigations indicate that the strong interactions among different surfactants in solutions make the bubble stable. However, whether to improve the stability of air voids in concrete remains to be investigated. This study discussed the interactions between anionic and nonionic surfactants. Coconut diethanol amide (CDEA) was chosen as a nonionic surfactant and the interactions with the anionic surfactant (sodium dodecyl polyoxyethylene ether sulfonate, AES) in solutions, cement mortars and concretes were respectively studied. CDEA and AES have the same hydrophobic part (dodecyl group), but different hydrophilic groups. We calculated the composition of mixed interfaces and the free energy of additional adsorption which are a measure of molecular interaction. The results clearly indicate that compounding of suitable nonionic surfactant in a certain proportion into the ionic surfactant is beneficial to decrease the interfacial free energy of the bubbles, thereby making the interface more stable. The improvement of this interfacial selfassembly behavior among different surfactants leads to a significant increasing of the air bubbles stability of cement mortars and concretes. 2. Experimental sections 2.1. Materials AES and CDEA from Aldrich Chemical Co. were used. The chemical structures of the two surfactants are shown in Fig. 1. PⅡ42.5 type Portland cement was purchased from Jiangnan-xiaoyetian Cement Co., Ltd., Nanjing China. Its chemical compositions were summarized in Table 1. Natural river sand with a nominal grain size of 0.5–1.5 mm was used as fine aggregate. It was used a higher performance polycarboxylate superplasticizer produced by Jiangsu Sobute New Materials Co., Ltd., Nanjing, China. 2.2. Test of surface tensions The surface tensions of the binary mixtures AES-CDEA and their pure constituents (AES and CDEA) were measured using a Krüss K100 surface tension meter. Different concentrations of the pure

AES were prepared in pure water. 50 mL of the different mole fractions of the binary mixtures were obtained from stock solutions containing different concentrations of AES and CDEA at 25 . The surface tension of the binary mixtures at each mole fraction was measured for three times. 2.3. Test of foam behavior The foam heights and bubble sizes of the solutions of AES, CDEA and the AES-CDEA binary mixtures were measured by a Krüss DFA100 dynamic foam analyzer. All samples were prepared as described in 2.2. Each sample solution was bubbled for 90 s and then stayed for another 1110 s to record the foam height (the bubbling rate: 0.2 L /min). Meanwhile, the bubble images were also recorded by the foam analyzer. 2.4. Test of fresh cement mortars The fresh cement mortars mixture consists of 900 g of cement, 1350 g of sand, 360 g of water (the water to cement ratio is 0.4). Amount of polycarboxylate superplasticizer was 0.16 wt% of the cement. Various dosages of multi-component surfactants were mixed with superplasticizer and mortars. These slurries were stirred by JJ-5 Cement Mortar Mixer (ISO 679) in low speed (autogiration 140 ± 5 r/min and revolution 62 ± 5 r/min) for 60 s, and stayed for 90 s then stirred in high speed (autogiration 285 ± 10 r/min and revolution 62 ± 5 r/min) for 60 s. The mortar mixing procedure was in accordance with the Chinese National Standard GB/T 17671– 1999. A portion of fresh cement mortars was taken out for air contents test using a SANYO direct reading air content tester after mixing and another portion was taken out for the bubble distribution of the fresh mortar test. The bubble distribution of the fresh mortar was tested using the AVA3000 fresh concrete pore structure analyzer. The bubbles in the mortar were released by stirring the above fresh mortar slurry (20 mL) for 30 s, buffered into the upper water column with the blue release liquid, and slowly rose to the balance of the water surface with the action of the blue release liquid. The blue release liquid was a mixture of glycerol and ethylene glycol (1:1.36) obtained from Danish Germann Company. According to the Stoke principle, the speed at which a bubble floats in liquids depends on the volume of the bubble, and the large bubble floats faster than the small one. The change in buoyancy is used to determine the change in weight, which gives the diameter distribution of the bubble in the fresh mortar. 2.5. Test of hardened mortars The fresh cement mortars were poured in moulds (10 cm  10 cm  10 cm) to obtain cubic specimens. Then the hardened specimens were incubated for 28 days at 20 °C. Cubic specimens were cut into three slices (thickness of 1.0 ± 0.2 cm), grinded, washed, filled with fluorescer and then were measured by a MIC-840–01 hardened concrete pore structure analyzer to obtain the air-void parameters of the hardened cement mortar (air content, mean air-void diameter, air-void spacing factor, airvoid distributions), the results were averaged. The images of the air-voids introduced by all the samples were also recorded by the hardened concrete pore structure analyzer. 2.6. Test of concretes

Fig. 1. Molecular structure of target surfactants with different charge performance.

Concrete mixing proportion was designed according to the methods stipulated in the Chinese National Standard GB/T 8076–2008, 5.04 kg of cement, 14.88 kg of sand, 2.16 kg of fly ash, 6.42 kg of crushed limestone (5–10 mm) and 14.98 kg of

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G. Shan et al. / Construction and Building Materials 237 (2020) 117625 Table 1 Chemical compositions of cement. Composition (% by mass)

SiO2 21.1

Al2O3 6.16

CaO 64.8

MgO 1.94

Fe2O3 4.41

SO3 2.52

Na2Oeq 0.48

Loss 2.59

crushed limestone (10–25 mm) were dry mixed. Polycarboxylate superplasticizer (0.2 wt% of the cement), various dosages of multi-component surfactants and 3.2 kg of water were mixed and stirred together with aggregate. The air contents of the concretes were tested according to the China National Standard GB/T 8076–2008. The fresh concretes were poured in moulds (10 cm  10 cm  10 cm) to obtain cubic specimens. Then the curing, treatment and test of the cubic specimens are the same as those of the hardened mortars. 3. Results and discussion 3.1. Effect of CDEA on surface activity Surface activity of the mixtures of AES and CDEA was firstly tested. The surface tension curves of the binary mixtures with different surfactant concentrations rates were given in Fig. 2. The surface tensions of all the solutions were decreased gradually by increasing surfactant concentrations, which reached their minimum values (cCMC) when the concentrations reached their critical micelle concentration values (CMC). By mixing two types of the surfactants, the self-assembly behavior of both surfactants needs to be considered. The theoretical CMC values for the coexistence of two surfactants were obtained from the Clint’s equation [18] as shown in Eq. (1), which was established on the premise that there was no interaction between the two surfactant molecules. The experimental CMC value is lower than the theoretical CMC value, indicating that strong attractive interactions take place between the surfactant molecules [16,19].

1 a1 1  a1 ¼ þ CMC CMC1 CMC2

ð1Þ

where CMC*, CMC1, and CMC2 are the theoretical CMC value of the binary mixture, the CMC value of AES, and the CMC value of CDEA, respectively, and a1 is the mole fraction of the AES. Fig. 3 and Table 2 show the relationship between the experimental CMC values and the theoretical CMC* values of the binary mixtures. The experimental CMC values show significant deviations and keep lower than the theoretical CMC* values, which is due to the interactions between the two surfactant molecules [19]. The results indicate that strong interactions take place between AES and CDEA.

Fig. 3. The relationship between the experimental CMC (red) and the theoretical CMC* values (black) in the binary mixtures (AES-CDEA). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 2 shows another three parameters of surface activity, including surface excess (Cmax), the minimum area per molecule (Amin) and standard Gibbs energy of adsorption (4Gads), which were calculated by the Gibbs adsorption equations Eq. (2), Eq. (3) [20] and Eq. (4), respectively [21]:

Cmax ¼  Amin ¼

  1 @c nRT @logc

ð2Þ

1 NA Cmax

DGads ¼ DGmic 

ð3Þ

pcmc

Cmax

¼ RTlnðXcmc Þ 

pcmc

ð4Þ

Cmax

where R, T, and NA are molar gas constant, degree Kelvin, and Avogadro’s constant (6.02  1023), respectively. (@ c/@ log c) is the slope of the surface tension curve. The n values are 2 and 3 for pure AES (or CDEA) and their binary mixtures, respectively. pcmc is the surface pressure at the CMC value, in which pcmc=c0-cCMC, c0 and cCMC are the surface tensions of distilled water (72 mN/m) and the solution at CMC, respectively. It can be observed that the binary mixtures have higher Cmax values than the pure AES, while lower Amin values than the pure AES. And the 4Gads values for all the binary mixtures were found to be lower than pure AES and CDEA. These results indicate that the binary mixtures molecules can adsorb and assemble on the gas-liquid interface more closely and then improve the surface activity of pure AES. 3.2. Interfacial parameters and interface co-assembly behaviors Different surfactants have different surface activities. Rosen [22] proposed his model to evaluate the interfacial composition (X1) for mixed interface, which considers the competitive selfassembly for binary mixtures on the mixed interface. X1 was evaluated from the equation Eq. (5):

 ðX1 Þ2  ln Fig. 2. Surface tension curve of the binary mixtures (AES-CDEA).

conca1 conc1 X1



 ¼ ðX2 Þ2  ln

conca2 conc2 X2

 ð5Þ

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G. Shan et al. / Construction and Building Materials 237 (2020) 117625

Table 2 Important interface parameters of the binary mixtures (AES-CDEA). Mole fraction of AES (a1)

CMC (mM)

CMC* (mM)

Ycmc (mN/m)

Cmax

1 0.69 0.43 0.2 0

0.223 0.066 0.042 0.033 0.034

/ 0.083 0.054 0.041 /

38.04 36.86 34.91 29.98 27.15

0.44 0.97 0.58 1.12 1.80

where X1, a1 are the mole fraction of AES at the mixed interface, the mole fraction of AES in the bulk, respectively; conc1, conc2 and conc are the mole concentrations of AES, CDEA, and their binary mixtures reduced the surface tension of water to 40 mN∙m1, respectively, (X1 + X2 = 1). The real ratio of the two surfactant molecules at the mixed interface was calculated from the above equation. Fig. 4 and Table 3 show the variation of interfacial mole fraction with the mole fraction of AES (a1). The interfacial mole fraction of AES (X1) was always lower than the ratio of AES (a1) in solutions and as the amount of AES in the solution increased, its participation in mixed interface also increased, which is due to the interactions between the two surfactant molecules. In order to better study the interaction and nonideality of the two surfactants in the mixed interface, Almgren [23], Maeda [24] and Das [25] introduced the interaction parameter, br, which is calculated by the following equation Eq. (6).

br ¼

ln ðconca1 =conc1 X1 Þ

ð6Þ

ðX2 Þ2

where X1, a1 are the mole fraction of AES at the mixed interface calculated by Eq. (2) (X1 + X2 = 1), the mole fraction of AES in the bulk, respectively; conc1 and conc are the mole concentrations of AES and the binary mixtures reduced the surface tension of water to 40 mN∙m1, respectively. Further, the negative br value means attraction interactions in the two molecules tend to form mixed interfaces; the positive br

(106 mol molecule/m2

Amin (Å2/chain)

4Gads (KJ/mol)

37.68 17.10 28.23 14.78 9.23

37.90 36.80 40.62 38.64 37.33

value means the interactions between the two molecules at mixed interfaces are weak and the whole system behaves as repulsion interaction; the br value is 0, which indicated that there is no interaction between two molecules that means ideal mixing [19]. The calculated values are depicted in Table 3. The br values of the system show negative, indicating that the two surfactant molecules exhibited an attraction interaction between the molecules when they are arranged at the mixed interface. Meanwhile, the br values are more negative with increase in a1. The excess free energy of adsorption changes due to the interaction was calculated from equation Eq. (7):

DGrex ¼ RTbr ðX1 ðX2 Þ2 þ X2 ðX1 Þ2 Þ

ð7Þ

r

where X1, b , R, and T are the mole fraction of AES at the mixed interface calculated by Eq. (5) (X1 + X2 = 1), the interaction parameter calculated by Eq. (6), the ideal gas constant, the temperature (298 K). Table 3 bears these values and Fig. 5 shows variation of the excess free energy of adsorption for the binary mixtures with the mole fraction of AES (a1). The DGr ex values of the binary mixtures were negative, indicating that the binary mixtures produced a more stable interface, which explains that there are more attractive or less repulsive interactions between two molecules than pure component. This conclusion was similarly obtained by comparing the interaction parameter, br values. Due to the target surfactants have hydrocarbon chains of the same length, the differences in DGr ex values owing to the differences in van der Waals interactions between the charges of target surfactants. Based on the above results, the mechanism of the interfacial coassembly behavior of the two surfactants was explained as shown in Fig. 6. The self-assembly of the anionic surfactant (AES) is greatly affected by the electrostatic repulsion between the negative charges, and the surfactant molecules can hardly arrangement at the interface tightly thus the interface was instable. When the nonionic surfactant (CDEA) molecules are introduced into the co-assembly interface, the hydrophobic interactions of the hydrophobic chains are effectively enhanced and the electrostatic repulsion between the anionic surfactant molecules is effectively diminished, thereby decreasing the interface energy and making the interface more stable. 3.3. Foaming stability of the mixed surfactants

Fig. 4. Variation of interfacial mole fraction (evaluated from Eq. (3)) with the mole fraction of AES (a1).

As we all know, foam is a thermodynamically unstable system and easily broken up, especially in a high salinity environment of cement paste [26]. Foam stability is mainly decided by the stability

Table 3 Important parameters of interface self-assembly behavior between the binary mixtures of AES and CDEA. Mole fraction of AES (a1)

X1

X2

br

4Gr ex (KJ/mol)

1 0.69 0.43 0.2 0

/ 0.18 0.08 0.05 /

/ 0.822 0.925 0.95 /

/ 0.32 0.39 1.11 /

0 0.27 0.15 0.30 0

G. Shan et al. / Construction and Building Materials 237 (2020) 117625

Fig. 5. Variation of the excess free energy for the binary mixtures (AES-CDEA) with the mole fraction of AES (a1).

of bubble liquid film, and the cracking and fusion of bubbles are the processes of the bubble liquid film from thinning to final cracking [27]. Therefore, a multi-component surfactant is introduced to stabilize the interface and thereby to make the bubble stable. The foaming properties of the binary mixtures and pure component were studied.

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Fig. 7 (a) shows the foam heights of all the samples against time. As can be seen, pure CDEA induced the foam height obviously lower than pure AES and other mixtures, indicating that CDEA has poor foam ability in solutions. The initial foam heights of the mixed systems were almost the same as that of pure AES, but the foam heights at time of 20 min were largely different, significantly increased with increasing the molar concentration of CDEA. Fig. 7 (b) shows the foam height ratios of each sample at times of 20 and 1.5 min (H20min/H1.5min). It can be observed that the binary mixtures have the H20min/H1.5min values higher than pure CDEA and AES, and the ratios increased with increasing the molar concentration of CDEA. The results indicated that the binary mixtures have much higher foaming ability compared with pure CDEA and AES, and the mixtures with higher molar concentration of CDEA have higher foam stability. The values of the bubble count and mean bubble area of each sample were given in Fig. 8. Fig. 8 (a) shows that pure CDEA induced the bubbles significantly less than pure AES and the binary mixtures. The binary mixtures induced the bubble counts almost same as pure AES during foaming time, but the bubble counts are much larger than that of pure AES at the end of incubated time and the bubble counts increased with increasing the molar concentration of CDEA. Fig. 8 (b) shows that compared with pure CDEA and AES, the binary mixtures have smaller mean bubble area at time of 20 min, and mixtures with higher molar concentration of

Fig. 6. Schematic structure of mixed interfaces for the binary mixtures (AES-CDEA).

Fig. 7. Changes in the foam heights of the solutions (a) and the H20min/H1.5min values (b) of the binary mixtures (AES-CDEA) ((A) CDEA-0.1 mmol/L, (B) AES-0.1 mmol/L, (C) AES-0.1 mmol/L + CDEA-0.02 mmol/L, (D) AES-0.1 mmol/L + CDEA-0.04 mmol/L, (E) AES-0.1 mmol/L + CDEA 0.08 mmol/L, (F) AES-0.1 mmol/L + CDEA 0.1 mmol/L).

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G. Shan et al. / Construction and Building Materials 237 (2020) 117625

Fig. 8. Bubble counts (a) and mean bubble areas (b) in a unit area of the solutions of the binary mixtures (AES-CDEA) at the times of 1.5, 10 and 20 min ((A) CDEA-0.1 mmol/L, (B) AES-0.1 mmol/L, (C) AES-0.1 mmol/L + CDEA0.02 mmol/L, (D) AES-0.1 mmol/L + CDEA0.04 mmol/L, (E) AES-0.1 mmol/L + CDEA0.08 mmol/L, (F) AES-0.1 mmol/ L + CDEA0.1 mmol/L).

Fig. 9. The bubbles photographs of the sample solutions at the time of 10 min ((a) CDEA-0.1 mmol/L, (b) AES-0.1 mmol/L, (c) AES-0.1 mmol/L + CDEA-0.02 mmol/L, (c) AES-0.1 mmol/L + CDEA-0.04 mmol/L, (d) AES-0.1 mmol/L + CDEA-0.08 mmol/L, (e) AES-0.1 mmol/L + CDEA-0.1 mmol/L, (f) CDEA-0.1 mmol/L).

CDEA has minimum mean bubble area. The photographs of bubbles induced by each sample at time of 10 min under the same scale were given in Fig. 9 to understand the shapes and sizes of the bubbles more directly. The binary mixtures induced the bubbles with smaller size and more count than pure CDEA and AES. All above studies indicated that CDEA has poor foaming ability, AES has good foaming ability and poor foaming stability and the binary mixtures have good foaming ability and foaming stability in aqueous solutions. 3.4. Applications of the mixed surfactants for fresh cement mortars The synergistic effect of CDEA and AES in cement mortars was further studied. As shown in Fig. 10, the samples of blank and pure CDEA induced the air contents of the fresh cement mortars markedly lower than those of pure AES and the binary mixtures. The initial air contents of pure AES and binary mixtures remained almost

Fig. 10. Air contents of the fresh cement mortar of the binary mixtures (AES-CDEA) ((A) blank (B) CDEA-0.5 wt%%, (C) AES-0.5 wt%%, (D) AES-0.5 wt%% + CDEA-0.1 wt% %, (E) AES-0.5 wt%% + CDEA-0.2 wt%%, (F) AES-0.5 wt%% + CDEA-0.4 wt%%, (G) AES0.5 wt%% + CDEA-0.5 wt%%).

constant at 15.0%, but the stability of air contents after 60 min introduced by the binary mixtures was markedly higher than that of pure AES, and the stability of air content increased with increasing the dosage of CDEA. The results indicated that for the mixtures, AES is as an air entraining component and CDEA is used to promote the performance of AES. The bubble size distribution of each sample in fresh cement mortar was also counted. Fig. 11 shows that the samples of pure CDEA and AES entrained a number of inhomogeneous bubbles with big size, which of the bubbles is mainly distributed between 500~2000 mm. However, for the mixtures, the addition of CDEA makes the bubble size much smaller (less than 200 mm) than pure CDEA and AES, and the bubble diameter was smaller and smaller with increasing the dosage of CDEA. The results of fresh cement mortars are highly consistent with the results of foam properties,

Fig. 11. Effect of the binary mixtures (AES-CDEA) on the bubble size distribution of the fresh mortar ((A) blank (B) CDEA-0.5 wt%%, (C) AES-0.5 wt%%, (D) AES-0.5 wt% % + CDEA-0.1 wt%%, (E) AES-0.5 wt%% + CDEA-0.2 wt%%, (F) AES-0.5 wt%% + CDEA0.4 wt%%, (G) AES-0.5 wt%% + CDEA-0.5 wt%%).

G. Shan et al. / Construction and Building Materials 237 (2020) 117625

which suggest that the binary mixtures having smaller bubble size in solutions also have smaller bubble size in fresh cement mortars. 3.5. Applications of the mixed surfactants for hardened cement mortars The hardened properties of the binary mixtures were also investigated. Fig. 12(a) shows the air contents of hardened cement mortars containing the binary mixtures are higher than pure CDEA and AES, and the air content increased with increasing the dosage of CDEA. Fig. 12(b) and (c) show that the air-void diameter and airvoid spacing factor containing the binary mixtures are both much

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smaller than pure CDEA and AES and the air-void parameters decreased with increasing the dosage of CDEA. The results show that the air voids in hardened cement mortars induced by the binary mixtures have smaller bubble sizes and larger amounts than the reference sample, and the size of the air voids reduced gradually with increasing the dosage of CDEA. That is to say, it is consistent with the conclusions in aqueous solutions and fresh mortars. As we know, tiny and homogeneous air voids smaller than 200 lm in hardened concrete have slight influence on the strength of concretes and are more beneficial for the workability and freeze–thaw durability of concretes [28–31]. Fig. 12(d) shows the percentages of the small air voids (0200 um) induced by the binary

Fig. 12. The performance of the hardened cement mortar of the binary mixtures (AES-CDEA) ((a) air content; (b) mean air-void diameter; (c) air-void spacing factor; (d) percentage of small air voids (0–200 um)) ((A) blank (B) CDEA-0.5 wt%%, (C) AES-0.5 wt%%, (D) AES-0.5 wt%% + CDEA-0.1 wt%%, (E) AES-0.5 wt%% + CDEA-0.2 wt%%, (F) AES0.5 wt%% + CDEA-0.4 wt%%, (G) AES-0.5 wt%% + CDEA-0.5 wt%%).

Fig. 13. Air-void images of the hardened cement mortar specimens ((a) blank (b) CDEA- 0.5 wt%%, (c) AES-0.5 wt%%, (d) AES-0.5 wt%% + CDEA-0.1 wt%%, (e) AES-0.5 wt% % + CDEA-0.2 wt%%, (f) AES-0.5 wt%% + CDEA-0.4 wt%%, (g) AES-0.5 wt%% + CDEA-0.5 wt%%).

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Table 4 Air contents and air-void parameters of the concrete specimens containing different samples ((A) blank (B) CDEA-0.4 wt%%, (C) AES-0.4 wt%%, (D) AES-0.4 wt%% + CDEA-0.1 wt%%, (E) AES-0.4 wt%% + CDEA-0.2 wt%%, (F) AES-0.4 wt%% + CDEA-0.3 wt%%, (G) AES-0.4 wt%% + CDEA-0.4 wt%%). Samples

A B C D E F G

Air contents (%) Initial

1h

Hardened

2.9 3.1 6.1 6.0 6.1 6.2 6.2

2.0 2.3 3.5 4.1 4.5 5.3 5.8

1.8 2.0 2.8 3.0 3.7 3.8 4.2

Mean air-void diameters (mm)

Air-void spacing factors (mm)

103.2 101.1 92.7 86.6 82.4 77.9 68.5

215.5 212.1 190.3 187.8 181.3 177.4 153.6

mixtures in the hardened cement mortars. Similarly, the results point out that the binary mixtures introduced smaller air voids than the reference sample and the percentage increased with increasing the dosage of CDEA. Fig. 13 shows the photograph of each hardened cement mortar specimen, which was more directly observed that the binary mixtures entrained smaller air voids with larger amounts than the reference sample. The above results clearly suggest that the binary mixtures have higher performance in air-void stability compared with pure CDEA and AES , and increasing the dosage of CDEA contributed to the hardened properties of the binary mixtures, which are also consistent with the results in aqueous solutions and fresh mortars.

Acknowledgements

3.6. Applications of the mixed surfactants for concretes

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The binary mixtures were also tested in concretes. Table 4 shows that the air contents of the fresh and hardened concretes containing pure CDEA were almost the same as those of blank, which means that pure CDEA has only slight air entraining ability in concretes. The binary mixtures have higher air content stability and smaller air-void parameters compared with pure AES, which are quite consistent with the results tested in aqueous solutions and cement mortars. 4. Conclusion In summary, the properties of the anionic-nonionic mixed surfactants (AES-CDEA) were studied. The surface activity and foaming properties in aqueous solutions were tested, and the air contents and void parameters of both cement mortars and concretes were also investigated. Our study has several important conclusions. First, there are strong interactions on the interfaces of multi-components by calculating the important parameters of surface activity. Second, when CDEA are introduced into AES for co-assembly, the electrostatic repulsions between the anionic surfactant molecules is effectively diminished, thereby decreasing the interfacial free energy and making the interface more stable. Third, the multi-component surfactants induced smaller bubbles with larger amounts in aqueous solutions, which also have higher air content stability in both cement mortars and concretes. Fourth, pure CDEA has only slight air entraining performance, thus in the binary mixtures, AES is the air entraining component and CDEA is used to promote the performance of AES. Our study shows great benefit to understanding the influence of mixed surfactants on the interfaces and stability of air bubbles in concrete, which may also help to develop ideal mixed surfactants for high performance air entraining agents. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This work was financially supported by the National Key R&D Program of China (NO. 2017YFB0310100), National Science Fund for Distinguished Young Scholars (51825203), the 15th batch of six talent peak projects (JZ-058), Jiangsu 333 talents project (BRA2018202), the Key Consulting Project of Chinese Academy of Engineering (2016-XZ-13) and Technology Research and Development Project of China Railway Corporation (N2018G030).

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