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Effects of salts and adsorption on the performance of air entraining agent with different charge type in solution and cement mortar
Construction and Building Materials 242 (2020) 118188
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of salts and adsorption on the performance of air entraining agent with different charge type in solution and cement mortar Min Qiao a,b, Guangcheng Shan b, Jian Chen b, Shishan Wu a,⇑, Nanxiao Gao b, Qianping Ran b,c,⇑, Jiaping Liu c a
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, PR China State Key Laboratory of High Performance Civil Engineering Materials, Jiangsu Sobute New Materials Co. Ltd., Nanjing 211103, PR China c School of Material Science and Engineering, Southeast University, Nanjing 211189, Jiangsu, China b
h i g h l i g h t s Failure mechanism of air entraining agent in cement and concrete is investigated. High valence cations weaken the surface activity of anionic surfactant. Anionic surfactants have strong adsorption on surface of cement. Cationic and nonionic surfactants have strong adsorption on surface of montmorillonite and granite.
a r t i c l e
i n f o
Article history: Received 22 May 2018 Received in revised form 25 November 2019 Accepted 14 January 2020
Keywords: Air entraining agent Cement mortar Surfactant Adsorption Charge type
a b s t r a c t In this paper, the effect of salts in solution and the adsorption of surfactant are studied to give the answer why the air entraining agents fail. The high valence cations (e.g. Ca2+) weaken the surface activity of anionic surfactant solution but have no effect on the surface activity of cationic and nonionic surfactant solution. The calcium induces the decreased surface activity of anionic surfactant solution, the low foamability, the big bubble size, and the depressed the air-entraining performance. Anionic surfactant has stronger adsorption on surface of cement than that on surface of montmorillonite, granite and limestone. But cationic and nonionic surfactants have much stronger adsorption on surface of montmorillonite and granite than that on surface of cement. When montmorillonite, granite and limestone are added into cement mortar, the air-entraining performance of anionic surfactant is not depressed, but the air-entraining performance of cationic and nonionic surfactants are obviously depressed. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction Air entrainment has been regarded to be highly desirable for concrete since the 1930s [1–3]. Entraining homogeneous, tiny and stable air voids in fresh concrete can effectively improve both workability and freeze-thaw durability of concrete [4,5]. Air entraining agents are important concrete admixtures that can help to intentionally create a number of effective air voids in concrete. All air entraining agents can be divided into three kinds [6–8]: anionic surfactants, cationic surfactants and nonionic surfactants. Anionic surfactants such as sodium dodecyl sulfate (SDS) [9], sodium dodecylbenzene sulfonate (SDBS) [10] and saponified rosin ⇑ Corresponding authors at: School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, PR China (S. Wu); State Key Laboratory of High Performance Civil Engineering Materials, Jiangsu Sobute New Materials Co. Ltd., Nanjing 211103, PR China (Q. Ran). E-mail addresses: [email protected] (S. Wu), [email protected] (Q. Ran). https://doi.org/10.1016/j.conbuildmat.2020.118188 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
[11] is used as the most common air entraining agents in cement and concrete. Besides, nonionic surfactants such as alkyl polyoxyethylene ether (AEO) [12] and cationic surfactants such as N, N,N-trimethyl-1-dodecyl-ammonium bromide (DTAB) [13] are also used as air entraining agents in cement and concrete. Recent years the composition of cement and concrete become complicated and varied, so it is more and more difficult for air entraining agents to entraining air into cement and concrete [14,15]. It is strange that the air entraining agents fail suddenly to entrain tiny and stable air bubbles into cement and concrete without any effable reasons. As a kind surfactant, the performance of air entraining agents has important relationship with the salts in solution because that the salts can reduce the surface activities of surfactants, which are widely reported by the researchers in area of surfactants and detergents [16–18]. Similar phenomenon takes place when the surfactant is used in cementitious materials because of the high salinity environment. Tunstall [11] investigated the surface activities of different anionic surfactants in cement pore solution and
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calcium hydroxide solution. The introduction of Ca2+ depressed the surface activities of anionic surfactants, which induced the failure of air entrainment. On the other hand, cement and other inorganic components may cause some adsorption of surfactants, which decrease the concentration of surfactants remaining in the solution. But there is little research reported about the adsorption behavior of surfactants onto the surface of cement and other inorganic components. Ahmed et al. [19] report that anionic air entraining agents have strong adsorption onto the surface of fly ash in concrete, which indicates that, the adsorption of air entraining agents play an important role in reducing of the performance of air entraining in concrete. Merlin et al. [20] report that polyoxyethylene alkyl ether, which is commonly used as air entraining agent, can be absorbed onto the surface of calcium silicate hydrate (C-S-H) due to the hydrogen-bonding interaction. Naden et al. [21] investigates the interaction between surfactants and montmorillonite and understands the intercalation adsorption behavior of surfactants into the interlayer structure of montmorillonite. In this work, we pay attention to investigate the failure mechanism of air entraining agent in cement and concrete. The effect of salts in solution and the adsorption of surfactant are studied scientifically to give the answer why the air entraining agents fail. Here three surfactants are chosen as the target air entraining agents. The three surfactants include sodium dodecyl sulfate (SDS), N,N,N-
trimethyl-1-dodecanaminium bromide (DTAB) and alkyl polyoxyethylene ether (AEO) whose chemical structures are shown in Fig. 1. As shown in Fig. 1, SDS, DTAB and AEO have anionic group, cationic group and no charge group, respectively. Moreover, three salts are chosen to investigate the effect of salts in solution, and four inorganic powders are chosen to investigate the different adsorption of surfactants. All the information of inorganic powders is listed in Table 1. 2. Experimental sections 2.1. Materials Sodium dodecyl sulfate (SDS), N,N,N-trimethyl-1-dodecylammonium bromide (DTAB) and alkyl polyoxyethylene ether (AEO) are obtained from Aldrich Chemical Co. in chemical purity and used as received. The sodium chloride, calcium chloride and sodium sulfate are obtained from Sinopharm Chemical Reagent Co. in chemical purity and used as received. Cement is PII42.5 Portland cement from Jiangnan-xiaoyetian Cement Co., Ltd., Nanjing, China. Montmorillonite is obtained from Guangzhou Yifeng Chemical Technology Co., Ltd, Guangzhou, China. Granite powder and limestone powder are obtained from Lingshou raoxin mineral processing plant, Shijiazhuang, China. The component, particle size and the surface charge of all the inorganic powders are tested and listed in Table 1. In cement mortar, river sand was used with a nominal grain size of 0.5–1.5 mm as fine aggregate. The cement filtrate was obtained following the procedure: the mixture of cement and water (weight ratio of cement vs. water is 1:2) was stirred for 30 min then filtrated, and the concentrations of Na+, K+ and Ca2+ were 26, 1411 and 1334 mg/L, respectively. 2.2. Test of zeta-potential Test of zeta-potential is used to investigate the charge of the inorganic particles at the shear plane. Zeta potential of inorganic particles suspensions is tested in 0.01 mol/L KCl solution as background. 4.8 wt% of inorganic particles suspensions is prepared by adding 4.8 part of inorganic particles into 95.2 parts of 0.01 mol/ L KCl solution. Then Zeta Potential is determined at 20 ± 2 °C using a Colloidal Dynamics electroacoustic-based ZetaProbe analyzer (USA). 2.3. Test of surface tensions
Fig. 1. Chemical structure of target surfactants with different charge performance.
Materials
Component
Average mean particle size/lm
Zeta-potential (mV) in 4.8% aqueous suspensions
Cement
C3S 52.17%; C2S 21.16%; C3A 7.49%; C4AF 8.21% SiO2 50.95%; Al2O316.54%;MgO 4.65% SiO2 72.04%; Al2O3 14.42%; K2O 4.12%; Na2O 3.69% CaO 50.65%
22.46
+81.95
Surface tensions are obtained with a Krüss K100 surface tension meter. The samples are dissolved in pure water (or salt solution as shown in Table 2) to prepare the solutions in different concentration, and the concentration of surfactants is from 0.003 to 100 mili mole per liter (mM). Then 50 mL of the solution is put in a quartz detecting pool. A platinum ring is put into the liquid and then elevated (elevation speed is 3 mm/min). The pulling force is detected when the platinum ring separated from the liquid to get the surface tension.
7.66
147.86
2.4. Test of foam performance
22.08
108.17
17.52
+6.67
Table 1 The component, particle size and zeta-potential of all the inorganic powders.
Montmorillonite
Granite powder
Limestone powder
Here, C3S, C2S, C3A and C4AF represent dicalcium silicate, tricalcium aluminate, tricalcium silicate and tetracalcium aluminate ferrite, respectively. VMD is the average mean particle size.
Foam and bubble results is obtained with a Krüss DFA100 dynamic foam analyzer. The foam heights and bubble size are measured as follows: 100 mL of a surfactant solution is placed in a quartz cylinder with a 50 mm internal diameter. The bubbling rate is 0.2 L/min by introducing fresh air. The solution is bubbled for 100 s and then stayed for another 700 s to record the foam height. Meanwhile, camera takes the photos of the foam and the software can give the distribution of the bubble size and the average bubble
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M. Qiao et al. / Construction and Building Materials 242 (2020) 118188 Table 2 The CMC, cCMC, Cmax and Amin values of the surfactants measured in different solution. Surfactants
Solution
CMC (mM)
cCMC (mN/m)
Cmax (106 mol/m2)
Amin (Å2)
SDS SDS SDS SDS SDS SDS DTAB DTAB DTAB DTAB DTAB DTAB AEO AEO AEO AEO AEO
Pure water 0.1 M NaCl 0.5 M NaCl 0.1 M Na2SO4 0.1 M CaCl2 Cement filtrate Pure water 0.1 M NaCl 0.5 M NaCl 0.1 M Na2SO4 0.1 M CaCl2 Cement filtrate Pure water 0.1 M NaCl 0.5 M NaCl 0.1 M CaCl2 Cement filtrate
1.80 0.35 0.15 0.32 0.025 0.033 0.76 0.65 0.58 0.62 0.60 0.72 0.10 0.11 0.095 0.11 0.10
33.03 29.54 26.78 29.72 41.68 37.64 32.40 31.75 30.95 30.89 32.15 32.89 30.92 31.03 30.76 30.85 31.46
1.93 1.67 1.61 1.22 0.91 1.21 1.87 1.82 1.77 1.73 1.80 1.74 1.92 1.89 1.87 1.92 1.89
86.02 99.55 103.36 133.27 181.68 137.34 88.85 91.04 94.00 96.01 92.29 95.33 86.57 87.70 88.66 86.39 87.88
Fig. 2. The changes of surface tension as a function of logarithms of surfactant concentration in different solution.
size. In this paper, the bubble size is given in unit of pixel2, and 1 mm occupies 156 pixels in the camera photos as conversion 2.5. Test of adsorption The adsorption ratio of air entraining agent is determined by means of a total organic carbon analyzer, Multi N/C3100 (analytik-
jene AG, Germany). 20 g of solution containing various amount of air entraining agent (the range of concentration is from 0.05 mM to 4.5 mM) and 10 g of cement or granite powder or limestone powder (or 2 g of montmorillonite) are mixed by a magnetic stirrer for 30 min at 20 °C. The sample solution is separated by suction filter. The aqueous phase is separated by centrifuging at 13,000 rpm for 5 min by using a centrifuge. The supernatant is immediately
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Fig. 3. The changes of foam height and size of bubbles in condition of different surfactants in different solution: (a, b) SDS; (c, d) DTAB; (e, f) AEO (all the concentration of surfactant is 1.8 mmol/L which is more than the value of CMC of all surfactants).
decanted and filtrated by a filter with pore size of 50 lm. The filtrate is diluted 100 times with deionized water for test of Total Organic Carbon (TOC). 20 mL of each sample solution are measured. The difference in the concentration before and after contact with the inorganic powders is assumed to be adsorbed polymer.
2.6. Test of performances of the cement mortars Air contents of fresh cement mortars were obtained with a SANYO direct reading air content tester. The procedure is as below: 900 g of cement, 1350 g of sand, 459 g of water (the ratio of water
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to cement is 0.51) and various air entraining agents were mixed, stirred in low speed for 60 s, and stayed for 90 s followed by stirred in high speed for 60 s respectively. Then the air contents of the fresh cement mortars were measured. 3. Results and discussion 3.1. The effect of salts on the surface activity of surfactant solution The surface activity of the target air-entraining agent is firstly studied. The surface tensions of the solutions containing the surfactants and salts with various concentrations are tested. As shown in Fig. 2, the surface tensions of all the sample solutions decreased gradually upon the gradual increase of surfactant concentrations. Among the three air-entraining agents, SDS has the highest critical micelle concentration (CMC) when AEO has the lowest CMC. Meanwhile, all of the three air-entraining agents have the similar surface tension at CMC (ccmc). For anionic SDS as shown in Fig. 2b, the adding of NaCl and Na2SO4 reduce CMC and ccmc of SDS solution, and the more NaCl adding the more reduction CMC and ccmc will have. But there is something different when adding CaCl2. When adding CaCl2, the CMC of SDS solution is decreased as expected, but the ccmc of SDS solution is apparently increased, which indicated that the surface activity of the SDS solution is weakened. Similar results can be got when using cement filtrate instead of pure water. But for cationic DTAB and nonionic AEO as shown in Fig. 2c and d, the adding of different salts take no obvious change in the CMC and ccmc. The above results give clear rule that the high valence cations (e.g. Ca2+) weaken the surface activity of anionic surfactant solution, which are also reported by the researchers in area of surfactants and detergents [16–18]. The reason is mainly the chelation between Ca2+ and sulfate group in SDS which is also reported by Souza [9]. Thermal motion results in dynamic equilibrium between adsorption and desorption of amphiphiles at interface. So Gibbs adsorption equation can be used to get various parameters, such as the maximum surface excess at the air/water interface (Cmax) and minimum area per amphiphile molecule at the air/water interface (Amin) [22–24]. Here Cmax means the maximun amount of a surfactant adsorbed per unit area of the water-air interface, and Amin means the minimum area occupied by the ionic groups of a surfactant molecule. As Amin is inversely proportional to Cmax, trend of Amin is opposite to that of Cmax. In the Gibbs concept it is considered that surface excess concentration of water is zero and Cmax is the relative surface excess concentration which is calculated by Formula (1). (oc/olog c) is the slope of c vs. log c plot in pre-cmc region and represents the capacity of surfactants for decreasing the surface tension. R and T are, respectively, the universal gas constant and temperature (in Kelvin). n is the number of species produced by surfactant (for SDS and DTAB n is taken as 2 and for AEO it is 1). Minimum area occupied by amphiphile head group (Amin) is calculated by Formula (2). NA is the Avogadro’s constant. Cmax and Amin are in mol*m2 and m2, respectively.
Cmax ¼
1 @c nRT @logc
ð1Þ
Amin ¼
1 NA Cmax
ð2Þ
Here Cmax represents the arrayed density of surfactant molecule at the air/water interface, and Amin represents area occupied by one surfactant molecule at the air/water interface. The higher the value of Cmax, the denser the arrangement of surfactant molecule at the air/water interface is. The denser the arrangement of surfactant molecule at the air/water interface is, the more stable air/water
interface can be achieved. The more stable air/water interface is achieved, the better performance of air entrainment can be achieved. The values of Cmax and Amin of all surfactants are listed in Table 2. For DTAB and AEO, the adding of salts take no obvious changes of Cmax and Amin, which indicated that the salts cannot destroy the self-assembly of cationic and nonionic surfactant at the water-air interface. But it is quite different in situation of SDS, the adding of salts, especially CaCl2, destroy the self-assembly of SDS at the water-air interface apparently. When adding CaCl2, the value of Cmax drops by more than 50% compared with that in pure water, which indicates the great decrease in the density of SDS molecule at water-air interface. The lower density of surfactants at water-air interface induces the more unstable interface, which will cause failure of stable foam in solution or stable bubble in concrete. 3.2. The effect of salts on the foam and bubble size The foam is an important application of the surfactants. Meanwhile, the foam in the surfactants solution can predict the air entraining performance of surfactants in cement or concrete. So the tests of foam height and bubble size are carried out to realize the effect of salts on the performance of surfactants. The changes and values of foam height and size of bubbles in condition of different surfactants in different solution are given in Fig. 3 and Table 3. The results of surface activity tell that the high valence cations (e.g. Ca2+) weaken the surface activity of anionic surfactant solution. Here there is evidence from the results of foam and bubble size to verify the above conclusion. As shown in Fig. 3a, when adding NaCl and Na2SO4 in SDS solution, the foam height is slightly decreased. But after adding CaCl2 or using cement filtrate instead of pure water, the foam heights of SDS solution have a sharp decrease, which is in consistent with the results of Section 3.1. As shown in Fig. 3b, when adding NaCl and Na2SO4 in SDS solution, the distribution of bubble size and the average bubble size take no obvious changes. But after adding CaCl2 or using cement filtrate instead of pure water, obvious changes happen for the distribution of bubble size and the average bubble size. The average bubble size increases nearly thrice as shown in Table 3. It can be concluded that the decreased surface activity of SDS solution induces the low foamability and the big bubble size, which may also depress the air-entraining performance of SDS in cement or concrete. The results are quite different for cationic DTAB and nonionic AEO as shown in Fig. 3c–f. The adding of different salts doesn’t take any changes. The results are also consistent with the results of surface activity. Based on the above results, a mechanism of the calcium Table 3 The maximal foam height and average bubble size of the surfactants measured in different solution. Surfactants
Solution
Hmax/mm
Average bubble size (px2)
SDS SDS SDS SDS SDS SDS DTAB DTAB DTAB DTAB DTAB DTAB AEO AEO AEO AEO AEO
Pure water 0.1 M NaCl 0.5 M NaCl 0.1 M Na2SO4 0.1 M CaCl2 Cement filtrate Pure water 0.1 M NaCl 0.5 M NaCl 0.1 M Na2SO4 0.1 M CaCl2 Cement filtrate Pure water 0.1 M NaCl 0.5 M NaCl 0.1 M CaCl2 Cement filtrate
138.2 131.5 122.8 123.5 96.4 105.2 132.5 131.8 129.6 129.7 125.4 126.7 122.5 123.2 121.1 120.6 120.4
111.7 105.3 116.4 128.2 418.7 356.2 220.3 235.8 231.2 225.7 248.2 245.3 305.8 310.2 308.5 305.8 310.7
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induced failure of anionic surfactants can be concluded as shown in Fig. 4. First, calcium ion has strong ionic attraction with the sulfate ion in SDS. The strong ionic attraction can be considered as the calcium chelation. The calcium chelation makes SDS unstable at water-air interface and decreases the packing density of SDS at
Fig. 4. Illustration of the calcium induced failure of anionic surfactants: (a) calcium chelation; (b) decreased packing density; (c) the fusion of bubbles.
water-air interface. Then the unstable interface can make the fusion of tiny bubble to induce the big bubble. 3.3. The adsorption of surfactant on different inorganic powders There are many kinds of inorganic components in concrete which have high surface energy. So these inorganic components can absorb some surfactants, and the adsorption is mainly due to the electrostatic attraction. The adsorption of water-reducing agent on the surface of cement and other inorganic components are widely reported [25–27], while there is little research about the adsorption of air-entraining agent. Here four kinds of inorganic powders are employed to investigate the adsorption of airentraining agent as shown in Table 1. Cement has strong positive zeta potential, limestone powders has slight positive zeta potential, and montmorillonite and granite powders have strong negative zeta potential. All these inorganic powders have similar particle size from 5 to 30 lm. Fig. 5 gives the changes of adsorption ratio (the ratio of Adsorbed surfactant amount/introduced surfactant amount) as a function of surfactant concentration on surface of different inorganic powders. The adsorption ratios of surfactants increase and then reach a platform when increasing the concentration of surfactants, which is quite different from the waterreducing agent. For water-reducing agent, adsorption ratios decrease when increasing the concentration [25,27]. The difference can be attributed to the saturation adsorption. The surface of
Fig. 5. The adsorption ratio of surfactant onto the surface of inorganic materials: (a) cement; (b) montmorillonite; (c) granite powder; (d) limestone powder.
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cement adsorbs the water-reducing agent in saturation when the dosage of water-reducing agent is more than 0.1% in weight of cement. But the concentration of surfactants as air-entraining agents is commonly less than 0.01% in weight of cement, and in this situation the surface of cement is not adsorbed in saturation. Fig. 6 gives the changes of adsorption amount as a function of surfactant concentration on surface of different inorganic powders. It can be clearly seen than the adsorption isotherms are in linear tendency, which is the evidence of unsaturated adsorption. As shown in Figs. 5a and 6a, on the positive charged surface of cement, anionic SDS has the highest adsorption ratio, and the
cationic DTAB and nonionic AEO have little adsorption. On the negative charged surface of montmorillonite and granite powders as shown in Figs. 5b, c and 6b, c, cationic DTAB has the highest adsorption ratio, while anionic SDS has little adsorption. Nonionic AEO has high adsorption ratio on surface of both montmorillonite and granite powders. On the surface of limestone powders as shown in Fig. 5d and 6d, the anionic SDS can absorb onto the limestone surface more easily because of the electrostatic attraction. On the other hand, anionic SDS has stronger adsorption on surface of cement than that on surface of montmorillonite, granite and limestone. So when montmorillonite, granite and limestone are
Fig. 6. The adsorption amount of surfactant onto the surface of inorganic materials: (a) cement; (b) montmorillonite; (c) granite powder; (d) limestone powder.
Table 4 The adsorption amount and adsorption ratio of the surfactants on the surface of montmorillonite, granite and limestone in pure water and cement filtrate (the concentration of surfactant is 2 mM). Surfactants
Powder
Adsorption Amount (mg/g)
Adsorption Ratio (%)
In pure water
In cement filtrate
In pure water
In cement filtrate
SDS DTAB AEO
Montmorillonite
0.06 5.12 8.76
0.08 4.98 8.70
8.2 92.5 81.7
10.9 90.0 81.1
SDS DTAB AEO
Granite
0.31 1.12 1.78
0.33 1.09 1.79
31.2 90.8 91.6
33.2 88.4 92.1
SDS DTAB AEO
Limestone
0.43 0.14 0.59
0.51 0.12 0.57
45.2 10.5 26.7
53.6 9.0 26.2
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on surface of cement. So when montmorillonite and granite powders are added into concrete, the total adsorption of cationic DTAB and nonionic AEO will be increased obviously. The obvious increased adsorption of surfactants may induce the great decrease of the concentration of surfactants in solution, which can depress the performance of air-entraining. The adsorption behavior of three surfactants at concentration of 2 mM on the surface of montmorillonite, granite and limestone is repeated in cement filtrate instead of pure water. The results are shown in Table 4. Compared to the adsorption behavior in pure water, in condition of cement filtrate, the adsorption amount and adsorption ratio of anionic SDS on all there powders are slightly increased, the adsorption amount and adsorption ratio of cationic DTAB on all there powders are slightly decreased, and the adsorption amount and adsorption ratio of nonionic AEO on all there powders are almost the same. 3.4. The failure of air-entraining because of the calcium and the adsorption In order to verify the effect of adsorption and salts on the performance of air-entraining, the air-entraining tests in cement mortar in different conditions with different surfactants are carried out. The results are given in Fig. 7. When using anionic SDS as air-entraining agent, the air content of cement mortar is 15.1%. By adding NaCl, limestone powders, granite powders and montmorillonite, the air contents have no obvious changes (Fig. 7a). By adding CaCl2, the air content of cement mortar drops by 17.2%. In Sections 3.1 and 3.2, it is known that the calcium depresses the surface activity of SDS solution and makes the bubbles unstable. So calcium is actually the key factor of the failure of anionic surfactants for air-entraining. It is quite different for cationic and nonionic surfactants as shown in Fig. 7a and b. By adding NaCl and CaCl2, the air contents have no obvious changes. But when adding montmorillonite, granite powders and limestone powders, the air contents decrease obviously. The cationic DTAB and nonionic AEO have much more adsorption on surface of montmorillonite, granite powders and limestone powders than that on surface of cement. So when montmorillonite, granite powders or limestone powders are added into cement mortar, the total adsorption of cationic DTAB and nonionic AEO will be increased obviously. The obvious increased adsorption of surfactants may induce the great decrease of the concentration of surfactants in solution, which can depress the performance of air-entraining. Among the three inorganic powders, montmorillonite and granite powders can strongly absorb cationic and nonionic surfactants and greatly depress the performance of air-entraining. So for cationic and nonionic surfactants, the adsorption is the most important reason of the failure of anionic surfactants for air-entraining. 4. Conclusions
Fig. 7. The air-entraining test in cement mortar in different conditions with different surfactants: (a) SDS; (b) DTAB; (c) AEO (LP: limestone powders; GP: granite powders; MMT: montmorillonite).
added into concrete, the total adsorption of anionic SDS will not be increased. But cationic DTAB and nonionic AEO have much stronger adsorption on surface of montmorillonite and granite than that
In this paper, the effect of salts in solution and the adsorption of surfactant are studied scientifically to give the answer why the air entraining agents fail. The high valence cations (e.g. Ca2+) weaken the surface activity of anionic surfactant solution but have no effect on the surface activity of cationic and nonionic surfactant solution. The calcium induces the decreased surface activity of SDS solution, the low foamability and the big bubble size, which depress the airentraining performance of SDS in cement or concrete. Anionic SDS has stronger adsorption on surface of cement than that on surface of montmorillonite, granite and limestone. When montmorillonite, granite and limestone are added into concrete, the air-entraining performance of anionic SDS is not be depressed. But cationic DTAB and nonionic AEO have much stronger adsorption on surface of
M. Qiao et al. / Construction and Building Materials 242 (2020) 118188
montmorillonite and granite than that on surface of cement. When montmorillonite and granite powders are added into concrete, the air-entraining performance of cationic DTAB and nonionic AEO are obviously depressed. Based on the above research results, some strategies for optimize the application of surfactants as air-entraining agent can be concluded. When the high valence metal cations (e.g. Ca2+, Fe3+ and Al3+) are at very high concentration, it is hard for anionic surfactant to get good performance of air-entraining by itself. Cationic and nonionic surfactants can be used as supplement to solve the problem. When montmorillonite and granite powders are introduced into concrete in spite of deliberately adding or unconsciously introduction, it is better to stop to use any cationic and nonionic surfactants as air-entraining agent, and the anionic surfactant is the best choice for air-entraining agent. CRediT authorship contribution statement Min Qiao: Writing - original draft. Guangcheng Shan: Investigation. Jian Chen: Investigation. Shishan Wu: Project administration, Supervision. Nanxiao Gao: Investigation. Qianping Ran: Funding acquisition. Jiaping Liu: Methodology, Supervision. 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. Acknowledgements 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) and Technology Research and Development Project of China Railway Corporation (N2018G030). References [1] S. Chatterji, Freezing of air-entrained cement-based materials and specific actions of air-entraining agents, Cem. Concr. Compos. 25 (2003) 759–765. [2] S. Riyazi, J.T. Kevern, M. Mulheron, Improving the stability of entrained air in self-compacting concrete by optimizing the mix viscosity and air entraining agent dosage, Constr. Build. Mater. 148 (2017) 531–537. [3] J. Plank, E. Sakai, C.W. Miao, C. Yu, J.X. Hong, Chemical admixtures – chemistry, applications and their impact on concrete microstructure and durability, Cem. Concr. Res. 78 (2015) 81–99. [4] F. Matalkah, P. Soroushian, thaw and deicer salt scaling resistance of concrete prepared with alkali aluminosilicate cement, Constr. Build. Mater. 163 (2018) 200–213.
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[5] N.P. Mayercsik, M. Vandamme, K.E. Kurtis, Assessing the efficiency of entrained air voids for freeze-thaw durability through modeling, Cem. Concr. Res. 88 (2016) 43–59. [6] J. Chen, M. Qiao, N.X. Gao, Q.P. Ran, S.S. Wu, S. Qi, Sulfonic gemini surfactants: synthesis, properties and applications as novel air entraining agents for concrete, Colloid Surf. A 522 (2017) 593–600. [7] M. Qiao, J. Chen, C. Yu, S.S. Wu, N.X. Gao, Q.P. Ran, Gemini surfactants as novel air entraining agents for concrete, Cem. Concr. Res. 100 (2017) 40–46. [8] J. Chen, M. Qiao, N.X. Gao, Q.P. Ran, J.Z. Wu, G.C. Shan, S. Qi, S.S. Wu, Cationic oligomeric surfactants as novel air entraining agents for concrete, Colloid Surf. A 538 (2018) 686–693. [9] M.T. Souza, C.K. Maykot, A.C.Z. Araújo, F. Raupp-Pereira, A.P. Novaes de Oliveira, Electrolytes’ influence on foamability and foam stability of cement suspensions, Constr. Build. Mater. 157 (2017) 363–371. [10] F.A. Rodrigues, I. Joekes, Water reducing agents of low-molecular-weightsuppression of air entrapment and slump loss by addition of an organicsolvent, Cem. Concr. Res. 24 (1994) 987–992. [11] L.E. Tunstall, G.W. Scherer, R.K. Prud’homme, Studying AEA interaction in cement systems using tensiometry, Cem. Concr. Res. 92 (2017) 29–36. [12] Z.M. He, J.Z. Liu, T.H. Wang, Influence of air entraining agent on performance of inorganic thermal insulating mortar, Appl. Mech. Mater. 71–78 (2011) 490– 493. [13] X.P. Wang, J.D. Ye, X. Li, H. Dong, Production of in-situ macropores in an injectable calcium phosphate cement by introduction of cetyltrimethyl ammonium bromide, J. Mater. Sci. Mater. Med. 19 (2008) 3221–3225. [14] L. Du, K.J. Folliard, Mechanisms of air entrainment in concrete, Cem. Concr. Res. 35 (2005) 1463–1471. [15] F.K. Cui, H.S. Shang, T.J. Zhao, G.X. Fan, G.S. Ren, Mechanical and failure criteria of air-entrained concrete under triaxial compression load after rapid freezethaw cycles, Adv. Mater. Sci. Eng. (2017) 6786270. [16] S. Javadian, J. Kakemam, Intermicellar interaction in surfactant solutions: a review study, J. Mol. Liq. 242 (2017) 115–128. [17] D. Michler, N. Shahidzadeh, M. Westbroek, R. van Roij, D. Bonn, Are antagonistic salts surfactants?, Langmuir 31 (2015) 906–911. [18] Y. Zhang, L. Chen, J.X. Xiao, J.M. Ma, Effect of salt with high concentration on surface activities of equimolar mixtures of cationic-anionic surfactants, Acta Chim. Sinica 62 (2004) 1491–1494. [19] Z.T. Ahmed, D.W. Hand, Direct adsorption isotherms of AEAs and fly ash: aolefin sulfonate and combination admixtures, ACS Sustainable Chem. Eng. 3 (2015) 216–223. [20] F. Merlin, H. Guitouni, H. Mouhoubi, S. Mariot, F. Vallée, H. Van Damme, Adsorption and heterocoagulation of nonionic surfactants and latex particles on cement hydrates, J. Colloid Interface Sci. 281 (2005) 1–10. [21] B.J. Naden, Competitive adsorption of surfactant foaming agents to nanoclays added to cement foams for enhanced strength, Mater. Struct. 49 (2016) 1667– 1675. [22] F.M. Menger, J.S. Keiper, Gemini surfactants, Angew. Chem. Int. Ed. 39 (2000) 1906–1920. [23] A.Z. Naqvi, S.N. Kabir-ud-Din, Effect of surfactant structure on the mixed micelle formation of cationic gemini-zwitterionic phospholipid systems, Colloid Surf. A: Physicochem. Eng. Asp. 477 (2015) 9–18. [24] J. Eastoe, S. Nave, A. Downer, A. Paul, A. Rankin, K. Tribe, Adsorption of ionic surfactants at the air-solution interface, Langmuir 16 (2000) 4511– 4518. [25] Y.H. Yu, J.P. Liu, Q.P. Ran, M. Qiao, N.X. Gao, Interfacial interaction between comb-like copolymer dispersants and cement component minerals, Polym. Polym. Compos. 21 (2013) 299–306. [26] M. Qiao, Q.P. Ran, J.P. Liu, Impact of linkage group in comb-like polymer on dispersion properties of cement pastes, Polym. Polym. Compos. 21 (2013) 43– 50. [27] Q.P. Ran, M. Qiao, J.P. Liu, C.W. Miao, Impact of molecular size of SMA-g-MPEG comb-like polymer on the dispersion of CaCO3 suspensions, Colloid Polym. Sci. 290 (2012) 435–443.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Effects of salts and adsorption on the performance of air entraining agent with different charge type in solution and cement mortar Min Qiao a,b, Guangcheng Shan b, Jian Chen b, Shishan Wu a,⇑, Nanxiao Gao b, Qianping Ran b,c,⇑, Jiaping Liu c a
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, PR China State Key Laboratory of High Performance Civil Engineering Materials, Jiangsu Sobute New Materials Co. Ltd., Nanjing 211103, PR China c School of Material Science and Engineering, Southeast University, Nanjing 211189, Jiangsu, China b
h i g h l i g h t s Failure mechanism of air entraining agent in cement and concrete is investigated. High valence cations weaken the surface activity of anionic surfactant. Anionic surfactants have strong adsorption on surface of cement. Cationic and nonionic surfactants have strong adsorption on surface of montmorillonite and granite.
a r t i c l e
i n f o
Article history: Received 22 May 2018 Received in revised form 25 November 2019 Accepted 14 January 2020
Keywords: Air entraining agent Cement mortar Surfactant Adsorption Charge type
a b s t r a c t In this paper, the effect of salts in solution and the adsorption of surfactant are studied to give the answer why the air entraining agents fail. The high valence cations (e.g. Ca2+) weaken the surface activity of anionic surfactant solution but have no effect on the surface activity of cationic and nonionic surfactant solution. The calcium induces the decreased surface activity of anionic surfactant solution, the low foamability, the big bubble size, and the depressed the air-entraining performance. Anionic surfactant has stronger adsorption on surface of cement than that on surface of montmorillonite, granite and limestone. But cationic and nonionic surfactants have much stronger adsorption on surface of montmorillonite and granite than that on surface of cement. When montmorillonite, granite and limestone are added into cement mortar, the air-entraining performance of anionic surfactant is not depressed, but the air-entraining performance of cationic and nonionic surfactants are obviously depressed. Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction Air entrainment has been regarded to be highly desirable for concrete since the 1930s [1–3]. Entraining homogeneous, tiny and stable air voids in fresh concrete can effectively improve both workability and freeze-thaw durability of concrete [4,5]. Air entraining agents are important concrete admixtures that can help to intentionally create a number of effective air voids in concrete. All air entraining agents can be divided into three kinds [6–8]: anionic surfactants, cationic surfactants and nonionic surfactants. Anionic surfactants such as sodium dodecyl sulfate (SDS) [9], sodium dodecylbenzene sulfonate (SDBS) [10] and saponified rosin ⇑ Corresponding authors at: School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, PR China (S. Wu); State Key Laboratory of High Performance Civil Engineering Materials, Jiangsu Sobute New Materials Co. Ltd., Nanjing 211103, PR China (Q. Ran). E-mail addresses: [email protected] (S. Wu), [email protected] (Q. Ran). https://doi.org/10.1016/j.conbuildmat.2020.118188 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.
[11] is used as the most common air entraining agents in cement and concrete. Besides, nonionic surfactants such as alkyl polyoxyethylene ether (AEO) [12] and cationic surfactants such as N, N,N-trimethyl-1-dodecyl-ammonium bromide (DTAB) [13] are also used as air entraining agents in cement and concrete. Recent years the composition of cement and concrete become complicated and varied, so it is more and more difficult for air entraining agents to entraining air into cement and concrete [14,15]. It is strange that the air entraining agents fail suddenly to entrain tiny and stable air bubbles into cement and concrete without any effable reasons. As a kind surfactant, the performance of air entraining agents has important relationship with the salts in solution because that the salts can reduce the surface activities of surfactants, which are widely reported by the researchers in area of surfactants and detergents [16–18]. Similar phenomenon takes place when the surfactant is used in cementitious materials because of the high salinity environment. Tunstall [11] investigated the surface activities of different anionic surfactants in cement pore solution and
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calcium hydroxide solution. The introduction of Ca2+ depressed the surface activities of anionic surfactants, which induced the failure of air entrainment. On the other hand, cement and other inorganic components may cause some adsorption of surfactants, which decrease the concentration of surfactants remaining in the solution. But there is little research reported about the adsorption behavior of surfactants onto the surface of cement and other inorganic components. Ahmed et al. [19] report that anionic air entraining agents have strong adsorption onto the surface of fly ash in concrete, which indicates that, the adsorption of air entraining agents play an important role in reducing of the performance of air entraining in concrete. Merlin et al. [20] report that polyoxyethylene alkyl ether, which is commonly used as air entraining agent, can be absorbed onto the surface of calcium silicate hydrate (C-S-H) due to the hydrogen-bonding interaction. Naden et al. [21] investigates the interaction between surfactants and montmorillonite and understands the intercalation adsorption behavior of surfactants into the interlayer structure of montmorillonite. In this work, we pay attention to investigate the failure mechanism of air entraining agent in cement and concrete. The effect of salts in solution and the adsorption of surfactant are studied scientifically to give the answer why the air entraining agents fail. Here three surfactants are chosen as the target air entraining agents. The three surfactants include sodium dodecyl sulfate (SDS), N,N,N-
trimethyl-1-dodecanaminium bromide (DTAB) and alkyl polyoxyethylene ether (AEO) whose chemical structures are shown in Fig. 1. As shown in Fig. 1, SDS, DTAB and AEO have anionic group, cationic group and no charge group, respectively. Moreover, three salts are chosen to investigate the effect of salts in solution, and four inorganic powders are chosen to investigate the different adsorption of surfactants. All the information of inorganic powders is listed in Table 1. 2. Experimental sections 2.1. Materials Sodium dodecyl sulfate (SDS), N,N,N-trimethyl-1-dodecylammonium bromide (DTAB) and alkyl polyoxyethylene ether (AEO) are obtained from Aldrich Chemical Co. in chemical purity and used as received. The sodium chloride, calcium chloride and sodium sulfate are obtained from Sinopharm Chemical Reagent Co. in chemical purity and used as received. Cement is PII42.5 Portland cement from Jiangnan-xiaoyetian Cement Co., Ltd., Nanjing, China. Montmorillonite is obtained from Guangzhou Yifeng Chemical Technology Co., Ltd, Guangzhou, China. Granite powder and limestone powder are obtained from Lingshou raoxin mineral processing plant, Shijiazhuang, China. The component, particle size and the surface charge of all the inorganic powders are tested and listed in Table 1. In cement mortar, river sand was used with a nominal grain size of 0.5–1.5 mm as fine aggregate. The cement filtrate was obtained following the procedure: the mixture of cement and water (weight ratio of cement vs. water is 1:2) was stirred for 30 min then filtrated, and the concentrations of Na+, K+ and Ca2+ were 26, 1411 and 1334 mg/L, respectively. 2.2. Test of zeta-potential Test of zeta-potential is used to investigate the charge of the inorganic particles at the shear plane. Zeta potential of inorganic particles suspensions is tested in 0.01 mol/L KCl solution as background. 4.8 wt% of inorganic particles suspensions is prepared by adding 4.8 part of inorganic particles into 95.2 parts of 0.01 mol/ L KCl solution. Then Zeta Potential is determined at 20 ± 2 °C using a Colloidal Dynamics electroacoustic-based ZetaProbe analyzer (USA). 2.3. Test of surface tensions
Fig. 1. Chemical structure of target surfactants with different charge performance.
Materials
Component
Average mean particle size/lm
Zeta-potential (mV) in 4.8% aqueous suspensions
Cement
C3S 52.17%; C2S 21.16%; C3A 7.49%; C4AF 8.21% SiO2 50.95%; Al2O316.54%;MgO 4.65% SiO2 72.04%; Al2O3 14.42%; K2O 4.12%; Na2O 3.69% CaO 50.65%
22.46
+81.95
Surface tensions are obtained with a Krüss K100 surface tension meter. The samples are dissolved in pure water (or salt solution as shown in Table 2) to prepare the solutions in different concentration, and the concentration of surfactants is from 0.003 to 100 mili mole per liter (mM). Then 50 mL of the solution is put in a quartz detecting pool. A platinum ring is put into the liquid and then elevated (elevation speed is 3 mm/min). The pulling force is detected when the platinum ring separated from the liquid to get the surface tension.
7.66
147.86
2.4. Test of foam performance
22.08
108.17
17.52
+6.67
Table 1 The component, particle size and zeta-potential of all the inorganic powders.
Montmorillonite
Granite powder
Limestone powder
Here, C3S, C2S, C3A and C4AF represent dicalcium silicate, tricalcium aluminate, tricalcium silicate and tetracalcium aluminate ferrite, respectively. VMD is the average mean particle size.
Foam and bubble results is obtained with a Krüss DFA100 dynamic foam analyzer. The foam heights and bubble size are measured as follows: 100 mL of a surfactant solution is placed in a quartz cylinder with a 50 mm internal diameter. The bubbling rate is 0.2 L/min by introducing fresh air. The solution is bubbled for 100 s and then stayed for another 700 s to record the foam height. Meanwhile, camera takes the photos of the foam and the software can give the distribution of the bubble size and the average bubble
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M. Qiao et al. / Construction and Building Materials 242 (2020) 118188 Table 2 The CMC, cCMC, Cmax and Amin values of the surfactants measured in different solution. Surfactants
Solution
CMC (mM)
cCMC (mN/m)
Cmax (106 mol/m2)
Amin (Å2)
SDS SDS SDS SDS SDS SDS DTAB DTAB DTAB DTAB DTAB DTAB AEO AEO AEO AEO AEO
Pure water 0.1 M NaCl 0.5 M NaCl 0.1 M Na2SO4 0.1 M CaCl2 Cement filtrate Pure water 0.1 M NaCl 0.5 M NaCl 0.1 M Na2SO4 0.1 M CaCl2 Cement filtrate Pure water 0.1 M NaCl 0.5 M NaCl 0.1 M CaCl2 Cement filtrate
1.80 0.35 0.15 0.32 0.025 0.033 0.76 0.65 0.58 0.62 0.60 0.72 0.10 0.11 0.095 0.11 0.10
33.03 29.54 26.78 29.72 41.68 37.64 32.40 31.75 30.95 30.89 32.15 32.89 30.92 31.03 30.76 30.85 31.46
1.93 1.67 1.61 1.22 0.91 1.21 1.87 1.82 1.77 1.73 1.80 1.74 1.92 1.89 1.87 1.92 1.89
86.02 99.55 103.36 133.27 181.68 137.34 88.85 91.04 94.00 96.01 92.29 95.33 86.57 87.70 88.66 86.39 87.88
Fig. 2. The changes of surface tension as a function of logarithms of surfactant concentration in different solution.
size. In this paper, the bubble size is given in unit of pixel2, and 1 mm occupies 156 pixels in the camera photos as conversion 2.5. Test of adsorption The adsorption ratio of air entraining agent is determined by means of a total organic carbon analyzer, Multi N/C3100 (analytik-
jene AG, Germany). 20 g of solution containing various amount of air entraining agent (the range of concentration is from 0.05 mM to 4.5 mM) and 10 g of cement or granite powder or limestone powder (or 2 g of montmorillonite) are mixed by a magnetic stirrer for 30 min at 20 °C. The sample solution is separated by suction filter. The aqueous phase is separated by centrifuging at 13,000 rpm for 5 min by using a centrifuge. The supernatant is immediately
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Fig. 3. The changes of foam height and size of bubbles in condition of different surfactants in different solution: (a, b) SDS; (c, d) DTAB; (e, f) AEO (all the concentration of surfactant is 1.8 mmol/L which is more than the value of CMC of all surfactants).
decanted and filtrated by a filter with pore size of 50 lm. The filtrate is diluted 100 times with deionized water for test of Total Organic Carbon (TOC). 20 mL of each sample solution are measured. The difference in the concentration before and after contact with the inorganic powders is assumed to be adsorbed polymer.
2.6. Test of performances of the cement mortars Air contents of fresh cement mortars were obtained with a SANYO direct reading air content tester. The procedure is as below: 900 g of cement, 1350 g of sand, 459 g of water (the ratio of water
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to cement is 0.51) and various air entraining agents were mixed, stirred in low speed for 60 s, and stayed for 90 s followed by stirred in high speed for 60 s respectively. Then the air contents of the fresh cement mortars were measured. 3. Results and discussion 3.1. The effect of salts on the surface activity of surfactant solution The surface activity of the target air-entraining agent is firstly studied. The surface tensions of the solutions containing the surfactants and salts with various concentrations are tested. As shown in Fig. 2, the surface tensions of all the sample solutions decreased gradually upon the gradual increase of surfactant concentrations. Among the three air-entraining agents, SDS has the highest critical micelle concentration (CMC) when AEO has the lowest CMC. Meanwhile, all of the three air-entraining agents have the similar surface tension at CMC (ccmc). For anionic SDS as shown in Fig. 2b, the adding of NaCl and Na2SO4 reduce CMC and ccmc of SDS solution, and the more NaCl adding the more reduction CMC and ccmc will have. But there is something different when adding CaCl2. When adding CaCl2, the CMC of SDS solution is decreased as expected, but the ccmc of SDS solution is apparently increased, which indicated that the surface activity of the SDS solution is weakened. Similar results can be got when using cement filtrate instead of pure water. But for cationic DTAB and nonionic AEO as shown in Fig. 2c and d, the adding of different salts take no obvious change in the CMC and ccmc. The above results give clear rule that the high valence cations (e.g. Ca2+) weaken the surface activity of anionic surfactant solution, which are also reported by the researchers in area of surfactants and detergents [16–18]. The reason is mainly the chelation between Ca2+ and sulfate group in SDS which is also reported by Souza [9]. Thermal motion results in dynamic equilibrium between adsorption and desorption of amphiphiles at interface. So Gibbs adsorption equation can be used to get various parameters, such as the maximum surface excess at the air/water interface (Cmax) and minimum area per amphiphile molecule at the air/water interface (Amin) [22–24]. Here Cmax means the maximun amount of a surfactant adsorbed per unit area of the water-air interface, and Amin means the minimum area occupied by the ionic groups of a surfactant molecule. As Amin is inversely proportional to Cmax, trend of Amin is opposite to that of Cmax. In the Gibbs concept it is considered that surface excess concentration of water is zero and Cmax is the relative surface excess concentration which is calculated by Formula (1). (oc/olog c) is the slope of c vs. log c plot in pre-cmc region and represents the capacity of surfactants for decreasing the surface tension. R and T are, respectively, the universal gas constant and temperature (in Kelvin). n is the number of species produced by surfactant (for SDS and DTAB n is taken as 2 and for AEO it is 1). Minimum area occupied by amphiphile head group (Amin) is calculated by Formula (2). NA is the Avogadro’s constant. Cmax and Amin are in mol*m2 and m2, respectively.
Cmax ¼
1 @c nRT @logc
ð1Þ
Amin ¼
1 NA Cmax
ð2Þ
Here Cmax represents the arrayed density of surfactant molecule at the air/water interface, and Amin represents area occupied by one surfactant molecule at the air/water interface. The higher the value of Cmax, the denser the arrangement of surfactant molecule at the air/water interface is. The denser the arrangement of surfactant molecule at the air/water interface is, the more stable air/water
interface can be achieved. The more stable air/water interface is achieved, the better performance of air entrainment can be achieved. The values of Cmax and Amin of all surfactants are listed in Table 2. For DTAB and AEO, the adding of salts take no obvious changes of Cmax and Amin, which indicated that the salts cannot destroy the self-assembly of cationic and nonionic surfactant at the water-air interface. But it is quite different in situation of SDS, the adding of salts, especially CaCl2, destroy the self-assembly of SDS at the water-air interface apparently. When adding CaCl2, the value of Cmax drops by more than 50% compared with that in pure water, which indicates the great decrease in the density of SDS molecule at water-air interface. The lower density of surfactants at water-air interface induces the more unstable interface, which will cause failure of stable foam in solution or stable bubble in concrete. 3.2. The effect of salts on the foam and bubble size The foam is an important application of the surfactants. Meanwhile, the foam in the surfactants solution can predict the air entraining performance of surfactants in cement or concrete. So the tests of foam height and bubble size are carried out to realize the effect of salts on the performance of surfactants. The changes and values of foam height and size of bubbles in condition of different surfactants in different solution are given in Fig. 3 and Table 3. The results of surface activity tell that the high valence cations (e.g. Ca2+) weaken the surface activity of anionic surfactant solution. Here there is evidence from the results of foam and bubble size to verify the above conclusion. As shown in Fig. 3a, when adding NaCl and Na2SO4 in SDS solution, the foam height is slightly decreased. But after adding CaCl2 or using cement filtrate instead of pure water, the foam heights of SDS solution have a sharp decrease, which is in consistent with the results of Section 3.1. As shown in Fig. 3b, when adding NaCl and Na2SO4 in SDS solution, the distribution of bubble size and the average bubble size take no obvious changes. But after adding CaCl2 or using cement filtrate instead of pure water, obvious changes happen for the distribution of bubble size and the average bubble size. The average bubble size increases nearly thrice as shown in Table 3. It can be concluded that the decreased surface activity of SDS solution induces the low foamability and the big bubble size, which may also depress the air-entraining performance of SDS in cement or concrete. The results are quite different for cationic DTAB and nonionic AEO as shown in Fig. 3c–f. The adding of different salts doesn’t take any changes. The results are also consistent with the results of surface activity. Based on the above results, a mechanism of the calcium Table 3 The maximal foam height and average bubble size of the surfactants measured in different solution. Surfactants
Solution
Hmax/mm
Average bubble size (px2)
SDS SDS SDS SDS SDS SDS DTAB DTAB DTAB DTAB DTAB DTAB AEO AEO AEO AEO AEO
Pure water 0.1 M NaCl 0.5 M NaCl 0.1 M Na2SO4 0.1 M CaCl2 Cement filtrate Pure water 0.1 M NaCl 0.5 M NaCl 0.1 M Na2SO4 0.1 M CaCl2 Cement filtrate Pure water 0.1 M NaCl 0.5 M NaCl 0.1 M CaCl2 Cement filtrate
138.2 131.5 122.8 123.5 96.4 105.2 132.5 131.8 129.6 129.7 125.4 126.7 122.5 123.2 121.1 120.6 120.4
111.7 105.3 116.4 128.2 418.7 356.2 220.3 235.8 231.2 225.7 248.2 245.3 305.8 310.2 308.5 305.8 310.7
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induced failure of anionic surfactants can be concluded as shown in Fig. 4. First, calcium ion has strong ionic attraction with the sulfate ion in SDS. The strong ionic attraction can be considered as the calcium chelation. The calcium chelation makes SDS unstable at water-air interface and decreases the packing density of SDS at
Fig. 4. Illustration of the calcium induced failure of anionic surfactants: (a) calcium chelation; (b) decreased packing density; (c) the fusion of bubbles.
water-air interface. Then the unstable interface can make the fusion of tiny bubble to induce the big bubble. 3.3. The adsorption of surfactant on different inorganic powders There are many kinds of inorganic components in concrete which have high surface energy. So these inorganic components can absorb some surfactants, and the adsorption is mainly due to the electrostatic attraction. The adsorption of water-reducing agent on the surface of cement and other inorganic components are widely reported [25–27], while there is little research about the adsorption of air-entraining agent. Here four kinds of inorganic powders are employed to investigate the adsorption of airentraining agent as shown in Table 1. Cement has strong positive zeta potential, limestone powders has slight positive zeta potential, and montmorillonite and granite powders have strong negative zeta potential. All these inorganic powders have similar particle size from 5 to 30 lm. Fig. 5 gives the changes of adsorption ratio (the ratio of Adsorbed surfactant amount/introduced surfactant amount) as a function of surfactant concentration on surface of different inorganic powders. The adsorption ratios of surfactants increase and then reach a platform when increasing the concentration of surfactants, which is quite different from the waterreducing agent. For water-reducing agent, adsorption ratios decrease when increasing the concentration [25,27]. The difference can be attributed to the saturation adsorption. The surface of
Fig. 5. The adsorption ratio of surfactant onto the surface of inorganic materials: (a) cement; (b) montmorillonite; (c) granite powder; (d) limestone powder.
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cement adsorbs the water-reducing agent in saturation when the dosage of water-reducing agent is more than 0.1% in weight of cement. But the concentration of surfactants as air-entraining agents is commonly less than 0.01% in weight of cement, and in this situation the surface of cement is not adsorbed in saturation. Fig. 6 gives the changes of adsorption amount as a function of surfactant concentration on surface of different inorganic powders. It can be clearly seen than the adsorption isotherms are in linear tendency, which is the evidence of unsaturated adsorption. As shown in Figs. 5a and 6a, on the positive charged surface of cement, anionic SDS has the highest adsorption ratio, and the
cationic DTAB and nonionic AEO have little adsorption. On the negative charged surface of montmorillonite and granite powders as shown in Figs. 5b, c and 6b, c, cationic DTAB has the highest adsorption ratio, while anionic SDS has little adsorption. Nonionic AEO has high adsorption ratio on surface of both montmorillonite and granite powders. On the surface of limestone powders as shown in Fig. 5d and 6d, the anionic SDS can absorb onto the limestone surface more easily because of the electrostatic attraction. On the other hand, anionic SDS has stronger adsorption on surface of cement than that on surface of montmorillonite, granite and limestone. So when montmorillonite, granite and limestone are
Fig. 6. The adsorption amount of surfactant onto the surface of inorganic materials: (a) cement; (b) montmorillonite; (c) granite powder; (d) limestone powder.
Table 4 The adsorption amount and adsorption ratio of the surfactants on the surface of montmorillonite, granite and limestone in pure water and cement filtrate (the concentration of surfactant is 2 mM). Surfactants
Powder
Adsorption Amount (mg/g)
Adsorption Ratio (%)
In pure water
In cement filtrate
In pure water
In cement filtrate
SDS DTAB AEO
Montmorillonite
0.06 5.12 8.76
0.08 4.98 8.70
8.2 92.5 81.7
10.9 90.0 81.1
SDS DTAB AEO
Granite
0.31 1.12 1.78
0.33 1.09 1.79
31.2 90.8 91.6
33.2 88.4 92.1
SDS DTAB AEO
Limestone
0.43 0.14 0.59
0.51 0.12 0.57
45.2 10.5 26.7
53.6 9.0 26.2
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on surface of cement. So when montmorillonite and granite powders are added into concrete, the total adsorption of cationic DTAB and nonionic AEO will be increased obviously. The obvious increased adsorption of surfactants may induce the great decrease of the concentration of surfactants in solution, which can depress the performance of air-entraining. The adsorption behavior of three surfactants at concentration of 2 mM on the surface of montmorillonite, granite and limestone is repeated in cement filtrate instead of pure water. The results are shown in Table 4. Compared to the adsorption behavior in pure water, in condition of cement filtrate, the adsorption amount and adsorption ratio of anionic SDS on all there powders are slightly increased, the adsorption amount and adsorption ratio of cationic DTAB on all there powders are slightly decreased, and the adsorption amount and adsorption ratio of nonionic AEO on all there powders are almost the same. 3.4. The failure of air-entraining because of the calcium and the adsorption In order to verify the effect of adsorption and salts on the performance of air-entraining, the air-entraining tests in cement mortar in different conditions with different surfactants are carried out. The results are given in Fig. 7. When using anionic SDS as air-entraining agent, the air content of cement mortar is 15.1%. By adding NaCl, limestone powders, granite powders and montmorillonite, the air contents have no obvious changes (Fig. 7a). By adding CaCl2, the air content of cement mortar drops by 17.2%. In Sections 3.1 and 3.2, it is known that the calcium depresses the surface activity of SDS solution and makes the bubbles unstable. So calcium is actually the key factor of the failure of anionic surfactants for air-entraining. It is quite different for cationic and nonionic surfactants as shown in Fig. 7a and b. By adding NaCl and CaCl2, the air contents have no obvious changes. But when adding montmorillonite, granite powders and limestone powders, the air contents decrease obviously. The cationic DTAB and nonionic AEO have much more adsorption on surface of montmorillonite, granite powders and limestone powders than that on surface of cement. So when montmorillonite, granite powders or limestone powders are added into cement mortar, the total adsorption of cationic DTAB and nonionic AEO will be increased obviously. The obvious increased adsorption of surfactants may induce the great decrease of the concentration of surfactants in solution, which can depress the performance of air-entraining. Among the three inorganic powders, montmorillonite and granite powders can strongly absorb cationic and nonionic surfactants and greatly depress the performance of air-entraining. So for cationic and nonionic surfactants, the adsorption is the most important reason of the failure of anionic surfactants for air-entraining. 4. Conclusions
Fig. 7. The air-entraining test in cement mortar in different conditions with different surfactants: (a) SDS; (b) DTAB; (c) AEO (LP: limestone powders; GP: granite powders; MMT: montmorillonite).
added into concrete, the total adsorption of anionic SDS will not be increased. But cationic DTAB and nonionic AEO have much stronger adsorption on surface of montmorillonite and granite than that
In this paper, the effect of salts in solution and the adsorption of surfactant are studied scientifically to give the answer why the air entraining agents fail. The high valence cations (e.g. Ca2+) weaken the surface activity of anionic surfactant solution but have no effect on the surface activity of cationic and nonionic surfactant solution. The calcium induces the decreased surface activity of SDS solution, the low foamability and the big bubble size, which depress the airentraining performance of SDS in cement or concrete. Anionic SDS has stronger adsorption on surface of cement than that on surface of montmorillonite, granite and limestone. When montmorillonite, granite and limestone are added into concrete, the air-entraining performance of anionic SDS is not be depressed. But cationic DTAB and nonionic AEO have much stronger adsorption on surface of
M. Qiao et al. / Construction and Building Materials 242 (2020) 118188
montmorillonite and granite than that on surface of cement. When montmorillonite and granite powders are added into concrete, the air-entraining performance of cationic DTAB and nonionic AEO are obviously depressed. Based on the above research results, some strategies for optimize the application of surfactants as air-entraining agent can be concluded. When the high valence metal cations (e.g. Ca2+, Fe3+ and Al3+) are at very high concentration, it is hard for anionic surfactant to get good performance of air-entraining by itself. Cationic and nonionic surfactants can be used as supplement to solve the problem. When montmorillonite and granite powders are introduced into concrete in spite of deliberately adding or unconsciously introduction, it is better to stop to use any cationic and nonionic surfactants as air-entraining agent, and the anionic surfactant is the best choice for air-entraining agent. CRediT authorship contribution statement Min Qiao: Writing - original draft. Guangcheng Shan: Investigation. Jian Chen: Investigation. Shishan Wu: Project administration, Supervision. Nanxiao Gao: Investigation. Qianping Ran: Funding acquisition. Jiaping Liu: Methodology, Supervision. 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. Acknowledgements 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) and Technology Research and Development Project of China Railway Corporation (N2018G030). References [1] S. Chatterji, Freezing of air-entrained cement-based materials and specific actions of air-entraining agents, Cem. Concr. Compos. 25 (2003) 759–765. [2] S. Riyazi, J.T. Kevern, M. Mulheron, Improving the stability of entrained air in self-compacting concrete by optimizing the mix viscosity and air entraining agent dosage, Constr. Build. Mater. 148 (2017) 531–537. [3] J. Plank, E. Sakai, C.W. Miao, C. Yu, J.X. Hong, Chemical admixtures – chemistry, applications and their impact on concrete microstructure and durability, Cem. Concr. Res. 78 (2015) 81–99. [4] F. Matalkah, P. Soroushian, thaw and deicer salt scaling resistance of concrete prepared with alkali aluminosilicate cement, Constr. Build. Mater. 163 (2018) 200–213.
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