Improving the chloride binding capacity of cement paste by adding nano-Al2O3: The cases of blended cement pastes

Construction and Building Materials 232 (2020) 117219

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Improving the chloride binding capacity of cement paste by adding nano-Al2O3: The cases of blended cement pastes Zhiqiang Yang a,b, Shiyu Sui a,b, Liguo Wang a,b, Taotao Feng a,b, Yun Gao a,b, Song Mu c, Luping Tang d, Jinyang Jiang a,b,⇑ a

School of Materials Science and Engineering, Southeast University, Nanjing 211189, China Jiangsu Key Laboratory of Construction Materials, Southeast University, Nanjing 211189, China Jiangsu SOBUTE New Materials Co., Ltd., Nanjing 211103, China d Department of Architecture and Civil Engineering, Chalmers University of Technology, Gothenburg 412 96, Sweden b c

h i g h l i g h t s
NA can be used as reactive alumina resource in cement paste.
Influence of NA on the chloride binding capacity of blended cement pastes were investigated.
NA is beneficial for improving the chloride binding capacity of cement paste blended with SCMs.

a r t i c l e

i n f o

Article history: Received 18 March 2019 Received in revised form 6 October 2019 Accepted 10 October 2019

Keywords: Chloride binding capacity Nano-Al2O3 Friedel’s salt C-A-S-H SCMs

a b s t r a c t Chloride ingress is one of the main causes for the degradation of reinforced concrete structures. Increasing the chloride binding capacity of concrete is generally thought as a feasible way to restrain the chloride ingress. In our previous study, the c-phase nano-Al2O3 (NA) was found to be beneficial for improving the chloride binding of plain Portland cement paste as a result of the formation of additional Friedel’s salt. Herewith, the cases of blended cement pastes were further investigated, into which supplementary cementitious materials (SCMs) were incorporated, including fly ash (FA), blast furnace slag (SL) and silica fume (SF). NA with a dosage of 1% and 2% was introduced to blended cement paste, and the chloride binding capacity of which were determined with the conventional equilibrium method. The results showed that the use of NA was even viable to improve the chloride binding capacity of blended cement pastes. X-ray diffraction (XRD)/Rietveld refinement method and thermogravimetric analysis (TGA) were performed to unravel the phase assemblages change upon exposure. It was revealed that besides the formation of more Friedel’s salt, the addition of NA could allow the enhanced physical binding of chloride as a result of the formation of C-A-S-H, i.e., the substitution of Si by Al in C-S-H gel. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Chloride-induced steel corrosion is one of the common deterioration mechanisms for concrete structures service in marine environment or exposed to de-icing salts. Such issue has thus been discussed a lot during past decades [1–4]. When the chloride concentration reaches a critical level, the depassivation of steel reinforcement might be induced which further causes a fast and serve corrosion. Consequently, the protective layer of steel reinforcement is destroyed and concrete structures lose their load carrying capacity soon. ⇑ Corresponding author. E-mail address: [email protected] (J. Jiang). https://doi.org/10.1016/j.conbuildmat.2019.117219 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

As concrete is a porous composite material, the ingress of chloride is mainly controlled by two factors when exposed to external chlorides, i.e., the microstructure and solid phase composition. Since chloride diffuses through the pore solution into the inner areas of concrete, extensive researches on refinement of the pore structure so as to restrain the transport of chloride have been published [5–7]. Moreover, except for free chlorides that lead to the corrosion of steel reinforcement, some chlorides can also be captured by the solid phases of concrete, which are classified as bound chlorides [8–11]. Despite the fact that free chloride is exclusively responsible for the steel depassivation, the amount of free chlorides and transport rate of free chlorides were both demonstrated to be decreased with the chloride binding process [12]. Therefore,

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Z. Yang et al. / Construction and Building Materials 232 (2020) 117219

it is significant to improve the chloride binding capacity, and further promote the chloride resistance of concrete. It has been reported that alumina-rich supplementary cementitious materials (SCMs) are beneficial for chloride binding of hardened cement pastes because of the formation of extra Friedel’s salt (3CaOAl2O3CaCl210H2O) and Kuzel’s salt (3CaOAl2O31/2CaSO41/2CaCl211H2O) [13]. For instance, the addition of meta-kaolin, CaO2Al2O3 (CA2) and blast furnace slag (SL) all contribute to a higher amount of bound chlorides [14–16]. Recently, there has been an increasing interest in using nano materials in cement-based materials to achieve higher mechanical properties and durability performances [17]. As reactive alumina source is thought to be helpful for chloride binding process, the addition of nano-Al2O3 (NA) seems to be effective for improving the chloride binding capacity of cement-based materials. In our previous study, the influence of NA on the chloride binding capacity of pure Portland cement paste was investigated [18]. It was found that proper dosage of NA is beneficial for chloride binding, which is believed to be mainly caused by the formation of Friedel’s salt. The present study is designed to bring the understanding of the improvement of NA on chloride binding capacity of cement paste a step further compared with previous study by adding NA into cement paste blended with SCMs including fly ash (FA), SL and silica fume (SF) as they are frequently used in concrete for a better mechanical and durability performances. Likewise, the bound chloride contents of blended cement pastes with the addition of 1% and 2% NA were measured through the equilibrium method. The reaction between NA and pure tricalcium silicate (C3S) clinker, and the reaction between NA and chloride were firstly studied to explore the chemical activity of NA in cement system. In addition, the phase composition of paste was obtained by X-ray diffraction (XRD)/Rietveld refinement method and thermogravimetric analysis (TGA). The contribution of Friedel’s salt and C-(A)-S-H to chloride binding of blended cement pastes were evaluated independently. Based on these investigations, the impact of NA on chloride binding capacity of blended cement pastes and the mechanisms were discussed.

Table 2 Mineral composition of cement (wt%). C3S

C2S

C3A

C4AF

C$

65.8

11.3

4.7

10.7

1.2

Fig. 1. XRD patterns of SCMs.

2. Materials 2.1. Cement and SCMs Portland cement TypeⅡ complying to the Chinese standard GB 175–2007 was used in this study. Class-Ⅱ fly ash (FA), grade-S95 blast furnace slag (SL) and silica fume (SF) were commercial products. The chemical composition of cement and SCMs are listed in Table 1, which were determined by X-ray fluorescence (XRF). The mineral composition of the cement was calculated by XRD/Rietveld refinement method and the result is shown in Table 2. The XRD patterns of SCMs are presented in Fig. 1.

Fig. 2. XRD pattern of C3S.

75 lm. Gypsum (C$) and calcium hydroxide (CH) were purchased and used as received.

2.2. C3S 2.3. Nano Al2O3 C3S was synthesized by high-temperature calcination method according to [19] and the XRD pattern of prepared C3S is shown in Fig. 2. Before being used, it was crushed and then sieved at

The physical properties of Nano Al2O3 (NA) are provided in Table 3. The XRD pattern of NA is shown in Fig. 3. The five

Table 1 Chemical composition of the cementitious materials used (wt%).

Cement (C) FA SL SF

CaO

SiO2

Fe2O3

Al2O3

K2O

Na2O

MgO

SO3

TiO2

61.73 10.2 42.9 0.62

19.97 48.6 31.5 97.2

2.82 4.46 1.11 –

4.52 30.6 13.4 0.41

0.58 1.08 0.54 –

0.12 0.36 0.30 –

1.98 0.75 6.32 0.19

2.95 1.30 2.12 0.29

0.27 1.35 0.66 –

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Z. Yang et al. / Construction and Building Materials 232 (2020) 117219 Table 4 The mix proportions of 4 groups of pastes blended with C3S, NA and C$.

Table 3 Physical properties of NA. Average diameter (nm)

Specific surface value (m2/g)

Density (g/ cm3)

Purity (%)

10

>180

3.9

99.9

1 2 3 4

C3S (g)

NA (g)

C$ (g)

H2O (g)

4.00 4.00 4.00 4.00

0 0 0.4 0.4

0 0.4 0 0.4

1.20 1.32 1.32 1.44

Table 5 The mix proportions of 2 groups of mixture prepared with NA, CH and C$. NA (g)

1 2 *

1.0 1.0

CH (g)

1.5 1.5

C$ (g)

6 0

Mixing water Deionized water (g)

NaCl solution* (g)

15 0

0 15

0.5 mol/L NaCl solution.

Table 6 Mix proportions of blended cement pastes.

Fig. 3. XRD pattern of NA.

diffraction peaks (2h = 37.59°, 39.88°, 46.06°, 61.01° and 67.14°) identify that c Al2O3 was used in this experiment. 3. Experiments

Binder system

Cement (g)

FA (g)

SL (g)

SF (g)

NA (g)

Water (g)

PCA (g)

C C1NA C2NA CFA CFA1NA CFA2NA CSL CSL1NA CSL2NA CSF CSF1NA CSF2NA

600 594 588 480 474 468 420 414 408 540 534 528

0 0 0 120 120 120 0 0 0 0 0 0

0 0 0 0 0 0 180 180 180 0 0 0

0 0 0 0 0 0 0 0 0 60 60 60

0 6 12 0 6 12 0 6 12 0 6 12

240.00 239.04 238.08 240.00 239.04 238.08 240.00 239.04 238.08 239.20 238.24 237.28

0 1.2 2.4 0 1.2 2.4 0 1.2 2.4 1.0 2.2 3.4

3.1. Chemical reactivity test of NA In order to keep clear of the alumina source from cement as C3A is one of the main mineral phases of cement clinker, here, pure C3S and C$ were chosen to mix with NA and the hydration products were tested by XRD. Table 4 shows the mix proportions of the 4 groups of pastes. The water cement ratio (w/c) of the four mixtures were fixed at 0.3. The sample was put in a bottle and mixed at high-speed for 2 min. After curing for 3 days, the hydration products were ground and prepared for XRD analysis. When the cement paste incorporating NA is exposed to chloride ions, NA may not react with the hydration products entirely. Hence, in order to investigate the reaction between NA, CH and C $, and the reaction between the NA, CH and chloride, another 2 groups of mixtures were performed, as shown in Table 5. The first mixture was prepared with deionized water, while the second mixture was prepared with a 0.5 mol/L NaCl solution. The mixtures were blurred for 5 min and cured for 1 day. After that, the solids were collected, dried and ground, prepared for XRD analysis. 3.2. Immersion test Table 6 shows the mixture proportion of the blended cement pastes in this study. 12 groups of cement pastes were prepared with a w/c of 0.4. The replacement of cement with FA, SL and SF were 20%, 30% and 10% by weight, respectively. For pure and blended cement pastes, NA with a dosage of 1% and 2% were introduced. Because of the high specific surface area of NA, the introduction of NA would reduce the workability of paste. Therefore, polycarboxylate water-reducing admixture (PCA) was added into the cement sample, and the amount of PCA was proportional to the mass of NA to keep an appropriate workability.

First of all, NA and superplasticizer were added to deionized water, stirred and dispersed by ultrasonic waves for 10 min to obtain a uniform suspension. Later, the suspension was mixed with binder and stirred at high-speed for 5 min. Next, mixtures were cast in a PVC tube, and both top and end of the tube were covered with plastic sheet. The samples were left in a standard curing room (20 °C and RH > 95%) for 1 day before demolding, and the demolded samples were further cured in saturated lime solution for 2 months (at 20 °C). After the curing period, the central portion of the hardened cement pastes were crushed into particles with a diameter of 0.3–3 mm, then the particles were vacuum dried for 3 days prior to tests. Equilibrium approach was used for the determination of the chloride binding capacity in this experiment [20,21]. Detailed to our case, 20 g of hydrate binder was placed in a plastic bottle and then filled with 60 ml NaCl solution with 5 different initial chloride concentrations (0.05, 0.1, 0.3, 0.5 and 1 mol/L). Next, the bottles were sealed and stored at 20 °C environment. In order to ensure the equilibrium was reached between the pore solution of the paste and the host NaCl solution, the samples were shaken regularly and stored for 8 weeks [22]. After that, the chloride of host NaCl solution were analyzed by Ion Chromatography (IC).

3.3. Characterization of hydrated cement pastes Upon the completion of equilibrium test, the solid was taken out of the chloride solution immediately and dried with a desiccator under vacuum for 3 days. Some tests were followed using the dry samples.

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3.3.1. XRD analysis XRD was applied to identify the crystalline phases in cement pastes before and after immersion in NaCl solution. Moreover, Rietveld refinement method was performed for quantitative analysis [23–25]. The sample was first sieved at 75 lm. Then, TiO2 was chosen as the internal standard to quantify the total amount of amorphous and crystalline phases, i.e., 0.12 g TiO2 and 1.08 g dried cement powder was mixed with appropriate isopropanol. After that, the mixture was dried again in 40 °C vacuum oven for another day. The mineral composition was analyzed by XRD on a Bruker 08 Advance, with 2h varies from 5° to 70°. The actual contents of crystalline phases were calculated with TOPAS 4.0 software. As C-(A)-S-H is amorphous phase, this paper deems all other phases as C-(A)-S-H except crystalline phases, thus the content of C-(A)-S-H can be calculated. 3.3.2. Thermogravimetric analysis (TGA) The samples used for TGA were identical with those used for XRD analysis. A STD Q600 instrument was employed in this paper. The samples were loaded in a 150 lm alumina crucible and heated up to 1000 °C from room temperature at a rate of 10 °C/min. Due to the reason that DTG curve can show the inflection points and plateaus, while both of which are absent in the TG curve, DTG curve can offer a better understanding on the different reactions during the heating process. In detailed, Friedel’s salt can be identified in the temperature ranges 100–150 °C and 230–410 °C, which are associated with the release of 4 interlayer water molecules and the release of 6 main layer water molecules, respectively [26,27]. 4. Results 4.1. The chemical reactivity of NA Fig. 4 presents the XRD patterns of hydration products of mixtures made of C3S, NA and C$. It can be seen that no AFt diffraction peak was found when C$ was added to C3S alone. However, when NA and C$ were introduced to C3S simultaneously, Eq. (1) occurred and the diffraction peak of AFt was traced, which indicates that NA reacted with C3S and C$. Therefore, it can be concluded that NA can be used as reactive alumina source in cement paste.

Fig. 4. XRD patterns of hydration products of the 4 groups of pastes blended with C3S, NA and C$.

C3 S þ H2 O þ Al2 O3 þ CaSO4 ! C  S  H þ CaðOHÞ2 þ AFt=AFm

ð1Þ

Fig. 5 shows the XRD patterns of hydration products of mixtures prepared with NA, CH, C$ and deionized water, and prepared with NA, CH and NaCl solution. When NA, CH and C$ were mixed with deionized water, AFt and AFm was formed with the reaction expressed in Eq. (2). However, when NA and CH were mixed with a 0.5 mol/L NaCl solution, the diffraction peak of Friedel’s salt could be detected from the XRD pattern, indicating that reaction (see Eq. (3)) occurred. It implies that when NA is added into cement paste, not only AFt/AFm phase can form with the reaction between NA and hydration products, but also Friedel’s salt can form when the cement paste is exposed to external chloride ions with the reaction between NA, CH and chloride.

Al2 O3 þ CaðOHÞ2 þ H2 O þ CaSO4 ! AFt=AFm

ð2Þ

Al2 O3 þ CaðOHÞ2 þ H2 O þ CaCl2 ! C3 A  CaCl2  10H2 O þ CaOH

ð3Þ

4.2. Chloride binding isotherms According to the mechanism of the equilibrium method, the reduction of chloride concentration after immersion test results from the chloride binding process. Accordingly, the amount of bound chloride can be calculated through the following formula:

C b ¼ 35:453  V  ð C i  C f Þ = W

ð4Þ

wherein, Cb: the amount of bound chloride ions, mg/g of the dry specimen; V: the volume of host NaCl solution, mL; Ci: the initial chloride concentration, mol/L; Cf: the final chloride concentration of the suspension, mol/L; W: the mass of the dry cement paste specimen, g. The adsorption isotherm can be drawn with the data of Cb and Cf through the tests with different initial chloride concentrations. Moreover, three parallel tests were carried out for each sample and the average value was presented as the final result. Generally, bound chlorides were fitted by Langmuir isotherm and Freundlich isotherm that were expressed as follows [28]:

Langmuir isotherm : C b ¼ a  C f = ð1 þ b  C f Þ

ð5Þ

Fig. 5. XRD patterns of hydration products of mixtures prepared with NA, CH, C$ and deionized water, and prepared with NA, CH and NaCl solution.

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Freundlich isotherm : C b ¼ a0  C bf

0

ð6Þ

where a b, a’ and b’ in the formulas are binding capacity constants. It has been reported that Langmuir isotherm predicted better the relationship between Cb and Cf for lower free chloride concentration (<0.05 mol/L), whereas Freundlich isotherm predicted better for higher free chloride concentration (>0.01 mol/L) [20,29]. Table 7 presents the binding constants (a, b, a0 and b0 ) and coefficient of determination (R2) for 12 samples subjected to Langmuir isotherms and Freundlich isotherms. Although Langmuir isotherm provides a reasonable fit to the results of certain samples, the curves can be more satisfyingly fitted through Freundlich isotherm than through Langmuir isotherm on the whole (higher R2), which is in accordance with our previous studies [16,22,28]. Specifically, Fig. 6 performs the Freundlich isotherms for NA-free pastes and samples incorporating with 2% NA. It is seen that the addition of NA samples presents little influence on type of the chloride binding isotherm comparing with NA-free samples. 4.3. Effect of NA on bound chlorides

Fig. 6a. Freundlich isotherms for blended cement pastes. (a):NA-free samples.

The bound chlorides of all pastes exposed to NaCl solutions are illustrated in Fig. 7. In general, paste with NA shows a higher chloride binding, compared with NA-free paste when tested under the same conditions. Moreover, the increased binding capacity of paste containing NA is considerable for both pure and blended cement pastes. For instance, the bound chlorides increased by 37% with 30% SL added, furthermore, it increased by 68% when 2% NA and 30% SL were added into cement paste together. These results suggest that the introduction of NA is beneficial for chloride binding of blended cement paste. The effect of used SCMs on chloride binding capacity can also be reflected from Fig. 7. It is suggested that the physical adsorption of chloride is closely related to the molar ratio of CaO to SiO2 (C/S), and that a higher C/S indicates more physical binding of chlorides [30]. When FA was added to cement paste, the chloride binding capacity decreased compared with pure cement paste, which is controversial to most researches [31,32]. While for paste specimens blended with SF or SL, the consistent pattern on bound chloride is in consistency with literatures [13,16,22,33,34]. Fig. 6b. Freundlich isotherms for blended cement pastes. (b): samples with 2% NA.

4.4. Effect of binder Al2O3 content on bound chlorides

more (bigger slopes of fitted lines) at higher chloride concentration than lower chloride concentration, which suggesting that the amount of alumina holds a more important role in controlling the amount of bound chloride at higher chloride concentration.

The total binder Al2O3 content is calculated through Eq. (7), and the influence of binder Al2O3 content on bound chlorides of pastes are shown in Fig. 8. Even though only 3 binder Al2O3 contents were designed for each type of paste in this experiment, the bound chlorides still show a linear increase in proportion to the increase of the binder Al2O3 content. In addition, the bound chlorides increase

W At ¼

4 X

W i  W Ai

ð7Þ

1

Table 7 Best fit parameters for Langmuir and Freundlich isotherms of cement pastes. Binder system

C CFA CSL CSF C1NA CFA1NA CSL1NA CSF1NA C2NA CFA2NA CSL2NA CSF2NA

Langmuir isotherm

Freundlich isotherm

a

b

R2

a’

b’

R2

63.08 50.49 83.81 66.04 77.34 72.44 97.59 62.25 84.53 70.10 87.74 61.29

9.01 8.11 9.30 11.81 10.69 10.36 9.61 10.46 9.56 8.84 7.88 9.01

0.94068 0.73881 0.96018 0.97029 0.95315 0.94268 0.98896 0.95092 0.97117 0.94895 0.96297 0.99423

6.78 6.04 8.70 5.52 7.10 6.82 9.81 5.81 8.58 7.61 10.52 6.53

0.32 0.34 0.30 0.27 0.29 0.29 0.29 0.29 0.30 0.31 0.33 0.31

0.99218 0.92530 0.96457 0.94952 0.99069 0.95348 0.95275 0.93763 0.97883 0.95089 0.93905 0.95109

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Z. Yang et al. / Construction and Building Materials 232 (2020) 117219

Fig. 7a. Influence of NA on bound chlorides of pastes among 5 NaCl solutions. (a):0.05 mol/L.

Fig. 7b. Influence of NA on bound chlorides of pastes among 5 NaCl solutions. (b):0.10 mol/L.

Fig. 7c. Influence of NA on bound chlorides of pastes among 5 NaCl solutions. (c):0.30 mol/L.

Fig. 7d. Influence of NA on bound chlorides of pastes among 5 NaCl solutions. (d):0.50 mol/L.

Fig. 7e. Influence of NA on bound chlorides of pastes among 5 NaCl solutions. (e):1.00 mol/L.

Fig. 8a. The influence of binder Al2O3 content on bound chloride. (a): pure cement paste.

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Z. Yang et al. / Construction and Building Materials 232 (2020) 117219

wherein, W: the total binder Al2O3 content, %; Wi: the weight percentage of cement or SCMs in binder, %; W: the Al2O3 content of cement or SCMs, %. 5. Discussions 5.1. Chloride bound as Friedel’s salt

Fig. 8b. The influence of binder Al2O3 content on bound chloride. (b): cement-FA paste.

Since indicated in the papers of Balonis [35], Kuzel’s salt was formed at a quite low chloride concentration. However, the chloride concentration in this study is a little bit high and the quantity of Kuzel’s salt is quite low. So, the contribution of bound chlorides in Kuzel’s salt to total bound chlorides content is just ignored. Fig. 9 illustrates the phase composition of pastes before immersion in NaCl solution. As shown in the figure, the content of amorphous phase for pure cement paste is lower than that for paste blended with SCMs, which is in line with other researches [16,31]. Specially, the amount of CH for blended cement pastes are shown in Table 8. It is clearly that the addition of NA lowers the amount of CH for pure cement paste as well as samples blended with FA or SL. Nima Farzadnia [36] and A. Nazari [37] also reported the reduction in CH quantity when NA was added to cement paste. They proposed that NA can act as seed at early ages, by which C-(A)-S-H is uniformly distributed and the growth of CH crystal is restricted. Afterwards, the poor crystalline formation of CH leads its easier consumption by FA and SL at later ages. Meanwhile, as shown in Eq. (2), CH is consumed due to the reaction between NA, C$ and CH when NA is added into cement paste. However, as for paste blended with SF, the content of CH shows a slight increase with the addition of NA. As is well known, the particle size of SF varies over three orders of magnitude from a few tens of nm to a few tens of lm, which is closer to the size of NA comparing with FA, SL and cement. Thus, the SF in cement paste may be enveloped by NA particles, leading to a lower hydration of SF. As a result, the quantity of CH increases with the addition of NA in cement paste that blended with SF. Taking 0.5 mol/L NaCl solution as an example, the phase composition of pastes after exposed to chloride were analyzed by QXRD method, and the results were shown in Fig. 10. Compared with NA-free samples, it is noticed that samples incorporating

Fig. 8c. The influence of binder Al2O3 content on bound chloride. (c): cement-SL paste.

Fig. 9. The QXRD analysis results of pastes before immersion in NaCl solution.

Table 8 The amount of CH in blended cement paste calculated by QXRD analysis (wt%).

Fig. 8d. The influence of binder Al2O3 content on bound chloride. (d): cement-SF paste.

C 17.09

C1NA 16.93

C2NA 14.90

CFA 11.22

CFA1NA 11.09

CFA2NA 10.21

CSG 8.92

CSG1NA 8.36

CSG2NA 7.59

CSF 8.92

CSF1NA 9.56

CSF2NA 9.60

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Z. Yang et al. / Construction and Building Materials 232 (2020) 117219

C3A from cement clinker, hence, with 10% SF replacement in cement paste, the content of Friedel’s salt decrease slightly comparing with pure cement paste. DTG curves for cement pastes after immersion in 0.05 mol/L NaCl solution are shown in Fig. 12. Three major mass losses are observed for the samples, i.e, AFt (60–80 °C), portlandite (410– 520 °C) and Friedel’s salt (120–150 °C, 300–350 °C). It is evident that Friedel’s salt content improves with the increasing content of NA in cement paste, which indicates that NA is helpful for the chemical binding of chloride. In addition, the reduction in the quantity of portlandite with the addition of NA is expected to be caused by the reaction between NA, portlandite and chloride, by which the content of Friedel’s salt increases. All these results are in good agreement with analysis above based on the XRD results. Fig. 10. The QXRD analysis results of pastes after immersion in a 0.5 mol/L NaCl solution.

NA shows a higher content of Friedel’s salt. Besides, Friedel’s slat shows a higher content in cement-FA paste and cement-SL paste than pure cement paste, while a slight decrease of Friedel’s salt is observed in cement-SF paste. Two main changes may occur by partial substitution of cement with SCMs: (1) a dilution effect of C3A content; (2) an increase of C-(A)-S-H content with a lower C/S ratio. It is known that C3A can react with chloride and form Friedel’s salt. Thus, the dilution of C3A content of cement paste results in a lower amount of Friedel’s salt. The influence of C-(A)-S-H phase on chloride binding will be discussed in Section 5.2. Fig. 11 shows the relationship between Friedel’s salt and binder Al2O3 content. It is observed that the formation of Friedel’s salt generally increases with the binder Al2O3 content, which indicates that the reactive alumina in SCMs contributes to the formation of Friedel’s salt. More details are given as follows. In cement-FA paste, although some alumina in FA is present in crystalline component such as mullite, dissolved alumina, as a result of pozzolanic reaction, can form AFm structure ([Ca2Al(OH)62H2O]+), and further react with chloride to produce Friedel’s salt [38]. Therefore, the content of Friedel’s slat in cement-FA paste is higher than binder pure cement paste in this study. SL, which comprises 13.4% Al2O3, most of which is assumed to be ‘‘available” to create calcium chloro-aluminates, thus more Friedel’s salt forms with the addition of SL when paste is exposed to chlorides. As SF contains a negligible amount of alumina, the main alumina source of cement-SF paste is

Fig. 11. The relationship between Friedel’s salt and binder Al2O3 content.

5.2. Chloride adsorbed by C-(A)-S-H According to the Electrical Double Layer (EDL) theory, chloride can easily reach the interlayer of amorphous phase (C-S-H) and then replace OH to maintain the charge balance or electroneutrality of the system [39]. In fact, rather than being pure phase,

Fig. 12a. DTG curves of cement pastes exposed to 0.5 mol/L NaCl solution. (a): pure cement paste. Ettr.: ettrigite; Fs: Friedel’s salt; CH: portlandite.

Fig. 12b. DTG curves of cement pastes exposed to 0.5 mol/L NaCl solution. (b): cement-FA paste. Ettr.: ettrigite; Fs: Friedel’s salt; CH: portlandite.

Z. Yang et al. / Construction and Building Materials 232 (2020) 117219

9

Fig. 14. Schematic adsorptive models of chlorides on the surface of C-S-H (a) and CA-S-H (b).

Fig. 12c. DTG curves of cement pastes exposed to 0.5 mol/L NaCl solution. (c): cement-SL paste. Ettr.: ettrigite; Fs: Friedel’s salt; CH: portlandite.

Fig. 12d. DTG curves of cement pastes exposed to 0.5 mol/L NaCl solution. (d): cement-SF paste. Ettr.: ettrigite; Fs: Friedel’s salt; CH: portlandite.

alumina can be incorporated into the structure of C-S-H to produce C-A-S-H, and the content of C-A-S-H increases if an alumina-rich SCM is added [40]. Fig. 13 presents the chlorides adsorbed by C(A)-S-H after immersion in a 0.5 mol/L NaCl solution. These data are gained by subtracting the chlorides bound as Friedel’s salt (QXRD result) from total bound chlorides. From Fig. 13 one can find that the cement-SL pastes exhibit a higher content of the physical binding of chloride than pure cement, cement-FA as well as cement-SF pastes. Partial substitution of cement with SL mainly leads to three effects on amorphous phase: (1) a lower C/S ratio; (2) the formation of C-A-S-H; (3) a higher amorphous phase. It was suggested that a lower C/S ratio of amorphous phase decreases the physical binding of chloride, whereas a higher C-A-S-H content is beneficial for it. When NA is introduced to cement paste, high specific surface of NA accelerates the hydration process and results in the formation of clusters of C-A-S-H gel [41]. Based on the experimental results, it is proposed that the substitution of Si by Al in C-S-H leads to a higher content of the physical adsorption of chloride, which can be explained by the EDL theory. According to the studies of G. Plusquellec [42] and D.A. Kilcoyne [43], the residual charge of C-S-H can be negative, neutral or positive that based on the C/S. An increase in C/S indicates that excessive calcium within the interlayer structures can shift the overall charge balance of C-S-H from negative to positive. Beyond a certain C/S, overall charge of C-S-H becomes positive and thus chloride is adsorbed to compensate for the net positive charge caused by excessive calcium when C-S-H is exposed to chlorides environment, as shown in Fig. 14(a). The replacement of SiO2 by AlO 2 leads to additional sorption of calcium onto the interlayer structures of negatively charged C-A-S-H, which results in a more positively charged C-A-S-H. As shown in Fig. 14(b), when C-A-S-H is exposed to chloride ions, this positively charged surface is compensated by chlorides in the diffuse layer of the C-A-S-H. Thus, C-A-S-H shows a higher physical binding capacity of chloride than C-S-H. 6. Conclusions In this paper, the effects of NA on chloride binding of blended cement pastes made of Portland cement with fly ash, blast furnace slag and silica fume were studied by an equilibrium method. The phase composition of cement pastes before and after immersion in NaCl solution were analyzed by XRD and TG analysis. From the results the following conclusions can be drawn:

Fig. 13. Chlorides bound by C-(A)-S-H in cement paste after immersion in a 0.5 mol/L NaCl solution.

(1) NA can be used as reactive alumina source in cement paste, since NA can transfer to Friedel’s salt when cement paste containing NA is exposed to chloride environment with the reaction between NA, CH and chloride.

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(2) Regardless of the addition of NA in blended cement pastes, Freundlich isotherm shows a better description for the relationship between bound chloride and free chloride than the Langmuir isotherm on the whole. (3) The introduction of NA helped in improving the chloride binding capacity of cement paste, i.e., compared with NAfree paste, bound chlorides increase by 35% with 2% NA in. The introduction of NA leads to a higher amount of Friedel’s salt, which is proven by the QXRD and TGA results. (4) More chlorides would be physically adsorbed when Si in the structure of C-S-H gel is substituted by Al with the addition of NA in cement paste, which is supported by the EDL theory.

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 research is sponsored by the National Natural Science Foundation of China (Nos. 51578143, Nos. 51608111) and NSFCSTINT project (Nos. 51711530039). References [1] W. Sun, R. Mu, X. Luo, C. Miao, Effect of chloride salt, freeze-thaw cycling and externally applied load on the performance of the concrete, Cem. Concr. Res. 32 (12) (2002) 1859–1864. [2] D. Chen, S. Mahadevan, Chloride-induced reinforcement corrosion and concrete cracking simulation, Cem. Concr. Compos. 30 (3) (2008) 227–238. [3] Y. Fan, S. Zhang, S. Kawashima, S.P. Shah, Influence of kaolinite clay on the chloride diffusion property of cement-based materials, Cem. Concr. Compos. 45 (2014) 117–124. [4] P. Hou, X. Cheng, J. Qian, R. Zhang, W. Cao, S.P. Shah, Characteristics of surfacetreatment of nano-SiO2 on the transport properties of hardened cement pastes with different water-to-cement ratios, Cem. Concr. Compos. 55 (2015) 26–33. [5] P. Halamickova, R.J. Detwiler, D.P. Bentz, E.J. Garboczi, Water permeability and chloride ion diffusion in portland cement mortars: relationship to sand content and critical pore diameter, Cem. Concr. Res. 25 (4) (1995) 790–802. [6] V.G. Papadakis, Effect of supplementary cementing materials on concrete resistance against carbonation and chloride ingress, Cem. Concr. Res. 30 (2) (2000) 291–299. [7] C.C. Yang, On the relationship between pore structure and chloride diffusivity from accelerated chloride migration test in cement-based materials, Cem. Concr. Res. 36 (7) (2006) 1304–1311. [8] J. Tritthart, Chloride binding in cement II. The influence of the hydroxide concentration in the pore solution of hardened cement paste on chloride binding, Cem. Concr. Res. 19 (5) (1989) 683–691. [9] T.U. Mohammed, H. Hamada, Relationship between free chloride and total chloride contents in concrete, Cem. Concr. Res. 33 (9) (2003) 1487–1490. [10] J. Tritthart, Pore solution of concrete: the equilibrium of bound and free chloride, Mater. Corros. 60 (8) (2009) 579–585. [11] R. Loser, B. Lothenbach, A. Leemann, M. Tuchschmid, Chloride resistance of concrete and its binding capacity-comparison between experimental results and thermodynamic modeling, Cem. Concr. Compos. 32 (1) (2010) 34–42. [12] B. Martı´n-Pérez, H. Zibara, R.D. Hooton, M.D.A. Thomas, A study of the effect of chloride binding on service life predictions, Cem. Concr. Res. 30 (8) (2000) 1215–1223. [13] A. Dousti, J.J. Beaudoin, M. Shekarchi, Chloride binding in hydrated MK, SF and natural zeolite-lime mixtures, Constr. Build. Mater. 154 (2017) 1035–1047. [14] K. Behfarnia, N. Salemi, The effects of nano-silica and nano-alumina on frost resistance of normal concrete, Constr. Build. Mater. 48 (2013) 580–584. [15] Y. Lee, M. Kim, Z. Chen, H. Lee, S. Lim, Chloride-binding capacity of portland cement paste blended with synthesized CA2 (CaO2Al2O3), Adv. Mater. Sci. Eng 2018 (6) (2018).

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