Leakage behavior of toxic substances of naphthalene sulfonate-formaldehyde condensation from cement based materials

Journal of Environmental Management 255 (2020) 109934

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Research article

Leakage behavior of toxic substances of naphthalene sulfonate-formaldehyde condensation from cement based materials Linan Gu a, b, d, e, Haoxin Li a, b, *, Xiaojie Yang a, b, Biqin Dong c, Zhaoyin Wen d, e a

Key Laboratory of Advanced Civil Engineering Materials Ministry of Education, Tongji University, Shanghai, 201804, China School of Material Sciences and Engineering, Tongji University, Shanghai, 201804, China c School of Civil Engineering, Guangdong Province Key Laboratory of Durability for Marine Civil Engineering, The Key Laboratory on Durability of Civil Engineering in Shenzhen, Shenzhen University, Shenzhen, 518060, China d The Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China e University of Chinese Academy of Sciences, Beijing, 100049, China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Naphthalene sulfonate-formaldehyde conden­ sation Leakage ratio Adsorption behavior Adsorption film

Naphthalene sulfonate-formaldehyde condensatation (NSF) is the main component of the naphthalene based water reducers for cement based materials, as well as an organic substance with potential toxicity. However it is still uncertain whether it can leak from the cement based materials. In this work, the leakage ratio and adsorption behavior of NSF from various cement based materials such as the different water/cement (w/c) ratio, NSF content, types of cementitious materials as well as at different hydration time were evaluated. The product components of the cement based materials cured for different times were also quantified to explore the mech­ anisms which are responsible for the leakage and adsorption behaviors. The results indicate that more NSF, lower w/c ratio and less mineral admixture decrease the NSF leakage ratio. The leakage ratio of NSF from cement paste mixed 0.3% NSF is up to 50.8% at 0.5 h, and it decreases to 31.0% at 28 d. The leakage ratio of NSF from cement paste decreases as the hydration time prolongs. The lower leakage ratio corresponds to the higher adsorption capacity. Less adsorption capacity and thinner adsorption film imply that lower temperature and mineral admixture decrease the NSF adsorption behavior. When 0.3% NSF is added into the cement paste, the adsorption amount and NSF layer thickness are 5.53 mg/g and 0.98 nm, 5.87 mg/g and 4.7 nm at 0.5 h and 28 d respec­ tively. The result demonstrates that the adsorption behavior of NSF in cement significantly increases at the initial several hours and gradually stabilizes after the first day. The X-ray powder diffractometer (XRD) results show that the contents of tricalcium silicate (C3S) and dicalcium silicate (C2S) continuously decline and the amorphous phases and ettringite (AFt) increase rapidly in the early stage. NSF adsorption and leakage behaviors are closely related to the hydration process of cement. These results indicate that NSF can definitely leak from the cement based materials and thus the NSF potential environmental pollution cannot be ignored. At least, it should be restricted or cautious to produce the water tower and pipe concrete structure with it. These results will sever as a theoretically reference for the pollution control as well as better application of NSF in cement-based materials.

1. Introduction Along with the emphasis on environmental protection and health, people pay more attention to the leakage behaviors of toxic substances, especially to guarantee the environment quality of the directly contacted with human or drinking water. The leakage of toxic inorganic heavy �n et al., 2019; Purwanti metal ions has caught concern (Diaz-Alarco et al., 2019). According to the Standards for drinking water quality

(GB5749-,2006), the concentration of cadmium, mercury and lead should not exceed 0.01 mol/L, 0.001 mg/L and 0.01 mg/L respectively. Meanwhile, the methods of removal heavy metal ions were studied (H. Li et al., 2014; Z. Li et al., 2014). However, little attention has paid to the organic one as compared to the attraction for inorganic leakage. And the only focus was on pesticides and dyes (Aksu, 2005; Yukseler et al., 2017), but fewer on the organic admixtures in the concrete structures. In general, cement and concrete superplasticizers (Zou et al., 2017; Yang

* Corresponding author. Key Laboratory of Advanced Civil Engineering Materials Ministry of Education, Tongji University, Shanghai, 201804, China. E-mail address: [email protected] (H. Li). https://doi.org/10.1016/j.jenvman.2019.109934 Received 10 October 2019; Received in revised form 24 November 2019; Accepted 25 November 2019 Available online 26 December 2019 0301-4797/© 2019 Elsevier Ltd. All rights reserved.

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Journal of Environmental Management 255 (2020) 109934

et al., 20191)contain organic components. These organic admixtures may leak from the preparing cement based materials or the existing concrete structures that can have a potential interaction with drinking water (Gu� erandel et al., 2011; H. Li et al., 2014; Z. Li et al., 2014). Naphthalene formaldehyde sulfonate (NSF) (Huynh et al., 2006), an early superplasticizer, is widely used in building materials industry for low price and high water reduction ratio (Winnefeld et al., 2007). The estimated world-wide use of NSF as concrete admixture and dye dispersant was about 300, 000, 000 kg/year in 2000 (Crescenzi et al., 2001). Besides, NSF is a registered toxic substance (Chemical Toxicity Database, 1997). 50% of rats can be died of poisoning after oral administration of water with NSF concentration of 3.5 mg/L. There are also some reports that it is harmful to the respiratory system (Kovalchuk et al., 2017). In view of the toxicity of NSF, it is necessary to assess whether it can leak from concrete structures. It is generally physically and chemically stable, and most of them can be adsorbed in cementitious materials such as cement and hydration products by physical or chemical action (Huynh et al., 2006; Zhang and Kong, 2015). However, this absorption capacity is limited. In addition, these adsorbed NSF, have the potential risk of desorption and stripping under changing environmental condi­ tions. If it migrates into water contact with the concrete structures, the survival of animals and plants in the water environment will be affected (Crescenzi et al., 2001). Moreover, if it leaked into drinking water from the concrete structures such as water tower and water pipes, it may directly affect people’s health and life safety. Although the use of NSF is relatively reduces in the key project in China, it is promising in the alkali-activated materials which is a potential alternative to the current cementitious system. In addition, there are a large number of existing concrete structures that have used NSF before. Therefore, it is necessary to study the potential leakage behavior of NSF from cement based ma­ terials in order to reduce the pollution of NSF and prevent the leakage of it. However, it is still uncertain whether NSF can leak from the cement based materials. The present work aims to evaluate the potential leakage behavior of NSF from cement based materials. The leakage behavior and adsorption capacity of NSF in the cement based materials were measured by the ultraviolet visible (UV–vis) spectrophotometry, and the adsorptive film thickness were tested by X-ray photoelectron spectroscopy (XPS). Be­ sides, the hydration products characteristics were investigated by X-ray diffraction (XRD), and the mechanisms of NSF leakage behavior were also discussed in this work.

ultraviolet region (Hsu et al., 1999; Tkaczewska, 2014). Therefore, the wavelength at 296 nm is usually selected to determine the relation be­ tween its content and ultraviolet absorption. U–3310UV–vis spectro­ photometer was used to measure the absorbance of the solution. Different concentrations of NSF were selected to determine the absorbance-concentration curve. The obtained data were analyzed sta­ tistically with three replicates in this section for their significance (p < 0.05). The NSF concentration and the corresponding ultraviolet absor­ bance are linearly fitted to achieve NSF absorbance-concentration curve of NSF solution. The linear regression coefficient (R2), standard error of the slope and intercept are 0.9994, 0.0050 and 0.0006 respectively, and the absorbance-concentration curve is show in Fig. 1. In this work, 0.2, 0.3, 0.4 and 0.5 water-cement (w/c) ratio, 0.1%, 0.2%, and 0.3% (mass, by the cement) NSF, cementitious materials of GGBS and fly ash were selected to evaluate the leakage behavior of NSF from cement based materials. Meanwhile, the leakage behavior of cement paste mixed 0.3% NSF and 0.3 w/c ratio at different hydration times was also assessed. The test temperature was controlled as 20 � 2 � C. The paste was diluted 1000 times before ultraviolet absorption test. To reduce the error caused by uneven sampling, three samples of each diluted paste were taken and recorded. It was ground to the same fine­ ness as cement if it was hardened. The leakage ratio of NSF from cement based materials was accepted if the three values did not exceed 95–105% of the average values. At the end of the test, the mass of NSF in diluted solution mi was converted from the concentration multiplied by the total volume. And the concentration was calculated according to the absorbance-concentration curve shown in Fig. 1. The leakage ratio Li was calculated according to Eq. (1). Li ¼

(1)

where mi is the mass of NSF in diluted solution; m0 is the mass of NSF before leaking; Li is the leakage ratio of NSF. 2.2.2. Adsorption capacity In this work, 1 g cementitious material was weighed and placed in 100 ml NSF solution with concentration of 0.1%, 0.2% and 0.3% respectively. Three samples of a group were taken and recorded. The absorbency of the diluted supernatant was tested by UV–vis spectro­ photometer. The test was carried out at different temperature, compo­ sition of cementitious materials and hydration ages. The adsorption amount was obtained by the concentration difference of NSF in the so­ lution before and after adding cementitious materials. The adsorption amount of NSF by cementitious materials was accepted if the three values did not exceed 95–105% of the average values. The NSF con­ centration was calculated according to the absorbance-concentration curve shown in Fig. 1. The adsorption amount Ai was calculated ac­ cording to Eq. (2)

2. Experimental programs 2.1. Materials The cementitious materials used in this work were P�I 42.5 Portland cement, granulated ground blast furnace slag (GGBS) and fly ash. They were produced and offered by China United cement Co., Ltd and Baotian New Materials Co., Ltd respectively. Their specific surface areas are 348, 402 and 517 m2/kg respectively and their chemical components are given in Table 1. The NSF used in this study was naphthalene based superplasticizer with 92% solid content and it was provided by Chinese Sinopharm Chemical Reagent Co., Ltd.

Ai ¼

ci c0

(2)

where ci is the concentration difference of the NSF in the solution; c0 is the concentration of the NSF in the solution before adsorption; Ai is the adsorption amount of NSF. 2.2.3. Adsorption film thickness In this test, X-ray photoelectron spectroscopy (XPS) was applied to determine the NSF adsorption film thickness. The cement pastes were dried at 50 � C for 24 h and ground into the powder if they were hard­ ened. Aluminum was used as the anode target and the energy resolution was 0.10 eV. The calculation method refers to previous report (Tan et al., 2017), and the thickness of NSF adsorption layer was obtained according to Eq. (3). � � Ib b ¼ 16 � ln (3) I0

2.2. Methods 2.2.1. Leakage ratio of NSF from various cement based materials (1) Determination of NSF absorbance-concentration curve NSF has a well-correlated absorption peak at 296 nm in the 1

mi m0

Yang et al., 2020. 2

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Table 1 Chemical components of cementitious materials (wt, %). Cementitious material

SiO2

CaO

Al2O3

Fe2O3

SO3

MgO

f-CaO

Na2O

TiO2

K2O

LOI

cement GGBS fly ash

21.73 32.2 55.80

64.55 41.5 12.06

4.60 11.7 14.33

3.45 – 4.14

0.46 3.04 0.46

3.56 5.28 –

0.96 – –

0.59 – 2.72

– 0.77 0.72

– 0.29 2.12

0.1 – 0.74

60

0.8 0.7

53.6%

Leakage ratio (%)

0.6

50

0.4

44.6%

45

0.3

y=0.0474+0.00588x R2=0.9994

0.2 20

40

60

C (mg/L)

80

100

50

40 35

120

Fig. 1. NSF absorbance-concentration curve.

60

Leakage ratio (%)

Absorption

0.5

0.1

58.2%

55

38.9%

y =26.01+65.9x R2=0.9849

40 0.2

0.2

0.3 w/c ratio

0.3

0.4

w/c ratio

0.4

0.5

0.5

Fig. 2. Leakage ratio of NSF from cement pastes with different w/c ratios.

(2) Leakage ratio determination

evaluation of leakage ratio was carried out at 1 h after the water was mixed, and there were fewer hydration products (He et al., 2019a, 2019b; Qin and Gao, 2019; Scrivener et al., 2015). Therefore, NSF mainly appears in solution and adsorbed on the surface of cement par­ ticles. Along with the increment of w/c ratio, the same amount of dis­ solved NSF appears in more water. Thus, the amount of NSF adsorbed on the surface of cement particles decreases, and the NSF content in pore solution increases. When the w/c ratio increases from 0.4 to 0.5, the leakage ratio increases by only 4.6%, and does not increase linearly with the increase of w/c ratio. The linear regression coefficient (R2), standard error of the slope and intercept are 0.9849, 4.69,787 and 1.72,611 respectively. This interesting phenomenon indicates that there is always some NSF adsorbed on the surface of cement particles. Overall, the leakage ratio of NSF increases with higher w/c ratio.

where I0 is the initial photoelectron intensity of the phase material was 2313.5; Ib is the photoelectron intensity after passing through the adsorption film; b is the thickness of the NSF layer. 2.2.4. Phase composition and content The phase composition of hardened cement paste with NSF was characterized by D/max 2550 X-ray powder diffractometer (XRD). These samples were ground into powder and dried at 50 � C for 24 h. The scanning range is 5� –80� in 0.02� steps, counting by 4 s per step. The radiation was CuKa at a wavelength of 0.1541 nm. Then Rietveld method was used to calculate the mineral content by GSAS-EXPGUI software and internal standard method. 2.2.5. Statistical analysis All the experiments were executed according to the standard pro­ cedures. The obtained data were analyzed statistically with three rep­ licates in the leakage behavior section and adsorption behavior section for their significance (p < 0.05). In the determination of NSF absorbance-concentration curve section, the NSF concentration was good linearly fitted with the corresponding ultraviolet absorbance with the determination coefficient (R2) 0.9994.

3.1.2. Effect of NSF content on leakage ratio In general, different contents of NSF are added to improve the workability and fluidity of fresh concrete. As more NSF is added, the concentration of NSF in solution raises, and its adsorption amount on the cement particles surface also increases. In order to determine whether the leakage behavior increases with NSF content, the leakage ratio of cement pastes prepared with 0.1%, 0.2% and 0.3% NSF were tested and the result is shown in Fig. 3. It is interesting that the leakage ratio de­ creases as more NSF is mixed. The leakage ratios of the pastes with 0.1%, 0.2% and 0.3% NSF are 53.6%, 50.8% and 44.6% respectively. This result shows the leakage ratio of NSF from cement paste with 0.3% NSF reduces about 9% by that with 0.1% NSF. Overall, the leakage ratio decreases as the rise of NSF content, and the higher NSF content cor­ responds to the lower leakage ratio. This result may because the hy­ dration products encapsulate a part of NSF, leading to a relative reduction of leakage ratio. Besides, it is possible that the cement parti­ cles are positively charged, whereas the NSF is negatively charged. The electrical attraction of NSF to cement particles increases when more NSF is added. As a result, the increment of NSF adsorbed amount on cement particles is more than that in aqueous solution, and then the leakage ratio varies this way.

3. Results and discussion 3.1. Leakage behavior of NSF from cement based materials 3.1.1. Effect of w/c ratio on leakage ratio The leakage ratios of NSF from cement pastes prepared with 0.2, 0.3, 0.4 and 0.5 w/c ratios were evaluated and the result is illustrated in Fig. 2. Fig. 2 shows that the leakage ratio of NSF from cement paste increases with the increment of w/c ratio. When the w/c ratio is 0.2, the leakage ratio of NSF is 38.9%. The leakage ratio of NSF is 44.6% when the w/c ratio is 0.3 and it increases about 5.7% by that of 0.2 w/c ratio. That is to say, the leakage ratio of NSF increases with w/c ratio. Finally, when the w/c ratio reaches to 0.5, the maximum leakage ratio is 58.2%, and it increases about 20% by that of 0.2 w/c ratio. In this work, the 3

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leakage ratio of cement paste mixed 0.3% NSF at different hydration times was tested, and the result is given in Fig. 5. As the result shown in Fig. 5, the leakage ratio of NSF from cement paste is up to 50.8% at 0.5 h, and it decreases rapidly to 44.6% at 1 h. This ratio is stable at 34.4% from 1.5 h to 2.5 h, and then it reduces to 32.1% at 3 h. The leakage ratio of NSF at 1 d, 7 d and 28 d are 32.1%, 31% and 31% respectively. These results indicate that the NSF leakage ratio decreases significantly in the first 3 h and remains stable over time after one day of cement hydration. It is generally known that in the first hours, the cement rapidly dissolves and some hydrated products such as calcium silicate hydrate (C–S–H) and ettringite (AFt) form (Scrivener and Nonat, 2011). Then it contin­ uously hydrates at a slower ratio (Scrivener et al., 2015). And the hy­ drated products such as calcium hydroxide (CH), C–S–H and AFt produced in this stage can adsorb NSF better than cement particles (De Laat and Van den Heuvel, 1995). At the same time, NSF is also encap­ sulated by cement hydration products, such as AFt and C–S–H. There­ fore, the leakage ratio decreases slightly. However, in this process, tricalcium silicate (C3S) may be covered by hydration products, possibly unable to continue hydration and wrap more NSF. Thus, the leakage ratio of NSF in cement decreases rapidly at early age.

54 53.6%

Leakage ratio (%)

52

50.8%

50 48 46 44

44.6%

0.1

0.2 Content of NSF (%)

0.3

Fig. 3. Leakage ratio of NSF from cement pastes with different NSF content.

3.1.3. Effect of cementitious materials on leakage ratio The leakage ratio of NSF from cement, GGBS and fly ash with 0.3% NSF and 0.3 w/c ratio were evaluated and the result is illustrated in Fig. 4. As shows in Fig. 4, the leakage ratios of NSF from cement, GGBS and fly ash are 44.6%, 50.8% and 54.2% respectively. These leakage results indicate that the composition of cementitious materials affects the leakage ratio of NSF. This ratio of fly ash or GGBS paste is higher than that of cement, and it is the highest in fly ash paste. It may relate to their surface properties and hydration characteristics. Normally, the surface of fly ash is rich in oxides such as Al2O3 and SiO2 (Ameh et al., 2016; Chen et al., 2020; Yu et al., 2019). These oxides can form a pro­ tective layer on the surface, which hinders the adsorption of NSF on its surface. Meanwhile, the early activities of GGBS and fly ash are lower than that of cement ( X. Liu et al., 2020; Y. Liu et al., 2020; Tan et al., 2018; Wang and Lee, 2010) and it is impossible to participate in hy­ dration reaction to form hydration products at early age. In addition, the adsorptive capacity of cement, GGBS and fly ash particles to NSF is not as good as that of hydration products (Huynh et al., 2006; Zhang and Kong, 2015). Hence, the leakage ratio of NSF in fly ash is the highest, followed by GGBS, and this ratio in cement is the smallest.

3.2. Adsorption behavior of NSF in cement based materials In general, leakage and adsorption behaviors of NSF from/by the cement based materials are related to each other. Thus the adsorptions of NSF in various cement based materials were studied in order to illu­ minate the leakage of NSF from them. 3.2.1. Effect of temperature on adsorption capacity of NSF Curing temperature affects the hydration process of cement based materials (Lothenbach et al., 2007) and hence it, to some extent, also affects the adsorption capacity of NSF. In order to illustrate the effect of curing temperature on adsorption behavior of cement paste on NSF, adsorption capacity of cement paste with 0.1%, 0.2% and 0.3% NSF at 20 � C, 40 � C and 60 � C were assessed and the results are given in Fig. 6. Fig. 6 shows that at 20 � C, the adsorption amounts of cement pastes with 0.1%, 0.2% and 0.3% NSF are 6.50 mg/g, 6.55 mg/g and 6.60 mg/g respectively. This result implies that at same temperature, the adsorp­ tion capacity of cement paste on NSF increases as its content increases. At 20 � C, 40 � C and 60 � C, the adsorptions amounts of cement paste with 0.1% NSF are 6.5 mg/g, 7.2 mg/g and 7.2 mg/g respectively. At these temperatures, the cement paste with 0.3% NSF provides 6.6 mg/g, 7.35 mg/g and 7.8 mg/g adsorption amounts respectively. This result in­ dicates the adsorption capacity of cement paste on NSF increases with temperature rising. In addition, the curing temperature plays a more

3.1.4. Leakage ratio of NSF at different hydration time The cement hydration products increase with longer hydration time, and the adsorption of NSF by hydration products is better than that of cement particles (Tkaczewska, 2014; Zhang and Kong, 2015). In order to further explore the leakage behavior of NSF with hydration time, the

52

60 54.2%

48

50.8%

Leakage ratio (%)

Leakage ratio (%)

55 50 45

44.6%

40

A: Initial several hours B: After one day of cement hydration

44.6%

44 40

34.4%

36

34.4%

34.4% 32.1% 32.1%

32

35 30

50.8%

A

Cement

GGBS

0.5h

Fly ash

Fig. 4. Leakage ratio of NSF from different cementitious materials.

1h

1.5h

31.0% 31.0% B

2h

2.5h Time

3h

1d

7d

28d

Fig. 5. Leakage ratio of NSF from cement pastes at different hydration time. 4

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Adsorption of NSF (mg/g)

8.0

7.6

7.2mg/g

and produces the hydration products such as AFt and C–S–H (Lang et al., 2020). The adsorption of hydration products on NSF are better than that of cement phases such as C3S, dicalcium silicate (C2S) and tricalcium aluminate (C3A), as well as these mineral admixtures such as fly ash and GGBS (Huynh et al., 2006; Zhang and Kong, 2015). As a result, adsorption of cement on NSF increases as more hydration products form, whereas the adsorption capacity in GGBS and fly ash are relatively less than that in cement.

7.8mg/g

20°C 40°C 60°C

7.3mg/g

7.35mg/g

7.2

6.8

6.55mg/g

6.5mg/g

3.2.3. Adsorption capacity of cement pastes with fly ash and GGBS on NSF The adsorption capacity of NSF in the cement pastes with 10%, 20%, 30%, 60% and 70% GGBS and fly ash were assessed respectively and the results are given in Fig. 8. As shown in Fig. 8, although there are several differences in the type of mineral admixtures and their content, adsorption capacity of cement pastes with fly ash and GGBS on NSF increases as the content of NSF increases. It is obvious that the adsorp­ tion capacity of cement paste with fly ash decreases as more fly ash is mixed. Usually, SiO2 and Al2O3 in fly ash make it easily form a protective layer on the surface and this layer hinders the adsorption behavior (Gu and Chen, 2020; Wang et al., 2017a, 2017b). As a result, fly ash presents a relatively lower adsorption capacity. Thus the cement paste with it presents a decreased adsorption capacity as fly ash is introduced. As compared the cement pastes with fly ash, the cement pastes with GGBS display a different adsorption capacity and higher adsorption capacity is observed in the cement pastes with GGBS (as shown in Fig. 8). In generally, the GGBS particles have more adsorption sites on its rough surface as comparing with the fly ash (Ameh et al., 2016; Li et al., 2019). And yet for all that, the adsorption capacities of cement pastes with more GGBS do not continuously increase when the same content of NSF is added. The reason for the changing adsorption behavior may come from the GGBS has lower adsorption capacity than cement. As comparing with cement, GGBS also displays a slow hydration at early ages. The cement paste with relatively less GGBS can still hydrate as the cement at early ages. But the paste hydration is significantly delayed as more GGBS is mixed. As a result, the less hydration product form and hence, the adsorption capacities of cement pastes with more GGBS do not continuously increase when the same content of NSF is added.

6.6mg/g

6.4 0.1

0.2 Content of NSF (%)

0.3

Fig. 6. Adsorption capacity of NSF at different temperature.

significant influence on the adsorption capacity of cement paste on NSF as more NSF is added. The increased adsorption capacity at higher temperature of cement paste on NSF may relate to the more hydrated products. The elevated temperature promotes the hydration of cement and more hydration products are produced (Lothenbach et al., 2007). The cement hydration products have better adsorption capacity on NSF than un-hydrated cement (Huynh et al., 2006; Zhang and Kong, 2015). Thus the adsorption capacity of cement paste on the NSF increases as the increments of NSF content and curing temperature. 3.2.2. Adsorption capacity of NSF in fly ash and GGBS The adsorption capacity of NSF on the fly ash and GGBS with 0.1%, 0.2% and 0.3% NSF were evaluated and the results are given in Fig. 7. It can be seen from Fig. 7 that adsorption amounts of NSF on the fly ash and GGBS pastes with 0.1% NSF are 2.83 mg/g and 2.99 mg/g respec­ tively. These values are less than 6.5 mg/g of cement. As shown in Fig. 7, the adsorption amount increases as more NSF is mixed. The adsorption amount in fly ash and GGBS with 0.2% and 0.3% NSF are 3.00 mg/g and 3.04 mg/g, 3.11 mg/g and 3.13 mg/g respectively. Likewise, these values are also less than that of cement. In general, adsorption and leakage are correlative each other. As shown in Fig. 4, leakage ratio of NSF from cement paste is lowest, followed by GGBS, and the highest in fly ash. Accordingly, the adsorption capacity results further demonstrate the effect of the three cementitious materials on the leakage behavior. Compared with the GGBS and fly ash, cement hydrates rapidly in water

3.2.4. Adsorption capacity of NSF at different hydration time The adsorption capacity of cement paste mixed with 0.3% NSF at different hydration ages were tested, and the results are shown in Fig. 9. It can be found from Fig. 9 that the adsorption amount of NSF by cement increases gradually from 5.53 mg/g at 0.5 h to 5.67 mg/g at 3 h. The

6.5

3.2 fly ash GGBS

3.0

2.99mg/g

3.04mg/g

Adsorption of NSF (mg/g)

Adsorption of NSF (mg/g)

3.1

3.13mg/g 3.11mg/g 3.00mg/g

2.9 2.8 2.7

2.83mg/g

6.0 5.5 5.0 4.5 4.0

0.1

0.2

Content of NSF (%)

0.3

Fig. 7. Adsorption capacity of NSF in fly ash and GGBS with 0.1%, 0.2% and 0.3% NSF.

fly ash 10% fly ash 20% fly ash 30% fly ash 60% 0.1

0.2 Content of NSF (%)

GGBS 10% GGBS 20% GGBS 30% GGBS 70% 0.3

Fig. 8. Adsorption capacity of NSF in the cement pastes with different contents of fly ash and GGBS. 5

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6.0

5.8

5.65

5.85

5.60

5.50

5.5

5.86

5.87

y =5.508+0.061x

5.55

R2=0.9149 0

1

2 Time (h)

5.7 5.6

20 20 20 60 60 60

5.70

Adsorption of NSF (mg/g)

Adsorption of NSF (mg/g)

5.9

Journal of Environmental Management 255 (2020) 109934

3

5.65

5.67

5.67

-0.1% -0.2% -0.3% -0.1% -0.2% -0.3%

5.61 5.53

A: Initial several hours B: After one day of cement hydration B

5.56 A

0.5h

1h

1.5h

2h

2.5h Time

3h

1d

7d

28d

Fig. 9. Adsorption capacity of NSF at different hydration time.

96

linear regression coefficient (R2) is 0.9149 of the initial several hours. This value increases to 5.85 mg/g, 5.86 mg/g and 5.87 mg/g at 1, 7 and 28 d respectively. These results indicate that the adsorption amount increases continuously as the hydration age prolongs during the hy­ dration age of the initial 3 h. The cement rapidly dissolves and some hydration products formed in the initial several hours (Scrivener and Nonat, 2011), and the NSF may be also partially adsorbed or enveloped by the hydrated products. Compared with the adsorption result at 0.5 and 3 h, the adsorption amount at 1, 7 and 28 d slightly increases. In general, NSF in cement paste partially adsorbs on the surface of cement particle or hydrated products, whereas it partially dissolves in solution. These dissolved NSF can be continuously absorbed by the hydrated products as the hydration proceeds. Thus the adsorption amounts of cement paste on NSF slightly increase at 1, 7 and 28 d.

98

100 102 104 106 Binding Energy (eV)

108

110

Fig. 10. XPS data of silicon at different temperatures. Table 2 XPS data of silicon and thickness of NSF at different temperatures. Temperature (� C)

NSF (%)

Si2p Binding Energy (eV)

Si2p Atoms percent (%)

Adsorbed film thickness (nm)

Standard error

20

0.1 0.2 0.3 0.1 0.2 0.3 0.1 0.2 0.3

102.45 102.47 102.32 102.17 102.31 102.2 102.47 102.79 102.46

8.3 7.05 7.33 3.79 4.78 5.05 7.5 7.99 7.18

2.13 3.51 3.55 2.28 3.55 4.57 2.33 3.72 4.74

0.148 0.134 0.145 0.093 0.141 0.140 0.111 0.099 0.149

40 60

3.3. Adsorption film thickness of NSF in cementitious materials The higher adsorption capacity corresponds to the lower leakage ratio. However, the NSF in cementitious materials does not only adsorb on the surface of cementitious materials or hydrated products, but also enclosed by hydrated products. In order to further illuminate the adsorption of NSF physically or chemically, NSF adsorption films in various cementitious materials were measured to illustrate the NSF adsorption behavior.

becomes more significant as more NSF content is mixed. Generally, the hydration of cement is promoted at higher temperature, and more hy­ dration products are produced (Chen and Gao, 2019; Lothenbach et al., 2007; Ren et al., 2019; Wang et al., 2020). The hydration products have better adsorption capacity than cement particles. As a result, the adsorption film thickness of NSF on the surface of cement increases with temperature rising.

3.3.1. Effect of temperature on adsorption film thickness of NSF in cement XPS data of silicon and adsorption film thickness of NSF in cement pastes at 20, 40 and 60 � C were evaluated and the results are shown in Fig. 10 and Table 2. These results indicate that the NSF film appears and adsorption film thickness is related to NSF content and temperature. As shown in Table 2, the pastes with 0.1%, 0.2% and 0.3% NSF display 2.13, 3.51 and 3.55 nm layer thickness respectively at 20 � C. It is obvious that the layer thickness increases as the increment of NSF content. In general, the more NSF leads to the increased adsorption amount and hence, the thicker adsorption film is found. At 60 � C, these pastes with 0.1%, 0.2% and 0.3% NSF provide 2.33, 3.72 and 4.74 nm NSF film thickness respectively. It shows that even at higher tempera­ ture, more NSF still results in the thicker adsorption layer. Besides, it is also found that cement pastes with 0.1% NSF has 2.13 nm, 2.28 nm and 2.33 nm adsorption film thickness at 20 � C, 40 � C and 60 � C respectively. This result reveals that temperature has the important influence on the adsorption film thickness of NSF and higher temperature means the thicker NSF adsorption film. It can also be found from Table 2 that cement paste with 0.3% NSF presents 3.55 nm, 4.57 nm and 4.74 nm adsorption thickness at 20 � C, 40 � C and 60 � C, respectively. It demon­ strates that the effect of temperature on the adsorption film thickness

3.3.2. Adsorption film thickness of NSF in fly ash and GGBS The adsorption film thickness of NSF in fly ash and GGBS were measured and the results are shown in Fig. 11 and Table 3. As shows in Table 3, the NSF adsorption film thickness in the fly ash or GGBS pastes increases as more NSF is mixed. It can also be seen from Table 3 that NSF film is thicker in GGBS pastes than in fly ash pastes. The NSF film layers in fly ash and GGBS with 0.1% and 0.3% NSF are 1.15 nm and 1.75 nm, 1.67 nm and 2.31 nm respectively and these values are lower than that of the cement. This difference may be attributed to the different hy­ dration or reaction ratios of these three cementitious materials. It is well known that fly ash and GGBS relatively hydrate or react slowly and thus the relatively less hydrated or reacted products in the same ages (De Weerdt et al., 2011; X. Liu et al., 2020; Y. Liu et al., 2020). But these hydrated or reacted products generally present the better adsorption capacity. Thus GGBS and fly ash display the relatively thinner adsorp­ tion film. Besides, cement contains a lot of CaO and the cement hydra­ tion is accompanied by the presence of Ca2þ in the water. Ca2þ can combine with the anions of NSF and then the adsorption of NSF by cement particles is promoted (Huang et al., 2019; Li et al., 2020; Zhang 6

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Journal of Environmental Management 255 (2020) 109934

3.3.3. Adsorption film thickness of NSF in cement pastes at different hydration times In order to determine the adsorption film thickness of NSF in the cement paste at different hydration times, the thickness of NSF adsorption layer in cement paste at 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 1 d, 7 d, and 28 d were analyzed and the results are shown in Fig. 12 a) and Fig. 12 b). Fig. 12 b) shows the thickness of NSF adsorption film in­ creases significantly at the first several hours and it increases from 0.98 nm at 0.5 h to 4.63 nm at 3 h. However, at 1 d, 7 d and 28 d, the thickness of NSF adsorption film are 4.74 nm, 5 nm and 5.7 nm respectively. The result shows that the NSF adsorption film thickness increases slightly after the first day. As mentioned above, the adsorption film thickness is related to its adsorption capacity. These results of adsorption capacity indicate that at the initial several hours, the cement paste has the significantly increased adsorption capacity and then this adsorption capacity is gradually stabilized. For the almost stable adsorption ca­ pacity and adsorption film thickness, the leakage ratio of NSF from cement paste thus becomes stable.

fly ash-0.1%NSF fly ash-0.2%NSF fly ash-0.3%NSF GGBS-0.1%nSF GGBS-0.2%nSF GGBS-0.3%nSF

96

98

100 102 104 106 Binding Energy (eV)

108

110

3.4. Hydration of cement with NSF

Fig. 11. XPS data of silicon in fly ash and GGBS with different amount of NSF.

NSF can adsorb on the surface of cement particles and its hydration products. It can also be encapsulated by cement hydration products. In general, the phase compositions of cement always present different adsorption capacity to NSF. Tricalcium aluminate (C3A) has the greatest adsorption capacity and the followings are ferrite phase (C4AF), trical­ cium silicate (C3S) and dicalcium silicate (C2S) (Huynh et al., 2006; Zhang and Kong, 2015). In addition, the adsorption of NSF by cement hydration products such as CH and C–S–H gel is generally higher than that of cement particles themselves. Thus the adsorption and leakage behavior of NSF in cement based materials are closely related to the surface characteristic, hydration process and hydration products. In order to further illuminate the mechanism of leakage behavior of NSF from cement based materials, the hydration of cement with SNF at 0.5 h, 1 h, 3 h, 1 d, 7 d and 28 d was characterized by XRD and these XRD patterns and the phase compositions and contents of the cement paste at different ages are given in Fig. 13 and Table 4 respectively. The hydration products produced by C3S and C3S hydration are calcium hydroxide (CH) and amorphous C–S–H gel (X. Liu et al., 2020; Y. Liu et al., 2020; Tkaczewska, 2014). C–S–H gel is the main component of amorphous phase. In the initial fast reaction period, the cement dis­ solves quickly and the amorphous C–S–H gel and AFt precipitate and form quickly (Scrivener and Nonat, 2011). Thus the C3S and C2S con­ tents decrease rapidly from 71.01% at the beginning to 63.35% at 3 h and the contents of the amorphous phase and AFt increase from 8.23% to 15.91% and from 0% to 1.85% respectively within 3 h (as shown in Table 4). At the beginning, NSF is mainly adsorbed by the un-hydrated

Table 3 XPS data of silicon and thickness of NSF in fly ash and GGBS. Mineral admixture

NSF (%)

Si2p Binding Energy (eV)

Si2p Atoms percent (%)

Adsorbed film thickness (nm)

Standard error

fly ash

0.1 0.2 0.3 0.1 0.2 0.3

102.19 102.24 102.14 102.06 102.02 102.07

10.66 9.46 8.91 8.81 8.69 8.05

1.15 1.34 1.67 1.75 2.02 2.31

0.073 0.083 0.108 0.090 0.124 0.135

GGBS

et al., 2018). However, fly ash and GGBS do not react with water immediately. And the abundant SiO2 and Al2O3 in fly ash will form a dense structure on the surface of fly ash, which hinders its adsorption on NSF. The dense structure also prevents the NSF adsorption on the surface of fly ash. As a result, fly ash provides the lowest NSF adsorption film thickness. However, as compared with fly ash, GGBS usually provides the rough surface. For the rough surface, GGBS has the relatively thicker adsorption film.

0.5h 1h 1.5h 2h 2.5h 3h 1d 7d 28d

Adsorption film thickness (nm)

28d 7d 96

98

5.7

6

100 102 104 Binding Energy (eV)

106

108

110

5

A: Initial several hours B: After one day of cement hydration

4

3.72

3 2.04

2 1

4.74

5

2.26

1.56 0.98 0.5h

A

1h

1.5h

B

2h

2.5h Time

Fig. 12. Adsorption film thickness of NSF in cement pastes at different hydration times. 7

4.63

3h

1d

7d

28d

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Journal of Environmental Management 255 (2020) 109934

a-C2S b-C3S c-Ca(OH)2 d-AFt

c

a d

10

b c

b

d

20

30

40 2 Theta

50

60

cement hydration time and it is up to 50.8% at 0.5 h and decreases to 31.0% at 28 d. ● The adsorption capacity of NSF in cement decreases at lower tem­ perature and it also declines as more content of mineral admixtures are mixed. The adsorption amount and NSF layer thickness of cement paste with 0.3% NSF are 5.53 mg/g and 0.98 nm, 5.87 mg/g and 5.7 nm at 0.5 h and 28 d respectively. ● The lower leakage ratio corresponds to the higher adsorption ca­ pacity. The leakage and adsorption behaviors of NSF in cement change significantly at the initial several hours and gradually stabi­ lize after the first day. ● The phase composition results show that the contents of C3S and C2S decline continuously and amorphous phase and AFt increase rapidly at the initial several hours. NSF adsorption and leakage behaviors are closely related to the hydration process of cement.

blank 0.5h 1h 3h 1d 3d 7d 28d

a bbc

70

80

The results from this work indicate that NSF can leak from the cement based materials and the potential environment pollution which leads from the NSF leakage should not be neglected. At least, it should be restricted or cautious to produce the water tower and pipe concrete structure with it.

Fig. 13. XRD pattern of cement with NSF. Table 4 Phase composition and content of different ages. Phase (%)

0h

0.5 h

1h

3h

1d

7d

28 d

C2S þ C3S C3A C4AF Amorphous phase AFt

71.01 5.71 6.79 8.23 –

70.29 2.83 10.05 5.96 1.40

67.6 3.45 8.56 9.31 1.63

63.35 2.89 8.63 15.91 1.85

33.36 0.89 6.41 36.66 5.94

38.23 0.79 4.6 34.41 5.46

36.11 2.78 5.08 37.61 4.54

Author contributions section Haoxin Li and Xiaojie Yang designed experiments; Linan Gu carried out the experiments; Zhaoyin Wen and Biqin Dong analyzed experi­ mental results. Haoxin Li and Linan Gu analyzed sequencing data and developed analysis tools and wrote the manuscript.

cement particles. However, the hydrated products are produced as the cement hydration proceeds and NSF can be also adsorbed or enveloped by the hydrated products. In general, C–S–H gel and AFt have the relatively greater adsorption capacity than the cement particles. Thus the adsorption capacity of NSF in cement paste increases and the leakage ratio of NSF from cement paste decreases at the initial several hours. As a result, the cement paste with 0.3% NSF provides 5.67 mg/g adsorption capacity and 32.1% leakage ratio after 1 h hydration. Table 4 also illustrates that the phase content in cement paste hardly alter after the hydration proceeds for 1 d. C3S and C2S contents decrease rapidly from 63.35% at 3 h to 33.36% at 1 d. However, this value be­ comes stable around 30%–40% from 1 d to 28 d. The amorphous phase content keeps at about 35% after the hydration proceeds for 1 d. Con­ trasting these results of adsorption and leakage behavior of NSF, as well as the phase content in cement paste at different hydration ages, it can be found that NSF adsorption and leakage behaviors are closely related to the hydration process of cement. In general, the water solution in the pore decreases as the hydration continuously proceeds (Scrivener and Nonat, 2011). As a result, the contact between the solution and cement particles or hydrated products is blocked and then the adsorption behavior of NSF is no longer increased rapidly. Besides, the cement paste has hardened as the hydration proceeds for several hours and NSF has been also partially enveloped by the hydrated products. Hence, the leakage ratio of NSF does not significantly changes after the hydration proceeds for 1 d.

Declaration of competing interest We declare that we do not have any commercial or associative in­ terest that represents a conflict of interest in connection with the word submitted. Acknowledgements The authors gratefully acknowledge the financial supports provided by National Natural Science Foundation of China (51678442, 51578412, 51478348, 51508404, 51878480 and 51878479), and the Fundamental Research Funds for the Central Universities. References Aksu, Z., 2005. Application of biosorption for the removal of organic pollutants: a review. Process Biochem. 40, 997–1026. https://doi.org/10.1016/j. procbio.2004.04.008. Ameh, A.E., Musyoka, N.M., Fatoba, O.O., Syrtsova, D.A., Teplyakov, V.V., Petrik, L.F., 2016. Synthesis of zeolite NaA membrane from fused fly ash extract. J. Environ. Sci. Heal. - Part A Toxic/Hazardous Subst. Environ. Eng. 51, 1–9. https://doi.org/ 10.1080/10934529.2015.1109410. Chemical Toxicity Database, 1997. Chen, T., Gao, X., 2019. Effect of carbonation curing regime on strength and microstructure of Portland cement paste. J. CO2 Util. 34, 74–86. https://doi.org/ 10.1016/j.jcou.2019.05.034. Chen, Z., Nong, Y., Chen, J., Chen, Y., Yu, B., 2020. A DFT study on corrosion mechanism of steel bar under water-oxygen interaction. Comput. Mater. Sci. 171 https://doi. org/10.1016/j.commatsci.2019.109265. Crescenzi, C., Di Corcia, A., Marcomini, A., Pojana, G., Samperi, R., 2001. Method development for trace determination of poly(naphthalenesulfonate)-type pollutants in water by liquid chromatography-electrospray mass spectrometry. J. Chromatogr. A 923, 97–105. https://doi.org/10.1016/S0021-9673(01)00964-5. De Laat, A.W.M., Van den Heuvel, G.L.T., 1995. Molecular weight fractionation in the adsorption of polyacrylic acid salts onto BaTiO3. Colloids Surfaces A Physicochem. Eng. Asp. 98, 53–59. https://doi.org/10.1016/0927-7757(95)03096-V. De Weerdt, K., Haha, M. Ben, Le Saout, G., Kjellsen, K.O., Justnes, H., Lothenbach, B., 2011. Hydration mechanisms of ternary Portland cements containing limestone powder and fly ash. Cement Concr. Res. 41, 279–291. https://doi.org/10.1016/j. cemconres.2010.11.014. Diaz-Alarc� on, J.A., Alfonso-P� erez, M.P., Vergara-G� omez, I., Díaz-Lagos, M., MartínezOvalle, S.A., 2019. Removal of iron and manganese in groundwater through

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