Effects of zinc oxide nanoparticles on early-age hydration and the mechanical properties of cement paste

Construction and Building Materials 217 (2019) 352–362

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Effects of zinc oxide nanoparticles on early-age hydration and the mechanical properties of cement paste Jintao Liu, Hang Jin, Chunping Gu, Yang Yang ⇑ College of Civil Engineering and Architecture, Zhejiang University of Technology, Hangzhou 310023, China Key Laboratory of Civil Engineering Structures and Disaster Prevention and Mitigation Technology of Zhejiang Province, Hangzhou 310023, China

h i g h l i g h t s  Nano-ZnO particles prolonged the setting times of cement paste.  Nano-ZnO affected the hydration reaction of cement at the early age.  The induction period of cement hydration was extended by nano-ZnO.  Nano-ZnO improved long-age strength of cement paste.

a r t i c l e

i n f o

Article history: Received 29 December 2018 Received in revised form 6 April 2019 Accepted 5 May 2019 Available online 17 May 2019 Keywords: Nanoparticles Resistivity Mechanical properties Cement hydration

a b s t r a c t Zinc oxide nanoparticles (nano-ZnO) were dispersed uniformly into cement paste by using ultrasonic treatment, and the effects of nano-ZnO on the setting time, strengths, hydration, and microstructure of the cement paste were investigated. The addition of nano-ZnO to cement paste showed a remarkable influence on the early hydration process of the cement paste, whose setting time increased drastically when the content of nano-ZnO was only 0.2 wt% (by weight of cement). The isothermal calorimetry and electrical resistance tests showed that the addition of nanoparticles can prolong the induction period of cement hydration and influence the hydration rate during the acceleration period. The mechanical test results indicated that long-age strengths of cement paste were improved with the increase of nano-ZnO concentrations. Furthermore, nano-ZnO refined the pore diameter distribution and formed a compact microstructure of the cement paste at 28 days. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction The performance of cement-based materials depends largely on structural elements that are effective on a micro and nanoscale. The size of the calcium silicate hydrate (C-S-H) phase, the primary component responsible for strength and other properties in cementations systems, falls in the few-nanometers range. Hence, nanoparticles have broad application in improving the mechanical and durability performance of cement-based materials due to their unique characteristics [1,2], which originate from extremely small size. Compared with conventional supplementary cement materials, they have a much higher specific surface area and reactivity. In general, the small size of nanoparticles provides a larger surface area for the reaction, so the smaller the particle sizes, the higher the rate of pozzolanic reaction. A variety of nanoparticles have

⇑ Corresponding author. E-mail address: [email protected] (Y. Yang). https://doi.org/10.1016/j.conbuildmat.2019.05.027 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

been used to modify the properties of cementitious materials, for example, nano-SiO2 [3,4], nano-CaCO3 [5], nano-Al2O3 [6], nanoTiO2 [7,8], and so on. Most of these nanoparticles can accelerate early-age hydration and provide nucleation sites for C-S-H hydration, and can densify the microstructure of the matrix [9,10]. Nano-ZnO with unique optical and electronic properties has emerged as a good candidate for many applications, such as semiconductor, catalysts and chemical absorbent [11–13]. SalavatiNiasari et al. have synthesized uniform nano-ZnO particles with the size of 12 nm, which can be used in large-scale production [14,15]. Also, different morphologies of ZnO nanostructures have been developed [16–19]. Some research on the use of ZnO in cementitious materials has indicated that it modifies cement hydration kinetics at early ages. For example, it have been reported that a formation of a surface layer of Ca(Zn(OH)3)2
2H2O was formed during the hydration of the C3S phase in the presence of Zn2+, which retarded the transport of water to the C3S phase [20– 23]. Fernandez Olmo et al. [20] demonstrated that the initial and final setting times of cement paste containing 15 wt% (by weight

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of cement) ZnO were 5650 and 8000 min, respectively. Moreover, ZnO decreased the compressive strength of specimens at early ages (7 days), but its effects decreased with the sample age [20]. Recently, nano-ZnO with semiconducting and piezoelectric dual properties have been added to various materials and products, including amongst others, plastics, ceramics and glass. Nevertheless, few studies have reported on the incorporation of nano-ZnO in cement-based materials have been reported. Arefi and RezaeiZarchi [24] observed that the addition of 0.5 wt% nano-ZnO was the optimal content, and the early age strengths were improved. Liu et al. [6] showed that the setting time of cement mortar was more than 2 days with 0.5 wt% nano-ZnO particles, but the 28day compressive strength of specimen showed no significant change. In a later work [21], the addition of nano-ZnO improved the compressive strength of mixes by 15 wt% at 28 days, moreover, Fourier-transform infrared spectroscopy and X-ray powder diffraction (XRD) supported the presence of CaZn2(OH)6
2H2O, which resulted in retardation of the hydration reaction. However, the effects of nano-ZnO on the early age hydration process of cement paste were still unclear. In previous studies, two hypotheses, i.e. the formation of the low- or non-permeable layers and the poison effect, was proposed to explain the retarding effect of Zn2+ in the cement based materials. However, it is still a controversial issue. Therefore, different amounts (0.05 wt%, 0.1 wt%, and 0.2 wt%) of nano-ZnO were added into cement paste in this study, and the strengths of samples were tested at 1, 3, 7, 14, 28, and 56 days, respectively. The early age hydration process of cement paste with nano-ZnO was evaluated with isothermal calorimetry and electrical resistance tests. Moreover, the microstructure of cement pastes with nano-ZnO was examined using mercury intrusion porosimetry (MIP) and a scanning electron microscope (SEM). The influences of nano-ZnO on the cement hydration products were studied with XRD instrumentation. 2. Experimental programs 2.1. Materials In this study, Portland cement (P O 42.5) was used, with its chemical composition listed in Table 1. Nano-ZnO particles used in this experiment were provided by Hangzhou Wanjing New Material Co., Ltd, the properties of which are shown in Table 2. SEM image of nanoparticles is shown in Fig. 1(a), and the XRD pattern of nano-ZnO particles is provided in Fig. 1(b). 2.2. Specimen preparation There are large Van der Waals force and electrostatic force between the nanoparticles, and untreated nanoparticles are prone to aggregate without any dispersion [25]. Furthermore, theses aggregations may emerge later as defects in the matrix, reducing the mechanical performance of the composites [6]. In order to

obtain a homogeneous paste, the ultrasonic dispersion processing is usually used to improve the dispersibility of nanoparticles in cementitious composites [10,26,27]. Our previous studies also indicated that ultrasonic dispersion process could produce good dispersing effect [28,29]. Therefore, an ultrasonic disrupter was employed to reduce the agglomeration of nanoparticles in this study, and the total sonication time was set as 15 min. Firstly, nano-ZnO particles were added to water and stirred evenly until they were completely wetted. Then, an ultrasonic disrupter (20 kHz, 750 W) was employed to reduce the agglomeration of nanoparticles in water, and the total sonication time was set as 15 min. In the process of dispersion, the suspension was placed in ice water to prevent heating and foaming caused by sonication process. In this study, nano-ZnO replaced cement weight by 0, 0.05 wt%, 0.1 wt% and 0.2 wt%, which was abbreviated by R, Z1, Z2 and Z3, respectively. Details of the mix proportions for cement paste were given in Table 3. The mixing process was conducted following ISO 679:1989 [30], and JJ-5 type cement mixer (Wuxi Jiangong Test Facility Co. Ltd., China) with rotation rates of 140 rmp and 285 rmp was used. Although nano-ZnO particles were dispersed into water by using ultrasonication, they were prone to re-unite during the mixing process due to high specific surface area. Therefore, high speed stirring was implemented to improve the dispersibility of nanoparticles in the matrix. Cement particles and nano-ZnO suspension were added in the mixing pot and stirred at 140 rmp for 120 s. In order to further disperse the nanoparticles in an effective manner, cement paste was then stirred at highspeed (285 rmp) for another 120 s. After the mixing process, the fluidity of the cement paste was measured by slump tests according to Chinese standard GB/T 8077-2000 (Methods for testing the uniformity of a concrete admixture). The cone mould had the bottom diameter of 60 mm, the top diameter of 36 mm, and the height of 60 mm. The well-mixed paste was poured into moulds to form the cubes with a size of 40 mm  40 mm  40 mm for compressive tests and prisms with a size of 40 mm  40 mm  160 mm for flexural tests. All samples were placed in the standard curing chamber (20 °C, 95% humidity) for 24 h with exposed surfaces sealed by plastic sheet to prevent moisture loss.

2.3. Setting time and strengths According to ISO 9597-2008 (Cement test methodsDetermination of setting time and soundness), a manual Vicat apparatus was used to determine the initial and final setting times of cement pastes. The penetration of the Vicat needle into cement paste at 36 and 0.5 mm was determined as the initial and final setting time, respectively. The compressive and flexural strength tests were conducted at different ages according to ISO 679:1989 (Method of testing cements – determination of strength), and six specimens were prepared for each group.

Table 1 Chemical composition of Portland cement (wt%). Material

Fe2O3

Al2O3

CaO

MgO

SiO2

K2O

Na2O

SO3

Loss

Cement

2.9

4.7

62.6

3.2

19.5

0.7

0.3

2.5

3.6

Table 2 Properties of the nano-ZnO particles. Species

Color

Average Diameter/nm

Surface volume ratio (m2/g)

Purity (%)

PH value

Nano-ZnO

White

50

50

99.0

9.0

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J. Liu et al. / Construction and Building Materials 217 (2019) 352–362

(b)

˄a˅

500

Intensity

400

300

200

100

0

500 nm

10

20

30

40

50

60

70

80

Degree (2 theta) Fig. 1. SEM micrograph (a) and XRD pattern (b) of nano-ZnO particles.

Table 3 Mix proportion of Portland cement paste blended with nano-ZnO. Mix

Proportion (g) Cement

Nano-ZnO

Water

R Z1 Z2 Z3

1000 999.5 999.0 998.0

0 0.5 1.0 2.0

380 380 380 380

avoided the problems in this technology due to the electrodes in the form of polarisation and cracking. To conduct such test, the freshly blended cement paste was casted into the mould to a specific height. Then, the mould was gently compacted by hand for expelling air from the mixture. At last, the mould was sealed by plastic cover and the electrical resistivity of cement paste was monitored for 3 days. The measuring equipment was in a temperature controlled room with the temperature kept constantly at 20 °C, and the resistivity changes of the cement paste were recorded.

2.4. Isothermal calorimetry test 2.6. SEM, MIP and XRD Isothermal calorimetry is a commonly used method to monitor the early hydration process. In this research, isothermal calorimetric analysis was performed to study the early age hydration kinetics of cement pastes with and without nano-ZnO. For this purpose, an 8-channel TAM Air isothermal micro calorimeter was adopted. Samples of cement paste were placed in glass ampoules of 20 ml, followed by immediate placement of the ampoules in the calorimeter chamber. The mass of cement paste was 15 g, and the samples were tested twofold for 48 h and at 20 °C. 2.5. Electrical resistivity The electrical resistivity of cement paste with different nanoZnO particles were monitored by an electrodeless fresh cement resistivity analyser, CCR-3, as shown in Fig. 2. This apparatus is manufactured by Hong Kong Brilliant Concept Technologies, and the principle of the measurement is given in Ref. [31–33]. There was no electrode used in the measurement, which completely

Transformer core

Current sensor

Computer

Mould and sample

Generator and amplifie

Fig. 2. The non-contacting resistivity measurement system.

Fragmented pieces from the mechanical test were saved for microscopic characterization. The paste samples at different age were selected for the SEM (FEI Quanta 650) investigation, and all samples were coated with gold for SEM observation. MIP was used to study pore structure and pore size distribution of specimens, and the test was carried out using AutoPore IV 9500, which can theoretically determine a pore diameter of up to 3 nm under high pressure. XRD analysis was performed for the identification of the hydration products at different ages, and a Bruker D8 Advance Xray diffractometer with monochromatic CuKa radiation was employed. 3. Results and discussions 3.1. Workability and setting time Nano-ZnO posed little influence on the workability of fresh cement paste in terms of slump values. Due to the small particle size, nano-ZnO has much larger surface area than cement particles. The large surface area increased the adsorbed water on the surface, which decreased the workability of the cement paste. Fig. 3 shows that the fluidity of the cement paste decreased with the increasing addition of nano-ZnO particles. The particle diameter of nano-ZnO is in nanometer scale, and cement particle size is usually in the scale of micron meters. Nanoparticles tend to impact on the paste workability due to their high specific surface area. Previous studies indicated that the mixture fluidity decreased by more than 20% when 2.0 wt% nano-ZnO was added [6]. However, the addition of nanoparticles did not exceed 0.2 wt%, which had less influence on the workability of cement paste. As shown in Fig. 3, the slump value of the Z3 mix only dropped by 4%. Fig. 4 gives the setting time for the cement paste with nano-ZnO particles. The initial setting time for the blank group mix was

J. Liu et al. / Construction and Building Materials 217 (2019) 352–362

Slump value 110 105

Slump value (mm)

103

102

101

100

90

80

R

Z1

Z2

Z3

355

inhibited the early hydration of cement, the initial and final setting time of cement paste rose with the increasing of nano-ZnO. Thus, nano-ZnO had an adverse impact on the development of early age strength of the cement paste. It explained why the compressive and flexural strength decreased markedly with the addition of nano-ZnO after 1-day curing. However, the strengths of cement pastes with and without nano-ZnO are almost the same after 3 days of curing. At 7 days, the compressive strength of Z1, Z2, and Z3 increased by 13.0%, 5.8%, and 9.1%, respectively, compared with the blank group. Similarly, the 7-day flexural strength of Z1, Z2, and Z3 increased by 25.0%, 7.4%, and 10.3%, respectively. At 56 days, the compressive and flexural strengths of cement paste with 0.2 wt% nano-ZnO were higher than that of the blank group by 7.2% and 12.5%, respectively.

Mix design 3.3. Heat evolution

Fig. 3. Mini-slump test results of mixtures.

1600

Setting time (Min)

1400

Initial setting time Final setting time

3.54 4.00

1200 2.49

1000 2.71

800 600 400

1.36 1.00

1.39

1.00

200 0

R

Z1

Z2

Z3

Mix design Fig. 4. Initial and final setting time of cement paste with and without nano-ZnO particles.

310 min and the final setting time was 390 min. The nano-ZnOmodified cement paste in this research exhibited longer initial and final setting times compared to the control. In general, ZnO retards cement hydration via the formation of a layer of crystalline at the onset of hydration products. On the basis of hydration heat and XRD results, the addition of nano-ZnO inhibited the hydration of C3S and C2S in the cement paste and prolonged the induction period, which further contributed to the increase of setting time. Moreover, the initial and final setting times of cement paste increased dramatically as the nano-ZnO was further added. The retardation times compared with the R mix are shown in Fig. 4. The greatest setting time was observed for the Z3 mix, which had an initial and final setting time of 1240 and 1380 min, respectively.

3.2. Mechanical properties The compressive and flexural strengths were measured on 1, 3, 7, 14, 28, and 56 days, with the average of at least 6 specimens for each group. The average compressive strength of the cement paste at different curing ages is shown in Fig. 5. It can be observed that the strength of mixtures containing nano-ZnO was improved with curing age. The results generally indicated that the compressive and flexural strengths decreased markedly with the addition of nano-ZnO after 1-day curing. Because the addition of nano-ZnO

The amount of heat released as cement hydrates is referred to as the heat of hydration and can serve as an indication of the early age hydration process. The incremental and cumulative curves of heat flow versus hydration time obtained from the isothermal calorimetry are shown in Fig. 6. The results indicated that compared with the hydration heat evolution process of the blank R sample the addition of nano-ZnO particles can elongate the induction period and increase the exothermic rate of the second hydration heat evolution peak. For the R sample, the acceleration period began at approximately 2.9 h after the introduction of the water and ended at approximately 14.3 h. When the nano-ZnO content raised from 0% to 0.2 wt%, the occurrence of the hydration heat peak was retarded. For the Z3 mixture, the acceleration period began at 24.4 h and ended at 37.3 h. The duration of the induction period was prolonged significantly. As shown in Fig. 6(a), slope calculation of the linear portion of the acceleration period reveals that increased nano-ZnO results in a higher rate of acceleration, and nano-ZnO particles act as a ‘‘delayed accelerator.” Meanwhile, the second exothermic peak of cement paste became narrower and higher with the increasing addition of nanoparticles. Although peak height after retardation increased with the addition of nano-ZnO, the cumulative heat of samples containing nano-ZnO was slightly lower than in the R sample. Thus, the use of nano-ZnO can be advantageous in retarding the heat reaction and reducing the heat evolution at the early age of hydration.

3.4. Electrical resistivity The resistivity of fresh cement paste changes during the hydration process, and it can be applied to reflect the early hydration process of the cement paste. Electrodeless resistivity curves of cement paste with different nanoparticles are shown in Fig. 7, and the setting time determined by Vicat needle measurement is also labeled in the resistivity-time curves. It is apparent that the start of the acceleration period was postponed when nano-ZnO was incorporated, and the resistivity of the cement paste with nano-ZnO was obviously lower than that of the R group at the same age. The increase rate of the resistivity of the hydrating cement paste with and without nano-ZnO is shown in Fig. 8. In accordance with the findings of Wei and Li [33], setting time and critical points (Pm, Pa, and Pi) are labeled in Fig. 8, and four stages (I, II, III, and IV) are divided based on these points. The electrical properties of these 4 periods are described as follows:

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(b) 12

90

R Z1 Z2 Z3

Compressive strength (MPa)

80 70

R Z1 Z2 Z3

10

Flexural strength (MPa)

(a)

60 50 40 30 20

8

6

4

2 10 0

1d

3d

7d

14d

28d

0

56d

1d

3d

7d

14d

28d

56d

Age

Age

Fig. 5. Compressive and flexural strength of cement pastes with and without nano-ZnO.

(b)

7

R Z1 Z2 Z3

6

Heat flow (mW/g)

5

300

R Z1 Z2 Z3

250

Cumulative heat (J/g)

(a)

4 3 2 1

200 150 100 50

0

0

0

5

10

15

20

25

30

35

Time from mixing (hour)

40

45

50

55

0

5

10

15

20

25

30

35

Time from mixing (hour)

Fig. 6. Heat evolution of cement paste blended with nano-ZnO: (a) heat flow; (b) cumulative heat of hydration.

Fig. 7. Resistivity-time curves of cement paste with nano-ZnO (a) and partial enlargement (b).

40

45

50

55

J. Liu et al. / Construction and Building Materials 217 (2019) 352–362

357

Fig. 8. The resistivity differential curves of the cement pastes.

(1) The dissolution stage (Stage I) is from the mixing time to the minimum point Pm, which corresponds with the minimum point on the electrical resistivity curve. In this period, the electrical resistivity of cement paste decreases due to the increase of ionic concentrations, such as K+, Na+, Ca2+, SO24 and OH . The dissolution of these ions improves the conductivity of composites. (2) The setting stage (Stage II) is from point Pm to the acceleration point Pa. Point Pa relates to the end of the setting period, and it correspond to the first peak point on the differential curve. In this period, the initial cement hydration reaction consumes some ions and the resistivity of the sample increases a little [34]. At the same time, cement paste starts to stiffen and gain strength. (3) The acceleration stage (Stage III) is from point Pa time to the second peak point (Pi) time. In this period, the rate of cement hydration increases sharply because of the rapid growth of hydration production. (4) The deceleration stage (Stage IV) is after the critical point of Pi. In this stage, the hydration rate of the cement deceased with the increase of ages, and the reaction is diffusion controlled. Fig. 9 shows that the addition of nano-ZnO increased the duration of Stages I, II, and III significantly. Notably, the duration of Stage I for the R group lasted about 1.9 h, which was approximately 16.8 h lower than that of the Z3 group. Also, Fig. 7 shows the contribution of each stage to the total time of Stages I, II, and III. It shows that the duration of Stage I increased along with the increase of nano-ZnO dosage. Meanwhile, the proportion of Stages

Fig. 9. Contribution of each stage to the total time of stage I, II, and III.

II and III duration decreased with the addition of nano-ZnO over 0.05 wt%. It can be concluded that the addition of nano-ZnO delays the dissolution of cement particles. Furthermore, the initial and final setting times of the cement paste incorporating nano-ZnO were also postponed. As the hydration proceeds, the fractions of the hydration products are increased to form skeletons. In addition, these products would block the path of ion movements and improve the bulk electrical resistivity of the mixture. Therefore, electrical resistivity development is comparable with setting time for the paste. Fig. 10 shows that there was a linear relationship between point Pa and the setting time.

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Fig. 10. Relationship between the point Pa and setting times.

3.5. X-ray diffraction analysis The crystal phases of the cement paste with different nano-ZnO at 1 day and 28 days were tested using XRD (Fig. 11). It was found that the R1 and Z3 mixes were composed mainly of unhydrated cement phase at 1 day. Moreover, the diffraction peak intensity of C3S and C2S in the Z3 sample was higher than that of the blank sample, which was due to the retardation of the hydration reaction by nano-ZnO. However, the XRD peak of Z3 mix at 1 day did not show an appearance of the ZnO phase, which could have been due to the low addition of nano-ZnO in this study. Furthermore, the Ca(OH)2 phase was detected in Z3 and R samples at 1 day and 28 days. The XRD patterns of R and Z3 were almost the same at 28 days, which implied that the addition of nano-ZnO did not significantly influence the hydration degree of the cement paste at a later age. 3.6. Scanning electron microscopy Scanning electron microscopy was utilized to analysis the morphology of samples from designed mixes, and the chemical composition of the crystals was detected using Energy-dispersive X-ray spectroscopy (EDX) technique (Figs. 12, 13 and Table 4). After 1 day of curing, ettringite needle-shaped crystals were identified in the Z3 samples, particularly around large pores and air voids. The microstructure represented poor hydration and unhydrated cement grains. It resulted in a decrease in the mechanical properties of the sample. By contrast, the cement paste microstructure without nano-ZnO particles was found to be denser. This suggested that the early hydration of cement paste was stunted by the addition of nano-ZnO. Fig. 12(f) shows the micrographs of the mixture, which contain 0.2 wt% nano-ZnO. Compared to the control paste, the microstructure was denser and compact at 28 days. In addition, a relatively homogeneous matrix was observed with more small pores distributed in it. Table 4 shows the element content of various regions in Fig. 12, measured by energy dispersive spectroscopy (EDS). The results showed that nano-ZnO had good dispersion, and no aggregation was found. 3.7. MIP analysis The results of MIP of the R and Z3 samples at different ages are shown in Table 5 and Fig. 14. Compared to the blank samples, the cement paste with 0.2 wt% nano-ZnO was found to have a much

Fig. 11. Comparison of XRD patterns of mix at 1 and 28 d.

higher average pore diameter (63.7 nm) and total porosity (39.3%) after 1 day of curing. In general, ZnO retards cement hydration via the formation of a layer of crystalline at the onset of hydration products. On the basis of hydration heat and XRD results, the addition of nano-ZnO inhibited the hydration of C3S and C2S in the cement paste and prolonged the induction period, which further contributed to the increase of setting time. Evidence from electrical resistivity test showed that nano-ZnO delayed the dissolution of cement particles and restrained the dissolving of C3S at early age. Therefore, the addition of nano-ZnO delayed the early age hydration of the cement paste. With the increase of curing age, the total porosity of the R and Z3 samples at 28 days were 18.3% and 15.9%, respectively. In addition, the average pore diameter of the Z3 sample was slightly lower than that of the blank sample. This indicated that the pore structure of the cement paste was refined by the presence of nano-ZnO at 28 days. It also explained why these nanoparticles had a positive impact on the long-term strength development of the cement paste.

4. Discussion In this work, the retardation of early cement hydration in the presence of nano-ZnO was assessed in detail by isothermal calorimetry and resistivity tests, providing evidence for evaluating the kinetics of the early hydration. From Fig. 6, the duration of the induction period increased with the further addition of nano-ZnO, and the acceleration period of Z3 mixture even began at 24.4 h. Similarly, the results of electrodeless resistivity test (Fig. 7) also showed that the start of the acceleration period of cement paste was postponed more than 20 h when 0.2 wt% nano-ZnO was incorporated. Furthermore, XRD test results indicated that nano-ZnO inhibited the hydration of C3S and C2S in the cement paste. Based on the above results, the addition of nano-ZnO retarded the early hydration reaction, and prolonged the setting times of the nanoZnO modified cement paste. As a result, the strength of cement paste decreased markedly with the addition of nano-ZnO after 1day curing, as shown in Fig. 5. In the induction period, the rapid hydration rate decreased within the first several minutes, and then the hydration rate remained at a very low rate for hours [35]. The mechanisms of the induction period are still being debated. It was once widely accepted that the protective layer formed on the cement grain is the cause for the induction period [36]. The dissolution of the cement grain is inhibited by the layer. But in recent years, a geochemical theory of dissolution was proposed to explain the induc-

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Fig.12. SEM images of R sample at 1 d (a), 7 d (b) and 28 d (c), and Z3 at 1 d (d), 7d (e) and 28 d (f).

Fig. 13. EDS spectra of area I, II, and III in Fig. 12.

Table 4 EDS test results. Area

Element (At%)

I II III

Ca

O

Si

Al

Mg

Zn

42.6 30.8 31.8

40.1 52.6 50.0

15.2 10.9 9.7

1.2 2.6 5.2

0.5 1.2 1.8

0.1 1.9 1.3

Table 5 MIP analysis of R and Z3 samples. Sample

R Z3

Nnao-ZnO (wt%)

0 0.2

Total pore area (m2/g)

Average pore diameter (nm)

Total porosity (%)

1d

28 d

1d

28 d

1d

28 d

34.7 63.7

24.9 23.9

35.3 39.3

18.3 15.9

25.8 15.4

16.2 15.0

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(a)

(b) 0.30

R at 1 day Z3 at 1 day R at 28 days Z3 at 28 days

0.4

Cumulative Intrusion (mL/g)

dV/dlogD Pore Volume (mL/g)

0.5

0.3

0.2

0.1

R at 1 day Z3 at 1 day R at 28 days Z3 at 28 days

0.25 0.20 0.15 0.10 0.05 0.00

0.0 10

100

1000

10000

10

Pore Diameter (nm)

100

1000

10000

Pore Diameter (nm)

Fig. 14. Pore size distribution for R and Z3 samples.

tion period, which was in agreement with the experimental observations [37]. In geochemical theory, the dissolution rate is only related to the undersaturation of the solution with respect to the dissolving phase. During the first several minutes, the concentration of the ions in the pore solution increased rapidly, so the undersaturation was decreased and could not overcome the activation energy for the creation of etch pits on the surface of cement grains. Hence, the dissolution was slowed down. At the end of the induction period, a critical supersaturation for Ca(OH)2 or C-S-H was achieved, and then the hydration products started to precipitate and grow rapidly [35,36]. Macro-properties of cement-based materials are closely related to the structure of hydration products (mainly C-S-H) at nanoscale. In this study, nano-ZnO particles had a high surface area and chemical reactivity, and they could dissolve in the pore solution or could react rapidly with cement hydration products. Fig. 6 shows that the addition of nano-ZnO particles prolonged the induction period. However, the underlying mechanism is also controversial. Some researchers have proposed that the presence of Zn2+ in the pore solution would lead to the formation of an amorphous Zn(OH)2 layer on the surface of cement grains [38– 40]. This layer would reduce the dissolution rate of cement and thus inhibit hydration. When the concentrations of Ca2+ and OH in pore solution are high enough, the amorphous Zn(OH)2 layer will be transferred into a crystalline calcium zinc hydroxide (CaZn2(OH)6
2H2O), which will destruct the amorphous Zn(OH)2 layer and provoke further hydration [40]. Other researchers found that crystalline CaZn2(OH)6
2H2O existed in 28-day cured cement paste incorporating Zn2+ using FTIR [21,22,41,42], and they believed that it was the CaZn2(OH)6
2H2O layer that retarded the hydration of the cement paste. No matter what kind of layer it was, all these studies considered that the formation of the lowor non-permeable layers was the cause of the retarding effect of Zn2+. However, Ataie et al. [38] found that, when Zn2+ was presented, the Ca2+ concentration was even higher than that in the pore solution of plain cement paste during the induction period, which meant that the dissolution of the cement grains was not inhibited, but actually promoted. Hence, it was proposed that the retarding effect of Zn2+ was caused by the poison effect, which delayed or blocked the nucleation of the hydration products. Compared with the plain cement paste, the poison effect improved the supersaturation of Ca(OH)2 and C-S-H in pore solution, hence once the poison effect was overcome, the nucleation and growth rate would be faster than that in plain cement paste. In our study, this phenomenon was also observed with isothermal

calorimetry (Fig. 6). The heat peak height was increased and the hydration peak width was narrowed, which indicated a faster nucleation and growth of hydration products. Nevertheless, the results the electrical resistance tests can’t be interpreted with the poison effect. Figs. 7 and 8 show that during the dissolution stage (Stage I), the ions dissolved from the cement grains and the electrical resistance of the pastes reduced. The lowest point of the electrical resistance corresponded to the age when the ion concentration in the pore solution reaches the critical (super) saturation for generation of hydration products. Then, the hydration began at a very low rate, because the aforementioned two possible mechanisms: inhibiting layer and geochemical theory [35]. Fig. 7 shows the dissolution process was inhibited by the addition of nano-ZnO particles. With the increase of nano-ZnO particles content, the lowest resistance (end of Stage I) was raised, which implied that, after initial dissolution, the ions concentrations in the pore solution actually reduced when Zn2+ was presented. Moreover, when 0.1 wt% and 0.2 wt% nano-ZnO particles were added in the paste, the resistance rose a little once the cement initially contacted with water, after which the resistance started to decline slowly. This finding suggested that an inhibiting layer might be formed right after the mixing, and this layer reduced the dissolution rate of the cement grains. The results of isothermal calorimetry and resistance tests were quite different, with respect to the hydration rate during the acceleration period. The results of isothermal calorimetry tests (Fig. 6) showed that the heat release rate of the pastes was improved with the addition of nano-ZnO particles, while the results of resistance tests (Fig. 8) presented that the resistance increase rates of the pastes with nano-ZnO particles were lower than that of the reference one. The results were rather contradictory. The hydration heat was an index of the hydration degree, and the resistance was determined by the microstructure of the cement paste and the ion concentration in the pore solution. The hydration process of the pastes in isothermal calorimetry and resistance tests was almost the same, since both temperatures were controlled at 20 . Hence, during the acceleration period, the hydration rate (or the microstructure development) of the pastes with nano-ZnO particles was actually higher than the reference one according to the isothermal calorimetry results. Nevertheless, the resistance increase rates of the pastes with nano-ZnO particles were lower than the reference one. This suggested that the ion concentration in the pore solution of the pastes with nano-ZnO particles was higher than that in reference one, which resulted in the lower resistance increase rate.

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In general, the layer mechanism and poison mechanism may both take effect during the hydration of the cement paste with Zn2+. Due to the presence of Zn2+, a Zn(OH)2 layer or CaZn2(OH)6
2H2O layer was formed on the surface of the cement grains after the mixing, and the dissolution of cement grains was inhibited. Hence, the resistance of the pastes reduced very slowly. The layer mechanism was dominant during the dissolution period. At the end of the induction period, the poison mechanism may take over. Fig. 8 shows that at the beginning of Stage III (the acceleration period), the resistance increase rate of the pastes with nano-ZnO particles was below 0 for 1–2 h, which meant that the resistance of this paste was actually reducing during this period. The rise of the ion concentrations in pore solution should be the reason for this reduction. The rise of the ion concentrations in pore solution may aim to overcome the poison effect of the Zn2+. For the reference paste, the resistance increase rate was reduced close to 0, but not minus, and the resistance raised constantly. The reason why the Zn2+ poisoned the nucleation sites is still not well understood. Ataie et al. [38] suggested the underlying mechanism was similar to that for sucrose [43]. Zn2+ could be absorbed on the nucleation sites, preventing further precipitation of the hydration products. If the nucleation sites were poisoned, Ca(OH)2 and C-SH had to nucleate independently in pore solution, which required more energy than nucleating at the existing nucleation sites. Hence, a higher supersaturation had to be achieved for the further hydration of cement pastes with Zn2+. 5. Conclusions The addition of nano-ZnO hinders the early hydration of cement paste dramatically. The initial and final setting times of the cement paste with 0.2 wt% nano-ZnO were improved by 4 times and 3.5 times, respectively. Although the early age compressive and flexural strength of cement paste with nano-ZnO was rather low, it was found to gain higher strengths after 7 days. Nanoparticles improved the pore diameter distribution and formed a compact microstructure in the cement paste at 28 days. Nano-ZnO particles show typical characteristics of a cement hardening retarder, and the greater the amount of nanoparticles, the greater the retardation effect. Both the resistivity tests and the isothermal calorimetry results revealed that the induction period was extended by adding nano-ZnO particles. These results indicate that the layer mechanism and poison mechanism may both take effect during the hydration of cement paste with Zn2+. Declaration of Competing Interest None. Acknowledgements The authors would like to acknowledge the financial supports provided by National Natural Science Foundation of China (No. 51708501, 51708502 and 51778583). References [1] Y. Reches, Nanoparticles as concrete additives: review and perspectives, Constr. Build. Mater. 175 (2018) 483–495. [2] V. Vishwakarma, D. Ramachandran, Green concrete mix using solid waste and nanoparticles as alternatives – a review, Constr. Build. Mater. 162 (2018) 96– 103. [3] D. Kong, S. Huang, D. Corr, Y. Yang, S.P. Shah, Whether do nano-particles act as nucleation sites for csh gel growth during cement hydration?, Cem Concr. Compos. 87 (2018) 98–109. [4] Q. Ye, Z. Zhang, D. Kong, R. Chen, Influence of nano-sio addition on properties of hardened cement paste as compared with silica fume, Constr. Build. Mater. 21 (3) (2007) 539–545.

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