The mechanical properties and electrochemical behavior of cement paste containing nano-MgO at different curing temperature

Construction and Building Materials 164 (2018) 663–671

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

The mechanical properties and electrochemical behavior of cement paste containing nano-MgO at different curing temperature Shiqi Song a, Linhua Jiang a,b,c,⇑, Shaobo Jiang a, Xiancui Yan a, Ning Xu a,d a

College of Mechanics and Materials, Hohai University, Nanjing 210098, PR China Engineering Research Center on New Materials and Protection in Hydraulic, Jiangsu Province, 1 Xikang Rd, Nanjing 210098, PR China c National Engineering Research Center of Water Resources Efficient Utilization and Engineering Safety, PR China d Nanjing Hydraulic Research Institute, Nanjing, PR China b

h i g h l i g h t s  Nano-MgO was used as admixture to improve the mechanical properties and electrochemical behavior.  The flexural and compressive strength of cement paste containing nano-MgO cured at 60 °C was higher than that of 20 °C.  Hydration feature of cement paste containing nano-MgO cured at different temperature was studied by EIS test.  A suitable electrochemical equivalent model was applied to fit the measured electrochemical impedance data of cement paste.

a r t i c l e

i n f o

Article history: Received 27 June 2017 Received in revised form 27 November 2017 Accepted 2 January 2018

Keywords: Nano-MgO Electrochemical impedance spectroscopy Mechanical properties Cement paste Curing temperature

a b s t r a c t The purpose of the research work is to investigate the mechanical properties and electrochemical behavior of cement paste containing nano-MgO (NM) which cured at different temperature. The measurements of flexural strength, compressive strength and electrochemical impedance spectroscopy were conducted. A suitable electrochemical equivalent mode was applied to fit the measured electrochemical impedance data (Nyquist curve), therefore the electrochemical property for cement paste containing NM can be quantitatively obtained. Results demonstrated that the flexural and compressive strength gradually increased with the increasing of NM contents ranged from 0 wt% to 9 wt% in cement paste. Moreover, the strength of the cement paste containing NM was increased gradually when the curing temperature rising from 20 °C to 60 °C. In addition, with the increasing of NM content and the rising of curing temperature, the electrolyte solution resistance Rs and the ion transport processes resistance Rct1 showed increasing tendency. And XRD, TG-DSC and SEM were used to characterize the phases, mass/heat changes and microstructure of the hardened cement pastes, which provided some evidence on these research findings. Ó 2018 Published by Elsevier Ltd.

1. Introduction It is generally accepted that the durability of concrete is the major factor affecting the service life of engineering structures, and crack is one of the main reasons which causes the premature deterioration of concrete [1]. In recent years, the durability of cement-based composites has become more and more prominent [2–5]. Durability and service life of concrete structure will be greatly reduced with the generating of cracks, which will cause ⇑ Corresponding author at: College of Mechanics and Materials, Hohai University, Nanjing 210098, PR China. E-mail address: [email protected] (L. Jiang). https://doi.org/10.1016/j.conbuildmat.2018.01.011 0950-0618/Ó 2018 Published by Elsevier Ltd.

possible huge economic losses, even loss of human lives. It is an effective measure to prevent shrinkage and cracking of concrete by using the volume expansion caused by the swelling component in the hydration process to compensate for shrinkage, such as magnesium oxide (MgO) [6]. It was reported that the carefully calcined and sized MgO powders had the potential of being used as expansive agents for compensating thermal shrinkage and thus preventing cracks in mass concrete due to thermal stresses [7,8]. However the presence of excessive amounts of MgO in hardened cementbased materials may lead to expansion and crack formation, which will lead to the reduction of mechanical performances. In order to avoid the possible deleterious effects, most specifications have

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Table 1 Chemical compositions (wt%) of ordinary Portland cement.

*

Composition

CaO

SiO2

Al2O3

Fe2O3

MgO

SO3

Na2O

K2O

LOI*

Portland cement

64.65

21.71

5.06

4.33

0.94

1.07

0.20

0.54

0.86

LOI: loss on ignition.

placed a limit on the content of MgO that can be present in cement, and the MgO content does not exceed 6% generally [9–12]. In recent years, the technology of nano-structured material is developing astonishing rapidly and will be applied extensively to many materials in the future. Although cement is a kind of common building material, its main hydrate C-S-H gel is a natural nano-structured material [9]. The use of nanoparticles in concrete materials significantly modifies their behavior not only in the fresh but also in the hardened conditions, as well as the mechanical and microstructure development. Moradpour [13] studied the effects of nanoscale expansive agents on the mechanical properties of nonshrink cement-based composites, the results indicated that the mechanical strength of the composites mixed with the nanoMgO were higher than those of a plain composite and the autoclave expansions of Portland cement paste containing 3% and 5% nano-MgO were 0.10% and 0.12%, respectively. The researches indicated that the thermal conductivity of cement paste containing NM is higher than that of pure cement paste after heated [14]. Riza [15] reported the effects of different percentages of pre-saturated expanded perlite aggregate micro and nano size MgO on autogenous shrinkage of the mortars, and the results showed NM has appeared to be more effective than micro size MgO. A research results also indicated that the addition of the micro and nano MgO particles to poly methyl methacrylate have improved the quality of bone-cement union [16]. Ye [9] reported the maximum content of NM added into ordinary Portland cement reaches up to 8% for soundness, and the shrinkages of dam concrete with NM and light burnt MgO may be completely compensated in safety. In order to more effectively analyze microstructural changes include the hydration process and the durability experiment, electrochemical impedance spectroscopy (EIS) has been used to estimate the properties of concrete [17,18]. The cement-based material is a complex nonuniform multiphase system which consists of the solid phase, gas phase and liquid phase. In principle, the solid phase is generally considered to be an insulator, and the conductivity of a cement system is mainly contributed by the ions in the solution of continuous microspore network such as Ca2+ and OH [19–22]. With the action of a variable frequency electric field, these phases in the cement-based material are interconnected to constitute an integral circuit. Therefore, it was possible to estimate the composition and intrinsic microstructure of the cement paste by EIS. Based on the researches of the use of NM in cement-based composites, few studies have been reported on the mechanical performances and electrochemical behavior in assessment of cementbased composites containing NM at different curing temperature, but the effect of temperature on concrete such as dam concrete is very huge in practical engineering. In this paper, we propose to add NM into the ordinary Portland cement, and take different curing temperature (20 °C, 40 °C, 60 °C) for the cement samples, then the mechanical strength, electrochemical impedance spectroscopy and equivalent electrochemical circuit simulation are provided in the paper. In addition, the hydration phases, the mass/heat change and the microstructure of the hardened cement pastes are characterized by X-ray powder diffraction (XRD), ThermogravimetryDifferential Scanning Calorimeter (TG-DSC) and Scanning Electron Microscopy (SEM), respectively.

2. Materials and methods 2.1. Raw materials A commercial ordinary Portland cement (42.5 grade, Blaine specific surface area 310 m2/kg complying with Chinese standard (GB 175–2007) was used, its compressive and flexural strengths at the age of 28 days were 46.4 and 7.2 MPa, respectively. The chemical compositions of the cement are given in Table 1. NanoMgO (NM) particles were supplied by Hangzhou Wanjing New Material Co., Ltd. in China, with specific surface area 25 m2/g, average diameter 50 nm and bulk density 0.25 g/ml. Its mineral composition is presented in Fig. 1. 2.2. Preparation of cement paste For the cement paste, cement with NM was fully mixed under dry conditions beforehand. The cement paste was prepared using a planetary mixer (ISO 9597). A cement (containing NM) to water ratio 0.5 was used, together with different NM content of 0%, 3%, 5%, 7% and 9%, the mixture proportions of specimens are presented in Table 2. The fresh paste was cast into prismatic moulds 40 mm  40 mm  160 mm on a jolting table. The paste samples which were cured at 20 ± 2°C and about 95% RH moisture were demoulded at 24 h and then stored in saturated limewater at 20 °C, 40 °C and 60 °C until required for testing. 2.3. Strength test for cement containing nano-MgO The flexural and compressive strengths were obtained by Microcomputer electro- hydraulic servo pressure testing machine (HG-HY-300F). The span for flexural strengt was 100 mm and the area for compressive strength was 40  40 mm2. Three prisms were tested for each sample at the designated ages. The flexural strength test was carried out by a group of three samples, and then

Fig. 1. XRD patterns of nano-MgO.

S. Song et al. / Construction and Building Materials 164 (2018) 663–671 Table 2 Mixture proportion of nano-MgO particles blended cement paste (g). Mixture number

Water

Cement

NanoMgO

Nano-MgO content (%)

W/ C

PC NM3 NM5 NM7 NM9

600 600 600 600 600

1200 1164 1140 1116 1092

0 36 60 84 108

0 3 5 7 9

0.5 0.5 0.5 0.5 0.5

the compressive strength test was carried out by a group of six specimens which was broken in flexural strength test.

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station, a logarithmic sweep at 30 points was made over the frequency range 0.1 HZ to 100 KHZ at a sinusoidal potential perturbation of 5 mV. For each specimen, two stainless steel mesh electrodes (40 mm  50 mm) were embedded at two sides of the test samples as conductive device. And the schematic diagrams of samples for EIS tests are illustrated in Fig. 2. Before the EIS tests, the specimen was subjected to water retention treatment. During the test, the surface was wiped with a rag until saturated surface-dry state, then immediately followed by EIS tests. The Nyquist diagrams of specimens were analyzed by equivalent electrochemical circuit method with ZsimpWin software. 2.5. XRD analysis

2.4. Electrochemical impedance spectroscopy test The Electrochemical Impedance Spectroscopy (EIS) of cement containing NM was tested by PARSTAT 2273 electrochemical work-

For the XRD test, the cement pastes were selected from the broken pieces owing to the compressive strength measurement at some ages samples which were taken to grind into powder which could go through 0.16 mm screen, and then the powder was dried at the temperature of 60 °C for 24 h for further experiments. In the XRD test, the D8 ADVANCEX X-ray powder diffractometer from BRUKER AXS Company of German, Cu Ka, 40 kV voltage, 40 mA current, scanning angle range from 5° to 80°, scanning speed of 10°/min and step length of 0.02° were used. 2.6. TG-DSC analysis

Fig. 2. Schematic diagrams of samples for EIS tests.

The preparation of powder samples for TG-DSC test was the same as Section 2.5. The measurement of TG-DSC (STAR 409 PC,

Fig. 3. The strengths of cement pastes with different contents of nano-MgO (a: 28 d flexural strength, b:28 d compressive strength; c: 90 d flexural strength, d: 90 d compressive strength).

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NETZSCH, Germany) was carried out in N2 atmosphere at a heating rate of 20 °C/min ranging from room temperature to1000°C. 2.7. SEM analysis A small flake sample was typically taken from the cement paste specimen with 0.5 w/c ratio. With the JSM-6360LV Scanning Electron Microscope (SEM), the micro morphology of the cement hydration was analyzed in order to acquire the effects of the addition of NM particles on the micro structure behavior and the differences in the microstructure of the hardened pastes. 3. Results and discussion

when the content of NM was 5 wt%, the flexural strength of the composite pastes cured at 40 °C was 8.24 MPa, which was 18.2% higher than that of pure paste at the age of 28 days. As a matter of comparison, at the age of 90 days, the flexural strength of paste enriched with 5 wt% NM can reach 8.74 and 8.96 MPa at curing temperature of 40 °C and 60 °C, which was 20.7% and 23.8% higher than that of composite pastes cured at 20 °C, respectively. The results seemed to indicate that the hydration of NM does not harm strengths and the strengths develop regularly within 90 days cured at 20 °C, 40 °C and 60 °C. Appropriately increased the curing temperature can promote NM hydration reaction, and then generate more magnesium hydroxide, so that the pore structure of the samples will be improved.

3.1. Flexural and compressive strengths

3.2. Results of EIS tests

Fig. 3 presents the strengths of cement pastes containing NM cured at 20 °C, 40 °C and 60 °C in saturated limewater at the age of 28 days and 90 days. The flexural and compressive strengths increased with the content of NM increasing in generally at the same curing temperature. That was mainly because NM hydration products gradually increased with the increasing of NM contents, which could lead to a certain expansion that can improve the internal structure of cement paste, and reduce the pores, so that the structure will be more denser, so as to improve the mechanical strength of cement paste. In the present paper, the flexural and compressive strengths of the samples which was cured at 20 °C slightly increased by addition of NM, but the strengths were greatly improved by raising the curing temperature from 20 °C to 40 °C and 60 °C. For example,

The content of NM and curing temperature both have a considerable influence on EIS curves of cement paste. The Nyquist and the Bode diagrams of cement paste with different NM contents (0 wt%, 5 wt%, 9 wt%) which cured at 20 °C for 28 days are provided in Fig. 4. Moreover, the Nyquist and the Bode diagrams of cement paste containing 5 wt% NM with different curing temperature (20 °C, 40 °C, 60 °C) shown in Fig. 5. In the Nyquist plots, there is a capacitive loop which expressed as a small part of arc in the high frequency, it can be seen that the low frequency region is a straight line of 45°. Increasing of NM contents and changing the curing temperature both significantly vary the impedance properties of the samples. In the electrochemical system of cement paste, OH has a significant impact on the impedance. When NM hydration reaction is carried out in the cementitious materials, it can reduce

Fig. 4. The Nyquist and the Bode diagrams of cement paste with different nanoMgO contents (a: Nyquist diagrams, b: Bode diagrams).

Fig. 5. The Nyquist and the Bode diagrams of cement paste with different curing temperature at age of 28 days (a: Nyquist diagrams, b: Bode diagrams).

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the content of OH ions [6]. Also the impedance is linked to the content of unhydrated NM. In the Bode plots, the Bodes’ angle is described as the angle between the horizontal part of high frequency impendence and the oblique part of the low frequency impendence. For the plots, it can be seen that the impedance is significantly increasing with the increasing of NM content, and with the decreasing of the curing temperature, the impedance also significantly increasing. Furthermore, with NM content increasing and the curing temperature decreasing, the Bodes’ angle gradually reduced, that means the pore structure of the cement paste samples containing NM improved. The results indicated that the hydration of NM is helpful to improve the pore structure of cement paste.

3.3. Interpretation of EIS results Concerning the analysis of impedance data, equivalent electrochemical circuit is a convenient way to interpret the electrical parameters. In order to analyze the relationship more sufficiently between the topological structure of EIS and the hydration time, the equivalent electrochemical circuit which can be described as Rs(Q1(Rct1W1))(Q2 (Rct2W2)) according to the Circuit Description Code (CDC) was used in this paper, where the Rs is the resistance of the electrolyte solution, Q1 corresponds to the double layer capacitance between the solid/liquid phases, Rct1 stands for the resistance caused by ion transfer procedure inside the cement paste sample, Q2 stands for the double layer capacitance between cement mortar and electrodes, Rct2 stands for the resistance caused by the charge transfer procedure on the surface of the electrodes, W1 stands for the Warburg resistance caused by the ion diffusion procedure inside the cement paste, W2 stands for Warburg resistance caused by the ion diffusion procedure on the surface of the electrodes. In the equivalent electrical circuit, ZF1 = Rct1 + W1 represents the Faraday impedance caused by the Faraday’s procedure inside the paste, while ZF2 = Rct2 + W2 represents the Faraday impedance caused by the Faraday’s procedure between the paste and electrodes [20,23]. And the schematic diagrams of the equivalent electrochemical circuit is illustrated in Fig. 6. The Nyquist curve and the fitting curve of electrochemical impedance measurement shown in Fig. 7. It’s observed that the model of Rs(Q1(Rct1W1))(Q2 (Rct2W2)) is able to give a good fitting to the experimental points. In addition, the total error is less than 1.26%, illustrating the reasonable of the model to monitor the microstructure of cement paste containing NM. For this study, two parameters include Rs and Rct1 were used to estimate the changes of the porous microstructure of the cement paste. During the hydration process, the formation of amorphous C-S-H occurred in the pores, which leads to the ratio of connected pores to the total micro-pores changed. The resistance of the electrolyte solution Rs was used to characterize the total porosity, and

Fig. 6. The equivalent electrochemical circuit for cement paste.

Fig. 7. A typical Nyquist curve and the fitting curve by model Rs(Q1(Rct1W1))(Q2 (Rct2W2)).

the ion transport processes resistance in cement internal structure Rct1 which was inversely proportional to hydrated electrons in cement internal structure and OH ion concentration in pore solution, it also could represent the hydration degree. In order to study the hydration process of the cement paste containing NM with time which cured at different temperature, the Nyquist diagrams of cement paste with different NM content at various ages (28 and 90 days) during the hardening process are provided in Fig. 8. And the Nyquist diagrams of these specimens were analyzed by equivalent electrochemical circuit, the equivalent electrochemical circuit parameters Rs and Rct1 which was acquired by ZsimpWin software shown in Table 3. It can be seen that the value of Rs is gradually increasing with the increasing of hydration time, but also with the NM content increasing, and the increasing tendency of Rs is found out with the rising of curing temperature, which represents that the total porosity decreasing obviously with the NM content and hydration time increasing an the rising of curing temperature. Moreover, the value of Rct1 is also gradually increasing with the hydration time, the NM content increasing and the rising of curing temperature, which means that the ion concentration in cement paste internal structure reducing, the structure of the specimens becoming more dense.

3.4. XRD analysis Fig. 9 illustrates the XRD patterns of cement pastes containing 5 wt% NM which cured at 20 °C for 28 and 90 days. In addition, the XRD patterns of cement pastes containing 5 wt% NM after cured at different temperature for 28 days shown in Fig. 10. In the hydration process, the cement and NM in water tend to react and recombine to give a new mixture of phases, It is reported that MgO hydrates in solution of higher pH value to form brucite with tiny crystals, after cured in saturated limewater at 20 °C for 90 days, crystal hydrates such as portlandite, and brucite existed, and a-quartz and periclase(nano-MgO) also existed. The result shows that the hydration rate of NM cured saturated limewater at 20 °C was low, and a little of NM existed till the age of 90 days. Furthermore, with the rising of curing temperature, the speed of cement hydration was great improved, an obvious characteristic peak of brucite was found out when the curing temperature was 60 °C compared to 20 °C and 40 °C, the a -quartz had reacted with portlandite, and a little periclase still existed.

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Fig. 8. The Nyquist diagrams of cement paste with different nano-MgO content cured at different temperature (a: 20 °C 28 d, b: 20 °C 90 d; c: 40 °C 28 d, d: 40 °C 90 d; e: 60 °C 28 d, f: 60 °C 90 d).

Table 3 The parameters of Rs and Rct1 in the cement paste containing nano-MgO cured at different temperature. Paste code

Rs (ohms)

Rct1 (ohms)

20 °C

PC NM3 NM5 NM7 NM9

40 °C

60 °C

20 °C

40 °C

60 °C

28 d

90 d

28 d

90 d

28 d

90 d

28 d

90 d

28 d

90 d

28 d

90 d

1459 1889 1962 1895 2114

2953 3098 3132 3014 3276

1913 2180 2253 2196 2460

3085 3276 3462 3595 3614

2048 2223 2350 2346 2652

3153 3498 3532 3414 3776

358 365 396 419 435

1788 1932 2360 2423 2758

629 631 752 697 763

2448 2754 3492 3553 3672

701 737 756 768 786

2765 2914 3623 3406 3857

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Fig. 9. XRD patterns of cement pastes containing 5 wt% nano-MgO cured at 20 °C.

Fig. 11. TG/DSC curves of specimens containing 5 wt% nano-MgO cured at different temperature. (a: NM5-20 °C; b: NM5-40 °C).

Fig. 10. XRD patterns of cement pastes containing 5 wt% nano-MgO cured at different temperature.

As a matter of comparison, total weight loss of the specimen cured at 20 °C was 3.43%, which was slightly higher than cured at 40 °C. That may be caused by more magnesium hydroxide existed in the hydration of specimen cured at 20 °C. Meanwhile, the total weight losses of the composite pastes cured at 20 °C and 40 °C were 24.68% and 22.44%, respectively. 3.6. SEM analysis

3.5. TG/DSC analysis Fig. 11 shows the TG/DSC curves of cement paste containing 5 wt% NM particles at the age of 90 days which cured at 20 °C and 40 °C, the measurement temperature ranged from room temperature to 1000 °C in nitrogen atmosphere. The hydration products in cement system are quite complex. For both specimens, the endothermic effects at 20 °C-200 °C, 400 °C-480 °C, 650 °C-750 °C are clearly observed in DSC curves. The endothermic peaks at 20 °C-200 °C, 400 °C-480 °C, 650 °C-750 °C correspond to the dehydration of hydration products such as calcium silicate hydrates, calcium aluminium hydrates and ettringite; the decomposition of calcium hydroxide and magnesium hydroxide; and the decomposition of calcium carbonate and magnesium carbonate, respectively [6,24–30]. Based on the TG results indicated in Fig. 11, both specimens exhibit a remarkable mass loss at temperatures below 200 °C. The specimens cured at 20 °C and 40 °C had a crystal water content of 8.66% and 7.04%, respectively. Moreover, the mass losses are associated with the decomposition of magnesium hydroxide and calcium hydroxide at the temperature range of 400 °C to 480 °C.

The SEM analysis of the cement samples are performed to elucidate the microstructure of the hydrated cement paste containing NM, and the SEM images of pure cement paste (PC) and paste containing 5 wt% NM (NM 5) after cured at different temperature for 28 days shown in Fig. 12. First observation of the cement pastes cured at 20 °C, the cement hydration was normal, it can be found that lots of calcium silicate hydrate (C-S-H) and calcium hydroxide (Fig. 12(a)) were formed. A lot of magnesium hydroxide, ettringite, C-S-H and a little of calcium hydroxide were formed in the composite paste after the addition of NM particles (Fig. 12(d)). NM has resulted in the unusual features for the composite pastes compared with pure paste. Then the images of PC and NM5 specimen after cured at 40 °C shown in Fig. 12(b) and (e). In Fig. 12(b), the hydration products mainly contained C-S-H, calcium hydroxide and ettringite. At the same time, lots of magnesium hydroxide and C-S-H, little ettringite and calcium hydroxide were formed in Fig. 12(e). Due to the curing temperature up to 60 °C, lots of C-S-H and ettringite existed in the image of PC specimen, and the amount of brucite increased greatly and the stick-shaped crystal already could be seen very clearly in

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Fig. 12. SEM images of specimens PC and NM5 after cured at 20 °C, 40 °C and 60 °C for 28 days.

the image of NM 5 specimen, and the length of brucite was about 0.5–1 lm. The images of NM5 specimen with different curing temperature shown in Fig. 12(d–f), which was in accord with the XRD patterns shown in Fig. 10. The NM reacted to water to form magnesium hydroxide that cause d an expansion and compensated the shrinkage of cement paste [31], which explained one of the reasons of the mechanical and electrochemical properties increase reported above. 4. Conclusion The addition of NM particles of Portland cement paste significantly affected mechanical and electrochemical properties, and different curing temperature also have a great influence on flexural and compressive strength and electrochemical impedance spectroscopy of the specimens containing NM particles. The following conclusions may be drawn from the obtained experimental data: (1). The addition of NM particles to cement paste could improve the mechanical properties, which was mainly contributed by a certain expansion the production of hydrated product

magnesium hydroxide, and resulted an expansion simultaneously. In general, with the increasing of NM content in cement paste, the flexural and compressive strength gradually increased. When the NM content was 5 wt% which cured at 20 °C for 90 days, the flexural and compressive strength was 7.1% and 13.2% higher than that of pure paste, respectively. (2). Curing temperature had a huge effect to cement paste containing NM on mechanical properties. Because with the curing temperature increased appropriately, NM hydration product magnesium hydroxide gradually increased, which could lead to a certain expansion that can improve the internal structure of cement paste, so that the pore structure of the samples will be improved. When the cement samples cured at 60 °C for 90 days, the flexural and compressive strength of paste enriched with 5 wt% can reach 8.96 and 50.29 MPa, which was 19.20% and 3.50% higher than that of samples cured at 20 °C, respectively. (3). The resistance of the electrolyte solution Rs and the ion transport processes resistance Rct1 increased with the increasing of NM content and the rising of curing tempera-

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ture, which represents that in the macro performance, the flexural and compressive strength of cement paste increased with the addition of NM, and NM could decrease the total porosity and improve structure of the cement paste obviously in the micro performance. Along with hydration time increasing, the value of Rs and Rct1 also gradually increased. (4). The hydration rate of NM cured in saturated limewater at 20 °C was low, and a little of NM existed till the age of 90 days. So it could react to water to form magnesium hydroxide that caused a delayed expansion and compensated the shrinkage of cement in later age. Consequently, NM can be used as a new expansive agent for cement and concrete in the future. On the basis, the work of adding NM to improve the durability of cement and concrete still need to be carried out in the future.

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