In situ-grown carbon nanotubes enhanced cement-based materials with multifunctionality

Journal Pre-proof In situ-Grown Carbon Nanotubes Enhanced Cement-Based Materials with Multifunctionality Mimi Zhan, Ganghua Pan, Feifei Zhou, Renjie Mi, Surendra P. Shah PII:

S0958-9465(20)30009-3

DOI:

https://doi.org/10.1016/j.cemconcomp.2020.103518

Reference:

CECO 103518

To appear in:

Cement and Concrete Composites

Received Date: 24 July 2019 Revised Date:

5 November 2019

Accepted Date: 7 January 2020

Please cite this article as: M. Zhan, G. Pan, F. Zhou, R. Mi, S.P. Shah, In situ-Grown Carbon Nanotubes Enhanced Cement-Based Materials with Multifunctionality, Cement and Concrete Composites, https:// doi.org/10.1016/j.cemconcomp.2020.103518. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.

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In situ-Grown Carbon Nanotubes Enhanced Cement-Based Materials with

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Multifunctionality Mimi Zhan a,b, Ganghua Pan*a,b, Feifei Zhou a,b, Renjie Mi a,b, Surendra P. Shah c,d

3 a

4

School of Materials Science and Engineering, Southeast University, Nanjing 211189, P. R. China

b

5

Jiangsu Key Lab of Construction Material, Southeast University, Nanjing 211189, P. R. China

6

c

7

d

Center for Advanced Construction Materials, University of Texas at Arlington, Arlington, TX 76019, USA

Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208, USA

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Abstract: Smart cementitious materials integrated with carbon nanotubes (CNTs) have potential

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applications as sensors in structural health monitoring (SHM). The sensitivity to strain (gauge factor)

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and strength of such materials are limited by the difficulty in dispersing CNTs. Here we synthesized

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CNTs in situ on the surface of fly ash (FA) to significantly improve the CNT dispersibility and enable

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the cement mortar to exhibit an outstanding strain-sensing capability. The mortar with CNT-coated FA

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(CNT@FA) at 2.0 wt.% CNT concentration had a gauge factor of 6544, about one order of magnitude

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higher than that of mortar with commercial CNTs under the same condition. Its electrical resistivity can

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reversely vary as high as 69% upon cyclic compressive loading. The great self-sensing ability of

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cement mortars reinforced with in situ-grown CNTs was explained by two mechanisms: 1) the high

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possibility of the breakup/formation of CNT conductive paths provided by the unique morphology of

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CNT@FA; 2) high ratio of tunneling resistance with respect to the total resistance caused by the good

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dispersion of CNTs, which is demonstrated by optical microscopy measurements. The compressive and

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flexural strength values of the mortars with CNT@FA are also higher than those of the plain mortar at

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an age of 28 days. The CNT@FA mortars with enhanced electrical and mechanical properties have

*

Corresponding author. Tel: +86-13357827675, E-mail: [email protected] 1

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potential applications in assessing the conditions of civil engineering structures.

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Keywords: in situ-growth, carbon nanotubes, cement mortar, piezoresistivity, compressive strength,

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flexural strength

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1. Introduction

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With low cost and high compressive strength, cement-based materials have become the most

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commonly used structure materials for the construction of civil infrastructure, including buildings,

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bridges, and roads [1–6]. However, cementitious materials are characterized as quasi-brittle materials.

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Therefore, structures made from these materials are susceptible to cracking during their service life.

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Furthermore, due to environmental loads, fatigue, caustic effects and material aging, civil engineering

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structures easily suffer damage [7]. Thus, the strength of these structures gradually decreases over their

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service life. To assess these damages and make appropriate decisions to keep structures in good service,

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structural health monitoring (SHM) has been introduced especially for crucial components. SHM is

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able to provide real-time data about the condition of structures by using a network of sensors that

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measures parameters, such as displacement, strain, and temperature [8]. For a conventional SHM

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system, the sensors, which are important components in SHM and are generally made of materials

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vastly different from concrete, are embedded or applied in the structures to form a self-sensing network

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for health monitoring. Such sensors are usually incompatible with cementitious materials, thereby

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leading to reduced strength and durability. Therefore, developing cement-based sensors seems to be a

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feasible way to solve the incompatibility problem; if the sensors are made of the same materials as the

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structures they are monitoring, the sensors would possess certain advantages, such as low cost, high

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durability, large sensing volume and no mechanical property loss [9–10].

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Generally, self-sensing functionality can be given to cement-based materials through the addition of 2

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conductive fillers. With conductive fillers, the electrical conductivity of cement-based materials

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significantly increases and could vary with external stress levels or deformations, i.e., the materials

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have piezoresistivity [11]. Carbon fibers (CFs), steel fibers, nickel powders and carbon nanotubes

47

(CNTs) are usually used as conductive fillers to produce self-sensing cement-based materials [12–14].

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Among these fillers, CNTs are considered one of the most promising fillers for fabricating cement

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sensors due to their excellent mechanical, electrical, and other physical properties [9, 15–17]. It has

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been reported that adding CNTs into conventional concrete not only enables the resulting concrete to

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have the ability to sense strains, stresses, cracks, damage, or temperature but also maintains or even

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improves the mechanical properties of the concrete [18–22].

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Li et al. [23] used multiwalled CNTs (MWCNTs) treated with acid solutions to fabricate piezoresistive

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MWCNT/cement composites and measured the piezoresistivity of these composites under uniaxial

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compression. Later, Azhari [8] investigated the piezoresistive behavior of CF and MWCNT hybrid

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cement composites with different CF/MWCNT ratios under uniaxial compression and studied the

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relationship between the fractional change in electrical resistance and compressive stress/strain. Han et

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al. [24] developed self-sensing CNT/cement composites to study the feasibility of traffic monitoring,

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such as vehicle detection, weighing, and speed measurement. More recently, Konsta-Gdoutos [19, 25,

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26] reported that cement composites reinforced with 0.1 wt.% CNTs and CNFs exhibited excellent

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piezoresistivity, as indicated by the amplified sensitivity of the material in strain sensing. Although it

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has been demonstrated that self-sensing cement can be produced through the addition of CNTs, the

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piezoresistive properties of CNT-reinforced nanocomposites vary over a wide range due to the complex

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piezoresistive mechanism and unstable dispersion of CNTs in cement matrix. Uniformly distributed

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CNTs in cementitious systems are vital to creating a continuous electrical network that can exploit the 3

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excellent electrical properties of CNTs to stably capture or sense strain changes in real-life structures

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[27]. Many different CNT dispersion technologies have been reported, and these technologies can be

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classified into chemical or physical techniques [28]. The basic physical approach is ultrasonication,

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which is often used in combination with surfactants. Chemical approaches contain covalent treatment

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(CNT functionalization) and noncovalent treatment (CNTs are physically attached to dispersing agents)

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[29, 30]. However, long-term ultrasonication for good dispersion is prone to inducing CNT damage

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[31], surfactant methods could influence the formation of conductive networks in the matrix because

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surfactant molecules block the connectivity of CNTs [29], and functionalization may introduce

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structural defects to CNTs [19, 28, 32]. Therefore, improving the dispersion of CNTs without

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introducing a detrimental effect on CNT properties and developing more stable and sensitive CNT

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cement sensors are still in demand.

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Consequently, the strain-sensing ability of cement mortars reinforced with in situ-grown CNTs was

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investigated in this study. There have been some studies on cement composites with CNT/cement or

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CNT/sand hybrids, but they all focused on mechanical improvement [1, 33–36]. Investigation of the in

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situ-grown CNT/cement sensors is still at an early stage. Therefore, CNTs were first synthesized in situ

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on the surface of fly ash (FA) particles in 30–40 s using a one-spot microwave heating method, and

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then these particles were incorporated into cement mortars to explore the feasibility of strain sensing.

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Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetric

84

analysis (TGA) were used to characterize the CNT-coated FA (CNT@FA). The conductivity and

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dispersion state of the CNT@FA were measured and compared with commercial CNTs to illustrate

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their possible influences on piezoresistivity. Then, monotonic and cyclic compressive loading were

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exerted on CNT@FA/mortar composites to observe the electrical resistivity variations with stress/strain. 4

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The same experiments were also carried out on plain mortars without CNTs and commercial

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CNT/mortar composites to better evaluate the enhancement effect of in situ-grown CNTs in terms of

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piezoresistivity. Finally, the working mechanism behind the piezoresistive behavior of CNT

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nanocomposites was explored, and the effects of CNT@FA on the mechanical properties of mortars

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were investigated.

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2. Experimental Procedure

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2.1 CNTs

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The CNT@FA was first fabricated with a modified poptube method [37, 38] and then incorporated into

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the mortar matrix to enhance the multifunctionality of the CNT/mortar composites. In a typical

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fabrication process, conductive polymer-polypyrrole (PPy) was first synthesized with pyrrole to fully

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cover the FA and form PPy-coated FA. Then, the conducting PPy-coated FA was blended with

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ferrocene and metal wires. Upon microwave irradiation, the PPy-coated FA and copper wires (i.e., the

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conductive materials) would serve as triggers, and their coupled effect could produce an intensive spark,

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which rapidly increased the temperature above 1100℃, where the ferrocene could be decomposed to an

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iron catalyst and cyclopentadienyl served as a carbon source for CNT growth. The CNT@FA was

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successfully synthesized within 30–40 s in a microwave oven without the need for any inert protection

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and additional feed stock gases, which are usually required in the conventional method such as

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chemical vapor deposition (CVD) [37]. The detailed synthesis procedure can be seen in ref. [37–39].

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The as-received CNT@FA was characterized using SEM (Sirion) and TEM (Tecnai G20) techniques,

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and the results are shown in Figure 1. Figure 1 (A) shows that highly dense and fluffy CNTs that were a

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few micrometers long were successfully grafted onto the surface of the FA. This phenomenon is more

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obvious in the TEM image presented in Figure 1 (B). The high-resolution TEM images (inset) 5

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confirmed the hollow inert nature and well-developed graphitic sheets of the synthesized CNTs. A

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statistical analysis showed that the outer diameters of the synthesized CNTs ranged between 30 and 100

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nm.

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Figure 1 SEM images of (A) the as-received CNT@FA and (C) commercial MWCNTs, wherein the inset shows

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high-magnification SEM images. TEM images of (B) the as-received CNT@FA and (D) commercial MWCNTs, wherein the

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inset shows high-resolution TEM images.

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The morphologies of the carboxyl-functionalized MWCNTs (C-MWCNTs) purchased from Aladdin are

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shown in Figure 1 (C) and (D). SEM inspection typically reveals agglomerated C-MWCNTs whose

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geometry can be approximately defined as ellipsoidal due to the attractive van der Waals forces. The

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TEM images in Figure 1 (D) show the multiwalled characteristics of C-MWCNTs. The outer diameters

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of the C-MWCNTs are greater than 50 nm with lengths ranging from 10 to 20 μm.

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TGA was conducted in an atmosphere of air after SEM and TEM characterization to investigate the

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purity of the C-MWCNTs and the weight ratio of the in situ-grown CNTs with respect to the CNT@FA.

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The mass variations in the FA, CNT@FA and C-MWCNTs with respect to temperature are depicted in 6

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Figure 2. As the temperature increased to 1000℃, physically adsorbed water was initially evaporated at

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100℃, followed by the burning of amorphous carbon particles at approximately 200–500℃. The CNTs

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were decomposed at 500–600℃, and the subsequent mass loss in the range of 600–800℃ was caused by

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the decarbonation of calcium carbonate (CaCO3). Therefore, according to the weight loss at 500–600℃

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in the TGA curves, it can be concluded that the purity of the C-MWCNTs is above 90%, and the

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concentration of synthesized CNTs is 33% by weight of the CNT@FA. This value will be used in the

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calculation of CNT usage during mortar preparation.

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Figure 2 TGA results of FA, commercial MWCNTs and CNT@FA.

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2.2 Dispersion of CNTs and Preparation of Mortar Specimens

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C-MWCNTs and in situ-grown CNTs were added to the mortar matrix at 0.4 wt.%, 0.8 wt.%, 1.2 wt.%

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and 2.0 wt.% by weight of the binder. The detailed usages of CNT@FA to achieve concentrations of

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0.4 wt.%, 0.8 wt.%, 1.2 wt.% and 2.0 wt.% of in situ-grown CNTs were calculated with the weight

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ratio of the in situ-grown CNTs with respect to the CNT@FA obtained from the TGA test. Due to the

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key role of the dispersion of CNTs in the formation of conducting networks, ultrasonication was

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adopted to achieve a homogeneous dispersion in the mortar matrix. The C-MWCNTs and CNT@FA

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were first mixed with water and then sonicated at 20 kHz using an ultrasound processor (IL100–6/1–1) 7

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at maximal power (700 W) for 30 min. TEM examination showed that several CNTs were shortened

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after ultrasonication (see Note 1 and Figure S1 in the supplementary information). The dispersion states

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of the C-MWCNTs and CNT@FA were quantitively evaluated by using a combination of optical

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microscopy and ImageJ software (National Institutes of Health, USA) on the 2.0 wt.% C-MWCNTs

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and CNT@FA suspensions after ultrasonication. For each suspension, 25 optical microscopy images

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(100× magnification) were taken with 5 representative drops on clean glass slides, resulting in a total

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investigated area of 17 mm2. The obtained optical microscopy images were processed with ImageJ to

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analyze the CNT agglomerates [40]. The dispersion sate of the CNTs was evaluated by plotting the area

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ratio of the CNT agglomerates to the total investigated area for each area class.

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The ready-dispersed CNT suspensions were used for mortar preparation. During mortar casting, P. O

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42.5 Portland cement was used, and the same amount of FA in the CNT@FA mortar proportion was

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also added to the C-MWCNT mortar as a reference. The fine aggregate used to prepare the mortars was

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dry river sand with dimensions of 0.15–4.75 mm, a specific gravity of 2.61 g/cm3 and a water

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absorption of 1.2%. Note that 0.1–0.3 wt.% of polycarboxylic acid-based superplasticizer (by weight of

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the binder) was used to compensate for the workability loss due to the different additions of CNTs. In a

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typical fabrication process, cement, FA and sand were first added to a rotary mixer and mixed in a dry

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state at low speed. Then, the ready-dispersed CNT suspensions and superplasticizer were added into the

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cement and sand at a constant water-to-binder ratio of 0.6 and a sand-to-cement ratio (s/c) of 3.0. All

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the materials were mixed in a rotary mixer at high speed. The ready-mixed materials were poured into

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40×40×160 mm3 plastic molds. To measure the electrical properties, two brass foils (60 × 20 × 0.2 mm3)

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were immediately embedded into the freshly mixed specimens to provide the external electrodes for the

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four-probe technique, which was implemented via Kelvin four-terminal sensing with Kelvin clips. The 8

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DC electrical resistance of each specimen was then measured with a Keithley 2100 multimeter. A

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detailed setup for the electrical property measurement and circuit diagram for the four-probe technique

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with Kelvin clips is shown in Figure 3. It is clear that the current and voltage were separately measured

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in outer and inner circuits with Kelvin clips so that the contact resistance was eliminated [41–45]. To

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ensure the test accuracy, for each CNT addition, three samples with brass foils were cast for electrical

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property testing, and another three samples without brass foils were cast for mechanical testing. All

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samples were placed in an electric vibrator for good compaction, surface-smoothed and covered with

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plastic film. After curing at room temperature for 24 h, the specimens were demolded and cured in a

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moist room at 20±1℃ and a relative humidity ≥95% for 28 days.

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Figure 3 (A) Setup for electrical property measurement. (B) Schematic illustration of circuit diagram for four-probe technique

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with Kelvin clips.

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3. Results

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3.1 Conductivity of CNTs

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The conductivities of the CNT-coated FA and commercial CNTs measured by the four-probe technique

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are shown in Table 1. The conductivities of these two types of CNTs are on the same order of

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magnitude, which are 4.7 S/cm for the CNT@FA and 8.1 S/cm for the commercial CNTs. It has been

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reported that the electrical properties of CNTs are related to their structures, which were elucidated via

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Raman spectroscopy in this study [46]. The spectra information and corresponding numerical results 9

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are presented in Figure 4 and Table 1, respectively. As Figure 4 shows, the Raman spectra of

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carbonaceous materials exhibit two characteristic peaks at approximately 1345 cm-1 (D-band) and 1576

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cm-1 (G-band). The G-band is assigned to the tangential (in-plane) mode of a well-ordered structure

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associated with sp2 carbon atoms in the graphene sidewalls, whereas the D-band is induced in a double

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resonance process, which is caused by disordered sp2 carbon defects (e.g., sp3 carbon atoms) in the

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CNT sidewalls and may also indicate the presence of amorphous carbon or graphitic materials [47, 48].

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In principle, the content and types of defects in the CNTs were evaluated based on the locations of the

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D- and G-bands and the ratio of ID/IG [46]. Table 1 clearly shows that the CNT@FA produced by

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microwave heating has a higher ID/IG ratio (0.72) than the commercial CNTs (ID/IG=0.68), which is also

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clearly indicated by the D-band intensity in Figure 4. Microwave heating also leads to in situ-grown

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CNTs with much wider D- and G-bands than the commercial CNTs (see Figure 4). These results

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suggest that the CNTs produced by microwave heating have a greater number of defects than the

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commercial CNTs. It is known that the wall of CNTs is built by π electron delocalization from the sp2

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hybrid orbital [49]. The defects caused by the rehybridization (from sp 2 to sp3) could result in the

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localization of π electrons and are responsible for the reduced conductance of the CNTs [49, 50]. A

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recent article also correlated conductivity with the Raman spectroscopy-derived ID/IG ratios of CNTs

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and found that an increased ID/IG ratio led to a decrease in conductivity [47, 51]. However, the

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contribution of defects in the tube walls and other forms of carbon, such as rings, to the D-band is still

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not completely understood and requires further in-depth study [52].

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Table 1 Electrical properties and structural information of CNT@FA and commercial CNTs. Full width at half FWHM of G-band Sample

Conductivity (S/cm)

maximum (FWHM) -1

(cm-1)

ID/IG

of D-band (cm ) Commercial CNT

8.1

55.56

52.72

0.68

10

CNT@FA

200

4.7

159.88

88.36

0.72

Figure 4 Raman spectra of CNT@FA and commercial CNTs.

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3.2 Dispersion of C-MWCNTs and CNT@FA in Aqueous Solution

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As the electrical properties of CNT-filled composites are dependent on the conductive network and the

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intrinsic CNT conductivity, the investigation of the dispersion/agglomeration of CNTs controlling the

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formation of conductive networks is important [53–55]. Therefore, the dispersion state of C-MWCNTs

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and CNT@FA in aqueous solutions was quantitively compared by a combination of optical microscopy

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and ImageJ. Typical optical microscopy images of C-MWCNT and CNT@FA suspensions with 2.0 wt.%

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CNTs and their quantitative information are presented in Figure 5. Figure 5 (A) shows that several

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large CNT agglomerates can be observed when 2.0 wt.% C-MWCNTs were used to prepare the

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suspension. Correspondingly, the quantitative analysis in the right figure shows that most of the

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C-MWCNTs reagglomerated into fragments with an area greater than 500 μm2 after sonication,

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accounting for 10.25% of the total area; the average area of CNT agglomerates is approximately 1367

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μm2. These results suggest that the C-MWCNTs are difficult to homogenously disperse in water with

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ultrasonication because the highly attractive van der Waals interactions enable the CNTs to

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reagglomerate into bundles and aggregates. However, Figure 5 (B) shows that the large agglomerates 11

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disappear, and only dozens of small black stains that are uniformly dispersed in the solution can be

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observed in the CNT@FA suspension. The ImageJ analysis in the right figure shows that there are no

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CNT particles larger than 300 μm2 after the sonication process. Most of the observed CNT@FA

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agglomerates have an area less than 100 μm2. Furthermore, the area ratio of CNT@FA agglomerates

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greatly decreased with increasing area, suggesting a low content of large agglomerates. The average

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area of CNT@FA agglomerates is approximately 52 μm2, which is two orders of magnitude lower than

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that of the C-MWCNTs. The above results suggest that the CNT@FA has a better dispersion state and

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stability than C-MWCNTs. For a better evaluation, the dispersion results of CNT@FA were then

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compared with ref. [56], where 0.038 wt.% of functionalized CNTs were dispersed in water by using a

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combination of polycarboxylate-based cement superplasticizer (as a surfactant) and sonication. The

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optical microscopy image of CNTs obtained under the optimal dispersion condition in ref. [56] was

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also processed with ImageJ by using the same method, and the results are shown in Figure 5 (C). The

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optical microscopy image qualitatively shows that the CNTs were distributed mainly on the right area

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of the image with several large agglomerates. ImageJ analysis revealed that CNT particles larger than

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500 μm2 have the highest area ratio (1.9% of the total area) among all the area classes. With a much

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lower CNT concentration, the average area of CNT agglomerates is approximately 179 μm2, which is

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higher than that of the CNT@FA. These data further confirm the great dispersion of CNT@FA. The

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reduced contact area between adjacent CNTs due to the presence of FA particles and the strong

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interfacial strength between CNTs and FA particles are the main reasons for the improved dispersion of

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CNTs in solution. The CNT@FA acted as a whole and homogenously dispersed in the water due to the

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good dispersibility of FA. Therefore, it is reasonable to speculate that the CNT@FA is more likely to

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form a good conducting network from the above optical microscopy results. 12

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Figure 5 Optical microscopy images (left) and corresponding histograms (right) of the CNT agglomerate area with respect to the

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total area for each area class: (A) 2.0 wt.% C-MWCNT suspension, (B) CNT@FA suspension with 2.0 wt.% CNTs, and (C)

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0.038 wt.% CNT suspension from ref. [56] (Reprinted from Carbon, 85, Bo Zou, Shu Jian Chen, Asghar H. Korayem, Frank

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Collins, C.M. Wang, Wen Hui Duan, Effect of ultrasonication energy on engineering properties of carbon nanotube reinforced

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cement pastes/3.1. Effect of UE on the dispersion of CNTs in aqueous solution, Pages 215, Copyright (2014), with permission

242

from Elsevier).

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3.3 Piezoresistive Properties of the Mortar Specimens

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The piezoresistive behaviors of the mortar specimens at an age of 28 days were investigated by

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measuring the resistivity change with the four-probe method (to eliminate contact resistance, as shown 13

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in Figure 3) under monotonic uniaxial compressive loading. Before piezoresistive testing, the

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polarization effect on all mortar specimens was investigated without load, and the results are shown in

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Figure S2 (see the supplementary information). The piezoresistive experiments were then conducted

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after the electrical resistance became stable (approximately 1 h for most specimens and 2 h for some

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specimens in this study) to prevent unwanted fluctuations induced by polarization. During

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piezoresistive testing, the compressive loading was exerted on the specimens in the direction

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perpendicular to the embedded electrodes with an electrohydraulic servo fatigue testing machine

253

(INSTRON 8802–10T). The experimental setup is shown in Figure 6 (A). The applied compressive

254

loading gradually increased up to 7.5 MPa at a rate of 120 N/s to ensure the elastic deformation of the

255

specimens during the sensing measurement (Figure 6 (B)). The strain change was recorded with two

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foil stain gauges attached to the opposite sides of the specimens along the loading direction during

257

testing. The electrical resistance was measured with a Keithley 2100 multimeter. To eliminate the

258

influence of the nature and geometry of the sensors, the resistance measurements were converted to

259

electrical resistivity (ρ) calculated as resistance per unit length:

260

ρ = 𝑅𝑆⁄𝑙 ,

(1)

261

where ρ is the electrical resistivity (Ω·cm), R is the electrical resistance (Ω), S is the cross-sectional

262

area (cm2), and 𝑙 is the length between the inner electrodes (cm). Then, the fractional change in

263

resistivity (FCR) is calculated by dividing the difference between the values of resistivity at each time

264

point and the initial resistivity, Δρ, by the value of the initial resistivity, ρ0.

14

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Figure 6 (A) Experimental setup for piezoresistivity testing. (B) Stress-strain relationship of plain mortar specimen under

266

monotonic compressive loading with an amplitude of 7.5 MPa.

267

The relationship between strain and electrical resistivity variations in the mortar specimens under

268

monotonic compressive loading is presented in Figure 7. For the plain mortar without CNTs, Figure 7

269

(A) clearly shows that the FCR with respect to strain is nearly zero, indicating that plain mortar is not

270

applicable for strain sensing. With the addition of CNTs, the resistivity of the mortars apparently

271

decreases as the strain increases, showing a visible piezoresistive response to compressive loading.

272

When the stress reaches 7.5 MPa, the resistivity of the mortars reinforced with different concentrations

273

of CNT@FA decreases by 26% to 71%, much higher than the 0.92% to 5.72% resistivity change

274

exhibited by the mortars reinforced with C-MWCNTs. The electrical resistivity of the CNT@FA

275

mortars seems to vary more sensitively than that of the C-MWCNT mortars. The strain-sensing

276

sensitivity is generally measured using the gauge factor (GF), which represents the relative change in

277

electrical resistivity due to mechanical resistance. The resistance change appears to arise from the

278

summation of the dimensional change (∆𝑅𝐷 ) and the relative change in the intrinsic resistivity of the

279

composite (∆𝑅𝑙 ) [57]. Therefore, the GF can be described as follows:

15

280

GF =

𝑑𝑅/𝑅 𝑑𝑙/𝑙

= ∆𝑅𝐷 + ∆𝑅𝑙 = 1 + 2𝜈 +

𝑑𝜌/𝜌 𝜀𝑙

,

(2)

281

where 𝑅 is the steady-state material electrical resistance before deformation, 𝑑𝑅 is the resistance

282

change caused by the deformation in length 𝑑𝑙, 𝜈 is the Poisson’s ratio of the composite, 𝜌 is the

283

resistivity, and 𝜀𝑙 is the strain [8, 11, 28, 58, 59]. The piezoresistive effect is represented by 𝑑𝜌/𝜌 to

284

eliminate the geometric contribution. The GF is then obtained by applying Eq. (2) via fitting with a

285

linear regression in the linear part of the resistivity change-strain curve. Consistent with the low

286

resistivity change, the fitted GF of plain mortar is only 8.36. The incorporation of CNTs significantly

287

increases the GF, as shown in Figure 7 (A)–(D). This finding further demonstrates that the visible

288

piezoresistivity stems from the added conductive CNTs. The good linear relationship between the FCR

289

and compressive strain of the mortars with C-MWCNTs provides GF values of 23.62, 96.90, 163.24

290

and 124.33 at CNT concentrations of 0.4 wt.%, 0.8 wt.%, 1.2 wt.% and 2.0 wt.%, respectively. When

291

the concentration of C-MWCNTs is 1.2 wt.%, the GF reaches a maximum value of 163.24.

292

Nevertheless, the relationships between the FCR and compressive strain of the CNT@FA mortars are

293

nonlinear. The linear fitting of the initial part response within 100 με offers GF values of 740.1, 874.32,

294

1082.51 and 6544.25 for CNT@FA mortars with 0.4 wt.%, 0.8 wt.%, 1.2 wt.% and 2.0 wt.% CNTs,

295

respectively. These GF values are much higher than those of the mortars with C-MWCNTs,

296

demonstrating the much higher sensitivity of CNT@FA mortars when the strain is less than 100 με. The

297

highest GF value of 6544.25 for the CNT@FA mortar with 2.0 wt.% CNTs indicates its excellent

298

strain-sensing capability under the initial compression. This phenomenon is likely due to the better

299

contacts between the CNTs and the matrix when the specimens are initially deformed [60]. To evaluate

300

the general sensitivity of the CNT@FA mortars during the entire loading process, the average GF

301

values were calculated with the total electrical resistivity change and deformation with Eq. (2), and the 16

302

results are shown in Table 2. Table 2 shows that the fitted GF values of the initial response are slightly

303

lower than the calculated average GF values due to the lower initial sensitivity, except for the mortar

304

with 2.0 wt.% CNTs. However, the difference is within the margin of error. This finding means that the

305

CNT@FA truly endows cement mortars with a highly sensitive strain-sensing ability in the whole

306

elastic regime. However, for CNT@FA mortars with 2.0 wt.% CNTs, the difference between the

307

average GF value and fitted GF value is significant. This phenomenon occurs because the FCR-strain

308

relationship of the CNT@FA mortar with 2.0 wt.% CNTs can be approximated by a bilinear curve. The

309

initial linear region at a low strain within 100 με has a steep gradient, but the gradient drops by nearly

310

an order of magnitude when the strain exceeds 100 με. Therefore, the GF value is remarkably high,

311

wherein the GF is on the magnitude of 103 for the CNT@FA mortar with 2.0 wt.% CNTs at low strains

312

and then drops to a magnitude of 102 due to the small further improvement in the contact between

313

CNTs [60]. Nevertheless, the average GF value of 1170.63 is still the highest among these mortars,

314

indicating the great potential of the CNT@FA mortar with 2.0 wt.% CNTs in strain measurements

315

within elastic deformation. The high GF value of 6544.25 in the initial part suggests the outstanding

316

strain-sensing capability of this mortar when the strain is less than 100 με.

17

317

Figure 7 FCR versus strain for mortar specimens at different CNT concentrations under a monotonic uniaxial compressive

318

loading with an amplitude of 7.5 MPa.

319

Table 2 Average GFs and fitted GFs of CNT@FA mortars with different CNT concentrations. GF

0.4 wt.%

0.8 wt.%

1.2 wt.%

2.0 wt.%

Average GF

743.70

887.20

1101.54

1170.63

Fitted GF

740.10

874.32

1082.51

6544.25

320

After monotonic compressive loading, 18 cycles of repeated compressive loading with an amplitude of

321

7.5 MPa were exerted on the mortars at a loading rate of 120 N/s to investigate the piezoresistive

322

stability and repeatability. The resistivity change in the mortars under cyclic compressive loading and

323

the corresponding average results of three samples for each CNT concentration are shown in Figure 8

324

and Table 3, respectively. Figure 8 shows that the electrical resistivities of all mortars decrease with an

325

increase in compressive load upon loading and increase to the initial values upon unloading during

326

each compressive loading cycle. The average maximum FCR of plain mortar in Table 3 is 0.28±0.02%

327

under cyclic loading, further indicating negligible piezoresistive behavior. However, the FCR of 18

328

C-MWCNT mortars can reach maximum values of 0.90±0.52%, 2.44±0.69%, 4.23±1.25%, and

329

3.65±0.90% at 0.4 wt.%, 0.8 wt.%, 1.2 wt.% and 2.0 wt.% CNT concentrations, respectively. The FCR

330

of the CNT@FA mortars reached maximum values of 34.82±1.01%, 24.36±1.61%, 41.93±4.33%, and

331

69.11±0.86% at 0.4 wt.%, 0.8 wt.%, 1.2 wt.% and 2.0 wt.% CNT concentrations, respectively. These

332

large resistivity changes in the CNT-reinforced mortars further demonstrated the enhancement effect of

333

CNTs in terms of piezoresistivity. At the same concentration of CNTs, the maximum FCR values of the

334

CNT@FA mortars are one order of magnitude higher than those of the C-MWCNT mortars. The FCR

335

values can reach the highest value of approximately 70% at a 2.0 wt.% CNT concentration. These high

336

resistivity changes under cyclic loading further indicate the great piezoresistivity of the CNT@FA

337

mortars. In addition, the large resistivity change of 70% suggests the amplified sensitivity of the

338

CNT@FA mortar with 2.0 wt.% CNTs in strain sensing, which is consistent with the result of

339

monotonic compressive loading. In addition, it is clear in Figure 8 that, compared with plain mortar and

340

mortars with C-MWCNTs, the mortars reinforced with CNT@FA exhibit more constant resistivity

341

changes in each loading-unloading cycle without the interference of noise. It should be noted that the

342

obtained data were not processed by filtration. This finding indicates that the mortars reinforced with

343

CNT@FA have more stable piezoresistive behaviors than the other mortars; therefore, the mortars

344

made with CNT@FA are more suitable to act as sensors for strain measurement. The noise signal in

345

mortars with 0.8 wt.% and 1.2 wt.% C-MWCNTs may be caused by the unstable conductive network in

346

the matrix [61–65]. Reports have shown that the weak nanotube-matrix interface can lead to unstable

347

conductive paths during loading-unloading cycles [66, 67]. The high amount of superplasticizer added

348

in the mortars with 0.8 wt.% and 1.2 wt.% C-MWCNTs also possibly blocks the contacts among CNTs

349

and causes insufficient CNT connectivity [68, 69]. With increasing CNT content, a stable conductive 19

350

network can be formed by CNT-CNT contacts [66]. For the mortar with CNT@FA, the stable

351

piezoresistive response can be attributed to the strong nanotube-matrix interface and the great

352

dispersion of the CNTs. The monotonic and cyclic loading experiments demonstrate that CNT@FA

353

provides a much better piezoresistivity than normal commercial CNTs.

354

Figure 8 Piezoresistive behaviors of mortars with 0 wt.%, 0.4 wt.%, 0.8 wt.%, 1.2 wt.% and 2.0 wt.% CNTs under cyclic

355

compressive loading.

356

Table 3 Average of the maximum FCR of the mortars upon cyclic compressive loading from three measurements. Average of the maximum absolute FCR obtained in cyclic loading (%) Mortar type 0 wt.% Mortar with C-MWCNTs

0.4 wt.%

0.8 wt.%

1.2 wt.%

2.0 wt.%

0.90±0.52

2.44±0.69

4.23±1.25

3.65±0.90

34.82±1.01

24.36±1.61

41.93±4.33

69.11±0.86

0.28±0.02 Mortar with CNT@FA

357

To further confirm the great ability of CNT@FA in inducing piezoresistivity, Table 4 clearly presents

358

the comparison result of the sensitivity among different cement-based materials obtained from the

359

literature. We can see that the mortar with CNT@FA exhibited the highest GF, outperforming all other

360

specimens, not only the C-MWCNT mortars in this study. When comparing the results of the 20

361

C-MWCNT mortars in this study with those in ref. [70] and [28], it seems that the lower values of FCR

362

of the C-MWCNT mortars may be due to poor dispersion of C-MWCNTs. It should be noted that the

363

results in ref. [28] shows that only 0.05% well-dispersed CNTs can result in a GF of 240 as opposed to

364

a GF of 163.24 for 1.2 wt.% C-MWCNTs. This finding means that the poor dispersion of C-MWCNTs

365

was an important factor. The efficiently changeable conductive network induced by well-dispersed

366

CNT@FA leads to the outstanding piezoresistive response of mortars to strain. In general, the working

367

mechanism behind the piezoresistive behaviors of the CNT/cement strain sensors has been explained in

368

terms of three main aspects: (1) the destruction/formation of CNT conductive paths; (2) the variations

369

in the distance between adjacent CNTs, inducing changes in the tunneling resistivity; and (3) the

370

inherent piezoresistivity of the CNTs [71, 72]. When subjected to compressive strains, the number of

371

CNT-to-CNT contacts increases to form more conductive paths, and the gaps between the CNTs

372

decreases, leading to the occurrence of the tunneling effect, thereby causing a decrease in electrical

373

resistance, i.e., the piezoresistive phenomenon [71, 72]. Therefore, increasing the possibility of the

374

breakup/formation of CNT contacts or the occurrence/absence of the tunneling effect during the

375

deformation leads to a higher sensitivity. The unique morphology of CNT@FA provides many possible

376

locations for triggering the CNT contacts and the occurrence of tunneling effect upon deformation. As

377

Figure 9 shows, in a complete conductive path formed by CNT@FA, there should be much more CNT

378

junctions or contacting points compared with a conductive path constructed by commercial CNTs.

379

Therefore, the probability of breakup of this path or the tunneling effect for the path containing the

380

CNT@FA should be higher than that of the commercial CNTs. Consequently, it can be estimated that a

381

nanocomposite sensor made with the CNT@FA can possess a higher sensitivity than a sensor made

382

with normal commercial CNTs. On the other hand, with the uniform distribution of CNT@FA (as 21

383

shown in Figure 5), it is highly possible that a large number of CNTs on the surface of FA are separated

384

with a close distance, increasing the ratio of tunneling resistance with respect to the total resistance of

385

the CNT@FA nanocomposites. This behavior is based on Simmons’s theory for tunneling resistance,

386

which is expressed as follows [73]: 𝑅𝑡𝑢𝑛𝑛𝑒𝑙 = 𝑉 ⁄𝐴𝐽 =

387

ℎ2 𝑑 𝐴𝑒 2 √2𝑚𝜆

𝑒𝑥𝑝 (

4𝜋𝑑 ℎ

√2𝑚𝜆),

(3)

388

where 𝐽 is tunneling current density, 𝑉 is the electrical potential difference, 𝑒 is the quantum of

389

electricity, 𝑚 is the mass of an electron, ℎ is Plank’s constant, 𝑑 is the distance between CNTs, 𝜆

390

is the height of barrier (for a cement-based material, this is 0.36 eV according to Wen and Chung) [54,

391

74], and 𝐴 is the cross-sectional area of the tunnel. Eq. (3) shows that the tunneling resistance between

392

CNTs decreases with decreasing distance 𝑑 of two CNTs as 𝑅𝑡𝑢𝑛𝑛𝑒𝑙 ∝ 𝑑𝑒𝑥𝑝(𝑐 × 𝑑), where 𝑐 is

393

constant. It is easily inferred that the tunneling resistance among CNTs decreases in an exponential

394

form with the compressive strain, which would be much more sensitive compared with the linear

395

piezoresistivity caused by the conductive paths of CNTs in contact [71, 75]. Therefore, the high

396

possibility of breakup/formation of CNT junctions and the high tunneling resistance ratio contribute to

397

the outstanding sensitivity of CNT@FA sensors. This finding agrees with those reported in Alamusi’s

398

study [71].

399

Table 4 Comparison of the sensitivity of different piezoresistive cement-based materials. Filler

Concentration

GF

Source

Commercial CNTs

1.2 wt.%

162.24

This paper

CNT@FA

2.0 wt.%

6544.25fitted/1170.63ave

This paper

CNT/nanocarbon black

2.4 vol%

704

[11]

CF

15 vol%

445

[8]

0.2 wt.%

626.05

[70]

0.1 wt.%

592.18

[26]

0.5 vol%

54

[76]

2.0 wt.%

220

[77]

0.05 wt.%

240

[28]

CNT

22

400

Figure 9 Schematic comparison of the possible CNT-to-CNT contacts or tunneling occurring in (A) CNT@FA/mortar composites

401

and (B) commercial CNT/mortar composites under compression.

402

3.4 Mechanical Properties of the CNT@FA-Modified Mortar Specimens

403

The mechanical properties should also be considered in developing cement sensors because the low

404

strength decreases the durability of sensors and restricts their application. Since there have been

405

inconsistent conclusions about the mechanical properties of CNT-reinforced cement composites, the

406

flexural and compressive strength of CNT@FA-reinforced mortars were measured at an age of 28 days.

407

The samples used for mechanical testing were cast without embedment of brass foils, and the results

408

were averaged from three measurements.

409

The average increases in compressive strength and flexural strength of the mortar specimens reinforced

410

with CNT@FA at an age of 28 days are presented in Figure 10. With the addition of CNT@FA, the

411

compressive and flexural strength of the mortars are enhanced, wherein these values first increase and

412

then decrease as the CNT content increases. For compressive strength, the mortars reinforced with

413

CNT@FA at concentrations of 0.4 wt.%, 0.8 wt.%, 1.2 wt.% and 2.0 wt.% exhibit 8.8%, 16.7%, 27.3%

414

and 6.2% higher strength than plain mortar, respectively. The flexural strength of the mortar specimens

415

reinforced with CNT@FA at concentrations of 0.4 wt.%, 0.8 wt.%, 1.2 wt.% and 2.0 wt.% are 11%,

416

17.5%, 3.2% and 1.6% higher than that of the plain mortar specimens, respectively. It is clear that the

417

incorporation of CNT@FA not only enables excellent piezoresistivity but also good mechanical

418

properties, which would largely widen the application of this cement sensor in SHM. Examples of 23

419

specific applications include monitoring the dynamic strain/stress of bridges and concrete structures

420

[78], monitoring traffic flows and weighing passing vehicles [24, 79, 80], and detecting damage

421

initiation and identifying fatigue damage in nanoconcrete beams [81–84]. An integrated system of

422

nanomodified cement-based sensors could also be used to monitor the real stress conditions before and

423

after a seismic event to promptly assess structural safety [61, 85, 86]. The nanopore filling effect and

424

the strong interfacial bonding strength between CNTs and the surrounding hydration product are

425

considered the main reasons for the improved strength of the nanocomposite [15, 87, 88]. The low

426

increase in mechanical strength of mortars containing 2.0 wt.% in situ-grown CNTs may be caused by

427

incomplete hydration as more water adhered to the CNTs [89, 90].

428

Figure 10 Effect of concentrations of in situ-grown CNTs on the (A) compressive strength and (B) flexural strength of mortar

429

specimens at an age of 28 days.

430

4. Conclusions

431

CNT@FA is successfully fabricated and incorporated into the mortar matrix to endow cement mortars

432

with great piezoresistivity. Conductivity testing shows that the conductivity of CNT@FA and

433

commercial CNTs are on the same order of magnitude. Upon monotonic compressive loading in the

434

elastic range, mortars containing CNT@FA exhibit a better sensitivity in strain sensing than the mortars

435

containing commercial CNTs. The linearity of the FCR-strain curves of the mortars with C-MWCNTs

436

shows that the GFs of these mortars are in the range of 23.62 to 163.24. However, for the mortars with 24

437

CNT@FA, the GFs provided by the linear regression within 100 με range from 740.1 to 6544.25. The

438

average GF values of the mortars with CNT@FA are in the range of 743.70 to 1170.63 during the

439

whole monotonic loading process. Furthermore, the CNT@FA mortars exhibit a stable and high

440

signal-to-noise piezoresistive response through 18 cycles of cyclic compressive loading within the

441

elastic range, wherein the maximum FCR values range from 24% to 69%. In contrast, the maximum

442

FCR values of the mortars reinforced with commercial CNTs are in the range of 0.9% to 4.23% due to

443

the poor dispersion of C-MWCNTs. The highest GFs are 6544 within 100 με and 1170 within the

444

whole elastic range and the maximum FCR value is 69%, which are the highest values currently

445

reported for CNT-reinforced mortars, are achieved by the mortars reinforced with 2.0 wt.% in

446

situ-grown CNTs; these findings suggest that this mortar exhibits the best available sensitivity among

447

mortars in terms of strain sensing. In addition, the mortars containing in situ-grown CNTs exhibit

448

higher compressive and flexural strength than plain mortar at an age of 28 days. The outstanding

449

piezoresistivity (GF of 6544 and FCR of 69%) and enhanced mechanical strengths (6.2% higher

450

compressive strength and 1.6% higher flexural strength) indicate the promising application of in

451

situ-grown CNTs in developing highly sensitive cement sensors as candidates for SHM in civil

452

infrastructure. Possible applications for these sensors include monitoring static and dynamic

453

stress/strain levels in civil infrastructure (e.g., dams, wind turbine bases, and transportation

454

infrastructure) [78], rapid assessment of post-earthquake damage and weigh-in-motion sensing for

455

traffic and crowd management [24, 61, 79, 80, 82, 91].

456

Acknowledgement:

457

This work was supported by the National Key Research and Development Program of China [grant

458

numbers 2017YFC0703100]. 25

459

The support from ARC Nanocomm Hub, Monash University to Northwestern University is

460

appreciated.

461

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Supplementary Information

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In situ-Grown Carbon Nanotubes Enhanced Cement-Based Materials with

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Multifunctionality Mimi Zhan a,b, Ganghua Pan

693

†a,b

, Feifei Zhou a,b, Renjie Mi a,b, Surendra P. Shah c,d

a

694

School of Materials Science and Engineering, Southeast University, Nanjing 211189, P. R. China

b

695

Jiangsu Key Lab of Construction Material, Southeast University, Nanjing 211189, P. R. China

696

c

697

d

Center for Advanced Construction Materials, University of Texas at Arlington, Arlington, TX 76019, USA

Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208, USA

698

Note 1:

699

Figure S1 shows the typical TEM images for commercial CNTs and in situ-grown CNTs (CNT@FA)

700

before and after ultrasonication. The dried un-sonicated CNT powder and sonicated CNT suspension

701

are imaged using transmission electron microscopy (TEM, here: Tecnai G20). Figure S1 shows that

702

several nanotubes were shortened after ultrasonication process. The high-magnification image of

703

C-MWCNT displays the damage process of commercial CNTs under ultrasonication that one-side walls

704

has been cut off after ultrasonication. The TEM image of CNT@FA shows that one single CNT has

705

open tip after sonication, indicating that it was damaged after ultrasonication. These results

706

demonstrate that ultrasonication process shortened CNTs to some extent.

†

Corresponding author. Tel: +86-13357827675, E-mail: [email protected] 37

707

Figure S1 TEM images of C-MWCNT and CNT@FA before and after ultrasonication.

708

Figure S2 Polarization effect on mortar specimens with/without CNTs under unloading.

38

The authors declare no conflict of interest